Oxo-degradation

Oxo Biodegradable (OXO) plastic is polyolefin plastic to which has been added amounts of metal salts. 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 process

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.

Illustration of the Oxo-Degradation: A two-step process whereby the conventional polyolefin plastic is first oxo-degraded to short-chain oxygenated molecules (typically 2–4 months exposed) .

The 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 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 because they are stabilized to control the service-life of the product. They will nevertheless degrade more quickly than nature's wastes such as twigs and straw (c10 years) and much more quickly than ordinary plastic (many decades).

Standards applicability

OXO plastic degrades in presence of UV and heat but cannot be recycled with normal plastic. It is not compostable in industrial composting facilities according to the norm ASTM D6400.

This oxidation process takes 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 standard due to the 180 day limit in ASTM D6400.

There is a Standard Guide (ASTM D6954-04) available which specifies procedures to test the degradability and to which some OXO complies. The ASTM 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 the 1-tier of the testing process for verifying a OXO plastic is degradable. After the 1-tier of testing 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 2-tier procedure. The ASTM Standards used for 2-tier testing are either ASTM D5338 or ASTM D5526. The specifications are both testing for the conversion of carbon in the polymer to CO₂ in a control environment. After 60% carbon is converted from the sample the OXO plastic is said to have biodegraded. Although ASTM D6954-04 is only 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 compostable plastics. There is no need to refer to a Standard Specification unless a specific disposal route (e.g.: composting), is envisaged.

Environmental issues

While OXO plastics, especially in the form of plastic bags, are promoted and now widely used as a "environmentally sound" solution of the plastic littering problem, there are several environmental issues related to their use. Metal salts used as the catalyst for OXO degradation carry risks of environmental pollution with heavy metals. A study by the Biodegradable Products Institute found significant amounts of lead and cobalt in their "EcoSafe" OXO bags samples. In case of lead, the amounts found had drastically exceeded the allowed concentrations: "The lead level are 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)"

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.

Sources

• Polyolefins with controlled environmental degradability. David M. Wiles and Gerald Scott, Polymer Degradation and Stability 2006; 91; 1581–1592.
• A Study of the Oxidative Degradation of Polyolefins. Alan J. Sipinen and Denise R. Rutherford, Journal of Environmental Polymer Degradation 1993; 1(3); 193-202.
• Accelerated Photo-Oxidation of Polyethylene (I). Screening of Degradation-Sensitizing Additives. Lynn J. Taylor and John W. Tobias, Journal of Applied Polymer Science 1977;21;1273–1281.
• Accelerated Photo-Oxidation of Polyethylene (II). Further Evaluation of Selected Additives. Lynn J. Taylor and John W. Tobias, Journal of Applied Polymer Science 1981; 26; 2917–2926.
• Biodegradation of polyethylene films with prooxidant additives. Marek Koutny, Jaques Lemaire and Anne-Marie Delort, Chemosphere 2006; 64; 1243–1252.
• Evaluation of degradability of biodegradable polyethylene (PE). Ignacy Jakubowicz*, Polymer Degradation and Stability 2003; 80; 39–43.
• "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).
• Acquired biodegradability of polyethylenes containing pro-oxidant additives. Marek Koutny, Martine Sancelme, Catherine Dabin, Nicolas Pichon, Anne-Marie Delort, and Jacques Lemaire, Polymer Degradation and Stability 2006; 91; 1495–1503.
• Polyethylene biodegradation by a developed Penicillium–Bacillus Biofilm. Gamini Seneviratne, N. S. Tennakoon, M. L. M. A. W. Weerasekara, K. A. Nandasena. Current Science, 2006; 90(1).
• Biodegradation of thermally-oxidized, fragmented low-density polyethylenes. Emo Chiellini, Andrea Cortia, and Graham Swift. Polymer Degradation and Stability 2003; 81; 341–351.
• A Review of Plastic Waste Biodegradation. Ying Zheng, Ernest K. Yanful, and Amarjeet S. Bassi. Critical Reviews in Biotechnology 2005; 25; 243-250.
• Biodegradation of Degradable Plastic Polyethylene by Phanerochaete and Streptomyces Species. Ungtae Lee, Anthony L. Polmetto III, Alfred Fratzke, and Theodore B. Bailey Jr, Applied and Environmental Microbiology 1991; 57(3); 678-685.
• 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. 13.

External Links

• Comparison Oxo-degradable with other technologies