Biodegradable plastic

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For information on plastics derived from renewable raw resources, see Bioplastic.
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Utensils made from biodegradable plastic.
Utensils made from biodegradable plastic.

Biodegradable plastics are plastics that will decompose in the natural environment. 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. Bioplastics are biodegradable plastics whose components are derived from renewable raw materials. 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] Advantages and disadvantages

The advantage of biodegradable plastics is that, in the proper conditions (sun, moisture, oxygen, etc), the plastics degrade to the point where organisms can digest them. This reduces problems with litter and reduces harmful effects on wildlife. Composting is a good methodology.

The main disadvantage with oil-based biodegradable plastics is that their degradation may contribute to global warming through the release of carbon dioxide as a main end product. This does not apply to starch-based plastics as they are formed from carbon which is already in the ecosystem (via photosynthesis). Another disadvantage with biodegradable plastic is that degradation occurs very slowly, if at all, in a sealed landfill. Also, biodegradable plastics cannot be mixed with other plastics when sent for recycling; this damages the recycled plastic and reduces its value.

[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.

Other types are semi-biodegradable, but avoid increased costs by using existing manufacturing processes and are based mainly on conventional non-biodegradable resins. These plastics can be manufactured to be clear or opaque, and in any color. A disadvantage of this approach is that the products of degradation of the conventional material will remain in the environment for years.

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[edit] Environmental concerns

Over 200 million tonnes of plastic are manufactured annually around the world, according to the SPE[citation needed]. 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 [1] [2], which coincides with another estimate by Akiyama, et al[3], 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[4] [5], 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[6], 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[7]. While this assessment is valid, it is important to realize the feedstock for PP continues to be fossil fuel-based, and in the light of limited fossil based resources, production of polymers with a slight increase in total energy could be advantageous by lowering dependence on fossil fuels. 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,[8] or by utilizing food waste as feedstock.[9] 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.[10]

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[11]. 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] References

  1. ^ Gerngross, T. U. Nature Biotechnology 1999, 17, 541-544.
  2. ^ Gerngross, T. U.; Slater, S. C. Scientific American 2000, 283, 37-41.
  3. ^ Akiyama, M.; Tsuge, T.; Doi, Y. Polymer Degradation and Stability 2003, 80, 183-194.
  4. ^ Vink, E. T. H.; Rabago, K. R.; Glassner, D. A.; Gruber, P. R. Polymer Degradation and Stability 2003, 80, 403-419.
  5. ^ Bohlmann, G. Biodegradable polymer life cycle assessment, Process Economics Program, 2001.
  6. ^ Frischknecht, R.; Suter, P. Oko-inventare von Energiesystemen, third ed., 1996.
  7. ^ Gerngross, T. U.; Slater, S. C. Scientific American 2000, 283, 37-41.
  8. ^ Metabolix
  9. ^ Microbes manufacture plastic from food waste Technology News, April 10, 2003. Last retrieved June 13, 2007.
  10. ^ PHB Industrial, Brazil
  11. ^ Vink, E. T. H.; Glassner, D. A.; Kolstad, J. J.; Wooley, R. J.; O'Connor, R. P. Industrial Biotechnology 2007, 3, 58-81.

[edit] See also

Sustainable development Portal

[edit] External links

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