Substantially thermographic recording materials with improved stability

A thermographic recording material comprising a support and a thermosensitive element containing a substantially light-insensitive silver salt of an organic carboxylic acid, a reducing agent therefor in thermal working relationship therewith and at least one proteinaceous binder, wherein the thermosensitive element contains between 700 ppm and 5 ppm of a non-fluoro-halide ion with respect to the proteinaceous binders in the thermosensitive element and the thermographic recording material is thermally developable under substantially water-free conditions; and a process for the production thereof.

FIELD OF THE INVENTION
 The present invention relates to thermographic recording materials with
 improved stability to incident light and improved archivability.
 BACKGROUND OF THE INVENTION
 Thermal imaging or thermography is a recording process wherein images are
 generated by the use of thermal energy. In direct thermal thermography a
 visible image pattern is formed by image-wise heating of a recording
 material containing matter that by chemical or physical process changes
 colour or optical density. Such recording materials become
 photothermographic upon incorporating a photosensitive agent which after
 exposure to UV, visible or IR light is capable of catalyzing or
 participating in a thermographic process bringing about changes in colour
 or optical density.
 Examples of photothermographic materials are the so called "Dry Silver"
 photographic materials of the 3M Company, which are reviewed by D. A.
 Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43,
 published by Marcel Dekker in 1991.
 In U.S. Pat. No. 2,910,377 the following statement is made in the
 description in column 7, lines 23-27: "Stability towards exposure to light
 is improved by selecting highly purified materials; freedom from halides
 and sulphides is particularly important in the case of compositions
 involving silver salts". The disclosure in U.S. Pat. No. 2,910,377
 concerned thermographic recording materials coated from solvent media.
 WO 94/16361 discloses a multilayer heat-sensitive material which comprises:
 a colour-forming layer comprising: a colour-forming amount of finely
 divided, solid colourless noble metal or iron salt of an organic acid
 distributed in a carrier composition; a colour developing amount of a
 cyclic or aromatic organic reducing agent, which at thermal copy and
 printing temperatures is capable of a colour-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
 colour-forming layer. WO 94/16361 concerns thermographic materials coated
 from aqueous media.
 Ever tighter solvent emission regulations and measures to avoid solvent
 explosions, make the avoidance of solvent coating desirable. However,
 thermographic materials of the type disclosed in WO 94/16361 while being
 coatable from aqueous media exhibit an inadequate archivability for many
 applications. Furthermore, the presence of chloride ions in the
 ingredients has been found to cause poor light stability. There is
 therefore a need for thermographic recording materials coatable from
 aqueous media based on substantially light-insensitive organic silver
 salts with improved shelf-life and stability to light, whose prints
 exhibit improved archivability and stability to incident light.
 OBJECTS OF THE INVENTION
 It is therefore an object of the present invention to provide thermographic
 recording materials coated from aqueous media with improved stability to
 incident light.
 It is therefore another object of the present invention to provide
 thermographic recording materials which are capable of producing
 thermographic prints with improved archivability and stability to incident
 light.
 Further objects and advantages of the invention will become apparent from
 the description hereinafter.
 SUMMARY OF THE INVENTION
 It is known that conversion of organic silver salts into silver
 non-fluoro-halides renders thermographic materials photosensitive, since
 this is the basis of photothermographic materials. This conversion would
 be expected to occur more readily in aqueous media due to the
 non-fluoro-halide ions being more mobile in a highly polar medium such as
 water. The statement made in U.S. Pat. No. 2,910,377 to the effect that
 the use of highly purified materials improves the light-stability of
 thermographic materials and in particular freedom from halides and
 sulphides, concerns thermographic materials coated in solvent media in
 which the mobility of non-fluoro-halide ions is much lower than in water.
 It is therefore surprising that in the presence of gelatin and despite the
 greater potential for silver halide formation in aqueous media, the
 expected light instability due to non-fluoro-halide ions only becomes
 significant, relative to the general stability of the material concerned
 (dependent upon choice of reducing agent and other ingredients), at
 non-fluoro-halide ion concentrations above 700 ppm with respect to the
 gelatin present. This invention enables the use of ingredients in
 thermographic materials without the exhaustive removal of
 non-fluoro-halides.
