a study on the effect of hardener on the mechanical properties of epoxy resin

Republic of Iraq Ministry of Higher Education and Scientific Research University of Technology Chemical Engineering Department

A STUDY ON THE EFFECT OF

HARDENER ON THE MECHANICAL

PROPERTIES OF EPOXY RESIN

A THESIS

Submitted to the Chemical Engineering Department of the University of

Technology in Partial Fulfillment of the Requirements for the Degree of

Master of Science in Chemical Engineering/Unit Operation

BY

MARIAM EMAD AZIZ

(B.Sc. In Chemical Engineering, 2004)

2010

CERTIFICATION We certify that we have read this thesis titled “A Study On The Effect Of

Hardener On The Mechanical Properties Of Epoxy Resin” which is being

submitted by Mariam Emad Aziz ; and as an examining committee, we

examined the student, and in our opinion it meets the standard of a thesis for

the degree of Master of Science in Chemical Engineering.

Signature: Signature:

Name: Asst. Prof. Dr. Najat J. Saleh Name: Dr.Adnan A. Abdul Razak

(Supervisor) (Supervisor)


Name: Asst. Prof. Dr. Qusay F. Alsalhy Name: Asst. Prof. Dr. F. S. Matty

(Member) (Member)

Signature:

Name: Prof. Dr. Mohammed H. AL-Taie

(Chairman)

Approved by the University of Technology.

Signature:

Name: Prof. Dr. Mumtaz A. Zablouk

(Head of Chemical Engineering Department)

Date: / /2010

CERTIFICATION

We certify that the preparation of this thesis titled “A Study On The Effect Of Hardener On The Mechanical Properties Of Epoxy Resin” was made by Mariam Emad Aziz under our supervision at the Department of Chemical Engineering in the University of Technology, as a partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering.


Name: Asst. Prof. Dr. Najat J. Saleh Name: Dr.Adnan A. Abdul Razak

(Supervisor) (Supervisor)

Date: / /2010 Date: / /2010

In the view of the available recommendation. I forward this thesis for the debate by the Examining Committee.

Signature:

Name: Dr. Muhammad Ibrahim

(Head of Post Graduate Committee)

(Chemical Engineering Department)

Date: / /2010

CERTIFICATION

This is to certify that I have read the thesis titled “A Study On The Effect Of

Hardener On The Mechanical Properties Of Epoxy Resin” and corrected any

grammatical mistakes I found. This thesis is, therefore, qualified for debate.

Signature:

Name: Prof. Dr. Mumtaz A. Zablouk

(Head of the Chemical Engineering Department)

Date: / / 2010

II

ABSTRACT

Diglycidyl ether of bisphenol-A (DGEBA) epoxy resin and two hardeners;

triethylene tetramine (TETA) and diamino diphenyl methane (DDM) were

prepared with different hardener/resin ratios, (under stoichiometry, stoichiometry

and above stoichimetry) and their mechanical properties; cure kinetics and

rheology were investigated by using mechanical tests, thermal and rheological

analysis.

Impact strength, tensile strength, hardness, flexural strength, compression

strength and bending strength were measured through using mechanical tests

instruments. The tests were carried out at room temperature. For DGEBA/TETA

system the tests were done on four hardener/resin ratios (10, 13, 15 and 20) phr and

for DGEBA/DDM system the hardener/resin ratios were four also; (24, 27, 30 and

34) phr. The results showed that the above stoichiometry ratio formulation (15 phr

for DGEBA/TETA system and 30 phr for DGEBA/DDM system) gave the best

mechanical properties. While the DGEBA/DDM system showed better mechanical

properties than the DGEBA/TETA system.

From dynamic and isothermal runs of the DGEBA/TETA system for three

hardener/resin ratios (5, 13and 20) phr, the cure kinetics at four temperatures (30,

45, 60 and 80) °C was analyzed by a differential scanning calorimetry (DSC). The

isothermal cure process was simulated with the four-parameter autocatalytic with

diffusion model (modified Kamal’s model). The fitted results agreed well with the

experimental values in the late and early cure stages. The results showed that the

stoichiometric ratio (13 phr) reaches complete cure (α =1) at 80 °C.

III

Viscosity )η ( of DGEBA/TETA system was measured through curing

using a Brookfield viscometer at four different temperatures (30, 45, 60 and 80)

°C. The measurements were carried out for three hardener/resin ratios (5, 13 and

20) phr. The gel time (tRgelR ) was calculated for each hardener/resin ratio

formulation; from the viscosity experimental data. The results showed that the gel

time decrease with increasing curing temperature for each hardener/resin ratio

formulation. Viscosity profiles were described by a model based on the Boltzmann

function. The fitted results agreed well with the experimental values.

MARIAM EMAD AZIZ . A STUDY ON THE EFFECT OF THE HARDENER ON THE MECHANICAL PROPERTIES OF THE EPOXY RESIN. UNIVERSITY OF TECHNOLOGY Department of Chemical Engineering. M.Sc. Supervisors: Dr. Najat. J. Saleh and Dr. Adnan A. AbdulRazaq. 2010. 131p.

Abstract Diglycidyl ether of bisphenol-A (DGEBA) epoxy resin and two hardeners; triethylene tetramine (TETA) and diamino diphenyl methane (DDM) were prepared with different hardener/resin ratios, (under stoichiometry, stoichiometry and above stoichimetry) and their mechanical properties; cure kinetics and rheology were investigated by using mechanical tests, thermal and rheological analysis. Impact strength, tensile strength, hardness, flexural strength, compression strength and bending strength were measured through using mechanical tests instruments. The tests were carried out at room temperature. For DGEBA/TETA system the tests were done on four hardener/resin ratios (10, 13, 15 and 20) phr and for DGEBA/DDM system the hardener/resin ratios were four also; (24, 27, 30 and 34) phr. The results showed that the above stoichiometry ratio formulation (15 phr for DGEBA/TETA system and 30 phr for DGEBA/DDM system) gave the best mechanical properties. While the DGEBA/DDM system showed better mechanical properties than the DGEBA/TETA system. From dynamic and isothermal runs of the DGEBA/TETA system for three hardener/resin ratios (5, 13and 20) phr, the cure kinetics at four temperatures (30, 45, 60 and 80) °C was analyzed by a differential scanning calorimetry (DSC). The isothermal cure process was simulated with the four-parameter autocatalytic with diffusion model (modified Kamal’s model). The fitted results agreed well with the experimental values in the late and early cure stages. The results showed that the stoichiometric ratio (13 phr) reaches complete cure (α =1) at 80 °C. Viscosity )η ( of DGEBA/TETA system was measured through curing using a Brookfield viscometer at four different temperatures (30, 45, 60 and 80) °C. The measurements were carried out for three hardener/resin ratios (5, 13 and 20) phr. The gel time (tRgelR )was calculated for each hardener/resin ratio formulation; from the viscosity experimental data. The results showed that the gel time decrease with increasing curing temperature for each hardener/resin ratio formulation. Viscosity profiles were described by a model based on the Boltzmann function. The fitted results agreed well with the experimental values.

Keywords: epoxy resin . mechanical properties . DSC . rheology

I

“Acknowledgement”

Above all, I have to thank Allah who created us and gave us the

mind to think and the ability to work.

I wish to express my gratitude to both my supervisors Dr. Najat J. Saleh

and Dr. Adnan A. Abdul Razaq for their patience, guidance, encouragement,

positive criticism, and supervision throughout this study.

Also, I wish to express my thanks to Prof. Dr. Mumtaz A. Zablouk, Head of

Chemical Engineering Department / University of Technology for his help in

providing facilities.

Thanks are due to the staff of the Chemical Engineering Department for

their valuable support, especially Dr. Zaydoon Muhssen. I also acknowledge

the great help and assistance of the Technical staff of the Central Library in

the University of Technology, especially Mrs. Vivian, Miss. Thaorah and Mr.

Saleh.

Special thanks are expressed to Dr. Balqis M. Deya and Dr. Mufeed Ali in

the Department of Applied Materials Science / University of Technology for

helping and providing facilities to perform part of this work.

Thanks are due to Mr. Sa’ad Michelle, Miss. Dalia and Mr. Bashar in the

Department of Materials Engineering / University of Technology for their help to

perform part of this work.

Finally, many thanks are due to all people who encouraged me, gave

me the will to work and the desire to continue, especially my parents and my

uncle Dr. Wadah Al-Mosawy, asking Allah to save them all.

IV

List of Contents

Contents Page

Acknowledgment I

Abstract II

List of Contents IV

Notations VII

Chapter One: Introduction

1.1 Introduction 1

1.2 Objective and scope 4

Chapter Two: Literature Review

2.1 Epoxy Resins 6

2.2 Curing Agents (Hardeners)

2.3 Curing Reactions

2.4 Selection of Curing Agents

2.5 The Stoichiometry

2.6 The Mechanical Properties of Epoxy Resin

2.6.1 The Impact Test

2.6.2 The Tensile Test

2.6.3 The Hardness Test

2.6.4 The Flexural Test

2.6.5 The Compression Test

2.6.6 The Bending Test

2.7 Differential scanning Calorimetry (DSC) Analysis

2.7.1 Cure Kinetics Models

2.8 Rheological Analysis

2.8.1 Rheology Models

2.8.1.1 Viscosity Model

7

11

14

14

15

16

17

18

19

20

21

22

23

27

28

28

V

2.8.1.2 Gel Time Model

2.9 Literature review of experimental Work on Epoxy Resin

2.9.1 Literature Review on The Mechanical Properties of The Epoxy

Resin

2.9.2 Literature Review on The Kinetice of the Epoxy Resin Using

(DSC)

2.9.3 Literature review on The Rheology of the Epoxy Resin

33

35

35

39

41

Chapter Three: Experimental Work

3.1 The Materials 44

3.1.1 Epoxy Resin

3.1.2 The Hardeners

3.2 The hardener/Resin Ratio

3.3 The Mold

3.4 The Mechanical Test

3.4.1 The Impact Test





3.4.6 Three point Bending Test

3.5 DSC Measurement

3.6 Viscosity Measurement

44

45

47

49

49

49

51

53

53

55

56

57

59

Chapter Four: Results and Discussion

4.1 The mechanical Properties 61

4.1.1 The Impact Test Results 62

4.1.2 The Tensile Test Results 65

4.1.2.1 Effect on the Elastic Modulus 65

4.1.2.2 Effect on the Ultimate Tensile strength 68

VI

4.1.2.2 Effect on the Elongation at Break 70

4.1.3 The Hardness Test Results 72

4.1.4 Flexural Strength Test Results 74

4.1.5 The Compression Test Results 77

4.1.6 The Bending Test Results 79

4.2 DSC Cure Analysis 81

4.2.1 Dynamic Cure Analysis 82

4.2.2 Isothermal DSC Cure Analysis 82

4.2.2.1 Analysis of Reaction Heat 83

4.2.2.2 Degree of Cure and Cure Rate 84

4.2.2.3 Cure Reaction Modeling 86

4.3 Isothermal Scanning Rheological Cure Analysis 99

4.3.1 Gel Time and Apparent Activation Energy (Ea 99 )

4.3.2 Viscosity Modeling 103

Chapter Five: Conclusions and Suggestions

5.1 Conclusions 114

5.2 Suggestions for Future Work 116

References 118 Appendix

VII

Notations

A Arrhenius frequency or Cross sectional area

A Initial viscosity η (mPa.sec) or (cp)

A Apparent rate constant k sec-1 A Arrhenius frequency t

ASTM American standard for testing and

materials

B Thickness of specimen mm

b Width of specimen mm C Empirical constant

c1, c2, c3,c4, c5 and c6

Constants

D Thickness of specimen mm D Width of specimen mm

DDM 4,4’- Diamino Diphenylmetane DGEBA Diglycidyl ethers of bisphenol A

DSC Differential scanning calorimetry E Young’s modulus MPa Eη Viscous flow activation energy KJ/mol Ek Kinetic activation energy KJ/mol Et Activation energy for kinetic model KJ/mol ΔEa Activation energy for kinetic model KJ/mol ΔEi Activation energy for kinetic model KJ/mol

% EL Percentage elongation F Applied force N

F. S Flexural strength MPa f Free volume of farction gf Fractional free volume

G Gravity m/sec-2

rH Total heat of reaction J/g Ht Accumulative heat of reaction J/g

Htotal Total heat released during reaction J/g

VIII

I Engineering bending momentum ISO International standard organization

K Reaction rate constant sec-1

Ki Reaction rate constant for kinetic model sec-1 ∞K Kinetic parameter of viscosity

Mw Weight average molecular weight g fm Weight of fiber g

m&n Empirical exponents in the cure kinetic model

L Specimen length mm Lf Final length mm lo Initial length mm P Load applied N R Universal gas constant J/mol. K t Time sec or min tc Critical time sec

tgel Gel time sec T Temperature °C

TETA TriethyleneTetramine

Tg Glass transition temperature °C Tr Reference temperature °C

Th Thermal conductivity Kcal/h .°C

IX

Greek Symbols

ζ Stress MPa ε Strain α Degree of conversion α Critical degree of reaction αmax Maximum degree of conversion at a specific

temperature

αgel Degree of cure at gel time ∗α Critical degree of conversion when resin gels. fα The thermal expansion coefficient of free volume.

ηo Initial viscosity (mPa.sec) or (cp)

η∞ Final viscosity (mPa.sec) (cp)

oµ Viscosity at infinite temperature (mPa.sec) or (cp)

gσ Conductivity W/K.m ψ Empirical expression ζ Empirical expression

CHAPTER ONE INTRODUCTION

1

CHAPTER ONE

INTRODUCTION

1.1 Introduction

Epoxy resins are one of the most versatile polymers under use today. Their

use ranges from matrix in high performance composite materials for aerospace

structures, to organic coatings and common adhesives for domestic applications [1-

3]. This versatility is a consequence of the many epoxy systems that can be

fabricated by using different chemical compounds to open the epoxy ring and set

the epoxy monomers. Therefore, by the use of anhydrides and aromatic or aliphatic

amines as hardeners, different epoxy systems with a large range of chemical and

physical properties can be obtained [3- 6].

Among the most widely used epoxy resin systems, those that can be cured at

room temperature are largely applied [3]. The epoxy resin system based on the

reaction of the difunctional epoxy monomer diglycidyl ether of bisphenol-A,

DGEBA, with aliphatic amines is such an example. Some other epoxy resin

systems, those which need elevated temperature to be cured, an example for this is

the epoxy resin system based on the reaction of the difunctional epoxy monomer

diglycidyl ether of bisphenol-A, DGEBA, with aromatic amine. The properties of

this and other epoxy systems can be varied as a function of the molecular weight of

the hardener molecule [7-10] by variations in processing conditions [11-13] or by

the use of different hardener to monomer ratios [8, 11]. This last variable

introduces off-stoichiometric mixtures. For the particular systems made of the

triethylene tetramine, TETA, hardener and the DGEBA monomer, and the 4, 4-

diamino diphenlmethane, DDM, hardener and the DGEBA monomer, the variation


2

of the hardener to monomer ratio promotes strong changes on the mechanical

behavior [14].

The problem of working with off-stoichiometric mixtures is that latent

reaction sites could remain on the macromolecular structure developed and under

the proper conditions the structure can evolve, resulting in changes on the

mechanical performance of the material. Temperature is clearly one external

parameter that could cause changes to the system.

Epoxy resin can be molded to the desired shape according to the needs of

the final products, and cured by the application of heat. These applications involve

curing cycles of the epoxy resin, in which different isothermal and dynamic curing

processes are applied. Curing cycles determine the degree of cure of the epoxy

resin and have an important effect on the mechanical properties of the final

products. Optimal curing schedules and hardener/resin ratios are the keys to

achieve efficiently the desired properties of the cured materials [15]. Although

companies manufacturing the commercial epoxy resin materials usually suggest

curing cycles and hardener/resin ratios for custom applications, their curing cycles

and hardener/resin ratios may not be the optimal ones for special applications. In

order to optimize the curing cycles and hardener/resin ratios for epoxy resin, it is

necessary to understand the cure kinetics and characteristics of epoxy resin in more

detail.

Number of methods was used to analyze the cure kinetics and physical

properties of epoxy resin in this study. Mechanical test methods were used to

investigate the mechanical properties of epoxy resin. These tests are made to

evaluate the general performance and behavior of the epoxy resin system, where its

response to applied stresses or strains are used to determine its mechanical

properties. This response depends markedly on the structure of the epoxy resin

system [16, 17].


3

Differential Scanning Calorimetry (DSC) analysis is based on the heat flow

change of the epoxy resin sample during the cure process. It is assumed that the

heat of cure reaction equals the total area under the heat flow-time curve. The

degree of cure is proportional to the reaction heat [18]. It was calculated either by

the residual heat or by the reaction heat at a particular time. Different kinetic

models for DSC cure analysis are available. The simpler model applied to DSC

data was the model from the mechanism of an nth order reaction. This model gave

a good fit to the experimental data only in a limited range of degree of cure [19].

More complicated models for isothermal and dynamic curing process assumed an

autocatalytic mechanism [20]. The autocatalytic model may have different forms

depending on whether the value of initial cure rate is zero or not. At isothermal

conditions, the rate constant and reaction orders are determined at each cure

temperature.

Applications of epoxy resin require understanding its rheological

properties during the cure process, as well as the cure kinetics. Rheological

analysis has been used to study the cure process of epoxy resin [21, 22] and is also

essential to the optimization of cure cycle and hardener/resin ratio. Like polymers,

epoxy resin is a viscoelastic material. During a curing process under continuous

stresses or strains, its viscoelastic characteristics change, which is reflected in the

variations of the viscosity η. Viscosity, measures the fluidity of the epoxy resin

system. Higher viscosity means the lower fluidity of the epoxy resin systems. It is

used to evaluate the viscoelasticity of epoxy resin. The flow behavior of reacting

system is closely related to the cure process. In the early cure stage, the epoxy resin

is in a liquid state. Cure reaction takes place in a continuous liquid phase. With the

advancement of the cure process, a crosslinking reaction occurs at a critical extent

of reaction. This is the onset of formation of networking and is called the gel point

[23]. At the gel point, epoxy resin changes from a liquid to a rubber state. It


4

becomes very viscous and thus difficult to process; so the gelation has an important

effect on the application process of epoxy resin. Although the appearance of the

gelation limits greatly the fluidity of epoxy resins, it has little effect on the cure

rate; so the gelation cannot be detected by the analysis of cure rate, as is the case in

the DSC. The gel time may be determined by a rheological analysis of the cure

process.

