Infrared laser heat sensitive recording material

Abstract

 An infrared laser heat sensitive recording material is provided which includes a support and a heat sensitive layer provided thereon. The heat sensitive layer includes an organic silver salt, a developer for the organic silver salt, a water soluble binder, and a tricarbocyanine dye having at least two acidic groups whose maximum absorption wave length is at least 50 nm longer than a maximum absorption wave length of an aqueous solution of the tricarbocyanine dye and is in a range of 650 to 1300 nm. The present invention increases infrared laser light absorption efficiency, reduces degree of coloration of background, and enables recording of images with good quality.

Description

[0001] The present invention relates to a heat sensitive recording material, and more particularly to a non-contact type infrared laser heat sensitive recording material on which an image is recorded by utilizing an infrared laser beam.

[0002] Heat sensitive recording methods, in which a thermal head is made to contact and is scanned on the surface of a heat sensitive recording material having a heat sensitive recording layer on a support such that whereby thermal energy is transferred directly or through a protective layer to the heat sensitive layer to record a colored image, have widely been known and applied to facsimiles and printers.

[0003] However, in such heat sensitive recording mathods, the thermal head may be abraded or components of the heat sensitive recording material may adhere the surface of the thermal head due to contact and scanning of the thermal head on the heat sensitive recording material, whereby a recorded image may not be able to be obtained accurately or the thermal head may be damaged. Further, in such heat sensitive recording methods using thermal heads, there are limits with respect to high speed control of heating and cooling of heater elements and increasing the density of the heater elements, due to the structural character of the thermal head. Therefore, application of such heat sensitive recording methods to high speed, high density and/or high quality recording has been limited.

[0004] In order to overcome the above-mentioned disadvantages of heat sensitive recording methods using thermal heads, other methods for heat sensitive recording have been proposed wherein an image is recorded at a high speed and a high density by using a laser beam without the thermal head contacting the heat sensitive recording material. Examples of such methods are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 50-23617, 54-121140, 57-11090, 58-56890, 58-94494, 58-134791, 58-145493, 59-89192, 60-205182 and 62-56195.

[0005] However, in such heat sensitive recording methods using a laser beam, since it is usually difficult for the heat sensitive layer to absorb light of the visible and near infrared regions, the thermal energy needed for color formation cannot be obtained unless the laser power is considerably high, and it is difficult to manufacture a compact and inexpensive device. Japanese Patent Application Publication (JP-B) No. 50-774 proposes a method for recording an image on a recording material comprising coating the raw paper with ink-filled microcapsules and irradiating strong light onto the microcapsules to cause the microcapsules to burst and the ink to be released. However, this method has very low sensitivity and has not been actually reduced to practice.

[0006] There have been proposed many methods for having a heat sensitive layer absorb laser beams efficiently, wherein generally a substance which absorbs laser beams is added into the heat sensitive layer. However, when the added light absorbing substance is not white, the background of the recording material may be colored, and only images with low contrast and poor quality are obtained.

[0007] Many of white light-absorbing substances are inorganic compounds, and almost all of these compounds have poor light-absorbing efficiency. On the other hand, organic compounds suitable for absorption of laser beams generally absorb light in the visible light region as well and are often colored. Further, when a light-absorbing substance of a deeper color, and therefore a higher light-absorbing efficiency, is added to the heat sensitive layer, it is difficult to obtain a recording paper with a good degree of whiteness although the sensitivity may be increased.

SUMMARY OF THE INVENTION

[0008] The present invention has been developed in consideration of the above described facts, and the object thereof is to provide an infrared laser heat sensitive recording material which has a high infrared laser beam absorbing efficiency, whose background after forming an image has a sufficient degree of whiteness, and which can record an image with high quality.

[0009] The present inventors have found that very good recording results can be obtained by using an infrared laser and a heat sensitive recording material contains at least an organic silver salt, a developer for the organic silver salt, a specific tricarbocyanine dye and a water soluble binder.

[0010] Accordingly, the present invention provides an infrared laser heat sensitive recording material comprising: a support; and a heat sensitive layer which is provided on the support and comprises: an organic silver salt; a developer for the organic silver salt; a water soluble binder; and a tricarbocyanine dye having at least two acidic groups whose maximum absorption wave length is at least 50 nm longer than a maximum absorption wave length of an aqueous solution of the tricarbocyanine dye and is in a range of 650 to 1300 nm.

[0011] The carbocyanine dye has a maximum absorption wave length in the visible light region as well and therefore is colored when it is in an aqueous solution. However, when the water of the aqueous solution is removed and thus the dye is in a dry state, the maximum absorption wave length in the visible light region is shifted to a longer wave length, i.e., to an infrared region, due to the association of the carbocyanine dye or other reasons. As a result, the heat sensitive layer in the infrared laser heat sensitive recording material of the present invention will be colorless after drying even though it may be colored when coated or applied. Therefore, the background of the infrared laser heat sensitive recording material can be rendered white or nearly colorless, and furthermore the infrared laser beam absorbing efficiency thereof can be improved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention will be described hereinafter in more detail.

[0013] The organic silver salt contained in the heat sensitive layer of the infrared laser heat sensitive recording material according to the present invention is any colorless or white silver salt which is stable with respect to light and generates silver through redox reaction when heated together with a developer. Such an organic silver salt is a silver salt of an organic compound having an imino, mercapto or carboxyl group and examples thereof include:

1) silver salts of organic compounds having an imino group such as saccharin silver, phthalazinone silver and benzotriazole silver;

2) silver salts of organic compounds having a mercapto or thione group such as silver salt of 3-(2-carbonylethyl)-4-oxymethyl-4-thiazoline-2-thione and silver salt of 3-mercapto-4-phenyl-1,2,4-triazole; and

3) silver salts of organic compounds having a carboxyl group such as silver stearate and behenate.

[0014] Among these organic silver salts, silver behenate is most preferred since it is white, is stable with respect to light, has excellent moisture resistance, can be used in combination with a mild developer, and an excellent tone modifier therefor is known.

