The group of thermosets, often also referred to as duromers, includes plastics that harden through a chemical reaction. They typically cure via polycondensation and form a tightly cross‑linked three‑dimensional network of macromolecules, which gives thermosets their high mechanical strength. This curing process, caused by structural changes in the molecule, is irreversible; in other words, the material cannot be remelted or reused.
In contrast to thermoplastics, thermosets cannot be melted, as they remain rigid up to their decomposition temperature. Examples of thermosets include phenolic resins, melamine resins, polyurethanes, and surface coatings for plastic components.