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This is to certify that the

thesis entitled

Effects of Thermal Aging and Moisture Absorption on
Mechanical and Physical Properties of Cyanate Ester Polymers

presented by

Aparna Venkatramani

has been accepted towards fulfillment
of the requirements for

Master's degree in Materials Science

 

 

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EFFECTS OF THERMAL AGING AND MOISTURE ABSORPTION ON
MECHANICAL AND PHYSICAL PROPERTIES OF CYANATE ESTER

POLYMERS

By

Aparna Venkatramani

A THESIS
Submitted to
Michigan State University

in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Material Science and Mechanics

2000

ABSTRACT
EFFECTS OF THERMAL AGING AND MOISTURE ABSORPTION ON
MECHANICAL AND PHYSICAL PROPERTIES OF CYANATE ESTER
POLYMERS
By

Aparna Venkatramani

The aim of this research is to study the effects of thermal aging and hygrothermal effects
on the mechanical and physical properties of cyanate ester (CE) and siloxane modified
cyanate ester (SMCE) polymers. Thermal aging was conducted on these polymers in air
at temperatures of 60°C, 80°C, 100°C in order to assess their susceptability to thermo-
oxidation processes, since thermo-oxidation can degrade the properties of some
polymers. Also, the properties of polymers can be degraded by absorption of moisture,
which can pose significant problems in CB and SMCE. Spectrographic techniques such
as, fourier transform infrared spectroscopy (F TIR) and x—ray photoelectron spectroscopy
(XPS) and thermal analysis techniques such as differential scanning calorimeter (DSC)
were employed to evaluate and interpret changes observed in the physical properties.
Environmentally induced changes in the mechanical properties of CE and SMCE were
determined by conducting flexure and microhardness tests. Cyanate ester tended to gain
weight when aged in air at elevated temperatures ranging from 60°C-1000C due to
thermo-oxidation. In sharp contrast, siloxane modified cyanate ester showed essentially

no weight gain in when aged at the same temperature range. Differential scanning

calorimeter tests showed that the glass transition temperature of CE remained unchanged
even afier aging in air. Thermomechanical analysis (TMA) showed a slight decrease in
glass transition temperature on testing the same sample twice by the instrument. The
flexural strength of cyanate ester was reduced by 20% after thermal aging in air for ~
1000 hours. FTIR analysis revealed the formation of carbonyl groups as well as other
significant spectral changes due to the thermo-oxidation process. XPS was utilized to
study the elemental composition and binding energies. XPS spectroscopy showed an
increase in surface concentration of oxygen with thermal aging for CE polymers.
Microhardness results obtained by using a Vicker’s indentor showed an increase in the
hardness of CE but showed no change in the microhardness of SMCE polymers when
aged in air. Moisture absorption studies revealed that SMCE absorbed substantially more
water than CE resin regardless of how the samples were pre-conditioned. Absorbed
moisture affected the Tg of both CE and SMCE. Microhardness tests showed the changes
in the microhardness of CE and SMCE due to moisture absorption. An increase in the
microhardness of CE was found due to moisture causing crosslinking of the polymer
chains on the surface of the sample while the hardness in case of SMCE remained
unchanged due to moisture absorption at all temperatures. The present investigation
reconfirms that there are performance differences between CE and SMCE resins. The

primary focus of this research is to discern the reasons for such observations.

Copyright by

Aparna Venkatramani

2000

To My Family and Friends

ACKNOWLEDGEMENT

I would like to thank the following people. Firstly, I would like to thank my master’s
thesis advisor, Dr. James P. Lucas, for his guidance, friendship, and suggestions
throughout my graduate studies and research work. His support is greatly appreciated.
It has been a great pleasure to work with him.

I would also like to thank Dr. K.N. Subramanian and Dr. Andre Lee for being on my
graduate committee. Thanks are also extended to my colleagues Fu Guo and Rajbala
Makar for their contributions, guidance and friendship.

I also acknowledge the Composite Materials and Structure Center for the use of much of
the equipment necessary for conducting my research. Finally, I thank my parents,

brother, and sister-in- law for their love, support and encouragement.

vi

TABLE OF CONTENTS

LIST OF TABLES .......................................................................... x
LIST OF FIGURES ........................................................................ xi
LIST OF ABBREVIATIONS ............................................................ xvi
CHAPTER 1
INTRODUCTION ........................................................................... 1
CHAPTER 2
LITERATURE REVIEW .................................................................. 6
2.1 Cyanate Ester Based Polymer Materials ................................... 6
2.2 Thermo-Oxidation of Polymer Materials .................................. 13
2.2.1 Thermo-Oxidation Mechanisms ....................................... 14
2.2.2 Thermo-Oxidative Kinetics in Polymer Materials .................. 18
2.2.3 Chemical Change in Polymers Due to Thermo-Oxidation ......... 20
2.2.4 Effects of Thermo-Oxidation on Chemical and Physical
Properties ................................................................. 21
2.3 Hygrothermal Effect of Polymer Materials ................................ 22
2.3.1 Moisture Absorption Kinetics .......................................... 23

2.3. 2 Effects of Moisture Absorption on Mechanical Properties.........26

CHAPTER 3
THERMO-OXIDATION OF CYANATE BASED POLYMER MATERIALS..28
3.1 Experimental Procedure ...................................................... 28
3.1.1 Materials Description ................................................... 28
3.1.2 Preparation of the Samples ............................................. 33
3.1.3 Gravimetric Experiments ............................................... 33

3.1.4 Glass Transition Temperature (Tg) Measurement Using DSC....34
3.1.5 Glass Transition Temperature Measurement Using TMA ......... 36

vii

3.1.6 FTIR Testing .............................................................. 37

3.1.7 X-ray Photoelectron Spectroscopy (XPS) Testing ................... 37
3.1.8 Three-point Bend Testing ................................................ 38
3.1.9 Microhardness Testing .................................................... 39
3.2 Results and Discussions ..................................................... 40

3.2.1 Assessment of Thermal Aging Effects of CE and SMCE

Resins ....................................................................... 40

3.2.1.1 Gravimetric Analysis ................................................... 40
3.2.1.2 Glass Transition Temperature (Tg) Using DSC .................... 55
3.2.1.3 Glass Transition Temperature (Tg) Using TMA ................... 64
3.2.1.4 Thermo-oxidation Mechanism in CE Using FTIR ................. 69
3.2.1.5 Therrno-oxidative Mechanism in CB Using XPS .................. 74
3.2.1.6 Three-point Bend Test .................................................. 84
3.2.1.7 Microhardness Testing .................................................. 88
3.3 Summary and Conclusion .................................................... 92

CHAPTER 4

HYGROTHERMAL EFFECT ON CYANATE BASED POLYMER MATERIALS

4.1 Experimental Procedure ........................................................ 94
4.1.1 Moisture Absorption Methods ........................................... 94
4.1.2 Gravimetric Experiments ................................................. 95
4.1.3 Glass Transition Temperature (Tg) Experiments .................. ....96
4.1.4 Microhardness Testing .................................................... 96

4.2 Results and Discussion ......................................................... 97

4.2.1 Assessment of Hygrothermal Effects of CE and SMCE Resins...97
4.2.1.1 Gravimetric Analysis .................................................... 97
4.2.1.2 Glass Transition Temperature Using DSC .......................... 112
4.2.1.3 Microhardness Testing ................................................. 125

4.3 Summary and Conclusion .................................................... 127

viii

CHAPTER 5

CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions ................................................................... 128
5.2 Recommendations ........................................................... 130
BIBLIOGRAPHY ........................................................................... 131

LIST OF TABLES

Table 3.1 The nominal physical properties of cyanate ester at room temperature.

(p. 32)

Table 3.2 The nominal physical properties of siloxane modified cyanate ester at room
temperature. (p. 32)

Table 3.3 Glass transition temperature of cyanate ester (CE) aged in ambient air for
different aging time. (p. 58)

Table 3.4 Glass transition temperature of siloxane modified cyanate ester (SMCE)
aged in ambient air for different aging time. (p. 58)

Table 3.5 X-ray photoelectron spectroscopy (XPS) results showing the % atomic
concentration of elements in CB material. (p. 76)

Table 3.6 X-ray photoelectron spectroscopy (XPS) results showing the % atomic
concentration of elements in SMCE material. (p. 76)

Table 4.1 Glass transition temperature for moisture absorbed SMCE polymer
material. (p. 113)

LIST OF FIGURES

Figure 2.1 Typical examples of commercial and developmental CE monomers. (p. 8)
Figure 2.2 Cyclotrimerization process of cyanate ester. (p. 9)
Figure 2.3 Schematic structures of polysiloxanes. (p. 12)
Figure 3.1 Schematic structure of cyanate ester network. (p. 30)
Figure 3.2 Schematic structure of siloxane modified cyanate ester (SMCE). (p. 31)
Figure 3.3 Thermo-Oxidation curves of cyanate ester aged in ambient air

for 1200 hours with square root of time. These curves compare the thermo

-oxidation of CE at temperatures of 60°C, 80°C and 100°C respectively. Q). 43)

Figure 3.4 Thermo—Oxidation curves for SMCE aged in ambient air for 1200

hours with square root of time. These curves compare the thermo
-oxidation of SMCE at temperatures of 60°C, 80°C and 100°C respectively.

(P- 44)

Figure 3.5 Comparison of thermo-oxidation curves of CE and SMCE aged in ambient
air at 100°C for 1200 hours. (p. 45)

Figure 3.6 Comparison of thermo-oxidation curves of CE and SMCE aged in ambient
air at 80°C for 1200 hours. (p. 46)

Figure 3.7 Comparison of thermo-oxidation curves of CE and SMCE aged in ambient
air at 600C for 1200 hours. (p. 47)

Figure 3.8 Linear graphs showing Thermo-Oxidation in CB materials aged in
air for 1200 hours. These plots compare the thermal aging of CE
polymers at temperatures 60°C, 80°C and 100°C respectively. (p. 52)

Figure 3.9 Linear graphs showing thermo-oxidation in SMCE materials aged in
air for 1200 hours. These plots compare the thermal aging of SMCE
polymers at temperatures 60°C, 80°C and 100°C respectively. (p. 53)

Figure 3.10 Linear graphs comparing the thermal aging in CB and SMCE polymers
aged in air at 60°C, 80°C and 100°C for 1200 hours. (p. 54)

Figure 3.11 Changes in glass transition temperature of CE with aging time. (p. 59)

Figure 3.12 Changes in glass transition temperature of SMCE with aging time. (p. 60)

xi

Figure 3.13 DSC scan showing the Tg for CE aged in air at 100°C for a period of
0 hours, 200 hours, 500 hours and 900 hours respectively. (p. 61)

Figure 3.14 DSC scan showing the Tg for SMCE aged in air at 1000C for a period of
0 hours, 200 hours, 500 hours and 900 hours respectively. (p. 62)

Figure 3.15 TMA scans showing the glass transition temperature range and relaxation
occurring in CE aged in air at 100°C for a period of 0 hours, 200 hours, 500
hours and 900 hours respectively. (p. 66)

Figure 3.16 TMA showing the Tg for CE obtained by rerun of the same CE sample for

the second time. The scan shows a decrease in Tg for CE in the second run
of TMA. (p. 67)

Figure 3.17 Comparison of Tg of CE aged in air at 1000C for a period of 0 hours, 200
hours, 500 hours and 900 hours respectively and obtained from DSC and
TMA. (p. 68)

Figure 3.18 FTIR spectra (766-1036 cm") of CE aged in ambient air at 100°C. Change
in spectra with aging time show the decrease in 911 cm'1 and 966 cm'1

peaks. (p. 71)

Figure 3.19 FTIR spectra (1661-2340 cm") of CE aged in ambient air at 100°C. Change
in spectra with aging time show the increase in the 1726 cm'1 peaks. (p. 72)

Figure 3.20 FTIR spectra (1661-3977 cm“) of SMCE aged in ambinet air at 100°C.
Change in spectra with aging time show the increase in the 1726 cm'1 peaks.

(p. 73)

Figure 3.21 XPS results showing the atomic concentration for CE and SMCE aged in air
at 1000C for a period of 0, 500 and 900 hours respectively. (p. 77)

Figure 3.22 XPS spectra showing the % atomic concentration and binding energies for
CE aged in air for 0 hours. (p. 78)

Figure 3.23 XPS spectra showing the % atomic concentration and binding energies for
CE aged in air for 500 hours. (p. 79)

Figure 3.24 XPS spectra showing the % atomic concentration and binding energies for
CE aged in air for 900 hours. (p. 80)

Figure 3.25 XPS spectra showing the % atomic concentration and binding energies for
SMCE aged in air for 0 hours. (p. 81)

Figure 3.26 XPS spectra showing the % atomic concentration and binding energies for

xii

SMCE Aged in air for 500 hours. (p. 82)

Figure 3.27 XPS spectra showing the % atomic concentration and binding energies for
SMCE Aged in air for 900 hours. (p. 83)

Figure 3.28 Degradation mechanism of thermally aged CE polymer by three-point bend
test. (p. 85)

Figure 3.29 Illustration of the oxide layer formed due to the thermo-oxidation for a 3-
point bend specimen. During 3-point bend testing, microcracks readily
nucleate and grow in the oxide layer at low applied loads. Such

microcracking at low loads leads to lower flexural strength in this material.

(p 85).

Figure 3.30 Illustration showing the oxide layer which forms on the surface of CE.
specimens during thermal aging in air. Microhardness was determined on
such specimens. (p 89)

Figure 3.31 Microhardness results showing the changes in the microhardness
for CE and SMCE aged in ambient air at 100°C for 0 hours, 200 hours
500 hours and 900 hours respectively. (p. 91)

Figure 4.1 Water absorption curves of CE at different environmental temperature with
square root of time. The plots compare the moisture absorption for CE aged
in air and at 100°C for 1000 hours and baked out in argon for 5 days.

(p. 100)

Figure 4.2 Water absorption curves of SMCE at different environmental temperature
with square root of time. At 1000C for 1000 hours and baked out in argon for
5 days. Q). 101)

Figure 4.3 Comparison of water absorption curves for comparing the weight change for
CE and SMCE exposed to environmental chambers containing distilled
water at 45°C with square root of time. (p. 102)

Figure 4.4 Comparison of water absorption curves for comparing the weight change for
CE and SMCE exposed to environmental chambers containing distilled
water at 60°C with square root of time. (p. 103)

Figure 4.5 Comparison of water absorption curves for comparing the weight change for
CE and SMCE exposed to environmental chambers containing distilled
water at 750C with square root of time. (p. 104)

Figure 4.6 Comparison of water absorption curves for comparing the weight change for

CE and SMCE exposed to environmental chambers containing distilled
water at 900C with square root of time. (p. 105)

xiii

Figure 4. 7 Comparison of diffusion rates in CB and SMCE polymer
when exposed to moisture absorption at 450 C for 2000 hours with square
root of time. (p. 108)

Figure 4. 8 Comparison of diffusion rates in CE and SMCE polymer
when exposed to moisture absorption at 600 C for 2000 hours with square
root of time. (p. 109)

Figure 4.9 Comparison of diffusion rates in CE and SMCE polymer
when exposed to moisture absorption at 75°C for 2000 hours with square
root of time. (p. 110)

Figure 4.10 Comparison of diffusion rates in CB and SMCE polymer
when exposed to moisture absorption at 90°C for 2000 hours with square
root of time. (p. 111)

Figure 4.11 Comparison of 0glass transition temperature for SMCE samples
aged in air at 100 C for 1000 hours and baked out in argon for 5 days and
exposed to water maintained at 45°C, 60°C, 75°C and 900 C
respectively. (p. 115)

Figure 4.12 DSC scan showing the Tg for SMCE aged for 0 hours. (pg. 116)

Figure 4.13 DSC scans showing the Tg for SMCE aged 1n air at 100°C and exposed to
water at 45°C. q). 117)

Figure 4.14 DSC scans showing the Tg for SMCE aged 1n air at 1000 C and exposed to
water at 600C. (p. 118)

Figure 4.15 DSC scans showing the Tg for SMCE aged 1n air at 1000 C and exposed to
water at 75°C. (p. 119)

Figure 4.16 DSC scans 0showing the T8 for SMCE aged 1n air at 1000C and exposed to
water at 900 C. (p. 120)

Figure 4.17 DSC scans showing the Tg for SMCE baked out in argon at 1000 C for 5
days and exposed to watergat450C. (p.121)

Figure 4.18 DSC scans showing the Tg for SOMCE baked out in argon at 1000 C for 5
days and exposed to waterg at 600 C. (p. 122)

Figure 4.19 DSC scans showing the Tg for SMCE baked out in argon at 1000C for 5
days and exposed to waterg at 75°C. (p.123)

Figure 4.20 DSC scans showing the Tg for SMCE baked out in argon at 1000C for 5

xiv

days and exposed to water at 900C. (p. 124)
Figure 4.21 The results determining the microhardness for CE and SMCE exposed to

preaging conditions and submerged in water for 2000 hours at 45°C, 60°C,
70°C and 900C respectively. (p. 126)

XV

CE
SMCE
BMI
HSCT
DSC
FTIR

PMS

UP

M0

UV
Aw°/o

W0

TMA
FPA

CHA
XPS

LIST OF ABBREVIATIONS

Cyanate ester

Siloxane modified cyanate ester
Bismaleimide

High-speed civil transport

Differential scanning calorimeter
Fourier transform infrared spectroscopy
N-phenylmaleimide styrene
Glass transition temperature
Unsaturated polyester

Moisture content
Initial moisture content

Equilibrium moisture content
Ultra-violet

Percentage by weight change

Initial weight of samples
Measured and final weight of the samples

Thermomechanical analysis

Focal plane array

Concentric hemispherical analyzer

X-ray photoelectron spectroscopy

xvi

CHAPTER 1

INTRODUCTION

During the last few decades, polymeric materials have played an important role in
replacing metals and metal composites in various applications. In the automobiles,
aerospace, space and sporting goods industries, polymers and their composites are highly
desirable, due to their lightweight, dimensional stability, good mechanical strength
properties that are desirable in such applications [1]. In such applications, polymer
composite materials are exposed to a wide range of temperatures, radiation, mechanical
fatigue and extreme environmental conditions like oxygen and moisture [2,3]. Several
authors [2-7] have reported the environmental degradation of mechanical and physical
properties of polymer matrices.

