Cure Thesis

8/2/2019 Cure Thesis

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RELATIONSHIP BETWEEN THE PHYSICAL PROPERTIES

AND CURING SYSTEM OF AN EPOXY MATRIX MATERIAL

by

Robert Rainer Pittroff

Submitted in partial fulfilment of the requirements for the degree

MAGISTER TECHNOLOGIAE: ENGINEERING: MECHANICAL

in the

Department of Mechanical Engineering

FACULTY OF ENGINEERING AND THE BUILT ENVIRONMENT

TSHWANE UNIVERSITY OF TECHNOLOGY

Supervisor: Prof O.C. Vorster

Co-Supervisor: Mr. D Louwrens

December 2007


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DECLARATION BY CANDIDATE

I hereby declare that the dissertation submitted for the degree M Tech:

Engineering: Mechanical, at Tshwane University of Technology, is my own

original work and has not previously been submitted to any other institution of

higher education. I further declare that all sources cited or quoted are

indicated and acknowledged by means of a comprehensive list of

references.

Robert R Pittroff

Copyright Tshwane University of Technology 2007


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Acknowledgement

I would like to express my sincere gratitude and appreciation to the following:

My supervisor, Prof. O.C. Vorster for his patience and guidance.

My co-supervisor, Mr. D. Louwrens for his positive attitude and guidance.

Tshwane University of Technology for financial assistance.

National Research Foundation for financial assistance.

Plastomax Supplies for supplying the material at no cost for this study.

Members of the Tshwane University of Technology, specifically the

Department of Polymer Technology and Department of Mechanical

Engineering for their assistance.

My family for their continued support


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Abstract

The effect of curing conditions such as time and temperature on the physical

properties of an epichlorohydrin-derived, aliphatic amine cured epoxy system

has been investigated.

The cure kinetics and thermal behaviour of a setting system was monitored

experimentally. The effect of the reaction temperature on gelation (tgel) and

vitrification times (tvit) were determined using parallel plate rheology and

Differential Scanning Calorometry (DSC) respectively. Rheology was used to

determine tgel, by monitoring the complex viscosity, complex modulus and tan

against time, whereas the effect of the curing temperature (Tcure) on the glass

transition temperature (Tg) of the system was determined from DSC.

The cure kinetics and thermal behaviour as determined were used to calculate atime-temperature-transformation (TTT) isothermal cure diagram. The TTT

isothermal cure diagram was used as a framework to monitor and characterize

changes during the curing process of a thermosetting system.

The gelation contour corresponds to the point where viscosity tends to infinity,

while the vitrification contour represents Tg rising to Tcure. Rheology

measurements under isothermal conditions indicate that Tgel decreases with an

increase in Tcure.The effect of the curing temperature (Tcure) and curing time (tc) on Tg, dynamic

mechanical properties, and indentation hardness of the system was determined

on cured samples characterized with the TTT isothermal cure diagram. Values

for Tg, and information on the dynamic mechanical properties of the epoxy

system was determined by means of dynamic mechanical thermal analysis

(DMTA). Indentation hardness of the cured epoxy system was determined by

means of measurements on samples using a Hand-Held Portable Hardness

Tester (Barcol Impressor, Model No. 934-1).

It is shown that there is a relationship between the cure conditions and the

mechanical properties of an epoxy matrix material.


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Samevatting

Die effek van die verharding kondisies soos tyd en temperatuur op die fisiese

einskappe van n epichlorohydrin-derived, aliphatic amine verhardingsreaksie

epoksie systeem word in hierdie studie pepaal.

Die verhardingreaksiekinetika en termiese verloop verharding systeem word

deur eksperimente bepaal. Die effek op jelpunt (tjel) en vitrifikasie (tvit) punt word

deur parallelplaatspanningsreometrie en differensiele skanderingskalorimetrie

eksperimente bepaal. Die komplekse viskositeit asook komplekse modulus wat

deur spanningsreometrie pebaal is, word gebruik om tjel te definer, terwyl

differensile skandeerkalorimetrie gebruik word om glas transformasie

temeratuur uitg te werk, deur bepaling van verhardingstye en verhardings

temperature.

Die verhardingsreaksiekinetika en termiese verloop wat bepaal is was gebruik

om n tyd-temperatuur-transformasie (TTT) isotermiese verharding diagram te op

te trek. Die TTT diagram is gebruik as n raamwerk om die verharding reaksie

van n reaktiewe systeem te beskryf.

Die effek op die verharding temperatuur en verhading tyd op die

glastransformasie temperatuur (Tg) sowel as dinamiese meganiese einskappe is

deur dinamiese meganiese termiese analise (DMTA) bepaal. Die hardheid van

verharde stelsels is bepaal deur hardheid analise met n Hand-Held Portable

Hardness Tester (Barcol Impressor, Model No. 934-1).

Daar is bewys dat daar wel n verhouding tussen die verharding kondisies en

meganiese einskappe van n epoksie system bestaan.


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CONTENTS

PAGE

ACKNOWLEDGEMENTS. iii

ABSTRACT. ivEKSERP.. v

LIST OF TABLES vi

LIST OF FIGURES. vii

GLOSSARY. viii

CHAPTER 1

1.1 INTRODUCTION ................................................................. 1

1.2 BACKGROUND INFORMATION.................................................... 21.3 OBJECTIVES.. 3

CHAPTER 2

2.0 LITERATURE SURVEY................................................................... 4

2.1 EPOXY RESIN................................................................................. 4

2.2 KINETICS OF CURE......................................................... 5

2.2.1 FACTORS INFLUENCING CURE................................................... 8

2.2.1.1 FUNCTIONALITY............................................................................ 9

2.2.1.2 MOLECULAR MASS....................................................................... 9

2.2.2 COMPETITION BETWEEN CURE AND DEGRADATION.. 10

2.3 GEL POINT..................................................................................... 10

2.3.1 MICROSCOPIC GELATION........................................................... 11

2.3.2 MACROSCOPIC GELATION.......................................................... 12

2.3.3 GELATION STUDY.................................................................. 12

2.3.3.1 CURING ANALYSIS: RHEOLOGICAL ANALYSIS......................... 13

2.4 VITRIFICATION.............................................................................. 15

2.4.1 CURE TEMPERATURE.................................................................. 16

2.4.2 GLASS TRANSITION TEMPERATURE......................................... 16

2.4.3 VITRIFICATION STUDY................................................................. 17

2.4.3.1 CURING ANALYSIS: DIFFERENTIAL SCANNING

CALORIMETRY............................................................................... 17


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2.5 CHARACTERIZATION OF THE CURE PROCESS......................... 18

2.6 MECHANICAL PROPERTIES.......................................................... 19

2.6.1 MECHANICAL PROPERTIES: MOLECULAR INFLUENCES.. 20

2.6.1.1 MOLECULAR WEIGHT.................................................................... 20

2.6.1.2 CROSS-LINKING DENSITY............................................................. 20

2.6.2 MECHANICAL PROPERTY ANALYSIS TECHNIQUES.................. 21

2.6.2.1 DYNAMIC MECHANICAL ANALYSIS.............................................. 21

2.6.2.1.1 INSTRUMENTATION....................................................................... 22

2.6.2.1.2 DMA AND DMTA ANALYSIS........................................................... 23

2.6.2.2 INDENTATION HARDNESS............................................................ 24

CHAPTER 3

3.0 RESEARCH METHODOLOGY....................................................... 25

3.1 MATERIALS USED AND THEIR PREPARATION......................... 25

3.2 GELATION SYUDY........................................................................ 25

3.3 VITRIFICATION STUDY................................................................ 27

3.3.1 SAMPLE PREPARATION.............................................................. 27

3.4 DYNAMIC MECHANICAL THERMAL ANALYSIS......................... 28

3.4.1 SAMPLE PREPARATION............................................................. 28

3.5 INDENTATION HARDNESS......................................................... 313.4.2 SAMPLE PREPARATION............................................................. 31

3.4.3 EXPERIMENTAL PROCEDURE 31

CHAPTER 4

4.0 RESULTS AND DISCUSSION........................................................ 32

4.1 INTRODUCTION............................................................................. 32

4.2 GELATION STUDY......................................................................... 324.3 VITRIFICATION STUDY................................................................. 34

4.3.1 DETERMINING Tg.......................................................................... 35

4.3.1.1 TgVERSUS TCure............................................................................ 36

4.3.1.2 TgVERSUS tCure............................................................................. 37

4.3.1.3 ISO- TgCONTOURS ...................................................................... 39


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4.3.1.4 PREDICTION OF tvit........................................................................ 40

4.4 TIME TEMPERATURE TRANSFORMATION DIAGRAM 45

4.5 MECHANICAL PROPERTY VS. CURE RELATIONSHIP.............. 46

4.5.1 AREAS FOR ANALYSIS................................................................. 46

4.5.2 DYNAMIC MECHANICAL THERMAL ANALYSIS (DMTA)............. 48

4.5.2.1 STORAGE MODULUS..................................................................... 49

4.5.2.2 GLASS TRANSITION TEMPERATURE (Tg)................................... 51

4.5.2.3 STORAGE MODULUS (E') RELATED TO STIFFNESS 52

4.5.2.4 LOSS MODUDLUS RELATED TO IMPACT RESISTANCE. 53

4.5.3 HARDNESS ANALYSIS............................................................... 56

CHAPTER 5

5.0 CONCLUSIONS AND RECOMMENDATIONS............................... 58

5.1 CONCLUSIONS.. 58

5.2 RECOMMENDATIONS.. 60

BIBLIOGRAPHY. 61

APPENDIX A: Material data sheet for Epon Resin 826.


