Thermal degradation of polyesters

Poly(ethylene terephthalate) decomposes upon heating through a series of different reactions. These run either concurrently or consecutively. The result is a complex mixture of volatile and nonvolatile products. It was found that when poly(ethylene terephthalate) is maintained in molten condition under an inert atmosphere at 282-323°C, it slowly converts to a mixture of gaseous low molecular weight fragments [581]. The major products from pyrolysis of poly(ethylene terephthalate) are carbon dioxide, acetaldehyde and terephthalic acid. In addition, there can be detected trace amounts of anhydrides, benzoic acid, p-acetylbenzoic acid, acetophenone, vinyl benzoate, water, methane, ethylene, acetylene, and some ketones [505]. The following mechanism of degradation was postulated [505]  

The vinyl end groups that form from cleavage of the ester groups decompose further in anumber of ways  

The thermal degradation of poly(butylene terephthalate) was examined with the aid of a laser microprobe and mass spectrometry [506]. A complex multistage decomposition mechanism was observed that involves two reaction paths. The initial degradation takes place by an ionic mechanism. This results in an evolution of tetrahydrofuran. This is followed by concerted ester pyrolyses reactions that involve intermediate cyclic transition states and result in formation of 1,3-butadiene. Simultaneous decarboxylations occur in both decomposition paths. The latter stages of decomposition are 

The subsequent decomposition, shown below, can actually take place at lower temperatures  

There are indications that there is moismre among the decomposition products. This may imply that acid hydrolysis plays a part in tetrahydrofuran formation  

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Luderwald, I., Thermal degradation of polyesters in the mass spectrometer. Developments in Polymer degradation, Vol. 2, Grassie, N., Ed. Applied Science Publisher, London, 1979. 

Many other companies have patented additives which they claim to be useful in preventing thermal degradation of polyesters, and are discussed helow. 

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. 

Figure 13.8 A mechanism to describe the thermal degradation of polyesters [22].

Polyesters are important thermoplastic resins, which can be obtained by the condensation reaction of glycols and acids or anhydrides 798163. For example, the condensation polymerisation of ethylene glycol or butylene glycol and terephthalic acid produces the well-known polyesters, poly(ethylene terephthalate) (PET) or poly(butylene terephthalate) (PBT), respectively. The mechanism of thermal degradation of polyesters has been studied for many years 817822 776328 756176 670828 with the help of model compounds. However, there is still some current controversy on the crucial point whether the primary thermal decomposition reactions that occur in polyesters involve radical or ionic processes 777330 490068. ... 

Aliphatic hyperbranched polyesters, 56 Aliphatic isocyanate adducts, 202 Aliphatic isocyanates, 210, 225 Aliphatic polyamides, 138 Aliphatic polyesteramides, 56 Aliphatic polyesters, 18, 20, 29, 32, 87 degradable, 85 hyperbranched, 114-116 melting points of, 33, 36 structure and properties of, 40-44 syntheses of, 95-101 thermal degradation of, 38 unsubstituted and methyl-substituted, 36-38... 

Table V. Analysis of Gases Evolved During Thermal Degradation of Brominated Polyester Resin Samples...

The color of the polymer can also be affected by inappropriate reaction conditions in the polymerization process, such as temperature, residence time, deposits of degraded polymer or the presence of oxygen. Degradation of polyesters and the generation of chromophores are thermally effected [29b, 29c, 39], The mechanism of thermal decomposition is based on the pyrolysis of esters and the formation of unsaturated compounds, which can then polymerize into colored products. It can be assumed that the discoloration takes place via polymerization of the vinyl ester end groups or by further reaction of AA to polyene aldehydes. 

Pyrolysis is the process of thermal degradation of a substance into smaller, less complex molecules. Many processes exist to thermally depolymerize tires to salable products. Almost any organic substance can be decomposed this way, including rice hulls, polyester fabric, nut shells, coal and heavy crude oil. Pyrolysis is also known as destructive distillation, thermal depolymerization, thermal cracking, coking, and carbonization. 

H. Sato, K. Kondo, S. Tsuge, H. Ohtani, and N. Sato, Mechanisms of thermal degradation of a polyester flame retarded with antimony oxide/brominated polycarbonate studied by temperature programmed analytical pyrolysis. Poly. Degr. Stab., 62, 41-48 (1998). 

The highest rate of degradation of polyester (i) is observed between 500 and 550°C. Differential thermal analysis indicates that decomposition occurs around 465°C. CO and C02 are the major products formed in this... 

Sato, H., Kondo, K., Tsuge, S., Ohtani, H., and Sato, N., Thermal Degradation of Flame-Retarded Polyester with Antimony Oxide/Brominated Polycarbonate Studied by Temperature Programmed Analytical Pyrolysis Techniques, Polym. 

As the temperature increases, sequences containing totally aromatic polyester accumulate in the residue, indicating that the overall thermal reaction has resulted in the formation of the most thermally stable compound, i.e., a totally aromatic polyester. As a consequence, the last decomposition stage is solely due to the evolution of the cyclic dimer (ion at m/z 717) formed by tiie thermal degradation of the aromatic polyester sequences (Scheme 5.11c), as confirmed by the DPMS analysis of pure poly(bisphenol A terephthalate). ... 

Othani, H., Kimura, T, and Tsuge, S., Analysis of thermal degradation of terephthalate polyesters by high-resolution pyrolysis-gas chromatography. Anal. Sci, 2, 179, 1986. 

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