A lot of terminolgy has been floating around these days- Biodegradable Plastic, Bioplastic, Compostable Plastic- what does it all mean? Let's start with Compostable Plastic. This is a plastic that undergoes biological degradation when it is composting that yields carbon dioxide, water, and inorganic compounds that leaves no visual distinguishable or toxic residues. You may be familiar with ASTM D6400 which covers plastics and products made from plastics that are designed to be domposted in industrial and municipal aerobic composting facilities.
Bioplastics are plastics that are considered 'environmentally friendly'. They are made from natural materials such as corn starch. Since bioplastics are made from corn starch, some can break down in a few weeks. The cornstarch molecules absorb water and swell up thus causing them to break apart into small fragments that baceria can more readily digest.
Biodegradable plastics (also called photodegrable, oxydegradable) are made from traditional petrochemicals which are engineered to break down much more quickly. Biodegradable plastic contain additives that cause them to decay more rapidly in the presence of light and oxygen. The down side is that biodegradable plastics don't always break down into harmless substances and can sometimes leave behind a toxic substance that can make them unsuitable for composting.
There are varying opinions as to whether these forms of plastic are actually good for the environment.