Engineering materials (polymers)

Polymeric materials now encompass a vast range of biological and synthetic materials that are used in a wide range of applications. The market for polymers involves many large commercial organisations that compete with one another in different market sectors. Thus, polymer nomenclature involves chemistry, from various eras, plus commercial trade names, all of which are used synonymously. The vast majority of the items we call ‘plastic’ are made from one of five ‘families’:

1. PE: polyethylene (low density = LDPE, high density = HDPE), e.g. carrier bags.

2. PP: polypropylene, e.g. drinks cups, kettles, shampoo bottles

3. PS: polystyrene and expanded polystyrene (EPS), e.g. insulating cups, packaging

4. PVC: poly(vinyl chloride), e.g. window frames, insulation cables for domestic wiring.

5. PET: poly(ethylene terephthalate), e.g. transparent water bottles.

The precise polymer chosen for any application depends upon the combination of properties required, such as its glass transition temperature (below which the polymer becomes brittle), its melting point, its toughness, its aging properties, whether it absorbs the chemicals it is to contain, how easily it can be moulded/ shaped and the cost.

The name of a polymer is derived from the monomer, for example propylene or ethylene terephthalate. If the monomer is a simple molecule such as ‘propylene’ then the name of the polymer is ‘polypropylene’ – with no brackets. If it is a combined molecule such as ‘ethylene terephthalate’ then the polymer is ‘poly(ethylene terephthalate)’ – with brackets.

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