Question

explain why thermoplastics become soft on heating while thermosetting plastics do not become soft on heating...

Answer 1

Thermoplastics become soft on heating because the arrangement of monomers in them are in straight line chain,whereas the arrangement of monomers in the thermosetting plastics is in a crossed link chain therefore the thermosetting plastics do not melt on heating.

Answer 2

“Thermoplastics can be softened by heat” but “thermosetting plastics cannot be softened by heat”.  

This reason behind this difference is the difference in the structure of molecule. The thermosetting plastics have irreversible chemical bonds in the structure which do not break or reform by heating whereas the thermoplastic have reversible bonding structure so they can change their ‘shape and size’ on heating.

As on heating the polymer chains will move easily over one another that makes the thermoplastic plastic soft on heating.