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LIST OF FIGURES

PAGE

2.1 A generalized time-temperature-transformation (TTT) cure

diagram. Times to gelation and vitrification during isothermal cure

against temperature demarks the boundaries of the four

distinct regions states of matter: liquid, gelled rubber, gelled glass,

and ungelled glass. (Taken from Prime (1997:1384)).......................... 7

3.1 Rheometric Dynamic Stress Rheometer (RS-500)............................... 26

3.2 Typical plot of the loss and storage moduli, tan and complex

viscosity (*) against time for Rheological measurements................... 26

3.3 Typical plot of a DSC scan of endothermic heat flow (mW) against

temperature(C)................................................................................... 28

3.4 Perkin-Elmer Differential Scanning Calorimeter (DSC-7).................... 28

3.5 Typical plot of the storage modulus (E') and tan against

temperature. (Taken from Laza et al. (1998:43)) ................................ 30

3.6 Rheometric Scientific Solid Analyzer RSA II (DMTA) in dual

cantilever mode................................................................................... 30

3.7 The Impressor: A Hand-Held Portable Hardness Tester

(Barcol Impressor, Model No. 934-1)................................................... 31

4.1 Time-temperature-transformation (TTT) diagram: Gelation contour.... 34

4.2 Tg (C) vs. Tcure (C) for different isothermal heating times (1 hour

through to 72 hours). The reference line of Tg = Tcure is also

included............................................................................................... 36

4.3 Tg (C) vs. Log Time (min) for different isothermal temperatures

(Tcure).................................................................................................... 37

4.4 Limiting Tg (C) vs. Tcure(C),............................................................... 38

4.5 Time-temperature-transformation (TTT) diagram: Iso-Tgcontours...... 39

4.6 Ln time (min) to fixed Tgvalues (from Figure 4.5) vs. 1/Tcure(K)......... 41

4.7 Ln time (min) to macroscopic gelation vs. 1/Tcure(K)........................... 44

4.8 Summary time-temperature-transformation (TTT) diagram................. 46

4.9 Detailed TTT-Isothermal cure diagram showing areas for DTMA

analysis................................................................................................ 47


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4.10 DMTAscans. Tan and storage modulus (E') vs. temperature (C)

for samples cured for different cure times (tcure, min) at different cure

temperatures (Tc).

4.10-a: DMTA scans forTcure= 70C.......................................... 494.10-b: DMTA scans forTcure= 90C.......................................... 494.10-c: DMTA scans forTcure= 110C.......................................... 50

4.11 Trends in Tg values for samples cured at different temperatures (Tc)

and for different times (tcure)................................................................. 51

4.12 Trends in E' values for samples cured at different temperatures (Tc)

and for different times (tcure)................................................................. 53

4.13 Determination of the area under the storage modulus curve............... 54

4.14 Trends in impact strength values for samples cured at different

temperatures (Tc) for different cure times (tcure)................................... 55

4.15 Trends in Barcol Hardness values for samples cured at different

temperatures (Tc) and for different times (tcure).................................... 56


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LIST OF TABLES

PAGE

2.1 Outline and classification of epoxy reactive groups, constituents, and

ammonia and amine reaction groups. Taken from Ellis

(1993:1,3&12) 5

2.2 Glossary of curing terms and characteristic curing parameters.

Taken from Prime (1997:1386)............................................................ 8

4.1 Average gel times (Log Time) for different frequencies at the

respective cure temperatures.............................................................. 33

4.2 Average glass transition temperatures for samples cured for

different times at different cure temperatures...................................... 35

4.3 Numerical relationship between Ln time to a fixed Tg and thereciprocal ofTcure(1/K)......................................................................... 40

4.4 Calculated times to vitrification for Tcure = Tg corresponding to

isothermal cure temperatures from Figure 4.2..................................... 43

4.5 Results from DMTA............................................................................. 50


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CHAPTER 1

1.1 Introduction

Thermoset systems possess good dimensional stability, thermal stability,

chemical resistance, and electrical properties. Prime (1997:1381) believes that

the increasing use of thermoset material as adhesives, matrix material in

reinforced composites, and protective coatings in aeronautical, automotive,

construction, electrical and medical application is consequence to thermosets

possessing these properties.

Using composite materials technology in design and manufacture in engineering

fields, as explained by Jones (1993:256), exploits the high specific moduli and

strength of reinforcing fibres, supported and held together by epoxy matrices to

produce low density high performing structures. According to Morgan

(1997:2097), epoxy resin systems are generally used for composite matrices,

due to the suitability in composite fabrication.

Optimal component design requires an in-depth knowledge of the properties and

mechanical capabilities and limitations of constituent materials of composites,

and their correct application. The reinforcing fibers provide strength and stiffness,while the epoxy accounts for the desired shape and load distribution between the

reinforcing fibers.

Morgan (1997:2198), Yasmin and Daniel (2004:8211), and Nicholson et al. (1)

are in general agreement that the failure or degradation of composite component

performance can be linked to matrix dominated properties. The weak link in the

composite is the structural integrity, mechanical, and physical response of this

interfacial region. Favorable response of composite structures to applied loads

requires competent structural design based on knowledge of the mechanical

properties of the constituent parts of the composite.


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1.2 Background information

Epoxy is a title given to the class of material that has an epoxide group or oxirane

ring in its chemical structure. Epoxies exist either as liquids with low viscosity or

as solids. Curing of an epoxy system as explained by Sun (2002:11), is through

the ring opening reaction of epoxides where addition of molecules produces a

higher molecular mass and finally resulting in a three-dimensional structure. The

active epoxide groups in uncured epoxy resins react with various curing agents

or hardeners that contain, amongst other, hydroxyl, carboxyl, amine, and amino

groups.

Epoxy resins can be divided into two major groups: epichlorohydrin-derived

resins and cycloaliphatic resins. This particular study will focus on the former

group, where there is a reaction with a common polyhydroxy compound, such as

bisphenol A, which is cured by means of an aliphatic amine curing agent.

Processing of thermosetting based composite structures involve curing cycles of

the epoxy systems at different isothermal curing temperatures. The degree of

cure of thermosetting systems is determined by these curing cycles and has an

important effect on the mechanical properties of the final product. To determine,

or to a certain extent predict, the mechanical properties through optimized curing

cycles requires thorough understanding of cure kinetics and characteristics of

epoxy systems. The kinetics of the curing process was studied by means of

parallel plate rheology (Halsz and Vorster (2000, 2004), Lange et al. (1996),

Sun (2002)) and differential scanning calorimetry (DSC) (Gan et al. (1989),

Gillham (1993), Gumen et al. (2000) and Nez et al. (2000)).

Information gained from the rheology and DSC analysis was then used to

generate additional information on the physical state of the epoxy resin. The

information was used to develop an isothermal time-temperature-transformation

(TTT) cure diagram (Gillham (1979, 1982, 1990 and 1993), Gan et al.(1989),

Jiawu et al. (2000), Nez et al. (2000), Sun (2002), and Reghunadhan Nair


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(2004:38)). The TTT cure diagram facilitates characterizing the thermosetting

system.

By making use of the isothermal TTT cure diagram epoxy samples were

characterized and cured according to specific cure temperatures (Tcure) and

curing times (tcure). These samples were then subjected to dynamic mechanical

thermal analysis (DMTA) scans as well as Barcol hardness test to determine

mechanical properties of the cured samples under different curing conditions.

1.3 Objectives

This study focuses on gaining insight into the properties and mechanical

capabilities by determining the relationship between the curing conditions and

physical properties of an epoxy matrix. It aims to:

Establish the kinetics of cure of the epoxy thermosetting system

through the use of rheology and differential scanning calorimetry.

Use the kinetics of cure to develop and construct an isothermal

time-temperature-transformation (TTT) cure diagram.

To ascertain the mechanical properties of the cured samplesthrough dynamic mechanical thermal analysis (DMTA) and Barcol

hardness indentation measurements.

Using the TTT diagram and mechanical properties from DMTA

scans and Barcol indentation tests to establish a relationship

between the curing conditions and physical properties of the epoxy

thermosetting system.


