A STUDY ON THE EFFECT OF HARDENER ON THE MECHANICAL PROPERTIES OF EPOXY RESIN

Transcription

1 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

2 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: Name: Asst. Prof. Dr. Najat J. Saleh (Supervisor) Signature: Name: Dr.Adnan A. Abdul Razak (Supervisor) Signature: Name: Asst. Prof. Dr. Qusay F. Alsalhy (Member) Signature: Name: Asst. Prof. Dr. F. S. Matty (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

3 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: Name: Asst. Prof. Dr. Najat J. Saleh (Supervisor) Signature: Name: Dr.Adnan A. Abdul Razak (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

4 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

5 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. II

6 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. III

7 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 p. 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

8 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. I

9 List of Contents Contents Page Acknowledgment I Abstract II List of Contents IV Notations VII Chapter One: Introduction 1.1 Introduction Objective and scope 4 Chapter Two: Literature Review 2.1 Epoxy Resins Curing Agents (Hardeners) Curing Reactions Selection of Curing Agents The Stoichiometry The Mechanical Properties of Epoxy Resin The Impact Test The Tensile Test The Hardness Test The Flexural Test The Compression Test The Bending Test Differential scanning Calorimetry (DSC) Analysis Cure Kinetics Models Rheological Analysis Rheology Models Viscosity Model 28 IV

10 Gel Time Model Literature review of experimental Work on Epoxy Resin Literature Review on The Mechanical Properties of The Epoxy 35 Resin Literature Review on The Kinetice of the Epoxy Resin Using 41 (DSC) Literature review on The Rheology of the Epoxy Resin Chapter Three: Experimental Work 3.1 The Materials Epoxy Resin The Hardeners The hardener/resin Ratio The Mold The Mechanical Test The Impact Test The Tensile Test The Hardness Test The Flexural Test The Compression Test Three point Bending Test DSC Measurement Viscosity Measurement 59 Chapter Four: Results and Discussion 4.1 The mechanical Properties The Impact Test Results The Tensile Test Results Effect on the Elastic Modulus Effect on the Ultimate Tensile strength 68 V

11 Effect on the Elongation at Break The Hardness Test Results Flexural Strength Test Results The Compression Test Results The Bending Test Results DSC Cure Analysis Dynamic Cure Analysis Isothermal DSC Cure Analysis Analysis of Reaction Heat Degree of Cure and Cure Rate Cure Reaction Modeling Isothermal Scanning Rheological Cure Analysis Gel Time and Apparent Activation Energy (E a ) Viscosity Modeling 103 Chapter Five: Conclusions and Suggestions 5.1 Conclusions Suggestions for Future Work 116 References 118 Appendix VI

12 Notations A Arrhenius frequency or Cross sectional area Aη Initial viscosity (mpa.sec) or (cp) Ak Apparent rate constant sec -1 At Arrhenius frequency ASTM American standard for testing and materials B Thickness of specimen mm b Width of specimen mm C Empirical constant c 1, c 2, Constants c 3,c 4, c 5 and c 6 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 E η Viscous flow activation energy KJ/mol E k Kinetic activation energy KJ/mol E t Activation energy for kinetic model KJ/mol ΔE a Activation energy for kinetic model KJ/mol ΔE i Activation energy for kinetic model KJ/mol % EL Percentage elongation F Applied force N F. S Flexural strength MPa f Free volume of farction f Fractional free volume g G Gravity m/sec -2 H r Total heat of reaction J/g H t Accumulative heat of reaction J/g H total Total heat released during reaction J/g VII

13 I ISO Engineering bending momentum International standard organization K Reaction rate constant sec -1 K i Reaction rate constant for kinetic model sec -1 K Kinetic parameter of viscosity Mw Weight average molecular weight g m f Weight of fiber g m&n Empirical exponents in the cure kinetic model L Specimen length mm Lf Final length mm l o Initial length mm P Load applied N R Universal gas constant J/mol. K t Time sec or min t c Critical time sec t gel Gel time sec T Temperature C TETA TriethyleneTetramine T g Glass transition temperature C Tr Reference temperature C Th Thermal conductivity Kcal/h. C VIII

14 Greek Symbols ζ Stress MPa ε Strain α Degree of conversion α Critical degree of reaction α max α gel Maximum degree of conversion at a specific temperature Degree of cure at gel time α Critical degree of conversion when resin gels. α f The thermal expansion coefficient of free volume. η o Initial viscosity (mpa.sec) or (cp) η Final viscosity (mpa.sec) (cp) µ o Viscosity at infinite temperature (mpa.sec) or (cp) σ Conductivity W/K.m g ψ ζ Empirical expression Empirical expression IX

