Crude oil is a mixture of hundreds of different chemicals. We burn some of them as fuels, but we can use others for making new materials, including plastics.
Oil has been used by humans for thousands of years, but it was during the 20th century that we began to use it on a massive scale. At first it was used mainly to provide us with fuel for engines, but then we learnt how to make plastics from it.
The first step to making plastics is to take the crude oil and distil it. This is done on a massive scale at an oil refinery in a fractionating column. It has this name because it splits the oil into groups of chemicals whose molecules are all nearly the same size. Each group is a fraction of the original oil — if you mixed them all together you would get the crude oil back. Fractions made from short molecules are referred to as being ‘light’ and the ones with long molecules are the ‘heavy’ fractions.
This process is also done at the oil refinery and is carried out on the heavy fractions. During cracking, long molecules are broken down into smaller molecules, many of which can be used to make plastics. With even more processing, the products from the cracking of crude oil can be turned into other compounds like styrene and chloroethene that can be used to make plastics too.
Plastics are substances made from very long molecules called polymers. They are constructed from much smaller molecules that we call monomers. The process of joining monomers together is called polymerisation. By choosing the monomers and the exact conditions of polymerisation carefully, we can produce different types of plastic.
During addition polymerisation, the monomers are joined together to make long chains. They are often, but not always, directly obtained from the process of cracking. All the monomers are the same. At the end of the reaction, there is no waste product, just the polymer and any monomers that didn’t get used up.
You can imagine it as being like when you make a daisy chain — the monomers are the daisies and the polymer they make is the daisy chain. Examples of polymers made like this are:
- poly(ethene) — usually called polythene and is made from the monomer ethene
- poly(propene) — sometimes referred to as polypropylene and is made from the monomer propene
- poly(chloroethene) — this is better known as PVC and is made from the monomer chloroethene
- poly(styrene) — can you work out the name of the monomer for yourself?
During this type of polymerisation, the two monomers are different. When they react to make the polymer, they produce a waste product, usually a small molecule like hydrogen chloride or water. An example of this that you may have seen at school is the making of nylon. Examples of plastics made by condensation polymerisation are:
- Acrylic polymers — example poly(methyl methacrylate), better known as Perspex
- Phenol-formaldehyde resins — example Bakelite
At some point, the Earth’s supply of oil will be used up, so scientists are working on finding other sources of these monomers such as vegetable oils.