Problem Plastics

The Naked Scientists spoke to Dr Peter Barham, University of Bristol
05 March 2006

Interview with 

Dr Peter Barham, University of Bristol


Chris - What actually is a polymer?

Peter - A polymer is a long molecule of atoms that are joined together to make a string of mostly carbon.

Kat - And how do you get different types of plastic? You get very hard plastics in bottles and flimsy plastics in bags.

Peter - Basically it depends on what you hang on the side of the chain. You can have different atoms on the side of your polymer chain. For instance you can make polyethylene, which is very floppy plastic.

Chris - Now what is a biodegradable plastic? Is it true to say that this could be in the ground for a hundred thousand years and therefore we should use a biodegradable plastic? What's the difference between the two and how they work?

Peter - A lot of this is definition and you have to be careful. Strictly speaking, a biodegradable plastic is one which will degrade, or disappear back to carbon dioxide and water if left in the atmospheric zone, whereas other plastics won't. Plastics like polythene is actually biodegradable because if you put it in the ground it will disappear in a hundred thousand years or so.

Chris - one thing I have actually noticed is that in the old days, plastic bags form supermarkets were actually quite reliable. You could put things in them and keep them for a long time and they wouldn't go off. But recently I've noticed that when I go to put stuff in the boot of my car and carry things around, that the ultraviolet light in the sun coming through the window are breaking the bags down into pieces of shrapnel. Is that true?

Peter - Well I don't know what you have in the back of your car! Most plastics can be degraded by ultraviolet but I'm surprised that much ultraviolet would get through the windows of your car in the first place. It's more likely that there's something in the back of your car.

Kat - What should we do about the huge number of plastic bags? Is there any solution to the plastics that we're putting into the environment?

Peter - The problem with plastic bags and all plastics is that recycling is not really a very good option. The trouble is that there are so many different plastics that unless you know exactly what you're recycling, you can't do much with it. So with plastic bottles, there are only a few different types. If they say PET on the bottom you know what they are and you can turn them into something else like fibre-fill for soft toys or duvets. But if you take plastic bags, they are made from a wide range of things. There's PVC, polyethylene, polypropylene and mixing up those makes an awful mess that's totally impossible to recycle.

Chris - So how are we now getting round the problem and making nags we can recycle? How do they work?

Peter - Basically any bag would be recyclable if you knew exactly what was in it. The only way you can make that work is if you are a supermarket that takes back the bags that you give out. This way, you know what's in them and you could send them to be recycled. As soon as you get one or two foreign bags in there from something else, it will completely ruin your machinery and the processing won't work. It's a terrible shame.


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