Biodegradable plastic

What if plastics could be made from biodegradable material? Just throw the old water bottles and fast-food cutlery in the garbage; bacteria at the dump will eat them, in short order, down to harmless substances.

That goal is still elusive. But we’re getting closer.

The first big step came more than a decade ago when scientists figured out how to make something called polylactic acid, or PLA, from corn. PLA is a polyester that can be used as a basic ingredient in manufacturing almost anything plastic.

By 2011, Coke and Pepsi were touting their decision to replace petroleum-based plastic in their bottles with plastics derived from plant waste and sugar cane. But the plastic still behaved like plastic: It survives for eons in a landfill or floating in the ocean, and leaches out noxious chemicals over time.

Next came bioplastics, made from a range of organic feedstocks. Some promoters call them “compostable,” implying that bioplastics can disintegrate harmlessly in your backyard compost heap along with your potato peels and lawn clippings.

That’s not necessarily so.

To be genuinely biodegradable, an object must be able to be broken down by microbes within a few months into nontoxic substances that other microbes can safely eat. Some bioplastics, but not all, can do that. Certain categories of bioplastics can’t biodegrade at all; others will do so only in industrial facilities dedicated to forcing the process. Sorting and managing bioplastics isn’t something that recycling centers, not to mention individual consumers, show a willingness to do.

So, hopes are moving on from bioplastics to “oxo-biodegradable” versions. These are conventional plastics – made from either petroleum or plant sources – to which some common, nontoxic metal salts have been added.

When the plastic is discarded, air and sunlight catalyze a reaction between the salts and the plastic. That process breaks down the plastic’s carbon chains until the plastic’s chemical nature is transformed. The resulting materials decompose further by the usual processes that return leaves and dead bugs to nature.

Many oxo-plastics already meet European and US criteria to be labeled as biodegradable.

Also, the formulation of an oxo-plastic can be fine-tuned to speed or slow decomposition. For example, a plastic sheet used to cover mulch and protect plants over a cold winter could be programmed to dissolve within a few months. Plastic shopping bags could be formulated to last much longer.

It’s a good step, but not the ultimate solution to plastic pollution.

Oxo-biodegradable materials need both oxygen and sunlight to decompose; if buried deeper than a few inches in a landfill, the dissolution can’t happen. Also, oxo-plastics need to be separated from the usual run of recycled materials. Because most towns and recycling operations haven’t stepped up to do that, the Oxo-Biodegradable Plastics Association is urging that all plastics be made as oxo-plastics. That’s also not likely to happen soon.   TJ