AN ABSTRACT OF THE THESIS OF

Aarushi Srivastava for the degree of Master of Science in Chemical Engineering

presented on March 16, 2017.

Title: Investigating the Effect of Tetrahydrofuran (THF) Plasticizer on the

Crystallization and Solvent Seaming of PET/NPG Copolymer Films

Abstract approved:

Travis W. Walker

Polyethylene terephthalate (PET) polymer has been the material of choice for packaging

industry as a result of its excellent mechanical and physical properties. These proper-

ties are highly dependent on the microstructure, chemistry, and processing conditions

of this material. PET is often oriented in the machine direction (MD), the transverse

direction (TD) or both MD and TD, depending upon the resultant properties required.

The orientation process causes polymer chain alignment and hence changes the original

crystallinity, thermal properties, and lattice structure. In this work, differential scan-

ning calorimetry (DSC) and X-ray diffraction (XRD) techniques were utilized to gain

microstructure knowledge about the orientation of PET.

PET is often copolymerized with different diacids or diols to produce desired film prop-

erties such as thermoforming, heat sealing, heat shrinking, solvent seaming, etc. In this

work, PET copolymerized with neopentyl glycol (NPG) was investigated. These films can

shrink about 75% in TD on appropriate heat treatment. Their ability to solvent seam

with tetrahydofuran (THF) solvent make them an excellent material for shrink-sleeve

bottle-label applications. The solvent induced crystallization of these copolymer films

was also studied at different exposure times varying from 0 min to 40 min. Tensile tests

of seals made from solvent exposed film samples indicated that increased crystallinity in

the film significantly decreases the solvent seaming ability. Hence, the hypothesis was

proposed that amorphous material provides better surface for solvent adsorption, which

results into stronger solvent seams and better printability.

c©Copyright by Aarushi SrivastavaMarch 16, 2017

All Rights Reserved

Investigating the Effect of Tetrahydrofuran (THF) Plasticizer onthe Crystallization and Solvent Seaming of PET/NPG Copolymer

Films

by

Aarushi Srivastava

A THESIS

submitted to

Oregon State University

in partial fulfillment of

the requirements for the

degree of

Master of Science

Presented March 16, 2017

Commencement June 2017

Master of Science thesis of Aarushi Srivastava presented on March 16, 2017.

APPROVED:

Major Professor, representing Chemical Engineering

Head of the School of Chemical, Biological and Environmental Engineering

Dean of the Graduate School

I understand that my thesis will become part of the permanent collection of OregonState University libraries. My signature below authorizes release of my thesis to anyreader upon request.

Aarushi Srivastava, Author

ACKNOWLEDGEMENTS

I would like to acknowledge my advisor Dr. Travis Walker for his constant guidance

through this industrial work thesis. He has been immensely supportive of my ideas all

through this work. I would also like to express my gratitude to Dr. Skip Rochefort for

numerous useful discussions for my thesis. This thesis would not have been possible

without their advice. My sincere thanks to the Walker research group members for their

assistance during my experimental work.

I would also like to thank Dr. Kieth Levien and Dr. Wade Marcum for serving on my

committee. I would like to express my sincere gratitude for Dr. Goran Jovanovic for

helping me with thesis format and numerous useful suggestions. Also, I would like to

thank Dr. Arijit Sinha for his help during tensile strength testing.

I would like to express my utmost gratitude to Dr. Steven Sargeant for giving me the

opportunity to learn and grow at Flex Films USA Inc., KY. The R&D team at Flex

Films assisted me at every point of this research. I would like to thank K.K. Sharma,

David Grob, Stella Dullaney, and everyone else for their help and support. I am grateful

to Western Kentucky University thermal laboratory and Dr. Houyin Zhao for assistance

in XRD data collection.

Most importantly I would like to dedicate this work to my parents and brother for

always being there and showering unconditional love and support. I would also like to

acknowledge my friends Revathy Narasimhan, Ritesh Sharma, Robin Garg and Ankita

Pujar for making my stay in Corvallis wonderful. My sincere heartfelt appreciation would

go out for my friends back home, Nikita Mathur, Kamini Pandey, Nitin Khurana, Nikita

Jain, and, Siddharth Chatterjee for their encouragement and unwavering support.

TABLE OF CONTENTSPage

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 PET Polymer 4

2.1 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Glass Transition Temperature and Free Volume . . . . . . . . . . . . . . . . 6

2.3 Biaxially Oriented Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Materials and Methods 12

3.1 Tenter Method for BoPET . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2 Heat-Shrink Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2.1 Shrink Sleeve Conversion Process . . . . . . . . . . . . . . . . . . 16

3.3 Crystallinity Measurement Techniques . . . . . . . . . . . . . . . . . . . . . 18

3.3.1 Differential Scanning Calorimetry (DSC) . . . . . . . . . . . . . . 19

3.3.2 X-ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.4 Tensile Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.5 Solvent Dispersion Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Experimental 26

4.1 Microstructure Analysis of PET Film Sequential Stretching . . . . . . . . . 26

4.2 Effect of NPG Copolyester on Thermal Properties of PET . . . . . . . . . . 27

4.3 Solvent-induced Crystallinity of PET/NPG Copolymer Shrink Films with

THF as a Function of Exposure Time . . . . . . . . . . . . . . . . . . . . . 28

4.4 Solvent Seam Strength Testing of Different THF Exposure on Shrink Films 29

5 Results and Discussions 31

5.1 Oriented PET Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.2 PET/NPG Copolymerized Films Analysis . . . . . . . . . . . . . . . . . . . 35

5.3 THF Plasticizer Exposure to PET/NPG Shrink Films . . . . . . . . . . . . 37

5.4 Tensile Testing Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6 Conclusions 45

TABLE OF CONTENTS (Continued)Page

Bibliography 45

LIST OF FIGURESFigure Page

1.1 Crystallization half-time curve of amorphous PET versus temperature.

The favorable modification temperature for PET is between Tm and Tg [3]. 2

2.1 Polycondensation reaction of PTA monomer with EG monomer to form

PET polymer. The polymer chains grow by reactions that occur between

these two monomer species; both monomers are bifunctional [7]. . . . . . 4

2.2 Polymer chain morphology before (left), during (center), and after orien-

tation (right). Uniaxial stretching aligns polymer chains in one direction

and biaxially stretching aligns polymer chains in two directions. Higher

order crystallization is obtained after biaxial stretching [7]. . . . . . . . . 5

2.3 Two different rotational conformations of PET: trans (extended form;

left) and gauche (relaxed form; right). With the rotation of the C-C bond

(-CH2-CH2-), the conformation changes from one form to the other. A

phenylene ring is also marked in the figure [3]. . . . . . . . . . . . . . . . . 6

2.4 Specific volume versus temperature for crystalline polymers (left) and

amorphous polymers (right). Amorphous polymers do not exhibit any

transition on melting [12]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.5 PET sequential orientation process flow diagram. Different zones are also

marked in the figure. Zone I is the extrusion zone, zone II is the machine

direction stretching zone, zone III is the transverse direction stretching

zone, and zone IV is the winding and finishing zone [7]. . . . . . . . . . . 9

2.6 Image of machine direction operation at Flex Films. The rollers are heated

to just below the glass transition temperature, Tg, of PET, and they are

rotating at about 100-400 rpm [7]. . . . . . . . . . . . . . . . . . . . . . . 10