[0015] In the infrared laser heat sensitive recording material according to the present invention, in order to record a high density image by using a high concentration of the organic silver salt, it is preferable to use an organic silver salt which has undergone salt removing purification. Here, salt removing purification means, for example, to remove nitrate salts from the reaction system which nitrate salts are by-products of the preparation of the organic silver salt by the addition of silver nitrate to an organic acid salt formed from an organic acid and an alkali. Such salt removing purification may preferably be carried out by ultrafiltration with a semipermeable membrane which is impermeable to any organic silver salt but permeable to the nitrate salts, or by centrifugation.

[0016] The developing agent or developer used in the present invention is a reducing agent which reduces the organic silver salt to produce silver. The properties required of such a developer include the reducing reaction proceeding rapidly at developing temperatures, the tone of an image after development not being affected, and the like.

[0017] Examples of such developers are hydroxycoumarones, hydroxycoumarans, sulfoamidophenols, sulfoamidonaphthols, hydrazones, hydroxamic acids, bis-β-naphthols, indane-1,3-diones, aminophenols, aminonaphthols, pyrazoline-5-ones, hydroxylamines, reductones, hydrazines, hydroquinones, polyphenols such as bisphenol A and bisphenol B, gallic acid, gallic acid esters, phenylene diamines, hydroxyindanes, 1,4-dihydroxypyridines, amidoximes, hydroxy-substituted aliphatic carboxylic acid arylhydrazides, N-hydroxyureas, phosphonamidophenols, phosphonamidoanilines, α-cyanophenylacetic acid esters, sulfonamideanilines. Among these developers, compounds represented by the following formulae and octyl, propyl, ethyl and methyl gallate are preferably used.

[0018] In order to prevent the reducing reaction of the organic silver salt with the developer at ordinary temperatures so as to prolong the shelf life of the infrared laser heat sensitive recording material and reduce fogging which occurs over time, it is preferred in the present invention to enclose at least one of the organic silver salt and the developer for the organic silver salt in a microcapsule. When only one of the organic silver salt and the developer for the organic silver salt is microencapsulated, the other in the form of a solid may be dispersed in a solvent, or may be dissolved in an organic solvent to form an oily phase followed by mixing with an aqueous phase containing a water soluble polymer to form an emulsified dispersion. In the latter case, the heat sensitive layer can be transparent. If both the organic silver salt and the developer for the organic silver are microencapsulated, they may be enclosed in separate microcapsules or in a single microcapsule.

[0019] A method for dispersing, in a solvent, an organic silver salt in the form of a solid which method is used in the present invention comprises, for example, dispersing the organic silver salt, a water soluble polymeric compound such as a polyvinyl alcohol, and optionally a tone modifier, an anti-fogging agent, a dispersing agent and water, by using a ball or sand mill, until the particle diameter is a few microns or less. A reducing agent may be dispersed in a solvent in a similar manner or may be completely dissolved in an aqueous solution of a polyvinyl alcohol. Both liquids thus obtained are blended to prepare a coating dispersion of solid particles which is then coated on a support to produce the recording material.

[0020] The microcapsule which may be used in the present invention is a heat responsive microcapsule which segregates or isolates the encapsulated material from the external environment at ordinary temperatures, and whose microcapsule wall is permeable when heated without being destroyed by pressure or heat.

[0021] Known methods for producing such microcapsules, such as interfacial polymerization, internal polymerization, and external polymerization, may be used as the method of producing microcapsules of the present invention. In particular, interfacial polymerization is preferred in which a core material obtained by dissolving or dispersing the organic silver salt or the developer for the organic silver salt in an organic solvent is emulsified in an aqueous solution of a water soluble polymer, and a capsule wall comprising a polymeric substance is formed around the oil droplet.

[0022] The organic solvent may preferably be a non-water solvent having a boiling point of 150 °C or lower such as a carboxylic acid ester (e.g., ethyl, butyl and isoamyl acetate), toluene, xylene, a phosphoric acid ester, or the like.

[0023] Reactants for forming the capsule wall are added to the inside and/or outside of the oil droplet.

[0024] Examples of the polymeric material forming the capsule wall (capsule wall-forming material) include polyurethanes, polyureas, polyamides, polyesters, polycarbonates, ureaformaldehyde resins, polyamic acids, polystyrenes, styrene-methacrylate copolymers, styrene-acrylate copolymers and the like. Preferable capsule wall-forming materials are polyurethanes, polyureas, polyamides, polyesters and polycarbonates. In particular, polyurethanes and polyureas are preferable since they provide a capsule which is difficult to damage. It is also possible to use two or more of these polymeric materials.

[0025] Examples of the water soluble polymer include gelatine, polyvinyl pyrrolidone, polyvinyl alcohol and the like.

[0026] For example, when polyurea or polyurethane is used as the capsule wall forming material, the microcapsule wall may be easily formed by reacting (a) a polyvalent isocyanate such as a diisocyanate, triisocyanate, tetraisocyanate or polyisocyanate prepolymer, and (b) a polyamine such as a diamine, triamine or tetramine, a prepolymer containing two or more amino groups, piperazine or its derivative, a polyvalent alcohol, or water, in an aqueous solvent by interfacial polymerization. The microcapsule obtained by this method is preferable because its wall is dense.

[0027] A composite wall comprising a polyurea or polyurethane and a polyamide may be prepared by, for example, using a polyisocyanate and an acid chloride or a polyamine and a polyvalent alcohol and adjusting the pH of an emulsified medium which is used as a reaction liquid, followed by heating. Methods for preparing such composite walls comprising polyureas and polyamides are described in detail in Japanese Patent Application Laid-Open (JP-A) No. 58-66948. A capsule comprising a polyamic acid may be formed from, for example, an interfacial reaction of a polystyrene-maleic anhydride copolymer and a polyvalent amine.

[0028] Particularly preferred capsule wall forming materials for microcapsules used in the infrared laser heat sensitive recording material according to the present invention include the following isocyanate compounds.

[0029] The isocyanate compounds used herein are selected from modified products of known isocyanate monomers such as (1) urethane modified products, (2) allophanate modified products, (3) isocyanurate modified products, (4) burette modified products, (5) carbodiimide modified products, and (6) blocked isocyanates, as well as (7) polymeric MDI, i.e., linear polymers of 4,4'-diphenylmethane diisocyanate (MDI).

