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Development of Initiator Systems for Ultraviolet
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presented by

Lindsay Scott Coons

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DEVELOPMENT OF INITIATOR SYSTEMS FOR ULTRAVIOLET
PHOTOPOLYMERIZATION OF THICK POLYMERS AND COMPOSITES

By

Lindsay Scott Coons

A DISSERTATION

Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemical Engineering

1996

ABSTRACT

DEVELOPMENT OF INITIATOR SYSTEMS FOR ULTRAVIOLET
PHOTOPOLYMERIZATION OF THICK POLYMERS AND COMPOSITES

By

Lindsay Scott Coons

The general objective of this research project is the development of novel curing
strategies based upon photopolymerizations for high-speed, low-cost production of
polymeric composites. Developing new technology based upon photopolymerizations
could have a signiﬁcant impact in current polymer and composite processing methods. A
host of products and processes could be improved by incorporation of this technology
into their development. Although photopolymerizations offer several advantages over
current processing methods such as cure on demand, its use has been limited largely to
the production of thin ﬁlms, inks and coatings due to problems arising from the fact that
severe light intensity gradients exist initially in the bulk of thicker systems. Two
strategies for overcoming the limitations imposed on thick systems by the existence of the
light intensity gradient are disclosed here. The ﬁrst strategy combines careful selection of
the initiating wavelength with the reactive system including the resin/monomer, initiator
formulation and their relative concentrations to produce light intensity gradients that may
be described as “self-eliminating”. In the second strategy, a hybrid photo/thermal
initiating mechanism is employed where the heat evolved from the highly exothermic
photochemical reaction elevates the temperature of the system causing a second, thermal

reaction which results in the production of additional active sites. A number of important

operating conditions such as initiator formulation including concentration of thermal and
photoinitiators as well as photosensitizers, initiating light including wavelength, intensity
and illumination time, and ﬁber selection including ﬁber type and degree of loading have
been investigate to validate these novel strategies. These studies have successfully
demonstrated the potential of these polymerization strategies for the production of
composites and thick polymers. Prototypical composites and thick polymers in excess of
two centimeters have been cured in cycle times ranging from less than a minute to ten

minutes or less depending on the thickness of the composite.

To Julie and The Kernel.

iv

ACKNOWLEDGMENTS

I would like to thank the members of my doctoral committee, Dr. Jack Giacin, Dr.
Martin Hawley, and Dr. Andre Lee for their role in this research both in fonning the path
of the research and in insuring that I never strayed too far from that path. I would
especially like to thank Dr. Alec Scranton for serving as my graduate advisor throughout
this project. His contributions to the advancement of my professional career have gone
above and beyond the scope of this research and I will always look to him as a mentor in
the area of photopolymerizations.

I would like to thank the staff in the Chemical Engineering Department including:
Candy, for always having a fresh supply of coffee and donuts for seminars; Julie, for
turning in our paperwork so that we always got paid; and Faith, for being a young
“mother” to all the graduate students.

I would also like to thank the graduate members of ABSLAB including Julie
Jessop, Arvind Mathur and Shail Moorjani for their ongoing contributions and mutual
support both in and out of the lab. I would also like to thank the numerous undergraduate
members whose path I have crossed throughout the years for making the laboratory a fun
place to work. David Godshall deserves special mention for the outstanding

contributions he has made to this project.

Bharath Rangarajan was my closest cohort in this research and together we
conducted some of the most important research in this laboratory. We also teamed up to
carry out some of the least successful experiments in the laboratory, but his support was
constant regardless of the successes or failures of the particular day. I will undoubtedly
miss the morning and late night trips we took in search of hash browns and endless cups
coffee and/or alternative beverages. Some of these trips provided intellectual stimulation,
but mostly they provided caffeine.

I would also like to thank my family for their understanding and support
throughout my graduate career. Never once did anyone suggest I think about “getting a
real job” even when everyone around me seemed to be doing just that.

Finally, I would like to acknowledge the person who I am most fortunate to share
my life with, my wife Julie. Four years ago she uprooted herself and moved to Michigan
without a job or any friends and offered me support and encouragement even at times
when I might not have appreciated it. She is truly special, and I am forever grateful for

her kindness, compassion and unconditional love.

vi

TABLE OF CONTENTS

LIST OF TABLES ..................................................................................................... x

LIST OF FIGURES .................................................................................................... xi

CHAPTER 1

INTRODUCTION ...................................................................................................... 1
1.1. Characteristics and Advantages of Photopolymerizations ............................. 2
1.2. Applications of Photopolymerizations ........................................................... 3
1.3. Composite Materials ...................................................................................... 6
1.4. Necessity for Development of Thick Photopolymerization Methods ............ 8
1.4.1. Impact in Technology ........................................................................ 9

1.4.2. Impact in Science ............................................................................... 10

1.5. References ...................................................................................................... 11

CHAPTER 2

BACKGROUND ....................................................................................................... 13

2.1. Polymers and Polymerization Reactions ........................................................ 13

2.1.1. Classifying Polymers ......................................................................... 14

2.1.1a. Step Polymerizations ............................................................. 15

2.1.lb. Chain Polymerizations .......................................................... 15

2.1.2. The Radical Mechanism .................................................................... 17

2.1.3. The Cationic Mechanism ................................................................... 20

2.2. Photopolymerizations .................................................................................... 25

2.2.1. General Attributes of Photopolymerizations ...................................... 27

2.2.2. Traditional Applications for Photopolymerizations ........................... 28

2.2.3. Radical Photoinitiation Processes ...................................................... 29

2.2.3a. Photoinitiator Compounds .................................................... 29

2.2.3b. Kinetics of The Radical Photoinitiation Step ........................ 35

2.2.4. Cationic Photoinitiation Process ........................................................ 36

2.2.5. Radical and Cationic Propagation and Termination Processes .......... 38

2.3. Composites ..................................................................................................... 39
2.3.1. Methods for Producing Composites ................................................... 41

2.3.1a. Filament Winding .................................................................. 42

2.3.1b. Resin Transfer Molding ........................................................ 43

2.3.1c. Reaction Injection Molding ................................................... 46

2.3.1d. Hand Lay-up .......................................................................... 48

vii

2.3.2. Materials used in the Fabrication of Composites ............................... 49

2.4. References ...................................................................................................... 52
CHAPTER 3
RESEARCH OBJECTIVES ...................................................................................... 57
CHAPTER 4
DEVELOPMENT OF THE THICK PHOTOINITIATION STRATEGIES:
FUNDAMENTAL EXPERIMENTS ......................................................................... 59
4.1. Introduction .................................................................................................... 59
4.1.1. Self-Eliminating Light Intensity Gradients ........................................ 61
4.1.2. Photo/Thermal Initiation Mechanism ................................................ 65
4.2. Experimental .................................................................................................. 66
4.3. Optimizing the Initiator Formulation ............................................................. 67
4.3.1. Developing an Operating Window .................................................... 67
4.3.1a. Radical Systems .................................................................... 68
4.3.1b. Cationic Systems ................................................................... 71
4.3.2. Selection of Suitable Photoinitiators .................................................. 73
4.3.3. Effects of Hydrogen Donors .............................................................. 77
4.3.4. Effect of Thermal Initiators ................................................................ 79
4.4. Conclusions/Chapter Summary ...................................................................... 80
4.5. References ...................................................................................................... 93
CHAPTER 5
CHARACTERIZATION OF THE DYNAMIC LIGHT INTENSTTY GRADIENT95
5.1. Simulation of the Dynamic Light Intensity Gradient ..................................... 95
5.1.1. Elementary Reactions for a Constant Light Intensity ........................ 95
5.1.2. Light Intensity Proﬁles as a Function of Position and Time .............. 97
5.1.3. Simulation Results ............................................................................. 99
5.1.4. Discussion of Simulation Results ...................................................... 103
5.2. Experimental Characterization of the Light Intensity Gradient ..................... 104
5.2.1. Developing an Experiment Setup for Actinometry Studies ............... 106
5.2.2. Experimental ...................................................................................... 108
5.2.3. Experimental Results ......................................................................... 108
5.3. General Conclusions/Chapter Summary ........................................................ 110
5.4. References ...................................................................................................... 123
CHAPTER 6
DIFFERENTIAL SCANNING CALORIMETRY .................................................... 124
6.1. Introduction .................................................................................................... 124
6.2. Experimental Procedure ................................................................................. 126
6.3. Results and Discussion .................................................................................. 127
6.3.1. In Situ Characterization of the Reaction ............................................ 127
6.3.2. Post-Cure Studies ............................................................................... 129
6.3.3. Variable Effects .................................................................................. 130

viii

6.4. Chapter Summary .......................................................................................... 132

6.5. References ...................................................................................................... 137
CHAPTER 7
THERMAL AND MECHANICAL CHARACTERIZATION
OF POLYMER SYSTEMS ....................................................................................... 139
7.1. Introduction .................................................................................................... 139
7.2. Experimental .................................................................................................. 143
7.3. Results and Discussion .................................................................................. 146
7.3.1. Flexural Testing ................................................................................. 146
7.3.2. Thermal Analysis ............................................................................... 150
7.4. Research Conclusions/Chapter Summary ...................................................... 153
7.5. References ...................................................................................................... 161
CHAPTER 8
INVESTIGATION OF ADDITIONAL POLYMER SYSTEMS .............................. 162
8.1. Ladder Polymers ............................................................................................ 162
8.1.1. Novel Ladder Synthesis Method ........................................................ 165
8.2. Ladder Polymer Research Experiments ......................................................... 167
8.2.1. Theoretical prediction of Top .............................................................. 167
8.2.2. Thermal proﬁles ................................................................................. 170
8.2.3. Computer Modeling ........................................................................... 172
8.2.4. Conclusions of Ladder Polymer Research ......................................... 175
8.3. Interpenetrating Polymer Networks ............................................................... 175
8.3.1. Classes of IPNs .................................................................................. 178
8.3.2. IPN Experiments ................................................................................ 181
8.3.3. Discussion/Conclusions of IPN Research .......................................... 182
8.4. References ...................................................................................................... 189
CHAPTER 9
RESEARCH SUMMARY AND RECOMMENDATIONS FOR
FUTURE WORK ....................................................................................................... 191
9.1. Research Summary ........................................................................................ 191
9.2. Recommendations for Future Work ............................................................... 194
9.2.1. Development of Radical Systems ...................................................... 195
9.2.2. Fundamental Research into Cationic Systems ................................... 196
9.2.3. Investigation and Development of Potential Applications ................ 197

ix

Table 2.1.

Table 4.1.

Table 4.2.

Table 6.1.

Table 6.2.

Table 7.1.

Table 8.1.

Table 8.2.

Table 8.3.

Table 9.1.

LIST OF TABLES

Chapter 2

Compatibility of polymeric resins with reinforcement materials ............... 50
Chapter 4

Absorbance decay characteristics for various cationic photosensitizers ....... 72

Effect of initiator formulation on polymer thickness .................................... 74
Chapter 6

The effect of initiator and illumination time on the degree of cure for
low viscosity vinyl ester resin ...................................................................... 129

The effect of initiator and illumination time on the degree of cure for a

moderate viscosity vinyl ester resin .............................................................. 129
Chapter 7

Thermal expansion coefﬁcient as a function of initiator formulation

and cure time for vinyl ester polymers ........................................................ 151
Chapter 8

Constant values for theoretical calculation of Rp ........................................ 169

Total energy for various compounds as predicted by computer

modeling ..................................................................................................... 174

Type of chain polymerization undergone by various unsaturated

monomers .................................................................................................... 178
Chapter 9

Resins, photo and thermal initiators studied in the development of
the initiation strategies. ............................................................................... 194

Figure 2.1.

Figure 2.2.

Figure 2.3.

Figure 2.4.

Figure 4.1.

Figure 4.2.

Figure 4.3.

Figure 4.4.

Figure 4.5.

Figure4.6.

Figure 4.7.

LIST OF FIGURES

Chapter 2

Dissociation of initiator to produce radical by hydrogen abstraction
(a) and fragmentation (b). ........................................................................... 30

Termination by chain transfer of growing polymer chains by amines
via a hydrogen abstraction mechanism ....................................................... 33

Incorporation of N,N-dimethylaminoethyl methacrylate amines into

acrylate polymer backbone .......................................................................... 33

Diagram of physical properties for a number of commonly-used

polymeric resins .......................................................................................... 51
Chapter 4

Benzoin ethyl ether photoinitiator (a) and corresponding radicals
produced via H+ abstraction (b) and fragmentation (c) ............................... 64

UV-Vis absorbance spectra for typical reactive system including
monomer (Derakane 470-45), photo (BEE) and thermal (BP)
initiators ...................................................................................................... 82

UV-Vis absorbance spectra for BEE radicals generated via hydrogen
abstraction (—) and photofragmentation (--) reactions ................................ 83

Absorbance spectra for the camphorquinone / tributylarnine
coinitiator system ........................................................................................ 84

Absorbance decay of radical photoinitiators benzoin ethyl ether (at 328

nm) and camphorquinone/TBA (at 472 nm) as a function of time ................ 85
Absorbance spectra for cationic photosensitizers including perylene,

pyrene, and anthracene (both before and after photosensitization) ............. 86
Absorbance decay of anthracene at 364 nm as a function of time .............. 87

xi

Figure 4.8. The effect of BEE weight fraction on flexural modulus for unﬁlled
vinyl ester polymers photocured for 4 minutes ........................................... 88

Figure 4.9. Intensity of light as a function of thickness for increasing periods of
irradiation .................................................................................................... 89

Figure 4.10. Average absorbance at 328 nm for BEE(-) and BEE/TBAO initiator
formulations irradiated as a function of time .............................................. 90

Figure 4.11. Amount of time required to cure samples of varying thickness for a
number of initiator formulations including BEE (0.2 wt%) (0), BEE
(0.2 wt%)/BP (0.2 wt%) (0) and BEE (0.2 wt%)/BP (1.0 wt%) (A) ......... 91

Figure 4.12. Effect of light intensity and thickness on cure time for 411-c50 vinyl
ester polymers containing 0.2 wt% BEE and 0.2 wt% BEE / 0.2 wt%

BP initiator formulations ............................................................................. 92
Chapter 5
Figure 5.1. Ground-state initiator concentration of benzoin ethyl ether as a
function of illumination time. ..................................................................... 112
Figure 5.2. Static light intensity gradient (non-eliminating system) ............................. 113

Figure 5.3. Proﬁle of the light intensity for BEE assuming all of the active
radicals are produced via photofragmentation ............................................ 114

Figure 5.4. Proﬁle of the light intensity for BEE assuming all of the active
radicals are produced via hydrogen abstraction .......................................... 115

Figure 5.5. Normalized light intensity proﬁle for a completely self-eliminating
system with a “normal” photoinitiator concentration ................................. 116

Figure 5.6. Normalized intensity proﬁle for a completely self-eliminating system

with a “high” photoinitiator concentration .................................................. 117
Figure 5.7. Effect of incident intensity on gradient elimination rate ............................. 118
Figure 5.8. Schematic of the experimental setup for actinometry studies ..................... 119

Figure 5.9. Experimental characterization of the light intensity gradient as a
function of exposure time and sample thickness for BEE initiation
reaction ........................................................................................................ 120

Figure 5.10. Measured light intensity after passing through BEE samples of
varying thickness before and after sample exposure for 10 minutes
(incident intensity = 1.2 uW/cmz) ............................................................... 121

xii

Figure 5.11. Normalized light intensity as a function of time for varying sample

Figure 6.1.

Figure 6.2.

Figure 6.3.

Figure 6.4

Figure 7.1.

Figure 7.2.

Figure 7.3.

Figure 7.4.

Figure 7.5.

Figure 7.6.

Figure 7.7.

Figure 8.1

Figure 8.2

thickness of 1 x 10-2 wt% anthracene/1 wt% UV9310 in hexanol as a

function of time (intensity monitored at 363.96 nm) .................................. 122
Chapter 6

Reaction proﬁle for photoinitiated and thermally initiated vinyl ester

system at 25°C ........................................................................................... 133

Effect of system temperature on the reaction rate proﬁle for samples

cured using 0.1 wt% BME to photoinitiate the polymerization .................. 134

Effect of system temperature on the reaction rate proﬁle for samples
cured using 0.1 wt% Irgacure 651 to photoinitiate the polymerization ...... 135

Effect of initiator formulation on the reaction rate proﬁle for
photocured (2 mm thick) vinyl ester polymers ........................................... 136

Chapter 7

The effect of irradiation time and degree of ﬁber loading on the
ﬂexural modulus of vinyl ester polymers cured using the SEG
strategy ........................................................................................................ 154

The effect of irradiation time and degree of ﬁber loading on the
ﬂexural modulus of vinyl ester polymers cured using the dual-cure

method ......................................................................................................... 155
The effects of light intensity and cure time on the ﬂexural modulus
of composites with a 25 wt% ﬁber loading ................................................ 156
The effect of cure time on the ﬂexural strength of vinyl ester
composites with a 50 wt% chopped e-glass ﬁber loading .......................... 157
TGA proﬁles for vinyl ester resins as a function of temperature ................ 158

Effect of ﬁbers on the thermal expansion coefﬁcient of vinyl ester

polymers ...................................................................................................... 159

Determination of the glass transition from TMA proﬁles for vinyl

ester polymers ............................................................................................. 160
Chapter 8

Chemical structure of glycidyl methacrylate. .............................................. 183

Potential structures of poly(glycidyl methacrylate) .................................... 184

xiii

Figure 8.3

Figure 8.4

Figure 8.5

Figure 8.6

Predicted propagation rates of MMA (—-) and propylene oxide (- -)
as a function of temperature ........................................................................ 185

Rp versus initiator intensity for GMA polymerized cationically at
73°C ............................................................................................................ 186

Thermal proﬁles of glycidyl methacrylate polymerized radically,
cationically, and simultaneously ................................................................. 187

Simulated stress/strain curves for poly(glycidyl methacrylate) ladder
structures ..................................................................................................... 188

xiv

CHAPTER 1.

INTRODUCTION

Polymer science is constantly changing and developing. Research in this area has
advanced from its primitive stages over a century ago when scientists were simply
studying natural polymers such as rubber, starch and some proteins, to the current
development of literally hundreds of advanced synthetic polymers. The trend to replace
traditional materials with synthetic polymers has provided a constant motivation to
develop new products without increasing costs and/or decreasing quality. Rather than
developing entirely new polymers to meet this demand, it is often more cost-effective to
modify or blend existing polymers using novel processes. As research progresses, it is
becoming more and more difﬁcult to develop “new” materials, thus a major research
objective is to now either modify current methods used to develop these polymers to
make them more cost-effective or to develop entirely new methods for producing these
polymers. Methods that focus on photoinitiated polymerization reactions have seen and
continue to see rapid expansion as scientists seek to reach the changing research

objectives of new and improved polymer processing methods.

1.1. Characteristics and Advantages of Photopolymerizations

Photopolymerizations are simply polymerizations of reactive mixtures initiated by
light. The unreacted mixtures are stable at room temperature but will polymerize readily
once exposed to light of the appropriate wavelength. This wavelength typically lies in the
ultraviolet or visible region of the light spectrum.l The active site produced may be either
a radical or a cation depending on the photoinitiator formulation present in the system.

Photopolymerizations provide excellent control over the reaction both spatially
because the light can be directed and/or focused to a particular area of the reactive
formulation and temporally because the light can be shuttered on and off during the
reaction. In addition, the rate of photoinitiation can be controlled by a combination of
variables including initiator formulation, incident source and intensity, and initial
temperature of the system. Photocuring is a simple procedure that uses highly targeted,
efﬁcient light sources to cure systems that are often environmentally “friendly” because
the polymerizations are typically carried out in bulk,2 hence no volatile solvents are
released during the course of the reaction.

While the exact value of the ﬁnancial savings varies as a function of the polymer
produced, photopolymerizations do present a number of economic advantages. First,
production of initiating species is efﬁcient because the light sources are highly targeted
and speciﬁc light sources may be selected for particular processes. Thus light sources are
typically selected for a particular process because they emit light at a wavelength where
the photoinitiator (or photosensitizer) absorbs appreciably. Second, the more expensive

components of the formulation, namely the photoinitiators or photosensitizers, are used in

much smaller quantities (typically 1 wt% or less as the thickness of the sample increases).
Third, because all components of the reaction may be premixed and stored for extended
periods of time and the reaction takes place under ambient conditions, there is no need for
expensive mixing or heating equipment. Lastly, most photopolymerization processes are
carried out in bulk, thus there is no cost associated with purchasing solvents and the need
for expensive ventilating systems is also greatly reduced or eliminated altogether

depending on the volatility of the monomer.

1.2. Applications of Photopolymerizations

Historically, commercial applications for photopolymerizations date back as far as
the 19405. A number of industrial processes based upon photopolymerizations have been
developed for the production of a variety of thin polymer applications including ﬁlms,
inks and coatings. More recently, photopolymerizations have been used for a number of
applications in the electronics industry including coating of optical ﬁbers, replication of
optical disks and printed copper circuitry. These applications make up what can be
described as “traditional” photopolymerization applications, namely thin polymeric
products which are cured using some type of photoinitiated polymerization method. The
number of products fabricated by photopolymerization methods is increasing at rate of
approximately 20-30 % per year, suggesting that the general public is beginning to realize
the potential of photopolymerizations for a wider variety of applications. However, the

majority of newer applications would still be considered “traditional” in the sense that

photopolymerizations are being used for the production of thin systems. Few examples
can be offered that use photopolymerizations to produce three-dimensional polymers.

For the most part, commercial use of photopolymerizations to produce thicker
polymers or composites has been limited to the production of composite dental materials
and additive or layering processes such as photolithography. Photolithography provides
an excellent working example for how the advantages of photopolymerizations can be
employed in the production of thicker systems. This particular process combines the
excellent spatial and temporal control afforded by photopolymerizations with the
computer-aided design capabilities of traditional stereolithgraphy to rapidly produce
three-dimensional prototypes and other complex structures. Focused light sources such
as helium-cadmium (HeCd) lasers may be employed to produce detailed structures such
as intricate electronics patterns or complex products. Brieﬂy, three-dimensional polymers
are produced by lowering a platform (which may support a substrate in some
applications) into a vat containing an excess of the reactive formulation. A pattern
predetermined by computer design is then photocured in a matter of seconds using a
point-focused light source capable of producing an intricate polymer. Once the pattern is
has been cured, the platform is lowered into the vat, thereby allowing a deliberate layer of
reactive monomer to ﬂow over the sample. The photocuring process is then repeated a
number of times until the desired speciﬁcations of the polymer are reached to produce a
detailed, three-dimensional polymer. Photolithography has become a standard and
accepted method for producing polymers and prototypes for a number of free-radical
systems. For example, Neckers3 reported polymerizing acrylates in three dimensions by

using an additive modeling process based on stereolithography while a patent was issued

in 1986 to Charles Hull4 for the invention of a three-dimensional photolithography
process similar to the one described herein. In the past ﬁve years, the scope of
photolithographic processes has expanded to include the production of three-dimensional
polymeric composites.5'7

As mentioned previously, photopolymerizable compositions have been utilized in
the production of dental composites. In particular, three patents pertaining to

-l .
1.8 0 An obvrous

photopolymerizable dental compositions were issued to Sasaki et a
advantage of a photocurable dental composite is the ability to cure the material directly in
the mouth of the patient quickly and at atmospheric conditions (no external heating
requirements). Also, these reactions are carried out in bulk so the patient is not subjected
to the release of volatile organic compounds at any time during the procedure. Clearly
photopolymerizable dental composites and photolithography demonstrate the potential of
photopolymerizations for “non-traditional” or three-dimensional applications. It can still
be said that with the exception of these two speciﬁc applications, photopolymerizations
have been limited to thin systems.

Until very recently, one could safely state that no method has been developed
which is capable of photocuring thick polymers and/or composite structures either
quickly or uniformly. That statement is now only partially true, thanks to a joint effort of
Sunrez Corporation (El Cajon, CA) and Film Technology Incorporated (Houston, TX).
The new venture, titled Synergistic Composites Systems, combines vacuum-assisted resin
transfer molding and UV-irradiation to produce polymeric composites.ll While the

process is an exciting advance for photopolymerizations, it still comes with a few bugs.

For example, the cure time of the process is relatively slow, ranging from 3 to 15 minutes

 

 

depending on the thickness of the laminate, and the process is limited to light-permeable
laminates, meaning that only glass ﬁbers may be used as reinforcing material.11 In
addition, the technology is currently limited to single cycle production of laminates whose
thickness is 0.5 inch or thinner and requires 7-10 minutes to cure. Curing thicker samples
requires at least two cycles which increases the overall time required to produe the
structures. Still, the development of this process is a signiﬁcant advance in the current
state-of-the-art used to produce thick parts and composites and could make sizable
inroads into mainstream composites processing technology with a few modiﬁcations and
improvements such as increasing the thickness of cured composites, improving the

number and type of usable reinforcing materials, and reducing the overall cycle time.

1.3. Composite Materials

A composite may be deﬁned as any two-phase material which consists of a
reinforcing phase supported by a binder phase. Composite materials offer signiﬁcant
advantages over other materials such as metals, alloys and wood. For example,
composite products are signiﬁcantly lighter than their steel counterparts and exhibit
outstanding strength to weight ratios. A number of methods have been used to produce
composites with the most common of these being hand lay-up and resin transfer molding
(RTM). A strength of RTM is its ability to produce composites in large quantities while
hand lay-up offers the ability to produce highly detailed composites for a number of

smaller, specialty applications.

Composites have been employed industrially for over half a century, with the ﬁrst
reported use occurring during World War II when ﬁber—reinforced plastics were used in a
variety of structural applications.l2 Composites were initially developed to solve the
problems associated with the lack of a single, lightweight material having desired
attributes such as resistance to fatigue and corrosion as well as excellent mechanical
properties. Currently, composites are used in a much broader array of applications
ranging for example from the transportation industry (automobile, marine and aerospace)
to kitchen appliances. When coupling their unique combination of high strength and light
weight with relatively inexpensive production costs, it is easy to understand why
composites are tested for and ultimately used in a wide variety of applications.

A variety of resin systems have been used in conjunction with an equally varied
assortment of reinforcing materials to produce composites. Common resin systems
include: epoxies, polyimides and polyesters, as well as a number of thermoplastic
resins.”16 Reinforcing materials range from short, discontinuous glass, carbon or ararnid
ﬁbers to continuous ﬁber strands to ﬁber mats and other fabrics and preforms.”20 While
they are not present for their reinforcing capabilities, inorganic ﬁller materials have also
been added to polyester resins. While addition of these ﬁllers often enhances the stiffness
of the polyester resin,'5 their main role is to reduce the overall cost of the cured material
and/or reduce its density. Inert ﬁllers are also added to various resins to alter the
physical, mechanical, thermal or electrical properties of the composite.

Through careful selection of the binder material and the type of reinforcement, a
composite can be designed for just about any application. The selection of each phase is

extremely important in composite design because the properties of the composite are

dictated by the type and orientation of the ﬁber and the resin used as the polymeric
binder. Thus it is difﬁcult to classify composites in any way other than by the type of
reinforcement used in the composite. Dividing composites in this manner typically
means that the composite will fall into one of three classes: particulate, ﬁber or

laminar.12

Particle composites are identiﬁed by the fact that all dimensions of the
reinforcing material are roughly equal or uniform while laminar composites consist of
reinforcing materials that are layered with two dimensions being far more signiﬁcant than
the third dimension. Fiber composites describe systems where the length of the
reinforcing material is much greater than its cross-sectional area. A ﬁber-reinforced

composite may be further classiﬁed as continuous or discontinuous depending on whether

or not its properties are dependent on ﬁber length.

1.4. Necessity for Development of Thick Photopolymerization Methods

The previous sections detail the advantages of photopolymerizations and its
potential for a variety of applications. In addition, it has been established that the sc0pe
of photopolymerizations has been restricted to thin systems and a few specialty
applications involving moderately thicker systems. However science is setting its sights
too low by limiting photopolymerizations to the domain of the aforementioned systems.
The beneﬁts associated with photopolymerizations are rapidly gaining industrial
acceptance as evidenced by the rapid growth rate of processes and products based on this
technology. Thus by expanding the realm of photopolymerizations to include three-

dimensional polymer systems, both academia and industry could realize signiﬁcant

technological advancements for a much broader array of applications as well as
improving current or developing altogether new polymer processing techniques.

There are a few potential explanations for the limited advancement of
photopolymerizations into the domain of three-dimensional systems. The most obvious
reason for this is the initial existence of severe light intensity gradients throughout the
bulk of the reactive system. It is widely accepted that this light intensity gradient virtually
eliminates the possibility of uniform degrees of cure throughout the bulk of the polymer.
While the existence of this light intensity gradient does affect thick polymerizations, it
does not preclude the possibility of polymerizing in three dimensions if careful attention

is paid to the selection of a wide variety of system variables.

1.4.1 Impact in Technology

This research could result in the development of novel photopolymerization
processes for high—speed, low-cost production of polymeric composites. Although
photopolymerizations offer tremendous advantages, problems associated with light
intensity gradients and scattering, have limited their use to the production of thin ﬁlms.
This research demonstrates that photopolymerizations may be used to produce thick and
ﬁber-ﬁlled specimens with proper system design, and development of applications based
on photocurable composite systems could have an important impact on commercial
composites processing. For example, in the realm of liquid molding,
photopolymerizations may allow decoupling of mold-ﬁlling and reaction, thereby
eliminating many problems and limitations of current liquid molding processes. Armed

with this fact, it is not inconceivable that photopolymerizations could be incorporated into

10

a continuous process for the production of ﬁber reinforced composites. This scheme, if
successful, represents an important breakthrough since it will allow composites to be
produced continuously at low pressure with a relatively small and inexpensive tool. A
second area where this research could have a signiﬁcant impact in current technology is
we will develop in this research is polymer prepregs. Since light rather than heat provides
the initiating energy, photocurable prepregs would have the unique characteristics of
having practically unlimited working time (without UV illumination), but short cure
times (upon illumination) at room temperature. Although these photocurable prepregs
may ﬁnd many applications, they are ideally suited for the production of perforated

composite acoustic panel face-sheets for the aerospace industry.

1.4.2. Impact in Science

This research may have a signiﬁcant impact in the area of radiation curing. To
date, nearly all of the new radiation curable products being developed are for thin
systems. While there is still remarkable growth in this area, it has been needlessly
limited. By providing experimental evidence supporting the possibility of photocuring
thick polymers and/or composites, possible radiation curing has been exposed to an
entirely new, undeveloped area. On a more basic level, it is hoped that this research will
serve as a springboard for future scientists to determine for themselves what are the limits
of photopolymerizations and not restrict their research to what has been established as

“acceptable” applications for radiation curing.

10.

11.

12.

13.

14.

11

1.5. References

G. Odian, Principles of Polymerization, Wiley—Interscience, New York, 222 (1991).

R. Sastre, M.Conde and J.L. Mateo, "Photoinitiated Bulk Polymerization of Lauryl
Acrylate by N-Acetyl-4-nitro-l-naphthylamine in the Presence of N,N-
Dimethylaniline," J. Photochem. and Photobiol., A: Chem, 44, 111 (1988).

DC. Neckers, “Architecture with Photopolymerization,” Polym. Eng. Sci., 32(20),
1481 (1992).

C. Hull, “Apparatus for Production of Three-Dimensional Objects by
Stereolithography,” US. Patent No. 4,575,330, issued March 11, 1986.

T. Renault, A.A. Ogale, R.L. Dooley, A. Bagchi, C.C. Jara-Almonte,
“Photolithography for Composites Manufacturing: 'Continuous Glass Fiber/-
Polyacrylate Composites,” SAMPE Quarterly, 22(2), 19 (1991).

A.A. Ogale, T. Renault, R.L. Dooley, A. Bagchi, C.C. Jara-Almonte, “3-D Photo-
lithography for Composite Development: Discontinuous Reinforcements,” SAMPE
Quarterly, 23(1), 28 (1991).

T. Renault, A.A. Ogale, M.J. Drews, “Influence of Reinforcements on Photocuring:
Photo Dynamic Mechanical Analysis,” Proceedings of Annual Technical
Conference, Society of Plastics Engineers, 2352 ( 1993).

1. Sasaki, N. Mukai, H. Ige, “Photopolymerizable Dental Composition Containing a
Hexafunctional Urethane Methacrylate Based on Isocyanuric Acid,” US. Patent No.
4,762,863, issued August 9, 1988.

