Improvements in and relating to the production of regenerated cellulose fibres from viscose

Regenerated cellulose fibres are produced by extruding viscose of salt figure below 3.5 into an aqueous coagulating bath containing below 5 per cent by weight of sulphuric acid and more than 20 per cent by weight of salts, including at least 5 per cent of magnesium sulphate, withdrawing the fibres from the bath at a rate equal to or lower than the extrusion rate of the viscose into the bath and subsequently completing the fixation of the fibres in an aqueous acid bath having a higher rate of concentration of acid to salt than the coagulating bath, but preferably less than the bisulphate ratio at which the concentration of ions is equivalent to a solution of sodium bisulphate. The salt figure is defined as the concentration of a solution of common salt necessary just to coagulate a drop of viscose, and is preferably 0.5-3.5. The preferred composition of the coagulating bath is 0.5-5 per cent of sulphuric acid, 10-25 per cent of sodium sulphate, 5-20 per cent of magnesium sulphate and a total salt content of about 30 per cent. The extrusion rate of the viscose is preferably 1-20 times the speed of withdrawal of the fibres from the bath, to allow the fibres to thicken on emerging from the jet. The fibres are preferably passed through the regenerating bath in a relaxed condition or under low tension. In an example, viscose containing 7.5 per cent of cellulose 7 per cent of caustic soda and 2.2 per cent of combined sulphur and aged to a salt figure of 1.2 is extruded into a coagulating bath at 55 DEG C. containing 1.5 per cent by weight of sulphuric acid, 15 per cent of magnesium sulphate and 15 per cent of sodium sulphate. The thread thus obtained is withdrawn from the bath at a rate of 40 metres per minute, the ratio of extrusion rate to take-up speed being 1 : 0.236, and is passed round a godet and through a second bath at 60 DEG C. containing 3 per cent of sulphuric acid and a small quantity of salts carried over from the coagulating bath. The thread then passes over a second godet and is collected in a centrifugal spinning-box. The second godet travels at a lower speed than the first godet so that the thread is under low tension in the second bath. The products give bulky yarns and fabrics and have a more irregular cross-section than have normal viscose fibres.