Greener Shopping Bags?

Consumers may find that the virtues of biodegradable plastics are really a mixed bag.

By Peter Fairley

Disappearing act: Novamont’s plant in Terni, Italy, turns out a polymer used in plastic bags. The polymer is a biodegradable blend of petroleum-based polyester and plant starch.

The San Francisco Board of Supervisors' vote last month to institute the first ban on polyethylene shopping bags in the United States may reduce the volume of plastic in landfills, but, despite many advocates' hopes, it is unlikely to dramatically reduce dependence on imported oil. That's because most biodegradable plastic bags (which San Francisco officials hope will take polyethylene's place) rely on a petroleum-based form of polyester.

San Francisco's ban will, however, create an important new market for biodegradable plastics that could bring plastics based on renewable feedstocks into the market. The best hope may be Metabolix, based in Cambridge, MA, which last year completed a $95 million initial public offering and signed a joint venture with agribusiness giant Archer Daniels Midland (ADM) to develop its corn sugar-based biodegradable polymer.

Standard polyethylene bags have multiplied (San Franciscans alone use 181 million a year) because they are cheap and easy to use. They also produce less pollution in their manufacture than paper bags do. Until recently, biodegradable plastic bags have cost at least three times more and fallen short on performance, but the picture has changed over the past decade. "Today you've got some products that work from a functionality standpoint--the price gap has come way down," says Keith Edwards, biopolymers business manager in North America for German plastics and chemicals giant BASF.

Most biodegradable plastic bags are produced by blending plant starch with petroleum-based polyesters, which improves the bag's strength and processibility with conventional film equipment. Leading producers are BASF and Italian polymers firm Novamont. Edwards estimates that biodegradable bags from these polymers could cost three to four cents more than the one-to-two-cents-per-bag cost of polyethylene. But he's betting that San Francisco consumers will demand them thanks to San Francisco's curbside organic-waste recycling program.

San Francisco's environmental officials are making the same bet. Currently, the program collects about 300 tons of food per day, contributing to a 67 percent recycling rate for its municipal waste overall. But that number must rise significantly if the city is to meet a self-imposed goal to recycle 75 percent of its waste by 2010.

BASF recently boosted capacity for its biodegradable resin from 8,000 metric tons to 14,000 metric tons per year. Overall, the company expects annual production of biodegradable and bio-based polymers to triple or quadruple by 2010 from an estimated 50,000 tons produced worldwide in 2005. Meanwhile, Novamont plans to scale up a process for producing its biodegradable form of polyester from vegetable oils; it could begin within the next two years.