15 CHAPTER ONE INTRODUCTION 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 1

16 CHAPTER ONE INTRODUCTION 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]. 2

17 CHAPTER ONE INTRODUCTION 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 3

18 CHAPTER ONE INTRODUCTION 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 Objective and Scope The specific objectives of the research are as follows: 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 timedependent 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 4

19 CHAPTER ONE INTRODUCTION complicated autocatalytic reaction models will be used and compared with experimental data. 5

20 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 2.1 Epoxy Resins 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]. 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 6

21 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 (Hardeners) 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. 7

22 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 8

23 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 9

24 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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] 10

25 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 11

26 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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. 12

27 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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]. 13

28 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 2.4 Selection of Curing Agents 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]. 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]. 14

29 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 Of Epoxy Resin 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 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 15

30 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW formation and propagation of defects that allows mechanical stress to be relaxed [40] The Impact Test 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 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/in 2 ); from the definition of the impact strength the following relation was proposed: Impact Strength = Energy of fracture Cross section area (2.1) 16

31 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW The Tensile Test 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]. 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 = A F (2.2) Where: F = applied Force (N) A= cross section Area (mm 2 ) 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 lo are the final and original length respectively. %EEEE = llll ll0 llll 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 17

32 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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] The Hardness Test 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]. 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 usedtype 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] The Flexural Test 18

33 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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: 3PL F.S = 2.. (2.4) 2bd Where: F.S = flexural strength (MPa). P = maximum load (N). L = distance between two fixed points (mm). b = width of the specimen (mm). 19

34 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW d = thickness of the specimen (mm). The most two popular flexural tests are the three point bending and the four point bending test 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 Three Point Bending Test 20

35 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 = ( mmmmmmmm dddddddddddddddddddd ) (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/sec 2 ) L = Specimen length (mm) ( mmmmmmmm dddddddddddddddddddd ): is the slope of linear part of mass deflection curve obtained from three points bending loads tests [16, 40]. 21

36 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 Cure Kinetic Models 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 22

37 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 = HH tt H total (2.8) Where ΔH t is the accumulative heat of reaction up to a given time t during the curing process, and ΔH total is the total heat released during a complete reaction. For an uncured resin, α = 0, whereas for a completely cured resin, α=1. The curing rate is assumed to be proportional to the rate of heat generation and is calculated by the following expression: dd = 1 dddd HH tttttttttt ( dddd dddd ) (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]: 23

38 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW dd dddd = kk(1 αα)nn (2.10) k = A exp ( ΔEa RRRR ) (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, ΔE a 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 ) nn (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. 24

39 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW Kamal's model [57] involves two rate constants and has been applied successfully to model a variety of resins: dd dddd = (k1 + k2r α ) (1- α ) n (2.13) m k i = A i exp (-ΔE i / RT) (i = 1,2) (2.14) where ΔE i are activation energies, R is the universal gas constant, m and n are material constants to be determined by experimental data, k 1 and k 2 have the same definition as in equation (2.11). 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. 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 = 1 1+exp (CC(α α cc ) ) (k 1 + k 2 αm ) (1- α ) n (2.15) 25

40 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW where C and α c are the two empirical constants which are temperature dependant. α c is called the critical degree of cure. One modified form of Kamal's model has been proposed as [59]: dd dddd = (k1 + k2r α ) (α m max - α ) n (2.16) where α max 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 k 1 and k 2 are the same as in equation (2.14). The modified Kamal model incorporates the term α fractional conversion will not exceed the degree of cure associated with vitrification at the specific temperature. Kenny et. al. [60] modified the model used by Pusatcioglu [59] accounting for diffusion effects by modifying equation (2.12): dd = k α m (α max - α ) n (2.17) dddd max, so that the where α max 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 α max on the isothermal cure temperature has been observed. 26

41 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW Michaud [61] found that the use of α max the autocataytic model. greatly improved the fit of 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. 2.8 Rheological Analysis 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]. 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 27

42 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW way that helps in understanding the rheological behavior of this material [66] Rheology Models Viscosity Model 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. 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) = η o and ζ(α) = 1 1 αα (2.19) where ηo is the initial viscosity which is a constant at isothermal cure conditions; α is the degree of cure. 28

