Aliphatic-aromatic copolyesters

These copolyesters combine the biodegradability of aliphatic polyesters with the excellent properties imparted by aromatic polyesters. While aliphatic polyesters are easily biodegradable, they lack thermal stability and mechanical properties needed for many applications. Aromatic polyesters, on the other hand, provide excellent use properties but are resistant to mierobial attack under environmental conditions.Both BASF and Eastman Chemicals produce aliphatic-aromatic copolyesters from terephthaUc acid, adipic acid, and 1,4 butane diol. Witt et al. reported on a new group of copolyesters, which combine both biodegradability and excellent properties. These copolyesters are synthesized by conventional bulk condensation techniques from various ahphatic diols with a defined mixture of different aliphatic dicarboxyhc acids and terephthaUc acid. The key to biodegradability is the blocklength of the aromatic unit, which should preferably be no more than a trimer. 

Copolymers of the polybutylene adipate-co-terephthalate type with a molar fraction of terephthalate of 42 mol-%, biodegrade completely to form compost in twelve weeks, whereas products with 51 mol-% of molar fraction of terephthalate show a percentage of biodegradation of less than 40% (42). This different behavior was attributed to the formation of a higher number of butylene terephthalate sequences with a length greater than or equal to 3, which are less easily biodegradable. 

If it would be possible to maintain suitable biodegradation properties, an increase in the percentage of aromatic acid in the chain is desirable in so far as it would enable an increase in the melting point of the polyester. In this way important mechanical properties, such as ultimate strength and elastic modulus are kept (43). 

The polymerization of aliphatic monomers with aromatic monomers such as terephthalic acid or their diester derivatives, such as dimethyl terephthalate (DMT), may improve the performance properties of aliphatic polyesters. 

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

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. 

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Hie ester linkage of aliphatic and aliphatic-aromatic copolyesters can easily be cleaved by hydrolysis under alkaline, acid, or enzymatic catalysis. This feature makes polyesters very attractive for two related, but quite different, applications (i) bioresorbable, bioabsorbable, or bioerodible polymers and (ii) environmentally degradable and recyclable polymers. 

M.D. Shelby, A.J. Matosky, C.M. Tanner, and M.E. Donelson, Blends of aliphatic-aromatic copolyesters with ethylene-vinyl acetate copolymers, US Patent 7 241838, assigned to Eastman Chemical Company (Kingsport, TN), July 10,2007. 

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. 

Compared with totally aliphatic copolyesters, aromatic copolyesters are often based on terephthalic diacid. BASF markets a product, Ecoflex, which is a copolyester of butanediol, adipic acid, and dimethyl terephthalate. Eastman s Eastar Bio Copolyester is similar aliphatic/aromatic copolyester. DuPont markets a modified PET known as Biomax [Chauhan, 2012]. 

Figure 21.9 The biodegradable polyester family poly(hydroxyalkanoates) (PHA), poly(hydroxybulyrate) (PHB), poly(hydro3qdiexanoate) (PHH), poly(hydrox3rvalerate] (PHV), polyflactic acid) (PLA), poly(caprolactone) fPCL), poly(butylene succinate) (PBS), poly(butylene succinate adipate) (PBSA), aliphatic-aromatic copolyesters (AAC), poly(ethylene terephthalate) (PET), poly(butylene adipate/terephthalate) (PBAT), poly(methylene adipate/terephthalate) (PTMAT). Adapted from [103].

What is the degradation mechanism for aliphatic-aromatic copolyesters, where aliphatic and aromatic structures are combined in one polymer chain ... 

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. 

The development of new aliphatic and aliphatic-aromatic copolyesters containing monomers from vegetable oils, covered by a range of Novamont s patents, have further improved and widened the performances of these products from an environmental and technical point of view. 

Monomers for aliphatic-aromatic copolyesters are summarized in Tables 4.3 and 4.4. 

Blends of Aliphatic-Aromatic Copolyesters with Poly(ethyl-ene-co-vinyl acetate)... 

Blends of aliphatic-aromatic copolyesters with poly(ethylene-co-vinyl acetate) blends have higher melt strength than the aliphatic-aromatic copolyester as such and exhibit an increased melt strength and a better thermal processability (50). An example of a branched basic aliphatic-aromatic material is made from poly(tetramethylene adi-pate-co-terephthalate) by transesterification. As branching agents pentaerythritol or pyromellitic dianhydride may be used. The... 

BasicaUy, the sulfonated aliphatic-aromatic copolyesters should contain up to 4 mol-% of sulfonate groups based on the total aUphatic dicarboxyUc acid component. 

The sulfonated aliphatic-aromatic copolyesters exhibit improved thermal properties, in particular, a desirable balance of high temperature properties and improved compostabUity Sulfonated polyesters can be prepared as follows (60) ... 

C. Bastioli, T. Milizia, G. Eloridi, A.S. Lallaro, G. Celia, and M. Tosin, Biodegradable aliphatic-aromatic copolyester, US Patent 8193300, assigned to Novamont S.p.A. (Novara, IT), June 5, 2012. 

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). 

Marten E, Muller R-J, Deckwer W-D (2005) Studies on the enzymatic hydrolysis of polyesters, n. Aliphatic-aromatic copolyesters. Polymer degradation and stability. 88(3) 371-381. 

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, Welzel K, VandenHeuvel J, Muller R-J, Deckwer, W-D (2005) Characterization of a new extracellular hydrolase from Thermobiftda fusca degrading aliphatic-aromatic copolyester. Biomacromolecules 6 262-270... 

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