Patent GB-794358-A (GB)
Title: Improvements in or relating to regenerated cellulose film and processes for producing the same
IPC: C08L1/08
Filed: 1955-07-29
Abstract: Regenerated cellulose films are made by casting a film of viscose, prepared with 35-50 per cent of carbon disulphide, based on the weight of the dry cellulose, and having a salt index of 10-15, preferably 11.5-13, into an aqueous coagulating, non-regenerating ammonium or alkali-metal salt bath buffered to a pH of at least 3, preferably at least 5, and substantially free from salts of metals having a specific gravity above 4, to form a gel film, and thereafter subjecting the film to the action of an aqueous acid-salt bath effective to regenerate the cellulose. When xanthation is carried out in a closed system the proportion of carbon bisulphide used is preferably not over 45 per cent, especially 38-42 per cent, based on cellulose.  When xanthation is carried out in vented systems, the proportion of carbon bisulphide used is preferably 42-48 per cent, based on cellulose. The viscose should preferably contain 2-2.6 per cent of xanthate sulphur at 2.5 hours after the completion of the mixing cycle. The viscose may contain about 7-10 per cent of cellulose and 4-10 per cent or more of sodium hydroxide, preferred concentrations being 8-9 and 5-6 per cent respectively.  The pH of the coagulating bath may be 10 or higher, but is preferably 7-9. Suitable buffer materials include the watersoluble ammonium, sodium and potassium salts of inorganic or organic acids such as sodium hydrogen sulphate, sodium dihydrogen phosphate, alone or mixed with sodium hydroxide, sodium hydrogen phthalate, sodium borate, diammonium phosphate, trisodium phosphate, potassium sulphate, sodium bisulphite, sodium and ammonium bicarbonate and especially ammonium sulphate.  The concentration of the buffering agent is preferably 25 per cent to saturation. The temperature of the coagulating bath is preferably 30-75 DEG  C. The regenerating bath preferably contains 4-12 per cent of sulphuric acid and 13-20 per cent of sodium sulphate, but other acids such as phosphoric acid and other salts such as ammonium or magnesium sulphate may be used.  The temperature of the regenerating bath is preferably 40-75 DEG  C.  The coagulating bath preferably contains 0.03-0.2 per cent of an anionic wetting agent.  Suitable agents include the sodium salts of long-chain hydrocarbon sulphonates, ammonium oleate, the sodium salt of sulphated oleic acid, the oleic acid esters of sulphonated aliphatic alcohols, sodium carboxymethylcellulose, aryl-alkyl sodium sulphonates, the ammonium salts of naphthalene sulphonic acidformaldehyde condensation products, the longchain aliphatic alcohol sulphates, sodium lignin sulphonate and sulphonated castor oil.  In an example, a viscose containing 8.5 per cent of cellulose and 5.3 per cent total sodium hydroxide, the cellulose xanthate in which had been prepared by xanthating alkali cellulose with carbon disulphide in a closed system, using 40 per cent of carbon bisulphide, based on the weight of dry cellulose, is cast at a salt index of 13.5 in a 40 per cent ammonium sulphate aqueous bath (pH 8) at 50 DEG  C.; the coagulated film is then regenerated in a 10 per cent sulphuric acid-25 per cent sodium sulphate aqueous bath at 50 DEG  C.  The films produced have high durability and tear strength without lack of clarity.