[0030] Examples of the isocyanate monomers forming the modified products include tolylene diisocyanate (TDI), 4,4'-diphenylmethane diisocyanate (MDI), xylylene diisocyanate (XDI), naphthylene diisocyanate (NDI), paraphenylene diisocyanate (PPDI), tetramethylxylylene diisocyanate (TMXDI), hexamethylene diisocyanate (HDI), dicyclohexylmethane diisocyanate (HMDI), isophorone diisocyanate (IPDI), lysine diisocyanate (LDI), isopropylidene bis(4-cyclohexylisocyanate) (IPC), hydrogenated xylylene diisocyanate (hydrogenated XDI), cyclohexyl diisocyanate (CHDI), tolidine diisocyanate (TODI) and the like. The structures of these isocyanate monomers are shown hereinbelow, but the isocyanate monomers used in the present invention are not limited thereto. These monomers are described in detail, for example, in "Syntheses, Formulations, Functionalization and Developments of Use for Advanced Polyurethanes" published by GiJutsu Joho Kyokai (1989).

[0031] These isocyanate monomers are converted into the above mentioned various isocyanate modified products by modification in order to eliminate undesirable properties such as toxicity, control the reaction rates and/or improve the formulation ratios, and the modified products are used as a capsule wall forming material in the present invention. The modified products will be described hereinafter. (1) The urethane modified products are obtained by modifying isocyanate monomers (or diisocyanates) with insufficient amounts of a polyol. (2) The allophanate modified products are produced by adding an isocyanate group to the urethane group. (3) The isocyanurate modified products have an isocyanurate ring in their molecule and tend to improve heat resistance. (4) The burette modified products are compounds which have an isocyanate group added to the urea bond and from which free isocyanate groups have been eliminated. (5) The carbodiimide modified products are obtained by subjecting 2 moles of isocyanate groups to decarboxylation to produce carbodiimide bonds. Further addition of isocyanate produces urethone imine, and the carbodiimide and urethone imine attain equilibrium. (6) The blocked isocyanates are obtained by reacting isocyanate with various blocking agents so as to temporarily mask the activity of isocyanate groups. The blocking agents may be active hydrogen containing compounds such as phenols (phenol, xylenol, and the like), oximes, lactams, alcohols and the like.

[0032] The reactants for forming the capsule wall of the infrared laser heat sensitive recording material according to the present invention may include isocyanate compounds represented by the following formulae (a) to (k) which are good compounds in view of safety and availability.

[0033] The reactant forming the capsule wall is preferably heated after emulsification and/or dispersion. Heating is preferably carried and at 30 to 80 °C for 30 minutes to 4 hours. At temperatures lower than 30 °C, the reaction would require a long period of time and furthermore would be incomplete. On the other hand, control of the reaction becomes difficult at temperatures higher than 80 °C.

[0034] For example, when an isocyanate compound is used as a reactant, the microcapsule wall can be formed in the following manner. The organic silver salt is dispersed in an organic solvent as described above, and at least one isocyanate compound is added to the dispersion to form an oil phase. Then, this oil phase is added into an aqueous solution containing a water soluble binder such as gelatine, followed by stirring. The resultant dispersion system comprising the oil phase dispersed in the aqueous solution can then be heated to, for example, about 40 to 90 °C to form a capsule wall forming material.

[0035] Preferably, the resulting microcapsule has an average particle diameter of 0.1 to 3.0 µm, more preferably 0.2 to 1.2 µm. Controlling the particle diameter to a value in this range is advantageous in that the transparency of the heat sensitive layer is increased, and therefore, an image can become visible by transmitted light when a light table is used.

[0036] The tricarbocyanine dye having at least two acidic groups (hereinafter referred to simply as "tricarbocyanine dye") used in the present invention is preferably represented by the following general formula:

wherein each of Z¹ and Z² is a non-metallic atomic group which forms a substituted or unsubstituted benzothiazole, benzoselenazole, indole, naphthothiazole, naphthoselenazole or benzindole ring; each of R¹ and R² is a substituted or unsubstituted alkyl group; each of R³ and R⁵ is a hydrogen atom or an atom needed to form a 5-membered ring when R³ and R⁵ are bonded to each other; R⁴ is a hydrogen atom or a monovalent group except any atomic group which forms a ring in a di-substituted amino group; X^- is an anion; and n is 1 or 2. If the tricarbocyanine dye molecule forms an intramolecular salt, then n is 1.

[0037] Substituents of the non-metallic atomic group which forms a benzothiazole, benzoselenazole, indole, naphthothiazole, naphthoselenazole or benzindole ring represented by Z¹ and Z² include sulfonic acid groups, carboxylic acid groups, hydroxyl group, halogen atoms such as fluorine, chlorine and bromine atoms, cyano group, substituted amino groups such as dimethylamino, diethylamino, ethyl-4-sulfobutylamino, di(3-sulfopropyl)amino groups and the like, as well as substituted or unsubstituted alkyl groups having 1 to 5 carbon atoms which link to the ring directly or through a divalent linking group wherein the alkyl groups may be methyl, ethyl, propyl, butyl groups and the like, the substituent may be sulfonic acid, carboxylic acid, hydroxyl groups and the like, and the divalent linking group may be -O-, -NHCO-, NHSO₃-, -NHCOO-, -NHCONH-, -COO-, -CO-, -SO₂- and the like.

[0038] The term "sulfonic acid groups" used herein means a sulfo group and salts thereof, and the term "carboxylic acid" means a carboxyl group or salts thereof. Examples of the salts include those of alkali metals such as Na and K, ammonium, and organic ammoniums such as triethylammonium, tributylammonium and pyridinium. Among the above described non-metallic atomic groups represented by Z¹ and Z², a benzindole ring having one or more sulfonic acid groups is particularly preferred.

[0039] Preferred examples of the alkyl group represented by R¹ and R² are lower alkyl groups having 1 to 5 carbon atoms, such as methyl, ethyl, n-butyl, isopropyl and n-pentyl groups, these alkyl groups having a substituent such as a sulfonic acid, carboxylic acid or hydroxyl group. Among these preferred examples, lower alkyl groups having 2 to 5 carbon atoms and a sulfonic acid group such as 2-sulfoethyl, 3-sulfopropyl, 3-sulfobutyl or 4-sulfobutyl group are particularly preferred.