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CHAPTER 2

2.0 Literature Survey

2.1 Epoxy Resin

Commercial epoxy resins were first marketed in the 1940s as result of

independent work by Pierre Castan in Switzerland, and Sylvan Greenlee in the

United States. Even though patents for similar resins had been in existence since

the 1930s, Ellis (1993:1) explains that products that would be called epoxy resins

in modern times, were synthesized as early as 1891. It is interesting to note that

early epoxy resins were reaction products of bisphenol A and epichlorohydrin,

and although various other types of resins are available, this is still the major

manufacturing route for most resins available today.

Epoxy resin is generally found in the form of liquids with low viscosities or as

solids. Prime (1997:1716) and Sun (2002:11) report that epoxy resins form part

of a class of material classified and characterized according to the reactive

epoxide group, or oxirane ring that forms part of the chemical structure. Ellis

(1993:1) points out that the term epoxy resin is generally applied to both the un-

reacted prepolymers and cured resins, even thought all the reactive groups may

have reacted in the cured resins.

Epoxy resins can be divided into two major groups: epichlorohydrin-derived

resins and cycloaliphatic resins. This particular study will focus on the former

group, where there is a reaction with a common polyhydroxy compound, such as

bisphenol A, which is cured by means of an aliphatic amine curing agent. The

reactive epoxy group, bisphenol A and epichlorohydrin reagents, as well as the

aliphatic amine group reaction is outlined and classified in Table 2.1.


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Table 2.1 Outline and classification of epoxy reactive groups, constituents, and ammonia and

amine reaction groups. Taken from Ellis (1993:1,3&12)

Ellis (1993:72) asserts that the cure of an epoxy resin involves reaction between

epoxy and hardener reactive groups, and that during cure, a liquid or fluid resin-

hardener mixture is converted to a solid.

2.2. Kinetics of cure

Thermosetting resin is an appropriate title in that it describes the final infusible

and insoluble state of a set, or cured system. Liquid resin is mixed with a

hardener or catalyst according to a prescribed ratio in order to initiate the curing

process. This process is known as polymerization, curing or cross-linking, and is

a permanent change from a viscous liquid to a viscoelastic fluid, to an elastic gel


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(or rubber), and finally to a glassy state. Heat or another form of energy is often

required to initiate cure. Proper formulation and complete processing will result in

a polymer with a highly cross-linked, infinite three-dimensional network structure,

a consequence of the chemical reactions that accompany cure. It is the

polymerization or curing process that separates thermosets from other polymer

materials such as thermoplastic materials.

According to Ellis (1993:72-74) and Prime (1997:1380-1388) the complexity of

the curing process of a thermoset is due to steps that are necessary during the

handling and storage of the material, prior to cure, and situational factors that

influence the process, and state or phase changes that the material undergoes

during cure.

The major changes to the properties of thermosetting resins from being a viscous

fluid to a glassy solid are a result of the cure process. These changes as cure

proceeds can be represented in a time-temperature-transformation (TTT)

diagram as represented in Figure 2.1 below.


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Figure 2.1.A generalized time-temperature-transformation (TTT) cure

diagram. Times to gelation and vitrification during isothermal cure against

temperature demarks the boundaries of the four distinct regions states of

matter: liquid, gelled rubber, gelled glass, and ungelled glass. (Taken from

Prime (1997:1384))

The first phase transformation from a viscoelastic fluid to an elastic gel is known

as the gel point, and occurs at the gel time (or gelation times), tgel. A distinction

must be made between molecular gelation and macroscopic gelation. Molecular

gelation occurs at a specific stage during the chemical reaction, and marks the

start of the formation of a cross-linked network, whereas the macroscopic

consequence of gelation shows a rapid increase in viscosity, and development of

elastic properties.

The next phase transition from a gel to a glassy solid is vitrification, with glass

transition measurements of Tg and times to vitrification, tvit. Prime (1997:1383)

points out that vitrification occurs as a result of the chemical reaction that

accompanies cure, and is independent of gelation. Vitrification of the system can

occur without gelation.


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Table 2.2 Glossary of curing terms and characteristic curing parameters. Taken from Prime

(1997:1386)

Chemical conversion (e.g., of epoxide groups), fraction reacted, degree ofcure

gel at gel pointult Maximum achievable extent of conversion 1

tgel Time to gelation (gel time)

tvit Time to vitrification

Tcure Cure temperature, a process parameter

Tcure,0Temperature below which no significant reaction of uncured resin mixtureoccurs in a reasonable time period (cf. storage temperature for uncured resinmixture)

gelTcure Lowest temperature at which gelation occurs in the liquid state

Tcure, Minimum temperature at which ultimate conversion occurs in a reasonabletime

Tg Glass transition temperature, a material property

Tg0 Tg for thermoset with degree of conversion = 0

gelTg Tg for thermoset with degree of conversion gel

Tg, Tg for thermoset with degree of conversion = 1

Prime (1997:1383) and Gillham (1989:804, 1993) repeatedly point out that the

onset of vitrification marks a decrease in the rate of reaction. This decrease is a

result of the change in reaction mechanism from being kinetically controlled to

diffusion control.

2.2.1. Factors influencing cure

Manufacturing and processing of composite parts destined for commercial

applications calls for the least amount of cycle/process time, yielding improved

properties. It is therefore imperative that the finished product demand set by themarket be met; Production and cost effective manufacture therefore compels the

use of additives such as catalysts, accelerators, initiators, hardeners, or fillers.

According to Prime (1997:1662), catalysts, accelerators, and initiators catalyze

the cure process, and in doing so reduce the processing times. Hardeners are

incorporated in the network structure of the polymer chains; thereby having an


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effect on the reaction rate, as well as the ultimate properties of the material, such

as the network structure, glass transition temperature, and dynamic mechanical

properties. Fillers are generally incorporated to provide or enhance physical

properties such as modulus, thermal expansion and dimensional stability.

2.2.1.1 Functionality

According to Odian (1991:40, 106-108), polymerization of bi-functional

monomers (f=2) will form linear polymers, whereas polymerization of monomers

with more than two functional groups per molecule (f>2) will result in the polymer

being cross-linked.

With certain monomers, cross-linking will take place, resulting in a network

structure in which one or more branches from one molecule become attached to

surrounding molecules.

2.2.1.2 Molecular Mass

Odian (1991:19-20) emphasises the importance of the molecular mass of a

polymer in its synthesis and application. Also that, the useful mechanical

properties uniquely associated with polymeric materials are a consequence of

their high molecular weight. It can generally be said that the use of polymer

material in practical applications require high molecular weights to obtain high

strengths. It should be noted that polymer chains with strong intermolecular

forces develop sufficient strength to be useful at lower molecular weights than

polymers having weaker intermolecular forces (with higher molecular weights).

There is a significant dependency of the properties (other than strength) of a

polymer on its molecular weight, albeit different quantities for different properties.

Optimum or maximum values of properties will be reached at different molecular

weights and can decrease with a further increase.

It is therefore necessary for the polymerization process to result in a compromise

molecular weight, where sufficient strength is obtained without sacrificing other


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properties. The control of the molecular weight of a cross-linking process is

essential for the practical application.

2.2.2 Competition between Cure and Degradation.

Prime (1997:1505) explains that the cure process can often become complicated

as a result of competition between degradation and chemical cross-linking

reactions. This can often occur as a result of the high cure temperatures required

to achieve high glass transition temperatures of high performance systems. Yuan

and Mazeika (1991) as reported by Prime (1997:1431), found that isothermal

degradation occurs in three stages; during the first stage, there is a decrease in

modulus and glass transition temperature (Tg), a probable result due to scission

in the epoxy network. The second stage marks the formation of char, and duringthe third stage char stabilizes, and shows an increase in modulus and Tg. The

increase may be caused by an increase in the cross-linking density of the

degraded network. The char region can be seen on Figure 2.1.

2.3 Gel point

Gelation is the first noticeable change that is encountered during the cure

process. Prime (1997:1496) describes gelation as an irreversible change from a

liquid to a rubber, and that this change marks the start of the formation of the

cross linked network. Malkin and Kulichikhin (1996:277) used gelation to denote

the process of fluidity loss, which is a result of the formation of a network of

chemical bonds.

Prime further defines gelation on a molecular basis, but states that the

phenomenon is measured as a macroscopic phenomenon. A distinction between

molecular gelation and macroscopic gelation may be drawn; macroscopic

gelation is the consequence of the microscopic gelation. Gelation will be

discussed on a microscopic and macroscopic level.


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2.3.1 Microscopic gelation

Microscopic gelation marks the start of the formation of the three dimensional

network structures that is typical of thermosetting systems, and will have a direct

influence on the final structure of the cured resin. It is vital to understand the

influence of the occurring changes, and their effect on the final properties of the

cured resin. Microscopic analysis of gelation focuses on molecular and chemical

changes that take place during polymerization.

Ellis (1993:4&72-74), Malkin and Kulichikhin (1996:277), and Prime (1997:1380-

1383, 1496) are in agreement that the cure of thermoset material on a

microscopic level is the formation and linear growth of molecular chains. These

linear chains form branches, cross-links with other branched chains, and result in

the formation of a rigid three dimensional molecular network, all due to the

chemical reaction between the epoxy and hardener reactive groups. Malkin and

Kulichikhin (1996:277) add that the microcrystals formed act as stable points of

the three-dimensional network. These strong physical bonds will cause fluidity

loss.