I. Sasaki, N. Mukai, H. Ige,“Photopolymerizable Dental Composition,” US. Patent
No. 4,933,376, issued June 12, 1990.

1. Sasaki, N. Mukai, H. Ige,“Photopolymerizable Dental Composition,” US. Patent
No. 4,977,197, issued December 11, 1990.

“Vacuum-Assisted RTM Increasingly Popular,” A. Hudson, ed., Composites
Technology, p.24, March/April 1996.

TJ. Reinhart, L.L. Clements, “Introduction to Composites,” in Composites, vol 1,
ASM International, Metals Park, OH, 27 ( 1987).

CA. May, “Epoxy Resins,” in Composites, vol 1, ASM International, Metals Park,
OH, 66 (1987).

D.A. Scola, “Polyimide Resins,” in Composites, vol 1, ASM International, Metals
Park, OH, 78 (1987).

15.

16.

17.

18.

19.

20.

12

CD. Dudgeon, “Polyester Resins,” in Composites, vol 1, ASM International, Metals
Park, OH, 90 (1987).

RB. Gosnall, “Thermoplastic Resins,” in Composites, vol 1, ASM International,
Metals Park, OH, 97 (1987).

J.C. Watson, N. Raghupathi, “Glass Fibers,” in Composites, vol 1, ASM
International, Metals Park, OH, 107 (1987).

NW. Hansen, “Carbon Fibers,” in Composites, vol 1, ASM International, Metals
Park, OH, 112 (1987).

J .J . Pigliacampi, “Aramid Fibers,” in Composites, vol 1, ASM International, Metals
Park, OH, 114 (1987).

RR Magin, “Multidirectionally Reinforced Fabrics and Preforms,” in Composites,
vol 1, ASM International, Metals Park, OH, 129 (1987).

CHAPTER 2.

BACKGROUND

The purpose of this chapter is to provide a relatively detailed historical
background of polymerizations and composites. The ﬁrst half of this chapter will be
devoted entirely to polymers, polymerization mechanisms, and the compounds used to
produce these structures. General mechanisms for radical and cationic chain
polymerizations will be discussed before presenting a section dedicated to radical and
cationic photopolymerization processes. The second half of this chapter focuses on
composites including current applications and methods for producing composites, various
types of composites, and typical materials used in both the reinforcement phase and the
binder (polymer) phase. This section will provide a historical perspective on the
composites industry as well as establishing the current state of composites processing as

well as a possible direction for its future.

2.1. Polymers and Polymerization Reactions

The term polymer applies to a broad spectrum of chemical structures which can be
characterized into more speciﬁc groups based on a number of traits including the method

used to form the polymer, its physical properties such as thermal stability and physical

13

14

appearance, and its mechanical properties such as ﬂexural strength and modulus. The
task of classifying polymers can be a difﬁcult one since many polymers are actually
networks of several different polymer chains, thereby having one or several qualities
which make it difﬁcult to easily categorize the polymer accurately. While polymers have
been characterized using a number of methods including structural differences between
monomer and polymer, the most effective classiﬁcation method seems to be based on the

chemical reaction that occurs when monomer is converted to polymer.

2.1.1. Classifying Polymers

The original polymer classiﬁcation system was developed in the early 1900’s by
W.H. Carothers,l who deﬁned condensation polymers and addition polymers based on
compositional differences between the polymer and its monomers. Polymers formed by
elimination of a small molecule (i.e. water) were called condensation polymers, while
polymers formed without losing a molecule were known as addition polymers. A second
classiﬁcation system was developed by Flory in the 1950’s.2 In his scheme, Flory focused
on the mechanism of the polymerization and deﬁned two classes: step polymers and
chain polymers. Although many people tend to use the terms condensation and step as
well as addition and chain interchangeably, this is not always accurate because of the
fundamental differences in the basis of these classiﬁcations. For example, polyurethanes
would be deﬁned as addition polymers in the Carothers scheme but step polymers using
Flory’s classiﬁcation. All of the polymer systems studied experimentally in the course of

this research were formed as a result of a chain reaction, therefore the focus of this

15

section will be on chain polymerizations with a brief overview of step polymerizations

provided simply as a method of comparison.

2.1.1a. Step Polymerizations. Step polymerizations propagate through a series
of reactions between functional groups of two reactants. In this mechanism, monomers
will react with monomers to form dimers, dimers will react with monomers to form
trimers or with other dimers to form tetramers and so on until a long-chain polymer is
been formed. Step polymerizations can generally be characterized by the following six
statements made by Rudin3 in the early 1980’s: 1) a single reaction type is involved in the
reaction; 2) monomer units can react with each other, or a polymer of any size; 3) the
reactivity of a functional group is not dependent on polymer (or monomer) size; 4) in
order to produce high molecular weight polymers, high conversion of the monomer
species must occur; 5) an equilibrium may exist between reactants and macromolecules;

and 6) condensation polymers are usually produced in step polymerizations.

2.1.1b. Chain Polymerizations. In addition to the monomer(s), chain
polymerizations require the addition of a separate compound known as the “initiator”,
which will undergo some type of chemical reaction when subjected to a change in initial
conditions (heating, irradiation, microwave or ultrasound) to form a product which
becomes an active site capable of reacting with the monomer to initiate the reaction.
Depending on the initiator, a radical, cation, anion or a combination of the three may be
produced during the initiation reaction. Polymerization proceeds with propagation of the

active species by successive addition of large numbers of monomer molecules in a

l6

relatively short period of time, often on the order of one second or less. A signiﬁcant fact
in chain reactions is that monomer will not react with monomer unless an active site is
present on (at least) one of the monomer units. Termination of the growing polymer
chains may occur by a number of possible reactions including: chain transfer,
disproportionation or combination to name a few.

Chain polymerizations are classiﬁed as radical or ionic, with ionic mechanisms
further designated as cationic or anionic, depending on the active site responsible for
propagation of the polymer chain. Each of these chain polymerizations has a distinctive
reaction mechanism because of the differences in the active species. It should be stressed
that reactive species are not indiscriminate since all three reactive centers do not initiate
polymerization in all monomers. In fact, monomers show varying degrees of selectivity
to the type of reactive center that will initiate polymerization, therefore some monomers
may undergo ionic initiation exclusively while other monomers (such as styrene) will
polymerize in the presence of any type of reactive center. Radical chain polymers are
most prevalent in industry to date because most monomers will undergo radical
polymerization under modest reaction conditions without being overly sensitive to
impurities, while industrial use for ionic reactions has been limited mainly to the rubber
industry in part because of the selectivity of many monomers to reactive ionic centers.
The following subsections will detail the radical mechanism as well as the cationic

mechanism, elaborating on the similarities and differences of each.

17

2.1.2. The Radical Mechanism

There are two main classes of bonds that will undergo chain polymerization:
vinyl bonds and carbonyl bonds.4 Radical species will not polymerize through a carbonyl
group due to the polarity of the double bond. Although a vinyl bond in the monomer
does not guarantee that the monomer will polymerize radically because of the resonance
structures of the substituents present in the monomer, most vinyl bonds are likely to
undergo radical polymerization in part because the radical species is a neutral one which
does not have strict “requirements” for attacking the rt-bond. Also, nearly all substituents
are able to stabilize the radical by delocalizing the radical over two or more atoms.4

In the radical mechanism, as is common to each chain polymerization, there is
initiation, propagation, and termination. The initiation step is really a two-step process in
which the initiator is ﬁrst dissociated into two radicals (Equation 2.1) before it attacks a
single monomer unit to produce an active site on the monomer itself (Equation 2.2).

I = 2R. (2.1)

R0 + M => M10 (2.2)

A number of initiators are used in radical reactions including many peroxides and azo
compounds such as 2,2'-azobisisobutyronitrile. In addition to these initiator, which are
susceptible to thermal initiation, there are a number of photoinitiators, or initiators that
will undergo dissociation after absorbing light. The photoinitiation mechanism will be
detailed more extensively in the following sections of this chapter. Once the monomer
becomes “active” (as shown in Equation 2.2), it has the ability to add to any other

monomer that comes in close contact. This occurs in the same manner as the reactive

18

initiator added to the original monomer, making the second monomer an active site
(Equation 2.3). Successive additions of monomer units will

Ml. + M :9 M2. (23)
continue to increase the size of the polymer in such a way that each radical will be
identical to the previous one except for the fact that the polymer chain will have grown by
a single monomer unit. This process is repeated and the polymer chain continues to grow
until termination of the radical, which may occur when the active polymer reacts with an
active radical species of another polymer, resulting in termination by coupling.

Termination is the ﬁnal step in the radical mechanism, and can occur by a number
of reactions, although the most common is coupling of two active polymer chains

(Equation 2.4). A less common method of termination is disproportionation, shown in

Equation 2.5.
Mn' + Mme => Mn+m (2.4)
Mn- + Mme => Mn + Mm (2.5)

The method of termination is not always speciﬁed, hence it is often represented as a
general termination reaction, shown in Equation 2.6.

Mn. + Mme :9 Dead Polymer (2.6)

As one would expect, rate constants differ for each step of the reaction and are

generally represented as kd for dissociation of the initiator (Equation 2.1), ki for the
addition of the radical to the monomer (Equation 2.2), kp for propagation of the polymer

(Equation 2.3), ktc for termination by coupling (Equation 2.4), km for termination by

19

disproportionation (Equation 2.5), and kt to represent general termination of the polymer

(Equation 2.6).

The propagation constant, kp, and the termination constant, k., are used to
determine the rate of polymerization, Rp' In order to determine Rp, one begins by
deriving an expression for the rate of monomer disappearance, which Odian4 states is
synonymous with the rate of polymerization. Monomer is used up in both the initiation
step and the propagation step, therefore the rate at which monomer disappears is given by

—d[M]/dt = Ri + Rp (2.7)
where R, is the rate of initiation and [M] is the monomer concentration. An assumption
is then made that the amount of monomer reacting in the propagation step is much greater
than the monomer used up in the initiation step, therefore (2.7) may be simpliﬁed to

-d[M]/dt = Rp (2.8)
which is a very close approximation of (2.7). The rate of propagation consists of several
repetitions of Equation (2.3), which has as its constant kp. The single rate constant allows
the rate of propagation to be expressed as

R,=k,iMiiM-] (2.9)
where [M-] represents the concentration of all chain radical species.

Since it is extremely hard to experimentally measure the concentration of radicals
in a polymerizing system, Equation (2.9) is not in an acceptable form. Therefore the
quasi steady-state assumption (QSSA) is commonly made for the concentration of

radicals. Stating that the concentration of radicals remains constant throughout

20

propagation is equal to stating that Ri and RI (the rate of termination) are equal. This
means

Ri = 2k,[M-]2 (2.10)
This equation can then be rearranged to yield an expression for [M0]

[M0] = (Ri/2k.)"2 (2.11)
which can be substituted into Equation (2.9) to yield the following expression

Rp = kp[M](Ri/2kt)”2 (2.12)
which gives a general equation for determining the rate of polymerization.

It is clear from Equation (2.12) that the rate of polymerization is proportional to
the square root of the rate of initiation and directly proportional to the monomer
concentration. This leads to the expectation that polymerization rates would decrease with
conversion due to depletion of monomer and initiator; yet this is not always the case in
radical polymerizations due to an effect known as autoacceleration. Autoacceleration is
a process that occurs in radical polymerizations when there is a reduction in the rate of
diffusion-controlled termination as a result of reduced mobility in the growing radical
chains. This effect causes dramatic increases in the rate of polymerization as well as an

increase in the molecular weight with conversion.

2.1.3. The Cationic Mechanism
Cationic polymerizations occur in much the same fashion as their radical
counterparts with the exceptions that the propagating center is a cation and termination by

coupling will not occur. Termination may occur by chain transfer of the reactive center,

21

or occasionally by counterion combination. Termination by chain transfer will typically
decrease the average primary polymer chain length, although it will not affect the rate of
polymerization due to the continued propagation by the resulting cation.4 For this reason,
these reactions are well-suited for the formation of crosslinked structures or ladder
polymers which rely on network structure, not long linear chains, for their properties.
Classes of monomers that will not polymerize by a free radical mechanism but can be
cationically polymerized include a-oleﬁns, 1,3-dienes, vinyl ethers and epoxides to name
a few.

Each of these monomer systems has potential advantages that make them
industrially useful. Epoxides for example exhibit tremendous adhesion to a variety of
substrates. Nevertheless, these monomers are rarely used in applications where they are
polymerized via a cationic mechanism to date. A potential explanation for this is the lack
of understanding of the cationic mechanism in general.5 Despite this problem, scientists
are still able to formulate a reaction scheme for cationic polymerizations involving the
same three steps as radical polymerizations.

Like other chain polymerizations, the ﬁrst step is the initiation step. Commonly

used thermal initiators include Lewis acids such as AlCl3, BF3, SnCl4 and TiCl4 to name

a few; and to a lesser extent Bronsted acids and iodine. Another type of initiator that has
been investigated over the last ﬁfteen or so years has been the photoinitiator, ordinarily an
iodonium or sulfonium salt. In this initiation sequence, the onium salt undergoes
photolytic cleavage to yield a radical-cation which then reacts with HY to yield an active

cationic species.6 Aside from the photoinitiators, Lewis acids are widely considered to be

22

the most important initiator of cationic polymerizations because they yield high
molecular-weight polymers at low to intermediate temperatures.
In the initiation sequence, the Lewis acid either undergoes self-ionization

(Equation 2.13) or ionization via a coinitiator, XY (Equation 2.14)

2AlCl3 a) AlC12+(AlCl4)’ (2.13)

I + XY 4: Y"(IX)' (2.14)
Whether or not an initiator-coinitiator scheme or single initiator is used, the second step

of the initiation sequence involves the addition of the active species to the monomer, M
AlC12+(AlCl4)' + M = AlC12M+(AlCl4)' (2.15)

At this point, the active species is a carbonium ion in the monomer, which is then
capable of propagating with unreacted monomers.

A1C12M+(A1Cl4)' +M => A1C12M2+(A1Cl4)' (2.16)
Successive addition of monomers will yield high molecular-weight molecules which will
terminate through chain transfer, or more rarely spontaneous termination or counterion
combination. Chain transfer occurs when a proton is transferred from the reactive center
to a monomer, thus terminating the growing polymer while at the same time creating a
reactive monomer capable of undergoing polymerization the same way as the terminated
polymer

AlClen+(AlCl4)' + M => Mn + AlC12M+(AlCl4)' (2.17)

Like radical polymerizations, a general expression describing the kinetics of a

cationic polymerization (as seen in Equations 2.13 - 2.17) can be derived. there are rate

constants associated with these. The terms used to describe rate constants for initiation

23

and propagation are the same as those for the radical mechanism, speciﬁcally ki and kp.

However, chain transfer to monomer is almost always responsible for termination of
cationic chains, therefore a speciﬁc rate constant for termination by chain transfer to

monomer is given by kam-

Deriving a general expression for the rate of a cationic polymerization is nowhere
near as simple as the derivation for radically-initiated systems. There are a number of
variables associated with the cationic mechanism which make it virtually impossible to
derive a general equation capable of accounting for each of these variables. For example,
the rate of polymerization is dependent on the method of initiation. To illustrate this, one
need only examine initiation by ionizing radiation. This method of initiation yields an
equation in which the rate of polymerization is dependent on the square root of the rate of
initiation, while other methods used to initiate cationic polymerizations tend to produce
ﬁrst-order dependence of the polymerization rate on the rate of initiation. Another
variation in the rate of polymerization is the method of termination.

Polymers that undergo termination by counterion combination will exhibit a rate
expression that can be derived in the same manner as the rate expression for radical
polymers.4 However, the most common method of termination in cationic
polymerizations involves chain transfer. Thus the following derivation focuses

speciﬁcally on termination by chain transfer, either to monomer (kmM) or to a chain
transfer agent (kms).

The rate of initiation is given by Equation (2.18) where K is the rate constant for

the ionization step, while the rate of propagation is given by Equation (2.19)

24

R.= KkillllXYllMl (2.18)
Rp = kp[YM+(IX)'][M] (2.19)
where [YM+(D()'] is the concentration of all reactive centers. The rate expression for

termination must be divided into two transfer reactions, one for transfer to monomer

(Equation 2.20) and one for transfer to chain-transfer agent (Equation 221)
RmM = k.,M[YM+(D()’][M] (2.20)
Rus = k..,s[YM*(IX)’][S] (2.21)
The formation of [YM+(D()'] is dependent upon the initiation step, therefore one
must know what the rate-limiting step in the initiation process is to derive an expression
for [YM+(D()']. Therefore, if one assumes that Equation (2.14) is rate-limiting and that

R. is equal to Rt (QSSA), then [YM+(IX)'] can be expressed by

[YM*(IX)'] = K1911][XYllMl/(kmmlMl+ 19.3151) (2.22)
Substituting Equation (2.22) into Equation (2.19) yields the following expression for the
rate of polymerization for a cationic reaction mechanism

R, = KkikpUHXYllMF/(RUMIMJ +k..,sisn (2.23)

This is a simple, straightforward method for deriving the rate of cationic
polymerization, however there are a number of inconsistencies and assumptions
associated with its derivation, and these inconsistencies have the potential to create large

discrepancies in the actual rate of polymerization versus the theoretical Rp determined

using Equation (2.23). As reported in Odian,4 various rate constants are calculated using

unsubstantiated reaction kinetics and mechanisms. Additionally, it is not known how to

25

interpret any rate constants because of the known multiplicity of propagating cationic
species.4 The net result of these discrepancies is Rp values calculated using Equation
(2.19) will be incorrect when the actual kinetics of the cationic reaction deviate from this
equation.

Cationic polymerizations exhibit unusually high reaction rates because they
exhibit higher propagation rate constants and lower termination rate constants than typical
free radical reactions. Furthermore, the concentration of propagating species is usually
two orders of magnitude higher for cationic polymerizations. In fact, these reactions
proceed so rapidly that they do not achieve steady-state under most conditions.7 This
makes the assumption that the concentration of active cationic species is maintained at
steady-state (QSSA) invalid, which furthermore renders the entire derivation invalid
because of its dependence on the cationic species achieving steady-state conditions. In
addition, when the rate of initiation is greater than the rate of termination more cations are
produced than are used up, consequently steady-state is not achieved during the course of
reaction. These disparities exemplify the problems associated with attempting to

mathematically describe the rate of polymerization in cationic reactions.

2.2. Photopolymerizations

Photopolymerizations are simply polymerization reactions initiated by light,
typically in the ultraviolet (UV) or visible region of the light spectrum.
Photopolymerizations are initiated by certain types of compounds which are capable of

absorbing light of a particular wavelength. The wavelength or range of wavelengths of

26

the initiating source is determined by the reactive system including the monomer(s), the
initiator(s), and any photosensitizers, pigments or dyes which may be present. An active
center is produced when the initiator absorbs light and undergoes some type of
decomposition, hydrogen abstraction, or electron transfer reaction. If necessary, the
effective initiating wavelength may be shifted by adding small amounts of a second
compound, termed a photosensitizer, to the reaction mixture. The photosensitizer absorbs
light and populates an excited state which may then react with the photoinitiator to,
produce an active cation or radical capable of initiating the polymerization. Upon
generation of active centers, photopolymerizations propagate and terminate in the same
manner as traditional (i.e. thermal) polymerizations.

There are two main types of radiation curing currently being used in industrial
applications: UV and electron beam (EB) processing. UV processes are based on the
generation of photons upon absorption of light, typically in the ultraviolet or visible
region of the light spectrum, while EB processes are based on electrons generated using
an e-beam accelerator. While both processes are extremely efﬁcient due to direct
interaction between the photon/electron and the chemical bonds, implementation of EB
technology is limited to an extremely small portion of the radiation curable market since
it is extremely expensive. Thus chain polymerizations initiated by radicals or cations
generated by a light-induced reaction are responsible for the majority of new products and

developing processes.

27

2.2.1. General Attributes of Photopolymerizations

There are a tremendous number of advantages inherent in photopolymerizations
which make them the technology of choice for a rapidly increasing number of industrial
applications. The initiating light can be directed to a speciﬁc area of the reaction,
providing excellent spatial control over the polymerization; and the start of the reaction
can be controlled as can the overall temperature of the reaction at any given point during
reaction by shuttering the light. The production of active sites begins immediately upon
exposure to light, hence once the initiation reaction begins, active sites may be produced
rapidly even at room temperature. The reactive formulation can be premixed and stored
for extended periods of time as long as it remains free of light, and the requirements for
initiating the process are easily automated. Hence the process is very simple.
Additionally, photopolymerizations quite often involve solvent-free reactions, thus they
are environmentally “friendly”. One fact often overlooked in photopolymerization
processes is the fact that they can be operated at room temperature, which is extremely
important when curing on substrates which are susceptible to damage at elevated
temperatures.

The ﬁnancial beneﬁts and energy efﬁciency attributed to UV-cured systems are
impressive. The fact that UV curing eliminates the need for expensive post-cure systems
combined with the fact that UV systems use approximately 20% of the energy consumed
by comparable thermal systems leads to a method which is both energy efﬁcient and less
expensive. One ﬁnal advantage of UV systems is their “portability”. Because UV
systems require only a fraction of space occupied by thermal curing systems, they free

valuable space for a variety of alternative uses.

28

2.2.2. Traditional Applications for Photopolymerizations

These attributes as well as additional advantages have established
photopolymerizations as standard technology in a number of “traditional” applications.
Traditional applications for photopolymerizations include ﬁlms, inks; and coatings, for a
variety of substrates including wood, metal, plastic, paper and glass.
Photopolymerizations have also been incorporated into traditional stereolithography
processes. Photolithography capitalizes on the excellent spatial control afforded by
photopolymerizations to produce detailed images either by “tracing” predetermined
patterns or by masking. An excellent example of the ﬂexibility afforded by
photopolymerizations is seen in their use for dental applicationsg‘9 Photopolymerizations
allow for in vivo curing of dental ﬁllers by using optical ﬁbers to direct light to the
uncured ﬁller material after it has been placed inside the mouth. The light carried
through the optical ﬁber to the ﬁller initiates a reaction in the mouth of the patient,
producing a solid ﬁlling in a matter of minutes or less without subjecting the patient to
potentially hazardous emissions from evaporating solvents or burns as a result of heating
sources. Recently, photopolymerizations have been used in a variety of high-tech
applications including coating of optical ﬁbers, replication of optical disks and production
of printed copper circuitry.”l4

UV curing methods are currently being employed in many industries with
saturation of the radiation curable market nowhere in sight. In fact, the number of new
polymer applications that utilize photopolymerization techniques are expanding at a rate

of approximately 20-30% per year. The growth of new photopolymer applications is an

excellent measuring stick for the potential of photopolymerizations and the ﬂexibility to

29

adapt UV curing into the fabrication of existing products and processes as well as new

products.

2.2.3. Radical Photoinitiation Processes
2.2.3a. Photoinitiator Compounds. Photopolymerizations involving radical
species have been around for many years and the initiation process has been well-
documented.”"6 A number of photoinitiators are available with absorption ranging from
the deep UV region well into the visible region of the light spectrum. The most common
UV-curable radical photoinitiators include ketones, benzoin ethers, and benzil ketals,

“7'” Initiator

along with well-established compounds such as Michler’s ketones.
formulations in the visible region of the light spectrum include many commonly used
dyes such as eosin20 and organic pigments and coinitiators such as
camphorquinone/aminez' formulations. In general there are some important advantages
of initiating in the visible region of the light spectrum. However the focus of this
research has been in the area of UV-initiated photopolymerizations, therefore attention
will be paid to initiators that absorb in this region of the light spectrum. Detailed studies

h22.23 and Pappasl6 providing some of

of various radical photoinitiators exist, with Ledwit
the more exhaustive studies regarding these compounds. In general, radical
photoinitiators yield radicals by one of two processes: fragmentation or hydrogen
abstraction. Understanding and being able to characterize (and potentially control) the
process used to produce the propagating radical is a focal point of this research.

The process by which a reactive radical is produced has a potentially dramatic

effect on the absorbance of the polymerizing system, and hence the rate of propagation

30

and thickness of the cured polymer. While production of radicals via abstraction alters
the structure only slightly and results in an initiator fragment having substantially the
same chemical structure as the unreacted initiator (see Figure 2.1a), fragmentation
produces two radicals whose chemical structures are signiﬁcantly different than the
original initiator (see Figure 2.1b). Thus initiator fragments resulting from the
abstraction reaction may continue to absorb initiating wavelengths while the absorbance
characteristics of the fragmentation reaction products are substantially different than those

of the unreacted initiator in many initiator formulations. In fact, for the systems

0 O * OH
ergo 3..., oi i» ociso + ..
(a)

*
ii i ii
oco 4m» are ——» c- + o
(b)

Figure 2.1. Dissociation of initiator to produce radical by hydrogen abstraction
(a) and fragmentation (b).

31

investigated in this research, the absorbance of initiator fragments resulting from
fragmentation proved to be essentially nonexistent in comparison with their unreacted
initiator counterparts.

While hydrogen abstraction is more efﬁcient than fragmentation, it will only occur
in the presence of hydrogen donors. The most common hydrogen donors are tertiary

amines, which have easily abstractable or-hydrogens. While not as efﬁcient as the

amines, alcohols and ethers may also be used as hydrogen donors with varying degrees of
effectiveness. Historically, tertiary amines have been used successfully in conjunction
with a number of photoinitiators such as benzil, benzoin ethers and benzil ketals, to

photocure thin polymeric systems_4.ie.24-2e

In general, photoinitiator formulations for
radical systems have incorporated some type of hydrogen donor in their initiator
formulation, thus serving two purposes. First, the presence of a hydrogen donor will tend
to promote hydrogen abstraction over fragmentation, therefore improving the overall
efﬁciency of the initiation reaction. Second, hydrogen donors, especially amines, tend to
act as oxygen scavengers, thereby removing oxygen molecules that will otherwise serve
as inhibitors for free radical polymerizations. The net result of the oxygen-scavenging
process is an increase in the rate of polymerization that is dependent on the ability of
compounds to serve as oxygen scavengers.

One well-known drawback of simple aromatic or aliphatic amines is the fact that
they act as chain transfer agents through a well-established hydrogen abstraction
mechanism. While this reaction does not result in direct or immediate termination of an

active site, it does terminate the growing polymer chain, resulting in a reduction in the

average molecular weight of the polymer chains. In practice, this is very signiﬁcant since

32

a number of mechanical and physical properties are directly related to the molecular
weight of the polymer. A simple representation of this mechanism is shown in Figure
2.2.

To eliminate or minimize any problems caused by chain transfer reactions
involving aromatic or aliphatic amines, scientists have attempted to incorporate the amine
functionality into a monomer which is capable of participating in the reaction.”29
Perhaps the study most relevant to this research is the work performed by Hoyle et al.28
reporting the effects of benzoin ether/amine photoinitiator formulations on the molecular
weight of photopolymerized acrylates. In this study, a simple aliphatic amine was
functionalized with an ethyl methacrylate monomer unit to produce N,N-
dirnethylaminoethyl methacrylate (DMAEMA).28 The authors reported minimal effects
on the (viscosity average) molecular weight for DMAEMA-promoted reactions, even
though this amine is still subject to chain transfer via a hydrogen abstraction mechanism
similar to the one seen in Figure 2.2. The increased molecular weight of functionalized
amines in comparison with simple amines can be attributed to the “crosslinking” nature
of the functionalized amines, which have the potential to crosslink because they become
“locked” into the polymer backbone upon reaction with an unsaturated monomer. This
occurs because polymerization can propagate through two spots on the compound (shown
in Figure 2.3): the radical formed in the chain transfer reaction; and through the
monomer unit incorporated into the backbone of the amine. In addition to these results,

the authors28 reported that the rate of polymerization passed through a maximum before

decreasing signiﬁcantly as the concentration of DMAEMA was increased from 0 - 2

33

mol/g. The importance of accelerator concentration was also addressed in studies by

Seretoudi and Sidericiou.29

(IZHZCH3 CHCH3
k
CH2CH3—N-CH2CH3 + opn ——‘L—> CHZCH3—N—CH2CH3 + PnH

Figure 2.2. Termination by chain transfer of growing polymer chains by amines
via a hydrogen abstraction mechanism.

CH3 CH3
HZC— CH3 H2C—(IJ
c= H2C=C|I ('3:
O + C=O —-> (I)
in, (I) (Eu,
/N\. R CH3—N CH3
H3C CH2 H2C—CH2-C'3-
(L:
(I)
(in,
1'2

Figure 2.3. Incorporation of N,N-dimethylaminoethyl methacrylate amines into
acrylate polymer backbone.

34

In contrast to the results published by Hoyle,28 Seretoudi and Sideridou29 reported
that DMAEMA emphatically increases the rate of reaction at the expense of achieving
high molecular weights. The authors of this study concluded that 10:1 benzil/DMAEMA
photoinitiator formulations increased the rate of polymerization by a factor of ~ 6 while
reducing the number-average molecular weight by a factor of ~ 5.29 The authors agreed
with previous studies in the sense that the relative concentrations of photoinitiator and
. hydrogen donor had a signiﬁcant effect on the overall reaction kinetics and the molecular
weight of the photocured samples.

These examples illustrate the widely accepted method of using amines and/or
other hydrogen donors to accelerate polymerizations for traditional (i.e. relatively thin)
systems. However, research in this laboratory has indicated that the presence of such
compounds may actually inhibit photopolymerizations involving thick (0.1 - 4 cm)
polymeric systems. The reasons for this were alluded to earlier in this section and involve
continuing absorption by certain compounds even after generation of reactive radicals.
Detailed explanations for this effect are offered in a later chapter.

While the aforementioned compounds are far and away the most prevalent UV-
curable radical photoinitiator systems, alternative formulations have been investigated.
Moderate attention has been focused on the development of photoinitiators based on

3032 According to Angiolini et al.,30 these compounds

phosphine oxide derivatives.
absorb at higher wavelengths than most of the better-known compounds, with peaks
centering around 380 nm and extending over 400 nm. This information is useful for

extending the Operating window of thick photopolymerizations into the near-visible

region of the light spectrum.

35

2.2.3b. Kinetics of The Radical Photoinitiation Step. The derivation used to
characterize the rate of polymerization for a radical photoinitiation process is simple and
straightforward. The ﬁrst step inthe derivation is to determine the rate of photoinitiation.
The general expression used to describe the rate of photoinitiation is shown in Equation
2.24 where it) can be described as either the quantum yield for the initiator or the

R, = 2t1)1a (2. 24)
number of propagating chains initiated per photon of light absorbed. The factor 2 is
included in the expression because two radicals are generated for every photoinitiator
molecule that undergoes reaction. The absorbed light intensity, 1,, can be expressed as a
function of the incident intensity, 10, concentration of photoinitiator, I, the extinction
coefﬁcient (or molar absorptivity) of the photoinitiator at the initiating wavelength, 3, and
the sample thickness, b. Incorporating these terms into Equation 2.24 yields an
alternative expression for Ri.

R, = 2¢elo[1]b (2.25)
Equations 2.24 and 2.25 can be substituted into Equation 2.12 to yield two alternative

expressions (Equations 2.26 and 2.27, respectively) for determining the rate of

polymerization for a photoinitiated system.
R.=k..tMi{i>I./k.}"2 (2.26)
R.=kptMii«pariah/1e}“2 (2.27)
A limitation of Equation 2.27 in regards to this research effort is the fact that it

assumes a relatively uniform distribution of incident light (i.e. little or no absorbance)

throughout the bulk of the sample. While this effect may not inhibit uniform

36

photochemical initiation in thin systems, it would have a severe effect on thick systems
because even slight absorbance will result in measurable light intensity gradients
throughout the bulk of thicker systems. Hence Equation 2.26 is a more useful equation
for calculating the rate of polymerization in thick systems because the amount of
absorbed light can be measured directly in polymerizing systems using chemical or
electrical actinometry. Experimental studies33 have shown that typical values for the
initiation rate constant can be expected to be on the order of approximately 109 M'l sec'l
for many photoinitiator formulations including several combinations of benzil/amine

derivatives.

