Classification

Two types of plastics are recognized. They are thermosetting and thermoplastics plastics. In general Plastics cannot stand higher temperatures. They becomes soft & lose their shapes at higher temperatures. Thermoplastics become gradually softer or their ability to deform plastically enhance as the temperature increases. They may be given any shape by moulding any number of times. Alternatively thermosetting plastics do not soften at higher temperature but lose their shape and might eventually burn or disintegrate. Therefore thermosetting plastics can be moulded only once. Both of the plastics might be machined or cut.

Structure of both plastics, being polymeric, having long chain molecules which are tangled as illustrated in Figure. Thermoplastics either carry very week cross linking bonds or no cross linkages between chains. Alternatively thermosetting plastics carry strong cross linking bonds & some unfinished polymeric reaction. At higher temperatures these incomplete polymeric reactions are completed and cross linking helps in distortion instead of melting.

                                      a) Thermoplastic                                                    (b) Thermosetting

                                                                    Figure: Two Types of Plastics