Stabilizers for use in substantially light-insensitive thermographic recording materials

A substantially light-insensitive black and white monosheet thermographic recording material comprising a support and a thermosensitive element, the thermosensitive element containing a substantially light-insensitive organic silver salt, an organic reducing agent therefor in thermal working relationship therewith, a binder and at least one stabilizer selected from the group consisting of 1-(5-mercapto-1-tetrazolyl)-acetyl compounds represented by formula (I): wherein R1 is —NR2R3, —OR4 or an optionally substituted aryl or heteroaryl group; R2 is hydrogen or an optionally substituted alkyl, aryl or heteroaryl group; R3 is an optionally substituted aryl or heteroaryl group; and R4 is an optionally substituted aryl group; and compounds with two or more groups represented by formula (II): where Q comprises the necessary atoms to form a 5- or 6-membered unsaturated heterocyclic ring, A is hydrogen, a counterion to compensate the negative charge of the thiolate group or two or more A groups provide a linking group between the two or more groups represented by formula (II).

FIELD OF THE INVENTION

The present invention concerns stabilizers for use in substantially light-insensitive thermographic recording materials.

BACKGROUND OF THE INVENTION

Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step. In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material.

EP-A 0 713 133 discloses a thermal imaging system consisting of (i) a donor element comprising on a support a donor layer containing a binder and a thermotransferable reducing agent capable of reducing a silver source to metallic silver and (ii) a receiving element comprising on a support a receiving layer comprising a silver source, capable of being reduced by means of heat in the presence of a reducing agent, a binder and a stabiliser selected from the group consisting of benzotriazoles, heterocyclic mercaptanes, sulphinic acids, 1,3,4-triazo-indinolines, 1,3-dinitroaryl compounds, 1,2,3-triazoles, phthalic acids and phthalic acid derivatives. EP-A 0 713 133 discloses that preferred heterocyclic mercaptanes are mercaptotetrazoles corresponding to the following general formula (C):
and discloses the following 1-(5-mercapto-1-tetrazolyl)-acetyl compounds:

EP-A 0 901 040 discloses a substantially light-insensitive monosheet recording material comprising a support and a thermosensitive element containing a substantially light-insensitive organic silver salt, an organic reducing agent therefor in thermal working relationship therewith and a binder, characterized in that said thermosensitive element further contains an unsaturated carbocyclic or heterocyclic stabilizer compound substituted with a —SA group where A is hydrogen, a counterion to compensate the negative charge of the thiolate group or a group forming a symmetrical or an asymmetrical disulfide and said recording material is capable of producing prints with a numerical gradation value defined as the quotient of the fraction (2.5-0.1)/(E2.5-E0.1) greater than 2.3, where E2.5is the energy in Joule applied in a dot area of 87 μm×87 μm of the imaging layer that produces an optical density value of 2.5, and E0.1is the energy in Joule applied in a dot area of the imaging layer material that produces an optical density value of 0.1.

WO 94/16361 discloses a multilayer heat-sensitive material which comprises: a color-forming layer comprising: a color-forming amount of finely divided, solid colorless noble metal or iron salt of an organic acid distributed in a carrier composition; a color-developing amount of a cyclic or aromatic organic reducing agent, which at thermal copy and printing temperatures is capable of a color-forming reaction with the noble metal or iron salt; and an image-toning agent; characterized in that (a) the carrier composition comprises a substantially water-soluble polymeric carrier and a dispersing agent for the noble metal or iron salt and (b) the material comprises a protective overcoating layer for the color-forming layer. Furthermore, WO 94/16361 discloses that suitable antifoggants are well-known photographic anti-foggants such as mercaptobenzotriazole, chromate, oxalate, citrate, carbonate, benzotriazole (BZT), 5-methylbenzotriazole, 5,6-dimethylbenzotriazole, 5-bromobenzotriazole, 5-chlorobenzotriazole, 5-nitro-benzotriazole, 4-nitro-6-chlorobenzotriazole, 5-nitro-6-chlorobenzotriazole, 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene, benzimidazole, 2-methylbenzimidazole, 5-nitrobenzimidazole, 1-phenyl-5-mercaptotetrazole, 2-mercaptobenzimidazole, 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptothiazoline, 2-mercapto-4-methyl-6,6′-dimethylpyrimidine, 1-ethyl-2-mercapto-5-amino-1,3,4-triazole, 1-ethyl-5-mercapto-1,2,3,4-tetrazole, 2,5-dimercapto-1,3,4-thiodiazole, 2-mercapto-5-aminothiodiazole, dimethyldithiocarbamate, and diethyldithiocarbamate.