1.2 The specific objectives of the research are as follows:

Objective and Scope

1. To study the mechanical properties of the DGEBA/TETA system and the

DGEBA/DDM system of different hardener/resin ratios and their effect on the

mechanical properties of the epoxy resin system, finding the best hardener/resin

ratio formulation and the best epoxy resin system.

2. To relate the heat flow or cure reaction heat to the degree of cure of

DGEBA/TETA system. The cure process of epoxy resin is an exothermic process.

The reaction heat released during the cure process can be calculated from the time-

dependent heat flow curve. The relationship between the degree of cure and time

will be determined for different hardener/resin ratios, finding the hardener/resin

ratio that gives the maximum degree of cure.

3. To relate the rheological properties such as the viscosity to the gel time and cure

process. When the cure process proceeds to a certain degree of cure, the molecular

mobility of the reacting system will be greatly limited and gelation occurs. The gel

point is usually determined by the rheological properties.

4. To test the existing kinetic and viscosity models. A number of kinetic and

viscosity models have been reported recently. The first and nth order reaction

models may be used with limited accuracy. To achieve better accuracy, the


5

complicated autocatalytic reaction models will be used and compared with

experimental data.

CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW

6

CHAPTER TWO

THEORETICAL CONCEPTS & LITERATURE REVIEW

2.1 Epoxy resins are thermosetting polymers that, before curing, have

one or more active epoxide or oxirane groups at the end(s) of the molecule

and a few repeated units in the middle of the molecule [24]. Chemically,

they can be any compounds that have one or more 1,2-epoxy groups and can

convert to thermosetting materials. Their molecular weights can vary

greatly. They exist either as liquids with lower viscosity or as solids.

Through the ring opening reaction, the active epoxide groups in the uncured

epoxy can react with many curing agents or hardeners that contain hydroxyl,

carboxyl, amine, and amino groups [24, 25].

Epoxy Resins

Compared to other materials, epoxy resins have several unique

chemical and physical properties. Epoxy resins can be produced to have

excellent chemical resistance, excellent adhesion, good heat and electrical

resistance, low shrinkage, and good mechanical properties, such as high

strength and toughness. These desirable properties result in epoxy resins

having wide markets in industry, packaging, aerospace, construction, etc.

They have found remarkable applications as bonding and adhesives,

protective coatings, electrical laminates, apparel finishes, fiber-reinforced

plastics, flooring and paving, and composite pipes.

Since their first commercial production in 1940s by Devoe-Reynolds

Company, the consumption of epoxy resins has grown gradually almost


7

every year [26, 27]. The three main manufacturers of epoxy resins are Shell

Chemical Company, Dow Chemical Company and Ciba-Geigy Plastics

Corporation. They produce most of the world’s epoxy resins. The United

States and other industrialized countries such as Japan and those in Western

Europe are the main producers and consumers of epoxy resins.

Since the 1930’s when the preparation of epoxy resins was patented,

many types of new epoxy resins have been developed from epoxides.

Tanaka [28] gave a complete list of epoxides and discussed their properties

and preparation. Most conventional epoxy resins are prepared from

bisphenol A and epichlorohydrin. For example, the most commonly used

epoxy resins are produced from diglycidyl ethers of bisphenol A (DGEBA).

Its properties and reaction mechanism with various curing agents have been

reported extensively [29, 30]. Other types of epoxy resins are glycidyl ethers

of novolac resins, phenoxy epoxy resins, and (cyclo) aliphatic epoxy resins.

Glycidyl ethers of novolac resins and phenoxy epoxy resins usually have

high viscosity and better high temperatures properties while (cyclo) aliphatic

epoxy resins have low viscosity and low glass transition temperatures. The

chemical structures of some epoxy resin types are shown in Table (2.1).

Although many accomplishments have been made in the field of epoxy

resins, researchers still make efforts to understand better their curing

mechanisms, to improve their properties, and to produce new epoxy resins.

2.2 Curing agents play an important role in the curing process of epoxy

resin because they relate to the curing kinetics, reaction rate, gel time, degree

of cure, viscosity, curing cycle, and the final properties of the cured

products.

Curing Agents (Hardeners)


8

Table (2.1) Chemical Structure of Some Epoxy Resins [26]

Mika and Bauer [31] gave an overview of the epoxy curing agents and

modifiers. They discussed three main types of curing agents:

1. The first type of curing agents includes active hydrogen compounds

and their derivatives. Compounds with amine, amides, hydroxyl, acid

or acid anhydride groups belong to this type. They usually react with

epoxy resin by polyaddition to result in an amine, ether, or ester.

Aliphatic and aromatic polyamines, polyamides, and their derivatives

are the commonly used amine type curing agents. The aliphatic

amines are very reactive and have a short lifetime. Their applications

are limited because they are usually volatile, toxic or irritating to eyes


9

and skin and thus cause health problems. Compared to aliphatic

amine, aromatic amines are less reactive, less harmful to people, and

need higher cure temperature and longer cure time. Hydroxyl and

anhydride curing agents are usually less reactive than amines and

require a higher cure temperature and more cure time. They have

longer lifetimes. Polyphenols are the more frequently used hydroxyl

type curing agents. Polybasic acids and acid anhydrides are the acid

and anhydride type curing agents that are widely used in the coating

field. Table (2.2) gives a list of commonly-used type 1 curing agents

and their chemical structures.

2. The second type of curing agents includes the anionic and cationic

initiators. They are used to catalyze the homopolymerization of epoxy

resins. Molecules, which can provide an anion such as tertiary amine,

secondary amines and metal alkoxides are the effective anionic

initiators for epoxy resins. Molecules that can provide a cation, such

as the halides of tin, zinc, iron and the fluoroborates of these metals,

are the effective cationic initiators. The most important types of

cationic initiators are the complexes of BF3.

3. The third type of curing agents is called reactive cross linkers. They

usually have higher equivalent weights and crosslink with the second

hydroxyls of the epoxy resins or by self-condensation. Examples of

this type of curing agents are melamine, phenol, and urea

formaldehyde resins.

Among the three types of curing agents, compounds with active

hydrogen are the most frequently used curing agents and have gained wide

commercial success. Most anionic and cationic initiators have not been used


10

commercially because of their long curing cycles and other poor cured

product properties. Crosslinkers are mainly used as surface coatings and

usually are cured at high temperatures to produce films having good physical

and chemical properties.

Table (2.2) Type 1 Curing Agents and Their Chemical Structures [31]


11

2.3

Curing Reactions

The curing reaction of epoxide is the process by which one or more

kinds of reactants, i.e., an epoxide and one or more curing agents with or

without the catalysts are transformed from low-molecular-weight to a highly

crosslinked structure. As mentioned earlier, the epoxy resin contains one or

more 1, 2-epoxide groups. Because an oxygen atom has a high

electronegativity, the chemical bonds between oxygen and carbon atoms in

the 1, 2-epoxide groups are the polar bonds, in which the oxygen atom

becomes partially negative, whereas the carbon atoms become partially

positive. Because the epoxide ring is strained (unstable), and polar groups

(nucleophiles) can attack it, the epoxy group is easily broken. It can react

with both nucleophilic curing reagents and electrophilic curing agents. The

curing reaction is the repeated process of the ringopening reaction of

epoxides, adding molecules and producing a higher molecular weight and

finally resulting in a three-dimensional structure. The chemical structures of

the epoxides have an important effect on the curing reactions. Tanaka and

Bauer [28] provide more details about the relative reactivity of the various

epoxides with different curing agents and the orientation of the ring opening

of epoxides. It was concluded that the electron-withdrawing groups in the

epoxides would increase the rate of reaction when cured with nucleophilic

reagents, but would decrease the rate of reaction of epoxides when cured

with electrophilic curing agents.

As discussed earlier, many curing agents may be used to react with

epoxides; but for different curing agents, there exist different mechanisms of

the curing reaction. Even for same epoxy resin systems, the cure mechanism


12

may be different for the isothermal and dynamic cure processes. Some of the

mechanisms are presented here for reference.

Many polyfunctional curing agents with active hydrogen atoms such

as polyamines, polyamides and polyphenols perform nucleophilic addition

reaction with epoxides. Tanaka and Bauer [28] gave the following general

cure reaction:

Where X represents NR2, O or S nucleophilic group or element and n is the

degree of polymerization, having a value of 0, 1, 2 …

Tanaka and Bauer [28] discussed in detail the curing mechanisms of

epoxides with several types of curing agents. For epoxy-1-propyl phenyl

ether/polyamines system they concluded that a primary amine would react

with epoxy-1-propyl phenyl ether to produce a secondary amine, and the

secondary amine would react with the same epoxide to produce a tertiary

amine. No evidence of tertiary-catalyzed etherification between the epoxide

and the derived hydroxyl was found.


13

On the other hand, Xu and Schlup [32] studied the curing

mechanism of epoxy resin/amine system by near-infrared spectroscopy and

derived the following equation of curing reaction:

They pointed out that the etherification during the epoxy resin cure

was significant only at certain reaction conditions such as at a high curing

temperature and for only some epoxy resin/amine systems. For the

tetraglycidyl 4, 4’-diaminodiphenylmethane and methylaniline system, they

found that the etherification reaction during cure is more significant. The

main curing reactions, similar to the above equations, were also used by

other researchers in the different epoxy resin/amine systems [33, 34].

Unlike mechanisms of polyaddition, the stepwise polymerization of

epoxy resin is initiated by anionic and cationic reagents. Anionic

polymerization of epoxides may be induced by initiators such as metal

hydroxides and secondary and tertiary amines. Cationic polymerization may

be induced by using Lewis acids as initiators. Many inorganic halids could

be used as cationic initiators. Tanaka and Bauer also discussed the

mechanisms of anionic and cationic polymerization of epoxy resin. They

pointed out that the products from anionic and cationic polymerization with

monoepoxides have relatively low molecular weights.

One important factor of polymerization is stoichiometry. It has effects

on the viscosity and the gel time of the epoxy resin system [35].


14

2.4 The selection of curing agents is a critical parameter. There are

numerous types of chemical reagents that can react with epoxy resins.

Besides affecting viscosity and reactivity of the formulation, curing agents

determine both the types of chemical bonds formed and the functionality of

the cross-link junctions that are formed. Thermal stability is affected by the

structure of the hardener [26, 36].

Selection of Curing Agents

2.5 The Stoichiometry The stoichiometric relationship between curing agents and resins has a

great effect on the physical and the mechanical properties of the epoxy resin

[37]. The different types of curing agents required addressing stoichiometric

balance between the reacting species. To evaluate the properties of the

epoxy resin the proportions of curing agents and resins must be calculated

and optimized.

Theoretically, a crosslinked thermoset polymer structure is obtained

when equimolar quantities of resin and hardener are combined. However, in

practical applications, epoxy formulations are optimized for performance

rather than to complete stoichiometric cures. This is especially true when

curing high molecular weight epoxy resins through the hydroxyl groups.

In primary and secondary amines cured systems, normally the hardener is

used in near stoichiometric ratio. Because the tertiary amine formed in the

reaction has a catalytic effect on reactions of epoxy with co-produced

secondary alcohols, slightly less than the theoretical amounts should be used

[26].


15

Often a commercial curing agent’s chemical structure is kept proprietary

or the amount of reactive functional group is ambiguous. In such cases, the

vendor provides an amine or active hydrogen equivalent from which an

appropriate mix ratio can be calculated. It is also important when performing

stoichiometric balances to be aware of reactive groups that may be

bifunctional (e.g., anhydride, olefin). The stoichiometric ratio (an example

of a stoichiometric calculation is shown in the appendix) of hardener/resin

doesn’t always produce a cured resin system having optimized properties,

where a specific application required properties have been developed

through the use of a defined hardener/resin ratio, is different from other

application which required different properties i.e. different hardener/resin

ratio.

2.6

The mechanical properties are often the most important properties

related for technology. This is because virtually all service conditions

involve some degree of mechanical loadings [38].

The Mechanical Properties Of Epoxy Resin

The selection of an epoxy Resin for a specific application is usually

based on the mechanical tests that applied on that particular resin such as

tensile, impact, compressive, bending, flexural and hardness tests [39]. From

a very general point of view, mechanical behavior is the response of a solid

to mechanical stress. The atoms of solid under load are displaced from

their equilibrium position, which induces restoring forces that are

opposed to the deformation and tend to restore the initial shape as the

load is removed. In the elastic region, usually for small deformations,

the behavior remains wholly reversible. Increasing load leads to


16

formation and propagation of defects that allows mechanical stress to

be relaxed [40].

2.6.1 The resistance to impact is one of the key properties of materials. A

tough polymer is one which has a high energy to break in an impact test

[41]. Impact strength depends on a range of variables including temperature,

geometry of article, fabrication conditions and environment [42].

The Impact Test

The simplest method which has been developed in both the Izod and

Charpy tests is to break the specimen with a pendulum and measure the

energy absorbed [43]. The Charpy test is essentially a high-speed three-point

bending test. In a brittle material, the force exerted by pendulum increases

linearly with deflection, and the crack begins to propagate. Once the crack

has initiated, no further energy is required from the pendulum, crack

propagation is maintained by energy already stored in the specimen,

therefore it is clear that the impact strength is basically a measure of the

energy absorbed in bending the Charpy bar to the point of crack initiation, in

addition, a small proportion of energy abstracted from the pendulum is

converted into kinetic energy of the two halves of the specimen [44, 43].

The energy required to break the specimen is determined from the

pendulum weight, the height from which it dropped and the height which it

reached after impact. The impact strength is defined as the energy to break,

with units such as (k J .m-2) or (ft .Ib/in2

Impact Strength = Energy of fractureCross section area

(2.1)

); from the definition of the impact

strength the following relation was proposed:


17

2.6.2

The ability of a material to withstand forces tending to pull it apart is

called tensile strength, also may be defined as the maximum tensile stress

sustained by the material being tested to its breaking point [45].

The Tensile Test

In the tensile test, the specimen was subjected to a continually

increasing uniaxial tensile force while simultaneous observations were made

on the elongation of the specimen [46]. The tensile strength is the maximum

tensile stress of the material and can be found by applying equation (2.2).

Stress = AF (2.2)

Where: F = applied Force (N)

A= cross section Area (mm2

It is also necessary to note the percentage elongation of the specimen.

This shows the relative ductility of the material. The percentage elongation,

%EL is the percentage of plastic strain at fracture point. The percentage

elongation can be found by applying the formula as shown in equation (2.3).

Where lf and l

)

o

are the final and original length respectively.

%𝐸𝐸𝐸𝐸 = 𝑙𝑙𝑙𝑙−𝑙𝑙0𝑙𝑙𝑙𝑙

× 100 (2.3)

Tensile tests are most widely used for defining both the quality of

production lots of polymeric materials, their design potential and their

engineering behavior. Tensile stress-strain measurements are generally made

under tension by stretching the specimen a uniform rate and simultaneously


18

measuring the force on the specimen [47]. The test is continued until the

specimen breaks. Often the change in length is determined by measurement

of the separation of the jaws or clamps holding the specimen. In tensile tests,

dumb-bell shaped specimens have been widely used. In stretching such

specimens at a uniform speed a uniform tensile stress exists within the gauge

section and the distance between the clamps measures the elongation [48].

2.6.3

Hardness is a mechanical property which represents the resistance of

the material to penetration and scratching, it is measured by the distance

of indentation and recovery that occurs when the indenter is pressed into

the surface under constant load [48, 49].

The Hardness Test

Hardness can be expressed in several ways. There are four methods

used to express the resistance of materials to indentation based on different

concepts of measurements, shore hardness, diamond pyramid hardness,

Brinell hardness and Rockwell hardness. Epoxy resins are tested for

resistance to penetration by the shore hardness method (shore Durometery).

The Durometer hardness tester consists of a pressure foot, an indentor, and

an indicating device. Two types of Durometers are most commonly used-

type A and type D. the basic difference between the two types is the shape

and dimension of the indentor. Type A- Durometer is used with relatively

soft material while type D- Durometer is used with slightly harder material

[50].



19

The flexural test measures the force required to bend a beam under

three point loading conditions. The data is often used to select materials for

parts that will support loads without flexing. Flexural modulus is used as an

indication of a material’s stiffness when flexed [17]. Since the physical

properties of many materials can vary depending on ambient temperature, it

is sometimes appropriate to test materials at temperatures that simulate the

intended end use environment.

Most commonly the specimen lies on a support span and the load is

applied to the center by the loading nose producing three points bending at a

specified rate. The parameters for this test are the support span, the speed of

the loading, and the maximum deflection for the test. These parameters are

based on the test specimen thickness and are defined differently by ASTM

and ISO. For ASTM D790 [51], the test is stopped when the specimen

reaches 5% deflection or the specimen breaks before 5%. For ISO 178, the

test is stopped when the specimen breaks. Of the specimen does not break,

the test is continued as far as possible and the stress at 3.5% (conventional

deflection) is reported.

Flexural strength is calculated from the maximum bending moment

by assuming a straight line stress-strain relation to failure. For a beam of

rectangular cross section, it is given by the following expression:

F.S = 223bdPL ………….. (2.4)

Where:

F.S = flexural strength (MPa).

P = maximum load (N).

L = distance between two fixed points (mm).

b = width of the specimen (mm).


20

d = thickness of the specimen (mm).

The most two popular flexural tests are the three point bending and

the four point bending test.

2.6.5

The Compression Test

Compression strength is the ability to resist force that tends to crush.

The crushing load at the failure of specimen is divided by the original

sectional area of the specimen, and the compressive stress is the

compressive load per unit area of original cross section carried by the

specimen during the compression test [52].