Flory (as quoted by Gillham (1989:804)) and Winter (as quoted by Prime

(1997:1381)) defines the microscopic gel point of a cross-linked system, as the

point where the weight average molecular weight of the network of chains

approaches infinity.

Molecular gelation is therefore a point that may be measured by the initial

identification of an insoluble gel.

Flory and Miller et al. (taken from Prime (1997:1382)) are in agreement that

molecular gelation generally occur during a well-defined and calculable stage in

the course of the chemical reaction. The only requirements are that the reaction

mechanism is not dependent on temperature and that there are no non-cross-

linking side reactions. Molecular gelation is dependent on the functionality,


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reactivity, and stoichiometry of the reactants, therefore the theoretical gel point

can be calculated if the chemistry of the reactants is known.

2.3.2 Macroscopic gelation

Ellis (1993:5&72) points out that the first critical feature of a curing system is

gelation and can be macroscopically described as the point where the resins

viscosity rapidly approaches infinity. This happens as a consequence of

microscopic gelation, which is also defined as such by Prime. Gan et al

(1989:804) explain that the transformation is from a viscous fluid to a state of

infinite viscosity, or a change from a fluid liquid to a rubbery state. Gillham

(1990:1) adds that there is a transformation of liquids with low molecular weight

to an amorphous solid with a high molecular weight by means of chemical

reactions.

This irreversible transformation from a fluid to a rubber means that beyond the

gel point a thermosetting resin loses all ability to flow. This marks a very

important stage, especially for manufacturing and processing.

2.3.3 Gelation Study

The measurements of isothermal times to gelation are important to cure studies

and will be used to form the basis of a cure diagram and is discussed in later

chapters of the study.

The progressive macroscopic change from the liquid state to the rubbery state

and/or glassy state which occurs during the setting of reactive systems has been

described on both a molecular and macroscopic level. Prime (1997:1496)

explains that the detection of an insoluble material, i.e. a cross-linked material

(gel) that forms from a reacting soluble material (sol), may be taken as the

starting point of measure of gelation.


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According to Winter, Holly et al., Feve, Boiteux et al. (as quoted by Prime

(1997:1496)), the point where the mechanical loss tangent becomes independent

of frequency can be used as an alternative measurement starting point.

Molecular gelation is taken as the point where the loss modulus (G') and storage

modulus (G'') cross each other, i.e., tan =1. The approximation of gelation

macroscopically is based on rheological and mechanical changes that occur

during the transition from liquid to rubber. The rheological aspect accounts for the

changing viscosity, while the mechanical aspect accounts for the evidence of an

equilibrium modulus.

Initial work by Gan et al. (1989) reported on the use of the torsional braid analysis

(TBA) technique to monitor and study gelation and vitrification of a setting

system. TBA is a resonant instrument, which according to Gallagher (1997:137),

was widely used in the past. The detailed mechanics of the system will not be

discussed here, but it is important to understand that the frequency of oscillation

is related to the modulus, and that the logarithmic decrement (ratio of the

amplitude of successive cycles) is related to tan .

Nez et al. (2000) proposed three methods to experimentally determine the time

at which gelation occurs (tgel), namely; solubility test, gel-timer and dynamic

mechanical analysis (DMA). The solubility test, as described by Hagnaver (1983),

is used to determine the time taken for the setting material to reach a fibriform

structure in tetrahydrofurane. The DMA measures the dynamic mechanical

properties of the epoxy.

2.3.3.1 Curing analysis: Rheological analysis

Malkin and Kulichikhin (1996:142), Lange et al. (1996), Kulichikin (1997), Halsz

et al. (2001), Vorster (1995), Vorster et al. (2004), and Sun (2002) used

rheology, the study of the change of viscosity, as a mean to determine the point

of gel formation. Ellis (193:87-89) explains that even though the initial rate of

reaction may be high, the molecular growth is slow. There is a relationship


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between the degree of cure and viscosity; this allows for cure monitoring by

measuring the change in shear viscosity of the setting system. The shear

viscosity of a resin increases during cure as a result of gelation and/or the onset

of vitrification.

According to Halsz et al. (2001:898), rheological analysis of the viscosity of

curing systems may be carried out using a small strain (5%) at different

frequencies and temperatures. The components of complex modulus, complex

viscosity and tan should be measured and recorded against frequency and

therefore as a function of time, as depicted in work performed by Halsz et al.

(2001:898). It is interesting to note that prior to gelation the behaviour of the liquid

resin can be characterized by the zero shear viscosity.

Vorsteret al. (2004) quoted Flory (1941 & 1959) and Stockmayer (1943 & 1944)

who predicted theoretical gel point values using the following equation;

( )( )

2

1

11

1

=

gfRg [2.1]

where Ris the molar ratio between the epoxy and hardener, fis the functionality

of epoxy and gthe functionality of the hardener.

From the data obtained from experimental rheological measurements the

gelation times (tg) can be determined using three methods:

Extrapolating the inverse of zero viscosity

Equilibrium (intersection) of G and G curves of the dynamic modulus

Intersection of the tan curves.

Halsz et al. (2001:899) obtained and compared gel times according to the three

methods discussed above. The time differences were found to be insignificant,

thereby permitting single test validity.


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Times to gelation (Log time in minutes) as described by Gan et al. (1989:807)

can be plotted against isothermal cure temperatures to obtain time temperature

transformation (TTT) diagrams with gelation contours. Gelation contours on a

TTT diagram shows the relationship between tgeland Tcure. The relationship that

exists between tgel(Ln time in minutes) to macroscopic gelation versus the

reciprocal temperature (1/K) is linear. The slope of the linear relationship

represents the apparent activation energy (Ea) for the system.

2.4 Vitrification

Wunderlich (1997:460) defines vitrification of a setting system as the transition

from a viscous liquid or rubber to an amorphous solid or glass. Gan et al.

(1989:803) describes vitrification as the point where the system solidifies. This

phase transition is a consequence of the cure reactions, and as Prime

(1997:1383 & 1569) explains, vitrification can occur at any stage during

polymerization. Glass formation (vitrification) is as a result of the glass transition

temperature (Tg) increasing with conversion. It is accepted that vitrification occurs

at times to vitrification, tvit, when Tgrises to the cure temperature (Tcure).

The measurements of the times to vitrification, similar to times to gelation, are

important and will be used in cure studies. It forms the basis of cure diagrams.

With reference to Figure 2.1, Malkin and Kulichikhin (1996:120), and Prime

(1997:1384) explain that in the glassy state beyond the vitrification line, the cure

reaction rate significantly drops. Wisanrakkit and Gillham (as quoted by Nezet

al. (2001:3581), Adabbo and Williams, Dillman and Seferis, and Wisanrakkit and

Gillham (taken from Prime (1997:1623)) explain that isothermal curing of

thermosets occur in two distinct stages; the first stage reaction mechanism is

kinetically controlled, but changes to diffusion controlled soon after vitrification.

There is an increase in the diffusion time scale, whereas segmental mobility, and

reacting groups decrease.


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According to Prime (1997:179), gel conversion is dependent on functionality and

stoichiometry, whereas the glass transition is dependent on the frequency of

measurements. Therefore indicating the dependence on the dynamic mechanical

technique used for monitoring it.

2.4.1 Cure temperature

According to Prime (1997:1381), most thermosetting formulations require heat or

irradiation to cure. Energy in the form of heat can be supplied to the reaction and

will be elevating the cure temperature of the system. The cure temperature (Tcure)

is a processing parameter and is the temperature at which the polymerisation or

cure process takes place.

2.4.2 Glass transition temperature

Scherer, Rekhson and Elliott (from Ellis, 1993:81) point out that glass-forming

systems exhibit a glass transition phenomenon. Prime (1997:1401) further

defines the point as a transition from a supercooled liquid or rubbery state to a

metastable glassy state.

According to Wunderlich (1997:226 & 380), there is a definite link between the

liquid state and glassy state of a thermosetting resin. The glass transition

temperature (Tg) is the main characteristic temperature of the two states, and is

the process where the molecular orientation of the liquid state is frozen in

position as a glass. Because there is no change in molecular order at Tg, the

macroconformation of the molecules in the glass state correspond to that of the

liquid.

Odian (1991:24 & 29) defines the glass transition temperature, Tg, of a polymeric

system, as the temperature where the amorphous or unordered regions of a

polymer take on the characteristic properties of the glassy state, typically

brittleness, stiffness, and rigidity that is associated with the solid phase.


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According to Gan et al. (1989:803), Gillham (1990, 1993), and Wang (1992), the

sensitivity and one-to-one relationship between Tg and the conversion for

thermosetting resins, implies that Tg can be used as an index to measure

conversion during cure. The glass transition temperature is a useful, sensitive,

easily measured macroscopic parameter for monitoring the cure process of

thermosetting systems.

