Waermeempfindliches Kopierblatt

摘要

947,633. Heat-sensitive copying sheets. MINNESOTA MINING & MANUFACTURING CO. Feb. 10, 1960 [Feb. 13, 1959], No. 4735/60. Heading G2C. A heat-sensitive copying sheet suitable for thermographic production of graphic original has a heat-sensitive layer comprising a water soluble compound of a cation and a difficulty reducible heteropoly acid complex having a central atom with an enclosing structure of atoms selected from molybdenum, tungsten or vanadium or mixtures thereof, a water soluble hydrolyzable salt of a strong acid anion and a metal cation not readily oxidisable in aqueous solution, and a water soluble reducing agent having a mild reducing action in acid solution, the heat-sensitive layer being capable of permanently converting to a visibly distinct form upon being momentarily pressed against a metal test bar at a temperature in the range 60 to 150‹C. Said heteropoly acid complex, preferably a phosphomolybdate, may be produced in aqueous solution for example by the interaction of a molybdate - e.g. ammonium molybdate - and a phosphate - e.g. disodium acid phosphate. Analogous reactions occur between molybdates and silicates or arsenates and between tungstates or vanadates and phosphates. Aluminium, chromium, iron, cobalt, manganese, antimony, germanium, boron, copper, tellurium, titanium, selenium, zirconium and thorium also form a heteropoly acid complex anion with molybdenum, tungsten or vanadium. Ascorbic acid and thiourea are the preferred reducing agents but itaconic acid, malic acid, para-amino-phenolsulphate, tartaric acid and sodium thiosulphate may also be used alone or in combination. Suitable hydrolyzable salts include magnesium chloride, calcium chloride, aluminium chloride, cadmium chloride, lithium chloride, magnesium bromide, magnesium nitrate, magnesium sulphate, manganese chloride, manganese sulphate, nickel chloride, strontium choride, zinc chloride zinc sulphate and cobalt chloride. A water soluble salt of a weak acid having an ionisation constant of less than 10-3 may be added to stabilize the compositions. Preferred examples include ammonium or alkali metal citrates, tartrates, acetates, formates, lactates, benzoates, phthalates, malates, carbonates and bicarbonates. Examples are given in which the reactants are dissolved in an aqueous solution for coating on to or impregnating into a suitable web material including aluminium foil and also (Example 8) in which they are dissolved in a film forming material which may be ethyl cellulose in acetane or water soluble cellulose or polyvinyl derivatives for producing self supporting sheets. The web material may be coated with a thin opacifying layer (Example 2) to increase contrast and may also have a thin transparent protective surface coating.