Introduction

Industrial revolution has resulted in many materials which are man-made and not resembling to natural ones. The term xenobiotic(stranger to life) is derived from the Greek word 'xenon' - a strange and 'bios' - life. For the environmental chemist, xenobiotic implies foreign to the biosphere. Anthropogenic is a specific term for man-made. These materials may be products, intermediates or wastes of our industrial and other activities. And at whatever stage they enter into nature, they can not become a part of cycle of matter. This is for the simple reason that microorganisms do not find any resemblance between these materials and their natural food habits. However simultaneously it is also true that all foreign to nature man-made substances need not be non-degradable.Those compounds which are not degraded in nature following their release into environment even when the conditions appear to be adequate for microbial growth, are termed as. Recalcitrant compounds persist in all natural environments regardless of whether they are inherently biodegradable or not. Without being bias to any industry, it is well accepted fact that xenobiotic compounds, toxic chemicals, hazardous wastes come from the petrochemical industry, pesticide industry, chemical industry, mining, metal processing etc. The list of pollutants which pose environmental and health hazard and are tough for biodegradation is long one and includes solvents, wood preservative chemicals, plasticizers, refrigerants, coaltar wastes, pesticides, biphenyls, polychlorinated and polybrominated biphenyls, synthetic fibers, plastics, polyvinyl chloride, polystyrene, detergents like alkyl-benzene sulphonates, oils etc

Biolac

Other bioplastic which is easily biodegradable are polylactides (PLA) and pol yglycolides (PGA). The earlier efforts in this field are by American Cyanamid Corporation which developed the first synthetic absorbable suture material. The product, Dexon was a polyglycolic acid homopolymer. Vicryl was developed then by Dupont which has 92:8 glycolic acid : lactic acid as copolymer. PLA and PGA are thermoplastics and are biodegradable polyesters. Low Mr polylactic acid and polyglycolic acid are made by direct polymerization of respective acids. The high Mr, PLA and PGA are made by ring opening polymerization of lactide and glycolide which are cyclic diesters of respective acids. Polyglycolic acid and polylactic aid have degradation time in few days and few weeks respectively while polylactides and polyglycolides have degradation time in few months to years. 

Researchers at the University of Wisconsin, USA, have produced biodegradable and photodegradable polymers from l-lactic acid. The researchers are working on a project to produce l-lactic acid from whey permeate. The technology to fer ment whey to lactic acid is in fact quite old but the end product is mixture of l-lactic acid and d-lactic acid. The pure l-lactic acid is worth nearly 3 times as much the mixture of l- and d- lactic acids. Pure l-lactic acid has several uses. It can be u sed to manufacture polylactides (plastics made from lactic acid that are bio- and photodegradable), as food preservative, as flavour enhancer and as acidulant and in pharmaceutical industry (in IV solution and drug delivery).

The Ecological Chemical Products Co. (ECOCHEM) has recently opened a $ 20 million commercial plant in Adel, Wisconsin to produce high-pure natural lactic acid and polylactide polymers for food and pharmaceutical applications. ECOCHEM is a joint venture of DUPONT and Con. Agra. Inc. ECOCHEM's manufacturing process has environmental advantages such as raw materials derived from natural byproducts of cheese industry, no new wastes generated during its production and recycled side streams producing either useful or environmentally acceptab le co-products. 

Production of lactic acid-based plastics from starchy food wastes and by-products is also on the way of commercialization. The Argonne National Laboratory has licensed key steps in its Biolac process to Kyowa Hakko USA Inc., a US subsidiary of Japan's Kyowa Hakko, which deals in fermentation products. Kyowa Hakko plans to carry out further research and development aimed to commercialization. PLA and PGA are having mainly medical applications, as sutures, as ligament replacements, for resorbable plates and screws in fracture fixation (i.e. in orthopedic repairs), for controlled drug release, for arterial grafts. The Biolac plastic has commercial applications for compost bags, coatings for paper, seeds, pesticides, fertilizers and agricultural mulch films for timed release of pesticides and fertilizers. In t wo key steps that have been licensed, one converts glucose of starchy wastes to lactic acid and the other converts lactic acid to polylactic acid.

