What are plastics?
Plastics are materials that can be moulded into required shapes by the application of heat and /or pressure.
Most plastics are derived from organic material, that is, substances made from things that have lived, including oil, cotton, sugar cane, coal, corn and many others. There are however exceptions such as silicone which is derived from sand.
At the point of processing plastics consist of granules, pre-formed tablets, powders, syrups or pastes.
Plastics have been traditionally classified as:
- Natural, a material that can be moulded in its natural form. Examples are amber, gutta percha, horn, rubber, and tortoiseshell.
- Semi-synthetic, that means, made of a chemically altered natural material. Examples are casein, cellulosic plastics and rubber.
- Synthetic, that is entirely laboratory made, as for example is the case with phenol formaldehyde and the many poly-plastics.
Plastics are based on polymers. That is a material made up of many smaller base units. The simplest plastic is polyethylene consisting of base units of carbon atoms with two hydrogen atoms to each carbon. The base unit is referred to as a monomer. Many monomer units linked together create a polymer, through a chemical process known as polymerization. Polymerization can be demonstrated by hooking together hundreds of paper clips (base units) to form chains. Chains in different configurations make plastics with different properties.
Plastics are divided into two distinct groups:
- thermosets, plastics that on being heated and moulded set permanently, and thus cannot be re-melted and re-formed.
- thermoplastics, plastics that can be re-melted after moulding again and again, and thus can be recycled by melting and reforming.