In Chapter 7, the conflict between recycling plastics and causing them to degrade in the environment was briefly discussed in the context of oil based plastics. This chapter is devoted to another naturally biodegradable set of materials called biopolymers.
The polymers discussed up until now have all been made synthetically, however, polymers also occur naturally and are produced by microorganisms, plants and animals. Polymers that are produced by living organisms are called biopolymers. The monomers in this case can be materials such as sugars, amino acids and nucleic acids and by polymerisation can produce starch based polymers or protein based polymers. We have biopolymers inside us all - nucleic acid which produces our own DNA is a biopolymer.
In Chapter 2, a polyethylene polymer was shown to be made of a carbon backbone. This structure is particularly resistant to biodegradability. Because a biopolymer by default must be naturally biodegradable in the environment (otherwise all plants, animals and microorganisms would be here for ever!), its backbone contains other elements such as nitrogen or oxygen. It is this structure that gives these materials their biodegradable nature. This makes them fundamentally different to the degradable plastics discussed in Section 7.6.1. Their chemical structure is also more complex as shown by Figure 11.1. Note the oxygen molecules within the structure which give the molecule its biodegradable nature.
Figure 11.1 Structural unit of cellulose
Three examples of biopolymers are cellulose, starch, and gelatin. Cellulose accounts for 40% of all organic matter and is found in plant cell walls. Starch is found in a number of plants such as corn, potatoes, wheat, rice, barley and peas. Gelatin on the other hand, is extracted from animal bones or animal skins rather than from plants.
To turn these biopolymers into bioplastics is the same as for synthetic polymers:
Biopolymer + additives = bioplastic
Therefore, a bioplastic is a non-toxic, biodegradable plastic made entirely or almost entirely of renewable raw materials.
As plants are more abundant than animals there has been considerable interest in creating modern plant-based bioplastics, especially from plant crop oils. Since crops can simply be replanted and harvested and therefore cannot be depleted, polymers made from these sources are called renewable. While renewable technology such as solar power and wind power is more environmentally friendly than fossil fuel technology, many believe that crops from renewable resources are also a more environmentally alternative to fossil fuels. However, in the case of crop-based plastics the evidence is not convincing at present, and it has been argued that the energy required to convert plants to polymers is actually larger. However, the need to find an alternative feed stock for future polymer production is a compelling one and one with which it is hard to disagree.
Although polymers made from crop-based plants currently make up only a small percentage of current polymer production, their use and market share is likely to expand rapidly in the next decade. Sales growth of between 20-30% per year suggests these materials will soon be competing with commodity materials . Many polymer manufacturers are actively researching replacing fossil fuel feed stocks, as potentially it removes the price restrictions and uncertainty imposed by the fluctuating price of crude oil. By 2007 forecasts estimate the worldwide manufacturing of these materials will reach 600,000 tonnes. Whilst most polymers are derived from fossil fuel, certain products are already based upon, or incorporate in their formulation, a number of vegetable oil-based derivative products. There appears to be considerable scope for an expansion in the use of vegetable oils and oilseed crops in polymer production. Therefore, throughout the world there are a large number of researchers active in this area, as well as in other product sectors such as paints, detergents, pharmaceuticals and lubricants which utilise oil crops.
One material dominates the worldwide oil crop market. This is rapeseed oil. Whilst this can obviously be used as food, a large percentage goes into non-food applications. Linseed oil is also used in a number of non-food applications and in order to support consumption and potential growth in the EU considerable quantities of vegetable oils have to be imported. Other oilseeds of importance include sunflower and soya.
The value and use of different vegetable oils in non-food applications depends on their composition. For polymer production this means long carbon chain lengths are needed. The major use of biopolymers has so far been in the packaging industry, with plastics made from using sugar beet (to produce polygonic acid) or starch to produce polylactic acid (PLA). Starch can also be used to produce polycaprolactone (PCL) or polyvinyl alcohol (PVA). If bioethanol is produced, plastics such as polyethylene can be produced. Products manufactured from these products include food trays and thin films for wrapping.
The main advantages in using biopolymers in the environment is biodegradablity, although some of these materials are also compostable.
To biodegrade they are broken down into carbon dioxide and water by microorganisms.
To be compostable the biomaterial requires a controlled microbial environment such as an industrial compost facility before they will degrade. This is because there are requirements of heat, moisture and aeration to activate and sustain the degradation process. To be considered compostable, a material must be able be put into an industrial composting process and breakdown by 90% within six months. Under the European Standard EN 13432  they can be labelled or marked with a 'compostable' symbol.
As an example a PLA film under 20 ^m thick is compostable and packaging made and marked with this symbol can be commercially composted. Thicker films above 20 ^m although still biodegradable do not qualify as compostable. A home composting logo has yet to be established. However, at some point in the future this will enable consumers to dispose of compostable packaging directly on their own compost heap. At the moment plastics can only be disposed of in industrial compost units.
When comparing the use of a fossil fuel derived polyethylene bag with a biodegradable vegetable-based bag, they are pros and cons for the use of both.
Oil-based polyethylene can be made biodegradable by the use of additives (Section 7.6.1). This is a well known, commercially used, proven technology and provides controlled degradation of waste. There is no performance difference in service compared to conventional bags, but they breakdown faster at the end of life. However, they are made of fossil fuels. If placed in landfill, degradation can be very slow, as degradation needs heat and sunlight. As the weather and climate is varied and unpredictable, so is degradation. They cannot be composted and if mixed in with recyclable plastics can decrease the value of such materials.
Commercially, biodegradable polyethylenes of this type are used in applications such as bin liners, carrier bags, agricultural film and mulch film.
An alternative is to use a biopolymer. In a similar film application for example, an option would be to use one that is starch-based and derived from corn (PCL, PVA or PLA). These biodegradable films, dependent on thickness, would meet the ASTM standard (American Standard for Testing Materials) and European [EN13432] for composting. However these materials require a controlled microbial environment such as an industrial compost facility before they will degrade.
In this case the waste would be both biodegradable and compostable and require no (or less) fossil fuel. However there are potential downsides. Generally, these materials have lower mechanical strength than conventional bags, a slow degradation in standard landfill sites, a limited shelf life (before degradation begins), and it would reduce the value of recycled material if material found its way into the supply chain and contaminated it. This is a major issue with keeping biodegradable plastics separate from recycling infrastructure. Further, any composting would need to be done in a special composting facility until composting infrastructure and legislation is finalised.
At the moment the current plastic recycling infrastructure is unable to cope with the extra demands caused by also having a waste stream of biodegradable materials. The two plastic recovery mechanisms cannot be mixed, since it is impossible to separate biodegradable from non-biodegradable materials at present.
Therefore the choice of which type of material to use for any application would need careful consideration.
As well as the direct production of polymers, vegetable oil derivatives have other uses in the polymer industry, for example as additives. Materials made from vegetable oil have many uses and are used to produce anti-static, slip, and plasticising agents, stabilisers, processing aids and as flame retardants (see Section 2.4.). They can also be incorporated into the manufacture of polyamides, polyesters and polyurethanes.
Markets already exist and are expanding for biodegradable plant derived polymers. Further growth areas include using plant fibres as fillers for production of biodegradable composite materials. The automotive industry especially is taking an interest in this area and a number of current commercially available models feature composite panels of this type.
What the future may hold is unclear but one idea that has been suggested is that of using genetic engineering to produce polymers directly within the plant itself. The potential of biopolymers has certainly captured public imagination and therefore we can expect to see massive growth in biopolymer sales worldwide. However, how it will co-exist with existing recycling infrastructure is yet to be seen.
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