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This is to certify that the

thesis entitled

The Effect of Surface Sulfonation on the Surface Free
Energy, and Peel Adhesion Strength of Polymer Films

Date June 30;

0-7 639

presented by

Insik Park

has been accepted towards fulfillment

of the requirements for

_Ma.a_t_ex__degree in Packag ing

dim

Major professor

1994

MSU is an Affirmative Action/Equal Opportuniry Institution

 

LIBRARY ‘
MIchIgan State
Unlverslty

 

 

 

PLACE ll RETURN BOXtonmovothbchockmnflom ywmcord.
TO AVOID FINES Mom on or baton dd. duo.

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSU IoAn Nflnnltlvo Adlai/Equal Opportunity Inotttutton
mm:

THE EFFCT OF SURFACE SULFONATION ON THE SURFACE
FREE ENERGY, AND PEEL ADHESION STRENGTH OF
POLYMER FILMS

By

INSIK PARK

A THESIS

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

MASTER OF SCIENCE

School of Packaging

1994

ABSTRACT

The effect of Surface Sulfonation on the Surface Free Energy,

and Peel Adhesion Strength of Polymer Films

BY

Insik Park

Surface sulfonation of polypropylene (PP), polyethylene (PE), and polyethyl-
ene terephthalate (PET) film was carried out by reaction with gaseous 803 in
this study. The effect of surface sulfonation on the surface free energy of the
these polymer films was determined by contact angle analysis. Electron Spec-
troscopy for Chemical Analysis (ESCA) of the polymer surfaces showed that
the effectiveness of the sulfonation process was dependent upon the chemical
structure of the polymer, with the sulfonation of the sulfonation of PP being

very effective.

The adhesion strength of pressure sensitive adhesive (PSA) tape on PP, and PE
films was determined by using the peel adhesion test. For PP film, the results
showed that sulfonation increased the polar component of the surface free
energy, and the peel adhesion strength of the treated film. Surface sulfonation
of PP film is therefore a suitable method to modify its surface properties,

improving surface free energy and peel adhesion strength.

To my parents and family

iii

ACKNOWLEDGEMENTS

I wish to express my sincere thanks and appreciation to Dr. Jack Giacin for his
constant guidance, continuous encouragement, valuable suggestions, and

financial support throughout this study.

Gratitude is also expressed to the members of my thesis committee, Dr. Susan
Selke, Ruben]. Hernandez, and James P. Lucas for their comments and sugges-

tions.

I am very grateful to Susan C. Jones, Brian Eriction, and Kitti Wangwiwatsilp

for their impressive help and encouragement.

Above all, I especially wish to take opportunity to express gratitude to my par-
ents and family for encouragement, patience, and understanding throughout

the course of this study.

iv

TABLE OF CONTENTS

List of Tables

List of Figures

Chapter 1. Introduction .................................................................................... 1

Chapter 2. Background

2.1.
2.2.

2.3.

The nature of interfacial forces in adhesion phenomena .................... 5
Mechanisms of adhesion .......................................................................... 7
2.2.1. Mechanical interlocking ............................................................... 7
2.2.2. Diffusion theory ............................................................................ 8
2.2.3. Electronic theory ........................................................................... 8
2.2.4. Adsorption theory ......................................................................... 8
Work of Adhesion ..................................................................................... 9

Chapter 3. Surface Energy of Polymer Solids

3.1.
3.2.

3.3.

Relationships between contact angle, wetting and adhesion ........... 12
Methods of estimating surface free energy of solid ........................... 16
3.2.1. The critical surface energy ......................................................... 16

3.2.2. Surface energy components to determine
' solid surface energy .................................................................... 17

3.2.3. The methods using the equations
for the work of adhesion ............................................................ 21

Calculations of dispersion and polar force contributions
to surface free energy of polymer ......................................................... 21

3.3.1. Determinant method .................................................................. 22

3.3.1(a) The least square method .............................................. 23
3.3.1(b) Derivation of least squares method ........................... 23
3.3.1(c) Computer program ”SEE” ........................................... 28

Chapter 4. Surface Modification Technology

4.1.
4.2.
4.3.
4.4.
4.5.

Introduction ............................................................................................. 33
Chemical etching [2] techniques ........................................................... 34
Flame Treatment ...................................................................................... 36
Corona and Plasma treatments ............................................................. 36
Surface Sulfonation Technique .............................................................. 39

Chapter 5. Materials and Experimental Process

5.1.

5.2.

5.3.
5.4.

Sulfonation Apparatus ........................................................................... 42
5.1.1. The Unit of Sulfonation System ................................................ 42
5.1.1(a) Sulfonating chamber .................................................... 43
5.1.1(b) Sampling Port ............................................................... 43
Materials using for sulfonation ............................................................. 47
5.2.1. Polymer films ............................................................................... 47
5.2.l(a) Oriented Polypropylene (OPP) film .......................... 47
5.2.1(b) Polyethylene (PE) ......................................................... 47
5.2.1(c) Polyethylene Terephthalate (PET) .............................. 47
5.2.2. Fuming Sulfuric Acid ................................................................. 47
5.2.3. Cleaning agent ............................................................................. 48
5.2.4. Neutralization Agent .................................................................. 48
Sulfonation Procedure ............................................................................ 48
Electron Spectroscopy for Chemical Analysis (ESCA) ...................... 49

vi

Chapter 6. Physical tests

6.1.

6.2.

Contact Angle Measurements of Polymers and Methods ................ 50
6.1.1. Test Apparatus of Contact Angle Measurements ................... 51
6.1.2. Sample Preparation and Contact Angle

Measurements of Polymers ....................................................... 51
6.1.3. Calculation of surface energy of solid ...................................... 54
Peel adhesion test .................................................................................... 56
6.2.1. Test Apparatus and conditions ................................................. 57
6.2.2. Pressure sensitive adhesive (PSA) test tape and

backing tape ................................................................................ 58
6.2.3. Test specimen preparation and scheme ................................... 58
6.2.4. Peel Adhesions Test Procedure ................................................. 60
6.2.5. Analysis of peel adhesion test ................................................... 61

Chapter 7. Sulfonation of Polypropylene

7.1.
7.2.
7.3.

7.4.
7.5.

Introduction ............................................................................................. 62
ESCA and Elemental Observations and Discussions ........................ 63
Results and discussions of contact angle measurements .................. 69
7.3.1. Contact angles of probe liquids on OPP films tested ............. 69
7.3.2. Determination of surface free energies of films ...................... 72
Results and discussions of peel adhesion test .................................... 77
Conclusions .............................................................................................. 81

Chapter 8. Sulfonation of Polyethylene

8.1.
8.2.
8.3.

Introduction ............................................................................................. 82
Sulfonation degree determined by the ESCA .................................... 83
Results and discussions of contact angle measurements .................. 85
8.3.1. Contact angles of probe liquids on PE films tested ................ 85

vii

8.3.2. Determination of surface free energies of films ...................... 87
8.4. Results and discussions of the peel adhesion test .............................. 90

8.5. Conclusion ............................................................................................... 93

Chapter 9. Sulfonation of Polyethylene Terephthalate (PET) film

9.1. Introduction ............................................................................................. 94

9.2. Results and Discussions ......................................................................... 94
Chapter 10. Conclusions .................................................................................. 99
Chapter 11. Possible Future Studies ............................................................ 101
Bibliography

viii

Table 1
Table 2

Table 3

Table 4

Table 5
Table 6

Table 7

Table 8

Table 9

Table 10

List of Tables

Types of physical attractive forces and typical bond energies ........... 6

The characteristics of typical liquids used as probes for the contact

angle measurements ............................................................................... 20
A comparison of sulfuric acid and 503 gas as sulfonation agents.

.................................................................................................................... 41
The surface energy of liquids, and the corresponding polar and

dispersive component, used for measuring contact angles on the

respective polymer samples and pressure adhesive tape.
.................................................................................................................... 50
Result of Contact angles of probe liquids on the PSA tape .............. 58

Atomic concentration for untreated and sulfonated OPP films
determined by ESCA analysis ............................................................... 65

Relative Atomic Ratios of Sulfonated OPP films ................................ 66

The comparison of sulfur content, measured by ESCA and
Elemental Analysis, in the film samples treated at various

sulfonation time. ..................................................................................... 66

Contact angle obtained on tested polypropylene films using

various liquids. ........................................................................................ 69

Surface Free Energy of PP films, Polarity, Atomic% of Sulfur by
ESCA, and Total% of Sulfur per gram of film sample for OPP
(untreated), SPPI (Sulfonated for 1 minute), SPP1.5 (Sulfonated for
1.5 min.), SPP2 (Sulfonated for 2 min.), and SPP3 (Sulfonated 3

min.) .......................................................................................................... 73

ix

Table 11

Table 12

Table 13

Table 14

Table 15

Table 16

Table 17

Table 18

‘ Peel adhesion strength for sulfonated film samples with the data of

surface free energies ............................................................................... 77

Surface composition of HDPE film samples before and after
sulfonation treatments at a various exposure time over 803 gas,
determined by ESCA .............................................................................. 84

Contact angle obtained on sulfonated polypropylene using various
liquids. ...................................................................................................... 85

Surface Free Energy of PE films, Polarity, and Atomic% of Sulfur
by ESCA for PEU (untreated), PE1.5 (Sulfonated for 1.5 minute),
PE3 (Sulfonated for 3 min.), PBS (Sulfonated for5 min.), and PEC
(Corona treated) ...................................................................................... 87

Peel adhesion strength for sulfonated film samples with the result

of surface free energies in polar dispersion and polar components
.................................................................................................................... 90

Surface composition of PET film samples before and after
sulfonation treatments at a various exposure time over 503 gas,
determined by ESCA ..... 95

Contact angle of liquids on the untreated PET film and sulfonated

PET samples for 1 min. and 3 min. reaction time. .............................. 96

Surface Free Energy of PET films, Polarity, and Atomic% of Sulfur
by ESCA for PETU (untreated), PETI (Sulfonated for 1.5 minute),
PET3 (Sulfonated for 3 min.) ................................................................. 98

Figure 1

Figure 2

Figure 3

Figure 4(a)

Figure 4(b)

Figure 4(a)

Figure 4(d)

List of Figures

The surface tension balance at a point of three-phase contact at
equilibrium for ideal surfaces. An ideal surface is a smooth

surface without interfacial reactions with a liquid drop. ............... 13

A plot of cosine of angles vs. the surface energy of liquid, as
proposed by Zisman. ........................................................................... 17

)0'5 versus

Presentation of typical individual relationships of (7:
(fl )05 deduced from one of equations [3.17 I or [3.18]. Plot shows
SPPl, sulfonated polypropylene for 1 min. The shaded part is in

the boundary ......................................................................................... 24

Program to calculate the average and standard deviation errors

of measured contact of each liquid angles on a polymer sample.
................................................................................................................. 29

Figure shows the input data file for the program shown in Figure
4(a). Columns in this data file present each liquid used for the
contact angle measurement. Rows in this file are the duplicate of
the measured contact angle of a liquid measured on different

points of a film sample. ...................................................................... 30

This program provides a graphic like the one presented in Figure
3, which is plotted (7:)0'5 versus (fl )0'5, deduced from one
equations [3.17] or [3.18]. ................................................................... 31

This program calculates surface free energy, corresponding
dispersion component, and polar component, of solid by solving
one of equations [3.27] or [3.28]. ....................................................... 32

xi

Figure 5

Figure 6
Figure 7

Figure 8

Figure 9(a)

Figure 9(b)

Figure 10
Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 1 7

A schematic representation of the interactions which are possible

in a gas plasma impinging upon a substrate. .................................. 38
Sulfonation reaction mechanism of HDPE ....................................... 39

The figure illustrates the formation of conjugated system of

double bonds as a result of sulfonation ............................................ 40

The figure illustrates the formation of alkene species during

sulfonation ............................................................................................ 4O

Diagram for the gas-phase SO3 generator unit(a) [Continued to

Figure 9(b)] ........................................................................................... 44
Schematic illustrates the 503 flow pattern in the sulfonating
chamber [Continued from Figure 9(a)] ............................................. 45
View of Sulfonating Chamber, samples, sample holder ................. 46

Diagram of contact angle specimen and sessile droplet form.
................................................................................................................. 51
A schematic of Goniometer (Model 100-00 115, Rame-Hart, inc).

OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 52

Figure illustrates the procedure of the contact angle measurement

with Goniometer ................................................................................... 55

The schematic of the SFM machine, which is controlled by the

computer ................... 57

Schematic diagram of sample-tape composite for testing peel
adhesion ................................................................................................ 59

Device to apply same force for laminating of the test film and
PSA tape ................................................................................................ 60

The molecular structure of sulfonated polypropylene ................... 64

xii

Figure 18

Figure 19

Figure 20

Figure 21 (a)

Figure 21(1))

Figure 22

Figure 23

Figure 24

Figure 25

Atomic ratio of O/ C, and S/ C determined by ESCA as a function

of sulfonation time (at 1% $03 concentration) ................................. 67

Sulfur content measured by ESCA, and Elemental Analysis, as a

function of sulfonation time. .............................................................. 68

The changes in contact angle of 4 liquids on polypropylene with

increasing time of sulfonation. ........................................................... 71

Variation of solid surface energy, dispersive and polar energy

components for untreated and sulfonated PP films ...................... 74

Variation of surface free energies, corresponding to the dispersive
and polar force components for untreated and sulfonated PP
films as function of sulfonation time. Note the visible changes in
polar force component of surface free energies of solid appeared

within the one minute of sulfonation time. ..................................... 75

Relationship between Permeability Coefficient of Ethyl Acetate in
Polypropylene films and Total Surface Free Energy as a function of
sulfonation time. Note the apparent changes occurred in the 90
sec, while the surface free energy changed within one minute of

sulfonating time. .................................................................................. 76

Feel adhesion strength of OPP films tested as a function of

.sulfonation time. In insert box, surface energies of the respective

film samples vs. sulfonation time are presented for comparison

with the increasing trend of the peel adhesion strength.
................................................................................................................. 79

The relationship of peel adhesion strength vs. adhesion-

interaction term. ................................................................................... 80

Atomic% of sulfur measured by ESCA as a function of

sulfonation time. .................................................................................. 84

xiii

Figure 26

Figure 27

Figure 27(a)

Figure 27(b)

Figure 28

Figure 29

The changes in contact angle of 4 liquids on polyethylene with
increasing time of sulfonation based on the untreated PE film as

reference. ............................................................................................... 86

The effect of sulfonation time on the change in total surface free
energy and the respective energy components for untreated,
sulfonated HDPE films as a function of reaction time, and corona
treated HDPE film. ............................................................................... 89

Peel adhesion strength of HDPE film samples as a function of

sulfonation time. ................................................................................. 91

Histogram of peel adhesion strength of untreated, sulfonated

HDPE film samples and corona treated HDPE film samples. ...... 92

Contact angle change of liquids for sulfonated PET samples with
increasing time of sulfonation. Based on the untreated PET film

as reference ............................................................................................ 96

Variation of solid surface energy corresponding dispersion and

polar components for untreated and sulfonated PET samples.

