Epoxy hardener is a chemical substance that is used to cure or harden epoxy resins. It reacts with the resin to form a strong and durable polymer that has excellent adhesion, mechanical strength, and chemical resistance. Epoxy hardeners come in different types, including amine based, acid based, and anhydride based, and the choice depends on the application and desired properties of the final product. They are commonly used in coatings, adhesives, composite materials, and repair applications. Proper handling and mixing with epoxy resin is important to achieve optimal performance.