Patent GB-741818-A (GB)
Title: An improved method of manufacturing film and sheet material
IPC: B29C48/08
Filed: 1952-08-25
Abstract: Films or sheets are formed by flowing a free unsupported stream of liquid film-or sheet-forming composition on to a casting surface from an applying device and subjecting the two sides of the stream flowing between the applying device and the casting surface to differential gaseous pressure which is lower on the side coming into contact with the casting surface, to prevent or minimize fluttering of said stream while moving the casting surface and applying device relatively to one another.  The film- or sheet-forming composition may consist of partially hydrolysed cellulose acetate dissolved in acetone or cellulose triacetate dissolved in a mixture of methylene chloride and methyl alcohol, e.g. in the proportions of 90:10 per cent by weight. Another composition consists of 4-5 parts by weight of a mixture of 90 per cent by weight of ethylene chloride and 10 per cent by weight of methyl alcohol in which is dissolved one part of a cellulose acetate-propionate, as disclosed in U.S.A. Specification 2,078,261, and 0.15 part <PICT:0741818/IV(a)/1> <PICT:0741818/IV(a)/2> <PICT:0741818/IV(a)/3> <PICT:0741818/IV(a)/4> by weight of triphenyl phosphate and about 0.15 part by weight of dimethoxyethyl phthalate as plasticizers.  Other examples of cellulose ester compositions which may be used are given in U.S.A. Specifications 1,833,136, 1,960,185, 2,006,361, 2,078,261, 2,492,977 and 2,492,978.  The apparatus used may be that described in Specification 741,295.  As shown in Fig. 1, the film-forming composition 7 flows from a hopper 4 through the slit between the lower edges of the blades 5, 6 on to the casting surface 1 of the wheel 2 turning on trunnions 3 (Fig. 4), and during its passage from the hopper to the casting surface is subjected to a vacuum maintained in the reduced pressure housing 11 (Fig. 1).  The latter comprises side plates 12 (Figs. 3 and 4) having substantially air-tight fit with the smooth peripheries 13 of the wheel 2, the side plates being fixed to a cross-bar 14 which is supported by a frame 10.  A second cross-bar 15 completely covered with plush 16 rides against the casting surface 1, thus making a substantially air-tight connection with the wheel and bar 14.  A top wall consists of a plate 17 (Fig. 3) having a conical top 18 from which a pipe 19 leads to the intake 20 of a suction fan 21 (Fig. 1). The side plates 12 extend beyond the hopper 4 and form the side walls of a chamber 22 (Figs. 1 and 4) spaced a short distance, such as 0.25 inch, from the casting surface.  The end 23 is open to the ambient atmosphere.  As the cellulosic film-forming material flows from the hopper blades 5 and 6 it is drawn out in a curve 25 (Fig. 4) which terminates in the parallel lateral edges 26 of the sheet 9, leaving a space 27 between the edges of the ribbon 80 (Fig. 1) and the side plates 12, through which the solvent vapours collecting in chamber 22 are drawn by the reduced pressure in chamber 11.  Thus sufficient solvent vapours pass the ends of th blades 5 and 6 to prevent slugging. The chamber 22 includes a top wall 28 hinged at 29 to the hopper 4.  As shown in Fig. 6, a hopper 56 supplies the film-forming composition in a stream 58 to the casting surface 50 consisting of an endless belt passing over a pair of wheels, of which one is shown at 51, having trunnions 52.  Idler rollers 53 support the upper run of the belt to prevent sagging.  A substantially air-tight reduced pressure casing 54 is provided on the incoming side of the belt and a pipe 55 leads to a source of reduced pressure.  A chamber 57 surrounds the belt and extends up to the wheel 51.