Oxo Biodegradable

Not to be confused with Oxo (food).

OXO Biodegradable OXO-biodegradation is defined by CEN (the European Standards Organisation) {CEN/TR 1535-2006} as "degradation resulting from oxidative and cell-mediated phenomena, either simultaneously or successively." Sometimes described as "OXO-degradable" this describes only the first or oxidative phase. These descriptions should not be used for material which degrades by the process of OXO-biodegradation defined by CEN, and the correct description is "OXO-biodegradable."

There are two different types of biodegradable plastic:

  1. Oxo-biodegradable plastic, made from polymers such as PE (polyethylene), PP (polypropylene), and PS (polystyrene) containing extra ingredients (not heavy metals) and tested according to ASTM D6954 or BS8472 or AFNOR Accord T51-808 to degrade and biodegrade in the open environment
  2. Vegetable based plastics (also loosely knows as bio-plastics "bioplastics" or "compostable plastics"). These are tested in accordance with ASTM D6400 or EN13432 to biodegrade in the conditions found in industrial composting or biogas facilities.

OXO-bio plastic is conventional polyolefin plastic to which has been added small amounts of metal salts, none of which are "heavy metals" which are restricted by the EU Packaging Waste Directive 94/62 Art 11. These salts catalyze the degradation process to speed it up so that the OXO plastic will degrade abiotically at the end of its useful life in the presence of oxygen much more quickly than ordinary plastic. At the end of that process it is no longer visible, it is no longer a plastic as it has been converted via Carboxylation or Hydroxylation to small-chain organic chemicals which will then biodegrade. It does not therefore leave fragments of plastic in the environment. The degradation process is shortened from decades to years and/or months for abiotic degradation and thereafter the rate of biodegradation depends on the micro-organisms in the environment. It does not however need to be in a highly microbial environment such as compost. Timescale for complete biodegradation is much shorter than for "conventional" plastics which, in normal environments, are very slow to biodegrade[1] and cause large scale harm.

The useful life of a product made using oxo-biodegradable plastic can be programmed at manufacture, typically 6 months for a bread wrapper and 18 months for a lighweight, plastic carrier bag to allow for re-use. Oxo-biodegradable plastic can be manufactured with the existing machinery and workforce in factories at little or no extra cost. They have the same strength and other characteristics as ordinary plastics during their intended lifetime.

Degradation process

Degradation is a process that takes place in many materials. The speed depends on the environment. Conventional polyethylene (PE) and polypropylene (PP) plastics will typically take decades to degrade. But OXO-biodegradable products utilize a prodegradant to speed up the molecular breakdown of the polyolefins and to incorporate oxygen atoms into the resulting low molecular mass molecules. This chemical change enables the further breakdown of the material by naturally-occurring micro-organisms.

Illustration of the OXO-Degradation: A process whereby the conventional polyolefin plastic is first oxidised to short-chain oxygenated molecules which are biodegradable (typically after two to four months of exposure)

The first process of degradation in OXO-treated plastic is an oxidative chain scission that is catalyzed by metal salts leading to oxygenated (hydroxylated and carboxylated) shorter-chain molecules .

OXO 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.) and the temperature and other factors in the environment.

OXO plastics are designed so that they will not degrade deep in landfill and they will not therefore generate methane (a powerful greenhouse gas) in anaerobic conditions.

OXO-biodegradable products do not degrade immediately in an open environment because they are stabilized to give the product a useful service-life. They will nevertheless degrade and biodegrade in nature if they are exposed to the environment as litter much quicker than natural waste such as twigs and straw and much more quickly than ordinary plastic. OXO-bio plastics will degrade indoors, but this is not their purpose. They are intended to degrade and biodegrade by a synergistic process in the open environment.

OXO-biodegradation of polymer material has been studied in depth at the Technical Research Institute of Sweden and the Swedish University of Agricultural Sciences. A peer-reviewed report of the work was published in Vol 96 of the journal of Polymer Degradation & Stability (2011) at page 919-928. It shows 91% biodegradation in a soil environment within 24 months, when tested in accordance with ISO 17556.

Standards applicability

OXO-biodegradable plastic degrades in the presence of oxygen, and the process is accelerated by UV and heat. It can be recycled during its useful life with normal plastic.[2] It is not designed to be compostable in industrial composting facilities according to ASTM D6400 or EN13432, but it can be composted in an in-vessel process.

