Post on 10-Nov-2014
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)
Signature: Signature:
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.
Signature: Signature:
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
3.4.2 The Tensile Test
3.4.3 The Hardness Test
3.4.4 The Flexural Test
3.4.5 The Compression 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
CHAPTER ONE INTRODUCTION
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].
CHAPTER ONE INTRODUCTION
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
CHAPTER ONE INTRODUCTION
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
CHAPTER ONE INTRODUCTION
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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)
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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]
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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.
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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].
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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].
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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:
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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].
2.6.4 The Flexural Test
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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).
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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].
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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]:
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
24
𝑑𝑑∝𝑑𝑑𝑑𝑑
= 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.
𝑑𝑑∝𝑑𝑑𝑑𝑑
= 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.
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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:
𝑑𝑑∝𝑑𝑑𝑑𝑑
= 11+exp (𝐶𝐶(α −α𝑑𝑑 ) )
(k1 + k2 αm ) (1- α )n
(2.15)
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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.
𝑑𝑑∝𝑑𝑑𝑑𝑑
= (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.
𝑑𝑑∝𝑑𝑑𝑑𝑑
= 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.
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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.
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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.
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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)
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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:
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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)
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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)
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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.
CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW
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
Chapter Three Experimental Work
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
Chapter Three Experimental Work
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.
Chapter Three Experimental Work
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.
Chapter Three Experimental Work
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
Chapter Three Experimental Work
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).
3.4.2 The Tensile Test
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).
Chapter Three Experimental Work
50
Fig. (3.2) Charpy Impact instrument.
Fig. (3.3) Specimen dimensions used in the Impact tests.
55 mm 10
5
Chapter Three Experimental Work
51
Fig.(3.4) Tensile test instrument
Fig. (3.5) Specimen dimensions used in the Tensile test
Chapter Three Experimental Work
52
3.4.3 The Hardness Test
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.
3.5.4 The Flexural Test
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.
Chapter Three Experimental Work
53
Fig. (3.6) Hardness Shore D tester
Chapter Three Experimental Work
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
Chapter Three Experimental Work
55
3.4.5 The Compression Test
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
Chapter Three Experimental Work
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.
Chapter Three Experimental Work
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
Chapter Three Experimental Work
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)
Chapter Three Experimental Work
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.
Chapter Three Experimental Work
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]:
CHAPTER FOUR RESULTS AND DISCUSSION
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.
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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].
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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].
CHAPTER FOUR RESULTS AND DISCUSSION
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)
hardener/resin ratio (phr)
DGEBA/TETADGEBA/DDM
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
hardener/resin ratio (phr)
DGEBA/TETA
DGEBA/DDM
CHAPTER FOUR RESULTS AND DISCUSSION
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].
CHAPTER FOUR RESULTS AND DISCUSSION
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
hardener/resin ratio (phr)
DGEBA/TETA
DGEBA/DDM
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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.
CHAPTER FOUR RESULTS AND DISCUSSION
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
)
hardener/resin ratio (phr)
DGEBA/TETA
DGEBA/DDM
CHAPTER FOUR RESULTS AND DISCUSSION
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].
CHAPTER FOUR RESULTS AND DISCUSSION
79
Fig. (4.7) Compression strength vs. hardener/resin ratio for the
DGEBA/TETA system and the DGEBA/DDM system
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)
hardener/resin ratio (phr)
DGEBA/TETA
DGEBA/DDM
CHAPTER FOUR RESULTS AND DISCUSSION
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).
CHAPTER FOUR RESULTS AND DISCUSSION
81
Fig. (4.8) Young’s modulus (E) vs. hardener/resin ratio for the
DGEBA/TETA system and the DGEBA/DDM system
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)
hardener/resin ratio (phr)
DGEBA/TETADGEBA/DDM
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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.
CHAPTER FOUR RESULTS AND DISCUSSION
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).
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
)
Experimental Linear fit
(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
CHAPTER FOUR RESULTS AND DISCUSSION
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
)
Experimental Linear fit
(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
CHAPTER FOUR RESULTS AND DISCUSSION
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,
CHAPTER FOUR RESULTS AND DISCUSSION
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.
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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).
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
111
0.0028 0.0030 0.0032 0.0034
1/ T (k-1)
-5
-4
-3
-2
-1
ln(k
) (se
c-1)
Experimental Linear fit
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)
Experimental Linear fit
Fig. (4.24) Rate Constant in equation (2.33) vs. Isothermal Cure Temperature for the 20
phr hardener/resin ratio
CHAPTER FOUR RESULTS AND DISCUSSION
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
CHAPTER FOUR RESULTS AND DISCUSSION
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.
CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS
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.
CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS
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.
CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS
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