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Oxo Biodegradable Oxo-biodegradation is defined by CEN (the European Standards Organisation) as "degradation resulting from oxidative and cell-mediated phenomena, either simultaneously or successively." Whilst sometimes described as "oxo-fragmentable" and 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."
(OXO-bio) plastic is polyolefin plastic to which has been added small amounts of metal salts (none of these are "heavy metals" restricted by the EU Packaging Waste Directive 94/62 Art 11). These salts catalyze the natural 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. At the end of that process it is no longer visible, it is no longer a plastic, and will then biodegrade. It does not therefore leave fragments of plastic in the environment. The degradation process is shortened from hundreds of years 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.
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 hundreds of years to degrade. But oxo-degradable products utilize a prodegradant to speed up the molecular breakdown of the polyolefins and incorporate oxygen atoms into the resulting low molecular mass. This chemical change enables the further breakdown of the material by naturally-occuring micro-organisms.
OXO plastic if discarded in the environment, will fragment and degrade to oxygenated low molecular weight (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.
An oxo-biodegradable PE plastic bag for example 30 µm thick with 2% prodegradant additive degrades within 3 months if left exposed in an open air environment in Thailand and a 150 µm thick PP container or sheet will degrade within 3–6 months. Oxo plastics are designed so that they will not degrade in a landfill due to insufficient oxygen present below a depth of approximately 15cm. They will not therefore generate methane, which is a powerful greenhouse gas.
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 biodegrade in nature much more quickly than nature's wastes such as twigs and straw (c10 years) and cannot be compared in speed with the degradation ordinary plastic (many decades).
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.
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. 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.
The oxidation process takes no longer than the 180 day period required by ASTM D6400 and similar standards for compostable plastics such as EN13432 Australian 4736 and ISO 17088. This short time is necessary for compostable plastics because industrial composting has a short timescale, and is not the same as degradation in the environment. A leaf is generally considered to be biodegradable but it will not pass the composting standards, due to the 180 day limit in ASTM D6400. (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.
There is a Standard Guide (ASTM D6954) which specifies procedures to test degradability, biodegradability, and non-toxicity, and with which a properly designed and manufactured oxo product complies. ASTM D6954 includes a 2-tier test procedure to determine whether a plastic could be marketed as "oxo-biodegradable". The Standard Guide references the ASTM D5510 Test Specification which determines the usable life of the OXO plastic through Thermal Degradation. Secondly it references ASTM D5208 which is used to determine the usable life based on UV degradation. These two test procedure in combination make up Tier 1 of the testing process for verifying that an OXO plastic is degradable. After Tier 1 is complete and the OXO plastic has been shown to be significantly degraded (usually done by FT-IR to show the new spectra peak at around the 1730 wavelength) the testing can move on to the Tier 2 procedure. The Standards used for Tier 2 testing are either ASTM D5338 or ISO 17556. These are both testing for the conversion of carbon in the polymer to CO2 in a controlled environment. After 60% carbon is converted from the sample the OXO plastic is said to be biodegradable. Although ASTM D6954-04 is a Standard Guide (as opposed to a Standard Specification) it does provide pass / fail criteria and therefore is useful in determining whether a plastic could be marketed as "oxo-biodegradable". 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 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 micro-organisms and measuring ATP and ADP by chimiluminescence. 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 micro-organisms are identified based on the environment where the plastic will be disposed which is not the case with the CO2-evolution method.
This French document is a very interesting innovation for predicting the behaviour of an oxo-biodegradable plastic in case of littering. This test method provides as well an ecotoxicity testing method to ensure that residues in the environment, pending complete biodegradation, are not toxic.
OXO-bio plastics, especially in the form of plastic bags, are now widely used as an environmentally sound solution for the problem of plastic litter in the open environment which cannot realistically be collected and they are now mandatory in the Middle-east, Asia and Africa. There are several environmental issues related to their use. Metal salts used as the catalyst for OXO-biodegradation carry no risks of environmental pollution. They do not contain heavy metals (see EU Packaging Waste Directive 94/62 Art 11).heavy metals. In case of lead,the BPI study is unhelpful, as the lead was not alleged to be in the oxo additive. A Life-cycle assessment made by INTERTEK in May 2012 (available at www.biodeg.org) classified oxo as the leading solution to address the problem of littering.
Other often discussed issues are the potential toxicity of the OXO plastic breakdown residue (but oxo-bio products have to pass the eco-toxicity tests in ASTM D69540) loss of degradable properties in landfills (but oxo-bio products are designed not to degrade deep in landfill so that they will not generate methane), the ability of plastic fragments to survive long enough to present danger to wildlife (there is no evidence of any danger to wildlife, and almost all the plastic fragments found in studies on the marine environment are fragments of conventional plastic) and discouragement of planned plastic bag phase-outs (because oxo-biodegradability removes the main rationale for plastic-bag bans ie 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 from a necessary by-product of the refining process. 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 of 5th December 2013 found that it can, and that bio-based "compostable" plastic cannot.
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. The stored energy potential of oxo-biodegradable plastic could be retrieved by thermal recycling if collected during its useful life, as it has the same calorfic value as the oil 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 of recycling. In the world only 3% of the plastics is recycled and oxo offers an interesting alternative to plastic pollution in case of littering.
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