GB640075A - Improvements in or relating to the manufacture of cellulose esters - Google Patents

Abstract

Cellulose is esterified under conditions such that an organic ester is obtained in solution and the cellulose ester is subjected to ripening in the solution; the solution is filtered and the ripened ester is precipitated from the filtered solution; the precipitated ester is stabilized by boiling it under pressure with water of alkalinity below 5 parts per million, reckoned as calcium carbonate, and the stabilized cellulose ester is subjected to extraction with an aqueous lower aliphatic alcohol containing 1-3 carbon atoms. The ester obtained has good moulding properties in that it is not only substantially free from brittleness, haze, and colour, but may be subjected to heat and pressure without developing a high degree of colour or suffering any substantial loss of viscosity. The cellulosic material may be cotton, cotton linters, purified wood-pulp, or regenerated cellulose. The ester obtained may be an acetate, propionate, butyrate, or other carboxylic ester or mixed ester. In the production of cellulose acetate, the cellulose may be pretreated with acetic acid containing a little sulphuric acid, and then acetylated with acetic anhydride in presence of sulphuric acid or other catalyst and sufficient acetic acid to dissolve the cellulose ester formed. Water may be added to the solution to destroy any unreacted acetic anhydride and to enable ripening to take place. Part or all of the sulphuric acid catalyst may be neutralized. Ripening may be effected at raised temperature. Before filtering, the solution may be diluted with 40-110 per cent of its weight of a 20-50 per cent acetic acid solution. The dilution may bring into solution inorganic materials, e.g. magnesium sulphate crystals, which have been formed during neutralization of the sulphuric acid. Filtration of the ripened ester may be effected at 30-60 DEG C. The cellulose acetate may be precipitated by adding a large amount of water or other non-solvent to the solution. After being washed, the cellulose acetate is stabilized by heating under pressure for 1/2 to 6 hours with distilled water or water which has been demineralized and which contains less than 5 parts per million alkalinity, as defined above. The stabilized cellulose acetate is then extracted, e.g. with aqueous methyl, ethyl or isopropyl alcohol. In an example, 90 per cent aqueous ethyl alcohol is used. The ethyl alcohol concentration is preferably not less than 40 per cent, and the extraction is best carried out between 30 DEG C. and the boiling point of the alcohol. The extraction with alcohol may be repeated several times or the treatment may be of a continuous nature, alcohol being added continuously or intermittently while corresponding amounts are removed.