Epoxy polymers have been widely used for many important applications such as
adhesives, insulating materials, reinforced plastics and as matrix for advanced composite
materials [7,8]. Unfortunately, epoxy resins have relatively high dielectric loss and a
high propensity for water absorption. Therefore, cyanate ester polymer materials were
developed. CE resins have low dielectric loss properties, low water absorption, excellent
heat resistance and low volume shrinkage; properties that are useful in both the aerospace
and electronic industries. These unique properties of CE materials make them attractive
composite matrices and direct competitors for standard epoxy and BMI resins in high
performance applications [1,9]. Other characteristics associated with cyanate esters

functionality are easy processibility, miscibility, and reactivity with epoxy resins and

miscibility with many amorphous therrnoplastics materials [10,11]. CE also offers an
attractive selection of physical and mechanical properties. The high aromatic content in
monomers and the cured network is responsible for the relatively high Tg, inherently low
smoke generation and good flame retardancy, and relatively low crosslink density. The
balanced dipoles in the cured state and absence of hydrogen-bonding functionality are
responsible for the low dielectric properties of the polymer material. Such characteristics
make CE resins prime candidates for use for a variety of applications from
microelectronics to aerospace. They also find use in primary and secondary structures in
military aircraft and have been evaluated for potential applications in the once proposed
but now defunct US high-speed civil transport (HSCT).

Chemically, this family of thermosetting monomers and their prepolymers contain
the reactive ring-forming cyanate (-O-CEN) functional groups. Conversion of
polyfunctional CE monomers to thermoset polymers occurs via cyclotrimerization to
form three-dimensional networks of oxygen-linked triazine and bisphenyl units, correctly
termed as polycyanurates or cyanate ester or triazine resins.

One of the major concerns of such polymeric based composite materials in industry
and commercial applications is their environmental degradation, which occurs in a wide
variety of service conditions such as moisture absorption, high temperature oxidation,
and photo-radiation [2]. Degradation occurs as a result of severe environmental
conditions, which involve chemical and physical attack, often caused by the combination
of degradation agents, and may involve several physical, chemical and mechanical

mechanisms. For example, many polymers are prone to degradation caused by

weathering in which photochemical reactions, involving solar ultraviolet photons and
atmospheric oxygen, leading to possible chain scission [2,3,12,13].

Hygrotherrnal effects of polymer materials involves the absorption of water by the
resin matrix, which often causes degradation, plasticization, swelling, blistering, and
lowering of glass transition temperature (T8) of the polymer matrix [3].

Most investigations [14-18] have shown that epoxy based composite materials
absorb significant amount of water which induces the degradation and dimensional
instability of polymer materials. Such changes tend to limit their application in high
technology industry, which usually demands for high environmental stability standards of
materials. Intensive research has been performed in an effort to provide thermosetting
polymers with a reduced tendency to thermal oxidation and moisture absorption.
Thermally stable and low moisture absorption materials were achieved by the
development of cyanate ester resin based polymeric materials.

To examine the degradation mechanisms of the family of cyanate ester, model
compounds like diphenyl cyanurate, triphenyl isocyanurate, cyanuric acid and triphenyl-
s-triazine were studied at elevated temperatures both in air and in vacuo [l]. Triphenyl
cyanurate was chosen as the model compound for the network structure as it contains the
principal bonds present in the three-dimensional structure of the polymer. The
mechanism of thermal degradation were studied using chromatographic and mass
spectroscopic methods and showed that, as in the degradation of the dicyanate polymers,
the principal decomposition products are carbon dioxide, carbon monoxide, phenol and
cyanuric acid. From the model study it was concluded that triphenyl-s-triazine exhibited

the highest thermal stability of the compounds investigated [8]. There have been many

studies [1] on the hygrothermal effect of cyanate ester based polymer materials. Results
show that the water absorption decreases the glass transition temperature and mechanical
strength of cyanate ester based polymer materials. Shimp and Ising [10] found that the
class of cyanate ester resins homopolymers absorbs less than 3% of water over a period
of 6 months as compared to the competing resins such as epoxies, BMI and condensation
polyirnides. The cyanurate linkage is very resistant to hydrolysis, withstanding ‘hundreds
of hours’ of immersion in boiling water [1].

Kasehagen et al. [1 9] studied moisture absorption in cyanate ester resins. They
found that hydrolysis of cyanate ester caused blistering of the network, which produced
gaseous products such as carbon dioxide, alcohol, etc. and lowered the T8 and strength of
the matrix. Cinquin et al. [20] also studied the water absorption in cyanate ester and
found that moisture absorption in cyanate ester materials followed Langmuir’s law. They
further determined that moisture absorption curves of these materials followed the Fick’s
second law of diffusion. Lee et al. [6] also studied the water absorption in cyanate ester
and showed that these materials absorbed small amounts of water when exposed to high
humidity and the moisture profiles followed F ick’s law in the early stage of moisture
absorption. As the material continued to undergo hydrolysis, it was observed that the
water absorption behavior became Non-Fickian after prolonged exposure to high
humidity.

This research focuses on the effects of thermal aging and effects of moisture
absorption on cyanate ester (CE) and siloxane modified cyanate ester (SMCE) using
gravimetric, analytical and spectroscopic techniques. The weight change of CE and

SMCE when exposed to ambient air at different temperatures and when exposed to 100%

humidity (submersion in water bath) maintained at different temperatures were studied.
FTIR was utilized to study the changes in the chemical structure of CE and SMCE during
thermo-oxidation process. Glass transition temperatures changes with thermal aging time
and with moisture absorption were determined to study the extent of degradation in CE
and SMCE resins. Three- point bend test and microhardness tests were used to study the
changes in flexural strength and microhardness of CE and SMCE with aging. XPS was
used to study the elemental surface concentration for each element making up the
chemical structure of the cyanate ester polymer materials following environmental
exposure. This research is an extension of the thesis work by Z. Liu, (1999), at Michigan
State University. This work clarifies and further explains the behavior of CE and SMCE

polymer materials due to thermal aging in ambient air and moisture absorption.

CHAPTER 2

LITERATURE REVIEW

2.1 Cyanate Ester Based Polymer Materials

The families of materials that make up the cyanate esters have been developed primarily
because of their high T8 and low dielectric properties [1,21]. These properties have made
cyanate ester an attractive alternative to epoxies and BMI in the manufacture of printed
circuit boards [9]. These materials are also potentially strong competitors to polyimides
and epoxy materials in structural composites, particularly in the aerospace industry.

Cyanate ester has a unique chemistry that makes them of interest for the study of
network formation. The primary polymerization reaction can be thought of as
cyclotrimerization to form a triazine ring with one functionality unlike epoxy-amine
system in which there are two or more distinct chemical functionalities with potentially
different reactivities [22]. A cyanate ester crosslink structure, which may be of a
thermoset or a thermoplastic nature, is derived from cyanate functional monomers and/or
oligomers. Some cyanate monomers can be advanced to a prepolymer stage wherein up
to about 50 percent of the functional groups react. Effectively, the product of such a
reaction contains the triazine moiety and may or may not contain unreacted cyanate
functional groups [22].

In particular, the cyanate functional monomers and/or oligomers consist essentially

of cyanate ester of one or more compounds of the general formula,

NCOAr(YxArm)qOCN

where, m = 0 to 5, q = 0 to 5, x = 0 up to 2, Ar is a single or fused aromatic or substituted
aromatic and combinations thereof linked in the ortho, meta and/or para position and Y is
a linking unit selected from the group consisting of oxygen, carbonyl, sulphur, sulphur
oxides, etc [22]. Commercial dicyanate monomers are divided into two categories: (1)
monomers that are currently commercial products, and (2) developmental monomers that
are previously, or currently, available as experimental products. The structures and
commercial trade names of these categories of materials is listed in Figure 2.1. The

cyclotrimerization process is explained by the chemical reactions as shown in Figure 2.2.

Commercial Cyanate Ester Monomers: Developmental Cyanate Ester Monomers:

NCO OCN NCO 0 OCN
M.P = 79°C M P: 87°C
AroCy B-IO (Ciba) ESR 288 (Ciba)
CH3 OCN
NCO oCN
11 OCN
M.P=290C M.P = 78°C
AroCy L-lO (Ciba) REX 370 (Ciba)
CF3
NCO OCN NCO OCN
CF3
M.P=87°C - '
_ L1 urd
AroCy F-10(C1ba) argx 370 (Ciba)

Figure 2.1 Typical examples Of commercial and developmental cyanate ester monomers

sec /\ /\OCN

 

{2,2-Bis(4-cyanatophenyl)propane}

 

Heat [ Catalyst {250ppm Al(acac) + 2phr nonyl phenol}

xl——l'—‘l

0

Figure 2.2 Cyclotrimerization Process of Cyanate Ester [4]

The formation of polycyanurates through polycyclotrimerization Of cyanate ester is a
typical step growth process. Starting from low molecular weight with at least two
cyanate groups, the reacting system passes through a pregel state with a broad
distribution of oligomers to reach the gelpoint at a certain value of the functional groups’
conversion. The chemical and physical structure of the network is closely related to the
underlying formation process. The CEN functional group in cyanate ester resin is Of
considerable importance since it is frequently used to quantify the conversion to
prepolymer and fully cured polymer [1,21,22].

One major property Of cyanate ester systems of concern is their inherent brittleness
[9], which is a major drawback in such materials. Toughening thermoset matrices
derived from the cyclotrimerization Of aryl dicyanates, modified with minor amounts of
thermoplastics resins is described, with emphasis on defining parameters contributing to
increase fracture toughness. Rubber toughening has been applied successfully to CE.
Rubbers, which have been used, include amine-terminated butadiene-acrylonitrile
copolymers [1,9], amine terminated polybutylene ethers and a hydroxyl- terminated
polydimethysiloxane-polyester block copolymer in addition to thermoplastic modifiers
like polysulphone, poly (ether sulphone), polyacrylates. Iijima et al. [24] found that N-
phenylmaleimide-styrene copolymer was an effective modifier and an alternative to
reactive rubber tougheners, for improving the brittleness of the resin, and from the
viewpoint Of maintenance Of mechanical and thermal properties of the resin. Inclusion of
10 wt % N-phenylmaleimide-styrene (PMS) led to 160% increase in the fracture

toughness for the modified resin with the slight loss of flexural strength and retention Of

10

flexural modulus and the glass transition temperature, compared to the values of the
unmodified resin [23].

Oligosiloxane elastomers are rubber tougheners, which have also been used to
toughen CE with little reduction in T8 up to 10% addition of the elastomer.
Organosiloxane polymers are known for their excellent thermal and thermo-oxidative
stabilities, very low glass transition temperature, high moisture resistance and good
electrical properties [6]. Microcracking is decreased relative to the unmodified resin.
Polysiloxanes are inorganic polymers, that is, there are no carbon atoms in the backbone
chains. The schematic structure of polysiloxane is shown in Figure 2.3. The backbone is
a chain of alternating silicon and oxygen atoms. Polysiloxanes possesses an unusual
combination of properties that are retained over a wide temperature range [24]. They
have very good low temperature flexibility because of the low Tg value. Silicones are
very stable to high temperature, oxidation, chemical and biological environments and
weathering and possess good dielectric strength. Thus, Silicones make good elastomers
because the backbone chain is very flexible. The bonds between a silicon atom and the
two oxygen atoms attached to it are very flexible. The angle formed by these bonds can
open and close like a scissor without much trouble. This makes the whole backbone

chain flexible.

11

CHZR CHZR

 

——-- Si— O—Si

CHZR CH2R

— —

R = H, n=10-1000

 

 

Figure 2.3 Schematic structure of Polysiloxane.

12

2.2 Thermo—Oxidation of Polymer Materials

Degradation of components made Of polymeric materials occurs in a wide variety Of
environments and service conditions, and very Often limits their service lifetime.
Degradation occurs as a result of environment-dependent chemical and physical attack,
often caused by the combination Of degradation agents, which may involve several
chemical and mechanical mechanisms. For example, many polymers are prone to
degradation caused by weathering in which photochemical reactions, involving solar
ultraviolet photons and oxygen lead to chain scission, but degradation of polymers is
much faster in the presence of oxygen at elevated temperatures [12,13]. In an aggressive
oxygen environment polymer molecular chains may break (chain scission), cross-link, or
suffer substitution reactions [2]. Substitution is the least common and causes the smallest
prOperty changes while, chain scission and cross-linking both occur under natural
weathering conditions. The initiation Of thermal aging proceeds by a radical chain
process initiated by dissociation. ThermO-oxidation reaction of polymer materials
usually causes the degradation Of polymer properties, that is, it lowers tensile strength,
lowers glass transition temperature, and lowers dimensional stability [3]. Chemical and
physical properties of the polymers depend on the composition and structure Of molecules
that make up the polymer matrix. The thermal stability of the polymer resin matrix is

greatly influenced by the strength Of chemical bonds [1,12].

13

2.2.1 Thermo-Oxidation Mechanism

The oxidative degradation of polymers takes place by free radical chain reaction. An
important difference from some other chain reactions is that besides the three steps, that
is, initiation, propagation, and termination, two additional important steps must be
considered here. The conversion Of the formed hydrocarbon radicals to peroxy radicals,
that is the main oxygen consuming reaction, and degenerate chain branching or
rearrangement, that is the reaction responsible for the autoacceleration character Of the
process [25,26].

The most important steps in polymer oxidation are as follows [26].

Initiation:
RH + 02 _> R‘ + HOO‘ (1.1)

where RH is a polymer monomer,

R'+RH ——>R'+RH (1.2)

where R' is a free radical,

HOO' + RH _’ R' + H202 (1.3)

Radical conversion (stabilization):

R' +02 ——> ROO' (II)

R00' is a peroxide radical,

14

Chain Propagation:
R02' + RH _, R‘ + ROzH (111.1)

ROz‘ + RH _"'> R’ + products (111.2)

Degenerate chain branching:
R02H —> OLR' (IV.1)

ROOH + ROOH—* BR“ (1V.2)

OR" and BR‘ are degenerated radicals.

