DMT process

Another approach is to use an easily oxidized substance such as acetaldehyde or methylethyl ketone, which, under the reaction conditions, forms a hydroperoxide. These will accelerate the oxidation of the second methyl group. The DMT process encompasses four major processing steps oxidation, esterification, distillation, and crystallization. Figure 10-16 shows a typical p-xylene oxidation process to produce terephthalic acid or dimethyl terephthalate. The main use of TPA and DMT is to produce polyesters for synthetic fiber and film. 

Scratch resistance depends on the hardness of the added particles. The problem of a lack of this property can be addressed by adding chemically identical particles of different crystal modification and Mohs hardness. The preferred additives are silica, alumina, layered silicates such as kaolin, titania, barium sulfate and calcium carbonate. The latter is only suitable for the DMT process owing to side reaction caused by acidity during the terephthalic acid (TPA) route. 

In the DMT process, the esterification is done by feeding a slurry of TPA crystals in methanol to a reactor with a catalyst of sulfuric acid at 220°F and 50 psi. DMT forms and can be purified by distillation. Yields exceed 95%, based on the TPA that ends up as DMT. In some later designs resulting in less severe operating conditions, MEK or acetaldehyde have been used as promoters in place of sodium bromide. 

The TA process is a modification of the DMT process. Terephthalic acid and an excess of ethylene glycol (in the form of a paste) are used to produce the bis(2-hydroxyethyl)terephtha-late, which is then polymerized as described above. The TA process has grown to exceed the DMT process. 

Heinicke, 1C, Novel developments of the Witten DMT process", ACS 164th National Meeting, New York (27 August/ Sept. 1972V... 

PBT is produced from both terephthalic acid and dimethyl terephtha-late with 1,4-butanediol. Most commercial PBT grades are initially developed with DMT because the TPA process generates a larger amount of tetrahydrofuran by-product and DMT is easier to purify than the TPA. Also, the higher solubility of the DMT allows a handling convenience and faster reaction rates in the transesterification stage with lower boiling point of methanol rather than water. The DMT process produces lower amounts of THF, a by-product formed by the irreversible acid-catalyzed dehydration of 1,4-butanediol, compared to the TPA process. However, in the commercial continuous process, more and more processes have shifted to TPA as the feedstock. In the newly developed TPA process. 

As has already been mentioned, the major drawback of the TPA-based route for the synthesis of PBT is that approximately double the amount of THF forms as a waste product, when TPA instead of DMT is used as a monomer. Consequently, there are two possible ways to make the TPA-based synthesis to PBT more lucrative than the DMT process - to reduce the amount of THF formed during the polymerization, or to elaborate an economical process for separation and purification of THF, which can be used further in production of, for example, polyftetramethylene oxide). Currently, DMT route is still used in most commercial plants for PBT production, with the exception of some plants in Asia, e.g. the new Zimmer AG plant in Nantong, China, which produces about 200 tones of PBT daily, starting from TPA as a raw material. Zimmer s process includes additional separation and rectification of the mixture of THF and water, and achieves THF of a purity of > 99.5 % [50]. 

Many DMT processes work better than electroless copper on substrates with difficult resin systems such as FIFE, cyanate ester, or polyimide. 

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