improving the long-term performance of pvc compositions/67531/metadc955074/...poly(vinyl chloride)...

IMPROVING THE LONG-TERM PERFORMANCE OF PVC COMPOSITIONS

Yu-Chia Yang, B.S.

Thesis Prepared for the Degree of

MASTER OF SCIENCE

UNIVERSITY OF NORTH TEXAS

December 2016

APPROVED:

Witold Brostow, Major ProfessorSamir Aouadi, Committee Member

Zhenhai Xia, Committee Member

Andrey Voevodin, Chair of theDepartment of Material Scienceand Engineering

Costas Tsatsoulis, Dean of the College of Engineering

Victor Prybutok, Vice Provost of theToulouse Graduate School

Yang, Yu-Chia. Improving the Long-Term Performance of PVC Compositions. Master of

Science (Materials Science and Engineering), December 2016, 37 pp., 4 tables, 15 illustrations,

references, 26 titles.

Poly(vinyl chloride) (PVC) are extensively applied in many fields, such as cables, pipes,

vehicles, shoes, toys and infusion bags. Generally, plasticizers are blended with PVC to improve

the ability of process in industrial production; however, these toxic plasticizers will gradually

migrate toward the surface of products and such a leakage results in brittleness of plasticized

PVC and environmental pollutions. In other words, humans are frequently exposed to the

potential risks. According to previous researches, cross-linked PVC was proved that it was able

to hinder the migration of plasticizer. Thus, in this research, we selected some commercially

used cross-linking agents and employed six different tests based on mechanical, tribological and

microscopy analysis in order to seek the best solution against plasticizer migration. Thus, we

expected to develop a cross-linked flexible PVC which performed improved long-term

performance and extended lifetime.

ii

Copyright 2016

by

Yu-Chia Yang

iii

ACKNOWLEDGEMENT

I would like to sincerely thank to my academic adviser, Dr. Witold Brostow, who provides

me such a precious opportunity to work in LAPOM. Dr. Witold Brostow is the best guider I have

ever met. He is a wise man who not only imparts knowledge regarding polymers to me, but also

teaches me some philosophies through his life experience. I would also like to thank to my

committee members including Dr. Samir Aouadi and Dr. Zhenhai Xia, who gave me useful

advices to make my thesis more completed.

I want to thank our laboratory manager Nathalie Hnatchuk who taught me how to build a

systematic and comprehensive research in order to valid the assumptions in a project and I was

trained to be extremely sensitive to experimental data. I would like to thank to all my laboratory

mates who have ever helped me to do variety experiments.

I appreciate what my beloved family members have done for me. I would not have earner a

Master’s degree without their support.

iv

TABLE OF CONTENTS

Page ACKNOWLEDGEMENT ........................................................................................................................... iii

LIST OF TABLES ....................................................................................................................................... vi

LIST OF FIGURES .................................................................................................................................... vii

CHAPTER 1. INTRODUCTION ................................................................................................................. 1

1.1 Introduction of poly(vinyl chloride) and Current Situation ................................................................ 1

1.2 Approaches of Inhibiting Plasticizer Migration .................................................................................. 2

1.3 Objectives ........................................................................................................................................... 3

CHAPTER 2. EXPERIMENTAL PROCEDURES ...................................................................................... 4

2.1 Materials ............................................................................................................................................. 4

2.2 Sample Preparation ............................................................................................................................. 4

2.2.1 Mixing Procedures ....................................................................................................................... 4

2.2.2 Heat Treatment .......................................................................................................................... 6

2.3 Theories, Apparatus and Test Procedures ........................................................................................... 8

2.3.1 Tensile Tests ................................................................................................................................ 8

2.3.2 Tribological Tests ........................................................................................................................ 9

2.3.3 Surface Profilometry Analysis ................................................................................................... 10

2.3.4 Scratching Tests ......................................................................................................................... 11

2.3.5 Water Absorption Tests ............................................................................................................. 12

2.3.6 Scanning Electron Microscope (SEM) Analysis........................................................................ 13

CHAPTER 3. RESULTS AND DISCUSSION .......................................................................................... 15

3.1 Mechanical Properties ....................................................................................................................... 15

3.2 Dynamic Friction .............................................................................................................................. 17

v

3.3 Wear Rates ........................................................................................................................................ 19

3.4 Viscoelastic Recovery ....................................................................................................................... 20

3.5 Water Absorption .............................................................................................................................. 24

3.6 SEM Analysis ................................................................................................................................... 26

CHAPTER 4. CONCLUSIONS ................................................................................................................. 32

REFERENCES ........................................................................................................................................... 35

vi

LIST OF TABLES

Page Table 2.1 Comparison of mechanical properties for samples with different concentrations of cross-linking

agents .......................................................................................................................................... 5

Table 2.2 Formulae of samples ..................................................................................................................... 5

Table 2.3 Samples and images of surfaces ................................................................................................... 7

Table 3.1 Average penetration depth, residual depth and viscoelastic recovery within 6.0 N to 15.0 N

applied load ............................................................................................................................... 22

vii

LIST OF FIGURES

Page Figure 3.1 Comparison of Young’s modulae .............................................................................................. 15

Figure 3.2 Comparison of strain at break .................................................................................................... 16

