2.1.2. Thermoset Polymers

These polymers possess a cross-linked network structure formed exclusively by a covalent bond. Based on their cross-link, thermosets are stiff and brittle materials but are stable at elevated temperatures and resistant to solvents and other chemicals [44]. Thermosets do not melt upon heating because the cross-link prevents the chains from sliding past each other. When heated, thermoset material softens under certain temperatures, not melting, but further heating will cause decomposition by breaking down the chain's covalent bonds. Unlike thermoplastics, thermosets are shaped by placing them into a mold. The chemical reaction is initiated to cause cross-links that cause the material to harden and take a permanent shape. The process of crosslinking is called curing. Thus, thermoset materials become healed or set with thermal energy. The nature of the curing of a thermoset material is similar to baking a cake. The ingredients, which include polymer or monomer (that is capable of forming cross-links), colorants, curing agents, fillers, and other additives, are mixed and placed into the mold of the desired shape. The mixture is heated to crosslink and then cooled to facilitate removal from the mold. Thermoset is widely used to insulate electric wiring, while thermosets (which are thermally stable) are used for switches, circuit breakers, light fittings, and handles. Table 3 shows applications of common thermosets in electrical applications [45].

**Table 3.** Common thermoset polymer for electrical application.

