Patent GB-609251-A (GB)
Title: Improvements in or relating to the manufacture of shaped articles of regenerated cellulose produced from viscose
IPC: C08L1/24
Filed: 1946-03-08
Abstract: Sheets, films, or tubes of regenerated cellulose are obtained by shaping viscose into the desired articles, removing water from the shaped viscose articles by evaporation at a rate sufficiently high to reduce the water-content to a value less than 45 per cent before irreversible gelation takes place, and thereafter subjecting the resulting viscose articles to the action of a non-swelling liquid regenerating medium until regeneration of cellulose from the viscose is substantially complete.  The cellulose employed is preferably cotton linters or wood pulp having good resistance to degradation and having a sufficiently high degree of polymerization to retain a degree of polymerization of at least 500 glucose units in the viscose. Preferably the alkali cellulose is unaged.  The viscose may have a salt index above 1.0 and preferably above 2.0.  The cellulose content of the viscose may be 7 to 15 per cent and the alkali content may be about 6 per cent.  A wetting agent such as a sulphonated oil may be present in the viscose.  The extruded or cast viscose film may be treated with water or a volatile alcohol to leach out some of the alkali, prior to removal of water by evaporation.  The viscose may be heated before extrusion.  It may be extruded on to a support of glass or steel plate or a drum surface or endless belt, and the support may be heated to a temperature not exceeding 120 DEG  C.  A rewetted regenerated cellulose film may be used as film-casting support, and a composite film may be obtained, or the top layer may be stripped, if desired after purification and drying.  Irreversible gelation of the cellulose occurs at the point when the combined xanthate in the alkali metal cellulose xanthate is present in the proportion of about 0.15 mols per glucose unit of the cellulose.  The rapid reduction in water content of the viscose film may be effected by passing dry air or nitrogen, or other inert gas over the surface of the film, preferably at a temperature of 75 DEG -150 DEG  C., and at a velocity in excess of 400 feet per minute relative to the movement of the film.  Before regeneration of the cellulose, the viscose film of reduced water content may be washed with methanol to remove some of the sodium hydroxide.  The non-swelling regenerating medium may be a mixture of an alcohol such as methyl, ethyl, isopropyl, or butyl alcohol, or ethylene glycol, and a strong mineral or organic acid such as hydrochloric, sulphuric, phosphoric, or trichloracetic acid. Water may be present, provided that the acid is strong enough to effect rapid regeneration and that substantially no swelling of the film takes place.  The regenerating medium may be heated.  A hot organic liquid may be used for effecting regeneration, e.g. hot glycerol, glycol, kerosene, tetrachlorethylene, or an aromatic hydrocarbon. The treatment with an alcoholic acid regenerating medium may be followed by a treatment with an aqueous acid regenerating medium. The films or sheets are washed and purified.  They may be treated with ammonium sulphate solution to remove salts liable to produce bubbles of gas, and may then be treated with dilute acid. The films may be dried under slight two-dimensional tension.  They may be used for wrapping articles of all kinds including machine parts with sharp corners, foods such as meat and vegetables.  They may be waterproofed and used as table-cloths, aprons, and floor-coverings.