 The above objects of the present invention are realized by providing a
 thermographic recording material comprising a support and a
 thermosensitive element containing a substantially light-insensitive
 silver salt of an organic carboxylic acid, a reducing agent therefor in
 thermal working relationship therewith and at least one proteinaceous
 binder, wherein the thermosensitive element contains between 700 ppm and 5
 ppm of a non-fluoro-halide ion with respect to the proteinaceous binders
 in the thermosensitive element and the thermographic recording material is
 thermally developable under substantially water-free conditions.
 A process for producing a thermographic recording material as described
 above is further provided by the present invention comprising the steps
 of: producing an aqueous dispersion of the substantially light-insensitive
 silver salt of an organic carboxylic acid; producing one or more aqueous
 coating compositions containing together the aqueous dispersion of the
 substantially light-insensitive silver salt of an organic carboxylic acid,
 the reducing agent and the proteinaceous binder(s); and applying the one
 or more aqueous coating compositions to the support thereby forming after
 drying the thermosensitive element.
 Preferred embodiments of the present invention are disclosed in the
 detailed description of the invention.
 DETAILED DESCRIPTION OF THE INVENTION
 In a preferred embodiment the substantially light-insensitive thermographic
 recording materials of the present invention are black and white
 thermographic recording materials.
 Definitions
 The term aqueous for the purposes of the present invention includes
 mixtures of water with water-miscible organic solvents such as alcohols
 e.g. methanol, ethanol, 2-propanol, butanol, iso-amyl alcohol etc.;
 glycols e.g. ethylene glycol; glycerine; N-methyl pyrrolidone;
 methoxypropanol; and ketones e.g. 2-propanone and 2-butanone etc.
 By substantially light-insensitive is meant not intentionally light
 sensitive. By substantially solvent-free aqueous medium is meant that
 solvent, if present, is present in amounts below 10% by volume of the
 aqueous medium.
 Heating in a substantially water-free condition as used herein, means
 heating at a temperature of 80 to 250.degree. C. The term "substantially
 water-free condition" 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.
 Non-fluoro-halide Ion Concentration in the Thermosensitive Element
 According to the present invention a thermographic recording material is
 provided comprising a support and a thermosensitive element containing a
 substantially light-insensitive silver salt of an organic carboxylic acid,
 a reducing agent therefor in thermal working relationship therewith and at
 least one proteinaceous binder, characterized in that the thermosensitive
 element contains between 700 ppm and 5 ppm of a non-fluoro-halide ion with
 respect to the proteinaceous binders in the thermosensitive element. In a
 preferred embodiment the non-fluoro-halide ion concentration in the
 thermosensitive element is between 500 ppm and 5 ppm of a
 non-fluoro-halide with respect to the proteinaceous binders in the
 thermosensitive element, with between 300 ppm and 5 ppm of a
 non-fluoro-halide ion with respect to the proteinaceous binders in the
 thermosensitive element being particularly preferred and between 150 ppm
 and 5 ppm being especially preferred. The non-fluoro-halide ion is
 preferably the chloride ion.
 Proteinaceous Binders
 The non-fluoro-halide ions present in the thermosensitive element may be
 non-exclusively or exclusively present in the proteinaceous binder(s) used
 in the thermosensitive element of the thermographic and photothermographic
 recording materials of the present invention. Therefore the proteinaceous
 binders in the thermosensitive element may together contain between 700
 ppm and 5 ppm of non-fluoro-halide ions and preferably between 500 ppm and
 5 ppm and particularly preferably between 300 ppm and 5 ppm and especially
 between 150 ppm and 5 ppm.
 The alkali metal ion concentration of the proteinaceous binder(s) used in
 the thermosensitive element of the thermographic and photothermographic
 recording materials of the present invention together of 100 ppm or less.