The compression strength test is an opposite of the tensile test and

it mainly deals with the brittle materials in which the tensile test doesn’t fit

it, where it is practically used in applications subjected to compressive

tensile stress.

The failure happens as a result of buckling mode and shear mode

which propagate through the internal surfaces of the material so the failure

will happen in sequence as a result to increase the shear stress [2]. The

reason of this failure is the presence of some defects in the material where

the stresses are concentrated, in which it is impossible to make a free

defect material.

2.6.6 Three Point Bending Test


21

Modulus measures the resistance of a material to elastic deformation,

for linear elastic materials the stress ζ is related to the strain ε by Young's

modulus Ε (Hooke's law).

ζ = Ε ε ……….. (2.5)

Hooke's law: The amount of change in the shape of an elastic body is

directly proportional to the applied force provided the elastic limit that

will not be exceeded.

In three points bending load, modulus of elasticity is calculated by using

the following relation:

E = ( 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑑𝑑𝑑𝑑𝑙𝑙𝑙𝑙𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑙𝑙𝑑𝑑

) (gL3

48I) (2.6)

I = DB3

12 (2.7)

Where: I = Engineering bending momentum

D = Width of specimen (mm)

B = Thickness of specimen (mm)

g = Gravity (m/sec2

L = Specimen length (mm)

)

( 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑑𝑑𝑑𝑑𝑙𝑙𝑙𝑙𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑙𝑙𝑑𝑑

): is the slope of linear part of mass deflection curve obtained

from three points bending loads tests [16, 40].


22

The bending test is the most appropriate for the brittle material, cause

in the case of any minor defect or a surface scratch; the stresses will be

concentrated in it and it will fail easily. This test is the best to get the (load –

deformation) curves and to define the elastic and ductile properties.

2.7

Differential Scanning Calorimetry (DSC) Analysis

DSC is a quantitative differential thermal analysis technique [53].

During measurement with DSC, the temperature difference between the

sample and reference is measured as a function of temperature or time. The

temperature difference is considered to be proportional to the heat flux

change.

In the study of curing kinetics of epoxy resins, it is assumed that the

degree of reaction (cure) can be related to the heat of reaction. Both

isothermal and dynamic methods can be adopted to determine the kinetic

parameters with DSC. For the isothermal method, the sample is quickly

heated to the preset temperature. The system is kept at that temperature and

the instrument records the change of heat flux as a function of time. For the

dynamic method, the heat flux is recorded when the sample is scanned at a

constant heating rate from low temperature to high temperature. The area

under the heat flux curve and above baseline is calculated as the heat of

reaction.

2.7.1 Phenomenological modeling (also called empirical modeling)

approach is commonly used to obtain analytical expressions for cure

kinetics, and it has been proved as an effective approach with simple

Cure Kinetic Models


23

procedure and satisfactory accuracy. In phenomenological modeling the

chemical details of the reacting system are ignored and an approximated

relationship is applied according to the reaction type, then the parameters in

the mathematical model are fitted with experimental data [54].

The cure process of a thermosetting resin results in conversion of low

molecular weight monomers or pre-polymers into a highly cross-linked,

three-dimensional macromolecular structure. The degree of cure, α , is

generally used to indicate the extent of the resin chemical reaction. α is

proportional to the amount of heat given off by bond formation, and is

usually defined as:

α = ∆𝐻𝐻𝑑𝑑

∆Htotal (2.8)

Where ΔHt is the accumulative heat of reaction up to a given time t during

the curing process, and ΔHtotal

The curing rate is assumed to be proportional to the rate of heat

generation and is calculated by the following expression:

is the total heat released during a complete

reaction. For an uncured resin, α = 0, whereas for a completely cured resin,

α=1.

𝑑𝑑∝𝑑𝑑𝑑𝑑

= 1∆𝐻𝐻𝑑𝑑𝑙𝑙𝑑𝑑𝑚𝑚𝑙𝑙

(𝑑𝑑𝐻𝐻𝑑𝑑𝑑𝑑

) (2.9)

A number of phenomenological models for cure kinetics have been

developed to characterize the curing process for different resin systems. The

simplest one is the nth order rate equation [55]:


24


= 𝑘𝑘(1 − 𝛼𝛼)𝑑𝑑 (2.10)

k = A exp (−ΔEa𝑅𝑅𝑅𝑅

) (2.11)

where n is the reaction order, and k is the reaction rate constant, which is

an Arrhenius function of temperature, A is the pre-exponential constant or

Arrhenius frequency factor, ΔEa

For autocatalytic thermosetting resin systems [56], the following

equation has been applied:

is the activation energy, R is the universal

gas constant, and T is the absolute temperature. The nth-order kinetics model

does not account for any autocatalytic effects and so it predicts maximum

reaction rate at the beginning of the curing.


= 𝑘𝑘 ∝𝑚𝑚 (1−∝)𝑑𝑑 (2.12)

where m and n are reaction orders to be determined by experimental data,

and k has the same definition as in equation (2.11). Rather than at the

beginning of the reaction process as in equation (2.10), the maximum

reaction rate takes place in the intermediate conversion stage for equation

(2.12), which results in a bell-shape reaction rate versus time curve for an

autocatalytic reaction process.

Both the nth order and autocatalytic model use a single rate constant

to model the whole curing process. In practice, multiple events may occur

simultaneously and lead to very complicated reaction; consequently, the use

of multiple rate constants can provide more accurate modeling results.


25

Kamal's model [57] involves two rate constants and has been applied

successfully to model a variety of resins: 𝑑𝑑∝𝑑𝑑𝑑𝑑

= (k1 + k2R αm ) (1- α )n

(2.13)

ki = Ai exp (-ΔEi/ RT) (i = 1,2)

(2.14)

where ΔEi are activation energies, R is the universal gas constant, m and n

are material constants to be determined by experimental data, k1 and k2

The various mathematical models described above have been widely

used. However, their validity is limited to reactions for which the kinetics of

bond formation is the only rate-controlling step in the curing process. While

this is usually true in the early stage, other factors may come into play as

reactants are consumed and crosslinking network is formed. As the

consequence, species diffusion can become very slow and govern the curing

reaction rate near and above the glass transition. To account for the different

cure rate controlling mechanisms and achieve greater accuracy at high

conversions, some modifications on the available cure kinetics models have

been introduced.

have

the same definition as in equation (2.11).

Chern and Poehlein [58] modified the species equation (2.13) by

adding a term to explicitly account for the shift from kinetics to diffusion

control in an autocatalytic isothermal thermosetting resin system; the

modified expression has the following form:


= 11+exp (𝐶𝐶(α −α𝑑𝑑 ) )

(k1 + k2 αm ) (1- α )n

(2.15)


26

where C and αc are the two empirical constants which are temperature

dependant. αc

One modified form of Kamal's model has been proposed as [59]:

is called the critical degree of cure.


= (k1 + k2R αm ) (αmax - α )n

(2.16)

where αmax is the maximum degree of cure at a given temperature due to the

vitrification phenomenon observed in isothermal cure. The constants m and

n are reaction orders to be experimentally determined, while k1 and k2

The modified Kamal model incorporates the term α

are

the same as in equation (2.14).

max

Kenny et. al. [60]

, so that the

fractional conversion will not exceed the degree of cure associated with

vitrification at the specific temperature.


= k α

modified the model used by Pusatcioglu [59]

accounting for diffusion effects by modifying equation (2.12):

m (αmax - α )n

(2.17)

where αmax denotes the final degree of reaction in isothermal DSC scans.

The final degree of reaction increases with the cure temperature, the

structural changes by the polymerization reaction are associated with

increase in glass transition temperature. When the increasing Tg approaches

the isothermal cure temperature the molecular mobility is strongly reduced,

and the reaction becomes diffusion controlled and eventually stops, linear

dependence of αmax on the isothermal cure temperature has been observed.


27

Michaud [61] found that the use of αmax

Liang [62] used the Kamal’s model as in equation (2.12) to develop

kinetic models for the soy-based epoxy resin system of different

formulations. The models developed can be readily applied to composite

processing.

greatly improved the fit of

the autocataytic model.

2.8 Rheology can be defined as ‘the science of the deformation

and flow of matter’, which means that it is concerned with

relationship between viscosity, stress, strain, rate of strain, and

time [64].

Rheological Analysis

In practice, rheology is concerned with materials whose

flow properties are more complicated than those of a simple fluid

(liquid or gas) or an ideal elastic solid, although it may be

remarked that a material whose behavior under same restricted

range of circumstances is simple, may exhibit much more complex

behavior under other circumstances. Many materials of industrial

interest behave in a way such as to bring their study within the

scope of rheology, and included in these epoxy resins [65].

Epoxy resins exhibit both viscous and elastic properties. During the

curing process, their viscosity increases quickly in the gel region. The

viscosity can be related to degree of cure. Rheological equipment can be

used to measure effectively the epoxy resin properties, such as Brookfield

viscometer, which provides a lot of information on the Epoxy resins in the


28

way that helps in understanding the rheological behavior of this material

[66].

2.8.1

2.8.1.1

Rheology Models

The viscosity of a curing resin system is determined by two factors:

the degree of cure and the temperature. As the cure proceeds, the molecular

size increases and so does the cross-linking density, which decrease the

mobility and hence increase the viscosity of the resin system. On the other

hand, the temperature exerts a direct effect on the dynamics of molecules

and so the viscosity.

Viscosity Model

Much work has been done to develop appropriate mathematical

models for the descriptions of the viscosity advancements for various

thermosetting resins during cure.

The variation of viscosity is the result of the combination of physical and

chemical processes and can be empirically expressed as [67]:

η = ψ (T) ζ (α) (2.18)

where ψ (T) is a function of curing temperature only; ζ (α) is a function of

degree of cure. Terms ψ (T) and ζ (α) can be empirically expressed with the

simple form respectively:

ψ (T) = ηo

where η

and ζ(α) = 11−𝛼𝛼

(2.19)

o is the initial viscosity which is a constant at isothermal cure

conditions; α is the degree of cure.


29

Substituting equation (2.19) into equation (2.18) to get the relationship of

viscosity vs. the degree of cure,

η = ηo

1

1−α (2.20)

The initial viscosity ηo

η

depends on cure temperature and can be further

expressed in Arrhenius equation,

o= Aη

𝑑𝑑−𝐸𝐸𝐸𝐸𝑅𝑅𝑅𝑅 (2.21)

Where Aη and Eη

Depending on the cure kinetics, the relationship of α versus time t may

have different forms. For the first order reaction, it can be expressed as:

are the initial viscosity at T = ∞ and the viscous flow

activation energy, respectively. The degree of cure α in equation (2.20) is a

function of cure time.

𝐝𝐝𝛂𝛂𝐝𝐝𝐝𝐝

= k (1- α) (2.22)

For the first order reaction with the isothermal cure process,

temperature T and rate constant k are constant:

η= ηo

ln η = ln A

𝑑𝑑𝑘𝑘𝑑𝑑 (2.23)

η + 𝐸𝐸 η 𝑅𝑅𝑅𝑅

+ t A

k 𝑑𝑑−𝐸𝐸 𝑘𝑘 𝑅𝑅𝑅𝑅 (2.24)

where Ak and Ek

Equation (2.24) is the empirical four-parameter model of viscosity

introduced by Roller [68].

are the apparent rate constant at T = ∞ and the kinetic

activation energy, respectively.


30

For the nth order reaction, it can be expressed as:

𝐝𝐝α𝐝𝐝𝐝𝐝

= k (1- α) n

So for the isothermal nth order (n≠ 1) reaction:

(2.25)

ln η = ln Aη + 𝐸𝐸 η 𝑅𝑅𝑅𝑅

+ 1𝑑𝑑−1

ln (1+ (n-1) t Ak 𝑑𝑑−𝐸𝐸 η 𝑅𝑅𝑅𝑅

R

Which is the empirical five-parameter model of viscosity for the nth (n≠1)

order reaction introduced by Dusi [69].

) (2.26)

The first and nth order viscosity models express viscosity as an

exponential function of the cure time. The first and nth viscosity models

have been frequently used in the rheological analysis of the cure process

(Dusi et al. [69]; Theriault et al. [70]; Wang et al. [71]). These models do not

incorporate the effect of gelation on the viscosity and the predication

accuracy is not good.

The modified Williams-Landel-Ferry (WFL) models for viscosity

(Tajima and Crozier [72]; Mijovic and Lee, [73]) describe viscosity as the

function of both cure temperature and glass transition temperature:

(2.27)

where η is the viscosity, MW

rT

is the weight average molecular weight of the

epoxy resin, g is the ratio for the radii of gyration of a branched chain to the

linear chain of the same molecular weight, is a reference temperature, Tg

))}(TTc/())(TT(cexp{)}TTc/()TT(cexp{

)M

)(Mg(

)T(),T(

grgr

gorgor.

w

w

α−+α−

−+−α=

ηαη

21

2143

00


31

is the glass transition temperature of the reacting system, and c1 and c2

Bidstrup and Sheppard [75]

are

constants. These models have been extensively used and they were reported

to achieve good accuracy (Karkanas and Partrige [74]). When applying the

WFL models, one needs to know the relationship between the glass

transition temperature and the cure time, which can be determined by

thermal analysis. showed that the temperature-dependence

of the ionic conductivity of a series of cured epoxy resin by varying

molecular weight can be modeled by the WLF equation if the constant c2

and the conductivity σg at the glass transition are taken as a function of Tg.

They assumed that c2 and log (σg) vary linearly with Tg

)(43

)(165log

gTTgTccgTTc

gTcc−++

−++=σ

; their model for

conductivity then gives a five-parameter equation, which can be written as

(2.28)

Where:

)log(65 σ=+ gTcc (2.29)

243 cTcc g =+ (2.30)

Sanford and McCllough [76] proposed a chemorheological model for

predicting the viscosity variation of epoxy resin during isothermal cure, using

the free volume concept. The underlying concept for this model is that the

ability of molecules or chain segments to rearrange themselves is dependent

on the existence of enough unoccupied space to accommodate motion. Where

there is relatively a large amount of free volume the chain may move

unhindered, however, as the free volume decreases, the chain becomes


32

crowded by their neighbors. They found the following empirical expression

for EPON828/PACM-20 resin system:

)]11(62.0exp[101.2 12

fwM w −−×= −η (2.31)

where wM is the number of average molecular weight, f fraction of free

volume which may be expressed as a linear function of the difference

between the resin temperature and the glass transition temperature as:

)( gfg TTff −+= α (2.32)

here gf is the fractional free volume at gT and fα is the thermal expansion

coefficient of free volume.

A percolation model for viscosity [77] expresses the variation of

viscosity with degree of cure by a power law. By introducing the degree of

critical reaction into the model, the gel effect on the cure process was taken

into account. It was reported that the percolation model fit the experimental

data quite well [77]. For the application of the percolation model, a kinetics

model is necessary in order to determine the relationship between the degree

of cure and time. The characteristics of other viscosity models for cure

applications were discussed by Halley and Mackay [78].

Sun et al. [79] predicted a model to describe the viscosity of the epoxy

prepreg, the model proposed based on a Boltzmann function to produce a

sigmoidal curve, which the viscosity profile for the isothermal cure process

seems to follow, especially in the gel region.

η = ηo−η∞1+𝑑𝑑𝑘𝑘(𝑑𝑑−tc) + η∞ (2.33)


33

where ηo is the initial viscosity, η∞ is the final viscosity; k is the rate constant

of cure reaction and tc is the critical time which follows Arrhenius behavior,

i.e.

tc = At 𝑑𝑑E t𝑅𝑅𝑅𝑅 (2.34)

where At is the pre-exponential factor and Et is the activation energy.

Equation (2.33) is just a fitting function which is based on the mathematical

knowledge instead of the rheological theory. It has a similar form as a

Boltzmann function, but each parameter in equation (2.33) has its own

physical meaning. The parameters in equation (2.33) are determined by the

multiple non-linear regression method.

2.8.1.2 Gel time, which was detected by the rheological measurement, varies

with the isothermal cure rate of reaction. Gonis et al. [80] expressed the

curing process as:

Gel Time Model

𝐝𝐝α𝐝𝐝𝐝𝐝

= k(T) g(α) (2.35)

where k(T) is the rate constant (which depends on the temperature T ), and

g(α ) is a function of α only. It may have different forms, depending on the

cure mechanism. The rate constant k(T) has the same definition as in

equation (2.11).

By integrating equation (2.35) from zero time to gel time tgel, the

relationship between tgel and cure rate is obtained:


34

tgel = 1K(T)

.∫ 1αgel

dααgel 0 (2.36)

where αgel is the degree of cure at gel time.

Substituting equation (2.11) into equation (2.36) and taking logarithm

on both sides to get the relationship between the gel time and isothermal

cure temperature:

ln(tgel ) = ln[ 1Ao

⟨∫ 1αgel

dααgel 0 ⟩] + Ea

R . 1𝑅𝑅 (2.37)

According to Flory’s expression [36], the degree of cure αgel at gel

time depends on the functionalities of the epoxy systems only. So it can be

considered a constant for a given epoxy systems regardless the cure

temperature.

By considering the first term on the left side of equation (2.37) as a

constant C, a linear relationship of ln(tgel) versus 1/T is obtained and

equation (2.37) can be rewritten as:

ln�tgel � = C + EaR

. 1T (2.38)

From equation (2.38), the apparent activation energy can be calculated from

the slope of the curve of ln(tgel) versus 1/T.

2.9 Many researches have been done on the epoxy resins, due to its

versatile applications. Some of these researches investigated the mechanical

properties of the epoxy resins, such as tensile strength, impact resistance,

Literature Review Of Experimental Work On Epoxy Resin


35

compression strength and others [81-90]. Others studied the kinetics of the

epoxy resins; using DSC technique in both isothermal and dynamic modes

[91-98]. Others made extensive effort to investigate the rheological

properties of the Epoxy resins, using rheometers, viscometers or others

which provided a way to analyze the material behavior better [99-103].

2.9.1

Selby and Miller [81] investigated the variation of fracture and

mechanical properties of epoxy resin Epikote 828 (DGEBA), cured with

diaminodiphenyl-methane (DDM) by variation of the resin/amine ratio.

