Ester bonds, thermal degradation

Figure 2.14 Mechanism for the thermal degradation of ester bonds...

The first reaction step of thermal degradation is scission of an ester bond. Esters containing at least one ft-hydrogen atom decompose pyrolytically to give olefins and acids via a cyclic transition state (Figure 2.14). 

The sensitivity to hydrolysis is a key issue in many applications. The ester bond in 4GT-PTMO copolymers is sensitive to hydrolysis however, it is fairly protected since most of the ester is contained in a crystalline structure. The addition of a small amount (1-2%) of a hindered aromatic polycarbodiimide substantially increases the lifetime of this material in the presence of hot water or steam (Brown et al., 1974). Polyurethanes are susceptible to hydrolytic attack, especially those with polyester soft segments. However, polyester soft segment polyurethanes are generally more resistant to oils, organic solvents, and thermal degradation. lonomers will swell when exposed to water in fact, a commercial hydrated perfluorosulfonic ionomer (Nation) is used as a membrane separator in chlor-alkali cells. Styrene-diene copolymers and polyolefin TPEs are insensitive to water. 

This PC is a product of either phosgenation or transesterification of 2,2-di(4 -oxyphenyl)propane (diphenylpropane of BPA) [45]. It has been established that the thermal degradation of PC under vacuum in the range 300-400 °C occurs with the random cleavage of ester bonds and elimination of carbon oxide. 

The thermal degradation of polyesters is generally described either by a ciy-elimination or a transesterification. The former process dominates with ester moieties with acidic C=H bonds due to activation via the carboxyl groups, as shown in Scheme 22.2, with the formation of polymer fragments [144]. Cw-elimination is a non-catalyzed process, almost uninfluenced by the presence of additives or impurities, and its rate is similar in the liquid or gas phase. 

Sato et al. reported that Vectra A950 has a 70/30 molar ratio of pHBA/2,6-HNA [18].TGA data show that the maximum degradation rate(9W/9t)m occurs at 520°C under both helium and quasi-air at 10°C min heating rate. The TGA curves under the two conditions almost superimpose each other below 550°C however, differential thermal analysis (DTA) results show much difference under two atmospheres. Under helium, a weak endothermic peak of ester bond rupture is observed at about 550°C, while no significant peaks exist above 550°C, and, finally, 40% weight remains. In quasi-air, a sharp exotherm happens at about 550°C, and an even larger endotherm occurs at about 600°C. Nearly no weight remains at the end. 

In the previous two sections we reported research on the block structures of PUs and their consequences for the phase transitions (recorded by the DMA and DSC methods) within Tg of the SS and HS, and for the melting temperatures of crystalline structures formed within the phases. The effects of those parameters are also essential for the PUs thermal stability. What is especially important in this respect is the initial degradation temperature which is dependent on the thermal history of the weakest points within the PU macromolecule - urethane bonds and ester bonds. 

The second important side reaction is thermal degradation of the ester bond, which results in formation of a double bond, and again, in a carboxylic end group (Figure 5.4). 

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