Potential market for PLA plastics and coatings as forcasted by Argonne National Laboratory, Illinois : -

• Controlled fertilizer and pesticide application - > 500 000 tonnes
• marine plastic applications - 250 000 tonnes
• degradable conditioner coatings for paperback stock - > 100 000 tonnes
• compost waste bags, sacs etc. - tens of thousands of tonnes
• agricultural mulch film - 75 000 tonnes

Polylactic acid can be produced in US since the US produces 5 million tonnes of food wastes in the manufacture of fried potatoes and 1-2 million tonnes wastes in the cheese industry. Kyowa Hakko USA Inc. want s to use this fact for producing polylactic acid by the Biolac process.

The 5000 tonnes / year capacity plant at a cost of $ 8 million will be operational at Port Cargill on Minnesota river near Minneapolis. It will make polylactic acid from lactic acid which is obtained by bacterial fermentation of sugars from corn, potato, milk, sugarbeet.

Cargill Dow Polymer (CDP) plans to produce 1,40,000 tonnes per year of commercial polymer from end of 2001 A. D. First plant will come up at Blair (Nebrasca, USA). Second plant will come up in Europe while the third is going to serve Asian markets. CDP is applying its Nature Works Technology to the processing of corn sugars to produce proprietary polylactide polymer (PLA) which is fully biodegradable. Corn starch is first converted to dextrose which is fermented to lactic acid. Lactic acid is then converted by condensation to lactide, a cyclic dimer. This lactide is purified through vacuum distillation. Ring opening polymerization of the lactide is accomplished with solvent free melt process. A wide range of products that vary in molecular weight a nd crystallinity can be produced. This enables wide range of applications. Wheat, maize, sugar beet and agricultural wastes are also being tried. CDP is working with several packaging and fibre manufacturers for development of applications. Clothing fibres, films, food containers and furnishings are amongst the many possible applications.

Archer Daniels Midland Co. and Warner Lambert Co. are among the companies gearing up to produce lactic acid from corn or starch that could be used in next generation biopolymer plastic.

Two Japanese companies Kobe steel Limited (Kobe) and Shimadzu Corp. (Kyoto) have perfected a low-cost continuous process for manufacture of biodegradable polymer - poly-2-lactic acid (PLLA). It can be produced at tens of thousands of capa city. PLLA melts at 170-180⁰Cand has vicat softening point of 58⁰C, a tensile strength of 700kg/cm and a transparency of 94%. It can be processed into a film of 10 to 500 Mm thickness and can be injection moulded. It can be produced at $2.54 to $4.23 per k g. It can be used for food containers, soil retention sheetings and agriculture film.

The physical properties of polylactic acid are similar to polystyrene. It can also be modified to make it similar to PE (Polyethylene) or PP (Polypropylene). It has performance benefits similar to petrochemical-based plastics but is biodegradable by composting. Applications of polylactic acid as plastic can be - disposable fast food, dairy and delivery containers, food service ware, medical garments, waste bags.

Asahi Che micals and Institute of Physical and Chemical Research in Japan have jointly discovered a new species of bacteria which synthesizes biopolyesters. The bacteria accumulate large quantities of polymers. The bacteria can use wide variety of compounds with 2- 22 carbon atoms as a source of carbon. In experiments, inexpensive oils and fats were used as carbon sources which produced polyester at a maximum yield of 45%. The yield will be increased to 60-70% when recovery process is optimized. High efficiency plast ic with low production cost is the aim of research work. The use of wide range (2-22 carbon atoms) compounds is advantageous. Product type and yield differ with type of feed used. The compound with 18 carbon atoms produces the highest recovery yields.

Biodegradable enviroplastic is developed by Planet Technologies, USA. It will be used for cosmetic, medical disposables, food service, personal hygiene product industries. It will dissolve in sludge compost or water without leaving any environmentally harmful residues.

Researchers at University of Iowa are investigating the synthesis of biodegradable plastic using enzymes in organic solvents. Sucrose and adipic acid are the substrates used and action of lipases and proteases is sought for to link sugar and d iacid into copolymer chain.