Termination:
R’ + R'—-—> inactive product (V .1)
R’ + RO'z —> inactive product (V .2)
R02' + R02“ __> Inactive product (V .3)

(1) Initiation
There are many factors that are responsible to initiate the free radicals, such as light
(photo-oxidation), thermal energy and thermo-oxidation, and will react most readily with
molecular oxygen. Initiation occurs by the direct reaction Of oxygen molecules with the
polymer material. This reaction is endothermic and very slow at low temperatures. The

probability of this reaction occurring is higher when the polymer contains reactive

15

hydrogen [25,26], and consequently tertiary-hydrogen and secondary hydrogen adjunct
electron-rich groups are very active.

Free radical are produced from non-radical intermediate by the process of chain
branching, as shown by steps (1.1), (1.2) and (1.3) in the oxidation process. Audouin et al.
[27-31] suggested that in the radical oxidation of hydrocarbon polymer, the initiation was
only due to unimolecular or bimolecular hydroperoxide decomposition. Their results of
analysis have been compared to the thermo-oxidative degradation of cross-linked
polyethylene, polypropylene and linear poly (vinyl chloride) when aged in 110-1700C-
temperature range. The results Obtained in these investigations show that the initiation
scheme is in good agreement with experimental results.

(2) Radical conversion
Radical conversion involves the important step wherein, conversion of the hydrocarbon
radical to a peroxy radical takes place during oxidation, namely, it is the step in which
majority of oxygen is absorbed by the polymer. This process is shown by step (11). As
soon as the alkyl radical is generated in an initiation reaction, it reacts with oxygen. Here
oxygen exists in a diradical state. This reaction is very rapid in nature as the alkyl radical
reacts spontaneously with oxygen. This type of reaction is essentially a radical coupling
reaction. The rate Of this reaction depends on the concentration of oxygen inside the
polymer, that is, on its pressure outside and its ease of diffusion [31]. Thus, the oxidation

process may become diffusion controlled with low pressure and/or with high sample

thickness.

16

(3) Chain propagation
Chain propagation in polymer oxidation consists of the hydrogen abstraction reaction of
the peroxy radicals. With increasing rate constant of this reaction, the rate of oxidation
increases and the kinetic chain length becomes greater. The chain propagation is shown
by steps (111.1) and (111.2). This will produce more free radicals, with increase in rate Of
oxidation. The reactivity Of hydrogen toward free radicals increase in the following
order: primary < secondary < tertiary, in accordance with the decrease in bond
dissociation energies. Because Of the high reactivity of the tertiary hydrogen in polymers
like polyprOpylene and polystyrene, intermolecular propagation takes place at a much

faster rate in oxidation.

(4) Degenerate chain branching
The decomposition Of hydroperoxides to radicals is the most important step in polymer
oxidation. Hydroperoxides, commonly the major product of oxidative degradation, are
potentially powerful initiators of further degradation. The thermal decomposition Of
hydroperoxides may proceed unimolecularly, bimolecularly, or with the participation of
the R-H group. The presence Of both carbonyl and carboxyl groups accelerate the
decomposition; acid catalyzed decomposition, however, favors the formation of
molecular products. Besides ketone and water, many other products can be formed in
this step. The formation of most of the volatile products like, alcohols, acids and
aldehydes is connected to the hydroperoxide decomposition reaction. This process is

explained by steps (IV.1) and (IV.2).

17

(5) Termination
Most common type of termination reactions includes unimolecular or bimolecular
termination Of free radicals in polymer oxidation. This occurs by participation of peroxy
radicals. Unimolecular termination takes place when solid polymers or polymer melts Of
high viscosity are oxidized. A bimolecular termination step is unlikely since it would
depend upon two very small concentrations requiring polymer-polymer interaction in a
medium Of very high density. This process is very important at the low oxygen pressures

and is shown in steps (V.l),(V.2) and (V.3).

2.2.2 ThermO-Oxidative Kinetics in Polymer Materials.

Many aging processes involve a reaction Of polymer with small molecules like 0;, H20
and ozone typically found in the environment. On thermal aging the bonds between the
atoms making up the polymer chains may break forming a variety of fragment radicals or
small molecules, which may mutually combine or break. For an aging process involving
the consumption of small molecules such as oxygen, there exists critical conditions of
reaction rate and thickness above which the process becomes kinetically controlled by the
diffusion Of the small molecules in the polymer. In many cases the thickness Of the
degraded layer is of the order of magnitude D / k, where D is the diffusion coefficient and
k is the pseudo-first-order rate constant for oxygen consumption [13]. Two processes
explain thermal aging in polymers, (i) diffirsion controlled and (ii) not diffusion
controlled. Based on the mechanism of diffusion Of oxygen into the polymer matrix there

are two possible domains.

18

(1) Domain 1: For low values of r and the sample thickness L, the kinetics is not diffusion
controlled. The low molecular weight diffusing species is consumed homogeneously
throughout the sample. Here, r is defined as the local reaction rate in a very thin
superficial layer of the sample exposed to thermal aging.

(2) Domain II: For higher values of r and L, the kinetics is diffusion controlled. Most of
the low molecular weight diffusing species is consumed in the superficial layers of the
sample. The diffusing species are heterogeneously distributed within the sample
thickness [13].

The thermo-oxidation of polymer materials is based on diffusion-controlled kinetics of
oxygen into the resin matrix. Most authors [27-31] have explained this process Of

thermo-oxidation in polymers fi'om Fick’s second law:

2
6C=D5 C

_ , 2.1
at 6x2 ( )

 

where, C = the concentration Of diffusing species,
t = time, and

x = depth Of penetration of reactive species.

When the reactant is consumed by the reaction at a rate r, the above reaction becomes,

6_C __ 62C
6! 6x2

 

— r , (2.2)

The local reaction rate r is a function Of the local reaction concentration: r _> r(C).

19

Furthermore, it can be concluded that the system comes to a stationary state when

aC/at = 0 and then,

02C

2

 

D -r(C)=0, (2.3)

It should be possible to determine the concentration profile C (x) with this differential

equation, provided that D and r(C) are known.

The diffusion coefficient, D, depends on the conversion of the aging process, that is, Of

time and on the reactant concentration.

(i) The diffusion coefficient depends on time, and in this case, the initially
homogeneous sample becomes heterogeneous. Thermo-oxidation in the
superficial layers leads to chain scission and a slight increase of the oxygen
permeability whereas crosslinking in the core zone results in decrease of the
oxygen permeability

(ii) The diffusion coefficient also depends on the concentration of diffusing species.
This anomalous behavior can be Observed in the case where high concentrations
Of the diffusing species can induce noticeable matrix plasticization. In these
cases, short-term physical aging processes are generally responsible for very
important property changes.

2.2.3 Chemical Change in Polymers Due to ThermO-Oxidation

The chemical composition and structure Of the molecules that make up the polymer resin

matrix determine the dimensional and thermal stability of the polymer. The diffusion Of

oxygen into such polymer matrix causes morphological degradation of the polymer

matrix. Vibrational analysis Of polymers provides information on three important

20

structural features: chemical composition, configurational and conformational structure
and interatomic forces associated either with valence bonding or intermolecular
interactions [32]. Any physical or chemical treatment of a polymer induces structural
change, the knowledge Of which is essential for a better understanding of polymer
properties. Chemical changes that occur during the thermo-oxidation process Of
hydrocarbon polymers can be determined using F TIR. It was established that carbonyl
formation after the end of the induction period occurred only in a superficial region
whose thickness was a decreasing function Of temperature as predicted by the simple
models Of the diffusion controlled process [26]. Infrared spectroscopic studies showed
that aldehydes (1735 cm'l) and ketones (1720 cm") are the main species present during
the early stages of degradation, although carboxylic acids (1710 cm!) are the species
dominating in the later stages. The presence of carboxylic and carbonyl group in the
degraded polymer indicates that oxidation has taken place and also warns that the

material is vulnerable to further deterioration [12].

2.2.4 Effects of Thermo-Oxidation on Chemical and Physical Properties Of CE

The chemical composition and structure of the polymer material determines its thermal
stability. A change in chemical, physical and mechanical properties during long-term
exposure can be due to various processes such as physical aging (volume relaxation),
crystallization, orientation or stress relaxation, crosslinking and chain scission [13]. It is
found that rate of oxygen diffusion is much greater in amorphous materials than in
crystalline domains, because the crystalline cores are inaccessible to molecular oxygen.

The oxidation rate of polyethylene is inversely proportional to the degree Of crystallinity

21

[27]. The mobility Of radicals is greater in amorphous regions. This implies that the rate
of diffusion Of oxygen is much faster in amorphous polymers than in crystalline domains.
It is also observed that the crystallinity increases with the degree of oxidation. This is
attributed to the breaking of chains in amorphous regions, facilitating their crystallization.

[33].

2.3 Hygrothermal Effect of Polymer Materials.

Hygrothermal effect on polymers is of great importance because water and temperature
are the most common environmental factors causing performance reduction in polymers.
The most prominent amongst the reasons is the fact that sorbed moisture promotes
material degradation, plasticization, swelling and lowered T8 of the polymer matrix.
Merdas et al. studied the consequences of water absorption on the physical properties of
polyetherimide [34]. They found that water absorption affected important resin
properties like glass transition temperature, tensile properties and viscoelastic behavior Of
the polymer resin. Tg decreased with exposure to moisture absorption, which indicates
that water had a significant plasticizing effect on polyetherimide. Tensile properties of
polyetherimide decreased with water absorption. Park et al. [3 5] studied the influence of
water absorption on the glass transition and the low temperature relaxations of
semiaromatic polyamides by means Of the calorimeter and dynamic mechanical
techniques. They found a depression in the glass transition temperature in diluted system
due to the presence of free volume provided by the diluent, which allowed high
segmental mobility at low temperatures. In addition to decrease in Tg, other useful

properties of the material are also affected with diffusion of moisture into the polymer

22

resin. Smith et al. [36] also studied the effects of water on Tg. Their study involved the
effects of water on Tg of poly methyl methacrylate polymers. They too found a
depression in the glass transition temperature by about 200K. Agarwal and Farris [3 8]
investigated the diffusion of water into acrylic-based latex blend films. They found a
significant change in the appearance and mechanical properties of the film. Many
researchers [8] have studied the mechanism Of moisture diffusion in epoxy resins. The
absorption showed an increase in weight during the initial stages of moisture absorption
followed by decrease at later times. This indicates that water diffusion is Fickian
followed by the Non-Fickian model governing the final stages Of the absorption process.
They also found a decrease in Tg and moisture absorption induced swelling which

increased with increase in moisture content.

2.3.1 Moisture Absorption Kinetics

Moisture induced property degradation tends to correlate well with the extent of
absorbed moisture in the polymer matrix. Moisture related degradation is associated with
chemisorptions and reactivity characteristics Of the permeating environment and matrix
material.

Typically, moisture transport in polymer resins can be described by classical Fickian
diffusion. The diffusion process is similar for both thermoset and thermoplastic polymers
and polymer composites.

The Fick’s law for diffusion Of moisture in resin materials is expressed as

2
x

23

where, M represents the concentration of diffusing medium, x is a dimensional
coordinate, and D is the diffusion coefficient that is temperature dependent. Applying
appropriate boundary conditions and integrating, we derive an expression predicting

moisture absorption with time,

44: _ i 91%
E—[hlinl ' (2'5)

Equation 2.5 represents the initial linear segment of the moisture diffusion profile, where

Mt is proportional to J; .

Further manipulation Of equation 2.5 shows that diffusion coefficient can be expressed as

D = 32(sz (MT (2.6)
16 Mm Jr: _ Jr,
where,

D = the diffusion coefficient,
h = the laminate thickness,
Mm = the maximum moisture content, a constant independent of temperature,
Mt = moisture content at time t.
Moreover, the entire moisture-time diffusion profile is determined theoretically by

the following expression.

24

M, = Mm{l—exp|:— 7363—21) ' J}. (2.7)

D Often exhibits a dependence on temperature that follows an Arrhenius relationship

given by

 

D(T) = D0 exp[ 33° 1 (2.8)

where,
D0 = pre-exponential factor,
A0 = the activation energy for diffusion,
R = the universal gas constant, and
T = the temperature measured on the absolute scale. (0K)

The kinetics of water absorption in polymers is usually assumed to be a concentration-in-
dependent on Fickian diffusion process [3 8-41 ]. This model, which is usually applicable
to the initial stages of moisture absorption process, fails to describe the whole
phenomenon [42]. This complete phenomenon is explained by means of a Langmuir type
diffusion process [43], in which the absorbed material can be divided into mobile and
strongly bound types. Langmuir’s model can be used to fit the experimental data. The

two equations, 2.9 and 2.10, express the model.

azn _6n 6N

—— ———+-—, 2.9
fix2 6: at ( )

25

%=yn—fln.. (2.10)

Here, n = number density Of mobile molecules,
N = number density Of bound molecules,
7 = probability per unit time, during which a bound molecule Of water becomes
mobile, and

B = probability per unit time that a mobile molecule Of water became bound.

2.3.2 Effect of Moisture Absorption on Mechanical Properties

Water absorption results in the depression Of morphological characteristics of plastic
materials and loss of usefirl mechanical properties. Mechanical degradation Of amine
cured epoxy resin matrix by water is due to the formation of microcavities, and long-term
exposure of the epoxy resin to high humidity [43]. Maggana and Pissis [42] concluded
that in neat epoxy resin, degradation begins with water uptake, swelling and leaching Of
non-bound substances. The hydrolytic degradation of epoxy results from the scission Of
the polymer chain, which leads to a progressive reduction in molecular weight. Although
relatively short-term exposure can lead to more or less reversible plasticization, more
prolonged exposure to moisture can lead to crack growth and loss of material. Loss in
tensile properties occurs rapidly after aging at higher temperatures and relative
humidities. Elongation shows similar trends, although the loss of elongation is even

more rapid. Lucas et al. [14,15] studied the variation in mechanical property of water

26

saturated epoxies with different exposure temperatures. They found that water absorption
affected the tensile and flexural modulus of the polymer. Their results showed a decrease
in the modulus after immersion in water for more than 1500 hours.

One primary advantage of thermoplastic resin over thermosetting composites is its
negligible moisture absorption and reprocessing capabilities. Further, comparing the
effect of moisture on the compressive properties of thermosetting and thermoplastic
composites, it was found that the compressive strength and modulus of graphite
reinforced thermoplastic APC-2 composites are comparatively higher than those Of T-3 00
epoxy composite [16]. The physical impact Of the absorbed moisture is to degrade the
mechanical properties of the polymer [45-48] mainly in compressive strength where,
failure is in the form of delamination, global specimen buckling, shear failure, and end

brooming. Moisture also modifies the damping properties of the material.

27

CHAPTER 3
THERMO-OXIDATION OF CYANATE BASED POLYMER MATERIALS
3.1 Experimental Procedures

3.1 .1 Materials Description

The materials used for this study were Hexcel 954-3, cyanate ester, (dark brownish-green
colored) and Hexcel 996, siloxane modified cyanate ester, (light orange-brown colored).
Hexcel 954-3 is a 3500F (1770C) curing cyanate ester resin with glass transition
temperature (190°C) and excellent resistance to moisture absorption, outgassing and
microcracking. Hexcel 954-3, cyanate ester chemically comprises Of one or more
formula of the general formula NCOAr(YxArm)qOCN, where m = 0 to 5, q = 0 to 5 and x
= 0 up to 2. Ar is a single or fused aromatic or substituted aromatic linked onto the ortho,
meta or para positions and Y is the linkage unit selected from the group consisting Of
oxygen,carbonyl, sulphur, sulphur oxides etc. Hexcel 954-3 also contains inorganic
species that are advantageously combined with and bonded to the cyanate ester resin. The
inorganic species may be amine, hydroxyl, epoxy, silicon, zirconium, aluminum, etc.
Hexcel 954-3 cyanate ester having this composition show enhanced oxidation resistance,
enhanced toughness and high glass transition temperature [22]. The schematic diagram

of Hexcel 954-3 CE is shown in Figure 3.1

Hexcel 996, SMCE contains inorganic species such as siloxanes, which combines and
bonds tO the cyanate ester system. The siloxane molecular weight was about 4500. The
inorganic species may be present in the resin body (cyanate ester functional monomers or
oligomers) from 0.1 tO 80 weight percent, preferably 1 to 50 weight percent and most

preferably l to 30 weight percent. The siloxane inorganic species contain about 50-70

28

mole present of methyl and 30-50 mole percent of phenyl groups present as substitutional

groups. The schematic diagram Of Hexcel 996, SMCE, is shown in Figure 3.2

29

Figure 3.1 Schematic Structure of Cyanate Ester Network [1].