 Suitable proteinaceous binders include gelatin, modified gelatins such as
 phthaloyl gelatin, zein etc, with gelatin being preferred. Table 1 shows
 that the chloride ion concentration present in gelatin as determined by
 ion chromatography using a DIONEX QIC ANALYSER ion chromatograph varies
 according to gelatin type from 5300 to 17 ppm:
 TABLE 1
 chloride ion sodium ion
 GELATIN general concentration concentration
 type description [ppm] [ppm]
 GEL01 low viscosity 5300 --
 GEL02 hydrolyzed gelatin 2900 1700
 GEL03 calcium-free, low viscosity 1270 --
 GEL04 calcium-free, medium 17 &lt;100
 viscosity
 GEL05 calcium-free, low viscosity &lt;40 2600
 GEL06* calcium-free, low viscosity &lt;40 &lt;100
 GEL07 calcium-containing, .ltoreq.250# --
 medium viscosity
 GEL08 calcium-free, high viscosity .ltoreq.200# --
 GEL09 calcium-free, medium .ltoreq.150# --
 viscosity
 GEL10 calcium-containing, low 150-300# --
 viscosity
 *type 17881, a gelatin with low potassium ion, sodium ion and chloride-ion
 concentrations from AGFA-GEVAERT GELATINEFABRIEK vorm. KOEPFF & SOHNE
 #specification
 Thermosensitive Element
 According to the present invention, a substantially light-insensitive
 thermographic recording material is provided comprising a thermosensitive
 element containing a substantially light-insensitive silver salt of an
 organic carboxylic acid, an organic reducing agent therefor in thermal
 working relationship therewith and a binder. The thermosensitive element
 may comprise a layer system in which the ingredients are dispersed in
 different layers, with the proviso that the substantially
 light-insensitive silver salt of an organic carboxylic acid and the
 organic reducing agent are in thermal working relationship with one
 another 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 silver salt of an organic carboxylic
 acid so that reduction of the silver salt of an organic carboxylic acid
 can take place. The thickness of the thermosensitive element is preferably
 in the range of 1 to 50 .mu.m.
 In a preferred embodiment of the present invention the thermosensitive
 element further contains a photosensitive silver halide, making
 thermographic recording material photothermographic.
 Silver Salts of an Organic Carboxylic Acid
 Preferred substantially light-insensitive silver salts of an organic
 carboxylic acid used in the present invention are silver salts of
 aliphatic carboxylic acids known as fatty acids, 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.
 Auxiliary Film-forming Binders of the Thermosensitive Element
 Suitable water-dispersible binders for use as auxiliary binders in the
 thermographic and photothermographic recording materials of the present
 invention may be any water-insoluble polymer It should be noted that there
 is no clear cut transition between a polymer dispersion and a polymer
 solution in the case of very small polymer particles resulting in the
 smallest particles of the polymer being dissolved and those slightly
 larger being in dispersion. Preferred water-dispersible binders for use
 according to the present invention are water-dispersible film-forming
 polymers with covalently bonded ionic groups selected from the group
 consisting of sulfonate, sulfinate, carboxylate, phosphate, quaternary
 ammonium, tertiary sulfonium and quaternary phosphonium groups. Further
 preferred water-dispersible binders for use according the present
 invention are water-dispersible film-forming polymers with covalently
 bonded moieties with one or more acid groups.
 Thermal Solvents
 The above mentioned binders or mixtures thereof may be used in conjunction
 with waxes or "heat solvents" also called "thermal solvents" or
 "thermosolvents" improving the reaction speed of the redox-reaction at
 elevated temperature.
 Organic Reducing Agents
 Suitable organic reducing agents for the reduction of silver salt of an
 organic carboxylic acid particles are organic compounds containing at
 least one active hydrogen atom linked to O, N or C.
 Catechol-type reducing agents, i.e. reducing agents containing at least one
 benzene nucleus with two hydroxy groups (--OH) in ortho-position are
 preferred with those described in EP-B 692 733 and EP-A 903 625 being
 particularly preferred. Other suitable reducing agents are sterically
 hindered phenols, bisohenols and sulfonamidophenols.
 Combinations of reducing agents may also be used that on heating become
 reactive partners in the reduction of the substantially light-insensitive
 silver salt of an organic carboxylic acid. 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 such 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.
 Toning Agents
 In order to obtain a neutral black image tone in the higher densities and
 neutral grey in the lower densities, the thermographic and
 photothermographic recording materials according to the present invention
 may contain one or more toning agents. The toning agents should be in
 thermal working relationship with the substantially light-insensitive
 silver salt of an organic carboxylic acid and reducing agents during
 thermal processing. Any known toning agent from thermography or
 photothermography may be used. Suitable toning agents are the phthalimides
 and phthealazinones within the scope of the general formulae described in
 U.S. Pat. No. 4,082,901 and the toning agents described in U.S. Pat. Nos.
