Second, the fine raw rubber granules are washed and the rubber is dried by vacuum filtration. Intial prepartion involves cutting up bales for the mixer and sometimes heating them. The mix is then shaped by extrusion or molding. Vulcanizing produces crosslinking of polymer chains. Without crosslinking, rubbers are thermoplastic and lack elasticity.
Elastomers are polymers with long flexible chains.
These chains are independant in the raw material or 'gum', which can be processed by the same methods as are employed for thermosetting plastics. Added 'vulcanising' agents introduce cross-linking on heating after, or in the later stages, of processing. Thermoplastic elastomers become fluid at elevated temperatures and can be processed by most of the same methods as themoplastics. The majority of rubbers used for engineering applications contain a filler, generally one of the many kinds of carbon black, which may comprise up to one-third of the total volume of vulcanisate. They can, however, be added in greater volumes than reinforcing blacks.