GB552498A - Improvements in or relating to the manufacture of cellulosic sheets and films - Google Patents

Abstract

552,498. Films ; drying apparatus. BRITISH CELLOPHANE, Ltd. Jan. 6, 1941, No. 192. Convention date, Jan. 12, 1940. [Class 2 (ii)] [Also in Group XIII] Cellulosic film or sheet in gel condition is submitted to a drying treatment, in the first stage of which it is passed over drying rolls but allowed to shrink lengthwise and in the final stage of which it is prevented from shrinking lengthwise. The process is carried out in such manner that the film still contains at least 100 per cent. of moisture after the preliminary drying step. The process is applicable to the drying of the film immediately after the coagulation, regeneration, and ancillary treatments such as desulphuring, bleaching, and softening. The shrinkage allowed in the first stage of drying may be as much as 60 per cent. of the potential shrinkage. The moisture content of the film may be reduced to 100-200 per cent. in the first stage and down to about 6 per cent. in the final stage. The film may be under substantially no tension in the first drying stage so that it can shrink freely, or it may be under a slight tension which permits a restricted amount of shrinkage. The process is applicable to films cast from viscose or from cuprammonium cellulose solution, or from aqueous alkaline solutions of lowly etherified or lowly esterified cellulose. As shown in Fig. 3, the film travels through a drying chamber 325 while passing round two sets of heated rollers, the first set 327, 328 being frictionally driven, and the second set 340 being positively driven by a chain drive. The rollers 327, 328 are provided with friction pads 392 (Fig. 3b) pressed by adjustable springs 398 into contact with the under sides of belt-driven pulleys. The amount of slip allowed is such that the film is allowed to shrink to a controlled extent. The rolls are heated internally by steam. Drying air is circulated by a fan 330. As shown in Fig. 4, the rollers 428, 427 are of diminishing diameter but are driven at the same angular velocity so as to enable shrinkage of the film to take place. The rollers 440 are of uniform diameter and prevent shrinkage during the final stage of the drying. Specification 471,641, [Group XIII], is referred to. The Specification as open to inspection under Sect. 91 comprises also a process in which the film during the preliminary drying stage is substantially unsupported. As shown in Fig. 1 (Cancelled), the wet gel film issues from the final wet treatment bath 15, passes between squeeze rolls 23, 24, and through the drying chamber 25 to a roller 26 rotating more slowly than rolls 23, 24 so as to allow the film to shrink freely. The chamber 25 is supplied with hot air jets or gas burners 28 and guide rods 27 for the film. Hot air is removed from the chamber through small openings 22 and conduits 31, 51. The partially dried film then passes over a system of heated drying rolls 40 which draw it along under tension and prevent it from shrinking. The dried film is wound up on roll 60. The process is also applicable to films which have been dried and re-wetted. In the preliminary drying stage, the moisture content may be reduced to 10 per cent. This subject-matter does not appear in the Specification as accepted.