Thermal degradation of PET

Acetaldehyde is formed during the degradation of PET. Vinyl ester endgroups formed during thermal degradation of PET liberate vinyl alcohol on transesterification with hydroxyethylterephthalate polymeric endgroups (Fig. 10.6). The vinyl alcohol tautomerizes to form acetaldehyde, which can affect the taste of foods in PET food contact applications.1... 

Table 13.5 Reaction rate constant0 and activation energy data for the thermal degradation of PET [29b, 29c, 39]. From Thermal degradation of PET. A kinetic analysis of gravimetric data , Covney, J. D., Day, M. and Wiles, D. M., J. Appl. Polym. Sci., 28, 2887 (1983), copyright (1983 John Wiley Sons, Inc.). Reprinted by permission of John Wiley Sons, Inc.

Table 18.4 Product yields for thermal degradation of PET and mixtures of PE and PET at 430°C. (Reproduced with permission from Elsevier)...

During polymerization, ester interchange between the vinyl ester and a glycol-terminated molecule eliminates acetaldehyde, the stable keto-form of the vinyl ester function. Activation energies for the thermal degradation of PET range [39] between 32 and 62 kcal mole . ... 

The thermal degradation of PET has been studied by Oudhuis et a/.85 using TGA experiments. The DTG curves of PET in argon show a peak around 420 °C whereas 82% of the initial mass is volatilized up to 500 °C. The products released were a complex mixture composed mainly of acetaldehyde, benzoic acid, ethylbenzoate and vinylbenzoate. 

Direct pyrolysis-MS (DPMS, an on-line technique) was used to investigate the thermal degradation of PET. The El mass spectra do not exhibit significant... 

From these early studies, it was considered that thermal degradation of PET does not involve a radical (homolytic) pathway, at least at the temperatures generally encountered by this polymer. The initial... 

It has been noted that initial work on the thermal degradation of PET suggested that homolytic (i.e., free radical-based) scission did not have a role, but this was not confirmed and, in any case, certain conditions may exist where such reactions might come into play. At very high temperatures (or under certain conditions in processing machinery where formation of mechanoradicals may occur), homolytic scission of the polyester chain may occur [7-10]. The weakest link in the PET chain would appear to be the sequence carbonyl-oxygen-methylene, which may be expected to homolytically cleave in two possible ways ... 

Buxbaum [5] proposed the following scheme for the main reactions involved in the thermal degradation of PET ... 

Further studies have been made on how acetaldehyde is generated during thermal degradation of PET [32-37]. Villain and co-workers [33] studied this problem with a particular regard to understanding... 

In 1991, McNeill and Bounekhel [38] studied the thermal degradation of PET using thermal volatilisation analysis (TVA), and subsequently made rather controversial assertions based on the results. 

Popoola [49] deduced from a study of the evolution of dioxane during thermal degradation of PET occurring in the polymer production process that this was due to diethylene glycol units. 

The authors tentatively suggest that this provides evidence that thermal degradation of PET may itself involve both pathways. 

The thermal degradation of PET (250-320 C) in an inert atmosphere gives carbon dioxide, acetaldehyde and methane, benzene, acetylene, 2-methyldioxane and water as the basic gaseous products, and these are released in considerable amounts. Terephthalic acid and oligomeric products (dimers, trimers, cyclic, tetra- and pentamers) are found among the poorly volatile products of the thermal degradation of PET [45, 53]. The half-life temperature for PET is 450 °C. 

The thermal degradation rate of PET has been determined by the change in the characteristic melt viscosity, by the concentration of end COOH and OH groups as well as by the rate of release of acetaldehyde. The data obtained show that the process proceeds by first-order kinetics and by random chain scission. The activation energy calculated for the thermal degradation of PET by changes in the intrinsic viscosity is 260.4 kj/mol. 

Temperatures higher than 290 °C result in thermal degradation of PET with the generation of micromolecular compounds that act like lubricants in melted polymers, decreasing the shear stress. 

Jabarin [785] showed a similar behavior for the PET melts. He mentioned that the thermal degradation of PET in static conditions is amplified four times in the interval 290-310°C, by comparison with the interval 270-290°C. 

Thermal degradation of PET occurs by a molecular mechanism via a cyclic transition state with random chain scission at the ester linkages [1] ... 

The development of colour in PET is also attributed to polyenes formed from those products of thermal degradation of PET which contain vinyl end groups. These products can also lead to the formation of gels and cause serious problems to processing machines and end product (film, fibre). 

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