Biodegradable plastic
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Biodegradable plastics are plastics that will decompose in the natural environment aerobic (composting) and anaerobic (landfill) environments. Biodegradation of plastics can be achieved by enabling microorganisms in the environment to metabolize the molecular structure of plastic films to produce an inert humus-like material that is less harmful to the environment. They may be composed of either bioplastics, which are plastics whose components are derived from renewable raw materials, or petroleum-based plastics which utilize an additive. The use of bio-active compounds compounded with swelling agents ensures that, when combined with heat and moisture, they expand the plastic's molecular structure and allow the bio-active compounds to metabolise and neutralize the plastic.
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[edit] Scientific definition of biodegradable plastic, and their limits
In the United States, ASTM International is the authoritative body for defining biodegradable standards. The specific subcommittee responsibility for overseeing these standards falls on the Committee D20.96 on Environmentally Degradable Plastics and Biobased Products [1]. The current ASTM standards are defined as standard specifications and standard test methods. Standard specifications create a pass or fail scenario whereas standard test methods identify the specific testing parameters for facilitating specific biodegradable tests on plastics.
Currently, there are three such ASTM standard specifications which mostly address biodegradable plastics in composting type environments, the ASTM D6400-04 Standard Specification for Compostable Plastics [2], ASTM D6868 - 03 Standard Specification for Biodegradable Plastics Used as Coatings on Paper and Other Compostable Substrates [3], and the ASTM D7081 - 05 Standard Specification for Non-Floating Biodegradable Plastics in the Marine Environment [4].
Currently the most accurate standard test method for anaerobic environments is the ASTM D5511 - 02 Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under High-Solids Anaerobic-Digestion Conditions [5]. Another standard test method for testing in anaerobic environments is the ASTM D5526 - 94(2002) Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under Accelerated Landfill Conditions [6], this test has proven extremely difficult to perform.
The current California legislation AB 1972 ensures accurate environmental advertising of plastics by allowing only the use of terms that can be verified by an American Society for Testing Materials (ASTM) standard specification. This legislation does not include ASTM standard test methods. The two ASTM standard specifications which are used in the legislation are ASTM D6400 and D7081. Products passing these ASTM specifications can use the term compostable on the product label [7].
A potential environmental disadvantage of all biodegradable plastics is that the carbon that is locked up in them is released into the atmosphere as a greenhouse gas. Biodegradable plastics from natural materials, such as vegetable crop derivatives or animal products, there is no net gain in carbon dioxide emissions, however these plastics require a very specific environment to biodegrade such as found in only professional composting facilities. There is much debate about the total carbon, fossil fuel and water usage in processing biodegradable plastics from natural materials and whether they are a negative impact to human food supply. Traditional plastics made from fossil fuels lock up much of the carbon in the plastic as opposed to being utilized in the processing of the plastic. Although concern will be for a worse greenhouse gas, methane release when the plastic biodegrades in an anaerobic (landfill) environment. Methane production from landfill environments are typically captured and burned to negate the relaease of methane in the environment. Many landfills today capture the methane biogas for use in clean inexpensive energy. Of course, incinerating non-biodegradable plastics will release carbon dioxide as well. Disposing of biodegradable plastics made from natural materials in anaerobic (landfill) environments will result in the plastic lasting for hundred of years.
It is also possible that bacteria will eventually develop the ability to degrade plastics. This has already happened with nylon: two types of nylon eating bacteria, Flavobacteria and Pseudomonas, were found in 1975 to possess enzymes (nylonase) capable of breaking down nylon. While not a solution to the disposal problem, it is likely that bacteria will evolve the ability to use other synthetic plastics as well. In 2008, a 16-year-old boy reportedly isolated two plastic-consuming bacteria.[8]
The latter possibility was in fact the subject of a cautionary novel by Kit Pedler and Gerry Davis (screenwriter), the creators of the Cybermen, re-using the plot of the first episode of their Doomwatch series. The novel, Mutant 59: The Plastic Eater, written in 1971, is the story of what could happen if a bacterium were to evolve—or be artificially cultured—to eat plastics, and be let loose in a major city.
[edit] Mechanisms
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Materials such as polyhydroxyalkanoate (PHA) biopolymer are completely biodegradable. Fully biodegradable plastics are more expensive, partly because they are not widely enough produced to achieve large economies of scale.
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[edit] Advantages and disadvantages
Under proper conditions biodegradable plastics can degrade to the point where microorganisms can metabolise them.
Degradation of oil-based biodegradable plastics may release of previously stored carbon as carbon dioxide. Starch-based bioplastics produced from sustainable farming methods can be almost carbon neutral but could have a damaging effect on soil, water usage and quality, and result in higher food prices.