2.2.4. Cationic Photoinitiation Process

Cationic photopolymerizations have not received as much attention as their
radical counterparts. A possible explanation for this is the fact that cationic
photopolymerizations are relatively new and have not been detailed to the same extent as
radical systems. Recent Discoveries of thermally stable cationic photoinitiators in the late
19705 and early 19805 have led to more concentrated efforts in understanding and
characterizing the cationic mechanism. Crivello has been instrumental in developing an
ever-increasing list of cationic photoinitiators including numerous diaryliodonium and

triarylsulfonium saltsf” 34"“

Much of Crivello’s early work seemed to focus on
developing initiator only so that he could polymerize important classes of monomers such
as epoxides and vinyl ethers, however he has been instrumental in developing many of

the onium salts that are now widely accepted as excellent initiators for many cationic

systems. There now exist a wide variety of onium salt initiators which are extremely

37

useful because they can be “tailored” to speciﬁc systems by manipulating the initiator
counterion.

The development of suitable initiators coupled with the rapid reaction rates of
vinyl ethers and other cationically polymerizable monomers has led to signiﬁcant
advances in the area of cationic polymerizations. For example, numerous compounds are
now being investigated as potential photosensitizers for cationic systems.
Photosensitizers such as perylene, pyrene, and numerous anthracene derivatives are added
in small amounts to cationic formulations to expand the effective initiating wavelength of
the photoinitiator from the deep UV to the visible region of the light spectrum. It is
widely accepted that photosensitization proceeds by an electron transfer

mechanismfmm‘43

However, there still exists some disagreement in the scientiﬁc
community regarding the source, or excited state (singlet or triplet), responsible for
electron transfer when anthracene photosensitizes diaryliodonium salts. Recent studies
by Nelson et al.44 have reported on the photosensitization mechanism for anthracene and
similar derivatives.

The system described by Nelson et al.44 includes a diaryliodonium salt initiator
and an anthracene photosensitizer. In systems studied during the course of this research,
the monomer (vinyl ethers, epoxies, acrylates) and the photoinitiator were transparent to
the initiating light. Therefore anthracene (the photosensitizer) is the only component of
the reactive system that absorbs light appreciably in the 350-400 nm range. This is
signiﬁcant because the absorbance of anthracene (and anthracene derivatives such as 9-

vinylanthracene and 9,10—dimethylanthracene) changes dramatically as it reacts due to a

signiﬁcant change in its chemical structure. Anthracene absorbance between 350-400 nm

38

is related to the highly conjugated nature of the compound. However, when anthracene
absorbs light and undergoes electron transfer to produce a cationic species, its structure
changes and is no longer highly conjugated,44 resulting in a structure that is transparent to

the initiating wavelength (in the 350-400 nm range).

2.2.5. Radical and Cationic Propagation and Termination Processes

Radical and cationic polymerizations proceed in a similar manner once an active
species has been produced. The radical (or cation) attacks a bond or group of bonds
which are susceptible to radicals (or cations). In either case the ﬁrst “link” of the polymer
chain is established when the active species adds to a monomer unit, resulting in the
creation of a new active site at the functionality on the monomer unit. Successive
additions via this method result in the formation of a long polymer chain.

Termination of growing radical chains occurs by coupling or disproportionation.
Coupling (Equation 2.4) occurs when one radical chain (length A) reacts with a second
radical chain (length B) to form a single, dead polymer (length A+B). Disproportionation
(Equation 2.5) occurs when radicals from two chains (length A and B, respectively) react
to produce two dead polymers (one of length A and the other of length B). These
methods of termination are different from termination in cationic polymerizations, which
are most commonly terminated by chain transfer to monomer (Equation 2.17) or more
rarely by spontaneous termination. Chain transfer occurs when a hydride is transferred
from the reactive center to a monomer, thus terminating the growing polymer while at the
same time creating a reactive monomer capable of undergoing polymerization in the same

manner as the terminated polymer. Thus while one polymer chain is terminated, there is

39

no reduction in active centers, hence cationic polymerizations may be considered “living”
because there is little or no reduction of active sites and more polymer could theoretically

be produced if additional monomer were added to the system.

2.3. Composites

Composites may be loosely deﬁned as any material consisting of two phases: a
discontinuous phase that composed of a reinforcement or ﬁller material; and a continuous
phase that may be described as the polymeric matrix, or binder phase. Composites have
been employed in industrial applications for over half a century, with the ﬁrst reported
uses occurring during World War II, when ﬁber-reinforced plastics were used in a variety
of structural applications.45 Composites were initially developed to solve the problems
associated with the lack of a single material having desired attributes such as resistance to
fatigue and corrosion , excellent mechanical properties, and light weight. Currently,
composites are used in a much broader array of applications ranging for example from the
transportation industry (automobile, marine and aerospace) to kitchen appliances. When
coupling their unique combination of high strength and light weight with relatively
inexpensive production costs, it is easy to understand why composites are tested for and
ultimately used in a wide variety of applications.

Composites are suitable for a large number of applications because the properties
of the composite are affected by such things as the type and orientation of the ﬁber and
the polymeric material used in the binder phase. Thus it is difﬁcult to classify composites

in any way other than by the type of reinforcement used in the composite. Composites

40

may be characterized as reinforced or “ﬁller” composites depending on the role of the
discontinuous, or ﬁber phase. In the case of ﬁller composites, the ﬁller materials are
added either to lighten the material, improve heat resistance, minimize creep, or to
reduce the overall cost of the material and not to reinforce the matrix, though that may
occur to a degree. Reinforced composites, as the name implies, utilize the strength of the
reinforcing material to enhance the polymeric binding phase. These composites may be
further divided into one of three subcategories: particulate, ﬁber or laminar.45
“Particulate composites” may be deﬁned as composites where all of the
dimensions of the reinforcing material are approximately equal. The reinforcing particles

are typically very small (around lu), thus they strengthen the composite by resisting

movement of the surrounding matrix phase.46 The particles should be small and well-
distributed throughout the matrix because they can interact with other particles only over
a limited area; and they should be approximately the same size because this results in
optimal use of the particles as reinforcement material. The most common method for
formulating particulate composites involves some type of powder metallurgy process
using either metallic or nonmetallic reinforcing materials.”46

“Laminar composites” are layered structures where two of the dimensions of the
reinforcing material are signiﬁcantly larger than the third.45 The layering effect makes it
difﬁcult to classify the continuous and discontinuous phases of the composite, since
neither the reinforced layers nor the matrix are truly “continuous”. Examples of

applications that may use laminar composites include low-performance products such as

household utensils and automotive trim components.“

41

The majority of composite processes and products are considered “high-
performance”, thus the focus of composite research is in the area of “ﬁber composites”,
although the knowledge gained in the generation of these materials may easily be
translated to the production of low-performance composites. Fiber composites offer
tremendous advantages over materials such as wood and metal because they combine
extremely high strength and stiffness and excellent resistance to corrosion and fatigue, all
the while maintaining a very low strength to weight ratio. High-performance, ﬁber
composites are used in a variety of applications ranging from the transportation industry
(automotive, aerospace and marine) to the production of various sporting equipment such

as ﬁshing rods, tennis rackets and golf clubs.

2.3.1 . Methods for Producing Composites
Selecting a composite fabrication method plays an important role in the overall
development of the composite. How the ﬁbers are placed in the mold affects the physical
properties of the part and ultimately impacts the decision of selecting a molding

process.48

A number of processes are currently being used in the fabrication of
composites. Composites are currently being produced on both large and small scales, in
continuous and batch processes, for both high and low-performance applications.
Depending on the particular application, a process might be entirely automated or highly
labor-intensive and it may use either closed or open molds to form the composite. While
the current state of the art for composite fabrication processes are labor-intensive to a

degree, the general consensus and goal in the composites industry is to move to the point

where even highly complex products such as aircraft components are almost entirely

42

automated.45 It is believed that the next-generation composites will compare favorably
with metallic structures, hence it is necessary to automate the processes used to fabricate
these systems in order to rapidly produce composites capable of meeting the increasing
demands such as material speciﬁcations and cost. By achieving complete or near-
complete automation, low-quality operations such as laying out the prepreg and

transferring the material between stations will be eliminated.45

2.3.la. Filament Winding. Continuous ﬁlament winding is one of the most
commonly used methods for continuous composite processing. There are a number of
advantages associated with ﬁlament winding including: repetitive placement of ﬁbers,
ability to use continuous ﬁbers over an entire part without forming any joints, high

degrees of ﬁber loading, elimination of expensive autoclaves.“9

Unfortunately, the
complexity of parts produced using ﬁlament winding is limited and the surface of the
cured composite is typically not ﬁnish quality.49 There are three methods used to unite
the binder and reinforcing phases of ﬁlament winding processes: preimpregnation
(prepreg), wet rerolled prepreg, and wet winding.

Prepregs are systems where the binder phase has been added to the reinforcement
phase and gelled to an intermediate stage before cure. This is the most expensive
ﬁlament winding method and must be cured at elevated temperatures, however it is also
the cleanest method and provides the best control of resin content and product quality. In
direct contrast to prepregs, wet winding involves addition of the resin to the

reinforcement at the time of winding.49 While this results in poor control of resin content

and product quality, it is the least expensive winding process and can be used with the

43

widest variety of ﬁbers available to ﬁlament winding.49 Wet rerolled prepregs lie
somewhere between prepregs and wet winding. This process involves a combination of
preimpregnation and wet winding. Thus it is less expensive than prepregs and provides
better control over resin content and product quality than wet winding.49 In general,
ﬁlament winding is a favorable alternative to many composite processing methods
because it effectively couples expensive ﬁbers with inexpensive resins to produce

relatively inexpensive composites.

2.3.lb. Resin Transfer Molding. Resin transfer molding (RTM) is a composites
fabrication technique in which a reactive resin is injected into a closed mold containing a
preplaced ﬁber mat, or preform. The fact that the dry reinforcement and resin are
combined within the mold distinguishes RTM from other liquid molding processes. The
curing of the resin (typically a thermoset) results in the formation of a ﬁber reinforced
composite which assumes the shape of the mold. Unlike well-developed processing
techniques such as hand lay-up, compression molding, and non-structural injection
molding, RTM is relatively new and undeveloped.47 However, RTM offers important
advantages since it is considerably less labor-intensive than hand lay-up methods, and
may be used to produce more complex reinforced composites than compression molding
or injection molding. For these reasons, RTM has already established itself as a viable
cost-effective alternative for the production of complex parts in quantities too large to
justify hand lay-up, and the production of less complex parts in quantities too small to
justify the capital costs associated with compression molding. 50 As RTM is developed it

will undoubtedly expand further into both processing domains.

44

Examples of the tremendous promise of RTM for composite fabrication in the
automobile industry were recently provided by Johnson50 and Chavka and Johnson.5|
These authors describe the production of complex composite support structures for the
Ford Escort and the Ford Aerostar, respectively, by HSRTM of a vinyl ester resin with
glass ﬁber preforms. In the former case, two composite parts can replace a 90 piece steel
structure with substantial weight savings and improved stiffness and strength. Production
cycle times for mass production were projected to be in the 6 to 9 minute range. However
it was concluded that mold ﬁlling, heat transfer, and gelation of resin in the transfer lines
are all issues that must be resolved before the process may be successfully used in
medium or large scale production.

Although it is a versatile method capable of producing composites of increasing
complexity, RTM is not without limitations or problems. For example, RTM processing
methods are complicated by the fact that the resin begins to cure before entering the mold.
In order to produce acceptable products, the resin must meet some rather stringent
requirements. The curing resin should exhibit a moderate viscosity plateau of 0.1 to 0.5
Pa - s (1 - 5 poise) for 20 seconds while ﬂowing into the mold, but should cure rapidly
once the mold has been completely ﬁlled. RTM has already found extensive application
for low volume production of a variety of specialty products,47 however further
developments are required in order to reach high volume RTM production for many
composite applications. A considerable challenge exists in meeting the two fundamental
requirements of mass production: low cost and high speed. The fact that a curing resin
enters the mold makes meeting both of these requirements extremely difﬁcult. In general,

RTM processes are operated at relatively high mold pressures to reduce the overall cycle

45

time of the process. While this is an effective method for obtaining more rapid cure rates,
it inevitably results in increased costs because the process requires more expensive
pumping systems to increase the ﬂow rate and more expensive, stronger molds to
withstand the increased pressure.

Many technical problems associated with resin transfer molding arise during mold
ﬁlling. The problem of mold ﬁlling is very complicated since a reacting liquid is being
forced through a porous medium (the preform). As the liquid reacts it becomes more
viscous (actually viscoelastic). Thermoset systems typically exhibit a tremendous
increase in viscosity as they cure due to branching and crosslinking in the system. Due to
the moderately high initial (uncured) resin viscosities which only increase during
reaction, most current RTM processes exhibit signiﬁcant mold ﬁlling problems
associated with high operating pressures required to ﬁll the mold and poor resin
impregnation into the ﬁbers. Moreover, preform displacement and/or compression often
occurs as the curing resin ﬂows into the mold. Preform displacement is an extremely
signiﬁcant problem in the sense that it undermines a major advantage of RTM: precise
control over ﬁber placement. A ﬁnal occurrence which can cause considerable delay and
downtime is gelation of the resin in the transfer lines before they enter the mold.

In addition to the mold ﬁlling problems discussed above, further complications
arise from the facts that the mold geometry may be highly irregular and the process- is
typically not isothermal. Furthermore, it has been found that it is important to
differentiate ﬂow on the microscale (within a ﬁber bundle) from ﬂow on a macroscale
(between ﬁber bundles). It is suggested52 that micro-ﬂow improves the wetting and the

bonding at the ﬁber-matrix interface, and therefore improves the strength of the ﬁnal

46

composite. The importance of the micro-ﬂow was underscored by Patel et al.,52 who
found that the best composite properties were under low-pressure conditions in which the
mold ﬁlled slowly, allowing ample time for micro-ﬂow within the ﬁber bundles. The
time required for micro-ﬂow depends upon the viscosity of the penetrating ﬂuid, but may
be on the order of hours for large parts using commercial resins. There are a large

number of studies dealing with the problem of mold ﬁlling on the macro— and

53,54 56

. . . . . S
rrucroscales, under rsotherrnal and non-isothermal conditions,5“‘

accounting for
compression of preforrnsf7 and for the viscoelastic behavior of the resin.58

In general, many RTM limitations in high volume production arise from the fact
that the resin system begins to react before it enters the mold, thereby creating high
viscosities, high operating pressures, poor "wet out," and preform displacement.
Additionally, attempts to decrease the cycle time by increasing the operating pressure to
accommodate faster reactions may actually decrease the quality of the composite by
allowing insufﬁcient time for micro-ﬂow. Finally, the high capital costs associated with

higher operating pressures create an inevitable trade-off between high speed and low cost

under the current technology.

2.3.1c. Reaction Injection Molding. Reaction injection molding (RIM) and
compression molding processes are used for molding polyurethanes, epoxies and other
liquid systems. In general, injection molding processes involve two to four component
systems that are mixed using some type of high-pressure, impingement-mixing technique.
In a typical RIM process the ﬂuid is forced into a closed mold. This is the fundamental

difference between RIM and compression molding, where the resin ﬁlls an open mold

47

which is then forcibly closed.59 Both RIM and compression molding can be readily
automated, although RIM is a superior alternative to compression molding for the
production of detailed parts. Injection molding processes are categorized as thermosets or
therrnoplastics based on the resin being cured. Thermosets are typically injected into a
warm mold where the resin undergoes further polymerization or crosslinking to produce a
solid polymer while thermoplastic resins are ﬁrst melted, then placed in a relatively cool
mold which allows the material to solidify and take the shape of the mold before
removal.59 It has been stated that compression molding is the most developed method in
terms of its ability to incorporate continuous or chopped ﬁbers into structural composites,
however some compression molding processes involving regions of high ﬂuid ﬂow rates
may result in orientation of ﬁber in the direction of ﬂuid ﬂow."0 This can be an
advantage if oriented ﬁbers are desired because ﬂow patterns could be studied and
developed in such a manner to achieve this effect.(’0

Unlike compression molding, RIM is not used to produce structural composites.
However, processing methods based on variations of reaction injection molding are
currently used to produce composites. These methods have been classiﬁed50 as i)
vacuum-assisted resin injection (VRIM), ii) structural reaction injection molding (SRIM),
and iii) high speed resin transfer molding (HSRTM). These methods are essentially
classiﬁed according to the pressure used to inject the resin, and the time required to ﬁll
the mold. VRIM uses very low mold pressures and very long ﬁll times with cycle times
approaching hours or days. Therefore relatively inexpensive molds may be used and slow
curing resins are required. SRIM uses more reactive resins and higher mold pressures

approaching 50 to 100 psi. Typically two highly reactive resins are held in separate

48

holding tanks before injection, and are impingement mixed immediately before entering
the mold. The resin ﬂows into the mold and "wets out" the preform while the reaction is
occurring. HSRTM is similar to SRIM except that higher mold pressures are used (100 -
500 psi), resulting in faster ﬁlling times; and the mold is maintained at elevated
temperatures to minimize cure times. HSRTM typically requires stronger and more
expensive metallic molds and has relatively short cycle times on the order of 1 to 10

minutes.

2.3.1d. Hand Lay-up. Hand lay-up is used in the production of highly detailed
composites, where the complexity of the part eliminates the possibility of molding; and
specialty composites in which the cost of setting up a molding process becomes
prohibitive. In either case, hand lay-up methods play an important role in the production
of composites. Hand lay-up has the advantages of great part complexity, pr_ec_i_s_e
placement of the ﬁber reinforcement, the ability to allow time for superior micro-ﬂow,
and production of ﬁnish-quality products in a single step. Unfortunately, each of these
advantages come at the expensive of time and are highly labor-intensive. Composites
produced by hand lay-up methods are very expensive because their production is highly
labor-intensive and time-consuming. An additional disadvantage of hand lay-up is the
fact that it cannot be used for large-scale production. However hand lay-up methods are

still the state of the art for producing highly complex, specialty composites.

49

2.3.2. Materials used in the Fabrication of Composites

Choosing a combination of reinforcing materials and binder matrices is indeed a
daunting task. The availability of materials which may be incorporated into the design of
a composite are so vast that a task as simplistic in theory as selecting materials for the
eventual composite design is in fact highly involved in practice. Recent developments in
the areas of ﬁber technology and resin formulation have only served to exacerbate this
situation. This section should not be thought of as a comprehensive review of all
materials available for composite design, rather it should serve as an illustrative guide as
to the wide range of materials which are currently being used in the design and fabrication
of composites.

Materials that are used as the binder phase for composites are selected for a
variety of reasons sUch as chemical resistance, heat resistance, cost, and general purpose
(or overall properties of the material). The ability to be used as an all-purpose binder
phase make polyester and vinyl ester resins the most commonly used resins in the

production of composites.6|

There are, however, a great number of alternatives to these
resins which afford tremendous ﬂexibility in designing a composite for a speciﬁc
application. For example, epoxy resins provide a broad range of physical properties
which make them possibly the most ﬂexible alternative for a variety of high-performance
applications while polyirnide resins are particularly well-suited for composites designed
for high temperature applications.62"r’3 Figure 2.4 is a schematic (adapted from ref. 64)

which provides a qualitative comparison of some physical properties for a number of

polymeric resins.

50

In general, the polymeric resin serves the role of matrix material while the
reinforcing material deﬁnes the overall performance of the composite. Scientists are
constantly investigating new reinforcing materials or modifying existing ﬁbers to meet
the ever-increasing demand for better, cheaper, lighter composites. While current state of
the art reinforcing ﬁbers may be made out of E-glass, S-glass, aramid, silicon carbide,
boron, alumina, fused silica, alumina-boria silica, or carbon graphite, the technology is
still evolving. A list of a few well-developed resin/ﬁller systems is shown in Table 2.1
(adapted from ref. 64). Rosato64 provides a far more comprehensive description of both
resins and ﬁller materials as well as the strengths and weaknesses of numerous

materials/composite formulations in his review.

Table 2.1. Compatibility of polymeric resins with reinforcement materials.

 

 

 

 

 

 

 

Calcium Carbon Glass

Alumina Carbonate Black Clay Fibers Quartz Talc
Alkyds NO YES NO YES YES NO YES
DAP NO YES NO YES YES NO YES
Epoxy YES YES YES YES YES YES NO
Phenolic YES YES YES YES YES N YES
Polyester NO YES NO YES YES NO NO
Silicone YES YES YES NO YES YES N O
Urethane YES YES YES YES YES YES NO

 

 

 

 

 

 

 

51

TOUGH

ABS l NYLON

    

   
       
   

POLYCARBONATE ACETAL

    

     

   

8 ”<2.
9 POLYPROPYLENE VINYL 7%
Q
l poLYETHYLENE DAP POLYSTYRENE (ti

21;

N 11. ES

TRE
BRITTLE

Figure 2.4. Diagram of physical properties for a number of commonly-used polymeric
resins.

10.

11.

12.

13.

14.

15.

52

2.4. References

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P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY,
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A. Rudin, Elements of Polymer Science and Engineering, Academic Press, New
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G. Odian, Principles of Polymerization, 3rd edition, Wiley & Sons, New York, NY,
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LP. Kennedy, E. Marechal, Carbocationic Polymerization, Wiley & Sons, New
York, NY, 1982.

J. Crivello, “Triarylsulfonium salts as photoinitiators of free radical and cationic
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SP. Lapin, “Radiation—induced cationic curing of vinyl-ether-functionalized
urethane oligomers,” Radiation Curing of Polymeric Materials, C.E. Hoyle, J.F.
Kinstle, Eds., ACS Symposium Series 417, ACS, Washington, DC, 363 (1989).

AD. Wilson, S. Crisp, H.J. Prosser, B.G. Lewis, S.A. Merson, “Aluminosilicate

glasses for polyelectrolyte cements,” Ind. Eng. Chem. Prod. Res. Dev., 19, 263
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EA. Wasson, J.W. Nicholson, “New aspects of the setting of glass-ionomer
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J .P. Fouassier, S.K. Wu, J. Appl. Polym. Sci., 44, 1779 (1992).

W. Radak, “Radiation Curing: New Market Rx,” Chemical Business, October
1990.

T. Yamaoka, Y.-C. Zhang, K. Koseki, J. Appl. Polym. Sci., 38, 1271 (1989).

DC. Neckers, O. Valdes-Aguilera, K.S. Raghuveer, D.G. Watson, “Production of
Three-Dimensional Bodies by Photopolymerization,” US. Patent No. 5,137,800,
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53

SP. Pappas in “Encyclopedia of Polymer Science and Engineering,” 11, HF. Mark,
N.M. Bikales, C.G. Overberger, G. Menges, Eds., Wiley-Interscience, New York,
186 (1988).

L. Angiolini, D. Caretti, C. Carlini, E. Corelli, P.A. Rolla, “Polymeric
photoinitiators having benzoin methyl ether moieties connected to the main chain

by the benzoyl group and their activity for ultra-violet curable coatings,” Polymer,
35, 5413 (1994).

M. Zheng, M. Yang, S. Liu, L. Zhang, “Photopolymerization of propargyl acetate
with Michler’s ketone as photoinitiator,” Chin. J. Polym. Sci., 13, 74 ( 1995).

B. Glad, K. Irgum, P. Horstedt, “Photopolymerization and characterization of
methacrylate-based reactive composite membranes,” J. Membrane Sci., 100, 171
(1995).

R. Mallavia, F. Amat-Guerri, A. Fimia, R. Sastre, “Synthesis and evaluation as a
visible-light polymerization photoinitiator of a new eosin ester with an O-benzoyl-
a—oxooxime group,” Macromolecules, 27, 2643 (1994).

J.L. Mateo, P. Bosch, A.E. Lozano, “Reactivity Of radicals derived from

dimethylanilines in acrylic photopolymerization,” Macromolecules, 27, 7794
(1994).

A. Ledwith, J. Oil Col. Chem. Assoc, 59, 157 (1976).
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NT. Lipscomb, Y. Tarshiani, “Kinetics and mechanism of the benzoin isobutyl
ether photoinitiated polymerization of styrene,” J. Polym. Sci. Polym. Chem. Ed,
26, 529 (1988).

M.V. Encinas, J. Garrido, E.A. Lissi, “Polymerization photoinitiated by carbonyl
compounds. VIII. Solvent and photoinitiator concentration effects,” J. Polym. Sci.
Polym. Chem. Ed., 27, 139 (1989).

M.V. Encinas, C. Majmud, J. Garrido, E.A. Lissi, “Methyl methacrylate
polymerization photoinitiated by pyrene in the presence of triethylamine,”
Macromolecules, 22, 563 (1989).

G. Jiang, Y. Shirota, H. Mikawa, Polym. Photochem, 7, 311 (1986).

CE. Hoyle, M. Keel, K.-J. Kim, “Photopolymerization of 1,6-hexanediol diacrylate:
the effect of functionalized amines,” Polymer, 29, 18 (1988).

29.

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G. Seretoudi, I. Sideridou, “ Benzil/N,N-dimethylaminoethyl methacrylate system
as photoinitiator for methyl methacrylate polymerization,” J.M.S. - Pure Appl.
Chem, A32(6), 1183 (1995).

L. Angiolini, D. Caretti, C. Carlini, N. Lelli, P.A. Rolla, “Polymeric photoinitiators
bearing side-chain acyldiphenylphosphinoxide moieties for UV surface coatings,” J.
Appl. Polym. Sci., 48, 1163 (1993).

T. Kimura, M. Kajiwara, “The synthesis of phosphinylphosphorirnidic hydroxyethyl
acrylate and the electrical properties of its polymer produced by ultraviolet-
irradiation-induced polymerization,” Polymer, 36, 713 (1995).

T. Surniyoshi, W. Schnabel, “On the reactivity of phosphonyl radicals towards
oleﬁnic compounds,” Makromol. Chem, 186, 1811 (1985).

J.C. Scaiano, “Photochemical and free-radical processes in benzil-amine systems.
Electron-donor properties of Ot-aminoalkyl radicals,” J. Phys. Chem, 85, 2851
(1981).

J.V. Crivello, J.L. Lee, D.A. Conlon, “New monomers for cationic UV-curing,”
Society of Manufacturing Engineers, 4, 28 (1982).

J.V. Crivello, “Photoinitiated cationic polymerization of coatings,” Organic
Coatings, Science and Technology, Vol. 5, GD. Parﬁtt, A.V. Patsis, Eds., Marcel
Dekker Inc., New York, NY, 35 (1983).

J.V. Crivello, D.A. Conlon, “Aromatic bisvinyl ethers: a new class of highly
reactive thermosetting monomers,” J. Polym. Sci., Polym. Chem, 21, 1785 (1983).

J.V. Crivello, J .L. Lee, D.A. Conlon, “Photoinitiated cationic polymerization with
multifunctional vinyl ether monomers,” J. Rad. Cur., 10, 6 (1983).

J.V. Crivello, “Cationic polymerization - iodonium and sulfonium salt
photoinitiators,” Adv. Polym. Sci., 62, 1 (1984).

J .V. Crivello, “Recent progress in the design of new photoinitiators for cationic
photopolymerization,” Cationic Polymerization and Related Processes, E.J.
Goethals, Ed., IUPAC, Oxford, UK, 289 (1984).

J .V. Crivello, D.A. Conlon, D.R. Olson, K.K. Webb, “Accelerators in UV cationic
polymerization,” Polym. Paint Colour 1., 178, 696 ( 1988).

J.V. Crivello, J.L Lee, “Alkoxy-substituted diaryliodonium salt cationic
photoinitiators,” J. Polym. Sci., Polym. Chem, 27, 3951 (1989).

42.

43.

45.

46.

47.

48.

49.

50.

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55

G. Manivannan, J.P. Fouassier, “Primary processes in the photosensitized
polymerization of cationic monomers,” J. Polym. Sci., Polym. Chem, 29, 1113
(1991).

J. Pelgrims, “Cationic UV polymerization of cyracure cycloaliphatic epoxies,”
Pigment and Resin Technology, 5, September, 1987.

E. W. Nelson, T. P. Carter, A. B. Scranton, “The role of the triplet state in the
photosensitization of cationic polymerizations by anthracene,” J. Polym. Sci.,

Polym. Chem, 33, 247 (1995).

T]. Reinhart, L.L. Clements, “Introduction to composites,” in Composites, Vol. 1,
ASM International, Metals Park, OH, 27 (1987).

2D. Jastrzebski, The Nature and Properties of Engineering Materials, 2nd edition,
Wiley & Sons, New York, NY, 1976.

P. Beardmore, “Manufacturing processes: consumer products,” in Composites, Vol.
1, ASM International, Metals Park, OH, 554 (1987).

AB. Strong, “Molding,” in International Encyclopedia of Composites, Vol. 3, SM.
Lee, ed., VCH, New York, NY, 420 (1990).

ST. Peters, W.D. Humphrey, “Filament winding,” in Composites, Vol. 1, ASM
International, Metals Park, OH, 503 (1987).

CF. Johnson, "Resin Transfer Molding," in Composites, Vol. 1, ASM International,
Metals Park, OH, 564 (1987).

NC. Chavka, C.F. Johnson, "The Taming of Liquid Composite Molding for
Automotive Applications," Advanced Composite Materials, New Developments and
Applications, Conference Proceedings, Detroit, MI, 115 (1991).

N. Patel, M.J. Perry and L]. Lee, "Inﬂuence of RTM and SRIM Processing
Parameters on Molding Mechanical Properties," Advanced Composite Materials,

New Developments and Applications, Conference Proceedings, Detroit, Michigan,
105 (1991).

A.W. Chan and R.J. Morgan, "Modeling Preform Irnpregnation and Void Formation
in Resin Transfer Molding of Unidirectional Composites," SAMPE Quarterly,
23(3), 48 (1992).

FR. Phelan, "Modeling of Microscale Flow in Fibrous Porous Media," Advanced
Composite Materials, New Developments and Applications, Conference
Proceedings, Detroit, MI, 175 (1991).

55.

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M.J. Owen, C.D. Rudd and K.N. Kendal, "Modeling the Resin Transfer Molding
Process," Advanced Composite Materials, New Developments and Applications,
Conference Proceedings, Detroit, MI, 187 (1991).

A. Bruno, G. Mollna, G. Bertacchi, A. Moroni, "Flow Behavior in Resin Transfer
Molding: Numerical Simulations," Advanced Composite Materials, New
Developments and Applications, Conference Proceedings, Detroit, MI, 149 (1991).

AC. Loos and M.H. Weldeman, "RTM Process Modeling for Advanced Fiber
Architecture," Advanced Composite Materials, New Developments and
Applications, Conference Proceedings, Detroit, MI, 209 (1991).

K. Jayaraman and ON. Satyadev, "The Role of Fluid Rheology in Resin Transfer
Molding," Advanced Composite Materials, New Developments and Applications,

‘ Conference Proceedings, Detroit, MI, 225 ( 1991).

AD. Murray, “Injection Molding,” in Composites, Vol. 1, ASM International,
Metals Park, OH, 555 (1987).

CF. Johnson, “Compression molding,” in Composites, Vol. 1, ASM International,
Metals Park, OH, 559 (1987).

I.H. Updegraff, “Unsaturated polyester resins,” in Handbook of Composites, G.
Lubin, Ed., Van Nostrand Reinhold, New York, NY, 17 (1982).

CA. May, “Epoxy resins,” in Composites, Vol. 1, ASM International, Metals Park,
OH, 66 (1987).

D.A. Scola, “Polyimide resins,” in Composites, Vol. 1, ASM International, Metals
Park, OH, 78 (1987).

D.V. Rosato, “Materials selection, polymeric matrix composites,” in International
Encyclopedia of Composites, Vol. 3, SM. Lee, ed., VCH, New York, NY, 148
(1990).

CHAPTER 3.

RESEARCH OBJECTIVES

The preceding chapters illustrate many of the advantages afforded by
photopolymerizations. Although they have been well—characterized and used extensively
for the production of thin ﬁlms and coatings, few attempts have been made to incorporate
photopolymerizations into the production of thick polymers and/or composites. It is
difﬁcult to hypothesize why the domain of photopolymerizations has not been expanded
to include these products, however there are at least two potential explanations for the
lack of research in the area: the importance of the initiation step in the overall reaction
mechanism has been overlooked and is therefore not completely understood; and there are
a number of logistical problems associated with light intensity gradients and light
scattering throughout the sample which make it difﬁcult to photopolymerize in three
dimensions. While chain polymerizations propagate and terminate in the same manner
once an active site has been produced, the initiation reaction is speciﬁc to the initiating
source be it heat, light, microwave, or ultrasonic frequency. Thus it is extremely
important in a photopolymerization to be able to predict with some degree of certainty
what exactly is occurring in the initiation step(s) and what is its signiﬁcance. However,
this is easier said than done, since it can be difﬁcult to characterize the products of a

polymerization in situ, which makes it difﬁcult to understand their relative importance in

57

58

the overall reaction. In addition, the fact that the existence of severe light intensity
gradients in thicker systems has been well-established may serve as a major deterrent
when trying to convince one of the possibility of using photopolymerizations for the
production of thick or composite structures.