Observations of the crack tip have shown that fracture toughness variations

can be attributed to the different blunting characteristics of the various

resin/amine compositions.

Literature Review On The Mechanical Properties Of The

Epoxy Resin

d’Almeida and Monteiro [82] investigated the role of the resin

matrix/hardener Ratio on the Mechanical Properties of low volume fraction

epoxy composites. The mechanical properties of the matrix where modified

by varying the amount of hardener. Experimental results showed that it is

possible to considerably vary the performance of low volume fraction

composites by the proper processing of the matrix. In particular, it was

observed that a significant change on the deformability of the composites

can be obtained.

Baraiya et. al. [83] investigated the mechanical properties of Bis

ester namely 1, 1'-(1-methylethylidene)bis[4-1-(1-imino-4-ethylbenzoate)-2-

pro panolyloxy]benzene which was synthesized by the reaction of epoxy

resin, diglycidyl ether bisphenol-A-(DGEBA) and 4-amino ethyl benzoate


36

(4-AEB) using triethyl amine as catalyst. The synthesized bisester was

reacted with two different aliphatic diamines viz., 1. 4-butylene diamine

(BDA) and 1. 6-hexamethylene diamine (HMDA) to obtain respective

polyamide resins (PAs) abbreviated as DGEBA-4-AEB: BDA and DGEBA-

4AEB:HMDA respectively. The PAs synthesized were used as a curing

agent for the difunctional epoxy resin, (DGEBA) and trifunctional epoxy

resin, (TGPAP) in three different ratios. Using triethylamine as a catalyst

and PAs as a curing agent. DGEBA and TGPAP were polymerized on mild

steel panels at 120°C for 1 hr. The coated panels thus obtained were tested

for scratch hardness, flexibility, impact strength and chemical resistancy. It

appears from the results that epoxy resins, DGEBA based polyamides can

successfully be used as a curing agent for the coating application.

Landingham et. al. [84] studied the changes in microstructure and

mechanical properties as a function of epoxy-amine stoichiometry. The

epoxy-amine system studied [DGEBA/Cycloaliphatic diamine bis (para-

amino cyclohexyl) methane] exhibits a two-phase structure consisting of a

hard microgel phase and a dispersed phase of soft, unreacted and/or partially

reacted material. The fracture toughness at room temperature increases with

increasing amine content. Changes in modulus values at 30°C with

stoichiometry are explained by considering the effective aspect ratio of the

polymer structure in the determination of sample rigidity.

d’Almeida and Cella [85] studied the epoxy systems which were

prepared by mixing proper quantities of the difunctional liquid epoxy

monomer diglycidyl ether of bisphenol –A, DGEBA, with respectively, an

aliphatic amine (triethylene tetramine, TETA), two aromatic polyamines

(diamino diaphenyl sulfone, DDS, and diamion diphenyl methane, DDM)


37

and a mixture of the tetrahydrophtalic anhydride, THPA, and a brominated

flame retardant (BFR). These four epoxy systems were fabricated using the

epoxy to hardener stoichiometric ratios. Where the effect of the

macromolecular network developed by reacting the same epoxy monomer

with different hardeners upon the efficiency of the thermal blunting

mechanism, the impact behavior and the topographic of the four epoxy

systems were investigated.

d’Almeida et. al. [86] investigated the room temperature ageing of

off-stoichiometric DGEBA/TETA epoxy formulations. The results obtained

show that the epoxy rich mixtures have their inherent brittleness increased

by the ageing treatment. The initial reaction steps dominated by the amine

addition reactions control the macromolecular structure and the mechanical

performance of the stoichiometric and near stoichiometric formulation with

excess of epoxy monomer. The amine rich mixtures have the more stable

structures.

Monteiro et. al. [87] investigated through mechanical tests and

scanning electron microscopy observation epoxy matrix composites, with

different phr (parts of hardener per hundred of resin), reinforced with 10, 20

and 30 wt.% diamond particles. The results have shown that the phr 17

epoxy; which has the highest tensile strength, is significantly stronger than

the stoichiometric phr 13. Moreover, the strength of the composite is

decreased with the amount of incorporated diamond.

Liu et. al. [88] studied the effects of curing agents, curing

temperature, epoxy/ESO ratio, and fiber loading on mechanical properties of

fiber-reinforced epoxidized soybean oil (ESO)/epoxy resin composites. The

curing agents that have been used are Jeffamine D-230


38

(polyoxypropylenediamine), Jeffamine EDR-148 (triethyleneglycoldiamine),

Jeffamine T-403 ( polyoxypropylenetriamine ), triethylenetetramine (TETA)

and diethylenetriamine(DETA). The flexural strength and the flexural

modulus for the Jeffamine curing agents were in the following order EDR-

148 > T-403 > D230. By comparison of triethylenetetramine (TETA) and

diethylenetriamine (DETA) to Jeffamine curing agents, TETA and

DETAcuring agents provide composites with better mechanical properties.

Sulaiman et. al. [89] investigated the effects of hardener on

mechanical properties of carbon reinforced phenolic resin composites.

Where carbon fibres are hot pressed with phenolic resin with various

percentages of carbon fiber and hardener contents that range from 5-15%.

Composites with 15% hardener content show an increase in flexural

strength, tensile strength and hardness.

Pandini et al. [90] studied the effects of the resin/hardener ratio on

the yield, post-yield and fracture properties of epoxy/layered-silicate

nanocomposites, using resin/hardener equivalent ratios (q) ranging between

0.75 (excess of hardener) and 1.1 (excess of resin). These tests revealed in

both neat and filled resins the highest modulus value, and thus the highest

cross-linking degree, for q = 0.93. In the fracture tests, the neat resins

exhibited either a ductile or a brittle behaviour in dependence on the value of

q, whereas all the nanocomposites broke in a brittle manner.

2.9.2

Yilgör et. al. [91] studied the kinetics of the curing reaction of an

epoxy resin based on bisphenol-A diglycidylether with a cycloaliphatic

Literature Review On the Kinetics Of The Epoxy Resin

Using (DSC)


39

diamine, bis(4-aminocyclohexyl)methane, it was done by differential

scanning calorimetry. The measurements were performed both under

isothermal and dynamic conditions. The cycloaliphatic amine used for this

study was demonstrated to be more reactive than analogous aromatic

systems and yet provided rigid networks with a desirably high Tg.

Nuiiez et. al. [92] investigated the variation of the epoxy/curing

agent ratio for a system containing a diglycidyl ether of a bisphenol A

derivative epoxy resin and the isophorone diamine (3-aminomethyl-3, 5, 5-

trimethylcyclohexylamine). Determination of the optimum value of the

epoxy/curing agent ratio was studied by means of differential scanning

calorimetry (DSC). The method is based on the search for the maximum

enthalpy change. It was found that this maximum corresponds to a 100/34

value.

Kiao and Caruthers [93] investigated the Epoxy-amine systems

which was prepared from diglycidylether of bisphenol-A (DGEBA) and 4,

4’-methylenedianiline (MDA) at amine-epoxy (A/E) ratios from 0.5 to 2.

Differential scanning calorimeter was used to measure the total heat of

reaction, and the extent of reaction was determined from the area of the DSC

exotherm as compared to the anticipated extent of reaction from the initial

stoichiometry. There was an increase in the extent of reaction with

increasing A/E ratio, there was a significant decrease in the ultimate

conversion in the vicinity of A/E=1.

Lisardo et. al. [94] studied the influence of the resin/diamine ratio on

the properties of the system diglycidyl ether of bisphenol A (BADGE

n=0/m-xylylenediamine) (m-XDA) . Variation of this ratio resulted in


40

significant effects on the cure kinetics and final dynamic mechanical

properties of the product material.

Rosu et. al. [95] investigated the curing kinetics of diglycidyl ether of

bisphenol A (DGEBA) and diglycidyl ether of hydroquinone (DGEHQ)

epoxy resins in presence of diglycidyl aniline as a reactive diluent and

triethylenetetramine (TETA) as a curing agent by using a non-isothermal

differential scanning calorimetry (DSC) technique at different heating rates.

The values of the activation energy for the (DGEBA/TETA) epoxy resin

system is less than (DGEHQ/TETA) epoxy resin system, the presence of the

reactive diluents leads to decrease of the activation energy for both the

studied epoxy resin systems.

Macan et. al. [96] studied the cure kinetics of epoxy resin based on a

diglycidyl ether of bisphenol A (DGEBA), with poly(oxypropylene) diamine

(Jeffamine D230) as a curing agent by means of differential scanning

calorimetry (DSC). Isothermal and dynamic DSC characterizations of

stoichiometric and sub-stoichiometric mixtures were performed. The

kinetics of cure was described successfully by empirical models in wide

temperature range. System with sub-stoichiometric content of amine showed

evidence of two separate reactions, second of which was presumed to be

etherification reaction. Catalytic influence of hydroxyl groups formed by

epoxy-amine addition was determined.

Costa et. al. [97] investigated the influence of aromatic amine

hardeners the diphenyl diaminosulfone (DDS) and the 4, 4’diamine

diphenylmetane (DDM) on the cure kinetics of epoxy resin diglycidyl ether

of bisphenol-A (DGEBA) used in advanced composites. The investigation

was carried out by using the DSC technique. It was found that the


41

polymerization temperature for the DGEBA/DDM mixture is lower than for

the DGEBA/DDS system. The DDM curing agent has a lower melting point

than DDS, and consequently, less energy is required to melt and start the

polymerization reaction. The DGEBA/DDM formulation has a higher

reaction rate than the DGEBA/DDS formulation.

Hasmukh et. al. [98] investigated epoxy-poly (keto-sulfide) resin

glass fiber-reinforced composites (GRC). Various epoxy/hardener poly(keto-

sulfide)s ( PKS) mixing ratios were used, and the curing of epoxy-PKS has

been monitored using differential scanning calorimetry (DSC) in dynamic

mode. Based on DSC parameters the GRC of epoxy-PKS were prepared and

characterized by thermal and mechanical methods. The variatio in

resin/hardener ratio led to variations in thermal and mechanical properties.

2.9.3

Velazquez et. al. [99] studied the changes in rheological properties

(gelation and vitrification) during non-isothermal curing of an epoxy resin

(DGEBA) with different aliphatic amines using different resin/hardener

ratio. A dynamic rheometer was used. It was found that the viscous modulus

(G”) represents two peaks. The first peak appears when the system reaches

the vitrification curve for the stoichiometric and amine rich systems, but the

epoxy rich systems don’t show peaks.

Literature Review On The Rheology Of The Epoxy Resin

Kim and Char [100] investigated the rheological behavior of

thermoset/thermoplastic blends of epoxy/polyethersulphone (PES) during

curing of the epoxy resin. During the isothermal curing of the mixture, a

fluctuation in viscosity just before the abrupt viscosity increase was

observed. This fluctuation is found to be due to the phase separation of PES

from the matrix epoxy resin during the curing. The experimentally observed


42

viscosity fluctuation is simulated with a simple two phase suspension model

in terms of the increase in domain size. The viscosity profiles obtained

experimentally at different isothermal curing temperatures are in good

agreement with the predictions from the simple model taking into account

the viscosity change due to the growth of PES domain and the network

formation of the epoxy matrix.

Grimsley et. al. [101] studied the cure kinetics and viscosity of two

resins, an amine-cured epoxy system, Applied Poleramic, Inc. VR-56-4, and

an anhydride-cured epoxy system, A.T.A.R.D. Laboratories SIZG- 5A, have

been characterized for application in the vacuum assisted resin transfer

molding (VARTM) of aerospace components. Simulations were carried out

using the process model, COMPRO, to examine heat transfer, curing

kinetics and viscosity for different panel thicknesses and cure cycles. Results

of these simulations indicate that the two resins have significantly different

curing behaviors and flow characteristics.

Ivancovic et. al. [102] investigated the chemorhelogy of a low-

viscosity laminating system, based on a bisphenol A epoxy resin, an

anhydride curing agent, and a heterocyclic amine accelerator. The steady

shear and dynamic viscosity are measured throughout the epoxy/ anhydride

cure. It was found that at the beginning of the cure, the viscosity slowly

increases with time. Then, at a certain point a very rapid increase of the

viscosity is observed. Gelation was assumed to occur when the rate of

viscosity increase reached a maximum. A chomorheological model that

describes the system viscosity as a function of temperature and conversion is

proposed.


43

Costa et. al. [103] investigated the rheological, structural properties and

cure kinetics of epoxy resin, prepared with diglycidyl ether of bisphenol-A

(DGEBA) and triethylenetetramine (TETA), for different ratios of hardener

(TETA) and epoxy (DGEBA), using a DSC and a rheometer. From the

experimental results, it was found that the higher the ratio, the higher the

onset temperature and the total heat of reaction and the lower the peak

temperature. The cure reaction follows an autocatalytic model. The dynamic

experiments showed that the complex viscosity and the elastic and loss

moduli increased with the curing times.

Chapter Three Experimental Work

44

CHAPTER THREE

EXPERIMENTAL WORK

3.1 The Materials

3.1.1 Epoxy Resin

Epikote 828 from Shell Co. was used as epoxy resin. Epikote 828 is an

unmodified liquid bisphenol A – epichlorohydrin epoxide resin of medium

viscosity. Combining reasonable ease of handling with high chemical resistance

and mechanical performance after cure, Epikote 828 is the standard liquid resin in

many applications. It’s used with room temperature and elevated temperature

curing laminating systems. Epikot 828 properties are shown in Table (3.1)

Table (3.1) Epikote 828 properties [104]

Property Test method Value Unit Epoxy group

content SMS 2062 5150-5490 m mol/kg

Epoxy equivalent weight

182-194 g

Viscosity ASTM D445 9-14 Pa.s Colour ASTM

D1544 3 max Gardner scale

Density at 25°C SMS 1374 1.16 Kg/l Flash

point(PMCC) ASTM D93 >150 °C


45

3.1.2 The Hardeners

The hardeners (curing agent) used in the experimental work was:

1. Araldite HY 951 (Triethelentetramine TETA) from Ciba Company, which is

a liquid of law viscosity of an aliphatic amine basis. Typical properties of

the hardener HY 951 are shown in Table (3.2).

2. 4, 4´-Diaminodiphenylmetane with Product No. 32950from Fluka AG

Company, which was tested for laboratory use only. It’s a solid state material of

an aromatic amine basis. Typical properties of this hardener are shown in Table

(3.3).

Table (3.2) Typical properties of the hardener HY 951 [105]

Property Value Unit Molecular weight 146.24

Viscosity (Hoeppler) at

25°C

450 mPa.s

Specific gravity at 25°C

0.973 g/cm3

Flash point DIN 51 758

129 °C

Vapor pressure at 20 °C

< 0.01 mmHg

Color Clear, pale yellow or

yellow liquid

Boiling point 284-287 °C


46

Table (3.3) Typical properties of the hardener 4,4´-Diaminodiphenymethane [106]

Property Value Unit

Molecular weight 198.27

Grade Purum

Flash point 230 °C

Melting point 88-92 °C

Color Brown

3.2 The Hardener/Resin Ratio

The epoxy resin and the hardener were mixed together in different

hardener/resin ratios. The ratio selected depended on the stoichiometry of the

epoxy resin system. The epoxy resin Epikote 828 (DGEBA) and the aromatic

amine hardener 4, 4´-Diaminodiphenylmethane (DDM) were prepared in four

hardener/resin ratios:

1. 24 phr (Under stoichiometry).

2. 27 phr (Stoichiometry).

3. 30 phr (Above stoichiometry).

4. 34 phr (Above stoichiometry).

These ratios were calculated based on the equivalent weight of the DGEBA

and DDM used to prepare samples in order to study the effect of changing the

hardener/resin ratio on the mechanical properties through applying the mechanical

tests on the DDM/DGEBA resin samples’ specimens. Three test samples from

each formulation were tested and the average values were reported.


47

The hardener 4, 4´-Diaminodiphenylmethane DDM is solid at the room

temperature so it must be melted in order to react with the DGEBA epoxy resin.

The formulations are prepared by mixing the DGEBA in the appropriate ratio with

DDM and then they were heated on a hot plate up to the DDM melting temperature

(90°C), for approximately 10 minutes. The mixture was poured into the mold and

was cured at 90°C for 1.5hr then post cured at 150°C for 1hr.

The DGEBA epoxy resin and the hardener HY 951 TETA were mixed in

four hardener/resin ratios:

1- 10 phr (Under stoichiometry).

2- 13 phr (Stoichiometry).

3- 15 phr (Above stoichiometry).

4- 20 phr (Above stoichiometry).

These ratios were based on the equivalent weight of the DGEBA epoxy

resin and the hardener TETA. Samples’ specimens were prepared in the above four

ratios and they were subjected to the mechanical properties tests. Three test

samples from each formulation were tested and the average values were reported.

The DGEBA epoxy resin and the hardener TETA were mixed together at

the room temperature; the mixing was slowly using a disposable stirrer; to avoid

making air bubbles. The mixing was carried out for about 20 minutes to ensure the

homogeneity of the mixer and the two cotenants were blended well together so that

the prepared sample have the same concentrations in all its part. Then the mixer

was poured into the mold and it was left for 24 hours at room temperature, then it

was post cured at 100°C for 1 hour.


48

The samples used to carry out the DSC tests and the viscosity tests were

prepared through mixing the DGEBA epoxy resin with the hardener TETA in three

different hardener/resin ratios:

1- 5 phr (Under stoichiometry).

2- 13 phr (Stoichiometry).

3- 20 phr(Above stoichiometry).

The prepared samples were mixed in a disposable container using a

disposable stirrer then they were poured into the chamber of the Brookfield

viscometer.

3.3 The Mold

The mold used to manufacture the composite material is rectangular with

the dimensions of 25×15 cm and 5 cm height as shown in Fig. (3.1).The mold is

made from carbon steel.

Fig. (3.1): The mold used for casting the composite

The mold was prepared for casting the epoxy resin, it was cleaned

thoroughly and a mold release wax ( Meguiar’s Mirror Glaze No.8 wax) which


49

contains carnauba wax was used as a release agent. It was applied for three times

on the mold surface to ensure all the porous of surface are covered well.

