03 August, 2017

A Study On Bio-Degradable Plastics

Since their creation, plastics have revolutionized the world. Being easily moldable and the ability to transform into any shape of any size, plastics have found uses in a wide application of household as well as commercial uses replacing glass, wood, steel. However, unlike conventional materials, plastics are immune to bio-degradation: a process where products are transformed into lesser compounds by bacteria in the soil. Some plastics may be able to break down plastics, however, it remains a remote and expensive method. The only real way to decompose plastics is by UV-radiation which breaks the bonds in the long molecular chain breaking down plastic into lots of little pieces. However, plastic buried in landfills rarely see light of day. Plastics dumped in the ocean are exposed to light, and do degrade in as less as an year. Yet, the resulting small bits of plastic end up in sea-animals and shorelines in toxic form. The solution proposed is biodegradable plastic. Currently, there are 2 types of of biodegradable plastics on the market –
  1. Plant-based hydro-biodegradable plastic
  2. Petroleum-based oxo-biodegradable plastic
In the Plant-based hydro-biodegradable plastic category, polylactic acid (PLA), a plastic made from corn, is the most popular alternative. In perfect conditions PLA is decomposable in water and carbon-dioxide in 47-90 days, i.e. 25% of the time taken by PET-based plastics.

These alternatives are collectively known as bioplastics, deriving their sources from renewable raw materials, petrochemicals and biodegradable additives which catalyze biodegradation.

Bioplastics are generally categorized in the following fashion.

Problems with bio-degradable plastics

Ideally, biodegradable plastics should completely metabolize to carbon-dioxide and water. For example, plastics made from starch could be carbon neutral, though widespread use poses a danger of food security.

Oxo-degradable plastics are plastics made using cobalt, iron or manganese which degrade in presence of oxygen and sunlight. Symphony, which supplies these bags to Co-op and Tesco, claims that its bags degrade 97% faster than standard bags. Tesco reckons that they decompose in half that time without leaving anything that could harm the environment. However, it has seldom been questioned where do these plastics gets the oxygen and sunlight from when they are buried in a landfill. Also, a study by the Biodegradable Products Institute found that the breakdown depends heavily on conditions like high temperature and low humidity. Also, manganese reacts with ammonia and other gases generated by microorganisms in the landfill which stops the breakdown. Besides, high quantities of lead and cobalt are used in some brands of these plastics raising questions about the toxicity of the leftovers. The European Plastic Recyclers Association further argues that plastics are manufactured with a lot of inputs from non-renewable resources; making it economically disastrous to create plastic bags made to self-destruct. In March 2008, the New York Times announced that it would not wrap its paper in bio-degradable plastics as they did not live up to their 100% biodegradable claims. Following this, the US Council of Better Business Bureaus ruled that makers should stop calling these bags “eco-friendly”. In contrast, the UK Periodical Publishers Association made a recommendation in 2007 that all its members should use oxo-biodegradable film to wrap their magazines. Symphony claims that Soil Association buys their bags. However, Clio Turton from the Soil Association says that they’re very frustrated with people making such claims. The British consume 8 billion plastic bags per year, hence for them these bags become a big environmental consideration.

GPC measurements demonstrate that biodegradation claims were confined to the additives and the PE and PVC polymers were not degraded. Carbon dioxide evolution is found to be a useful screening tool for plastic film biodegradation, but for films with additives, polymer biodegradation needs to be confirmed by GPC. Photochemical cross-linking of polymer strands reduces solubility and may interfere with GPC measurements of polymer degradation. It is observed that microbial growth occurs on the presence of PE samples that have been compressed to thick sections but had not been pre-oxidized. Molecular enlargement and broadening of molecular weight distribution occurs after preheating in the air at 60ᵒC but not at ambient temperatures but colonization of microorganisms occurs on all samples. Erosion of the film surface is observed in the vicinity of the microorganisms and the decay of oxidation products in the surface of the polymer film is measured and is found to be associated with the formation of protein and polysaccharides, attributed to the growth of microorganisms.

The Oxo-biodegradable Plastics Association argues that OBD plastics contain salts of metals, not metals themselves, which are not prohibited by law. Infact, they are necessary elements in the human diet. When tested in accordance with ISO 17556, a peer-reviewed report by the joint effort of the Technical Research Institute of Sweden and the Swedish University of Agricultural Sciences shows that 91% of OBD plastics are biodegraded in a soil environment within 24 months.

Costs of Production 

The fossil fuel energy required to produce PHA is 50.4 MJ/kg. Other estimates value between 50-59 MJ/kg. NatureWorks speculates that technological advancements using Wind power and biomass driven strategies will reduce the fossil fuel dependence to 16.6 MJ/kg. In contrast, polypropylene and high density polyethylene require 85.9 and 73.7 MJ/kg respectively. 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. Any biodegradable polymer alternative will need to take into account the priorities of society with regard to energy, environment, and economic cost. It must also be taken into consideration that the technology to produce PHA is still in its latent stage and elimination of the fermentation step may further reduce energy consumption. The dependence from corn may be shifted to sugar to lower energy requirements. 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. 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. Currently, it takes 2.65 kg of corn to make 1 kg of PLA. Since 270 MT of plastic are made every year, replacing conventional plastic with corn-derived PLA would remove 715 MT from the world's food supply. 

Environmental benefits

Methane is released when any biodegradable material degrades in an anaerobic environment. Methane production from 594 managed landfill environments is captured and used for energy.

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 non-recoverable 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.

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