Biodegradable materials aliphatic polyesters

For conventional technical applications aromatic polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are widely used. But these polymers are biologically inert and thus not directly applicable as biodegradable plastics. Combining both the excellent material properties of aromatic polyesters and the potential biodegradability of aliphatic polyesters has led to the development of a number of commercially available aliphatic-aromatic co-polyesters over the last decade or so. 

With the attempt to combine good material properties of aromatic polyesters and biodegradability of aliphatic polyesters, aliphatic aromatic copolyesters have been developed during the last years to be used as technical biodegradable plastics The BASF AG / Germany is now producing a biodegradable material based on a copolyester of 1,4-butanediol, adipic acid and terephthalic acid (BTA-copolyester) tmder the trade name Ecoflex in a several thousand tons per year scale. 

While the biological susceptibility of maity aliphatic polyesters has been known for maiy years, aromatic polyesters such as polyetltylene terephthalate (PET) or polybutylene tereph-thalate are regarded as non-biodegradable [39]. To improve the use properties of aliphatic polyesters, an attempt was made to combine the biodegradability of aliphatic polyesters with the good material performance of aromatic polyesters in novel aliphatic-aromatic copolyesters. 

In order to try to combine both the excellent material properties of aromatic polyesters and the potential biodegradability of aliphatic polyesters, several aliphatic-aromatic copolyesters have been developed over the last 20 years and are now, together with the aliphatic polyester polylactic acid (PEA), one of the most common biodegradable resins on the market with a production of several thousand tonnes per year. The... 

Ethoxylation of the carboxylic acid end groups of aliphatic polyesters significantly changes the biodegradation rate as well as the crystallinity of these materials (41). 

Pitt, C. G., Marks, T. A., and Schindler, A., Biodegradable drug delivery systems based on aliphatic polyesters application to contraceptives and narcotic antagonists, in Controlled Release of Bioactive Materials (R. Baker, ed.). Academic Press, New York, 1980, pp. 19-43. 

Common biodegradable polymers for medical devices are constructed from synthetic linear aliphatic polyesters. One material commonly used for internal sutures is poly(glycolic acid) (PGA). PGA is synthesized from the dimer of glycolic acid (Fig. 13.1.l). 1... 

Syntheses of aliphatic polyesters by fermentation and chemical processes have been extensively studied from the viewpoint of biodegradable materials science. Recently, another approach to their production has been made by using an isolated lipase or esterase as catalyst via non-biosynthetic pathways under mild reaction conditions. Lipase and esterase are enzymes which catalyze hydrolysis of esters in an aqueous environment in living systems. Some of them can act as catalyst for the reverse reactions, esterifications and transesterifications, in organic media [1-5]. These catalytic actions have been expanded to... 

However, most of the aliphatic polyesters presently commercially used for biodegradable materials exhibit serious disadvantages. Beside the relatively high price level, properties are often limited and exclude these materials from many applications. For example, PCL has a very low melting point of about 60 °C. 

Aliphatic polyesters like polycaprolactone (PCL) or polybutylene adipate (PBA) are readily biodegradable, but because of their melting points of 60 °C are unsuitable for many applications. On the other hand, aromatic polyesters like polyethylene terephthalate (PET) or polybutylene terephthalate (PBT) have high melting points above 200 °C and very good material properties, but are not biodegradable. 

The solution is a combination of aliphatic polyesters and aromatic polyesters. This involves modifying the crystalline structure of PBT by incorporating aliphatic monomer (adipic acid) in the polymer chain in such a way that the material properties of the polymer would remain acceptable (e.g., melting point of the crystalline range still around 100 °C), but the polymer would also be readily compostable/biodegradable. In this way it was possible to combine the degradability of aliphatic polyesters with the outstanding properties of aromatic polyesters. 

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