Figure 3.3 Comparison of tensile toughness results ................................................................................... 17

Figure 3.4 Comparison of dynamic friction results .................................................................................... 19

Figure 3.5 Comparison of wear rates .......................................................................................................... 20

Figure 3.6 Viscoelastic recovery as a function of applied load for samples without heat treatment .......... 22

Figure 3.7 Viscoelastic recovery as a function of applied load for samples with heat treatment at 100℃ 23



Figure 3.10 Weight increase as a function of time for samples without heat treatment and samples with

heat treatment at 100℃ .......................................................................................................... 25

Figure 3.11 Weight increase as a function of time for samples with heat treatment at 121℃ and 136℃ .. 26

Figure 3.12 SEM images of control sample and Samples I, II and III without heat treatment ................... 28

Figure 3.13 SEM images of control sample and Samples I, II and III with heat treatment at 100℃ ......... 29



1

CHAPTER 1

INTRODUCTION

1.1 Introduction of poly(vinyl chloride) and Current Situation

Poly(vinyl chloride) also known as PVC, is one of the most widely used synthetic plastics.

According to statistics from the Chemical Market Association, Inc., the worldwide consumption

of PVC in 2007 was 35.3 million metric tons while the average demand for PVC increases 4.8

percent every year.[1] When PVC first appeared in 1892, it was ignored by scientists because of

its very high brittleness and difficulties of processing. It was not until Waldo Semon applied

plasticizers to PVC in 1926 so that PVC became more flexible and easier to shape. [2] Nowadays,

flexible PVC is a kind of polymeric material which exhibits high mechanical properties, high

electrical resistivity, and most importantly, it is resistant to corrosive solutions and relatively fire

resistant. In addition to these advantages, low cost of PVC makes it a good option for

construction materials, pipes and wire insulators.

However, flexible PVC has a severe flaw: the migration of plasticizer is the main reason that

aging of PVC is accelerated. In other words, the service life of PVC is shortened. Plasticizer

leaching not only affects material properties, but also provides environmental pollution and

potentially diseases. First, heat and pressure are two common environmental factors that may

force the plasticizers to move out of PVC matrix and makes PVC-based materials brittle. [3] As

discussed below in Chapter 3, once the elongation at break become smaller, the brittleness B

increases. Such materials will not be able to withstand even relatively small deformations and

fractures will occur. In addition, the exuded plasticizers residing on the surface of PVC material

will lower the surface energy so that cracks will be easier to create. [4, 5] Such a vulnerable

2

characteristics will lead to dramatic effects in industrial applications. Second, exudation and

evaporation of plasticizer are serious issues while flexible PVC materials are extensively used.

Generally, 6.4 million tons of plasticizer are used annually and about 30% of plasticizers are

emitted into air in an ‘ideal’ experimental chamber.[6, 7] This is still a very large number that

people should face, although the real amount of emission may be lower depending on different

environments. To conclude, the problem of plasticizer migration must be controlled as soon as

possible in order to isolate humans from toxic substances.

1.2 Approaches of Inhibiting Plasticizer Migration

We need to figure out the mechanism of plasticizer diffusion, before choosing the most

effective method to mitigate such diffusion. There are two main factors which influence the

migration rate of plasticizer: its molecular mass and density of the barriers between free spaces

in the PVC matrix. [8–10] Numerical calculations based on the above factors have been made in

order to reduce the migration rate of the plasticizer.

(1) Increasing molecular mass of the plasticizer is a direct method for lowering the migration

rate, hence a polymeric plasticizer can be substituted for a conventional one.[11] However, PVC

blended with polymeric plasticizers shows poor mechanical properties because a polymeric

plasticizer has high viscosity. As a result, PVC chains are “frozen” and have difficulty to move

and stretch. Therefore, PVC loses its flexibility when polymeric plasticizers are added.

(2) Cross-linking is another option to limit the movement of plasticizer. Cross-links can act

as barriers while plasticizers are transferred from one free space to the others. Cross-links are

induced and formed between PVC chains by exposure to ultraviolet radiation [12]; nevertheless,

irradiation with ultraviolet at the initial step will lead to degradation of PVC. Thus, long-term

exposure to ultraviolet is necessary in order to compensate for degradation. According to the

3

experiments carried out by Duvis, it took at least 10 days of irradiation to complete the whole

process of cross-linking PVC specimens.[13] From the point of view of industry, long-term

ultraviolet irradiation is not time and cost efficient.

(3) Chemical reaction is another way to increase the density of the barrier.[14] Cross-linking

agents are employed and PVC chains are bonded by the bridging cross-links. This is currently

the simplest method to hinder the migration of plasticizer and there are no obvious disadvantages.

1.3 Objectives

Given the situation described above, I chose the method of chemical cross-linking and three

different kinds of cross-linking agents—with the objective of defining the best one. Heat

treatments at low, medium and high temperatures were carried out because temperature is a

significant factor that affects the rate of migration in polymers.

4

CHAPTER 2

EXPERIMENTAL PROCEDURES

2.1 Materials

In this research, samples are composed of different materials, including PVC resin,

plasticizer, three kinds of cross-linking agents and a thermal stabilizer. All of the components we

used are commercially available materials. In Chapter 1, the purpose of employing a plasticizer

and a cross-linking agent have been explained—except for a thermal stabilizer.

