Epoxy adhesives comprise a liquid or a fusible solid containing epoxies groups, and a curing agent containing functional groups with which the epoxies groups combine to form a cross linked polymer. A type of so-called curing agent, of little importance in adhesive, is that which catalyses homo polymerization of the epoxy compound.
When the reaction between resin and hardener is capable of taking place at room temperature the two component are mixed immediately before use, but if the mixture is relatively inactive under ordinary storage conditions the components may be mixed together and marketed as a 'single package' adhesive. The 'B-stage' adhesive systems also comprise a single component, but in this case the resin and hardener have been reacted to an intermediate, but still fusible stage.
The curing that takes place when the resin and hardener are brought into intimate contact, for example be melting the solid under appropriate conditions, is an irreversible reaction that result in a thermoset resin. This reaction does not involve condensation polymerization and therefore no low-molecular weight substance such as water is split off. For this reason the shringkage that takes place during cure is slight, and this is one of the properties that makes epoxy resins especially valuable as adhesives. Furthermore, a wide range of end products having a correspondingly wide range of properties can be obtained by use of different resin/hardener adhesive systems.
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