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31 March 2015updated 26 Sep 2015 7:01am

Scientists buried biodegradeable plastics for three years, found it doesn’t degrade

Common method of making plastic "biodegradeable" seems to be useless, in some types.

By Ian Steadman

Gardeners beware – those “compostable” bin bags might not so readily turn into soil. A study published in Environmental Science & Technology has found that many common biodegradeable plastics, aren’t – and that recycling is a much better bet.

There are typically two ways to make plastics degrade faster than they otherwise would: either add stuff to them to make them break down faster, or make them out of biological materials like vegetable oil or corn starch. The researchers, from Michigan State University, were specifically interested in the first set, and did what has to be done when testing how fast something rots – they buried a bunch of different plastics, for three years, and then dug them up again to see what had happened. The results don’t bode well for the efficacy of additives.

Typically, plastics made from petroleum can take hundreds of years to decompose, yet getting precise estimates for the timeframes involved can be difficult. After all, most modern plastics haven’t been around anything like the amount of time we think they’ll take to break down – and the thing that makes plastics so useful as packaging, that they’re unappetising to bacteria, makes them also an environmental terror. It’s not just about plastic bags caught in trees; the plastics that form the Great Pacific Garbage Patch, a vast gyre of debris, form an ongoing natural disaster that is entirely human-made.

Recycling is, of course, one option to prevent the problem in the first place; another is making plastics biodegradeable, so that they either break down into other, harmless molecules, or into something small enough that bacteria might consider it food. But designing material that is able to maintain its strength and durability during everyday use while also breaking down when exposed to sunlight is a challenge, as is creating a material that breaks down in both landfills (airless, warm, dark, compressed) and stuck in a hedge or river (light, airy, varying temperature).

For this study, the researchers looked at two specific types of plastic – polyethelene, and polyethylene terephthalate, both of which are extremely ubiquitous as packaging material, especially for containers like plastic bottles – with five different additives (that supposedly improve biodegradeability) in three different environments: buried with oxygen (as in compost); buried without oxygen (as in a landfill); and simply bured in soil.

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No matter the additive, and no matter the environment, they didn’t find anything that appeared to have made the plastics decompose quicker than without. In a statement, the authors recommend “the proper management of waste plastics” instead – which means, for now, if you want to do the right thing by the environment, either use something known to degrade properly, recycle it before it gets into the ground, or (and possibly the easiest option in many situations) not using plastic at all in the first place.