3.4 The Mechanical Tests

3.4.1 The Impact Test Charpy impact instrument is used in this test as shown in Fig. (3.2), often a

bar of material is supported as a beam and struck at the middle. The energy which

is absorbed by the blow can be determined by measuring the reduction in swing of

the pendulum compared with the swing with no sample, the specimens were cut

according to (ISO-179). The size of the tested specimens is shown in Fig. (3.3).


The tensile test is the test most commonly used to evaluate the mechanical

properties of materials.

The tensile properties were determined using Microcomputer controlled

electronic Universal testing machine. Model WDW-50 E made by Time Group

INC. as shown in Fig. (3.4). The cross head speed was 5mm/min and the applied

load was 1 KN. The size of the tested specimens is shown in Fig. (3.5).


50

Fig. (3.2) Charpy Impact instrument.

Fig. (3.3) Specimen dimensions used in the Impact tests.

55 mm 10

5


51

Fig.(3.4) Tensile test instrument

Fig. (3.5) Specimen dimensions used in the Tensile test


52


Shore D hardness was measured using Shore D Hardness tester TH210

made by Time Group INC. as shown in Fig. (3.6). Tests were carried out according

to ISO 868. The specimens were tested by pressing the indenter of the instrument

which is a needle of a sharp head into the specimen surface so that the result was

appeared on the digital screen attached with the instrument. The range of Shore D

measurement is (0-100), so the reading 100 means that no indentation happened,

while the reading 0 means that the indentation through the specimen surface is

2.54mm.


The flexural strength of the prepared specimens was measured by using

hydraulic piston type Leybold Harris No. 36110 was used as shown in Figure (3.7).

The specimens were cut according to ASTM-D790. Rectangular specimens with

dimensions of (100*10*5) mm, were used in this test as shown in Fig. (3.8). The

specimen was fixed from its two ends where the piston of the instrument was in the

middle, and the specimen was put on a moving base where the surface of the

specimen should be plain.


53

Fig. (3.6) Hardness Shore D tester


54

Fig.(3.7) Hydraulic piston type Leybold Harris No. 36110

Fig. (3.8) Specimen dimensions for the Flexural test

100 mm 10 mm

5 mm


55


Hydraulic piston type Leybold Harris No. 36110 was used as shown in Fig.

(3.7) to measure the compressive strength of the specimens.

The specimens were cut according to ASTM-D695 as shown in Fig. (3.9),

where the specimen length is double its thickness. The specimens were fixed

between the surfaces of the piston, the load was applied at a constant rate until

failure occurs, and the compressive strength is calculated as follows:

ndeformatiobeforeSampleofareationCross

LoadStrengtheCompressivsec−

=

8 10

Fig. (3.9) Specimen dimensions for the Compression test

5

5


56

3.4.6 Three Point Bending Test

Three point bending tester was used to determine the modulus of elasticity.

This test was carried out according to (ASTM –D790). Rectangular specimens

with dimensions of (100*10*5) mm were used in this test as shown in Fig. (3.8).

Specimens were fixed between two points; certain load (weight) was applied in the

middle of the specimens. Fig.(3.10) shows the three point bending instrument.

Fig. (3.10) Three point bending instrument.


57

3.5 DSC Measurements Differential Scanning Calorimetry (DSC) is an extensively used

experimental tool for thermal analysis by detection of heat flows from the samples.

It is the most commonly used device to characterize cure kinetics for thermosetting

polymer resins. The heat of reaction, the rate of cure and the degree of cure can be

measured by DSC. The experiments are categorized in two typical modes: (1)

isothermal scanning, during which the test is performed with the sample kept at a

constant temperature; and (2) dynamic scanning, during which the sample is heated

at a constant scanning rate.

In this work, a Model PYRIS 6 DSC from Perkin-Elmer; as shown in

Fig.(3.11), was used to study the cure kinetics of the three formulations of

DGEBA/TETA system. It consists mainly of a sample holder and a reference

holder, temperature controller, and a heating block. Resin samples weighing

approximately 10-20 mg were encapsulated in aluminum hermetic pans and then

subjected to isothermal calorimetry and dynamic DSC scanning. The reference was

an empty aluminum pan with cover. The purging gas was nitrogen. The flux of

nitrogen was set to 100 ml/min.

Dynamic runs at a heating rate of 5ºC/min were made in order to determine

the conversion profiles and the total heats released during the dynamic curing for

all the three DGEBA/TETA system formulations. The heat evolutions are then

monitored from 30ºC to 250ºC. The total reaction heat is then evaluated by:

Htotal = ∫ �𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑�tfd

0 d

dt (3.1)

where tfd is the time required for the completion of the chemical reaction during

the dynamic scanning, and (dQ/dt)d is the instantaneous heat flow during the


58

dynamic scanning. The integration baseline was obtained by drawing a straight line

connecting the baseline before and after the heat flow peak.

In accordance with the dynamic curing profiles obtained previously, four

temperatures, 30, 45ºC, 60ºC & 80ºC, are selected for the isothermal DSC

experiments for each DGEBA/TETA system formulations. Thermal curves are

recorded until the rate of heat flow approaches zero. The heat flow rates of all the

three resin formulations are found to approximate zero within 30 minutes during

the isothermal scans. The amount of heat released up to time t in an isothermal

measurement is determined by:

H = ∫ �𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑�t

0 𝑑𝑑𝑑𝑑 (3.2)

where (dQ /dt) is the instantaneous heat flow during the isothermal scanning.

Fig. (3.11)

Perkin Elmer Pyris 6 Differential Scanning Calorimetry (DSC)


59

3.6 Viscosity measurement

Viscosity instruments have been widely accepted as reliable tools for

obtaining meaningful rheological measurements on thermosetting polymer resins.

As for cure kinetics, the viscosity measurements can also be categorized as

dynamic viscosity measurement, during which the temperature of the resin is

changed according to some special cure cycle, and isothermal viscosity

measurement, during which the temperature of the resin is kept constant. The time-

temperature history of viscosity is recorded and then applied in the viscosity

modeling.

A Brookfield RV-II+ programmable rotational-type viscometer shown in

Fig.(3.12) is used to perform isothermal viscosity measurements at the

temperatures of 30, 45°C, 60°C & 80°C. For a given viscosity, the viscous

resistance is related to the spindle rotational speed and the spindle geometry. In

this study, the spindle used is disposable SC4-27, and the chamber used is

disposable HT-2DB, both of them are specially designed for measuring sticky

fluids. The clearance between the spindle periphery and the chamber inner wall is

3.15 mm. A temperature control unit maintains the sample at a fixed temperature.

It is a fully computer controlled device with a well-defined menu system. The

output data are viewed on a monitor in graphical and table form during the

measuring time. For isothermal measurements, the sample chamber was preheated

to the desired temperature and stabilized at that temperature for half hour. A water

bath system was used to control the temperature. The sample was put into the

chamber and measurement was started. The viscosity histories at different

temperatures for each resin formulation are recorded with time.


60

Fig.(3.12) RV+II Programmable Brookfield Viscometer

CHAPTER FOUR RESULTS AND DISCUSSION

61

CHAPTER FOUR

RESULTS AND DISCUSSION

4.1

The changes observed on the mechanical properties when the epoxy resin to

hardener ratio is varied may be considered a direct consequence of the different

macromolecular structures that are developed and/or the possible reactions that

could occur given a boundary condition (i.e., when a variable like temperature or

the amount of monomers is changed). For the studied epoxy resin system, the cure

reactions scenarios are led by the primary amino addition reaction, occurring

between the primary amines (~NH

The Effect of The Hardener/Resin Ratio on The Mechanical

Properties

2

) and the epoxide group according to the

following reaction [1,107]:

Which leads to the formation of strongly hydrophilic hydroxyl groups (-OH). For

non-stoichiometric formulations with excess of epoxy monomer the epoxy ring can

react with hydroxyls groups, leading to the formation of ether groups according to

the reaction [1, 12]:


62

Finally, homopolimerization reactions can be catalysed by steric hindered

tertiary amines [108], leading to the formation of the p-dioxane ring structure:

Or to the step like structure:

The formation of p-dioxane rings is, however, of minor relevance for non

-stoichiometric reactions [109], although it can be responsible for the consumption

of about 1/16 of all epoxy rings.


63

4.1.1

The resistance to impact is one of the determining properties of materials. A

tough polymer is one which needs high energy to break in an impact test.

The Impact Test Results

The principle of this test is based on the fact that some of the primary

energy which is kept as a potential energy in the hummer was absorbed by the

sample during the rupture. The energy of fracture is calculated by applying the

Charpy test.

The impact strength can then be calculated using the above equation for the

epoxy resin DGEBA with TETA and DDM as hardeners using different

hardener/resin ratios (under stoichiometry, stoichiometry & above stoichiometry).

Fig. (4.1) shows the variation of the impact strength of DGEBA/TETA and

DGEBA/DDM systems. The DGEBA/TETA system was analyzed for four

different hardener/resin ratios 10, 13, 15 and 20 phr. The epoxy rich formulation

10 phr, shows the lowest impact strength, this is due to the presence of a large

number of epoxy rings [110], and also a rigid and tight macromolecular structure is

developed, were the only expected mobile group is the dimethylene ether linkage

of bisphenol-A [108, 111]. These characteristics are a direct consequence of the

complete exhaustion of all the reactive sites on the hardener molecule, giving way

to a rigid and brittle structure [111].

While the stoichiometric ratio 13 phr shows a higher impact strength than the

under stoichiometry ratio 10 phr, which means that the stoichiometric formulation

is tougher than the epoxy rich formulation which indicates that it’s more flexible.

The amino rich formulation 15&20 phr and the stoichiometric formulation 13 phr

shows higher impact strength than the epoxy rich formulation 10 phr, this is due to


64

the large amount of amino hydrogen groups so that more epoxy rings would be

opened by the amino addition reaction making the material tougher .

The amino rich formulation 15 phr shows the highest impact strength of all

the hardener/resin ratio formulations, which indicates that this material can absorb

more energy before the break where the applied force is dissipated through the

molecular structure and the crack happens when the material can no more

withstand the applied load and the material’s chains began to break. The amino

rich formulation 20 phr is showing less impact strength than the amino rich

formulation 15 phr, this behavior was associated with the presence of non-reacted

points on the hardener molecule which leads to the fracture of the material [108].

The DGEBA/DDM system was analyzed via four different hardener/resin

ratios 24, 27, 30 and 34 phr. The amino rich formulation 30 phr shows the highest

impact strength, this is due to the fact that the amino addition reaction is dominated

and the crosslinking between the resin and the hardener proceed making the

material flexible and stable [86]. While the epoxy rich formulation 24 phr shows

the lowest impact strength, which indicates the presence of a large amount of

epoxy groups which leads to the formation of a brittle and fracture material.

These results are in good agreement with those obtained by d”Almeida and

Cella (85).

The DGEBA/DDM system shows higher impact strength than the

DGEBA/TETA system, this could be explained by the fact that the aliphatic

amines which include TETA is less stable than the aromatic amines which include

DDM and that’s due to the presence of benzene which has a low potential energy

making the epoxy resin system more stable [112]. There are two primary amine

groups located on primary carbon atoms at the ends of an aliphatic polyamine

chain in TETA molecule. At the same time, there are two secondary amine groups

in TETA molecule. Those secondary amine groups also take part in the reaction


65

and formulate a network structure of epoxy resin. The DDM has two amine groups

located on primary carbon atoms at the ends of an aliphatic polyamine chain in

DDM molecule. The primary amine groups are more reactive than the secondary

amine groups so that the DGEBA/DDM would show higher impact resistance than

the DGEBA/TETA system [25].

Fig.(4.1) Impact strength of DGEBA/TETA and DGEBA/DDM systems

4.1.2

4.1.2.1

Tensile Test Results

The elastic deformation which is due to intermolecular force of attraction

can be estimated in terms of Young’s modulus which describes tensile elasticity or

the tendency of an object to deform along an axis when opposing forces are

Effect Of The Hardener/Resin Ratio On The Elastic Modulus

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

10.5

11

0 5 10 15 20 25 30 35 40

Impa

ct S

tren

gth

(KJ/

m2)

hardener/resin ratio (phr)

DGEBA/TETA

DGEBA/DDM


66

applied along that axis. The elastic modulus of a sample is a measure of its

stiffness. The higher the modulus the stiffer the material. The value of elastic

modulus is normally derived from the initial slope of the stress-strain curve [46,

113].

Fig. (4.2) shows the elastic modulus of the DGEBA/TETA and

DGEBA/DDM systems. For DGEBA/TETA system, the amino rich formulation

shows the higher elastic modulus, which means that the higher the hardener ratio in

the epoxy resin the higher the Young’s modulus of it. The amino addition reaction

leads to the formation of a three dimensions network so the material will deform

linearly until it fails under the subjected load giving a higher Young’s modulus

than that for the epoxy rich formulation where the material is brittle and tight as

observed for the 10 phr formulation where the epoxy ring could be opened by the

hydroxyls groups leading to the formation of ether group and also

homopolymerization plays a role in the formation of this material, all these factors

affect the material ability to handle with the stretching force subjected to it making

the epoxy rich formulation chains to be broken in a brittle manner showing a low

Young’s modulus which indicates the poor cross-linking between the epoxy resin

and the hardener. The amino rich formulation 15 phr shows a Young’s modulus

higher than that for the stoichiometric formulation 13 phr, that’s due to the

presence of a larger amount of epoxy monomers in the stoichiometric formulation

which in turn leads to the epoxy ring opening reaction by the hydroxyls groups.

The amino rich formulation 20 phr is showing a Young’s modulus less than the

amino rich formulation 15 phr, that’s due to the presence of non-reacted hardener

molecules.

For DGEBA/DDM system, the Young’s modulus values varied from the

epoxy rich formulation to the amino rich formulation, as shown in Fig. (4.2) the


67

above stoichiometric ratio 30 phr gives the highest Young’s modulus which means

that it deforms linearly until the failure, giving way to the material chains to be

stretched and slide on each other to the point of breaking, where the amine

structure in the epoxy resin is dominated. The amino rich formulation 34 phr is

showing less Young’s modulus than the amino rich formulation 3o phr, that’s due

to the presence of the non-reacted hardener molecules which makes the material

brittle. For the under stoichiometric formulation 24 phr the presence of excess

epoxy monomer making the reaction proceed in the direction of epoxy ring

reaction with hydroxyls groups introducing the ether group which is less stable

than the carbon-amine nitrogen linkage so the material would be brittle and break

without yielding [24,87]. For the stoichiometric formulation 27 phr the Young’s

modulus is higher than that for the epoxy rich formulation 24 phr , this is due to the

amino addition reaction in which it dominates the curing process rather than the

homoplymerization or the epoxy ring opening by the hydroxyls groups making the

material more rigid and tougher. These results agree well with the results obtained

by Pandini et. al. [90] and Lee [114] where they found that the Young’s modulus

increase with the increase of the hardener/resin ratio.

The Young’s modulus for the DGEBA/DDM system is higher than that for

the DGEBA/TETA system; this is due to the aromatic structure in the backbone

which imparts better rigidity to the epoxy resin system making the material more

stable and showing higher resistance to the pulling load [115].


68

Fig. (4.2) Young’s modulus vs. hardener/resin ratio for the (DGEBA/TETA) system and the

(DGEBA/DDM) system

4.1.2.2 Effect Of Hardener/Resin Ratio On The Ultimate Tensile

Strength

Ultimate tensile strength (UTS) is a measure of stress applied to a

specimen until failure (break).

Fig. (4.3) shows the relation between the ultimate tensile strength (UTS)

and the hardener content (phr) for DGEBA/TETA and DGEBA/DDM systems.

For DGEBA/TETA system, the ultimate tensile strength increased with

increasing the hardener content where the amino rich formulation 15 phr exhibits

the higher stress at break. The higher degree of cross-linking makes the material

strong and rigid in which it performs a ductile behavior in comparison with the

epoxy rich formulation 10 phr that break in a brittle manner due to the presence of

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35 40

Youn

g's m

odul

us (M

Pa *

103 )

hardner/resin ratio (phr)

DGEBA/TETA

DGEBA/DDM


69

ether groups and homopolymerization, so the 10 phr formulation needs a lower

strength to be broken than the amino rich formulation 15 and 20 phr. The

stoichiometric formulation 13 phr shows a better resistance to the pulling load than

the epoxy rich formulation but still the amino rich formulation 15 phr is the best,

where the carbon-amine nitrogen linkage represents most of the structure but also

there’s a fairly amounts of ether groups and the products of homoplymerization

[86]. While the above stoichiometry formulation 20 phr is showing less resistance

to the pulling load, where there is a fairly amount of non reacted hardener

molecules making the material less stable

For the DGEBA/DDM system, the material shows a great resistance to the

stretching force till the failure of the specimen as the hardener/resin ratio increase.

the above stoichiometric formulations show high ultimate tensile strength

especially the 30 phr formulation due to the amino addition reaction which

develops a three dimensional network where the material’s chains slide on each

other and try to withstand the applied load where finally the stress relaxed when

these chains are broken [116], while the 34 phr is showing less ultimate tensile

strength than the 30 phr formulation, that’s due to the presence of non reacted

hardener molecules. As seen in Fig. (4.3) the stoichiometric formulation 27 phr

needs higher strength to be broken than the under stoichiometric formulation 24

phr which reveal the poor cross-linking between the DGEBA resin and the

hardener DDM.

The tensile strength for the DGEBA/DDM system is higher than that for

the DGEBA/TETA system, where the aromatic structure of the DDM hardener

through the presence of the benzene makes the material more stable and rigid,

where the linear structure of the aliphatic TETA hardener makes the material less

stable and brittle [112].


70

These results are in good agreement with the results obtained by Sulaiman

et. al. [89] and Rao [117], who found that the tensile strength increased with

increasing the hardener content.

Fig. (4.3) Ultimate tensile strength vs. hardener/ resin ratio for DGEBA/TETA

system and DGEBA/DDM system

4.1.2.3 Elongation, the increase in length of the sample at the breaking point is

also a useful property. Elongation gives a picture about how much the material will

be stretched before it breaks.

