GB914420A - Heat-sensitive recording material and process for the preparation thereof - Google Patents

Abstract

A process for preparing a heat-sensitive material adapted to receive an impression from a heat source, comprises the steps of preparing a solution of an organic thermoplastic polymer material, capable of forming a film, in a substantially anhydrous organic solvent for said organic material, adding to said solution a substantially anhydrous organic liquid which is a non-solvent precipitating liquid for the organic material and which has a boiling point which is at least 40 DEG C. higher than that of said substantially anhydrous organic solvent, the said non-solvent being added in such an amount that the pH of the resultant solution is lower than the pH of the isoelectric point of the solution of said organic thermoplastic polymer material in said organic solvent, but such that an incipient precipitation of at least a fraction of the solid phase takes place, spreading the resultant solution over a flat surface and evaporating all the liquid phases of the system until a continuous film is formed on said flat surface. The flat surface may be backing paper. The substantially anhydrous solvent is preferably a non-azeotropic mixture of substantially anhydrous organic solvents for the material, and the substantially anhydrous organic liquid is preferably a non-azeotropic mixture of substantially anhydrous organic liquid non-solvents for the material, the low-boiling component of the non-azeotropic mixture of non-solvent liquids having a higher boiling point than that of the high boiling component of the non-azeotropic mixture of substantially anhydrous organic solvents. Polymers specified are polyvinyl chloride and afterchlorinated polyvinyl chloride, natural and synthetic rubbers and chlorinated rubbers, e.g. polymerized chlorinated derivatives of isoprene and butadiene, polymers and copolymers of styrene, and polycarbonates. The films may be self-supporting or have a flexible backing. Besides paper other specified backing materials are flexible sheets of polyvinyl chloride, regenerated cellulose, polyethylene terephthalate, and polypropylene terephthalate. The solvents and non-solvents will vary according to the polymer but in the general examples of such materials given examples of solvents are dichloromethane, trichloromethane, trichloroethylene, 1,1,2-trichloroethane, tetrachloromethane, tetrachloroethylene, propylene oxide, ethyl formate, tetrahydrofurane, and tetrahydropyrane. General examples of non-solvent liquids are methanol, ethanol, propanol, butanols, amylalcohols, hexyl alcohols, octylalcohols, benzyl alcohol, cyclohexanol, furfural alcohol, tetrahydrofurfuryl alcohol, dipropylene glycol, polyglycol, 2-ethoxy ethanol and its methyl, butyl, dibutyl, and diethyl derivatives. Examples of plasticizers which may be included are dimethyl-, diethyl-, dibutyl-, diamyl- and dihexyl-phthalates, butyl-cyclohexyl phthalate, butyl-benzyl phthalate, tricresyl phosphate, alkyl-aryl phosphates, cresyl-diphenyl phosphate, diethylene glycol benzoate and dipropylene glycol dibenzoate. A secondary plasticizer, such as a chlorinated wax, may also be present. When stabilizers are used they should have melting points between 40 DEG C. and 140 DEG C. and have a multiple action as stabilizer, sequestrant, and lubricant. Examples given are metallic soaps, e.g. stearates of aluminium, calcium, magnesium, zinc, tin, lead, lithium and bismuth and ricinoleates of calcium, barium, cadmium and zinc. Small amounts, e.g. 1% of a phosphite or epoxy resin may be used in conjunction therewith. Several examples are provided illustrating the preparation of heat-sensitive films from the various materials mentioned above. The examples also illustrate the failure to obtain satisfactory heat-sensitive films if the requirements as to pH and gradient of boiling points of solvent and non-solvent are not complied with. In Example 5 the polymer is a mixture of polyvinyl chloride and polycarbonate; in Example 7 the polymer is a vinyl chloridevinylidine chloride copolymer, the solvent is ethyl formate and the stabilizer phenyl salicilate. The heat-sensitivity of the recording material produced according to the invention is due to its structure. The material comprises a plurality of translucent micellar particles of a thermoplastic polymeric organic material obtained by evaporating the solvent from a solution of the thermoplastic polymeric organic material, admixed to a second plurality of opaque particles of the thermoplastic polymeric organic material obtained by adding to the solution an organic liquid non-solvent for the material, the particles of the first and second plurality being close to one another substantially without discontinuity between adjacent particles of the same plurality and between adjacent particles of different pluralities, so as to form a continuous opaque film capable of changing its optical behaviour by the application of thermo energy. The material produced in Example 4 from a polycarbonate is tested with a recorder of the electro-cardiograph type. Specification 705,417 is referred to.