Chlorine atoms are covalently bonded to carbon atoms in PVC structure; however, such

bonds are not stable at elevated temperatures. This results in the decomposition of PVC when it

is heated prior to processing. A thermal stabilizer is added in order to prevent degradation of

PVC.[15] Thus, processing PVC at elevated temperatures becomes possible.

2.2 Sample Preparation

2.2.1 Mixing Procedures

Table 2.1 shows the comparison of mechanical properties and concentrations for PVC

samples blended with different cross-linking agents. According to Table 2.1, there are no

obvious changes of Young’s modulae as the concentrations of cross-linkers increase. However,

we can find that there are relatively larger differences in strain at break and tensile toughness

between concentrations at 3% and 4% for each kind of sample. Consider the relations of

mechanical properties and effectiveness of cross-linking, we chose to mix our PVC resin with 4

wt% of different cross-linking agents.

5

Table 2.1 Comparison of mechanical properties for samples with different concentrations of

cross-linking agents

Concentration Mechanical property Flexible PVC +

cross-linker 1

Flexible PVC +

cross-linker 2

Flexible PVC +

cross-linker 3

1%

Young’s modulus (MPa) 342±58 366±40 369±55

Strain at break (%) 309±34 299±33 320±45

Tensile toughness (N*m) 8.7±0.7 8.9±1.1 9.0±0.8

2%


Strain at break (%) 308±15 309±56 322±77


3%


Strain at break (%) 315±31 306±18 335±44


4%


Strain at break (%) 329±63 333±28 361±42


Table 2.2 shows individual formulae for each kind of sample. Control sample was prepared

without a cross-linking agent, while Samples I, II and III contained different cross-linking agents

such as Cross-linker 1, Cross-linker 2 and Cross-linker 3, respectively.

Table 2.2 Formulae of samples

Samples

Components Control I II III

PVC 100 phr 100 phr 100 phr 100 phr

Plasticizer 25 phr 25 phr 25 phr 25 phr

Thermal

Stabilizer 3 phr 3 phr 3 phr 3 phr

Cross-linker 1 N/A 4 % N/A N/A

Cross-linker 2 N/A N/A 4 % N/A

Cross-linker 3 N/A N/A N/A 4 %

6

In order to prepare homogeneous PVC mixtures, a heater (SCILOGEX MS-H280-Pro) and a

stirrer (Caframo Petite Digital Stirrer BDC250U1) were used. First, a batch of pure PVC resin

with a thermal stabilizer was preheated at 70℃ and stirred at a speed of 1500 revs/min for 5

minutes. After 5 minutes, temperature was raised to 90℃. Second, an appropriate amount of

plasticizer was added and stirred for another 5 minutes. Third, some droplets of cross-linker were

dripped into PVC mixture and kept stirring it for 5 minutes. Once the PVC mixture was well-

mixed, we fed it to an extruder (Thermo Electron Corporation HAAKE PolyDrive Extruder)

which has four zones and one die with different temperatures ranging from 350℉ to 400℉. A

sheet die with 50 mm gap was used in order to produce sheet samples.

2.2.2 Heat Treatment

As mentioned before, temperature is a critical factor affecting the migration rate. According

to the standard requirements of Underwriters Laboratories Inc., thermoplastic-insulated wires

and cables must meet the requirements of UL83 and UL2556 which state that samples that

underwent a 7-day heat treatment must maintain at least 45% tensile elongation at break when

compared to samples not subjected to heat treatment.[16, 17] The temperature of heat treatment

listed in UL83 is 136℃ and the temperatures defined in UL2556 are 121℃ and 100℃. Following

the requirements of UL83 and UL2556, our samples were placed in an oven (Shellab 1408

economy vacuum oven) so that they did not touch the walls of the chamber for 7 days at 100℃,

121℃ and 136℃. Table 2.2 lists the sample names corresponding to different temperatures of

heat treatment and surfaces of the samples. We can find that the control sample shows darker

surface color than the cross-linked samples, since degradation is more likely happened in the

control sample which has no combined PVC chains.

7

Table 2.3 Samples and images of surfaces

Sample Type of Cross-linker Heat Treatment Surface

RT-Control N/A N/A

100C-Control N/A 100℃



RT-I Cross-linker 1 N/A

100C-I Cross-linker 1 100℃



RT-II Cross-linker 2 N/A

8

100C-II Cross-linker 2 100℃



RT-III Cross-linker 3 N/A

100C-III Cross-linker 3 100℃



2.3 Theories, Apparatus and Test Procedures

2.3.1 Tensile Tests

Some dog-bone shaped specimens were cut from flatted PVC sheets which were made from

the extruder. The dimensions of dog-bone specimen are consistent with ASTM D638 type IV. In

this research, eight specimens were tested for each kind of sample with 5.0 mm/min tensile speed

9

and the MTS® Universal Testing Machine QTEST/5 was used to determine the mechanical

properties such as Young’s modulus, strain at break and tensile toughness.