[0040] The 5-membered ring formed by the bonding of R³ and R⁵ includes indene and cyclopentyne rings. Preferred examples of the monovalent group represented by R⁴ include lower alkyl groups such as methyl group; substituted or unsubstituted phenyl groups, and aralkyl groups such as benzyl group; lower alkyl groups such as methoxy group; di-substituted amino groups such as dimethylamino, diphenylamino and methylphenylamino groups; alkylcarboxyloxy groups such as acetoxy group; alkylthio groups such as methylthio group; cyano group; nitro group; and halogen atoms such as fluorine, chlorine and bromine atoms. Examples of the anion represented by X are halogen ions such as Cl^- and Br^-, p-toluenesulfonic acid ion and ethylsulfuric acid ion. Among the tricarbocyanine dyes described above in detail, particularly preferred are those in which Z¹ and Z² are sulfo-substituted benzindole rings and R¹ and R² are sulfoalkyl groups.

[0041] Examples of the tricarbocyanine dyes represented by the above general formula are compounds described in Japanese Patent Application Laid-Open (JP-A) No. 3-226736 as well as the following compounds:

[0042] The tricarbocyanine dye used in the present invention should be contained in the heat sensitive layer in a state in which the tricarbocyanine dye has a maximum absorption wave length which is at least 50 nm longer than the maximum absorption wave length of an aqueous solution of the dye and is in the range of from 650 to 1300 nm, in order to increase the whiteness of the infrared laser heat sensitive recording material and to enhance the infrared laser absorption efficiency thereof.

[0043] To this end, various methods may be used, for example, adsorption of the tricarbocyanine dye onto non-photosensitive halogenated particles; dissolution of the tricarbocyanine dye in a high boiling point oil and emulsification thereof in an aqueous phase; dispersion of a finely divided tricarbocyanine in a solvent; and use of an aggregate or associate of the tricarbocyanine dye. Among these methods, a method in which a tricarbocyanine dye is dissolved in water to form an aqueous solution, and, after removal of water, the resultant aggregate is introduced into a heat sensitive layer is preferable in view of suitability to the production of the infrared laser heat sensitive recording material.

[0044] The tricarbocyanine dye may be included in the core material of the microcapsule, and may of course be present outside of the microcapsule or in the microcapsule wall. Further, the tricarbocyanine dye may be present simultaneously in two or more of these locations.

[0045] The water soluble binder used in the present invention binds the developer and the microcapsules contained in the heat sensitive layer, and adheres the heat sensitive layer to a support. Examples of such water soluble binders are gelatine and/or gelatine derivatives such as phthalized gelatine; water soluble polymers such as polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose and hydroxypropyl cellulose; various emulsions of gum arabic, polyvinylpyrrolidone, casein, styrene-butadiene latex, acrylonitrile-butadiene latex, polyvinyl acetate, polyacrylate ester and ethylene-vinyl acetate copolymer and the like.

[0046] The amount of the binder used is preferably 0.5 to 5 g/m² as calculated as solids.

[0047] In the present invention, it is preferable to include a tone modifier in the heat sensitive layer in order to promote the reducing reaction of the organic silver salt with the developer during heating so as to adjust the image tone to carry out development rapidly.

[0048] The tone modifier is preferably used when dark colored, particularly black colored images are desired. The amount of the tone modifier used is about 0.0001 to about 2 moles, preferably about 0.0005 to about 1 mole, per mole of the organic silver salt. An appropriate tone modifier is to be selected in accordance with the natures of organic silver salt and developer used, and the most common tone modifiers are heterocyclic organic compounds containing at least two hetero atoms including at least one nitrogen atom in the heterocyclic ring.

[0049] Such tone modifiers are described in, for example, U.S. Patent No. 3,080,254. Examples of such tone modifiers are phthalazone (phthalazinone), phthalic anhydrides, 2-acetylphthalazinone, 2-phthalylphthalazinone, and substituted phthalazinones as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 50-67132. These tone modifiers are preferably used in the present invention.

[0050] Examples of other effective tone modifiers include pyrazolin-5-ones, cyclic imides and quinazolinone as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 46-6077. Specific examples are phthalimide, N-hydroxyphthalimide, N-potassium phthalimide, and phthalimide silver. Phthalazinones are also effective as tone modifiers.

[0051] Still other effective tone modifiers include mercapto compounds as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 49-5019 and 49-5020. In addition, also usable are oxazinediones as described in Japanese Patent Application Laid-Open (JP-A) No. 50-2542, phthalazinediones as described in Japanese Patent Application Laid-Open (JP-A) No. 50-67641, uracils as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 58-114217, N-hydroxynaphthalimides as disclosed in U.S. Patent No. 3,782,941, substituted phthalimides as described in West Germany Patent Application Publication Nos. 2,140,406, 2,141,063 and 2,220,597, and phthalazinone derivatives as disclosed in West Germany Patent Application Laid-Open No. 2,220,618.

[0052] In the infrared laser heat sensitive recording material of the present invention, in order to prevent thermal fogging and stabilize the background after image formation, use of the following compounds is preferred.

[0053] First, it is well known that mercury compounds are very effective in preventing thermal fogging and stabilizing the background after image formation. However, such compounds are not preferable from the standpoint of environmental contamination. In the present invention, N-halogeno compounds such as N-halogenosuccinic acid imides, N-halogenoacetamides, N-halogenooxazolinones, N-halogenobenzotriazoles and N-halogenobenzimidazoles as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 49-10724, 48-2842 and 48-8194 are preferably used.

[0054] Further, the following compounds disclosed in U.S. Patent No. 3, 645,739, and Japanese Patent Application Laid-Open (JP-A) Nos. 49-125016, 49-130720, 50-57619 and 50-39264 may be used as acid stabilizers: higher fatty acids; tetrahalogeno phthalic acid or its anhydride; arylsulfonic acid salts such as benzenesulfonic acid and p-toluenesulfonic acid; aryl sulfinic acids such as benzenesulfinic acid and p-toluenesulfinic acid or salts thereof; and higher fatty acid lithium salts such as lithium myristate, stearate, behenate, palmitate and laurate.