43 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW Substituting equation (2.19) into equation (2.18) to get the relationship of viscosity vs. the degree of cure, η = η o 1 1 α (2.20) The initial viscosity η o depends on cure temperature and can be further expressed in Arrhenius equation, η o= A η ee EEEE RRRR (2.21) Where A η and E η 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. 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: ddαα dddd = k (1- α) (2.22) For the first order reaction with the isothermal cure process, temperature T and rate constant k are constant: η= η o ee kkkk (2.23) ln η = ln A η + EE η RRRR + t A k ee EE kk RRRR (2.24) where A k and E k are the apparent rate constant at T = and the kinetic activation energy, respectively. Equation (2.24) is the empirical four-parameter model of viscosity introduced by Roller [68]. 29

44 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW For the nth order reaction, it can be expressed as: ddα dddd = k (1- α) n (2.25) So for the isothermal nth order (n 1) reaction: ln η = ln A η + EE η + 1 ln (1+ (n-1) t A k ee EE η RRRR R RRRR nn 1 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: η(t, α ) η(t ) 0 = ( g M w ( α ) ) M w exp{ c exp{ c 1 1 (T r (T r T T g go ) /( c ( α )) /( c T r + T r T T go g )} ( α ))} (2.27) where η is the viscosity, M W 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 T r 30

45 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW is the glass transition temperature of the reacting system, and c 1 and c 2 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. Bidstrup and Sheppard [75] 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 c 2 and the conductivity σ g at the glass transition are taken as a function of T g. They assumed that c 2 and log (σ g ) vary linearly with T g ; their model for conductivity then gives a five-parameter equation, which can be written as logσ = c + c T 5 6 g c ( T T ) 1 g + c + c T + ( T T ) 3 4 g g (2.28) Where: c = log( 5 + c6tg σ ) (2.29) + (2.30) c g = 3 c4t c2 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 31

46 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW crowded by their neighbors. They found the following empirical expression for EPON828/PACM-20 resin system: where 12 1 η = M w wexp[ 0.62(1 )] (2.31) f M w 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: f = f + α T T ) (2.32) g f ( g here f g is the fractional free volume at T g and α f 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+ee kk(tt tc) + η (2.33) 32

47 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW where η o is the initial viscosity, η is the final viscosity; k is the rate constant of cure reaction and t c is the critical time which follows Arrhenius behavior, i.e. t c = A t ee E t RRRR (2.34) where A t is the pre-exponential factor and E t 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 Gel Time Model 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: ddα dddd = 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 t gel, the relationship between t gel and cure rate is obtained: 33

48 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW α gel t gel = 1. 1 K(T) 0 α gel dα (2.36) where α gel 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(t gel ) = ln[ 1 Ao 0 α gel 1 α gel dα ] + E a R. 1 TT (2.37) According to Flory s expression [36], the degree of cure α gel 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(t gel ) versus 1/T is obtained and equation (2.37) can be rewritten as: ln t gel = C + E a R. 1 T (2.38) From equation (2.38), the apparent activation energy can be calculated from the slope of the curve of ln(t gel ) versus 1/T. 2.9 Literature Review Of Experimental Work On Epoxy Resin 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, 34

49 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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] Literature Review On The Mechanical Properties Of The Epoxy Resin 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. 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 35

50 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW (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 (paraamino 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) 36

51 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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

52 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW (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 = 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 Literature Review On the Kinetics Of The Epoxy Resin Using (DSC) Yilgör et. al. [91] studied the kinetics of the curing reaction of an epoxy resin based on bisphenol-a diglycidylether with a cycloaliphatic 38

53 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 39

54 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 40

55 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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(ketosulfide)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 Literature Review On The Rheology Of The Epoxy Resin 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. 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 41

56 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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 lowviscosity 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. 42

57 CHAPTER TWO THEORETICAL CONCEPTS & LITERATURE REVIEW 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. 43

58 Chapter Three Experimental Work CHAPTER THREE EXPERIMENTAL WORK 3.1 The Materials 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 SMS m mol/kg content Epoxy equivalent g weight Viscosity ASTM D Pa.s Colour ASTM 3 max Gardner scale D1544 Density at 25 C SMS Kg/l Flash point(pmcc) ASTM D93 >150 C 44

59 Chapter Three Experimental Work 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 from 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 Viscosity 450 mpa.s (Hoeppler) at 25 C Specific gravity at g/cm 3 25 C Flash point DIN 129 C Vapor pressure at < 0.01 mmhg 20 C Color Clear, pale yellow or yellow liquid Boiling point C 45

60 Chapter Three Experimental Work Table (3.3) Typical properties of the hardener 4,4 -Diaminodiphenymethane [106] Property Value Unit Molecular weight Grade Purum Flash point 230 C Melting point 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: phr (Under stoichiometry) phr (Stoichiometry) phr (Above stoichiometry) 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. 46

61 Chapter Three Experimental Work 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) phr (Stoichiometry) phr (Above stoichiometry) 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. 47