3.1 Image of transverse direction operations at Flex Films. A long heating

oven is set and maintained at different processing temperatures [7]. . . . . 13

3.2 Commercially available examples of shrink-sleeve bottle-labels. Full color

and 360◦ design coverage has reported increase in sales of products by

upto 20% [1]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

LIST OF FIGURES (Continued)Figure Page

3.3 Percent shrinkage at different temperatures for PET/NPG shrink films in

MD and TD directions. Shrink films are capable of shrinking in the TD

direction to upto 75%. MD and TD direction of sleeve label on bottle is

also shown (right) [7]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.4 the figure shows neopentyl glycol (NPG) molecule. NPG is a constituent

of shrink films [13]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.5 Co-monomers of PET/NPG shrink films. The ratio of TPA:EG:NPG is

5:4:1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.6 Flow diagram representing the shrink sleeves conversion process. The

main steps are printing the sleeve, creating the sleeve, and, applying the

sleeve [8]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.7 TA Instruments Q2000 DSC equipment at Oregon State University Poly-

mers Lab. The nitrogen supply is attached, and the flow rate is maintained

at 50 ml/min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.8 Image shows Instron Universal Tensile tester located at Wood Science &

Engineering department of Oregon State University. The figure shows 1

kN load cell, grips at 25 mm separation and test setup. . . . . . . . . . . . 23

3.9 Solvent dispersion device used for preparing test specimen for seam strength

testing. The syringe and motion controlled devices are illustrated. . . . . 25

4.1 Depression in glass transition temperature caused by acetone solvent treat-

ment time in PET film of thickness 0.23 mm can be seen in DSC thermo-

grams heating curves [5]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.1 DSC heating thermograms for PET films A, B, C, and D. Glass transition

temperature, crystallization temperature, and melting temperature can be

observed. This heating run was conducted from 0-300◦C, with 10◦C/min

heat rate in nitrogen atmosphere. . . . . . . . . . . . . . . . . . . . . . . . 32

5.2 The plot shows DSC cooling thermogram for PET films A, B, C, and D.

Melt crystallization temperature can be observed here. This cooling run

was conducted from 300-70◦C, with 20◦C/min cooling rate in nitrogen

atmosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

LIST OF FIGURES (Continued)Figure Page

5.3 XRD scans of films A, B, C, and D. The samples were analyzed using a

Thermo ARL X’tra X-ray diffractometer (at WKU) using Cu Kα radiation

from a tube energized at 45 kV and 40 mA. The specimens were scanned

from 2 -100◦ 2θ at 1.2◦/min with a 0.02◦ step size . . . . . . . . . . . . . 34

5.4 XRD scans for PET films samples A, B and C. The crystal lattice of films

A and B is very similar. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.5 DSC heating thermogram for PET/NPG film and film C. Glass transition

temperature, crystallization temperature, and melting temperature can be

observed. This heating run was conducted from 0-300◦C, with 10◦C/min

heat rate in nitrogen atmosphere. . . . . . . . . . . . . . . . . . . . . . . . 35

5.6 XRD scans comparing PET and PET/NPG films. Film C is more crys-

talline and ordered than PET/NPG film. . . . . . . . . . . . . . . . . . . 36

5.7 DSC cooling thermogram for PET/NPG film and film C. Melt crystalliza-

tion temperature can be observed here. This cooling run was conducted

from 300-150◦C, with 20◦C/min cooling rate in nitrogen atmosphere. . . . 37

5.8 Percent mass change versus soak time (minutes) for THF absorption in

shrink films. Saturation around 40 minutes can be seen. . . . . . . . . . . 38

5.9 DSC curve comparing shrink films of thickness 40 microns, exposed to

THF at different time intervals. Melting temperatures can be observed

here. These runs were conducted from 0 to 250◦C at 20◦C/min in nitrogen

atmosphere. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.10 Percent crystallinity of PET/NPG copolymer films versus soak time du-

ration from DSC analysis. Increase in percent crystallinity with soak time

duration can be observed here. . . . . . . . . . . . . . . . . . . . . . . . . 39

5.11 Load versus extension plot for shrink film samples exposed to THF at

different time intervals. The thickness of these films were 40 microns and

the grip separation was 25 mm. . . . . . . . . . . . . . . . . . . . . . . . . 40

5.12 A solvent seam strength test in progress using Instron Universal Tensile

Tester with shrink seam sample (at OSU Wood Science department). . . . 41

LIST OF FIGURES (Continued)Figure Page

5.13 Image of the observed crazing phenomena for the 10-minute THF soaked

sample during tensile strength testing. The crazing was followed by break-

ing/failure of film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5.14 Tensile Strength versus soak time duration of PET/NPG copolymer films.

Declining tensile strength of PET/NPG shrink films can be observed with

increasing soak time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.15 Seam strength plot for seams formed from shrink films samples exposed

to THF at different time intervals. Shrink film with no THF exposure

shows maximum tensile strength. . . . . . . . . . . . . . . . . . . . . . . . 43

5.16 Seam Strength versus soak time duration of PET/NPG copolymer films.

Seam strength is seen to be decreasing with increasing solvent soak time. 44

LIST OF TABLESTable Page

3.1 The table shows the treatment details of films A, B, C and D that were

used for this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.1 DSC Heating Run data for PET film samples A, B, C, and D. . . . . . . . 32

5.2 DSC heating run data for PET Film C and PET/NPG films. . . . . . . . 35

5.3 DSC heating run data for shrink films exposed to THF at different time

intervals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Chapter 1: Introduction

Oriented polymer films such as biaxially oriented polypropylene (BoPP) and biaxially

oriented poly(ethylene terephthalate) (BoPET) films are crucial materials for packaging

industries and electric display industries [11]. The modification temperatures for these

materials are generally either near their melting point, Tm, or the glass transition tem-

perature, Tg. Figure 1.1 depicts the crystallization behaviour of PET around Tg and

Tm [16, 18, 19, 24]. Crystallinity refers to the degree of structural order in a solid. The

atoms or molecules are arranged in a regular and periodic manner in a crystal. The degree

of crystallinity affects the material properties such as hardness, density, transparency,

and diffusion. Thermal-induced crystallization and stretch-induced crystallization (SIC)

are the essential physics involved during the orientation process, which ultimately de-

termines the final mechanical, electric and optic properties [21, 29]. SIC around the

melting point is caused by chain alignment or orientation, which leads to increased order

of initial melt and enhancement of crystallization kinetics [23,30,31]. The mechanism of

SIC around Tg differs in various ways from that around Tm. The primary reason is that

polymer chains are restricted around Tg [2, 22,25] with low diffusion ability [28].

Uniaxial stretch around Tg, as shown in Figure 2.2, can induce chain orientation and order

in the film, which contributes in crystallization of amorphous PET [10,15,17]. The rate of

crystallization (inverse of crystallization half-time) increases between the glass transition

temperature, Tg, and melt temperature, Tm, shown in Figure 1.1. Therefore, the ideal

deformation temperature should be between Tg and Tm. With differential scanning

calorimetry and wide angle X-ray diffraction (WAXD) measurements, the crystallization

of PET film were studied in this work.

Furthermore, the introduction of a specific interactive organic solvent of low molecular

weight can induce crystallization in amorphous PET by plasticizing the material. The

significant effect that results from solvent exposure is that the chains inside the polymer

overcome the activation barrier and attain a more stable state of crystallization. The

degree of plasticization depends on the amount of solvent absorbed and the duration

that it is exposed to solvent environment.