[0055] Other effective acid stabilizers include salicylic acid, p-hydroxybenzoic acid, tetrabromobenzoic acid, tetrachlorobenzoic acid, p-acetamidebenzoic acid, alkyl substituted benzoic acids (such as p-t-butylbenzoic acid), phthalic acid, isophthalic acid, trimellitic acid, pyromellitic acid, dicitric acid, 5',5'-methylenebissalicylic acid, citric acid, tartaric acid, oxalic acid, boric acid, phosphoric acid and pyrophosphoric acid. These acid stabilizers do not only prevent thermal fogging but also improve the shelf life and prevent discoloration of the background upon exposure to white light after image formation.

[0056] Still other compounds effective in preventing thermal fogging and photodiscoloration include benzotriazole and its derivatives; thiouracils; mercapto compounds such as 1-phenyl-5-mercaptotetrazol; azolethioethers or blocked azolethiones as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 47-318; tetrazolylthio compounds as disclosed in U.S. Patent No. 3,700,457; photosensitive halogeno organic oxidizing agents as disclosed in U.S. Patent Nos. 3,707,377 and 4,108,455; polybrominated organic compounds such as 2,4-bis(tribromomethyl)-s-triazine and polybromoalkylsulfonyl compounds as disclosed in U.S. Patent No. 3,874,946; trihalomethyltetrazole derivatives, trihalomethylbenzimidazole and corresponding benzoxazole or benzothiazole derivatives as disclosed in U.S. Patent No. 4,546,075; compounds represented by R^a-CX₂-R^b as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 59-57234 wherein X is a halogen and R^a and R^b denote acyl, oxycarbonyl, oxysulfonyl, alkylsulfonyl, arylsulfonyl, aralkylsulfonyl, carboxyl, sulfo and sulfamoyl groups; organic halogen compounds as disclosed in U.S. Patent No. 4,465,761; 2-trihalomethyloxazole derivatives as disclosed in U.S. Patent No. 4,452,885; heterocyclic compounds having a trihalomethyl group as disclosed in U.S. Patent No. 4,756, 999; and compounds represented by the following formula as disclosed in European Patent No. 622,666:

wherein R⁶, R⁷ and R⁸ independently denote a hydrogen or halogen atom, or an alkyl, alkoxy, thioalkyl, perhalogenated alkyl or cycloalkyl group.

[0057] The combined use of the thermal fogging and photodiscoloration preventing agent together with the heat responsive capsule provides particularly remarkable effects of the present invention.

[0058] Further, it is also possible to add a sensitizer in an emulsion-dispersed or solid-dispersed state to increase the heat sensitivity, thereby swelling the microcapsule wall during infrared laser heating for forming the capsule wall forming material.

[0059] The sensitizer may be any plasticizer selected from plasticizers for the polymers used as the capsule wall forming material, which plasticizer has a melting point of greater than or equal to 50 °C and preferably less than or equal to 150 °C and is solid at ordinary temperatures. For example, when the capsule wall forming material comprises polyurea or polyurethane, a hydroxy compound, a carbamate ester compound, an aromatic alkoxyl compound, an organic sulfonamide compound, an aliphatic amide compound or an arylamide compound is preferably used as the sensitizer.

[0060] It is also possible to use a color formation assistant in the present invention. The color formation assistant which may be used in the present invention is a substance which increases the color formation density during laser heating recording or lowers the minimum color formation temperature, and provides conditions which allow for ease of reaction of the organic silver salt and developer by, for example, lowering the softening point of the capsule wall.

[0061] Examples of the color formation assistant are phenolic, alcohol, amide and sulfonamide compounds, more particularly p-tert-octylphenol, p-benzyloxyphenol, phenyl p-oxybenzoate, benzyl carbanilate, phenethyl carbanilate, hydroquinone dihydroxyethyl ether, xylylene diol, N-hydroxyethylmethanesulfonic acid amide and N-phenyl-methanesulfonic acid amide. Such a compound may be present within the core material or as an emulsified dispersion outside of the microcapsule.

[0062] The support used in the present invention may be transparent or opaque. Opaque supports include paper, synthetic paper, paper laminated with a polymeric film, aluminum deposited bases, and polymeric films coated with a white pigment. In these cases, a support having a high laser beam reflectivity is preferably used so that the laser beam irradiated into the heat sensitive layer can be efficiently absorbed in the heat sensitive layer.

[0063] On the other hand, a transparent support used in the present invention is preferably a support which does not absorb the irradiated laser beam and which has dimensional stability such that it does not deform due to heat generated during laser irradiation. In this case, a laser beam can be irradiated through the transparent support to record an image. The thickness of such a support is 10 to 200 µm.

[0064] Such transparent supports include, for example, polyester films such as polyethylene terephthalate and polybutylene terephthalate; cellulose derivative films such as cellulose triacetate films; polystyrene films; polyolefin films such as polypropylene and polyethylene films; polyimide films, polyvinyl chloride films; polyvinylidene chloride films; polyacrylate copolymer films; and polycarbonate films. The films may be used alone or laminated to each other. These films which may be used as a transparent support preferably have a haze of 3% or less.

[0065] It is particularly preferable that the support used in the present invention is a polyester film which has been subjected to heat resistance treatment and antistatic treatment.

[0066] It suffices that the methods for preparing such a film are carried out under conditions capable of attaining the above mentioned purpose, and these methods are not particularly limited. For example, the film may be prepared by subjecting the materials used for the preparation thereof to heat fusion, extrusion, cooling, solidification, stretching and heat setting.

[0067] The support may further contain inorganic fine particles, antioxidants, antistatic agents and pigments in so far as they do not interfere with the object of the present invention.

