A separate factor that can also have an important effect on the end-product is the temperature of curing; in certain cases the reaction between resin and hardener will not take place at all, or only to a limited extent, unless heat is applied. At the same time complete cure, that is to say theoretically complete cross linking, is virtually impossible because of strict restrictions in the growing macromolecule.
Theoretically, a stoichiometric and therefore calculable amount of hardener is required, but the amount giving optimum properties may be greater or less than this. To achieve the optimum in one respect may mean downgrading in another, hence the “correct” amount of hardener is an arbitrary quantity that depends to some extent on what is required of the cured polymer; this is in some ways an advantage.
The range of chemical substances that react and harden bisphenol A resins, as distinct from those that catalyze homopolymerisation, is wide, and consist primarily of compounds containing active and alkalies, are numerous and include a number of other synthetic resins, such as melamine, urea and phenolic resins. In adhesive applications the most valuable hardeners and phenolic resins.
In the polyamine class both aromatic and aliphatic are important, in the anhydride class both aromatic and aliphatic are important, in the anhydride class aromatic and cycloaliphatic are more important than aliphatic. Another class of hardener that has a limited importance in adhesive comprises the boron triflouride complexes. Each class of hardener is relation to the curing of bisphenol A resin will now be discussed separately.