1376869 Thermographic and thermochromic materials NCR CORP 4 June 1973 [16 June 1972] 26477/73 Heading G2C A thermographic or thermochromic material comprises a support carrying at least two layers thereon, one layer containing a first binder and an organic sulphonic acid and another layer (immediately over- or under-lying said one layer) containing a second binder and a compound which reacts with the sulphonic acid on heating to form a colour and which is a spiropyron, a phthalide, or a fluoron or a spiro-oxazine, and the layer further from the support being applied to the dried preceding layer in a non-solvent for the binder of the preceding layer. Preferably the sulphonic acid layer is nearer the support and a binder which is water or alcohol-soluble (e.g. gelatine, polyvinyl alcohol, polyamides or cellulose nitrate) is used as the first binder. The second binder may be polystyrene, polyvinyl acetate, a styrene copolymer, polymethylmethacrylate, a polyvinyl acetal, a cellulose acetate, a cellulose ether or dicyclopentadiene resin using toluene, benzene, cyclohexanone or chlorobenzene as solvent. A second sulphonic acid layer may be applied over the colour-forming compound layer. The material may contain meltable materials such as p (phenyl or t -buty) phenil, bisphenol A, diphenylamine, acetanilide, 4-methoxybenzoin and 2, 3 dihydrexynaphthalene.
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