WO 96/10213 discloses a thermographic imaging element comprising a substrate having coated on at least one surface thereof a thermographic imaging system comprising at least one layer comprising light-insensitive organic silver salt; reducing agent for silver ion; binder; toner; and a dye which absorbs radiation in the wavelength range of 750-1100 nm, wherein said at least one layer comprising said light-insensitive organic silver salt forms an image density greater than about 1.0 when exposed to 0.10-2.0 joules/cm2of said radiation in 0.20 to 200 microseconds. WO 96/10213 does not disclose a stabilizer against the influence of light, but mentions the optional incorporation of benzotriazole in the thermographic imaging element, but only exemplifies the incorporation of benzotriazole.

Substantially light-insensitive thermographic recording materials contain the imaging-forming components both before and after image formation and unwanted image-forming must be hindered both during storage prior to printing and in prints exposed to light on light-boxes e.g. during examination by radiologists. Furthermore, such stabilization must take place without adverse effects upon the image quality particularly the image tone. Thermographic printers are being introduced with ever higher throughputs, which require thermographic recording materials able to provide stabilization without an adverse effect on the image quality at such faster throughputs. There is therefore a need for stabilizers which fulfil these requirements.

ASPECTS OF THE INVENTION

It is therefore an aspect of the present invention to provide stabilizers for use in substantially light-insensitive thermographic recording materials suitable for use in high throughput thermographic printers without adverse effect on the image tone.

Further aspects and advantages of the invention will become apparent from the description hereinafter.

SUMMARY OF THE INVENTION

It has been surprisingly found that specific types of 5-mercapto-tetrazole compounds provide effective stabilization in substantially light-insensitive thermographic recording materials suitable for use in high throughput thermographic printers without an adverse effect on the image tone as characterized by CIELAB a* and b* values. The L*, a* and b* CIELAB-values were determined by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90.

Aspects of the present invention are realized with a substantially light-insensitive black and white monosheet thermographic recording material comprising a support and a thermosensitive element, the thermosensitive element containing a substantially light-insensitive organic silver salt, an organic reducing agent therefor in thermal working relationship therewith, a is binder and at least one stabilizer selected from the group consisting of 1-(5-mercapto-1-tetrazolyl)-acetyl compounds represented by formula (I):
wherein R1is —NR2R3, —OR4or an optionally substituted aryl or heteroaryl group; R2is hydrogen or an optionally substituted alkyl, aryl or heteroaryl group; R3is an optionally substituted aryl or heteroaryl group; and R4is an optionally substituted aryl group; and compounds with two or more groups represented by formula (II):
where Q comprises the necessary atoms to form a 5- or 6-membered unsaturated heterocyclic ring, A is hydrogen, a counterion to compensate the negative charge of the thiolate group or two or more A groups provide a linking group between the two or more groups represented by formula (II).

Preferred embodiments of the present invention are disclosed in the detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

The term alkyl means all variants possible for each number of carbon atoms in the alkyl group i.e. for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethylpropyl, 2,2-dimethylpropyl and 2-methyl-butyl etc.

The term acyl group as used in disclosing the present invention means —(C═O)-aryl and —(C═O)-alkyl groups.

The L*, a* and b* CIELAB-values are defined in ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90.

Substantially light-insensitive means not intentionally light is sensitive.

Heating in association with the expression a substantially water-free condition as used herein, means heating at a temperature of 80 to 250° C. The term “substantially water-free condition” as used herein means that the reaction system is approximately in equilibrium with water in the air, and water for inducing or promoting the reaction is not particularly or positively supplied from the exterior to the element. Such a condition is described in T. H. James, “The Theory of the Photographic Process”, Fourth Edition, Macmillan 1977, page 374.

Thermosensitive Element

The term thermosensitive element as used herein is that element which contains all the ingredients which contribute to image formation. According to the present invention, the thermosensitive element contains one or more substantially light-insensitive organic silver salts, one or more reducing agents therefor in thermal working relationship therewith and a binder. The element may comprise a layer system in which the above-mentioned ingredients may be dispersed in different layers, with the proviso that the substantially light-insensitive organic silver salts are in reactive association with the reducing agents i.e. during the thermal development process the reducing agent must be present in such a way that it is able to diffuse to the particles of substantially light-insensitive organic silver salt so that reduction to silver can occur. Such materials include the possibility of one or more substantially light-insensitive organic silver salts and/or one of more organic reducing agents therefor being encapsulated in heat-responsive microcapsules, such as disclosed in EP-A 0 736 799 herein incorporated by reference.

The substantially light-insensitive black and white monosheet thermographic recording material of the present invention can contain at least one 1-(5-mercapto-1-tetrazolyl)-acetyl compound represented by formula (I):
wherein R1is —NR2R3, —OR4or an optionally substituted aryl or heteroaryl group; R2is hydrogen or an optionally substituted alkyl, aryl or heteroaryl group; R3is an optionally substituted aryl or heteroaryl group; and R4is an optionally substituted aryl group. Preferred substituents for the alkyl, aryl and heteroaryl groups include alkyl, ester, acyl, carbonato-alkyl, alkoxy, —S-alkyl, hydroxy and mercapto groups.