30

 

i ‘1
NC -— Si— O Si CN
1 .

 

 

R = Pendant group
n= 4500

Figure 3.2 Schematic structure of siloxane modified cyanate ester [1 ].

31

Typical properties Of Hexcel 954-3 cyanate ester resin and Hexcel 996 siloxane modified
cyanate ester Obtained from Hexcel’s data sheet, are listed in Table 3.1 and Table 3.2

below.

Table 3.1 Typical physical properties of 954-3 CE resin at room temperature 1‘.

 

 

 

 

 

 

 

 

PROPERTIES RT
Tensile Strength 57 MPa
Tensile Modulus 2.8 GPa
Tensile Ultimate Strain, % 2.4
Flexural Strength 119 MPa
F lexural Modulus 3.0 GPa
Density 1.19 g/cc
Table 3.2 Typical physical properties of 996 SMCE resin at room temperature ..
PROPERTIES RT
Tensile Strength 51 MPa
Tensile Modulus 3.0 GPa
Strain (%) 1.7
Flexural Strength 132 MPa
Flexural Modulus 3.2 GPa
Density 1.146 g/cc

 

 

 

 

* Hexcel’s Manufacturer Data Sheet dated 3/23/99 and 3/20/98 respectively.

32

3.1.2 Preparation Of the samples

Cyanate ester and siloxane modified cyanate ester were Obtained in the form Of neat resin
panels from Hexcel Inc. The panels were polished on a lapping wheel to remove
contaminants such as grease, mold release agents from the surfaces of the laminates. The
laminates were further cut into samples Of sizes 25.4 mm x 25 .4 mm x 2.2 mm
dimensions and 12.5 mm x12.5 mm x 2.2 mm dimensions respectively using a diamond
saw. The samples Of size 12.5 mm x 12.5 mm x 2.2 mm dimensions were used for the
thermo-oxidation experiments. The edges of the samples were subsequently ground
using 600 grit abrasive papers to achieve consistently smooth edge surfaces. The sample
surfaces were polished on polishing wheel containing silicon carbide and alumina as the
polishing medium. All samples were marked with a vibrating pen and further cleaned in
methanol using ultrasonic cleaner. The initial weight of the samples ranged from 2 tO 2.5
gm. The samples were handled very carefully during the experiments to avoid any

surface contaminations.

3.1 .3 Gravimetric Experiments
Samples were tested in environmental test chambers, consisting Of gaseous aging
environment Of ambient air maintained at 60°C, 80°C and 100°C with a fluctuation range
Of iloC. Gravimetric experiments were used to monitor weight change of specimens

during thermal aging. Polymer samples were exposed to the aging environment for ~

1200 h.

33

Samples were taken out Of the environmental test chamber periodically during the aging
experiment. Samples were weighed using the analytical electronic balance with 0.01 mg
resolution. The experimental error of the measurement is i0.02%.

The percentage weight change is calculated as follows:

 

Aw%=[W’—w°]x 100 (3.1)

W0

3.1.4 Glass Transition Temperature Measurement Using DSC

The glass transition temperature Of CE and SMCE for different stages of
thermal aging was determined using a TA Instruments” 930 differential scanning
calorimeter (DSC) system [49]. DSC is used to determine the temperatures Of thermal
events such as glass transition , melting, phase transformations, Curie point transitions,
reactions (including decomposition, devolatization, crosslinking, etc). The heat liberated
or consumed during these events, such as heat of fusion and heats Of reaction, can also be
accurately determined along with the specific heat capacity. The measurements from
DSC provide quantitative as well as qualitative information about the physical and
chemical changes that the sample undergoes. They include both endothermic (heat
absorbed) and exothermic (heat released) processes. The DSC uses a two pan system,
where the sample is placed in one pan and the other pan is left empty as a reference.
Sample size is usually between 5-15 grams Of the material. In the power compensated
DSC the two pans are heated separately in a way that they achieve the same temperature

simultaneously. In heat flux DSC the two pans are exposed tO the same heat source and

34

the temperature difference between the two is measured. In either case, the Objective is
to measure the amount Of energy absorbed or released from the polymer sample. The
resulting exothermic or endothermic energy requirements for the sample are plotted
against the temperature [50]. Modulated DSC method was utilized to Obtain the Tg of CE
and SMCE samples. Modulated DSC produces three signals consisting Of heat flow,
reversible heat flow and non-reversible heat flow. The Tg is obtained from the transition
Observed in reversible heat flow. The advantage Of temperature modulated DSC is
improved resolution and sensitivity, and the ability to separate overlapping phenomena.
The sample size used in this glass transition temperature (Tg) experiment varied from
3-15 mg. The samples were sealed in small aluminum pans to prevent any weight loss
during the experiment, and the weight Of each sample pan was measured before and after
each test. The samples were put into testing chamber protected by pure nitrogen gas.
The temperature range for heating was maintained between 25°C to 2500, and the
temperature increase rate was SOC/min. The Tg values were obtained by the intersection
of two tangential lines of the DSC curves, which was automatically determined by the
instrument’s software. These measurements were Obtained before and after the glass

transition temperature regime [49].

35

3.1.5 Glass Transition Temperature Measurement Using TMA
Thermomechanical analysis (TMA) measures variations in the vertical displacements Of a
probe resting on top of a sample and is used to obtain physical property changes as a
function Of temperature and time. Some of the properties Obtained by using TMA are
thermal expansion, phase changes, glass transition temperature, melting temperatures,
dimensional stability, modulus, compliance and deflection temperature under load and
vicat temperature. TMA used for thermoset materials show typically two linear region,
the first one associated with the glassy state and is followed by a change tO a second
linear region Of higher slope associated with the rubbery state because of Tg [50]. The
coefficient Of thermal expansion and Tg of a thermoset are closely related to the degree of cure
of the resin. Size of the samples used in this glass transition temperature experiment had
the dimensions of 12 mm x 8 mm x 2 mm. The samples was held by the TMA probe in
the vertical position. Precautions were taken to see that the sample did not touch the
thermocouple during each test. They were enclosed in a furnace having an oxygen
environment. The temperature range for heating was maintained between 25°C to 2500,
and the temperature increase rate was 40C/min. TMA profile of the material exhibits stress
relief. Thermal cycling or annealing above ”I“g will smooth the curve but will not elevate T3.
Ideally, Tg is Observed as an abrupt change in the slope of the linear expansion versus
temperature curve. However, as Often, relaxation occurs near Tg, the transition can be

broad, depending upon such factors as cure state, internal stresses and test conditions.

36

3.1.6 FTIR Testing

Fourier transform infrared spectroscopy was used to determine the change Of
chemical structure Of cyanate ester and siloxane modified cyanate ester materials
associated with bonding characteristics at various stages [51]. A Bio-Rad, Excalibur
Series FTIR was used for this study. The samples of dimensions 12.5 mm x 12.5 mm x
2.2 mm were polished to 0.25 mm film thickness. These thin films were aged in
environmental thermal aging chambers in ambient air at 100°C for different periods Of
time up to 500 h. The sample was periodically removed from the chambers and allowed
to cool to room temperature before transferring to a holder designed for FTIR testing.
The FTIR scans for each sample in the thermal aging experiment were Obtained. The
peaks for (-C=C-H-) at 966 cm'1 and at 911 cm"1 were studied. Skeletal stretching at
1017 cm'1 peak and between 2220-2250 cm'l for (-RNCO-), and changes in the aromatic
ring at 810-820 cm'1 were also studied. Thermal aging also showed changes in the

carbonyl group (-O-C=O-) at 1726 cm'1 peak.

3.1.7 X-ray Photoelectron Spectroscopy (XPS) Testing

X-ray photoelectron spectroscopy is a surface sensitive technique, which allows the
determination of the elemental composition and binding energies of the elements in a
depth of a few nanometers. XPS is based on photoelectric effect. When the polymer
material is exposed to electromagnetic radiations of sufficient photon energy,
photoelectrons are emitted. The XPS technique is highly surface specific due to the short
range of the photoelectrons that are excited from the solid. The ejected electrons are then
separated in an electrostatic spectrometer according to their energies ranging from 0-1100

eV. The energy of the photoelectrons leaving the sample was determined using a

37

concentric hemispherical analyzer giving rise to arbitrary count intensity versus binding
energy spectrum. The binding energy of the peaks was characteristic of each element.
The peak areas can be used (with appropriate sensitivity factors) to determine the
composition Of the materials surface. The shape Of each peak and the binding energy can
be slightly altered by the chemical state of the emitting atom [52]. Surface analysis was
performed using a Perkin-Elmer Physical Electronics PH15400 ESCA Spectrometer
equipped with a standard magnesium X-ray source Operated at 300 W (15 kV and 20
mA). Data was collected in the fixed analyzer transmission mode utilizing a position
sensitive detector and hemispherical analyzer. The elemental composition Of the surface
was determined from survey spectra collected using pass energy of 89.45 eV. High-
resolution spectra of the elements were Obtained using pass energy of 22.5 eV and a step
size of 0.1 eV. Binding energies were referenced to adventitious carbon and were

measured with a precision Of i 0.1 eV.

3.1.8 Three-point Bend Test

The three point bend flexure testing was performed to study the effects of thermo-
oxidation on mechanical properties of both CE and SMCE. The standard procedure
(ASTM D790-90) was used as a guide for specimen preparation and testing. A
representative sample size of six specimens was tested for each aging conditions
consisting Of as-received samples that were aged for 200, 500 and 900 hours respectively.
Bend tests were conducted using Instron® screw driven mechanical test machine, with a
2000-1b load cell. The nominal specimen dimensions used in this study were 25.4 mm
long and 7-8 mm wide and 2 mm thick. A crosshead speed of 10 mm/min was used in

the test. The flexural strength was calculated by formula

38

3P1.

: 26d2 (32)

Where, S= flexure strength (MPa)
P= breakload (KN)
L= outer (support) span (mm)
b = specimen width (mm)

d = specimen thickness (mm)

3.1 .9 Microhardness testing

Hardness is the resistance Of a material to local plastic deformation. Microhardness
testing is an indentation method for measuring the hardness Of a material on a
microscopic scale. Microhardness has been successfully used to characterize metals and
ceramics, where it has been shown to be sensitive to the chemical and microscopic
structure Of materials studied. A precision diamond indentor is impressed into the
material at loads from 1 to 1000 gm. The indenter was forced into the specimen until a
predetermined load Of 10 gm was achieved. The impression length, measured
microscopically, and the test load are used to calculate a hardness value. The
indentations are made using a square-based pyramid indenter, on Vickers hardness scale.
The indent can be precisely located with the microscope and its dimensions can be
determined. Typically, the samples used for this study had the dimensions of 6.5 mm x
6.5 mm x 2.2 mm. The sample size Of six specimens was tested for each aging condition.

The Vickers hardness number was then computed using

39

HV=1.854§PZ— (3.3)

where, P = applied indentation load (grams)

S= diagonal Of indentation (um)

3.2 Results and Discussions

3.2.1 Assessment of Thermal Aging Effects of CE and SMCE Resins

3 .2. 1.1 Gravimetric Analysis

Gravimetric experiments were used to monitor the weight change of the specimens
during thermal aging. The weight change of CE and SMCE are shown in Figure 3.3-3.7.
CE specimens showed a weight reduction in the initial stages of thermo-oxidation and
followed with weight gain after 9 hours Of exposure. The initial weight loss is due to
desorption of the moisture absorbed mostly on the specimen surface, while the gain in
weight is due to the absorption of oxygen across the ester linkage. Shimp et al. [1 1]
studied the thermo-oxidative degradation mechanism in CB polymer materials.
The following schematic diagram explains the thermal aging mechanism in CB

Step 1: Thermal initiation Of CE

LhflkbR-O-CN —> KL“KR-00 'N

40

Step 11: Radical conversion

, NH
«9% M e2
R-OC' 'N + H20 —> R-OC- OH

/ SlOW

Step III: Rearrangemer;\H—It and Termination reactions

WNH S’\¢\m\x\10Rearrang

They proposed that the degradation mechanism began with the hydrolytic cleavage Of the
ester linkage, as shown in step I, and the subsequent attachment of oxygen across this
ester linkage, as shown in step 11, which resulted in gain of weight due to the absorption
of oxygen by the polymer matrix. The radicals generated in steps I and II rearrange
quickly generating a stable form of carbamate group. Here, the decomposition of
hydroperoxides and the rearrangement step is a very important step in the polymer
oxidation because the formation of oxidation products is caused by the degradation Of

these radicals. The thermal degradation and rearrangement Of the hydroperoxides can

41

break down the polymer chain, forming new chemical groups such as carbonyl as shown
in step 111.

On the other hand, SMCE did not gain a significant amount of weight during thermal
aging but the initial loss in weight seen in the Figure 3.4 for SMCE, is due to the
desorption of moisture absorbed on the specimen surface. Figure 3.3 and Figure 3.4
show the thermo-oxidation curves for CE and SMCE respectively. These curves also
compare the thermo-oxidation process in CE and SMCE at different temperatures.
Figure 3.5-3.7 compare the thermo-oxidation process for CE and SMCE at each aging

temperature.

42

% Weight Change

0.4

 

 

r 1 r I I r I 1 r I l f 1 I I I I r l 1 T 1 1

 

+ . . . . i) j .1
( l. aged 111111111161) (

"— CE aged in air at 80°C I. 7
"- C13 aged in air at 1000C

 

 

 

 

 

Square root of Time (hm)

Figure 3.3 Comparison of % weight change for CE polymer when
aged in air at 60°C, 80°C and 100°C, respectively for 1200 hours
with square root of time.

43

% Weight Change

 

 

. SMCllaged 1111111111601 1
0.4 “l— SMCE aged in air at 80°C
'O- SMCL aged in air at 1()0fl('

 

 

0.2 7

 

 

 

 

Square Root of Time (hm )

Figure 3.4 Com arison of % weight change for SMCE polymer when
aged in air at 60 C, 80°C and 100°C, respectively for 1200 hours
with square root of time.

44

 

T I L 1 4 1 1 1 I’ x r I I 1 I T Y

+(‘li aged in air at 6011' for 1200 hours
0-4 + SMCE aged in air at 60°C for 1200 hours

 

0.2

% Weight change

 

 

 

 

Square root of time ( hm)

Figure 3.5 Comparison of weight change in CB and SMCE aged
in ambient air at 60°C for 1200 hours with square root of time.

45

% Weight Change

 

 

. (‘1: aged in air at 80'1" 1111‘ 1200 hours
0-4 I SMCE aged in air at 800C for 1200 hours

 

0.2 7

 

 

 

 

.Lm1r11.1.riiirrriiiir
0 10 20 30 40 50

 

 

Square root of time (hm)

Figure 3.6 Comparison of weight changes in CE and SMCE
aged in ambient air at 80°C for 1200 hours with square root of time.

46

% Weight Change

 

I l l I 1 I I 4 I I l 1 1 I l 1 T T I I

 

. ('IC aged in air at 1001‘1‘ 1111‘ 120011011118 ‘

0.4 + SMCE aged in air at 100 0C for 1200 hours

 

 

 

1 I I l l 1 1 _ 1,,714 l I 1 l l l l 1 l 1 1 1 L

0 10 20 30 40 50

 

Square root of time (hm)

Figure 3.7 Comparison of weight change in CB and SMCE
aged in ambient air at 60°C for 1200 hours with square root of time.

