Aliphatic-aromatic polyesters

The aliphatic-aromatic polyesters have petrochemical origin, and are generally produced through traditional polycondensation reactions. The structure of two of them, PBAT and PBST. They consist of aliphatic chain segments (residues of 1,4-butanediol, and of adipic or succinic acid), which provide flexibility, toughness, and biodegradability and aromatic segments (residues of terephthalic acid and 1,4-butanediol) , which impart mechanical strength and rigidity.

 PET, an aromatic polyester, decomposes very slowly in recalcitrant aromatic oligomers. The degradation of the aliphatic-aromatic polymers may be oxidative, hydrolytic and enzymatic. The oxidative degradation occurs in the presence of oxygen gas and heat, ultraviolet radiation and/or mechanical stresses. The hydrolytic degradation occurs in presence of water, and is self-catalyzed by the acidity of the carboxylic acids, being more intense inside the part.

  The enzymatic hydrolysis uses non-specific enzymes, such as hydrolases and lipases, produced by an enormous variety of organisms, especially the mycelium-forming microorganisms (fungi and actinomycetes). The amorphous regions are degraded preferentially over the crystalline regions, both chemically and biologically.

Interestingly, there is an inverse relationship between the melting temperature of the polyester and its rate of biodegradation, indicating that the crystalline characteristics are a very important factor in its biodegradability. The polyesters that behave like elastomers at the degradation temperature, undergo enzymatic degradation from the moment in which they are placed in the disposal environment, showing surface erosion. The polyesters that behave like glass at the degradation temperature are enzymatically degraded only at the end of the degradation process, from the by-products of the preliminary abiotic degradation. Although the aliphatic-aromatic polyesters present high degradability in industrial composting, their rate of degradation in soil and water bodies is much lower, and their degradability under anaerobic conditions is even lower.

  Some applications are the same typical for LDPE: transparent blown films, mulch films for agriculture, films for package and bags, and also blown bottles, filaments, injection moulded and thermoformed products. Poly(butylene adipate-co-terephthalate) (PBAT): Some of its main applications are films (mulch, containers, bags), filaments, thermoformed and injection moulded products, and blown bottles. Two products in the market are Ecoflex (BASF, 14,000 t per year) and MaterBi (former EasterBio/Eastman, now Novamont, 15,000 t per year). For some applications, PBAT has a very low stiffness, and may be mixed with PHB or PLA, for example. It may also be mixed with thermoplastic starch.

Poly(butylene succinate-co-terephthalate (PBST): Some of its main applications are blown films, filaments, blown and injection moulded containers, thermoformable cups and trays, paper coating, etc. A product in the market is Sorona/Biomax). PBST has good mechanical properties, reasonable processing and biodegrades slowly.