Epoxy resins – an introduction

After unsaturated polyester resins, epoxies are the most widely-used thermosetting resins used in reinforced plastics. Because of their superior properties epoxy resins are frequently used with high performance fibre reinforcement such as carbon.

Epoxy resins are characterised by their very good electrical properties and chemical resistance, good strength and low absorption of moisture.

They are versatile resins, offering particularly excellent resistance to corrosion (solvents, alkalis and some acids), high strength/weight ratio, dimensional stability and adhesion properties. They are linear polymers produced by condensing epichlorhydrin with bisphenol A. Other formulations are glycidyl esters (for vacuum impregnation, lamination and casting), glycidyl ethers of novolac resins, and brominated resins. They differ from polyesters and vinyl esters in that they do not contain any volatile monomer component. Different resins are produced by varying the ratios of the components.

The resins are relatively high in viscosity, so that they are usually moulded at temperatures in the region of 50-100°C, or dissolved in an inert solvent to reduce viscosity to a point at which lamination at room temperature becomes possible. Curing agents, also referred to as catalysts, hardeners or accelerators, are used, acting either by catalytic action or directly reacting with the resin.

With correct additives, epoxy resins can exhibit outstanding resistance to heat (some up too 290°C) and electrical insulation properties. They can be either liquid or solid form and can be formulated to cure either at room temperature of with the aid of heat.

Heat curing is more common for situations where maximum peformance is required. Epoxies generallly cure more slowly than other thermset resins. Cold-cure types are available, but performance is usually better when curing at 40-60°C.

Epoxies are frequently used in aerospace and defence, chemical plant and high performance automotive applications.