Epoxy-urethane reaction mechanism

Thermoset plastics are cross-linked by chemical reaction so that their molecules cannot slip by one anotber. They are rigid when cool and cannot be softened by the action of heat. If excessive heat is applied, thermoset plastics will degrade. Consequently, they are not weldable. Because of their chemical resistance, they cannot be solvent cemented. Thermoset plastics are usually joined by either adhesive bonding or mechanical fastening. Typical thermosetting plastics are epoxy, urethane, phenolic, and melamine formaldehyde. 

Therefore, in general, a thermal curing resin contains a radical curable function that is activated by light and a hardening agent Acrylic resins such as urethane acrylate and epoxy acrylate are mainly used for the radically curable resin, and an epoxy resin is mainly used for thermal curing. The reaction mechanism due to the heat and UV irradiatiOTi are shown in Fig. 7.16. 

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