Epoxy hardener, also known as epoxy curing agent, can be defined as a substance able to participate in the chemical crosslink reaction between epoxy oligomers to achieve the polymerization process and provide the final epoxy film.  Reaction of hardener with epoxy resins results in hard thermoset materials.  It will stay in the final film and participate to its chemical and mechanical properties. There are two types of epoxy hardener that most widely used, polyamides and polyamines.

Polyamides epoxy hardeners are based on aliphatic chains connected via amide linkages. This unique backbone imparts toughness and substrate adhesion into the final form. The cured resin features high tensile, compression, and flexural strengths, while it is stiff, strong, and excellent in shock resistance. Polyamide curing agents also offer excellent water resistance in the final thermosets. Excellent water resistance, combined with toughness, make polyamide agents preferred choices in applications such as adhesives, primer coatings, and moisture barrier coatings.

Polyamines are made up of an organic molecule containing two or more amine groups. Polyamines are classified into aliphatic, cycloaliphatic, and aromatic polyamines according to the types of hydrocarbons involved. Aliphatic and cycloaliphatic amines in general are low viscous and have a low color impact on the film.  Cycloaliphatic amines are one of the most commonly used curing agents for their excellent mechanical properties and chemical resistance. Aromatic polyamines, on the other hand, have even greater chemical resistance, but  they are much darker in color and less environment friendly.