2

Figure 1.1: Crystallization half-time curve of amorphous PET versus temperature. Thefavorable modification temperature for PET is between Tm and Tg [3].

1.1 Motivation

The motivation for this study came from investigating the PET/NPG shrink films and

their ability to solvent seam. These films are used in the bottle-labelling industry and

are expected to seam to itself on solvent exposure.

In the solvent seaming process, a solvent is applied on the film surface, which temporarily

dissolves the polymer at room temperature. Upon dissolution, the polymer chains are

free to move in the liquid environment and can attach with other similarly dissolved

chains in the other component. After sufficient time, the solvent will permeate through

3

the polymer and out into the environment so that the chains lose their mobility and

become rigid. This process leaves a solid mass of entangled polymer chains that forms a

solvent seam.

The significance of crystallinity and morphology on ultimate properties of these films

motivated the investigation of crystallinity at greater depths.

4

Chapter 2: PET Polymer

Polyethylene terephthalate (PET) is a thermoplastic polyester with excellent mechan-

ical properties. It is produced via condensation polymerization upon reacting purified

terephthalic acid (PTA) with ethylene glycol (industrially referred as monoehtylene gly-

col, MEG) as shown in Figure 2.1. The largest volume application of PET is in the fiber

and packaging industry. Linear PET is semi-crystalline, and PET processing parameters

can induce additional crystallinity in the material.

Figure 2.1: Polycondensation reaction of PTA monomer with EG monomer to form PETpolymer. The polymer chains grow by reactions that occur between these two monomerspecies; both monomers are bifunctional [7].

Longitudinal uniaxial stretching of PET can form strong fibers, and biaxial stretching

can form strong films (Figure 2.2). PET favors crystallinity because of its structural

regularity and phenylene ring’s symmetry (as shown in Figure 2.3). The presence of the

asymmetrical iso-phenylene ring acts to counter structural regularity. The presence of

the asymmetrical iso-phenylene ring also decreases the chain regularity and the tendency

of the polymer to crystallize. During the tenter-frame process (discussed in detail in

Chapter 3), PET tends to crystallize under strain and form tiny crystallites (10 - 200

A), making the film transparent. PET displays a relatively slow crystallization rate,

and this rate of crystallization (inverse of crystallization half-time) increases around

the glass transition temperature (Tg) and the melt temperature (Tm), shown in Figure

1.1. Crystallization half-time is defined as the time taken for reaching half of the final

5

crystalline quantity for the specific temperature and process conditions. It depends on

properties such as, intrinsic viscosity (I.V.), resin chemistry (catalyst, co-monomers),

and moisture content. Researchers have found that aging (enthalpic relaxation) speed of

PET decreases with the increase in crystallinity, because the amorphous content trapped

between the crystalline regions increases along with activation energy [3].

Figure 2.2: Polymer chain morphology before (left), during (center), and after orientation(right). Uniaxial stretching aligns polymer chains in one direction and biaxially stretch-ing aligns polymer chains in two directions. Higher order crystallization is obtained afterbiaxial stretching [7].

2.1 Microstructure

PET has been under significant focus by researchers for investigating its structural prop-

erties. These structural properties impart desirable physical properties to PET polymer

films, fibers, bottles, etc. PET is a semi-crystalline polymer which contains both amor-

phous and crystalline phases. Increased crystallinity imparts increased hardness or stiff-

ness in the polymer, while increasing the amorphous structure makes it more ductile.

Therefore, analyzing the crystalline and amorphous phases in the polymer can signifi-

cantly help in predicting the final physical properties.

The presence of glycol linkage in the structure gives rise to two possible structural con-

formations, i.e., trans (extended form) and gauche (relaxed form). The conformations

6

Figure 2.3: Two different rotational conformations of PET: trans (extended form; left)and gauche (relaxed form; right). With the rotation of the C-C bond (-CH2-CH2-), theconformation changes from one form to the other. A phenylene ring is also marked inthe figure [3].

change with the rotation of C-C bond. In the initial spectroscopic studies of PET struc-

ture, the trans conformation was related to the crystalline phase, while the gauche confor-

mation was related to the amorphous phase. However, both trans and gauche were later

revealed to exist in the amorphous phase. Therefore, “an increase in crystallinity can

be correlated with a decrease in the ethylene glycol units in gauche conformation” [14].

Moreover, a proportion of amorphous material is now recognized to exists in a distinct

state, which is known as the rigid amorphous fraction (RAF). The RAF is believed to be

associated with the interface between the crystalline and amorphous phases, exhibiting

intermediate physical properties. The RAF has properties that are similar to the crystal

region. Researchers have established that different micro-structures can be obtained by

varying the process parameters.

2.2 Glass Transition Temperature and Free Volume

In its amorphous state, PET has low mechanical strength, and it can become brittle.

Thus, it has little commercial value in the amorphous state. The concept of free volume

must be introduced to understand such behavior of PET. Free volume is defined as the

7

volume in the polymer matrix that is not occupied by the polymer (i.e., void volume).

The Williams, Landel, and Ferry (WLF) theory predicts that this free volume will attain

a value of 2.5% at temperatures below Tg. This prediction is based upon infinitely small

cooling rates- the rates are slow enough to allow equilibrium relaxation of the polymer

chains to occur during cooling. In reality, cooling rates are much faster, preventing this

equilibrium relaxation from occurring and resulting in practical free volumes that are

greater than the 2.5% that is predicted.

The glass transition temperature, Tg, of polymer is the temperature at which the material

transitions from a glassy state to a rubbery state. Glass transition in polymers is a second

order transition, and it is observed in the amorphous region of a polymer only. In this

region, the long polymer chains are oriented randomly and have more freedom to move.

The glass transition temperature is related to segmental chain motion when polymers

are heated.

Starting from the melt, as the polymer is cooled quickly, the motion of polymer chain

segments is greatly reduced. If the cooling rate is much larger than the re-orientation

time of the polymer (also called the relaxation time), it is unable to volumetrically relax;

therefore, excess free volume is ‘frozen’ inside the polymer. This state is called the glassy

or amorphous state. The discontinuity in the specific volume versus temperature plot,

as shown in Figure 2.4, is associated with the temperature at which the polymer changes

from a flexible rubbery material to a brittle glassy material. Thus, it is called the glass

transition temperature, Tg. At temperatures above Tg, polymer chains have greater

mobility, and larger segments of the polymer chain are able to re-orient. Therefore, they

are able to achieve their equilibrium conformations, which corresponds to an increased

free volume. Below Tg, polymer chain mobility is greatly decreased corresponding to a

reduction in polymer free volume. The microstructure of a polymer has a huge impact

on the Tg of a specific polymer. Rigid polymers such as PET have high Tg because of

the lower polymer chain motion, while polymers with extremely flexible backbones like

polydimethylsiloxane (PDMS) have low Tg’s below room temperature. This behavior has

a significant effect on polymer properties, and it makes them very versatile materials.

8

Figure 2.4: Specific volume versus temperature for crystalline polymers (left) and amor-phous polymers (right). Amorphous polymers do not exhibit any transition on melt-ing [12].

2.3 Biaxially Oriented Films

Biaxial stretching of film is the process of forming plastic films in both MD and TD

directions to obtain stronger films. Stretching is commonly known as orientation in

industry. Orientation is the alignment of polymer chains on the molecular level in a par-

ticular direction. The cause of this molecular orientation is a force on polymer molecules

that pulls them in a specific direction, and then, molecules are frozen in that place by

quenching from the molten state.

