Oxo-degradation

OXO-degradation is the degradation of polymers such as plastic by an oxidative process. Such degradation breaks down plastic into small pieces that are easier to biodegrade (but not necessarily enough to be called "biodegradable"). When biodegradation does happen fast enough, either simultaneously or successively, the combined process becomes OXO-biodegradation and would break the polymer down to simple molecules.

A range of additives can be used to make conventional plastics OXO-degradable, but OXO-biodegradability is harder to obtain and characterize. If a plastic is merely OXO-degradable but the fragments remain non-biodegradable, it can instead worsen pollution by creating microplastics. In 2019, by the EU directive 2019/904 (Article 5) of the European Parliament and of the Council (5 June 2019) the introduction of single use plastics made from oxo-degradable plastics was prohibited.

Background

There are two different types of biodegradable plastic:

• Vegetable-based plastics, which are also loosely known as "bioplastics" or "compostable plastics", are tested in accordance with ASTM D6400 or EN13432 as to their ability to biodegrade under conditions found in industrial composting or biogas facilities.
• Oxo-biodegradable plastics—which are made from polymers such as polyethylene (PE), polypropylene (PP), or polystyrene (PS)—contain a prodegradant catalyst—often a salt of manganese or iron, and are tested in accordance with ASTM D6954 or BS8472, or AFNOR Accord T51-808, as to their ability to degrade and then biodegrade in the open environment. The prodegradant catalyzes the abiotic degradation process so that Oxo-biodegradable plastic will degrade in the presence of oxygen much more quickly than ordinary plastic.
  The plastic material has then been converted into small-chain organic chemicals, such as ketones, alcohols, carboxylic acids, and low molecular mass hydrocarbon waxes. The remaining chemicals are no longer plastic and are biodegradable by bacteria, which are ubiquitous in the terrestrial and marine environments. The timescale for complete biodegradation at any time or place in the open environment is much shorter than for "conventional" plastics, which in normal environments are very slow to biodegrade and cause large scale harm.

"Oxo-degradable" plastics fragment but do not biodegrade except over a very long time; "Oxo-biodegradable" plastics degrade and then biodegrade. The European Committee for Standardization (CEN, for Comité Européen de Normalisation) has established the following definitions, in TR 15351:

1. Oxo-degradation is degradation resulting from "oxidative cleavage of macromolecules";
2. Oxo-biodegradation is "degradation resulting from oxidative and cell-mediated phenomena, either simultaneously or successively.

With regard to definition 2, an oxo-biodegradable polyolefin plastic file (e.g. polyethylene, polypropylene, and all their combinations), incorporating a catalyst that ensures fast oxidative cleavage of its macromolecules, will become biodegradable by cell-mediated phenomena (bacteria and fungi) in the environment and much more quickly than conventional ordinary plastic.

Degradation process

Illustration of the OXO-Degradation: A process whereby the conventional polyolefin plastic is first oxidised to short-chain oxygenated molecules.

Degradation is initially prevented by the presence of polymer stabilizers in the plastic, which ensure a useful service-life for the article. Once the stabilisers have been exhausted OXO-biodegradation will begin. The chemical mechanism is that of autoxidation but it is greatly accelerated by the presence of metal-catalysts, which promote the homolysis of hydroperoxides into free radicals which drive the degradation process. Access to oxygen is essential and OXO-degradable plastics will not degrade if buried deep in landfill.

Conventional polyethylene (PE) and polypropylene (PP) plastics will typically fragment quite quickly, but will then take decades to become biodegradable. OXO-degradable plastic, if discarded in the environment, will degrade to oxygenated low-molecular-weight chains (typically MW 5–10,000 amu) within 2–18 months, depending on the material (resin, thickness, anti-oxidants, etc.), temperature, and other factors in the environment.

biodegradation of up to 91% has been obversed in a soil environment within 24 months, when tested in accordance with ISO 17556. OXO-degradation has been studied at the Eurofins laboratory in Spain, where on 25 July 2017 they noted 88.9% biodegradation in 121 days,

The statements about biodegradation of oxo-degradable plastics have however been disputed by the EU. In a 2017 report it was stated that the biodegradation of the fragmented pieces is only partially supported

Standards applicability

Oxo-biodegradable plastic degrades in the presence of oxygen. Heat and UV light will accelerate the process, but neither they nor moisture are necessary. Such plastic is not designed to be compostable in open industrial composting facilities, according to ASTM D6400 or EN13432; but it can be satisfactorily composted in an in-vessel process.

