Bisphenol A-epichlorohydrin based epoxy

RA928 (difunctional bisphenol A/epichlorohydrin-based epoxy) 3.13... 

Figure 5.1 Typical epoxy resin and hardening reaction using a primary diamine hardener, (a) A bisphenol A-epichlorohydrin based epoxy resin. The most important type is when n 0.2 and the liquid resin is essentially (b) a simple diglycidyl ether of bisphenol A (DGEBA). (c) A primary diamine hardener, (d) a crosslinked, thermosetting epoxy adhesive.

Compare the properties of epoxy resins based on bisphenol A-epichlorohydrin with those based on epoxides that can be made without the use of epichlorohydrin. 

History The first commercial epoxies were prepared in 1927 in the USA by reacting epichlorohydrin with bisphenol A. The name epoxy is based on the Greek words epi, which means upon and oxy, which means sharp/acidic. 

Liquid epoxy resin based on the reaction product of epichlorohydrin and bisphenol A or bisphenol F mild to moderate irritants mild to moderate sensitizers low volatility, exposure unlikely unless heated, sprayed, or spread over large unventilated surface low toxicity... 

This reaction is quite general and, since the organic group R can be aliphatic, cycloaliphatic, or aromatic, there is wide scope for variation in the composition of epoxy resins. In practice, however, the most frequently used materials are those based on bisphenol A and epichlorohydrin, which represent over 80% of commercial resins. 

Virtually all epoxy resins are made with starting materials based on epichlorohydrin, principally by reaction with bisphenol A. It is also used for the preparation of resins to increase the wet strength of paper. Both epichlorohydrin and glycidol are used in the manufacture of pharmaceuticals. 

Synthesis Epoxy resins consisting ofglycidyl ether, ester and amines are generally prepared by the condensation reaction between diol, dibasic acid or amine and epichlorohydrin in the presence of sodium hydroxide with the elimination of hydrochloric acid. The commercially available epoxy resins are, however, made by the reaction of epichlorohydrin and bisphenol-A. Cashew nut shell liquid (CNSL)-based novolac epoxy resins have also been reported [342]. 

A very useful group of adhesives and plastics is based on condensation polymers of bisphenol A and chloromethyloxacyclopropane (epichlorohydrin, CH2—CHCH2C1). The first step in the formation of epoxy resins is to form a... 

Epoxy resins, which are used as adhesives, are also thermoset polymers that form by cross-linking when the two components of the resin are mixed. One component is a low-molecular-mass linear polymer formed by the reaction of the conjugate base of bisphenol A with epichlorohydrin. The nucleophilic oxygens of the phenolate dianion can either displace the chlorine or open the epoxide ring of epichlorohydrin. A slight excess of epichlorohydrin is used to keep these polymer chains short and to ensure that the linear molecules have epoxide groups at their ends. 

Bisphenol F based epoxy resins. Instead of reacting bisphenol A with epichlorohydrin to form a liquid resin, a similar reaction can be conducted with bisphenol F (Figure 2.2). Bisphenol F is composed of a mixture of isomers - ortho-ortho, ortho-para and para-para linkages -whereas bisphenol A is composed mainly of para-para linkages. If n is less than about 0.2, the resins are called bisphenol F epoxides. If n is higher they are referred to as epoxy phenol novolac (EPN) resins (Helfad, 1996). 

The most widely used epoxy resins and adhesives are based on a prepolymer made from bisphenol A and epichlorohydrin. On treatment with base under carefully controlled conditions, bisphenol A is converted into its anion, which acts as a nucleophile in an S142 reaction with epichlorohydrin. Each epichlorohydrin molecule can react with two molecules of bisphenol A, once by S 2 displacement of chloride ion and once by opening of the epoxide ring. At the same time, each bisphenol A molecule can react with two epichlorohydrins, leading to a long polymer chain. Each end of a prepolymer chain has an unreacted epoxy group, and each chain has numerous secondary alcohol groups. 

Other phenol derivatives that are used to form the backbone of an epoxy resin include bisphenol E, bisphenol F, resorcinol, brominated bisphenols, and more highly functionalized molecules such as tetrakisphenylolethane. Alcohols, amines, and carboxylic acids may be combined with epichlorohydrin to give a range of diglycidyl ether based epoxy resins. Non-aromatic, commercially available epoxides are produced by peracid epoxidation of alkenes and dienes, such as vinyl cyclohexene and esters of cyclohexane carboxylic acids [23]. The chemical formulas for two common uncured polymers are shown below ... 

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