Thermoset adhesives are thermosetting polymers, or plastics, that are used as adhesives. They’re supplied in an uncured state consisting of unlinked monomers. A chemical hardener may be used to cause hardening or curing may be induced by the application of some other stimulus, such as heat or light.
Thermoset adhesives have very high strength, excellent gap filling ability, and resistance to moisture and heat. Examples include epoxy and polyester resins.
Most thermoset adhesives are supplied as a two-component system although one-part adhesives are used as well. Two-component adhesives are typically made up of a resin and a hardener, in liquid or gel form, which are mixed to initiate the curing process.
Curing typically takes between 10 minutes and a few hours. Two-component adhesives often provide quality bonds, easy storage, and relatively simple processing.
One-component systems are usually supplied as pastes, which are heat cured and must be cold-stored to delay curing. One-part resins are also used in pre-impregnated (prepreg) composites. For certain applications, one-component systems can offer process advantages.
The most widely used class of thermoset adhesives are epoxies. These offer several distinct advantages in different applications. When used to bond wood, epoxy provides better moisture resistance and gap-filling that alternatives such as polyurethane adhesives. Epoxy resin is also unusual in that it creates strong bonds on metals and glass. Unmodified epoxy is a hard and brittle solid. To improve toughness, flexibility and fatigue resistance, modifiers are added.
Other thermoset adhesives include:
- Polyester resin is often used as a lower-cost alternative to epoxy resin for composite structures, especially fiber-glass. It does not cure well as a thin film, limiting its use as an adhesive to gap-filling applications. Bond strength is lower than epoxy, it is less resistant to moisture and it off-gasses VOCs.
- Phenol formaldehyde resins (PF) or phenolic resins are one of the earliest forms of plastic. They are typically used as an adhesive for laminating materials such as paper and wood, in, particular, plywood for external use.
- Polyimide adhesives can resist very high temperatures of up to 500° C (930° F) and are sometimes used to bond metal aerospace components.