Lifetime of a Plastic Bag

450 years into the future - that’s the agreed lifetime of a uniform piece of plastic like a bag. But who’s arguing? Man-made plastics have only been around for about 50 years: we don’t know for sure how long it takes a plastic bag to decompose: so where does this number come from?

A floating plastic bag. Like so many, it has ended up in the ocean. Image credit: Andrew (Flickr).

Plastic readily breaks down into smaller and smaller pieces (microplastics) under environmental conditions: it can now be found anywhere, a splash of colour amongst sand particles viewed beneath a microscope. But this isn’t the same as biodegradation, defined as the bacteria-driven chemical transformation of a material into other compounds.

Natural plastics do exist and do biodegrade: rubber and cellulose, for example. But these don’t make a good comparison with man-made plastics: cellulose is eaten by many animals, and enzymes and microorganisms exist in nature to catalyse its break down, which normally happens by about 6 months^[1]. There are some man-made plastic-eating bacteria, but these have only recently been discovered: how fast they act or what products the plastics are broken down into (and whether they are safe and useful) is still a mystery^[2].


Biodegradation is normally estimated using scientific methods to accelerate the chemical reactions. To estimate the decomposition rate of organic materials like food waste, they are isolated in an industrial composter and flushed with air, moisture and microorganisms. CO₂ levels are measured to determine the extent of degradation. Although common microorganisms will not break down classic man-made plastics, they do undergo photodegradation under ultraviolet light, eventually going brittle and cracking. Above 50 °C, some plastics will even biodegrade within one year, but it varies depending upon what stabilising additives are in the plastic and how brittle it already is^[3]. Once the breakdown is complete, scientists then have to work out how much higher temperatures and better conditions have sped up the natural process. The figure 450 years is just a guess – a sensible scientific guess.

References
why don't all references have links?

[1] Golley, F. B., An Index to the Rate of Cellulose Decomposition in the Soil, Ecology, Vol. 41, No. 3 (Jul., 1960), pp. 551-552.
[2] Zaikab, G. D., Marine microbes digest plastic, Nature, (2011) doi:10.1038/news.2011.191.
[3] Billingham, N., University of Sussex, Polymers and their Environmental Degradation, Royal Society of Chemistry Environmental Chemistry Group Bulletin (July 2014).