Materials that undergo a chemical crosslinking reaction going from liquid to solid or semi-solid. This reaction is irreversible. Typical thermosets are polyesters, acrylics, epoxies, and phenolics.
Materials that may not be reheated and softened again. Once the structural framework is set, these plastics cannot be reformed.
Resins or plastic compounds which in their final state as finished articles are substantially infusible and insoluble.