Because of these advantages, one of the leading users of epoxies is the aerospace industry. Airliners rely on epoxy resins for structural parts as well as interior equipment. Epoxies are also commonly used in automobiles, sporting goods equipment, pipes and storage tanks, roads and bridges, electronic circuit boards and high-voltage electrical insulation.
Another promising market for epoxy is wind energy. As wind blades get increasingly longer – growing from 60 meters to 100 meters in length – they need to be stiffened so they don’t buckle or collapse on themselves. Many customers are turning to specially-formulated epoxy resins for high-speed production of structural components for large wind turbine blades.
Any discussion of epoxies must include mention of curatives. “Unlike some other chemistries, epoxy resins as a nomenclature only refers to about half of the total resin system because epoxies need a curing agent,” says Mishra. “The second component – the curing agent – is actually a significant portion of the final cured composite resin when compared with polyester or vinyl ester resins.” The choice of curing agent, he adds, determines how the epoxy resin system will be cured, how it will behave and what kinds of applications it will be best suited for.
Historically, most epoxies have been made from bisphenol A (BPA) with common curing agents including aliphatic amines and aromatic amines. However, aromatic amines – an organic compound comprising an aromatic ring attached to an amine – have faced some scrutiny because they can be hazardous. “Epoxy resins are relatively safe by themselves. It’s the curing agent that can change the safety rating of the system,” says Mishra. “So more and more people are looking at other amines, hydrides and other curing agents with low volatile organic compounds (VOCs).”
Increased demand for enhanced performance in composites also is leading end users to consider new combinations of epoxies and curing agents, including anhydrides, dicyandiamide and imidazole. For instance, Dixie Chemical recently developed a proprietary line of pre-catalyzed curing agents for epoxy pultrusions that offer long pot life, fast cures and high Tg. These anhydride curing agents, included in the company’s ECA 607 and NMA 407 products, open up opportunities for a variety of applications, including leaf springs, bridge pilings, cable stays and composite rebar.
“Epoxy anhydride systems are now rivaling the traditional line speeds of polyester and vinyl ester resin systems, thereby narrowing their traditional productivity gap while offering a non-styrene containing, higher performing system,” says Mishra. “Combined with the increased use of carbon fiber, such advancements are enabling the growth of advanced composite applications.”