The thermal behavior of polymers is a very interesting question that involves the chemical composition of these materials. To make a test it would be very simple: place a plastic children's toy (ball or doll) under the blazing sun, what happens? It acquires a smooth consistency as if it were melting isn't it?
Thermoplastic polymers are composed of long linear or branched strands. The advantage of this material is in remoulding, as these plastics can be recycled several times.
Thermosets, on the other hand, as the name implies, have a more rigid structure, everything is explained by the structure that composes them: cross-links unite the polymer threads. During the preparation of this type of plastic, it is heated to form fixed bridges in the polymeric structure.
Bakelite is an example of thermoset plastic, it is used to compose frying pan handles because it is hard, resistant and non-conductive (the handle does not heat up in a fire).
By Líria Alves
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/polimeros-termoplasticos-termofixos.htm