Question: Does plastic decompose at all?

Biodegradable plastic




Biodegradable plastics are plastics that can be decomposed by the action of living organisms, usually bacteria.[1]
Two basic classes of biodegradable plastics exist:[2] Bioplastics, whose components are derived from renewable raw materials, and plastics made from petrochemicals containing biodegradable additives which enhance biodegradation.
Examples

While aromatic polyesters are almost totally resistant to microbial attack, most aliphatic polyesters are biodegradable due to their potentially hydrolysable ester bonds:
Naturally Produced: Polyhydroxyalkanoates (PHAs) like the poly-3-hydroxybutyrate (PHB), polyhydroxyvalerate (PHV) and polyhydroxyhexanoate (PHH);
Renewable Resource: Polylactic acid (PLA);
Synthetic: Polybutylene succinate (PBS), polycaprolactone (PCL)...
Polyanhydrides
Polyvinyl alcohol
Most of the starch derivatives
Cellulose esters like cellulose acetate and nitrocellulose and their derivatives (celluloid).
Polyethylene terephthalate
Enhanced biodegradable plastic with additives.[3]
Home Compostable Plastics
No international standard has been established to define home-compostable plastics, however, national standards have been created; in Australia: AS 5810 “biodegradable plastics suitable for home composting”, and in France: NF T 51-800 “Specifications for plastics suitable for home composting”. The French standard is based on the “OK compost home certification scheme”, developed by Belgian certifier TÜV Austria Belgium.[4] The following are examples of plastics which have conformed to an established national standard for home compostability[5]:
Ecopond Flex 162 resin, maximum thickness 65 micron
BWC BF 90A resin, maximum thickness 81 micron
BioPBS FD92 resin, maximum thickness 85 micron
Torise TRBF90 resin, maximum thickness 43 micron
HCPT-1 triple laminate, maximum thickness 119 micron
HCFD-2 duplex laminate, maximum thickness 69 micron
Controversy
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.[6]  Many plastic manufacturers throughout Canada and the US have released products indicated as being compostable. The waste management infrastructure currently recycles regular plastic waste, incinerates it, or places it in a landfill. Mixing biodegradable plastics into the regular waste infrastructure poses some dangers to the environment.[7] However this claim is debatable, if the manufacturer was minimally conforming to the now-withdrawn American Society for Testing and Materials (ASTM  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) [8] 
There is a major discrepancy between this definition and what one would expect from a backyard composting operation.  With the inclusion of "inorganic compounds", the above definition allows that the end product might not be humus, an organic substance.  The only criterion the ASTM standard definition did outline is that a compostable plastic has to become "not visually distinguishable" at the same rate as something that has already been 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 had been referencing to attain credibility in labelling their products as compostable.  The withdrawn description was as follows:
 "This guide covered suggested criteria, procedures, and a general approach to establish the compostability of environmentally degradable plastics."[9] 
As of 2014, the ASTM has yet to replace this standard.
Advantages and disadvantages
Under proper conditions, some 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.
There are allegations that biodegradable plastic bags may release metals, and may require a great deal of time to  degrade in certain circumstances [10] and that OBD (oxo-biodegradable) plastics may produce tiny fragments of plastic that do not continue to degrade at any appreciable rate regardless of the environment.[11][12] The response of the Oxo-biodegradable Plastics Association (www.biodeg.org) is that OBD plastics do not contain metals. They contain salts of metals, which are not prohibited by legislation and are in fact necessary as trace-elements in the human diet. 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 shows 91% biodegradation in a soil environment within 24 months, when tested in accordance with ISO 17556.[13]
Environmental benefits
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. To make 1kg (2.2lb) of polylactic acid, the most common commercially available compostable plastic,  2.65kg (5.8lb) of corn is required.[14] Since 270 million tonnes of plastic are made every year, replacing conventional plastic with corn-derived polylactic acid would remove 715.5 million tonnes from the world's food supply, at a time when global warming is reducing tropical farm productivity.[15]
"Although U.S. corn is a highly productive crop, with typical yields between 140 and 160 bushels per acre, the resulting delivery of food by the corn system is far lower. Today’s corn crop is mainly used for biofuels (roughly 40 percent of U.S. corn is used for ethanol) and as animal feed (roughly 36 percent of U.S. corn, plus distillers grains left over from ethanol production, is fed to cattle, pigs and chickens). Much of the rest is exported.  Only a tiny fraction of the national corn crop is directly used for food for Americans, much of that for high fructose corn syrup."[16]
Traditional plastics made from non-renewable fossil fuels lock up much of the carbon in the plastic, as opposed to being burned 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 through a process 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 non-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. 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.[17]
Environmental concerns and benefits

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%.[18]
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), leading to accumulation of waste in landfills and natural habitats.[19] Current usage also makes 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. As of 2015, food scraps and wet, non-recyclable paper respectively comprise 39.6 million and 67.9 million tons of municipal solid waste[20]. 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 portion 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. This move away from the use of landfills may help alleviate the issue of plastic pollution.
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,[21][22] which coincides with another estimate by Akiyama, et al.,[23] 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,[24][25] 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,[26] but these values include the embedded energy of the feedstock because it is based on fossil fuel.
Gerngross reports a 2.65kg total fossil fuel energy equivalent (FFE) required to produce a single kilogram of PHA, while polyethylene only requires 2.2kg FFE.[27] 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.[28] 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.[29]  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.
Regulation
United States
In terms of  ASTM industrial standard definitions, the U.S.Federal Trade Commission and the U.S. EPA set standards for biodegradability. 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 Bio based Products.[30] 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.
Two testing methods are defined for anaerobic environments: (1) ASTM D5511-12 and (2) ASTM D5526 - 12 Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under Accelerated Landfill Conditions,[31] Both of these tests are used for the ISO DIS 15985 on determining anaerobic biodegradation of plastic materials. 

See also

Biodegradable bags
Biodegradable waste
Biodegradable additives
Bioplastic
BioSphere Plastic
Economics of plastics processing
Microplastics
Photodegradation
Plastic bag
Plastics 2020 Challenge

Further reading