Polyesters, degradation

Weathering tests, 245 Wholly aromatic liquid crystalline polyesters, degradation of, 38 Wholly aromatic polyamides, 136-137, 139 synthesis of, 184-189 Wholly aromatic polyesters, 25-26, 32 copolymerization and, 35 synthesis of, 71-72... 

In order to control polyester degradation, we must understand the processes involved in how polyesters degrade. The following section presents a brief summary of degradation mechanisms that are involved. 

Polyesters offer multiple options to meet the complex world of degradable polymers. All polyesters degrade eventually, with hydrolysis being the dominant mechanism. Degradation rates range from weeks for aliphatic polyesters (e.g. polyhydroxyalkanoates) to decades for aromatic polyesters (e.g. PET). Specific local environmental factors such as humidity, pH and temperature significantly influence the rate of degradation. 

Liu ZQ, Gosser Y, Baker PJ et al (2009) Stmctural and functional studies of Aspergillus oryzae cutinase enhanced thermostability and hydrolytic activity of synthetic ester and polyester degradation. J Am Chem Soc 131 15711-15716... 

Aliphatic polyesters degrade chemically by hydrolytic cleavage of the backbone ester bonds [38,92,93,143-145] and by enzymatic promotion [35,146]. Hydrolysis can be catalyzed by either acids or bases [38]. Polyester hydrolysis is schematically illustrated and exemplified for PLA in Fig. 5. Carboxylic end groups are formed during chain scission, and this may enhance the rate of further hydrolysis. This mechanism is denoted autocatalysis [147] and makes polyester matrices truly bulk eroding [38,43]. Degradation products are resorbed by the body with a minimal reaction of the tissues [8,15,95,148]. 

The present paper reports on a study which was conducted to investigate the effect of NaOH on the in vitro release profiles of microspheres prepared with polylactides (2) Since these polyesters degrade by hydrolysis (4), it is possible that the molecular weight of the polymers can be decreased by the alkaline pH of the sodium oleate emulsifier solution (pH 10) during fabrication. This in turn could affect the release kinetics of the microspheres. 

Polymers do not normally biodegrade until they are broken down into fairly low molecular weight chemical species that can be assimilated by microorganisms. Aliphatic polyesters degrade in the presence of water by sequential hydrolytic cleavage of the backbone ester bonds leading to production of monomeric hydroxyl carboxylic acids which in turn can be metabolized by bacteria (5). 

After exposure to and equilibration in an aqueous environment, polyesters degrade by hydrolysis that occurs homogeneously throughout the bulk of the... 

The NMR analysis of polymer A tensile bars aged In methanol and Isooctane-methanol solutions showed a systematic decrease In the amount of polyester present with aging time. The same behavior was observed for polymer B except that the loss of polyester was less systematic. Subsequent analysis of the residue confirms that the low molecular weight polyester degradation products are dissolved Into the solution In both cases. 

Kaminsky90,92 has reported the product distribution obtained in the fluidized bed pyrolysis of different condensation polymers (polyesters, polyurethanes, polyamides, etc.). Polyester degradation led to 51% of gases, with a high proportion of CO and C02, and 40% of oil rich in benzene, toluene and naphthalene, the formation of water also being detected. On the other hand, polyurethane and polyamide decomposition led to the formation of about 40% gases and 55% oil. In both cases, the gases obtained contained certain amounts of HCN. 

The exoeellular enzymes produced by three different polyester-degrading bacteria have been characterized using native granules of PHB as the substrate. The bacteria studied were the following (1) two Pseudomonas strains which... 

In recent years, biodegradable plastics have attracted as environmentally fHendly materials to solve the problem of the waste plastics. This paper reports the microbial degradation of polyesters in the naarine environments and the properties of extracellular depolymerases from some polyester-degrading microorganisms. 

While some controversies remain, the study of aromatic polyester degradation in the last few years has begun to untangle the mechanisms involved, particularly through the use of analytical techniques previously unavailable. The study of stabilisation has also moved on, with examination of why the established commercial stabilisers (largely developed for use in polyolefins and rubbers) are not as effective in aromatic polyesters, and also attempts to develop new stabilisers specifically for use in such polymers. 

Simple chemical hydrolysis of the hydrolytically unstable backbone is the prevailing mechanism for polyester degradation, which depends on the pH of the solution and may be catalysed by an acid or a base.The acid-catalysed reaction mechanism is given in Fig. 5.2." The reaction is reversible and is displaced towards hydrolysis by an excess of H2O. This mechanism is characterised by acyl-oxygen cleavage, and the removal of the alcohol is the slow stage of the reaction. 

Poly(e-caprolactone) Aliphatic polyester Degraded by hydrolysis or bulk erosion Slow degrading Degradation products incorporated in the tricarboxylic acid cycle Low chemical versatility Some problems related with withstanding mechanical loads... 

Figure 15.2 Water uptake as function of time (in week) monitoring polyester degradation using Pick s law Eq. (15.9).

Figure 15.3 Molecular weight reduction as a function of time monitoring polyester degradation according to a generalised Pick s law Eq. (15.10).

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