Biodegradable polyesters mechanism

Ecoflex FS A Biodegradable Polyester with New Mechanical Properties and a High Content of Renewable Resources... 

A controllable biodegradability, desirable mechanical properties, suitable gas permeability and selectivity would extend the potential application areas of aliphatic polyesters not only in agriculture or in the greenhouse or packaging industry but also as a substitute for human skin. There is a need for such focused studies in the future. 

Most of the commonly used degradable polymer scaffolds are mechanically strong, but for certain applications such as engineering muscles and tendons, which require considerable elasticity, these polymers are not optimal. Novel biodegradable polyesters have been developed with superior elasticity and strength that resemble vulcanized rubber and are hence termed as biorubber. Scaffolds made with these mechanically functional materials may be useful especially in engineering elastic tissue such as muscular-skeletal tissues and blood vessels. 

Kim HK, Park TG. Microencapsulation of human growth hormone within biodegradable polyester microspheres protein aggregation stability and incomplete release mechanism. Biotechnol Bioeng 1999 65(6) 659-667. 

Pitt, C., Non-microbial degradation of polyesters mechanisms and modifications, in Proceedings of the Second International Scientific Workshop on Biodegradable Polymers and Plastics, Vert M., Feijen J., Albertsson A., Scott G. and Chiellini E., Eds., Royal Society of Chemistry, Cambridge, UK, 1992, page 7. 

Figure 9.4 illustrates the chemical stmctures of the different biodegradable polyesters. They can be classified into bio-based or non-renewable-resouree-based polymers. Table 9.1 hsts the main physico-chemical and mechanical properties of commercially used polyesters. 

In order to preserve the final compostabihty, different blends of biodegradable materials have been developed. There is a vast body of literature available in this domain. We find certain associations with agropolymers such as proteins [ARV 99, FIS 00, OTA 99] or pectins [FIS 00], but most research focuses on blends of plasticized starches and biodegradable polyesters PCL, PHA, PBSA, PBAT, etc. These polyesters, described previously, are produced industrially. They exhibit interesting properties such as a more hydrophobic natiue, limited water permeability and improved mechanical properties, in comparison to polysaccharides. However, the cost of biodegradable polyesters is generally higher than that of starch... 

Due to the poor mechanical properties and instability of native starches, it is frequently reinforced with fibers [134—136] or blended with synthetic polymers [137-140]. Recently, much emphasis has been put on blending starch with biodegradable polyesters like PLA [141, 142], PCL [120, 143-145] and PBS [143, 144]. The injection conditions depend on the polyesters incorporated generally, blending with these synthetic polymers decrease the overall viscosity of starch. 

Multilayer co-extrusion is another technique used in the preparation of starch/ synthetic sheets or films [164, 263-266], in which TPS is laminated with appropriate biodegradable polymers to improve the mechanical, water-resistance and gas-barrier properties of final products. These products have shown potential for applications such as food packaging and disposable product manufacture. Three-layer co-extrusion is most often practiced, in which a co-extrusion line consists of two single-screw extruders (one for the inner starch layer and the other for the outer polymer layers) a feedblock a coat-hanger-type sheet die and a three-roll calendering system [164]. Biodegradable polyesters such as PCL [164, 264], PLA [164, 263], and polyesteramide, PBSA and poly(hydroxybutyrate-co-valerate) [164] are often used for the outer layers. These new blends and composites are extending the utilization of starch-based materials into new value-added products. 

Wachsen Reichert Kruger, R. R, Much, H., and Schultz, G., Thermal Decomposition of Biodegradable Polyesters. 3. Studies on the Mechanism OUgo-L-lactide,... 

Table 8.5 Critical thickness (Lcritiail) of biodegradable polyesters where hydrolytic degradation mechanism changes from bulk erosion to surface erosion [200], (Reprinted from Biomaterials, vol. 23, von Burkersroda et al., Why degradation polymers undergo surface erosion or bulk erosion, pp. 4221-4231. Copyright Elsevier, 2002.)...

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