62 Chapter Three Experimental Work 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) phr (Stoichiometry) 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 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 48

63 Chapter Three Experimental Work 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 The Impact Test Charpy impact instrument is used in this test as shown in Fig. (3.2), often a bar of material is supported as a beam and struck at the middle. The energy which is absorbed by the blow can be determined by measuring the reduction in swing of the pendulum compared with the swing with no sample, the specimens were cut according to (ISO-179). The size of the tested specimens is shown in Fig. (3.3) The Tensile Test 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). 49

64 Chapter Three Experimental Work Fig. (3.2) Charpy Impact instrument mm 10 Fig. (3.3) Specimen dimensions used in the Impact tests. 50

65 Chapter Three Experimental Work Fig.(3.4) Tensile test instrument Fig. (3.5) Specimen dimensions used in the Tensile test 51

66 Chapter Three Experimental Work 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 The Flexural Test The flexural strength of the prepared specimens was measured by using hydraulic piston type Leybold Harris No 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. 52

67 Chapter Three Experimental Work Fig. (3.6) Hardness Shore D tester 53

68 Chapter Three Experimental Work Fig.(3.7) Hydraulic piston type Leybold Harris No mm 100 mm 10 mm Fig. (3.8) Specimen dimensions for the Flexural test 54

69 Chapter Three Experimental Work The Compression Test Hydraulic piston type Leybold Harris No 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: Compressive Load Strength = Cross section area of Sample before deformation Fig. (3.9) Specimen dimensions for the Compression test 5 55

70 Chapter Three Experimental Work 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. 56

71 Chapter Three Experimental Work 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 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: tfd dddd dddd H total = 0 d dt (3.1) where t fd 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 57

72 Chapter Three Experimental Work 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: t 0 H = dddd dddd 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) 58

73 Chapter Three Experimental Work 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 timetemperature 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. 59

74 Chapter Three Experimental Work Fig.(3.12) RV+II Programmable Brookfield Viscometer 60

75 CHAPTER FOUR RESULTS AND DISCUSSION CHAPTER FOUR RESULTS AND DISCUSSION 4.1 The Effect of The Hardener/Resin Ratio on The Mechanical Properties 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 (~NH2) 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]: 61

76 CHAPTER FOUR RESULTS AND DISCUSSION 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. 62

77 CHAPTER FOUR RESULTS AND DISCUSSION The Impact Test Results 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 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 63

78 CHAPTER FOUR RESULTS AND DISCUSSION 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 64

79 CHAPTER FOUR RESULTS AND DISCUSSION 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]. Impact Strength (KJ/m2) DGEBA/TETA DGEBA/DDM hardener/resin ratio (phr) Fig.(4.1) Impact strength of DGEBA/TETA and DGEBA/DDM systems Tensile Test Results Effect Of The Hardener/Resin Ratio On The Elastic Modulus 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 65

80 CHAPTER FOUR RESULTS AND DISCUSSION 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 66

81 CHAPTER FOUR RESULTS AND DISCUSSION 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]. 67

82 CHAPTER FOUR RESULTS AND DISCUSSION DGEBA/TETA DGEBA/DDM Young's modulus (MPa *10 3 ) hardner/resin ratio (phr) Fig. (4.2) Young s modulus vs. hardener/resin ratio for the (DGEBA/TETA) system and the (DGEBA/DDM) system 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 68

83 CHAPTER FOUR RESULTS AND DISCUSSION 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]. 69

84 CHAPTER FOUR RESULTS AND DISCUSSION 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. Ultimate Tensile Strength (MPa) DGEBA/TETA DGEBA/DDM hardener/resin ratio (phr) Fig. (4.3) Ultimate tensile strength vs. hardener/ resin ratio for DGEBA/TETA system and DGEBA/DDM system Effect Of Hardener/Resin Ratio On The Elongation At Break 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. 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 70

85 CHAPTER FOUR RESULTS AND DISCUSSION 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 71

86 CHAPTER FOUR RESULTS AND DISCUSSION 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. 7 6 DGEBA/TETA DGEBA/DDM 5 % Elongation at break hardener/resin ratio (phr) Fig. (4.4) %Elongation at break vs. hardener/resin ratio for the DGEBA/TETA system and the DGEBA/DDM system The Hardness Test Results Hardness 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 72

87 CHAPTER FOUR RESULTS AND DISCUSSION 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-ch 2 -O-CH 2 -), 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]. 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 73

