How Do They Work?
Now for the science bit. The two main types of biodegradable plastic bags available now, are oxo degradable, and photo degradable.
With oxo degradable bags, degradation of plastic takes place when larger molecules are broken down into smaller molecules that have oxygen present in them. The degradation can be initiated by either heat, physical stress or exposure to UV light. As the process continues, the hydrogen and some of the carbon from the polyethylene element of plastic begin to convert to water and carbon dioxide.
Photo degradable bags work through the additives included in small proportions into the polymer when the bag is made. Once this additive is exposed to UV light, it releases free radicals that travel through the bags material. This eventually causes the bag to become brittle. As with oxo degradation, the process of degradation continues through exposure to heat, stress and UV light. It continues to break down into smaller, and smaller pieces, until it's reduced to dust-size particles.
It can take anywhere from 18 months to 5 years for a biodegradable bag to degrade, but 3 years is more usual. The speed at which a bio bag degrades depends on the type of landfill site the bag ends up on, and what amount of heat, stress and exposure to sunlight occurs. Moisture levels have also been shown to alter the speed at which a bag will degrade.