The definition of Young’s modulus is the slope in stress-strain diagram within the region of

elastic deformation and it is written as:

E = 𝜎

𝜀(1)

where σ and ε represent elastic stress and elastic strain, respectively. Larger Young’s modulus

means more external force is needed to elastically deform the sample. For strain at break, it

refers to the degree of deformation at break and the expression is:

ε = ∆𝐿

𝐿(2)

where ΔL and L represent the elongation at break and the initial length of sample, respectively.

Obviously, the higher strain at break the sample has, the larger elongation takes place. Lastly,

tensile toughness indicates how much energy is required to tear the sample and is represented by

the area under the stress-strain curve.

2.3.2 Tribological Tests

Tribology is a very broad area that includes the studies of friction, lubrication, wear,

adhesion, scratch resistance and any interactions of multiple surfaces. [18, 19] In this research,

tribological tests focus on the process which converts kinetic energy into thermal energy by

friction. In classical physics, friction is divided into static friction and kinetic (dynamic) friction.

Static friction is a friction between the surfaces of two stationary objects and it is generated to

react against the external force. The static friction can be described by an expression:

10

𝐹𝑠 = 𝜇𝑠 ∙ 𝑁 (3)

where μs is static friction which is related to property of surface and type of material and N

represents the normal force exerted on the contact area. On the other hand, dynamic friction

appears while two objects are sliding and it can be expressed as:

𝐹𝑘 = 𝜇𝑘 ∙ 𝑁 (4)

where μk is dynamic friction and it significantly depends on surface property which is changed

by residual plasticizer in this research.

In order to prove the effectiveness of cross-linking, dynamic friction is an important

indicator and it is determined by using tribological tester produced by Nanovea Inc. The testing

mode we chose was called “pin-on-disk” mode. As the name implies, a specimen is secured on a

spinning disk and it is contacted with a stationary pin which is subjected to normally 5.0 N force

while the machine is running. A SS302 stainless steel ball with 3.2 mm diameter is used as a pin

in this work. During the testing, the total sliding distance is 75.36 m (6000 revolutions and a

track with 2 mm radius) and the spinning speed is 200 revs/min.

2.3.3 Surface Profilometry Analysis

The purpose of profilometry analysis is to determine one variable in the equation of wear

rate and the wear rate is an index that describes the degree of abrasion. In other words, lower

wear rate refers to a stronger ability to resist against abrasion. The wear rate[18 - 20] can be

expressed as:

Wear Rate = 𝑉𝑙𝑜𝑠𝑠

𝑁𝑥 (5)

11

where N and x are force and total distance in tribological test, respectively, and Vloss is the

volumetric loss of a specimen. In this case, Vloss refers to the volume of a track which was

created during a tribological test. Thus, Vloss is approximately described as:

𝑉𝑙𝑜𝑠𝑠 = πRA (6)

where R refers to track radius and A is the cross-sectional area of a track. By combining

equations (5) and (6), one obtains:

Wear Rate = 𝜋𝑅𝐴

𝑁𝑥(7)

So far, there is only one unknown term which is called cross-sectional area (A) and it will be

determined by profilometry analysis.

In this work, Dektak® 150 stylus surface profilometer produced by Veeco Instruments Inc. is

used to construct the surface topology or surface metrology of a sample. It is a contact-type

scanning machine which has a stylus moving across a sample with 1.0 mg force applied and the

tip radius of the stylus is 12.5 μm. The scanning length is 1200 μm and measurement depth is

524 μm that must be chosen larger than the dimensions of every track. After 60-second scanning,

the surface features are displayed as a result of two-dimensional graph and the surface

information including cross-sectional area is calculated by a specific program.

2.3.4 Scratching Tests

Viscoelastic recovery is a property that represents the ability of healing for a polymer after it

is deformed by external forces in scratch resistance determination. The percentage viscoelastic

recovery[18 - 20] can be written as:

12

Viscoelastic Recovery = 𝑃𝑑−𝑅𝑑

𝑃𝑑× 100% (8)

where Pd and Rd are the penetration depth and the recovery depth, respectively. These two

quantities are defined in the scratching test. Penetration depth is measured simultaneously by the

indenter while the indenter is scratching a specimen. On the other hand, recovery depth is

detected by the indenter after a specific time in order to assure the specimen is fully recovered.

The process of measuring recovery depth is very similar to profilometry analysis. The tip of the

indenter is exerted by 0.03 N force to slightly contact to the groove and the recovery depth

encountered by the indenter will be presented afterwards.

The apparatus we employed for scratching test is Micro Combi Tester produced by CSM®

Instruments and this is a multifunctional instrument so that scratching tests and hardness tests

can be carried out. The scratch length is 5.0 mm and scratch speed is 5.0 mm/min. During the

scratching process, the load of the indenter linearly increases from 0.03 N to 15.0 N while the

allowed recovery time is 2 minutes. As we have mentioned, fully recovered status is a basic

criterion to determine the recovery time. We chose 2 minutes for recovery time, since we did not

observe apparent differences of recovery depth and percentage recovery for a recovery time

longer than 2 minutes.

