The Truth About Biodegradable Plastics

Image Gallery Only one of the bioplastics we tested, Novamont's Mater-Bi (top left), was compostable in typical home compost pile conditions. PHOTO: GRETCHEN STUART

Most of us are aware of how long-lived petroleum-based plastic bags and packaging are — we’ve seen the trash along roadsides and in our lakes and oceans. Some new “bioplastics” claim to be “100 percent compostable,” but testing commissioned by Mother Earth News reveals that most of these claims are misleading at best.

Basically, there are two kinds of “composting.” Composting at home usually involves small-scale piles with low temperatures and less-than-optimum humidity. Then there’s large-scale commercial composting, in which materials are shredded, mixed and maintained at 140 degrees Fahrenheit — a much higher temperature than that of typical home compost piles. Many cities compost yard waste, but only a few sites — about 100 in the entire nation — will also accept these “degradable” plastics.

We tested five types of bioplastic bags to see how well they would compost. None of them broke down completely after 25 weeks in home compost conditions (77 degrees). A product from Italian bioplastic manufacturer Novamont came closest to what we would call truly compostable (see photo in the Image Gallery), with a product called Mater-bi. Mater-bi is “made of corn starch, vegetable oil derivatives and biodegradable synthetic polyesters” In our tests, only Mater-Bi was compostable at typical home compost pile conditions.

Three other brands did fairly well in commercial composting conditions, but they showed little or no degradation in home compost conditions. One type, Oxo-Biodegradable, did not begin to break down even after 25 weeks at 140 degrees.