How we talk about plastics

Materials World magazine
,
28 Nov 2018

Plastics can be recyclable, compostable biodegradeable – but what do these really mean? Stuart Patrick breaks down single-use plastics.

Bioplastics are mainly based on carbohydrate-rich plants, such as corn or sugar cane, and so the ingredients are, in essence, from renewable sources. European Bioplastics, the association representing the interests of the bioplastics industry in Europe, defines different categories of polymers as follows.

Biodegradable

The property of biodegradability does not depend on the resource basis of a material. This feature is directly linked to the chemical structure of the polymer and can benefit particular applications, in particular packaging. Biodegradable plastic types offer new ways of recovery, including organic recycling.

These materials can be completely biodegraded, or bio-assimilated, by micro-organisms such as bacteria, fungi and algae. On its own, the term is obsolete to a degree, as most materials will biodegrade given enough time. There are no defined time limits for the term ‘biodegradable’, thus the use of this word can be potentially confusing to the general public, both regarding where to dispose of the plastic and what happens to it once discarded.

However, not all biodegradable materials or products are compostable, as the time needed for them to biodegrade may be beyond that allowable in both home and industrial compostable facilities. This is coupled by the fact that the temperatures often needed to induce biodegradation may not be reached through a non-composting environment. If a packaging or plastic material is described or labelled as simply ‘biodegradable’, this does not convey sufficient meaning about suitable ways to recover it after it has become waste.

Compostable

If certified compostable according to international standards, such as the EN 13432, these plastics can be composted in industrial composting plants, located at local authority sites, depending on each authority. This is achieved through the action of naturally occurring micro-organisms, performed to a high extent within a specified timeframe. Industrial composters operate at about 600°C with controlled humidity.

This level of certification indicates that the product can be industrially composted, and not only the plastic but all other components of the product, including colours, labels, glues and in the case of packaging products any residues of the content.

Testing for certification according to EN 13432 encompasses:

  • Chemical test. Disclosure of all constituents, threshold values for heavy metals are to be adhered to
  • Biodegradability in controlled composting conditions (oxygen consumption and production of CO2). Proof must be made that at least 90% of the organic material is converted into CO2 within six months
  • Disintegration. After three months’ composting and subsequent sifting through a 2mm sieve, no more than 10% of residue may remain, compared with the original mass
  • Practical test of compostability in a semi-industrial (or industrial) composting facility. No negative influence on the composting process is permitted
  • Ecotoxicity test. Examination of the effect of resultant compost on plant growth (agronomic test).

Home compostability

If certified home compostable, i.e. for direct inclusion in a home composting bin along with other organic waste, the same testing criteria applies but the test is carried out at 20-30°C for 12 months for biodegradation and at the same temperature for six months for disintegration. All other test criteria are the same as for industrial composting.

Anaerobic digestion

Anaerobic digestion (AD), an alternative to industrial composting, is the breakdown of organic material by micro-organisms in the absence of oxygen. AD produces biogas, a methane-rich gas that can be used as a fuel, as well as digestate, a source of nutrients that can be used as a fertiliser. Increasingly, AD is being used to make the most of consumer waste by turning it into renewable energy.

Marine environment

It should now be obvious that almost all biodegradable plastics are designed to biodegrade in soil under specific conditions, though not in water. All plastics will degrade over time in water but well outside of the ‘accepted’ timescale for breakdown.

According to a recent review paper in Elsevier’s journal, Polymer degradation and stability, few studies have been carried out into degradation of a common bioplastic, polylactic acid (PLA) in aquatic systems, in contrast to terrestrial systems. In both static and dynamic seawater, no evidence of microbial degradation was found after 10 weeks, leading the authors to suggest that marine microbes have a limited ability to degrade PLA, while another study reported little change in molecular weight of PLA rods after three months’ immersion in seawater at 200°C. However, a 48% reduction was found when immersed in seawater at 400°C for three months, which was attributed to hydrolysis rather than microbial degradation.

Microplastics

Commonly, the term ‘microplastics’ is used to describe plastic particles of less than 5mm in diameter, which includes particles as small as 10nm. There is currently much debate and investigation into the fate of microplastics resulting from the biodegradation process.


Stuart Patrick is Interim Chair of the IOM3 Polymer Society and a member of the Sustainable Development Group. He would like to thank Professor Stephen Rimmer at the University of Bradford, UK, for his help in supplying material for this article.