Friday, 10 August 2018

Why I am pulling 100% degradable plastic bags from my compost bin

My organic veg supplier has recently started using plastic bags with this logo on. It says they are '100% degradable'. Perhaps I was naive in assuming that meant it was OK to put the bags in the compost bin. It turns out that they are 'oxo-degradable', a technology which is highly controversial.  They are currently banned in France and Spain [1]. 150 organisations including the World Wildlife Fund, Unilever and British Plastics Federation Recycling Group have signed a statement calling for them to be banned completely [2]. Now the European Commission has called for all member countries to take measures against them [3].

What are oxo-degradable plastics
Oxo-degradable plastics are ordinary plastics with special additives that allow them to degrade in the environment. EPI is one company that makes them. The process happens in several phases [4].

  1. When exposed to UV light, they physically degrade. This takes from a few weeks to two years. It is faster at higher temperatures and under mechanical stress.
  2. This leaves very small pieces of plastic (often known as microplastic) which can be degraded by biological organisms to carbon dioxide and water in the open environment or compost bin.

However EPI has another clever trick to control how long the plastic lasts. The start of the degradation is delayed by adding anti-oxidants at the same time as the other additives. The anti-oxidants are slowly consumed, the time taken depending on how much is added and how much sun they get, and while they are present any degradation is very slow indeed. This is an extremely useful feature from the point of view of ensuring your plastic bag does not disintegrate in use.

They take too long to decompose for normal composting
Unfortunately, although these plastics will degrade eventually, the process takes far too long for industrial composting processes and, from the sound of it, too long for my home composting too. If I don't turn my compost heap regularly so that everything gets some sunlight it might take a very long time indeed.

Then even when they do degrade, the first phase leaves particles that are too small to notice but at this stage they are still plastic. The effects of microplastic on marine organisms are starting to be understood. For example small animals that eat microplastics grow less rapidly. This may be simply because the plastic takes up room in their gut but there are other concerns; for example it may be partly because the plastic surface accumulates toxins from the environment and these are eaten at the same time [5]. Microplastics may be less toxic for soil organisms, and maybe oxo-degradable plastic is not as bad as other microplastics, but I'm not convinced (and neither is the European Commission [6]).

People are more likely to drop these bags as litter
There is a concern, too, that people will see the 100% degradable logo and assume that it is OK to drop them in the street, or the park or wherever, like a tissue. Well I am not advocating dropping tissues either: although they degrade rapidly when it rains they can last a long time if it does not, as we have experienced recently. In any case, oxo-degradable plastic litter could take months to degrade even if it does rain.

They can contaminate recycled plastics
I might accidentally put these plastic bags in my recycling bin. In fact I would say it is almost certain that I will at some point do this, since I don't generally look for recycling logos on plastic bags. If I do, then any plastic made from them by recycling will also contain the additives, and so they could degrade at the wrong time. The manufacturers claim that oxo-degradable plastics can be identified and separated but the recycling companies disagree [6].

From now on, in our household, we will try to remember to put these bags in the black bin, the one that goes to landfill.

So often, what appears to be an easy solution to a sustainability issue proves to have unintended consequences.

[1] Oxo-degradable bags increasingly under fire in Europe (European Bioplastics) Feb 2017
[2] BBIA and Ellen MacArthur Foundation call for oxo plastics ban ( Nov 2017
[3] EU takes action against oxo-degradable plastic (European Bioplastics) Jan 2018
[4] How does Oxo-degradable plastic work (EPI)
[5] The impact of microplastics on marine life (Exeter university)
[6] The impact of the use of oxo-degradable plastic, including oxo-degradable plastic carrier bags, on the environment (European commission) Jan 2018

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