The types of hardeners that can be used for epoxy resins include aromatic amines, aliphatic amines, cycloalipathic amines, novolacs, anhydrides, polyamides, polyamido-amines, and polymercaptans (thiols). An indication of the market shares for the various type of hardeners is provided below.
Multifunctional Amines
Multifunctional amines represent the largest segment based on offering high Heat Distortion Temperatures (HTDs >150 °C), high thermal and chemical resistance, excellent mechanical properties, and well-controlled cure rates. Because of the wide diversity of amine curatives available, the breadth of performance imparted by amine hardeners is unmatched. Their major industry segments include composites, heavy-duty flooring, syntactic foam, and other performance applications.
Aliphatic Amines
Aliphatic amines offer faster cure rates and lower cure temperatures. Aliphatic amine curatives include polyether amines, ethyleneamines such as DETA, TETA, etc, and cycloaliphatic amines such as PACM – H12MDI. Exemplary aromatic amines include DETDA, MBOEA, and other substituted methylene dianiline derivatives.
Novolac Resins
Novolac resins react with epoxy resins at higher temperatures in the presence of a catalyst to form ether linkages. The resulting structures display excellent thermo-oxidative and chemical resistance vs. epoxies cured with amines or anhydrides. This class of hardeners is used in powder coatings as many novolacs are solids.
Anhydrides
Anhydrides are low-viscosity hardeners exhibiting high latency and low exotherms in epoxy cure, and excellent mechanical and dielectric characteristics. This makes anhydride cured epoxy resins suitable for applications such as potting and bonding of electrical components, high voltage transformers, switches, resistors, and capacitors, intergrade circuit impregnation, and casting. Other applications are in fiberglass-reinforced parts. Methyltetrahydrophthlic anhydride (MTHPA) is an exemplary anhydride hardener.
Polyamides
Polyamides are primarily used in coatings and adhesives where they provide good adhesive characteristics, toughness, flexibility, chemical and water resistance, and longer pot lives. Polyamide hardeners are amine functional, produced by the condensation of di- and polybasic acids with diamines. Applications include anticorrosion coatings, for example, with storage tank linings and structural adhesives.
Mercaptan
Mercaptan end-functional curatives afford fast cure times. Gel times can be under one minute at ambient temperatures. Thiol cured epoxy resins are known for excellent resistance to acids, bases solvents, and good transparency. Applications include coatings, adhesives, sealants, patching compounds, cementitious repair, and road marking paints.