88 CHAPTER FOUR RESULTS AND DISCUSSION 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] DGEBA/TETA DGEBA/DDM Hardness shore D value hardener/resin ratio (phr) Fig. (4.5) Hardness shore D value vs. hardener/resin ratio for the (DGEBA/TETA) system and the (DGEBA/DDM) system The Flexural Test Results 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 74

89 CHAPTER FOUR RESULTS AND DISCUSSION 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 75

90 CHAPTER FOUR RESULTS AND DISCUSSION 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. 76

91 CHAPTER FOUR RESULTS AND DISCUSSION 150 DGEBA/TETA 130 DGEBA/DDM Flexural strength(mpa) hardener/resin ratio (phr) Fig. (4.6) Flexural strength vs. hardener/resin ratio for (DGEBA/TETA) and (DGEBA/DDM) systems The Compression Test Results 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]. 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 77

92 CHAPTER FOUR RESULTS AND DISCUSSION 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]. 78

93 CHAPTER FOUR RESULTS AND DISCUSSION DGEBA/TETA DGEBA/DDM Compression Strength (MPa) hardener/resin ratio (phr) Fig. (4.7) Compression strength vs. hardener/resin ratio for the DGEBA/TETA system and the DGEBA/DDM system The Bending Test Results: 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. 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 79

94 CHAPTER FOUR RESULTS AND DISCUSSION 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). 80

95 CHAPTER FOUR RESULTS AND DISCUSSION DGEBA/TETA DGEBA/DDM Young's modulus (E) (MPa) hardener/resin ratio (phr) Fig. (4.8) Young s modulus (E) vs. hardener/resin ratio for the DGEBA/TETA system and the DGEBA/DDM system 4.2 DSC Cure Analysis 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]. 81

96 CHAPTER FOUR RESULTS AND DISCUSSION 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 Onset Peak temperature Total reaction ratio (phr) Temperature ( C) ( C) heat (J/g) Isothermal DSC Cure Analysis 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. 82

97 CHAPTER FOUR RESULTS AND DISCUSSION Analysis of Cure Reaction Heat 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]. 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 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 83

98 CHAPTER FOUR RESULTS AND DISCUSSION 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 Degree of Cure and Cure Rate 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. 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 84

99 CHAPTER FOUR RESULTS AND DISCUSSION 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 ). As temperature increases (~ 45-80) C the dddd 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 85

100 CHAPTER FOUR RESULTS AND DISCUSSION the maximum conversion rate ( dd ). As temperature increases (~ 45-80) C the dddd 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 Cure Reaction Modeling 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. 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 χ 2, decreases and the parameters change. The regression stops when χ 2 is minimum. The parameters thus obtained achieve the best values for the model. 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 αc 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. 86

101 CHAPTER FOUR RESULTS AND DISCUSSION 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 k 1 and k 2 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. The rate constants k1 and k 2 increase with the increment of temperature and follow the Arrhenius law as equation (2.10). By equation (2.10), the plots of ln (k 1 ) and ln (k 2 ) 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 A 1 and A 2 and activation energies E a1 and E a2 can be determined. Their values are also given in Tables (4.2) and (4.4). The cure temperature has more effect on rate constant k 1 than k 2. 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 Ea1 and E a2 for the stoichiometric formulation are lower than the above stoichiometry ratio, which means a lower heating rate is required (92). 87

102 CHAPTER FOUR RESULTS AND DISCUSSION Table (4.2) Kinetic Parameters of the Autocatalytic Model for Isothermal Cure Process of 13 phr DGEBA/TETA system Temperature ( C) k 1 (sec -1 ) 10-3 SE(sec ) k2(sec -1 ) 10-3 SE(sec ) m SE E a1 (KJ/mol) SE (KJ/mol) 1.2 A 1 (sec -1 ) 2416 SE (sec -1 ) E a2 (KJ/mol) SE (KJ/mol) 0.86 A 2 (sec -1 ) 0.43 SE (sec -1 ) n SE 10-2 Table (4.3) Values of Constant C and Critical Degree of Cure α c Temperature ( C) for Autocatalytic Model for Isothermal Cure Process of 13 phr hardener/resin ratio of DGEBA/TETA system C SE αc SE ( 10-4 )

103 CHAPTER FOUR RESULTS AND DISCUSSION Table (4.4) Kinetic Parameters of the Autocatalytic Model for Isothermal Cure Process of Temperature ( C) k 1 (sec -1 ) 10-3 SE(sec phr DGEBA/TETA system -5-1 ) k2(sec -1 ) 10-3 SE(sec ) m SE E a1 (KJ/mol) SE (KJ/mol) 1.34 A 1 (sec -1 ) 1671 SE (sec -1 ) E a2 (KJ/mol) SE (KJ/mol) 0.71 A 2 (sec -1 ) 1.81 SE (sec -1 ) n SE 10-2 Table (4.5) Values of Constant C and Critical Degree of Cure α c for Autocatalytic Model for Isothermal Cure Process of 20 phr hardener/resin ratio of DGEBA/TETA system Temperature ( C) C SE αc SE ( 10-4 )