The general objective of this research project has been to verify the applicability
of photopolymerizations for the high-speed, low-cost production of thick polymers and
composites by developing novel polymerization strategies based on ultraviolet
photoinitiation methods for uniform cure in three dimensions. Speciﬁc objectives of this
research were: i) to develop a set of guidelines which would predict whether or not a
particular photoinitiator (or class of initiators) could be used in the production of three-
dimensional polymers; ii) to thoroughly characterize the initiation step of free-radical and
cationic photopolymerizations which could provide an explanation for why some systems
are susceptible to three-dimensional polymerization while other formulations fail to
polymerize in thick or 3-D geometries; iii) to determine an optimal initiator concentration
and formulation for a representative system including photoinitiator, photosensitizer and
thermal initiator which can be used to demonstrate how photopolymerizations afford
control over the general kinetics of the polymerization reaction as well as how minute
alterations in the initiation mechanism affect the entire polymerization; iv)
characterization of the mechanical properties of the composites cured using the novel
methods which would provide experimental evidence as to the potential of
photopolymerizations as a method for the production of high-performance composites; v)
determine the effects of a number of processing conditions on the kinetic and mechanical

properties of the cured polymers and composites.

CHAPTER 4.
DEVELOPMENT OF THE THICK PHOTOINITIATION STRATEGIES:

FUNDAMENTAL EXPERIMENTS

4.1. Introduction

A great number of compounds have been studied as initiators for free radical
photopolymerizations."12 Benzoin ethers are the most commonly used class of these
compounds, and their use as free-radical photoinitiators has been well-documented”9
There are a number of advantages associated with photopolymerizations that make them a
practical alternative to current methods for industrial applications. Included among these
advantages are excellent spatial and temporal control over the reaction and superior
energy efﬁciency. These and other advantages are largely responsible for the rapid
growth rate of photopolymerizations, roughly 20-30% per year, for industrial
applications. This growth rate is even more signiﬁcant when one takes into consideration
the fact that the majority of applications for photopolymerizations have focused on thin
systems, thus targeting only a small portion of the polymer industry.

Although photopolymerizations are used in a number of applications involving
thin ﬁlms, inks, coatings and electronics, their development for thick polymers and
polymeric composites has basically been restricted to a few specialty applications that

utilize techniques such as photolithography and screen printing.'3"5 While

59

6O

photolithography is an excellent method for creating detailed polymeric structures, its
industrial usage is largely restricted to the production of prototypes and electronics
applications such as printed copper circuitry and [replication of optical disks due to the
fact that the products typically have relatively low mechanical and physical properties.

One industry where it is common practice to use photopolymerizations to produce
composites is the dental industry. Photocurable dental composites based on acrylate
resins have been used for years as long-term ﬁllings for teeth that have been damaged by
cavities or other dental problems. This process demonstrates both the ﬂexibility and the
spatial and temporal control afforded by photopolymerizations. This application, while it
is an excellent example of the potential of photopolymerizations for the production of
composites, differs from the research conducted in this project in at least two areas. First,
it uses point focused light sources (lasers) to cure the sample whereas the light sources
used in this research were less expensive arc lamps and other sources which produce light
over a much larger wavelength range at a sacriﬁce of intensity at any given wavelength.
Second, the method of cure is based on “overexposing” the reactive resin (using enough
light so that even with losses due to absorption, a sufﬁcient amount of light is able to
penetrate through the bulk of the sample) with initiating light, which can be done in this
case because the thickness of samples is relatively limited (less than 1 cm).

Alternatives to current photoinitiator systems have coupled some of the traditional
photoinitiators with well-known hydrogen donors such as tertiary amines in an attempt to
accelerate the polymerization processm’l7 While these efforts have proven to be effective
in thin-system photopolymerizations,l7 the presence of tertiary amines may in fact inhibit

efforts to photopolymerize thicker systems. The possible interference Of certain

Str.
usi

the.

COm

initial

4

61

accelerators in thick systems coupled with the fact that severe light intensity gradients
initially exist in thick samples are likely responsible for the general consensus in the
scientiﬁc community that photopolymerizations cannot be employed to cure thick
polymers.18 However, detailed investigations of common photoinitiator systems have
revealed that it is in fact possible to use photopolymerizations to produce thick polymers
and composites if every component of the reactive system is chosen judiciously.

In this chapter, two strategies for photocuring thick polymers and composite
structures in relatively short cycle times using moderate light sources are presented. In
the ﬁrst strategy, samples in excess of two centimeters have been polymerized using
relatively low intensity light sources (less than 1 mW/cmz) by carefully selecting the
initiator (both formulation and concentration) and the incident wavelength. The second
strategy capitalizes on the fact that the polymerization reactions are highly exothermic by
using the heat generated by polymerization to “kick off” a second reaction where a
thermal initiator undergoes some type of dissociation reaction which results in the
generation of additional active sites. In essence, the heat caused by the
photopolymerization causes the thermal initiator to dissociate, and radicals are produced
concurrently by thermal and photoinitiation, allowing for more complete cure, especially
in partially opaque systems or complex geometries where it is difﬁcult or impossible for

the incident light to reach the sample in every comer or crevice of the mold.

4.1.1. Self-Eliminating Light Intensity Gradients
By carefully selecting the initiator formulation and concentration along with the

initiating wavelength and intensity, vinyl ester polymers as thick as 3-4 cm can be cured

62

in relatively short cycle times (ranging from 2 - 7 minutes) depending on the light source.
These polymerizations have been possible due to the existence of a light intensity
gradient that is “self-eliminating”. In these systems, a compound absorbs light at a
particular wavelength and is excited to a higher energy state before reacting to produce an
active site (either a cation, radical or anion). A result of this reaction is the production of
a compound which no longer absorbs light at the initiating wavelength, thereby allowing
incident light to penetrate into the bulk of the sample. In short, the light which is initially
absorbed at or near the surface of the system and thus responsible for any light intensity
gradients in this system is also responsible for initiating the reaction to produce active
sites, which results in a substantial decrease in the light intensity gradient in the system,
thus the system can be described as “self-eliminating”. The idea of a decreasing light
intensity gradient has been discussed in previous literature,'9 however the decay is
attributed to “photobleaching”. It can be argued that a self-eliminating gradient is simply
a photobleaching effect. However, the term photobleaching is a generic statement for all
events, chemical or otherwise, which result in a decrease such as the one that occurs in
self-eliminating systems. What distinguishes this type of system from other
photobleaching effects is that it can be speciﬁcally attributed to a chemical reaction; and
in direct contrast to other photobleaching effects,‘9 it is a very efﬁcient method of
photobleaching. While this effect may not occur in all photoinitiating systems, a set of
loose guidelines to determine which systems will likely exhibit this self-eliminating
characteristic has been developed.

The characteristic of a self-eliminating gradient is likely to be found in systems

which contain initiators (photoinitiator, photosensitizer, or co-initiator) with the

63

following characteristics. The initiator system should consist of at least two aromatic
rings which are either conjugated or otherwise coupled in such a manner that the resulting
molecule has a considerable or continuous pi—electronic structure. This excess of pi
electrons, in this highly coupled aromatic structure, enables the molecule to absorb light
in the ultraviolet or near ultraviolet region of the spectrum. Upon irradiation, the excited
state of this molecule should initiate either a free radical or an ionic polymerization
reaction by some type of electron transfer or dissociation process. The resulting initiator
molecule should lose its characteristic absorbance in the near-UV region of the spectrum,
due to the loss of the coupled aromatic pi structure. If a compound maintains its general
structure even after it has reacted to produce a radical, its characteristic absorbance could
remain largely unchanged. In such situations, the light intensity gradient would not be
sufﬁciently reduced even after long periods of irradiation. Hence the reacted initiator
would continue to absorb initiating light, thus limiting the absorption and subsequent
reaction of initiators in the bulk of the sample.

As stated earlier, the success of any photocurable system is highly dependent on ‘
the initiator formulation. An example of this is an initiator system consisting of BEE and
a tertiary amine (tributylamine, TBA). While TBA and other compounds have been

successfully employed as accelerators for thin photocurable systems,'7"8'2°'23

experiments
performed during the course of this research have shown that in fact some hydrogen
donors have a negative, inhibiting effect on the polymerization reaction when thicker
systems are photocured.

The addition of a tertiary amine will likely promote hydrogen abstraction as the

dominant method for producing radicals as opposed to fragmentation. HI abstraction is

56

[hi

Fig

HUm
lnClu

math.

64

typically more efﬁcient than fragmentation and thus serves to enhance cure in thin
systems, however radicals produced via HI abstraction maintain a structure very similar
to that of the unreacted initiator (see Figure 4.1, a-b), thus they may continue to absorb
incident light appreciably. In this instance the light intensity gradient is not signiﬁcantly
eliminated and the incident light is not able to effectively penetrate the bulk of the
sample, thereby limiting the depth of polymerization. However, when BEE fragments to
produce active radicals, its structure is altered drastically (Figure 4.1 c) and its absorbance
of the initiating light decreases almost completely. In this instance, light is able to
penetrate deeper into the system allowing for more uniform and deeper polymerization in

the bulk of the sample.

ll) TCHZCH on ocnzcn, R (|)CH2CH
Che-GHQ Q‘s-”£2 0-0 WC.)

a) Benzoin ethyl ether b) BEE radical produced c) BEE radicals produced
via H+ abstraction via fragmentation

Figure 4.1. Benzoin ethyl ether photoinitiator (a) and corresponding radicals produced
via H+ abstraction (b) and fragmentation (c).

This research has focused not on developing novel photoinitiators but on
thoroughly characterizing commonly used photoinitiators and initiator formulations to
determine if they have any utility for photocuring thick polymeric systems. To this end, a
number of well-reported radical photoinitiator formulations have been investigated,

5

including: camphorquinone / triethylamine; several benzoin ethers such as benzoin

methyl ether (BME), benzophenone and benzoin ethyl ether (BEE);7'9 and 2-benzyl-2-

Cll

SV

[tn
lit.
the:

16m

65

(dimethylamino)- l -[4-(4-morpholinyl)phenyl]- 1 -butanone and or,a-dimethoxy-a-phenyl-
acetophenone, marketed by Ciba-Geigy (Hawthorne, NY) under the names Irgacure 639
and Irgacure 651, respectively. However, the bulk of this research has focused on
photoinitation based on benzoin ether initiator formulations, and this will be reﬂected in

the results presented in this chapter.

4.1.2. Photo/1‘ hermal Initiation Mechanism

Addition of a thermal initiator to the reactive formulation can greatly enhance the
degree of cure in systems where the sample is largely opaque such as a composite with a
high degree of ﬁber loading. This initiating mechanism capitalizes on the highly
exothermic nature of these systems by using a photoinitiator to initiate reaction at or near
the surface of the sample and “using” the heat generated by this reaction to cause some
type of dissociation reaction involving a second, thermal initiator which is then capable of
creating active sites in areas where light may not be able to sufﬁciently penetrate the
system.

This initiating method is favorable to traditional, thermally-cured systems because
it is more energy-efﬁcient, highly targeted, and can be initiated at or near room
temperature. The thermal initiator should not absorb the initiating light as it is far less
likely to exhibit self-eliminating characteristics than the photoinitiator. Additionally, the
thermal initiator should be susceptible to rapid dissociation at relatively moderate

temperatures (80 - 100°C). As our results will show, the observed effect of a thermal

initiator such as benzoyl peroxide (BP) on the mechanical properties of cured polymers

can be dramatic.

66

4.2. Experimental

Benzoin methyl ether (BME), benzoin ethyl ether (BEE), benzil, benzophenone,
camphorquinone, tributylarnine (TBA), Eosin B, anthracene, 9,10-diphenylanthracene,
9,10—dihyroanthracene and 9-phenylanthracene were obtained from Aldrich (Milwaukee,
WI). Benzoyl peroxide (BP) was obtained from Polysciences (Warrington, PA). Irgacure
639 and 651 were obtained from Ciba-Geigy (Hawthorne, NY). Derakane resins 470-45,
411c-50 and 441-400 were obtained from Dow Chemical (Midland, MI). The cationic
photoinitiator UV9310C (GE Silicones, Schenectady, NY) had a composition of 5-10
wt% linear alkylate dodecylbenzene, ~50 wt% 2-ethyl-1,3-hexanediol, and ~50 wt%
bis(4-dodecyclphenyl)iodoniumhexaﬂuoroantimonate. Cationic initiator concentrations
speciﬁed in the remainder of this chapter correspond to the overall UV9310C con-
centration. 3,6,9,12-tetraoxatetradeca—1,13-diene (DVE-3) (GAF Chemicals Corp.) was
also used as a cationic monomer. All materials were used without further puriﬁcation.

Absorption Experiments. UV-Vis absorption experiments were conducted in a
Hewlett-Packard Diode Array Spectrophotometer (Model 8452A). Hexanol was used as
the solvent unless otherwise speciﬁed. Absorbance decay experiments were conducted
by irradiating samples in a quartz cuvette for a predetermined amount of time before
mixing the sample and placing it in the spectrometer to measure the absorbance. By
mixing the sample, an average absorbance for the entire sample is collected.

Irradiating Light Sources. The primary light source for this research was a 1000
watt Hg(Xe) arc lamp (model 6293) produced by Oriel Corporation (Stratford, CT). The

light source was ﬁtted with a water ﬁlter obtained from Oriel Corporation to eliminate

67

infrared (IR) light and reduce thermal effects. The ﬁlter was attached to a circulating
water bath to maintain constant temperature in the ﬁlter so as to avoid possible effects
caused by evaporation of water inside the ﬁlter during reaction. Additional light sources
included a 200 watt Hg(Xe) arc lamp (model 6291), also produced by Oriel Corporation,
and a 100 watt Black-Ray long wave UV lamp (model BIOOAP) produced by UVP (San
Gabriel, CA). A hand-held UV meter designed by UVP was used to monitor the incident
intensity to insure that it remained constant for given sets of experiments.

Monitoring Thickness of Photocured Samples. A method was devised for
measuring polymer thickness that consisted of placing 5 gram samples in a glass cylinder
( 1 cm inner diameter). The sample was then placed in an enclosure in such a manner that
it could only be irradiated from light entering the enclosure through a circle having a
cross-sectional area approximately equal to that of the glass cylinder. This setup
guaranteed that light could only penetrate the sample from a single direction, thus
insuring that only one surface of the sample would be directly exposed to initiating light

throughout the course of the polymerization.

4.3. Optimizing the Initiator Formulation

4.3.]. Developing an Operating Window
There must exist at least one region in the light spectrum where only the unreacted
initiator (or sensitizer) absorbs light appreciably in order for a reactive formulation to be
successfully photocured. In addition, the initiator absorbance in this region must decrease

to the extent that it does not absorb appreciably after excitation and subsequent

68

production of radicals. Consequently, light is able to penetrate into the bulk of the
sample because it becomes transparent to the initiating light once it reacts. If these
criteria are met, then there is a region in the light spectrum where the polymerization can
be photoinitiated without interference from species that do not participate in the initiation
reaction. It should be noted that the operating window is speciﬁc to each reactive
formulation, hence the existence of an operating window must be determined before a

reactive formulation can be photocured in three dimensions.

4.3.1a. Radical Systems. Figures 4.2-4.3 show an Operating window for a
typical vinyl ester system studied in this research including thermal and photoinitiators,
monomer/resin, and initiator fragments produced by photofragmentation and hydrogen
abstraction mechanisms. This system consists of an oligomer (470-45) and an initiator
formulation (BEE and BP) present in a 99.6 / 0.4 ratio by weight (initiator formulation is
0.2 wt% each for BEE and BP). It should be noted that the absorbance of benzil, whose
structure is essentially that of a BEE radical produced by a fragmentation reaction, does
not absorb appreciably above 310 nm. Figure 4.2 clearly shows that the only species in
this system that absorbs light appreciably in the 320-360 nm region is the benzoin ether
photoinitiator. Figure 4.3 shows the absorbance spectra for BEE radicals generated via
hydrogen abstraction (illustrated in Figure 4.1 b) and photofragmentation (illustrated in
Figure 4.1c). Figures 4.2-4.3, when observed together, make it abundantly clear that it
should be possible to initiate the polymerization by irradiating the sample using light in
this region of the spectrum. In fact, a more broad description of the operating window for

a vinyl ester based system would simply be that the photoinitiator must absorb light above

69

310 nm. This simple strategy will prevent any light intensity gradient as a result of
monomer absorption. However, absorption of light by the initiator (at the initiating
wavelength) will still result in the formation of light intensity gradients, even though the
initiator is present in relatively small quantities (~O.l wt. %). A strategy for reducing or
. eliminating even these gradients is required for the successful photoinitiated
polymerization of thick parts.

A strategy to reduce or even eliminate the light intensity gradients arising from the
initiator has been developed. In most systems, light intensity gradients arise not only
from the initiator molecules before they have reacted to produce active centers, but also
from the residual initiator fragments after reaction. If these residual initiator fragments
absorb light at substantially the same wavelengths as the unreacted initiator, the light
intensity gradient will persist even after initiation of the polymerization reaction.
However, if the absorbance (at the initiating wavelength(s)) of the residual initiator
fragment is signiﬁcantly lower than that Of the parent molecule, then the light intensity
gradient will disappear at a rate proportional to the rate of disappearance of parent
molecules. Using this criteria, photoinitiators which become non-absorbing at the
initiating wavelength upon reaction have been identiﬁed. It has been found that these
initiators may be used to polymerize systems of considerable thickness since light
intensity gradients that exist at the beginning of the reaction are self eliminating.
Therefore these initiators allow continuously increasing light penetration in the sample,
and consequently allow the relatively rapid and uniform polymerization of thick
polymeric structures. In this study, three separate photoinitiator formulations were tested

for the self-eliminating characteristic: benzoin ethers (BBB and BME), camphor-

70

quinone/TBA (abbreviated as C/T BA), and Eosin B. Benzoin ethers absorb UV light,
C/TBA absorb in the UV and the visible regions, and Eosin B absorbs in the visible
region of the light spectrum. Absorbance spectra for BEE and C/T BA photoinitiator
formulations are shown in Figures 4.2-4.

To test for a self-eliminating characteristic, absorbance lifetime experiments were
performed for each photoinitiator. Two of the initiators (C/T BA and BEE) exhibited
marked decreases in their average absorbance the longer they were exposed to the
incident light. In the case of OT BA, the absorbance decrease occurred between 440-490
nm, while the BEE absorbance decrease took place between 320-360 nm. The
wavelengths that were monitored and compared (472 nm for C/TBA and 328 nm for
BEE) in Figure 4.5 were selected because they represent the maximum initial absorbance
in the decaying region. In order to directly compare the two absorbance proﬁles in Figure
4.5, the data was normalized to an initial value of one. The initial concentration for BEE
was 0.2 wt% and the initiator concentration for C/TBA was 0.2/0.2 wt%
camphorquinone/TBA, while the solvent for both samples was benzene. The shape of the
absorbance proﬁles observed in Figure 4.5 show that the average absorbance of both
samples decrease exponentially as the exposure to the light source is prolonged. This can
be explained by the fact that the initiator fragments do not absorb light in the operating
window, hence the average absorbance of each sample decreases as more and more initiator
molecules react to produce active sites. The net effect of this situation is a continuing
increase in the ability of the incident light to penetrate deeper into the sample as the
initiation reaction proceeds towards equilibrium. Unlike the other two formulations, the

absorbance proﬁle for Eosin B did not decrease signiﬁcantly over prolonged periods of

71

irradiation. This led to the conclusion that Eosin B would not be as effective as either BEE
or CfTBA as a photoinitiator for thick polymerizations, hence no further studies involving

this compound were conducted in the course of this research.

4.3.1b. Cationic Systems. Like radical systems, cationic operating windows are
dependent on the particular system being investigated. One difference between the two
systems is the introduction of an additional component into the cationic system. The
absorbance of the initiator in the cationic system occurs almost entirely below 300 nm,
and this can cause a number of problems including more expensive optics and molds
(made out of quartz), potential absorbance by the monomer or resin (which would
virtually eliminate any possibility of a thick photopolymerization), and a reduction in the
number of commercially available light sources. To alleviate these problems, small
quantities of a compound termed a “photosensitizer” are added to the reactive mixture,
making it possible to initiate polymerization in the near UV or visible region of the light
spectrum. In brief, the photosensitizer absorbs near UV or visible light and is excited to a
higher energy state. From this state, the photosensitizer undergoes direct interaction with
the initiator to produce an active cation. Absorbance spectra for some common
photosensitizers are shown in Figure 4.6. In addition, this ﬁgure shows the absorbance
for anthracene both before and after photosensitization and makes it very clear that there
is a region in the light spectrum where the reaction can be photoinitiated (340-380 nm).
Because many commercially available lamps utilize mercury bulbs, anthracene is a
particularly good choice as a photosensitizer for this system because mercury has strong

emission peaks in the 340-380 nm region.

72

Each of the photosensitizers shown in Figure 4.6 was tested to determine if it
would exhibit the self-eliminating characteristic. Individual samples for each
photosensitizer were prepared in hexanol containing 1 wt% UV9310 and 10-2 wt%
photosensitizer. Absorbance decay experiments for each sample were conducted using
the method described in the previous section. Results for the three systems (shown in
Table 4.1) clearly establish that neither perylene nor pyrene absorbance decays
sufﬁciently. In fact, pyrene absorbance actually increases slightly (likely a result of
sample degradation). In direct contrast to these photosensitizers, anthracene absorbance

decayed rapidly to approximately zero even at a low light intensity (0.75 uW/cmz). The

decay constant, 1rd in Table 4.1, is the rate at which the absorbance decays according to
the expression

A = Ce'md (4.1)
where A is the absorbance at time t, and C is a constant of the equation. Figure 4.7
shows the rate of decay for the anthracene system at 364 nm. The exponential decay
observed in Figure 4.7 shows that the average absorbance of this sample decreases even
more substantially than the radical system, suggesting that an initiator formulation including

anthracene could be extremely effective for cationic photopolymerization of thick polymers.

Table 4.1. Absorbance decay characteristics for various cationic photosensitizers.

Photosensitizer Normalized Absorbance (@ t= 1 hour) Decay Constant, 1d (min")

 

Anthracene ~ 0.00 1.8
Perylene 0.98 NA
Pyrene > 1.0 NA

 

 

73

4.3.2. Selection of Suitable Photoinitiators

Since all of the monomers/resins, hydrogen donors (TBA), and thermal initiators
(BP) investigated in this research all absorb below ~ 310 nm, it is theoretically possible to
initiate a radical photopolymerization consisting of any combination of monomer/
accelerator/thermal initiator using any photoinitiator that absorbs above 310 nm.
However, the selected photoinitiator must be self-eliminating. The C/TBA initiator
formulation and the benzoin ethers both demonstrated this characteristic, thus they were
tested further to determine which initiator system was best suited for this research. Each
initiator system was added to the resin such that the weight percent of the formulation
consisted of 0.2 initiator / 99.8 470-45 resin.

The effectiveness of these formulations were compared by direct observation and
thermal proﬁles obtained using a computerized thermocouple system (Omega Systems,
Stamford, CT). By direct observation it was determined that the OT BA formulation was
not suitable for producing thick polymers from vinyl ester resins because it proved
incapable of producing a polymer thicker than roughly 2 mm after a reasonably long
period of irradiation (15 minutes) at a moderate incident intensity (approximately 1.2
uW/cmz). Thermal proﬁles of this system indicated that the bulk of the reaction had
occurred after 15 minutes by showing that the temperature of the system peaked at 4.9

minutes and subsequently cooled to roughly ambient temperature (40-50°C) after ﬁfteen

minutes. This suggested that reaction had occurred only on or near the surface of the

sample while the bulk of the sample remained uncured. One possible explanation for this

74

is the absorbance of camphorquinone in the initiating region (440—490 nm) is extremely
modest, thus even slight absorption in this region by initiator fragments or other species is
enough to keep additional initiator sites from being excited to a higher energy state and
the molecule is not able to dissociate to produce a reactive radical.

The benzoin ethers proved to be far more successful than C/T BA at polymerizing
thick polymers, even upon exposure to relatively low intensity (less than 1 uW/cmz) light
sources. Direct observation and thermal proﬁles of the BEE system indicated irrefutably
that the bulk sample had polymerized after ﬁfteen minutes. The 2 cm thick sample had
completely solidiﬁed, leaving no discernible traces of uncured monomer at any point in
the sample; and the thermal proﬁle indicated that the temperature of the system peaked at
4.1 minutes and had returned to ambient conditions before ﬁfteen minutes. Repeating
this set of experiments yielded virtually the same results, making it clear that the benzoin
ethers were the most effective class of photoinitiators for this system. Table 4.2 shows
the effects of various initiator formulations on polymer thickness for samples irradiated

for 5 minutes using the 1000 watt source (measured intensity of 0.40 uW/cmz).

Table 4.2. Effect of initiator formulation on polymer thickness.

 

Initiator Formulation I % Monomer Cureda I Polymer Thickness (mm)b
Camphorquinone/TBA 1 1.4 i- 0.2 1.7 i 0.1
Irgacure 651 41.9 i 0.6 12.9 i 0.2

BEE 43.4 i 0.8 13.5 i 0.2

BME 42.6 :t 1.2 13.4 i 0.2
BME/BP 58.6 i 1.4 22.4 i 0.3
BME/TBA 38.5 i 0.5 12.2 i 0.2

3compares the mass of the sample before and after irradiation
bmeasures the polymer thickness at the largest point in the sample

75

The process of optimizing the photoinitiator formulation began as soon as it was
determined that the benzoin ethers were the best class of photoinitiators for this research.
Two separate studies were conducted to quantify the optimal initiator formulation. The
ﬁrst study involved monitoring the average absorbance of BEE samples in hexanol with
concentrations of photoinitiator ranging from 0.05 - 2 wt%. The second study measured
the ﬂexural modulus for polymers photoinitiated using the same range of photoinitiator
concentrations. Results from the ﬁrst set of experiments showed that the average
absorbance for initiator concentrations greater than ~ 0.5 wt% were not signiﬁcantly

reduced after irradiation for a period of 15 minutes at moderate (~ 1.0 uw/cmz) light

intensities. This meant that initial light intensity gradients for samples having initiator
concentrations above 0.5 wt% would not decrease signiﬁcantly during this period of time.
Thus few if any active sites would be produced in the bulk of these samples and
polymerization would therefore by limited everywhere except at or near the surface.

The second study showed that the maximum value for the ﬂexural modulus for
neat (no reinforcing ﬁber added) 470-45 polymers photocured for 4 minutes occurred
around 0.2 wt% of photoinitiator. Figure 4.8 shows the ﬂexural modulus as a function of
initiator weight percent for samples up to 1 wt% photoinitiator. Samples containing
greater than 1 wt% of photoinitiator were not completely cured after four minutes of

irradiation at this light intensity (~1.3 uw/cmz). As seen in Figure 4.8, there is an

optimum amount of initiator for achieving maximum strength in a given cure time, and
further increases in the initiator concentration actually undermine the mechanical
properties. In fact, he shape of the curve in Figure 4.8 can be divided into three regions.

In the ﬁrst region (0.05 - 0.15 wt%), the ﬂexural modulus increases as function of

76

initiator concentration. This suggests that an insufﬁcient number of active sites are
produced at very low initiator concentrations (0.05 wt%). The maximum values for the
ﬂexural modulus occur in the second region (0.15-0.25 wt%). The third region (> 0.25
wt%) shows that as the weight fraction of initiator increases the ﬂexural modulus actually
decreases as a result of an optical effect. At high initiator concentrations, there is a
dramatic reduction in the penetration of initiating light, since it is strongly absorbed by
the large amount of initiator present. This results in non-uniform initiation and the
formation of a polymeric ﬁlm at or near the surface. It should be state that the optimal
initiator concentration depends on a large number of factors such as initiator type,
temperature, and light intensity and is speciﬁc to each photocurable system.

BEE and other benzoin ethers proved to be the most successful photoinitiators for
the vinyl ester resins investigated in this research because they meet all of the criteria
necessary for efﬁcient elimination of the light intensity gradient. For example, the
absorbance of the unreacted (initiator) and reacted (active radical) states in the operating
window is dramatically different. As the initiator reacts to form active sites, its
absorbance in the operating window decreases severely, corresponding to a decrease in
the overall absorbance in the system due to a reduction in the initiator concentration.
Thus the transverse light intensity gradient that exists initially in the bulk of the sample
will decrease gradually as the reaction proceeds and more initiators absorb light and react
to form active sites. This effect is shown in Figure 4.9. This ﬁgure shows that the
severity of the light intensity gradient decreases as a function of irradiation time.

An important facet of Figure 4.9 is that it provides in situ measurements of the

light intensity gradient. Being able to monitor the light intensity gradient during reaction

77

allows one to extract important information about the system. For example, one can
easily study the effects of initiator concentration and type on the gradient as well as
qualitatively determine the importance of incident intensity on the intensity gradient.
Figure 4.9 shows that the light intensity gradient decreases even at moderate light

intensities (1.0 uw/cmz) for the optimal (in terms of ﬂexural properties) photoinitiator

formulation. Figures 4.8-9 help to emphasize the importance of the self-eliminating light
intensity gradient. Figure 4.8 shows that the initiator concentration affects the ﬂexural
properties of the resultant polymer while Figure 4.9 shows that the light intensity gradient
does decrease at optimal initiator concentrations. In fact, the intensity increased by ~ 9%
after 10 minutes at a depth of 1.1 cm, while the intensity at the bottom of the sample (~
4.7 cm) increased by ... 505%. The rate and the degree of elimination are dependent on

the intensity of the initiating light and the initial concentration and type of photoinitiator.

4.3.3. Eﬂects of Hydrogen Donors

As stated in Chapter 2, a number of compounds are added to photocurable
systems in order to enhance the degree of cure or to increase the kinetics of the
polymerization. Hydrogen donors are commonly employed as accelerators to increase the
polymerization kinetics. The basic principle behind these compounds is that they
enhance the likelihood that a photoinitiator will undergo hydrogen abstraction, which
enhances the polymerization kinetics. However, a look back at Table 4.2 shows an
unexpected effect. Addition of 0.2 wt% TBA to the 0.2 wt% BEE photoinitiator actually
decreases both the degree of cure and the depth of polymerization. At the very least, this

result is surprising considering the reputed success of this initiator formulation in the

78

literaturen'lg‘u'23 Additional experiments performed during the course of this research
have shown that promoting H+ abstraction (by addition of traditional hydrogen donors
such as TBA) results in a decrease in the rate of absorbance decay in the operating
window.

Figure 4.10 demonstrates how the presence of a strong hydrogen donor, in this
case TBA, affects the average absorbance of the system at a wavelength (328 nm) used to
initiate the polymerization. In this experiment, two initiator formulations were irradiated
for a predetermined period of time, removed from the light and placed in a UV-Vis
spectrometer to obtain absorbance spectra. This process was repeated several times with
the same sample until it had been irradiated for a total of ﬁfteen mintues and
corresponding absorbance data had been collected. Each initiator formulation consisted
of 0.2 wt% BEE dissolved in hexanol with one sample also containing 0.2 wt% TBA.
Upon irradiation, the samples absorbed light and were excited to higher energy state
before reacting to produce active radicals. TBA was added to the system to promote one
method of radical production (W abstraction) over the other method (fragmentation).

It is clear from Figure 4.10 that addition of TBA to the system slows the rate at
which absorbance in the initiating window decays. Therefore it can be concluded from
this data that, while it serves as an accelerator in thin-ﬁlm photopolymerizations and
thermal polymerizations, strong hydrogen donors may and in some cases do inhibit thick
photopolymerizations by decreasing the rate at which light intensity gradients in the

system decrease.

