Biodegradation of Aliphatic-aromatic Copolyesters

Polyesters which solely contain aromatic acid components, such as PET or PBT, are used for many technical applications and are commonly regarded as quite resistant to any hydrolytic degradation. Only by applying very drastic chemical treatments, (e.g., sulfuric acid at 150 °C), very far from any physiological conditions, can the hydrolysis of such polymers be achieved at reasonable rates and used for recycling 

Trade name Ecoflex Origo-Bi BicKosafe S-F.nPol 

Producer BASF AG, Germany Novamont, Italy Xinfu Pharm, China Samsung Fine Chemicals, Korea 

Chemical basis Modified copolyesters from 1,4- butanediol, adipic acid and terephthalic acid Modified copolyesters from 1,4-butanediol, terephthalic acid and aliphatic dicarboxylic acid Copolyester from 1,4-butanediol, adipic acid and terephthalic acid Copolyester from 1,4-butanediol, adipic acid and terephthalic acid 

Elongation at break (%) 560 (ISO 527) 700 (ASTM D882) 650 675 (ASTM D638) 

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However, questions have been raised with regard to the complete biodegradability of aliphatic-aromatic copolyesters because aromatic copolyesters such as poly(ethylene terephthalate) (PET)... 

Witt U et al. (2001) Biodegradation of aliphatic-aromatic copolyesters evaluation of the final biodegradability and ecotoxicological impact of degradation intermediates. Chemosphere 44 289-299. 

Kleeberg I., Hetz C., Kroppenstedt R.M., Muller R.-X, Deckwer W-D. Biodegradation of aliphatic-aromatic copolyesters by Thermonospora fusca and other thermophilic compost isolates, Appl Environ. Microbiol. 64 (1998) 1731. 

Kleeberg, L, Hetz, C. et al. Biodegradation of aliphatic-aromatic copolyesters by Thermo-monospora fusca and other thermophilic compost isolates, AppUed Environ. Polymer Degradation, A 64 (1995) 5, p. 1731-1735... 

Witt, U., R.J. Muller, and W.D, Deckwer, Studies on sequence distribution of aliphatic/aromatic copolyesters by high-resolution 13C nuclear magnetic resonance spectroscopy for evaluation of biodegradability. Macromolecular Chemistry and Physics, 197(4) p, 1525,1996,... 

Since nonbiodegradable aromatic polyesters like PET provide excellent material properties [55], with respect to easily degradable aliphatic polyesters, a number of aliphatic/aromatic copolyesters were studied and developed in order to produce materials which combined good mechanical properties and biodegradability. Major polyester producers in Europe and the USA brought aliphatic/aromatic copolyesters for biodegradable applications to the market. 

In biodegradable PBTA aliphatic/aromatic copolyesters available on the market, the amount of aromatic acids in the polymer chain is maintained below 49 mol%, in... 

In recent years, Novamont developed a family of aliphatic-aromatic copolyesters with its aliphatic dicarboxylic acid component predominantly based on long chain dicarboxylic acids of natural origin (sebacic acid, azelaic acid and brassylic acid) [56-58]. Compared with aliphatic-aromatic polyesters where the aliphatic dicarboxylic component is a shorter carbon chain length, such as BTA polyesters, these copolyesters do not show the sudden decrease of biodegradation properties above... 

Furthermore, novel polyfpropylene terephthalate-co-adipate) random copolymers showed that they can be degraded via hydrolysis, especially in the presence of enzymes (Rhizopus delemar and Pseudomonas cepacia lipases), even for a terephthalate content as high as 66 mol% [38]. In contrast to hydrolysis rates, mechanical properties increased upon increasing the terephthalate content. The main advantage of aliphatic-aromatic copolyesters over pure terephthalates is their biodegradability. 

Starch-based materials represent the largest class of biodegradable polymer with 44,800 tonnes (including loose-fill foam packaging) consumed in 2005. Excluding loose-fill, starch-based materials amounted to 21,700 tonnes in 2005. Polylactic acid (PLA) is the second largest material class with 35,800 tonnes in 2005, followed by synthetic aliphatic-aromatic copolyesters with 14,000 tonnes. The embryonic PHA category amounts to around 250 tonnes. 

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. 

On the other hand, it has been established that aliphatic-aromatic copolyesters are indeed biodegradable and that the biodegradability of these copolyesters is related to the length of the aromatic sequence. Block copolyesters with relatively long aromatic sequences are not rapidly degraded by microorganisms. 

Instead of the conventional binder pol5miers described above, biodegradable polymers may be used advantageously, such as aliphatic-aromatic copolyesters. A preferred aliphatic dicarboxylic acid is adipic acid and as aromatic dicarboxylic acid terephthalic acid can be used. As a glycol component 1,4-butanediol has been suggested. The natural cellulose fibers are selected from hemp, sisal, flax, kenaf, cotton, jute, or coconut (32). Commercial natural cellulose containing fibers are summarized in Table 5.10. 

Compositions have been developed that include a biodegradable aliphatic-aromatic copolyester, i.e., poly(tetramethylene adi-pate-co-terephthalate) and a plasticizer. The composition exhibits a moisture vapor transmission rate of at least 400 (1). 

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. 

Generally it seemed that many polyesters composed of aliphatic monomers were degradable by lipases, while most aromatic polyesters were characterized as biologically inert [95]. In aliphatic-aromatic copolyesters the tendency was found that biodegradability decreases with the content of aromatic constitnents. For copolyesters composed from adipic acid, terephthalic acid and 1,4-butanediol a maximum content of about 50-60% terephthalic acid in the diacid component was reported to be the hmit for biodegradability. 

Biodegradation of natural and synthetic copofyesters in two different natural environments, i.e. in compost with activated sludge at a sewage farm and in the Baltic Sea, was studied by Rutkowska et al. [142]. The results revealed that the natural aliphatic copolyester 3-hydroxybutyrate-co-3-lydroxyvalerate (PHBV) and its blends with the synthetic aliphatic-aromatic copolyester of 1,4-butanediol with adipic and terephthalic acids degraded faster in compost than in seawater. In both natural environments, blends degraded faster than aliphatic-aromatic copolyester, but at a slower rate than natural component PHBV... 

Zhang E, Huang E, Wang B. Characterization of biodegradable aliphatic/aromatic copolyesters and their starch blends, Polym.-Plast Technol Eng. 41 (2002) 273. 

Poly (butylene adipate-co-terephthalate) (PBAT) is an interesting polymer [116,117] and has attracted more attention. PBAT is also a biodegradable poljmier, which has proper viscosity and elasticity. Moreover, the balance of biodegradability (e.g., life time) and its physical properties (e.g., thermal and mechanical properties) can be adjusted by controlling the molar ratio of comonomers in the copolymer [118,119]. It has been reported that the aliphatic/aromatic copolyester with aromatic units within the range of 35 - 55 mol% offers an optimal compromise of its biodegradability and physical properties. 

Indeed, a large number of biodegradable polyesters are based on petrolenm resources, obtained chemically from synthetic monomers and polycaprolactone, poly-esteramide, or aliphatic/aromatic copolyesters can be distinguished. Generally, these petroleum-based polyesters are soft at room temperature since their glass transition temperatures are lower. 

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