GB985933A - Improvements in or relating to methods of thermographic copying - Google Patents

Abstract

985,933. Printing plates. KODAK Ltd. April 14, 1961 [April 16, 1961], No. 13538/61. Heading B6C. [Also in Division G2] A copy is made thermographically by forming a heat pattern of an original to be reproduced and conducting this pattern to a layer of high jelly strength gelatin, having a Bloom number greater than 200, carried on a support, thereby increasing the temperature of the gelatin patternwise by an amount which causes an increase in the rate of swelling of the gelatin in water, and thereafter moistening the gelatin in an aqueous moistening bath to cause differential swelling. Preferably the heat pattern is produced by thermographic exposure (see Division G2) or by a heated stylus and causes imagewise heating of the gelatin to a temperature between 50‹ and 200‹ C., preferably above 100‹ C., and such that immersion in water at 20‹ C. for two minutes causes swelling in the exposed areas of at least 1À5 times that in the other areas. With soft gelatin most of the gelatin in the exposed areas may be washed away in the moistening bath but typically the exposed gelatin is softened so that by pressing it on an absorbent receiving sheet a thin stratum of gelatin is transferred from the swollen areas of the gelatin. Either the gelatin may contain an infra-red transparent dye or a pigment such as Prussian Blue, colloidal silver, silver halide, or copper ferrocyanide or the moistening bath may contain a dissolved dye, so that the transferred gelatin is coloured. If desired, the gelatin may be transferred in the presence of a hardening agent either in the moistening bath or in the receiving sheet. If the gelatin is initially so hardened that it is not itself transferable even after swelling, the aqueous moistening bath is a solution and this solution, e.g. of a dye, is transferred to the receiving sheet from the swollen areas to form an image. Alternatively, if the gelatin has been hardened with a metal salt, it may be treated in the moistening bath with an unhardening agent, such as citric or tartaric acid, which complexes the cations of the metal and so softens, and permits transfer of, the swollen gelatin. Moistening of the heat-affected gelatin may be delayed for several days without affecting the production of transfer copies. Softening agents such as urea, glycerol, thiourea, sorbitol, tartaric acid, citric acid or sodium nitrate may be included in the gelatin layer to keep it in unhardened condition. Small amounts of low jelly strength hydrolysed gelatins may be used in admixture with high strength gelatins such as photographic gelatin and gelatin derivatives as in Specification 649,544. A gelatin coating containing glycerol urea, poly-(α-methylallylaminoguanidine), glycollic acid and a butadieneacrylonitrile copolymer may also be employed. Lithographic printing plates may be produced by transfer of gelatin to a grained zinc or aluminium sheet or casein-coated paper and hardening the transferred gelatin. Examples are given of colouring the gelatin layer before or after transfer, e.g. by dusting the transferred gelatin with a powdered dye or pigment. Luminescent materials, pH sensitive dyes or metal salts may be included in the gelatin layer to be transferred.