[0068] Inorganic fine particles which may be used include oxides, hydroxides, sulfides, nitrides, halides, carbonates, acetates, phosphates, phosphites, organic carboxylates, silicates, titanates and borates of elements of IA, IIA, IVA, VIA, VIIA, VIII, IB, IIB, IIIB and IVB groups, and hydrated compounds thereof, composite compounds based thereon, as well as naturally occurring mineral particles. Examples include compounds of IA group elements such as lithium fluoride and borax (hydrated sodium borate); compounds of IIA group elements such as magneium carbonate, magnesium phosphate, magnesium oxide (magnesia), magnesium chloride, magnesium acetate, magnesium fluoride, magnesium titanate, magnesium silicate, hydrated magnesium silicate (talc), calcium carbonate, calcium phosphate, calcium phosphite, calcium sulfate (gypsum), calcium acetate, calcium terephthalate, calcium hydroxide, calcium silicate, calcium fluoride, calcium titanate, strontium titanate, barium carbonate, barium phosphate, barium sulfate and barium phosphite; compounds of IVA group elements such as titanium dioxide (titania), titaniummonoxide, titanium nitride, zirconium dioxide (zirconia) and zirconium monoxide; compounds of VIA group elements such as molybdenum dioxide, molybdenum trioxide and molybdenum sulfide; compounds of VIIA group elements such as manganese chloride and manganese acetate; compounds of VIII group elements such as cobalt chloride and cobalt acetate; compounds of IB group elements such as cuprous iodide; compounds of IIB group elements such as zinc oxide and zinc acetate; compounds of IIIB group elements such as aluminum oxide (alumina), aluminum fluoride and aluminosilicate (alumina silicate, kaolin, kaolinite); compounds of IVB group elements such as silicon oxide (silica, silica gel), graphite, carbon, and glass; and particles of naturally occurring minerals such as carnallite, kainite, and mica (phlogopite). Among these inorganic fine particles, silica, talc, titania, alumina, calcium carbonate, calcium oxide, calcium chloride and mixtures thereof are preferred from the viewpoint of ease of handling. Examples of organic fine particles are fine particles of crosslinked polystyrene or crosslinked polymethylmethacrylate. Any of antioxidants, antistatic agents and pigments which are known as additives for resins may be added depending upon the final use and/or purpose of the infrared laser heat sensitive recording material.

[0069] When a polymeric film or a paper laminated with a polymeric film is used as a support or a transparent support is used in the present invention, it is preferable to provide a primer layer between the support and the heat sensitive layer to increase adhesion therebetween.

[0070] Examples of materials for the primer layer are gelatine, synthetic polymer latex, nitrocellulose, acrylate ester copolymer, polyvinylidene chloride, SBR, aqueous polyester and the like. The amount of the primer layer applied is preferably in the range of from 0.1 to 2.0 g/m², particularly from 0.2 to 1.0 g/m².

[0071] When a heat sensitive layer is applied on the primer layer, the primer layer may swell with the water contained in the heat sensitive layer such that the image quality of the heat sensitive layer deteriorates. Therefore, the primer layer should preferably be hardened by means of a hardener.

[0072] The hardener may be any of hardeners described in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2-141279. The amount of the hardener added may suitably be selected from the range of 0.20 to 3.0% by weight based on the weight of the primer layer, in accordance with the application method and the intended degree of hardening.

[0073] Depending upon the hardener used, if necessary, sodium hydroxide may be added to adjust the pH of the liquid to an alkaline range, or the pH may be made acidic by the addition of citric acid or the like. Further, an antifoaming agent may be added to eliminate the foam produced upon application of the primer layer, or an activator may be added to prevent the generation of application stripes by improving the liquid leveling.

[0074] Prior to application of the primer layer, it is desirable to treat the surface of the support in a known manner to activate the surface. Examples of methods of activation treatment are etching with an acid, flame treatment with a gas burner, and corona or glow discharge treatment. However, the corona discharge treatments described in U.S. Patent Nos. 2,715,075, 2,846,727, 3,549,406 and 3,590,107 are most preferably used in view of cost and convenience.

[0075] In the present invention, a protective layer containing a pigment is preferably provided on the heat sensitive layer in order to protect the heat sensitive layer from sticking or from solvents.

[0076] Examples of such a pigment are mica, talc, calcium carbonate, zinc oxide, titanium oxide, aluminum hydroxide, kaolin, agalmatolite, synthetic silicate, amorphous silica and urea-formalin resin powder. Among these pigments, calcium carbonate, aluminum hydroxide, kaolin, silica, mica and talc are particularly preferred.

[0077] The binder contained in the protective layer of the present invention is preferably a fully saponified polyvinyl alcohol, carboxy modified polyvinyl alcohol or silica modified polyvinyl alcohol in order to retain the pigment and improve the transparency.

[0078] The coating liquid of the protective layer is obtained by mixing a solution of the above binder with the pigment. If desired, other various additives such as lubricants (e.g., zinc stearate, calcium stearate, paraffin wax and polyethylene wax), dispersants, fluorescent whitening agents, crosslinking agents, surfactants containing sulfosuccinic acid alkali metal salt and fluorine, and polyoxyethylene surfactants may further be added.

[0079] The infrared laser heat sensitive recording material according to the present invention may be prepared by, for example, dispersing an organic silver salt represented by the general formula RCOOAg in an organic solvent, adding a polyvalent isocyanate to produce an oil phase, adding the oil phase into an aqueous solution of a water soluble polymer and stirring this mixture to emulsify and disperse the oil phase, heating to prepare a microcapsule mixture containing microcapsules encapsulating the organic silver salt, further adding a developer for the organic silver salt and other additive(s) to prepare a coating liquid for producing a heat sensitive layer, separately preparing a coating liquid for producing a protective layer such as that mentioned above, and applying the coating liquids on a support by bar coating, blade coating, air knife coating, gravure coating, roll coating, spray coating or dip coating followed by drying so as to provide a heat sensitive layer with a solid content of 2.5 to 25 g/m² and a protective layer with a solid content of 0.2 to 7 g/m².

[0080] The amount of organic silver salt and developer applied as the Ag component in the heat sensitive layer is preferably 0.5 to 3.0 g/m², and an amount of 0.8 to 2.0 g/m² is particularly preferable. The thickness of heat sensitive layer is desirably 1 to 20 µm.

[0081] The coating liquids used in the present invention may optionally contain a pigment dispersing agent, thickener, flow modifier, anti-foaming agent, foam suppresser, mold releasing agent, coloring agent, wax and/or hardener, if necessary, in so far as the characteristic properties of the present invention are not affected.

[0082] Further, if necessary, a backcoat layer may be provided on the side of the support of the infrared laser heat sensitive recording material which side is opposite the side at which the coloring layer is provided. The backcoat layer may be any of known backcoat layers for conventional infrared laser heat sensitive recording materials.

[0083] The laser beam used in the present invention has wave lengths in the near infrared region. Examples thereof are helium-neon, argon, carbonic acid gas, YAG and semiconductor lasers.