ASTM D6400 and EN13432, the standards for industrial composting, require oxo-biodegradable material to convert to carbon dioxide (CO₂) gas within 180 days by industrial composting, which is faster than degradation in the open environment. A leaf is generally considered to be biodegradable, but it will not pass the ASTM composting standards, due to the 180-day limit. Indeed, materials which do comply with ASTM D6400, EN13432, Australian 4736, and ISO 17088 cannot properly be described as "compostable". This is because those standards require them to convert substantially to CO₂ gas within 180 days. You cannot therefore make them into compost—only into CO₂ gas. This contributes to climate change, but does nothing for the soil.

OXO-biodegradable plastic conforms to the American Standard (ASTM D6954) and a British Standard (BS8472), which specify procedures to test degradability, biodegradability, and non-toxicity, and with which a properly designed and manufactured OXO product must comply. These standards contain pass/fail criteria.

There is no need to refer to a standard specification unless a specific disposal route (e.g., composting), is envisaged. ASTM D6400, EN13432, and Australian 4736 are standard specifications appropriate only for the special conditions found in industrial composting.

According to an EU report the oxo-degradable plastics do not biodegrade on a landfill neither should they be regarded as compostable.

Environmental issues

Oxo-degradable plastic, including plastic carrier bags, may degrade quicker in the open environment than conventional plastic. However, according to a report from the European Commission, there is no evidence that oxo-degradable plastic will subsequently fully biodegrade in a reasonable time in the open environment, on landfills, or in the marine environment. According to the 2018 report:

Sufficiently quick biodegradation is in particular not demonstrated for landfills and the marine environment. A wide range of scientists, international and governmental institutions, testing laboratories, trade associations of plastics manufacturers, recyclers and other experts have therefore come to the conclusion that oxo-degradable plastics are not a solution for the environment and that oxo-degradable plastic is not suited for long-term use, recycling or composting. There is a considerable risk that fragmented plastics will not fully biodegrade and a subsequent risk of an accelerated and accumulating amount of microplastics in the environment, especially the marine environment. The issue of microplastics is long acknowledged as a global problem in need of urgent action, not just in terms of clean-up of littering, but also of plastic pollution prevention.

One major problem with testing oxo-degradable plastics for safety is that current standards and test methods can't realistically predict the biodegradability of plastic carrier bags within natural ecosystems. Moreover, existing biodegradability standards and test methods for aquatic environments do not involve toxicity testing or account for the potentially adverse ecological impacts of carrier bags, plastic additives, polymer degradation products, or small (microscopic) plastic particles that can arise via fragmentation.

A 2019 EU report describes oxo-degradable plastics as:

Oxo-degradable plastic’ means plastic materials that include additives which, through oxidation, lead to the fragmentation of the plastic material into micro-fragments or to chemical decomposition

Controversy

On 6 November 2017, the Ellen MacArthur Foundation issued a paper supported by 150 organisations—including M&S, PepsiCo, and Unilever—backing a call to ban oxo-biodegradable plastics. The report had support from industry associations including the British Plastics Federation Recycling Group and the Gulf Petrochemicals and Chemicals Association, NGOs such as the World Wildlife Fund (WWF), scientists including those based at Plymouth Marine Laboratory, and ten MEPs from nine EU countries.

The Oxo-Biodegradable Plastics Association (OPA) however, claimed the report was inaccurate. It argued many of the 150 organisations aggressively promoted a rival bio-plastic technology, while many of the others whose logos appeared in the document were themselves producers of the plastic items that get into the open environment as litter. The paper's conclusions were rejected by Professor Ignacy Jacubowicz, who said the degradation process was not merely a fragmentation, but a change from a high molecular weight polymer to a material that can be bio-assimilated.

