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:
- 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).
- Enhanced biodegradable plastic with additives.
ASTM industrial standard definitions
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.
Method was define 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 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.
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. 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 allegations that "Oxo
Biodegradable (OBD)" plastic bags may
release metals, and may require a great deal of time to degrade in certain
circumstances and that OBD plastics may produce tiny fragments of plastic
that do not continue to degrade at any appreciable rate regardless of the
environment. 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 . A peer-reviewed report of the work was published in Vol
96 of the journal of Polymer Degradation & Stability (2011) at page
919-928, which shows 91% biodegradation in a soil environment within 24 months,
when tested in accordance with ISO 17556.
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. 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
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 opp
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