[0084] In accordance with the present invention, if the heat sensitive layer of the infrared laser heat sensitive recording material of the present invention contains the "tricarbocyanine dye" inside and/or outside of the microcapsule and/or within the wall of the microcapsule, the "tricarbocyanine dye" absorbs the irradiated laser beam and converts the energy into heat energy, whereby the microcapsule is heated to become permeable and the pressure therein is increased. As a result, the reactive materials inside and outside of the microcapsule pass through the microcapsule wall so as to contact each other, resulting in color formation.

[0085] The present invention will be further illustrated by way of the following examples, which are not to be construed as limiting the present invention. Unless otherwise specified, "percents" and "parts" are by weight.

[Example 1]

Preparation of silver behenate

[0086] Behenic acid (25.59 g) was added to 480 g of water, and the mixture was heated to 90 °C. An aqueous solution of 3.0 g NaOH in 45 g water was added thereto, and this mixture was stirred thoroughly and cooled to 50 °C.

[0087] To the resultant solution, an aqueous solution of 12.9 g AgNO₃ in 75 g water was added dropwise over 5 minutes, and the mixture was stirred for 30 minutes and the reaction was carried out.

[0088] The resultant reaction mixture was filtered through a filter cloth, and the solid remaining on the filter was placed in a container 800 ml of water was added to the container, and the resultant dispersion was stirred. This dispersion was filtered through the filter used previously. Such rinsing of the solid was repeated two more times and the resultant solid remaining on the filter was dried in a blast drier at 50 °C for 3 days to yield a dried solid of silver behenate.

Preparation of co-dispersion of silver behenate/phthalazone

[0089] From the thus prepared dry solid of silver behenate, 21.1 g thereof was weighed out. To this 21.1 g of silver behenate, 3.26 g of phthalazone, 52.8 g of 15% aqueous solution of polyvinyl alcohol (tradename: PVA205, manufactured by Kuraray Co., Ltd.) and 103 g of ion-exchanged water were added. The resultant mixture was dispersed by a paint shaker for 3 hours to yield a behenate/phthalazone co-dispersion with an average particle diameter of 0.6 µm or less.

Preparation of aqueous developer solution

[0090] An aqueous solution of a developer was prepared by adding 4.5 g of methyl gallate and 35.0 g of ion-exchanged water to 50.0 g of 22% polyvinyl alcohol (tradename: PVA203, manufactured by Kuraray Co., Ltd.) and stirring the mixture at 60 °C for one hour.

Preparation of heat sensitive layer coating liquid

[0091] A coating liquid for a heat sensitive layer was prepared by stirring and mixing 8.0 g of the silver behenate/phthalazone co-dispersion, 4.0 g of the aqueous developer solution, 1.9 g of 3% solution of benzotriazole in methanol, and 0.12 g of the tricarbocyanine dye representing above formula [A].

Preparation of protection layer coating liquid

[0092] 5 g of 2% aqueous solution of polyoxyethylene (surfactant) and 4 g of 20% dispersion of zinc stearate (tradename: Hydrin Z, manufactured by Chukyo Yushi KK) were added to a mixture of 32 g water, 32 g of 10% aqueous solution of carboxy modified polyvinyl alcohol (tradename: PVA-KL-318, manufactured by Kuraray Co., Ltd.), and 8 g of 30% dispersion of epoxy modified polyamide (tradename: FL-71, manufactured by Toho Kagaku KK), to prepare a coating liquid for a protective layer.

Preparation of infrared laser heat sensitive recording material

[0093] Onto a transparent polyethylene terephthalate support (130 µm in thickness) provided with a primer layer, the coating liquid for a heat sensitive layer was applied by means of a wire bar to a dry film thickness of 10 g/m² and dried at 50 °C for 20 minutes.

[0094] Onto the coating, the coating liquid for a protective layer was applied to a solid content of 2.0 g/m² and dried to prepare an infrared laser heat sensitive recording material.

[0095] The heat sensitive layer of the thus prepared infrared laser heat sensitive recording material was imagewise irradiated with a semiconductor infrared laser (GaAs junction laser) at a wave length of 950 nm to obtain a black recorded image. The laser power was adjusted such that the energy was 40 mJ/mm² per millisecond at the surface of the heat sensitive layer.

[0096] The transmission density of the colored portion of the recorded image was measured to be 2.52 by a Macbeth densitometer. The background of the infrared laser heat sensitive recording material was hardly colored.

[Example 2]

Preparation of capsule liquid enclosing silver behenate

[0097] 27 g of the previously prepared dry solid of silver behenate was weighed out, and 1.10 g of polyvinyl butyral and 110 g of isoamyl acetate were added to the 27 g of silver behenate. The resultant mixture was dispersed by a paint shaker for 3 hours. The dispersion was further subjected to a motor mill to prepare a silver behenate dispersion with an average particle diameter of 0.4 µm.

[0098] To 10 g of the resultant dispersion, 6 g of Takenate D-110N (tradename, manufactured by Takeda Chemical Industries, Ltd., the isocyanate compound having the above formula (a)) as a reactant was added to prepare an oil phase. This oil phase was added to a solution (aqueous phase) of 32 g of 10% aqueous solution of polyvinyl alcohol (tradename: PVA217E, manufactured by Kuraray Co., Ltd.) and 80 g of water. The resultant mixture was finely emulsified by means of a homogenizer (Ace Homogenizer, manufactured by Nippon Seiki Co., Ltd.) at 8,000 rpm for an additional 10 minutes.

[0099] The finely emulsified dispersion was heated to 65 °C while being stirred, and was maintained at this temperature for 3 hours to conduct the capsulating reaction. The solid weight ratio of the reactant to silver behenate was about 2.7. A microcapsule liquid having an average particle diameter of 0.8 µm and encapsulating silver behenate was obtained.

Preparation of coating liquid

[0100] A coating liquid was obtained by mixing 15 g of the capsule liquid encapsulating silver behenate, 4 g of water, 0.6 g of 3% solution of benzotriazole in methanol, 2.8 g of the aqueous developer solution and 0.4 g of the tricarbocyanine dye representing above formula [B].