The biaxial stretching is conducted in two directions, i.e., the machine direction (MD)

and the transverse direction (TD). MD is the direction in which the film moves through

the machine from start to finish. TD is the direction perpendicular to the MD. The TD

width is the same as the width of the finished film (7-10 meters).

Biaxial orientation of films can take place via two methods, simultaneous and sequential.

In the simultaneous stretching method, film is stretched at the same time in both direc-

tions in a single step. In this method stretch ratios in both MD and TD are kept equiv-

alent; hence, similar physical properties are obtained in both directions. This method

9

produces more balanced films than the sequential method.

Figure 2.5: PET sequential orientation process flow diagram. Different zones are alsomarked in the figure. Zone I is the extrusion zone, zone II is the machine directionstretching zone, zone III is the transverse direction stretching zone, and zone IV is thewinding and finishing zone [7].

In the sequential method, on the other hand, the film is first oriented in the machine

direction. The MD orientation induces some crystallinity in the polymer film. After the

MD orientation, film is stretched in the transverse direction independently. The changes

induced in the crystallinity of the film during the MD stretching process require that

the temperature of the TD oven is kept higher than the MD process. Also, the stretch

ratios are generally kept different in both MD and TD; hence the physical properties of

final film are different in MD and TD. Depending on the type of desired final product,

preference can be given to either simultaneous or sequential process.

Biaxial stretching of the film is achieved in the industry by two main processes, namely,

10

the double-bubble method and the tenter-frame method. The bubble method is an

example of simultaneous orientation, where a circular die from the extruder is used to

form a thick-walled polymer tube. This tube is then blown under air pressure, thereby

orienting the film in transverse direction. Simultaneously, a similar orientation in the MD

is achieved by adjusting the speed at which the tube is pulled downwards and collapsed.

The bubble process produces films that are balanced in the physical properties in both

MD and TD. This method is not very common in the industry. In the tenter-frame

method, polymer is directly extruded in the form of thicker sheets on a chilled chrome

roller. Then, the film is passed through a stretching unit over rollers that are moving

faster than the rate at which material is extruded. This setup orients the polymer chains

in the machine direction. Figure 2.6 shows an image of machine direction operations at

Flex Films USA, Inc.

Figure 2.6: Image of machine direction operation at Flex Films. The rollers are heatedto just below the glass transition temperature, Tg, of PET, and they are rotating atabout 100-400 rpm [7].

11

Next, film is fed into the tenter-frame for transverse direction stretching. The film is

gripped along the edges by clamps that are attached to moving chains. These chains

move outwards for stretching the film in TD. The stretch ratio typically ranges from

3 to 4 for PET polymer. The tenter method is an example of a sequential stretching

method. This method is favored in industries for producing biaxially oriented films of

polyetheylene terephthalate, polypropylene, polyethylene, and other polymers.

12

Chapter 3: Materials and Methods

3.1 Tenter Method for BoPET

The tenter-frame is a more favored method in industry compared to the bubble method.

BoPET production is no exception. About 80-90% of BoPET is produced by the ten-

ter method. A conventional tenter-frame machine consists of an extruder, some heated

rollers for MD orientation, a TD oven comprising of at least three different sections, and

finally a winder section. Orientation in the MD is normally achieved by passing the film

over heated rollers. Increasing the temperature of heated rollers directly corresponds

to the higher stretch ratios without causing film breakage. The MD stretching ratio is

directly proportional to reduction in film thickness. Hence, an MD stretch ratio of four

can be expected to reduce the film thickness by a factor of four as well.

Post MD, the film is fed into the TD section for transverse stretching. The TD oven

generally consists of three to four different sections that are operating at different temper-

atures: (1)pre-heating, (2) stretching, and, (3) heat setting. Figure 3.1 shows transverse

direction oven at Flex Films USA, Inc. The length of each segment is determined by

the factor of speed at which the production line is running. Faster lines require longer

residence times for films to reach the set temperature. The pre-heat section is used to

bring the film temperature to its melting point, which is the temperature that orientation

takes place in TD. For PET, this temperature is about 250◦C. The oven temperature is

commonly set at a few degrees higher than the desired film temperature because of the

heat transfer issues.

After passing through the oven, the film is fed into the stretching section, where the

actual stretching and crystallization takes place in TD. The entering film from MD is

about 2-3 meters wide, and the post stretching film width is about 8-9 meters. Hence,

a draw ratio of 3-4 is achieved here. This orientation is conducted using rails that pull

13

Figure 3.1: Image of transverse direction operations at Flex Films. A long heating ovenis set and maintained at different processing temperatures [7].

the film outwards whilst stretching the film. The film is gripped using clamps at its

edges during orientation. The distance between these rails determines the stretch ratio

achieved in this step. After the stretching section of the oven, the film is passed to

the final section of the tenter-frame orientation, annealing or heat-setting section. Here,

the temperature is maintained higher than the orientation temperature of film because

polymer chains are moved far beyond their equilibrium state during stretching. Hence,

a tendency exists for these chains to return to their equilibrium condition, which causes

undesired film shrinkage. Therefore, higher temperature treatment of film allows ‘freez-

ing’ of the chains orientations which preserves better mechanical properties. Heating

the film at a higher temperature also induces memory in the film of having been heated

to the most recent temperature. Hence, exposure of the film to lower temperatures has

minimal effects on its properties.

14

The BoPET films are produced via sequential stretching with different process con-

ditions. Film samples were collected after each process to quantify crystallinity changes

in the microstructure. From the BoPET extrusion line, the samples collected were (Fig-

ure 2.5):

Cast film, the unstretched PET that is mostly amorphous, is referred in this work as

film A;

Machine direction stretched film, which is semi-crystalline, is referred in this work as

film B;

Transverse direction stretched film which is semi-crystalline is referred in this work as

film C; and

Biaxially oriented PET film, which is semi-crystalline, is referred in this work as film D.

The Table 3.1 shows the films used for crystallinity investigation and the various zones

treatments conducted on these films (Figure 2.5).

Table 3.1: The table shows the treatment details of films A, B, C and D that were usedfor this study

Film type Orientation Zones treatment

Film A Unoriented I

Film B MD oriented I, II

Film C TD oriented I, III, IV

Film D MD and TD oriented (sequential BoPET) I, II, III, IV

3.2 Heat-Shrink Films

Shrink films are generally uniaxially oriented films that can shrink when exposed to a

specified temperature. These films are used to produce shrink sleeves for the packaging

industry. Sleeve labels can be found on all supermarket shelves on cans, bottles and

jars, attracting consumers to products as diverse as baby food, air fresheners, shampoo,

and shaving cream. Figure 3.2 shows different end applications for these films. The

shrink label can best be described as a film label printed on an oriented plastic sheet or

tube, which, when heat is applied to the label, conforms or shrinks to the contours of

the container. Two different types of shrink labels exist: the shrink sleeve label and the

rollfed, wrap-around shrink label. These films are generally made of polyvinyl chloride

15

(PVC), polyester (PET), or polyolefin (PS/OPS). PVC has been widely used for this

application; however, recent environmental concern about PVC films has caused its us-

age to decline. PS/OPS films are very sensitive to heat, and they can start shrinking at

temperatures as low as 40◦C. Therefore, PET shrink film market has expanded.

Figure 3.2: Commercially available examples of shrink-sleeve bottle-labels. Full colorand 360◦ design coverage has reported increase in sales of products by upto 20% [1].