2.3.5 Water Absorption Tests

For these tests, average weight changes of three specimens of each sample were recorded

until no obvious weight increments were observed. The weight change can be simply described

as:

Weight change = 𝑊−𝑊𝑖

𝑊𝑖× 100% (9)

13

where W and Wi represent the current weight and the initial weight of a specimen. Typically, all

specimens stopped absorbing more water on the seventh day of tests; thus, the measurements of

weight were carried out at some particular time within 7 days such as 15 minutes, 30 minutes, 1

hour, 3 hours, 6 hours, 12 hours and first day to seventh day. During water absorption tests, each

specimen was immersed in a separate beaker filled with hot water at 90℃. These beakers were

covered with aluminum foil in order to prevent the evaporation of water and they were placed in

a furnace (ThermolyneTM Atmosphere Controlled Ashing Furnaces) to maintain at the same

temperature for one week.

2.3.6 Scanning Electron Microscope (SEM) Analysis

Quanta 200 SEM, a product from FEI Company, was used to observe the cross-sectional

areas after the specimens were torn by the tensile tester. By observing these areas at 1000x

magnification, one is able to see how close-packed the PVC chains are and the orientations of

chains. In this work, the working distance of Quanta 200 SEM ranges from 9.4 mm to 12.8 mm

and the Everhart-Thornley detector is used—operated in the high vacuum mode with the electron

accelerating voltage of 12.50 kV. We could not use higher voltage for higher resolution and

magnification since PVC samples could not withstand the irradiation of such high-energy

electron beam.

The major damages to polymers resulting from electron beams can be categorized into two

types: heat and radiolysis. [21] For electron beam heating, the temperature of specimen raises due

to the inelastic collisions of incident electrons and molecules in a specimen. The heating damage

becomes worse for polymers since polymers generally have low thermal conductivity. Thus, heat

dissipation is restricted and degradation or even melting may take place at elevated temperatures.

For radiolysis damage, ionization decomposition is the mechanism. As the name ionization

14

indicates, the electrons in the molecules are excited to higher energy states but do not return to

stable states. Consequently, the breakages of chemical bonds are generated and cause mass loss

of light atoms such as hydrogen, nitrogen and oxygen.

15

CHAPTER 3

RESULTS AND DISCUSSION

3.1 Mechanical Properties

Figure 3.1 shows the Young modulae of all samples after different degrees of heat treatment.

There are two factors that influence Young’s modulus: temperature and cross-linking.[22, 23] From

Figure 3.1, the first thing that attracts attention is that Young’s modulae increase as the

temperature of heat treatment increases. This trend is natural, given the evaporation of plasticizer

which results in the PVC samples becoming more brittle. However, adding cross-linking agents

could prevent the exudation of the plasticizer and maintaining the flexibility of PVC—as

demonstrated below. In Figure 3.1, we find some samples containing cross-linking agents that

show higher Young’s modulae than the control sample; apparently the separate PVC chains were

cross-linked and formed a network structure. The network structure leads to an increasing

Young’s modulus.[24] Generally, the cross-linked sample not only conserve high flexibility but

also show improved Young’s modulae.

Figure 3.1 Comparison of Young’s modulae

0

100

200

300

400

500

600

700

800

900

1000

Control Sample I Sample II Sample III

Yo

ung's

mo

dulu

s (M

Pa)

No heat treatment 100℃ 121℃ 136℃

16

Figure 3.2 presents the comparison of tensile strain at break for each case. For the same

reasons as we have stated in the Young modulae discussion, our samples lost flexibility as a

result of heat treatment. Thus, a decreasing of strain at break is seen in Figure 3.2. On the other

hand, if the cross-linked samples are compared with the control sample, we find that cross-linked

samples have relatively higher strain at break than the control sample after being subjected to the

same heat treatment. This can be explained by the formation of cross-links which improve the

ability of stretching between PVC chains.

By recalling section 2.2.2, the standard requirements UL83 and UL2556 state that the

remaining strain at break must be at least 45% after heat treatment at 100℃, 121℃ and 136℃.

Those numbers embedded in Figure 3.2 represent the remaining strain at break based on the

values for samples without heat treatment. According to Figure 3.2, all samples studied fulfill the

requirements of standards.

Figure 3.2 Comparison of strain at break

84%

80%80%

82%

87%

88%

80%67%

70%

84%

67%

78%

0%

50%

100%

150%

200%

250%

300%

350%

400%

450%


Str

ain a

t b

reak

(%

)


17

Tensile toughness is defined as the area under the stress vs. strain curve. In Figure 3.3, the

fact is proved again that tensile toughness (and flexibility along with it) decreases as temperature

increases. In addition, the toughness increases after application of cross-linking agents. We

conclude that the mechanical properties including Young’s modulae, strain at break and tensile

toughness were improved when the flexible PVC samples were blended with these three chosen

cross-linking agents.

Figure 3.3 Comparison of tensile toughness results

3.2 Dynamic Friction

As mentioned in Section 2.3.2, the friction is mainly affected by two factors: nature of the

surface and type of the material. The fundamental components of control sample and Sample I, II,

and III were the same commercial PVC and plasticizer; thus, the differences of dynamic friction

displayed in Figure 3.4 are caused by changes of surfaces. Plasticizer is a kind of viscous fluid

and it increases the adhesion between two moving parts.[14] When the migration of plasticizer

0

2

4

6

8

10

12

14


Ten

sile

to

ughnes

s (N

*m

)


18

occurs, a large amount of plasticizer resides on the surface of sample. Therefore, the residual

plasticizer changes the surface and increases the dynamic friction.