Polyglutamic acid (PGLU) is a water soluble polymer produced by Bacillus species. Bacillus subtilis releases polyglutamate in growth medium. Fermentations to produce PGLU have been patented some years ago but commercial product ion is not reported. Yields to the tune of 40 g/l of PGLU in 5 days are reported.

Japanese researchers at National Food Research Institute have developed wate r-resistant, biodegradable plastic films from corn protein. This newly developed plastic is expected to have wider applications such as food trays, because of low material cost and fabricability. The process is based on a protein called Thujene. It remain s in corn after removal of starch. Thujene is spread into thin transparent film after being dissolved in acetone. When it has thickness of around 70 �m the film is as strong as commercial wrapping film with respect to boring. The film rarely permeates water. It will be enzymatically decomposed in one month in ordinary soil.

Biodegradability of PHAS

One of the properties that distinguishes PHAs from petroleum-based plastics is their biodegradability. Produced naturally by soil bacteria, the PHAs are degraded upon subsequent exposure to soil, compost, or marine sediment. Despite their biodegradability the PHAs still have good resistance to water and moisture vapor, and are stable under normal storage conditions and during use.
Biodegradation of PHAs is dependent upon a number of factors such as the microbial activity of the environment and the exposed surface area. In addition, temperature, pH, molecular weight and crystallinity are important factors. Biodegradation starts when micro-organisms begin growing on the surface of the plastic and s ecrete enzymes that break down the polymer into its molecular building blocks, called hydroxyacids. The hydroxyacids are then taken up by the micro-organisms and used as carbon sources for growth. In aerobic environments the polymers are degraded to carbo n dioxide and water, whereas in anaerobic environments the degradation products are carbon dioxide and methane.
A number of reports have demonstrated that PHAs are compostable over a wide range of environmental conditions. In one report, the maximum biodegradation rates were observed at moisture levels of 55% and temperatures of around 60⁰C -- conditions similar to those used in most large-scale composting plants. Up to 85% of the samples degraded within 7 weeks, and PHA coated paper was rapidly degraded and incorporated into the compost. In another study, the quality of PHA compost was determined by measuring seedling growth relative to a control. Seedling growth of around 125% of the control was found for a 25% PHB copolymer compost indicating that the com post can support a relatively high level of growth.
Biodegradation of PHAs has also been tested in various aquatic environments. In one study in Lake Lugano, Switzerland, items were placed at different depths of water as well as on the sediment surface. A l ife span of 5-10 years was calculated for bottles under these conditions (assuming no increase in surface area), while PHA films were completely degraded in the top 20 cm of sediment within 254 days at temperatures not exceeding 6⁰C.

About Plastic Pollution

Plastic is one of the few new chemical materials which pose environmental problem. Polyethylene, polyvinyl chloride, polystyrene is largely used in the manufacture of plastics. Synthetic polymers are easily molded into complex shapes, have high chemical resistance, and are more or less elastic. Some can be formed into fibers or thin transparent films. These properties have made them popular in many durable or disposable goods and for packaging materials. These materials have molecular weight ranging from several thousands to 1,50,000. Excessive molecular size seems to be mainly responsible for the resistance of these chemicals to bio degradation and their persistence in soil environment for a long time.

Plastic in the environment is regarded to be more an aesthetic nuisance than a hazard, since the material is biologically quite inert. The plastic industry in the US alone is $ 50 billion per year and is obviously a tempting market for biotechnological enterprises. Biotechnological processes are being developed as an alternative to existing route or to get new biodegradable bio-polymers . 20% of solid municipal wastes in US is plastic. Non-degradable plastics accumulate at the rate of 25 million tonnes per year. According to an estimate more than 100 million tonnes of plastic is produced every year all over the world. In India it is only 2 million tonnes. In India use of plastic is 2 kg per person per year while in European countries it is 60 kg per person per year while that in US it is 80 kg per person per year.

Processing of Bio-plastics

Presence of nucleating agents (which facilitate crystallization) or the use of plasticiser shortens the processing cycles during the moulding operations. There are two main points about processing of PHBV bio-plastics - (i) The limited thermal stability of the polymer and so it degrades rapidly above 195 degree centi. (ii) The need to optimize conditions to allow a maximum crystallization rate (which reduces cycle times). The maximum rate of crystallization is reported to be at about 55-60 degree centi. which is significantly closer to Tg than the Tm. Processing temperatures should not exceed 180 degree centi. and duration of time when the material is in melt state should be kept minimum.



