Because the hardeners or curing agents become part of the finished structure, they are chosen to provide desired properties in the molded part. Epoxies have outstanding adhesive properties and are widely used in laminated structures. The cured resins have better resistance than polyesters to solvents and alkalies, but less resistance to acids. Phenolics, the oldest of the thermoset plastics, have excellent insulating properties and resistance to moisture. Chemical resistance is good, except to strong acids and alkalies.
Water absorption is low, and dielectric properties are excellent.
Glass is the most widely used reinforcing material in thermoset composites. All forms of glass fibers are produced in the standard E-glass reinforcement type. Cost of S-glass is considerably higher. Carbon fibers in composites can be long and continuous, or short and fragmented, and they can be directionally or randomly oriented. Milled fibers are the shortest carbon fibers used for reinforcement.