Biodegradable plastic, now often found in plastic bags and bottles, contains additives that are supposed to get microbes to break down tough plastic faster. But a new study from Michigan State University finds that some of these additives may actually doing, well, jack shit.
Aside from generally sounding virtuous (thanks, greenwashing!), "biodegradable plastic" is a bit of a catch-all term, so let's get more specific. There are bioplastics, usually made from renewable plant materials, and there is ordinary plastic made from petrochemicals with chemicals added to speed up degradation — supposedly, anyway. The current study looks at the latter category.
Researchers at Michigan State University tested three additives in two common types of plastic: low-density polyethylene (aka the stuff of plastic bags) and PET sheets (aka the stuff of plastic containers). In theory, these additives help attract microbes that will gobble up the plastic.
To test that, the team put the plastics through a battery of tests designed to simulate different landfill conditions. First, there was an anaerobic, or oxygen-less, condition, much like being buried at the bottom of a big landfill. Second, they mixed it with compost. And lastly, the researchers buried the plastics under soil for three years.
In all of these conditions, the plastic treated with biodegradable additives fared no better than those without. "Thus, no evidence was found that these additives promote and/or enhance biodegradation of PE or PET polymers," the authors concluded.
An earlier study from 2013 found that biodegradable plastics with additives had a similarly unimpressive records. These studies are limited in length for logistical reasons, but they do tell us that we really don't know much about happens to so-called "biodegradable plastics". They might just be too good to be true. [Environmental Science & Technology, KQED]