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Heat shrink
Find out what happens if you heat rubber, and what it has to do with crisp packets and shrink wrap.
What you need
| A rubber band |
| Something to hang it from (you can use something smaller!) |
| A hot air gun or hair drier |
| A weight (you can use something smaller!) |
What to Do
Hang your rubber band from the hook.
If you stretch a rubber band it starts off quite easily and then it gets more difficult. You want to add weight to the band to stretch it until it starts to get more difficult to stretch.
Watch the weight very carefully, you will probably have to keep your head in one place by putting it on the table and sighting the weight onto something behind.
Heat up the rubber band to see what happens.
What may Happen
You should find that as you heat the band the weight lifts up and as the band cools down again it will stretch again.
What is going on?
Most materials expand in all directions as they heat up and unstretched rubber is no exception, however due to its structure stretched rubber behaves very differently. Rubber is a polymer, it is made up of long chains of repeating units. These chains are occasionally attached to one another chemically.
| Rubber is made up of long polymer molecules occasionally joined together. |
| When you stretch the rubber the molecules are forced to become a lot straighter. |
The molecules are normally very randomly arranged and so very wiggley. If you stretch the rubber it will tend to straighten out the chains. As you heat up these chains they get more energy and vibrate more violently. If you vibrate a real piece of chain it will become more wiggley and therefore wider but shorter.
A rubber molecule is the same so when it is stretched the more you heat it up, the shorter the rubber gets!
What has this got to do with crisp packets and shrink wrap?
The plastic film in crisp packets and shrink wrap has a similar structure to rubber, and when it was made the plastic was stretched into the thin film. The big difference is that the plastic freezes below about 100°C and the polymer molecules cease to be able to move across one another. If you heat the plastic up above about 100°C then it is free to shrink back to its unstretched state. This is why crisp packets shrink if you put them on a fire, and how they shrink the plastic in shrink wrap goods.
Written by Dave Ansell
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