Shrink films start with different monomers, wherein neopentyl glycol (NPG) is also a

constituent of the polymer. These films can be manufactured on the same operation line

as BoPET. However, as these films are typically required to shrink only in the transverse

direction, they are generally not oriented in the machine direction section of the line.

Figure 3.3 shows a plot for the shrinkage of these films in MD and TD at different

temperatures. These films can shrink up to 75% in TD. Hence, after the extruded cast

film in zone I (Figure 2.5), the MD section (zone II) is by-passed, and the film is fed

directly into the transverse direction oven (zone III). An additional antiblock coating

can be applied in some cases to avoid undesired self-sticking in the shrink film.

Research has also suggested that the incorporation of neopentyl glycol (NPG) with two

methyl side groups, as shown in Figure 3.4, could be effective in improving the hydrolytic

stability of polyesters.

16

Figure 3.3: Percent shrinkage at different temperatures for PET/NPG shrink films inMD and TD directions. Shrink films are capable of shrinking in the TD direction toupto 75%. MD and TD direction of sleeve label on bottle is also shown (right) [7].

Figure 3.4: the figure shows neopentyl glycol (NPG) molecule. NPG is a constituent ofshrink films [13].

The films used for studying the PET/NPG behavior were supplied by Flex Films USA

Inc. These films constitute of ethylene glycol and neopentyl glycol as glycol components

of the copolymer. The NPG to EG ratio was maintained at 1:4. Figure 3.5 represents

the composition of these films.

3.2.1 Shrink Sleeve Conversion Process

The converting process is generally determined by the end user requirements, which

entails the container type of product, properties such as heat/water resistance, the sub-

strate type, solvent used for seaming and the finishing process that is needed. Figure 3.6

shows the step procedure for converting shrink film rolls into shrink sleeve labels. This

17

Figure 3.5: Co-monomers of PET/NPG shrink films. The ratio of TPA:EG:NPG is 5:4:1.

process could be categorized into following three stages.

1. Pre-printing

The shrink sleeves are typically printed on the internal side or reverse side of the trans-

parent film, therefore ensuring that printed substrate is not subjected to any scratching

or scuffing during the sleeving process. Mirror printing is used for the internal printing

purpose. After printing, these films are varnished to further avoid scratching during

finishing stages.

2. Creating the Sleeve

The sleeve creation process is conducted after printing. A slitting operation follows

printing and a slitter is used to split the film roll into one label width size, with some

18

empty area for solvent seaming. This is followed by seaming process. These film rolls are

typically center seamed. The seamer seams the edges of the substrate using a specified

solvent.

3. Applying

By this stage the sleeve material is rigid, and it can be easily slid on the bottle surface.

Finally, heat treatment is completed at 70-80◦C, which results in shrinkage of the roll

sleeves on the contours of bottle.

Figure 3.6: Flow diagram representing the shrink sleeves conversion process. The mainsteps are printing the sleeve, creating the sleeve, and, applying the sleeve [8].

3.3 Crystallinity Measurement Techniques

Crystallinity is widely accepted as an extremely important property for determining the

physical properties of a polymer. PET properties are also governed by its crystallinity

levels. For oriented film samples, strain-induced crystallinity is most significant [3]. Sev-

eral methods have been researched and developed to determine crystallinity in polymers.

The most common methods that are used are X-ray diffraction, differential scanning

calorimetry (DSC) and fourier transform (FTIR). Recently, atomic force microscopy

19

(AFM) and scanning electron microscopy (SEM) techniques have also shown to be help-

ful in gaining information about surface details of polymers. While bulk information can

be gained from:

Differential Scanning calorimetry

X-ray diffraction

The DSC and the XRD techniques were utilized to investigate crystallinity in this work.

These techniques and their working principles are explained in the next section.

3.3.1 Differential Scanning Calorimetry (DSC)

DSC is one of the most reliable techniques for the thermal analysis of polymers. It is

based on the principle that a polymer response varies with changing temperature. It

comprises of a reference pan, which is used to profile a caloric heat change in the sample

pan. A DSC thermogram is a quantitative plot that helps the user analyse phase changes

like glass transition, crystallization, melting, or stress release in polymers. The power

(mW) required to maintain the sample pan at the same temperature as the reference

pan is monitored, and hence, endothermic and exothermic heat flow is measured.

DSC is capable of monitoring even small heat flow changes during the heating and cool-

ing process. If the polymer sample absorbs heat during the heating cycle and becomes

plastic as chains gain mobility, an increase in heat capacity is displayed on the thermo-

gram. This temperature is the glass transition temperature. On further heating, when

amorphous regions of polymers become crystalline, they exhibit energy release, which

is an exothermic heat of crystallization. When the polymer reaches its melting tem-

perature, the crystals melt, an endothermic melting peak is observed, and the polymer

chains become disordered, resulting in an amorphous melt. A recrystallization could be

observed if slow cooling is allowed.

A TA Instruments Q2000 DSC, as shown in Figure 3.7 was used to obtain the thermo-

grams. The instrument was calibrated for its heat flow and temperature using an indium

reference. All experiments were conducted in a nitrogen atmosphere. Crystallinty of a

20

Figure 3.7: TA Instruments Q2000 DSC equipment at Oregon State University PolymersLab. The nitrogen supply is attached, and the flow rate is maintained at 50 ml/min.

polymer sample can be calculated using equation 3.1,

Xc =∆Hm − ∆Hc

∆Hofusion

(3.1)

where Hm is the melting enthalpy and Hc is the crystallization enthalpy ∆Hofusion is the

heat of fusion for 100% crystalline polymer. For this work, ∆Hofusion is taken as 140.1

J/g for PET.

3.3.2 X-ray Diffraction

X-ray diffraction is a non-destructive technique that can be used for gaining information

regarding the crystal lattice changes at the molecular level. One of the earliest crys-

tallography studies of PET was reported in the mid-20th century by Johnson et al. [9]

21

Further studies confirmed that crystalline PET has a triclinic unit cell. Diffraction scans

of a semi-crystalline PET samples have shown mainly three reflections at 17◦, 22.5◦,

and 25.5◦ angles for copper radiation corresponding to (100), (010), (110) crystal planes,

represented in terms of d-spacing 5.2 A, 3.9 A, and 3.4 A. A large broad peak, created

from the scattering of X-rays by the amorphous content present in the sample, is over-

lapped by the crystalline peaks and the shape is “independent of degree of crystallinity”,

yet the relative area under the hump to the diffraction peaks is proportional to the

crystallinity [9]. The crystallinity of a sample can be calculated using equation 3.2,

Xc =

∑Ac∑

Ac +∑Aa

× 100%, (3.2)

where Ac is the area under the crystalline peaks and Aa is the area under amorphous

peak.

Researchers like Mahendrasingam et al., Jabarin et al., and Schmidt et al. [3] have used

X-ray diffraction to study the molecular orientation and also the effect of factors such as

temperature and strain rate on the crystallization of PET. Diffraction can be conducted

in situ, which allows a clear visualization of the chain packing during the strain-induced

crystallization process.

3.4 Tensile Strength

Tensile testing measures the ability of a test material to withstand a force that is pulling

it apart and the distance that the material will stretch before it breaks. An Instron

Universal Tester (Figure 3.8) was used to tensile testing results in this work.

The properties of a polymeric material depend on its structural arrangement, which

closely aligns with its morphology. Orienting polymers can significantly alter this mor-

phological structure. Proper orientation conditions can greatly improve mechanical and

thermal properties of PET.

