Common epoxy resins

The diglycidyl ether of bisphenol-A (EXjEBA), shown in Fig. 5.4, represents the most common type of epoxy resin. It is the product of the reaction between a large excess of epichlorohydrin (Fig. 5.5) with bisphenol-A (Fig. 5.6) with a stoichiometric amount of sodium hydroxide at about 65°C. The excess of epichlorohydrin is needed to limit the production of higher molecular weight products in fact DGEBA can react with bisphenol-A, generating 

DGEBA resins are available on the market with 0 n 14. The basic commercial version of these resins has a molecular weight of 380 while purified versions (n = 0) show molecular weights equal to 344. On the other hand, epoxy resins with higher molecular weight n = 1-14) can be produced by reducing the amount of epichlorohydrin and reacting under more alkaline conditions. 

By reacting epichlorohydrin with aromatic diamines, it is possible to manufacture epoxy resins with higher functionality than DGEBA or DGEiBF resins. As shown in Fig. 5.9, the structure of tetraglycidyl methylene dianiline epoxy, the most important glycidyl amine, is characterized by four epoxy 

7 General formula of diglycidyl ether of bisphenol-A epoxy (DGEBA). 

These epoxy resins can be manufactured by reacting epichlorohydrin with tetrakis(4-hydroxyphenyl) ethane. As shown in Fig. 5.10, as in the TGMDA epoxy resins, the polynuclear phenol epoxy resins have four epoxy rings in their molecule. 

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Cross-Linked Polymers. In the 1980s, not only glass and BPA-PC but also uv-curable cross-linked polymers, eg, epoxy resins, were used as substrate material for optical mass storage disks with laige diameters (12 in., 14 in.) (219). The epoxy resins consisted of compounds containing one or several highly reactive epoxy or hydroxyl groups. The common epoxy resins (EP) mainly are reaction products of bisphenol A and epichlorohydrin ... 

The most common epoxy resins are formed by the reaction of bisphenol A with epichlorohydrin. 

Because of the highly polar nature of common epoxy resins and their curatives, most formulations can be cured via a microwave environment. A further advantage is that the uncured material absorbs microwave energy more strongly than that which is cured, so the cure occurs uniformly within the material. 

TABLE 18.1 Permissible Exposure Limits (PELs) for Some Common Epoxy Resin System Chemicals (California/OSHA)1... 

The most common epoxy resins use a prepolymer made from bisphenol A and epichlorohydrin. 

Figure 3.4 illustrates the structures of the most common epoxy resins in use today. Two or more epoxide groups per molecule are required to form a cross-linked network. 

DGEBA diglycidyl ether of bisphenol-A, a common epoxy resin... 

Fig. 2. Chemical structures of common epoxy resins a) 2,2-Bis[4-(glyddyloxy)phenyl] propane (DGEBA) b) 3,4-Epoxycyclohexylmethyl-3,4-epoxycydohexanecarboxylate (ECC) c) DGEBA oligomer, n = 0.2 typically.

The solubility characteristics of two common epoxy resin types are given in Table 3.3. 

Most commonly, epoxy resin is used because the process lends itself to demanding applications which justily the expense. However, any of the thermosets can be, and are, used commensurate with the application requirements. 

Another class of widely used amine curatives for heat curing epoxy adhesives are the imidazoles. Imidazole (33), like 10, is a crystalline material which only shows appreciable solubility in common epoxy resins at elevated temperatures. There are many derivatives of 33 available, but improvements in room temperature latency and speed of cure, in addition to final adhesive properties, are always desired. 

Fig. 7. Comparison of relative properties of common epoxy resins. L, low M, medium H, high.

The most common epoxy resins are the reaction products of bisphenol A with excess epichlorhydrin and have the structure shown in Structure 9.5. Because the simple diepoxide... 

To improve the apparent lower clarity of emerald, fissures may be hidden by impregnating the stones with some more or less permanent colorless substances. Although colored oils and resins are also used to fill emeralds, these materials are not covered here. Effective fillers have refractive properties close to that of emerald. Such substances comprise oils (natural or synthetic), resins (natural or on the basis of natural resins), as well as glues or artificial resins. Fissure treatments have been practiced for a long time, and mention of these treatments reach back to Plinian times [29-38]. The identification of the nature of filler substances in emeralds started to become an issue when artificial resins, commonly epoxy resins, were used as fissure-filling substances. These substances are commercially available or consist of a mixture of commercially available substances. Therefore, their properties are well known and spectra readily available [39]. 

The most common epoxy resin is based on the diglycidyl ether of bisphenol A. Epoxy resins are sometimes esterified with fatty acids or modified with polyurethane intermediate chains for elastification. 

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