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Most plastics today are produced from oil. They are used widely in engineering because they are cheap and have a resistance to atmospheric corrosion. Their behaviour when heated makes it easy for us to divide them into two main categories.
The first group, called thermoplastics, includes well-known plastics such as PVC, nylon and polythene. These can be heated, softened and cooled several times, changing their shape repeatedly. This process is possible because only weak bonding is present between their molecules, and when warm they slide very easily past one another. Pipes, bags and combs are made using this method.
The second group, called thermosetting plastics, becomes rigid on heating. Examples are bakelite, melamine and polyurethane. These materials consist of polymer chains which react with one another at points of contact becoming strongly linked together. The intermolecular bonds then prevent movement of the original chains. Thermosetting plastics are used for plugs, foams and tableware.