79

4.3.4. Effect of Thermal Initiators

As seen in Table 4.2, the degree of curing can be signiﬁcantly enhanced by adding
small amounts (less than 0.2 wt% of the total formulation) of thermal initiators such as
benzoyl peroxide (BP). BP does not absorb in the operating window (see Figure 4.2)
thus it will not interfere with the production of active sites via photopolymerization. In
fact, the highly exothermic reaction associated with propagation elevates the temperature
of the system to the extent that BP dissociates to produce additional active sites that are
capable of reacting with monomers to produce new polymer chains. This method is
exceptionally useful in systems that are semi-opaque or systems where light can not travel
in a direct path through the bulk of the sample such as composites with high degrees of
ﬁber loading. It is also a useful method for systems where it is difﬁcult or impossible for
direct light to reach the entire sample (i.e. complex mold designs).

The addition of thermal initiators can greatly enhance the rate at which thick
polymers are cured. Figure 4.11 shows the effect of BP on the amount of time required to

cure samples of predetermined thickness at a modest light intensity (2.15 uW/cmz). It is

clear from Figure 4.11 that addition of BP increases the rate of propagation signiﬁcantly.
Furthermore, cure time increased only slightly as sample thickness was increased from
2.5 to 15 mm for samples initiated using the dual-cure method while the amount of time
required to cure samples containing only photoinitiator increased by ~ 250% as the
thickness of the sample was increased from 2.5 to 6.5 mm. Interestingly, there is virtually
no change in the amount of time required to cure the polymer as the concentration of BP

is increased from 0.2—1 wt%.

80

Employing a dual cure initiation mechanism to photocure polymers also reduces
the effects of incident intensity on polymer cure time. Figure 4.12 compares cure times
for single-initiation (pure photoinitiator formulation) and dual-cure (photo and thermal
initiator) mechanisms at predetermined thicknesses. This ﬁgure shows how the effect of
light intensity is reduced when a thermal initiator is added to the system. In cases where a
single-initiation mechanism is employed, increasing the light intensity from 0.78 to 2.15

j.tW/cm2 signiﬁcantly decreased the amount of time required to cure samples as the

polymer thickness was increased. However, polymers cured using the dual-cure initiation
mechanism were essentially independent of light intensity as it was increased from 0.78

to 2.15 uW/cmz, indicating that signiﬁcance of light penetration for completeness of cure

is reduced when a thermal initiator is added and active centers are produced from both
photo and thermal initiation reactions. Interestingly, there is virtually no change in the

amount of time required to cure the polymer as the concentration of BP is increased from

0.2-1 wt%.

4.4. Conclusions/Chapter Summary

Photopolymerizations can be used to cure thick systems if careful attention is paid
to: the system formulation including initiator(s), monomer(s) and photosensitizer; the
wavelength or wavelengths of light used to irradiate the system; the mechanism
responsible for production of active sites; and the use of so-called accelerators or
catalysts. These polymerizations can be initiated using low-cost UV light sources such as

are lamps and low-pressure mercury lights. The kinetics and cure times of photocurable

81

polymer systems are enhanced by addition of small amounts of non-interfering thermal
initiators such as benzoyl peroxide to create a so-called dual-cure system. Purely
photoinitiated systems and dual-cure photo/thermal systems are more energy-efﬁcient
than thermally cured systems. In addition, they react immediately upon absorbance of
light of the appropriate wavelength to produce active sites capable of initiating reaction.
The experiments presented in this chapter validate the potential for each of the
curing strategies developed in this research as alternatives to current processing methods
for the production of composites and thick polymers. Both radical and cationic
photoinitiators which exhibit the self-eliminating characteristic have been disclosed and
clear operating windows based on these initiators have been established. Additional
experiments have shown that careful attention must be paid to every compound in the

system because their addition may signiﬁcantly affect the polymerization.

82

 

 

 

 

 

4 « i
3.55 i
. i
35
32.5?
E 23
a
D Z
< 1.5:}
1
11
: — BEE
; ——Derakane
0.53 _____ BP
0 ‘ . . . v 4% r . v
190 440 490

Wavelength (nm)

Figure 4.2. UV-Vis absorbance spectra for typical reactive system including monomer
(Derakane 470-45), photo (BEE) and thermal (BP) initiators.

83

3.5

 

ABSORBANCE

    

------ photofragmentation

—— hydrogen abstraction

.
.0
......
.0
.....
0..
o
o
o
o
0..

 

 

 

I90 240 290 340 390 440 490
WAVELENGTH (nm)

Figure 4.3. UV-Vis absorbance spectra for BEE radicals generated via hydrogen
abstraction (-) and photofragmentation (--) reactions.

84

 

3.00

2.50 7” I: .
a) i 1, Camphorquinone
g 2.00 ‘ I """" TBA

 

I
‘0

j

91.501
0

I

3
(1.00 I

0.50 ‘

 

 

 

 

0.00
1 90 240 290 340 390 440 490 540 590

Wavelength (nm)

Figure 4.4. Absorbance spectra for the camphorquinone / tributylamine coinitiator
system.

85

 

 

 

‘1
1“ \ Light Intensity:
\ \ 0.25 uW/cm2
0.9 ‘.
0.8 ‘\ \‘\
o 0.7 \\
O \ \
S ‘ ‘.
e 0.6 ‘P a
o . .
B ‘. \x W
< 05 ‘.
'8 ' \ ‘..\ oCampho/TBA (472nm)
Q ' ‘x. oBEE(328nm)
E 0.4 0‘. o
o \‘ ‘ -
b\ \\
2 0.3 ‘x\ o‘~c\\
\o‘\ T“‘\
0.2 D run“
0 \“~-..,‘_ ‘0‘“~--_\ 0
or ° 7““‘~~---_--
o.ﬁj.-.r+ﬁj4r....t....i.-..T
o 5 10 15 20 25 30
Time (min)
Figure 4.5. Absorbance decay of radical photoinitiators benzoin ethyl ether (at 328 nm)

and camphorquinone/TBA (at 472 nm) as a function of time.

86

 
 
  

 

 

anthracene (before) - -- - perylene
- - ‘ anthracene(atter) - — - -pyrene
IMP-m

 

 

 

-CO-I-..—..
o.-

 

 

 

".
’a

4 \a—u-uui‘:

440 490 540
WAVELENGTH (nm)

 

590

Figure 4.6. Absorbance spectra for cationic photosensitizers including perylene, pyrene,
and anthracene (both before and after photosensitization).

87

 

1.61

IAJ

1.2-

0.8-

A

0.6~~

0A~

0.2“

 

i."- ’ ":‘ 1;

Sample: .
11)"2 wt% unthrucene/ 1 wt% UV9310
Solvent:
l-hexunol
Monitored Wavelength:
364 nm
Incident Intensity:
~ 0.75 nW/cm2

 

Time (sec)

 

Figure 4.7. Absorbance decay of anthracene at 364 nm as a function of time.

 

88

 

 

 

 

4
3.5 7* l» m
3 1*
A .. 400 ..,
5 2.5 i ' g
E
“l i 300 g
a E
5 in“
5 1.5 ‘
E i ‘* 2w 2:
I *t
r 100
0.5 ‘t
o 4 ¥ ‘r i , v 7% v V o
o 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Weight % of BEE

Figure 4.8. The effect of BEE weight fraction on ﬂexural modulus for unﬁlled vinyl
ester polymers photocured for 4 minutes.

89

 

800” \. C>305ec limin A3min

 

700" “ x5min l37min .10min
600"
500”
400”

3m .1,

Intensity (Counts)

200‘t

100"

 

 

 

Thickness (cm)

Figure 4.9. Intensity of light as a function of thickness for increasing periods of
irradiation.

90

 

 

 

 

 

 

 

 

 

1
0.9 ‘r. a Initiator Formulation
r... (wt% of solution)
3 0.8 oBEE(0.2)
5 0,7 oBEE(0.2)/TBA(0.2)
I-
8 0.6
.D
g 0.5 ..
g 0.4 ...........
a ..........
E 0.3 ~~~~~~~ i
0
Z 0.2
0.1
‘9
0 I r I 1 v v r r r f 4 . § * , 1
o 3 6 9 12 15

Time (min )

Figure 4.10. Average absorbance at 328 nm for BEE(0) and BEE/TBAO initiator
formulations irradiated as a function of time.

91

 

l4

Intensity - 2.15 uwmn’

 

 

 

 

.s 8 ‘*
E
E
g 6 a r
4 d >
+ BEE (0.2)
2 ‘ ’ ' ' . ' ' BEE (0.2”? (0.2)
+ BE (02”? (1.0)
o 3 4 A. 5 ; f f
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

Thickness (mm)

Figure 4.11. Amount of time required to cure samples of varying thickness for a number
of initiator formulations including BEE (0.2 wt%) (0), BEE (0.2 wt%)/BP
(0.2 wt%) (0) and BEE (0.2 wt%)/BP (1.0 wt%) (A).

92

 

 

 

 

30
‘ BEE (0.78) A BEFJBP (0.78)
0 BE (2.15) ' BEUBP (2.15)
25 4. (incident intensity, uwmn‘) 1
20 l
S
E 15 L A . .
cu . . '
E
E-- ’“o . '9
o o I
10 .. ,-’
. a ‘ I
n ’ n
’ A
O ’ I _________________
5 9 O ------------- A 2.: - ...... I
A _______ A ————— L.“- ...................
I. -------- I ° ' " 'l‘ '
0 . 7 v +
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00

Thickness (mm)

Figure 4.12. Effect of light intensity and thickness on cure time for 411-c50 vinyl ester
polymers containing 0.2 wt% BBB and 0.2 wt% BEE / 0.2 wt% BP initiator

formulations.

10.

11.

93

4.5. References

M. Zheng, M. Yang, S. Liu, L. Zhang, “Photopolymerization of propargyl acetate
with Michler’s ketone as photoinitiator,” Chin. J. Polym. Sci., 13, 74 (1995).

L. Thijs, S.N. Gupta, D.C. Neckers, “Photochemistry of perester initiators,” J. Org.
Chem, 44, 4123 (1979).

CB. McArdle, J. Burke, J .6. Woods, US Patent No. 5,084,490, issued 1/28/1992.

0. Nuyken, J. Stebani, A. Lang, “Synthesis and characterization of photopolymers
with azo units,” Angew. Makromol. Chem, 226, 143 (1995).

J.L. Mateo, P. Bosch, A.E. Lozano, “Reactivity of radicals derived from

dimethylanilines in acrylic photopolymerization,” Macromolecules, 27, 7794
(1994).

R. Mallavia, F. Amat-Guerri, A. Fimia, R. Sastre, “Synthesis and evaluation as a
visible-light polymerization photoinitiator of a new eosin ester with an O-benzoyl-
a-oxooxime group,” Macromolecules, 27, 2643 (1994).

L. Angiolini, D. Caretti, C. Carlini, E. Corelli, P.A. Rolla, “Polymeric
photoinitiators having benzoin methyl ether moieties connected to the main chain

by the benzoyl group and their activity for ultra-violet curable coatings,” Polymer,
35, S413 (1994).

B. Glad, K. Irgum, P. Horstedt, “Photopolymerization and characterization of

methacrylate-based reactive composite membranes,” J. Membrane Sci., 100, 171
(1995).

L. Angiolini, D. Caretti. C. Carlini, N. Lelli, P.A. Rolla, “Polymeric photoinitiators
bearing side-chain acyldiphenylphosphinoxide moieties for UV surface coatings,” J.
Appl. Polym. Sci., 48, 1163 (1993).

H. Kaczmarek, L.A. Linden, J.F. Rabek, “Reactions of hydroxyl (H00) and
hydroperoxyl (H020) radicals generated chemically and photochemically with
poly(ethylene oxide),” J. Polym. Sci., Polym. Chem, 33, 879 (1995).

S. Knaus, H.F. Gruber, “Photoinitiators with functional groups. III. Water-soluble

photoinitiators containing carbohydrate residues,” J. Polym. Sci., Polym. Chem, 33,
929 (1995).

12.

13.

14.

15.

16.

17.

18.

19.

21.

21.

22.

23.

94

T. Kimura, M. Kajiwara, “The synthesis of phosphinylphosphorimidic hydroxyethyl
acrylate and the electrical properties of its polymer produced by ultraviolet-
irradiation-induced polymerization,” Polymer, 36, 713 (1995).

DC. Neckers, “Architecture with photopolymerization,” Polym. Eng. Sci., 32, 1481
(1992).

C. Hull, US. Patent No. 4,575,330, issued March 1 l, 1986.

I. Rohm, W. Kunnecke, U. Bilitewski, “UV-polymerizable screen-printed enzyme
pastes,” Anal. Chem, 67, 2304 (1995).

E. Bortel, A. Kochanowski, W. Rys, E. Witek, “Polymerization initiated by the
system triethanolamine/hydrogen sulﬁte with and without the cooperation of
hydrogen peroxide. 1. Polymerization of acrylamide,” J.M.S. - Pure Appl. Chem,
A32, 1197 (1995).

G. Seretoudi, I. Sideridou, “Benzil/n,n-dimethylaminoethyl methacrylate system as
photoinitiator for methyl methacrylate polymerization,” J.M.S. - Pure Appl. Chem,
A32, 1183 (1995).

G. Odian, “Principles of Polymerization,” 3rd Ed., Wiley, New York, NY, 223
(1991).

DC. Neckers, O. Valdes-Aguilera, K.S. Raghuveer, D.G. Watson, “Production of
Three-Dimensional Bodies by Photopolymerization,” US. Patent No. 5,137,800,
Issued August 11, 1992.

SP. Pappas in “Encyclopedia of Polymer Science and Engineering,” 11, H.F. Mark,
N.M. Bikales, C.G. Overberger, G. Menges, Eds., Wiley-Interscience, New York,
186 (1988).

NT. Lipscomb, Y. Tarshiani, “Kinetics and mechanism of the benzoin isobutyl
ether photoinitiated polymerization of styrene,” J. Polym. Sci. Polym. Chem. Ed.,
26, 529 (1988).

M.V. Encinas, J. Garrido, E.A. Lissi, “Polymerization photoinitiated by carbonyl
compounds. VIII. Solvent and photoinitiator concentration effects,” J. Polym. Sci.
Polym. Chem. Ed., 27, 139 (1989).

M.V. Encinas, C. Majmud, J. Garrido, E.A. Lissi, “Methyl methacrylate
polymerization photoinitiated by pyrene in the presence of triethylamine,”
Macromolecules, 22, S63 (1989).

CHAPTER 5.

CHARACTERIZATION OF THE DYNAMIC LIGHT INTENSITY GRADIENT

To validate the self-eliminating light intensity gradient approach for
photopolymerizations of thick polymers and composites, the time-evolution of the light
intensity gradient was characterized both experimentally and theoretically. The time
evolution of the dynamic light intensity gradients was simulated by first solving the
elementary kinetic differential equations for a constant light intensity (to yield the
effective kinetic constant for a rate expression which is ﬁrst order in both the initiator
concentration and the light intensity) then solving the governing set of coupled
differential equations for the light intensity and the initiator concentration as functions of
both position and time. Experimentally, actinometry was used to characterize the light
intensity as a function of both time and position. These studies provide insight on the
effects of factors such as sample thickness, incident intensity, and initiator formulation

(type and concentration) on the time-evolution of the light intensity gradient.

5.1. Simulation of the Dynamic Light Intensity Gradient

5.1.1. Elementary Reactions for a Constant Light Intensity
The discussion in the previous chapter illustrates that benzoin ethyl ether (BEE) is

an appropriate initiator for polymerizations of thick systems if care is taken to ensure that

95

96

the active centers are produced by a photofragmentation mechanism rather than by
hydrogen abstraction. The reaction mechanism for generation of active centers by
photofragmentation is described by three elementary reaction step, as illustrated by

Equations 5.1-3:

kabs
hv + A G) 1A (5.1)
lA => A (5.2)
d
lA => 2R0 (5.3)

where A is the ground state initiator, lA represents the excited singlet state, k, is the rate

.tbs
constant for absorption (the calculation of km based upon the incident light intensity is
described below), k... is the rate constant for nonradiative deactivation (from the excited

state to the ground state) and kd is the rate constant for dissociation of the excited state

initiator to produce active radicals (R0). Therefore, BEE in the ground state absorbs a

photon and is promoted to the excited singlet state, then undergoes an excited-state
dissociation reaction to yield radical centers.
Based upon this set of elementary reactions, differential balances on the ground

and excited-state initiator yield the following kinetic differential equations:

9% = k...(['A1-tA]>+k..t'A1 (5.4)

1[A | I l
gal = k...([Al - [ Al) - k... [ A] - k. [ A] (5-5)

These equations may be readily solved numerically once the rate constants have been

evaluated. The rate constant for absorption, k is equal to the product of the absorption

abs

97

cross-section of the initiator, 0', and the photon ﬂux of the incident light, (1). For BEE, our
absorption experiments revealed that o is equal to 1.8 x 10''8 cm2, while the value of (1) is
easily calculated as the incident light power divided by the energy of a photon (hv). For

our simulations, these calculations yielded a value of 1.509 x 10'5 for km. A value of 1.01
x 10° sec'l was used for the dissociation constant, k d based upon ﬂash photolysis studies
on benzil reported in the literature.1 The rate constant for the return to the ground state

through non-radiative processes, k is readily calculated from the initiator quantum yield,

nr’

f, as shown below:
1
k"r = kdq- 1) (5.6)

This equation is derived from the consideration that the excited-state initiator either
returns to the ground state nonradiatively, or fragments to produce radical centers. For
BEE, a typical value of f is 0.90, yielding a value for k... equal to 1.12 x 105 sec". These
values for the elementary rate constants were then substituted into Equations 5.4-5, which
were then solved simultaneously using the software package Mathcad. Simulation results
for the ground-state initiator concentration as a function of time are shown in Figure 5.1.
This ﬁgure reveals that, for a constant light intensity, the initiator follows a ﬁrst-order
decay with an effective rate constant, km, equal to 0.112 sec'l for photopolymerizations

initiated by benzoin ethyl ether.

5.1.2. Light Intensity Proﬁles as a Function of Position and Time
In these reaction systems the initiator concentration and the light intensity in a
given location are coupled to one another because the light drives a reaction which

consumes the initiator, and the initiator (and its reaction products) in turn absorb light to

98

reduce its intensity. This interrelationship is described by the set of coupled differential

equations and boundary conditions shown below:

9‘5? = - keff [A(z,t)] I(z,t) (5.7)
(II

a; = -eA[A(z,t)]I(z,t) - ER [R(z,t)]I(z,t) (5.8)
initial condition: @ t = O, A = A0 for all z (5.9a)
boundary condition: @ z = O, I = I“ for all t (5%)

where R(z,t) = [Ao(z,t)] - [A(z,t)]
where EA and ER are the molar absorptivities of the original initiator and the initiator
fragments, respectively. 1 represents the transmitted intensity at a particular wavelength
and is a function of both position and irradiation time.

In a completely self-eliminating system, the initiator fragments do not absorb in
the operating window, therefore light is able to penetrate deeper into the sample as
initiator absorbs light and subsequently reacts to produce active sites (initiator
fragments). In this case, the second term on the right hand side of Equation 5.8 will equal
zero, therefore the intensity will be dependent only on the photoinitiator (or
photosensitizer) concentration which decreases monotonically throughout the course of
the reaction. If, however, both the photoinitiator and the initiator fragments absorb in the
operating window, the light intensity gradient will be either partially eliminated or
completely static depending on the absorptivity of the initiator fragment(s). In reality, all

of the radical photoinitiator systems investigated in this research lie somewhere between

99

complete elimination and static light intensity gradients, therefore the second term in
Equation 5.8 must be included in any model.

Equations 5.7-8 were solved using ﬁnite differences to yield a numerical solution
for the light intensity proﬁle. Microsoft Excel software was used to create a spreadsheet
based upon the ﬁnite difference method to determines the normalized light intensity at a
given point in the sample at any given time. The simulations allow the effects of a
number of variables on the dynamic light intensity proﬁles to be determined, including:
the effective rate constant, k

W, the initial concentration of photoinitiator, incident

intensity, sample thickness, and the total time for illumination.

5.1.3. Simulation Results

For illustrative purposes, simulation results were obtained for three types of
reactive systems: i) systems for which the molar absorptivities of the initiator fragments
and the original initiator are essentially the same, (representative of many initiators used
for thin-ﬁlm polymerizations); ii) systems for which the molar absorptivity of the initiator
fragments is lower than that of the original initiator, but non—zero (representative of
benzoin ethers such as BEE); and iii) systems for which the molar absorptivity of the
initiator fragment is zero (a desirable characteristic for photopolymerizations of samples
exhibited by a few systems such as anthracene/iodonium salt cationic initiators).

The light intensity gradient remains static in‘cases where the molar absorptivity of
the initiator fragments is equal to or greater than the molar absorptivity of the original
initiator. An illustrative example of such a system is shown in Figure 5.2. In this case,

light is not able to penetrate deeper into the sample over time which means that

100

polymerization will occur at or near the surface while the bulk of the sample remains
uncured. If a system has a static light intensity gradient similar to the example shown in
Figure 5.2 then it will not produce thick polymers or composites based on the SEG
initiation strategy developed in this research because light will not be able to penetrate
into the bulk of the sample efﬁciently.

In many photoinitiator systems, the molar absorptivity of the initiator fragments,
while smaller than the molar absorptivity of the original initiator, is non-zero. In fact,
this is the situation when benzoin ethers such as those used most frequently in this
research. For BEE, the molar absorptivity of the original initiator is 474 L mol'| cm’l at
328 run while the molar absorptivity of the initiator fragments ranges from ~ 18 - 48 L
mol'l cm'l depending on the initiator fragments generated during the dissociation reaction.
Hence the light intensity gradient will never be completely eliminated even if all of the
initiator reacts because the initiator fragments will continue to absorb (slightly) in the
operating window. However, Figure 5.3 shows that the severity of the gradient decreases
signiﬁcantly over time and there is a noticeable increase in the amount of light that
reaches the bottom of the sample as initiator reacts to produce initiator fragments. Hence
the system is more likely to polymerize throughout the bulk of the sample than a system
where the light intensity gradient is completely static. Figures 5.3-4 demonstrate the
effect the molar absorptivity of initiator fragments can have on the elimination of the
light intensity gradient. When radicals are generated solely by photofragmentation
(illustrated in Figure 5.3), the molar absorptivity of the initiator fragments is equal to 18
L mol'l cm". However, when the initiator undergoes hydrogen abstraction, the molar

absorptivity of the initiator fragments is equal to ~ 48 L mol'l cm’| (illustrated in Figure

101

5.4). Thus this system is not as efﬁcient as the system where the initiator fragments are
generated by photofragmentation because the light intensity gradient does not eliminate
as rapidly or completely.

In the ﬁnal system, the molar absorptivity of the initiator fragments is zero. This
is by far the most idyllic situation for photocuring thick polymers and composites because
the light intensity gradient is dictated entirely by the concentration of original
photoinitiator in the system. Thus as the initiator absorbs light and reacts, the elimination
of the light intensity gradient will eventually be complete if all of the initiator dissociates.
Figure 5.5 is an illustrative example of an initiator system that is completely self-
eliminating. This ﬁgure clearly shows that the light intensity gradient is completely
eliminated, meaning that the amount of light that penetrates through the bulk of the
sample is equal to the amount of light that reaches the surface of the sample (i.e. the
sample eventually becomes transparent to the incident light). While the molar
absorptivity governs the amount of elimination that will ultimately occur, there are
consequences associated with initiator concentration.

The importance of initiator concentration is simulated in Figure 5.6. This ﬁgure
simulates the light intensity proﬁle for a system where the molar absorptivity of the
initiator fragments is zero, however the original initiator concentration is ~700% larger
than the original initiator concentration in Figure 5.5. While the light intensity gradient
is initially very severe even for “normal” concentrations of photoinitiator (Figure 5.5), it
decreases quickly until the gradient is completely eliminated. However, when the
concentration of photoinitiator becomes excessive (as is the case in Figure 5.6), the light

intensity gradient remains severe much longer because a greater number of compounds

102

absorb the initiating light, demonstrating an optical effect which has a devastating effect
on the gradient elimination rate. In this case, the initiator at or near the surface absorbs
light and reacts, becoming transparent to the initiating light. However, the shear
concentration of initiator in this sample dramatically slows the rate at which the gradient
eliminates, creating a system where there is no longer efﬁcient penetration of light into
the bulk of the sample. Hence it is unlikely that these systems would completely
polymerize in reasonable periods of illumination time.

In addition to obtaining three-dimensional proﬁles of the light intensity as a
function of position and time, simulations proved to be useful in predicting how the
intensity of the incident light affects the amount of time required for a given amount of
light to penetrate to a predetermined depth. This is signiﬁcant because the amount of
light required to initiate a reaction resulting in the production of radical centers depends
on a number of variables including: the temperature of the system; initiator type; and
presence (or lack thereof) of additional initiator compounds such as thermal initiators,
accelerators or photosensitizers. Figure 5.7 compares the amount of time required for
varying amounts of light to penetrate to a depth of 1.5 cm as a function of light intensity
for a system where the molar absorptivity of the initiator fragments is zero. This ﬁgure
compares three different “percentages of light” (60, 75 and 90%), where “percentage of
light” represents the amount of light that penetrates to a depth of 1.5 cm divided by the
amount of light that reaches the surface of the sample. From the ﬁgure it is clear that
even at low incident intensities, 60% of light reaches the sample immediately. In cases
where more energy (thus more light) is required, the sample will have to be exposed to

light for longer periods of time. Figure 5. 7 also shows the effects of incident intensity on

103

the amount of time required to reach a given percentage of light penetration. For
example, while one hour of exposure time is required before 90% of the incident light
penetrates through the bulk of the sample for a 1 mW/cm2 light source, 90% of the
incident light will reach the bottom of the sample in less than 5 minutes if a 20 mW/cm2
light source is employed. Higher light intensities reduce the amount of time necessary to
attain the desired levels of energy throughout the bulk of the sample because more intense
light sources carry more energy into the sample, thus more molecules absorb photons and

react in a given period of time.

5.1.4. Discussion of Simulation Results

Clearly, the molar absorptivity of the initiator fragments has a dramatic impact on
the degree of gradient elimination. In an ideal system such as the anthracene/iodonium
salt cationic initiator formulation, the molar absorptivity of the initiator fragments would
be zero and the light intensity gradient would exist only as long as the original initiator
remained in the system, eventually eliminating completely if all of the initiator
dissociated. However, when the molar absorptivity of the initiator fragments is non-zero
(representative of initiator systems such as BEE), a light intensity gradient will always
exist, although it will decrease as the initiator dissociates and it does not prohibit these
initiator systems from being used in self-eliminating gradient (SEG) initiation strategies.
In these initiator systems, it is often favorable to control the method responsible for
generating initiator fragments as some initiator fragments exhibit higher molar
absorptivity values than other initiator fragments (illustrated in Figures 5. 3-4). In the

worst case, the molar absorptivities of the original initiator and the initiator fragments are

104

equal (representative of most initiators used in thin-ﬁlm photopolymerizations), creating
a light intensity gradient which is static. In this case, the initiator system will likely be
ineffective for SEG initiation strategies because there will not be efﬁcient penetration of
light into the bulk of the sample.

The simulation results indicate that system variables such as initiator formulation
and incident intensity can have dramatic effects on the gradient elimination rate. While
the molar absorptivity of the components in the system, particularly the initiator
fragments, dictates the degree to which elimination will occur, the incident intensity and
initiator formulation govern the rate at which elimination occurs. Increasing the light
intensity and/or reducing the concentration of initiator in the system increases the rate at
which the light intensity gradient is eliminated. Conversely, high initiator concentrations
can effectively decrease the gradient elimination rate, thereby reducing the efﬁciency of

light penetration into the bulk of the sample and negatively impacting the polymerization.

5.2. Experimental Characterization of the Light Intensity Gradient

Actinometry is a general term for techniques which are used to monitor the
absorbed light intensity, 1,, of a system during polymerization. It is a valuable technique
for photopolymerizable systems because it can be used to directly characterize the rate of
polymerization (using Equation 2.26) if the propagation and termination rate constants
are known.” Three types of actinometers have been used to monitor 1. during
polymerization: chemical, thermal and electrical. In general, thermal and electrical

actinometers are more convenient than chemical actinometers, possibly because there are

105

a wider variety of thermal and electrical actinometers. Some of the more common
thermal and electrical actinometers include thermocouples which convert photon energy
to electrical or thermal energy, photomultipliers and semiconductor photodetectors.2
Chemical actinometers present a more complicated system than thermal or
electrical actinometers because they are based on using chemical compounds which
undergo a reaction where the quantum yield is known. Chemical actinometry could pose
a host of problems for thick photopolymerizations including: possibility of the chemical
actinometer serving as a photoinitiator, thus distorting the measured In value; introduction
of additional species into the operating window, thus affecting the elimination of the light
intensity gradient; and difﬁculty getting light to the actinometer. To emphasize some of
these problems, one need only examine common actinometers. Although potassium
ferrioxalate is used most frequently,3 benzophenone is also employed as a chemical
actinometer.2 In this function, the benzophenone undergoes a reduction to produce
benzhydrol.2 However, benzophenone is one of the most commonly used radical
photoinitiators,"5 therefore it would be difﬁcult or impossible to determine if
benzophenone were being reduced or was initiating a polymerization reaction. In
addition, in order for a chemical actinometer to be effective it must be able to absorb
light. However, if its primary absorbance is in the same region as the monomer or
another species which is present in excess, its effectiveness will be minimized or
eliminated completely. One ﬁnal disadvantage of chemical actinometers is that they must
be placed directly in the reaction vessel where they are subject to the same system
variations as the rest of the system. This can be a problem since potassium ferrioxalate is

sensitive to changes in temperature and wavelength.2

106

5.2.1 . Developing an Experiment Setup for Actinometry Studies

The inherent difﬁculties associated with chemical actinometers made them a poor
option for studying intensity in this research. Consequently, it was determined that an
electrical actinometer would be the best option for this research in lieu of thermal or
chemical actinometers. The fact that electrical actinometers are non—intrusive and are
therefore not sensitive to environmental changes proved to be the deciding factors when
selecting between thermal and electrical actinometers. Once the type of actinometer to be
used was decided, a setup had to be devised that would be capable of providing in situ
measurements of the absorbed light intensity. This proved to be an arduous task, more so
than it appeared at ﬁrst glance.

The initial setup employed a highly sensitive photomultiplier (PMT) used in
conjunction with ﬁber optics and a monochromator in such a fashion that a single
wavelength could be monitored during reaction. Unfortunately, the sensitivity of the
PMT meant that its environment had to be blanketed in total darkness. What made this a
problem is that measuring devices attached to the PMT such as a voltmeter and the
monochromator also had to be blanketed in darkness. This made it nearly impossible to
accurately read the data. While this setupproved to be an inferior one for monitoring the
intensity during reaction, it did lead to the setup which was ultimately used to collect
intensity data.

A schematic of the experimental setup used in. this research is shown in Figure
5.8. This setup provided a non-intrusive method for obtaining in situ measurements of
the light intensity for samples of thicknesses ranging from 0-5 cm. In fact, this setup

allows one to construct three-dimensional proﬁles for the light intensity as a function of

107

both sample thickness and irradiation time by combining light intensity proﬁles for
samples of different thickness. Developing a setup capable of monitoring the light
intensity after it has passed through reactive samples provides physical data that supports
the importance and the impact of self-eliminating light intensity gradients. The
experimental results produced from dynamic light intensity gradient studies can be
interpreted to determine if a self-eliminating gradient is both feasible and signiﬁcant in
producing thick polymers and composites using photopolymerization techniques.

A series of optical (bandpass) ﬁlters were employed to restrict the wavelength
range to the operating window for the system being investigated. A bandpass ﬁlter

centered around 330 nm (range i 5 nm) was used to study the benzoin ether
photoinitiators while a 360 nm (range i 5 nm) ﬁlter was used to monitor the anthracene

photosensitization reaction. Neutral density ﬁlters were employed to insure that the
spectrometer responsible for “counting” photons was not bombarded with potentially
damaging levels of energy. Neutral density ﬁlters “cut” the intensity through a series of
equations that relate density, D, incident intensity, IO, transmitted intensity, 1,, and
transmittance, T.° The relationship between these variables is shown in Equation 5.10.

T=I,/10= 10'” (5.10)
The effects of neutral density ﬁlters are additive, meaning that they can be placed in
series to attenuate a beam to a desired factor via Equation 5.11

D = - logm(1/AT) (5.11)

where A, is the desired attenuation.