[edit] Environmental concerns
Over 200 million tons of plastic are manufactured annually around the world, according to the Society of Plastics Engineers.[9][unreliable source?] Of those 200 million tons, 26 million are manufactured in the United States. The EPA reported in 2003 that only 5.8% of those 26 million tons of plastic waste are recycled, although this is increasing rapidly.
[edit] Energy Costs For Production
Various researchers have undertaken extensive life cycle assessments of biodegradable polymers to determine whether these materials are more energy efficient than polymers made by conventional fossil fuel-based means. Research done by Gerngross, et al. estimates that the fossil fuel energy required to produce a kilogram of polyhydroxyalkanoate (PHA) is 50.4 MJ/kg,[10][11] which coincides with another estimate by Akiyama, et al.[12], who estimate a value between 50-59 MJ/kg. This information does not take into account the feedstock energy, which can be obtained from non-fossil fuel based methods. Polylactide (PLA) was estimated to have a fossil fuel energy cost of 54-56.7 from two sources[13][14], but recent developments in the commercial production of PLA by NatureWorks has eliminated some dependence fossil fuel based energy by supplanting it with wind power and biomass-driven strategies. They report making a kilogram of PLA with only 27.2 MJ of fossil fuel-based energy and anticipate that this number will drop to 16.6 MJ/kg in their next generation plants. In contrast, polypropylene and high density polyethylene require 85.9 and 73.7 MJ/kg respectively[15], but these values include the embedded energy of the feedstock because it is based on fossil fuel.
Gerngross reports a 2.65 total fossil fuel energy equivalent (FFE) required to produce a single kilogram of PHA, while polypropylene only requires 2.2 kg FFE[16]. Gerngross assesses that the decision to proceed forward with any biodegradable polymer alternative will need to take into account the priorities of society with regard to energy, environment, and economic cost.
Furthermore, it is important to realize the youth of alternative technologies. Technology to produce PHA, for instance, is still in development, and energy consumption can be further reduced by eliminating the fermentation step,[17] or by utilizing food waste as feedstock.[18] The use of alternative crops other than corn, such as sugar cane from Brazil, are expected to lower energy requirements- manufacturing of PHAs by fermentation in Brazil enjoys a favorable energy consumption scheme where bagasse is used as source of renewable energy.[19]
Many biodegradable polymers that come from renewable resources (i.e., starch-based, PHA, PLA) also compete with food production, as the primary feedstock is currently corn. For the US to meet its current output of plastics production with BPs, it would require 1.62 square meters per kilogram produced[20]. While this space requirement could be feasible, it is always important to consider how much impact this large scale production could have on food prices and the opportunity cost of using land in this fashion versus alternatives.
[edit] See also
[edit] References
- ^ http://www.astm.org/COMMIT/SUBCOMMIT/D2096.htm
- ^ http://www.astm.org/Standards/D6400.htm
- ^ http://www.astm.org/Standards/D6868.htm
- ^ http://www.astm.org/Standards/D7081.htm
- ^ http://www.astm.org/Standards/D5511.htm
- ^ http://www.astm.org/Standards/D5526.htm
- ^ http://www.cawrecycles.org/issues/current_legislation/ab1972_08
- ^ WCI student isolates microbe that lunches on plastic bags
- ^ The Hazards of Plastics Julia Mackiewicz
- ^ Gerngross, Tillman U. (1999). "Can biotechnology move us toward a sustainable society?". Nature Biotechnology 17 (6): 541–544. doi:.
- ^ Slater, S. C.; Gerngross, T. U. (2000). "How Green are Green Plastics?". Scientific American. http://www.sciam.com/article.cfm?id=how-green-are-green-plast.
- ^ Akiyama, M.; Tsuge, T.; Doi, Y. Polymer Degradation and Stability 2003, 80, 183-194.
- ^ Vink, E. T. H.; Rabago, K. R.; Glassner, D. A.; Gruber, P. R. Polymer Degradation and Stability 2003, 80, 403-419.
- ^ Bohlmann, G. Biodegradable polymer life cycle assessment, Process Economics Program, 2001.
- ^ Frischknecht, R.; Suter, P. Oko-inventare von Energiesystemen, third ed., 1996.
- ^ Gerngross, T. U.; Slater, S. C. Scientific American 2000, 283, 37-41.
- ^ Metabolix
- ^ "Microbes manufacture plastic from food waste". Technology News. April 10, 2003. http://pubs.acs.org/subscribe/journals/esthag-w/2003/apr/tech/rp_plastic.html. Retrieved on June 13, 2007.
- ^ PHB Industrial, Brazil
- ^ Vink, E. T. H.; Glassner, D. A.; Kolstad, J. J.; Wooley, R. J.; O'Connor, R. P. Industrial Biotechnology 2007, 3, 58-81.
