Biodegradable polymers biopolymers

Keywords Biodegradable polymers, biopolymers, ceUulose, hemicellulose, polyphenols, dex-tranes, excipients, drug delivery, tissue engineering... 

There exist an important number of biodegradable polymers (biopolymers) that are derived from both synthetic and natural sources. The utiUzation of agricultural products in plastic applications is considered an interesting way to reduce surplus farm products and to develop non-food applications. 

More than a dozen biocompatible and biodegradable polymers have been described and studied for their potential use as carriers for therapeutic proteins (Table 13.5). However, some of the monomer building blocks such as acrylamide and its derivatives are neurotoxic. Incomplete polymerization or breakdown of the polymer may result in toxic monomer. Among the biopolymers, poly-lactide cofabricated with glycolide (PLG) is one of the most well studied and has been demonstrated to be both biocompatible and biodegradable [12]. PLG polymers are hydrolyzed in vivo and revert to the monomeric forms of glycolic and lactic acids, which are intermediates in the citric acid metabolic pathway. 

In addition to synthetic biodegradable polymers discussed so far, naturally occurring biopolymers have also been used for fabricating implantable dmg delivery systems. Examples of natural biopolymers are proteins (e.g. albumin, casein, collagen, and gelatin) and polysaccharides (e.g. cellulose derivatives, chitin derivatives, dextran, hyaluronic acids, inulin, and starch). 

There are around thirty suppliers actively involved in the world biodegradable polymers market in 2005. The synthetic biopolymers market is dominated by large, global and vertically integrated chemical companies such as BASF, DuPont, and Mitsubishi Gas Chemicals. The starch and PLA sectors contain mainly specialist biopolymer companies such as Novamont, NatureWorks LLC, Rodenburg Biopolymers and Biotec, which were specifically established purely to develop biodegradable polymers. 

Other leading starch-based biodegradable polymer manufacturers are Biotec and BIOP Biopolymers. 

In 2005, there were very few biodegradable polymer production plants operating on a fully commercial scale. NatureWorks LLC, Novamont, Rodenburg Biopolymers and BASF are currently the only major operators with significant production capacity. Nevertheless, the world biopolymers market has shown significant growth during the last five years or so, albeit from a very small base. 

The leading biodegradable polymer suppliers are Novamont, NatureWorks, BASF and Rodenburg Biopolymers, which together represent over 90% of the European market for biodegradable plastics. 

Starch-based biopolymers are lower cost materials than some other biodegradable polymer types such as synthetic co-polyesters and PLA. They are produced from relatively cheap agricultural feedstock and have simpler manufacturing processes compared with synthetic biopolymers. 

Starch-based biodegradable polymers also have a better environmental image than synthetic biopolymers as they are based on sustainable resources, which open up marketing opportunities for brand owners who wish to promote their products as being packaged in materials based on sustainable resources. 

Biodegradable polymer prices are generally much higher than commodity polymers for a number of reasons. Most biopolymers have only been commercially available for a couple of years and production volumes are very low compared with the mass produced polyolefins. Initial development costs are also very high. 

Procter Gamble is the other leading pioneer on the field of PHA biodegradable polymers. The Nodax biopolymers are based on the copolymer PHBH, a copolymer polyester of 3-hydroxybutyric and 3-hydroxyhexanoic acid. The higher the 3-hydroxyhexanoic acid comonomer component, the more flexible... 

The price of synthetic biodegradable polymers has come down a little during the last three years. In 2003, for example, the average price of Eastar Bio and BASF s Ecoflex was around 3.5-4.0 per kg. In 2005, the average cost of an aliphatic aromatic polyester biopolymer was between 2.75-3.65 per kg. The more specialised polymers, such as DuPont s Biomax, cost as much as 5-6 per kg. Polycaprolactones cost between 4-7 per kg. Synthetic biodegradable polymer prices are expected to fall further over time as production volumes increase and unit costs fall further. 

During the period 2000 to 2005, world consumption of synthetic biodegradable polymers has increased from 3,900 tonnes to 14,000 tonnes. In 2010, world consumption of synthetic biopolymers is projected to reach 32,800 tonnes. This represents a compound annual growth rate of 18.6% during the period 2005-2010. These forecasts assume that producers are successful in lowering the cost of production and that the price differential between synthetic biopolymers and standard thermoplastics continue to narrow. 

Serviceware made with biodegradable polymers such as PLA is particularly valued at outdoor events such as sports stadiums, concerts, universities, amusement parks, shopping malls and other venues that benefit from the disposal options available with biodegradable polymers. Over the next five years, biopolymers are expected to make further inroads into these markets. 

Biodegradable polymers and biopolymers can be produced by a wide variety of technologies, both from renewable resources of animal or plant origin, and from fossil resources. A number of different types are already available on the market. 

In 2005, starch-based materials were the largest class of biodegradable polymer with just over 47% of total market volumes. Loose-fill foam packaging accounts for more than a half of starch biopolymer volumes. Polylactic acid (PLA) is the second largest material class followed by synthetic aliphatic-aromatic co-polyesters. The PHA category is at an embryonic stage of market development with very low market tonnage at the moment. 

The most fundamental classification of polymers is whether they are naturally occurring or synthetic. Common natural polymers (often referred to as biopolymers) include macromolecules such as polysaccharides e.g., starches, sugars, cellulose, gums, etc.), proteins e.g., enzymes), fibers e.g., wool, silk, cotton), polyisoprenes e.g., natural rubber), and nucleic acids e.g., RNA, DNA). The synthesis of biodegradable polymers from natural biopolymer sources is an area of increasing interest, due to dwindling world petroleum supplies and disposal concerns. 

In recent years starch, the polysaccharide of cereals, legumes and tubers, has acquired relevance as a biodegradable polymer and is becoming increasingly important as an industrial material (Fritz Aichholzer, 1995). Starch is a thermoplastic polymer and it can therefore be extruded or injection moulded (Balta Calleja et al, 1999). It can also be processed by application of pressure and heat. Starch has been used successfully as a matrix in composites of natural fibres (flax, jute, etc.). The use of starch in these composites could be of value in applications such as automobile interiors. An advantage of this biopolymer is that its preparation as well as its destruction do not act negatively upon the environment. A further advantage of starch is its low price as compared with conventional synthetic thermoplastics (PE, PP). 

The blending of different polymers is a frequently used technique in industrial polymer production to optimize the material s properties. The biodegradable polymer poly(3-hydroxybutyrate) (PHB) [45, 46], for example, which can be produced by bacteria from renewable resources, has the disadvantage of being stiff and brittle. The mechanical properties of PHB, however, can be readily enhanced by blending with another biopolymer, poly(lactic acid) (PLA) [47]. In order to prepare the optimum blend, it must be noted that the miscibility of different polymers depends on their concentration, the temperature, and their structural characteristics [48]. 

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