Patent GB-1108361-A (GB)
Title: An engraving foil with a light-absorbing layer for electronic engraving
Inventor: HELLMIG EHRHARD
IPC: G03F1/00
Filed: 1965-03-19
Abstract: An engraving foil comprises a transparent film support and a light-absorbing layer comprising a light-absorbing material which is capable of being removed or decolourised without destroying the layer and which is uniformly distributed in a water permeable binding agent.  The supports may be: foils of polycarbonates e.g. polycarbonates of bis-phenylol alkanes; polyesters e.g. polyethylene terephthalate; cellulose esters e.g. cellulose acetate, aceto-butyrate; polystyrene; polymers of vinyl chloride e.g. copolymers with vinyl acetate, vinylidene chloride, maleic acid esters, fumaric acid esters or esters of acrylic or methacrylic acid say with aliphatic alcohols having up to 5 carbon atoms a matted support may be used or a matted layer may be provided on the back of the foil or between the light absorbing layer and the support.  The support may be provided with an antistatic laver or may contain antistatic effecting substances. The light-absorbing layer may be applied by means of a subbing layer.  Organic or inorganic dyes may comprise the light absorbing materials e.g. finely-divided metals such as silver, colloidal manganese dioxide or pyralusite, "antihalo dyes", dyes known from the silver azo dye-bleaching process.  The binding agent may be a hydrophilic water-permeable colloidal material e.g. gelatine, colloidal albumin; a cellulose derivative e.g. cellulose ester such as acetate, carboxyethyl cellulose, a synthetic resin e.g. polyvinyl alcohol, partially hydrolysed polyvinyl acetate polyacryl amide, vinyl alcohol, vinyl chloride copolymers, alginic acid or derivatives thereof such as salts or esters or polyvinyl pyrrolidone.  The light absorbing layer may be plasticized. Plasticizers mentioned are: hydrosols, in particular, for gelatine layers, of copolymers of esters of acrylic acid or methacrylic acid with acrylonitrile, acrylamide, styrene or butadiene; graft polymers of gelatine derivatives with esters of acrylic acids and other polymerisable compounds; glycerine or other polyhydric alcohols, fatty acid amides, cyclic amides such as caprolactam and acylation products thereof; polyethylene glycol and derivatives thereof such as esters, ethers and condensation products with dibasic carboxylic acids; phosphoric acid esters of pentaerythritol or polyethylene glycol or mixed esters thereof, and ethers of pentaerythritol. Suitable products are also described in British Patents Nos.  The engraving foil may have a transparent protective layer comprising a binding agent.  Suitable binding agents are a cellulose ester of inorganic or organic acids such as cellulose nitrate or acetate, polystyrene, or copolymers of styrene with vinyl monomers or hardened gelatine.  The binding agent may be applied from a solution or a dispersion.  If the light absorbing layer consists of a binding agent capable of being hardened, the protective layer is not needed. Hardening agents for gelatine layers may be a solution of chrome alum, chromyl acetate, stearate or polymers such as poly acrolein bisulphite or dialdehyde starch.  The engraving foil is suitable for the reproduction of tone corrected, half tone-screened images for reproducing screened images by photographic processes by making a relief image in the engraving foil by chipping out portions by photoelectric engraving in a regular screen pattern of discrete separate indentations, the area of the indentations in each elemental image being representation of the tone valve of such elemental area in the original image, treating the raised portions of the engraved relief image, the tone values of which are to be corrected, to remove or decolourize the light absorbing material from the light absorbing layer of the raised portions, washing and drying the corrected engraved relief image.