................................................................................................................. 98

xiv

Chapter 1. Introduction

Polymer films, especially polyolefins, are widely used as a packaging material,
as well as for industrial applications, due to their low specific weight, compat-
ibility with other materials and relatively low cost. The low surface energy of
polyolefins, and consequently their poor wetting and adhesion to other mate-
rials, requires enhancement of surface properties such as printability, wettabil-
ity, and adhesion for new uses and technical applications. Several techniques
have been developed to modify polymer surfaces in an attempt to improve
adhesion, wettability, and other surface characteristics, with the aim of incor-
porating polar groups onto the polymer surfaces!” These techniques include
plasma treatment/2H” flame treatment/’0’ chromic acid etching/1” and corona
treatment."2"”4’ Although the precise mechanism of enhanced adhesion on
surface modified polymers is not completely understood, the experimental
evidence points to changes in the chemical nature of the surface, such as the

formation of polar groups1 1", elimination of weak boundarieslmvm’

Sulfonation, using gaseous 803, has been evaluated as an effective technique
to improve the surface properties as well as the barrier properties of polymer
films. Studies involving the sulfonation on polymer films have been per-
formed to evaluate the effect of sulfonation on the physical and mechanical
properties of polymer films to include: mechanical propertiesnsl' “9’; electrical

properties”“; barrier properties’onzz’; and surface propertiesml.

Studies on the chemical changes of polyethylene during sulfonation, reported

2

by Ihata’241'125’, indicated that introduction of polar sulfonic acid groups (-
SO3H-), with unsaturated C=C bonds on the polyethylene surface is a result of
chemical reactions taking place during surface treatment. Ihata’z‘” subse-
quently inferred that any polymer containing C-H or N-H bonds can be sul-
fonated. ErictionI 19’ also reported that the sulfonic acid group, neutralized with
ammonium hydroxide (NH4OH), had penetrated into the bulk of the polymer,

resulting in changes on the surface as well as within polymer bulk phase.

Wangwiwatsilpm‘ has studied the effect of surface sulfonation on the barrier
properties of polypropylene films, by using permeation and sorption measure-
ments involving acetate and toluene. The author observed distinctively
improved barrier properties by reduction in the diffusion and sorption coeffi-
cient of ethyl acetate following the surface sulfonation of oriented polypropy-
lene (OPP) film. He also investigated the effect of surface sulfonation on the
barrier properties of polyethylene terephthalate (PET) and Nylon 6.6 films, but
found no significant improvement for either film, under the conditions

employed.

The effects of surface treatments are usually evaluated by the surface nature
changes through X-ray Photoelectron Spectroscopic (XPS) Analysis’m 123’, Fou-
rier-Transform Infra-red (FI‘IR) Analysis and Atomic Force Microscopy
(AFM)‘29'. Spectroscopic measurements are complementary, as they afford
information related to chemical structure changes on the surface and bulk of
polymers, but provide little information related directly to adhesion interac-
tions. For example, Asthana'23' attempted to investigate the effect of sulfona-

tion on the adhesion properties of polypropylene (PP) and polystyrene (PS)

3

films by observations of X-ray Photoelectron Spectroscopic (XPS) Analysis,
Fourier-Transform Infra-red Analysis, and by Contact angle analysis with dis-
tilled water. Most of Asthana’s work was directed to developing a better
understanding of the surface chemical structural changes due to sulfonation of
PP, and PS films, and a detailed analysis of the changes in polymer surface
properties was not reported.

Knowledge of the surface energies and of the wetting characteristics of poly-
mers, as well as information related to surface energy changes of polymers, is
essential to evaluate the effect of surface treatment and the processibility of
surface treated polymer film structures. Traditionally, contact angle measure-
ments ’30" ’3“ have been performed to obtain information on surface energies
and on the wetting characteristics of solids. For example, the empirical method
was described by Zismanm’. Recently, Inverse Gas Chromatography(’33’"’35’)
analysis has been proposed as an alternative method to determine polymer

surface energy data.

In the present study, Contact Angle Measurements, and Electron Spectroscopy
for Chemical Analysis (ESCA) techniques were employed to evaluate the
effect of sulfonation on the surface free energy of polymer film and the associ-
ated relationship with the surface adhesion properties. Contact angle measure-
ments were carried out with four liquids of known surface properties, on both
the sulfonated and untreated polymer films. The level of sulfonation was
determined by the ESCA analysis. The surface free energy of the surface mod-
ified polymer film, to include both dispersion and polar components, was

determined from contact angle measurements by the method proposed by

Kinloch et all36'.

A Peel Adhesion (PA) test, with pressure adhesive tape, was then carried out
the respective film samples, which were sulfonated and untreated. Pressure
Sensitive adhesive (PSA) tape was also characterized by the determination of
the surface tension, prior to the PA test. The values of surface energy, deter-
mined for the respective polymer samples and the PSA tape, were then used to

determine a correlation with the results of the PA tests.

In addition to surface energy characteristics, attempts were also made to inves-
tigate surface topographical changes due to sulfonation of polymers by using
Atomic Force Microscopy (AFM). Unfortunately, due to the soft and flexible
nature of the polymer surface, AFM was found to be an ineffective method to

evaluate the rheology of the polymer films under the conditions employed.
The main objectives of this study are summarized;

1. To determine the effect of surface sulfonation of the oriented polypropy-
lene (OPP), polyethylene (PE) and polyethylene terephthalate (PET)
films on surface energies, as well as the corresponding to dispersion and
polar components, by measuring contact angle of various liquids on the

respective film samples.

2. To determine peel adhesion strength of PP and PE films, which were
untreated, and sulfonated as a function of sulfonation time. The PA test

followed the standard test method '37] with minor modification.

3. To evaluate and analyze the obtained data.

Chapter 2. Background

Adhesion is defined as the phenomenon in which two different surfaces (for
example, solid-solid, and solid-liquid) are held together by interfacial forces.
Polymer adhesion is fundamentally dependent on a variety of factors includ-
ing interfacial forces which are related to morphology, rheology, barrier prop-
erties or mechanical properties of polymers. Brief descriptions of the nature of
interfacial forces, mechanisms of adhesion, and work of adhesion related to

surface free energies are included in this chapter.

2.1. The nature of interfacial forces in adhesion phenomena

Interfacial forces are closely related to surface tensions, which occur as a result
of the attraction of the bulk material to the surface layer. This attraction tends
to reduce the number of molecules in the surface region, resulting in an
increase in intermolecular distance.“" The forces of establishing intrinsic
molecular contact are decisive in determining adhesion strength. These forces
of attraction include ionic forces, covalent bonds, hydrogen bonds and Van der
Waals’ forces of various types. These forces and their bonding energies are rep-
resented in Table 1 (page 6)”. From those forces of attraction, Van der Waals
forces are the most common. Different effects are attributed to the overall Van
der Waals attractions; they are (a) dispersion forces (or London forces),

(b)induction forces (or Debye forces), and (c) Polar forces (or Keesom forces)

Dispersion forces are non-polar forces which arises from internal atom

motions, whether atoms are charged or not. Dispersion forces exist between all

5

6

atoms”, because all atoms consist of positive nuclei around which negative
electrons are moving. They are dependent on the total number of electrons and
the positive charge to which these electrons are bound. Dispersion forces are
short-range forces, and these forces display the important property wherein
the energy of interaction due to such forces between unlike atoms is at most

equal to the geometric mean of the energies between forces‘39'.

 

 

 

 

 

 

 

 

 

 

 

 

Table 1 Types of physical attractive forces and typical bond energiesa
Types of Forces Bond energy [kg/mouj
Primary bonds
Ionic 600~1100
Covalent 60~7OO
Metallic 110~350
Donor-accepted bonds
Bronsted acid-base interactions Up to 1000
Lewis acid-base interactions Up to 80
Secondary bonds: Hydrogen bonds
Hydrogen bonds involving fluorine Up to40
Hydrogen bonds excluding fluorine 10~25
Van der Waals bonds
Permanent dipole-dipole interactions 4~20
Dipole-induced dipole interactions Less than 2
Dispersion(London) forces 0.08~40

 

 

 

a. From Ref, Kinloch, AJ.,” Adhesion and Adhesives: Science and Technology”I39], p. 79

 

Induction forces originates from an asymmetry of charge distribution. Induc-

tion forces are considered as polar forces but are very negligible compared to

dispersion forces'381.

Polar forces (or Keesom forces) arises from the orientation of permanent elec-
tric dipoles and the induction effect of permanent dipoles on polarized mate-
rial. Dispersion forces are usually weaker than the polar forces but dispersion

forces are found in all materialsfil.

2.2. Mechanisms of adhesion

Many adhesion models have been proposed to account for a wide range of
related experimental observations, but there is no plausible single mechanism
to cover all circumstances. There are four major mechanisms which are gener-
ally accepted as explaining adhesion phenomena including; mechanical inter-
locking, diffusion theory, electronic theory, and adsorption theory. Lately, the
adsoprtion theory is the most widely accepted in understanding adhesion
phenomena. Each of these theories has certain support in some cases, but also
many weaknesses; at present, none of the above theories taken alone can ade-
quately account for all of the experimental observations relating to improved
adhesion propertiesl‘“. More details are presented with examples in the refer-

ence of [39], and [41].

2.2.1. Mechanical interlocking

The basic concept of this mechanism is that mechanical interlocking of adhe-
sives with irregular substrate surfaces promotes intrinsic adhesion!” In certain
instances mechanical interlocking may contribute to the intrinsic adhesion
mechanisms. However, since strong adhesion also can be attained between

smooth surfaces, the concept of interlocking is not adequate to explain adhe-

8

sion. Kinloch‘39‘ stated that the increases in adhesion with increasing surface
roughness might be related to other factors such as the removal of weak sur-

face layers, or improved interfacial contact to make better wetting conditions.

2.2.2. Diffusion theory

The diffusion theory, originated by Voyutskii'38’, supposes that the adhesion of
polymers is caused by the mutual diffusion of polymer molecules across the
interface, which conveys adhesive strength. Interdiffusion of polymer chains
across the interface requires the polymers to be mutually soluble. This fact
may limit this theory to the autohesion of elastomers or materials with similar
solubility parameters. If the materials are not similar, diffusion across the
interface will be very slow, and then interdiffusion will be an unlikely mecha-

nism of adhesion.

2.2.3. Electronic theory

Deryagin et al'4‘“ suggested that electrostatic forces arising from such contact
or junction potentials may contribute significantly to adhesion. In their work,
an electric double layer was observed in the interface of pressure-sensitive
tapes when the bond was broken. The concept of this theory is based on the
adhesive/ substrate system acting as a capacitor which can store electric
energy charged due to the contact of two different substrates. From this theory,
adhesion is postulated to occur due to the attraction of forces across the electri-
cal double layer in the interface. The main argument made against this theory,
however, is that any electrical double layer generated at the broken joint

emerges from the failure of surface, rather than adhesion between the materi-

als.I39‘

2.2.4. Adsorption theory

The adsorption theory is the most widely applied in understanding adhesion
phenomena with the base of attractive force. Attractive forces, arising from the
interface in a joint between two materials, provide cohesive strength between
the atoms and molecules. These attractive forces of interfaces are closely
linked with chemical adsorption, and surface free energy. Since the nature of
such forces was summarized in a previous chapter, the contributions of inter-
molecular forces to the thermodynamic work of adhesion, to the surface free

energies involved in adhesion will be detailed below.

2.3. Work of Adhesion

The involvement in adhesion of surface and interfacial energies is explained
by the equation for the thermodynamic work of adhesion. W A is defined as a
unit area of two phases from the interface across which forces are acting’43’.

This yields the Dupré'43I equation:
WA = st + 710 — 75! [2'1]

where Y, , y, , and Y5! are the surface free energy of the solid, liquid phase,

and interface, respectively.

Good and Girifalco'46" ”71 have evaluated this interfacial surface free energy by
the ratio, R, of the free energy of adhesion to the geometric mean of the free

energies of the solid and liquid phases:

lO

(yS + y, — ‘YsI)/2(IY5.YI)1/2 = R [2.2]

where 75 , y, , and 75, are the surface free energy of the solid, liquid, and inter-
face, respectively. By assuming R is approximately unity for the simplest cases,
the equation I 2.2 I can be simplified as:

1/2
751 = Y; + Y; - 2(7571) [2.3]

And then WA is expressed I4”;

/2

WA = 2 (757,) 1 [2.4]

Fokwesl‘sl proposed that this work of adhesion was due to the addition of con-
tributions from many interfacial interactions such as dispersion forces, hydro-
gen bonds, dipole/ dipole, dipole/ induced forces, acid / base interactions, and

so on, yielding:

WA = Wf‘ + (Wfi + Wff + Wj‘) + W? [2.5]

where the superscripts represent d-(dispersion forces), h-(hydrogen bonds), dd-
(dipole/ dipole), di- (dipole/ induced forces), ab-(acid / base) interactions,
respectively. Recently, the work of adhesion has been recognized as effectively
arising from two major components: the dispersion forces and the polar forces

(or acid / base interactions). This can be expressed as follows:

w, = w; + we, [2.6]

11

where superscript d-dispersion force components, and p-polar forces compo-

nents, respectively.

Chapter 3. Surface energy ofpolymer solids

The effects of surface modification are commonly characterized by the estima-
tion of surface free energies of solid. The surface free energy of solid cannot be
directly measured. The contact angle analysis is one of traditional method to

estimate the surface free energy of solid.

3.1. Relationships between contact angle, wetting and adhesion

Wetting is defined as the extent to which a liquid makes contact with a solid
surface‘39'. The wettabih'lty of liquids with respective surface can be character-
ized by measuring the peripheral contact angles between the surface of a small
sessile drop of the liquid and the horizontal surface of the solid. Adhesion is
the intimate sticking together of surfaces on which interfacial forces are acting.

The work required to pull the interface apart to two faces can be estimated.“

Contact angle measurements provide a simple, inexpensive method to obtain
direct information on wetting by the contact angle of a liquid, and can estimate
the surface free energy terms of a solid as described in Figure 1 (page 13). At
equilibrium and in the absence of interfacial reactions, the extent of wetting on
the surface by the liquid is determined by the force balance between the three
phase contacts as illustrated in Figure 1. This balance of interfacial forces is

generally described by Young’ s equation'491:

‘st = 751+ 710C059 [31]

where the terms 71v and st are the surface tension of the liquid and the

12

13

solid in equilibrium with the vapor, respectively, and 751 represents the

interfacial tension between liquid and solid with the contact angle 0.

 

 

 

 

 

 

st
#
ideal solid surface
FigureI The surface tension balance at a point of three-phase

contact at equilibrium for ideal surfaces. An ideal surface
is a smooth surface without interfacial reactions with a
liquid drop.

