The ecologically friendly version of polyethylene might not be so friendly after all.
Introducing transition metals such as iron and cobalt can increase the oxidation of the ethylene polymers and claims for the degradability or biodegradability of these substances are widespread on food packaging and plastic-type bags. However, a review published last week in Environmental Science & Technology, remarks that there is no evidence that 'degradable polyethylenes' are actually all they suggest.
Although it is clear that 'degradable' polymer bags, for example, will fall apart in the natural environment, the producing fragments can easily remain for a long period of time, and there are no long-term studies on these pieces. An important issue is that items can be accepted as biodegradable without reference to the timescale it takes them to completely biodegrade.
"There are a tremendous number of papers about degradable polyethylene but no one has really shown a high degradation," says Ann-Christine Albertsson, a polymer researcher at the Swedish Royal Institute of Technology (KTH) in Stockholm and lead author on the critical review. "Of course they degrade in one way - they are losing part of their properties. But if you mean it as a positive for nature, that has not been proved."
Developing-world countries such as China are also starting to use 'degradable' polyethylene, says Albertsson. Indian authorities have taken an awareness in the topic, and recently sent a postdoc to work with her. Although some cities are already attempting to move to naturally degradable products based on starch-based polylactide, this is still highly-priced. And paper-based products may not be a suitable replacement because of the necessity to cut down trees.
Adapted from Nature