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Biodegradable plastics are plastics that are capable of being decomposed by bacteria or other living organisms.
Two basic classes of biodegradable plastics exist: Bioplastics, whose components are derived from renewable raw materials and plastics made from petrochemicals with biodegradable additives which enhance biodegradation.
Examples of biodegradable plastics
- While aromatic polyesters are almost totally resistant to microbial attack, most aliphatic polyesters are biodegradable due to their potentially hydrolysable ester bonds:
- Polyvinyl alcohol
- Most of the starch derivatives
- Cellulose esters like cellulose acetate and nitrocellulose and their derivatives (celluloid).
- Enhanced biodegradable plastic with additives.
ASTM industrial standard definitions
In the United States, the Federal Trade Commission and the EPA are the authoritative body for biodegradable standards. The EPA brought to congress the 6-pack legislation for photo-degradable plastics to be used. The Diamond shape on all 6-pack holders are used today for a symbol of photo-degradation of plastics.
ASTM International defines methods to test for biodegradable plastic, both anaerobically and aerobically as well as in marine environments. The specific subcommittee responsibility for overseeing these standards falls on the Committee D20.96 on Environmentally Degradable Plastics and Biobased Products. 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 time frames and toxicity of biodegradable tests on plastics.
There are two testing methods for anaerobic environments they are the ASTM D5511-12 or ASTM D5526 - 12 Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under Accelerated Landfill Conditions, Both of these tests are used for the ISO DIS 15985 on determining anaerobic biodegradation of plastic materials.
Many people confuse "biodegradable" with "compostable". "Biodegradable" broadly means that an object can be biologically broken down, while "compostable" typically specifies that such a process will result in compost, or humus. Many plastic manufacturers throughout Canada and the US have released products indicated as being compostable. This practice, however, can be debatable if the manufacturer is submitting to the, now withdrawn, American Society for Testing and Materials standard definition of the word, as it applies to plastics:
"that which is capable of undergoing biological decomposition in a compost site such that the material is not visually distinguishable and breaks down into carbon dioxide, water, inorganic compounds and biomass at a rate consistent with known compostable materials." (ASTM D 6002) 
There is a major discrepancy between this definition and what one would expect from a backyard composting operation. With the inclusion of "inorganic materials", the above definition allows that the end product might not be humus, an organic substance. The only criteria the ASTM standard definition does outline is that a compostable plastic has to disappear at the same rate as something that we have already established as being compostable, under the traditional definition.
Withdrawal of ASTM D 6002
In January 2011, the ASTM withdrew standard ASTM D 6002, which many plastic manufacturers referenced to attain credibility in labelling their products as compostable. Its description is as follows:
"This guide covered suggested criteria, procedures, and a general approach to establish the compostability of environmentally degradable plastics."
The ASTM has yet to replace this standard.
Advantages and disadvantages
Under proper conditions biodegradable plastics can degrade to the point where microorganisms can completely metabolise them to carbon dioxide (and water). For example, starch-based bioplastics produced from sustainable farming methods could be almost carbon neutral (although widespread adoption might result in higher food prices).
There are concerns that "Oxo Biodegradable (OBD)" plastic bags may release metals, and may require a great deal of time to degrade in certain circumstances. Furthermore, OBD plastics may produce tiny fragments of plastic that do not continue to degrade at any appreciable rate regardless of the environment.
There is much debate about the total carbon, fossil fuel and water usage in manufacturing bioplastics from natural materials and whether they are a negative impact to human food supply. It takes 2.65 kg (5.8 lb) of corn to make 1 kg (2.2 lb) of polylactic acid, the most common commercially compostable plastic. Since 270 million tonnes of plastic are made every year, replacing conventional plastic with corn-derived polylactic acid would remove 715 million tonnes from the world's food supply, at a time when global warming is reducing tropical farm productivity.
Traditional plastics made from non-renewable fossil fuels lock up much of the carbon in the plastic as opposed to being utilized in the processing of the plastic. The carbon is permanently trapped inside the plastic lattice, and is rarely recycled, if one neglects to include the diesel, pesticides, and fertilizers used to grow the food turned into plastic.
There is concern that another greenhouse gas, methane, might be released when any biodegradable material, including truly biodegradable plastics, degrades in an anaerobic (landfill) environment. Methane production from 594 managed landfill environments is captured and used for energy; some landfills burn this off, called flaring, to reduce the release of methane into the environment. In the US, most landfilled materials today go into landfills where they capture the methane biogas for use in clean, inexpensive energy. 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 hundreds of years.
Bacteria have developed 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 have developed the ability to consume hydrocarbons. In 2008, a 16-year-old boy reportedly isolated two plastic-consuming bacteria.
Environmental concerns and benefits
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According to a 2010 EPA report, 12.4%, or 31 million tons, of all municipal solid waste (MSW) is plastic. 8.2% of that, or 2.55 million tons, were recovered. That is significantly lower than the average recovery percentage of 34.1%.
Much of the reason for disappointing plastics recycling goals is that conventional plastics are often commingled with organic wastes (food scraps, wet paper, and liquids), making it difficult and impractical to recycle the underlying polymer without expensive cleaning and sanitizing procedures.
On the other hand, composting of these mixed organics (food scraps, yard trimmings, and wet, non-recyclable paper) is a potential strategy for recovering large quantities of waste and dramatically increasing community recycling goals. Food scraps and wet, non-recyclable paper comprise 50 million tons of municipal solid waste. Biodegradable plastics can replace the non-degradable plastics in these waste streams, making municipal composting a significant tool to divert large amounts of otherwise nonrecoverable waste from landfills.
Compostable plastics combine the utility of plastics (lightweight, resistance, relative low cost) with the ability to completely and fully compost in an industrial compost facility. Rather than worrying about recycling a relatively small quantity of commingled plastics, proponents argue that certified biodegradable plastics can be readily commingled with other organic wastes, thereby enabling composting of a much larger position of nonrecoverable solid waste. Commercial composting for all mixed organics then becomes commercially viable and economically sustainable. More municipalities can divert significant quantities of waste from overburdened landfills since the entire waste stream is now biodegradable and therefore easier to process.
The use of biodegradable plastics, therefore, is seen as enabling the complete recovery of large quantities of municipal sold waste (via aerobic composting) that have heretofore been unrecoverable by other means except land filling or incineration.
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, which coincides with another estimate by Akiyama, et al., 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, but recent developments in the commercial production of PLA by NatureWorks has eliminated some dependence of 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, 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. 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 today, and energy consumption can be further reduced by eliminating the fermentation step, or by utilizing food waste as feedstock. The use of alternative crops other than corn, such as sugar cane from Brazil, are expected to lower energy requirements. For instance, manufacturing of PHAs by fermentation in Brazil enjoys a favorable energy consumption scheme where bagasse is used as source of renewable energy.
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. 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.
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