Bio-degradable plastics     The eco-friendly alternative Durability and resistance to degradation have made plastic materials an integral part of contemporary life. The standard plastic formulation used includes polyolefins, polyesters and polyurethanes, all of which are petroleum-based and non-degradable. Hazardous chemicals are needed for their production and disposal. A steady increase in plastics production and their persistence in the environment long after their intended use pose serious environmental problems. These non-degradable plastics accumulate in the global environment at a rate of 25 million tonnes per year, posing a major challenge to solid waste management. To regulate global plastic disposal problems several laws were recently enacted, including the Maritime Pollution Treaty (MARPOL) which, since 1994, prevents the disposal of plastics at sea, and the US Plastic and Pollution Research and Control Act, 1994. The environmental problems mean there is an increased demand for biodegradable polymers - materials that can be degraded by enzymes. To be biodegradable, a plastic needs to have chemical groups at the ends of its polymer chains that are susceptible to oxidation by microbial enzymes. Initial efforts to develop biodegradable plastics was focussed on incorporating new materials into conventional plastics that could generate these oxidisable chain-ends. Based on this principle, photodegradable polymers (which degrade when exposed to the sun's UV light) and copolymers of a plastic with cellulose or starch were developed. Recently, scientists at the Central Tuber Crops Research Institute (CTCRI), Thiruvananthapuram, developed an inexpensive method to develop biodegradable plastics by adding tapioca starch to it. Starch made from tapioca, available in plenty in Kerala and Tamil Nadu, and certain other tubers are added to plastic at the production stage using a soluble chemical agent. While it takes centuries for ordinary plastics to dissolve normally, this material degrades in six months. The technology has been transferred to a Delhi-based company, and commercial production is expected to start in three months. States such as Jammu and Kashmir and Himachal Pradesh, and one metro - Mumbai - have already banned non-degradable plastics. They may prefer to switch over to biodegradable plastics in the near future. For the first time, biodegradable plastic will enter the Indian market, which has recently registered a manifold increase in the demand for conventional plastics, which threaten the ecological balance due to their non-degradable nature. Biodegradable polymers developed the CTCRI have many important end uses. Successful applications include surgical implants, agricultural mulch, controlled release formulations of pharmaceutical drugs and agro-chemicals. However, this plastic could be 15-20 per cent costlier than conventional plastic. Photo self-destruction uses a photo-stabilising agent incorporated into the polymer, which is active for a predetermined length of time. When it becomes inactive, it causes photo-oxidation. This forms carbonyl ends on the polymer chains, allowing microbial breakdown of the plastic film. An interesting spin-off from this idea combines biodegradability with the photodegradability of polymers containing azo-aromatic units and UV-sensitive keto-groups. During use the azo-groups protect the light-sensitive keto groups, but when the plastic is disposed of, the azo-groups are attacked by bacteria, releasing the ketones and activating photodegradation. Copolymers work on the principle that mixing plastic polymers with natural polymers such as cellulose or starch improves biodegradation. However, the percentage of natural polymer needed to achieve significant copolymer biodegradation is around 30-50 per cent, which makes it difficult to process the copolymer without compromising its physical properties. In addition, copolymers are only partially degraded by biologically mediated processes, leaving the undegraded fragments unchanged in the environment. Clearly, neither route to biodegradability is ideal. Biopolymers - polymeric materials produced by living organisms - offer an alternative to the current biodegradable plastics. They are biodegradable and can be generated using renewable resources like microorganisms or plants. Some microorganisms in nature produce biopolymers as their storage products (like carbohydrates). For example, lactic acid polymers can be derived from lactic acid bacteria via a fermentation process, and these bacteria have been used to make biodegradable plastics. Robert Coleman at Argonne National Laboratory, has produced biodegradable plastics from high carbohydrate food waste like potato and cheese whey waste. Potato waste is enzymatically converted into glucose by a two-enzyme process which is used in fermentation to produce lactic acid, this is harvested