47

Figure 3.3, shows the thermal aging curves for cyanate ester (CE) polymer aged at
different temperatures of 60°C, 80°C, and 100°C respectively. Gravimetric results show a
greater increase in weight for CE aged in air at 100°C as compared to the CE samples
aged in air at 800C and 60°C. Mckellop et al. [53] studied the oxidation in irradiated
polyethylene. They Observed that oxidation in polyethylene was confined only to the
amorphous regions as crystalline cores were inaccessible to molecular oxygen. As a
result amorphous regions continued to undergo oxidative chain scission accompanied by
increase in oxidation. The high temperature also results in chain scissions of the polymer
chains, which encourage the diffusion of oxygen at a faster rate. Thus, thermal aging Of
cyanate ester involves two basic mechanisms: (a) the diffusion of oxygen into the cyanate
ester matrix and, (b) the chemical reaction associated with the free radicals. The thermal
stability of polycyanurates varied depending on the nature of the bridging group.
Polycyanurates of higher molecular weight showed excellent thermal stability and
thermal degradation or crosslinking Of the molecular chain, in polycyanurates containing
alkoxy, piperidyl, chloro or phenoxy substituents in the s-triazine nucleus [56]. The resin
was found to lose weight completely only at higher temperatures Of 360-3750C. Here the
loss in weight indicates the total degradation of the resin above the glass transition
temperatures. This degradation process proceeds in several stages and is accompanied by
weight loss amounting gradually to 95% of the initial value. At very high temperatures,
almost or above glass transition temperature, degradation products of cyanate ester
consist mainly of volatile products like carbon dioxide, carbon monoxide, hydrogen,
phenols and bisphenols. In our experiment, thermo-oxidation takes place at a

temperature of 100°C, which is much lower than the glass transition temperature. At this

48

temperature, the molecules generated by the thermo-oxidation reaction are likely to stay
in the polymer, increasing the weight Of the samples. In this case the weight gain is due
to the consumption and retention of absorbed oxygen molecules. Similarly, we observed
the thermal aging in siloxane modified cyanate ester polymer in Figure 3.4. SMCE also
tends to lose weight at the initial stages of thermo-oxidation process, which indicates that
there is desorption of moisture during the early stages of the aging. Later, as the thermo-
oxidation process proceeds for 1200 hours we Observe that there is no change in the
weight of the sample with aging time. SMCE also remained unaffected by changes in
aging temperatures. The thermal properties Of SMCE depend on the nature of the
pendant groups on the silicon atom [53]. Substitutional groups like phenyl group and
methyl group present on the backbone of SMCE polymer chains raises the oxidative
stability of the polymers. Pendant groups on the SMCE backbone increase the
intermolecular forces because of its weak polarity in addition to steric hindrance. The
intermolecular forces between the chains facilitate rigid crosslinking between the chains,
thereby increasing the stability Of SMCE to thermal aging. Figures 3.5-3.7 compare the
thermal aging for CE and SMCE for each of the aging temperatures. These plots indicate
that the weight of the CE samples progressively increased with aging time for all the
aging temperatures. This showed that oxygen consumption increased with increase in
aging temperature. On the other hand, SMCE showed no gain in weight as compared to
CE pure resin. Furthermore, surface color Of both the samples changed with aging.
Further Optical examination of the specimen showed that the discoloration started on the
surface and moved gradually to the specimen’s interior indicating that thermal aging is a

surface phenomenon. As the aging time increase more and more chain decomposed

49

causing scission and thus increasing the diffusion Of oxygen. Only initial weight loss was
Observed in SMCE, which indicated that the loss in weight was due to the evaporation of
moisture absorbed on the specimen’s surface.

Results Of thermal aging of CE and SMCE materials are presented in Figures 3.8-3.10.
These figures represent the change in weight for CE and SMCE materials due to thermal
aging with square root of time. Figure 3.8 shows the change in weight due to thermo-
oxidation in case Of CE polymers. The weight change depends on diffusion rates of
oxygen into the polymer, which is dependent on the temperature and aging time. The
slope for CE at 100°C is higher as compared to the slopes for CE aged at 80°C and 60°C
respectively. This also indicates that the diffusion rate is higher in CB at higher
temperatures. Uzomah et al. [55] studied the effects Of time and temperature on the
tensile yield properties of polypropylene. They found that, higher aging temperature
needed a smaller time interval to cause significant reduction in the properties. At longer
time intervals and higher temperature, faster rates Of chain scission and volatilization Of
products occurred decreasing the crosslinking density leading to deterioration Of the
polymer. Figure 3.9 shows the changes in weight due to thermal aging of SMCE
polymer materials. NO change in the slopes Of the plots, indicating that the diffusion
rates of oxygen in SMCE polymers maintained between 60°C and 100°C remained
relatively flat throughout the thermal aging process. Figure 3.10 shows the comparison in
weight change for CE samples aged in air at 60°C, 80°C and 100°C respectively, and for
SMCE polymer samples aged in air at 600C for 1200 hours. CE showed change in

weight with aging time and temperature during thermal aging indicating that diffusion

50

rates Of oxygen into the CE polymer depended on aging temperature, while SMCE

showed no change in weight for all aging times and temperatures used in this study.

51

% Weight Change

 

 

 

 

 

 

 

 

I T I T I I I I I I T I I
. (‘12 aged in air :11 (mt ‘
0.4 I CE aged in air at 800C ) ’ T
-O- (‘13 aged in air at 100%? .
’ -1
0.2 ” 7
r- -1
r a
0 _ _
-02 ” 7
[— —4
r- -<
-0.4 ” 7
l L 1 l l 1 1 l l 1 l 1 l L 1 I L L L L l 1
0 10 20 30 40 50

Square root of Time (hi/2)

Figure 3.8 Comparison Of diffusion rates Of oxygen in CB polymer
when aged in air at 60°C, 80°C, and 100°C respectively for 1200
hours with square root Of time.

52

 

 

 

 

 

 

 

I I I L I I I I I I I I I I I I I I I I I I
. S\i(‘15 aged in air at 100 ”(‘ J
0.4 "' SMCE aged in air at 80 0C 7
'O- SMCE aged in air at ()0 “C
0.2 ’ 7
a)
DD e _
I:
a
a ,_ -4
U
in 0 O
g P W _
_ . —
O
O
*— .~-------w-LI —<
-0 2
b O .0 O. O _
-0.4 ” 7
L l L l l L 1 1 l L 1 l I l I I l L l l I I L
0 10 20 30 40 50

Square root of Time (hm)

Figure 3.9 Comparison Of diffusion rates of oxygen in SMCE polymer
when aged in air at 60°C, 80°C, and 100°C respectively for 1200
hours with square root of time.

53

% Weight Change

 

 

 

 

 

 

 

 

T I I fi Ti 7 I I
. ' CE aged in air at 600C 4
0 4 'I— CE aged in air at 80 OC . —
"" CE aged in air at 100 OC . V -
"au’TFFSE .Jf.“‘:.i ’i‘ 31" :1: 6; C I J
' ‘ V

0.2 f a
0 h —4
-0.2 “ ~
-0.4 7 —

I I l 1 i I 1 I 1 l I I 1 A l 1 1 1 l l l 1 I l

0 10 20 30 4O 50

Square root of Time (h 1’2)

Figure 3.10 Comparison of diffusion rates in CB when aged in air
at 60°C, 80°C and 100°C respectively for 1200 hours and SMCE aged in
air at 600C for 1200 hours with square root of time.

54

Basically, thermo-oxidation of polymers involves two steps
(i) The diffusion of oxygen into the polymer material and

(ii) The reaction of oxygen with the polymer network groups.

Thus, the rate of thermo-oxidation is usually controlled by one of these two steps. For
the aging process the weight change is found to be proportional to the square root of time,
to“5 [56]. Nam et al. [56] showed in their investigations that the thermo-oxidation is
controlled by the rate of diffusion of oxygen into the polymer network. Their study was
based on the weight change Observed in bismaleimide/carbon fiber composite during the
thermo-oxidation process, which was controlled by oxygen diffusion, and the reaction
rate was found to be proportional to tO'5 . Similarly, results of the current study indicate
that the thermo-oxidation was definitely controlled by the rate Of oxygen diffusion into

the resin matrix.

3.2.1.2 Glass Transition Temperature Using DSC

To evaluate the thermal aging of SMCE and CE samples in this investigation, we
aged the samples at 1000C over a period Of 900 hours and measured the glass transition
temperature over this period Of time. For this purpose differential scanning calorimeter
(DSC) was utilized. The results are shown in Table 3.3 and 3.4. Table 3.3 illustrate that
the glass transition temperature Of CE remained the same when aged in air at 1000C for
900 hours. Table 3.4 shows that the glass transition temperature of SMCE also remained

virtually unchanged when aged in ambient air at 100°C for 900 hours. SMCE shows no

55

degradation when aged in air. Paravatareddy er al. [2,5] also conducted numerous
experiments on the physical aging of cyanate ester polymers. Their results showed a
decrease in glass transition temperature of CE by 10-30% when aged in air for 3 months
at 150°C. There is a decrease in glass transition temperature for CE polymers on the
surfaces of the samples but as we proceed towards the interior of the samples from the
surface the Tg remained unchanged. This difference can be well explained by the changes
in the chemical structure of CE polymers on the surface of the sample. The CE
molecules on the surface undergo a chemical change, which is attributed to the chain
scission occurring in the amorphous regions of the polymer matrix due to diffusion and
consumption of oxygen across the surface. Another possible reaction could occur here.
These polymers would have long chains with free end groups, which have highly reactive
polar groups that are susceptible to oxidation. On chain scission the polymer molecules
Obtained higher mobility, which resulted in the decrease in the Tg Of the polymer on the
surface of the polymer sample.

Shanahan et al. [59] studied the influence of gaseous environments on the thermal
degradation of structural epoxy adhesive. Their experiments showed that the initial glass
transition temperature Of the adhesive decreased very rapidly in the initial stages Of aging
in air strongly suggesting that rapid molecular chain scission occurred in the polymer
adhesive. Gravimetric experimental results showed that CE gained weight. The gain in
weight was attributed to the absorption of oxygen by the amorphous region entrapped in
the rigid crosslinked chains in the polymer matrix, which means, that chain scission Of
the polymer molecules present on the surface Of the sample had to occur in order tO allow

oxygen to enter the chains and chemically react with the chains [54]. Chain scission of

56

the long polymer chains results in the decrease of cross-linking density in the initial
stages Of thermal aging, and subsequent increase in segmental mobility, which results in
the decrease Of glass transition temperature for the polymer molecules on the surface Of
the polymer. The chain scission of CE polymer chains produces more flexible chains that
are able to move, thus lowering Tg. As the thermal aging proceed for a longer duration of
time, more and more polymer backbone chains attain mobility, resulting in decrease in
glass transition temperature of the polymer resin on the surface. Erikkson et a1. [60]
studied the effects of thermo-oxidation aging on mechanical, chemical and thermal
properties Of recycled polyamide 66. They showed that the surface area of aged
polyamide samples underwent more severe degradation than the interior. To confirm
this, they studied the elongation at break for aged and unaged samples. They found a
decrease in elongation properties as compared with unaged samples. Oxidation in
thermosets was generally known to occur in the amorphous regions, but oxidations also

occurred in the crosslinked regions, on their surfaces and boundary regions.

57

Table 3.3 Glass transition temperature Of CE aged in ambient air at 100°C "

 

 

 

 

 

 

 

 

Materials (CE) Time for aging (hours) Tg (0C)

As Received 0 196.29°C

Heated in air at 100°C 100 194.76°C

Heated in air at 100°C 300 190.47°C

Heated in air at 100°C 900 196.21°C

Table 3.4 Glass transition temperature of SMCE aged in air at 1000C "

Material (SMCE) Time for aging (hours) i, (°C)
As Received 0 158.60°C
Heated in air at 100°C 100 161.52°C
Heated in air at 100°C 300 158.77°C

Heated in air at 100°C 900 160.84°C

 

 

 

 

.. The glass transition temperature (Tg) of the samples CE and SMCE were Obtained for

only one set of samples.

58

 

 

8

Glass Transition Temperature ( T , 0C)

 

 

 

 

 

 

 

1 1 I I 1 1 I 1 I 1 I I T r 4

220 . (ilass Transition temperature with aging time(‘l‘ .OC) —_

11 r

210 L L

200 L j

_ . . r

190 L o i

Z i

180 L L

r —t

170 L L

160 L L

150i... i..i...i i..*
-200 0 200 400 600 800

Aging Time (hours)

Figure 3.11 Changes in the Glass Transition Temperature Of CE polymer

with aging time

59

l 000

8

Glass Transition Temperature (T ,OC)

200

180

160

140

120

. Glass Transition Temperature (T8,°C) with aging time

 

 

 

 

-200 0 200 400 600 800

Aging Time (hours)

1 000

Figure 3.12 Changes in the glass transition temperature of SMCE with
Aging time.

60

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61

Figrorre 3.13 DSC scans showing the glass transition temperature for CE aged in air at

100

Rev Heat Flow (Wig)

 

 

   

        
    

 

 

 

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Figlolre 3.14 DSC scans showing the glass transition temperature for SMCE aged in air at

100

62

Figures 3.11 and 3.12 show the changes in glass transition temperature of CE and SMCE
polymer materials during the thermo-oxidation process. Figure 3.11 indicates that the
glass transition temperature of CE polymer is not affected by the thermo-oxidation
occurring on the surface of the polymer sample. Higher diffusion of oxygen with
increasing aging time, into the resin matrix causes chain scission on the surface of the
sample, thus increasing the mobility of the backbone chain of the matrix and further
resulting in decrease of glass transition temperatures in the surface regions of the polymer
[59]. Figure 3.12 shows no change in glass transition temperature for SMCE with aging
time. This indicates that SMCE does not undergo oxidation. The difference in thermo-
oxidation degradation in SMCE and CE is related to their chemical structure. The
chemical structure difference of SMCE and CE suggests that the thermal degradation of
CE is due to the oxidation occurring across the (-OCN-) group in the triazine structure,
which involves the chain scission of the molecules, while in case of SMCE the polymer
forms a rigid structure thus impeding the diffusion of oxygen into the matrix. The DSC
scans on Figures 3.13 and Figure 3.14 show the glass transition temperatures for CE and

SMCE determined by the DSC sofiware.

63

3.2.1.3 Glass Transition Temperature Measurement Using TMA
To study the thermal aging of SMCE and CE samples in this investigation, we aged the
samples at 100°C over a period of 900 hours and measured the glass transition
temperature over this period of time. For this purpose thermomechanical analysis was
utilized. The results of glass transition temperature studies are shown in Figure 3.15 and
Figure 3.16. Figure 3.15 shows that the glass temperature of CE remained virtually
unchanged when aged in air at 1000C for up to 900 hours. In order to study the effects of
thermal aging on glass transition temperature for CE aged in air at 100°C with thermal
aging time, the experiments were further modified. The CE sample of size 12 mm x 8
mm x 2 mm was heated up to atemperature of 200°C with a ramp rate of 4°C / min and
later cooled within the furnace to 100°C. The sample was once again heated to 250°C
with a ramp rate of 40C / min. The results for glass transition temperature obtained by the
first run indicate that the Tg for CE remained virtually unchanged with aging time.
Figure 3.15 shows the glass transition temperature for CE obtained by the first run in
TMA instrument. The results for glass transition temperature obtained by the second run
indicate that the Tg for CE decreased with aging time. Figure 3.16 shows the glass
transition temperature of CE polymer obtained by the second run of the experiment. CE
samples showed a decrease in Tg with aging time. The Tg decreased because the samples
may be undergoing further thermal degradation in the unprotected oxygen environment
present in the TMA instrument. This oxygen may have caused further degradation of the
already aged and degraded samples. In this case oxygen further reacts with the degraded

polymer chains. Figure 3.17 compares the Tg for aged CE polymer obtained from DSC

64

and TMA scans. TMA studies show that CE polymer undergoes further thermal aging in

the TMA environment during the experiment resulting in Tg.

65

Dimension Change (pm)

 

 

 

 

 

 

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00 Lo 00 00 ..00 30 .30 400 400 000
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0C for a period of 0 hours, 200 hours,500

Figure 3.15 TMA scans showing the glass transition temperature range and the
hours and 900 hours respectively and obtained by first run.

relaxation occurring in CB aged in air at 100

66

Dimension Change (pm)

0008

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Figure 3.16 TMA scans showing the glass transition temperature for CE obtained by
rerun of the same CE sample for the second time. The scans show a decrease in Tg for CE

in the second run of TMA.