 3,074,809, 3,446,648 and 3,844,797. Particularly useful toning agents are
 the heterocyclic toner compounds of the benzoxazine dione or naphthoxazine
 dione type described in GB-P 1,439,478, U.S. Pat. Nos. 3,951,660 and
 5,599,647.
 Stabilizers and Antifoggants
 In order to obtain improved shelf-life and reduced fogging, stabilizers and
 antifoggants may be incorporated into the thermographic recording
 materials of the present invention.
 Polycarboxylic Acids and Anhydrides Thereof
 According to the recording material of the present invention the
 thermosensitive element may comprise in addition at least one
 polycarboxylic acid and/or anhydride thereof in a molar percentage of at
 least 15 with respect to all the silver salt of an organic carboxylic
 acid(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.
 These acids may be substituted e.g. with alkyl, hydroxyl, nitro or
 halogen. They 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 are surface active agents which are soluble compounds which
 reduce the interfacial tension between a liquid and a solid. The
 thermographic and photothermographic recording materials of the present
 invention may contain anionic, non-ionic or amphoteric surfactants e.g.:

Surfactant Nr. S01 = ammonium dodecylphenylsultonate;
 Surfactant Nr. S02 = N, N-dimethyl-N-hexadecyl-ammonio-acetic acid;
 Surfactant Nr. S03 = MARLON .TM. A-365, supplied as a 65%
 concentrate of a sodium alkyl-phenylsulfonate by
 HULS.
 Surfactant Nr. S04 = AKYPO .TM. OP 80, supplied by CHEMY as an
 80% concentrate of an octyl-phenyl-oxy-
 polyethylene-glycol (EO 8) acetic acid;
 Surfactant Nr. S05 = hexadecyl-dimethylammonium acetic acid;
 Surfactant Nr. S06 = acid form of ULTRAVO .TM. W from
 CIBA-GEIGY;
 Surfactant Nr. S07 = ULTRAVON .TM. W, an aryl sulfonate from
 CIBA-GEIGY
 Surfactant Nr. S08 = ARKO .TM. N060 (previously
 HOSTA .TM. W), a nonylphenylpolyethylene-
 glycol from HOECHST
 Surfactant Nr. S09 = SAPONINE QUILAYA, containing 10% of
 saponines, 15% of tannins, 11% of calcium oxalate
 and 64% of starch from SCHMITTMANN;
 Surfactant Nr. S10 = NIAPROOF ANIONIC .TM. 4, supplied as a 27%
 concentrate of a sodium 1-(2'-ethylbutyl)-4-
 ethylhexylsulphate by NIACET;
 Surfactant Nr. S11 = ammonium salt of perfluoro-octanoic acid.
 Suitable dispersants are natural polymeric substances, synthetic polymeric
 substances and finely divided powders, for example finely divided
 non-metallic inorganic powders such as silica.
 Other Ingredients
 In addition to the ingredients the substantially light-insensitive
 thermographic recording material may contain other additives such as free
 fatty acids, silicone oil, ultraviolet light absorbing compounds, white
 light reflecting and/or ultraviolet radiation reflecting pigments, silica,
 and/or optical brightening agents.
 Support
 The support for the substantially light-insensitive thermographic recording
 material according to the present invention may be transparent,
 translucent or opaque and is preferably a thin flexible carrier made e.g.
 from paper, polyethylene coated paper or 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. The support may be subbed with a
 subbing layer. It may also be made of an opacified resin composition.
 Protective Layer
 In a preferred embodiment of the thermographic recording material according
 to the present invented the thermosensitive element is provided with a
 protective layer. A protective layer 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. In a
 particularly preferred embodiment of the thermographic recording material
 of the present invention, the protective layer is exclusive of
 proteinaceous binders.
 The protective layer may comprise a dissolved lubricating material and/or
 particulate material, e.g. talc particles, optionally protruding
 therefrom. Examples of suitable lubricating materials are a surface active
 agent, a liquid lubricant, a solid lubricant or mixtures thereof, which
 may be used with or without a polymeric binder.
 Layer on Opposite Side of the support to the Thermosensitive Element
 The thermographic recording material according to the present invention may
 be provided with a layer containing a second proteinaceous binder on the
 opposite side of the support to the thermosensitive element protective
 layer.
