Patent GB-650896-A (GB)
Title: Improvements in or relating to the production of viscose rayon
IPC: D01F2/06
Filed: 1948-05-11
Abstract: Viscose rayon filaments, having in the air-dry condition a tenacity greater than 4.8 gm. per denier and an elongation at the breaking point greater than 8 per cent, are obtained by extruding into a bath of sulphuric acid at 10-30 DEG  C. a viscose prepared from cellulose containing less than 25 per cent by weight of molecular chains with a degree of polymerization below 500 and less than 4 per cent by weight of molecular chains with a degree of polymerization below 250, said viscose being prepared via alkali-cellulose of which the mean degree of polymerization after ripening is greater than 450, the viscose having a degree of xanthogenation (g ) between 40 and 50, and the sulphuric acid having a concentration between 57.5 and 73 per cent and being defined by the formula: 1.26g +8\sB3, and subjecting the extruded filaments to stretching to increase their length by 150-280 per cent while they are still wet from the spinning bath, washing the filaments and allowing them to relax, and then winding or winding and twisting.  The highest quality products are obtained when the viscose has a g -value of about 46 and the acid concentration in the bath is about 66 per cent.  The degree of xanthogenation is defined as the number of hydroxyl groups substituted by -CSS- groups per 100 glucose units of the cellulose.  Cellulose of the required polymerization characteristics is provided by carefully purified cotton linters. Wood pulps as ordinarily prepared are not suitable but in cases where they conform to the required standard they may be used.  The degree of polymerization is calculated from observations of the viscosity of a solution of cellulose nitrate prepared from the cellulose under conditions in which degradation is avoided as far as possible.  The cellulose nitrate may be obtained by treating cellulose for 3 hours at 14 DEG  C. with a nitrating mixture which has been obtained by mixing acetic acid and acetic anhydride in a refrigerating bath and adding fuming nitric acid.  The cellulose nitrate is separated from the nitrating mixture, washed with water, stabilized by extraction with ethyl alcohol, and then dried in vacuo at 50 DEG  C. Solutions of the cellulose or cellulose nitrate may be fractionated and the degree of polymerization of the fractions may be determined.  In the preparation of the viscose conditions are preferably such that the viscose contains not more than 7 per cent of viscose and not less than 6.5 per cent of caustic soda. The viscosity of the viscose should be greater than 150 c.g.s. units, preferably greater than 250. In an example, bleached cotton linters having an a -cellulose content of 98.2 per cent, mean degree of polymerization 850, with 75.2 per cent of chains of polymerization above 500, 22 per cent of polymerization between 250 and 500, and 2.8 per cent of polymerization below 250, is formed into alkali-cellulose which is shredded at 18 DEG  C. with an addition of pine oil.  The alkali-cellulose is ripened for 6 hours after which it has a degree of polymerization of 500.  The alkali-cellulose is xanthated under vacuum at 25 DEG  C.  The xanthate obtained is dissolved to form a viscose containing 6.3 per cent of cellulose and 7.8 per cent of caustic soda.  Towards the end of the dissolution period tricresyl phosphate or a mixture of branched chain aliphatic alcohols is added to prevent scum forming.  The viscose is degassed under vacuum, and then matured for 40 hours.  The degree of xanthogenation is 43.5. The viscose is spun into sulphuric acid of 62.8 per cent strength. The filaments are stretched by means of rollers and are then passed through a dilute sulphuric acid bath and allowed to relax.  They are finally wound in a spinning pot.  The yarns obtained are suitable for making cords to be used in the production of pneumatic tyres, as well as for ordinary textile purposes.  The yarns dye readily and evenly. They have good flexibility, knot strength, and work-till-rupture values.  Specifications 274,521, 274,690, 281,351, 317,608, 321,679, [all in Class 2 (ii)], and 652,645 are referred to.