Prime (1997:1506&1414) points out that softening of a thermoset is affiliated with

Tg, defines the upper use temperature of the system. This information is critical

when designing and manufacturing composite structures to be used at elevated

temperatures.

2.4.3 Vitrification study

As stated before, onset of vitrification of a thermosetting system is when there is

a change from a liquid to a glassy solid. This transition is further defined as the

point where Tgrises to Tcure. It is therefore important to be able to determine Tg.

Ellis (1993:72) points out that when Tcure is too low, vitrification may occur before

gelation and inhibit further reaction.

2.4.3.1 Curing analysis: Differential scanning calorimetry

Differential scanning calorimetry (DSC) has been used in work by various groups

such as Gillham (1990 & 1993), Nez et al. (2001) and Mafi et al. (2005) to

determine the glass transition temperature (Tg) of setting systems. According to

Turi (1981) and Wunderlich (1990) as quoted by Prime (1997:1388-1390), DSC

forms part of a family of measuring techniques that record the response of a

material to temperature scans. This is achieved by measuring and recording the

heat flow into a material (endothermic) or out of a material (exothermic). DSC

can therefore be used to determine Tg for thermosets; by assuming that the

measured heat flow (dH/dt) is proportional to the reaction rate (d/dt), or after

integration, the total heat detected is identical to the heat evolved by the curing


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reaction. Other characteristics that can be determined through the use of DSC

are cure, and physical ageing of systems.

This technique monitors the setting system during the change from a liquid to

rubbery state and finally glass state. According to Gan et al(1989:804) Tgcan be

measured in three types of experiments:-

1 Times to vitrification are obtained when Tgrises to Tcure

2 The progress of setting before vitrification at Tcure can be obtained

from measurements ofTgobtained during intermitted cooling

3 Temperature scans first to a temperature below and then to

temperatures above Tcure, give Tg values which reflect the state of

cure obtained at Tcure, and the stability of the material to

temperatures higher than Tcure.

Tg values for samples cured at Tcure fortcure determined from DSC scans can be

used to monitor cure conversion simply by plotting Tg against Tcure for isothermal

heating times. The relationship is linear at lower isothermal cure temperatures.

All Tcure

versus Tg

curves features a maximum. Gan et al. (1989:807) explain that

this inflection is due to the competition between cure (which raises Tg) and

thermal degradation (which decreases Tg).

2.5 Characterization of the cure process

Gillham (taken from Prime (1997:1568)) provides a useful frameworks for the

understanding and conceptualization of the changes that occur during the curing

process of a thermosetting resin. This is through the development of the

isothermal time-temperature-transformation (TTT) cure diagram as depicted in

Figure 2.1. The diagram depicts the various physical states of a thermoset (liquid,

rubber, and glass) in relation to the two critical phenomena: gelation and

vitrification. Sun (2002:91) proposes that the characteristics of the cure process

can be understood by combining and describing the rheological analysis and


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DSC thermal analysis by a time-temperature-transformation (TTT) isothermal

cure diagram.

Construction of a TTT isothermal cure diagram requires that the relationship

between degree of cure and cure time at isothermal cure temperatures be

established.

The relationship between gel times (tgel), defined as the elapsed time to achieve

equal storage and loss moduli, and isothermal cure temperature is represented

by the gelation contour. Earlier discussions verify that Tg is a function of degree

of cure and dependent on cure temperature. The relationship between Tgand the

isothermal cure time can be determined. This allows that the time taken for

isothermal cure, or vitrification (tvit), when Tg rises to the cure temperature (Tcure),

can be determined. The relationship between tvit and Tcure is represented by the

vitrification curve. By plotting the gelation and vitrification curves on the same

graph, the isothermal TTT cure diagram for epoxy resin system is constructed.

2.6 Mechanical properties

Proper application of epoxy resins requires that the material be a rigid solid at the

use temperature Tuse

. The use temperature in all these applications is below the

glass transition temperature of the cured resin, TuseTg.

Work by various authors (Nielsen (1969:69-103), Ellis (1993:159) Paul

(1989:187), Gillham (1993), Lange et al. (1996), Morgan (1997:2103), Fabrice

and Redford (2002:340), Ahamed et al. (2004), and Mafi et al. (2005)) confirm

the influence of molecular weight and cross-linking density on the final

mechanical properties of thermosetting systems. Ellis explains that epoxies with

higher cross-linking densities have higher Tgs, which often succumbs to brittle

failures, whereas epoxies with low cross-linking densities exhibit structures with

more mobility that allows for greater plastic deformation. Higher levels of

deformation often result in higher fracture toughness. Misev (1991:196-197)

substantiates that the mechanical properties are directly proportional to the


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molecular weight. But only to a point, as the molecular weight is dependent on

the resin and cross-linker stoichiometry.

Bell as well as Misra et al. (as reported by Ellis (1993:159)) found significant

increases in impact strength with an increase in molecular mass between cross-

links. A higher molecular masss between cross-links denotes a lower cross-link

density.

2.6.1 Mechanical properties: Molecular influences

2.6.1.1 Molecular weight

According to Misev (1991:176), the molecular weight of a thermoset influences

the following mechanical properties; tensile strength, impact resistance and melt

viscosity. This was confirmed through studies performed by Nielsen, Matsuoka,

Walsh and Termonia (taken from Nicholson et al. (date:1)), the molecular weight

distribution effects fracture toughness, and impact strength of the polymer.

The critically molecular weight dependent property is the glass transition

temperature (Tg), which represents the temperature region where there is a

phase change, either form a glass to a rubber, or from a rubber to a glass. Tg

generally increases with the extend of cure, which denotes molecular growth, and

confirms the relationship.

Good mechanical properties after curing requires that the molecular weight be

high. There is however a constraint; thermosetting systems with higher molecular

weights consequently have high viscosities, which causes production problems.

2.6.1.2 Cross-linking density

The cross-linking density is representative of the number of bonds or cross-links

that has formed as result of the chemical reaction between the constituents

reagents during the cure. Ellis (1993:159), Nielsen (1969:69-103), Paul

(1989:187) and Morgan (1997:2103) are in agreement that the fundamental


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physical and mechanical properties of epoxy resins are determined by the nature

and density of its cross-linked network. There is influence on Tg of the polymer,

the ability to undergo plastic deformation, therefore its toughness characteristics

at a given temperature. Gillham (1993) explains that the maximum cross-linking

density for a fully-cured material will yield the minimum internal stress, and

maximum modulus.

There is of course a limit to the cross-linking density, explained by Reghunadhan

Nair (2004:406) and Mimura and Ito (2002:7559), as epoxies with high cross-

linking densities tend to have high Tgs, but often exhibit brittle structures.

Almeida and Monteiro (1996:330) add that higher cross-linking densities

generally result in stiffer structures with higher ultimate strengths.

Excessive cross-linking reactions result in a rigid, brittle network structure with a

diminished load bearing capacity.

2.6.2 Mechanical property analysis techniques

2.6.2.1 Dynamic mechanical analysis

Methods such as torsional braid analysis (TBA), dynamic mechanical analysis

(DMA), and dynamic thermal mechanical analysis (DMTA) are methods of

measuring changes in complex modulus, compliance, and viscosity of samples

during oscillatory deformation of material samples. The different modes of

deformation that a sample material can be subjected to are as follows; flexure,

tensile, shear, bending, and compression deformations. Principal applications of

DMA for thermosetting material is the measurement of Tg, analysis of cross-

linking, mechanical properties developed during the curing process and

characterizing cured or partially cured materials.

Prime (1997:1464-1465) states that Tg for thermosetting materials can be defined

to occur at the maximum value of the loss modulus (E'), the loss compliance, or


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tan . Mechanical properties such as storage and loss moduli or compliances for

samples during or after cure can be measured.

The dynamic mechanical modulus, E*, is defined as the ratio of the stress, *, to

the strain, *, in the material being tested. The strain is a response to the stress

applied, or the stress is a response to the applied strain. E*is a complex quantity

and is defined as:

'''**

*

iEEE +==

[2.2]

'

''tan

E

E= [2.3]

where E' is the storage modulus, a measure of the stress stored in the sample

as mechanical energy; E''is the loss modulus, a measure of stress dissipated as

heat; tan is the phase lag between the stress and strain.

2.6.2.1.1 Instrumentation

o Torsional braid analysis (TBA)

TBA is a dynamic mechanical technique where the specimen for a freelyoscillating torsion pendulum experiment consists of an inert substrate or braid

coated with the reactive system. The changes from the liquid or rubbery state to

the glassy state can be monitored during cure. Gan et al. (1989) and Jiawu et

al.(2000) used the TBA to study the cure behaviour and measure macroscopic

gelation times tgel and glass transition measurements such as Tg and times to

vitrification (tvit) to characterize reactive coatings.

o Dynamic mechanical analysis (DMA)

DMA is a thermal analysis system module that is designed for four modes of

oscillation: fixed frequency oscillation, resonant frequency oscillation, stress

relaxation, and creep. Direct measurements of frequency, temperature, time,

stress, stain, and phase angles are taken during scans. Measurements can be


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made in the following modes; three-point bending, dual cantilever, fiber

extension, film extension, and parallel plate.