104 CHAPTER FOUR RESULTS AND DISCUSSION Degree of cure (α) C 45 C 60 C 80 C Time (min) Fig. (4.9) Degree of cure vs. Time for 13 phr hardener/resin ratio of DGEBA/TETA system Degree of cure (α) C 45 C 60 C 80 C Time (min) Fig. (4.10)Degree of cure vs. Time for 20 phr hardener/resin ratio of DGEBA/TETA system 90

105 CHAPTER FOUR RESULTS AND DISCUSSION 2.53E C 30 C Cure rate (dα/dt) (S -1 ) 2.03E E E E E Time (min) (a) 3.53E E C 80 C Cure rate (dα/dt) (S -1 ) 2.53E E E E E E Time (min) (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 91

106 CHAPTER FOUR RESULTS AND DISCUSSION 1.82E C 45 C 1.62E-04 Cure rate (dα/dt) (S -1 ) 1.42E E E E E E E E Time (min) (a) 1.60E E C 80 C Cure rate (dα/dt) (S -1 ) 1.20E E E E E E E Time (min) Fig. (4.12) Cure rate vs. Time for 20phr hardener/resin ratio of the DGEBA/TETA system (b) (a) at 30 and 45 C and (b) 60 and 80 C 92

107 CHAPTER FOUR RESULTS AND DISCUSSION 6.E-05 5.E-05 experimental model Cure rate (dα/dt) (S -1 ) 4.E-05 3.E-05 2.E-05 1.E-05 0.E Degree of cure (α) (a) 3.E-04 Experimental Model Cure rate (dα/dt) (S -1 ) 2.E-04 2.E-04 1.E-04 5.E-05 3.E Degree of cure (α) (b) 93

108 CHAPTER FOUR RESULTS AND DISCUSSION 8.00E E-04 Experimental Model 6.00E-04 Cure rate (dα/dt) (S -1 ) 5.00E E E E E E Degree of cure (α) (c) 3.50E E-03 Experimental Model Cure rate (dα/dt) (S -1 ) 2.50E E E E E E Degree of cure (α) Fig.(4.13) Cure rate vs. Degree of cure for 13 phr hardener/resin ratio of DGEBA/TETA (d) system at: (a)30 C, (b) 45 C, (c) 60 C and 80 C 94

109 CHAPTER FOUR RESULTS AND DISCUSSION 5.00E E-05 Experimental Model Cure rate (dα/dt) (S-1) 3.00E E E E Degree of cure (α) (a) 2.E-04 1.E-04 experimental model 1.E-04 Cure rate (dα/dt) (S -1 ) 1.E-04 8.E-05 6.E-05 4.E-05 2.E-05 0.E Degree of cure (α) (b) 95

110 CHAPTER FOUR RESULTS AND DISCUSSION 4.E-04 4.E-04 Experimental Model 3.E-04 Cure rate (dα/dt) (S -1 ) 3.E-04 2.E-04 2.E-04 1.E-04 5.E-05 0.E Degree of cure (α) 2.E-03 2.E-03 1.E-03 (c) Experimental Model Cure rate (dα/dt) (S -1 ) 1.E-03 1.E-03 8.E-04 6.E-04 4.E-04 2.E-04 0.E Degree of cure (α) (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 96

111 CHAPTER FOUR RESULTS AND DISCUSSION -5-6 Experimental Linear fit -7 ln (k 1 ) (sec -1 ) / T (K -1 ) (a) -5 Experimental Linear fit -5.5 ln(k2) (sec -1 ) / T (K -1 ) (b) Fig. (4.15) Rate constant in equation (2.14) as a function of Temperature for 13 phr of DGEBA/TETA system: (a) k 1 and (b) k 2 97

112 CHAPTER FOUR RESULTS AND DISCUSSION -5-6 Experimental Linear fit ln(k 1 ) (sec -1 ) / T (K -1 ) (a) -5 Experimental Linear fit -5.5 ln(k 2 ) (sec -1 ) / T (K -1 ) (b) Fig. (4.16) Rate constant in equation (2.14) as a function of Temperature for 20 phr of DGEBA/TETA system: (a) k 1 and (b) k 2 98