The surface free energy( 75 ) of a polymer can also be expressed by the equi-
librium spreading pressure (we) of a test vapor due to the adsorption of vapor

molecules '39]:
75 = 751+ 7te [3'2]

where 1t.e is defined as the reduction of 7., due to the adsorption of vapor by the

surface, when the vapor obeys the ideal gas laws as in equation [3.3]:

14
«9 = Rrjo" r d(ln p) [3.3]

where p is the vapor pressure, p0 is the saturation vapor pressure; R and T are
the gas constant and temperature, respectively, and F is the surface concentra-
tion of absorbed vapor?” For a polymer with low surface energy, It, can usu-

ally be neglected so that equation [3.1] is approximated:
Y5 z 751 + 710C059 [3'4]

Equation [3.4] provides a way to indicate the spreadability (or wettability) of
liquids on the solids. The criteria of the spreading of liquid on a solid can be

expressed by defining a parameter of the equilibrium spreading coefficient, 8,:
Se = 75 ‘ (751+ 71v) = 71,, (5056 — 1) [35]

When 6 is zero, i.e. cos 9 = 1, S, = 0, the liquid spontaneously spreads over the
surface because of the negative free energy association with the process. When
0 is not zero, i.e. C08 0 <1, Se< 0, then the liquid is non-spreading over the sur-

face.

The work of adhesion separates two phases which are originally in intimate
contact with each other. The work of adhesion, WA, can be combined with

equation [3.4] to give a direct relationship between WA and wetting, yielding;
WA = 75v + 710—75, = y,v(1 + c050) [3.6]

Equation [3.6] indicates that WA can be maximized when the liquid exhibits a

15

zero or near zero contact angle.

In addition to the concept of the wetting equilibria described by thermody-
namic relationships with contact angles, the rate at which the contact angle
equilibrium is approached depends on such factors as the driving force for

wetting, the viscosity of the liquid, and roughness of the solid.[391

3.2. Methods of estimating surface free energy of solid

Surface energies of solids are usually estimated from the contact angle mea-
surements using probeliquids of known surface characteristics. Most of the
contact angle methods to estimate surface free energy of solid are based on the
Young’s equation (Equation [3.1]) by using a liquid drop on the smooth and
undeformed surface of solid. Various methods are proposed to estimate the
surface energy of solid. Since the surface energy of solid cannot be directly
measured, the methods of estimating surface energy solids have been
attempted by many authors. The methods which have been proposed are

reviewed in the following.

3.2.1. The critical surface energyflc

The critical surface tension, originated by Zisman’32’, was the first approach to
characterize low-energy solids by measuring contact angles of a homologous
series of liquids. He used a rectilinear relationship existing between the con-
tact angle and the surface tension of the wetting liquid. As shown in Figure 2
(page 17), which was obtained from contact angles of liquids for untreated
polymer films, a straight line is obtained when 0039 is plotted against the liq-

uid surface tension. The intercept of the line cos 9:1 is defined as critical sur-

l6

face energy, 7“,, which just spreads over the solid. Kinloch‘39' indicated that
ye" is not identical with the surface energy of solid, but is only an empirical
parameter with relative values which act as one would expect of the surface
free energy. From Zisman’s method, 7m can be expressed as an equation [3.7]

by substitution of c059 =1 into equation 2:

Ycrt = 75 — Ysl _ Re [3.7]

This equation reveals that ‘ch can only be identical with 75 when the interfacial
tension between solid and liquid/yd, and 1t, are negligible for the polymer sur-
face”. Hata et al'45' also stated that the critical surface energy, ‘ym, provides
minimum surface energy, and that homologues and nonhomologous series of
liquids should be selected to maximize 7m if ye" is to be used to as a measure

of surface free energy, 75.

Later, Dann‘sol observed the different values of 7m depending on the values of
75] by measuring the contact angles of various liquid series on the a number of
polymers and explained this feature by employing the concepts of Good‘“‘,
Girifalcol‘m and Fokweslm He also demonstrated the limitation of Zisman’s
approach, which was restricted to the range of surface tensions of a homolo-
gous series of liquids for 7,3,, measurements because no polar force compo-

nents were involved in the total surface tension contributions.

3.2.2. Surface free energy components to determine solid surface energy

This method starts from Fowkes’ suggestion'481 that surface free energy com-

ponents exist which are due to particular types of intermolecular forces,

l7

 

 

 

    

 

 

 

 

 

 

 

 

30°
CD
a . e
U 60
0.2 , PET (Untreated) —u—
’ PP (Untreated) we - -- 36.5 -
0.1 h, ---5 _______ ' _______
. PE (Untreated) —9— 34-3 5 g
- r f r r . . ' ' d o
0 ........ 5. ....... 5 ....... g ....... ; ...................... . ....... ---.,, 9°
.-..l.-..i....1...-i.1..1....i.-1-ll.-.l. ..
30 35 40 45 50 . 55 60 65 70 75
Surface energy of Inqund, Yr [dyne mm]
Figure 2 A plot of cosine of angles vs. the surface

energy of liquid, as proposed by Zisman.

including dispersion forces, dipole-dipole forces, or hydrogen-bonding inter-
action. It has been recognized as effectively caused by two major components;
the dispersion forces (or London forces) and the polar forces, which are con-

sidered as acid / base interactions, are expressed as follows:

7 = yd +~f [3.8]

18

where the superscripts d and p, respectively, are dispersion and polar force

components of solid surface energy.

Fowkes'431 then applied the geometric mean to the dispersive force compo-

nents for estimating the surface energies involving only dispersion forces:

d d
m = Y. +Yb-2JI 7,th [3.9]

where y, and 7b are the surface energies of the two phases, and superscript d is

dispersive component of the interfacial energy.

Owens et all”) presented an equation to describe the interfacial energy
between two surfaces by combining all interactions including dispersion
forces into a single 7" term. They assumed that the geometric mean expression
could be extended to polar interactions and subsequently to equation [3.9] in

the a solid / liquid system, as following;

d d _
is: = tum—2 v.7, -2 {it} [3.10]

where d and p denote dispersion and polar components of interfacial free

energy, respectively.

Finally an equation to manipulate estimating the surface free energy of solid
can be reached by combining equation [3.10] with equation[3.4] (on page 14)
and [3.8] (on page 18), which eliminates the interfacial energy of solid and liq-

uid:

19

Yz(l+0039) = 2 7377 +2 r5); [3.11]

where y, is the surface tension of the liquid, which is the sum of dispersion

and polar force components.

d
y, = 7, +7}; . [3.12]

By making contact angle measurements with two probe liquids of known

characteristics [Table 2 (on page 20)]; two equations can be set up on the com-
d

mon solid surface and solved for the unknown 75 , and VS) . Then the surface

energy of the solid,ys, is the sum of these two components.

d
ys = 75 + 1(5) [3.13]
This method is described in greater detail in the Chapter 3.3.1. (on page 22)

Dann‘sm evaluated the surface energy of many liquids used for contact-angle
measurements from the previously reviewed equations. Basically, he deter-
mined only the dispersion-force components of the liquid surface tension”? ,
by measuring contact angles against a solid that was intended to have only a
dispersion-type surface energy. He measured contact angles of various liquids
on paraffin, assuming 7: as zero, and evaluated the dispersion component of

the liquid using the equation [3 .14]:

d
d 4yp- (c039)
71 = ,Y
I

 

[3.14]

20

where y? is the dispersion force component of the liquid surface free energy;
7, is the total surface free energy of the liquid which can be directly measured
by using the Du Nouy ring method or Wilhemy plate technique; and y: is the
paraffin surface energy of the dispersion-force component which was known
as 25.5 [dynes/ cm]. Then the V; , the polar (or non-dispersion) component of

liquid surface tension was calculated by using Equation[3.8] (on page 18).

Some typical liquids used in the contact angle method to characterize the solid

d
surface are given with y, , V; and y, in Table 2.

Table 2 The characteristics of typical liquids used as probes for the contact
angle measurementsa

 

 

 

 

 

 

 

 

 

 

 

 

Liquid y? [Dyne/cm] 1(1) [Dyne/cm] y, [Dyne/cm]
Distilled Water 2.0 50.2 72.2
Glycerol 34.0 39.0 73
Formamide 32.3 26 58.3
Di-iodomethane 48.5 2.3 50.8
I—Bromonaphthaleneb 44.6 0.0 44.6
Dimethylsulphoxide" 36.2 4.5 40.7
Tricresylphosphate" 36.2 4.5 40.7
Polyglycol 152-200 28.2 15.3 43.5
Polyglycol 15-200 26.0 10.6 36.6
Hexadecane 27.6 0.0 27.6

 

 

a. Dann, unless otherwise noted
b. Fowkcs

 

 

 

21

3.2.3. The methods using the equations for the work of adhesion

This method starts from the surface tension that can be divided into two com-
ponents, the dispersion force and polar force components, and is represented
by 'y = ydrf (equation [3.8] (on page 18)). The equation [2.4] (on page 10),

1/2
)

which is W A = 2 (leI can be extended with the concept of Owens, et

all”), following;

WA = 2,II 7:7? I + 2,II 165/!) I [3.15]

Combining this equation [3.15] with Young-Dupre equation [3.6] (on page 14)
yields:

71(1 + c059) = 2,II y: 7I+ 2 16% [3.16]

3.3. Calculations of dispersion and polar force contributions to
surface free energy of polymer

Contact angle measurements have been widely used to calculate the values of
the dispersion force,y:, and polar force, “/5, , components to the total surface
free energy/ys , by using equation [3.11] (on page 19) or [3.15] (page 21). Kaelble
and Cirlin‘53‘ used two fluids to calculated the surface energy (Details in 3.3.1 .).
Carley et allm used four liquids rather than two liquids to characterize each
test surface and IKinloch et al’36’ have well developed the equation to reduce
errors during calculation of surface energies on the solids by using four liquids
(Details in 3.3.1(a)). The followings details the derivation of equations and

methods.

22

3.3.1. Determinant method

Kaelble, et all”) analyzed the experimental values in work of adhesion
acquired by using the equation W A = y, (l + c056) . In his analysis a pair of
simultaneous equations is derived for two liquids, m and n, on a common

solid surface:

(WA)m = 2(y:)1/2(y7);/2+2(£)1/2(%,);/2 [3.17]

(WA)n = 2(y:)1/2(Yf);/2+2(£)1/2(fi)l/2 [3.18]

where 6 is the contact angle of the liquid on the solid surface. Thus, if the val-
ues of 9 , 77, 7f for the two liquids are known, these equations may be solved
to yield the dispersion and polar force components to the surface free energy
of the solid surface. The total surface energy is then simply the sum of these
components. But this has some limitation for using contact angle data for only
two liquids, and suffers having errors when calculation of the surface energies

is performed to other liquids.

The equation [3.17] or equation [3.18] has a linear relationship in the schematic
representation of (y: )‘l-5 versus (7’; )0:5 as shown in (Figure 3) where four linear
relationships obtained from four liquids contact angles are illustrated. Previ-
ously reported work has solved for the two unknowns by solving for each
individual pair of lines and then averaging the results, but this direct approach

was found to lead to considerable errors in the values of y: and If; , which had

23

been calculated.

Kaelble‘53’ suggested that the pairs of lines have exceeding values of boundary

condition should be excluded to minimize errors, where Dboundary is given by:

abound”, = [(YIImIYIInII/z' [Iii My] Inf/22:10 [3.19]

Though this condition was shown to contribute somewhat to reducing the
scattering, the condition of Dboundary remains unverified condition. According
to this method, points A and B in (Figure 3) are disregarded in the calculation

of the surface energy of solids, which may lead to some errors.

3.3.1(a) The least square method

Kinloch’36’ provided this method to make possible not only to calculate the
surface energy values by accepting all the contact angle data, but also to
reduce errors in calculations. This technique is not depending on the individ-
ual intersections but on the slopes of the straight lines shown schematically in
Figure 3. It is possible to use equation [3.16] (on page 21), [3.17] and [3.18] (on
page 22) with method of least squares to obtain best values of the dispersion
force,y:, and polar force, '{5’ , components for each treated surface, based on the
data from several liquids instead of just two. The derivation of this procedure

and a computer program for solving equations are detailed below.

3.3.1(b) Derivation of least squares method

The equation [3.16] can be rewritten for each liquid used for the contact angle

measurement on the common film surface as an equation:

24

 

 

  
 
 
 
  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

20 r ‘ !
. .
(1): Distilled Water I
15 (2): Formamide ,.
g (3): Methane diiode
"E" (4): Tricresly Phosphate
O . .
2 10
>5
3
v3
DA 5
V?!)
0
-5
0 2 4 6 8 l0
0.5
(15) [dyne/ cm] 0'5
Figure3 Presentation of typical individual relationships of (790-5

versus W905 deduced from one of equations [3.17] or
[3.18]. Plot shows SPP1, sulfonated polypropylene for 1
min. The shaded part is in the boundary

d
Zéfl+zgfi=l+cme [3.20]
1 I

When k numbers of liquids are used for the contact angle measurement, equa-

tion [3.20] can be simplified as in the matrix form with two unknown compo-

25
nents of the surface free energy:

[Alkx2[x]2xl = [BIle+[e]kxl [3’21]

where matrix [A] represents the constant coefficients of the two unknowns, fl:
and If: , in Matrix [X]. Matrix [8] represents the constant values of right-hand
side of equation [3.20], i.e. 1 + case. And matrix [e] is the error involved in bal-

ancing the individual equation.

When m numbers of contact angle of each liquid on a film are recorded, then
equation [3.21 I can be extended as:

[Alkx2lxlzxm - [3'22]

[Blkxm+IeIkxm

Matrix [B] is taken to the left-hand side of the equation [3.22], and multiplied
by both sides of the equality with the transpose of the left-hand side:

{[Alkx2IX12xm- [Blkxm}T{ IA] lezxm- [Blkxm} [3.23]
= {[AlkleX12xm- [Blkxm}T{ [8]}
where superscript T presents the transpose of the matrix concerned.
The equation can be expanded as giving,
{[XITIAITIAI [X] Imxm“ { {XlTlAlTlBl}mxm [3.241

—{[B]T[Al [Xl}mxm+{[BlTlBl}mxm = [Elmxm

26

When the partial derivative of matrix [B] is employed for the two unknowns

(X1 and X2) and made to zero to minimize the error then:

815] /8x1 = [AlTlAl [X] + [XITIAITIAI - [AlTlB] — [BITIAl = o

[3.25]

ant/ax, = [AlTlAl [X] +[X1TlAlTlA] — [AlTlBl-[BJIIAI = 0

Since the above two equations are the same, rearranging one of the equations

gives:

{me [x] - [A1T[31}2xm+ {leTlAlTlAl - [sthmm 2 = o [3.261

X

The equation [3.26] is in the form of [2] + [le = o . In order to satisfy the
equality to zero, then both matrixes should be individually equal to zero.

