Patent GB-489928-A (GB)
Title: Manufacture of improved electrical insulating material from cellulose
IPC: H01B3/18
Filed: 1936-11-05
Abstract: Cellulose, free from salts of inorganic acids containing sulphur, selenium, or phosphorus, is subjected to an esterification process in presence of a catalyst that does not itself react with cellulose to form cellulose esters, and the esterified material is subjected to a washing operation in order to eliminate the electrolytes contained therein.  The cellulosic material may be dyed or undyed.  It may be freed from sulphates and like compounds by repeatedly washing with distilled water or other sulphate free water, or with soap solutions, or organic solvents, or by subjection to a bowking operation followed by washing with sulphate-free water.  If dyed cellulose fibres are used, it is preferable to select material which has been dyed in the absence of sodium sulphate and with pure dyes.  Paper, cotton, hemp, ramie, or regenerated cellulose may be used.  For the esterification there may be used methane sulphonic acid, benzene sulphonic acid, naphthalene sulphonic acids, ferric chloride, zinc chloride, tin chloride, perchloric acid, or an alkali metal acetate as catalyst. Diluents or non-solvents such as benzene, toluene, or petrol may be present.  Mono-di- or tri-esters may be produced.  The cellulose ester may be formed in solution or in suspension with retention of the fibrous structure of the cellulose.  After precipitation, if necessary, the cellulose ester is separated from the esterification mixture and subjected to a washing operation to remove electrolytes. The removal of the electrolytes may be determined by measuring the conductivity of the washing water and the freedom from sulphates may be tested by adding barium chloride to the used wash water.  The end point of the washing operation is attained when the wash waters are completely free from electrolytes or water-soluble substances which dissociate in presence of water.  The cellulose esters may be formed into artificial filaments or other materials.  They may be mixed with plasticizers, and softeners, and if they are to be used for electrical insulation any additions should also be free from electrolytes.  Paraffin oil, higher aliphatic alcohols, soaps of alkaline earth or heavy metals, may be used as softening agents.  The cellulose ester may be a formate, acetate, propionate, butyrate, or laurate.  Cellulose di-and tri-acetates produced with retention of the fibrous structure may be produced with a very high resistance to direct current and a low power factor for the very high frequencies of alternating current.  In an example, bowked and bleached cotton yarn is carefully washed with pure water to eliminate electrolytes, the process being controlled by measuring the conductivity or the sulphate content of the washing water.  The yarn is then treated with acetic anhydride, acetic acid, and methane sulphonic acid.  The yarn is hydro-extracted, immersed in ice-water, washed several times in hot and cold water, and then dried.  In other examples, ferric chloride, copper perchlorate, zinc perchlorate, or zinc chloride, is used as catalyst.  Cellulose triacetate may be treated with formic acid to render it soluble in acetone.  Specifications 363,700, 372,312, 395,737, 396,309, and 399,952 are referred to.