Biodegradable Plastics
Biodegradable Plastic is plastic which will degrade from the action of naturally occurring microorganism, such as bacteria, fungi etc. over a period of time. Note, that there is no requirement for leaving “no toxic residue”, and as well as no requirement for the time it needs to take to biodegrade.
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. They may be composed of either bioplastics, which are plastics whose components are derived from renewable raw materials, or petroleum-based plastics.
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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.
Advantages and disadvantages
Under proper conditions biodegradable plastics can degrade to the point where microorganisms can metabolise them. This reduces problems with litter and reduces harmful effects on wildlife. However degradation of biodegradable plastic occurs very slowly, if at all, in a sealed landfill. Proper composting methods are required to efficiently degrade the plastic, which may actually contribute to carbon dioxide emissions.
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Degradation of oil-based biodegradable plastics may contribute to global warming through the release of previously stored carbon as carbon dioxide. Starch-based bioplastics produced from sustainable farming methods can be almost carbon neutral.
Biodegradable plastics cannot be mixed with other plastics when sent for recycling; this damages the recycled plastic and reduces its value.
Mechanisms
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.
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.
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. 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, 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.
Photo: physorg.com
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.
Three generations of starch-based plastics are recognized. The first generation consists of a synthetic polymer. Starch is only used as a filling material it’s polymeric properties are not made use of. An example are “biodegradable” plastic bags. These bags are not fully biodegradable, though, since they consist of mainly non-biodegradable synthetic polymers like polyethylene or polypropene and only 5-20 percent starch. Under special conditions the starch degrades and the plastic falls apart into small particles, that will prevail for many years although they are not visible.
In the second generation the starch is used for its polymeric properties. It is blended with hydrophilic synthetic polymers and contributes to the strength of the material. 50-80% starch can be used in these plastics, but still a large part is not biodegradable.
The third generation is a truly biodegradable plastic, that does not contain synthetic polymers at all. To improve some of the properties of the plastic, the biopolymer may be modified, but no synthetic materials are necessary.
The barrier properties that are required for a film depend on it’s use. Fresh fruits or vegetables have to be able to breath, so a film with too low an oxygen and/or carbon dioxide permeability can not be used. Foods which are rich in polyunsaturated fat, however, are sensitive to oxygen and need a film with a high oxygen barrier. Often the barrier against water is the most important function of a film, since aw is an important factor for the shelf life of a product (microbial growth, chemical reactions, crispiness).
by: wikipedia.org, mindfully.org