According to Figure 3.4, if we focused on individual sample, we found that generally the

dynamic friction increased as the temperature of heat treatment increased, since more plasticizer

moved toward the surface of sample. However, if all the samples at the same temperature were

compared, we discovered the cross-linked samples exhibited lower dynamic friction than the

control sample. Moreover, Sample II exhibits the lowest dynamic friction in every case of heat

treatment. We find that the cross-links formed between PVC chains clearly mitigate the

migration of the plasticizer.

In addition to the above explanation in terms of the effectiveness of cross-linking, we can

interpret the dynamic friction results also from a different angle. The threshold temperature at

which a significant amount of the plasticizer begins to exudate is seen in Figure 3.4. Focusing on

the control sample, it is worth to notice that the dynamic friction values at 121℃ and 136℃

differ very little. Apparently the majority of the plasticizer starts to exudate around 121℃. On

the other hand, differences in dynamic friction values are seen between 121℃ and 136℃ for

Sample I, II and III. We infer that a large process of exudation was launched around 136℃ for

cross-linked flexible PVC. In short, the exudation of the plasticizer for control sample occurs at

lower temperatures than for cross-linked ones. This is another evidence of the ability of cross-

linking agents in restricting plasticizer exudation.

19

Figure 3.4 Comparison of dynamic friction results

3.3 Wear Rates

Wear is often an unexpected process from the industrial viewpoint; it costs large amounts of

money to maintain worn parts or replace damaged ones because people ignored the importance

of surface interaction in the past time. Nowadays, tribology has grown and some solutions for

decreasing wear have been provided. Cross-linking is one of the methods to decrease the wear

rate [25]; we consider the results reported below from this point of view.

According to Figure 3.5, an increase of wear rate with the treatment temperature is found for

all samples. It can be explained that the surface of each specimen is occupied by more plasticizer

as the temperature becomes higher. The high density of the plasticizer on the surface results in

higher dynamic friction acted and a larger amount of debris formed. Thus, controlling the

migration of the plasticizer is a crucial way to reduce the wear rate. Aside from the effect of

residual plasticizer, network structure resulting from cross-linking is another factor enhancing

wear resistance.[25] The tightly combined chains in cross-linked flexible PVC prevent the

0.3

0.35

0.4

0.45

0.5

0.55


Dynam

ic f

rict

ion


20

mobility and fractures of chains and the detachment of fragment from a specimen. Briefly

speaking, the results of wear rate reveal that Sample I, II and III achieve high enough wear

resistance, especially when they underwent high-temperature heat treatment. Thus, cross-linked

flexible PVC can survive longer without oozing plasticizers and generating damage.

Figure 3.5 Comparison of wear rates

3.4 Viscoelastic Recovery

PVC is a viscoelastic material, as are its plasticized versions. Such materials exhibit

simultaneously elastic and viscous flow behavior during deformation. Dynamic mechanical

analysis (DMA) allows determination of the storage modulus E' and the loss modulus E" to

quantify respectively the solid-like elastic behavior and liquid-like viscous behavior.[19] An

elastic material deforms immediately when it is subjected to an external force and it returns to

the original shape immediately once the force is removed. This while viscous materials exhibit

time-delay behavior of strain, which means strain always happens after stress.

Witold Brostow et al. provided a useful equation as definition of brittleness.[26] Namely:

0

2

4

6

8

10

12

14

16

18


Wea

r ra

te(x

10

-5m

m3N

-1m

-1)


21

B = 1

𝜀𝑏𝐸′ (10)

where E' is the storage modulus at 1.0 Hz and εb is the tensile strain at break. They have also

demonstrated that B is inversely proportional to the viscoelastic recovery defined by Eq. (8). [26]

The same group has also shown that B is inversely proportional to the tensile toughness. [27]

Therefore, return now to the results of tensile toughness reported in Section 3.1. Cross-linked

samples exhibit higher tensile toughness than the control sample. Particularly for the heat

treatment at 121℃ and 136℃, we observe large differences of tensile toughness between cross-

linked samples and control sample. Thus, from Figure 3.8 and Figure 3.9, we observe the control

sample exhibits relatively lower viscoelastic recovery than the cross-linked samples.

In Table 3.1, we display the average penetration depth, residual depth and viscoelastic

recovery corresponding to the applied load from 6.0 N to 15.0 N, since small applied load might

cause some inaccuracies in depths and results of recovery. For example, small applied load

generated a shallower groove on the surface of a specimen; therefore, it became more difficult to

precisely measure the small penetration depth and residual depth. According to Figures 3.6 to

Figure 3.9, such inaccuracies in depths leaded to obvious differences of viscoelastic recovery

between large applied force and small applied force. Returning to Table 3.1, we can clearly see

Sample I, II and III have not only smaller penetration and residual depths, but also show

relatively higher viscoelastic recovery than control sample. Moreover, it is worth to note that

Sample II has the best performance in scratching tests because it maintained over 90%

viscoelastic recovery and is better penetration resistant in each case of heat treatment. To

conclude, we verify that viscoelastic recovery is improved by cross-linked structure because

more plasticizers are contained inside the PVC matrix and the flexibility of PVC is maintained.