108

5.2.2. Experimental

BEE, BME and anthracene were obtained from Aldrich (Milwaukee, WI). The
onium salt cationic photoinitiator, UV9310C, was obtained from GE Silicones
(Waterford, NY). All materials were used without further puriﬁcation. Each of the
dynamic intensity experiments were conducted in l-hexanol instead of monomer. This
was done for practical reasons. The absence of monomer prevented either system from
polymerizing in the quartz cuvette. If monomer had been added and the samples were
allowed to polymerize, the quartz cuvettes would have been destroyed after each
individual run. This would have required multiple cuvettes in order to complete the
experiments, which would have substantially increased the cost for this study.

In order to construct a three-dimensional intensity proﬁle, samples ranging in
thickness from less than 1 cm to 5 cm were irradiated for a total of 10 minutes per sample

at a constant light intensity (1 uW/cmz). This method produced intensity proﬁles for a

given thickness as a function of time. By varying the thickness from sample to sample,
one can obtain intensity proﬁles over a wide range of sample thickness. A three-
dimensional proﬁle of intensity as a function of sample thickness and exposure time can

then be constructed by combining the individual proﬁles.

5.2.3. Experimental Results
Comparing Figure 5.3 and Figure 5.9, we see that there is a general agreement in
the trends of the model and the experimental results. For example, at a minimum
thickness in each ﬁgure, the intensity proﬁle shows a marked increase right from the

beginning and the intensity approaches the maximum value in each case. As the

109

thickness increases, each ﬁgure show increases in the intensity proﬁle, although the initial
increase is more dramatic experimentally than predicted by the model. And ﬁnally,
thicker samples in both ﬁgures seem to show a slight delay, more pronounced in Figure
5. 9, before the intensity begins to increase.

A simple explanation is offered to justify the delay, or induction period, observed
at the onset of irradiation in thicker samples. This trend stands to reason since the
severity of the initial light intensity gradient dramatically limits the amount of light that
penetrates through the bulk of the sample and few if any initiator molecules are able to
absorb enough energy (light) to undergo reaction. However, as the exposure time
increases, initiators in the deepest regions of the sample absorb enough energy to react,
resulting in the generation of active sites and in the process becoming transparent (or
signiﬁcantly less absorbent) to the incident light. Another signiﬁcant result of the
experimental study shows that the light intensity gradient is eliminated almost entirely for
samples approaching 2 cm in thickness. Were the intensity of the initiating source
higher, one would assume that the intensity gradient would be completely eliminated for
2 cm samples in less than 10 minutes.

Figure 5.10 shows intensity proﬁles for BEE samples at 328 nm before and after

irradiation at 1.2 uW/cm2 for a total of 10 minutes. It is clear from Figure 5.10 that the
severity of the intensity gradient does in fact decrease as initiator reacts to produce active
radicals. In fact 1,, the decay constant in the exponential expression relating intensity, I,

and thickness (denoted by the variable h in Equation 5.12), decreases by a factor of two

after the sample has been exposed for a total of 10 minutes.

110

I = exp(- h / In) (5.12)
One ﬁnal glance at Figure 5.10 shows that the amount of light that reaches the bottom of
the sample (~ 5 cm) after irradiation has increased by 433%, meaning that more than four
times as many photons reach the bottom of the sample as it reacts to produce active sites.

Results for the anthracene system were far less conclusive than for the BEE
system, however similar trends were observed for this system. For example, the amount
of 364 nm light intensity measured after passing through a 2 cm sample containing 0.1
wt% anthracene and 1 wt% UV9310C dissolved in hexanol increased by ~ 60% after the

sample had been exposed to 1.5 uW/cm2 for 20 seconds. However, monitoring the

intensity growth in the anthracene system is more difﬁcult than in the BEE system
because the reaction is far more rapid and the molar concentration of anthracene is
typically much lower than the BEE molar concentration. In order to alleviate this

problem, the intensity of the incident light was decreased to < 0.25 uW/cmz. The results

for this set of experiments are shown in Figure 5.11. This ﬁgure shows a plot of the light
intensity at the bottom of anthracene samples of varying thickness over a period of 900
seconds. The linear increase in light intensity at the bottom of the sample demonstrates

the self-eliminating light intensity gradient.
5.3. General Conclusions/Chapter Summary
Obtaining intensity proﬁles proved to be very important to the success of this

research. The modeling experiments veriﬁed the importance careful selection of

initiators, incident intensity and initiator concentration in the evolution of self-eliminating

111

light intensity gradients. The experimental results veriﬁed both the existence of an initial
light intensity gradient and its disappearance upon generation of active centers. Because
the experimental data report the amount of intensity that has passed through a sample of a
given thickness, the data can be directly related to a decrease in light intensity gradients.
In addition, modeling the light intensity gradient provided a framework which served as a
qualitative measuring stick for determining if the system would behave experimentally as
one would expect it to. Lastly, the model could be a useful tool if one were attempting to
determine the amount of energy or light penetration required for a system to polymerize if

it were to be photoinitiated.

112

 

0.006

1
I
0.005 «r

0.(X)4 -~

.4

0.m3 ‘1‘ k¢"=()‘112’su

BEE Concentration (mol/I.)

0.002 ..

0.(X)l «r

 

 

 

0 20 40 60 80 11X) 1 20

Figure 5.1. Ground-state initiator concentration of benzoin ethyl ether as a function of
illumination time.

113

Normalized Output Intensity

 

Figure 5.2. Static light intensity gradient (non-eliminating system).

114

Normalized Output Intensity

"I
I
I
I
I
I
I
I

 

Figure 5.3. Proﬁle of the light intensity for BEE assuming all of the active radicals are
produced via photofragmentation.

115

Nomalized Output Intensity

 

Figure 5.4. Proﬁle of the light intensity for BEE assuming all of the active radicals are
produced via hydrogen abstraction.

116

Intensity

Time (min)

 

Figure 5.5. Normalized light intensity proﬁle for a completely self—eliminating system
with a “normal” photoinitiator concentration.

ll7

,- '
yr ,
v' ,r

. V. .17.?“ pp]??? W" ’ . ..

Intensity

 

Figure 5.6. Normalized intensity proﬁle for a completely self-eliminating system with a
“high” photoinitiator concentration.

118

 

 

 

1
1
60 1- o,
.1
so :. w
40 1.. ‘.
A ‘ I.
c: i .p
E Z
V .
Q) 30 “'- f
.E 1
E" 1 '-._
q R
20 w \ ‘0.
. \ a. o 90%
: ‘ i ‘ + 75%
. \ . .
10 .. R ° . +60%
. -‘ . . o
I ‘ o .,, O .
. . 9 O o--. ..
“ t"~l~I—-. .....0--O
01 A ‘ A '1+."-2+I-I-I-I—I—I-!-I-
0 2 4 6 8 10 12 14
Incident Intensity (mW/cmz)
Figure 5.7. Effect of incident intensity on gradient elimination rate.

 

119

90” mirror & IR ﬁlter

1

Sample & jacketed
cuvette holder

     

 
 

 

 

 

 

 

 

 

 

 

 

\

Water Bath

 

 

 

T Hg(Xe) Arc Lamp

Dual spectrometer

Fiber optics

Figure 5.8. Schematic of the experimental setup for actinometry studies.

120

Intensity (Counts)

 

Figure 5.9. Experimental characterization of the light intensity gradient as a function of
exposure time and sample thickness for BEE initiation reaction.

121

 

0.9 ~~
0.8 ‘0 ‘~
I.
0.7 a
20.6 r "1.

m
5 0.5 ‘”
H

c ‘ i.
- 0.4 .- ~-

0.3 + _ I‘m“;
__ 0t = 0 min -- .. ._ I
0'2 tit: 10 min T

.0
.-~ g
>- ~~- -
-i ..-
0.1 u ‘00--
T v v T v r r v v 1 1' x

 

 

Thickness (cm)

Figure 5.10. Measured light intensity after passing through BEE samples of varying
thickness before and after sample exposure for 10 minutes (incident
intensity = 1.2 uW/cmz).

122

 

 

 

 

an
-l
II
I
I
5001- ._'
I25nmi .-
x2£nmi .
023um1 I
A I
3 A33nmi I
e
B "- H
II x
3 yml MM. g x
5 'I x
E II x
_ x
E I ‘ x
1
g ﬁn: .. x o °o°°l
I x °
I'I ‘ ‘ o000
. 00000
...I ‘ ooo
. H x 0°
x 00
roo~~ I I x 0ow" M
“ II ' x x o°°° “AA““
.. ‘ 0° ““‘
‘ x 090 ‘AAAA
. Ix 0 ‘AAA‘
II ! °°°°° AAAAA““
oJIOIISSpSSSIAAAAL_. i :7 :
0 1“) ﬂ” an «m N» am ﬂ» am> mm
Tmuama

Figure 5.11. Normalized light intensity as a function of time for varying sample thickness
of 1 x 10-2 wt% anthracene/1 wt% UV9310 in hexanol as a function of time
(intensity monitored at 363.96 nm).

123

5.4. References

J.C. Scaiano, “Photochemical and free-radical processes in benzil-amine systems.
Electron-donor properties of a—aminoalkyl radicals,” J. Phys. Chem, 85, 2851
(1981).

G. Odian, Principles of Polymerization, 3rd edition, Wiley & Sons, New York, NY,
1992.

NT. Lipscomb, Y. Tarshiani, “Kinetics and mechanism of the benzoin isobutyl
ether photoinitiated polymerization of styrene,” J. Polym. Sci., Polym. Chem, 26,
529 (1988).

J.L. Mateo, P. Bosch, A.E. Lozano, “Reactivity of radicals derived from

dimethylanilines in acrylic photopolymerization,” Macromolecules, 27, 7794
(1994).

M.V. Encinas, J. Garrido, E.A. Lissi, “Polymerization photoinitiated by carbonyl
compounds. VIII. Solvent and photoinitiator concentration effects,” J. Polym. Sci.,
Polym. Chem, 27, 139 (1989).

“Optics and Filters,” Oriel Corporation Product Catalog, Vol. III, 2-19 (1990).

CHAPTER 6.

DIFFERENTIAL SCANNING CALORIMETRY STUDIES

6.1. Introduction

In this chapter, a number of variables have been investigated using Differential
(DSC) and Photodifferential (PDSC) Scanning Calorimetry. PDSC has been a widely
used technique for in situ characterization of reaction kinetics and polymerization rate
constants involving photopolymerizations."6 The response time of PDSC is relatively
fast, on the order of 2-3 seconds,7 therefore it can be used to monitor the kinetics of most
free-radical photopolymerizations including vinyl esters and acrylates. In addition, DSC
can be used to determine the degree of conversion in cured polymers as well as to
determine the glass transition temperature, T3, for polymers (results for values of T1; for
various polymers are presented in Chapter 7 - Thermal and Mechanical Characterization
of Polymer Systems).

The fundamental operating principle of DSC is relatively straightforward. It
measures the difference between reference and sample cells during a thermal event such
as a chemical reaction or a controlled temperature change. The DSC typically uses
temperature sensors placed in heat sinks located immediately below the sample and

reference cells to monitor changes in temperature.8 Because the cells are maintained

124

125

independently, heat ﬂow can be controlled for each cell. Heat can either be removed or
supplied from the sample and reference cells depending on the system being tested which
allows DSC to characterize both exothermic and endothermic reactions.9 The exothermic
nature of these polymerization reactions mean that they are well suited to kinetic
characterization techniques afforded by DSC.'O'”

DSC can be used to glean a wealth of information for polymer samples in addition
to its ability as a kinetic analysis technique. For example, the degree of conversion for
polymeric samples can be measured by measuring the size of the exothermic peak after
the sample has been heated above its glass transition temperature.'2 Moreover, DSC has
been used to characterize thermodynamic constants; cure rates for monomers and resins;
and rates of physical and chemical change.8 There are a number of advantages afforded
by DSC including: the ability to measure all thermal events such as glass transition,
melting, and degradation; it responds rapidly to thermal transitions and chemical
reactions; and it uses small sample sizes. Unfortunately, DSC may be too slow to
characterize extremely rapid reactions such as vinyl ethers; and it can be sensitive to
slight deviations in sample size, heating rate, and sample morphology. However, it is
without a doubt one of the most versatile and ﬂexible techniques currently available for
thermal analysis during and after reaction.

The versatility of DSC was exploited in this research to study the effects of
temperature, initiator formulation, and initiator concentration on the reaction rate proﬁle
of vinyl ester polymers cured using both the self-eliminating gradient (SEG) and dual-
cure initiation strategies. In addition, DSC experiments are shown that provide

qualitative proof that the reaction is occurring as a direct result of the light source as

126

opposed attributing the reaction to a thermal effect. Finally, post-cure DSC analysis was
used to characterize the degree of cure for polymers that were cured using both initiation

strategies.

6.2. Experimental Procedure

Resins were selected from the DerakaneTM family (Dow Chemical Co., Midland,

MI). In this research, low viscosity (470-45), medium viscosity (411c-50) and high
viscosity (441-400) Derakane resins were investigated, although the majority of research
involved the lowest viscosity, 470-45, resin for the sake of convenience. Initiators used
in this study included benzoin ethyl ether (BEE, Aldrich Chemical Co., Milwaukee, WI)
and benzoyl peroxide (Aldrich Chemical Co.). An additional radical photoinitiator was
obtained from Ciba-Geigy (Hawthorne, NY), 0t,(x-dimethoxy-(x-phenylacetophenone,
which is marketed by Ciba as Irgacure 651. All materials were used without further
puriﬁcation.

Standard samples that were initiated based on the SEG strategy were prepared in a
bulk solution by mixing 99.8 wt% 470-75 with 0.2 wt% BEE. Upon mixing, the sample
was dark-stored in an amber bottle at room temperature. Standard samples cured using
the dual-cure initiation strategy consisted of 99.6 wt% 470—45 with the balance being
equal parts benzoyl peroxide thermal initiator and BEE photoinitiator. This bulk solution
was also dark-stored at room temperature after mixing. The initiator concentration has
been noted for cases where the initiator formulation differs from standard and the

concentration refers to the weight percent of the entire sample. Individual samples for

127

each reaction were weighed out from the bulk solution using a Mettler analytical balance
that measures to the nearest 0.1 milligram.

The DSC studies were performed using a Perkin Elmer (Norwalk, CT)
Differential Scanning Calorimeter (model DSC 7). For the PDSC studies, the calorimeter
was modiﬁed so that both the sample and reference cells could be irradiated using UV
light generated from a 200 watt Hg(Xe) arc lamp (Oriel Corporation - Norwalk, CT). The
samples were maintained isothermally using room temperature water that was stored in a
reservoir within the calorimeter. Individual samples were placed in aluminum DSC pans
and weighed 15-40 milligrams. Light intensities were measured using a UVP Blak-Ray
ultraviolet meter (model J 225), and the intensities were varied by either using the
condenser lens on the lamp or by altering the distance between the sample and the light
source. In separate experiments, a high-speed computerized thermocouple (produced by
Omega Systems) was used to obtain in situ temperature measurements of these photo-

initiated reactions.

6.3. Results and Discussion

6.3.1 . In Situ Characterization of the Reaction
The possibility that the reactions were being thermally initiated instead of
photoiniated was raised during this research. This raised an important question: whether
the reactions were being thermally or photoinitiated. To answer this question, isothermal

DSC experiments were conducted at a series of temperatures ranging from 25-45°C. This

was a straightforward series of experiments where two samples were maintained

128

isothermally for a predetermined period of time. The sole difference between the two
samples was that one sample was exposed to light from the 200 watt arc lamp. This
provided a controlled environment where the only variable between the two samples was
initiating light, therefore any differences in the reaction could be attributed to the
presence or absence of UV light. The effect of incident light proved to be the same for

samples cured at 25, 35 and 45°C, thus only the results for the system maintained at 25°C

are shown here (in Figure 6.1).

Figure 6.1 shows conclusively that UV light initiates the polymerization reaction,
and the absence of light virtually insures that reaction will not take place at or near room
temperature. The measured light intensity for this set of experiments was relatively low

(< l ItW/cmz) therefore it is highly unlikely that the light supplied enough heat to initiate

the polymerization. However, to insure that the reaction was in fact photoinitiated, a
simple experiment was performed to completely rule out the possibility of the light
heating the sample sufﬁciently to initiate a thermal polymerization.

DSC was used to monitor the reaction proﬁle for a solution containing 99.8 wt%
470-45 resin and 0.2 wt% benzoyl peroxide thermal initiator maintained isothermally at
25°C and irradiated using the same 200 watt arc lamp at the same intensity that resulted
in the photopolymerization shown in Figure 6.1. As expected, no reaction was observed
even after the sample was irradiated for a total of 15 minutes. Even after this time, the

sample remained a liquid upon its removal from the DSC.

129

6.3.2. Post-Cure Studies
DSC was used to determine the presence of uncured resin in the polymer and
composite specimens. These measurements show that after 5 minutes of exposure to
moderate light intensities (1.2 uW/cmz) the samples cured more than 95%. These studies
also showed that the addition of thermal initiator (BP) dramatically accelerates the degree of
cure of the sample. For example, addition of 0.2 wt% BP reduced the amount of time

required to achieve a 95% degree of cure by 60%. A summary of the post-cure studies for

the 470-45 and 41 lc-50 resins are presented in Tables 6.1-2, respectively.

Table 6.1. The effect of initiator and illumination time on the degree of cure for low
viscosity vinyl ester resin.

 

 

 

1 1 1
Sample : Initiator : Irradiation time : Degree of Cure %
T l I
DER 470-45 : 0.1 % BEE : 3 min : 50
------------ +—------------—I—--------—---+-—-—---------
DER 47045 : 0.1 % BEE : 5 min : 95
———————————— i--------------I-------------+------------:
DER 47045 : 0.1% BEE : 10 min : 100
------------ +--------------I-------------+------------*
DER 47045 : 0.1% BEE, 0.2% BP : 2 min : 95

 

 

Table 6.2. The effect of initiator and illumination time on the degree of cure for a
moderate viscosity vinyl ester resin.

 

 

 

l l I
Sample : Initiator : Irradiation time : Degree of Cure %

DER 411c-50 : 0.2 % BEE : 3 min : 60
------------ +----—---.------l-------------+--------——---

DER 41 lc-50 : 0.2 % BEE : 5 min : ~98
------------ +—-----—------—I—----—--———--+-—-—---------

DER 411c-50 : 0.2% BEE : 10 min : 100
------------ +--------------l-------------+-------------

DER 411c-50 : 0.2% BEE, 0.2% BP : 2 min : 100

 

 

130

6.3.3. Variable Eﬂects
The effect of temperature on the rate of reaction was monitored by PDSC. In
these studies, about 30 milligrams of reactive resin (containing 0.1 wt.% benzoin methyl
ether) were polymerized at 70, 90 and 110°C, by exposure to UV light (0.07 uW/cmz).
As expected, the location of the exotherrns show that the rate of reaction increases with
increasing temperature. In other words, the amount of time required for complete cure
decreases as the temperature of the system increases. For example, at this light intensity,

the reaction peaked in 3 minutes at 110°C, 6 minutes at 90°C, and 10 minutes at 70°C.
Figure 6.2 shows reaction rate proﬁles for photoinitiated samples cured isothermally at
70, 90 and 110°C. The initial shape of the 110°C curve can be attributed to equipment
problems associated with heating the sample to the reaction temperature and should be
not be considered an indication of reaction (the samples in this experiment were
preheated to 50°C, however further heating could have led to production of active

radicals through some type of thermal dissociation process).

A result similar to the BEE initiator system was observed for 411c-50 resins
photocured using 0.1 wt% Irgacure 651 as the photoinitiator. Figure 6.3 shows the
effects of temperature on the reaction rate proﬁle for samples irradiated using UV light

(0.6 uW/cmz) from the 200 watt arc lamp. The polymers cured much more rapidly than

in the previous system, however the intensity of the initiating light was approximately an
order of magnitude larger. Interestingly, as the temperature of the system was increased,
the reaction occurred more rapidly, but a comparison of the reaction rate proﬁles showed

that the cooler the sample, the higher the reaction rate. While this result was somewhat

131

surprising, it is not altogether unexpected, especially when one considers the fact that
initiation and termination rates can be assumed to be equal for radical polymerizationsl3
(recall kinetic derivation of radical mechanism in Chapter 2 - Background). Thus the
termination rate constant, k,, increases as the rate of initiation increases. It follows from
Equation 2.12 that an increase in kt would result in a decrease in the overall rate of
polymerization, RP.

One ﬁnal variable that was investigated using PDSC was initiator formulation. A
hypothesis of this research has been that the addition of polymerization enhancing
compounds such as strong hydrogen donors does not accelerate thick photopolymer-
izations but instead slightly inhibits them. While DSC would seem to be perfectly suited
to quantify the effect of hydrogen donors such as TBA on the reaction rate, there are a
number of problems which minimize its effectiveness. First and foremost among these
concerns is the fact that the sample thickness is also relatively thin because extremely
small samples are used in DSC. This is indeed a problem because TBA and other
hydrogen donors have previously shown that they do indeed enhance the kinetics of
photopolymerizations for thin systems, partially because radicals produced by abstraction
are more efﬁcient than radicals produced by fragmentation.”ls This minimizes the
effectiveness of DSC because there are competing effects (efﬁciency of radicals and self-
eliminating light intensity gradients) which can not be differentiated. However, PDSC
can at the very least be used to qualitatively determine if one effect will show dominance
over the other effect. The results of this experiment are shown in Figure 6.4. This ﬁgure

seems to suggest that the effect of the self-eliminating gradient is slightly more signiﬁcant

than the efﬁciency of the initiator fragment for 4 2mm thick samples because the reaction

132

occurs a little bit more rapidly for the BEE system than the BEE/TBA initiator

formulation.

6.4. Chapter Summary

The experiments discussed in this chapter reinforce the notion that thick photo-
polymerizations can be cured using ultraviolet light as the sole initiating source. In
addition, PDSC studies show that the ambient temperature of the system can be used to
increase or decrease the amount of time required for complete polymerization. Achieving
complete cure is a concern for any polymerization process. The degree of concern is
exaggerated when the process involves production of thick polymers and/or composites
based upon photopolymerizations in large part because the general consensus in the
scientiﬁc community all along has been that photopolymerizations can not be used to
produce thick polymers. The results of this research both validate and alleviate this
concern. In one sense, the results of this research validate this concern because it is clear
from the post cure studies that the degree of cure can be alarmingly low (around 50%) for
a photopolymerization if certain conditions are not optimized (initiator formulation, light
intensity, exposure time). However, the post cure studies also alleviate the concern
because they show that photopolymerization strategies do exist which be used to produce

polymers where the percentage of conversion approaches unity.

133

 

 

 

 

 

 

 

 

1
0.75 i
in
E
E 0.5 .
ii
2
Photo
0.25 ~ ------ Thermal
1-
0 I I I Y Y V
1.00 2.00 3.00 4.00 5 .00 6.00 7.00 8.00 9.00 10.00

Time (min)

Figure 6.1. Reaction proﬁle for photoinitiated and thermally initiated vinyl ester system
at 25°C.

134

 

0.750
0625 i’ 70 “C
0.500 4*

0375 J-

Heat How (W/g)
c
N
8

0.125 4*

 

 

 

 

 

Time (min)

Figure 6.2. Effect of system temperature on the reaction rate proﬁle for samples cured
using 0.1 wt% BME to photoinitiate the polymerization.

135

 

0.3
20"C

0.25 “b 300C

0.2 if

Heat Flow (Wig)
c
5‘.

0.1 4*

50°C

0.05 ‘r

 

 

 

 

 

 

Figure 6.3. Effect of system temperature on the reaction rate proﬁle for samples cured
using 0.1 wt% Irgacure 651 to photoinitiate the polymerization.

136

 

 

 

 

------ BEE
—— BEEJTBA

Reaction Exotherm

 

 

 

 

11

Time (min)

Figure 6.4. Effect of initiator formulation on the reaction rate proﬁle for photocured (2
mm thick) vinyl ester polymers.

10.

11.

12.

13.

137

6.5. References

T. Doomkamp, Y.Y. Tan, Polym. Comm, 31, 362 (1990).
J .G. Kloosterboer, G.F.C.M. Litjen, Polymer, 28, 1149 (1987),

J .G. Kloosterboer, “Network formation by chain crosslinking photopolymerizations
and its application in electronics,” Adv. Polym. Sci., 84, 1 (1988).

K.S. Anseth, C.M. Wang, C.N. Bowman, “Kinetic evidence of reaction diffusion

during the polymerization of multi(meth)acrylate monomers,” Macromolecules, 27,
650 (1994).

K.S. Anseth, C.M. Wang, C.N. Bowman, “Reaction behavior and kinetic constants

for photopolymerizations of multi(meth)acrylate monomers,” Polymer, 35, 3243
(1994).

R. Mallavia, F. Amat-Guerri, A. Fimia, R. Sastre, “Synthesis and evaluation as a
visible-light polymerization photoinitiator of a new eosin ester with an O-benzoyl-
a-oxooxime group,” Macromolecules, 27, 2643 ( 1994).

C. Decker, K.J. Moussa, “Real-time monitoring at ultrafast curing by UV-radiation
and laser beams,” Coatings Tech, 62, 55 ( 1990).

EA. Grulke, Polymer Process Engineering, Prentice Hall, Englewood Cliffs, NJ,
1994.

HE. Pebly, “Glossary of Terms,” in Composites, Vol. 1, ASM International, Metals
Park, OH, 3 (1987).

AB. Scranton, et. al. Macromolecules paper on using dsc to characterize the
polymerization rates of exothermic systems.

R.E. Sanders, S. Taha, “Bonding cure considerations,” in Composites, Vol. 1, ASM
International, Metals Park, OH, 702 (1987).

J .L. Courter, “Thermal Analysis,” in Composites, Vol. 1, ASM International, Metals
Park, OH, 779 (1987).

G. Odian, Principles of Polymerization, 3rd edition, Wiley & Sons, New York, NY,
1992.

138

14. E. Bortel, A. Kochanowski, W. Rys, E. Witek, “Polymerization initiated by the

15.

system triethanolamine/hydrogen sulﬁte with and without the cooperation of
hydrogen peroxide. 1. Polymerization of acrylamide,” J.M.S. - Pure Appl. Chem,
A32, 1197 (1995).

G. Seretoudi, I. Sideridou, “Benzil/n,n-dimethylaminoethyl methacrylate system as
photoinitiator for methyl methacrylate polymerization,” J.M.S. - Pure Appl. Chem,
A32, 1183 (1995).

CHAPTER 7.
THERMAL AND MECHANICAL CHARACTERIZATION

OF POLYMER SYSTEMS

7.1. Introduction

In the broadest sense, any structure having a combination of two or more distinct
materials where there exists a distinguishable interface between the materials can be
deﬁned as a composite.1 However, in general terms, composites are thought of as
materials having a reinforcing phase and a binder phase. In these terms, composites are
distinguished by, and used for, their structural properties. Moreover, there are a number
of applications where the working environment subjects the composite to thermal
stresses. Thus it is important to expose the polymeric composites to a variety of thermal
stresses in order to determine its thermal stability. In fact, most composites are used
based on their structural properties. It was therefore important that both the mechanical
properties and thermal stability of the polymeric composites developed in this research be
characterized to determine their potential for industrial applications.

The phrase “mechanical properties” is a catchall term that is used to lump many
different properties into one broad deﬁnition. For example, polymeric properties such as
solvent, chemical and electrical resistance may be grouped with tensile properties
including modulus, strength and elasticity into the deﬁnition of “mechanical properties”.2

However, there are more speciﬁc deﬁnitions of mechanical properties such as the one

139

140

offered by Pebly,3 where mechanical properties refer to “compressive and tensile
strengths, and modulus, that are associated with elastic and inelastic reaction when force
is applied...” that can result in confusion when it is unclear what someone is referring to
when they use the phrase “mechanical properties”. To avoid any confusion, the deﬁnition
offered by Pebly will be used to describe the phrase “mechanical properties” in this
contribution.

A number of techniques are available which use well-developed, standard testing
methods for characterizing the mechanical properties of polymers and composites. In
addition, there exist a tremendous array of thermal analysis techniques such as
therrnogravimetric (TGA), therrnomechanical (TMA) and dynamic mechanical (DMA)
analysis which are commonly used to identify the thermal properties of composites.
Through a combination of these techniques, one is able to obtain a detailed thermal and
mechanical “history” for polymers and composites that includes the glass transition
temperature, thermal stability, ﬂexural strength and modulus, loss modulus, and thermal
expansion coefﬁcient of polymers and composites. In this research, the properties which
were of most interest included the ﬂexural strength and modulus, the thermal stability,
and the dimension change. The techniques used most frequently in this research included
TGA, TMA, and flexural testing because the properties of immediate interest included
thermal stability, dimension change as a function of temperature, and ﬂexural strength
and modulus.

Therrnogravimetric Analysis (T GA) is a useful tool for analyzing polymers and
composites. In basic terms, TGA measures changes in weight as a sample is heated or

cooled. When used in conjunction with mass analyzers such as gas chromatography or

141

mass spectrometry, TGA can be used to determine the composition of composites.4 In
this research, however, TGA was used to study the degradation kinetics for several
polymer and composite systems. TGA is not ideally suited to monitoring the degradation
process because it does not allow for good environmental control during the degradation
process.4 However, it can be extremely useful in determining the onset temperature for
degradation, To, and regions where the composite is thermally stable. Finally, TGA
results can be applied to speciﬁc systems to determine their long-term stability in various
thermal environments under several atmospheric (oxygen, nitrogen, etc.) conditions.
Therrnomechanical analysis (TMA) measures dimensional changes in a polymer
as a function of temperature. Speciﬁcally, it measures minute changes in polymer length
or thickness as the sample is heated at a constant rate.5 TMA is an excellent method for
measuring or, the coefﬁcient of thermal expansion, which is critical to minimizing or
eliminating stresses between ﬁber-matrix interfaces depending on the composite design.

For example, the value of o: is a signiﬁcant factor when designing graphite composites.6

TMA does not require large sample sizes, typically on the order of 0.0625 inz, and it can
also be used to measure the glass transition temperature, Tg. Unfortunately, it is often
difﬁcult to measure Tg using TMA because it is corresponds to the point where or
changes. This change can be extremely minor depending on the sample, therefore it may
go undetected using TMA.

Flexural modulus can be deﬁned as the resistance to deformation while ﬂexural
strength can be deﬁned as the amount of stress required to break a sample. Flexural
modulus and strength values have units of force per area and are typically reported in

units of gigapascals and megapascals, respectively. To measure the ﬂexural modulus, a

142

stress is applied to a polymer of known dimension and the elongation resulting from the
applied stress is measured, often as a percent elongation. To determine the ﬂexural
strength, increasing stress loads are applied to the sample at a constant rate until the
sample breaks. The point at which the sample breaks is often described as the “ultimate
elongation”.2 The elasticity of a given sample can also be determined using ﬂexural
testing by measuring the amount of reversible elongation?3

There are a number of methods which can be used to obtain values for the ﬂexural
modulus and ﬂexural strength, however the most widely accepted technique for ﬂexural
testing is established by the American Society for Testing and Materials (ASTM) and is
called ASTM D790.7 The ASTM method is inexpensive, easy-to-use, and generally
provides good quality-control from one sample to the next.8'9 Unfortunately, ﬂexural
strength is not an intrinsic property, therefore careful attention has to be paid to the
method of preparation in order to ensure that one does not generate slight deviations from
sample to sample that result in slight variations in the ﬂexural strength values.8

The focus of the remainder of this chapter is to present the results of material
characterizations for polymers formed by the photoinitiation strategies (SEG and dual-
cure) which have been described throughout this contribution and to discuss the impact of

these results on present and future research in the area of photopolymerizations of thick

polymers and composites.

143

7 .2. Experimental

The resins used in this study were vinyl ester Derakane resins 411C50, 441 and
470 provided by Dow Chemical Company (Midland, MI). Numerous types of glass
reinforcements were used in this research including powdered (E) glass ﬁber, 5 mm long
chopped ﬁbers, Vetrotex Certainteed (Uniﬂow U750, Vetrotex) ﬁber roving and woven
glass mat. Initiators which were evaluated include benzoin ethyl ether (BEE), benzoin

methyl ether (BME), benzoyl peroxide (BP), (Aldrich Chem. Co.) and 01,01-dimethoxy-oc-

phenylaceto-phenone (sold by Ciba-Geigy under the trade name Irgacure 651). All
materials were used without further puriﬁcation. To prepare a photopolymerizable
mixture, a predetermined amount (~0.2 wt%) of the initiator was dissolved in the resin at
room temperature by stirring on a magnetic plate. These mixtures were remarkably stable
in the absence of initiating light (even at elevated temperatures ~60°C). Prior to use, the
reactive resins were stored in the dark at room temperature.