 Photosensitive Silver Halide
 The photosensitive silver halide used in the present invention may be
 employed in a range of 0.1 to 100 mol percent; preferably, from 0.2 to 80
 mol percent; particularly preferably from 0.3 to 50 mol percent;
 especially preferably from 0.5 to 35 mol %; and especially from 1 to 12
 mol % of substantially light-insensitive organic silver salt.
 The silver halide may be any photosensitive silver halide such as silver
 bromide, silver iodide, silver chloride, silver bromoiodide, silver
 chlorobromoiodide, silver chlorobromide etc. The silver halide may be in
 any form which is photosensitive including, but not limited to, cubic,
 orthorhombic, tabular, tetrahedral, octagonal etc. and may have epitaxial
 growth of crystals thereon.
 The silver halide used in the present invention may be employed without
 modification. However, it may be chemically sensitized with a chemical
 sensitizing agent such as a compound containing sulphur, selenium,
 tellurium etc., or a compound containing gold, platinum, palladium, iron,
 ruthenium, rhodium or iridium etc., a reducing agent such as a tin halide
 etc., or a combination thereof. The details of these procedures are
 described in T. H. James, "The Theory of the Photographic Process", Fourth
 Edition, Macmillan Publishing Co. Inc., New York (1977), Chapter 5, pages
 149 to 169.
 Spectral Sensitization
 The photosensitive silver halide in the photo-addressable thermally
 developable element of the photothermographic recording material,
 according to the present invention, may be spectrally sensitized with a
 spectral sensitizer, optionally together with a supersensitizer.
 Antihalation Dyes
 The thermographic recording materials used in the present invention may
 also contain antihalation or acutance dyes which absorb light which has
 passed through the photosensitive thermally developable photographic
 material, thereby preventing its reflection. Such dyes may be incorporated
 into the photosensitive thermally developable photographic material or in
 any other layer of the photographic material of the present invention.
 Coating
 The coating of any layer of the substantially light-insensitive
 thermographic recording materials of 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.
 Thermographic Printing
 Thermographic imaging is carried out by the image-wise application of heat
 either in analogue fashion by direct exposure through an image of 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, or by direct thermal imaging with a thermal head. Heating
 takes place in a substantially water-free condition.
 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 via the Joule effect into
 heat, which is transferred to the surface of the thermographic recording
 material wherein the chemical reaction resulting in the development of a
 black and white image takes place. Such thermal printing heads may be used
 in contact or close proximity with the recording layer. The operating
 temperature of common thermal printheads is in the range of 300 to
 400.degree. 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. 200-500 g/cm.sup.2 to ensure a good transfer
 of heat.
 In order to avoid direct contact of the thermal printing heads with a
 recording layer not provided with an outermost protective layer, the
 image-wise heating of the recording layer 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.
 The image signals for modulating the laser beam or current in the
 micro-resistors of a thermal printhead are obtained directly e.g. from
 opto-electronic scanning devices or from an intermediary storage means,
 optionally linked to a digital image work station wherein the image
 information can be processed to satisfy particular needs. Activation of
 the heating elements can be power-modulated or pulse-length modulated at
 constant power. EP-A 654 355 describes 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.
 When used in thermographic recording operating with thermal printheads the
 thermographic recording materials are not suitable for reproducing images
 with fairly large number of grey levels as is required for continuous tone
 reproduction. 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 thermographic recording material can also be
 carried out using an electrically resistive ribbon incorporated into the
 material. Image- or pattern-wise heating of the thermographic recording
 material may also proceed by means of pixelwise modulated ultra-sound,
 using e.g. an ultrasonic pixel printer as described e.g. in U.S. Pat. No.
 4,908,631.
 Photothermographic Printing
 Photothermographic recording materials, according to the present invention,
 may be exposed with radiation of wavelength between an X-ray wavelength
 and a 5 microns wavelength with the image either being obtained by
 pixel-wise exposure with a finely focused light source, such as a CRT
 light source; a UV, visible or IR wavelength laser, such as a He/Ne-laser
 or an IR-laser diode, e.g. emitting at 780 nm, 830 nm or 850 nm; or a
 light emitting diode, for example one emitting at 659 nm; or by direct
 exposure to the object itself or an image therefrom with appropriate
 illumination e.g. with UW, visible or IR light.
