Oxo degradable plastics

plastic bottle greenhouse at CATThere was a letter in today’s Guardian about oxo degradable plastics.  I’d never heard of the term before and decided to do a bit of research.  Conventional wisdom is that plastics do not degrade.  Or at least only on a geological timescale.  Conventional wisdom is largely right.  Polyvinylchloride (PVC) is degraded by heat and light and releases hydrochloric acid, which isn’t great.  Polypropylene is pretty much regarded as non biodegradable.  Most plastics do break down into small nano-particles.  Then progress stops.  Research this week has found these are present almost everywhere in supposedly pristine environments.  Other research has found that they are in the food chain and therefore in us.  The longterm biological effects of this are unknown.  They are unlikely to be too serious for the plastic itself, which is as alluded to above is biologically inert.  The problem is the effects of the plasticisers.  These are chemicals that give the same plastic different physical properties.   These plasticisers mimic hormones in the body.  Large pieces of plastic of course have detrimental effects on wildlife.

A number of solutions have been proposed to this problem.  The first has been so called bioplastics.  These are polymers largely made from natural polymers (e.g cellulose) using renewable feedstocks.  These are mostly, but not totally biodegradable.  Broadly speaking compounds are biodegradable when bacteria or fungi have seen them in evolution before and therefore possess enzymes capable of degrading them.  This is not the case for oil based plastics.  We outlined the problems with bioplastics in our book.  Basically in replacing our massive plastic use we need a lot of plant based feedstock.  This is not possible to do totally.  But these materials will have a place in a post oil world.  Another issue is that bioplastics do not have either the durability or entire range of properties of oil based plastics.

The next solution to the problem is the circular economy.  Recycle and reuse what we have in new products.  Some progress is being made on this.  One problem is that plastics do degrade when reused.  Not enough to destroy them but enough to make them unusable.  This is a particular problem in mixed recycling.  So this is not a total solution either.  Chemical methods of destruction using heat are being developed.  These would allow the resulting material to be reused perhaps de novo.

The next solution is to find bacteria or fungi that can breakdown conventional plastics.  Again some progress is is being made.  This is probably of more interest in bioreactors as a means of disposing what we produce at some point in the future.  But for what’s out there now it won’t work.  Bacteria and Fungi will degrade compounds that are simpler, easier nad more abundant first.

Of course we mustn’t forget the solution of using less in the first place.  Under public pressure companies are starting to alter the way that packaging is used.  Walkers Crisps and Guinness (this week) are too examples.  But the reader will know progress on this front is slow.

Finally this brings us to oxo degradable plastics.  These are plastics that contain small amounts of salt or metals.  These act as a catalyst and allow oxygen to break the plastic down to short chain molecules such as ketones or organic acids.  These bacteria will readily use.  It sounds to good to be true.  And to some people it is.  The EU don’t believe it works.  Other countries such as some in the middle east think it does and require its use.  Even if it does work its not a total solution.  Potentially it solves the litter in the environment bit. But not the use of non-renewable resources problem.

There is not one solution to the plastics issue.  All of the above are required.  We have become addicted to these highly convenient materials.  We need to reduce our use of plastics and look for alternatives, fast.


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