Therefore:
{lAlTlAl [x1 I2”. = {[AlTIBl I2... [3.271
and

{liTIAJTI/«mez = {lath/mm” [3.281

27

Equation [3.27] is the transpose of equation [3.28]. Therefore one of these two
equations is required for analysis for contact angle measurements. Matrix [X]

in the above equations can be solved by one of equations below:

[x1,,.,,, = {inverset [AITIAIH {[AlTlBl}2xm [3.291

and

1x1“, {inverseHAlTlAH} {mm/mm” [3.301

There are several advantages to using this method; this method yields the least
errors, it accept all the data, and it is very simple to employ into a computer
program. A computer program to solve this equation can be made by using
fortran, basic, or any software which can calculate the matrix. In the present
study, ”Matlab” (The MathWorks, Inc.) software has been used to solve the
matrix [X] in the equation [3.29].

28

3.3.1(c) Computer program ”SFE”

A program to calculate surface free energy by solving matrix [X] in equation
[3.29] has been presented from Figure 4 (a) to Figure 4 (d). A program ’SFE’ was
written using MATLAB software (The MathWorks, Inc., Natick, MA) which is
available for both the Personal Computer and the Unix Operating System

computer.

 

lrm diary:

clear;

clg:
%input(‘datafile=')
load test:

test=B;

Ag=mean(B);
Std_angle=std(B);

G= [21.8 51: 32.3 26; 48.5 2.3; 36.2 4.5];
T= G(:,1) + G (:12);

GL = [G T]

diary on

B

[Ag, Std_angle]
GL

 

 

diary off % [continued]

 

Figure 4 (a) Program to calculate the average and standard
deviation errors of measured contact of each liquid
angles on a polymer sample.

29

The average value and standard deviation of contact angles for each liquid are
expressed as in Ag and Std-angle matrix, [ m x k], respectively, where k is the
number of liquids used and m is a measured number on a common surface

with each liquid.

Applying this data, measured contact angles for various liquids on a film sur-
face should be in the matrix ”test”, [ m x k], where column, k, represents the liq-
uid used for measurements, and rows, m, are duplicates of recorded contact

angles.

Datafile ”test”

C:\ type test

86.2 68 44.

 

V

 

 

9 l

86 69 42.8 34.4
87 67 43.7 31.7
88.8 67.4 42.1 32.1
85.3 68.2 43 31.8
86.7 64.7 42.7 34.3
94.3 66.2 41.8 32.9
87.2 68.9 46.2 33.8
89.2 67.2 43.6 32.7
2 70.2 44.5 33.2

L o

Figure4(b) Figure shows the input data file for the
program shown in Figure 4 (a). Columns in
this data file present each liquid used for
the contact angle measurement. Rows in this
file are the duplicate of the measured
contact angle of a liquid measured on
different points of a film sample.

30

The surface energies of liquids used in contact angle measurement are pre-
sented in the matrix GL, [3 x 4] from the matrix G, [2 x 4], where the first column
is the dispersive component, the second column is polar component of liquid
surface energy, and the third column is total surface energy of the liquid
expressed in the matrix T, [4 x1], as the sum of the first and second columns of

matrix G.

31

 

Ca=cos(B*pi/180);

Aa=cos(Ag*pi/180)

C=1+Ca:

[A]=[2.*(GL(:,1).‘O.5)./GL(:,3)
2.*(GL(:,2).“O.5)./GL(:,3)];

x=(-4:.5:15)’;

y1=-A(1,1)./A(l,2).*X+C(l,l)./A(l,2),‘
y2=-A(2,l)./A(2,2).*x+C(l,2)./A(2,2):
y3=-A(3,1)./A<3,2).*x+c<1,3>./A<3,2);
y4=—A(4,l)./A(4,2).*x+C(1,4)./A(4,2);

plot(x,yl,'y-',X,y2,'-.’,x,y3,'--',x,y4)
axis([-5 15 -40 50])

axis('square=’)

grid

%gtext(’PET film (Untreated)'

title =(‘Surface Free Energy<sfe) of solid
by the plot’)

xlabel (‘dispersive component,(dyne/cm)“0.5')

ylabel (‘polar component,(dyne/cm)‘0.5')

print result

 

clg:

pause; %[continued]

 

Figure4 (c) This program provides a graphic like the one
presented in Figure 3 (on page 24), which is plotted
(yf)°'5 versus (1005, deduced from one equations [3.17]
or [3.18] (page 22).

 

A solution for either equation of [3.29] or [3.28] (page 26) can be obtained by

the following program;

32

 

 

L

Figure4(d) This program calculates surface

E = [inv([A]’*[A])*([A]’*[C]’)];

AA = E’;
XY_point=mean(AA);
Ans=B.“2;

s=Ans’;

n =size(S,l):
s_avg=sum(s)./n;

Sur_dp = [s_avg];

Total_se = s_avg(:,1)+s_avg(:,2)

end

V

J

energy,

corresponding dispersion component,(y:’) and polar
component,(£) of solid by solving one of equations

[3.27] or [3.28].

 

Chapter 4. Surface modification technology

4.1. Introduction

Over the years, several methods have been developed to modify polymer sur-
faces for improved adhesion, wettability, and printability. These include
mechanical treatments, wet-chemical treatments, exposure to flames, corona
discharge and glow discharge plasmas. The most commonly using modifica-
tion methods on the polymer surface are’“;

- Chemical etching

. Flame Treatment

- Corona and Plasma treatments

The basic objective of such treatments is to remove weakly bonded surface
contamination and to provide intimate contact between the two interacting
materials on a molecular scale, for molecular energies across an interface
decrease with increasing intermolecular distance!“ Kinloch’39’ summarized
that the purpose of any particular surface treatment might be various but the
main aims are usually increasing adhesion properties of the polymer by one or

more mechanisms as follows:

1. The removal of any weak boundary layer, such as may occur from low
molecular weight contamination, antioxidants, processing aids or the
tail of the polymer's molecular weight distribution.

33

34

2. The introduction of specific groups on the surface to aid the formation
of the interfacial polar interactions, hydrogen bonds, acid-base interac-
tions to increase the extent of interfacial contact and the degree of intrin-
sic adhesion. This is usually reflected in higher values for the surface
energy, thermodynamic work of adhesion.

3. The creation of a surface topography suitable for mechanical interlock-
ing to occur, thereby increasing the intrinsic adhesion.

In the following sections the various types of surface treatment techniques and

the effect of different treatments will be briefly described. The sulfonation

technique is considered separately since the principles involved are quite new

and are conveniently reviewed in a separate chapter.

4.2. Chemical etching techniques

Chemical etching is the treatment of polymer surfaces by exposure to solu-
tions of reactive chemical compounds, which may cause a chemical change on
the surface and also may introduce micro roughness by removing some mate-
rial!“ Thereby surface bondability and wettability are improved. This tech-
nique, however, has some limitations: it requires immersion for some time
period; the polymer surface must be rinsed and dried. Therefore, chemical
etching is used only when simpler and less expensive methods are not avail-
able. For example fluorocarbon polymers are often etched chemically because

other treatments do not produce sufficiently good bond ability. ’4“

Etching with sulfuric and chromic acids are suitable for modifying the surface
of polyethylene, polypropylene, ABS, polyether, and polystyrene!“ Chromic

acid etching removes amorphous areas, forming cavities which contribute to

35

the improved bondability. Some surface oxidation may also take place which

improves surface wettability.

Chromic etching of polyethylene has showed a combined mechanism for
enhanced bondability and thus adhesion. Blais et a1 observed fine surface
roughness after an etching treatment, which led to the increased bondability.
The calculated surface tension of a solid was 34.2 dynes/ cm for untreated, and
52.3 dynes/ cm for a treated polyethylene surface. However, Blais et a1
revealed extensive chemical changes with an increasing concentration of OH,
>C=O groups in the surface layer. They also suggested the main mechanism
for the increased adhesion to be due to the removal by the etchant of a weak

boundary layer covering the polyolefin surface.

Briggs et al[9] investigated treated low-density PE, and PP with ESCA and
showed that oxidation and sulfonation occurred in the outermost surface
regions of both materials with evidence for -COH, >C=O, -COOH, and -
S(=O)ZOH groups. These incorporated specific groups that are due to etching
treatment may have a major role in increasing surface polarity and thus

enhancing adhesion and bondability.

No single identifiable mechanism has yet been established for developing the
etching technique. One of chemistry changes due to the introduced group at
the surface of the polymer seems to be an essential mechanism for improving
bondability and adhesion, but with some polyolefins the removal of a weak
boundary layer may be a necessary step for modifying the make polymer sur-

face.

36

4.3. Flame Treatment

Flame treatment is one of the most common surface treatments for improving
the adhesion of such molded polyolefin products, as containers. While the
corona discharge is used for sheeting below a thickness of 0.6 mm, flame treat-

ment is applied to thicker sheeting!”

Flame treatment oxidizes the polymer surface and makes it more easily wetta-
ble. Because the flame treatment can readily induce to degradation of the poly-
mer surface, optima in process is quite critical. Important parameters for
optimum conditions are the position of the component relative to the flame;
the air, gas; the air/ gas flow rate; the nature of the gas; and the exposure

time.“‘”

Briggs et al extensively examined the effect of flame treatment and the mecha-
nism of surface modification on low density polyethylene(LDPE) by employ-
ing ESCA. They showed that flame treatment caused oxidation with a range
between 4 nm and 9 nm of the polymer surface. They assumed that a chain-
reaction free radical process takes place due to the thermal oxidation, which

may induce surface modification for the improved adhesion.

4.4. Corona and Plasma treatments

The plasma treatment has been recently introduced as an effective and eco-
nomic route for polymer surface modification. Like corona treatment, plasma
treating uses an electromagnetic energy to ionize gas molecules, which in turn
react with the polymer surface. But plasma treatment takes place in a vacuum,

unlike corona treatment, and plasma can use gases other than air to achieve a

37

wider variety of surface modification effects. (Plastics Technol, Feb. 1993)

Corona, also known as ”atmospheric” or ”nonvacuum” plasmas, are ionized
gases which can react with a polymer surface to improve adhesion for print-
ing, painting or laminating. Plasma treatment is a very effective method for
modifying the polymer surfaces. Corona and Plasma treatment do not usually
affect wettability but cross-linked the surface material, thereby improving
bondability and adhesion. A schematic represented in Figure 5 (page 38) illus-

trates various interactions which are possible in a gas plasma contact on a

polymer. ’4’

In plasmas, four main effects are normally observed with singly or combined
depending on the substrate and the gas chemistry, the reactor design, and the

operating parameters!“ They are:

1. Surface Cleaning, that is, removal of low molecular weight contamina-
tion, e.g. processing additives, which are present on the surface. Surface
cleaning is a major effects for improved bonding to plasma-treated
polymers because it does not leave any organic residual, the way most
other cleaning procedures do, which interferes with adhesion processes.

2. Degradation and ablation of material from the surface, which can
remove a weak boundary layer and increase the surface area. The abla-
tion effect causes a change in surface morphology which provides
mechanical interlocking and sites for chemical interaction. But some
polymers, such as PET, are more prone to these effects, which give a
porous surface due to overtreatment.

3. Cross-linking, which can cohesively strengthen the surface layer. The
free radical can be created on the polymer surface exposed to the novel
gas plasmas, and the free radicals resulting can react with other surface

38

radicals to form stable moieties. Cross-linking may have an advantage,
e.g. hamper the development of a weak boundary layer at the joint
interface. But lack of heat sealing due to Cross-linking is a disadvan-
tage.

Modification of surface chemical structure, which can occur during
plasma treatment. The plasmas often lead to the introduction of polar
groups, such as carbonyl groups, into the surface regions of the sub-
strate. Due to the polar groups, the polymer surface has an increased
surface energy which leads to improved wetting and thus adhesion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Process-gas:I Effluent
Supply Gas
A
Unexcited process gas or vapor
A l
Excitation Source Protons
Excited species .
* Atoms and molecules Ultravrolet
It- 10118 light
* Free radicals
* Free electrons
* Meta stables
Polymeric substrate
Figure 5 A schematic representation of the interactions which

are possible in a gas plasma impinging upon a
substrate. ’4’

39

4.5. Surface Sulfonation Technique

The surface modification by a chemical technique requires a repeatable and
controllable reaction through the modification process. The sulfonation pro-
cess, exposing the polymer to gaseous 503 or fuming sulfuric acid to form sul-
fonic acid groups, has this potential. Sulfonation has advantages which can
control the depth of surface modification by regulating concentration of 50;,
gas and reaction time!” These two factors, concentration and reaction time,
can be offered as good alternatives to form acid groups in contribution of
changing surface properies. The sulface sulfonation of a polymer can result in
changes of the physical and mechanical properties, such as adhesion’19l'123l,

electrical conductivity1 ’3’, and barrier properties!“ 122’

A broad range of polymers, (except a fluorocarbon-based polymer), can be
readily sulfonated?“ It was found that a PE film and 803 produced unsatur-
ated sulfonic acid with the highly conjugated C=C unsaturated bonds.”
Ihata’z‘” reported the reaction mechanism of sulfonation, that the reaction of PE
film with 803 was initiated by the obstruction of a hydrogen atom by 803 to
give a PE radical. This free radical could either react with 503 to give a sul-
fonic acid group (Figure 6a) or eliminate a hydrogen atom to form an unsatur-

ated bond (Figure 6 b).

 

so
-CH2 -cn2 -cn,- —3> -cn2 -CH2-QH- ——> -cn2 bug-cu-

50 H so H
3 (a) 3

‘——> 'CH2 'CH=CH'
“"2303 (b)

 

 

 

 

Figure 6 Sulfonation reaction mechanism of HDPE [Ihata]

40

Similar results have been also observed by Asthanam’ who investigated adhe-
sion properties in polypropylene and polystyrene films. He suggested, for
polypropylene, that the tertiary carbons in the molecules aremost probable to
be attacked, and that reaction continues until conjugated -C=C- unsaturated
bonds are formed due to desulfonation. For polystyrene, he confirmed that the
para position of the aromatic rings are responsible for the active sites of reac-
tion, and that the alkene species are formed during sulfonation. The reaction

schemes for PP and PS are presented in Figure 7 and Figure 8, respectively.