67

 

 

 

 

 

 

 

I T I T T T T T T ‘1' I T l T
+1 .obtuincd from DSC ,
-I— T obtained from TMA d
240 3
220 h 1
A ,— -4
U
C
v _ —d
E
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_ k ~ \ - __ __,, _ II ,
I60 _ 1
1 l l l i 1 L i #41 l i l i 1 I l 1 l l l 1 l
-200 O 200 400 600 800 l 000
Aging Time (hours)

Figure 3.17 Comparison of glass transition temperatures of CE aged in air at 100°C for a
period of 0 hours, 200 hours, 500 hours and 900 hours respectively, and obtained from
DSC and TMA

68

3.2.1.4 Thermo-oxidation Mechanism in CB Using F TIR
Infrared spectroscopy, Bio-Rad, Excalibur Series, was used to determine the mechanism
of thermo-oxidation and changes in the chemical structure of the specimens. The samples
were aged in air at 100°C for up to 500 hours. From the experimental results, shown in
Figure 3.18 and Figure 3.19, it is evident that the intensity of the peak at wavelength
1720-1730 cm'1 increases with thermal aging time. This peak represents the presence of
carbonyl group (-OC=O-) [51]. The growth of the 1720-1730 cm'1 band is interpreted as
resulting from the formation of carbonyl groups such as aldehydes, carboxylic acid and
ketones, different degrees of hydrogen bonding, conformations of the carbonyl groups
and their temperature dependence. In case of CE material, thermal aging in air results in
the cleavage of the ester linkage, which is accompanied by the subsequent decomposition
of the triazine ring via both heterolytic and hemolytic decompositions reactions [1].
These reactions are followed by trapping of cyanate molecules, which combine or react
with diffusing oxygen to form carbonyl groups in the polymer. During the thermal aging
process, number of change in the characteristic bonds occur, resulting in smaller products
like carbon dioxide, and volatile products. One such change is observed in (-C=C-H)
bond, which is indicated by the decrease in the intensity of the peaks at 911 cm'1 and 966
cm'1 with increase in aging time. The peaks of wavelength 911 cm'1 and 966 cm'l, is the
characteristic absorption of the (-C=C-H-) bonds of the isocyanate group. Other changes
in the CE polymer include the changes in the peaks between 2220-2250 cm'1 which
represent the stretching of (-RNCO-) groups in the matrix and between 810-820 cm'l

which represents changes in the aromatic ring structure within the polymer. From Figure

69

3.20, it is evident that SMCE does not change weight or undergoes oxidation, as there is
no change found in the F TIR spectra for the sample aged in air.

Scpulveda et al. [62] studied the structural changes such as the oxidation level of
LDPE due to thermo-oxidation and biological treatment. They used FTIR to estimate the
presence of similar groups like carbonyl. They also determined that increasing the
thermal duration treatment led to a significant increase in carbonyl group concentration
due to higher oxidation of the molecule. Singh et al. [64], also studied the photo
degradation and thermo-oxidation of SBS rubber using FTIR spectroscopic methods.
They found that the initial oxidation process took place by the decrease in the intensity of

absorption band of OH groups and consequent increase in the carbonyl concentration.

70

 

 

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Figure 3.18 FTIR spectra (766-1036 cm") of CE aged in ambient air at 100°C. Change in

and 966 cm'1 peaks.

-1

the spectra with aging time shows the decrease in the 911 cm

71

 

 

  

 

 

 

 

 

 

 

 

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Figure 3.19 FTIR spectra (1661-2340 cm") of CE aged in ambient air. Change in the
spectra with aging time shows the increase of the (1650-1950) cm'lpeak.

72

 

 

 

 

 

 

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Figure 3.20 FTIR spectra (1651-3977 cm“) of SMCE aged in ambient air at 100°C.
There is no change in the spectra with aging time.

73

3.2.1.5 Thermo-oxidative Mechanism in CE using X-ray Photoelectron Spectroscopy.

XPS techniques help to determine the composition of the materials surface. CE and
SMCE polymer materials that were as-received and the samples that were aged for up to
900 hours in ambient air at 100°C, were studied using XPS. XPS results revealed the
changes in the % atomic concentration of each element in the samples under study. It
was observed that % atomic concentration of oxygen increased in aged samples than in as
received samples for CE, and remained the same in SMCE. CE showed an increase in %
atomic concentration of oxygen, indicating that chain scission occurred on the surface of
the polymer sample. Chain scission was accompanied by diffusion of oxygen during the
thermal aging process resulting in the absorption and consumption of oxygen by the
polymer molecules in the resin matrix. SMCE showed no change in the atomic
concentration of oxygen on thermal aging, confirming that SMCE was remained
unaffected by thermal aging process. Figure 3.22 shows the XPS results for CE and
SMCE polymer aged in air at 100°C for a period of 0 hours, 500 hours and 900 hours
respectively. The figure clearly shows that the atomic concentration for CE increased
with aging time. Oxygen has diffused into the polymer matrix and hence we observe the
increase in atomic concentration of oxygen across the surface of the polymer sample.
SMCE showed no increase in the atomic concentration of oxygen, which indicates that
SMCE remained unaffected by thermal aging. XPS spectra for CE in Figures 3.23-3.25
show the consumption and diffusion of oxygen into the CE polymer materials. The
atomic concentration of oxygen on the surface of the CE polymer material increased. On
comparing the results for CE samples aged in air at 1000C for 500 hours and 900 hours,

the atomic concentration of oxygen for CE aged for 500 hours was found to be more than

74

the atomic concentration of oxygen for CE aged for 900 hours. The decrease in oxygen
content for CE aged for 900 hours may be due to the loss of some material from the oxide
layer during handling. On the other hand, the XPS spectra for SMCE in Figures 3.26-
3.28 show that the surface atomic concentration of oxygen in SMCE remained unchanged
throughout the thermal aging process. We observed a peculiar outlier in case of CE aged
for 500 hours. We observed an increase in % atomic concentration for oxygen and

silicon as compared to the % atomic concentration of oxygen and silicon for CE aged for

900 hours

75

Table 3.5 and Table 3.6, show the results in atomic concentration of oxygen and silicon
for as received and aged CE and SMCE, obtained from XPS.
Table 3.5: XPS results showing the atomic concentration for oxygen and silicon

elements in CE material.

 

Cyanate Ester (CE) Atomic Concentration ( %)

 

Oxygen (0) Silicon (Si)

1) As received 11.3 0.5
2) Aged for 500 hours 30.6 5.0
3) Aged for 900 hours 21.8 0.8

 

 

 

 

 

Table 3.6: XPS results showing the atomic concentration for oxygen and silicon

elements in SMCE material.

 

Siloxane Modified Cyanate Ester Atomic Concentration ( %)

 

Oxygen (0) Silicon (Si)

1) As received 13.5 8.2
2) Aged for 500 hours 14.5 9.6
3) Aged for 900 hours 14.6 9.3

 

 

 

 

 

76

 

 

35 I I T I r
+ Atomic Concentration ot'oxygcn for (‘13 (%)

-I— Atomic Concentration of oxygen for SMCE (%)

1

1

1

 

T

30?

25

T T

20

T

T

15

 

Atomic Concentration (%)

I I T fiT firfT

10

 

J g; 1 i 1 1 1 1 i 1 1 1 1 l 1 1 1 1 i 1 1 1 L i 1 1 J 1 i 1

 

 

-200 0 200 400 600 800 1000

Aging Time (hours)

Figure 3.21 XPS results showing the atomic concetration for CE and SMCE
aged in air at 1000C for a period of 0 hours, 500 hours and 900 hours respectively.

77

 

 

 

 

 

 

B‘
' -§
- 9' Jg
* -§§
- -'§§
‘ -§
‘ -§
‘ -§
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'9
“a .2. :13 ’ v1- é a?
up

Figure 3.22 XPS spectrum showing the atomic concentration and binding energies for
CE sample aged in air for 0 hours.

78

 

 

 

 

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19’
“" -§
.. 0'
-§
' -§

 

 

 

9
WWW)

 

 

 

- Jfi
” -§
' -§
’ -§
‘ -§
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x I 1 I
In

Figure 3.23 XPS spectrum showing the atomic concentration and binding energies for
CE aged in air at 100°C for 500 hours.

79

 

 

 

 

 

 

 

l l I I 1 °
- a a§
9
- -§
_ 9‘ -§
r » -§
- as
.. -gg
- -§
- -§
- -§
- “.5.
'9
x r 1 l i 8
'° 3 ~ .3 r s 0"
3p

Figure 3.24 XPS spectrum showing the atomic concentration and binding energies for
CE aged in air at 100°C for 900 hours.

80

 

 

 

 

 

 

19‘
- -§
_ 9": -3
_ ... ..g
° i
_ _,§
- -§
- -§
- .Jg
- 9,5.
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on

Figure 3.25 XPS spectrum showing the atomic concentration and binding energies for
SMCE aged in air for 0 hours.

81

 

 

 

 

 

 

' -§
” -§§
° 5
_ _§§
“ -§
‘ -§
‘ ~§
" -.§.
13
83 N é el- é 0:8:
IR

Figure 3.26 XPS spectrum showing the atomic concentration and binding energies for
SMCE aged in air at 100°C for 500 hours.

82

 

 

 

 

 

 

_ 5‘ _§
3.
' ‘5
.. 0- “fig
_. N' -5
' -§§
° E
_ _§§
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r ‘ -§
‘2
x _l I 1 8
on

Figure 3.27 XPS spectrum showing the atomic concentration and binding energies for
SMCE aged in air at 100°C for 900 hours.

83

3.2.1.6 Three-point Bend Test

To further study the effects of thermo-oxidation of cyanate ester and siloxane modified
cyanate ester polymeric materials, three-point bend test were used to characterize the
flexural strength of these polymeric resin materials before and after aging. The results
from three-point bend tests are shown in Figure 3.30. Flexural strengths were determined
for SMCE and CE samples aged in ambient air at 100°C for a period of 0 hours, 200
hours, 500 hours and 900 hours respectively. Three point bend test results show a
decrease in flexural strength of CE by about 20%. Figure 3.29 further explains the

decrease in flexural strength observed in CE.

84

2““:

   
  
    

Polymer
matrix

P (load) Oxide layer

microcracks

  

 

 

2mm

Figure 3.28 Illustration of the oxide layer formed due to the thermo-oxidation for a 3-
point bend specimen. During 3-point bend testing, microcracks readily nucleate and
grow in the oxide layer at low applied loads. Such microcracking at low loads leads
to lower flexural strength in this material.

85

Figure 3.28 shows the surface of polymer sample after exposure to thermal aging. An
oxide layer is formed on the surface of the polymer sample on thermal aging. The oxide
increases in thickness with increase in aging time and temperature. On application of
fairly low loads this oxide layer on the surface of the polymer cracks, as it is unable to
withstand the load due to its brittle structure [63]. In case of CE polymer, the flexural
strength trend shows a decrease in strength with aging time and temperature. Obviously,
as the aging temperature and time increased, the thickness of the oxide layer on the
surface of the sample increased with consequent decrease in the thickness of the interior
bulk polymer that has not undergone thermal aging. The flexural strength test performed
on this bulk polymer decreases with decrease in thickness of the bulk polymer.

The decrease in flexural strength is also attributed to the chain scission of the CE chains
on the surface of the polymer sample on thermal aging. The chain scission takes place
along the -OCN- bonds of the polycyanurate, which breaks into smaller molecules, and
recombines or crosslink later to form a brittle oxide layer. Larger decreases in fiexural
strength with increase in aging time, indicates that diffusion of oxygen and chain scission
increased with increase in aging time [53]. In comparison, the flexural strength of SMCE
remained the same afier it was aged in ambient air at 100°C for 900 hours. SMCE

polymer does not form any oxide layer on thermal aging.

86

Flexural Strength (MPa)

 

120'fj'l""i
' I CEagedinairathdJC

 

SMCE aged in air at 10(PC

 

 
   
 

 

Average of
six samples

 

 

 

 

 

 

 

 

200

Time (hours)

Figure 3.29 Three-point bend results showing the changes in flexural
strength for CE and SMCE aged in ambient air at 1000C for a period
of 0 hours, 200hours, 500hours and 900 hour respectively.

87

3.2.1.7 Microhardness Testing
Microindentation is a common and simple technique used to measure the
micromechanical behavior of polymers and its correlation with microstructure.
Microhardness test was used to characterize the effect of oxidation on the hardness of CE
and SMCE materials before and after the thermal aging. The results were plotted on Fig
3.31. Result from the microhardness test of CE and SMCE shows a gradual increase in
the hardness of CE on aging in ambient air at 100°C for 200, 500 and 900 hours
respectively. The increased microhardness in CE polymer can be explained based on the
structure of the oxide layer formed on the surface of CE polymer. Figure 3.31 shows the

microhardness for CE polymer.

88

2m:

Polymer
matrix

indenter

 

 

Figure 3.30 Illustration showing the oxide layer which forms on the surface of CE
specimens during thermal aging in air. Microhardness was determined on such
specimens.

89

Figure 3.30 shows the formation of oxide layer on the surface of thermal aging. The
depth of penetration of the indenter depends on the hardness and thickness of the oxide
layer on the surface of the polymer. The thickness of the oxide layer is found to increase
with aging time and temperature. As the thickness of the oxide layer increase, it becomes
more difficult for the indenter to penetrate the oxide layer and hence hardness increases.
Increased microhardness can be further explained on the basis of micro-structural
changes occurring on thermal aging on the surface of CE polymer. The surface of CE
polymer may be modified to form a carbon-rich surface consisting of distorted sp, sp2,
and sp3 carbon atoms that may interconnect to form a three-dimensional network and
thereby increase the hardness of CE polymer [65]. Thermal aging of the surface of CE
polymer results is absorption and consumption of oxygen into the polymer chains. The
oxide surface consists of two layers comprising outermost carbon layer, where carbon
atoms are combined with sp2 and sp3 bonds, and the other layer comprising of chemical
links with oxygen atoms, such as O-H and C=O as well as ester groups. The three-
dimensional crosslinking provided by these two layers account for the increased hardness
observed on the surface of CE polymer.

SMCE polymer does not undergo any change during thermal aging, no oxide layer is
formed on the surface and therefore, the microhardness remained unchanged for all aging

time and temperatures.

90

Microhardness (HV)

 

3 5 I I I 1 I I I I l I I I I l I I I l

 

 

 

     

 

 

i
I (I aged in .Iil at 10000 a
L] \'\1(‘| aged in .tirut 1001' :
30 _
Average of
six samples ‘
25

1J_1r

l 14111111

 

 

 

 

 

500 900

Time (hours)

Figure 3.31 Microhardness test results showing the changes in
microhardness for CE and SMCE aged in ambient air at 100°C for a
period of 0, 200, 500 and 900 hours respectively.

91

3.2 Summary and Conclusions

The thermo-oxidation effect on cyanate based polymer materials can be summarized as

follows:

(i)

(ii)

(iii)

0V)

(V)

(vi)

Gravimetric experiments show that cyanate ester gains weight when aged in
ambient air at 100°C due to the absorption and consumption of oxygen by the
polymer molecules on the surface of the sample. Siloxane modified cyanate ester
shows no significant weight gain when aged in ambient air at 100°C.

Differential scanning calorimetn'c results shows that glass transition temperature
of aged CE remained unchanged even after being aged in ambient air for 900
hours, indicating that glass transition temperature of the resin is unaffected by the
thermal aging process occurring on the surface of the polymer.

TMA results showed that Tg of CE polymer remained unaffected during the first
run of the experiment but in the rerun experiment showed a decrease in the Tg,
indicating that aged CE polymers was undergoing further degradation within the
TMA environment.

FTIR shows that thermo-oxidation process in CB results in formation if carbonyl
groups and subsequent rearrangement of other molecules within the matrix.
There is an increase in peak height for carbonyl group with aging.

XPS results show that surface concentration of oxygen in cyanate ester increases
with aging time indicating the consumption and absorption of oxygen on the
surface of the polymer sample.

Three-point bend test results show a decrease in flexural strength in the CE

material indicating that chain scission and consumption of oxygen during the

92

(vii)

thermo-oxidation process deteriorating the mechanical properties of the CE
polymers. The decrease in flexural strength could be related to the low-stress
induced microcracks in the oxide layer formed on the surface of the CE polymer.
There is no change found in the flexural strength of SMCE polymers.
Microhardness results show an increase in microhardness of CE with decrease in
depth of penetration of the indent, which indicating that difiusion of oxygen on
the surface of the polymer results in a surface layer containing carbon-rich bonds
and high contents of O-H and C=O groups, form a three-dimensional structure on
crosslinking. This structure leads to the increase in microhardness observed on

thermal aging.

