One of the properties that distinguishes PHAs from petroleum-based plastics is their biodegradability. Produced naturally by soil bacteria, the PHAs are degraded upon subsequent exposure to soil, compost, or marine sediment. Despite their biodegradability the PHAs still have good resistance to water and moisture vapor, and are stable under normal storage conditions and during use.
Biodegradation of PHAs is dependent upon a number of factors such as the microbial activity of the environment and the exposed surface area. In addition, temperature, pH, molecular weight and crystallinity are important factors. Biodegradation starts when micro-organisms begin growing on the surface of the plastic and s ecrete enzymes that break down the polymer into its molecular building blocks, called hydroxyacids. The hydroxyacids are then taken up by the micro-organisms and used as carbon sources for growth. In aerobic environments the polymers are degraded to carbo n dioxide and water, whereas in anaerobic environments the degradation products are carbon dioxide and methane.
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 600C -- 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 com post 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 l ife 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 60C.
Biodegradation of PHAs is dependent upon a number of factors such as the microbial activity of the environment and the exposed surface area. In addition, temperature, pH, molecular weight and crystallinity are important factors. Biodegradation starts when micro-organisms begin growing on the surface of the plastic and s ecrete enzymes that break down the polymer into its molecular building blocks, called hydroxyacids. The hydroxyacids are then taken up by the micro-organisms and used as carbon sources for growth. In aerobic environments the polymers are degraded to carbo n dioxide and water, whereas in anaerobic environments the degradation products are carbon dioxide and methane.
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 600C -- 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 com post 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 l ife 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 60C.
No comments:
Post a Comment