Science Interviews


Mon, 26th Jan 2015

What is plastic?

Dame Professor Athene Donald, Cambridge University

Listen Now    Download as mp3 from the show Lifting the Lid on Plastic

Plastics play an important role in almost every aspect of our lives. Look around you now: toothbrush, car steering wheel, toys, pens, kitchen white goods, tupperware,the list goes on! Making NylonPlastic is everywhere! Cambridge Universityís Athene Donald lifts the lid on what plastics are with Chris Smith...

Athene - Hello.

Chris - Now, I suppose the logical place to start should be first to address what actually is a plastic.

Athene - Okay, so to me as a physicist, I would probably call them polymers. They're long chain molecules. Most simple things around us are small molecules. So something like water is just H2O Ė two hydrogen atoms and an oxygen, whereas polymers are long chains, many, many atoms long as it were. Polymer means many units so that they have blocks of units which repeat all along the chain.

Chris - By changing the chemicals that are in that chain, you can get different things that behave differently or have different properties.

Athene - Absolutely. So, the simplest is polythene or polyethylene which is just the repeat of CH2. So, one carbon and two hydrogens and the carbon atoms join together to form these long chain molecules. But other polymers are much more complicated than that.

Chris - Now, youíve kindly agreed to show us a chemical reaction that revolutionised the lives of women everywhere with the creation of nylon stockings back in the sort of Ď40s and late Ď30s. You're going to do the chemistry Athene, here in the studio for us to make some nylon. We won't make stockings but can you talk us through actually how this works and what you're going to do.

Athene - Okay, so what I have in front of me are two glass bottles, each a litre in size, so like a litre of milk. These are clear transparent fluids I've got. One is hexamethylenediamine in sodium hydroxide and water.

Chris - Easy for you to say.

Athene - Well, I can read it off the label. The other one is sebacoyl chloride and hexane. So, solution A, that first one I said and I'm not going to repeat it, I've already got a small amount in this glass vile.

Chris - So, youíve got literally a centimetreís worth in a little bottle beside the big bottle.

Athene - Thatís right. I've just put a little in there and I've got a glass rod with a hook on it. Iíll explain why I've got the hook in a minute and what I'm going to do is pour some of the second solution in Ė if I can get the lid off Ė and I'm going to pour this down in the glass rod to try and form a stable layer between the two solutions. And then the point about the hook is thatÖ

Chris - I see. Weíve actually got a liquid sitting on top of a liquid now with the glass rod going between the two.

Athene - Thatís right. The second solution is less dense than the first one. So, it floats on top and at the junction between these two fluids, at the interface, there's a reaction going on and I shouldÖ

Chris - It looks like a spider web.

Athene - Thatís right and I should be able to pull out this rope of nylon.

Chris - Wow! So literally, itís coming from the layer between the two liquids. Weíve got what looks like a spider web material and you're drawing it out. Itís now Ė wow! Thatís 15, 16, 17 centimetres, still going.

Athene - Thatís right. I should be able to pull it for a very long way. It gets thinner and thinner and I have to pull it quite slowly because what I'm doing is picking up the nylon thatís forming at the interface between the two liquids where the reaction is occurring.

Chris - What is the chemical reaction thatís happening just in simple terms? What's going on here?

Athene - Itís known as condensation reaction. So, the two reagents mix together. They give out in fact in this case, hydrochloric acid, HCl. What is left is the two main bits of the chemicals joined together to make this long chain molecule. And there, itís broken. I'm afraid itís got to about half a metre.

Chris - Itís pretty impressive out of a tiny little bottle in the studio and this is just one type of polymer. So, what sorts of qualities and properties do these sorts of plastics and polymers made in this way have that makes them special?

Athene - They are mainly pretty strong. They are very flexible and perhaps the way we find them most useful is itís very easy to form them into different shapes. So, if you're trying to make Ė I'm old enough to remember enamel washing up bowls and they always had to be round. But you could make a square or rectangular washing up bowl out of plastic. You can form it into these shapes and thatís actually much less likely to tip or sort of collapse under the weight of the water. So, one of the key things is the ability to make into different shapes at quite low temperatures. Not the kind of hundreds and hundreds of degrees you need for instance with metals.

Chris - When you refer to temperature, thatís quite important too because plastics do deform or change their shape when you heat them. So, why is that and how can that be used?

Athene - Okay, so as I say, these are long chain molecules and in something like this example of the nylon rope, those molecules get all tangled up rather like knitting wool or something like a bowl of spaghetti. But that means the molecules can only move very slowly past each other. And so, if you go to high temperatures, there's quite a lot of mobility. But at room temperature, most of these molecules are essentially frozen into place. So perhaps, I can illustrate it with this second demonstration Iíve got which consists of a plastic cup, the kind of thing you would have at a drinking fountain. Itís just a regular cup like that. And itís formed into this shape. We all know what shape a plastic cup is. But if I heat it up, so I've got a quite powerful hairdryer here. If I heat it up, it will shrink back to the shape from which it was made. It will take a little while to get going and you can see itís crinkling up. Weíre giving enough thermal energy to the molecules. Giving enough heat to the molecules for them to retract back to the shape from which this cup was originally made and the cup was originally made just from a flat disk and you can see it there.

Chris - We have indeed got a flat sheet of plastic. Itís about 5 or 6 centimetres across.

Athene - So, itís the size of the base of that original cup, but the shape of the cup was made by shaping it around some kind of mould at high temperature and then cooling it to room temperature and it will stay in that shape. There's nothing thatís going to make it change shape at room temperature.

Chris - Because those chains of molecules cannot slip past each other until they're made hot enough again.

Athene - Thatís right, yes.

Chris - The one thing that people are always telling us though is that, if I dump that in the waste paper basket, I've got a problem because it then is not going to breakdown anytime soon. That must be one aspect of plastic Ė whilst they're very useful, they're very durable, we do have the downside, they donít biodegrade.

Athene - Thatís right. If you break it up into very small pieces, it sort of becomes much less visible. But most of the bugs can't digest them essentially. One of the ways of making biodegradable plastic bags is really, to mix in the polythene with starch and the starch which is natural does degrade. And so, you get just tiny fragments and you no longer see that bright orange Sainsburyís bag or whatever.

Chris - But the bottom line is, because the plastic contains molecules with chemical bonds in them that you donít naturally find in the environment, there are no bugs out there like fungi and bacteria that know how to eat that. So, it doesnít break down.

Athene - So, there are no common bugs. I wouldnít like to go quite so far as to say there aren't any because bugs are amazingly clever things.

Chris - So, there may be a plastic cup digesting bugs somewhere, but they're not that abundant or itís not as abundant as the plastic bag.

Athene - They're not yet in landfill. Yes, I think thatís right.


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