As all users of bisphenol A epoxy adhesives will know, the uncured (solid) resins have low mechanical strength, and by themselves are of little value simply as hot-melt adhesives. It is only when cured that they make really useful adhesives.
The valuable polymers are those by reaction with crosslinking substances, referred to as accelerators strength, curing agents or hardeners. Certain polymerization catalysts, in conjunction with curing agents, sometimes have a use in adhesives.
The normal method of crosslinking epoxy resins entails a resinification reaction whereby molecular growth takes place throught the linking of dissimilar molecues which, as in the case of the polyurethanes, involves a poly addition reaction with compounds having active hydrogen atoms.
There is, of course, no difference between the curing of an epoxy resin for adhesive use and any other application; nevertheless, one or two curing agents appear to be especially good for loading certain materials, in particular dicyandiamide for the bonding of certain metals.
If the curing agent is a substance of high molecular weight, or more correctly, if its equivalent weight is high with respect to the functional groups involved, a large mass of it will be required to cure the resin; the cured product will therefore contain a large percentage of it, perhaps more than one half. As a result, the mature of the curing agent itself will be built into the final resin molecule and contribute its own part towards physical (including mechanical) and chemical properties. Thus the subject of curing agents is at least as important as the subject of epoxy resins themselves.
As well as the physical properties of the curing agent, the number and the arrangement of its functional groups is important; on these depend the nature and speed of the reaction with the epoxide groups, the nature of the cured polymer, its hot-strength and so on. Equally of course, the epoxy compound itself has a similar influence. Collectively, a number of factors determine the properties of the cured resin; the functionalities of the epoxide and the hardener, the nature of the functional groups of the hardener, the overall structure of both the epoxy resin and the hardener molecules, and the ratio of hardener to resin.