GB693699A - Improvements in and relating to the production of cellulose esters - Google Patents

Abstract

A precipitated secondary cellulose ester, e.g. cellulose acetate, is stabilized by treating it with a dilute aqueous solution of a mineral acid at a temperature not less than 70 DEG C. and, either simultaneously with or subsequently to the stabilizing treatment, it is bleached by treating it with an aqueous sodium chlorite solution in presence of acid. Sulphuric acid is the preferred mineral acid, and a solution containing 0.02-0.10 per cent by weight of sulphuric acid may be used. The proportion of sodium chlorite used may be 0.1-2.5 per cent by weight based on the weight of the cellulose ester. The stabilizing treatment may be carried out at 125-150 DEG C. under superatmospheric pressure. When the bleaching is carried out separately from the stabilization, the temperature of bleaching may be 70-100 DEG C. at atmospheric pressure. The treated cellulose esters may be moulded into crystal-clear articles of good heat stability. In an example, freshly-precipitated cellulose acetate flakes are washed until the free acetic acid content is reduced to 0.02 per cent. The flakes, suspended in 10 times their weight of wash liquor, are pumped into a pressure vessel and the temperature is raised by live steam to 90 DEG C. Small proportions of sulphuric acid and sodium chlorite are added and then the slurry is heated to 140 DEG C. and maintained at this temperature for 30 minutes. The pressure is then released and the product is washed and dried. U.S.A. Specifications 1,948,517, 2,135,980 and French Specification 915,740 are referred to.