What's Biodegradable plastic?
Biodegradable plastic can be defined as a material that, due to the activity of microorganisms, can finally convert to carbon dioxide, water, and biomass. In the absence of oxygen, methane may also be produced.
Based on this definition, the degree of biodegradation of plastic can be quantitatively determined by measuring the generated carbon dioxide and methane. This is true nowadays for most, if not all, the standard test methods to determine the degree of biodegradations.
Biodegradation of plastics may take several weeks to several centuries or even longer depend on not only their chemical and physical structures but also the surrounding environments, such as the temperature, humidity, pH, etc. Even the same material, when they are placed in different environments, for example, one is in compost while another one in soil, the rate of biodegradation may differ significantly.
The biodegradation rate is often described as the percentage of biodegradation in a given time period. To reach a same degree of biodegradation, the shorter time period the faster biodegradation rate, or in a same period, the higher degree of biodegradation the faster biodegradation rate.
Biodegradable plastics are always expected to biodegrade quickly once they are placed in the right environment, for example, in compost or in soil after their end use. However, this is not always true because most of them take much longer time than expected to get fully biodegraded. This raises a question that how to define a plastic is biodegradable in terms to a measurable timescale.
There are many standards worldwide, such as EN 13432 and ASTM D 5526. In most cases, it is required that no less than 90% of biodegradation must be achieved within a maximum of 6 months in a given environment. Only the plastics meeting these requirements can get the relevant certificate and then can be claimed to be biodegradable.
This explains why the only a limited number of biodegradable plastics can be found in the markets because most of the plastics we are using everyday cannot biodegrade fast enough in that given environment to meet the requirements.
