GB736303A - Improvements in or relating to the treatment of cellulose ethers and the resulting products - Google Patents

Abstract

Colour- and viscosity-stable cellulose ethers are produced by contacting an organic solvent-soluble cellulose ether with an acidified aqueous swelling medium, separating the cellulose ether from the swelling medium, adding an unacidified aqueous swelling medium and a minor proportion of a phenol, and separating the cellulose ether from the unacidified swelling medium, the phenol being absorbed in the cellulose ether. The cellulose ethers used are soluble in solvents such as acetone, benzene, toluene-alcohol, methanol, ethanol, ethyl acetate or butyl acetate. Ethyl cellulose containing about 37-52 and preferably 43-48 per cent of ethoxyl is particularly suitable. The etherifying substituent(s) of the cellulose may also be propyl, ethyl propyl, ethyl butyl, methyl ethyl, benzyl or hydroxyethyl. Ester-ethers in which the ether grouping is the dominant substituent may also be used. The aqueous swelling medium may be water in which the cellulose ether may be digested at elevated temperature and pressure or in which it may be subjected to prolonged steeping at 0-3 DEG C., but is preferably an aqueous solution of an organic solvent for the ether, the concentration of the solution being such that the ether is substantially insoluble in the solution. The organic solvent may be acetone, dioxane, tetrahydrofuran, or methyl or ethyl acetate, but methanol, ethanol, propanol and particularly isopropanol are preferred. The aqueous solution will generally contain 10-50 per cent by weight of the organic solvent, e.g. 30-35 per cent of isopropanol or 40-50 per cent of ethanol. The swelling medium is acidified until its pH is substantially below 6 and preferably 1.5-3.5, the pH adjustment being preferably made with an inorganic acid such as hydrochloric, sulphuric or nitric acid. The contacting of the cellulose ether with the swelling medium is usually carried out at room temperature, but may be effected at 75 DEG C. or over. The contact time may be up to 20 hours but is preferably 0.25 to 5.0 hours. Either simultaneously with or subsequent to the above operation the cellulose ether may be treated with a water-soluble inorganic chlorite bleaching agent, e.g. a chlorite of an alkali metal or alkaline-earth metal or of ammonium, sodium chlorite being preferred. The proportion of chlorite may be 0.1-2.0, particularly 0.1-0.2, per cent by weight based on the cellulose ether. After separation from the swelling medium, e.g. by filtration or centrifuging, the cellulose ether is washed with water or preferably with successive portions of fresh swelling medium until the pH of the washings is 4.2-4.8. The phenolic stabilizer is added to the cellulose ether in the last wash and may be o-, p- or di - sec. - amylphenol, di - tert. - amylphenol, p-cyclohexylphenol, diisobutylphenol, p-tert.-amylphenol, methylphenol, tert. - butyl-or methylene bis-p-cresol, methylene bis-(tert.-butyl-p-cresol) or di-(hydroxymethylphenyl) methyl-p-cresol. The proportion of the phenol may be 0.5-1 per cent by weight of the cellulose ether. The cellulose ether is recovered by draining off the last wash liquid and drying the recovered ether. Before using the ether in a moulding operation, additional phenolic stabilizer is preferably added so that the resulting product contains 1-3 per cent by weight of the stabilizer, based on the ether. In an example, a mixture of 5000 ml. of 35 per cent by weight aqueous isopropanol adjusted to pH 2.2 with HCl and 681 grams of ethyl cellulose containing 46.5 per cent ethoxyl is maintained at 75 DEG C. for 1.5 hours and the liquor is then drained off. The ethyl cellulose is washed with five p successive 4000 ml. portions of fresh 35 per cent neutral isopropanol, the last wash containing 5 grams of di-tert.-amylphenol. The liquor is drained off and the cellulose ether is vacuum-dried. In other examples ethyl cellulose is similarly treated except that the aqueous ethanol or aqueous isopropanol swelling medium contains sodium chlorite. Specifications 580,359, 634,808, 643,348 and 736,304 are referred to.