A primer on biodegradable plastics

Pros and cons of two kinds of biodegradable plastics on the market today.

By Tony Azios | Correspondent

from the December 19, 2007 edition

'Biodegradable' can be a confusing term when applied to plastics. While biodegradation is defined as the breaking down of organic material by microorganisms, the industry tends to take a wider view: If it breaks down safely, quickly, and completely, it is biodegradable.

Despite efforts to standardize the industry, bioplastics are presently found in many forms with various degrees of degradability. Two common types of bioplastics are hydrobiodegradable and oxobiodegradable. Each has its pros and cons:

Hydrobiodegradables are made from food or plant starch, yet often contain some percentage of synthetic (oil-based) polymers as well. These plastics are broken down into water, carbon dioxide, methane, and biomass primarily through the enzymatic action of microorganisms. But for many brands, this can only occur within industrial composting conditions with their higher humidity and heat.

Pro: Starch-based plastics have become cheaper and more durable than their unpopular "bio" predecessors.

Con: Many compostable plastics must be sent to industrial facilities, and will not biodegrade in landfills, backyard compost piles, or open environments.

Con: Despite the initial "green feel" of plant-based plastics, energy intensive agricultural practices and increased demand for food reduce their benefits.

Oxobiodegradables, like traditional plastics, are often made exclusively from nonrenewable petroleum byproducts. But unlike traditional plastics, they degrade more quickly – after a preset period of time. Exposure to such things as sunlight, heat, and mechanical stress ultimately reduce oxobiodegradables to a mix of water, CO2, and biomass, making them easy and safe to compost.

Pro: Many oxobiodegradables will decompose in industrial plants, backyard compost piles, or open environments.

Con: As they are made from petroleum byproducts, the manufacture of oxobiodegradables contributes to greenhouse-gas emissions and fossil fuel dependency.

Con: Many oxobiodegradables will degrade into small fragments of polymer, which then persist in the environment for months or years until fully degraded.

Both types of plastics degrade best in hot, humid conditions where microbes flourish, meaning that consumers must consider where they store their bioplastics. Cool, dry areas are preferred. Most have a shelf life ranging from one to three years.

Although some brands claim to biodegrade in open air and saltwater, virtually no decomposition occurs within a closed landfill that has been capped, preventing air circulation.

Beware of "bio-based" plastics. This term refers to plastic that is at least partially made from plant or food starch. In terms of its degradability, it's on a par with ordinary plastic.

Further complicating proper disposal methods, bioplastics typically have a lower melting point than traditional plastics. This means that even a small amount entering the recycling stream can weaken the line, often leading to their removal at recycling facilities.

As a result, consumers must research proper disposal methods of their bioplastics, as the requirements for each brand and type can vary significantly.

– Tony Azios