A polymer is a compound formed of repeating structural units called mers, whose atoms share electrons to form huge molecules. Polymers usually consist of carbon plus one or more other elements, such as hydrogen, nitrogen, oxygen, and chlorine.
Polymers are divided into three categories:
- thermoplastic polymers,
- thermosetting polymers, and
Thermoplastic polymers can be subjected to multiple heating and cooling cycles without substantially altering the molecular structure of the polymer. Conventional thermoplastics include polyethylene, polystyrene, polyvinylchloride, and nylon.
Thermosetting polymers chemically transform (cure) into a rigid structure on cooling from a heated plastic condition, hence the name thermosetting. Members of this type include phenolics, amino resins, and epoxies.
Although the name thermosetting is used, some of these polymers cure by mechanisms other than heating.
Elastomers are polymers that exhibit significant elastic behavior; hence the name elastomer. They include natural rubber, neoprene, silicone, and polyurethane.