Yasmin and Daniel (2004:8213) determined thermomechanical properties such

as storage modulus (E), loss modulus (E), damping factor tan, and Tg of

samples through DMA scans. Park et al. (2004) used DMA purely to determine

the temperature dependence of the loss factor tan for samples modified through

synthesis of epoxidized soybean oil. Ahamed et al. (2004) used DMTA to study

the effect of stoichiometry on the dynamic mechanical properties of resins

systems. Cain et al. (2004:3) performed DMA in dual-cantilever mode on

samples.

o Dynamic mechanical thermal analysis (DMTA)

The DMTA is a fixed frequency thermal analysis module that measures time,

temperature, frequency, sample, and suspension stiffness (in-phase and out-of-

phase) directly. Three measurement modes are possible: bending mode, single-

cantilever (and double cantilever), and shear sandwich geometry.

Dynamic mechanical thermal analysis (DMTA) can be used to measure the

complex modulus, compliance, and viscosity of materials as functions of

temperature in several different modes of oscillatory deformation.

Mafi et al. (2005) and Remiro et al. (2001) used DMTA to gauge the effect of

curing conditions such as time and temperature on dynamic mechanical

behaviour (adhesion, viscoelasticity, and stiffness) of a thermoset system. Laza

et al. (1998) used DMTA to study the effect of amine concentrations on the

dynamic mechanical properties of resins systems.

2.6.2.1.2 DMA and DMTA analysis

DMA according to Prime (1997:1581-1597), is commonly used in characterization

of polymeric materials. This is due to the viscoelastic behaviour of the material

which changes with time. A reacting system therefore requires a description of

the kinetics of the system and the effect of the changing system on the

viscoelastic model. The non-isothermal dynamic mechanical characterization of

cured or partially cured systems will include effects of additional cure. The


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viscoelastic model is described by the dynamic mechanical behaviour shown

when the measured storage (E), and loss (E) moduli are plotted against

temperature.

2.6.2.2 Indentation hardness

The indentation hardness of cured epoxy samples can be measured by means of

a Barcol Impressor.

Pharr (1998) reviewed and discussed ultra-low load indentation (or

nanoindentation) techniques commonly used for measuring mechanical

properties of thin films and small volumes of material. The different indentation

techniques vary and can be classified according to the type of indenter used in

the measurement. Although the above mentioned work deals with small

quantities of material on a sub-micron scale, it still bears justice to this study as

mechanical properties can be determined by analysis of the indentation load-

displacement data.

According to Reghunadhan Nair (2004:89), enhanced modulus, compressive

strength and hardness are desirable improvements in epoxy resins. Good

mechanical performance offer immediate solutions for challenging problems.

Zhang et al. (2004) investigated the enhancement of wear resistance of epoxies

and found that the friction and wear behaviour can be improved through

enhancement of hardness, stiffness and compressive strength.


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CHAPTER 3

3.0 Research Methodology

3.1 Materials used and their preparation

The base resin used in this study was prepared from the reaction between

epichlorohydrin and bisphenol-A (Epon Resin 826 (ARH)) manufactured by

Resolution Performance Products, and supplied by Plastomax. The curing agent

used in this study was an aliphatic amine (Ancamine) also from Plastomax. Data

sheets representing the materials are presented in Annexure A.

The components of the system, the epoxy resin and hardener (curing agent),

were carefully weighed on an electronic scale and homogeneously mixed to a

ratio of 100:25 by mass (as per instruction from the supplier).

3.2 Gelation Study

A Rheometric Dynamic Stress Rheometer (RS-500) (Figure 3.1) equipped with

Orchestrator software was used under oscillatory dynamic mode to obtain the

experimental data reported in this study.

A parallel 25 mm diameter plate measuring head with a 0,2 mm gap was

employed. The measurements were carried out by using a small strain (5%) at

four frequency values (50, 15, 10 and 1 rad/s) and nine temperatures for each

frequency (30, 40, 50, 60, 70, 80, 90, 100 and 110C). The components of

complex modulus, complex viscosity and tan were measured and plotted

against frequency and therefore as a function of time.

The gel point was taken as the crossover value of the two components of the

complex modulus curves, namely the storage and loss modulus. (See Figure

3.2). Gelation occurs at time tgel=196 s, indicated by the crossover of the

components of complex modulus.


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Figure 3.1. Rheometric Dynamic Stress Rheometer (RS-500)

Figure 3.2. Typical plot of the loss and storage moduli, tan and

complex viscosity * against time for Rheological measurements

G'/G ' Crossover Point: 196


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The average time for the system to reaction to reach gelation, time Tgel,

was calculated from the data and plotted in Figure 3.2. This shows the

relationship between the time to gelation (tgel) and curing temperature

(Tcure) of the system.

3.3 Vitrification Study

3.3.1 Sample preparation

Small amounts of the initial reactive mixture as per section 3.1 (approximately 10

mg) were transferred into small DSC aluminium pans. These samples were

grouped in batches of 10 and sealed in plastic zip lock bags. The plastic bags

were stored in a freezer, at approximately -15C to prevent any cure taking place.

Sample pans were removed from the sealed plastic bags, and were allowed at

least 20 minutes to reach room temperature before placing in an oven for

isothermal curing at 60, 80, 100,120 and 140C for periods up to 4320 minutes.

A Perkin Elmer differential scanning calorimeter, Model DSC 7 (see Figure 3.4),

equipped with Pyris software was employed to determine the glass transition

temperatures. The heating profile consisted of a heating run from 25C to 150C

at 10C/min. The test was carried out in a nitrogen atmosphere with a flow rate of

20 5 ml/min. The parameters measured were the glass transition temperature

(Tg) and residual exotherm (Hr) of the reaction after the material had been

subjected to isothermal curing at the different curing temperatures.

Tgappears as an endothermic shift over a temperature interval in the DSC scan

as shown in Figure 3.3. In this study, Tg was taken as the mid-point of the step-

transition, while Hr of the remaining reaction appears as an exothermic peak in

the temperature range of the rubbery state of the material.


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Figure 3.3. Typical plot of a DSC scan of endothermic heat flow (mW) against temperature(C).

Figure 3.4 Perkin-Elmer Differential Scanning Calorimeter (DSC-7)

3.4 Dynamic Mechanical Thermal Analysis


The reactive mixture (prepared as described in section 3.1) was poured into

specially created flat moulds made from impression material (Zetalabor Platinum

Laboratory High Precision Addition Silicone. Manufactured by Zhermack).

These samples were stored in a freezer, at approximately -15C to prevent any

cure taking place.


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Samples were removed from the freezer, and were allowed at least 20 minutes to

reach room temperature before moving to an oven for isothermal curing.

The cured samples were cut to approximately 5mm x 25mm x 1.5 mm, carefully

measured and mounted in the DMTA.

A Rheometric Scientific Solid Analyzer RSA II (DMTA), equipped with

Orchestrator software was used to obtain the experimental data (see Figure 3.6).

A Dynamic Temperature Ramp using the dual cantilever mode was employed

and measurements of the storage modulus (E'), loss modulus (E) and loss

tangent (tan ) of fully cured samples were carried out using a small strain (0.01

%) at a frequency of=6.28 rad/s.

The cured samples were subjected to temperature scans from 40 to 160C at a

rate of 10C/min.

DMTA analyses were performed on samples cured at three different

temperatures (70, 90 and 110C) for three different periods (30, 60 and 120

minutes). A typical DMTA scan is shown in Figure 3.5


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Figure 3.5. Typical plot of the storage modulus (E') and tan against temperature. (Taken from

Laza et al. (1998:43))

Figure 3.6. Rheometric Scientific Solid Analyzer RSA II (DMTA) in dual cantilever mode


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3.5 Indentation hardness


The samples were prepared as described in Section 3.4 under Dynamic

Mechanical Thermal Analysis

3.5.2 Experimental procedure

A Hand-Held Portable Hardness Tester (Barcol Impressor, Model No. 934-1) was

used to obtain the experimental data (see Figure 3.7). The indentor consisted of

a hardened steel truncated cone with an angle of 26 with a flat tip of 0.157 mm

in diameter.

The test method used to determine the indentation hardness was performed in

accordance with Standard Test Method for Indentation Hardness of Rigid Plastics

by Means of a Barcol Impressor, ASTM Designation: D 2583 87.

Barcol indentation analyses were performed on samples cured at three different

temperatures (70, 90 and 110C) for three different times (30, 60 and 120

minutes). It must be noted that the impression hardness tests were carried out on

samples at room temperature.

