GB530889A - Improvements in or relating to the manufacture of transparent films - Google Patents

Abstract

530,889. Films. ROSE, R. C., and CEFOIL, Ltd. June 23, 1939, No. 18408. [Class 2 (ii)] Transparent alginic films are obtained by causing a solution of an alginic compound to flow through a slot to form a liquid film, coagulating the film and, at the same time or subsequently, subjecting the undried film to the action of an acid so as to induce substantial syneresis, i.e. expulsion of water from the film, and finally drying the film. The coagulating bath may be strongly acidic so as to effect coagulation and syneresis simultaneously, or syneresis may be effected in a subsequent bath containing sulphuric acid or a solution of calcium chloride acidified with hydrochloric acid or other acid. The process may be conducted so that the production of the film is completed under slightly acid conditions, the film thus containing free alginic acid. The film may be subjected to the syneresis treatment and dried under conditions which allow three-dimensional contraction. The alginate material used may be such that it has a viscosity of not less than 10 centipoises in a 1 per cent. solution in water at 15‹C. In an example, a solution of sodium alginate is extruded through a slot into a solution of calcium chloride and the film is subjected to syneresis and allowed to contract in a bath of calcium chloride containing 0.2 normal HCl. It is further converted to alginic acid by a solution of HCl, washed and dried. When sulphuric acid is used for the syneresis a strength of 0.1-0.5 N may be used. When sulphuric acid is used in the coagulating bath and simultaneous syneresis is desired, a strength of 0.5-2.0 N may be used. Instead of converting the film into alginic acid or calcium alginate it may be converted into another salt or an amine derivative of alginic acid. As shown in Fig. 1, sodium alginate solution passes from tank 11 to storage tank 12, and thence to hopper 13. It then issues from an adjustable slot as a liquid film 16. This is received on an endless band 17 running round rollers 18 in a coagulating bath 19. The film is stripped from the band and passed successively through a synerizing bath 21 containing calcium chloride and hydrochloric acid, a conversion bath 25 containing calcium acetate, or an acid such as sulphuric acid, or calcium chloride solution rendered alkaline with ammonia, a washing bath containing water, and a plasticizing bath containing e.g. glycerol. It then passes into a drying chamber where it passes over drying rollers rotating at progressively lower speeds so as to allow for shrinkage. Contraction in the syneresis bath 21 is permitted by rotating rollers 24 at a lower speed than roller 20. As shown in Fig. 2, the alginate solution is extruded from the hopper 13 and the film is received on and forwarded by a system of rollers 31 immersed in a bath 19 containing calcium chloride and strong mineral acid whereby coagulation and syneresis take place simultaneously. Specification 492,264 is referred to.