Resins with internal epoxy groups

While they are called resins, they are really monomers to low-molecular-weight oligomers, liquids to soluble fusible solids, with high reactivity in the epoxy rings, and fair reactivity in the internal hydroxyl groups. They are cured most often by room-temperature reaction with polyamines or polyamide amines. 

Classical combination of diepoxide monomers with diamine counterparts in conventional Araldite-type adhesives involves terminal oxirane groups, which are considerably more reactive than the internal counterparts present in epoxidised oils or fatty acids. This difference is probably why studies on the potential of coupling epoxidised oils and diamine for the development of epoxy resins based on renewable resources have been published. 

An example is Bisphenol-Epichlorohydrin Epoxy Prepolymer, in which the R groups may H, methyl, or ethyl, and n = 1 to 10. These are very miscible with epoxy resins, they provide non-migrating internal plasticization after curing, and they are useful in coatings, adhesives, and sealants. 

For raw materials of reactive adhesives, the chemical equivalent values are very important to specify the reactivity, and often tested. Epoxy equivalents for epoxy resins, and functional group equivalents for hardeners are experimentally determined. Methods to measure the epoxy equivalents are described by industrial standards as follows International organization for standardization (ISO) 3001, Japan industrial standards (JIS) K 7236, and American society for testing and materials (ASTM) D 1652. For instance, JIS K 7236 indicates the methods using titration. In this process, an epoxy sample is solved with chloroform, mixed with acetic acid, tetraethylammonium bromide, and crystal violet solution. The epoxy sample solution is finally titrated with perchloric acid—acetic acid solution. If automatic titrators can be used, the epoxy... 

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