GB794012A - Improvements in or relating to the manufacture of continuous, non-fibrous, regenerated cellulose sheet material which is subject to shrinkage during drying - Google Patents

Abstract

A continuous web of non-fibrous gel regenerated cellulose sheet material produced by the viscose process, and not previously dried and re-wetted, is passed through a drying zone in which it is subjected to a drying temperature maintained substantially uniform across the width of the web, the web being maintained under longitudinal tension just sufficient to prevent its wrinkling or distortion until the moisture content of the edge lanes of the web has been reduced to about 0.1 to 0.5 times the weight of the dry cellulosic material, and the moisture content of the centre section of the web has been simultaneously reduced to approximately 0.3 to 1.5 times the weight of the dry cellulosic material, the moisture content of the centre section being higher than that of the edge lanes. Thereafter the longitudinal tension on the web is increased to an amount sufficient to elongate the centre section to about the same length as the edge lanes, and the drying of the web is completed. The tension just sufficient to prevent wrinkling of the web is not sufficient to prevent shrinkage of the web as it dries. The edge lanes of the web are areas 3-6 inches wide extending lengthwise of the web and each bounded on one side by an edge of the web. The centre section constitutes the remaining area of the web between the edge lanes. In the illustrated example, wet regenerated cellulose film web, containing 3.0-3.5 times as much water as cellulose, enters a dryer, which contains a series of driven heated rolls, such as are described in Specification 488,137, [Group XIII]. In the first section of the dryer the web passes over a series of rolls operated at a speed which maintains the web under just sufficient longitudinal tension to prevent wrinkling. When the moisture contents of the edge lanes and the centre section of the web have been simultaneously reduced to 0.1-0.5 and 0.3-1.5 times the weight of the dry cellulose in the web, respectively, the web is passed over a second series of dryer rolls, the peripheral speed of which is about 0.4 to about 3.0 per cent above that of the preceding dryer rolls. The increase in web tension resulting from this increase in peripheral speed causes the moister and still plastic centre section of the web to elongate to approximately the same length as the already dried edge lanes. The drying is then completed with the centre section of the web having the same length as the edge lanes, and the film leaves the dryer and is collected as a roll having a moisture content about 0.06 times the weight of the dry cellulose in the roll. Fibres dried according to the invention have improved flatness, as shown by a reduction in the centre and edge droops of unsupported lengths of film. Specification 794,013 also is referred to.