These standards require the material to convert to CO2 gas within 180 days because industrial composting has a short timescale and is not the same as degradation in the open environment. A leaf is generally considered to be biodegradable but it will not pass the composting standards due to the 180-day limit. (Indeed, materials which do comply with AST 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.

There is an American Standard (ASTM D6954) and a British Standard (BS8472) which specifies procedures to test degradability, biodegradability, and non-toxicity, and with which a properly designed and manufactured OXO product complies. It also contains pass/fail criteria to exclude any significant gel content which might inhibit degradation.

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

Another reference document has recently been published by the French standards organisation AFNOR. This document AC.51 808[3] offers a well researched method to test OXO-biodegradable plastics based on usage and climate conditions. It introduces a new testing method for the biodegradation of polymer using selected microorganisms and measuring ATP and ADP by chemiluminescence. This method brings a new approach as tests are done at 37 °C which is much more relevant to outdoor conditions than ASTM D6400 or EN 13432 done at 58 °C plus the microorganisms are identified based on the environment in which the plastic is likely to be disposed, which is not the case with the CO2-evolution method.

This French document[3] is a very interesting innovation for predicting the behaviour of an OXO-biodegradable plastic in case of littering. This test method provides an ecotoxicity testing method to ensure that residues in the environment, pending complete biodegradation, are not toxic for the Rhodococcus rhodochrous ATCC 29672[4] bacterium strain.

Environmental issues

OXO-bio plastics, especially in the form of plastic bags, are now used in many places as a solution to the problem of plastic litter in the open environment. They are mandatory in some areas of the Middle-East, Asia and Africa. OXO-biodegradable plastics contain metal salts used as the catalyst for OXO-biodegradation. These carry no risks of environmental pollution as they do not contain heavy metals (see EU Packaging Waste Directive 94/62 Art 11) but rather transition metals such as iron, cobalt, manganese or nickel.[5] A Life-cycle assessment made by INTERTEK in May 2012[6] classified OXO as the leading solution to address the problem of littering.

OXO-bio products have to pass the eco-toxicity tests in ASTM D6954; they are designed not to degrade deep in landfill so that they will not generate methane. There is no evidence of any danger to wildlife; almost all the plastic fragments found in studies on the marine environment are fragments of conventional plastic, unsurprisingly as this still makes up the vast majority of plastics in circulation; some argue OXO-biodegradability removes the main rationale for bans on plastic-bag i.e. that conventional plastic can lie or float around in the environment for decades.

The other rationale is that oil-reserves should not be used to make plastic, but oil is extracted primarily to make fuels, and plastic is made as an inevitable by-product of the refining process. Some argue that the same amount of oil would therefore be extracted if plastic did not exist. Another issue often discussed is whether OXO-bio plastic can be safely recycled with other oil-based plastics. The Roediger report, commissioned by the European Plastic Converters trade association, of 5 December 2013 found that it can, and that most bio-based "compostable" plastic cannot.[7]

There is no evidence that OXO-biodegradable plastic of any kind encourages littering or discourages recycling, and it is in fact indistinguishable to the naked eye from conventional plastic.[8] Assessing the Environmental Impacts of Oxo-biodegradable Plastics Across Their Life Cycle]). The stored energy potential of OXO-biodegradable plastic could be retrieved by thermal recycling if collected during its useful life, as it has a similar calorific value to the raw product (fossil fuel) from which it was made.

It is clear that millions of tons of plastics are in the environment and a lot of countries do not have the capacity to recycle them. It is estimated that of the 300 million tons of plastic produced annually in the world only 3% is recycled. OXO offers an option for plastic in the environment which cannot realistically be collected, but should not be treated as a reason not to collect plastic if possible.

Sources

References

  1. ↑ http://cmore.soest.hawaii.edu/cruises/super/biodegradation.htm [Mote Marine Laboratory, 1993]
  2. ↑ Archived January 19, 2010, at the Wayback Machine.
  3. 1 2 AC T51-808 - Plastiques - Évaluation expérimentale de l'oxobiodégradabilité de matériaux polyoléfiniques sous forme de films - Méthodologie et exigences
  4. ↑ Rhodococcus ruber (Kruse) Goodfellow and Alderson ATCC ® 29672&tra
  5. ↑ Benefits and challenges of oxo degradable plastics
  6. ↑ available at WWW.biodeg.org)
  7. ↑ The Roediger report is available on www.biodeg.org
  8. ↑ content/uploads/2011/03/publications/EV0422_8858_FRP.pdf

External links

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