113 CHAPTER FOUR RESULTS AND DISCUSSION 4.3 Isothermal Scanning Rheological Cure Analysis Gel Time and Apparent Activation Energy (Ea) During the isothermal reaction, a phenomenon of critical importance can occur, which is gelation. 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, 99

114 CHAPTER FOUR RESULTS AND DISCUSSION 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 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 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. 100

115 CHAPTER FOUR RESULTS AND DISCUSSION Table (4.6) Gel Time for the 13 phr hardener/resin ratio at different Temperatures and the Activation Energy Temperature ( C) t gel (sec) E a (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) t gel (sec) E a (KJ/mol) SE (KJ/mol)

116 CHAPTER FOUR RESULTS AND DISCUSSION Experimental Linera fit ln(tgel) (sec -1 ) / T (K -1 ) Fig. (4.17) Gel time as a function of isothermal cure temperature for 13 phr hardener/resin ratio 10 9 Experimental Linear fit ln(t gel ) (sec -1 ) / T (K -1 ) Fig. (4.18) Gel time as a function of isothermal cure temperature for 20 phr hardener/resin ratio 102

117 CHAPTER FOUR RESULTS AND DISCUSSION Viscosity Modeling 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). 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 t c and final viscosity η. All the parameters η o, η, t c 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 t c 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). 103

118 CHAPTER FOUR RESULTS AND DISCUSSION 9.00E E E+06 experimental model Viscosity (mpa.s) 6.00E E E E E E E E Time (sec) (a) 1.40E E+07 Experimental Model 1.00E+07 Viscosity (mpa.s) 8.00E E E E E E Time (sec) (b) 104

119 CHAPTER FOUR RESULTS AND DISCUSSION 1.20E E+07 Experimental Model Viscosity (mpa.s) 8.00E E E E E E Time (sec) (c) 1.10E E+05 Experimental Model Viscosity (mpa.s) 7.00E E E E E Time (sec) (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 105

120 CHAPTER FOUR RESULTS AND DISCUSSION 9.00E+05 Experimental Model Viscosity (mpa.s) 7.00E E E E E Time (sec) (a) 1.10E E+05 Experimental Model Viscosity (mpa.s) 7.00E E E E E Time (sec) (b) 106

121 CHAPTER FOUR RESULTS AND DISCUSSION 1.20E E+06 Experimental Model viscocity (mpa.s) 8.00E E E E E E Time (sec) (c) 1.10E E+05 Experimental Model Viscosity (mpa.s) 7.00E E E E E Time (sec) 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 (d) 107

122 CHAPTER FOUR RESULTS AND DISCUSSION 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 and KJ/mol, respectively. Also, for the 20 phr hardener/resin ratio and 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) t c (sec) SE (sec) K (sec -1-2 ) 10 SE(sec -1 ) Pre-exponential factor (sec -1 ) At= SE (sec -1-8 ) Activation energy (KJ/mol) E t = SE (KJ/mol)

123 CHAPTER FOUR RESULTS AND DISCUSSION 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) t c (sec) SE (sec) K(sec -1-2 ) 10 SE(sec -1 ) Pre-exponential factor (sec -1 ) At= SE (sec ) Activation energy (KJ/mol) E t = SE (KJ/mol)

124 CHAPTER FOUR RESULTS AND DISCUSSION Experimental Linear fit ln(t c ) (sec) / T (K -1 ) Fig. (4.21) Critical time versus Isothermal cure temperature of 13 phr hardener/resin ratio of DGEBA/TETA system 10 9 Experimental Linear fit 8 ln(t c ) (sec) / T (K -1 ) Fig. (4.22) Critical time versud Isothermal cure temperature of 20 phr hardener/resin ratio of DGEBA/TETA system 110

125 CHAPTER FOUR RESULTS AND DISCUSSION -1 Experimental Linear fit -2 ln(k) (sec -1 ) / T (k -1 ) Fig. (4.23) Rate Constant in equation (2.33) versus Isothermal Cure Temperature for the 13 phr hardener/resin ratio 1 0 Experimental Linear fit -1 ln(k) (sec -1 ) / T (K -1 ) Fig. (4.24) Rate Constant in equation (2.33) vs. Isothermal Cure Temperature for the 20 phr hardener/resin ratio 111

126 CHAPTER FOUR RESULTS AND DISCUSSION 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]. 1.4E E E phr at 45 C 13 phr at 60 C 13 phr at 80 C Viscosity (mpa.s) 8.0E E E E E E Time (min) Fig. (4.25) Viscosity versus cure time for 13phr of DGEBA/TETA system at 45, 60 and 80 C 112