 

 

 

 

 

 

'CIH3 .CIH3 'CIH3
SO
-CH-Cl~|2 - 3 > 4: ..(:H2 - H2303) -C=CH-
|
H303
Figure 7 The figure illustrates the formation of conjugated

system of double bonds as a result of sulfonation’23’

 

 

 

 

 

 

ll-|803

SO -

-CH-CH2 - 3 > -CH-CH- $03 > -CH=CH-
l l I

Figure 8 The figure illustrates the formation of alkene species

during sulfonationm’

41

Since the 503 group is not stable, the neutralization step is required to stabilize
the 503 group on the surface. Aqueous ammonium hydroxide (NH4OH) and
ammonia gas (NH3) are usually used as neutralization agents, however vari-
ous bases can be selected, (such as Li*, Na* Cu“, Mg“), depending on the

polymer propertieslm

There are several sulfonating agents, including fluorosulfonic acid, chlorosul-
fonic acid and sulfur trioxide complexes, of organic compounds. Among them,
sulfuric acid and fuming sulfuric (oleum)’20" ’23], are most popular in the form
of stOrtzO. They are extremely hygroscopic and react with water. Some

important factors of those agents have been compared in Table1.’23’

Table 3 A comparison of sulfuric acid and 803 gas as sulfonation agents. 123’

Factor compared Sulfuric Acid Sulfuric trioxide

 

Reaction rate Slow Instantaneous

 

 

Heat Input Heat requires for reaction Strongly exothermic reaction

 

Side reaction Minor Extensive

Boiling point 290-317 °C 42-44 °C I

 

 

 

 

 

Extent of reaction Partial Complete I

-
r

 

Chapter 5. Sulfonation of Polymer films

The effect of surface sulfonation on the surface properties of three commodity
films was studied by Electron Spectroscopy for Chemical Analysis (ESCA),
Contact Angle Measurement Analysis (CAMA), and Peel Adhesion (PA) test.
The type of polymers and exposure time for sulfonation were evaluated as

experimental parameters.

5.1. Sulfonation Apparatus

The surface treatment using sulfonation was performed at the Composite
Materials and Structures Center (CMSC), at Michigan State University. The
unit used was designed and manufactured by Coalition Technologies, Ltd.,

Midland, Mi.

5.1.1. The Unit of Sulfonation System

The sulfonation generator is a novel system that produces sulfur trioxide (503)
gas from fuming sulfuric acid (H25207), or oleum. Schematic diagrams of the
sulfonation unit are presented in Figure 9 (a), and Figure 9 (b). The main unit is
divided into two sections, the sulfur trioxide generator shown in Figure 9 (a),

and the unit with sulfonating chamber set-up as shown in Figure 9 (b).

The gaseous SO3 concentration should be of constant composition and must
be repeatable. There are two crucial external factors affecting the 503 concen-
tration: moisture and temperature. Since 303 is a highly reactive chemical spe-

cies with moisture, the system must be designed so as to avoid contact with

42

43

moisture. Also the temperature of the oleum reactor should be maintained at a
constant level because the vapor pressure of gaseous 803 is very sensitive to

the temperature.

5.1.1(a) Sulfonating chamber

The sulfonating chamber is a stainless steel parallelopiped box with manifolds
at two of its narrow sides. The dimensions of the box are 15x15x1.75 inches.
Both the manifolds are welded to the narrow sides of the box and are made of
stainless steel fittings. Figure 10 shows a schematic of the sulfonating chamber,
samples, and sample holder. This chamber is designed to mount four film
sheets (6 x 13 in. each sheet) to be sulfonated with a sample holder, which
assures that the sheets are not touching each other, nor the sides of the cham-

ber.

5.1.1(b) Sampling Port
The sampling port, made up of stainless steel, is located in the main line which
carries the 803 gas into the sulfonating chamber, and functions to measure the

$03 concentration while sulfonating.

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45

 

 

 

 

(I;

 

 

 

 

 

 

 

 

O
Nitrogen D ing Flow
Cylinder Tu Meter From the reactor
Sampling Port

 

 

Sulfonating , < A p: .
Chamber v
To the reactor

p

 

 

 

 

axe
[See in Figure 10] %

Vacuum
Pump

 

 

 

Dewar

 

Figure9(b) Schematic illustrates the $03 flow pattern in the
sulfonating chamberm’ [Continued from Figure 9(a)]

 

46

 

 

Gaseous $03 outlet

I

L‘Q’.‘.‘.‘.‘.‘CJJ

 
 

I

Gaseous $03 inlet

4 Samples (6 In. x 13 in.)

 

 

 

 

 

 

 

 

 

,_

Sample Holder

 

Figure 10 View of Sulfonating Chamber, samples, sample holderm’

 

47

5.2. Materials using for sulfonation

5.2.1. Polymer films

Films of oriented polypropylene (OPP, 2 mil thickness, 45.7% crystallinity,
Mobile Company), polyethylene terephthalate (PET, 0.5 mil thickness, 31%
crystallinity, DuPont), untreated polyethylene (UPE, 1 mil thickness,
Tredgarm) were surface sulfonated. Corona discharged polyethylene (CPE, 1
mil thickness, Tredgarm) was also used in the present studies for the CAMA
and PA test.

5.2.2. Fuming Sulfuric Acid
Oleum (H25207), otherwise known as 30% fuming sulfuric acid, was used to
generate free SO3 gas during sulfonation. Oleum concentration was measured

by weight percent of SO3 in the mixture, which consists of 70% (wt.) of sulfuric

acid and 30% (wt.) of free 803.

5.2.3. Cleaning agent

Deionized water with 2% MicroTM

solution was used to remove any contami-
nations from manufacturing or handling of test samples prior to sulfonation.

MicroTM was supplied by the Cole-Parmer Instrument Company (Niles, IL)

5.2.4. Neutralization Agent
Ammonium hydroxide (NH4OH) solution (5% wt/v) was used to stabilize all
sulfonated film samples. The sulfonated film samples were placed in NH4OH

solution for five minutes, and rinsed under running deionized water.

48

5.3. Sulfonation Procedure

All the reactions are carried out under ambient conditions. All the sheets were
washed and rinsed in deionized water thoroughly and the samples were air-
dried. In order to minimize the effect of moisture and to ensure the repeatabil-
ity of the sulfonation on the polymer, the following procedure was followed in

each run:

1. Film samples were carefully cut from each polymer film roll, the size of
6 inches by 13 inches. They were cleaned with MicroT'“ solution, and
rinsed under running deionized water to eliminate contaminants from
manufacturing and handling the films. Then they were completely
dried at room temperature prior to further tests. Complete drying is a
crucial process, as moisture is extremely reactive with S03 to form sul-

furic acid, consequently it is harmful to the concentration of 803.

2. A vacuum of about 300 microns was applied to the sulfonating chamber
(Figure 10 (page 46)) with the samples for 10 minutes to remove the
moisture. Also the sulfonating chamber was flushed with dry nitrogen
gas at a rate of 32 liters per minute to avoid any reaction of active gases
in the chamber with 503 gases, which results in uniform sulfonation on

the polymers.

3. The samples were sulfonated for the desired time. Sulfur trioxide gas,
generated from the sulfonation unit (Figure 9 (a) (page 44)), was circu-
lated through the external circulation lines. The $03 generator tempera-
ture was adjusted to maintain a constant 803 concentration of about 1%
volume/ volume. The gas was continuously circulated through the
chamber for a predetermined time interval, to obtain various sulfona-
tion levels, which were controlled by exposure time to sulfur trioxide

gas.

49

4. Constant 503 concentration during circulation, being one of the most
important factors, must be controlled and monitored for each sulfona-
tion run. A pH method was applied in order to monitor the concentra—
tion of 503 (volume%) for each run. A 100 ml gas sample, during
circulation, was taken with a gas-tight syringe through the septa which
was placed in the external circulation sulfonation lines. The gas sample
was then injected into a 125 ml Erlenmeryer flask containing 20 ml of
deionized water. The flask was shaken to react the 803 and water and
then the pH of the acid solution was measured using a Corning model
M-250 pH / ISE meter, with an accuracy of +0.001 pH. The S03 concen-
tration was calculated using the following equation.”

$03 [%] = 209.94 x (—2.065 X [pH]) [5.1]

5. The system was then flushed with dry nitrogen for 5 minutes before
opening the sulfonating chamber. This ensures that all the residual

SOg/HZSO4 is purged out of the system through the vent tube.

6. The films were neutralized with a 5% ammonium hydroxide solution
(NH4OH) for five minutes and placed in a deionized water bath for
another five minute. Before drying the films at room temperature, they
were rinsed under running deionized water to remove the excess
NH4OH. After the films had dried, they were stored at ambient temper-
atures for further tests.

5.4. Electron Spectroscopy for Chemical Analysis (ESCA)

All film samples were submitted to the CMSC, at Michigan State University,

for determining the extent of sulfonation.

Chapter 6. Physical tests

6.1. Contact Angle Measurements of Polymers and Methods

One of the goals of the present study was to calculate the surface energy of the

polymer in order to compare the effectiveness of surface sulfonation. The sur-

face energies of both untreated and sulfonated polymers were computed from

contact angle analysis values, obtained with distilled water, formamide, di-

iodomethane, and tricresyl phosphate, on the respective polymer sample sur-

faces. The surface energy characteristics of the four liquids are given in Table 4.

The surface energy values of OPP, PET, PE films and corona treated PE films

determined by contact angle analysis were used as a reference to determine

the effects of sulfonation on the polymer films.

Table 4 The surface energy of liquids, and the corresponding polar and

dispersive component, used for measuring contact angles on

the respective polymer samples and pressure adhesive tape.

 

 

 

 

 

 

 

50

 

 

Liquid 7? [dyne/cm] if; [dyne/cm] y, [dyne/cm]
Distilled Water 2.0 50.2 72.2
Formamide 32.3 26 58.3
Di-iodomethane 48.5 2.3 50.8
Tricresyl phosphate 36.2 4.5 40.7

51
6.1.1. Test Apparatus for Contact Angle Measurements

Contact angles of liquids on test polymers were measured with a Goniometer
(Model 100-00 115, Rame—Hart, Inc., Mountain Lakes, NJ), which was available
at the Chemical Engineering Department, Michigan State University. A sche-

matic of the Goniometer is shown in Figure 12.

6.1.2. Sample Preparation and Contact Angle Measurements of Polymers

Unmodified films were cleaned by the procedure outlined above for sulfona-
tion, prior to measuring contact angles of test fluids. Clean and air dried films
(2 in. x 1 in.) were mounted very evenly on a glass slide by using double adhe-

sive tape as seen in Figure 11.

 

Sessile droplet of test liquid (3 ~ 5 pl)

 
   

 

 
  

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Glass slide (2.5 in. x 1.25 in.)

 

 

 

 

 

L Film sample Double Adhesive tape —-——

 

 

 

Figure 11 Diagram of contact angle specimen and sessile droplet
form.

52

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53

The sessile-drop method of measuring contact angles was used in this study,
in ambient air and at room temperature. Droplets of 3 ~ 5 ul size were formed
on the polymer film surfaces delivered from a Pipetman pipet (0~200 ul) in
such a way as to make the angles advance. A minimum of 10 contact angle
measurements were made for each liquid, within the error of 3 degrees. All
contact angles measured were used to calculate the surface energy of the test
polymer film by the computer program ’SFE’ (Chapter 3.3.1(c) on page 28).
Angles were read to the nearest degree by using a 10X microscope with a pro-
tractor eyepiece. The procedure followed for each specimen to measure the

contact angle’551 ia as follows:

1. The polymer sample mounted on the glass slide was placed on the spec-
imen stage of the Goniometer. The microscope was focused on the near-
est edge of the film surface and adjusted the film surface and ’base—line’
to achieve coincidence. This setting was not changed during the reading

of the contact angle [Figure 13(a)].

2. A liquid droplet of 3 ~ 5 pl volume, depending on the test liquids, was
deposited onto the film to form a sessile drop of about 2.5 mm diameter
with a 200 Ill pipet[Figure13(b)].

3. The specimen stage was adjusted to view the extreme left side of the
sessile drop, and the microscope was refocused for accurate drop pro-
file, by shifting the line of sight [Figure 13(c)].

4. The measuring cross line was adjusted to tangency above the base of the
drop to create a wedge of light bounded by the two cross lines and the
drop profile [Figure 13(d)].

54

5. The cross line was slowly rotated in order to measure it while adjusting
the cross travel of the specimen stage so that the wedge of light would
be gradually extinguished and the cross-line would attain tangency
with the drop profile at the base of the drop [Figure 13 (e)].

6. The contact angle value was then read directly from the measuring reti-
cle at the bottom of the eyepiece.

7. This process from (b) to (e) was repeated for measuring other contact
angles on the same sample. At least 10 droplets were used to measure

contact angles in this study.

For water, and formamide (having high surface tension which results in form-
ing relatively large contact angles on the film), the same contact angles were
observed after more than 2 minutes of forming the droplet. Therefore, read-
ings were generally taken within 20 ~ 30 seconds. But the liquids with low sur-
face tension, such as di-iomethane and tricrysyl phosphate, were able to
spread rapidly on the film so that contact angles were read as soon as the drop-
lets formed. They were usually spread out within 20 seconds of forming the
sessile drops. For tricrysyl phosphate on the PET films (untreated and sul-
fonated), the droplet spread too rapidly to allow a reading of the angles
formed. “Therefore, for PET films, three test liquids were used to measure con-
tact angles, and surface energy of PET films was calculated with the data from
then three contacting liquids.

6.1.3. Calculation of surface energy of solid
The measured contact angle values on the film surfaces from the four liquids
were used to calculate surface free energies, and the corresponding dispersion

and polar components, according to the method of solving the equation pro-

55

 

 

Sample specimen
\X ,

(a) Base line
Specimen stage

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

(e) (d)

 

Figure 13 Figure illustrates the procedure of the contact angle
measurement with Goniometer.

posed by Kaelblem’.

The equation [3.11] (on page 19) describing the interaction of liquids with a
solid surface was based on the geometric means of the force interactions and

the sum of the dispersive and polar terms.

p p)l/2

D D 1/2
7L(c030+1) = 2 75 '71.) +2I'yS-yL [6,1]

 

56

y? , and y: can be calculated by solving two equations which were set up by
contact angle measurements with two liquids of known surface tension val-
ues. (See ”Determinant method” on page 22 for more details) The least square
method was used to obtain the best values of the surface energy of the test

solid. (See ”The least square method ” on page 23)

Program ’SFE’ (Chapter 3.3.1(c) on page 28) was developed to make possible not
only calculating surface free energy of polymers with polar and dispersive
components, but also for providing a graphic representation of (y? )1’2 versus
(7: )“2, where the four linear relationships are plotted using one of the simul-
taneous equations given in Equation [6.1]. The procedures of Kaelble’53’, using
the determinant method, and of Kinloch et al’35’, using the least square

method, were fully detailed in the previous Chapter.

6.2. Peel adhesion test

Another purpose of the present study was to establish a correlation between
changes in surface energies of film and adhesion strength, in order to examine
the effects of surface sulfonation on polyethylene and polypropylene films.
The peel test (180° peel-test, See ”Schematic diagram of sample-tape composite for
testing peel adhesion” on page 60) is usually carried out to determine the force
required to peel a PSA tape from a film surface. The peel test for untreated film
was used as a reference. The ASTM D2578-67’37l, Adhesion Ratio of Polyethylene
film, test procedure was followed with some modification. However, the test
conditions under which adhesion of the tape to, and separation of the tape
from the surfaces were carried out, followed those specified in ASTM D2578-
67 standard procedure.

57

6.2.1. Test Apparatus and conditions

A PSA tape peel test was performed on the respective film samples by using a
computer controlled Tensile System (SFM, United calibration corporation) at
the CMSC, at Michigan State University. Since the program for a peel-adhesion
test was not set up in the computer, the program for tensile test of polymers
has been modified prior to the test. Figure 14 (on page 58) shows the schematic
of the test machine. A 500 gram load cell was used to maximize the sensitivity
of the machine during the peel test, and thus make possible the measurement

of the force with minimal fluctuations during the test.

