Having concluded the section dealing with hardeners, it is appropriate at this point to say a few words about circulating the stoichiometric amount of hardener to add to the resin.
With commercially available hardeners the recommendations of the resin manufacturer should be observed, but when it is desirable to evaluate a new hardening compound it is necessary to know something about the chemical structure of both resin and hardener in order to ascertain the proportions in which they should be mixed. Whereas the hardener is usually a substance of known chemical composition, the epoxy resin, so far as its epoxide content is concerned, is not. Therefore the epoxide equivalent of epoxide, must be determined experimentally, that viscosity and melting point give an indication of the value. Methods of finding the epoxide equivalent are available from number to react with all the epoxide equivalent are available from a number of source; in general they entail measuring the amount of acid required to react with all the epoxide groups.
Theoretically, the optimum ratio of hardener to resin should satisfy the requirement that one reactive hydrogen atom of the hardener is provided for each epoxide group of the resin. For example, if a diprimary amine (four reactive hydrogens) having a molecular weight of 100 is required to be used as hardener of a resin which has been found to have one gram equivalent of epoxide per 250 g, then the stoichiometric ratio of hardener to resin is 25:250.
The well-known technique of separate application of resin and hardener has not been overlooked, but it would not appear to be at all easy to obtain a satisfactory, much less a stoichiometric ratio without considerable care. The use of a catalytic hardener would, be a more reliable adaptation of this idea.