What are biodegradable plastics? The need for a clarified terminology
| Dhaval Tanna
Our Knowledge Team has put together a comprehensive summary on bioplastics. The article below gives an overview and clarity on the terminology surrounding biodegradable plastics, their different uses and standards used in various regions. A one-page summary is available at the end of the article.
What are biodegradable plastics?
Biodegradable plastic is plastic which degrades under biological (mainly microbial) action. Some biodegradable plastics are compostable (but not all), which means they degrade under controlled conditions, such as those occurring at compost (or anaerobic digestion) sites.
Different biodegradable plastics only degrade under certain specified and tested conditions (e.g. degradable in soil, in wastewater treatment plants, etc), but most need industrial composting facilities1, which, as we’ll argue further on, is their reasonable destination. They may biodegrade in open environment, but the speed at which this occurs is wide-ranging, and related test methods and standards are not sufficiently defined and universally accepted. Compostable plastics need to further adhere to a set of requirements such as fragmentation, absence of ecotoxicity and threshold concentrations of potential pollutants such as heavy metals.
The focus of this article is on biodegradable plastic that is compostable, using mostly, and whenever appropriate, the term “compostable”. The focus relates to the fact that what matters from the angle of sustainable management of discards/waste, is the end-of-life behaviour of the materials, i.e. the possibility to be incorporated in compost schemes, and therefore to support separate collection (e.g. compostable bags) or to replace plastic items which would end up in disposal sites (e.g. compostable throwaway cutlery, when reusable cutlery is not suitable for the site-specific conditions, as e.g. local events in the countryside or disaster relief operations).
Bioplastics, Bio-based, Biodegradable, Oxo-plastics: some clarification
Other terms that might be used and are sometimes confusing:
Bioplastics ̶ may mean either bio-based (see below) or biodegradable; it is therefore a confusing term, usually needing further clarification; it is not recommended to use because of that.
Bio-based plastic ̶ refers to the renewable sources used to make the particular plastic. For example, sugar cane is processed to produce ethylene, which can then be used to manufacture, for example, polyethylene (PE). Starch can be processed to produce lactic acid and subsequently polylactic acid (PLA). The properties of bio-based plastics can vary considerably from material to material.
They can be very different from biodegradable plastics in terms of applications and end-of-life management. They are often similar to regular plastic; just the fossil source is replaced with a biological one (meaning it can behave just like fossil-based plastic and should therefore go to recycling, preferably, or be disposed of). However, some bio-based plastics can be also biodegradable (e.g. PLA), but not all of them.
A specific case, worth separate consideration, is oxo-degradable plastics (sometimes also misleadingly called oxo-biodegradable) – they are neither bio-based nor biodegradable, but simply break into smaller fragments (hence, they can end up as micro-plastic). Therefore this term is generally regarded negatively from the environmental standpoint.
They are made from traditional polymers such as PE (polyethylene), PP (polypropylene), and PS (polystyrene) containing extra ingredients (metal salts). Oxo-degradable plastics are plastics made to break down into smaller fragments, not visible to the eye, in the presence of oxygen and sunlight, thanks to the addition of tiny amounts of metals like cobalt, iron or manganese2. As a result, they hamper composting (since they are not biodegradable) and recycling (since their breaking down harms mechanical properties of recycled plastics); they also represent a danger for the environment and the food chain, due to their fragmentation into micro-plastics. As a consequence, many NGOs have called on national institutions and international organisations for restrictions for such materials3; many countries, and the EU itself in the recently issued Plastic Strategy, have already formally expressed concerns about them, and will ban or restrict the use of this type of plastics4.
How to recognise compostable plastics? Standards and labels
Compostable plastics which are suitable for industrial composting have to meet specific requirements: compostability is defined by specific standards.
● ASTM D6400 is US standard for solid material biodegradation (by composting) required for labelling of plastics designed to be aerobically composted in municipal or industrial facilities5.
● EN 13432 is an EU standard where the key component is the need to recover packaging waste on the basis of industrial composting. The EN 13432 is considered as a solid reference for standards elsewhere as well; nowadays there is a trend to adopt similar requirements as EN 13432 also in the context of other standards, and ASTM standard itself was recently revised to align with EN (the same happened for the global standard ISO 18606). When related to non-packaging plastics (e.g. cutlery), the similar standard EN 14995 (which includes similar requirements) is used.
The key requirements of EN 13432 standard are:
✓ Chemical composition: the standard sets limit for volatile solids (i.e. organic matter) and heavy metal (Cu, Zn, Ni, Cd, Pb, Hg, Cr, Mo, Se, As).
✓ Biodegradation: chemical breakdown of materials into CO2, H20 and minerals. Pursuant to the standard, at least 90% of the materials have to be broken down by biological action in the testing conditions, within 6 months.
✓ Disintegration: the physical decomposition of a product into tiny pieces after 12 weeks; at least 90% of the products should be able to pass through a 2x2mm sieve.
✓ Quality of the final compost and ecotoxicity: the quality of the compost should not decline as a result of the added packaging material. The standard specifies checking this via ecotoxicity tests: this involves conducting an examination to see if the germination and biomass production of the plant are not adversely affected by the influence of composted packaging.
The EN 13432 standard specifies that packaging may be deemed to be compostable only if all the constituents and components of the packaging are compostable. Hence, all components must be tested. Compostable materials have to be labelled with specific logos, which guarantee that they are certified according to the compostability standard. A neutral competent third party has validated the conformity and guaranteed the monitoring of the product on the market.