Why do some plastics go from bendy to brittle over time?

James Tytko took on this mind-bending question from listener Jimmie with the help of Chemistry World's Phillip Broadwith.

Phillip - Plastics are polymers. That means they have molecules that are long chains inside them. If you want those polymers to be flexible, those chains need to be able to move over each other when you apply a force. They don't stay still. You can do that in a few different ways: you can either make the chain molecules branched and twiggy, that means they can't pack together so tightly which means that there's a bit of space between them for them to move as you apply a force. Or, some polymers, like silicones, have bonds between the chains which change how rigid they are. If you think about silicone, you can have it as almost a paste or liquid when you squeeze your bathroom sealant out of the tube. But then, as it cures, what's happening is there are more bonds being made between the chains, so the polymer gets harder and more rigid into a kind of rubbery state. But for the same polymer, if you take it even higher and make more links between the chains, you can make it into a completely rigid polymer. That's called cross-linking. The more cross-linking you have, the more rigid the polymer is going to be. The other way that you can do it is to add an additive called a plasticiser which sits in between the chains and acts almost like a lubricant. There's lots of different types of plasticiser, but the most common one that people will encounter is in PVC. If you think about window frames, they're made of unplasticised PVC; they're quite rigid. There's not very much plasticiser inside them. But if you think about softer PVC, like the fake leather that you might may have clothes or furniture made out of, or things like Barbie dolls with its kind of soft PVC, that will have a lot more plasticiser in it.

James - Now, under our forum post on this question, over at nakedscientists.com/forum, user Alan suggests that exposure to heat and UV radiation can change bendy plastics to brittle over time. Is he right to say that and, if so, why?

Phillip - That's exactly what's happening. Environmental conditions, heat, light, oxygen, all cause chemical reactions within the polymer. Those chemical reactions might break the chains down, which might make them more rigid or crumbly. It might make more cross links between the chains which is going to make them them more rigid, or it might evaporate or leach out the plasticiser molecules. We had a feature in Chemistry World recently about preserving Barbie dolls which are made of PVC which has a plasticiser in, and they have all sorts of problems with the plasticisers coming out or the materials degrading. So if you want to then preserve those plastics, it's quite a challenge.

James - Are all plastics susceptible to this loss of bendiness over time?

Phillip - Most flexible polymers will tend to become more rigid over time because they will age, but sometimes that's not the problem. Sometimes you can go the other way, the way the polymer degrades will make it more sticky or more flexible in some ways. There's quite an interesting story on that which is, in the 60's and 70's, there was a musical instrument called a clavinet. A favourite of Stevie Wonder. If you know the song 'Superstition,' that riff at the start, that's a clavinet. The hammers that hit the keys in that instrument - it's a keyboard - were tipped with a rubbery polymer. Over time, that polymer would become more sticky and that changed the sound of the instrument. It went from more hitting the strings to a slightly sticky plucked sound, which sounded slightly different. There were various jazz musicians and some people who really preferred that kind of sticky, plucked sound so they would start to seek out instruments where the hammers were degraded enough to make that sound.