According to a first embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the at least one stabilizer is

The 1-(5-mercapto-1-tetrazolyl)-acetyl (MTA) compounds, according to the present invention, can be prepared from readily available starting materials using standard organic chemistry techniques known to one skilled in the art and available in such reference books such as Houben-Weyl.

Suitable 1-(5-mercapto-1-tetrazolyl)-acetyl (MTA) compounds according to formula (I), according to the present invention, include:

The substantially light-insensitive black and white monosheet thermographic recording material of the present invention can contain at least one compound with two or more groups represented by formula (II):
where Q comprises the necessary atoms to form a 5- or 6-membered unsaturated heterocyclic ring, A is hydrogen, a counterion to compensate the negative charge of the thiolate group or two or more A groups provide a linking group between the two or more groups represented by formula (II).

According to a second embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, at least one of the 5- or 6-membered unsaturated heterocyclic ring is a pyridine, a pyrazine, a pyrimidine, a triazine, a pyrrole, a 1,2,3-triazole, a 1,2,4-triazole, a tetrazole, an oxadiazole, a thiadiazole, an oxazole, an iso-oxazole, a thiazole, an iso-thiazole or an imidazole ring. Such rings may also be annelated with an aromatic ring system.

According to a third embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the at least one compound according to formula (II) is represented by formula (III):
(X1)n(X2)m(X3)p(X4)qL  (III)
wherein L is a linking group linking the one or more X1, X2, X3and X4groups; X1, X2and X3are independently
n, m, p and q are independently 0, 1, 2, 3 or 4; n+m+p+q=2, 3 or 4; R5and R6are independently hydrogen, alkyl, aryl, acyl, —NR7R8, —SR9or —S-A; R7and R8are independently hydrogen or an alkyl or acyl group; R9is an alkyl group; and A has the same meaning as in formula (II). L is preferably an optionally substituted alkyl group, an optionally substituted alkyl group in which at least one of the non-terminal main chain carbon atoms is substituted with an oxygen or a sulfur atom, an optionally substituted aryl group, an optionally substituted alkyl-aryl-alkyl group, an optionally substituted alkyl-heteroaryl-alkyl group, an optionally substituted alkyl-COO-alkyl-OOC-alkyl group, an optionally substituted alkyl-O—COO-alkyl group, an optionally substituted alkyl-CONH-alkyl group, or an optionally substituted alkyl-COO-aryl-OOC-alkyl group. Preferred substituents for the alkyl, aryl and heteroaryl groups include alkyl, alkoxy, —S-alkyl, hydroxy and mercapto groups.

According to a fourth embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the at least one compound according to formula (II) is represented by formula (IV):
wherein R5has the same meaning as in formula (III); the two R5groups may be the same or different; and L1is a linking group. L1is preferably an optionally substituted alkyl group, an optionally substituted alkyl group in which at least one of the non-terminal main chain carbon atoms is substituted with an oxygen or a sulfur atom, an optionally substituted aryl group, an optionally substituted alkyl-aryl-alkyl group, an optionally substituted alkyl-heteroaryl-alkyl group, an optionally substituted alkyl-COO-alkyl-OOC-alkyl group, an optionally substituted alkyl-O—COO-alkyl group, an optionally substituted alkyl-CONH-alkyl group, or an optionally substituted alkyl-COO-aryl-OOC-alkyl group. Preferred substituents for the alkyl, aryl and heteroaryl groups include alkyl, alkoxy, —S-alkyl, hydroxy and mercapto groups.

According to a fifth embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the at least one compound according to formula (II) is represented by formula (V):
wherein R6has the same meaning as in formula (III); the two R6groups may be the same or different; and L2is a linking group. L2is preferably an optionally substituted alkyl group, an optionally substituted alkyl group in which at least one of the non-terminal main chain carbon atoms is substituted with an oxygen or a sulfur atom, an optionally substituted aryl group, an optionally substituted alkyl-aryl-alkyl group, an optionally substituted alkyl-heteroaryl-alkyl group, an optionally substituted alkyl-COO-alkyl-OOC-alkyl group, an optionally substituted alkyl-O—COO-alkyl group, an optionally substituted alkyl-CONH-alkyl group, or an optionally substituted alkyl-COO-aryl-OOC-alkyl group. Preferred substituents for the alkyl, aryl and heteroaryl groups include alkyl, alkoxy, —S-alkyl, hydroxy and mercapto groups.

