Biodegradable Polymers from Petroleum-Derived Products

Chitosan can be crosslinked using epochlorohydrin [42]. Chitosan has also been found to exist naturally, being synthesized by zygomycete fiingi as part of their ceU wall. Chitosan has been shown to be biodegraded by chitosanases [40, 41]. 

Polylactic Acid Polylactic acid (PLA) is a thermoplastic, ahphatic polyester that can be synthesized from biologically produced lactic acid. Currently, the major production of polylactic acid is from the ring-opening polymerization of the lactide [43, 44]. This material has been used extensively in the medical field for sutures, staples, and the like and as such is very expensive. Recently, April 2002, Cargill-Dow has opened a large-scale production facility whereby PLA is being produced at a low cost for nonmedical applications  

The poly-L-lactic acid shows high melting points and good mechanical properties. Polylactic acid degrades by hydrolysis, which has been shown to be accelerated by many enzymes. Recently, it has been found to be biodegradable in a compost environment [43, 45]. The thermoplastic material can be made stereo specific or racemic to yield different properties. 

BIODEGRADABLE POLYMERS FROM PETROLEUM-DERIVED PRODUCTS 

Polycaprolactone Poly-e-caprolactone (PCL) is a semicrystalline, thermoplastic, linear aliphatic polyester synthesized by the ring-opening polymerization of e-caprolactone and is produced commercially by Union Carbide-Dow (Midland, MI) and Rhone-Poulenc (Collegeville, PA). This polymer has a melhng point of approximately 62°C and a glass transihon temperature of approximately -60°C. PCL is readily degraded and mineralized by a variety of microorganisms [45]  

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BIODEGRADABLE POLYMERS FROM PETROLEUM-DERIVED PRODUCTS... 

Polylactic acid (PLA) is a biodegradable polymer derived from lactic acid. It is a highly versatile material and is made from 100% renewable resources like corn, sugar beet, wheat and other starch-rich products. Polylactic acid exhibits many properties that are equivalent to or better than many petroleum-based plastics, which makes it suitable for a variety of applications. 

The naturally biodegradable polymers such as starch, chitosan and cellulose derived from natural sources have produced a number of interesting NR blends and IPNs. These blended systems have an advantage in that they create fewer waste disposal problems compared to the petroleum based polymeric materials. The use of stareh blends to enhance the biodegradability of conventional plastics has been reported by many researchers in order to reduce the environmental impaet of petroleum based plastic products and waste. The NR/maize stareh blends exhibited a decrease in their mechanical strength due to the speeifie properties of starch. However, the blended polymers showed a low interfaeial interaetion between the two phases due to the different polarity behaviour of the hydrophobic NR and the hydrophilic starch. 

PHAs can substitute petroleum-derived polymers, can be produced from renewable resources and are harmless to the environment due to their biodegradability. However, the major hurdle facing commercial production and application of PHA in consumer products is the high cost of bacterial fermentation. It makes bacterial PHA production 5 10 times more expensive than the petroleum-derived polymers such polyethylene and polypropylene. The significant factor of the production cost of PHA is the cost of substrate (mainly carbon source). In order to decrease this cost, the use of cheap carbon sources as substrates have been developed. The researches have been carried out to develop recombinant strains utilizing a cheap carbon source, while corresponding fermentation strategies have been developed and optimized. 

Most of the plastics and synthetic polymers that are used worldwide are produced from petrochemicals. Replacing petroleum-based feedstocks with materials derived from renewable resources is an attractive prospect for manufacturers of polymers and plastics, since the production of such polymers does not depend on the limited supply of fossil fuels [16]. Furthermore, synthetic materials are very persistent in the environment long after their intended use, and as a result their total volume in landfills is giving rise to serious waste management problems. In 1992,20% of the volume and 8% of the weight of landfills in the US were plastic materials, while the annual disposal of plastics both in the US and EC has risen to over 10 million tons [17]. Because of the biodegradability of PHAs, they would be mostly composted and as such would be very valuable in reducing the amount of plastic waste. 

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