Plastics have names such as polypropylene, polyethylene, polyester ...

Plastics have names such as polypropylene, polyethylene,
polyester etc. What does the “poly” bit mean?
In the manufacture of plastics, for example polyethylene (see plastic milk bottle for
example), we start with an ethylene, which is a gas.
In picture 1 we have an atomic model of the gas ethylene which is made up of two atoms of
carbon and four atoms of hydrogen. The black spheres (balls) represent carbon and the
white spheres (balls) hydrogen and they are joined together by bonds. The red arrow is
pointing to a double bond joining the two carbon atoms together.
Picture 1:
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Let’s see what happens if we disconnect one of those bonds as in picture 2.
Picture 2:
Basically, this will leave us with two connections to which we can add two more molecules of
ethylene. If we keep repeating these additions we can form a long chain as in picture 3.
As the chain gets longer the ethylene changes from a gas to liquid which becomes thicker
and thicker until it reaches a point when the chains are long enough to form a solid, and
becomes the plastic that you and I know.
Picture 3:
Now we could call our chain “many ethylenes” but that is a bit clumsy. So instead of saying
‘many’, we use the term poly (which comes from the Greek ‘polloi’ meaning ‘many’) and finish
up with polyethylene.
© Packaging Council of Australia 2005
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