The primary light source used in this study was a 1000 watt Hg(Xe) arc lamp
(model 6293) from Oriel Corporation, (Stratford, CT) equipped with a water ﬁlter to
eliminate infrared light from the incident beam thereby reducing thermal effects.

Additional experiments were performed using a 100 watt Black-Ray® long wave

ultraviolet lamp (model B100AP), produced by UVP (San Gabriel, CA) and a 200 watt
Hg(Xe) arc lamp (model 6291) from Oriel Corporation. Light intensities were measured
using a UVP Blak-Ray ultraviolet meter (model 1225), and the intensities were varied
both by using the condenser lens on the lamp and by changing the distance between the

sample and the light source.

144

Samples of the reactive formulation were polymerized in both rectangular and
cylindrical geometries. These reactions were carried out in silicone molds with light
projected down onto the sample, in cylindrical glass molds irradiated either axially or
radially, and in rectangular acrylic (poly(methyl methacrylate)) molds irradiated either
axially or radially. Composite materials were prepared by two methods. In the ﬁrst
method, the ﬁber reinforcement was placed in the mold and the mold was subsequently
ﬁlled with the resin-initiator mixture. The ﬁlled mold was then placed under UV light for
measured periods of time. In the second method, the powdered glass ﬁber reinforcement
was mixed with the formulated resin to form a slurry with a desired ﬁber loading. This
slurry was poured into the mold and irradiated with UV light for a predetermined period
of time.

Flexural properties of various polymeric and composite specimens were measured using
a United STM-20 instrument according to the ASTM D 790 method. All mechanical
properties were measured by the three point ﬂexural strength test, using 50.8 mm (2 inch)
long samples with a length to thickness ratio of ~ 16. A 454 kg (1000 pound) load cell was
used with a downdrive rate of 0.0212 mm/sec (0.05 in min'l). The ﬂexural modulus was
calculated for these samples at elongations between 0.0013% to 0.0064% of a meter. The
STM-20 measures the load, P, and the elongation, 5, and using these values the ﬂexural
modulus, E, can be calculated. Equation 7.1 relates the load to elongation for cylindrical

rods of radius r and length L.

4

145

The ﬂexural modulus can then be calculated by computing the slope of the linear region of

P vs. 5, then using equation 7.2.

 

E = (310ml L3 ] (7.2)

A similar equation was used for rectangular samples (Equation 7.3) where b is the width, h

is the thickness, and L is the length of the rectangle.

3
p = [4:]: E] a (7.3)

 

E can be calculated for rectangular samples using Equation 7.4.

 

L3
E = (slope) (4&3) (74)

Average values of measurements from at least four different samples were used for each
specimen, with the error bars shown in corresponding ﬁgures representing absolute high
and low values.

Therrnomechanical (T MA) and thermogravimetric (TGA) analysis experiments
were performed using analyzers produced by TA Instruments. TGA samples (~ 20 mg
per sample) were placed in aluminum pans and analyzed in both nitrogen and air
atmospheres. TMA samples were cylindrical in geometry with a length of 12 mm and a
radius of ~2.7 mm. Samples that were subjected to post-cure conditions have been noted,

otherwise samples were tested after irradiation.

146

7.3. Results and Discussion

7.3.]. Flexural Testing

In order to determine the effects of various processing parameters on the
mechanical properties of the polymers and composites produced by photopolymerization,
a series of experiments were conducted in which operational parameters such as initiator
concentration, cure time and initiating light intensity were varied for each initiating (SEG
and dual-cure) strategy. The initial ﬂexural testing experiments were performed in order to
determine if photocured samples were capable of producing polymers whose ﬂexural
properties were comparable to polymers cured using traditional (i.e. thermal) curing
methods. The results of these experiments showed that values for ﬂexural modulus of UV

cured vinyl ester resins (cured for 300 seconds at an incident intensity of ~1.5 uW/cmz)

were indeed comparable to the properties of specimens cured thermally (at 363 K) for 24
hours, and were on the same order of magnitude as values reported in the literature for vinyl
ester polymers.lo For example, unﬁlled 411c-50 samples which were UV cured had a
ﬂexural modulus of 3.10-3.17 GPa, while thermally cured 411c-50 samples counterparts
had a ﬂexural modulus of 2.96-3.10 GPa and literature values for vinyl ester polymers were
reported as 3.90 GPa.lo These unﬁlled systems showed elastic behavior, as evidenced by a
linear load-displacement curve, and did not fracture at the maximum displacement of
0.0013 m (0.05 inches).
The photoinitiator concentration has an effect on the modulus of the polymers
fOl’rned by photopolymerization. In a sequence of experiments the initiator concentration

was Varied from 0.1 to 4 wt%, in neat resin. These unﬁlled samples were exposed to UV

147

light (with radial initiation of 8 mm diameter samples with a light intensity of 4.4
uw/cmz) for a period of 240 seconds. Samples containing 2 and 4 wt% of initiator did

not completely polymerize in this time frame. The ﬂexural modulus of the remaining
samples (0.1 to 1 wt% initiator) were measured and were shown previously in Chapter 4
(Figure 4.7). Figure 4.7 shows that the maximum ﬂexural modulus is achieved between
0.15-0.25 wt% of initiator. As the concentration of initiator is increased beyond this
point, there is a dramatic reduction in the penetration of initiating light and decreasing
percentages of the sample cure until eventually only a thin ﬁlm is cured at the surface of
the sample (an effect observed when the concentration of initiator in the sample reached 2
wt%). It should be noted that the optimal initiator formulation shown in Figure 4., 7 is for
the SEG strategy since the dual-cure system does not rely as heavily on efﬁcient
penetration of light as this system.

This result led to a series of experiments where the amount of time allowed for
cure was varied. It was believed that, at a given intensity, the sample would cure
completely if enough time was allowed for the intensity gradient to eliminate completely
or be reduced signiﬁcantly. If this were the case, this effect should manifest itself in the
form of increased ﬂexural modulus values. The relative importance of cure time was
investigated for both SEG and dual-cure systems and proved to be somewhat dependent
on the method of initiation. For these measurements the 470-45 was used with very
ﬁnely— chopped (powdered) e-glass ﬁbers. These studies were conducted on samples

which were 8 m (1/4 inch) thick, at a light intensity of 4.4 uW/cmz. A slurry consisting

of the desired ﬁber loading, resin, and initiator (~ 0.2 wt% BEE) was poured into a

cylindrical glass mold coated with a release agent (silicone spray). The samples, which

148

were 8 m (1/4 inch) thick, were exposed to the initiating light at an intensity of 4.4

uW/cmz. The light was incident to the radial direction, and had to pass through a

maximum distance of 8 mm. After exposure to the light for a predetermined period of
time the mold was removed from the light and allowed to cool to room temperature. The
sample was then removed from the mold and its ﬂexural properties were tested.

The results of these studies are shown in Figure 7.1. This ﬁgure shows the effects
of cure time on the ﬂexural modulus of glass-ﬁlled and unﬁlled vinyl ester
composites/polymeric materials. The results indicate that after 420 seconds (7 minutes)
of cure there is no further increase in the modulus of the material. It may also be noted
that there is a slight decrease in ﬂexural modulus at higher exposure times, although this
decrease falls close to the limits of experimental error. It is believed, however, that this
phenomena may be due to degradation of the polymer by UV light. As expected,
increasing the amount of ﬁber increases the ﬂexural modulus of the samples. It can be
seen that the ﬂexural modulus increases by a factor of 3 at 50 wt% ﬁber loading when
compared to the unﬁlled sample.

Figure 7.2 shows the effect of cure time on the ﬂexural modulus of composites
formed by the hybrid photo/thermal initiating mechanism. It may be observed from this
ﬁgure that the plateau in ﬂexural modulus is reached signiﬁcantly sooner with the dual-
cure mechanism, even at relatively high ﬁber loadings. This is attributable to the
completeness and uniformity of cure provided by the thermal reaction. In essence, Figure
7.2 shows that samples cured completely in less than two minutes using the dual-cure

strategy.

149

The relative importance of the incident light intensity is highly dependent on the
method of initiation. For example, when a polymer is cured using the dual-cure strategy,
the intensity of the initiating source is a minor consideration. However, when the self-
eliminating gradient (SEG) strategy is used to polymerize the composite, the mechanical
properties become highly dependent on the incident intensity (recall Figure 4.11). The
effects of initiating light intensity and cure time on the ﬂexural modulus of samples
initiated via the SEG strategy are shown in Figure 7.3. In these experiments samples
containing 470-45, 0.08 wt% BEE, 0.08 wt% BME and 25 wt% powdered e-glass ﬁbers
were polymerized under UV light at various light intensities, for different periods of time.
Each sample was 8 mm in diameter, and the initiating light intensity was varied between
2.2, 3.3 and 5.5 mw/cmz. As previously observed, increasing the time of cure increased
the mechanical strength of the composite. It was also observed that increasing the
intensity of the initiating light resulted in an increased flexural modulus of the ﬁnal
composite, and also reduced the cure time. This suggests that higher light intensities
allow the samples to cure more completely in shorter periods of time, resulting in a higher
ﬂexural modulus. Indirectly, this experiment also shows that, while they are effective
initiators for thick photopolymerization, benzoin ethers do not produce a completely self-
eliminating gradient. Figure 7.3 supports this contention because, in theory, the ﬂexural
modulus values at each light intensity should approach an asymptotic limit given enough
time to cure. However, this does not occur, therefore it is likely that low intensity light
sources will be less efﬁcient than moderate or high intensity light sources at curing thick

samples.

150

One ﬁnal set of experiments were conducted to determine the effect of cure time
on ﬂexural strength. The results of this experiment are presented in Figure 7.4. The
samples in this ﬁgure were cured using 0.2 wt% BEE as the photoinitiator with ﬁnely
chopped e-glass ﬁbers employed as the reinforcing material at a loading of 50 wt%. The
ﬁbers were similar in texture to baby powder and they provided an extremely low aspect
ratio. Therefore, they were not expected to provide a large amount of stiffening. Instead
they were selected for this set of experiments because they were the most challenging
system available because their presence in the reactive resin maximized the light
scattering and presented the highest number of resin/ﬁber interfaces. The result of this
formulation is a system that is perhaps the most difﬁcult photopolymerize. To emphasize
this, one need only compare the neat and ﬁber-ﬁlled polymers after each had been
photocured. While the unﬁlled polymers were clear with a light yellow color, the
composites obtained using the powdered ﬁbers were gray, opaque, and produced almost a
marble-like ﬁnish once they had been completely cured. Figure 7.4 showed that the
samples had reached a maximum value of ~ 0.1 GPa after being exposed to incident light

of ~ 1.5 p.W/cm2 for a total of 5 minutes. The plateau achieved for samples cured for ﬁve

minutes or longer indicated that the samples had cured completely after ﬁve minutes.

7.3.2. Thermal Analysis
Thermogravimetric analysis of the vinyl ester samples showed that there was
virtually no degradation at 573 K. Figure 7.5 shows TGA curves for each of the vinyl ester
resins used in this research. Each of the samples in Figure 7.5 was cured for eight minutes

at a light intensity of 3.13 p.W/cm2 using a 0.1/0.1 BEE wt%/BP wt% initiator formulation.

151

As this ﬁgure shows, the lowest viscosity resin begins to degrade ﬁrst (470—45, around
378°C) followed by the medium viscosity resin, 411c-50 (384°C) and ﬁnally 441-400, the
most viscous resin (~ 390°C). The results of this experiment support the contention that
these polymers are suitable for use temperatures < 275°C.

The ﬁnal set of experiments performed on the cured composites were designed to

characterize the thermal expansion coefﬁcient, 01, and the glass transition temperature, T ,

for neat polymers and composites. The effects of initiator formulation and cure time on

are shown in Table 7.1 for 470-45 and 441-400 vinyl ester resins.

Table 7.1. Thermal expansion coefﬁcient as a function of initiator formulation and cure
time for vinyl ester polymers.

 

 

 

 

 

 

 

 

 

Initiator Cure Time a
Resin Formulation (min) (mm/m°C)
470-45 BEE 3 .5 23 .52
470-45 BEE 8 23 .06
470-45 BEE/BP 3 .5 23 .46
470-45 BEE/BP 8 21.82
441-400 BEE 3 .5 34.23
441-400 BEE 8 26. 12
441-400 BEE/BP 3.5 30.44
441-400 BEE/BP 8 26.43

 

 

 

 

 

 

The results in Table 7.1 show that as the cure time was increased for each sample, the

thermal expansion coefﬁcient decreased for each sample regardless of initiator

152

formulation or resin type. This is likely a result of enhanced cure as the amount time
allowed for cure was increased. Further scrutiny of Table 7.1 shows that, for the most
part, the thermal expansion coefﬁcient was slightly smaller for samples cured using the
dual-cure method. Again this would seem to suggest that the dual-cure method cured
samples more completely than the SEG strategy. A ﬁnal observation of this table shows
that or was ~ 37% larger for the highly viscous (441-400) resin than the low viscosity
resin (47045) when the sample was cured for 3.5 minutes, however it was only 17%
larger when the samples were cured for a total of 8 minutes. This seems to suggest that
the effect of viscosity can signiﬁcantly affect the kinetics of the reaction.

The effect of ﬁber loading on or was also studied in addition to cure time, initiator
formulation, and resin type. Figure 7.6 compares or values for neat polymers and
composites having a 50 wt% ﬁber loading of powdered e-glass ﬁbers. As expected, 01

values decreased for each viscosity resin as ﬁbers were added to the polymer.

For the most part, both DSC and TMA proved to be somewhat ineffective at
determining the glass transition temperature for the polymers investigated in this research.
Due to the highly crosslinked structure of the polymers formed in these reactions, any
evidence of a T8 would be subtle. With this in mind, TMA did show Tg values between

l20-150°C for vinyl ester samples. Figure 7.7 shows TMA curves for two vinyl ester
polymers photocured for 3.5 minutes at a light intensity of 3.13 uW/cmz. T8 is

determined as the point where the slope of the speciﬁc volume curve changes. Figure 7. 7

This occurs at ~ 125°C for 441-400 and 145°C for 470-45. The higher Tg value for the

low viscosity resin may in part be due to a higher degree of cure (and an increased high

153

degree of crosslinking). In general, Tg values determined via TMA experiments were in

the 120-150°C range for each of the Derakane resins investigated in this research.

7.4. Research Conclusions/Chapter Summary

The results presented in this chapter show that the materials produced by SEG and
dual-cure strategies exhibit mechanical properties comparable to composites cured using
traditional (i.e. thermal) methods. The effects of basic operational variables such as cure
time, light intensity, photoinitiator concentration, ﬁber loading and temperature on the
performance of the composites have been established. Additional experiments have
shown that increasing the ﬁber loading of the samples reduces both the rate of reaction
and the temperature rise in the sample. Increasing the initiating light intensity not only
enhanced the reaction rate but also increased the ﬂexural modulus of the ﬁnal composite.
These studies demonstrate the tremendous potential of photopolymerizations for the
affordable manufacture of composite materials, and show that with careful attention to
detail, photopolymerizations can be used to rapidly produce products with outstanding

mechanical properties.

154

 

 

 

 

 

 

 

13
-» 2500
16 ~-
14 .. 4* 2000
E 12 J- 9
E E
3 10 ~- -~ 1500 E
.g 3
2 8.
s ~~ E
E E
a . 1000 9
5
I: 6 " :5.-
4 .
-~ 500
2 *r
o 4 t A. . . . . v ' 0
150 200 250 300 350 400 450 500 550 600 650

Cure Time (s)

Figure 7.1. The effect of irradiation time and degree of ﬁber loading on the ﬂexural
modulus of vinyl ester polymers cured using the SEG strategy.

18(X) -.

16(X) 4%

MIX) 4*

12(X) ll

IIXXJ ~>

8(1) *-

Flex mod. (kpsl)

600 it

41!) a»

2m 4..

155

 

 

Fiber loading
+0 wt% 0 ._
25 “q. ‘T W!- -P
+50 wt% “-

 

 

 

 

 

 

 

 

 

 

0

Figure 7.2.

The effect of irradiation time and degree of ﬁber loading on the ﬂexural
modulus of vinyl ester polymers cured using the dual-cure method.

156

 

 

 

 

 

 

 

 

 

900
l
+ 850
5.76 «r
4* 800
5.26 -«
4 750
2 '11
a. I"
g 4 700 E
.3 4.76 l 1
Z
'8 4). * 650 8
2 l =
'3 4.26 l E
3 4» 600 i?-
g E
3.76 11- 4. 550
+1 = 2.19 microW/cm"2 1 500
3.26 4 +1 = 3.29 micro W/cm‘Z
+1 = 5.48 micro W/cm‘2 4» 450
2.76 . 4 4 4 4 4 4 4 400
2 3 4 5 6 7 8 9 10 11

Exposure time (min.)

Figure 7.3. The effects of light intensity and cure time on the ﬂexural modulus of
composites with a 25 wt.% ﬁber loading.

157

 

 

 

 

 

 

0.12
-~ 16
0.1 -~ T
4 14
JI

? =1

» 12
@004 2
a ~~ w %
r: L 5'?
g 0.06 7” ‘ a
m _.,. 8 a
E E
§ 0.04 4 4 6 g
E :3

-~ 4

0.02 -
I
0 . + r; . ﬂ , o
0 100 200 300 400 500 600

Time (s)

Figure 7.4. The effect of cure time on the ﬂexural strength of vinyl ester composites
with a 50 wt% chopped e-glass ﬁber loading.

158

 

B

 

\O
o
1_

& LI! 8 \l 00
o o C o
1 1 1 1 J
r T r r r

Wt. % sample remaining

b)
O
I
r

 

 

 

0 1 4L 1 1 1 1 1 1 1
Y T Y T Y Y Y T Y

 

 

300 320 340 360 380 400 420 440 460 480
Temperature (°C)

Figure 7.5. TGA proﬁles for vinyl ester resins as a function of temperature.

159

 

a)

 

050%Fibcr
INoFiber

 

 

 

   

 

5

 

 

      

   

Coeff. of Therm Exp. (mm/m 0C)
8 S

 

 

 

 

 

 

 

 

 

 

 

 

470-45 410-45 ('13?) “I “I (Ir/3P) 411 411 (It/BF)
Resin Type

Figure 7.6. Effect of ﬁbers on the thermal expansion coefﬁcient of vinyl ester polymers.

 

160

 

 

 

300
250 T
’17.?
2
g 200 «-
E
.2
E
0
OD 150 1r
5
.1:
0
8 .
--,,,- 44M“ 47045
= 1004 :
O , l
' I
| I
50 ‘1- , 1
I l
' I
l l
. I
I l
0 -4.-44.....-.......4 ........... 1 ........ 4...-
20 40 60 80 100 120 I40 I60 180
Temperature (°C)

Figure 7.7. Determination of the glass transition from TMA proﬁles for vinyl ester
polymers.

 

10.

161

7.5. References

T.J. Reinhart, L.L. Clements, “Introduction to composites,” in Composites, Vol. 1,
ASM International, Metals Park, OH, 27 (1987).

G. Odian, Principles of Polymerization, 3rd edition, Wiley & Sons, New York, NY,
1992.

H.E.Pebly, “Glossary of terms,” in Composites, Vol. 1, ASM International, Metals
Park, OH, 3 (1987).

EA. Grulke, Polymer Process Engineering, Prentice Hall, Englewood Cliffs, NJ,
1994.

J.L. Courter, “Thermal analysis,” in Composites, Vol. 1, ASM International, Metals
Park, OH, 779 (1987).

R.H. Stone, “Dissimilar material selection,” in Composites, Vol. 1, ASM
International, Metals Park, OH, 716 (1987).

“Standard test methods for ﬂexural properties of unreinforced and reinforced
plastics and electrical insulating materials,” D 790, American Society for Testing
and Materials.

9’

NR. Adsit, “Mechanical properties and environmental exposure tests,” , in
Composites, Vol. 1, ASM International, Metals Park, OH, 295 (1987).

RB. Pipes, R.A. Blake, “Test methods,” in Composites Design Encyclopedia, Vol.
6, University of Delaware, 68 (1983).

CD. Dudgeon, “Polyester resins,” in Composites, Vol. 1, ASM International, Metals
Park, OH, 90 (1987).

 

 

 

CHAPTER 8.

INVESTIGATION OF ADDITIONAL POLYMER SYSTEMS

This chapter summarizes work performed at the initial stages of this research in
the area of polymer networks. Speciﬁcally, this work focuses on ladder polymers and
interpenetrating polymer networks (IPNs). This work served as the groundwork for the
more detailed research on photopolymerizations by providing valuable experience in the
areas of radical and cationic chain polymerizations. In addition, a number of experiments
used in the development of novel photopolymerization strategies were ﬁrst tested on
these polymer networks. Finally, the inclusion of thermal initiators in the dual-cure
system evolved in part from the working knowledge that had been established during this
introductory research.

This chapter will be divided into two main sections, one detailing research
performed on ladder polymers and one describing a potential method for producing
interpenetrating polymer networks. The majority of pre-photopolymerization research
was on ladder systems, thus the focus of experimental work in this chapter will be on

these types of polymeric structures.

8.1. Ladder Polymers

Ladder polymers, or fused-ring chain polymers, have received attention for their

ability to achieve both superior resistance to molecular weight losses and high thermal

162

163

stability. This class of polymers is so named because its chemical structure physically
resembles a ladder. Ladder polymers occur when several bifunctional monomers or
aromatic rings are linked together to form an uncrosslinked polymer. These polymers
exhibit strong resistance to decreases in molecular weight and superior thermal resistance
due to the fact that for chain scission to occur there must be at least two cleavages within
the same ring structure. This is a statistically unlikely condition unless massive oxidation
occurs.l Their unique structure allows ladder polymers to approach the maximum
serviceability temperature limits of organic polymers, widely reported to be in the 250-
300°C range,“3 while their superior properties make them industrially useful for
aeronautical applications, as ﬁber materials, and as composite parts."2 Additionally, the
excellent thermal resistance of ladder polymers makes them ideal for high-temperature
applications such as aerospace charring ablators.4

Ladder polymers are unique in the sense that every polymer contains bifunctional
monomers where both functionalities have been polymerized to form two distinct
polymeric backbones. The degree of polymerization for each functionality within each
individual ladder polymer determines whether or not the ladder structure is perfect or
imperfect. Structures having no cleavages in either backbone may be described as perfect
ladder polymers while a single cleavage in either polymer backbone results in an
imperfect ladder polymer. The production of imperfect ladder polymers is not a desired
reaction since they fail to enhance either the thermal or the mechanical properties when
compared with their linear chain analogs.’

Early attempts to produce ladder structures centered on sequentially polymerizing

bifunctional molecules. However, this technology has seen only limited success because

164

there does not appear to be a reaction that is capable of completely “closing up” each
individual ring without some degree of cleavage or crosslinking once one of the
functionalities have been reacted to produce a linear polymer with dangling chain ends.
As a result of this, the majority of ladders formed are imperfect with little or no
enhancement in mechanical or thermal properties while the perfect ladders typically have
very low molecular weights. For these and other reasons, ladder polymers synthesis has
focused primarily on two polymerization methods.

The ﬁrst method used to form ladders involves a Diels-Alder reaction. In this
reaction, each "rung" of the ladder is a single monomer unit, and the sides of the ladder
are formed when that monomer undergoes 1,3-dipolar addition with a second monomer,
thereby forming a ring where both sides of the monomer react at the same time. A
strength of the Diels-Alder reaction is the fact that either both of the functional groups
react or neither of the functional groups react, therefore no incomplete or imperfect rings
may be forrned.° However, a drawback of the Diels-Alder mechanism is rearrangement
into more thermodynamically stable, non-ladder structures due to a lack of stability in the
newly formed bonds.2 An additional problem with the Diels-Alder method is that it
typically forms low molecular-weight polymers with little or no enhancement in
properties that are directly related to molecular weight. These problems limit the
potential of this mechanism as an alternative for polymerizing ladder structures.

The second method used most often to produce ladders involves step
copolymerization of two aromatic molecules such as imides, benzimidazoles or
quinazolones.7‘lo The resulting polymers still fail to produce perfect ladder structures in

most instances when formed using this method. For example, Ghafoor and Still10 have

165

synthesized polymers from polyquinazolones and other aromatic molecules which were
then cyclized to form only partial ladder structures. The problem with these structures is
that only a single cleavage is required to cause signiﬁcant decreases in the molecular
weight. An additional problem with this mechanism is the cyclization step. Cycylization
may not always occur, thereby creating an imperfect ladder structure because of the single

linkages in the polymer.

8.1.1. Novel Ladder Synthesis Method

Scientists have been studying ladder polymers for a number of years with limited
success in an attempt to develop a process which is capable of producing perfect ladder
structures repeatedly. While some methods can guarantee that both functionalities of a
given monomer will react in the proper manner (i.e. they will not cyclize), these same
methods can not guarantee that the reaction will occur to a high degree of conversion. As
a result, low-molecular weight polymers are formed which exhibit only slight increases in
thermal and mechanical properties due to their (relatively) light weight. Although perfect
ladder polymers exhibit excellent thermal and mechanical properties, they have not been
used extensively in industrial applications because of the numerous difﬁculties inherent
in their formation.

The initial thrust of this research effort was to produce ladder polymers from the
monomer glycidyl methacrylate (GMA), also named 2,3-epoxypropyl methacrylate. The
structure of a GMA monomer is shown in Figure 8-1. GMA is a bifunctional monomer
(it contains an acrylate linkage and an epoxy functionality) that can be polymerized via a

number of chain polymerization reactions. This monomer was thought to be ideal for

166

ladder polymer systems because its functionalities are mutually exclusive, meaning active
cations will not initiate a propagation reaction in acrylate monomers while free-radicals
are likewise unable to react with epoxides. The novelty of this research was underscored
by the fact that it combined two chain polymerization mechanisms while insuring that
there would be no crosslinking or reaction between systems whether the systems were
reacted simultaneously or sequentially. This approach differed dramatically from
research on network systems which had been investigated previously. In fact, the
majority of networked polymer systems that require each system to be distinct from the
other involve a combination of step and chain polymerizations to maintain the
homogeneity of the polymers within the network.

Poly(glycidyl methacrylate) (PGMA) may form a number of ladder-like structures
ranging from perfect, two-dimensional ladders to three-dimensional “ladders” to
imperfect ladders. An additional structure which may be formed is a “crosslinked”
polymeric ladder. This type of network is not actually a crosslinked polymer, instead it
links poly(epoxy) homopolymers with poly(methacrylate) homopolymers via the
“crosslinking agent” glycidyl methacrylate. Its unique structure allows GMA to
polymerize via two distinct methods, therefore simultaneous polymerization of GMA
may result in a number of homopolymer chains which are linked together through the
backbone of the monomer. Some potential, ladder-like structures of PGMA are shown in

Figure 8-2.

167

8.2. Ladder Polymer Research Experiments

A large majority of this research involved attaining a better understanding of the
kinetics involved in these reactions. Speciﬁcally, a large amount of time was devoted to
optimizing an experimental setup such that experimental conditions such as temperature,
pressure, and method of initiation were repeatable. Although pressure changes during
these reactions were generally not signiﬁcant due to small-scale reactions and typically
unsealed vessels, the temperature of the reaction was prone to large ﬂuctuations
depending on a number variables. Thus the temperature was monitored over the course
of reaction in order to determine Optimal conditions for these particular simultaneous
reactions.

Determining the optimal reaction temperature, T p, for these reactions inVolved

multiple experiments and calculations. The ﬁrst step involved in this process was to
determine if the kinetics of the radical and cationic reactions were similar under usable
conditions (i.e. suitable initiating temperatures). Once a theoretical Tup was calculated,
thermal proﬁles for each reaction were obtained to determine an experimental Top. In

addition, the thermal proﬁles for each reaction were studied thoroughly to determine if

they could provide any evidence of simultaneous reactions.

8.2.1 . Theoretical prediction of T0,,
In order for the simultaneous mechanism to be successful, the reactions must behave
similarly, that is they must polymerize over similar time scales under identical conditions.

In situ temperature-monitoring of these reactions was important because it provided

168

qualitative information on the time requirements for each reaction both individually and
simultaneously. To determine if the epoxide functionality and the acrylate functionality
could be polymerized in similar times, two theoretical curves were calculated. To get an
approximation of this temperature, theoretical studies were conducted to predict
polymerization rates for the cationic and radical reactions as a function of temperature. A
general method for predicting the polymerization rate for radical reactions is shown in
Equations 8.1-5. An extensive derivation of Equations 8.1-5 is readily available in a

number of sources including Odian, therefore only the resulting equations are shown

here.3
R1 = 2fkd[l] (8.1)
R,=k,[M](fk.lIl/k.)'” (8.2)
In RP = ln{kp(kd/k,)'”} + ln{ [M](f[I])"2} (8.3)
In Rpm/k)“ = In A - E,/R'r (8.4)
In RP = ln{ AP(A,/A,)'”}+ ln{ [M](f[I])"2}- ER/RT (8.5)

A similar expression for the theoretical determination of Rp versus temperature for
cationic mechanisms can be derived in a manner similar to that performed in Equations
8.3-5. The resulting equation shows the dependence of cationic reactions on temperature

In R9 = ln(Kkikp/k,) 4» ln{ [I][ZY][M]2} (8.6)
where K, k,, kp, and k, are all temperature-dependent. Determining the effects of
temperature on these quantities is more difﬁcult than for radical reactions, and existing
data is much more limited. However, there are a few sources that list k values at various

11.12

temperatures, including Odian3 and Subira. These values are not extremely accurate,

however, therefore there are potentially large errors in these calculations.

169

After deriving the rate expressions for both mechanisms, experimental values
were reported in order to predict an optimal theoretical temperature. The values used to
make this determination were extremely liberal in their application, however it must be
emphasized that these temperatures were only used in order to make a ballpark estimation
for Top. With this in mind, the values shown below in Table 8.1 were substituted into
Equation 8.5 to predict Rp as a function of temperature. Table 8.1 shows literature values
for methyl methacrylate (the comparable structure to the radical functionality of GMA) as

a function of temperature.

Table 8.1. Constant values for theoretical calculation of Rpm.

 

Ap x 10-7 Ad x 10‘" At x 10‘9 [M] (mol/L) [1] (mol/L) f ER (KJ/mol)

 

 

0.087a 7.38lb 0.1 1' 7.04 0.006 0.8c 82.15

 

 

 

 

 

 

 

 

‘has units of liters/mol-sec.
bcalculated using reported k values at 60°C.3
ctypical values for f range from 0.3 - 0.8.

Predicting the propagation rate of the epoxide functionality provided an even less
reliable approximation for Rp versus temperature. Literature has reported that Ep typically
ranges from 20-80 KJ/mol and may vary greatly according to speciﬁc reaction
conditions.M Increases in temperature generally lead to increases in the rate of
polymerization. These facts were used in conjunction with typical ranges for some of the
reaction variables in Equation 8.6 to estimate an equation relating RP with temperature.
The point resulting from the intersection of the lines produced from Equations 8.5 and 8.6
was determined to be Top. This point ranged from approximately 25°C to 140°C when

varying the limits of cationic reaction variables, therefore median values for each cationic

 

 

170

reaction variable were used, resulting in a predicted optimal temperature of 82°C. This

intersection point is shown in Figure 8.3.

8.2.2. Thermal proﬁles

Polymerization of either functionality of glycidyl methacrylate results in a highly
exothermic reaction which gives off tremendous amounts of heat over very short periods
of time. The large amounts of heat liberated during reaction would be a source of
concern in any industrial application employing simultaneous reactions, hence it is
important to characterize the thermal proﬁles of these reactions. Although both the
cationic and radical reactions of glycidyl methacrylate are highly exothermic, they are not
identical. Therefore, one can gain insight into the actual kinetics of each reaction by
monitoring the temperature of the system throughout the polymerization. Moreover,
thermal proﬁles might lead to conclusive evidence of simultaneous reaction if any had
occurred. A computerized thermocouple system, developed by Omega Thermal Systems,
was used to monitor the temperature of these reactions in situ.