According to a sixth embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the at least one compound according to formula (II) is represented by formula (VI):
wherein L3is a linking group. L3is preferably an optionally substituted alkyl group, an optionally substituted alkyl group in which at least one of the non-terminal main chain carbon atoms is substituted with an oxygen or a sulfur atom, an optionally substituted aryl group, an optionally substituted alkyl-aryl-alkyl group, an optionally substituted alkyl-heteroaryl-alkyl group, an optionally substituted alkyl-COO-alkyl-OOC-alkyl group, an optionally substituted alkyl-O—COO-alkyl group, an optionally substituted alkyl-CONH-alkyl group, or an optionally substituted alkyl-COO-aryl-OOC-alkyl-group. Preferred substituents for the alkyl, aryl and heteroaryl groups include alkyl, alkoxy, —S-alkyl, hydroxy and mercapto groups.

According to a seventh embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the at least one compound according to formula (II) is represented by formula (VII):
wherein A has the same meaning as in formula (II) and L4is a linking group. L4is preferably an optionally substituted alkyl group, an optionally substituted alkyl group in which at least one of the non-terminal main chain carbon atoms is substituted with an oxygen or a sulfur atom, an optionally substituted aryl group, an optionally substituted alkyl-aryl-alkyl group, an optionally substituted alkyl-heteroaryl-alkyl group, an optionally substituted alkyl-COO-alkyl-OOC-alkyl group, an optionally substituted alkyl-O—COO-alkyl group, an optionally substituted alkyl-CONH-alkyl group, or an optionally substituted alkyl-COO-aryl-OOC-alkyl group. Preferred substituents for the alkyl, aryl and heteroaryl groups include alkyl, alkoxy, —S-alkyl, hydroxy and mercapto groups.

According to an eighth embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the at least one stabilizer is:

Compounds represented by formula (II), according to the present invention, can be prepared from readily available starting materials using standard organic chemistry techniques known to one skilled in the art and available in such reference books such as Houben-Weyl.

Suitable compounds represented by formula (II), according to the present invention, include:

Organic Silver Salt

According to a ninth embodiment of the substantially light-insensitive black and white monosheet thermographic recording material of the present invention, the organic silver salts are not double organic salts containing a silver cation associated with a second cation e.g. magnesium or iron ions.

According to a tenth embodiment of the substantially light-insensitive black and white monosheet thermographic recording material of the present invention, at least one of the organic silver salts is a substantially light-insensitive silver salt of an organic carboxylic acid.

According to an eleventh embodiment of the substantially light-insensitive black and white monosheet thermographic recording material of the present invention, at least one of the organic silver salts is a substantially light-insensitive silver salt of an aliphatic carboxylic acids known as a fatty acid, wherein the aliphatic carbon chain has preferably at least 12 C-atoms, e.g. silver laurate, silver palmitate, silver stearate, silver hydroxystearate, silver oleate and silver behenate, which silver salts are also called “silver soaps”. Other silver salts of an organic carboxylic acid as described in GB-P 1,439,478, e.g. silver benzoate, may likewise be used to produce a thermally developable silver image. Combinations of different silver salt of an organic carboxylic acids may also be used in the present invention, as disclosed in EP-A 964 300.

Organic silver salts may be dispersed by standard dispersion techniques. Ball mills, bead mills, microfluidizers, ultrasonic apparatuses, rotor stator mixers etc. have been found to be useful in this regard. Mixtures of organic silver salt dispersions produced by different techniques may also be used to obtain the desired thermographic properties e.g. of coarser and more finely ground dispersions of organic silver salts.

Reducing Agents

According to a twelfth embodiment of the black and white thermographic recording material, according to the present invention, the reducing agent is an organic compound containing at least one active hydrogen atom linked to O, N or C, such as is the case with, aromatic di- and tri-hydroxy compounds. 1,2-dihydroxybenzene derivatives, such as catechol, 3-(3,4-dihydroxyphenyl) propionic acid, 1,2-dihydroxybenzoic acid, gallic acid and esters e.g. methyl gallate, ethyl gallate, propyl gallate, tannic acid, and 3,4-dihydroxy-benzoic acid esters are preferred, with those described in EP-A 0 692 733 and EP-A 0 903 625 being particularly preferred.

Combinations of reducing agents may also be used that on heating become reactive partners in the reduction of the one or more substantially light-insensitive organic silver salt. For example, combinations of sterically hindered phenols with sulfonyl hydrazide reducing agents such as disclosed in U.S. Pat. No. 5,464,738; trityl hydrazides and formyl-phenyl-hydrazides such as disclosed in U.S. Pat. No. 5,496,695; trityl hydrazides and formyl-phenyl-hydrazides with diverse auxiliary reducing agents as disclosed in U.S. Pat. Nos. 5,545,505, 5,545,507 and 5,558,983; acrylonitrile compounds as disclosed in U.S. Pat. Nos. 5,545,515 and 5,635,339; and 2-substituted malonodialdehyde compounds as disclosed in U.S. Pat. No. 5,654,130.

