Patent GB-705196-A (GB)
Title: Improvements relating to the manufacture of thermoplastic sheet materials
IPC: B29C48/08
Filed: 1951-06-18
Abstract: <PICT:0705196/IV (a)/1> Films and like sheet materials are made by extruding a hot composition comprising a thermo-plastic organic derivative of cellulose and a plasticiser therefor, but substantially no volatile liquid, and forcing the hot composition, on its way to the die, through a filter comprising a layer of granules of such a nature as not to dissolve, coalesce or undergo chemical change during the process.  The filtering medium may comprise one or more layers of granules of sand, ceramic materials, glass, or metal, and at least one of the layers should preferably be of 40-100 mesh U.S. Standard Screen Scale.  Preferably the filtering medium comprises a first layer of 20-50 mesh granules, a second of 40-100 mesh and a third of 20-50 mesh.  The total depth of the layers should not exceed about 5 cm., and the dwell time of the thermoplastic material in the layers should not exceed about 200 seconds.  When plasticised cellulose acetate is used its temperature, when passing through the filter, may be 160 DEG -300 DEG C. The area of the filter may be 1 to 10 times the cross-sectional area of the extruder.  The filter medium may be wetted with a plasticiser (preferably the same as that present in the thermoplastic material) before starting the extrusion.  The thermoplastic material may be heated to 65 DEG -95 DEG C. at atmospheric pressure or at a pressure below 10 mm. of mercury in order to reduce its moisture content to below 0.10, preferably below 0.03 per cent., before extrusion.  The cellulose derivative used may be cellulose acetate, propionate, butyrate, acetate-propionate or acetate-butyrate or ethyl cellulose.  As shown in cross-sectional view in Fig. 2, the filter 11 is fastened to the face plate 12 of an extruder (not shown) by threaded bolts 13, and comprises a plate 14 having a passageway 15 extending therethrough and an annular projecting boss 16, by means of which it is spigoted to the plate 12. A plate 22 and tubular member 23 are fastened to the plate 14 by means of threaded bolts 25.  The plate 22 has a passageway 31 of frusto-conical cross-section leading from the passageway 15 to a chamber 32 in the tubular member 23.  The front end of the tubular member 23 has external threads 36 and has mounted thereon, in engagement with these threads, an internally threaded fitting 37 having a cylindrical bore leading from the chamber 32 to a passageway 41 of frustoconical cross-section. The passageway 41 opens into a passageway 42, through which the thermoplastic material flows from the filter 11 into a passageway 43 in a back plate 44 of a conventional extrusion nozzle.  A filtering medium 49 is positioned in the chamber 32 and comprises a first layer 50 of 30-40 mesh sand, a second layer 51 of 60-80 mesh and a third layer 52 of 20-30 mesh. On the inlet side the filtering medium is held in place by an 80 mesh screen 53, which is secured against the layer 50 by means of a split ring 54 in engagement with threads 55 on the interior of the tubular member 23.  On the outlet side, the filtering medium is held in place by a screen assembly 56 comprising a 120 mesh screen next to the filtering medium and a 10 mesh screen The screen assembly is edge-sealed by means of a gasket and is clamped between the front face of the tubular member 23 and a backing plate 61, which is positioned in the bore of the fitting 37 and has a number of holes 62 extending therethrough.  In an example, the filter medium is wetted with diethyl phthalate and the filter mounted with its axis horizontal in an extrusion apparatus.  A cellulose acetate material containing 27 parts by weight of diethyl phthalate for each 100 parts of cellulose acetate is air dried at 120 DEG -130 DEG C. and extruded through the filter at 270 DEG C. to produce a film.  The filtration of the heated thermoplastic material enables films of good clarity to be obtained from material contaminated with gels differing in refractive index from the rest of the material.