A thermoplastic polymer is in a solid (glassy) state at room temperature, but flows if heated above its glass transition temperature of about 100℃ (for molding or extrusion), and becomes solid again when cooled.
Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Examples include polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polytetrafluoroethylene (PTFE). The molecular weight of common thermoplastics range from 20000 to 500000 amu (unified atomic mass unit), while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as repeat units, derived from monomers; each polymer chain will have several thousand repeating units.
Thermosets can melt and take shape once. In the thermosetting process, a chemical reaction, cross linkage reaction, occurs that is irreversible.
The vulcanization of rubber is a thermosetting process. Before heating with sulfur, the polyisoprene is a tacky, slightly runny material, but after vulcanization the product is rigid and non-tacky. After they have solidified, they stay solid.