Biodegradable plastics are those that, in specific conditions, can be decomposed completely by living organisms within a reasonable timeframe, without harming the environment.
Biodegradable plastics must have the right conditions to biodegrade. In landfill or littered they will not biodegrade and instead pose similar risks to traditional plastics. Those that find their way into the ocean or other natural environments gradually splinter into microplastics, polluting ecosystems and making their way into the food chain.
The majority of biodegradable plastics currently available are compostable plastics. Compostable plastics are those that completely break down within a specified time frame (12 weeks according to European standards) in the right composting conditions. Most require industrial composting facilities of 60°C or higher, of which availability is limited the world over.
Neither the terms biodegradable nor compostable imply anything about the material’s ability to break down quickly in a natural environment.
Isabel Thomlinson, The Conversation
Most compostable plastics end up in landfill where, instead of turning into compost, they release methane, a greenhouse gas approximately 30 times more potent than carbon dioxide. Our peer reviewed Life Cycle Analysis showed that across their life cycle, compostable cups have worse carbon emissions than other single-use cups.
Bioplastics are plastics made from plants, or other biological materials, rather than crude oil. Not all bioplastics are biodegradable.
The two main types of bioplastics are PLA (polylactic acid), usually made from corn starch, cassava or sugar cane, and PHA (polyhydroxyalkanoate) made by microorganisms which are sometimes genetically engineered.
Polymer chains for PET plastic - commonly used to make single-use water bottles - can now be synthesised using renewable sources like sugar cane or wood. However, the resulting non-biodegradable bioplastic is chemically identical to the fossil counterpart, behaving in the environment just like conventional plastic, persisting for a long time and with limited recovery pathway for recycling.
PLA is technically recyclable, compostable and biodegradable however requires specialised systems to be processed, and these are rarely available for consumers. PLA is often indistinguishable from other plastics and compromises standard recycling streams.