Preparation of infrared laser heat sensitive recording material

[0101] As in Example 1, an infrared laser heat sensitive recording material was obtained by applying and drying said coating liquid to a dry thickness of 20 g/m² and applying and drying a protective layer to a dry thickness of 2.0 g/m² on the coating.

[0102] The heat sensitive layer of the thus prepared infrared laser heat sensitive recording material was imagewise irradiated with a semiconductor infrared laser (GaAs junction laser) at a wave length of 910 nm to obtain a blue recorded image. The laser power was adjusted such that the energy was 40 mJ/mm² per millisecond at the surface of the heat sensitive layer.

[0103] The transmission density of the colored portion of the recorded image was measured to be 2.38 by a Macbeth densitometer. The background of the infrared laser heat sensitive recording material was slightly colored to a light yellow-green, but not to an extent of creating problems in actual use.

[Example 3]

[0104] The procedures of Example 1 were repeated to prepare an infrared laser heat sensitive recording material except that the tricarbocyanine dye used in Example 1 was replaced by the tricarbocyanine dye representing above formula [C].

[0105] The heat sensitive layer of the thus prepared infrared laser heat sensitive recording material was imagewise irradiated with a semiconductor infrared laser (GaAs junction laser) at a wave length of 985 nm to obtain a black recorded image. The laser power was adjusted such that the energy was 40 mJ/mm² per millisecond at the surface of the heat sensitive layer.

[0106] The transmission density of the colored portion of the recorded image was measured to be 2.48 by a Macbeth densitometer. The background of the infrared laser heat sensitive recording material was hardly colored.

[Control 1]

[0107] The procedures of Example 1 were repeated to prepare an infrared laser heat sensitive recording material except that the tricarbocyanine dye used in Example 1 was replaced by the following infrared absorbing pigment. The transmission density of the colored portion of the recorded image was measured to be 1.94 by a Macbeth densitometer. The background of the infrared laser heat sensitive recording material was colored to a slight green.

[Control 2]

[0108] The procedure of Example 2 were repeated to prepare an infrared laser heat sensitive recording material except that the tricarbocyanine dye used in Example 2 was replaced by the following infrared absorbing pigment. The transmission density of the colored portion of the recorded image was measured to be 1.65 by a Macbeth densitometer. The background of the infrared laser heat sensitive recording material was colored to a slight blue-green.

[0109] Table 1 lists the maximum absorption wave lengths (λ_max, either in an aqueous solution or in the heat sensitive layer) and half band widths (absorption width at the maximum absorption wave length (λ_max)/2) of the dyes and pigments used, as well as the degrees of coloration of the infrared laser heat sensitive recording materials and transmission densities of the colored portions of the recorded images in Examples 1 to 3 and Controls 1 and 2.

Table 1

Sample	λmax in aqueous solution (nm)	λmax in heat sensitive layer (nm)	Half band width (nm)	Degree of coloration of background	Transmission density of colored portion of recorded image
Example 1	822	950	35	Almost no coloration	2.52
Example 2	780	910	33	Slight yellow-green but not to an extent of creating problems in actual use	2.38
Example 3	840	940	30	Almost no coloration	2.48
Control 1	940	940	115	Pale green	1.94
Control 2	920	920	108	Pale blue-green	1.65

[0110] As can be seen from Table 1, the Examples provide higher transmission densities (in other words, recorded images with better color formation) and lighter color of the background than Controls 1 and 2. That is to say, the Examples of the present invention provide recorded images with higher contrast. It can be seen from comparison between Example 1 and Control 1 or between Example 2 and Control 2, where only the dyes used are different from each other, that such effects are due to the tricarbocyanine dyes used. Further, from comparison of the maximum absorption wave length of the tricarbocyanine dye in an aqueous solution and after drying (i.e., in the heat sensitive layer), it can be seen that, for each dye, the maximum absorption wave length after drying is shifted toward the infrared region by at least 50 nm from the maximum absorption wave length in an aqueous solution, and that the maximum absorption wave length after drying is in the range of 650 to 1300 nm. Since the half band widths of the tricarbocyanine dyes used in the Examples are more narrow than those of the infrared absorbing pigments used in the Controls, the tricarbocyanine dyes used in the Examples provide sharper peaks at the maximum absorption wave lengths and better sensitivities.

Claims

1. An infrared laser heat sensitive recording material comprising:

a support; and

a heat sensitive layer which is provided on said support and comprises:

an organic silver salt;

a developer for said organic silver salt;

a water soluble binder; and

a tricarbocyanine dye having at least two acidic groups whose maximum absorption wave length is at least 50 nm longer than a maximum absorption wave length of an aqueous solution of said tricarbocyanine dye and is in a range of 650 to 1300 nm.

2. The infrared laser heat sensitive recording material of claim 1, wherein said tricarbocyanine dye is represented by the general formula:

wherein each of Z¹ and Z² is a non-metallic atomic group which forms a substituted or unsubstituted benzothiazole, benzoselenazole, indole, naphthothiazole, naphthoselenazole or benzindole ring; each of R¹ and R² is a substituted or unsubstituted alkyl group; each of R³ and R⁵ is a hydrogen atom or an atom needed to form a 5-membered ring when R³ and R⁵ are bonded to each other; R⁴ is a hydrogen atom or a monovalent group except any atomic group which forms a ring in a di-substituted amino group; X^- is an anion; and n is 1 or 2.

3. The infrared laser heat sensitive recording material of claim 1 or 2, wherein said tricarbocyanine dye is represented by the structural formula:

4. The infrared laser heat sensitive recording material of claim 1 or 2, wherein said tricarbocyanine dye is represented by the structural formula:

5. The infrared laser heat sensitive recording material of claims 1, 2 or 3, wherein said tricarbocyanine dye is represented by the structural formula:

6. The infrared laser heat sensitive recording material of any preceding claim, wherein at least one of said organic silver salt and said developer for said organic silver salt is encapsulated in a microcapsule.

7. The infrared laser heat sensitive recording material of claim 6, wherein said microcapsule is a heat responsive microcapsule.

8. The infrared laser heat sensitive recording material of claim 6 or 7, wherein said microcapsule is formed from a polyurethane.

9. The infrared laser heat sensitive recording material of claim 8, wherein said microcapsule is formed from a polyurethane obtained by a reaction of the following isocyanate and a polyvinyl alcohol: