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CHAPTER I Introduction 1.1 Thermosets Thermosetting Ph.D. Thesis Savitribai Phule Rune University CHAPTER I Introduction 1.1 Thermosets Thermosetting polymers/resins are pre-polymers in a soft solid or viscous liquid state. They change irreversibly into an infusible, insoluble three dimensional polymer network of covalent bonds as a result of a chemical reaction taking place in presence of a curing agent/hardener. The resultant structure is known as a thermosetting plastic or thermoset. They can be heated and shaped only once. If re-heated they cannot soften as polymer chains are interlinked. Thus, these polymers are synthesized as a final material of desired shape (polymerization and final shaping is done in the same process or mould is the true chemical reactor). They offer high thermal stability, good rigidity, hardness, and resistance to creep [1]. A good solvent can swell the thermoset but cannot dissolve it. Thermosets are usually amorphous (crosslinks inhibit the movement of polymer chains to pack or crystallize and restrict ordering of network structure) in nature. Thermosets do often exhibit high glass transition temperatures. Their glass transition temperature is higher than the temperature at which they are used (Tuse), therefore, they behave as glasses during their use [2]. The main thermosetting resins are epoxy, melamine formaldehyde, polyester, polyurethane and urea formaldehyde. Thermosets find applications where thermoplasfics cannot compete because of their properties and cost. When fire resistance is required, phenolics are the first choice. Urea formaldehyde is the material used as an adhesive for wood. Melamine formaldehyde is an excellent quality and low cost material used for furniture coating. Polyester resin is used to make fiber reinforced structural parts and epoxy resins constitute a very important class of thermosets used in aeronautical applications. The annual global market for different thermosets can be represented by a Pie chart as shown in the Figure 1.1 [3]. Jyoti A. Pandit Page 1 Ph.D. Thesis Savitribai Phule Pune University % Market Share • Polvurethane • Urea-Fomuldehyde • Phenolic resins 1 T • Unsaturated Polyester L 1^^"^^^ ^ • Epoxies % JL—^ • amino resins V ^^^^ Figure 1.1: Global market for different thermosets 1.2 History and Global Market of Epoxy Resin. Epoxy was first synthesized by Pierre Castan of Switzerland and S.O. Greenlee of United States in late 1930s. The epoxy resin was first marketed by CIBA Company in 1946. By late 1960 almost 25 types of epoxies were available. The present and fiiture global epoxy market by Zion research analysis is as shown in the Figure 1.2. Global bpoxy Kesiiis Market, 2014 - 2020 (Million Tons) (USD Billion) I unTfl2014 2015 2016 2017 2018 2019 2020 i i^"Volume -•-Revenue Sow«;ZionRM*afch An*lysii 2015 Figure 1.2: The present and future global epoxy market. Jyoti A. Pandit Page 2 Ph.D. Thesis Savitribai Phule Pune University Epoxy resins find applications in areas like coatings, composites, electrical and electronic insulation, wind turbines, construction and adhesives. The global demand of epoxy resin is represented as a pie chart (Figure 1.3) on the basis of applications by different sectors [3]. Figure 1.3: The global demand of epoxy resin by different sectors. (PAC, paint, adhesive, cements; AT, automotive and transportation; BC, building and civil engineering; E, electricity and electronics; F, furniture; C, consumer goods; MI, mechanical and industrial; P, packaging; O, others) The consumption of epoxy resin by different countries (by HIS market) is presented by a pie chart in Figure 1.4. China is the leading consumer of epoxy resin in the world. Jyoti A. Pandit Page 3 Ph.D. Thesis Savitribai Phule Pune University World Cor»sump>tion (xf Ep>oxy Resins 2013 Other Asia V Other Canada-14'^ livdia Certral'South America I^ntral' East«rr Europe Taiwan Chiria Japan MiMeEast Rep. of Koreai Western Europe Figure 1.4: The consumption of epoxy resin by different countries 1.3 Properties of Epoxy Resin Epoxy resins exhibit outstanding adhesion to various substrates due to the presence of polar hydroxyl groups and ether Hnkages. They show low shrinkage upon cure as epoxy molecule has a rather small reorientation during the curing process as compared with other polymers and as a result, contact between the epoxy resin and the substrate is not disturbed by tensions. The surface tension of epoxy is most often less than the critical surface energy for most materials which also result in good adhesion. Mechanical properties of epoxy are better in comparison to other plastics due to its low shrinkage upon cure. Epoxy resin shows high corrosion and chemical resistance especially to alkali. They have good electrical insulating properties; their volume resistivity is normally lO'^ Q cm"'. Epoxies with high aromatic content are sensitive to light in the UV range. Direct irradiation with ultraviolet light quickly causes yellowing. Even normal sunlight contains enough ultraviolet radiation for yellowing to occur. Aliphatic epoxy resins with anhydride or amine hardener are U.V. resistant. They have ability to be processed under a variety of conditions. Epoxy Jyoti A. Pandit Page 4 Ph.D. Thesis Savitribai Phule Pune University composites provide high strength to weight ratios, good dimensional stability and good moisture as well as thermal resistance. Electrical properties of epoxy resin Epoxy is electrically insulating in nature. Considering insulating properties of epoxy, this thesis focuses on the improvement in this property. Inducing electrical conductivity in such a versatile material would extend its application potential and will combine electrical properties of metals with advantages of polymers such as light weight, resistance to corrosion and chemicals, and lower cost. If electricity is induced in epoxy resin it can find applications such as protection of aircraft against lightning strike and electromagnetic interference of aircraft as epoxy is used as an adhesive and matrix material in the aerospace and automotive industry. Electrically conductive epoxy also finds application in fire retardant anfistatic regulation for mining equipments and storage of oil and gas and their transportation [4]. Electrically conductive epoxy adhesives are widely used in electronic packaging applications [5]. 1.4 Chemistry of Epoxy Resin Epoxy resins contain an oxirane ring in their structure [Figure 1.5]. Diglycidyl ether of bisphenol A (DGEBA) is the most common type of epoxy resin. It has glycidyl group in it [Figure 1.5]. -CH2— Oxirane ring Glycidyl group Figure 1.5: Oxirane ring and glycidyl group. Jyoti A. Pandit Page 5 Ph.D. Thesis Savitribai Phule Pune University DGEBA is synthesized by condensation reaction of bisphenol A and epichlorohydrin in presence of a base such as NaOH, as presented in Figure 1.6. An epoxy resin system requires two components, a diepoxide or equivalent (epichlorohydrin is equivalent to a diepoxide since another epoxide group is formed upon destruction of the first one) and a reactive diol. Epichlorohydrin is the most readily available and cheapest diepoxide equivalent. Bisphenol A is a reactive diol, widely used as its aromatic nature enhances the hydroxyl reactivity and adds strength to the resin formed. DGEBA has two terminal epoxide groups. 0 CH3 /\ NaOH OH -fyA-\j—0E__ + 2CICH2 ^ ^ • CH3 Bisphenol A Epichlorohydrin I CH3 -HCl ClCH2CHCH20-^-C—Q-OCH2CHCH2CI OH CH3 OH CH3 CH3 0 _ / \ ^^CH^fo^Q-C-QoCH^CHCH^ ^0©—?—QoCH^"^^ CH3 L CH3 Diglycidyl ether of bisphenol A (DGEBA) Figure 1.6: Reaction of bisphenol A and epichlorohydrin giving diepoxide Jyoti A. Pandit Page 6 Ph.D. Thesis Savitribai Phule Rune University The reaction involves nucleophilic attack of phenolic -OH on the least substituted carbon of the epoxide as shown in the Figure 1.7. Internal ring closure reforms the epoxide ring. Figure 1.7: Nucleophilic attack of phenolic -OH on epoxy ring Epoxy resins are available in two broad catagories: solid resins, which are usually used in coating applications and their degree of polymerisation i.e. n value is between 1 to 20. The other variety is available in liquid form having n value 0 /I. The commercial version of resin is the one having molecular weight of 380 ( n = 0). They find applications in structural plastics (reinforced plastics, adhesives and casting). Pure DGEBA is a solid (m.p. 43 °C). The amount of epoxide groups present in the resin is usually expressed as the weight per epoxide (WPE) or epoxide equivalent [6]. Epoxide equivalent is defined as the weight of the resin in grams which contain one gram equivalent of epoxide. For unbranched diepoxide, the WPE will therefore be half the number average molecular weight.The amount of epoxide present in the resin can also be expressed as 'epoxide content'. It represnts amount of epoxide present as equivalents per kilograms of resin. Epoxide content is reciprocal of WPE, multiplied by 1000. The other epoxy resin types are novolacs, peracid resins, hydantoin etc. 1.5 Curing Agents for Epoxy Resin The low molecular weight, viscous liquid epoxy resins find applicafions when cured with appropriate curing agent (hardener). In most of the applicafions of epoxide Jyoti A. Pandit Page 7 Ph.D. Thesis Savitribai Phule Pune University resins, they are converted into a three dimensional infiisible network held together by covalent bonds, by initiating a cross linking reaction. In context to epoxy resin technology, this conversion from liquid or friable brittle solid into a tough cross- linked polymer is known as curing or hardening. The curing agents are broadly of two types viz. polyfiinctional and catalytic. 1.5.1 Polyfunctional curing agents Polyfiinctional curing agents have fiinctional groups that can react with glycidyl type of epoxide. On curing, a three dimensional network structure is obtained. They are used in stoichiometric or near-stoichiometric amounts. The cross linking is facilitated by opening of the epoxide ring usually through ionic mechanism. They cure usually by polyaddition reaction. Normally, no byproduct is formed in curing reaction and it is an exothermic process.
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