93

CHAPTER 4
HYGROTHERMAL EFFECTS ON CYANATE BASED POLYMER MATERIALS

4.1 Experimental Procedure

4.1.1 Moisture Absorption Methods
All hygrothermal exposed samples were cut into the dimensions of 25 mm x 25 mm x 2
mm from the polymer panel with a diamond saw. The surface of the samples was ground
to a finish of 0.05 pm using the polishing wheel containing silicon carbide and alumina
as the polishing medium. The edges were subsequently ground using 600 grit abrasive
paper to maintain consistently smooth edge surfaces. The specimens were further
cleaned in methanol in ultrasonic cleaner and labeled with a vibrating pen. A set of
samples containing 24 specimens was aged in ambient air at 100°C for a period of 1000
hours before exposing them to moisture. Another set of samples containing 24 specimens
were baked out in argon for 5 days, to remove absorbed moisture on the surfaces and to
eliminate residual stress caused by sample fabrication. These two sets of samples were
removed from their ovens and their initial weights were obtained by using an analytical
balance with 0.01 mg resolution. Specimens were then placed in chambers containing
distilled water maintained at constant temperatures of 45°C, 60°C, 75°C and 90°C
respectively. The temperature variation is iZOC. Specimens were removed from the
water chambers periodically and weighed. Once the specimens were taken out of their
environmental chambers, the surface water was absorbed using clean dry filter paper.
Then, the specimens were left for 2-3 minutes at the ambient temperature and humidity

conditions before weighing. These specimens were again weighed using an analytical

94

balance with 0.01 mg resolution. The experimental error of the test is i 0.02%. The
percentage moisture absorbed in the resin material is calculated using equation 3.1
Gravimetric experiments, three-point bend test and microhardness were used to

investigate surface modifications and weight gain.

4.1.2 Gravimetric Experiments

Specimens were tested in environmental test chambers containing distilled water
maintained at 45°C, 60°C, 75°C and 90°C respectively. Gravimetric experiments as
mentioned earlier in section 3.1.3 were used to monitor weight of specimens during
moisture absorption. Specimens were exposed to distilled water for a period of 2000
hours. Specimens were taken out of the water chambers periodically during the moisture
absorption experiment. Specimens were weighed using the analytical electronic balance
with 0.01 mg resolution. The experimental error of the measurement is i 0.02 %.

The percentage weight change is calculated by using equation 3.1 given in section 3.1.3.

95

4.1.3 Glass Transition Temperature (Tg) Experiments

Differential scanning calorimeter was used to study the Tg of moisture absorbed
samples. Samples of size of 25.4 mm x 25.4 mm x 2 mm were exposed to environmental
chambers containing distilled water maintained at temperatures of 90°C, 75°C, 60°C and
45°C respectively. The Tg of the as-received samples was determined using a TA
Instrurnentsm 930 differential scanning calorimeter (DSC) system. The sample size used
in DSC instrument varied from 3-15 mg. Specimens of this dimension were cut from
material exposed to moisture at different temperatures for the same 2000 hours. The
' samples were sealed in small aluminum pans to prevent any weight loss during the
experiment, and the weight of each sample pan was measured before and after each test.
The samples were put into testing chamber maintained with pure nitrogen atmosphere.
The temperature range for heating was maintained between 25°C to 2500C, and the
temperature increase rate was 10°C/min. The Tg values were obtained by the intersection

of two tangential lines of the DSC curves, automatically provided by the instrument’s

sofiware.

4.1.4 Microhardness Testing

The specimens that were used for this test had a dimension of 25 mm x 25 mm x 2 mm.
A precision diamond indentor is impressed into the material at loads from 1 to 1000 gm.
The indenter was forced into the specimen until a predetermined load of 10 gm was
achieved. The impression length, measured microscopically and the test load are used to
calculate a hardness value. The indentations are made using a square-based pyramid
indenter, Vickers hardness scale. The hardness impressions can be precisely located with

the microscope to perform tests on microscopic features. A representative sample of six

96

specimens was tested for each moisture absorption conditions, aged in air for 1000 hours
and later then exposed to moisture and baked out in argon for a period of 5 days and later
exposed to moisture at different temperatures. For every experiment, the hardness of the
specimens was tested and the average data was plotted. The Vickers hardness number

was then computed using the equation 3.3

4.2 Results and Discussion

4.2.1 Assessment of Moisture Absorption Effects of CE and SMCE resins

4.2.1 .1 Gravimetric Analysis

Figures 4.1-4.6 show the weight change profiles for cyanate ester and siloxane modified
cyanate ester polymer materials. Two sets of samples were prepared for this experiment.
One set of samples were aged in ambient air at 100°C for 1000 hours and later exposed
water, and another set of samples were baked out in argon at 100°C for a period of 5 days
and later exposed to water absorption for 2000 hours. These materials were exposed to
environmental chambers containing distilled water, and maintained at different
temperatures of 90°C, 75°C, 60°C and 45°C respectively. The symbols in the plots
represent the experimental data points of water absorption under different conditions.
Each data point plotted was based on the average values from three samples. The
moisture absorption experiments of CE and SMCE polymer materials show that the
water-absorption curves follow Fick’s law in the early stage, and at later stages deviate

from the Fickian behavior, and become non-Fickian .

97

Some researchers [1,55] have observed the non-Fickian moisture absorption of
cyanate ester materials. They suggested that non-Fickian water-absorption is caused by
chemical interaction or reaction of water molecules with the cyanate ester polymer
network. The chemical interaction caused the sample to keep gaining weight even after
the pseudo-saturation moisture level in the resin matrix network. The chemically
absorbed water in the polymer network can be attributed to the non-Fickian behavior
observed in cyanate ester.

Hydrolysis was also observed in cyanate ester due to the presence of the chemical
interactions. Leo et al. [19] showed that cyanate ester laminates stored in humid
condition at high temperature exhibited hydrolysis and blistering phenomena. They
found that cyanate ester blisters much faster than epoxies, although cyanate ester
networks absorb less water than do epoxies. Figure 4.3 shows the curves for moisture
absorption in cyanate ester polymer that was exposed to moisture in environmental
chambers containing distilled water. The profiles compare the water absorption for CE at
different temperatures. As seen from Figure 4.3, CE absorbs moisture to a greater extent
at higher temperatures. These profiles also compare the diffusion of moisture for two
sets of CE samples, one aged in ambient air at 100°C and exposed to moisture, and the
other baked out in argon for 5 days before immersing the samples in 100% humidity.
Water uptake in CE samples aged in air at 100°C and subsequently exposed to water is
much higher as compared to the CE samples baked out in argon and then exposed to
water. The difference within the same material is seen because CE aged in ambient air at
1000C undergoes chain scission during thermal aging forming products of low molecular

weight, and high mobility within the resin. During the moisture absorption process,

98

water molecules diffuse at a faster rate in these aged samples and behave as a plasticizer
within the polymer matrix thus increasing the segmental mobility. The diffusion of
moisture in CE samples aged in air at 1000C and maintained in chambers containing
water at 90°C was also found to be higher than the diffusion of moisture into samples
aged in air at 100°C but maintained in water at lower temperatures. The rate of diffusion
of moisture molecules into the polymer matrix depends on temperature. At higher
temperatures of water, the diffusion of water molecules is faster, as the water molecules
attain the required activation energy for diffusion, much earlier as compared to the
molecules maintained at lower temperatures. On the other hand, Figure 4.4 shows the
moisture absorption curves for SMCE samples maintained in chambers containing
distilled water at temperatures of 90°C, 75°C, 60°C and 45°C respectively. In case of
SMCE samples, one can conclude that SMCE absorbs less moisture as compared to CE
polymer materials. SMCE samples aged in air at 100°C and later exposed to water
showed an increase in moisture absorption as compared to SMCE samples baked out in
argon for 5 days and later exposed to water. SMCE polymer is a rigid structure, which

impedes the diffusion of moisture molecules into the matrix.

99

% Weight change

2.5

1.5

0.5

 

 

 

 

 

 

 

I—l' r I I I fir 1 I T
.1
. At 90°C aged In air 7 : - ‘t‘. l'dht‘fid m argon ]
' At 75°C aged in air 0 1
.4
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‘ At 45°C aged in air 5’ At 45°C baked in argon :
o
O '1
_ v . . ' x —+
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Square Root of Time (h )

Ur
0

Figure 4.1 Water absorption curves of cyanate ester at different

environmental temperatures with square root of time.

The plots compare the moisture absorption for CE aged in air and

at 100°C for 1000 hours and baked out in argon for 5 days.

100

 

 

 

 

 

 

 

2.5 T T I 1 Y
. At 90°C aged in air ' At 90°C baked in argon
I At 75°C aged in air 0 At 75°C baked in argon
. At 60°C aged in air E At 60°C baked In argon
2 ‘ At 45°C aged in air '5 At 45°C baked in argon _,
o
E’ 1.5 — a
.r: P .
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E
.20 '
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gash E E a N
L- -+
0 EV fig 1 1 1 J 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 L L

0 10 20 30 40 50

Square Root of time (h I’2)

Figure 4.2 Water absorption curves of SMCE at different environmental
temperatures with square root of time. The plots compare the moisture

absorption for SMCE aged in air and at 100°C for 1000 hours and baked
out in argon for 5 days.

101

% Weight Gain (gms)

i"
m

N

—n
(It

_

.o
u.

C

 

 

 

 

 

 

I I T T I I l i T T I T

C SVlt‘l‘ aged in air 1

I SMC E baked in argon 1

O .' ;‘ i 1

A CE baked in argon “
r. _
r a
a o O “
. O o O O 00 ‘A —
r 0 © 0 O 00 A ‘ “ A
: 00A A A A “ A j
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0 10 20 3O 40

Square root of time (hm)

tit
C

Figure 4.3 Water Absorption curves comparing the weight change
of CE and SMCE exposed to environmental chambers containing distilled
water maintained at 45°C with square root of time.

102

% Weight change

 

 

 

 

 

 

O

3 T 1
C \M('l~ aged in air 0 CE aged in air
I SMCE baked in argon A CE baked in argon
2.5 h “
L.
2 F a
L.
[-
1.5 ” ‘
L . O
. . . o c .
l ” Q ‘ A ‘ 4
. . .’ “ A .
. O A .
. .0 M A ,
0'5 f e" ‘A I I I "
.(AI I..|I|II'.'.
i,— 1 1 J 1 1 1 l 1 41 1 1 1 1 1 1 1 1 1 1 1 1 1 4
0

10 20 30 40 50

Square root of time (hm)

Figure 4.4 Water Absorption curves comparing the weight change for CE
and SMCE exposed to environmental chambers containing distilled water
maintained at 60°C with square root of time.

103

% Weight change

 

 

 

 

 

 

3 I I g]; I I I I j IiT I I I I I I I I I T I I I
C \\l( ‘1 :wcrl in :nr 0 CT: aged in air :
I SMCE baked in argon A 1 l baked in dram! -
2.5 T F
. 1
2 C a
1 5 ”— . . . A A -:
c 9 . A A ,
A 1
r— . A
I L— O . A _
a O
. .0 ° A A A .
. . O. A? .
0.5 ~ $ I a
_ g I | I -
. 0’ I 0 II 5 I I I .
. 353 fl I l ,
0 ’- 1 4 1 1 L 1 l J. J 1 1 1 1 1 l 1 1 1 1 1 1 1 ‘4
0 10 20 30 40 50
Square Root of Time (hm)

Figure 4.5 Water Absorption curves comparing the weight change for CE
and SMCE exposed to environmental chambers containing distilled water
maintained at 75°C with square root of time.

104

% Weight change

 

 

 

 

 

 

3 r 1 r r L T 1 I r 1 1 1 r r 1 I r r r I r r r i
O S\1(‘l" ilng in air 0 ( l-L aged in air ‘
I SMCE baked in argon A a .: .w in A
2.5 L ‘
L e A
O O
A _
2 I . A A A X 8 9 <
c A u
L a
1.5 [- 0A “
. a
L- . .i
a A
t 0 A . a
I r ° A . o u
9 A I .
_ e 0 I -
e. A I I I I I
>- A ' -<
,_ . aA I ' I a
0.5 — ’A A ' A
~ , II:- «
r- . —
P O a
i. .
0 LI 1 1 1 1 l 1_ L 1 l P 1 1 1 l 1 1 1 1 l 1 1_ 1 1
0 10 20 30 40 50

Square Root of time (hm)

Figure 4.6 Water Absorption curves comparing the weight change for CE
and SMCE exposed to environmental chambers containing distilled water
maintained at 90°C with square root of time.

105

The comparison of water absorption profiles for CE and SMCE was further studied in
Figure 4.3-4.6. Comparisons were made between samples aged in ambient air at 100°C
for 1000 hours and then exposed to water and samples baked out in argon for a duration
of 5 days and then exposed to water. It was found that the samples, which were aged in
ambient air for 1000 hours showed higher water absorption rates as compared to the
samples, baked out in argon. The temperature also played an important role here. At
higher temperatures the rate of diffusion of water molecules is higher. The samples aged
in air and exposed to water at 900C, absorbed moisture at a faster rate as compared to the
samples maintained at the other temperatures. Further comparing the water absorption in
CE and SMCE materials, it was observed that CE absorbed more moisture than SMCE at
all temperatures. A valid reason given here was that thermo-oxidation of CE involved
chain scission of macromolecules and surface degradation, which made the material more
susceptible to moisture absorption.

Figures 4.7- 4.10 show the comparison of weight change in CB and SMCE polymer
aged in air at 1000C for 1000 hours and exposed to water maintained at 45°C, 60°C, 75°C
and 90°C respectively and CE and SMCE samples baked out in argon for 5 days and later
exposed to water maintained at 45°C, 60°C, 75°C and 900C respectively. Each figure
represents the diffusion rates of moisture absorption process for the initial 49 hours. CE
and SMCE samples preaged in air at 100°C and later exposed to water showed higher
diffusion rates as compared to CE and SMCE samples presaged in argon at 100°C for
periods of 5 day. CE samples preaged in air at 100°C and exposed to water also shows
higher diffusion rates as compared to CE samples preaged in argon at 1000C for periods

of 5 days and exposed to water. CE samples undergoes thermal aging, which causes

106

chain scission. On exposure to moisture absorption, the water molecules easily diffuse

into the degraded polymer samples.

107

% Weight change

 

 

 

 

 

 

1 I I I I I I I I I I I I I I I f I I I
. SMCI‘ (lfJCtl in air 0 (1‘ 1:111 in air «
I SMCE baked in argon A CE baked in argon *
0.8 ” ‘
0.6 r A
F o
I ‘ u
_ . I
0.4 7 I, ‘1
*- O -1
I O I
2’ j
0 2 F 0 o a ’
'1
O/ . 1
.. / A
0 I 4 1 l J I L L L 4 l I L 4L I l J
0 2 4 6 8 10

Square root of time (h I’2)

Figure 4.7 Comparison of diffusion rates in CE and SMCE polymer
when exposed to moisture absorption at 45°C for 2000 hours with square
root of time.

108

% Weight change

 

 

 

 

 

 

] L I X l I ‘l I l I f I r r J r I Y T I
. SM( ‘1‘ aged in air 0 ( l‘ aged in air 1
I SMC E baked in argon A CE baked in argo

0.8 ” T

r -<

. I

0.6 “ T

" . A .1

I . .

0.4 P 9 "

O
.. t . ..
_ o I _
0.2 ‘ .
.. ‘ <
.f
k ’ / r
F / / _
0 ’ L I l l l I L l L l l J l g l l l L

0 2 4 6 8 IO

Square root of time (hm)

Figure 4.8 Comparison of diffusion rates in CB and SMCE polymer
when exposed to moisture absorption at 60°C for 2000 hours with square
root of time.