At the end of a run the processing equipment should be purged with polyethylene. When blow moulding the blow-pin and the mould should be at about 60 degree centi. to optimise crystallisation rates. Similarly injection moulds are recommended at 55-65 degree centi. The low-hydroxyvalerate, unplasticised grades are most critical to process, requiring the higher processing temperatures. Conditions are slightly less critical with the higher hydroxyvalerate containing and plasticised grades. In addition to producing PHAs in dry powder form for melt processing, Metabolix is also developing PHA latexes. These materials have unique film forming properties, which are finding application in higher performance applications as well as in more traditional commodity uses. Metabolix company supplies PHA samples to companies under research and development agreements.

Causes Of Plastic Pollution

Plastics are used because they are easy and cheap to make and they can last a long time. Unfortunately these same useful qualities can make plastic a huge pollution problem. Because the plastic is cheap it gets discarded easily and its persistence in the environment can do great harm. Urbanization has added to the plastic pollution in concentrated form in cities. Plastic thrown on land can enter into drainage lines and chokes them resulting into floods in local areas in cities as experienced in Mumbai, India in 1998. It was claimed in one of the programmers on TV Channel that eating plastic bags results in death of 100 cattle's per day in U.P. in India. In stomach of one dead cow, as much as 35 kg of plastic was found. Because plastic does not decompose, and requires high energy ultra-violet light to break down, the amount of plastic waste in our oceans is steadily increasing. More than 90% of the articles found on the sea beaches contained plastic. The plastic rubbish found on beaches near urban areas tends to originate from use on land, such as packaging material used to wrap around other goods. 

THE SOLUTIONS FOR PLASTIC

Plastics may be either (a) photodegradable or (b) semi-biodegradable or (c) 100% biodegradable. Photodegradable plastics have light sensitive groups incorpora ted directly into backbone of the polymer as additives. This produces non-degradable smaller fragments which cause loss of material integrity. Example of semibiodegradable plastic can be blends of starch and polyethylene. PHB is an example of 100% biodegradable plastic. Approximately a dozen of inherently biodegradable plastics are now in the market, with range of properties suitable for various consumer products. Some examples of biodegradable films or other raw material (for biodegradable plastic articles) made by different companies are - 

(1) BASF - Ecoflex - for biodegradable garbage bags - made up of polyester
building blocks that offer properties similar to LDPE (10000 tonnes production
plant at Ludwigshafen, Germany.
(2) Bayer - BAKbiodegradable polyester amide. - contains ester and amide groups
similar to that in nature.- therefore biodegradable.
(3) Dupont's - BioMax
(4) Eastman's - EasterBio
(5) Cargill-Dow's - EcoPla
All of them are targetting market of 70000 tonnes by 2001.
(6) Arrow Coated Products Ltd. (ACPL) has started bioplastic watr soluble film
production in India in Gujrat State at Ankleshwar. But capacity is only 600
tonnes per annum. They do not export any of it.
(7) Om Engineers (India) - have also tested and about to introduce LDPE
modified biodegradable film (for which they have applied for patent even.
(8) Fasalex, The Institute for AgroBiotechnology, Kopfig, Austria have developed biodegradable material using sawdust and corn grit held together with cellulose based natural resin. This material can be processed by same machines which are otherwise used for normal plastic.
(9) Environmental polymers of Warrington make biodegradable polymer which can be melt - processed by film blowing or injection moulding etc. Product is water soluble and is useful for interior purpose. It is based on PVOH.
Estimate of current global market for biodegradable plastic is 1.3 billion kg per year. Estimated global market for biodegradable polymers will reach 1.4 billion tons by the year 2000. In response to increasing public concern over environmental hazard caused by plastic, many countries are conducting various solid waste management programmes incl uding plastic waste reduction by development of biodegradable plastic material. There is an intense research for making the biodegradable plastic. Some biodegradable plastic materials under development are -

1. PHAs
2. Polylactides
3. Aliphatic polyesters
4. Polysaccharides
5. Co-polymers and/or blends of above.