127 CHAPTER FOUR RESULTS AND DISCUSSION 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. viscocity (mpa.s) 1.20E E E E E E phr at 45 c 20 phr at 60 c 20 phr at 80 c 0.00E E Time (min) Fig. (4.26)Viscosity versus cure time for 20 phr DGEBA/TETA at 45, 60 and 80 C 113

128 CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS 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 C than the stoichiometic ratio (13 phr) of DGEBA/TETA system of 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. 114

129 CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS 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 k 1 and k 2 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. 6. The activation energies Ea1 and E a2 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. 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 115

130 CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS 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. 116

131 CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS 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). 117

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147 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 146( grams / mol) = 24.3grams / equivalent 6( equivalents / mol) 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/ grams of TETA per hundred grams of DGEBA.

148 ) الخلاصة باستخدام الفحوصات الميكانيكية التحليل الحراري و الريولوجي الخواص الميكانيكية حركيات الا نضاج rheological analysis) kinetics) ( cure و الريولوجي لراتنج الايبوكسي المحضر من تفاعل (DGEBA) مع مصلدين مختلفين (TETA) و (DDM) قد تم دراستها باستخدا م نسب مختلفة من المصلد/الراتنج (التكافو (stoichiometry) تحت التكافو و فوق التكافو ). قوة الصدمة قوة السحب الصلادة قوة متانة الانحناء قوة الانضغاط و قوة الانحناء تم قياسها من خلال استخدام اجهزة الفحوصات الميكانيكية علما الفحوصات الميكانيكية لنظام " ان الفح وصات تم اجراءها عند درجة حرارة الغرفة. (DGEBA/TETA) اجراو ها تم باستخدام اربع نسب من المصلد /الراتنج 10) & (20 phr و لنظام (DGEBA/DDM) ا يضا" باستخدام اربع نسب 24) & phr (34 نتاي ج الفحوصات اظهرت بان نسبة فوق التكافو (15 (phr لنظام ) (DGEBA/TETA و (30 (phr لنظام (DGEBA/DDM) ا عطت ا فضل الخواص الميكانيكية ميكانيكية ا فضل من نظام.(DGEBA/TETA). بينما ا ظهر نظام (DGEBA/DDM) صفات باستخدام جهاز المسح التفاضلي (DSC) تم عمل الفحوصات الديناميكية و الفحوصات بثبوت درجة الحرارة (isothermal) لثلاثة نسب من المصلد /الراتنج.phr (20 & 13 5) كما تم دراسة حركيات الانضاج لنفس النسب و لاربع درجات حرارية (30 & ) م. عملية الانضاج بثبوت درجة الحرارة تم محاكاتها باستخدام موديل رياضي يحتوي على ست محددات بضمنها عامل الانتشا ر( factor (diffusion و هو موديل كمال المعدل. و قد وجد ا ن هنالك تطابق المراحل المبكرة و المتاخرة من عملية الانضاج التام (1 =α) عند درجة حرارة 80 م. ال لزوجة جدا جيدا ا بين الموديل المقترح و النتاي ج العملية في. كما اظهرت النتاي ج بان نسبة التكافو تصل درجة النضوج لنظام (η) (DGEBA/TETA) تم قياسها من خلال عملية الانضاج باستخدام جهاز بروكفيلد للزوجة Viscometer) (Brookfield و لا ربع درجات حرارية (30 & ) م.الفحوصات تم اجراو ها لثلاث نسب من المصلد /الراتنج.phr (20 & 13 5) زمن تشكل المادة الهلامية (gel time)

149 R R (trgelr) تم حسابه لكل نسبة من نسب المصلد /الراتنج باستخدام نتاي ج اللزوجة العملية. النتاي ج ا ظهرت ا ن زمن تشكل المادة الهلامية يتناقص مع زيادة درجة حرارة الانضاج لكل نسبة من نسب المصلد /الراتنج. منحنيات اللزوجة تم محاكاتها بواسطة موديل رياضي مبني على دالة بولتزمان function) (Boltzmann و قد وجد ا ن هنالك ا تطابق ممتازا بين النتاي ج العملية و الموديل المقترح.

150 جمهورية العراق وزارة التعليم العالي و البحث العلمي الجامعة التكنولوجية قسم الهندسة الكيمياوية دراسة تا ثير المصلد لراتنج الايبوكسي على الخواص الميكانيكية رسالة مقدمة ا لى قسم الهندسة الكيمياوية في الجامعة التكنولوجية كجزء من متطلبات نيل درجة ماجستير علوم في الهندسة الكيمياوية من قبل مريم عماد عزيز (بكالوريوس في 2004) الهندسة الكيمياوية 2010