The thermal proﬁles obtained from the computerized thermocouple provided
valuable insight into the kinetics of both reactions. A number of variables proved to be
important when observing the thermal history of the reaction, thus these variables
required substantial examination to determine their effects on the rate of polymerization
and the amount of time required for reaction. Some of the variables we studied included
initiation temperature, sample size, and initiator concentration. These variables were
studied in order to determine their effects on the reaction kinetics of both (radical and

cationic) polymerization methods.

171

The effect of the initiator concentration on the reaction rates is shown in Figure
8.4, where the epoxide functionality of GMA samples was polymerized using three
different concentrations of FeCl,. This ﬁgure shows dramatically decreasing reaction
times as the amount of initiator increases. Intuitively this makes since a higher initiator
concentration means more active species present. Unfortunately, the concentration can
only be raised so high because higher concentrations will result in the formation of
oligomers or lower molecular-weight polymers, thereby diminishing the physical
properties which are directly proportional to molecular weight. Experimental
determination of the limits for the concentration of radical and cationic initiators were
inhibited by the fact that simultaneously polymerized samples were insoluble in all
solvents tested including alcohols, methylene chloride, and various acids. However,
many researchers including Dusek et al.'5 have used radical initiator concentrations
around 0.1 wt%, with similar concentrations being used for cationic initiation by Lewis
acids. This led to the decision to use both radical and cationic initiator concentrations of
approximately 0.1 wt% for simultaneous polymerizations.

A ﬁnal set of experiments were conducted using the thermocouple to determine if
it could be used to detect simultaneous polymerization to a noticeable degree. In order to
do this, three samples were made up from bulk solutions of 99.8 wt% GMA and 0.2 wt%
AIBN and 99.8 wt% GMA and 0.2 wt% FeC13. The three samples were a bulk radical
solution (AIBN as the initiator), a bulk cationic solution (Fed), and a simultaneous
solution (both). The simultaneous solution was prepared by mixing equal parts of the
radical and cationic solutions, thereby diluting each initiator concentration by 50 wt%

while maintaining the monomer concentration. The initiator concentration in the bulk

 

 

172

samples were diluted down to 0.1 wt% by adding GMA to the samples to insure that they
would be consistent with the simultaneous solution. These samples were then
polymerized at 769°C in a water bath. Figure 8.5 shows the accelerated polymerization
in the simultaneous sample relative to the radical and cationic samples. The simultaneous
polymer reaches its maximum temperature in roughly half the time of either the cationic
or radical polymers. This may occur because the reaction is non-isotherrnal, hence as
reaction occurs the system gets hotter, resulting in an increased production of radicals and

cations capable of initiating polymerization.

8.2.3. Computer Modeling

Computer modeling, or molecular modeling as it is sometimes called, can provide
a vast amount of useful information. Using the technology now available, researchers are
able to study many different aspects of polymers on a microscopic level that were once
impossible to simulate. For example, programs exist today which are capable of
predicting various physical properties, structures, surface composition, and volume of
polymers in solution. These examples are only a small percentage of what can be
predicted by various software packages, however they represent the broad area of
polymer technology which can now be studied electronically. Moreover, computer
modeling can be extremely useful when trying to determine likely structural
conformations of new polymers or when attempting to create a polymer having speciﬁc
properties. These experiments were carried out using Polygrafm, molecular simulation
software for the design, simulation, and analysis of polymeric materials, on a Silicon

Graphics Supercomputer (IRIS 4D/220GTX).

173

Polymeric structures based on GMA were created using POLYGRAFTM; and the
total energies of each structure were determined. Each structure was formed from the
same monomer and its energy was computed in order to make qualitative comparisons.
A POLYGRAFTM energy minimization package was used to transform each structure
from its original conformation to its most probable conformation (i.e. conformation
having the lowest total energy).

To ensure that the structures were in their "best" conformation, each structure was
ﬁrst subjected to an energy minimization process. Next, the structure was perturbed from
its minimum energy for an average of ﬁve picoseconds and minimized again to determine
if it had reached a local or global minimum energy. This was repeated n times until the
nth energy was approximately equal to the (n-1)th energy, meaning that the structure had
reached its global minimum. This value is reported as the total energy of that particular

structure. This process was repeated for each of the structures listed in Table 8.2.

 

 

174

Table 8.2. Total energy for various compounds as predicted by computer modeling.

 

 

 

 

 

 

 

 

Structure Total Energy (kcal/mol)
Glycidyl methacrylate monomer 129.878
Radical homopolymer 1450.818
Cationic homopolymer 193.326
Perfect ladder polymer 2539.952
Imperfect ladder polymer 2408.233
3 strand ladder with epoxy backbone 2894.584
3 strand ladder with acrylate backbone 1538.396

 

 

 

 

The energy contributions of bonds, angles, van der Waals forces, and hydrogen
bonds16 are totaled to obtain the energy values listed in Table 8-2. The energy values
produced during these simulations are the sum of the kinetic and potential energies for
each structure. To produce each structure, ten glycidyl methacrylate repeat units were
manipulated in order to produce energy values that could be quantitatively compared to
other polymeric structures (with the exception of the energy associated with the single
monomeric unit) to determine which structure is more likely to exist in nature. Structures
having lower energy values would more likely be formed in polymerization reactions.
For example, it is more likely that an imperfect ladder will be produced from glycidyl
methacrylate than a perfect ladder, because the imperfect ladder requires approximately
100 less kilocalories per mole than the perfect ladder structure. It is important to realize

that the energies listed in Table 8.2 are not related to the amount of energy required to

 

175

polymerize a functionality, rather they are the amount of energy associated with a

particular structure under standard conditions (25 °C, latm).

8.2.4. Conclusions of Ladder Polymer Research

The molecular modeling experiments led to the conclusion that it would be highly
unlikely that a high molecular weight perfect ladder would form in a simultaneous radical
and cationic chain polymerization scheme. In addition, it is very difﬁcult to “match” the
kinetics of two distinct reactions such that the rate of polymerization for each is identical
under identical conditions. Since the conditions of a simultaneous polymerization
involving two different reaction mechanisms are by deﬁnition identical, thus there exists
an extremely complicated logistical problem which must be solved in order for a perfect
ladder structure to be formed. These problems and others made it clear that the formation
of perfect ladder polymers in a simultaneous, one-step polymerization of two distinct

functionalities would meet with extremely limited success, at best.

8.3. Interpenetrating Polymer Networks

Interpenetrating polymer networks (IPNs) consist of at least two physically
intertwined polymer networks which are inseparable. IPNs have attracted attention as a
special type of polymer blend for crosslinked polymer systems which allows the
synergistic combination of two polymers, thereby creating a network with properties
superior to those of each individual constituent polymer.'7 In addition, IPNs may show

higher resistance to deformation as measured by initial stress divided by elongation.” An

176

example of this increased resistance is a rubbery polymer/glassy polymer IPN, which
exhibits both increased strength and impact resistance when compared with the strength
and impact resistance of its single chain analogs.l9

IPNs are particularly attractive to research scientists because their properties can
be tailored to speciﬁc applications by proper construction of the network. An additional
advantage of IPNs is the fact that they combine properties of existing polymer systems to
form one polymeric network whose properties are superior to any and all individual
polymers in the network. Hence new polymer technology is not required in the
development and production of IPNs. These advantages plus the fact that IPNs can be
produced with inexpensive components (polymers) make them potentially suitable for a
number of industrial applications. Despite these obvious advantages as well as other
beneﬁts of IPNs, they have not yet been used in many industrial applications due to
process limitations. For example, many of the previously reported methods involve the
use of large amounts of solvent and require signiﬁcant time for swelling.

A goal of this research was to to circumvent many of the problems associated with
the formation of IPNs by reacting two distinct, crosslinkable monomer systems
simultaneously to form an IPN. To test this strategy, the possibility of initiating one
monomer via a radical chain reaction while at the same time polymerizing the second
monomer via cationic chain reactions was investigated. Successful development of such
a process would yield at least two major advantages: it would drastically reduce time
requirements for both reactions to reach equilibrium; and the process would be

environmentally benign because it is solvent-free.

177

While such a process would have obvious advantages over current IPN processing
methods, there are a number of disadvantages which drastically reduce the rewards
associated with developing such a process. A signiﬁcant disadvantage of this method is
the fact that it greatly limits the number of IPNs which can be produced. There will be at
least two different active species present in measurable amounts due to the fact that there
are two reactions occurring simultaneously. This limits the availability of useful
monomers right from the start because no monomer may be initiated by more than one
active species during the course of the reaction or the structure most likely to form will be
a single copolymer as opposed to an IPN. For example, polystyrene/polyacrylate IPNs
would not be possible because an active radical could propagate through either the
acrylate or the styrene, thus resulting in a polystyrene/polyacrylate copolymer instead of
an IPN. The initial hurdle in selecting a system is determining if the selected monomers
will undergo the same type of chain polymerization. Table 8.3 (adapted from Odian’)
shows various monomers which will undergo radical or cationic polymerization
exclusively. In order for our process to be amenable to a speciﬁc system, the monomers
must come from separate columns (of Table 8.3). This table is by no means
comprehensive, however it affords one the opportunity to determine if their system is
fundamentally capable of producing an IPN based upon this technology. Furthermore,
selecting monomers from each column of Table 8.3 does not guarantee a successful IPN
since there are other compatibility questions such as miscibility and reaction kinetics

which were not contemplated during the course of this research.

 

 

 

178

Table 8.3. Type of chain polymerization undergone by various unsaturated monomers.

 

Type of Initiation

 

Radical Cationic

 

ethylene oz-oleﬁns

 

halogenated oleﬁns 1,1-dialkyl oleﬁns

 

 

 

 

vinyl esters vinyl ethers
acrylates aldehydes, ketones
acrylonitriles epoxides
acrylarnides

 

 

 

 

8.3.1. Classes of IPNs

Three methods are currently used to synthesize IPN’s. The ﬁrst method (A) is a
multiple-step process in which one network is formed, then swollen by the second
monomer, which is then polymerized and crosslinked in situ. The second method (B) is
also a multiple-step process in which both monomers are mixed initially and the IPN is
formed by sequentially polymerizing and crosslinking the monomers in the system. The
ﬁnal method (C) involves mixing all network ingredients at once, followed by
polymerization and crosslinking of both networks in a truly simultaneous, one-step
process in which one monomer typically undergoes chain polymerization while the other
undergoes a step polymerization.” Method C is best-suited for industrial applications
because it does not involve swelling of polymer networks, dramatically reducing the

overall cure time of the IPN. Despite the improved processability in the simultaneous

179

method, solvent capable of producing unwanted side reactions is still required, which can
result in complications in the formation of one or both polymer networks. Moreover, the
amount of solvent evaporated during reaction can be excessive and may be damaging to
the environment.

The classiﬁcation of IPNS is generally based on the method used to form the IPN.
Polymer networks produced by method A or method B are classiﬁed as sequential IPNs
while method C produces simultaneous IPNS which are more commonly known as SINS.
Both sequential and simultaneous IPNS may be further divided into subclasses based on
the number of crosslinked homopolymers within the network. The most signiﬁcant class
of interpenetrating polymer networks in terms of industrial potential are the simultaneous
IPNs, commonly referred to as SINS. SINs display the highest industrial potential for
many reasons ranging from their facile synthetic process to signiﬁcant time and energy
reductions. Interpenetrating polymer networks synthesized simultaneously are less time-
intensive in both the preparation of the monomeric solutions as well as in the production
of the SINs; hence parts produced using this method are more amenable to large-scale
processing methods such as resin transfer molding (RTM) and reaction injection molding
(RIM). Despite their promise, SINs are the least researched and therefore most poorly
characterized class of interpenetrating polymer networks.

Sle are formed when all components of the ﬁnal network, including monomers,
initiators and crosslinking agents, are mixed together either in bulk or in a solvent to form
a reactive formulation. This formulation is then exposed to a set of experimental
conditions such that two distinct polymerization mechanisms are able to proceed at

approximately equal reaction rates. Nearly all SINs involve one monomer system

180

proceeding via chain polymerization while a second monomer undergoes step
polymerization. These polymerizations occur in a manner such that each reaction will
progress towards completion without interference from the other. Monomers that
"overlap" (i.e. polymerization will occur via either reaction mechanism) will be
ineffective because they will likely yield one large, crosslinked polymer instead of two
intertwined, crosslinked homopolymers. Therefore the selection of restrictive reaction
schemes is vital to the formation of SINs. The choice of monomeric and/or oligomeric
precursors as well as the rates at which these components polymerize are also crucial to
the success of the network. For example, the rate of polymerization for each component
of the system must be equal or nearly equal to the rate of polymerization of other
components in the network or a SIN will not form.

Interpenetrating polymer networks synthesized from (at least) two monomers,
each polymerizing and crosslinking in the presence of the other, involve extremely
complex reaction kinetics. The crosslinking reaction for each component is complicated
by the added reaction that exists when a second monomer is synthesized and polymerized
at the same time. Researchers have tried to largely circumvent this problem by using low
molecular-weight polymers which may still be solubilized instead of monomers, and

crosslinking these homopolymers to form SINS.”23

In addition, IPNs may be sensitive to
environmental changes, both before and during polymerization.24

Composition, like temperature and pressure, has a large affect on the morphology
and physical properties of IPNS. For example, Frisch et al.” stated that an

elastomeric/glass IPN would be a reinforced rubber if the elastomeric phase dominated,

while an IPN having the same components present in different composition would yield

 

 

181

an impact resistant plastic if the glass phase dominated. The ability to design the IPN so
that the properties can be controlled by the composition of the IPN is considered by

Frisch” to be one of the biggest advantages of IPNs.

8.3.2. IPN Experiments

IPN research in this laboratory centered on epoxy/acrylate polymer networks.
The original model system selected for this study was propylene oxide / methyl
methacrylate. These monomers were chosen because they were the single-chain analogs
of glycidyl methacrylate, a monomer whose kinetics for both the radical and cationic
reactions were characterized in the ladder research. This model system proved to be
shortsighted from the beginning because the monomers were incompatible in a number of
areas ranging from the boiling point of each monomer to the density difference. Initial

experiments attempting to polymerize 50/50 mixtures at 65°C belied some of the

fundamental problems of this system. The results of these initial experiments showed
that roughly 40% of the solution had been converted to a solid after 20 minutes, an
extremely low yield when compared with ladder experiments. After attempting to
polymerize propylene oxide individually, the problem became clear almost immediately.
A reactive formulation consisting of 98.9 wt% propylene. oxide, 1.0 wt%
(triethylene glycol) divinyl ether, and 0.1wt% FeCl3 was mixed and stored in a
refrigerator. A 3g sample was placed in a plastic vial, capped, and placed securely in a

water bath maintained at 65°C. The sample was removed from the water bath after 20

minutes, only to ﬁnd that nearly all of the solution had vaporized. This process was

repeated seven times with improvements made each time to seal the vial and prevent the

 

 

182

vial from vaporizing. In each case, there was less than 2% of the monomer remaining at

the conclusion of the experiment.

8.3.3. Discussion/Conclusions ofIPN Research
The experiments with the propylene oxide/methyl methacrylate system led to the
immediate conclusion that production of an IPN based on a thermally-initated
simultaneous radical/cationic chain polymerization would be extremely difficult due to a
number of considerations including monomer compatibility, competing reactions, and
lack of (thermal) control over the reaction. In fact, the lack of control led to the initial

investigations using photoinitiators to polymerize the individual networks.

 

183

Figure 8.1. Chemical structure of glycidyl methacrylate.

 

 

184

 

-—-

""i

I I
I I
I I
I I

-—d

I I I I I I
I I I I I I
I I I I I I
I I I I I I

P---
--d
--q
---d
--

 

PERFECT 2-DIMENSIONAL LADDER

Inch-4

 

 

 

Side View Top View
3-DIMENSIONAL "LADDER"

 

 

---- methacrylate — polyether —— polyvinyl
linkage chain chain

"CROSSLINKED" LADDER

Figure 8.2. Potential structures of poly(glycidyl methacrylate).

 

185

1.75
1.65
1.55
1.45

1.35

 

In Rp
/,

1.25 I q,
1.15
1.05
0.95
0.85

 

m—
.1

0,75 , . w v f . v . . Y . - . . . .
2.83 2.85 2.87 2.89 2.91 2.93 2.95

III at 10*3(1/K)

Figure 8.3. Predicted propagation rates of MMA (—) and propylene oxide (- -) as a
function of temperature.

 

 

186

Exotherm

 

 

O 5 10 15 20 25 30 35 40
Time (min)

Figure 8.4. Rp versus initiator intensity for GMA polymerized cationically at 73°C.

45

187

 

I40

 

 

 

Temp to C)

 

 

 

 

 

20 ~r
simu
- - - -cation
m.-.” Meal
0 T v T v . . Y . . Y f ﬂ
0 500 1000 1500

'ﬁn: (s)

Figure 8.5. Thermal proﬁles of glycidyl methacrylate polymerized radically,
cationically, and simultaneously.

 

 

188

 

 

 

 

 

 

 

 

2'5 ;
4
1 A
1
1
2 up ‘
1 .
1.5 f x‘
‘ I
4 l .
l 1
1
1
4
0.5 *'
3. :
0 4»
-0.5 "
j I 3D
-| 1* 3%!
‘ A Perfect
-l.5 b .' """ Pcrfectﬁt
. 0 Partial
1
2 v v $a C1 5 - f g 1 Miami
—0. l S -0.l -0.05 I) 0.05 0.]

Figure 8.6. Simulated stress/strain curves for poly(glycidyl methacrylate) ladder
structures.

10.

11.

12.

13.

14.

15.

16.

17.

189

8.4. References

M.S.M. Alger, in "Specialty Polymers," R.W. Dyson, Ed., Chapman and Hall,
New York, 64 (1987).

W.J. Bailey, Polym. Mat. Sci. Eng., 60, 400 (1989).

G. Odian, "Principles of Polymerization," 3rd Ed., Wiley-Interscience, New York,
149 (1991).

J .1. Jones, Rep. Prog. Appl. Chem, 27, 544 (1968).
M.M. Tessler, J. Polym. Sci., Part A, l, 2521 (1966).

.I. McMurray, "Organic Chemistry," Brooks/Cole Publishing, Monterey,
California, 423 ( 1984).

W.R. Dunnavant, J. Polym. Sci., Part B, 6, 49 (1968).

EH. Lee, C.S. Marvel, J. Polym. Sci., Polym. Chem, 21, 83 (1983).
SA. Jenekhe, Macromolecules, 24(1), 1 (1991).

A. Ghafoor and RH. Still, J. Appl. Polym. Sci., 21, 2905 (1977).

F. Subira, G. Sauvet, J.P. Vairon, and P. Sigwalt, J. Polym. Sci. Symp., 56, 221
(1976).

F. Subira, J .P. Vairon, and P. Sigwalt, Macromolecules, 21, 2339 (1988).

J. Brandup and EH. Irnmergut, Eds. (with W. McDowell) in “Polymer
Handbook”, Wiley-Interscience, New York, II-316 (1989).

J .C.W. Chien, Y.-G. Cheun, C.P. Lillya, Macromolecules, 21, 870 (1988).
K. Dusek and J. Spevacek, J. Polym. Sci., Polym. Phys, 18, 2027 (1980).

POLYGRAF Reference Manual, v3.1, Molecular Simulations, Waltharn, MA, 5-
15 (1992).

DR. Paul, J.W. Barlow and H. Keskkula , in "Encyclopedia of Polymer Science
and Engineering," 12, H.F. Mark, N.M. Bikales, C.G. Overberger and G. , Eds.,
Wiley-Interscience, New York, 399 (1988).

 

‘

 

18.

19.

20.

21.

22.

23.

24.

25.

190

R. Pemice, K.C. Frisch and R. Navare, in "Polymer Alloys III," 20, D. Klempner
and KC. Frisch, Eds., Plenum Press, New York, 219 (1983).

A. Morin, H. Djomo and G.C. Meyer, Polym. Eng. Sci.,23(7), 394 (1983).
H. Djomo, A. Morin, M. Damyanidu, G. Meyer, Polymer, 24, 65 (1983).
DJ. Hourston and MG. Huson, J. Appl. Polym. Sci., 46, 973 (1992).
MS. Lin and KT. Jeng, J. Polym. Sci., Polym. Chem, 30, 1941 (1992).
MS. Lin and R.J. Chang, J. Appl. Polym. Sci., 46, 815 ( 1992).

DJ. Hourston and MG. Huson, J. Appl. Polym. Sci., 45, 1753 (1992).

H.L. Frisch, K.C. Frisch and D. Klempner, in "Chemistry and Properties of
Crosslinked Polymers," S.S. Labana, Ed., Academic Press, New York, 205
(1977).

CHAPTER 9.

RESEARCH SUMMARY AND RECOMNIENDATIONS FOR FUTURE WORK

Through fundamental “proof-of-concept” and characterization studies, this
research effort has demonstrated the tremendous potential of photopolymerization
processes for the production of composites and thick polymers. The initiation strategies
developed in this research can in fact be used to produce thick polymers and composites.
As described in the preceding chapters, it is clear that the advantages afforded by
photopolymerizations can be extremely advantageous in the production of thick and ﬁber-
ﬁlled polymer composites. A general summary of this research effort as well as
recommendations for the continuation of research in this area are presented in the

remainder of this chapter.

9.1. Research Summary

Two novel strategies for the production of thick polymers and composites based
on photoinitiation methods have been developed in this research effort. The ﬁrst strategy
employs careful selection of initiators, resins and system “extras” including
photosensitizers, accelerators, dyes and pigments to establish a system where the

transverse light intensity gradient that exists in thick samples is initially modest. This

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strategy relies on efﬁcient penetration of light into the bulk of the sample as the reactive
formulation polymerizes. The second strategy capitalizes on the fact that these reactions
are highly exothermic to use a hybrid photo/thermal initiation mechanism. In this
method, ultraviolet light excites a photoinitiator, causing it to undergo some type of
decomposition reaction resulting in the generation of active sites. These active sites
initiate polymerization in the sample while simultaneously elevating the temperature of
the system. This causes a second, thermal initiator to undergo some type of
decomposition reaction which results in the generation of additional active sites
throughout the bulk of the sample, thereby enhancing the reaction.
A reactive formulation consisting of a radical photoinitiator (benzoin ethyl ether),
a vinyl ester resin, and occasionally a thermal initiator (benzoyl peroxide) was employed
to demonstrate the initiation strategies developed in this research. A number of resin and
initiators were investigated in addition to the materials used in the representative system.
A summary of investigated materials is shown in Table 9.1. A series of fundamental
investigations were conducted in order to establish the validity of these initiation
strategies and to establish their potential as new methods for producing composites and
thick polymers. Fundamental “proof-of-concept” studies that were completed to validate
the concepts of “self-eliminating gradients” and hybrid “photo/ thermal” initiation
methods consisted of a number of important system variables including:
o optimization of initiator concentration and overall initiator formulation
including concentration of photoinitiator, thermal initiator and photosensitizer;
0 optimization of operating window including careful selection of initiator

formulation, incident light intensity and wavelength range;

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- investigating the effects of ﬁber type, treatment, and loading on the cure time

and mechanical properties of resultant composites;

0 mechanical and thermal characterization of the resulting composite parts.

Combined, these studies present a picture that clearly shows the domain of
photopolymerization applications can be expanded to include thick polymers and
composites using either the concept of self-eliminating light intensity gradients or the
notion of a hybrid photo/thermal initiation scheme. Widely accepted photoinitiator
classes which have proven to be successful catalysts for either strategy developed in this
research include benzoin ethers and the so called “Irgacure” initiators produced by Ciba
Geigy. These initiators can be used to cure important classes of monomers and resins
such as vinyl esters, acrylates and methacrylates. Additional experiments have
demonstrated the ability to decouple mold ﬁlling and reaction, resulting in the rapid
production of thick and ﬁber-ﬁlled polymer composites which exhibit good mechanical
properties.

In conclusion, these initial studies have conﬁrmed the potential of these novel
processing schemes for the production of photocurable composites. This research
represents a signiﬁcant breakthrough in UV technology. It has used novel strategies
based on photoinitiation processes to cure products that are considerably thicker than
what was once thought possible. However, before employing either of the strategies
outlined in this research commercially, considerable work must be put in to optimize the

composite products and to develop applications.

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Table 9.1. Resins, photo and thermal initiators studied in the development of the
initiation strategies.

 

 

 

 

 

 

 

 

 

 

Monomers/Resins Photoinitiators Thermal Initiators
Derakane Benzoin ethers Benzoyl Peroxide
470-45 Benzophenone 2,2’-azobisobutyronitrile
411c-50 Benzoin Methyl Ether Wako V-70
441—400 Benzoin Ethyl Ether Wako VA-044
51 1 Camphorquinone/TBA
8084 Irgacure 639
Methyl Methacrylate Irgacure 651
Glycidyl Eosin B
Methacrylate

 

 

 

 

9.2. Recommendations for Future Work

Although photopolymerizations offer several advantages when compared with
traditional thermal polymerizations, their use in the production of thick and ﬁber-ﬁlled
composite parts has been limited due to problems associated with poor mechanical
properties arising from non-uniform and incomplete cure. In order to fully exploit the
ﬁndings of this research, it is recommended that this research branch off in three
directions: further development of the radical system; research into cationic systems; and

investigations and development of industrial applications.

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9.2.1. Development of Radical Systems

A general objective of this research has been to validate the concept of using
photopolymerizations to produce thick polymers and composites. As a result of this, only
minor attention has been paid to characterizing the kinetics of these systems. While
effects of system variables have been qualitatively analyzed and radical propagation and
termination constants are well-established for vinyl ester resins, it would still be
beneﬁcial to quantify the overall kinetics of these reactions.

Although the mechanical properties of these systems have been characterized,
they were optimized for an extremely limited number of glass-ﬁber reinforcement
materials which were chosen to provide the most optically challenging systems possible
instead of providing the best mechanical properties. It is therefore worthwhile to
investigate available reinforcement materials in order to optimize the composites to
achieve the highest performance levels possible (although the level of performance is
dictated somewhat by the particular application). While it is highly unlikely that they
would be well-suited for SEG strategy due to the opaque nature of the composite, carbon
ﬁbers should be investigated in addition to the aforementioned glass ﬁber systems as
reinforcement materials for composites cured using the dual-cure strategy. Initial
polymerizations involving relatively modest degrees of carbon ﬁber loading (~ 25 wt% or
less) have met with some success. However, by no means can any conclusion be drawn
as to their ability or inability to be used as reinforcing materials for the photocurable
composites.

Characterization of the chemical resistance for the vinyl ester polymers and

composites has recently been initiated and should be continued. Preliminary results from

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swelling studies indicate that most of the Derakane polymers are stable in a number of
chemical environments ranging from water to methanol to 10 % solutions of sodium
hydroxide and hydrochloric acid. Still, these experiments have been initiated very
recently and are highly limited in scope.

The ﬁnal area of research which merits attention for the Derakane vinyl ester
resins is to determine how different variables such as initiator concentration and
formulation, incident intensity, resin type and ﬁber type and degree of loading affect the
extent of cure for these polymers and composites. Techniques for determining the extent
of cure have been discussed in this contribution and could be performed to better
characterize the effects of operational variables on the ultimate properties of resultant
polymers and composites. Infrared (IR) spectroscopy is an additional technique that can
be used to quantitatively characterize the extent of cure for polymeric systems. IR
experiments performed in this research proved to be inconclusive, however there are a
number of peaks in the IR spectrum such as the vinyl bond and the ether linkage which

can be monitored to quantitatively determine the extent of cure.

9.2.2. Fundamental Research into Cationic Systems
Preliminary studies have indicated that the cationic initiation systems may be
successfully used to cure thick polymers and composites, however further development
and characterization is necessary. Development of a strategy based on photoinitiation of
cationically polymerizable systems would allow one to cure important classes of
commercially available resins such as epoxies. This research may serve as a guide for the

future efforts in cationic systems. Speciﬁc efforts in this area should focus on

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establishing the following operating conditions for a representative epoxy system:
development of usable photoinitiator systems including photoinitiator, photosensitizer
and thermal initiator; optimizing the monomer/resin formulation for desirable viscosity,
curing rate and temperature control; and characterizing the effects of cure time, incident
intensity and wavelength range on the reaction kinetics. In addition, photoinitiator
formulations based on diaryliodonium salt/anthracene systems have been investigated and
it has been established in this research that these systems exhibit the self-eliminating
characteristic even more dramatically than the radical systems employed in this research
(see Chapter 4 - Developing the Polymerization Strategies: Fundamental Experiments).
These fundamental studies will provide an understanding of the relationships
among reaction variables, thereby allowing efﬁcient process development. Because
general strategies have been developed which can serve as guidelines for this research and
potential initiator formulations have already been established, research efforts should
focus primarily on developing suitable epoxy systems and not on investigating new
initiator formulations (although investigations of new initiators should not be completely

overlooked).

9.2.3. Investigation and Development of Potential Applications
It is strongly recommended that research in pursuit of applications based on this
technology be performed in tandem with industrial contacts. The input of industrial
collaborators in terms of both ﬁnancial and technological support is invaluable and offers
insight into many of the problems which one might face when trying to take a product

from the prototype stage to production. With these comments in mind, a number of

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potential applications which merit further attention have been investigated in the course
of this research.

Potting Compounds. Prototype samples for photocurable potting compounds
have already been developed and tested at three industrial sites. Based upon the industrial
feedback, adjustments in the formulation should be made to make the compounds
industrially feasible for speciﬁc applications. An objective of research in this area should
be to develop several formulations which can be used to meet a variety of industrial needs
depending on the particular application. For example, highly adhesive formulations may
be desired for repair applications while some applications such as systems where two or
more structures are bonded temporarily may require formulations which exhibit fair or
even poor adhesion.

Photocurable prepregs. This application exploits the rapid response and high
degree of control provided by photoinitiation to partially polymerize a resin-impregnated
ﬁber mat to a precise, predetermined degree of cure. In this partially polymerized,
intermediate state, the resin is cured to an extent that it will not ﬂow, but the prepreg will
be pliable (i.e. tacky but shapeable) and can be worked with and formed into the desired
shape before photocuring to form to a rigid composite. Since light rather than heat
provides the initiating energy, these prepregs will have the unique characteristics of
superior working times in the absence of UV light, but short cure times upon illumination
at room temperature.

This novel design of these photocurable prepregs is based upon fundamental
aspects of the photoinitiation and free radical termination processes. Due to the

underlying photochemistry, the production of active centers occurs rapidly, consuming a

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portion of the photoinitiator upon illumination by the appropriate wavelength of light.
When the prepreg is removed from the light, the active center concentration decreases
rapidly due to radical termination reactions and will decay to zero in a few seconds,
allowing the part to cure to an intermediate state that is both predictable and reproducible.
The degree of intermediate conversion is readily manipulated by changing the
illumination time, hence the system can be tailored to achieve an intermediate prepreg
with the desired characteristics. The ﬁnal photocure (after forming) utilizes the
unconsumed portion of photoinitiator to rapidly form a stiff, ﬁber-reinforced composite.
If necessary, this cure could readily be achieved using a second photoinitiator and a
different wavelength of light than the initial partial cure.

Perforated, Composite, Acoustic Panel Face Sheets. Although the
photocurable prepregs may ﬁnd many applications, they are ideally suited for the
production of perforated composite acoustic panel face-sheets for the aerospace industry.
Perforated, cylindrical composite structures are used to surround jet engines to provide
acoustic dampening. These acoustic panel face-sheets are difﬁcult to manufacture due to
the curvature of the structure and the ordered pattern of perforations. Since these radial
perforations must extend through the thickness of the part, it is impossible to remove
these structures from a cylindrical mold containing pins to produce the perforations. Due
to this problem, current methods of production are tedious and inefﬁcient. A
photopolymerizable prepreg offers an efﬁcient and elegant method to produce such
structures. For this application, the resin—impregnated ﬁber mat may be partially
polymerized in a ﬂat mold equipped with ordered pins to produce the perforations. The

semi-cured prepreg can be easily removed since the mold is ﬂat. The perforated prepreg

 

 

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will then be shaped around a cylinder to impart the proper curvature, and the cure will be
completed. The perforations will remain intact, therefore no pins are required in the

cylindrical geometry and the ﬁnished part is easily removed.

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