from the bacteria and purified to make biodegradable plastics. The lactose in cheese whey waste is fermented directly into lactic acid. The plastic's decay rate is manipulated during its preparation by either varying the isomers of lactic acid or by adding other compounds. The most useful of all the microbially derived biodegradable plastics are the polyhydroxybutyrates (PHBs) and are stored as an energy reserve in the cells of many bacteria including Alcaligenes eutropus. When deprived of nitrogen, phosphate, magnesium or sulphate, A. eutropus produces polymer up to 90 per cent of its dry weight. PHB stays flexible from sub-zero temperatures up to 130 degree Celsius, and completely breaks down into water and carbon-dioxide in a few months. Results on biodegradation rates showed that copolymer degraded faster than the homopolymer. Both are degraded by a wide variety of microorganisms flourishing in the soil. Producing polymers by fermentation is, however, not cost- effective when compared with petroleum-based polymers. Protein-based polymers (PBPs) occur in nature as materials with extraordinary mechanical properties, such as spider silks that are tougher than steel, or the elastic fibres in the mammalian cardiovasculature, which can last almost a century without loss of function. Some natural polymers, like soyabean PBPs, can be used in aqueous-based dues as an alternative to solvent-based dyes, which generate hazardous fumes. Developments in the field of protein chemistry, genetic engineering and biotechnology have enabled a better understanding of nature's idea in protein designing and engineering novel proteins in living organisms. PBPs offer a wide range of materials similar to that of oil-based polymers, such as hydrogels, elastomers and plastics. PBPs can created in a variety of designs and compositions, and can be made biodegradable with chemical clocks to determine their halflives. One important application of PBPs is in the field of bioelastic materials in the medical field. The non-medical application of bioelastic materials include biodegradable plastics, transducers, molecular machines. Biodegradable plastics made from PBPs not only break down in the environment but also can play useful roles. Since saline solution can break down PBPS, the plastic products can be disposed of in oceans and gulfs - and, as they degrade, the plastics can provide proteins for oceanic animals, thus entering the food chain and benefiting the marine ecosystem. A major problem in modern agriculture is the over- production and under-utilisation of agricultural products. The long-term sustainability of agriculture depends critically on finding new uses for surplus agricultural products. Efforts are, therefore, being made to synthesise high-value industrial products in plants using genetic engineering. The production of polymers in plants could allow biodegradable plastics to be synthesised on the million-tonne scale. If biodegradable polymers could be synthesised in plants to a level comparable to stored lipids or starch, they could be produced at a cost similar to vegetable oil or corn starch and thus be more competitive with petroleum-derived plastics. Expressing PBPs in a tissue-specific manner in leaves could lead to crop plants being used as renewable resource, in addition to harvesting the cobs, tubers, seeds, fruits for food. Chris Somerville's group at the Carnegie Institute of Washington has successfully engineered the polyhydroxybutyrate (PHB) pathway, containing three genes coding for a biodegradable polyester, into Arabidopsis. The transgenic plants produced as much as 10 mg/g fresh weight of PHB or 14 per cent of the dry weight in old leaves. Unfortunately, PHB could not be produced in non-oil crops. In contrast, expressing elastomeric PBPs from synthetic genes may not have any species-specificity because it relies on the availability of amino acid pools, common to all plant species. Recently, scientists expressed an elastomeric PBP in tobacco cells and polymer-like inclusions in tobacco leaves. This was the first demonstration of expression of a synthetic gene in plant cells. Several biopolymer products have reached the market recently and some of them are made in plants and bacteria. Several non-medical biopolymer products have also reached the market. Japan's Fuji Spinning is blending a chitin derivative, chitosan, with the natural fibre, polynosic, to create chitipoly, a fibre that is woven with cotton fibres for use in underwear to limit the growth of bacteria and fungi. Similarly, another company is field- testing `cellulose/polyester natural fibre' created by genetically engineering enzymes involved in the production of polyesters, like PHBs, in cotton to improve the thermal properties of cotton fibres. Source : Mahendra Pandey. The Business Line. May 26 1999     Copyright 2002 Earthsoul India. All Rights Reserved