GB657342A - Improvements in or relating to the manufacture of films of hygroscopic, non-fibrous,organic material - Google Patents

Abstract

Liquid aqueous dispersions of silica are applied to films of hygroscopic, non-fibrous, organic material, the silica particles of the dispersions carrying an electric charge of opposite sign to that of the electric charge developed by the film in contact with the dispersions, and the films are then dried. The film may be of regenerated cellulose, lowly substituted cellulose ethers, esters or etheresters, albuminous materials such as gelatine, alginic acid derivatives or polyvinyl alcohol. The silica may be applied to the film by passing the film, as it comes from the casting machine, through an aqueous impregnating bath containing the dispersion of silica and, if desired, a softening agent, such as glycerine, for regenerated cellulose film and/or a water-soluble cationic surface-active agent. The pH value of the bath may be 3-7.5 and should preferably be kept at 7. As normally prepared from a water-soluble silicate and an acid, the particles of colloidal silica dispersed in an aqueous medium carry a negative charge and are therefore suitable for application to gelatine films, which become positively charged when immersed in the dispersion, or to regenerated cellulose films which have been treated with a cationic surface-active agent and consequently also become positively charged when immersed in an aqueous medium. By treating aqueous dispersions of colloidal silica, as normally prepared, with a suitable water-soluble cationic surface-active agent, the particles of colloidal silica, dispersed in the aqueous medium, can be made to acquire a positive charge and thus become suitable for application to films which acquire a negative charge when immersed in aqueous media, e.g. films of regenerated cellulose or of alginic acid derivatives. The cationic surface-active compound may be a water-soluble organic compound, the molecule of which comprises a quaternary ammonium or phosphonium group or a ternary sulphonium group and, attached directly to the nitrogen, phosphorus, or sulphur atom respectively, an alkyl group comprising at least 8 carbon atoms; it may be added to the finished purified liquid aqueous dispersion of colloidal silica after or at any stage of its preparation. Thus an aqueous solution containing 0.01 per cent of cetyl rimethyl ammonium bromide and 1.0 per cent of hydrochloric acid may be added to an aqueous solution containing 1.0 per cent of sodium silicate to yield an aqueous dispersion of colloidal silica, the colloidal particles of which carry a positive charge. The proportion of silica on the treated and dried film should preferably be below 0.5 per cent and may be as low as a few hundredths of one per cent based upon the oven-dry weight of the film. The aqueous dispersion used for treating the film should preferably contain 0.007-0.2 per cent of silica particularly 0.03-0.08 per cent. The concentration of surface-active agent in the treating bath containing silica may range from 0.025-1 per cent, and preferably from 0.3-0.7 per cent. The silica may be applied to regenerated cellulose film that has been dried and then rewetted. In an example of the treatment of gel regenerated cellulose film with an aqueous dispersion of positively charged colloidal silica particles, the film is passed through a bath containing 6 per cent of glycerine, 0.5 per cent of a 15 per cent liquid aqueous dispersion of colloidal silica, 0.6 per cent of octadecyl triethyl ammonium bromide (80 per cent paste) and 93 per cent of water (pH adjusted to 6.5) at such a speed as to give a contact period of 15 seconds. Excess liquid is removed from the surface of the film by passage between squeeze rolls and the treated film dried by passage over drying rolls heated to 80-90 DEG C. Additional examples are given of the treatment of gel regenerated cellulose film in similar manner using as cationic surface-active agents tetradecyl pyridinium bromide, cetyl trimethyl ammonium bromide, cetyl triethanol ammonium iodide, cetyl ethyl methyl sulphonium iodide, and cetyl triethyl phosphonium iodide. The treatment is said to reduce the friction between adjacent films and their tendency to stick together when stacked.