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 polythene 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.

Thermosets referred to on this site are:

Bois durci
Casein formaldehyde
Glass reinforced polyester
Hard rubber
Melamine formaldehyde
Phenol formaldehyde
Polyester (some types)
Polyurethane foam
Shellac (some times)
Silicones (some times)
Urea formaldehyde

Thermoplastics referred to on this site are:

Acrylonitrile butadiene styrene (ABS)
Cellulose acetate
Cellulose nitrate
Gutta percha
Horn
Polyamide
Polycarbonate
Polyester (some types)
Polyethylene terephthalate
Polylactide
Polymethyl methacrylate
Polypropylene
Polystyrene
Polythene
Polyurethane (some types)
Polyvinyl chloride
Shellac
Silicones (some times)

Recognising whether plastics are thermosets or thermoplastics is relevant for the curator as certain production techniques, for example those that rely on reforming plastic sheet, can only be done with thermoplastics.

Increasingly plastics are copolymers, that is made up of two or more polymers, in order to increase the range of performance of the resulting material, e.g. Lycra.

The role and nature of additives

The performance, appearance and stability of a specific plastic can be greatly modified by a mix of additives in its recipe. They are used for a wide range of reasons including to:

  • give additional strength or dimensional stability
  • act as plasticisers or lubricants
  • provide decoration or pigmentation
  • improve chemical resistance
  • act as fire-retardants
  • protect against ultra-violet degradation
  • as fillers to reduce cost

Commonly found additives include:

  • calcium carbonate
  • camphor and phthalates (as plasticisers)
  • pigments
  • cotton flock
  • gas/air in foams (as expanders)
  • glass and other fibres
  • mica
  • minerals
  • stabilisers
  • talc
  • wood flour

It would be impossible to process most polymers into useful objects without additives. Additives can be added in different quantities and can affect the long term stability of the plastic. It is as likely to be the additive contributing to a plastic object’s degradation as the plastic itself. Please go to Degradation for more information.

For information on particular plastics please go to the A-Z of Plastics or to an individual letter within it.

< A curator's guide to plastics

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