GB763445A - Process for the manufacture of films and film coatings of polyvinyl alcohol - Google Patents

Abstract

<PICT:0763445/IV(a)/1> Polyvinyl alcohol films are obtained by an immersion method in which a band is coated on one or both sides and guided vertically through a bath of polyvinyl alcohol solution, and the solution is pressed against the band by rollers rotating in the bath. The band continues to move vertically after it has left the solution and during the drying of the film layer or layers. The dried films are stripped from the band. Preferably, the band is guided between rollers located in the bath at different heights and in staggered relation so that the band contacts the rollers over a substantial part of their surfaces. After the band leaves the immersion bath it passes over a short distance in which a smooth surface is allowed to form on the coatings. This may be assisted by spraying water or a thin solution of polyvinyl alcohol on to the surface or by treating it with wet wiping rollers. The coated band is then passed through a drying zone, drying being effected by means of warm gases, radiant heat, infra-red radiation, or other means. The band may then pass to a cooling zone. Films of greater thickness may be obtained by repeated immersion of the band. The dry films are continuously removed from both sides of the band, and if desired they may be stretched. The expression "polyvinyl alcohol" includes derivatives of polyvinyl alcohol which still contain free hydroxyl groups and which are soluble in water or aqueous liquids. The forming-band may be of polyvinyl chloride or a polyvinyl acetal, or of steel, chrome-nickel steel, nickel or chrome-plated steel, brass or copper. The surface of the band may be smooth or polished or may have a relief or profile pattern. Coloured polyvinyl alcohol solutions may be used or the surface of the band may be prepared by applying a colouring matter uniformly or in pattern so that the surface of the film becomes coloured. In an example, an endless band of polyvinyl chloride is coated with a solution of a wetting agent and dried. It is passed through a solution of polyvinyl alcohol and the solution is pressed on both sides of the band by staggered rollers. The band is passed vertically upwards out of the bath and dried by exposure to infra-red radiation. The dry films are stripped from the two sides of the band. As shown, an endless belt B is driven by drum R and guided by rollers W1, W2 to a dipping bath T where it is immersed in a solution of polyvinyl alcohol. Immersed rollers W3, W4 press the solution against the belt. The films F1 and F2, formed on opposite sides of the belt, are stripped off as the belt is deflected round the rollers W5, W6.