Thermoplastics:

A plastic material, which can be repeatedly melted-molded again and again by simple heating, is called a thermoplastic. Another name of thermoplastic material is thermoplastic polymer. Thermoplastics become soft and moldable on heating. They can then be recast in different shapes and cooled. On cooling the thermoplastic material hardens.

Polyethylene with a series of chains of (----CH2---CH2----)n and is obtained by polymerization of ethene.

Thermoplastic polymers require no further chemical processing before molding.

Thermosetting plastics:

A plastic material, which cannot be repeatedly melted-molded again, is called a thermosetting plastic. Another name of thermosetting plastic material is thermosetting polymer. Thermosetting plastics are long chain molecules, which have crosslinks to other long chains. In case of thermoplastics, the long chain molecules are held to other long chain molecules by Van Der Waal’s forces. When heated, these chains slip off from each other and the material melts. When cooled, the long chain molecules return back to be held together again. On the other hand, in case of thermosetting plastics, the cross-links prevent displacement of the polymer chains on heating. Thus, thermosetting plastics do not become soft on heating and they do not change their shape on heating. Bakelite is one example of a thermosetting plastic.

Thermoset polymers require a two-stage polymerization process. The first is done by the material supplier, which results in a linear chain polymer with partially reacted portions. The second is done by the molder, who controls final cross-linking. Short chains with many cross-links form rigid thermosets, while longer chains with fewer cross-links form more flexible thermosets. With all thermosets, the polymerization is permanent and irreversible.