Effect Of Hardener/Resin Ratio On The Elongation At Break

Fig. (4.4) shows the elongation of DGEBA/TETA and DGEBA/DDM

systems for different hardener content.

For the DGEBA/TETA system, the above stoichiometry formulations 15

phr is showing the highest elongation, that’s due to the carbon- amine nitrogen

0

20

40

60

80

100

0 5 10 15 20 25 30 35 40

Ulti

mat

e Te

nsile

Str

engt

h (M

Pa)


DGEBA/TETADGEBA/DDM


71

linkage which imparts better flexibility to the material in the way that the chains

are stretched to a high extent before it breaks. The under stoichiometry formulation

10 phr is showing the lower elongation, where the ether groups and the

homopolymerization reaction result are affecting the amino addition reaction

between the DGEBA epoxy resin and the TETA hardener making the material

brittle and less flexible than the other formulations. The stoiciometric formulation

13 phr is showing better elongation than the epoxy rich formulation 10 phr, that’s

due to the amino addition reaction which is the dominated in spite of the presence

of a fair amount of the ether groups and the results of the homopolymerization, and

that makes the material more flexible and ductile so that it would withstand the

applied load. The amino rich formulation 20 phr is showing less elongation than

the amino rich formulation 15 phr, that’s due to the non reacted hardener

molecules which makes the material brittle [90].

For the DGEBA/DDM system, the presence of a high amount of the

hardener DDM in the amino rich formulation 30 phr enhance the material ductility

so that it shows a high elongation before the failure. The stoichiometric

formulation 27 phr is showing higher elongation than the epoxy rich formulation

24 phr which imply that the larger amount of the hardener DDM give the

superiority to the epoxy ring opening by the amino-hydrogen groups rather than

the epoxy ring opening by the hydroxyl groups, in which it gives the material a

higher degree of cross-linking making the material flexible and rigid [115], while

the amino rich formulation 34 phr shows less elongation than the 30 phr

formulation, that’s indicated the presence of non reacted molecules, which it

makes the material less flexible and brittle [89].

The DGEBA/DDM system, in general, exhibits higher elongation than

the DGEBA/TETA system, that’s due to the aromatic structure of the DDM


72

hardener where it makes the epoxy resin more stable and more flexible in order to

stand the pulling force that tends to break the material [112].

The results obtained here are in good agreement with the results

obtained by Tricca [118] where he found that the elongation of the epoxy resin

system increased with increasing the hardener/resin ratio.

Fig. (4.4) %Elongation at break vs. hardener/resin ratio for the

DGEBA/TETA system and the DGEBA/DDM system

4.1.3 The Hardness Test ResultsHardness is a property of penetration strength, deformation strength, etc, but

in most, hardness test depends on the penetration strength of the material surface

[40].

The impartibility experiments are used to measure the material resistance to

the elastic distortions in the surface area, usually fine head used from rigid

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40

% E

long

atio

n at

bre

ak


DGEBA/TETA

DGEBA/DDM


73

materials which could penetrate in the given rigid material and when the sharp

head penetrates then the elastic distortion happens first followed by plastic

distortion. Hardness tests are one of the best properties giving an indication of the

ability of material to resist scratching, abrasion, or penetration. In the present work

hardness Shore D method was used to measure the hardness of the DGEBA/TETA

and the DGEBA/DDM systems.

Fig. (4.5) shows the variation of hardness values with the different

hardness/resin ratios for DGEBA/TETA and DGEBA/DDM systems.

For the DGEBA/TETA system the hardness values for the four

hardener/resin ratios 10, 13, 15& 20 phr indicate that the amino rich formulations

15 shows the highest values that’s due to the amino addition reaction which

dominates the cross-linking process leading to the formation of a stronger material

which exhibits better hardness. The formation of the three dimensional network

and the high degree of crosslinking, the material tends to be more flexible than the

epoxy rich formulation [47]. Fig.(4.5) shows that the epoxy rich formulation 10 phr

exhibits the lower hardness, where the epoxy ring is opened by the hydroxyl group

(-OH) leading to the formation of ether group(R-CH2-O-CH2

For the DGEBA/DDM system, the amino rich formulation 30 phr shows the

highest hardness that indicates the higher degree of crosslinking making the

material more flexible and needs higher force to be penetrated. The lower hardness

-), also the

homopolymerization reactions could participate in the formation of the epoxy resin

structure, in which the material would be rigid and brittle and easy to be penetrated

(89).the highest hardness value is at 15 phr, where the three dimensional network

groups formulated from the reaction of the amino hydrogen groups with epoxy

groups so the material would have a hard structure to resist scratching [12]. These

results are in good agreement with the results obtained by Sulaiman et. al. [89].


74

observed at the epoxy rich formulation 24 phr, which suggests that the higher the

epoxy monomer ratio to the hardener monomer the more brittle the material

become and easy to be scratched.

The DGEBA/DDM system shows higher hardness shore D value than that

for the DGEBA/TETA system. The presence of the benzene in the DDM curing

agent provides the DGEBA/DDM system with better resistance to the penetration

load than the DGEBA/TETA system [119].

Fig. (4.5) Hardness shore D value vs. hardener/resin ratio for the (DGEBA/TETA) system and the (DGEBA/DDM) system

4.1.4

Flexural strength tests are carried out on the proposed sample to find out the

ability of the specimens to resist deformation under a load. Three-point test is

designed for materials that break at relatively small deflection [120]. In this test the

flexural strength was determined for both DGEBA/TETA and DGEBA/DDM

The Flexural Test Results

70

75

80

85

90

95

100

0 5 10 15 20 25 30 35 40

Hard

ness

shor

e D

valu

e


DGEBA/TETA

DGEBA/DDM


75

systems, the specimens that have been tested have different hardener/resin ratios

(under stoichiometry, stoichiometry and above stoichiometry).

Fig. (4.6) shows the flexural strength of DGEBA/TETA system and

DGEBA/DDM system.

The DGEBA/TETA system has the highest flexural strength at the

hardener/resin ratio of 15 phr, which indicates the higher degree of crosslinking

which imparts high toughness to the sample’s material in order to resist the force

that tends to break it. It was observed that the epoxy rich formulation 10 phr has

the lowest flexural strength values; this is due to the large amount of epoxy groups

which leads to the brittleness of the materials through the reaction with the

hydroxyl groups or with each other through homopolymerization [108]. While the

stiochiometric formulation 13 phr seems to have higher flexural strength than the

epoxy rich formulations 10 phr; that’s because more epoxy rings has been opened

by the amino addition reaction which makes the material more stable and flexible.

The best flexural strength was accomplished at above stoichiometric formulations

15, the amino rich formulations; that could be due to the amino addition reaction

where the DGEBA monomer will develop into stronger and more rigid solid by the

reaction with excess hardener TETA than other formulations, but the amino rich

formulation 20 phr, on the other hand exhibits a lower flexural strength than the 15

phr formulation, which could be related to the non reacted hardener molecules

making the material less flexible and brittle.

For the DGEBA/DDM system, the epoxy rich formulation 24 phr shows the

lowest flexural strength, in which it bends and breaks under a small load indicating

the brittleness of the material and the weak linkages between the hardener and the

resin [25,121], so that the material chains will not flex well in response to the

applied load. 27 phr; the stoiciometric formulation shows better resistance to the


76

flexing load where the specimen required higher strength to be bended and finally

to be broken. That indicates the strong linkages between the hardener DDM and

the DGEBA resin so as one could expect it would withstand a higher load and that

load would be dissipated through the material’s chains in which it would be flexed

until the break of the specimen. The amino rich formulation 30 phr shows the best

result, which indicates the high degree of crosslinking among all the formulations

where the carbon-amine nitrogen linkage gives the material more rigidity and

toughness than the others so that the chains would be flexed and withstand the

force that tends to break it through bending it. Also the 34 phr shows lower

flexural strength than the 27 phr formulation, where a fairly amount of hardener

molecules still without reacting, so it will lead to the fracture of the material.

When a comparison is made between the DGEBA/TETA system and

DGEBA/DDM system based on their flexural strengths, the results show that the

DGEBA/DDM system formulations have higher values than those the aliphatic

ones DGEBA/TETA system formulations. That’s because the aromatic amine

curing agent DDM make the DGEBA monomer tougher than the aliphatic amine

curing agent TETA. That’s because the aromatic structure in the backbone in the

DDM imparts better rigidity to the finally cross-linked network [122].The presence

of thermally stable linkages within the aromatic nuclei is also responsible for

superior properties [123].

The results obtained here are in good agreement with those obtained by Liu

et. al. [88] and Kamlesh et. al. [115], where they found that the type of the curing

agent has a direct effect on the flexural strength and by using different types of

curing agents we have different values of flexural strength.


77

Fig. (4.6) Flexural strength vs. hardener/resin ratio for (DGEBA/TETA) and

(DGEBA/DDM) systems

4.1.5

On the continuous increase of load the specimen’s thickness decreases

(cross- section) because of the Poisson effect. This leads to the appearance of

lateral expansion distributed isotropically around the specimen [124].

The Compression Test Results

Fig. (4.7) Shows the compression strength for both the DGEBA/TETA and

the DGEBA/DDM systems with different hardener/resin ratios.

For the DGEBA/TETA system the compression strength for the amino rich

formulations 15 is higher than the epoxy rich formulation 10 phr. Two kinds of

mechanisms occur in different sites at the same time and are responsible for the

50

70

90

110

130

150

0 5 10 15 20 25 30 35 40

Flex

ural

stre

ngth

(MPa

)


DGEBA/TETA

DGEBA/DDM


78

occurrence of this kind of failure in the material; the failure is because of

compression stresses and shear stresses. It was found that it is probable that the

failure will occur in the epoxy resin material by the effect of compressive stresses,

which will lead to the occurrence of buckling phenomenon in the material [125].

Where the presence of a large amount of amino groups lead to the formation of

stronger material that struggles against the compressive load and inhibits the

buckling, so that the amino rich formulation needs higher strength to be

compressed. The stoichiometric formulation 13 phr also demands higher

compressive strength than the epoxy rich formulation 10 phr, that’s due to the

formation of three dimensional network and the strong chains making the material

hard and tough. The above stoichiometry formulation 20 phr is showing less

resistance to the compressive load, which indicates the brittleness of the material

and that could be due to the non reacted hardener molecules [126].

For the DGEBA/DDM system, the compressive strength for the amino rich

formulations 30 and 34 phr is higher than the epoxy rich formulation 24 phr, where

the excess amount of epoxy groups leads to the formation of the ether groups and

the hompolymerization, making the material weak and easy to be compressed. The

epoxy rich formulation 24 phr, where the amino groups lead to the formation of

three dimensional networks in the amino addition reaction, in which the material

become harder and stronger. These results are in good agreement with those

obtained by d’Almeida [126].

The compression strength for the DGEBA/DDM system is higher than that

for the DGEBA/TETA system, that’s due to the aromatic structure of the hardener

DDM which makes the epoxy resin system more stable and stronger than the

hardener TETA, where its linear structure makes the DGEBA/TETA system less

strong and can’t handle a high compressive load [122].


79

Fig. (4.7) Compression strength vs. hardener/resin ratio for the


4.1.6 The values of Young's modulus (E) were determined by using three-point

bending test. The specimen usually retains its original shape after removing the

applied load, so there’s no failure happens in this test, where the test is carried out

in the elastic state only.

The Bending Test Results:

Fig. (4.8) represents the Young’s modulus values of the DGEBA/TETA

system and the DGEBA/DDM system for different hardener/resin ratios.

For the DGEBA/TETA system, the Young’s modulus increased as the

hardener content TETA increased. The amino rich formulations 15 shows the

higher elastic modulus value, that’s due to the material’s stiffness indicating

60

70

80

90

100

110

120

0 5 10 15 20 25 30 35 40

Com

pres

sion

Str

engt

h (M

Pa)


DGEBA/TETA

DGEBA/DDM


80

the ductility of the material in which the material is requires high load to bend.

The epoxy rich formulation 10 phr shows lower Young’s modulus than the

stoichiometric formulation 13 phr, which could be explained in the view of the

lower stiffness which indicates the lower stress and strain exhibited by the

epoxy rich formulation leading to a lower rigidity and elasticity where the

material is to be bend at a low load. The amino rich formulation 20 phr shows

less Young’s modulus than the amino rich formulation 15 phr, which suggests

the brittleness of the material due to the presence of non reacted hardener

molecules[50].

For the DGEBA/DDM system, the amino rich formulations 30 and 34 phr

and the stoichiometric formulation 27 phr show better results for the Young’s

modulus than the epoxy rich formulation 24 phr, that’s due to the higher

degree of cross-linking which imparts better ductility and rigidity to the

material, so that means the higher elasticity of the material.

The DGEBA/DDM system is showing higher Young’s modulus values

than the DGEBA/TETA system, where the aromatic structure of the DDM

imparts better ductility and flexibility and more stability to the epoxy resin

system. Where the aliphatic structure of the TETA and its simple formulation

makes the epoxy resin system less stable and less flexible, so the elasticity

would be lower [119]. These results are in good agreement with the results

obtained by Rao (117).


81

Fig. (4.8) Young’s modulus (E) vs. hardener/resin ratio for the


4.2 The isothermal method can identify two types of reaction: n order or

autocatalytic order [127, 128]. If the maximum peak of the isotherm is close to

t = 0, the system obeys kinetics of n order and it can be studied either by

dynamic or isothermal methods [129]. In the case where the maximum peak is

formed in between 20 and 40% of the total time of the analysis, the cure is

autocatalytic and it should be studied exclusively by isothermal method [18,

127, 129].

DSC Cure Analysis

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35 40

Youn

g's m

odul

us (E

) (M

Pa)


DGEBA/TETADGEBA/DDM


82

4.2.1 Dynamic Cure Analysis Dynamic results can be seen in Table (4.1). As expected, the peak

temperature is lower for higher ratio, because the reaction is more effective due to

the fact that there are more amine groups when the ratio is increased. The amine is

responsible for the crosslink reaction.

Being known the dynamic run of the 13 and 20 phr hardener/resin ratio,

three specific temperatures for the isothermal runs were chosen. The

isothermal temperatures were chosen between the beginning of the reaction

and peak temperature, because main kinetic events of the reaction occur in that

area. Another isothermal run was obtained at a temperature of one forth

distance from the initial temperature and the peak temperature, which gives

information on the kinetic order of the studied formulation system [127]. The 5

phr hardener/resin ratio is showing no significant peak during the dynamic run

at 10 °C/min.

Table (4.1) Total dynamic cure reaction heat of DGEBA/TETA system at

(10°C/min) heating rate for 13 and 20 phr

hardener/resin

ratio (phr)

Onset

Temperature (°C)

Peak temperature

(°C)

Total reaction

heat (J/g)

13 70.96 99.71 201

20 68.15 97.58 240.5

4.2.2 During the isothermal curing measurements, the variation of the heat flow

of the epoxy resin sample is caused by the cure reaction. The instrument records

the heat flow change with respect to the cure time based on the sample size.

Isothermal DSC Cure Analysis


83

4.2.2.1 Both dynamic and isothermal measurements were done to obtain more

information about the curing process. For dynamic measurements, the sample was

scanned from 30 to 250 °C. With the information from the dynamic cure, a series

of isothermal measurements were performed, starting from 30 °C. To achieve

almost constant heat flow in the late cure stage, the measurement time was set long

enough, for 13 phr hardener /resin ratio of DGEBA/TETA system, it was set from

15 minutes at 80 °C to 235 minutes at 30 °C. For 20 phr hardener/resin ratio of

DGEBA/TETA system, the measurement time was set from 20 minutes at 80 °C

and 300 minutes at 30 °C. For the 5 phr hardener/resin ratio of DGEBA/TETA

system, there was no observed curing (no peak) as long as the time was set. That’s

due to the low amount of the hardener TETA so that no appreciable curing is

happened [103].

Analysis of Cure Reaction Heat

At different cure temperatures, the isothermal cure heat is different. Its value

increases with the increment of temperature. Also the isothermal cure heat is

different with the different hardener/ resin ratio, its value seems to increase with

increasing the hardener/resin ratio. For 13 phr hardener/resin ratio, when the cure

temperature was raised to 80 °C, the cure heat was 200 J/g. This value is thought to

be the total reaction heat of isothermal cure because it is very close to the total

dynamic cure heat of 201 J/g at heating rate of 10 °C/min, also it is close to the

isothermal cure heat of reaction at 60 °C, which means that no additional cure heat

was released and the cure reaction was completed at 80 °C. All of the other values

for reaction heats cured isothermally below 80 °C were considered as the partial

isothermal reaction heats [130]. For the 20 phr hardener/resin ratio, the cure heat

was 240 J/g at 80°C, which thought to be the total reaction heat of isothermal cure

because it is very close to the total dynamic cure heat of 240.5 J/g at a heating rate

of 10 °C/min, also it is close to the isothermal cure heat of 238 at 60 °C; which


84

means that no additional cure heat was released and the cure reaction was

completed at 80 °C. The experimental result obtained by the isothermal cure was

confirmed by the combination of dynamic and isothermal cure. Several

measurements with the combination of dynamic and isothermal cure were done.

4.2.2.2 The curing process is an exothermic reaction. The cumulative heat

generated during the process of reaction is usually related to the degree of cure. It

is assumed that the degree of cure is proportional to the reaction heat. In our

experiments, the sample used is fresh and uncured. Its reaction heat at each

sampling time is determined by integrating the curve of heat flow from the

beginning to the determined time, so the degree of cure can be directly calculated

from the partial reaction heat.

Degree of Cure and Cure Rate

Once the partial reaction heats at each sampling time and temperature have

been measured, the degree of cure can be easily calculated by equation (2.8). The

degree of cure versus cure time at the temperature range from 30 to 80 °C for the

13 and 20 phr hardener/resin ratios are shown in Figs. (4.9) and (4.10) respectively.

