Oxo Biodegradable (OXO) plastic is polyolefin plastic to which has been added amounts of metal salts (none of these are "heavy metals" except Cobalt—see heavy metals and * ) These catalyze the natural degradation process to speed it up so that the OXO plastic will degrade resulting in microfragments of plastic and metals which will remain in the environment but will not be seen as a visual contaminant. The degradation process is shortened from hundreds of years to years and/or months for degradation and thereafter biodegradation depends on the micro-organisms in the environment.
Degradation is a process that takes place in some 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 micro breakdown of the plastics.
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.
OXO plastics will not degrade in a landfill environment due to insufficient oxygen present below a depth of approximately 15 cm. A 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 treated products do not degrade rapidly in an open environment because they are stabilized to control the service-life of the product. 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 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 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. The ASTM D6954 Standard Guide is a 2-tier test procedure to determine whether a plastic could be marketed as "biodegradable". The Standard Guide references ASTM D5510 Test Specification which determines the usable life of the OXO plastic through Thermal Degradation. Secondly it also 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 compost 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 deciding whether a plastic could be marketed as "biodegradable".
ASTM D6400 is a Standard Specification, but is appropriate only for the special conditions found in industrial composting. There is no need to refer to a Standard Specification unless a specific disposal route (e.g.: composting), is envisaged.
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 so far the most interesting one for predicting the behaviour of an oxo plastic in case of littering. This test method provides as well an ecotoxicity testing method to ensure that fragments in the environment, pending complete biodegradation, are not toxic.
OXO plastics, especially in the form of plastic bags, are promoted and now widely used as an environmentally sound solution of the plastic littering problem, and they are now mandatory in the Middle-east and Africa. There are several environmental issues related to their use. Metal salts used as the catalyst for OXO degradation carry no risks of environmental pollution with heavy metals. A recent LCA made by INTERTEK in May 2012 classified oxo as the leading solution to address the problem of littering www.biodeg.org In case of lead,the BPI study is questionable as the amounts found had drastically exceeded the allowed concentrations, and the lead was not alleged to be in the oxo additive.: "The lead level was 4 times higher than those allowed by ASTM D6400-99 in the US and 12 times higher than the concentrations permitted in Europe (EN 13432) and Japan (GreenPLA)". But still there is a possibility that this heavy metals get taken under ground by rain water and finish in the water reservoirs where the city water is taken.
Other often discussed issues are the potential toxicity of the OXO plastic breakdown residue, loss of degradable properties in landfills, the ability of plastic fragments to survive long enough to present danger to wildlife and discouragement of planned plastic bag phase-outs. Although OXO plastic are considered recyclable, the additives could produce unpredictable quality of the secondary product, and there are few independent studies which describe their recycling efficiency and properties. It's also been said that the "biodegradable" public image is discouraging people from recycling, which leads to increased littering and the loss of the stored energetic potential (and therefore the value) of the material, which could be retrieved by recycling and other means of use. However 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 plastics pollution in case of littering.
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