MAKING OF PLASTICS

The raw material for plastics is crude oil, a complex mixture of thousands of compounds. To become useful, it must be processed. Around 4% of the world’s production is turned into plastics.Because the compounds in crude oil, have different masses, and therefore boil at different temperatures, it is possible to separate them by a process known as fractional distillation. The mixture is separated into fractions, not into individual compounds. Fractions contain a mixture of compounds whose boiling temperatures are similar.
Cracking breaks large molecules into smaller ones which are more useful - and therefore of greater value. For example, very high boiling point fractions are cracked to produce gasoline and gas oil fractions. Today most cracking uses catalysts, but some heat treatment still occurs.

A feature central to all plastics is that they are constructed from polymers or macromolecules. This applies to both natural and synthetic plastics. A polymer is a long chain molecule consisting of many (usually thousands) of small units called monomers joined end to end. The relationship between polymers and monomers is that of a chain being made up of many individual links.

Reforming changes the internal structure of molecules to produce different compounds with a greater usefulness – and therefore higher value. By altering conditions – such as temperature, pressures and the catalyst – cracking and reforming techniques can now be controlled to produce exactly the blend of compounds which will be most useful at a particular time.

At the heart of all plastics manufacture is synthesis (or joining together). Monomers are joined or synthesised together to form polymers, this process is called polymerisation.