Orientation of any polymer material may involve three deformations: (1) elastic or

Hookean deformation, which can be recovered on removal of stress, (2) viscous flow

deformation, which is and caused by slippage of molecules and making it nonrecover-

22

able, (3) uncoiling of polymer chains deformation, which can be slowly recovered [6]. As

already discussed, PET undergoes strain induced crystallization during orientation. [6].

The ideal orientation should achieve maximum uncoiling of tangled polymer chains and

chain alignment as well as minimum chain slippage or viscous flow [6].

23

Figure 3.8: Image shows Instron Universal Tensile tester located at Wood Science &Engineering department of Oregon State University. The figure shows 1 kN load cell,grips at 25 mm separation and test setup.

24

3.5 Solvent Dispersion Device

The process of preparing test specimens for seam strength testing required a setup for dis-

persion of solvent onto film substrate. Industrially, the seaming process is completed by

needle tip method, top felt method, or bottom felt method. The dripping-onto-substrate

(DOS) device was utilized to replicate the industry needle tip solvent dispersion method

in this work. The advantage of using this device was to prepare uniform solvent volume

test specimens for seam strength tests. Figure 3.9 illustrates the device used for this

study.

The solvent dispersion setup comprised of a syringe containing THF solvent, a motor

controlled stage with an inverted L-shaped syringe dispenser, and another motor con-

trolled stage translating the film substrate. These motor controlled devices also had

variable speed adjustment option. Therefore, the speed at which the syringe dispenser

dispenses the solvent and the quantity of solvent dispersed on the substrate could be

controlled in an efficient manner. The amount of solvent dispersed for seaming test

sample preparation was 1 ml for each specimen.

25

Figure 3.9: Solvent dispersion device used for preparing test specimen for seam strengthtesting. The syringe and motion controlled devices are illustrated.

26

Chapter 4: Experimental

4.1 Microstructure Analysis of PET Film Sequential Stretching

The orientation of PET film can alter its strength, stiffness properties and induce higher

levels of crystallinity, thereby, enhancing barrier and chemical inertness properties of

PET. Vieth and Sladek (1965) [26] observed approximately six times reduced carbon

dioxide gas diffusivity in crystalline oriented PET film. Salame (1986) [20] also reiter-

ated that orientation can significantly decrease permeability because of the increase in

tortuosity created by crystals alignment in the film.

The aim of this study is to investigate the changes in crystalline structure of PET film

when it is stretched in the machine direction, in the transverse direction, and in both

directions sequentially to form biaxially-stretched PET. The thermal analysis was con-

ducted using DSC. Microstructure analysis was also observed by X-ray diffraction.

In this work, PET films supplied by Flex Films USA Inc. were investigated. Four dif-

ferent films, i.e., films A, B, C, and D (Table 3.1). The zone treatment of these films

is described in Figure 2.5. The processing parameters for these films can induce crys-

tallinity via two different modes, i.e., strain-induced crystallization and a combination of

strain-induced and thermal-induced crystallization. These induced crystallization modes

are explained below.

1. Strain-induced crystallization [4]. If PET is stretched under appropriate conditions

(such as, temperature, draw ratio, and extension rate), it exhibits strain-induced crystal-

lization morphology. This phenomenon has been widely studied. After the orientation

process, crystal sizes generally remain small enough for optical transparency to be main-

tained. This method of crystallization is commercially important for processing PET

into products such as films and bottles as crystallinity effects the overall property of the

material.

2. Combination of strain-induced and thermal-induced crystallization. PET can be

exposed to additional thermal energy after it has undergone strained induced crystal-

27

lization (e.g., the TD operation in the tenter-frame oven). In its strained state, PET

is kinetically predisposed for further crystallization [4]. Moreover, the increased heat

treatment facilitates the relaxation of stresses induced by the processing, leading to the

final product with improved thermal resistance.

In this work, we have determined crystallinity values and investigated crystallinity changes

at each step of tenter-frame orientation method.

4.2 Effect of NPG Copolyester on Thermal Properties of PET

Polyethylene terephthalate (PET), synthesized from terephthalic acid (TPA) and ethy-

lene glycol (EG), has excellent mechanical and thermal properties, owing to the incor-

poration of the benzene ring in the main chain of the polymer. However, copolyesters

of PET are regularly manufactured for different applications in packaging industry. The

characterization of these materials is crucial to enhance the desired properties accord-

ingly.

This work aims to characterize one such industrially important copolyester: a uniaxially

oriented mono-film copolyester of PET and neopentyl glycol (NPG). This copolyester

is a heat shrinkable film that shrinks to almost 75% in transverse direction upon heat

treatment at a specified temperature. This film comprises of glycol monomers EG and

NPG in the ratio of 4:1.

These films are manufactured by stretching only in the transverse direction. There-

fore, after extrusion, the MD stretching section is by-passed, and the film is passed to

the TD oven. The processing steps are similar to film C. For that reason, the thermal

properties of these films were compared with the film C, that was stretched in the trans-

verse direction, to understand the effects of NPG on the PET polymer backbone using

DSC analysis. Also, the crystal lattice structure of the film C was compared with the

PET/NPG copolyester film using XRD. The effects of the addition of a bulky methyl

group to PET backbone were studied. The glass transition temperature, the melting

temperature, and the crystallinity values were compared to examine NPG effects on

PET backbone.

28

4.3 Solvent-induced Crystallinity of PET/NPG Copolymer Shrink

Films with THF as a Function of Exposure Time

The PET/NPG films are used for bottle-labeling applications because, firstly, they can

shrink on heat application, and secondly, they can solvent seam on solvent exposure.

For PET/NPG films used in this study, the seaming solvent is tetrahydrofuran (THF).

Introduction of THF on a PET/NPG film surface plasticizes the surface, thereby induc-

ing crystallinity. This process is known as solvent-induced crystallinity. Solvent-induced

crystallinity (SINC) can be used to improve crystallinity of PET films by solvent treat-

ment. These solvents are absorbed in the amorphous region of PET and they facilitate

chain mobility, thereby increasing crystallinity [27]. Structural reorganization after sol-

vent treatment induces crystallinity in the film.

For PET, the amount of solvent that is used [5] and solvent treatment time are found

to be directly proportional to the induced crystallinity. Figure 4.1 shows the effect of

acetone solvent treatment time on the induced crystallinity in PET [5]. In the presented

work, we have investigated the effect of solvent exposure time on PET/NPG copolyester

shrink films.

We soaked PET/NPG film samples in THF for different time durations ranging from 5

minutes to 40 minutes. The weight of each sample was plotted against duration time,

and at around after 40 minutes, the saturation was attained. The thermal properties of

each of these samples were investigated using DSC analysis.

29

Figure 4.1: Depression in glass transition temperature caused by acetone solvent treat-ment time in PET film of thickness 0.23 mm can be seen in DSC thermograms heatingcurves [5].

4.4 Solvent Seam Strength Testing of Different THF Exposure on

Shrink Films

The seam strengths of THF soaked PET/NPG films were obtained and compared to

investigate the relationship between seaming ability and solvent treatment. American

Standard Testing Method (ASTM) F88 method was used for this purpose. ASTM F88

specifies a method for testing the seam strength of a thin plastic sheet or similar flexible

material. These types of seals are used extensively in the food and beverage sector as

well as in bio-medical applications.