109

% Weight change

 

 

 

 

 

 

l I 1 1
O SMCE aged in air O (‘I4 aged in air
I ‘0“ i mi ct: m .agwn A CE baked in argon
0.8 ” ‘
0.6 - o ‘ ~
0
A
P O
0.4 ” A "
i- ‘ -<
' I
0.2 _ ‘ O a
0 O I
O
I .4
O — 1 l L 1 1 l I I 4*
0 2 4 6 8 10
Square Root of Time (hm)

Figure 4.9 Comparison of diffusion rates in CB and SMCE polymer
when exposed to moisture absorption at 75°C for 2000 hours with square
root of time.

110

% Weight change

 

 

 

 

 

 

 

l i I I " ‘fi— 7 r r 1 ‘I—r‘—
I SMH aged m air 0 ('I aged in air
I SMCE baked in argon - ~ : i g . m imnn
0.8 ‘
l i
i O
0.6 '
O
O
O
i ’ “
0.4 ' - I 4
o H” ., ,,
,n; i
o L7 19”"
I 911"
o 171/" "1‘
0.2 --. O
, I' 4
i a» , .
O wl{ III 7 J 4 fi i_.. -1 ‘4. __i_.._+____ ____LL_ i_____l_-—.L__._-l—_.—j
0 2 4 6 8 10

Square Root of time (hm)

Figure 4.10 Comparison of diffusion rates in CB and SMCE polymer
when exposed to moisture absorption at 75°C for 2000 hours with square
root of time.

111

4.2.1.2 Glass Transition Temperature Using DSC

Glass transition temperature is an important parameter that determines the
polymer materials usage to commercial and industrial applications. The glass transition
temperature is lowered or decreases when the polymer absorbs moisture. This affects a
number of useful mechanical properties.

To evaluate the thermal aging in our SMCE and CE samples, we aged the samples at
100°C over a period of 1000 hours and immersed these samples in chambers containing
distilled water maintained at 45°C, 60°C, 75°C and 90°C respectively for a period of 2000
hours and finally measured the glass transition temperature. For this purpose differential
scanning calorimeter (DSC) was utilized. The results are shown in Table 4.1. Table 4.1,
shows that the glass transition temperature of SMCE remained unchanged for most of the
temperatures. The Tg for SMCE sample exposed to water at 90°C showed a decrease in
the glass transition temperature indicating that SMCE undergoes degradation when
exposed to water at higher temperatures. Figures 4.8- 4.16 show the DSC scans for glass
transition temperature for SMCE samples aged in ambient air at 100°C for a period of
1000 hours and baked out in argon at 1000C for a period of 5 days. Problems were
encountered in obtaining the glass transition temperature of moisture absorbed CE
samples. Glass transition temperature for moisture absorbed CE samples were not
comparable to the results obtained by researchers [1]. Further tests would be required in

future to determine the Tg for moisture absorbed samples by proper analytical techniques.

112

Table 4.1 Glass transition temperature for moisture absorbed SMCE polymer materials ..

 

 

Siloxane Modified Cyanate Ester Glass Transition Temperature
For SMCE polymer
(1) As received 157.590C

(2) SMCE aged in air at 1000C and exposed to water
at 45°C for 2000 hours 152.09°C
(3) SMCE aged in air at 1000C and exposed to water
at 60°C for 2000 hours ‘ 158.99°C
(4) SMCE aged in air at 100°C and exposed to water
at 75°C for 2000 hours 156.29°C
(5) SMCE aged in air at 100°C and exposed to water

at 90°C for 2000 hours 133.95°C

 

 

(6) SMCE baked in argon for 5 days at 1000C and
exposed to water at 45°C 156.42°C
(7) SMCE baked in argon for 5 days at 1000C and
exposed to water at 60°C 152.930C
(8) SMCE baked in argon for 5 days at 1000C and
exposed to water at 750C 156.510C
(9) SMCE baked in argon for 5 days at 1000C and

exposed to water at 90°C 135.630C

 

 

 

.. The glass transition temperatures are the Tg results taken for one sample at each
temperature.

113

Table 4.1 shows the glass transition temperature (Tg) for SMCE samples aged in air at
100°C for 1000 hours and later exposed to chambers containing distilled water at
different temperatures and the other set of samples containing SMCE again, only this
time, they were baked out in argon for a period of 5 days. SMCE showed a decrease in
the Tg for samples exposed to water at 90°C. Shanahan and his coworker [64] studied the
irreversible hygrothermal effects on epoxy resin. Their experiment involved the exposure
of epoxy polymer to water maintained at 50°C, 70°C and 90°C respectively. They found
that diffusion of water into the polymer led to the introduction of carbonyl groups in the
resin and chain scission of crosslinked structure. The water diffusion also led to the
decrease in Tg. The decrease in Tg is swelling dependent. The temperature increase also

facilitates water diffusion and swelling of the polymer.

114

Glass Transition Temperature (°C)

200

180

160

140

120

100

 

 

 

 

 

 

7 I I Y T r I l rT l
O l .. ul \‘Mt‘l, agcd in air at It)” ”(‘ for 1000 hours
I T ot'SMCE baked out in argon for 5 days
8
T for SMCE aged for 0 hours = 157.59)C ‘
g
,— O —
r I ®
I
O -1
O l
)— —1
r-
1 4 L l 1 l l 1 l 1 Pl 4 L 14 l l l L l l L L i l l l l l l 1 l i L4 4 l L
20 30 40 50 6O 70 80 90 l00

Temperature of Distilled water (°C)

Figure 4.11 Comparison of glass transition temperature for SMCE samples
Aged in air at 1000C for 1000 hours and baked out in argon for 5 days and
Exposed to water maintained at 45°C, 60°C, 75°C and 90°C respectively.

115

Heat Flow (Wig)

mmBEQ mzom
93 8.38 So

32:09 2930 01.9338 80.0

00330:" mzom mm $87.8

UmO

 

0.8

0.00 1

-o.om 1

-081

 

-0451

    

5E6
339%

 

Seawd

 

 

-owo .

06 Cu

mo

 

— q d a

—
So 3.0 woo wmo
422.2683 aoov c328. 5 2,; 532328

Figure 4.12 DSC scan showing the Tg for SMCE aged in air for 0 hours

116

mmBEQ 0:988 Zoaaoa 933$ mmfloq
mfiou 8.88 30

00338" E a mzom 393:8 8838 :63 8.063

000

 

0.8 1

0N0 1

 

0.401

Heat Flow (Wig)

0.81

0.00 1

0.00 1

   

Gmmwoo

 

 

mxo Cu

_ _ d . _
Go woo “mo
422.2983 0.9

.

 

an o
c2382 5.0)...) 53.5338

Figure 4.13 DSC scan showing the Tg for SMCE aged in air at 100°C for 1000 hours and
°C for 2000 hours

exposed to water at 45

”7

mmBQQ mzom

was“ ~88 so 000
7338” .3888

093.6% 0:988 26038 98:08 mama 30588 00868

 

   
    

 

 

 

 

 

0.50 1
one 1
320.0

M 088.03
:1. 50.8.0
w as -
H

0 50 1

q _ 1 _ a _ . q 1 _ .
0 00 000 80 ~00 8.0
me 8 4232683 0.9

000

c368». 50> 0.) 3.0.5338 .

ged in air at 1000C for 1000 hours and

for SMCE a

8

exposed to water at 60°C for 2000 hours.

Figure 4.14 DSC scan showing the T

118

 

 

 

 

 

 

d
m
00306” 029800 Z0038 96:08 mam:
meow 3.88 an 000 W
00330:" .8 oq mZOm 30586 8838 303 8.203 W
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a
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H e 0
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co 1m m
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use
m B
. :0 2.0 m t
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50.8.03 C a
303.0 m M.
5
J m
IOA q d d d u _ 4 q q 4 d
0 00 000 30 ~00 ~00 m m
me E. 4.0332633 Ace c=2oao_<wo>§so§3oau We m.
cl x
F e

119

00329 mzom 39.....qu 00.868

0.3 8.0000 30

000

02:30:" ...0 9. 32088 00838 020m 903 8888

 

0.0

0&1

0; 1

Heat Flow (Wig)

0.0 1

 

$3.0
8.... 8.09

 

$0.390

 

 

 

-0;

ma Cu

00

_ q _

80 80
40302683 0.9

 

woo . woo

c366». <00) 4) 53:32.8

Figure 4.16 DSC scan showing the Tg for SMCE aged in air at 100°C for 1000 hours and

0C for 2000 hours

exposed to water at 90

120

mmagm mzom

was” 8.88 so 000
7398” £933

Ooaaoan 9.988 26038 96:08 mmfioq 399:6 mcmoaoa

 

0.00 1

0.8 - 4_

Heat Flow (Wig)

0.05 1

0.0w 1

 

 

 

 

ma c. 42.60383 0.9

 

~00
c268». 50> ...) 53:338

C for 2000 hours

Figure 4.17 DSC scan shosiving the T8 for SMCE baked out in argon at 100°C for 5 days

and exposed to water at 45

121

003an wzom

mfiou 3.0000 30 UmO
26069 $0303

9330:.” 0:983 Z838 9088 mam. 30.088 8888

 

 

 

 

 

 

 

 

57.23. 50> .2, 593335

0.00
0.~0 1
W .
m 0.8 1
F
H l
0.~A 1
31111.11) 0 :2 .86
a $8.2:
$8.0
CNN 4 _ a _ . u a J
0 00 80 80 ~00 ~00
03 Co 4633883 09

Figure 4.18 DSC scan showing the Tg for SMCE baked out in argon at 100°C for 5 days
°C for 2000 hours

and exposed to water at 60

122

00329 0.608 78058 90:08 mama

mam 0.800 30

000

023.03 .6 o“ mz0m 390an 0009.000 :63 8.203

 

ohm

050 1

0.00 1

0.00 1

Heat Flow (Wig)

0.8 1

0&0 1

 

 

 

0.8

ms Cu

8 so 30 ~00 N 0
4.0330383 Adov c3330. 503.) 505.3030

 

C for 2000 hours

Figure 4.19 DSC scan shogving the T8 for SMCE baked out in argon at 100°C for 5 days

and exposed to water at 75

123

03.29 9983 3858 9388 mag.

9N9 0.0000 :6 UmO

003303. .8 on mzom 30888 00888 :63 8589

 

0.0m

0.00 1

0.00 1

0N0 1

Heat Flow (Wig)

0.0m:

    
  

0.3L _

$0 0590

$3.00

 

 

0.8 . _ . _ .

me E .3333qu 0.9

W 1 —‘ -
30 ~00 Mm 0

9.203». <00) ._.> 532328

 

20 DSC scan showing the Tg for SMCE baked out in argon at 100°C for 5 days
0 hours

and exposed to water at 900C for 200

Figure 4.

124

4.2.1.3 Microhardness Test
The results obtained by microhardness testing for moisture absorbed samples are shown
in Figure 4.16. From the figure we observed that CE shown an increase in microhardness
as compared to SMCE materials. On further comparing within the CE materials we
observed that CE aged in air at 1000C and later exposed to water at different temperatures
showed higher microhardness results than the CE materials baked out in argon at 1000C
for 5 days and later exposed to water at different temperatures. The difference in
microhardness is explained by the changes occurring to the chemical structure of the CE
polymer due to moisture absorption process. CE samples aged in air and later exposed to
water absorb moisture,which acts as a plasticizer and lubricates the molecules on the
surface of the polymer, reorienting them and allowing to crosslink. The crosslinked
surface results in the increased hardness observed in CB polymer. In case of SMCE
polymers, we observed a very significant change in microhardness with moisture
absorption. SMCE polymer contains pendant groups in the backbone of the polymer
chain. These pendant groups increase the intermolecular force of attraction between the
polymer chains in the matrix in addition to its steric hindrance. This structure of SMCE
impedes the diffusion of water into the polymer matrix and hence the micorhardness of

SMCE remained unchanged.

125

4o ~~ 00-0

 

 

 

 

 

 

 

 

 

 

 

 

I Water at 45? I
35 D Water :1! (WC I Water at 90"C
CE preaged in CE preaged in l
30 . air for 1000 argon for 5 .
hours days "
i SMCE ”"3398" SMCE preaged 7
9 In air for 1000 in argon for ~‘
5 25 hours 5 days i
m J
a l
15 l '
E 20
.r:
0
.§ .
E 15
10 1
5
0 4).- i

   

Figure 4.21 The results determining the microhardness for CE and SMCE exposed to
preaging conditions and submerged in water for 2000 hours at 45°C, 60°C, 75°C and
90°C respectively.

126

(i)

(ii)

(iii)

4.3 Summary and Conclusions

Gravimetric experiment results show that the water diffusion in cyanate ester
polymer materials was F ickian in the initial stages of moisture absorption process
and becomes non-Fickian in the later stages. CE aged in air at 100°C and later
exposed to water showed higher moisture absorption as compared to CE baked
out in argon and later exposed to water. Diffusion of moisture molecules thus
depends on temperature and concentration of diffusing species. SMCE showed a
very significant increase in weight during moisture absorption process.

Glass Transition temperature results showed that T8 of SMCE decreased by 20%,
at 90°C and remained the same for most of the water absorption temperatures.
Therefore, SMCE was unaffected by moisture absorption for most of the
temperatures.

Microhardness results showed that the hardness of CE based polymer materials
increased for samples aged in air at 100°C and exposed to water as compared to
samples baked out in argon for 5 days and exposed to water. The microhardness
results for SMCE showed significant changes in microhardness due to moisture

absorption process.

127

CHAPTER 5

CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

The major findings of this thesis are concluded as follows :

(1) Cyanate ester gained weight when aged in ambient air indicating the diffusion and
consumption of oxygen molecules, while siloxane modified cyanate ester showed no
gain in weight when aged in ambient air for a long period of time.

(2) Glass transition temperature of aged cyanate ester remained unchanged when aged in
ambient air at 100°C as thermal aging was occurring on the surface of the polymer
sample and the Tg for the entire sample was not dependent on the aging effects on the
surface, while Tg of aged SMCE remained unchanged when aged in ambient air at
100°C for the same duration of time.

(3) Changes in the chemical structure of CE polymer materials clearly indicate that CE
underwent thermo-oxidation to form carbonyl groups. The presence of this carbonyl
group indicated the consumption and diffusion of oxygen into the matrix.

(4) The % atomic concentration of oxygen in CB polymers increased with thermal aging.
The increase in % atomic concentration in oxygen indicates the consumption and
diffusion of oxygen by the polymer resin. SMCE remained unaffected by thermal
aging.

(5) Mechanical properties include changes in flexural strength of CE with thermal aging.
Flexural strength of CE decreased by 20% after being aged for 1000 hours, while the

flexural strength of SMCE remained unchanged.

128

(6) Microhardness of CE increased with thermal aging. SMCE showed very significant
change in microhardness on thermal aging.

All the above results indicate that the polymer network degrades due to thermo-
oxidation, a process that is governed by the oxygen diffusion in the polymer matrix.

(7) Water diffusion is CE follows the F ickian mode in the initial stages of diffusion
process but becomes non-Fickian in the later stages after a time period of square root
of 6 hours. CE samples aged in air absorbs water to a greater extent as compared to
CE samples baked out in argon. CE samples also absorb more moisture when
exposed to higher temperatures. SMCE absorbs moisture to a lesser extent as
compared to CE materials.

( 8) Tg remained unchanged for SMCE exposed to water at lower temperatures but
decreased at when exposed to water absorption at higher temperatures.

(9) Microhardness for CE based polymer materials increased for samples aged in air at
1000C and exposed to water as compared to samples baked out in argon for 5 days
and exposed to water. The microhardness results for SMCE showed very significant

change in microhardness during moisture absorption process.

129

5.2 Recommendations
Although the current study illustrates the effect of thermal degradation and moisture
absorption in CE and SMCE polymers, the following investigations are recommended to
develop a clear understanding of the operative mechanisms for the same.
(1) A study of thermal aging and hygrothermal effects on the molecular weight and
molecular weight distribution of polymers.
(2) A deep understanding of the synthesis and/or cure of these polymers is often critical
for their successful application in different environments. Studying the degradation of
such polymers with different synthesis and curing rate in different environment
(3) To utilize DSC to further monitor the changes in the polymer over aging time.
Changes in the degree of curing and melting peak temperatures for samples cut from the
superficial layers of thermally aged samples
(4) Chromotagraphy techniques could be utilized for further elemental analysis for

molecular structure identification

130

10.

BIBLIOGRAPHY

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