 

 

500g load cell

 

 

 

 

°\\\‘W\\\V

 

    

[5655-33.

 

 

 

 

 

 

computer system
' \Sa 1 ta t
e- e com osi e
lo/wer jaw (see mi’igureplS) p

 

 

 

 

 

 

Figure 14 The schematic of the SFM machine, which is
controlled by the computer.

58

6.2.2. Pressure sensitive adhesive (PSA) test tape and backing tape

Three rolls of PSA tape (2 in. width), provided by Kochiom International Inc.,
(Seoul, Korea) were used as a test tape and backing tape. Contact angle deter-

minations of the surface of the PSA tape were also carried out. The results are

 

   
   
  
   
 

 

 

 

 

   

 

presented in the Table 5.
Table 5 Result of Contact angles of probe liquids on the
PSA tape
Liquid Mean 9 [degrees]
Distilled Water 96.5 i 1.5
Formamide 85.0 i 1.9
Di-iodomethane 79.9 i 2.8
Tricresly phosphate 54.6 i 1.8

 

 

 

   

The total surface free energy of the adhesive tape was then calculated, which
are 21.46 dyne/cm. Polar and dispersion components contribution to the total

surface energy was 2.8 dyne/ cm, and 18.66 dyne/ cm, respectively.

6.2.3. Test specimen preparation and scheme

The test specimens, shown in Figure 15 (on page 60), consist of strips not less
than 1.5 in. width and 6 inches in length, having their edges approximately
parallel and free of tears or creases. The greater dimension of the test specimen
was in the direction of extrusion (machine direction). The schematic diagram,
presented at Figure 15 (on page 60), illustrates the sample-tape composite with
dimensions of film sample and PSA tape used for the peel adhesion test. A test
specimen was prepared by laminating with constant force applied. A device

which consisted of a rubber roller, and spring system was made in order to

59

obtain the same conditions. Figure 16 (on page 61) is a schematically presented

diagram of this device.

 

Force applied

I

Pressure adhesive tape

/

 

 

 

 

 

 

Test film
specimen
Backing tape
Bottom grip (fixed)
Polymer film (6 X in.) Backing tape (6 x 12.5 in.)

 

Pressure adhesive tape (6 x 0.5)

 

 

 

Figure 15 Schematic diagram of sample-tape composite for
testing peel adhesion

60

 

Supporter by wood

 

Rubber Roller

 

”at:

//

I/I/
/

 

Sample film ‘1)

\
WWW

Smooth and h t wood surface

I

 

 

 

Spring

 

Figure 16 Device to apply same force for laminating of the test
film and PSA tape.

6.2.4. Peel Adhesions Test Procedure

At least three specimens of the respective film samples were tested in the
present study. Because the test requires control of the time between lamination
and peel measurement, specimens were prepared just before the peel adhesion

test. The following describes the test procedure;

1. The tape was peeled about 0.5 in. from the specimen at the doubled end
to allow a total of l in. of the specimen to remain exposed, and this por-

tion of the exposed specimen was clamped to the fixed lower jaw.

2. The other end of the tape was carefully clamped to the loose upper jaw
so not to disturb the sample.

6.2.5.

61

The machine was calibrated with a 500 gram load cell, and set at a rate
of 20 inch/ minute speed by using a computer program. Since the adhe-
sion peel test requires the force to be applied to the peel tape over the

sample, a specific program was not developed to carry out this test.

The tape was peeled from the specimen, and the data of distance versus
the force was automatically recorded, to assess the average load
required to peel the test tape from the sample specimen. The data in the
first and last 1 inch of tape was not considered in calculating the aver-

age load to avoid unexpected results.

Analysis of peel adhesion test

The average adhesion strength for each tested surface was calculated in
grams per 0.5 inch of PSA tape width by dividing the average load in
grams required to peel PSA tape (0.5 inch width) from the film surface.
The average load was determined from the graphic data, which plotted
values of force as a function of distance. An example of such a plot is
shown in the appendix #. From the plot of distance vs. force, any sharp

peaks or troughs were disregarded to calculate the average load.

An adhesion-interaction term, Apt, was also determined for each tested
sample on which tape adhesion was measured. This interaction term,
proposed by Carley et. all”), is the sum of the geometric mean of polar
and dispersion forces across the tape-polymer film interface and is

shown as follows;

Ap, = ,II 7:75 I + ,II if: I [6.2]

where subscripts s and t denote the test polymer and PSA tape, respec-

tively.

Chapter 7. Sulfonation ofPolypropylene

7.1. Introduction

Polypropylene is an extremely versatile material in the packaging industry.
The reason for its adaptability is the ease with which its polymer structure and
additive packages can be tailored to meet diverse requirements. Many useful
properties are inherent in polypropylene. It has low density (high yield), a rel-
atively high melting point, and good strength at a modest cost. However, like
most polymers, polypropylene has poor adhesion properties due to its low
surface energy. In recent experiments, surface modification of polypropylene
using sulfonation has been found to improve its adhesion properties’19l'123’ as

well as its barrier propertieslm.

In the present study, the PP film surface was sulfonated by using gaseous 503.
The level of sulfonation, as a function of reaction time, was characterized by
ESCA and by Elemental analysis. Untreated OPP film was used as a control to
evaluate the effect of sulfonation on the surface properties of OPP films as a
function of reaction time. The effects of sulfonation on the OPP films were
investigated by analyzing the surface properties of the film samples, while
included changes of the surface free energy, and peel adhesion test as a result

of sulfonation. Results and discussions are reported in the following chapters.

7.2. ESCA and Elemental Observations and Discussions

Electron Spectroscopy for Chemical Analysis (ESCA) provides a global evalua-

tion of the surface composition of the untreated OPP film sample as compared

62

63

to sulfonated OPP samples of a various sulfonation time (1 minute to 3
minute). The results obtained are summarized in Table 6, and presented graph-

ically in Figure 18.

Summarized in Table 6 are the atomic concentration for carbon, oxygen, nitro-
gen and sulfur determined for the respective film samples. The oxygen
detected in the non-sulfonated OPP film is assumed to be the result of corona
treatment“, or oxidation of the outer layer during the film-making process.
From Table 6, it can be seen that the respective atomic concentration values
approach constant levels following one minute reaction time, under the reac-
tion conditions employed. As shown in Table 7, the atomic ratios obtained for
the respective sulfonated films were in agreement with the theoretical molecu-
lar structure of the sulfonate group. For the ammonium sulfonate group (-SO3'
NHf), the atomic ratios for S/O and O/ C reported were based on corrected
values for the oxygen atomic concentration level, where the initial oxygen
atomic concentration was subtracted from the total atomic concentration to
yield a corrected value. The respective atomic ratio of O/ C, and S/ C, as a
function of reaction time, are presented graphically in Figure 18. Figure 18
shows good agreement with the stoichiometry of the ammonium sulfonate
group (_so;,NH;-) on the surface of film samples; i.e. the ratio of O/ C is almost
three times as high as that of S/ C, which is the same as the stoichiometry of

the grafted sulfonated group.

Recently, Asthanalm proposed that the surface of polymers, to include
polypropylene, could not be sulfonated beyond a limit. The author also pro—

posed that there was chain movement within the polymer at the molecular

64

level, that did not allow insertion of additional sulfonate groups after a sul-
fonation limit. For polypropylene, the sulfonation limit was reported by Ast-
hanaml to be one sulfonate group per three repeat monomer units, on average,
which is in good agreement with the results obtained in the present studies.
Asthanam] confirmed that the site of reaction in polypropylene would be at
the tertiary carbon, as described previously. From the present study, the aver-
age ratio of C/S is about 10, which means that 1 atom of sulfur is present for
every 10 atoms of carbon. Each repeating unit of polypropylene contains 3 car-
bons; therefore, on average, one sulfonate group is present for at least 3 repeat-
ing monomer units. The proposed repeat structure is presented below (Figure

17).

 

- +
— H. CH3 5°9NH4 CH3 H.‘ CH3 -—

/ / /

_\,/’\,/’\,/‘\,_

Figure 17 The molecular structure of sulfonated polypropylene

 

 

 

 

 

In addition to ESCA, elemental analysis was also per-formed on the respective
film samples to determine the total percent sulfur per gram of polymer. The
comparison of sulfur content measured by ESCA analysis and elemental anal-

ysis, as a function of sulfonation time, are summarized in Table 8 and Figure 19,

65

respectively. As shown by the results of ESCA analysis, the atomic percent sul-
fur approaches a constant value within the first minute of treatment. It should
be noted that ESCA is a surface technique which can determine the composi-
tion on the material surface within 50~6O angstroms. As shown in Figure 19,
while the atomic percent sulfur determined by ESCA seems to approach a con-
stant value, the total weight percent sulfur, as determined by elemental analy-
sis, increases in a linear fashion with sulfonation time. This is assumed to be
the result of SO3 diffusion and subsequent reaction beyond the surface and

within the film bulk phase, with extended treatment times.

 

 

 

 

 

 

 

 

Table 6 Atomic concentration for untreated and sulfonated
OPP films determined by ESCA analysis a
Sample Reaction C (%) O (%) N (%) S (‘7)
time [sec]

OPP o 93.1 6.9 o o I
OPPI 60 66.5 23.1 5.9 4.5 I
OPP1.5 90 65.5 24.3 5.2 5.1 I
OPP2 120 58 28.9 6.6 6.5
OPP3 180 60.4 27.9 5.4 6.3 I

 

 

 

 

 

 

 

 

 

 

a. Observed by Wangwiwatsilpml

 

66

 

 

 

 

 

Table 7 Relative Atomic Ratios of Sulfonated OPP filmsa
Sample Reaction

time [sec] 05 O/S N/S
OPP O - - -
OPPI 60 14.7 3.6 1.3
OPP1.5 90 12.8 3.4 1.02
OPP2 120 8.9 3.4 1.01
OPP3 180 9.5 3.4 0.86

 

 

 

 

 

a. Determined by Wangwiwatsilp

[21]

 

Table 8

The comparison of sulfur content, measured by ESCA and
Elemental Analysis, in the film samples treated at various
sulfonation time. a

 

 

 

 

 

 

 

 

 

 

 

Atomic% of Sulfur Total% Sulfur per gram
Sample Sulfonation of film sample
time [see]
ESCA Analysis Elemental Analysis

OPP O 0 O

SPPl 60 4.5 0.062
SPP1.5 1 90 5.1 0.11 I

SPP2 120 6.5 0.15 I

SPP3 180 6.4 0.24 I

 

 

 

 

a. Determined by Wangwiwatsilp

[21]

 

67

 

 

0.6 ; , , , : 0.20

 

0.15

 

0.10

Atomic ratio of O/C
Atomic ratio of SIC

0.05

 

 

 

‘ i 0.0
O 60 120 180
Sulfonated time [seconds]

 

 

 

 

Figure 18 Atomic ratio of O/C, and S/C determined by ESCA as a
function of sulfonation time (at 1% 803 concentration)

68

 

 

 

 

 

 

 

8 r 1 0.4
7 ‘ """"" s """"" r """"""""""""" r """"" a """""" I
.I a
6 '5.
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8
5 EA
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52‘s 2 a?“
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r E E"
1.. ........ J ........ E; .................... u ......... a .......... I
O I l O
O 60 120 180

Sulfonation time [seconds]

 

 

 

Figure 19 I Sulfur content measured by ESCA, and Elemental Analysis,
as a function of sulfonation time.

69

7.3. Results and discussions of contact angle measurements

7.3.1. Contact angles of probe liquids on OPP films tested

The contact angles were measured directly using a Rame-Hart Goniometer
Model 100-00 115 with liquids having strong polar properties, such as distilled
water and formamide, and with liquids having weak polar properties like di-
iodomethane, and tricresylphosphate as the test liquids. The results of the con-
tact angle measurements on the oriented polypropylene film and for film sam- f
ples sulfonated for l, 1.5, 2 and 3 minutes, respectively, are tabulated in Table 9

and presented by histogram in Figure 20.

Table 9 Contact angle obtained on tested polypropylene films using
various liquids.

 

 

 

 

 

 

 

 

- Contact Angle Measured, mean 0 [degrees]a
Sample Distilled Water Formamide Di-iodomethane Tricresyl
phosphate

OPP 78.1 i 2.3 (90.7)b 61.5 i 2.2 42.4 i 2.4 26.3 i 1.8
SPP1 31.6 i 1.3 (82.8)b 22.5 i 2.4 39.5 i 3.2 20 i 1.8
SPP1.5 17.6 i 1.8 10.1 i 1.6 34.8 i 1.6 14.2 i 1.5
SPP2 16.2 i- 2.4 (14.5)b 10.6 i 2.5 35.1 i 1.7 12.8 i 2.2
SPP3 12.5 i 1.5 8.5 :t 1.5 33.9 i 3.1 10.4: 1.5

 

 

 

 

 

 

 

 

a. Averaged value over at least 10 different measurements, performed in different positions of the sample
surface. The typical standard error was within :30.

b. Asthana measured contact angle of distilled water on the PP film.

 

Seen from the data in Table 9 and the histogram in Figure 20 (page 71) that larg-

 

70

est apparent contact angle changes of probe liquids appeared with the water
and formamide. This indicates that sulfonation resulted in an increase in sur—
face polar properties. This has been confirmed by ESCA which shows that sul-
fur and oxygen concentrations are rapidly increased on the surface within the
first minute of sulfonation time. These findings were not in agreement with
observations of contact angles of distilled water on the polypropylene film
measured by Asthanam‘. His results are presented in Table 9 (page 69) for com-
parison. The author has reported that the first minute of sulfonation did not
change the surface properties of polypropylene appreciably. But as mentioned
earlier, a cause for the discrepancy lies in the fact that the conclusions of Ast-
hana have been based on studies carried out with a different source of
polypropylene which was found to have a different content of oxygen in the
untreated PP film (See ”ESCA and Elemental Observations and Discussions” on
page 62). From this fact, it may be concluded that initially present oxygen on
the surface would be one factor in promoting sulfonation of polypropylene. It
is of importance to note that the OPP film sulfonated for 2 minutes showed

contact angle values with untreated, which agreed with those of Asthanam].

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72

7.3.2. Determination of surface free energies of films

Whereas ESCA and Elemental Analysis provide an evaluation of the surface
concentration of sulfonate groups for sulfonated OPP films, the surface free
energies of the sulfonated OPP films determined from contact angle measure-
ments, consider only the outer accessible layer of the treated surface. Surface
free energies, and their corresponding polar and dispersive components, were
calculated using the computer program SFE, in accordance with the method
described in Chapter 3.3.1(a)”The least square method” on page 23. The polarity,
defined’“ as the ratio of the polar component to the total surface energy,

{577$ , was also determined. The results of contact angle analysis are pre-
sented with the results of ESCA analysis and Elemental analysis in Table 10.
The atomic% of sulfur and total sulfur concentration of the respective films are
also summarized in Table 10 to allow comparison with the surface free energy
values of the test films. The histogram of surface free energy is presented in
Figure 21 (b) (page 75). The effect of sulfonation time on the polar, dispersion
and total surface free energy values is also shown graphically in Figure 21 (b)
(page 75), where the respective surface free energy values are plotted as a func-

tion of sulfonation time.