The seal strength test is a combination of both an adhesive test and a tensile test on the

thin plastic material. A few failure modes can exist- either the adhesive can fail, or the

flexible barrier material can be ripped apart. Seam strength testing is a uniform way to

30

understand the strength of two materials that are seamed together. Tensile testing also

helps in understanding the strength that is required to break the seam apart. Knowing

the maximum strength of a seam according to the application is important for any

manufacturer. ASTM F88 seam strength testing method is used for uniform testing and

results reporting in the industry.

This test was conducted by cutting a strip of 0.5 inches of seamed shrink films by

clamping each end of the test specimen in a Instron Universal Tensile Tester (Figure

3.8). The more rigid side of the sample was clamped in the top grip. The seal was tested

at a rate of grip separation of 50 mm/min. For each cycle, the maximum force was

reported as the specimen was stressed to failure, and the type of failure was identified.

31

Chapter 5: Results and Discussions

5.1 Oriented PET Analysis

In this section, we have presented the results of oriented PET films microstructure anal-

ysis. DSC and XRD were the main techniques that were utilized for this purpose. Figure

5.1 shows the DSC heating curves of PET film samples A, B, C, and D (Table 3.1) at

10◦C/min. The sample weight was around 6-7 mg. The steps for obtaining the DSC

heating were as follows

1. Equilibrate at 0◦C,

2. Ramp 10◦C/min to 300◦C,

3. Ramp 10◦C/min to 0◦C, and

4. Equilibrate at 40◦C.

The detailed data that was collected from these experiments, such as, crystallization

temperature, melting temperature, and corresponding enthalpies, are shown in Table

5.1. We found that films A and B show crystallization peaks, while crystallization peaks

were not observed for films C and D. The reason for the difference was attributed to

the heat treatment on these films. The slowly cooled PET films C and D were provided

sufficient time to adequately arrange its polymer chains into ordered crystalline regions

in zone IV (see Table 3.1). Ordered crystalline regions are a lower energy configuration.

Therefore, the slowly cooled films C and D did not absorb any more energy upon re-

heating; hence, no crystallization peak was observed.

Film A, the unoriented film sample, was found to have a crystallinity of about 11%,

and the percent crystallinity was shown to increase after each process of stretching. The

glass transition temperature was observed to be at about 70◦C. The broader melting

peaks of films A, B, and C indicate that these samples are less crystalline than film

D, as the sizes of crystallites in films A, B, and C are broadly distributed. Generally,

the melting temperature increases with the degree of crystallinity, as shown by our film

samples (Figure 5.1).

The heating DSC data shows that the combination of strain-induced crystallinity and

32

Figure 5.1: DSC heating thermograms for PET films A, B, C, and D. Glass transitiontemperature, crystallization temperature, and melting temperature can be observed.This heating run was conducted from 0-300◦C, with 10◦C/min heat rate in nitrogenatmosphere.

Table 5.1: DSC Heating Run data for PET film samples A, B, C, and D.

Glass Transition Crystalline Melting Difference

Sample

Tg

(◦C)Onset(◦C)

Tc

(◦C)∆Hc

(J/g)Onset(◦C)

Tm

(◦C)∆Hm

(J/g)∆Hm − ∆Hc

(J/g) χc(%)

Film A 70.3 129.0 135.6 32.2 241.9 255.2 47.5 15.3 10.9Film B 85.5 103.2 111.1 20.6 242.2 255.6 52.0 31.4 22.5Film C 85.0 85.0 87.5 2.6 245.7 256.2 44.9 42.3 30.2Film D - - - - 246.6 257.3 52.7 52.7 37.6

thermal-induced crystallinity generates the most crystalline behavior in PET films.

Figure 5.2 shows the cooling curve of films A, B, C, and D at 20◦C/min cooling rate.

At this cooling rate, crystallization peaks are clearly seen for all four samples at around

200◦C. This data indicates that films A and B have higher heat of melt crystallization

than films C and D. Also the the asymmetrical nature of these peaks could be attributed

to different size crystals present in the film and different rates at which these crystallize.

Figure 5.3 shows the X-ray diffraction scans for films A, B, C and D. The crystal lattice

changes occurring at the microstructure level can be observed from this data. The

33

Figure 5.2: The plot shows DSC cooling thermogram for PET films A, B, C, and D.Melt crystallization temperature can be observed here. This cooling run was conductedfrom 300-70◦C, with 20◦C/min cooling rate in nitrogen atmosphere.

long and sharp peaks are generated when X-ray beams diffracts after colliding with the

crystalline region of the film, while broader and shorter peaks are observed when the

X-rays are diffracted by less ordered amorphous region.

Figure 5.4 shows the crystal lattice XRD scan for film A, B, and C. Film A is the

unoriented amorphous film, while film B is machine direction oriented film. The crystal

lattice structures for films A and B are found to be very similar. However, a sharp

crystalline peak was found for film C, which is transverse direction oriented, when it

was scanned in the XRD. These results reiterate the results from the DSC that film C

is more crystalline than film B.

34

Figure 5.3: XRD scans of films A, B, C, and D. The samples were analyzed using aThermo ARL X’tra X-ray diffractometer (at WKU) using Cu Kα radiation from a tubeenergized at 45 kV and 40 mA. The specimens were scanned from 2 -100◦ 2θ at 1.2◦/minwith a 0.02◦ step size

Figure 5.4: XRD scans for PET films samples A, B and C. The crystal lattice of filmsA and B is very similar.

35

5.2 PET/NPG Copolymerized Films Analysis

In this section, the results from DSC and XRD analysis are shown for the case when

PET is copolymerized with NPG. This copolymer film is stretched in the TD direction,

similar to film C treatment. The DSC analysis is a very useful tool for determining the

compatibility of two copolymers. The PET/NPG copolymer was shown to have only

one melting peak in Figure 5.5, confirming that this material is not blended but that it

is copolymerized completely and compatibly.

Figure 5.5: DSC heating thermogram for PET/NPG film and film C. Glass transitiontemperature, crystallization temperature, and melting temperature can be observed.This heating run was conducted from 0-300◦C, with 10◦C/min heat rate in nitrogenatmosphere.

The Tg of PET was found to drop by about 10◦C after it was copolymerized with NPG.

The incorporation of NPG in the PET backbone has significantly altered the thermal

transition of PET polymer. The bulky methyl groups on NPG copolymer partially

Table 5.2: DSC heating run data for PET Film C and PET/NPG films.

Crystalline Melting Difference

Sample

Tg

(◦C)Onset(◦C)

Tc

(◦C)∆Hc

(J/g)Onset(◦C)

Tm

(◦C)∆Hm

(J/g)∆Hm − ∆Hc

(J/g) χc(%)

Film C 85.0 85.0 87.5 2.6 245.7 256.2 44.9 42.3 30.2PET/NPG Film 75.2 91.88 114.46 15.0 183.15 209.5 21.9 6.9 4.9

36

disrupt the regularity of the PET chain. As shown in Figure 5.5, PET melts at around

250◦C, while PET/NPG melted at about 210◦C. The DSC data for comparison of thermal

transitions in PET film C and PET/NPG films are tabulated in Table 5.2.

XRD scans for PET/NPG film and film C are illustrated in Figure 5.6. The crystalline

peak of film C is appearing sharper and longer, indicating higher crystallinity in film C

than PET/NPG film. This confirm the data collected from the DSC analysis.

