4.2d Plastics

Structure of thermoplastics

Thermoplastics are linear chain molecules, sometimes with side bonding of the molecules but with weak secondary bonds between the chains. Between the long chain molecules are secondary bonds which are weak forces of attraction between the molecules.

In a thermoplastic the long chains of molecules are held to each other with weak secondary bonds. When heated the heat energy overcomes these weak bonds and the material softens, allowing it to be reshaped. When cooled the bonds reform. This process can be repeated almost indefinitely and makes thermoplastics relatively easy to recycle.

To summarise: Thermoplastics can be heated and reformed. Their polymer chains do not form cross links. Thus, the chains can move freely each time the plastics are heated.

Structure of thermosetting plastics

Thermosets are linear chain molecules but with strong primary bonds between adjacent polymer chains (or cross links). This gives thermosets a rigid 3D structure.

On first heating, the polymer softens and can be moulded into shape under pressure. However, the heat triggers a chemical reaction in which the molecules become permanently locked together. As a result the polymer becomes permanently ‘set’ and cannot be softened again by heating. Examples of thermosetting plastics are polyurethane, urea formaldehyde, melamine resin and epoxy resin.

To summarise: In a thermoset the long chains of molecules are joined to other chains with strong, primary cross-links which gives a rigid 3D structure. When heated these plastics do not soften as there is not sufficient heat energy to overcome covalent bonds. This makes them useful for applications where heat resistance is important i.e. saucepan handles, kitchen work tops and electrical fittings like plugs, sockets and light housings. However too much heat will cause combustion.