The evidence for and against oxo-biodegradable plastic was also reviewed in November 2018 by Peter Susman QC, a deputy judge of the High Court on England, who had over 25 years experience of adjudicating cases in the technology and construction branch of the high court, involving the evaluation of expert evidence. He declared the scientific case in favour of oxo-biodegradable plastic to be "clear and compelling". Susman examined the processes of abiotic and biotic degradation of plastics, and then looked specifically at degradation in air and degradation in seawater. He concluded, in a 15-page written opinion that

It is no longer tenable to conclude that there is 'no firm evidence either way' whether oxo-biodegradable is effective. I consider that recent research provides clear and compelling evidence that oxo-biodegradable plastic is indeed effective in facilitating very significantly speedier degradation than is the case when that technology is not used.... [I] cannot imagine that such significantly speedier final degradation occurs later than 'within a reasonable time', however that the expression might be defined.... [I regard the idea that biodegradable plastics might encourage littering as] "fanciful and unreasonable".

Peter Susman's report was criticized by others.

European strategy for plastics in a circular economy

On 16 January 2018, the European Commission published its report on the use of oxo-degradable plastic. The document forms part of the European strategy for plastics in a circular economy, which was released the same day.

The Commission focused on three key issues relating to oxo-degradables: the biodegradability of oxo-degradable plastics in various environments; the environmental impacts in relation to littering; and recycling.

The Commission found there was no conclusive evidence that, in the open environment, oxo-degradables fragmented to a sufficiently low enough molecular weight to enable biodegradation. There was no conclusive evidence about the time needed for oxo-degradable plastics to fragment in marine environments, nor about the degree of fragmentation. It said there was a considerable risk that fragmented plastics would not fully biodegrade, leading to subsequent risk of an accelerated and accumulating amount of microplastics, especially in the marine environment. Rapid fragmentation was found to increase the risk of microplastic ingestion by marine animals.

In relation to littering, the report found that, although it appeared the oxo-degradable plastics industry could create products with minimal toxic impact on flora and fauna, it had not been conclusively proven that there were no negative effects. Marketing oxo-degradables as a solution for plastic waste in the environment may make it more likely that items are discarded inappropriately and in marine environments; the fragmentation process made oxo-degradable plastic less likely to be recovered during clean-up exercises.

The report was criticised by the Oxo-Biodegradable Plastics Association (OPA), which said the European Commission had failed to understand the difference between oxo-degradable and oxo-biodegradable plastics. The OPA accused the Commission of not listening to evidence relating to the breakdown of oxo-plastics, which it maintained showed the plastic broke down to a molecular level that could be bioassimilated. In relation to timescales for biodegradation, the OPA said it was not useful to examine how long it took for particular specimens to breakdown in particular conditions, due to the variability of environmental conditions. It said the key point was that oxo-biodegradable plastics would breakdown faster than conventional plastics under the same conditions. Regarding recycling, it said its members had been successfully recycling oxo-biodegradable plastics for more than ten years, with no adverse reports. It rejected the Commission's view on littering and said that, as oxo-degradable plastics were indistinguishable from other plastic products, they were unlikely to cause any additional levels of littering. It criticised the Commission's use of external reports, including that of the Ellen MacArthur Foundation, the findings of which it previously disputed.

A more recent report, of October 2018, is in line with the previous one. It states that micro-plastics need to be restricted, including oxo-degradable plastics.

The EU directive 2019/904 of the European Parliament and of the Council (5 June 2019) prohibits the introduction in the market of products made from oxo-degradable plastic (Article 5)

Response of the Oxo-biodegradable Plastics Association to the EU

In its Proposal (2018/0172(COD)) for a Directive on "Reduction of the impact of certain plastic products on the environment", the EU Commission proposed various measures for reducing the quantity of plastic goods being produced, and measures for encouraging collection for recycling. Most people would support those measures, but plastic will still escape into the open environment in unacceptable quantities until such time as plastic waste has been eliminated. This is not likely to happen any time soon.

According to the Oxo-biodegradable Plastics Association oxo-biodegradable technology is the only way to prevent the accumulation of plastic waste in the environment; and if oxo-biodegradable technology were severely restricted in the EU, there would be unintended consequences. There would be a distortion of markets, if European companies were effectively prevented from manufacturing for countries where oxo-biodegradable plastic is mandatory. Alternatively, some countries could follow Europe's lead with disastrous consequences, and much of their accumulated plastic waste would eventually find its way to the shores of Europe.

