These are polymers, which on heating change irreversible into hard and rigid materials.
The melt of this polymer when set into a mould to form an article, is almost a permanent set.
On reheating the article does not soften again, thus inhibiting irreversibility.
They are hence known as thermo hardening plastics or permanent setting resins and during moulding acquire three dimensional cross linked structure with strong covalent bonds.
On reheating, these bonds retain their strength and hence such a plastic does not soften on reheating.
Thermo setting plastics can not be reclaimed from waste due to their irreversibility.
They are hard, strong and brittle than thermo plastics.
The method by which these are form is called condensation polymerization.
They are insoluble in almost all organic solvents, due to their cross linked three dimensional structure.
Phenol formaldehyde/Bakelite ,amino plastics, alkyl plastics, epoxy plastics, silicon plastics etc. are the best known examples of thermosets