Polymers are classified into four groups: thermoplastics, thermosets, elastomers and thermoplastic elastomers (TPEs).
Thermoplastics consist of polymer chains and are highly entangled. They are kept together by interactions between molecules such as van der Waals forces. Thermal effects and shear forces allow them to be processed by injection molding. When cooled down the material solidifies again. As this process is purely physical, it can be repeated at any time (example: polypropylene).
Thermosets such as epoxy resins are polymers that are tightly crosslinked via chemical bonds and do not melt. They maintain their crosslinked structure even at high temperatures and are characterized by high strength and stiffness. Under stress, they are subject to brittle fracture (example: epoxy resins).
Elastomers, often referred to as rubbers, are chemically crosslinked polymers. They are considerably less densely crosslinked than thermosets, leaving larger elastic areas between the individual cross-links. These areas are crucial to their elastic properties. Elastomers cannot be melted again. Once they have been crosslinked they will maintain their shape. Examples of elastomers are natural rubber or synthetic rubber such as EPDM and NBR.
Basically, thermoplastic elastomers can be divided into two types: reactor-made TPEs (e.g. TPA, TPU, and TPC) and TPE compounds (e.g. TPS or TPV). The properties of reactor-made TPEs are implemented in one polymer. The properties of TPE compounds result from mixing two polymers to form a so-called compound.