Explain why, thermoplastics become soft on heating but thermosetting plastics do not become soft on heating. Draw labelled diagrams to illustrate your answer.

Thermoplastic materials are made of polymers which are linked by intermolecular forces, forming linear or branched structures. On the other hand, thermosetting plastics are those materials that are made by polymers joined together by chemical bonds, having a highly-crosslinked polymer structure. In case of thermoplastics, the shape hardens when it is cooled, but can be reshaped when heated up again. Polythene is a thermoplastic polymer. Its tangled polymer chains can uncoil and slide past each other, making it a flexible material. On the other hand, Thermosetting polymers, once moulded, cannot be softened when heated. They cannot be reshaped. Vulcanised rubber is a thermosetting polymer which is used to make tyres. Its polymer chains are joined together by cross-links, so they cannot slide past each other easily.

Thermoplastic polymers

Thermosetting polymer with cross links.