Polymeric materials are conventionally referred to as ‘plastics’. Polymers are formed by combining together a large number of basic chemical units (monomer molecules) to form long chain molecules (polymers). Carbon is the main building block of polymer materials but one or more other elements such as hydrogen, nitrogen, chlorine and oxygen are part of this building block.
Polymers have unique and diverse properties and are increasingly replacing metallic materials in all types of applications including cars, aircraft, general office and domestic equipment. They provide many advantages including good strength to weight ratio, resistance to corrosion, ease of shaping and low cost of manufacture. However, compared with metals, they tend to have low strength and stiffness and are limited to relatively low temperature applications.
Polymeric materials can be grouped into three general categories:
These materials can be softened and re-hardened indefinitely, as often as they are reheated providing the temperature is not high enough as to cause decomposition. Thermoplastics have linear or branched molecular chain structures with few links, if any between chains. Typical examples include nylon, polyethylene, polycarbonate, and PVC.
These materials are rigid and not softened by the application of heat. Such polymers have molecular structures which are extensively cross-linked. Because of this, when heat causes the bonds to break, the effect is not reversible on cooling. Typical examples of thermosetting polymers are phenolics, epoxies and resins.
These are polymers, which as a result of their molecular structure allow considerable elastic behaviour. Such materials are lightly cross-linked polymers. Between the cross-links the molecular chains are fairly free to move. These include rubber, silicone, and polyurethane.