22

Table 3.1 Average penetration depth, residual depth and viscoelastic recovery within 6.0 N to

15.0 N applied load

Sample Penetration

depth (μm)

Residual

depth (μm)

Viscoelastic

recovery Sample

Penetration

depth (μm)

Residual

depth (μm)

Viscoelastic

recovery

RT-

Control 154 20.3 87%

100C-

Control 257 105 87%

RT-I 156 23.3 84% 100C-I 129 10.5 91%

RT-II 128 8.9 92% 100C-II 127 10.0 92%

RT-III 167 23.4 86% 100C-III 145 16.3 89%

Sample Penetration

depth (μm)

Residual

depth (μm)

Viscoelastic

recovery Sample

Penetration

depth (μm)

Residual

depth (μm)

Viscoelastic

recovery

121C-

Control 143 42.7 68%

136C-

Control 209 77 62%

121C-I 127 9.2 93% 136C-I 122 8.4 93%

121C-II 137 11.5 91% 136C-II 137 3.3 97%

121C-III 141 4.4 96% 136C-III 130 8.6 94%

Figure 3.6 Viscoelastic recovery as a function of applied load for samples without heat treatment

0%

20%

40%

60%

80%

100%

120%

0 2 4 6 8 10 12 14 16

Vis

coel

asti

c re

cover

y

Applied load (N)

RT-Control RT-I RT-II RT-III

23

Figure 3.7 Viscoelastic recovery as a function of applied load for samples with heat treatment at

100℃


121℃

0%

20%

40%

60%

80%

100%

120%

0 2 4 6 8 10 12 14 16

Vis

coel

asti

c re

cover

y

Applied load (N)

100C-Control 100C-I 100C-II 100C-III

0%

20%

40%

60%

80%

100%

120%

0 2 4 6 8 10 12 14 16

Vis

coel

asti

c re

cover

y

Applied load (N)


24


136℃

3.5 Water Absorption

In general, flexible PVC is classified as a hydrophobic material, hence water repellent.

However, Figures 3.10 and 3.11 reveal that a small amount of water was absorbed by non-cross-

linked flexible PVC, about 2% to 6% of its initial weight. Apparently water molecules were

attracted by C-Cl bonds in flexible PVC. The polarity of C-Cl bonds is the main reason for water

absorption because Coulombic attraction is created between C-Cl bonds and water molecules

which are well known as polar.[28]

In addition to polarity, the issues of free spaces and cross-links between PVC chains are

other factors that influence the results of water absorption tests.[29] As we have already known,

cross-links are thought as barriers while the plasticizer molecules are moving. For the same

reason, water molecules were repelled by such barriers and found difficult to occupy the free

spaces between PVC chains. Although a few water molecules could overcome the barriers and

0%

20%

40%

60%

80%

100%

120%

0 2 4 6 8 10 12 14 16

Vis

coel

asti

c re

cover

y

Applied load (N)


25

occupy the free spaces, the relatively small free spaces limited the saturated amount of water

absorbed by flexible PVC with significant cross-linking.

Consider Figure 3.10; the weight increments are: for the control sample we have 2.10% and

2.94%; for Sample I we have 0.16% and 1.33%; for Sample II we have 0.56% and 1.75%; for

Sample III we have 0.27% and 1.79%. For samples without heat treatment and with heat

treatment at 100℃, Figure 3.10 shows that all cross-linked flexible PVC samples show less water

absorption than the control sample does. We infer that there were more free spaces left by the

migration of plasticizer and no cross-links were present to hinder the ‘invasion’ of water

molecules for the control sample. On the other hand, in Figure 3.11 we notice that Sample I and

II maintain low weight increments compared to the control sample, while the weight increments

for Sample III exceed those for the control sample at high degrees of heat treatment. Thus, we

conclude that Sample III performed not as well as Sample I and Sample II did in water

absorption tests.

Figure 3.10 Weight increase as a function of time for samples without heat treatment and

samples with heat treatment at 100℃

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

3.5%

0 1 2 3 4 5 6 7

Wei

ght

incr

ease

Time (days)

100C-Control

RT-Control

100C-III 100C-II

100C-I

RT-II

RT-III RT-I

26

Figure 3.11 Weight increase as a function of time for samples with heat treatment at 121℃ and

136℃

3.6 SEM Analysis

SEM is a convenient method to understand how cross-linking agents change the structure of

flexible PVC; however, the limit of using high energy electron beam and high magnification is a

disadvantage when polymers are concerned. We still can find some evidences of cross-linking,

although the cross-links cannot be directly seen under a microscope. If a specimen is highly

cross-linked, it means that there are more chains combined together. As a consequence,

numerous wide and white lines are seen in the images of SEM. The other evidence of cross-

linking is the orientations of chains. We find that the chains in cross-linked flexible PVC are

oriented along a specific direction. As the results of a plasticizer, chains were able to move and

stretch along the direction of tensile force before fracture occurred. If such phenomenon of

particular orientation was shown in the images which contain specimens with heat treatment, we

can conclude that the plasticizers were kept in flexible PVC matrix due to the effectiveness of

cross-linking.