Binder of the Thermosensitive Element

The film-forming binder of the thermosensitive element may be all kinds of natural, modified natural or synthetic resins or mixtures of such resins, in which the at least one organic silver salt can be dispersed homogeneously either in aqueous or solvent media: e.g. cellulose derivatives, starch ethers, galactomannan, polymers derived from α,β-ethylenically unsaturated compounds such as polyvinyl chloride, after-chlorinated polyvinyl chloride, copolymers of vinyl chloride and vinylidene chloride, copolymers of vinyl chloride and vinyl acetate, polyvinyl acetate and partially hydrolyzed polyvinyl acetate, polyvinyl alcohol, polyvinyl acetals that are made from polyvinyl alcohol as starting material in which only a part of the repeating vinyl alcohol units may have reacted with an aldehyde, preferably polyvinyl butyral, copolymers of acrylonitrile and acrylamide, polyacrylates, polymethacrylates, polystyrene and polyethylene or mixtures thereof.

Suitable water-soluble film-forming binders for use in thermographic recording materials according to the present invention are: polyvinyl alcohol, polyacrylamide, polymethacrylamide, polyacrylic acid, polymethacrylic acid, polyvinylpyrrolidone, polyethyleneglycol, proteinaceous binders, polysaccharides and water-soluble cellulose derivatives. A preferred water-soluble binder for use in the thermographic recording materials of the present invention is gelatine.

The binder to organic silver salt weight ratio is preferably in the range of 0.2 to 7, and the thickness of the thermosensitive element is preferably in the range of 5 to 50 μm. Binders are preferred which do not contain additives, such as certain antioxidants (e.g. 2,6-di-tert-butyl-4-methylphenol), or impurities which adversely affect the thermographic properties of the thermographic recording materials in which they are used.

Toning Agent

According to a thirteenth embodiment of the black and white monosheet thermographic recording material, according to the present invention, the thermosensitive element contains a toning agent, which enables a neutral black image tone to be obtained in the higher densities and neutral grey in the lower densities.

According to a fourteenth embodiment of the black and white monosheet thermographic recording material, according to the present invention, the thermosensitive element further contains a toning agent selected from the group consisting of phthalimides, phthalazinones, benzoxazine diones and naphthoxazine diones e.g. phthalimides and phthalazinones within the scope of the general formulae described in U.S. Pat. No. 4,082,901; the toning agents described in U.S. Pat. Nos. 3,074,809, 3,446,648 and 3,844,797; and the heterocyclic toner compounds of the benzoxazine dione or naphthoxazine dione type as disclosed in GB 1,439,478, U.S. Pat. Nos. 3,951,660 and 5,599,647, herein incorporated by reference.

According to a fifteenth embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the substantially light-insensitive thermographic material contains a thermosensitive element, the thermosensitive element containing one or more toning agents selected from the group consisting of phthalazinone, benzo[e] [1,3]oxazine-2,4-dione, 7-methyl-benzo[e] [1,3]oxazine-2,4-dione, 7-methoxy-benzo[e] [1,3]oxazine-2,4-dione and 7-(ethylcarbonato)-benzo[e] [1,3]oxazine-2,4-dione.

According to a sixteenth embodiment of the black and white monosheet thermographic recording material, according to the present invention, the thermographic recording material further contains an auxiliary antifoggant to obtain improved shelf-life and reduced fogging.

According to a seventeenth embodiment of the black and white monosheet thermographic recording material, according to the present invention, the thermographic recording material further contains an antifoggant selected from the group consisting of benzotriazole, substituted benzotriazoles and aromatic polycarboxylic acid such as ortho-phthalic acid, 3-nitro-phthalic acid, tetrachlorophthalic acid, mellitic acid, pyromellitic acid and trimellitic acid and anhydrides thereof.

According to an eighteenth embodiment of the black and white monosheet thermographic recording material, according to the present invention, the thermosensitive element further contains an optionally substituted benzotriazole.

Polycarboxylic Acids and Anhydrides Thereof

According to a nineteenth embodiment of the black and white monosheet thermographic recording material, according to the present invention, the thermosensitive element further contains at least one polycarboxylic acid and/or anhydride thereof in a molar percentage of at least 15 with respect to all the organic silver salt(s) present and in thermal working relationship therewith. The polycarboxylic acid may be aliphatic (saturated as well as unsaturated aliphatic and also cycloaliphatic) or an aromatic polycarboxylic acid, may be substituted and may be used in anhydride form or partially esterified on the condition that at least two free carboxylic acids remain or are available in the heat recording step.

Surfactants and Dispersants

Surfactants and dispersants aid the dispersion of ingredients which are insoluble in the particular dispersion medium. The substantially light-insensitive thermographic material used in the present invention may contain one or more surfactants, which may be anionic, non-ionic or cationic surfactants and/or one or more dispersants. Suitable dispersants are natural polymeric substances, synthetic polymeric substances and finely divided powders, e.g. finely divided non-metallic inorganic powders such as silica.

