GB794013A - Improvements in or relating to the manufacture of regenerated cellulose sheet material - Google Patents

Abstract

A continuous web of non-fibrous gel regenerated cellulose sheet material or film produced by the viscose process, and not previously dried and re-wetted, is passed through a drying zone wherein it is subjected to a drying temperature maintained uniform across the width of the web, the web being maintained under longitudinal tension just sufficient to prevent wrinkling or distortion until the moisture content of the edge lanes of the web has been reduced to 1.5 to 2.5 times the weight of dry cellulosic material, the centre section of the web having a higher moisture content than the edge lanes. The longitudinal tension on the web is then decreased and the web is maintained under decreased tension until the moisture content of its edge lanes has been reduced to 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 0.3 to 1.5 times the weight of the dry cellulosic material. Finally, the longitudinal tension on the web is increased to an amount sufficient to elongate the centre section to the same length as the edge lanes and the drying of the web is completed. The edge lanes are the areas at each edge of the web bounded on the inner side by a line 3-6 inches from the edge of the web. The centre section constitutes the remaining area of the web between the edge lanes. In the illustrated example, wet gel regenerated cellulose film web, after leaving the softener tank, passes between squeeze rolls or scraping devices and enters the first section of the dryer at about the same linear speed as in the softener tank the water content of the web being about 3.0-3.5 times its dry cellulose content. In the first stage of the dryer the web passes over a series of conventional dryer rolls driven at a speed sufficient to maintain the web under just enough longitudinal tension to prevent wrinkling or distortion. The dryer rolls throughout the dryer are uniformly heated and heated air is circulated across the web so as to maintain it at uniform drying temperature. When the moisture content of the edge lanes of the web has been reduced to 1.5-2.5 times its dry cellulose content, the web is passed over a second series of conventional dryer rolls driven at a peripheral speed 0.5-2 per cent below that of the preceding rolls, to decrease the longitudinal tension on the web. In the second stage of the dryer the moisture contents of the edge lanes and of the centre section of the web are reduced to 0.1-0.5 and 0.3-1.5 times, respectively, their dry cellulose content, after which the web is passed over a third series of conventional dryer rolls driven at a peripheral speed 0.5-3.0 per cent above that of the preceding rolls, to increase the longitudinal tension on the web and cause the moist and still plastic centre section of the web to elongate to the same length as the edge lanes, which are already dry. Finally the film leaves the dryer and is collected on a mill roll with a moisture content about 0.06 times its dry cellulose content. The increase and decrease in the peripheral speeds of the dryer rolls may be effected at one point between two successive dryer rolls, or incrementally, with the speed differential distributed over two or more dryer rolls, or between the entrance and exit of a section of freely rotating rolls driven by the web, the moisture contents of the web at the entrance to this section being in the specified ranges. Films produced according to the invention have improved flatness, as shown by a reduction in centre and edge droop in unsupported lengths of the film. Specification 794,012 is referred to.