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Author: Peter Taylor
  • Video
  • 5 minutes

Fantastic plastic

Updated Monday, 10th August 2009
It's such a key part of our lives - but what exactly is plastic, and how does it work?

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Fantastic Plastic

Peter Taylor
In the Bang Goes the Theory programme you saw how plastics can be designed to meet all kinds of needs.  Flexible and sponge like, solid, or even like putty.  Plastic or polymers are long chain-like molecules that have these properties because they intermingle and knot with each other just like a bowl of spaghetti.  If we build other properties into the chain at the molecular level, we can link up the chains to give a solid, or ensure they flow over each other to give flexibility. 

The long polymer chains become entangled like this bowl of spaghetti.  Like the polymer, if I suddenly pull one part of the spaghetti the whole mass comes with it, whereas if I slowly pull one strand it can become untangled.  Whether a polymer is a solid or has more fluid properties depends on two factors: the length of the chains, the longer chains will be much more entangled, and the ease by which the chains can flight over each other. 

Let’s examine this latter factor at the molecular level.  Here’s a short polythene chain.  It is made up of a chain of carbon atoms with hydrogen atoms attached. If it is not too entangled the chains can move past each other.  This is polypropylene, again there is a chain of carbon atoms but this time there are short side chains.  These side chains stick out from the chain and stop the chains from easily moving past each other.  If we link our chains by cross-links, as shown by the yellow highlighted atoms…….the chains can no longer move independently and the polymer is a solid. 

The putty you saw is not made from carbon like most plastics which is nylon and polythene, but has a silicone backbone; it’s completely made-made.  Here’s some elemental silicone.  It’s a bit like a lump of coal.  Here’s some silica, sand.  The product of silicone reacting with oxygen in the air.  Even the glass container is made from silica.  The materials used for sealing baths and showers are usually silicones.  They’re used in the car industry for gaskets and grease, in medicine and in the film industry they use it for prosthetics and they are present in all kinds of personal care products such as hair conditioner, shampoo and hair gel products, although I don’t have much need of them. 

I can’t make any silicones here but I can make something similar.  I’ve got two liquids, one has got a bit of dye in it and if I mix these two liquids together I get some slime.  See how it’s nice and slimy now.  We’ve gone from two perfectly running liquids to a much more solid slime.  


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