Support

According to a twentieth embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the support is transparent or translucent. It is preferably a thin flexible carrier made transparent resin film, e.g. made of a cellulose ester, e.g. cellulose triacetate, polypropylene, polycarbonate or polyester, e.g. polyethylene terephthalate. The support may be in sheet, ribbon or web form and subbed if needs be to improve the adherence to the thereon coated thermosensitive element. The support may be dyed or pigmented to provide a transparent coloured background for the image.

Protective Layer

According to a twenty-first embodiment of the substantially light-insensitive black and white monosheet thermographic recording material, according to the present invention, the thermosensitive element is provided with a protective layer. In general this protects the thermosensitive element from atmospheric humidity and from surface damage by scratching etc. and prevents direct contact of printheads or heat sources with the recording layers. Protective layers for thermosensitive elements which come into contact with and have to be transported past a heat source under pressure, have to exhibit resistance to local deformation and good slipping characteristics during transport past the heat source during heating. A slipping layer, being the outermost layer, may comprise a dissolved lubricating material and/or particulate material, e.g. talc particles, optionally protruding from the outermost layer. Examples of suitable lubricating materials are a surface active agent, a liquid lubricant, a solid lubricant or mixtures thereof, with or without a polymeric binder.

Coating Techniques

The coating of any layer of the substantially light-insensitive thermographic material used in the present invention may proceed by any coating technique e.g. such as described in Modern Coating and Drying Technology, edited by Edward D. Cohen and Edgar B. Gutoff, (1992) VCH Publishers Inc., 220 East 23rd Street, Suite 909 New York, N.Y. 10010, USA. Coating may proceed from aqueous or solvent media with overcoating of dried, partially dried or undried layers.

Thermographic Processing

Thermographic imaging is carried out by the image-wise application of heat either in analogue fashion by direct exposure through an image or by reflection from an image, or in digital fashion pixel by pixel either by using an infra-red heat source, for example with a Nd-YAG laser or other infra-red laser, with a substantially light-insensitive thermographic material preferably containing an infra-red absorbing compound, or by direct thermal imaging with a thermal head.

In thermal printing image signals are converted into electric pulses and then through a driver circuit selectively transferred to a thermal printhead. The thermal printhead consists of microscopic heat resistor elements, which convert the electrical energy into heat via Joule effect. The operating temperature of common thermal printheads is in the range of 300 to 400° C. and the heating time per picture element (pixel) may be less than 1.0 ms, the pressure contact of the thermal printhead with the recording material being e.g. 200-1000 g/linear cm, i.e. with a contact zone (nip) of 200 to 300 μm a pressure of 5000 to 50,000 g/cm2, to ensure a good transfer of heat.

In order to avoid direct contact of the thermal printing heads with the outermost layer on the same side of the support as the thermosensitive element when this outermost layer is not a protective layer, the image-wise heating of the recording material with the thermal printing heads may proceed through a contacting but removable resin sheet or web wherefrom during the heating no transfer of recording material can take place.

Activation of the heating elements can be power-modulated or pulse-length modulated at constant power. EP-A 654 355 discloses a method for making an image by image-wise heating by means of a thermal head having energizable heating elements, wherein the activation of the heating elements is executed duty cycled pulsewise. EP-A 622 217 discloses a method for making an image using a direct thermal imaging element producing improvements in continuous tone reproduction.

Image-wise heating of the recording material can also be carried out using an electrically resistive ribbon incorporated into the material. Image- or pattern-wise heating of the recording material may also proceed by means of pixel-wise modulated ultra-sound.

Industrial Application

Thermographic imaging can be used for the production of reflection type prints and transparencies, in particular for use in the medical diagnostic field in which black-imaged transparencies are widely used in inspection techniques operating with a light box.

The invention is illustrated hereinafter by way of comparative examples and invention examples. The percentages and ratios given in these examples are by weight unless otherwise indicated.

Subbing Layers on the Emulsion Side of the Support:

COMPARATIVE EXAMPLES 1 TO 3

The substantially light-insensitive thermographic materials of COMPARATIVE EXAMPLES 1 to 3 were prepared by coating a dispersion with the following ingredients in 2-butanone onto a 175 μm thick blue-pigmented polyethylene terephthalate support with CIELAB a*- and b*-values of −9.5 and −17.9 respectively subbed on the emulsion-coated side with subbing layer 01 giving layers after drying at 50° C. for 1 h in a drying cupboard with the compositions given in Table 1.