As shown, the increase in the values of the polar component of the surface free
energies appears to have approached maximum levels within one minute’s
exposure time for the sulfonated OPP film. Also the observed changes in the
total surface free energy can be attributed to the increased contribution of the
polar component. Further sulfonation resulted in little or no changes in the

polar and dispersion component of the film surface free energy.

 

73

Recently, Wangwiwatsilp“ observed a significant decrease in the permeabil-
ity coefficient fro ethyl acetate in OPP film following sulfonation. This is illus-
trated in Figure 22 (page 76), where the permeability coefficient for ethyl acetate
in OPP film is plotted as a function of sulfonation time. Superimposed in Fig-
ure 21 (b) is a plot of the total surface free energy as a function of sulfonation

time. As shown, as the total surface free energy approaches a constant value,

the permeability coefficient also approaches minimum level.

Table 10

 

Surface Free Energy

of PP films, Polarity, Atomic% of Sulfur by

ESCA, and Total% of Sulfur per gram of film sample for OPP
(untreated), SPPl (Sulfonated for 1 minute), SPP1.5 (Sulfonated for
1.5 min.), SPP2 (Sulfonated for 2 min.), and SPP3 (Sulfonated 3

 

 

 

 

 

 

 

 

min.)
Surface free energies of solid Total%
[dyne/cm] Atomic% Sulfur
Samples Polarity of per gram
Dispersive Polar Sulfur“) of film
Component Component Total sample“)
OPP 32.61 5.11 37.72 0.14 0 0
SPP 1 22.69 35.94 58.63 0.61 4.5 0.062
II SPP 1.5 22.75 41.22 63.96 0.64 5.1 0.11
SPP 2 22.66 41.67 64.33 0.65 6.5 0.15
SPP 3 22.86 42.22 65.08 0.65 6.4 0.24

 

 

 

 

 

 

 

 

 

 

 

a Observed by K. Wangwiwatsilpml

Wittenbeck‘6| observed, that for plasma treatments on PP surfaces, the func-
tional group introduced on the surface of PP during treatments has consider-
able mobility. He proposed that mobility of the functional groups allowed
them to rotate into the interior of the OPP. Occhillo‘sl also proposed that

74

plasma treated surface of PP possessed the tendency to minimize its interfacial

energy by macromolecular movement into the polymer bulk phase.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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70

Surface Energy of Polymer Films [dyne / cm]

 

 

 

- Y — ‘

 

 

 

 

 

 

 

 

 

 

 

 

 

Sulfonation Time [second]

 

Figure 21 (b) Variation of surface free energies, corresponding to the

dispersive and polar force components for untreated and
sulfonated PP films as function of sulfonation time. Note
the visible changes in polar force component of surface
free energies of solid appeared within the one minute of
sulfonation time.

 

76

 

Permeability Coefficient, P, x 1018 “"
[Kg-m / mZ-s-Pa]

 

 

 

 

   

 

 

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0 60 120 180

Sulfonating time [sec.]

 

(a) Shade line shows the deviation errors.

Figure 22

Relationship between Permeability Coefficient of Ethyl Acetate
in Polypropylene films and Total Surface Free Energy as a
function of sulfonation time. Note the apparent changes
occurred in the 90 sec., while the surface free energy changed
within one minute of sulfonating time.

 

 

 

77

7.4. Results and discussions of peel adhesion test

The results of ESCA and contact angle analysis of various liquids on the
respective film sample clearly indicate that surface treatment using sulfona-
tion on the OPP film changes the surface chemistry, resulting in an increase of
surface polarity, and of polar components contributing to the surface energies
of the respective film samples. The changes of surface chemistry and surface

energies due to the sulfonation have been discussed in the previous chapter.

Table 11 Peel adhesion strength for sulfonated film samples with the data
of surface free energies

 

 

 

 

 

 

 

 

 

 

 

 

 

Surface free energies of solid Peel Adhesion Adhesion-
[Dyne/cm] Strength interaction
term,
Dispersive Polar [Gram/0.5 inch] Apt
Component Component
18.66 2.8 -
32.61 5.11 6336:2102 28.45
22.69 35.94 165.97i7.4 30.61
22.75 41.22 178441-1174 31.35
22.66 41.67 184.12i6.7 31.36
22.86 42.22 184.11_+_11 31.2
l_—.___._J.——_.——

 

The peel adhesion test was then carried out to determine whether changes in
surface chemistry and surface free energy would result in increased adhesion
strength of the surface treated OPP films. Peel adhesion strength values
reported in the present study are the average of the three replicates for each

respective film sample. The results are summarized in Table 11. Also summa-

 

78

rized Table 11 are the corresponding surface free energy and adhesion-interac-
tion values. The results are also presented graphically in Figure 23 (on page 79),
where the peel adhesion strength as a function of sulfonation time is plotted.
The trend in the observed increase in the adhesion strength with increased sul-
fonation time follows the increase in polar components contribution to the
total surface energies of film, as shown in the insert box in Figure 23. This sug-
gest that, up to a certain level of surface treatment, the adhesion strength will
increase at a high rate, due to the increase in the polar components of the sur-

face free energy of the respective film samples.

The adhesion-interaction terms, Apt, are calculated by using equation [6.2] (on
page 61) to correlate the surface energy changes due to the sulfonation and
adhesion properties. The relationship of the peel adhesion strength to the
adhesion-interaction term is nearly linear in the range studied, as shown in
Figure 24 (page 80). These results indicate that the increased surface free energy
following sulfonation treatment is primarily responsible for the increase in the

adhesion strength between the treated film surface and the applied PSA tape.

78

rized Table 11 are the corresponding surface free energy and adhesion-interac-
tion values. The results are also presented graphically in Figure 23 (on page 79),
where the peel adhesion strength as a function of sulfonation time is plotted.
The trend in the observed increase in the adhesion strength with increased sul-
fonation time follows the increase in polar components contribution to the
total surface energies of film, as shown in the insert box in Figure 23. This sug-
gest that, up to a certain level of surface treatment, the adhesion strength will
increase at a high rate, due to the increase in the polar components of the sur-

face free energy of the respective film samples.

The adhesion-interaction terms, Apt, are calculated by using equation [6.2] (on
page 61) to correlate the surface energy changes due to the sulfonation and
adhesion properties. The relationship of the peel adhesion strength to the
adhesion-interaction term is nearly linear in the range studied, as shown in
Figure 24 (page 80). These results indicate that the increased surface free energy
following sulfonation treatment is primarily responsible for the increase in the

adhesion strength between the treated film surface and the applied PSA tape.

 

79

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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b ' ‘5 :
. 00 A 510 A 130 ‘ 1so A 200
' _’ E Sulfonation Time [second]
I E . r ; ,
50 l j i - i .
~ 0 1 2 3
Sulfonation Time [minutes]
Figure 23 Peel adhesion strength of OPP films tested as a function of

sulfonation time. In insert box, surface energies of the
respective film samples vs. sulfonation time are presented
for comparison with the increasing trend of the peel

 

80

 

 

200 . I . u . u

150

100

Peel Adhesion Strength, [Gram/0.5 inch]

 

 

 

 

28 29 30 31 32

Jiv‘s’vfldiflj

 

 

 

Figure 24 The relationship of peel adhesion strength vs. adhesion-
interaction term.

81

7.5. Conclusions

The above results show that sulfonation of PP film surface is a suitable method
to progressively increase the polar components of the surface free energy,
while results in an increase in the adhesion strength of the film surface. A
treatment time of 1 minute, for the given sulfonating conditions and parame-
ters, results in a reproducible surface state, exhibiting dramatically increased
values for the polar components of the surface energy and the polarity of the
surface. This increase in surface free energy is due to the insertion of polar, sul-
fonic acid groups, on the film surface. However, further reaction time with
gaseous 503, though effective in enhancing the. barrier properties of the film
by interdiffusion of the sulfonic group through the bulk phasem‘, results in
minimal changes in surface properties, but lead to color changes from light

brown to dark brown.

Adhesion depends fundamentally on forces of attraction across an interface,
which is directly related to the surface energy properties of sample film.
Results of peel adhesion studies show that surface modification of OPP films,
using sulfonation is undoubtedly an effective method. Comparing polar and
dispersion components at the interface between the tape and the treated poly-
mer surface, and peel adhesion strength, a good correlation was found. This is
mostly attributed to the increase in the polar component of the surface free
energy, which results in an increase in the adhesion strength between the

treated film surface and the applied PSA tape.

 

Chapter 8. Sulfonation ofPolyethylene

8.1. Introduction

Polyethylene (PE) film is known to be readily sulfonated by reaction with gas-
eous $03, with fuming sulfuric acid, or with 803 in chlorinated hydrocarbons,
resulting in changes in its surface properties, and increasing commercial appli-
cations and uses. Ihatam’ evaluated the structure of the PE film sulfonated by
using gaseous 503 and found that sulfonation resulted in the formation of
alkanesulfonic acid, C-SO3H groups, with highly conjugated C=C unsaturated
bonds on the PE surface. Fonseca et. al. also ’1“ observed that fuming sulfuric
acid etching of PE film results in the formation of sulfonic acid groups in the

polyethylene chain.

Among the packaging applications of PE film, a main consideration is the
enhancement of its adhesion properties. For the present study, ESCA analysis
was carried out to determine the extent of PE sulfonation under the experi-
mental conditions used. Thus, the aim of the present study was to determine
the effect of sulfonation on the adhesion properties of HDPE film by means of
estimating the surface energies of the film, since the adhesion properties are
directly related to changes in the films surface energy values. Specifically, peel
adhesion strength values have been determined to correlate the changes in
surface free energies of sulfonated polyethylene film with film adhesion prop-

erties.

82

83

8.2. Sulfonation degree determined by the ESCA

The surface composition of an untreated HDPE film, corona treated HDPE
film, and sulfonated HDPE samples were determined by the ESCA technique.
The results are summarized in Table 12, and presented graphically in Figure 25,
where the increase of the atomic percent of sulfur on the surface of sample
films as a function of reaction time is plotted. The extent of sulfonation for
each film given in Table 12 was estimated by an S/ C value based on the results
of ESCA analysis, although degree of the sulfonation on the surface layer may

be different from that of the inner layer.

All the experiments in the present study were run with a constant 803 gas con-
centration of approximately 1% at room temperature. The S/ C ratio value of
0.3% for HDPE film sulfonated for 1.5 minutes suggests that the sulfonation
conditions in the present test were not adequate to graft sufficient level of sul-
fonic groups onto the HDPE film surface. Little or no changes in the sulfur
content of PE film surface following extended reaction times, under our sul-
fonation condition, makes it evident that reaction time is not the only variable
for introducing the sulfonic groups onto the surface of PE film. For example,
Ihata’25’ observed progressively increased sulfur content from 1.26% to 11.55%
in the PE film after 1 minute, and 5 minute treatment, respectively, and the

color changes of samples from pale green to dark brown.

Table 12

 

84

Surface composition of HDPE film samples before and after
sulfonation treatments at a various exposure time over 503 gas,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

determined by ESCAa
Percentage Atomic Concentration
Ammk
Reaction time C (%) O (%) S (%) ration
[sec] (SIC x 102)
0 98.51 1.49 0 O
90 98.23 1.47 0.3 0.3
180 95.3 4.09 0.61 0.64
300 92.78 6.59 0.63 0.69
- 91.3 8.7 O -
a. UPE:Untreated polyethylene film
PE1.5:Sulfonated PE film for 90 seconds
PE3:Sulfonated PE film for 180 seconds
PES: Sulfonated PE film for 300 seconds
PEC: Corona treated PE film
0.8 . a.
g: ..........................................................
g; g. 0.6 ............................ - .......................... .
“a g t ---------- ----------- . -------------------- , ---------------------
' E 0.4 —-~------' ------------------------------------------------------ -
a 5 ..................................................................
3 a o 2 p ---------------------------------------------------------------- q
< V ' _______
0'0 160 260 300
Sulfonation time [seconds]
Figure 25 Atomic% of sulfur measured by ESCA as a

function of sulfonation time.

 

 

85

8.3. Results and discussions of contact angle measurements

8.3.1. Contact angles of probe liquids on PE films tested

The contact angle measurements were made on the untreated, sulfonated and

corona treated HDPE films with liquids having strong polar properties, such

as distilled water, and formamide, and with liquids having weak polar proper-

ties like di—iodomethane, and tricresyl phosphate as the probe liquids. The

results are summarized in Table 13 and a histogram of the contact angles mea-

sured on the HDPE film samples is given in Figure 26.

 

 

 

 

 

 

Table 13 Contact angle obtained on sulfonated polypropylene using various
liquids.
Contact Angle Measured, mean 9 [degrees]3
Sample Distilled Water Formamide Di-iodomethane Tricresyl
phosphate
UPE 90.6 2.6 69.6 2.4 48.4 2.8 31.9 2.4
PE1.5 85.2 2.0 69. 2.29 45.7 1.8 32.3 1.9
PE3 80.4 2.9 67.4 1.7 43.2 2.2 35.7 1.9
PBS 83.7 2.1 67.1 1.8 41.1 2.2 31.3 2.0
PEC 64.7 2.6 46.3 1.9 34.5 2.1 10.7 2.0

 

 

 

 

 

a. Averaged value over at least 10 different measurements. performed in different

positions of the sample surface. The typical standard error was within i3°.

 

As shown, no significant changes of the contact angle for the liquids used were

observed for the HDPE film following sulfonation. This implies that the sul-

fonic groups are rarely introduced onto the surface of HDPE, under the

 

 

 

86

present sulfonation conditions.

 

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87

8.3.2. Determination of surface free energies of films

The surface free energies, and corresponding polar and dispersive compo-
nents, were calculated using the computer program SFE in accordance with
the method described in Chapter 3.3.1(a)”The least square method” on page 23.
The surface free energy values obtained from the contact angle of various liq-
uids are summarized in Table 14. The polarity, atomic% of sulfur for the sul-
fonated film sample and atomic% of oxygen for the corona treated film sample
are also presented in Table 14. The variation of surface energy, dispersive and
polar energy components, for untreated, sulfonated HDPE films as a function

of reaction time, and corona treated HDPE film is shown in Figure 27.

