Biodegradable plastic

For information on plastics derived from renewable raw resources (biomass), see Bioplastic. For information on plastics designed to biodegrade in human bodies, see Biodegradable polymer.

Utensils made from biodegradable plastic.

Biodegradable plastics are plastics that will decompose in natural aerobic (composting) and anaerobic (landfill) environments. Biodegradation of plastics occurs when microorganisms metabolize the plastics to either assimilable compounds or to humus-like materials that are 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 contain additives.

Biodegradable plastics typically are produced in two forms: injection molded (solid, 3D shapes), typically in the form of disposable food service items, and films, typically organic fruit packaging and collection bags for leaves and grass trimmings, and agricultural mulch.

Scientific definitions of biodegradable plastic

In the United States, the Federal Trade Commission is the authoritative body for biodegradable standards.

ASTM International defines appropriate testing 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.

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, ASTM D6868 - 11 Standard Specification for Labeling of End Items that Incorporate Plastics and Polymers as Coatings or Additives with Paper and Other Substrates Designed to be Aerobically Composted in Municipal or Industrial Facilities, and the ASTM D7081 - 05 Standard Specification for Non-Floating Biodegradable Plastics in the Marine Environment.

One of the testing methods for determining anerobic biodegradation is the ASTM D5511 - 12 Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under High-Solids Anaerobic-Digestion Conditions. The most accurate test method for testing in anaerobic environments is the 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.

Examples of biodegradable plastics

Development of biodegradable containers

• 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).

Environmental benefits of biodegradable plastics depend upon proper disposal

Biodegradable plastics are not a panacea, however. Some critics claim that a potential environmental disadvantage of certified biodegradable plastics is that the carbon that is locked up in them is released into the atmosphere as a greenhouse gas. However, biodegradable plastics from natural materials, such as vegetable crop derivatives or animal products, sequester CO₂ during the phase when they're growing, only to release CO₂ when they're decomposing, so there is no net gain in carbon dioxide emissions, except those caused by the intensive farming process which uses petroleum based diesel, fertilizers, and pesticides. .

However, '"certified" biodegradable plastics require months of elevated heat, moisture and oxygen to biodegrade, conditions found in only in professionally managed 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. It takes 2.65 pounds of corn to make a pound of polylactic acid, the commonest 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 you neglect 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 specially managed landfill environments is captured and used for energy or burnt off to reduce the release of methane in the environment. In the US, most landfilled materials today go into landfills where they 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.

The latter possibility was in fact the subject of a cautionary novel by Kit Pedler and Gerry Davis, 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.

Mechanisms

Biodegradable plastics may be composed of either bioplastics, which are plastics derived from renewable raw materials, or petroleum-based plastics containing additives that facilitate the biodegradation process.

One method of facilitating biodegredataion is the use of hydrophilic swelling agents that, when combined with heat and moisture, expand the plastic's molecular structure and allow microbes to secrete acids to break down the macromolecular chains into CH4, CO2, Biomass and air.

• "Oxo Biodegradable (OBD)" which breakdown by light, heat, mechanical stress and moisture. These products are made by oxides, such as zinc and magnesium when breaking down the molecular chain they reduce the molecular weight to diols which are not plastic polymers. Diols are regulated in California under Prop 65

• Materials such as a polyhydroxyalkanoate (PHA) biopolymer are completely compostable in an industrial compost facility.

Polylactic acid (PLA) is another 100% compostable biopolymer which can fully degrade above 60C in an industrial composting facility. Fully biodegradable plastics are more expensive, partly because they are not widely enough produced to achieve large economies of scale.

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

Environmental concerns; benefits

Over 200 million tons of plastic are manufactured annually around the world, according to the Society of Plastics Engineers 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.

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 increase community recycling goals. Food scraps and wet, non-recyclable paper comprises 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.

Certified biodegradable plastics combine the utility of plastics (lightweight, resistance, relative low cost) with the ability to completely and fully biodegrade in a 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.

Confusion over proper definition of terms

Until recently there were few legal standards regarding marketing claims surrounding the use of the term 'biodegradable'. In 2007, the state of California passed regulation banning companies from claiming their products are biodegradable without proper scientific certification from a third-party laboratory.

The Federal Court of Australia declared on March 30, 2009 that a director of a company that manufactured 'biodegradable' disposable diapers (who also approved the company's advertising) had been knowingly making false and misleading claims about biodegradability.

In June 2009, the Federal Trade Commission charged two companies with making unsupported marketing claims regarding biodegradability.

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 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- 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.

See also

• Biodegradable bags
• Biodegradable waste
• Bioplastic
• Microplastics
• Photodegradation
• Plastic bag
• Plastics 2020 Challenge