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

7.0%

0 1 2 3 4 5 6 7

Wei

ght

incr

ease

Time (days)

136C-III 136C-Control

121C-III 136C-II 136C-I

121C-Control

121C-II 121C-I

27

By examining Figure 3.12, we find that Sample II and Sample III show wide and white

cross-linked chains in the case without heat treatment. Although Sample I does not exhibit such

wide cross-linked chains like Sample II and III do, it has denser packing characteristic than the

control sample has. In Figure 3.13, in addition to close-packed chains, oriented chains are present

in Samples I, II and III but not in the control sample. As for Figure 3.14 and Figure 3.15 with

severe degrees of heat treatment, Sample I and Sample II still maintain some characteristics of

cross-linking. However, we are not able to observe dense chains and orientation in the control

sample and Sample III. In summary, Sample I and II can survive in high temperature situations

without losing plasticizers and flexibility, while Sample III can only withstand medium

temperatures.

28

Figure 3.12 SEM images of control sample and Samples I, II and III without heat treatment

RT-Control RT-I

RT-II RT-III

29

Figure 3.13 SEM images of control sample and Samples I, II and III with heat treatment at 100℃

100C-I

100C-II 100C-III

100C-Control

30


121C-I

121C-II 121C-III

121C-Control

31


136C-I

136C-II 136C-III

136C-Control

32

CHAPTER 4

CONCLUSIONS

Being one of the most popular plastics, PVC is worth for scientists to do further researches.

Humans developed plasticized PVC about a century ago but were aware of the danger of exuded

plasticizers only in recent decades. The migration of plasticizer results in brittleness of flexible

PVC and environmental pollution. In order to solve this problem, we must determine the most

feasible way to hinder plasticizer diffusion. Given the disadvantages of polymeric plasticizer and

ultraviolet-induced cross-linking, the chemical cross-linking is the best approach to enhance the

degree of cross-link and restrict the migration of plasticizer. In this research, three types of cross-

linking agent were carefully selected and blended with commercial PVC, plasticizer and thermal

stabilizer. Our purpose is to achieve the most effective formula in order to improve the long-term

performance of flexible PVC. According to the experimental results including mechanical

properties, tribological properties, water absorption and SEM analysis, the effectiveness of cross-

linking for different PVC compositions was clearly presented.

From the results of mechanical properties, we found that the cross-linked structure could not

only prevent the evaporation of plasticizer, but also slightly decrease the relative movement

between PVC chains. Consequently, the mechanical properties such as Young’s modulus, strain

at break and tensile toughness were improved when the flexible PVC samples were blended with

cross-linking agents which we have chosen.

As for the tribological properties, they include dynamic friction, wear rate and scratch

recovery. The summary is as follows:

(1) The dynamic friction depended on the surface structure of a specimen—altered by

plasticizer because plasticizer increased the adhesion between two moving parts. According to

33

the results of dynamic friction, we discover the cross-linked samples have lower dynamic friction

while Sample II exhibits the lowest dynamic friction. Besides, the plasticizer migration of

control sample happened before the plasticizer migration of cross-linked samples. These results

indicate that the plasticizer migration is reduced by cross-links.

(2) The results of wear rate reveal that Samples I, II and III achieved higher wear resistance,

since less plasticizer resided on the surfaces of cross-linked samples. Besides, the cross-links

restricted the mobility of chains and prevented fracture in the specimens.

(3) Considering the results of viscoelastic recovery, cross-linked samples exhibited higher

viscoelastic recovery than the sample without cross-linkers, since more plasticizers were limited

inside the cross-linked PVC matrix and the flexibility was maintained. According to Table 3.1,

Sample II shows the highest average viscoelastic recovery in scratching tests.

According to the results of water absorption tests, all cross-linked samples have better water

resistance than the control sample without heat treatment and with heat treatment at 100℃. This

indicates that significant plasticizer migration generated many free spaces which can be occupied

by water molecules in a specimen without cross-linking. However, the Sample III shows the

highest weight increments at high degrees of heat treatment, while Sample I and II remain highly

water repellent.

From Figure 3.12 and 3.13, the cross-linked Samples I, II and III exhibit the evidences of

cross-linking including wide cross-linked chains and specific orientations of chains. On the other

hand, in Figure 3.14 and Figure 3.15 with heat treatment at high temperature, we are unable to

observe close-packed chains and orientations in Sample III. In summary, Samples I and II

preserve the characteristics of cross-linking in high temperature situation, while Sample III can

only withstand medium degree of heat treatment.

34

Finally, we formulate a general conclusion: our cross-linked flexible samples including

Samples I, II and III exhibit good mechanical properties and tribological properties; moreover,

Sample II performs outstandingly in tribological tests. Nevertheless, for Sample III, given the

inferior performances in water absorption tests and SEM analysis, we think the Sample III

cannot maintain the effectiveness of cross-linking and hinder sufficiently the leakage of

plasticizer after it underwent heat treatment at a temperature above 100℃. Generally, flexible

PVC blended with cross-linker 1 (Sample I) or cross-linker 2 (Sample II) provide highly

satisfactory solutions for improving the long-term performance of PVC.

35

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