There are two main types of biodegradable plastics in the market: hydro-biodegradable plastics (HBP) and oxo-biodegradable plastics (OBP). Both will first undergo chemical degradation by hydrolysis and oxidation respectively. This results in their physical disintegration and a drastic reduction in their molecular weight. These smaller, lower molecular weight fragments are then amenable to biodegradation.
OBPs are made by adding a small proportion of compounds of specific transition metals (iron, manganese, cobalt and nickel are commonly used) into the normal production of polyolefins such as polyethylene (PE), polypropylene (PP) and polystyrene (PS). The additives act as catalysts to speed up the normal oxidative degradation, increasing the overall process by up to several orders of magnitude (factors of 10).
The products of the catalyzed oxidative degradation of the polyolefins are precisely the same as for conventional polyolefins because, other than a small amount of additive present, the plastics are conventional polyolefins. Many commercially useful hydrocarbons (e.g., cooking oils, polyolefins, many other plastics) contain small amounts of additives called antioxidants that prevent oxidative degradation during storage and use. Antioxidants function by ‘deactivating’ the free radicals that cause degradation. Lifetime (shelf life + use life) is controlled by antioxidant level and the rate of degradation after disposal is controlled by the amount and nature of the catalyst.
Since there are no existing corresponding standards that can be used directly in reference to plastics that enter the environment in other ways other than compost – i.e. as terrestrial or marine litter or in landfills, OBP technology is often attacked by the HBP industry as unable to live up to the standards (which are actually the standards for composting). It has to be understood that composting and biodegradation are not identical. OBP can however be tested according to ASTM D6954, and (as from 1.1.2010) UAE 5009:2009.
HBPs tend to degrade and biodegrade somewhat more quickly than OBP, but they have to be collected and put into an industrial composting unit. The end result is the same – both are converted to carbon dioxide (CO2), water (H2O) and biomass. OBP are generally less expensive, possess better physical properties and can be made with current plastics processing equipment. HBP emits methane in anaerobic conditions, but OBP does not.
Polyesters play a predominant role in hydro-biodegradable plastics due to their potentially dydrolysable ester bonds. HBP can be made from agricultural resources such as corn, wheat, sugar cane, or fossil (petroleum-based)resources , or blend of the two. Some of the commonly-used polymers include PHA (polyhydroxyalkanoates), PHBV (polyhydroxybutyrate-valerate), PLA (polylactic acid), PCL (polycaprolactone), PVA (polyvinyl alcohol), PET (polyethylene terephthalate) etc. It would be misleading to call these “renewable” because the agricultural production process burns significant amounts of hydrocarbons and emits significant amounts of CO2. OBPs (like normal plastics)are made from a by-product of oil or natural gas, which would be produced whether or not the by-product were used to make plastic.
HBP technology claims to be biodegradable by meeting the ASTM D6400-04 and EN 13432 Standards. However, these two commonly quoted standards are related to the performance of plastics in a commercially managed compost environment. They are not biodegradation standards. Both were developed for hydro-biodegradable polymers where the mechanism including biodegradation is based on reaction with water and state that in order for a production to be compostable, the following criteria need to be met:
- Disintegration, the ability to fragment into non-distinguishable pieces after screening and safely support bio-assimilation and microbial growth;
- Inherent biodegradation, conversion of carbon to carbon dioxide to the level of 60% and 90% over a period of 180 days for ASTM D6400-04 and EN 13432 respectively; There is therefore little or no carbon left for the benefit of the soil, but the CO2 emitted to atmosphere contributes to climate-change.
- Safety, that there is no evidence of any eco-toxicity in finished compost and soils can support plant growth; and
- Toxicity, that heavy metal concentrations are less than 50% regulated values in soil amendments