Explain why , thermoplastics become soft on heating but thermosetting plastics do not become soft on heating . Draw labelled diagrams to illustrate your answer .
“Thermoplastics can be softened by heat” but “thermosetting plastics cannot be softened by heat”.
Thermoplastics can be softened by heat but thermosetting plastics cannot be softened by heat. This is due to the difference in their structure. Both, thermoplastics and thermosetting plastics are made up of long chain molecules called polymers. In thermoplastics, the long polymer chains are not cross-linked with one another [Figure (a)]. Due to this, on heating, the individual polymer chains can slide over one another and thermoplastic material becomes soft and ultimately melts.
On the other hand, in thermosetting plastics, the long polymer chains are cross linked with one another [Figure (b)]. These cross-links prevent the displacement (or sliding) of individual polymer chains on being heated. Due to this, thermosetting plastics do not become soft on heating (or change their shape on heating) once they have been
set into a particular shape.
For example, polythene is a thermoplastic having linear polymer chains with no cross-linkages, so it becomes soft on heating. On the other hand, bakelite is a thermosetting plastic having long polymer chains connected through cross-links (or held strongly through cross-links), due to which it does not become soft on heating.
Hope it is clear