Hardener

A substance that initiates or speeds a resin's curing reaction without being consumed in stoichiometric amounts. In the GRP trade the organic peroxide added to polyester resin, usually MEKP, is called the catalyst, although strictly it acts as an initiator.

The proportion of resin to hardener, by weight or by volume, specified by the manufacturer for a two-part system. Because the components react stoichiometrically, deviating from the stated ratio leaves unreacted material in the joint and permanently degrades the cure; it must never be used as a way to speed or slow curing.

The chemical process by which a liquid resin or adhesive hardens into its final solid state through crosslinking, driven by a hardener, catalyst, heat, moisture or radiation depending on the chemistry. Full properties develop only when cure is complete, which can take considerably longer than the initial set.

A thermosetting resin based on molecules containing epoxide groups, cured by reaction with a hardener such as an amine or anhydride. Valued for strong adhesion, low cure shrinkage, chemical resistance and high mechanical performance, it is widely used in structural adhesives, laminating resins, coatings and composite matrices.

An adhesive or resin supplied as two separate components, typically a resin and a hardener, that begin to cure only once mixed. Two-part systems cure through the full depth of the joint without needing air or moisture, but demand accurate proportioning, thorough mixing and use within the pot life.