Figure 3.7. The Impressor: A Hand-Held Portable Hardness Tester(Barcol Impressor, Model No. 934-1)


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CHAPTER 4

4.0 Results and discussion

4.1 Introduction

The results obtained from the experimental work will be dealt with individually in

this chapter. Results from gelation and glass transition temperature studies are

used to monitor the extent of cure of the thermoset system, and forms the basis

of characterizing the material. The results will be summarized in the form of an

isothermal time-temperature-transformation (TTT) diagram.

The results obtained from dynamic mechanical thermal analysis (DMTA) and

hardness tests will be used to illustrate the relationship between mechanical

properties and the molecular structure of the cured material as the molecular

structure of the system is dependent on curing conditions.

4.2 GelationStudy

The aim of the gelation study is to determine at what point during the curing

process of the thermosetting system gelation (Tgel) occurs. The experimental

results obtained from parallel plate rheology tests, as per Section 3.1., were

used to obtain crossover values of the storage and loss modulus curves as a

function of time. A derived summary of the average gel times (Log Time) for

tests performed at the four frequencies (50, 15, 10 and1 rad/s) and seven cure

temperatures (30, 40, 50, 60, 70, 80, 90, 100 and 110C) are shown in Table

4.1. Times to gelation for samples tested at low curing temperatures (30, 40

and 50C) are extrapolated from storage and loss modulus curves, as the

system solidified prior to reaching gelation. We can classify these as having

vitrified before the curing system could form a gel. The greyed out cells

represent measurements that could not be recorded due to premature

solidification of the resin system. The values of pre-gelation and premature

solidification will not form part of the calculations.


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Table 4.1.Average gel times (Log Time) for different frequencies

at the respective cure temperatures

Frequency (rad/s)

Gelation

Times

(Log t) 50.00 15.00 10.00 1.00

Average

(s)

Average

(min)

Log

Time

(min)

30 8326.95 8273.35 8973.95 10009.80 8896.01 148.27 2.17

40 4628.70 4830.05 4597.20 5722.45 4944.60 82.41 1.92

50 3007.05 3107.15 3273.30 3615.80 3250.83 54.18 1.73

60 1504.70 1677.15 1727.45 1661.25 1642.64 27.38 1.44

70 1020.21 1126.38 893.51 1008.60 1012.18 16.87 1.23

80 663.49 688.20 650.42 680.04 670.54 11.18 1.05

90 443.39 435.13 416.90 409.87 358.65 5.98 0.78

100 344.07 297.42 312.99 318.16 5.30 0.72

110 224.10 243.00 289.00 252.03 4.20 0.62

Further analysis of data from Table 4.1 presented in Figure 4.1 shows the

decrease in gelation times (Tg) for samples cured at higher cure temperatures

(Tcure). This indicates that there is a definite relationship between the time taken

for the polymer system to react and form a three dimensional network structure

and the temperature at which the system is cured. It is obvious that the time

taken for the system to form a gel decreases as the cure temperature

increases. The oscillation frequency at which the tests were conducted does

not have a real effect on gel times.


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20

30

40

50

60

70

80

90

100

110

120

0.40 0.60 0.80 1.00 1.20 1.40 1.60

Log Time (min)

Tcure

(C)

Figure 4.1. Time-temperature-transformation (TTT) diagram: Gelation contour.

4.3 Vitrification Study

The aim of the vitrification study is to determine at which point the

thermosetting system vitrifies during the curing process. Vitrification is a

transition from a viscous liquid or rubber to a glass and according to Gan et al.

(1989:803), vitrification is the point where the system solidifies. According to

Prime (1997:1383 &1569), vitrification can occur at any stage during

polymerization, either as a consequence of solvent evaporation or chemical

reaction. It is therefore important to be able to track and monitor the cure

reaction and define the phase changes that occur during cure and in doing so,

demonstrate the conditions where vitrification takes place.

Work by Gan et al. (1989:803), Gillham (1990, 1993), and Wang (1992),

illustrated that there is a sensitive one-to-one relationship between the glass

transition temperature, Tg, and the conversion in thermosetting resins. This

implies that Tg can be used as an index for measuring conversion during cure.

The glass transition temperature is a useful, sensitive, easily measured

macroscopic parameter for monitoring the cure process of thermosetting

systems. Measurements taken on samples cured at different cure

temperatures, Tcure, for different periods, tcure, were measured by differential


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4.3.1.1 Tg versus Tcure

Values ofTg determined by DSC and the corresponding Tcure values for cure

times tcure (1 to 72 h) are plotted in Figure 4.2. The contours show the

relationship between Tg, and Tcure.

Figure 4.2.Tg (C) vs. Tcure (C) for different isothermal heating times (1 hour through to72 hours). The reference line ofTg= Tcure is also included.

The reference lines included are ofTcure = Tg, and Tg= 107.6C. The value for

Tg is determined in Section 4.3.1.2.

By examining Figure 4.2, especially Tgvs. Tcure for the tcure=10 hours contour,

we note a definite increase in glass transition temperature with an increase in

cure temperature. It is important to note that samples cured at a constant Tcure

for increased time increments, show continuous increases in Tg values.Keeping in mind that Tg is used to monitor cure, we can see from the positive

contour slopes ofTgvs. Tcure at lower curing temperatures, that the cure is not

complete. Contours at higher curing temperatures, for longer curing times,

exhibit a flexure point in the slope of the plot and a definite maximum is

observed. This is a result of the competition between cure (which increases Tg)

Tg=107,6C

Tg=Tcure


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and thermal degradation (which decreases Tg) and is indicative of a

deteriorating molecular structure. The change is possibly due to de-vitrification,

and molecular breakdown as a result of thermal influences on the molecular

structures.

According to Gan et al. (1989:807), a system that exhibits contrasting values of

Tg>Tcure forTcure >Tg, even for prolonged cure times (tcure = 72 hours) indicates

that the system is highly reactive.

4.3.1.2 Tgversus tcure

The variation of Tg with log time at different isothermal temperatures is

illustrated in Figure 4.3. It shows that samples cured at constant temperaturesfor increasing cure times exhibit a general increase in Tgwhich is a result of the

curing reaction taking place and, is in accordance with that reported by Gan et

al. (1989:808).

Figure 4.3.Tg (C) vs. Log Time (min) for different isothermal temperatures (Tcure).


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4.3.1.3 Iso-Tgcontours

The data derived from Figure 4.3 was used to develop iso-Tg contours in the

format of a time-temperature-transformation (TTT) diagram and is shown in

Figure 4.5.

Figure 4.5. Time-temperature-transformation (TTT) diagram: Iso-Tgcontours.

As previously stated, the glass transition temperature is a useful tool to monitor

the reactions that occur during cure. The iso-Tgcontours can therefore be seen

as an extention of conversion curves. Vitrification of the system occurs when Tg

rises to Tcure. Vitrification will be discussed in the next paragraph.

Tcure (C)against LogTime (min)


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4.3.1.4 Prediction oftvit

Data from Figure 4.5 was used to compile Table 4.3, which, in turn, was used

to compile Figure 4.6. This illustrates the relationship between Ln time to a

fixed Tgand the reciprocal ofTcure (1/K).

The apparent activation energies, E, can be obtained from the slopes of the

linear relationships in Figure 4.6 (as in Section 4.1.2): E values are 18.19,

16.90, 26.80, and 46.82 kJ/mol, for the iso-Tg values 95, 98, 100, and 102C,

respectively.

Table 4.3 Numerical relationship between Ln time to a

fixed Tgand the reciprocal ofTcure (1/K).

Ln Time (min) Iso-Tg

Tcure

(C)

Tcure

(K) 1000/Tcure 93 95 98 100 102 104 105

60 333 3.00 5.73 8.01

80 353 2.83 3.73 4.14 5.00 6.26 7.71

100 373 2.68 2.58 2.95 3.82 4.49 4.79 5.66 7.44

120 393 2.54 2.30 3.09 3.73 3.89 4.37 5.07

140 413 2.42 2.60 3.89 4.31 4.81 5.04


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1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

2.30 2.40 2.50 2.60 2.70 2.80 2.90

1000/Tcure (K)

Ln

Time

(min)

Tg: 95C

Tg: 98C

Tg: 100C

Tg: 102C

Figure 4.6. Ln time (min) to fixed Tgvalues (from Figure 4.5) vs. 1/Tcure (K).

According to Gan et al. (1989:810) and Prime (1997:1616-1617) (Arrhenius

Reference) the slope of the linear part of the Arrhenius plot, Figure 4.6,

represents the apparent activation energies, E, of an epoxy (thermoset)

system. Prime further explains that the activation energy may be viewed as a

time-temperature shift factor where the cure process can be described by a

single overall activation energy. The activation energies, E, as determined

from the slopes of curves from Figure 4.6 were used to determine vitrification

times as illustrated below in equations 4.1 to 4.4.


