Biosynthesis & Degradability
Degradation to carbon dioxide and water will occur only when they expose the polymer to microorganisms found naturally in soil, sewage, river bottoms, and other similar environments. The rate of degradation is dependent on the material thickness and the amount of bacteria present. Landfill simulations over a 19week period show test bottles experienced a weight loss ranging from 30% with oxygen present to 80% with no oxygen present. The fact that Biopol decomposes more rapidly without oxygen present is significant because oxygen is not present in modern landfills.
A number of reports have demonstrated that PHAs are compostable over a wide range of environmental conditions. In one report, the maximum biodegradation rates were observed at moisture levels of 55% and temperatures of around 60°C — conditions similar to those used in most large-scale composting plants. Up to 85% of the samples degraded within 7 weeks, and PHA coated paper was rapidly degraded and incorporated into the compost. In another study, the quality of PHA compost was determined by measuring seedling growth relative to a control. Seedling growth of around 125% of the control was found for a 25% PHB copolymer compost indicating that the compost can support a relatively high level of growth.
Biodegradation of PHAs has also been tested in various aquatic environments. In one study in Lake Lugano, Switzerland, items were placed at different depths of water as well as on the sediment surface. A life span of 5-10 years was calculated for bottles under these conditions (assuming no increase in surface area), while PHA films were completely degraded in the top 20 cm of sediment within 254 days at temperatures not exceeding 6°C.