Compared to the value of 1 at 80 °C, the final degree of cure at 30 °C is only about

0.67 for the 13 phr hardener/resin ratio. While for the 20 phr hardener/resin ratio;

the final degree of cure is 0.95 at 80°C, and only about 0.62 at 30 °C. The time

needed to reach the final degree of cure is also much different, depending on the

isothermal cure temperature and the hardener/resin ratio.

The cure rate at each sampling time and temperature can be calculated by

differentiating the degree of cure to time. The changes of cure rate with time at

each isothermal temperature from 30 to 80 °C for 13 phr hardener/resin ratio are

shown in Fig. (4.11) and for 20 phr hardener/resin ratio are shown in Fig. (4.12). In

the early stages of cure reaction, the cure rate at a higher temperature is faster than


85

that at a lower temperature; but in the late stages, the cure rate is slower at the

higher cure temperature. That’s because the reaction is being controlled by

diffusion [63]. In the late stage of the curing process, the sample approaches the

solid state. The movement of the reacting groups and the products is greatly

limited and thus the rate of reaction is not controlled by the chemical kinetics, but

by the diffusion of the reacting groups and products.

It is observed that the maximum heat evolution (the maximum reaction rate)

occurs in between 20 and 40% of the total reaction time, i.e., at a conversion α ≠ 0

[128, 132]. Therefore the DGEBA/TETA system obeys the autocatalytic cure

kinetics. Autocatalytic cure kinetics implies that the formulation obeys equation

(2.11). The constant m is related to the autocatalytic concentration of the reaction,

i.e., the concentration of hydroxyls groups that are being generated as cure

proceeds and the constant n is related to the consumption of epoxy groups.

Besides, m influences the initial rate of reaction and controls the symmetry of the

curve [132] and the constant n defines the reaction type, i.e., by the shape of the

curve. Figs. (4.11) and (4.13) show the influence of the temperature on the reaction

rate of the 13 phr hardener/resin ratio. The curve obtained at the lowest

temperature 30 °C presents the lowest slope, and the reaction take longer to reach

the maximum conversion rate (𝑑𝑑∝𝑑𝑑𝑑𝑑

). As temperature increases (~ 45-80) °C the

curves become steeper, reaching the maximum reaction rate in a short time. As

seen in Fig. (4.13), the maximum reaction rate, for the 13 phr hardener/resin ratio;

occurs at nearly 30% of conversion, suggesting that, when the cure reaction

reaches its highest conversion rate, 30% of the total epoxy groups have already

been consumed. Figs. (4.12) and (4.14) show the influence of the temperature on

the reaction rate of the 20 phr hardener/resin ratio. The curve obtained at the lowest

temperature 30 °C presents the lowest slope, and the reaction take longer to reach


86

the maximum conversion rate (𝑑𝑑∝𝑑𝑑𝑑𝑑

). As temperature increases (~ 45-80) °C the

curves become steeper, reaching the maximum reaction rate in a short time. As

seen in Fig. (4.14), the maximum reaction rate, for the 20 phr hardener/resin ratio;

occurs at nearly 25% of conversion, suggesting that, when the cure reaction

reaches its highest conversion rate, 25% of the total epoxy groups have already

been consumed.

4.2.2.3 The autocatalytic model for the isothermal cure process as the modified

Kamal’s model in equation (2.16) was used to model the curing process of

DGEBA/TETA system.

Cure Reaction Modeling

An easier and efficient way to analyze the data is by the nonlinear

regressions of the experimental data. For this data analysis, an origin software was

employed to do nonlinear least squares curve fitting to the experimental data. To

obtain the six parameters in the autocatalytic model successfully, the selection of

initial values for the parameters and ranges of experimental data is very important.

During the process of nonlinear regressions, the sum of the squares of the

derivations of the theoretical values from the experimental values, which is called

χ2, decreases and the parameters change. The regression stops when χ2

The values for the rate constants and reaction orders for 13 and 20 phr

hardener/resin ratio of the DGEBA/TETA system are listed in Tables (4.2) and

(4.4), respectively. The values for constant C and critical degree of cure α

is

minimum. The parameters thus obtained achieve the best values for the model.

c for

13 and 20 phr hardener/resin ratios at different temperatures are listed in Tables

(4.3) and (4.5) respectively. The critical values for degree of cure increase with the

increment of temperature.


87

The fitting curves and experimental data for 13 and 20 phr hardener/resin

ratios of DGEBA/TETA system are provided in Figs. (4.13) and (4.14), the fitting

curves agree well with the experimental data.

It is observed that, as for 13 and 20 phr hardener/resin ratios of the

DGEBA/TETA system, the rate constants k1 and k2

The rate constants k

and the kinetic exponent n

increase proportionally as a function of temperature. Then, a higher number of

molecules acquire enough energy for collision, reaching the reaction activation

barrier and, consequently, increasing the reaction rate [133,134]. On the other

hand, the kinetic exponent m decreases as temperature increases due to the effect

of the thermal catalysis, superseding the autocatalytic effect of m [128]. It should

be noted that, a value nearly constant for the total reaction order (m + n =2) is

obtained throughout the polymerization reaction.

1 and k2 increase with the increment of temperature

and follow the Arrhenius law as equation (2.10). By equation (2.10), the plots of

ln (k1) and ln (k2) vs. 1/T with their linear regression curves, shown in Figs.(4.15)

and (4.16), are provided. From the intercepts and slopes of the regression curves,

the pre exponential factors A1 and A2 and activation energies Ea1 and Ea2 can be

determined. Their values are also given in Tables (4.2) and (4.4). The cure

temperature has more effect on rate constant k1 than k2

Through this analysis of 13 and 20 phr hardener/resin ratios of

DGEBA/TETA system, the stoichiometric ratio formulation 13 phr seems to show

better results than the above stoichiometric ratio 20 phr, where it gives a higher

degree of cure at all the isothermal temperatures and reaches the complete degree

of curing (α = 1) at 80 °C. Also the activation energies E

.

a1 and Ea2

for the

stoichiometric formulation are lower than the above stoichiometry ratio, which

means a lower heating rate is required (92).


88

Table (4.2) Kinetic Parameters of the Autocatalytic Model for Isothermal Cure Process of 13 phr DGEBA/TETA system

Temperature

(°C)

k1(sec-1)

×10

SE(sec-3

-1)

× 10

k-5

2(sec-1)

× 10

SE(sec-3

-1)

× 10

m -5

SE ×

10

n -2

SE ×

10-2

30 0.40 1 1.22 2 1.325 0.73 0.675 1.3

45 2.02 0.8 2.51 4 1.287 0.72 0.713 1.23

60 6.27 0.3 3.58 7 1.277 0.86 0.723 1.40

80 19.4 0.1 5.14 16 1.224 1.18 0.776 1.70

Ea1 55.42 (KJ/mol)

SE (KJ/mol) 1.2

A1(sec-1 2416 )

SE (sec-1 62.54 )

Ea2 17.85 (KJ/mol)

SE (KJ/mol) 0.86

A2 (sec-1 0.43 )

SE (sec-1 0.231 )

Table (4.3) Values of Constant C and Critical Degree of Cure αc

Temperature

for Autocatalytic Model

for Isothermal Cure Process of 13 phr hardener/resin ratio of DGEBA/TETA system

(°C)

C SE α SE c

(× 10-4)

30 41.4 0.139 0.6247 0.9

45 49.7 0.396 0.7276 2.1

60 39.3 0.380 0.8485 2.8

80 88.3 0.876 0.999 3.5


89

Table (4.4) Kinetic Parameters of the Autocatalytic Model for Isothermal Cure Process of

20 phr DGEBA/TETA system Temperature

(°C)

k1(sec-1)

×10

SE(sec-3

-1)

× 10

k-5

2(sec-1)

× 10

SE(sec-3

-1)

× 10

m -5

SE ×

10

n -2

SE ×

10-2

30 0.55 2.12 1.5 3 0.566 1.01 1.434 1.8

45 1.44 5 2.04 6 0.552 1.43 1.478 2.14

60 3.45 0.40 2.82 4 0.446 0.60 1.554 1.13

80 10.32 0.15 3.88 5 0.428 0.52 1.572 0.53

Ea1 67.61 (KJ/mol)

SE (KJ/mol) 1.34

A1(sec-1 1671 )

SE (sec-1 75.27 )

Ea2 28.43 (KJ/mol)

SE (KJ/mol) 0.71

A2 (sec-1 1.81 )

SE (sec-1 0.43 )

Table (4.5) Values of Constant C and Critical Degree of Cure αc

Temperature

for Autocatalytic Model

for Isothermal Cure Process of 20 phr hardener/resin ratio of DGEBA/TETA system

(°C)

C SE α SE c

(× 10-4)

30 50 0.167 0.5980 0.8

45 38.2 0.180 0.6783 1.6

60 42.3 0.314 0.7637 2.3

80 75.7 3.584 0.9523 6.7


90

Fig. (4.9) Degree of cure vs. Time for 13 phr hardener/resin ratio of DGEBA/TETA system

Fig. (4.10)Degree of cure vs. Time for 20 phr hardener/resin ratio of DGEBA/TETA system

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 50.00 100.00 150.00 200.00 250.00

Degr

ee o

f cur

e (α

)

Time (min)

30 °C

45 °C

60 °C

80 °C

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 50 100 150 200 250 300 350

Degr

ee o

f cur

e (α

)

Time (min)

30 °C

45 °C

60 °C

80 °C


91

(a)

(b)

Fig. (4.11) Cure rate vs. Time for 13 phr hardener/resin ratio of the DGEBA/TETA system

(a) at 30 and 45 °C and (b) 60 and 80 °C

3.26E-06

5.33E-05

1.03E-04

1.53E-04

2.03E-04

2.53E-04

0 50 100 150 200 250

Cure

rate

(dα/

dt) (

S-1)

Time (min)

45°C

30 °C

2.60E-05

5.26E-04

1.03E-03

1.53E-03

2.03E-03

2.53E-03

3.03E-03

3.53E-03

0 10 20 30 40 50 60

Cure

rate

(dα/

dt) (

S-1)

Time (min)

60 °C

80 °C


92

(a)

(b)

Fig. (4.12) Cure rate vs. Time for 20phr hardener/resin ratio of the DGEBA/TETA system

(a) at 30 and 45 °C and (b) 60 and 80 °C

1.50E-06

2.15E-05

4.15E-05

6.15E-05

8.15E-05

1.02E-04

1.22E-04

1.42E-04

1.62E-04

1.82E-04

0 50 100 150 200 250 300

Cure

rate

(dα/

dt) (

S-1)

Time (min)

30 °C45°C

0.00E+00

2.00E-04

4.00E-04

6.00E-04

8.00E-04

1.00E-03

1.20E-03

1.40E-03

1.60E-03

0 10 20 30 40 50 60

Cure

rate

(dα/

dt) (

S-1)

Time (min)

60°C

80 °C


93

(a)

(b)

0.E+00

1.E-05

2.E-05

3.E-05

4.E-05

5.E-05

6.E-05

0 0.2 0.4 0.6 0.8 1

Cure

rate

(dα/

dt) (

S-1)

Degree of cure (α)

experimental

model

3.E-06

5.E-05

1.E-04

2.E-04

2.E-04

3.E-04

0.0 0.2 0.4 0.6 0.8 1.0

Cure

rate

(dα/

dt) (

S-1)

Degree of cure (α)

Experimental

Model


94

(c)

(d)

Fig.(4.13) Cure rate vs. Degree of cure for 13 phr hardener/resin ratio of DGEBA/TETA

system at: (a)30 °C, (b) 45 °C, (c) 60 °C and 80 °C

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

0.0 0.2 0.4 0.6 0.8 1.0

Cure

rate

(dα/

dt) (

S-1)

Degree of cure (α)

Experimental

Model

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

2.50E-03

3.00E-03

3.50E-03

0.0 0.2 0.4 0.6 0.8 1.0

Cure

rate

(dα/

dt) (

S-1)

Degree of cure (α)

ExperimentalModel


95

(a)

(b)

-6.78E-21

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

0.0 0.2 0.4 0.6 0.8 1.0

Cure

rate

(dα/

dt) (

S-1)

Degree of cure (α)

Experimental

Model

0.E+00

2.E-05

4.E-05

6.E-05

8.E-05

1.E-04

1.E-04

1.E-04

2.E-04

0.0 0.2 0.4 0.6 0.8 1.0

Cure

rate

(dα/

dt) (

S-1)

Degree of cure (α)

experimental

model


96

(c)

(d)

Fig. (4.14) Cure rate vs. Degree of cure for 13 phr hardener/resin ratio of DGEBA/TETA

system at: (a)30 °C, (b) 45 °C, (c) 60 °C and 80 °C

0.E+00

5.E-05

1.E-04

2.E-04

2.E-04

3.E-04

3.E-04

4.E-04

4.E-04

0.0 0.2 0.4 0.6 0.8 1.0

Cure

rate

(dα/

dt) (

S-1)

Degree of cure (α)

Experimental

Model

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

1.E-03

1.E-03

1.E-03

2.E-03

2.E-03

0.0 0.2 0.4 0.6 0.8 1.0

Cure

rate

(dα/

dt) (

S-1)

Degree of cure (α)

Experimental

Model


97

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

-11

-10

-9

-8

-7

-6

-5

ln (k

1) (s

ec-1

)

Experimental Linear fit

(a)

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

-7

-6.5

-6

-5.5

-5

ln(k

2) (s

ec-1

)


(b)

Fig. (4.15) Rate constant in equation (2.14) as a function of Temperature for 13 phr of

DGEBA/TETA system: (a) k1 and (b) k2


98

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

-10

-9

-8

-7

-6

-5

ln(k

1) (s

ec-1

)

Experimental

Linear fit

(a)

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

-7

-6.5

-6

-5.5

-5

ln(k

2) (s

ec-1

)


(b)

Fig. (4.16) Rate constant in equation (2.14) as a function of Temperature for 20 phr of

DGEBA/TETA system: (a) k1 and (b) k2


99

4.3 4.3.1

Isothermal Scanning Rheological Cure Analysis

During the isothermal reaction, a phenomenon of critical importance can

occur, which is gelation.

Gel Time and Apparent Activation Energy (Ea)

Gelation is characterized by the incipient formation of a material of an

infinite molecular weight and indicates the conditions of the processability of the

material. Prior to gelation, the system is soluble, but after gelation, both soluble

and insoluble materials are present. As gelation is approached, viscosity is

increased dramatically and the molecular weight goes to infinite; gelation doesn’t

inhibit the curing process [135].

The gel point of the cure process is closely related to rheological properties.

It indicates the beginning of cross-linking for the cure reaction, where the resin

system changes from a liquid to a rubber state. The gel time can be determined

according to different criteria [136,137]. The commonly used criteria for gel time

are as follows:

• Criterion 1, the gel time is determined from the crossing point between the base

line and the tangent drawn from the turning point of storage modulus G' curve [67,

34].

• Criterion 2, the gel time is thought as time where the tangent of phase angle (tan

δ) equals 1, or the storage modulus G' and the loss modulus G" curves crossover

[34, 138].

• Criterion 3, the gel time is taken as the point where tan δ is independent of

frequency [139, 140].

• Criterion 4, the gel time is the time required for viscosity to reach a very large

value or tends to infinity [141].

In this study, the determination of gel time was based on the forth criterion.

The values for gel time, determined from Fig. (4.19) and Fig. (4.20) by criterion 4,


100

are listed in Tables (4.6) and (4.7). As the isothermal temperature increases, the gel

time decreases, where the temperature increase the crosslinking [34].

The relationship between gel time and temperature is analyzed by cure

kinetics. The kinetic model as equation (2.35) is used for the gelation analysis.

Equation (2.38) shows the relationship between the gel time and isothermal cure

temperature.

According to equation (2.38), the semi-logarithmic plot of gel time vs. the

reciprocal of the absolute temperature for the 13 phr hardener/resin ratio is drawn

in Fig. (4.17). A linear fit of the experimental data gives a value for the apparent

activation energy of 63.636 KJ/mol. The semi-logarithmic plot of gel time vs. the

reciprocal of the absolute temperature for the 20 phr of hardener/resin ratio is

shown in Fig. (4.18). A linear fit of the experimental data gives a value for the

apparent activation energy of 67.192 KJ/mol.

The gel time for the above stoichiometric ratio 20 phr at all the

temperatures is higher than that for the stoichiometric ratio 13 phr, that’s due to the

higher amount of amine groups which will speed the crosslinking, resulting in

reaching the gelation in a shorter time.


101

Table (4.6) Gel Time for the 13 phr hardener/resin ratio at different

Temperatures and the Activation Energy

Temperature (°C) 30 45 60 80

tgel 7560 (sec) 2380 840 360

Ea 63.64 (KJ/mol)

SE (KJ/mol) 0.95

Table (4.7) Gel Time for the 20 phr hardener/resin ratio at different

Temperatures and the Activation Energy

Temperature (°C) 30 45 60 80

tgel 6600 (sec) 1360 330 150

Ea 67.19 (KJ/mol)

SE (KJ/mol) 0.41


102

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

5

6

7

8

9

10

11

ln(tg

el) (

sec-1

)

Experimental Linera fit

Fig. (4.17) Gel time as a function of isothermal cure temperature for 13 phr

hardener/resin ratio

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

4

5

6

7

8

9

10

ln(t g

el) (

sec-1

)

Linear fitExperimental

Fig. (4.18) Gel time as a function of isothermal cure temperature for 20 phr

hardener/resin ratio


103

4.3.2 The viscosity profile of the DGEBA/TETA epoxy resin system with

different hardener/resin ratio, as a function of time at different temperatures, is

shown in Figs. (4.19) and (4.20).

Viscosity Modeling

The viscosity increased slowly at the beginning of each curing process, and

then rose faster because of crosslinking reaction. At higher temperatures, the

viscosity of the epoxy resin was initially lower, but then increased earlier due to

the faster curing.

Based on the extent of the viscosity measurements, a model of viscosity for

isothermal cure process of epoxy resin system is proposed and used to fit the

experimental viscosity as shown in equation (2.33).

