Plastics from Plants?

At the moment, plastics are produced industrially using oil as a raw material. But, in future, it might, quite literally, grow on trees! Karnik Tarverdi, Professor of Materials Processing at Brunel University spoke to Kat Arney about the plastics of the future...

Karnik - Hi.

Kat - So first of all, do we really need all the kinds of plastic that we have in our lives at the moment? So, if I go to the supermarket, I might get a nice salad, I pick up a lettuce and a cucumber, and they're shrink wrapped in plastic. Why are they packed like that?

Karnik - Yes, I'm glad you mentioned cucumber because that has some very interesting statistics. First, I'd like to mention that regarding cucumbers, there's a very interesting informative book written by Laurel Miller and Stephen Aldridge. They're veterans in this area and the title of their book is, guess what, "Why Shrink-Wrap a Cucumber?"

Kat - There you go. Why should we? Why do I need to use this plastic to do it?

Karnik - Now hopefully, I'll try and convince you why we do it, why the industry does it. Now, if we shrink-wrap the cucumber, it loses 1.5% of its weight in 14 days. But if it's not shrink-wrapped, it loses 3.5% in just 3 days.

Kat - So basically, it's going off in my fridge faster if it's not wrapped up.

Karnik - Yes, exactly. And then with shrink wrap, it has much lower carbon footprint. It has a longer life in the fridge and also, wherever it's kept really.

Kat - I guess this is a bit counterintuitive because people may think, plastic packaging, it's always bad. Why do we need it? If it does keep our foods cleaner and safer and better for longer, it's a good thing. But can we recycle this kind of packaging?

Karnik - Yes. First, let me mention that in the UK, we use about 10 million tons of packaging per annum. Yes, it's a lot of material, but what we need to do is make sure that this material has a recycling lifecycle. It's produced, it's used, and it can be professionally recycled and reused again.

Kat - But is there a limit to the number of times say, that the shrink wrap from my cucumber can be recycled?

Karnik - Yes, there is. But what we do is you can always replenish it with some virgin polymer. When you're recycling, it needs tender,  love and care. That means you need to have the temperature just right with some flow promoters and also some anti-oxidants which will then extend the longevity of these polymers.

Kat - But assuming that no one is going to recycle all their shrink wrap ever and people will just chuck it in the bin, tell me about the work that you're doing to try and make plastics that are actually more biodegradable from natural products. Tell me about these. How do they work?

Karnik - Yes. This is interesting area but there's quite a lot of work being done and also of course, at Brunel University, I've got PhD students working in this area. There are quite a range of sugars that can be used to recycle and also, make new polymers.

Kat - We had Athene talking about starch earlier, is that...

Karnik - Yes. In fact, Brunel was one of the pioneers in processing starch and there are a couple of large companies that make loose fill plastic made out of starch. So, it's excellent packaging and it's taken over, around 25% of polystyrene loose fill which is excellent. But the polymers that are made out of plastic, recycled and also, biodegradable is quite a big range now. But PET and polypropylene play a big role. PET is from sugarcane. It's sugar but the big advantage of that is that when you process this material, it has good barriers against volatiles, against carbonation material inside a liquid.

Kat - So, you can store all kinds of things, all sorts of food and sorts of products in this kind of plastic.

Karnik - Yes, very much so and again, it has a good longevity as some of the bilayer materials that are being used nowadays. But the biodegradable polymers are also playing a big role. There is a special polymer called polyhydroxyalkanoates, it has polyhydroxybutyrate and polyhydroxyvalerate. These are again, sugar-based from plants, but generated through bacteria sources and they are biodegradable. They will do degrade but also, from nature works, they generate a polylactic acid from cornstarch. That is again biodegradable, but at much higher temperatures. So, if it's in very large compost bins then it will degrade in municipalcompost bins.

Kat - It will get a bit warmer.

Karnik - Yes, exactly.

Kat - So, the kind of the lifecycle of a biodegradable plastic, a farmer might grow some sugar beet or some of the other plants that are used. You'd extract the material, polymerise it, turn it into these plastics, and then I guess, do I just chuck it in the bin?

Karnik - Yes, most of it can actually be mixed with say, if it's PET with PET and recycled. But the new technology that is being developed is to identify, which is from plants and which is from mineral petroleum. At the moment, you can definitely mix both PETs and end up using it again for bottles and for anything else that anyone wants to use.

Kat - Now, one thing that always concerns me about biodegradable plastics, I've got a nice drinking glass here in the studio. I assume this is not biodegradable. It's filled with water, I'm drinking out of it. I worry that it will just disintegrate on me. How stable are these plastics in their use?

Karnik - Yes, they are. They are very stable. In fact, the biodegradables from polyhydroxyalkanoates, which one of my students is working and there are other scientists in the UK and overseas that are working on these, they are very stable materials and you need to treat them carefully. You need to process them at the right temperature. But once they are processed, they're as good as any polymer.

Kat - But then when you get them in the compost heap, that's when they degrade.

Karnik - Yes. When the bacteria starts attacking it and breaking down, taking all the nutrients in the way, and eventually it all ends up like soil itself.