When we refer to plastics we usually mean polymers, since plastic is a condition or state of a material. (metals when heated can be plastic: can be moulded into shapes).
Types of plastics
Natural Plastics
- Amber.
- Animal Horn.
- Natural rubber.
- Shellac.
Modified natural plastics
- Cellulose (made from cotton fibre and wood pulp).
- Casein (made from cows milk).
Synthetic plastics
Produced entirely by human-controlled chemical processes:
- By-products from the production of gas from coal.
- By-products from the distillation of crude oil.
Polymerisation
The word polymer comes from the Greek word for “many parts”. They are the result of joining a number of basic units known as “monomers”. Within the polymer molecule there is a repeated unit known as a “mer”. Mers are the building blocks of all plastics.
Classification of Polymers
Polymers can be classified under two main headings:
Thermoplastics
Can be heated and formed, then re-heated and re-formed repeatedly. The shape of the polymer molecules is generally linear, or slightly branched, allowing them to flow under pressure when heated above the effective melting point. All of the bonds within the polymer molecule are primary bonds which are very strong chemical bonds. The bonds between adjacent molecules are secondary bonds or Van der Waals forces they have relatively weak forces of attraction. They can be weakened even further by raising the temperature, this is why they flow under pressure.
Thermosetting
Undergo a chemical as well as a phase change when they are heated. Their molecules form a three-dimensional cross-linked network, this is known as curing. Once they are heated and formed they can not be reprocessed – the three- dimensional molecules can not be made to flow under pressure when heated. (normally condensation polymerisation). Primary bonding occurs
Additives used in plastics
Polymers are often combined with other materials to give them certain desired properties. They include:
Fillers
Used to improve polymers mechanical properties, can also reduce the amount of polymer present to make them cheaper.
Wood flour – gives strength and good mouldability.
Cloth fibre – gives good impact strength.
‘whiskers’ of metal – gives very high strength.
Mica – good electrical insulating properties.
Plasticisers
Added to improve flexibility, the plasticiser separates the molecules and reduces the forces of attraction between them.
Stabilisers and antioxidants
Help to prevent the degrading effects of heat, ultraviolet light and oxidation on the polymer.
Colour pigments
Give the plastic the desired colour.
Flame retardants
Used when plastic is to be used as a building material or for clothing or furniture.
Lubricants
Added to make it easier to mould. Waxes and soaps are examples of lubricants. Usually very little is used as they affect the engineering properties of the moulded material.
Recycling Plastics
Most thermoplastic polymers can be recycled – that is converted from their initial use as a consumer, business, or industrial product, back into a raw material from which some other product can be manufactured. Recycled materials are often classified as Post-Industrial and Post-Consumer. Post-Industrial includes such things as manufacturing scrap, containers and packaging. Post-Consumer is basically any product, container, packaging, etc. that has passed through the hands of a consumer, e.g. plastics bags, beverage containers, carpeting, home appliances, toys, etc.
Thermosetting polymers can only be recycled for use as an inert filler (something to take up space) in another material.
The keys to effective recycling are:
- an efficient infrastructure for collecting used materials
- ease of separation and low levels of contamination
- an established market for reprocessing/reusing the materials