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The linear equations for the iso-Tgplots emanating from Figure 4.6 are listed

below:

186.181000.8827.7)(

=Tcure

tLn (r = 1) for iso-Tg= 95C [4.1]

904.161000

.7311.7)(

=

TcuretLn (r = 1) for iso-Tg= 98C [4.2]

803.261000

.672.11)(

=


824.461000

.252.19)(

=


The times to vitrification, tvit, were calculated by setting Tg = Tcure in the iso-Tg

equations, since vitrification is define to occur at Tg = Tcure. ForTcure = 95, 98,

100 and 102C, the logarithm10 of times to vitrification are 1.447, 1.750, 2.077,

and 2.26 minutes respectively.

The calculated values were used to construct the isothermal TTT cure diagram

simply because of the long experimental times for the system to vitrify.

The method used by Gan et al. (1989) to calculate vitrification times for a

thermosetting system, assumes the absence of diffusion control, solvent

evaporation, and thermal degradation. Their method describes the relationship

between the rate of reaction (-dx/dt), reactant concentration (x), activation

energy (E), temperature (T,K) and time (t) with the Arrhenius equation:

( ) )(/exp xfRTEAdt

dxa = [4.5]

or by integrating

( ) ( ) tRTEAxF a = /exp [4.6]


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A being the Arrhenius pre-exponential factor, E the activation energy, R the

gas constant (8,314 J/(mol.K)), and F(x), f(x) functions of x.

The analysis of thermal data obtained in this study, with support from work

performed by other researchers is summarized in an isothermal time-

temperature-transformation (TTT) cure diagram. The vitrification curve in a TTT

system cure diagram corresponds to a stage in the reaction at temperature Tg,

for time tvit.

The iso-Tgcontour for any corresponding pair of values ofTgand tvit is given by

the following equation;

( ) )vitgacurecureatRTEtRTE = /exp/exp [4.7]

Or

( ) curegcure

avit tTT

REt ln11

/ln +

= [4.8]

The use of equation [4.8], in conjunction with data fortcure = 3 hours from Figure

4.2, and E/R= 7882.7, obtained from time temperature conversion behaviour

forTg = 95C (assumed to be constant for lower conversions), indicates thatcure at 60C for 3 hours gives Tg = 92.5C, with log tvit = 1.88 min forTcure =

92.5C. Vitrification times were calculated for isothermal cure and presented in

Table 4.4.

Table 4.4 Calculated times to vitrification forTcure = Tg

corresponding to isothermal cure temperatures from Figure 4.2.

Tcure(C,

From Fig 4.2)Tg (C)

Tcure

(C)

tvit (Log

min)

60 92.5 92.5 1.88

70 95.2 95.2 1.95

80 97.7 97.7 2.02

90 99.1 99.1 2.05

100 101.4 101.4 2.11

110 103.2 103.2 2.15


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Figure 4.7 indicates the relationship between Ln time to gelation and the

reciprocal temperature (K);

583,111000.9374,4)(

=Tcure

tLn (r = 0.986) [4.9]

for which the apparent activation energy is 11.583 kJ/mol. The value of Tg at

gelation is expected to vary with Tcure since the measurement of gelation in this

study was carried out by means of parallel plate rheology.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

2.5 2.6 2.7 2.8 2.9 3 3.1

1000/Tcure (K)

Ln

Time

(min)

Figure 4.7. Ln time (min) to macroscopic gelation vs. 1/Tcure (K).

The isothermal temperature at which the time to vitrification equals the time to

macroscopic gelation (extrapolated) is indicated as gelTg. This is calculated

using Eq. (8), where tvit corresponds to reaction temperature gelTg, equation

[4.9], and equation [4.10] which summarizes isochronal Tg versus Tcure data for

tcure= 3 hours of Figure 4.2.

5,77.25,0 += gcure TT (Tg


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The relevant expression becomes:

193.5

5,77'.25,0

1

'

17.7882583,11

'

4,4937+

+

=ggelggelggel TTT

[4.11]

Solving the expression, gelTg = 40.204C, log tvit = 1.82 min for Tcure =

40.204C.

4.4 Time-Temperature-Transformation Diagram

The results of the work done in the first two parts of this chapter can be

summarized in the form of a time-temperature-transformation (TTT) isothermal

cure diagram, as in Figure 4.8 where the progress of transformation of liquid to

glass or to rubber is shown by the gelation, vitrification and iso-Tgcontours.


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Figure 4.8. Summary time-temperature-transformation (TTT) diagram.

4.5 Mechanical property Vs. cure relationship

4.5.1 Areas for Analysis

The present investigation is aimed at indicating the relationship between cure

and mechanical properties. To facilitate this, a TTT isothermal cure diagram

(Figure 4.8) was used to determine regions or zones to illustrate the


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relationship and is presented in Figure 4.9 below. Figure 4.9 is a detailed

version of Figure 4.8.

Zone 1 through to zone 3 represents samples cured at Tcure = 70C for cure

times (tcure) of 30, 60, and 120 minutes, zones 4 to 6 are representative of

samples subjected to Tcure = 90C, and zones 7 to 9 to Tcure = 110C, for cure

times of 30, 60, and 120 minutes respectively.

Figure 4.9. Detailed TTT-Isothermal cure diagram showing areas

for DTMA analysis.

With reference to the generalised time-temperature-transformation (TTT) cure

diagram (taken from Prime (1997:1384)), Figure 2.1, zones 1 and 2 fall into a

conversion area beyond gelation but ahead of vitrification, zone 3 is after

vitrification.


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4.5.2 Dynamic Mechanical Thermal Analysis (DMTA)

As it would have been very time and material consuming, DMTA was chosen

as method of determining mechanical properties instead of tensile, impact and

bending tests. The results for storage modulus, E', and loss modulus, E as

obtained from DMTA scans in this study are dealt with separately for samples

cured isothermally at differentcure times (tc). The values forTcure and tc were

determined by means of the time-temperature-transformation (TTT) diagram

constructed in the previous section of this chapter.

The DMTA temperature scans were performed at a rate of 10C/minute, for

temperatures between 25 and 160C.

4.5.2.1 Storage Modulus

The scans show that systems cured at different temperatures (Tcure) display

different moduli but broadly similar dynamic mechanical behaviour.

Distinct changes in log(E') are evident and coincide with the glass transition

temperature (Tg) which is represented by the peak tan values.


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DMTA Graphs : 70C

0

0.2

0.4

0.6

0.8

1

1.2

20 40 60 80 100 120 140 160

Temperature (C)

TanDelta

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

StorageModulusLogE'

Tan Delta 30 min

Tan Delta 60 min

Tan Delts 120 min

30 min @ 70C

60 min @ 70C

120 min @ 70C

Figure 4.10-a. DMTA scans forTcure = 70C

DMTA Graphs for 90C

0

0.2

0.4

0.6

0.8

1

1.2

1.4

20 40 60 80 100 120 140 160

Temperature C

TanDelta

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

StorageModulusL

ogE'

Tan Delta 30 min

Tan Delta 60 min

Tan Delta 120 min

30 min @ 90C

60 min @ 90C

120 min @ 90C

Figure 4.10-b. DMTA scans forTcure = 90C


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DMTA Graphs for 110C

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

20 40 60 80 100 120 140 160

Temperature (C)

TanDelta

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

StorageModulusLogE'

Tan Delta 30 min

Tan Delta 60 min

Tan Delta 120 min

30 min @ 110C

60 min @ 110C

120 min 110C

Figure 4.10-c. DMTAscans forTcure=110C. Tan and storage modulus (E') vs. temperature

(C) for samples cured for different cure times (tcure, min) at different cure temperatures (Tc).

The results from Figure 4.10 are tabulated in Table 4.4 below. Subsequent

Tables and Figures were derived from this Table.

Table 4.4. Results from DMTA

Tan E' (Pa) Tg (C) Time (min) Tan E' (Pa) Tg (C) Time (min) Tan E' (Pa) Tg (C) Time (min)

70 0.74 1.69E+08 91.30 7.95 1.10 7.12E+07 96.06 8.45 0.83 8.45E+07 98.14 8.65

90 0.99 6.01E+07 109.82 9.75 0.83 5.65E+07 111.13 9.93 1.15 8.60E+07 113.37 10.27

110 0.75 1.08E+08 110.71 9.83 0.95 1.05E+08 114.84 10.42 0.79 5.51E+07 112.39 10.05

Cure

Temperatures

(Tcure,

C)

30 min 60 min 120 min


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4.5.2.2 Glass transition temperature (Tg)

A dynamic mechanical thermal analyser (DMTA) was used to determine the

modulus and tan as a function of temperature and/or frequency. The samples

were tested under three-point bending mode, known as dual cantilever. According to Laza et al. (1998), and Mafi et al. (2005), the glass transition

temperature, Tg, is generally defined as the maximum in the tan curve.

The contours of Tg values against tc (log time) as in Figure 4.11 below,

indicates the relationship between Tg, Tcure, and tcas determined by DMTA.

80

90

100

110

120

0 20 40 60 80 100 120 140

Cure Time (min)

Tg

(C)

Tcure : 70C

Tcure : 90C

Tcure : 110C

Figure 4.11. T

Cure Thesis

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