The proposed viscosity model introduces two new parameters, the critical

time tc and final viscosity η∞. All the parameters ηo, η∞, tc and k in equation

(2.33) are determined at the same time by fitting experimental viscosity with

respect to time by nonlinear least square approach. The fitted curves are shown in

Figs. (4.19) and (4.20). The predicted viscosities have very good agreement with

the experimental data, even in the gel region. It seems clear that the viscosity

profile at each temperature for a specific hardener/resin ratio can be well described

by the proposed viscosity model. The regressed values of critical time tc and rate

constant k in equation (2.33) for every hardener/resin ratio at each temperature are

listed in Tables (4.8) and (4.9). The variation in critical time with respect to

temperature is the same as one observed in gel time and can also be described by

an Arrhenius law as equation (2.34).


104

(a)

(b)

-1.00E+06

0.00E+00

1.00E+06

2.00E+06

3.00E+06

4.00E+06

5.00E+06

6.00E+06

7.00E+06

8.00E+06

9.00E+06

0 2000 4000 6000 8000

Visc

osity

(mPa

.S)

Time (sec)

experimentalmodel

-2.00E+06

0.00E+00

2.00E+06

4.00E+06

6.00E+06

8.00E+06

1.00E+07

1.20E+07

1.40E+07

0 200 400 600 800 1000 1200 1400

Visc

osity

(mPa

.S)

Time (sec)

Experimental

Model


105

(c)

(d)

Fig. (4.19) Experimental and calculated viscosity for DGEBA/TETA of 13 phr

hardener/resin ratio at isothermal temperatures: (a) 30°C, (b) 45°C, (c) 60°C &(d) 80°C

-2.00E+06

0.00E+00

2.00E+06

4.00E+06

6.00E+06

8.00E+06

1.00E+07

1.20E+07

0 200 400 600 800 1000

Visc

osity

(mPa

.S)

Time (sec)

Experimental

Model

-1.00E+05

1.00E+05

3.00E+05

5.00E+05

7.00E+05

9.00E+05

1.10E+06

0 100 200 300 400

Visc

osity

(mPa

.S)

Time (sec)

Experimental

Model


106

(a)

(b)

-1.00E+05

1.00E+05

3.00E+05

5.00E+05

7.00E+05

9.00E+05

0 2000 4000 6000 8000

Visc

osity

(mPa

.S)

Time (sec)

Experimental

Model

-1.00E+05

1.00E+05

3.00E+05

5.00E+05

7.00E+05

9.00E+05

1.10E+06

0 300 600 900 1200 1500 1800

Visc

osity

(mPa

.S)

Time (sec)

ExperimentalModel


107

(c)

(d)

Fig. (4.20) Experimental and calculated viscosity of the DGEBA/TETA system for 20 phr

hardener/resin ratio at isothermal temperatures: (a) 30°C, (b) 45°C, (c) 60 °C & (d) 80°C

-2.00E+05

0.00E+00

2.00E+05

4.00E+05

6.00E+05

8.00E+05

1.00E+06

1.20E+06

0 100 200 300 400

visc

ocity

(mPa

.S)

Time (sec)

ExperimentalModel

-1.00E+05

1.00E+05

3.00E+05

5.00E+05

7.00E+05

9.00E+05

1.10E+06

0 50 100 150 200

Visc

osity

(mPa

.S)

Time (sec)

Experimental

Model


108

As seen in Figs. (4.21) and (4.22), there is a very linear relationship

between the logarithmic critical time and the reciprocal of absolute temperature.

The rate constant in equation (2.32) also obeys an Arrhenius equation as a function

of temperature. The relationship of ln k versus 1/T and the linear fit are given in

Figs. (4.23) and (4.24). The fitted values of pre-exponential factor and activation

energies are listed in Tables (4.8) and (4.9). It is interesting to note that the

activation energies obtained from gel time in equation (2.38) and the critical time

in equation (2.34) for the 13 phr hardener/resin ratio are close to each other, with

the values of 63.64 and 62.309 KJ/mol, respectively. Also, for the 20 phr

hardener/resin ratio 67.19 and 69.778 KJ/mol.

Table (4.8) Kinetic parameters in equation (2.33) of the viscosity model for 13

phr hardener/resin ratio of DGEBA/TETA system

Temperature (°C) tc SE (sec) (sec) K (sec-1) ×10 SE(sec-2 -1)×10-3

30 7510.99 2.18 1.80 0.60

45 2202.684 1.74 4.10 1.20

60 765.989 1.26 5.34 1.40

80 327.9047 0.31 12.93 1.30

Pre-exponential

factor (sec-1 A) t= 2.05052 × 10-7

SE (sec-1 6.005 × 10) -8

Activation energy

(KJ/mol)

Et=62.309

SE (KJ/mol) 0.542


109

Table (4.9) Kinetic parameters in equation (2.33) of the viscosity model for 20

phr hardener/resin ratio of DGEBA/TETA system

Temperature (°C) tc SE (sec) (sec) K(sec-1) ×10 SE(sec-2 -1)×10-3

30 6371.664 5.27 1.5 4.12

45 1586.236 2.18 8.2 3.26

60 290.225 0.20 15.32 2.84

80 155.282 0.10 43.81 1.42

Pre-exponential

factor (sec-1 A) t=1.16563 × 10-9

SE (sec-1 7.62 × 10) -10

Activation energy

(KJ/mol)

Et=69.778

SE (KJ/mol) 2.06


110

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

ln(t c

) (se

c)

Linear fitExperimental

Fig. (4.21) Critical time versus Isothermal cure temperature of 13 phr

hardener/resin ratio of DGEBA/TETA system

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

4

5

6

7

8

9

10

ln(t c

) (se

c)

Linear fit Experimental

Fig. (4.22) Critical time versud Isothermal cure temperature of 20 phr

hardener/resin ratio of DGEBA/TETA system


111

0.0028 0.0030 0.0032 0.0034

1/ T (k-1)

-5

-4

-3

-2

-1

ln(k

) (se

c-1)


Fig. (4.23) Rate Constant in equation (2.33) versus Isothermal Cure Temperature for the 13 phr hardener/resin ratio

0.0028 0.0030 0.0032 0.0034

1/ T (K-1)

-7

-6

-5

-4

-3

-2

-1

0

1

ln(k

) (se

c-1)


Fig. (4.24) Rate Constant in equation (2.33) vs. Isothermal Cure Temperature for the 20

phr hardener/resin ratio


112

Figure (4.25) presents the transient profile of the viscosity for a 13 phr of

hardener/resin ratio at 45, 60 and 80 °C. When curing at 45 °C, the viscosity starts

to increase after 16 minutes of cure. It can be noticed that after 10 minutes of cure

at 60 °C, the viscosity starts to increase, and after some minutes, there is a sharp

increase in the viscosity. While at 80 °C, the viscosity increased rapidly after

several minutes of the cure. At this time, it was observed during the experiment

that the resin became a gel-like material. The sharp increase in the viscosity,

noticed at all temperatures, is due to the crosslink reaction.

Therefore, this behavior occurs earlier at higher temperatures [135].

Fig. (4.25) Viscosity versus cure time for 13phr of DGEBA/TETA system at

45, 60 and 80°

C

-2.0E+06

0.0E+00

2.0E+06

4.0E+06

6.0E+06

8.0E+06

1.0E+07

1.2E+07

1.4E+07

0 2 4 6 8 10 12 14 16 18 20 22 24

Visc

osity

(mPa

.S)

Time (min)

13 phr at 45°C13 phr at 60°C13 phr at 80°C


113

The transient profile of the viscosity for the 20 phr hardener/resin ratio at

45, 60 and 80 °C is shown in Fig. (4.26). At 45 °C, the viscosity increased after 20

minutes of cure, several minutes a sharp increase in the viscosity is noticed. At 60

°C, the viscosity increases much faster than that for 13 phr hardener/resin ratio;

within several minutes of cure it increases rapidly, in seconds of cure there’s a

sharp increase in the viscosity of the 20 phr hardener/resin ratio at this temperature;

this is due to the higher amount of amino groups in this formulation where the

crosslinking takes place in a short time depending also on the temperature which

accelerate the curing process [103]. At 80°C, the viscosity increases in just 2

minutes of the cure, that’s faster than at 60 °C and 45 °C which indicates that as

the temperature was increased the epoxy resin reached the gel point faster and that

means that the curing happens faster.

Fig. (4.26)Viscosity versus cure time for 20 phr DGEBA/TETA at 45, 60 and

80°C

-2.00E+05

0.00E+00

2.00E+05

4.00E+05

6.00E+05

8.00E+05

1.00E+06

1.20E+06

0 5 10 15 20 25 30

visc

ocity

(mPa

.S)

Time (min)

20 phr at 45°c

20 phr at 60°c

20 phr at 80°c

CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS

114

CHAPTER FIVE

CONCLUSIONS AND SUGGESTIONS

5.1 Conclusions

In this work the mechanical properties of DGEBA/TETA system and

DGEBA/DDM system for different hardener/resin ratios, the thermal kinetics

properties and the rheological properties of the DGEBA/TETA system for different

hardener/resin ratios were investigated.

The following conclusions were dawn:

1. The above stoichiometric ratio (15 phr) of DGEBA/TETA system shows the

highest mechanical properties among the other hardener/resin ratio formulations.

The above stoichiometric ratio (30 phr) of DGEBA/DDM shows the highest

mechanical properties among the other hardener/resin ratio formulations. The

DGEBA/DDM system shows higher mechanical properties than the

DGEBA/TETA system.

2. The dynamic DSC measurements show that the above stoichiometric ratio (20

phr) of DGEBA/TETA system has the lower peak temperature of 97.58 °C than

the stoichiometic ratio (13 phr) of DGEBA/TETA system of 99.71 °C. The

dynamic DSC measurements show no peak (no curing) during the measurement

from 30 °C to 250 °C for the under stoichiometric ratio (5 phr) of DGEBA/TETA

system. From the dynamic DSC measurements, four temperatures were chosen to

carry out the isothermal DSC measurements of the DGEBA/TETA system; which

are 30, 45, 60 and 80 °C.


115

3. The isothermal DSC measurements show that the complete degree of cure (α =1)

is accomplished at 80 °C for the stoichiometric ratio (13 phr) of DGEBA/TETA

system, where it’s (α = 9.5) for the above stoichiometric ratio (20 phr) of

DGEBA/TETA system.

4. For both the stoichiometric and above stoichiometric ratios (13 and 20 phr), the

relationship between cure rate and degree of cure was simulated by the

autocatalytic six-parameter model (the modified Kamal’s model) including the

diffusion factor. The simulated results with the modified model show a very good

agreement with experimental data.

5. The kinetic rate constants k1 and k2

6. The activation energies E

and the rate of reaction n increase with the

increment of cure temperature, while the rate of reaction m decrease with

temperature, for both the 13 and 20 phr hardener/resin ratios.

a1 and Ea2

7. The isothermal rheological measurements show that the gel time decrease with

increasing temperatures for both stoichiomeric and above stoichiometric ratio (13

and 20 phr) of DGEBA/TETA system. The isothermal rheological measurements

show that the gel time for the above stoichiometric ratio (20 phr) is lower than the

stoichiometric ratio (13 phr) of DGEBA/TETA system at the four temperatures

(30,45,60and 80) °C, that’s due to the higher amount of amine groups. The

relationship of gel time vs. temperature follows the Arrhenius law and thus the

apparent activation energy can be obtained. The isothermal rheological

measurements show that the viscosity increased slowly at the beginning of each

curing process, and then rose faster because of crosslinking reaction. At higher

for the stoichiometric ratio (13 phr) are

lower than the above stoichiometric ratio (20 phr) of the DGEBA/TETA system,

which means a lower heating rate is required.


116

temperatures, the viscosity of the epoxy resin was initially lower, but then

increased earlier due to the faster curing.

8. During the curing process, the variation of viscosity vs. time is predictable by a

model based on the Boltzmann function and it agrees very well with the

experimental data, for both the stoichiometric and above stoichiometric ratios (13

and 20 phr). The critical time in the viscosity model decreases with the increment

of the isothermal temperature and the relationship can be described by an

Arrhenius equation, for both the 13 and 20 phr hardener/resin ratios. The activation

energies determined by the gel time and critical time are close to each other.

5.2 Suggestions for Future Work

With the knowledge from the cure analysis of epoxy resin, this study may be

extended as follows:

1. Using the same epoxy resin systems (DGEBA/TETA & DGEBA/DDM) and

reinforcing them with fibers at different percentages and study their effects on the

mechanical properties.

2. Studying the effect of changing temperature and time on the mechanical

properties of the same epoxy resin systems (DGEBA/TETA & DGEBA/DDM)

with the same hardener/resin ratio; using the dynamic mechanical analysis

technique.

3. Studying the thermo kinetics properties of the DGEBA/DDM system for

different hardener/resin ratios, by providing the appropriate measurements

conditions of cooling system by nitrogen. The DGEBA/DDM system required high

temperatures to be cured so the cooling system with water is not functionalized.


117

4. Studying the rheological properties of the DGEBA/DDM system for different

hardener/resin ratios. The heating device that must be used with such system

should provide high temperatures (~ 150 °C).

5. Studying the structure changes during the cure reaction of epoxy resin systems

(DGEBA/TETA & DGEBA/DDM) with different hardener/resin ratios; by using

the FTIR analysis (Fourier Ttransform Infrared Spectrometry).

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APPENDIX

Example of a Stoichiometric Calculation

Resin: DGEBA

Amine Curing Agent: Triethylene Tetramine (TETA)

Molecular weight of amine:

6 carbons = 6x12 = 72 (g/mol)

4 nitrogens = 4x14 = 56 (g/mol)

18 hydrogens = 18x1 = 18 (g/mol)

____

Molecular weight = 146 (g/mol)

There are 6 amine hydrogen functionally reactive with an epoxy group.

Therefore

equivalentgramsmolsequivalent

molgrams /3.24)/(6

)/(146=

Thus, 24.3 grams of TETA are used per equivalent of epoxy. If the DGEBA has an

equivalent weight of 190 (380 g/mol/2 eq./mol), then 24.3 grams of TETA are used

with190 grams of DGEBA, or 24.3/190 ≈ 13 grams of TETA per hundred grams of

DGEBA.

الخالصة

؛)rheological analysis( ام الفحوصات الميكانيكية، التحليل الحراري و الريولوجيدباستخ

جي لراتنج االيبوكسي، المحضر من والريولو ) cure kinetics(حركيات األنضاج الخواص الميكانيكية،

م نسب مختلفة من قد تم دراستها باستخدا (DDM) و (TETA) مختلفين مع مصلدين) DGEBA(تفاعل

). ، تحت التكافؤ و فوق التكافؤ)stoichiometry( التكافؤ( الراتنج/المصلد

قوة الصدمة، قوة السحب، الصالدة، قوة متانة االنحناء، قوة االنضغاط و قوة االنحناء تم قياسها من خالل

. وصات تم اجراءها عند درجة حرارة الغرفةان الفح" استخدام اجهزة الفحوصات الميكانيكية، علما

الراتنج /ب من المصلدسباستخدام اربع ن اجراؤهاتم ) DGEBA/TETA(الفحوصات الميكانيكية لنظام

)10 ،13 ،15 &20(phr و لنظام)DGEBA/DDM (باستخدام اربع نسب " أيضا)30، 27، 24 &

34 (phr .

لنظام ) phr 30(و ) DGEBA/TETA( لنظام ) phr 15(فوق التكافؤ نسبةنتائج الفحوصات اظهرت بان

)DGEBA/DDM (بينما أظهر نظام . أعطت أفضل الخواص الميكانيكية)DGEBA/DDM ( صفات

). DGEBA/TETA(ميكانيكية أفضل من نظام

الحرارة تم عمل الفحوصات الديناميكية و الفحوصات بثبوت درجة) DSC(باستخدام جهاز المسح التفاضلي

)isothermal (الراتنج /لثالثة نسب من المصلد)20& 13، 5 (phr . كما تم دراسة حركيات االنضاج

عملية االنضاج بثبوت درجة الحرارة تم . °م) 80& 60، 45، 30(الربع درجات حرارية و لنفس النسب

و ) diffusion factor(رمحاكاتها باستخدام موديل رياضي يحتوي على ست محددات بضمنها عامل االنتشا

بين الموديل المقترح و النتائج العملية في جداً اً جيد اً و قد وجد أن هنالك تطابق. هو موديل كمال المعدل

كما اظهرت النتائج بان نسبة التكافؤ تصل درجة النضوج . المراحل المبكرة و المتاخرة من عملية االنضاج

. °م 80عند درجة حرارة ) 1α =(التام

تم قياسها من خالل عملية االنضاج باستخدام جهاز بروكفيلد ) DGEBA/TETA(لنظام ) η(لزوجة ال

الفحوصات تم .°م) 80& 60، 45، 30(و ألربع درجات حرارية ) Brookfield Viscometer(للزوجة

) gel time(زمن تشكل المادة الهالمية . phr) 20 & 13، 5(الراتنج /اجراؤها لثالث نسب من المصلد

)tRgelR (النتائج أظهرت أن زمن . الراتنج باستخدام نتائج اللزوجة العملية/تم حسابه لكل نسبة من نسب المصلد

منحنيات . الراتنج/تشكل المادة الهالمية يتناقص مع زيادة درجة حرارة االنضاج لكل نسبة من نسب المصلد

و قد وجد ) Boltzmann function(ولتزمان اللزوجة تم محاكاتها بواسطة موديل رياضي مبني على دالة ب

R R. بين النتائج العملية و الموديل المقترح اً ممتاز اً أن هنالك تطابق

دراسة تأثير المصلد لراتنج االيبوكسي على الخواص الميكانيكية

رسالة مقدمة إلى

قسم الهندسة الكيمياوية في الجامعة التكنولوجية كجزء من متطلبات نيل درجة

الهندسة الكيمياوية في ماجستير علوم

من قبل

مريم عماد عزيز

)2004الكيمياوية الهندسةبكالوريوس في (

2010

جمهورية العراق وزارة التعليم العالي و البحث العلمي

الجامعة التكنولوجية قسم الهندسة الكيمياوية

a study on the effect of hardener on the mechanical properties of epoxy resin

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