Reporter 440, 11 October 1999


Starch team stringing new polymers together to give plastics a rotten future

Since polyethylene was first created in the 1940s, thousands of novel uses have been found for plastic. Mother Nature, however, can spend up to fifty years trying to get rid of the resulting rubbish. An inexpensive plastic that simply rots away has been the Holy Grail of polymer scientists for decades. University researchers believe they could now have the answer...and it all starts with the humble potato.


Professor Tony Johnson and Dr Wendy Xia are working on a 1m project to develop biocompostable plastic materials using starch

Plastics now account for a fifth of the rubbish dumped in landfill sites. Environmental concerns are driving the global search for green alternatives. Last month biotechnology company Monsanto announced they had genetically engineered plants to produce a plastic that can be broken down more easily. University researchers in chemistry and physics aim to go one better and develop a polymer plastic that will rot away completely.

Strength and security come through numbers and long chain polymer chemicals involve some very, very big numbers. Polystyrene and polythene are now household names and both contain hundreds of thousands of identical chemical units repeated end upon end. Advances in technology mean entirely man-made synthetic chains can now be created and tweaked to alter the behaviour of the resulting space-age polymer plastics, but the basic principles date back millions of years; natural rubber and cellulose are both natural polymer chains.

Synthetic polymers are made deliberately difficult to break down: car tyres that easily degrade are a neat concept until they start to dissolve in the fast lane of the M1. The same is true of polymer plastics used for packaging - their very inertness makes them useful. The life span of food packaging, however, is much shorter, as the contents are eaten or thrown away within weeks. This means if a plastic could be designed to begin rotting after a mere couple of months, then burgers and bagels may be the first benefactors.

"Nobody would ever suggest using a biocompostable plastic to contain, say, engine oil as it could need to sit on the shelf for years," said Professor Tony Johnson of the chemistry department and IRC (Interdisciplinary Research Centre). "But a yoghurt pot is a different matter as we know its useful life ends when the yoghurt begins to go off." In an attempt to create a strong and flexible polymer material that can be thrown on the compost heap and rot with the vegetable peelings, Professor Johnson's team have gone back to nature. The keys to the problem are water and starch, a common natural polymer found in potatoes.

Starch dissolves readily in water but a plastic duck will bob away happily long after the bathwater is cold. To produce a compromise polymer material that will dissolve away slowly over a few months, Professor Johnson's team is chemically building starch into the plastic chains. It is a delicate process. Too little starch and the plastic will not rot, too much just produces a soggy mess.

"Creating the material is just the first part of the problem," said Professor Johnson. "To be of any use it needs to be easily processed, have the required physical properties and, of course, be cost-effective."

Hundreds of experimental starch-plastic graft materials are currently being subjected to a battery of tests in the chemistry department. Once Professor Johnson is happy they have a material to meet the required criteria they will be passed onto Professor Geoffrey Davies and Dr Alan Duckett in physics for further analysis of the polymers' performance.

The project is being carried out in partnership with Dr Rukmal Abeysekera at York University - who first developed the raw 'Starplast' material the researchers are investigating. Several companies in the packaging industry are also supporting the research including Marks and Spencer, National Starch and Pira International (through the White Rose consortium). The three-year scheme has received over 1m in funding, including a recent 375,000 grant for the Leeds researchers from the Biotechnology and Biological Sciences Research Council.

Professor Johnson said the inter-disciplinary team are aiming to produce a material that will rot away completely after a few months in the ground - hence the suffix 'compostable' instead of the more commonly used biodegradable.

"Biodegrading just means breaking the material up into smaller units, but it is all still there - we are aiming to go one better and leave absolutely no trace."

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