Biodegradation of plastics in the open environment

Biodegradation of plastics in the open environment

The need to assess and characterise the biodegradation of plastics which are lost to the open environment, through littering (intentional or otherwise), commonly referred to as “fugitive” plastics, has been recognised for a long time, with the use of several established methods being successfully employed for this purpose for many years.

The conditions which plastics are exposed to in the open (natural) environment are complex and variable; temperature range, moisture levels, UV exposure, bioactivity levels of the terrestrial or marine environments. Consequently, unlike conditions found in synthetic environments such as industrial composting, it is impossible to standardise any single set of conditions which are representative of the open (natural) environment.

The biodegradation of plastics, in general, requires that the high molecular weight polymeric material be structurally and chemically altered to produce lower molecular weight carbonaceous molecules, which is typically achieved by the introduction of (additional) oxygen into the molecular structure. This oxidation is produced by one or more processes such as thermal/photo-oxidative degradation or hydrolysis reactions.

The resulting substances, structurally degraded and chemically altered from the original polymeric material, can then be used as a food source by bacteria etc., resulting in the biodegradation of the lower molecular weight materials.

Several standards, specifications or test guides have been developed to assess and characterise the biodegradation of plastics in the open (natural) environment. A detailed comparative summary of three of these documents, ASTM D6954-18, BSi 8472:2011 and PAS 9017:2020, is provided after the overview discussed below.

ASTM D6954-18: The (first) standard test guide, initially published in 2004, has been revised and developed several times since, most recently in 2018. This standard test guide recognises the variability of the conditions in the open environment by not imposing artificial time-frames on the various aspects of the biodegradation processes characterised in the laboratory testing environment.

Biodegradation of plastics in the open (natural) environment is characterised through a three-tier process (a system also used in all other national and international testing regimes, including BSi 8472:2011 and PAS 9017:2020, for biodegradation of plastics in the open (natural) environment).

Tier 1 involves the abiotic degradation of the plastic; various conditions of UV level, temperature and moisture conditions are permitted with the time to achieve certain parameters of reduction in Mw (<5000 Daltons) recorded, thus reflecting the variability of the exposure conditions fugitive plastic would encounter in the open environment, and are thus independent of sample thickness. Carbonyl Index and mechanical property measurements may be used to follow the degradation process.

The degraded material is subjected to a standard laboratory-based biodegradation process, Tier 2, which may be selected from various methods, representative of the different conditions found in the open environment. The methodologies of the standard biodegradation tests are utilised, with the time required to achieve specified minimum levels (60% or 90% dependant on polymer type) of biodegradation recorded.

The resulting biodegradation residues within the test inoculum are assessed for ecotoxicity to flora and fauna in Tier 3 through the use of international standards and methodologies. Additionally, there is a requirement for materials to comply with the heavy metal and toxic substance lists of the applicable jurisdictions.

BSi 8472:2011: This British Standard utilises a similar three-tier methodology as ASTM D6954- 18.

Tier 1 abiotic degradation is assessed by the exposure of the plastic under standard conditions of UV and temperature, independent of sample thickness. The level of structural degradation is assessed through empirical “mechanical” testing methods with the exposure time required to meet the subjective, empirical requirements recorded.

The degraded material is assessed for biodegradation in Tier 2, using the standard methodology of BS EN ISO 17556 (soil environment), with the time required to achieve 50% (minimum) biodegradation recorded.

The residues within the test inoculum once 50% biodegradation has been achieved are assessed for ecotoxicity to flora in Tier 3, through the use of international standards and methodologies. However, there are no requirements for materials to be assessed for ecotoxicity to fauna or heavy metal and toxic substances.

PAS 9017:2020: This publicly available specification utilises a three-tier methodology to assess the biodegradation of, specifically, polyolefins in an open-air terrestrial environment.

Tier 1 involves the abiotic degradation of the polyolefin-based product under defined conditions of UV exposure level and temperature for specified times. The time of, and the exposure conditions, are dependent on the material thickness; counter-intuitive to the variable exposure conditions fugitive plastic would encounter in the open environment.

Molecular weight parameters determined; Mn <5000 Daltons, Mz <30,000 Daltons and Mw >90% reduction, with carbonyl index (methodology defined in PAS) >1, to be achieved within the specified time frames applicable to the different thicknesses of polyolefin samples.

Note: the methodology for determining carbonyl index defined in this PAS utilises the determination of the area under the curve between two sets of wave numbers which include characteristic peaks of typical polymer additives found in commercially available polymers, which may affect the validity of the carbonyl index measurement.

Tier 2 testing involves the assessment of the surface of undegraded material and the degraded material from Tier 1 for ecotoxicity to flora and fauna through the use of international standards and methodologies.

The degraded material is assessed for biodegradation in Tier 3, using the standard methodology of BS EN ISO 17556 or ASSTM D5988-18 (soil environment), with a maximum time of two years to achieve 90% (minimum) required.

There are no requirements for biodegradation residues to be assessed for ecotoxicity to flora or fauna, or for any material at any stage of the biodegradation process to be assessed for heavy metal and toxic substances.

In conclusion, from reviewing the testing performed in the biodegradation characterisation techniques detailed in these methodologies, it is clear that the analysis of molecular weight parameters (Mw, Mz & Mn) and FT-IR analysis (carbonyl index) utilised by ASTM D6954-18 and PAS 9017:2020 demonstrate the molecular structural changes that occur (molecular weight reduction) and chemical alteration of the plastic/polyolefin materials (carbonyl index growth), thus confirming that the abiotic degradation of the materials (characterised by these methods) do not generate microplastics.

ASTM D6954-18 is commonly referred to as the standard test guide to assess and characterise Oxo-Biodegradable Plastics. The extensive and obvious similarities between ASTM D6954-18 and PAS 9017:2020 demonstrate that these methodologies are different ways to assess and characterise the same types of materials.

Comparison of the requirements for ASTM D6954-18, BS 8472:2011 & PAS 9017:2020

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