Plastics are synthetic materials with a macromolecular structure that under certain temperature and pressure conditions can change shape permanently. There are two types of plastics: thermoplastics and thermosets.
Thermoplastics: These become malleable when heated, and can be molded and shaped into objects which become rigid when cooled. This process can theoretically be repeated several times, depending on the type of plastic.
Thermosets: With this group of plastics, after an initial phase of malleability caused by heating, they harden due to a three-dimensional cross-linking of the molecular chains. During the softening phase, the combined effect of heat and pressure makes them moldable, but if these plastics are heated after hardening, they decompose and burn.