TABLE 1stabilizerR01R02T01T02S01S02Comparativeconc.AgBehmol %mol %mol %mol %mol %mol %examplemol %coverageBL5HPvsvsvsvsvsvsVLOilnr.typevs AgB[g/m2][g/m2]AgBAgBAgBAgBAgBAgB[g/m2][g/m2]1S03103.8915.1250305102250.170.0352MTA-C1103.8915.1250305102250.170.0353MTA-C2103.8915.1250305102250.170.035
The thermosensitive elements were then coated with an aqueous composition with the following ingredients, which was adjusted to a pH of 3.8 with 1N nitric acid, to a wet layer thickness of 85 μm and then dried at 50° C. for 15 minutes to produce a protective layer PRO-L with the composition:

Thermographic Printing

The substantially light-insensitive thermographic recording materials of COMPARATIVE EXAMPLES 1 to 3 were printed using a DRYSTAR™ 4500 printer from AGFA-GEVAERT with a resolution of 508 dpi which had been modified to operate at a printing speed of 14 mm/s and a line-time of 3.5 ms instead of 7.1 ms and in which the 75 μm long (in the transport direction) and 50 μm wide thermal head resistors were power-modulated to produce different image densities.

The maximum densities of the images (Dmax) measured through a visible filter with a MACBETH™ TR924 densitometer were all greater than 2.2.

Evaluation of Thermographic Properties

The image tone of fresh prints made with the substantially light-insensitive thermographic recording materials of COMPARATIVE EXAMPLES 1 to 3 was assessed on the basis of the L*, a* and b* CIELAB-values at optical densities, D, of 1.0 and 2.0 and the results given in Table 2.

Simulated long-term archivability tests were performed by heating prints made with the substantially light-insensitive thermographic recording materials of COMPARATIVE EXAMPLES 1 to 3 at 57° C. in 34% relative humidity in the dark for 3 days and determining the shifts in CIELAB a*- and b*-values. The results are also given in Table 2.

Light-box tests were performed by exposing the substantially light-insensitive thermographic materials of COMPARATIVE EXAMPLES 1 to 3 for 3 days on top of the white PVC window of a specially constructed light-box placed in a Votsch conditioning cupboard set at 30° C. and a relative humidity of 85%. Only a central area of the window 550 mm long by 500 mm wide was used for mounting the test materials to ensure uniform exposure.

The stainless steel light-box used was 650 mm long, 600 mm wide and 120 mm high with an opening 610 mm long and 560 mm wide with a rim 10 mm wide and 5 mm deep round the opening, thereby forming a platform for a 5 mm thick plate of white PVC 630 mm long and 580 mm wide, making the white PVC-plate flush with the top of the light-box and preventing light loss from the light-box other than through the white PVC-plate. This light-box was fitted with 9 Planilux? TLD 36W/54 fluorescent lamps 27 mm in diameter mounted length-wise equidistantly from the two sides, with the lamps positioned equidistantly to one another and the sides over the whole width of the light-box and with the tops of the fluorescent tubes 30 mm below the bottom of the white PVC plate and 35 mm below the materials being tested. The shifts in CIELAB a*- and b*-values at an optical density, D, of 1.0 and the shift in the CIELAB b*-value were determined for COMPARATIVE EXAMPLES 1 to 3 and the results are also given in Table 2.

COMPARATIVE EXAMPLES 4 TO 7 AND INVENTION EXAMPLES 1 TO 2

The substantially light-insensitive thermographic materials of COMPARATIVE EXAMPLES 4 to 7 and INVENTION EXAMPLES 1 to 2 were prepared by coating a dispersion with the following ingredients in 2-butanone onto the support described for COMPARATIVE EXAMPLES 1 to 3 giving layers after drying at 85° C. for 3 minutes in a drying cupboard with the compositions given in Table 3.

The thermographic properties of the substantially light-insensitive thermographic recording materials of COMPARATIVE EXAMPLES 4 to 7 and INVENTION EXAMPLES 1 to 2 were evaluated as described for COMPARATIVE EXAMPLES 1 to 3. The results are given in Table 4.

The results reported in Table 4 were generated with substantially light-insensitive thermographic recording materials with a different compositions from the substantially light-insensitive thermographic recording material on which the results reported in Table 2 were based, However, by comparing the results in the two table generated with substantially light-insensitive thermographic recording materials with the same stabilizer, the influence of this difference in composition can be estimated. The results for the substantially light-insensitive thermographic recording materials of COMPARATIVE EXAMPLE 1, COMPARATIVE EXAMPLE 4 and COMPARATIVE EXAMPLE 5 using stabilizer S03, benzotriazole are given in Table 5.

This comparison shows that the composition of substantially light-insensitive thermographic recording material used for COMPARATIVE EXAMPLES 7 to 18 and INVENTION EXAMPLES 3 to 18 gives:more negative b*-values at D=1.0;higher shifts in CIELAB values after 3 d/57° C./34% RH in the dark; andca. 2.0 lower shifts in CIELAB b*-values after light-box exposure;
compared with the composition of substantially light-insensitive thermographic recording materials used for COMPARATIVE EXAMPLES 1 to 6 and INVENTION EXAMPLES 1 and 2. Bearing this information in mind the results reported in Tables 2 and 4 can be considered as a whole.

