GB1000045A - Method of testing terephthalic acid to be converted into polyethylene terephthalate - Google Patents

Abstract

A batch of terephthalic acid is tested to determine its suitability for conversion into polyethylene terephthalate by measuring the light transmittance of a solution of a small portion of the batch and rejecting the batch if its light transmittance does not correspond to a transmittance of 90.98%. The light transmittance method comprises dissolving 7.5 grams of the acid in 50 ml. of aqueous 2N potassium hydroxide solution, measuring the light transmittance of the solution in a spectrophotometer using a wavelength of 340 mm and a cell of length 10 mm. ml. comparing the transmittance with that of an aqueous solution of 2N potassium hydroxide as standard. Terephthalic acid of the desired purity may be made by treating an aqueous solution of the potassium or ammonium salt of the acid with potassium permanganate and active carbon and precipitating the acid from the solution (see Division C2). The purified terephthalic acid may then be converted into polyethylene terephthalate by heating the acid with ethylene glycol under atmospheric or superatmospheric pressure and polymerizing the resulting product. Specification 747,204 is referred to.ALSO:A batch of terephthalic acid is tested to determine its purity by dissolving 7.5 grms. of the batch in 50 ml. of 2N aqueous potassium hydroxide solution, and comparing its light transmittance with that of a 2N potassium hydroxide aqueous solution at standard; the measurement being made with a spectophotometer using a wave length of 340 m m and a cell of length 10 mm. If the transmittance is 90-98%, the batch is converted into polyethylene terephthalate (see Division C3). In example, purification to the desired light transmittance is effected as follows (1) potassium terephthalate prepared from potassium phthalate by the process of Example 1 of Specification 747,204 is dissolved in water and the solution, after filtration, is treated with potassium permanaganate and active carbon whereupon the resulting solution, again after filtration, is acidified to liberate the terephthalic acid which is agitated with water and then dried; (2) crude terephthalic acid prepared from p-xylene by the CRC process is dissolved in aqueous ammonium hydroxide and the solution treated with potassium permanganate and active carbon; the solution is then centrifuged to give wet terephthalic acid which is heated in an autoclave with aqueous sodium hydroxide solution, the solution allowed to cool, and the purified terephthalic acid precipitated by the addition of hydrochloric acid.