Recital (3) to the draft Directive says "Marine litter is of a transboundary nature and is recognized as a global problem." The Reis report to the European Parliament (11 October 2018) says "Every year in Europe, 150,000 tonnes of plastic are dumped into the sea. The situation is even more alarming at [the] global level, with 8 million tonnes ending up in the sea each year." Recital (5) to the draft Directive says "In the Union, 80 to 85 % of marine litter, measured as beach litter counts, is plastic, with single-use plastic items representing 50%." This is why the plastic needs to be urgently upgraded so that it will convert into biodegradable materials much sooner than ordinary plastic, if it does escape into the open environment, especially the oceans.

Microplastics being recovered from the oceans are from "oxo-degradable" plastics, which degrade and fragment but do not biodegrade except over a very long period of time. These are conventional plastics which undoubtedly create persistent microplastics, and this is why they have been banned for a wide range of products in Saudi Arabia and 11 other countries, where oxo-biodegradable technology for making these products is now mandatory. The products have to comply with strict standards, based on ASTM D6954.

Sources

• "Environmentally Degradable Plastics Based on Oxo-biodegradation of Conventional Polyolefins". Norman C. Billingham, Emo Chiellini, Andrea Corti, Radu Baciu and David M Wiles, Paper presented in Cologne (can be obtained from Authors).
• Chiellini, Emo; Cortia, Andrea; Swift, Graham (2003). "Biodegradation of thermally-oxidized, fragmented low-density polyethylenes". Polymer Degradation and Stability. 81 (2): 341–351. doi:10.1016/s0141-3910(03)00105-8.
• Report from CIPET (India) test on Renatura OxoDegraded PE Film using ASTM D5338 demonstrates 38,5% Bio-mineralization of PE in 180 days 1991; 57(3): 678–685.
• Jakubowicz, Ignacy (2003). "Evaluation of degradability of biodegradable polyethylene (PE)". Polymer Degradation and Stability. 80: 39–43. doi:10.1016/s0141-3910(02)00380-4.
• Jakubowicz, Ignacy; et al. (2011). "Kinetics of abiotic and degradability of low-density polyethylene containing prodegradant additives and its effect on the growth of microbial communities". Polymer Degradation & Stability. 96 (5): 919–928. doi:10.1016/j.polymdegradstab.2011.01.031.
• Koutny, Marek; Lemaire, Jaques; Delort, Anne-Marie (2006). "Biodegradation of polyethylene films with prooxidant additives" (PDF). Chemosphere. 64 (8): 1243–1252. Bibcode:2006Chmsp..64.1243K. doi:10.1016/j.chemosphere.2005.12.060. PMID 16487569.
• Koutny, Marek; Sancelme, Martine; Dabin, Catherine; Pichon, Nicolas; Delort, Anne-Marie; Lemaire, Jacques (2006). "Acquired biodegradability of polyethylenes containing pro-oxidant additives" (PDF). Polymer Degradation and Stability. 91 (7): 1495–1503. doi:10.1016/j.polymdegradstab.2005.10.007.
• Seneviratne, Gamini; Tennakoon, N. S.; Weerasekara, M. L. M. A. W.; Nandasena, K. A. (2006). "Polyethylene biodegradation by a developed Penicillium–Bacillus Biofilm". Current Science. 90: 1.
• Sipinen, Alan J.; Rutherford, Denise R. (1993). "A Study of the Oxidative Degradation of Polyolefins". Journal of Environmental Polymer Degradation. 1 (3): 193–202. doi:10.1007/bf01458027.
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• Ungtae Lee, Anthony L. Polmetto III; Fratzke, Alfred; Bailey Jr, Theodore B. (1991). "Biodegradation of Degradable Plastic Polyethylene by Phanerochaete and Streptomyces Species". Applied and Environmental Microbiology. 57 (3): 678–685. Bibcode:1991ApEnM..57..678L. doi:10.1128/AEM.57.3.678-685.1991. PMC 182779. PMID 16348434.
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External links

• Oxo Biodegradable Plastics Federation