In the CIELAB-system a negative CIELAB b*-value indicates a bluish tone which becomes increasingly bluer as b* becomes more negative and a positive b*-value indicates a yellowish image-tone becoming more yellow as b* becomes more positive. In terms of the visual perception of an image as a whole, the image tone of elements of the image with a density of 1.0 have a stronger effect than the image tone of elements with lower or higher optical density.

In evaluating image tone the image tone of the SCOPIX™ LT2B silver halide emulsion laser medical hardcopy film from AGFA-GEVAERT has been used as a benchmark:

If the results for substantially light-insensitive thermographic recording materials containing 1-(5-mercapto-1-tetrazolyl)-acetyl compounds (MTA's) reported in Tables 2 and 4 are considered as a whole, the following conclusions can be drawn:the substantially light-insensitive thermographic recording materials of COMPARATIVE EXAMPLE 2, 3 and 6 to 8 containing MTA-C1 to MTA-C5 respectively exhibited poor stability to light in the light-box test as shown by high shifts in CIELAB b*-values at D=1.0 and Dmin.
No prohibitive imaging characteristics were found for the thermographic recording materials of INVENTION EXAMPLES 1 and 2 containing MTA-1 and Compound 2 respectively, according to the present invention.

INVENTION EXAMPLES 3 AND 4

The substantially light-insensitive thermographic materials of INVENTION EXAMPLES 3 and 4 in which S03, benzotriazole, was used in the thermosensitive element in combination with various 1-(5-mercapto-tetrazolyl)-acetyl stabilizers were prepared by coating a dispersion with the following ingredients in 2-butanone onto a 175 μm thick blue-pigmented polyethylene terephthalate support with CIELAB a*- and b*-values of −9.5 and −17.9 respectively subbed on the emulsion-coated side with subbing layer 02 giving layers after drying at 50° C. for 1 h in a drying cupboard with the compositions given in Table 6. The thermosensitive elements were then provided with a protective layer as described for COMPARATIVE EXAMPLES 1 to 3.

The thermographic properties of the substantially light-insensitive thermographic recording materials of INVENTION EXAMPLES 3 and 4 were evaluated as described above for COMPARATIVE EXAMPLES 1 to 3. The results are given in Table 7.

The results of Table 7 show that the 1-(5-mercapto-1-tetrazolyl)-acetyl compound, MTA-1 and compound 1 with two groups represented by formula (II), according to the present invention, upon incorporation into the thermosensitive elements of substantially light-insensitive thermographic recording materials provide stabilization to exposure to light and acceptable image tone when used in combination with S03, benzotriazole.

COMPARATIVE EXAMPLES 8 AND 9 AND INVENTION EXAMPLES 5 TO 8

The substantially light-insensitive thermographic materials of COMPARATIVE EXAMPLES 8 and 9 and INVENTION EXAMPLES 5 to 8 were prepared by coating a dispersion with the following ingredients in 2-butanone onto the support as described for COMPARATIVE EXAMPLES 1 to 3 giving layers after drying at 85° C. for 3 minutes in a drying cupboard with the compositions given in Table 8.

The thermographic properties of the substantially light-insensitive thermographic recording materials of COMPARATIVE EXAMPLES 8 and 9 and INVENTION EXAMPLES 5 to 8 were evaluated as described for COMPARATIVE EXAMPLES 1 to 3. The results are given in Table 9.

The substantially light-insensitive recording materials of INVENTION EXAMPLES 5 to 8 containing the stabilizers Compound 3 to Compound 6 exhibited improved light box stability i.e. reduced shifts in CIELAB-values compared with the substantially light-insensitive recording materials of COMPARATIVE EXAMPLES 8 and 9, without a stabilizer and with benzotriazole respectively, particularly with respect to lower shifts in the CIELAB b*-value for Dmin.

The image tone characteristics of the fresh substantially light-insensitive recording materials of INVENTION EXAMPLES 5 to 8 and COMPARATIVE EXAMPLES 8 and 9 are comparable.

The marginally higher shifts in CIELAB b*-values for the substantially light-insensitive recording materials of INVENTION EXAMPLES 5 to 8 compared with the substantially light-insensitive recording materials of COMPARATIVE EXAMPLES 8 and 9, without a stabilizer and with benzotriazole respectively, is non-critical in medical applications.

The substantially light-insensitive recording materials of INVENTION EXAMPLES 5 to 8 containing the stabilizers Compound 3 to Compound 6 clearly exhibit improved overall stabilizing properties compared with the substantially light-insensitive recording materials of COMPARATIVE EXAMPLES 8 and 9, without a stabilizer and with benzotriazole respectively.

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Having described in detail preferred embodiments of the current invention, it will now be apparent to those skilled in the art that numerous modifications can be made therein without departing from the scope of the invention as defined in the following claims.