GB695555A - Improvements in or relating to the manufacture of films of hygroscopic, non-fibrous,organic material - Google Patents

Abstract

Films of hygroscopic, non-fibrous, organic material, of the type which normally develop when immersed in water an electric charge of negative sign, are coated by applying to them a liquid aqueous dispersion of wax, the wax particles of which carry, as a result of previous treatment to reverse the sign of the electric charge normally developed by the films or developed by the wax particles of the liquid aqueous wax dispersion as normally made, an electric charge of opposite sign to that of the electric charge developed by the films in contact with water. The treated films are then dried. The films may consist of regenerated cellulose produced from viscose or from cuprammonium cellulose, of alginic acid derivatives, of lowly substituted cellulose ethers or esters, or of polyvinyl alcohol. The waxes which may be used include ester waxes such as beeswax and carnauba wax and hydrocarbon waxes such as paraffin wax, ceresin, ozokerite and microcrystalline waxes. The term "wax" also includes wax-like substances having wax-like physical properties such as octadecanol, ceryl alcohol, dioctadecyl ether, stearamide, stearic acid, cerotic acid, laurone, stearone, chlorotriacontane, octadecyl and ceryl mercaptan, ceryl cerotate, cetyl palmitate, and pentaerythritol monostearate. Normally made liquid aqueous wax dispersions are defined as having been made with the aid of anionic surface-active agents, the dispersed wax particles being negatively charged. The charge normally developed by the film or the dispersed wax is reversed by treating the film or wax with a dilute aqueous solution of a cationic surface-active agent such as a water-soluble quaternary ammonium or phosphonium compound or a ternary sulphonium compound containing, attached directly to the nitrogen, phosphorus or sulphur atom, an alkyl group comprising a straight chain of at least 8 carbon atoms. Preferred agents are cetyl trimethyl ammonium bromide, tetradecyl pyridinium bromide, and octadecyl trimethyl ammonium sulphate. Cetyl triethyl phosphonium iodide or cetyl ethyl methyl sulphonium iodide may alternatively be used. An electrophoretic method of determining the sign of the charge on the film and on the wax particles is described. In the case of regenerated cellulose film the latter is preferably in the gel state, i.e. in the swollen hydrated state which the film initially possesses when coagulated in manufacture before it is dried. The wax may be applied to the regenerated film, as it comes from the casting machine, by passing the film continuously through an aqueous bath containing the dispersed wax and a known softening agent for regenerated cellulose, e.g. glycerine. The bath may also contain a liquid aqueous dispersion of colloidal silica and/or alumina, the particles of which carry a charge of the same sign as that on the wax particles. The aqueous dispersion of colloidal silica may be prepared in the usual manner by the acidification of an aqueous solution of sodium silicate, and the negative charge on the silica particles may be changed to positive by treating the dispersion with a water-soluble cationic surface-active agent such as one of those previously listed. The charge sign of particles of dispersed colloidal alumina may be similarly reversed. In an example (all parts by weight) gel regenerated cellulose film is passed through an aqueous bath at 60 DEG C. containing 7 parts of glycerol, 0.20 of dispersed paraffin wax, 0.05 of octadecyl trimethylammonium sulphate and 93 of distilled water. The bath is prepared by stirring the wax (melting-point 60 DEG C.) into an aqueous solution of the sulphate in about 2 parts of water at 65 DEG C., allowing the mixture to cool to 45 DEG C. while continuing stirring and then diluting the resulting wax dispersion with the remainder of the water and the glycerol. Excess liquid is removed from the surface of the film by passage through squeeze rolls and the film is passed over drying rollers at 80-90 DEG C. In a second example, stearamide is used instead of paraffin wax. In a third example gel regenerated cellulose film is similarly treated with an aqueous bath having the same composition as that of the first example but containing also 0.07 parts of a 15 per cent aqueous dispersion of colloidal silica, which has been added to the wax dispersion before dilution. In a fourth example gel regenerated cellulose is similarly treated with an aqueous bath of similar composition except that the concentrated aqueous wax dispersion added to the bath contains colloidal alumina instead of colloidal silica. In a fifth example gel regenerated cellulose film is first passed through a dilute aqueous solution of octadecyl triethyl ammonium bromide and then through an aqueous bath containing glycerol and paraffin wax dispersed by means of sodium octadecyl sulphate. Specification 657,342 is referred to.