Figure 5.7 shows the cooling curve of DSC for PET/NPG film and film C at cooling rate

of 20◦C/min. A crystallization peak was not observed in the PET/NPG copolymer film,

while film C showed a clear crystallization peak for both cooling runs. This indicates

that the melt crystallization rate of PET is higher than the melt crystallization rate of

the PET/NPG copolymer film. The melt crystallization peak for PET/NPG copolymer

film could have been observed at very slow cooling rates, which was not conducted during

this study.

Figure 5.6: XRD scans comparing PET and PET/NPG films. Film C is more crystallineand ordered than PET/NPG film.

37

Figure 5.7: DSC cooling thermogram for PET/NPG film and film C. Melt crystallizationtemperature can be observed here. This cooling run was conducted from 300-150◦C, with20◦C/min cooling rate in nitrogen atmosphere.

5.3 THF Plasticizer Exposure to PET/NPG Shrink Films

Generally, if a liquid encounters an unstressed amorphous polymer, the small solvent

molecules may disrupt the intermolecular cohesive forces between the polymer chains

and enhance the chain mobility, thereby lowering the glass transition temperature. Con-

sequences of polymer-liquid interactions include swelling, crystallization, solvent stress

cracking or crazing, and possibly dissolution. When the interactions are strong enough

to suppress the Tg below the exposure temperature, crystallization becomes kinetically

favorable. Figure 5.8 shows a plot of the percent weight change in the film sample against

the time duration that it was exposed in THF. The saturation level was reached after

about 40 minutes of soaking.

We studied the thermal effects of solvent exposure by conducting DSC tests on these

soaked films samples. The DSC thermograms for shrink films samples soaked in THF

for different time intervals is illustrated in Figure 5.9. Table 5.3 shows the increase in

percent crystallinity value with soak time duration. Stress relaxation due to solvent

adsorption was observed for all the soaked film samples. Cold crystallization peaks were

not observed because of chain re-alignment in the film as a result of solvent THF action.

38

Figure 5.8: Percent mass change versus soak time (minutes) for THF absorption in shrinkfilms. Saturation around 40 minutes can be seen.

The crystallinty was observed to be increasing even after mass saturation at 40 minutes.

Although the rate of crystallinity increment is slow from 15-minute sample t0 40-minute

sample.

Figure 5.9: DSC curve comparing shrink films of thickness 40 microns, exposed to THFat different time intervals. Melting temperatures can be observed here. These runs wereconducted from 0 to 250◦C at 20◦C/min in nitrogen atmosphere.

39

Table 5.3: DSC heating run data for shrink films exposed to THF at different timeintervals.

Soak Time (min) Enthalpy (J/g) % Crystallinity

0 6.9 4.95 15.58 11.115 22.42 16.0430 23.81 17.0240 28.05 20.03

From this study we can infer that the time duration of solvent exposure is directly

proportional to the induced crystallinty of the PET/NPG film. Figure 5.10 shows the

growth in percent crystallinity with increasing soak time duration. This data was plotted

from Table 5.3.

Figure 5.10: Percent crystallinity of PET/NPG copolymer films versus soak time dura-tion from DSC analysis. Increase in percent crystallinity with soak time duration canbe observed here.

5.4 Tensile Testing Results

The objective of these tests was to investigate the tensile strength of PET/NPG film

samples when soaked in THF for different time intervals. This study can help in predict-

ing the mechanical properties of PET/NPG films under solvent THF exposure. Figure

5.11 show the load versus extension curve for PET/NPG copolymer film samples that

40

were exposed to THF for 5 minutes, 10 minutes, and 40 minutes. These results were

collected from Universal Tester from Instron (5582), the test setup is shown in Figure

5.12.

Figure 5.11: Load versus extension plot for shrink film samples exposed to THF atdifferent time intervals. The thickness of these films were 40 microns and the gripseparation was 25 mm.

Figure 5.11 was plotted from data obtained from tensile tests. It shows significant

decrease in tensile strength of the sample soaked in THF for 40-minutes as compared

with the other samples. The breaking in the 5-minute and 10-minute samples was seen

after crazing had occurred during the test run. Crazing is the formation of surface cracks

on the polymer film surface, and it has a hazy or foggy appearance, as shown in Figure

5.13.

Crazing has a weakening effect on the film, reducing the aesthetic qualities. Cracks

formed by crazing are indicative of the beginning of a fracture or failure. Crazing is

caused by solvent molecules interacting with the polymer chains. The cracks are found

at right angles to the direction of the tensile stress.

Figure 5.14 shows the plot of maximum load that a soaked film sample could bear against

the soaking duration time for that sample. The duration of solvent exposure is found to

be inversely proportional to the tensile strength of the PET/NPG films.

Figure 5.15 represents the data from tensile strength testing from solvent seamed spec-

41

Figure 5.12: A solvent seam strength test in progress using Instron Universal TensileTester with shrink seam sample (at OSU Wood Science department).

42

Figure 5.13: Image of the observed crazing phenomena for the 10-minute THF soakedsample during tensile strength testing. The crazing was followed by breaking/failure offilm.

imens of PET/NPG films that were seamed with THF. The data indicates that the

strongest seam was formed when THF solvent soaking was absent. This data shows

that exposure of solvent on polymer chains is not a favorable condition for stronger seam

strength. Figure 5.16 illustrates the decline of seam strength with the soak time duration

of the PET/NPG copolymer films. The solvent-induced crystallinity in the PET/NPG

samples decreases the solvent seaming ability of these films.

A hypothesis can be thus proposed from these results that an amorphous surface of shrink

films can be linked with better solvent seaming ability as the amorphous phase allows

better solvent adsorption than ordered crystalline phase. Hence, higher crystallinity

could negatively impact the strength of seams.

43

Figure 5.14: Tensile Strength versus soak time duration of PET/NPG copolymer films.Declining tensile strength of PET/NPG shrink films can be observed with increasingsoak time.

Figure 5.15: Seam strength plot for seams formed from shrink films samples exposedto THF at different time intervals. Shrink film with no THF exposure shows maximumtensile strength.

44

Figure 5.16: Seam Strength versus soak time duration of PET/NPG copolymer films.Seam strength is seen to be decreasing with increasing solvent soak time.

45

Chapter 6: Conclusions

An important conclusion of this work is that the orientation of PET film in MD and

TD direction can induce crystallinity in the film via stretch-induced crystallinity and

thermal-induced crystallinity. From the XRD results, the crystal lattice structure was

also shown to be altered in the process of stretching.

TD orientation was observed to possess greater percent crystallinity than unstretched and

MD stretched samples, as it combines thermal-induced crystallinity and strain-induced

crystallinity. Maximum crystallinity was observed in BoPET films, which were oriented

in MD and TD sequentially.

Copolymerization of NPG with PET was studied in this work. The application and

properties of these films were described. Copolymerizing PET with NPG significantly

decreases PET melting temperature and glass transition temperature. The copolymer-

ization also slows the rate of crystallization of the polymer melt. The addition of NPG

as a constituent monomer in PET polymerization process, allows shrinkage of film on

heat application, and also solvent seaming using THF plasticizer. These properties make

PET/NPG shrink films excellent material for shrink-sleeve bottle-label application.

Seaming strength of PET/NPG films using THF was investigated in this work. We

found that exposure of PET/NPG films to THF reduces the tensile strength of these

films. Also, the THF solvent exposure decreases solvent seam strength. Maximum ten-

sile strength of PET/NPG shrink films was found when solvent exposure was absent.

Also, maximum seam strength was seen when no prior THF solvent treatment of shrink

films was conducted.

46

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