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Changing Course: Applications and Standards of Biodegradable Polymers

Key Highlights:

Biodegradable polymers are finding evermore applications within society, from carrier and food bags, food packaging, single-use cutlery and beverage cup lids, to agricultural applications such as mulching film. Whilst there has been, and continues to be, significant development in biodegradable polymers, with new grades of PHA, PBS & PLA for instance, entering the market, the same cannot be said of the associated standards which are referenced as suitable for the applications or anticipated disposal routes.

These standards have typically been developed to reflect what the material can do rather than what it needs to do, and by and large do not reflect actual conditions, as evidenced by consideration of the most commonly recognised biodegradable polymers; those marketed as “compostable”.

These are usually only suitable for industrial composting disposal routes as per EN13432 [Packaging. Requirements for packaging recoverable through composting and biodegradation] or ASTM D6400-21 [Standard Specification for Labelling of Plastics Designed to be Aerobically Composted in Municipal or Industrial Facilities]. These polymers add no value to the compost, rather, a minimum of 90% of the organic carbon with the polymer needs to be converted to CO2 within 6 months at 58°C in a standardised test compost. However, industrial composting facilities generally operate at lower temperatures, with much shorter periods of time and the mix of compost ingredients being highly variable.

This unsatisfactory situation with “compostable” polymers is further complicated with the development of “home compostable” test methods which attempt to reflect the varied conditions and management of home compost heaps, which are more akin to natural environments. How to standardise the natural environment for laboratory testing to produce a definitive result applicable to all natural environments?

The issues of assessing the polymer biodegradation in a natural environment such as soil, is illustrated by consideration of the test criteria in ISO17556 [Ultimate Aerobic Biodegradability of Polymers in Soil. Degradation of Polymers and Plastics in Soil]. Test material may be micronised to powder, increasing the surface area of the polymer to the test inoculum, mixed and tamped to increase surface contact with a highly prescribed standard soil composition and tested at (typically) 25-28°C for up to 2 years. A minimum of 90% of the organic carbon of the polymer is required to be converted to CO2 within two years for the polymer to be classed as soil biodegradable.

How reflective of real, natural soil environments are these highly prescribed test conditions? Especially when ISO17566 is used within EN17033 [Plastics - Biodegradable mulch films for use in agriculture and horticulture - Requirements and test methods] to determine whether a material may be classed as a biodegradable mulching film, and thus allow the polymeric film to be ploughed into the arable ground at the end of the growing season.

ASTM D6954-24 [Standard Guide for Exposing and Testing Plastics that Degrade in the Environment by a Combination of Oxidation and Biodegradation] recognises the issues of the relevancy of standardised test conditions to a variety of natural environments with regards to biodegradability of polymers by prescribing a tiered approach and a requirement for a highly prescribed reporting of test results caveated with test conditions. This brings its own issues, with no simple message proffered compared to compostability and soil biodegradation testing.

It is therefore necessary for standards bodies to develop more appropriate standards which better define material performance to realistic conditions, thus safeguarding the public and environment from those materials which do not perform as advertised.

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