GB650715A - Method and apparatus for forming particles of thermoplastic material into a sheet orfilm or impregnating flexible sheeting with thermoplastic material - Google Patents

Abstract

<PICT:0650715/IV (a)/1> Thermoplastic material in powdered form is distributed in a uniform layer on an endless belt 21 on which it passes through an oven 27 containing infra-red radiators 25. The softened sheet or film emerging from the oven is pressed between a roller 36 and the belt 21 or by a pair of rolls disposed above and below the belt 21 and passes along the under-run of the belt where it may be cooled, e.g. by cooling the belt with air blasts, and is finally stripped off the belt by roller 43 and draw-off rolls 41, 42. A scraper 44 removes any adherent material after the formed sheet has been removed. The gap between roller 36 and belt 21 may be adjustable. The roll 37 of material which, as shown, is coated with the thermoplastic, is omitted when plain thermoplastic sheet or film is required. The powder feeding means may comprise a hopper 12 leading to a constant rate of feed device 13 which feeds via a hopper 16 to a tapering board 15 which together with the hopper 16 is vibrated. The powder falls over the edge 17 of the board 15 and through a slot between members 18, 19 on to the belt 21. Alternatively a trough with a preferably adjustable slot in its bottom or side may be disposed across the belt 21. The belt, which may be of polished metal, may be heated from above and/or below before entering the oven. The roller 36 may be heated internally by steam, electricity, &c., or by an extension of the oven 27 or by conduction from the belt 21. The belt and roller 36 may be lubricated. The Specification lists a large number of materials, in particular cellulose derivatives, which can be sheeted in the above manner and also lists plasticizers for the materials and resins which can be added to improve the properties of the materials. In examples: (i) a composition of 67.5 parts cellulose acetate, 26 parts methyl phthalyl ethyl glycollate and 6.5 parts triphenyl phosphate in fine powder form was passed into the oven, the temperature of the plastic sheet leaving the oven being about 420 DEG F. The sheet was pressed into cotton duck, cooled to 120 DEG F. and stripped from the belt; (ii) similar to (i); (iii) cellulose acetate (69 parts) was colloided on a mill at 65 DEG C. with dimethylphthalate (15.5 parts) and diethyl-phthalate (15.5 parts) in presence of 90-10 acetone-alcohol (85 parts). The colloidal material was chipped and dried and passed through the apparatus to form a coating for cotton duck; (iv) ethyl cellulose (85 parts) was colloided with tricresyl phosphate (15 parts) on a two-roll mill at 150 DEG C. and treated as in (iii). The Specification as open to inspection under Sect. 91 states that induction heating or high-frequency heating may be employed instead of infra-red radiation. This subject-matter does not appear in the Specification as accepted.