Table 14 Surface Free Energy of PE films, Polarity, and Atomic% of Sulfur

by ESCA for PEU (untreated), PE1.5 (Sulfonated for 1.5 minute),
PE3 (Sulfonated for 3 min.), PE5 (Sulfonated for5 min.), and PEC
(Corona treated)

 

 

 

_ — J

 

 

 

 

 

 

 

 

 

 

l: - __ i = i
Surface free energies of solid
[dyne/cm]
Polarity Atomic%
Sample film Dispersive Polar Total of Sulfur
Component Component

PEU 33.69 1.58 35.27 0.045 0 (1.49)a
PE1.5 32.97 2.43 35.4 0.069 0.3 (1.47)
PE3 31.48 3.96 35.44 0.112 0.61(4.09)
PE5 32.7 3.78 36.5 0.104 0.63 (6.59)

PEC 32.37 11.7 44.07 0.265 . 0 (8.7)

 

 

 

 

 

 

F

 

a. The atomic% of oxygen is presented to compare the with result of the corona
treated HDPE film samples.

 

88

Table 14 and a graphical presentation (Figure 27) show that sulfonation of
HDPE film under the experimental conditions used is much less efficient than
corona treatment in modifying the surface free energy of the HDPE film. No
significant changes of surface energy, of the corresponding polar and disper-
sion components, were found between untreated and sulfonated HDPE film
samples. This is due to the limited sulfonic acid functional group content on
the film surface achieved, resulting in no increase in surface polarity following

sulfonation.

Apparent changes in the surface free energy of the PEC film were the result of
an increase in the polar components of the surface free energy, due to the intro-
duction of oxygen onto the film surface as a result of corona treatment. From
this result, it is apparent that increased polarity by introduction of functional
groups contributes to the surface energy and consequently enhances adhesion

strength.

 

89

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

50 I ‘ I ‘ fl I ‘ I r I I
5- 5:3
“é 4° ' """""""" >3:
3 '9‘.
w 7 Ir:
5 T v. . / .9;
D O D Q
E 30 -- - go: °°°°°°°°°°°°°°°° ¢ ' .3; '
0 t... / 5‘- '
E. m / V-
8 t: / to:
9: > .; % »:.;
o 20 - - 4.0. --------------------- , / a...
a :e / to:
3 6'1 % ’z’: t
m 'o’. »/ 'o‘. ' '
8 s: / to:
.. mm a. -------------------- ./ .9.-
:s ’o’. / 5‘.
D O D O
”‘ e a to-
r 02 / r 02
0 ’. 4 I l 4\ /t. 5.?
Dispersive Polar force Total
Component Component Surface Energy
PEU: Untreated PE =
PBS 1.5: Sulfonated for 1.5 min. =
PBS 3: Sulfonated for 3 min. m
PBS 5: Sulfonated for 5 min. m
PEC: Corona treated m
Figure 27 The effect of sulfonation time on the change in total surface

free energy and the respective energy components for
untreated, sulfonated HDPE films as a function of reaction
time, and corona treated HDPE film.

90

8.4. Results and discussions of the peel adhesion test

The peel adhesion test results for the sulfonated and corona treated HDPE film

samples are shown in Table 15 with surface free energies, and graphically pre-

sented in the Figure 27 (a) (page 91), where peel adhesion strength as a function

of sulfonation time is plotted. In Figure 27 (b) (page 92), the histogram of the

peel adhesion strength of untreated, sulfonated HDPE film samples, and

corona treated HDPE film sample is presented to evaluate the effect of sulfona-

 

 

 

 

 

 

 

tion on the HDPE film.

Table 15 Peel adhesion strength for sulfonated film samples with the
result of surface free energies in polar dispersion and polar
components

Surface free energies of solid Peel Adhesion
[Dyne/cm] Strength “
Sample Dispersion Polar [Gram/0.5 inch]
Component Component
PEU 33.69 1.58 15.08 i 1.94
PE1.5 32.97 2.43 43.86 i 3.21
PE3 31.48 3.96 43.5 i 2.97
PE5 32.7 3.78 44.9 i 1.23 I
PEC 32.7 11.7 107.54 i 6.87

 

 

 

 

 

 

a. Averaged value over three test results of each respective film samples.

 

As expected from the results of the surface free energy values determined, an

improvement of peel adhesion strength was found between untreated and sul-

91

fonated film samples. However, the peel adhesion strength for the sulfonated
PE films was approximately 50% lower than that of the corona treated film.
This suggests that the peel adhesion strength is directly associated with the
polar component of the surface free energy, and implies that an increased
polar contributions to the surface energy of the polymer film results in an
enhanced peel adhesion strength between treated film surfaces and applied
PSA tape.

 

 

 

 

 

Peel Adhesion Strength, [Gram/0.5 inch]

 

 

 

O 100 200 300

Sulfonation Time [seconds]

 

 

 

Figure 27 (a) Peel adhesion strength of HDPE film samples as a
function of sulfonation time.

 

 

 

120

—

c

- 100

“2

o

\

E

a 80

H

4:

on

t»

I:

g 60

in

c

.2

in

o 40

11

i 20
o

 

 

 

 

 

 

 

 

 

 

 

l I l ‘
l
J 0000000000000000000000000000
4
. ......................................
p
------------------------------- I: ~---o--a
E
C
t-‘I
,___.__ g
t ................... g ....... -
In
........... -' 8 mm.
8 it!
a
a 4
III
a

 

 

 

Sulfonated PE

 

Figure 27 (b) Histogram of peel adhesion strength of untreated,
sulfonated HDPE film samples and corona treated
HDPE film samples.

 

93

8.5. Conclusion

The above results show that sulfonation of polyethylene film under the test
conditions employed does not provide an alternative method to increase the
surface free energy and the surface properties, such as the peel adhesion
strength. For HDPE film under the treatment conditions used, a limited num-
ber of sulfonic acid groups were substituted onto the film surface, as a result of

the sulfonation of reaction.

an:

 

‘WI 1

Chapter 9. Sulfonation of Polyethylene terephthalate (PET)
film

9.1. Introduction

In the present study, the sulfonation of polyethylene terephthalate (PET) was
investigate to determine the effect of sulfonation treatment on the surface free
energy of the polymer. Surface characteristics were based on contact angle
analysis and ESCA analysis. ESCA analysis of the PET surface suggests that

sulfonation was minimal on the PET film surface under the conditions used.

9.2. Results and Discussions

The atomic composition of the polymer surface of the untreated and sul-
fonated PET film samples was determined by ESCA analysis. The results are
summarized in Table 16. The atomic composition of the PET surface is 69.6%
and 30.4% for carbon and oxygen, respectively. For PET film samples subjected
to sulfonation treatment, no increase in the sulfur concentration on the surface
of the sulfonated polymer samples was observed with increased treatment
time. This implies that only a limited number of sulfonic groups can be grafted
onto the film surface. For PET, the site for the substitution of sulfonic groups
would be on the aromatic ring positionlz” Since the aromatic ring is already

stable, the hydrogen extraction is not easily achieved during treatment of the

polymer.

94

 

95

Table 16 aSurface composition of PET film samples before and after
sulfonation treatments at a various exposure time over 803 gas,

 

 

 

 

 

 

 

 

 

 

 

 

determined by ESCA
Percentage Atomic Concentration
Sample Reaction time C (%) O (%) N (%) S (%)
[minutes]
PET 0 69.6 30.4 0 0
PET3 3 78.1 21.1 0 0.4
PETS 5 70.2 29.4 0 0.4
T _____J

 

_—

 

a. PET: Untreated polyethylene terephthalate film
PET]: Sulfonated polyethylene terephthalate film for 1 minute
PET3: Sulfonated polyethylene terephthalate film for 3 minutes

 

Contact angles of liquids (distilled water, formamide, and di-iodomethane)
were measured on the untreated PET samples and film samples sulfonated for
1 minute and 3 minutes, respectively. The values obtained for the correspond-
ing samples are summarized in Table 17, and presented by a histogram in Fig-
ure 28. Table 17 shows that most of the contact angle changes following the
sulfonation of PET film occurred with distilled water, and formamide (both
have high polar properties). This suggests that sulfonation of the PET film is
mostly associated with changes in the polar properties of the polymer surface.
Different reaction timed, under the conditions used, had little effect on the sur-

face properties. This finding was in agreement with the ESCA analysis.

 

96

Table 17 Contact angle of liquids on the untreated PET film and
sulfonated PET samples for 1 min. and 3 min. reaction
time.

    

 

 

 

 

Contact Angle Measured, mean 9 [degrees]a
Distilled Water Formamide Di-iodomethane
66.9 1.5 50.6 2.4 20.9 2.4
55.0 1.9 30.1 2.1 18.7 2.4
56.6 2.7 24.9 2.0 24.8 2.6

 

 

 

 

 

 

 

 

a. Averaged value over at least 10 different measurements, performed in different positions of
the sample surface. The typical standard error was within :30.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.0 00
» PETU 1:1 "'5 300
0.8 _, PETl. [:3
PET3.
a: 0.6 --------------- , ................ 0
° 60°
K.)
0.4 ~ ---------
0.2 t -----
l
0.0 900
Distilled Formamide Di- ido
Water Methane

 

Figure 28 Contact angle change of liquids for sulfonated
PET samples with increasing time of sulfonation.
Based on the untreated PET film as reference.

97

The surface energy, 75, of a solid is the sum of the dispersion component, 7: ,
and the polar component, 34’. These values are obtained by measuring the con-
tact angle of the strong polar liquid (distilled water, formamide,), and weak
polar liquids (di-iodomethane). Similar surface free energy values have been
reported by Cueff, et. al.1541 for PET films, 40 dyne / cm and 3 dyne/ cm respec-
tively, for the dispersion component and the polar component (typically from

the 39 to 47 dyne / cm for total surface energy of PET film).

The results given in Table 18 show the surface free energy and the correspond-
ing dispersion and polar components. For PET samples with different reaction
times of treatment, the atomic% of sulfur on the sample surface from ESCA
analysis and film polarity values are summarized in Table 18. The effect of sul-
fonation time on the change in total surface free energy, and the respective free

energy components, of PET samples is shown in Figure 29. From Table 18, and

the graphical analysis of the surface energy parameters, the surface treatment
of PET film by sulfonation did not change, significantly, the dispersion compo-
nent of the surface free energy of the PET. On the other hand, sulfonation
appeared to the increase the polar component contribution. The content of sul-
fur (i.e. sulfonic acid groups) on the surface following sulfonation seems to be

the predominant factor in changing surface properties of PET film.

98

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  
 
  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 18 Surface Free Energy of PET films, Polarity, and Atomic% of Sulfur
by ESCA for PETU (untreated), PET1 (Sulfonated for 1.5 minute),
PET3 (Sulfonated for 3 min.)
Surface free energies of solid
[dyne/cm]
Polarity Atomic%
Sample film Dispersive Polar Total of Sulfur
Component Component
PETU 40.85 6.94 47.79 0.15 0
PET1 38.84 14.83 53.67 0.28 0.4
PET3 37.38 15.86 53.24 0.3 0.4
__ 60 . .
é PETU: Untreated
5. 50 _.-. PETS1:Sulfonated for l min.
5 PETS3: Sulfonated for 3 min.
8 _
E 40 ~ --------
E
g 30 - --------
a?
"5
Q 20 - --------
2
m
8
a 10
0
g L
(2 0 ............
Dispersive energy Polar energy Solid (Total)
Component Component Surface Energy
Figure 29 Variation of solid surface energy corresponding

dispersion and polar components for untreated and
sulfonated PET samples.

 

Chapter 10. Conclusions

1. Surface sulfonation of OPP film, using gaseous $03, was found to be a
very effective method of enhancing the surface free energy of the film,
which results in an increase in peel adhesion strength. For polypropy-
lene, the tertiary carbons in the molecule were found to be the site for

the insertion of sulfonic acid groups onto the polymer backbone.

2. It was found that surface sulfonation exhibited varying levels of effec-
tiveness for polymers studied, depending on their molecular structure.

The reason for this behavior can be attributed to the steric hinderance to

 

substitution of sulfonic acid groups onto the polymer backbone. As
large functional groups such as the sulfonic acid group are'introduced
onto the polymer surface beyond a limit, the spatial restrictions will not
allow the substitution of additional sulfonic acid group. Further, the
number of active sites for substitution are limited.

3. An additional reason for a limited sulfonation can be attributed to the
repulsion of adjacent sulfonic acid groups. The inserted sulfonic acid
group, which is considered to exhibit a negative charge, may require a
distance limit for the insertion of additional sulfonic acid groups onto
the polymer surface. For polypropylene the limit was reached in about 1
minute of reaction time, and was equivalent to one sulfonic acid group
per three repeating monomer units.

4. Sulfonation of OPP film increased by almost one order of magnitude of
the polar component contribution to the surface free energy within the
first minute of reaction time, as compared to the untreated OPP film.

99

100

Further sulfonation of OPP film showed little or no changes on the sur-
face properties, as a result of the spacial restrictions for additional sul-

fonic acid group insertion to tertiary carbon bonds.

Sulfonating OPP film significantly increased the films peel adhesion
strength, as compared to the untreated OPP film.

Surface free energy and adhesion strength results indicated that sul-
fonation of OPP film is a suitable and alternative method to other tech-
niques currently employed to modify the surface properties of polymer
fihn.

Under the sulfonation conditions employed, ESCA showed that sul-
fonation of was ineffective in modifying the PE film surface properties,
of PE film. Sulfonation of PE film was ineffective in increasing the sur-
face free energy and peel adhesion strength of treated film, as compared
to the untreated PE film.

Sulfonation was found to be less effective than corona treatment in
increasing the surface free energy and peel adhesion strength of PE. The
polar component of the surface free energy of the corona treated PE film
was about three times the observed values for the sulfonated PE film.
The peel adhesion strength of the corona treated PE film was twice that
of the sulfonated PE film.

Sulfonation of PET was found to have little or no effect on the surface
free energy of treated PET film. This can be attributed to spatial restric- '
tions for sulfonic acid group substitution into the ortho positions of the

terephthalate group.

Chapter 11. Possible Future Studies

11.1. Sulfonation method

In this study, sulfonating HDPE and PET film was not successful in signifi-
cantly changing the surface properties of these film. It would be interesting to
investigate different sulfonating method on those films to increase the extent
of sulfonic acid group substitution onto the polymer surface. Under the same
conditions, the sulfonating LDPE and/ or LLDPE would be useful to investi-

gate the problem of the sulfonating conditions on HDPE film.

11.2. Surface free energy

In performing contact angle analyses, different types of testing liquids can be
selected for determining the effect of surface sulfonation of the surface free
energy of the treated film. It may be considered important to evaluate related
variables of the test procedure, such as temperature, since the viscosity of the

is the function of temperature.

Furthermore the application of a related method, such as the ICC analysis,
would be useful to investigate the effect of the surface sulfonation on the sur-

face properties of treated film.

11.3. Ink printability

In this present study, sulfonating OPP film increased the surface free energy

and peel adhesion strength. It would be useful to study the effect of sulfona-

tion on ink printability. It Oxidation following sulfonation may be possible to
101

 

102

investigate the aging effect on the surface free energy and ink printability that

may be changing due to oxidation following sulfonation.

 

[1]

[2]

[3]

[4]

[5]

[6]

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MICHIGAN STRT

 

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