Patent GB-603974-A (GB)
Title: Improvements in or relating to the manufacture of shaped aticles of regenerated cellulose produced from viscose
IPC: D01F2/22
Filed: 1945-11-19
Abstract: Films, foils or tubes of regenerated cellulose are produced by shaping viscose into the desired form, e.g. by extruding on to a support, removing water from the film or other article by evaporation at such a rate that the water content is reduced to less than 45 per cent before irreversible gelation occurs, and heating the so-formed viscose article in an inert atmosphere until regeneration is substantially complete.  The finished sheet or film is tough and tear-resistant, has substantially uniform tensile properties in all directions and is characterized by three-dimensional random X-ray patterns.  Any natural cellulose may be used as long as it has good resistance to degradation by oxidation and a sufficiently high degree of polymerization to retain at least 500 and preferably 550-600 glucose units in the viscose.  The viscose should be unaged and have a salt index of over 1.0 and preferably over 2.0.  The higher the salt index the hazier and stronger the finished films.  A small proportion of wetting agents may be present.  The support on to which the viscose is extruded may be a flat polished surface or it may be a highly polished drum.  It may be heated, but not above 120 DEG  C., and preferably not above 110 DEG  C.  To obtain rapid reduction of water content, an inert gas is forced over the sheet at a rate of circulation greater than 400 feet per minute relative to the movement of the film.  The best results are obtained when the rate of circulation of gas is in excess of 1000 feet/min.  Air may be employed for this purpose.  At room temperature, 50 per cent relative humidity and air circulation 1000 feet/min. relative to the movement of the film, a drying time of 30 mins. is required. In a hot drum casting process with the drum at 90-110 DEG  C. temperature of air 100-130 DEG  C., and the same rate of circulation 1-3 mins. will suffice.  The regeneration may be carried out in combination with the water-removal, but preferably the sheet is removed from the support prior to the regenerative heating.  A temperature of from 100-150 DEG C., but preferably within the range 130-150 DEG  C., is preferred. Higher temperatures cause degradation of the cellulose.  Regeneration may be brought about by passage over heated rolls.  The dry, regenerated cellulose sheets may be purified by known methods and dried under slight two-dimensional tension.  In an example, viscose, containing 7 per cent of cotton linters cellulose of degree of polymerisation 550 glucose units and 6 per cent of sodium hydroxide and salt index 1.0 was cast as a film on to a plate glass support, and dried in an air-stream at 105 DEG  C. circulating at just above 400 feet/min. for 12 minutes. The film was then heated in an inert atmosphere to regenerate the cellulose, washed in water to remove soluble salts, then treated in an acid bath to decompose sulphur compounds, washed again, desulphurized in 0.3 per cent sodium hydroxide, washed and dried at 70 DEG  C. under slight tension.  The tearing strength was 17 grams, whereas a film of the same thickness (0.00088 inch) produced by the wet-regeneration process, has a tearing strength of 2 grams.  In another example a viscose containing 8.5 per cent cellulose of polymerisation degree 500-550 glucose units, 6.5 per cent sodium hydroxide and salt index 5.0 was extruded at room temperature through a slot in a hopper on to the polished surface of a stainless steel drum at 90 DEG  C. rotating at a surface speed of 2.6 feet/min.  The drum was enclosed in a jacket to permit the circulation of heated air at 110 DEG  C. at a speed of about 1500 feet/min.  When the water content of the film had been reduced to about 30 per cent, the film was stripped from the drum and passed over heated rolls at 150 DEG  C.  The film was washed, purified and dried.  Details of the tearing test are given.