Patent GB-1002752-A (GB)
Title: Method and apparatus for producing regenerated cellulose film
IPC: B29C48/10
Filed: 1962-07-16
Abstract: 1,002,752. Producing regenerated cellulose film. E. I. DU PONT DE NEMOURS & CO. July 16, 1962 [July 17, 1961], No. 27291/62. Heading B5B. Regenerated cellulose film is produced by extruding viscose in the form of a tubular film into a coagulating bath, immediately expanding the tubular film in the bath while simultaneously advancing it through the bath, the rate of advancement and the amount of expansion being sufficient to stretch the tubular film to at least 1À5 times, preferably 1À5 to 3 times, its " extruded dimensions " in the longitudinal and transverse directions, expanding the twoway stretched tubular film in the transverse direction an additional amount up to 50% of its circumference, slitting the tubular film longitudinally to provide at least one flat sheet of film, completing regeneration of the film and purifying and drying the film. " Extruded dimensions " are defined as the dimensions the tubular film would have if it were not stretched in either direction. In the simultaneous advancing and two-way expansion steps, it is preferred to stretch it equally in both directions. As shown in the drawing, the viscose is extruded through an annular die 12 into a tank 13 containing coagulating liquid 14 to form a tubular film 11. The die may be stationary, or can be rotated continuously in one direction or oscillated to and fro to reduce variations in thickness across the width of the film. The coagulating liquid may be an aqueous solution containing 40-55% ammonium sulphate and up to 5% sulphuric acid at 80-95 C., or containing 4-15% sulphuric acid and 5-20% sodium sulphate at 25-60 C. The outside surface of the film is coagulated by liquid fed into the tank 13 through inlet 15 and the inside surface by liquid entering the tank through inlet 16. Outlets 17 and 18 maintain the desired level of coagulating liquid around and within the tubular film. The film may be expanded by hydraulic pressure due to the liquid level within the film being maintained above the level outside the film, as shown in the drawing, or by gas pressure within the tubular film, by combinations of gaseous pressure within the tubular film and hydraulic pressure outside the tube, or hydraulic and gaseous pressure within the tubular film combined with annular restraining means around the tubular film. After leaving tank 13 the film is passed over guide roll 19 through the nip of rolls 20 and 21 and then through the nip of positively driven rolls 22 and 23. Rolls 20 and 21 are driven rolls which advance the film at a rate 1À5 to 3 times the extrusion rate. Between the two sets of nip rolls, the tubular film is further expanded to stretch it transversely an additional degree such that its circumference is increased by up to 50% above the circumference which it has after the first two-way stretching step. A tube 24 admitted through a circumferential groove in the surface of roll 23 conveys a gas at superatmospheric pressure to effect the additional expansion of the tubular film. As described in Specifications 983,325 and 1,002,753, it is desirable to provide in the film at least one lane having a wall thickness less than that of the remainder of the film. This may be effected by endless belts 26 and 27 contacting the periphery of the tubular film except for the longitudinal lane or lanes and moved at the same rate as the advancing tubular film by the driven rolls 28 to 31. After the second expansion step, the expanded film is collapsed by nip rolls 22 and 23 and slit by knife 32 to provide two sheets of film. The resulting two flat sheets of gel film 56 and 57 are then led as superimposed sheets by rolls 33 to 43 through a sulphuric acid-metal sulphate regenerating bath 44, a purification bath 45 and a softening bath 46. The film sheets are then led through the drying chamber 47 and wound on rolls 48 and 49.