Terephthalic acid, production 432 Subject

Terephthalic acid (TA) (diacid) is now the major monomer used in the production of PET. It is produced on a world-wide scale and is available in a number of purities. These can be generally referred to as PTA (pure TA), MTA (medium purity TA) or CTA (cmde TA). These descriptions are subjective and used in different ways by manufacturers. The purity of TA is the self-limiting process in PET manufacture. The major impurity in any TA grade is 4-carboxybenzaldehyde and it is a chain growth inhibitor. As the level of impurity increases less reaction can take place. From a food contact point of view the purity or impurities in the TA is important. Both US and EU law state that monomers should be of good technical quality but do not define what that technical quality should be. If no other information is available the... 

Polyester fibres, specifically PETP, possess high thermal stability and sufficient resistance to light effect and atmospheric action. However, effect of high temperatures, to which PETP is subjected in the process of synthesis and processing into products, may cause thermal destruction of polymer and in the absence of oxygen. Decrease of molecular mass, increase of end groups quantity, extraction of volatile products, the main of which are terephthalic acid, acetaldehyde and carbon oxide are observed at PETP thermal destruction. Besides these substances, anhydride groups, benzoic acid, p-acetyl-benzoic acid, ketones, acetals are identified in the compound mixture of the products of thermal decay [34]. 

The repeat unit of the PET molecule, the product of a condensation reaetion of terephthalic acid and ethylene glycol, was shown in Fig. 2.3 and is repeated here in Fig. 5.20. For their study, Illers and Breuer chose a commercial product with no detectable initial crystallinity, and removed all traces of residual stress as well as any previously existing water by appropriate prolonged thermal treatments above the glass-transition temperature of 67 °C. Different levels of crystallinity in samples were then obtained by nine separate isothermal crystallization protocols at temperatures ranging from 70 to 245 °C for pre-selected times until equilibrium crystallinities ranging from 0 to 46% were achieved in each case. Only samples subjected to temperatures above 86 °C showed X-ray evidence of crystallinity. 

The decarbonylation of aromatic aldehydes catalyzed by Pd on carbon seems to be fairly general. Anisole was obtained in 94% yield by the treatment of p-methoxyben-zaldehyde at 243 °C for 1 h using 1% Pd on carbon. In order to remove p-formylben-zoic acid formed in a smaU amount in the oxidation of p-xylene to terephthalic acid, the oxidation product containing the aldehyde is subjected to Pd-catalyzed decarbonylation and formylbenzoic acid is converted to benzoic acid, which can be separated from terephthalic acid easily (Scheme 14). 

Polyfethylene terephthalate) 2GT.— The formation of 2GT from the direct interaction of terephthalic acid with ethylene glycol rather than the transesterification process formerly used, has been the subject of several papers. These have dealt with the production of high purity terephthalic acid, the rheological aspects of its processing and analysis of reaction mixtures by gel permeation chromatography. The formation of 2GT and characterization of the derived fibres from the direct interaction of ethylene oxide with terephthalic acid has also been described. ... 

Such polymers contain repeat units based on ethylene glycol (and indeed other glycols such as 1, 4-butane diol) and terephthalic acid and proportions of isophthalic acid. Allen and co-workers [118] developed a precise quantitative method for determining such units in terylene. The sample is subject to an alkaline hydrolysis and glycol and acidic products after conversion to the trimethylsilyl derivatives are analysed by gas chromatography. 

Poly(butylene terephthalate) is also subject to analogous side reactions [128], the formation of 1-butenyl end groups (Scheme 3.4) followed by splitting-off of 1,3-butadiene or tetrahydrofuran (Scheme 3.5). This latter reaction is not affected by the metallic catalyst, but is catalyzed by the carboxyl end groups, making production of PBT by direct esterification of terephthalic acid and 1,4-butanediol disadvantageous. 

Mitsubishi Chemical Industries, Ltd. practiced a Henkel II technology starting with toluene to produce benzoic acid. Reaction of benzoic acid with potassium hydroxide resulted in potassium benzoate, which was subjected to a disproportionation reaction to produce dipotassium terephthalate and benzene. Dipotassium terephthalate reacted with sulfuric acid, and the resulting terephthahc acid was recovered by filtration and drying (65,66). Here, dipotassium sulfate was the by-product. 

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