 For the thermal development of image-wise exposed photothermographic
 recording materials, according to the present invention, any sort of heat
 source can be used that enables the recording materials to be uniformly
 heated to the development temperature in a time acceptable for the
 application concerned e.g. contact heating, radiative heating, microwave
 heating etc.
 Industrial Application
 Thermographic recording materials according to the present invention may be
 used for both the production of transparencies, for example in the medical
 diagnostic field in which black-imaged transparencies are widely used in
 inspection techniques operating with a light box, and reflection type
 prints, for example in the hard copy field. For such applications the
 support will be transparent or opaque, i.e. having a white light
 reflecting aspect. Should a transparent base be used, the base may be
 colourless or coloured, e.g. with a blue colour for medical diagnostic
 applications.
 The following examples and comparative examples illustrate the present
 invention. The percentages and ratios used in the examples and
 compositions of the ingredients are by weight unless otherwise indicated.
 i) backing layer ingredients:
 KELZAN.TM. S, a xanthan gum from MERCK & CO., Kelco Division, USA, which
 according to Technical Bulletin DB-19 is a polysaccharide containing
 mannose, glucose and glucuronic repeating units as a mixed potassium,
 sodium and calcium salt;
 PERAPRE.TM. PE40, a 40% aqueous dispersion of polyethylene latex from BASF;
 LATEX02, a 20% by weight dispersion of polymethyl methacrylate with an
 average particle size of 88.8 nm prepared as described in U.S. Pat. No.
 5,354,613;
 LATEX03, a 15% dispersion of a terpolymer of 18 mol % methyl acrylate, 79
 mol % potassium salt of acrylic acid and 3 mol % TAOE;
 LATEX04, a 20% dispersion of a 1 .mu.m polymethylmethacrylate latex;
 KIESELSOL.TM. 100F, a colloidal silica from BAYER;
 KIESELSOL.TM. 300F, a colloidal silica from BAYER;
 PLEXIGUM.TM. M345, a polymethylmethacrylate type from ROHM;
 ii) thermosensitive element ingredients (in addition to those mentioned
 above):

INVENTION EXAMPLES 1 TO 3
 Thermographic Composition I
 Preparation of Silver Behenate Dispersions
 Silver behenate was added with stirring to an aqueous solution of ammonium
 dodecylsulfonate (Surfactant Nr. SOI) and the mixtures stirred for 30
 minutes with a KOTTHOFF.TM. stirrer. The resulting dispersions were then
 ball-milled to obtain a finely divided 18.5% by weight aqueous dispersion
 of silver behenate with 1 g of a 0.1 g of ammonium dodecylsulfonate per g
 silver behenate.
 Preparation of the Thermographic Recording Materials
 3.23 g of GEL05 (gelatin) was allowed to swell in 15.986 g of deionized
 water for 30 minutes and the swollen GEL05 was heated up to 36.degree. C.
 The following ingredients were then added with stirring: 4.434 g of a 20%
 aqueous solution of T02 followed by 5 minutes stirring, then 24.20 g of
 the silver behenate dispersion at a temperature of 36.degree. C. followed
 by 10 minutes stirring, then 11.150 g of an aqueous solution containing
 5.55% of boric acid, 8.17% of R01 and 15.23% of ethanol was added and
 finally 1.0 g of an aqueous solution containing 19.2% of formaldehyde and
 6.75% of methanol. The dispersions for INVENTION EXAMPLES 1 to 3 contained
 the concentrations of chloride and sodium ions with respect to the gelatin
 present given in table 2.
 The resulting silver behenate dispersions were then doctor blade-coated
 onto a 175 .mu.m thick subbed polyethylene terephthalate support to
 produce the coating weights of silver given in table 2.
 Thermographic Printing
 The printer was equipped with a thin film thermal head with a resolution of
 300 dpi and was operated with a line time of 19 ms (the line time being
 the time needed for printing one line). During this line time the
 printhead received constant power. The average printing power, being the
 total amount of electrical input energy during one line time divided by
 the line time and by the surface area of the heat-generating resistors was
 1.6 mJ/dot being sufficient to obtain maximum optical density in each of
 the substantially light-insensitive thermographic recording materials of
 INVENTION EXAMPLES 1 to 3.
 The maximum densities, D.sub.max, and minimum densities, D.sub.min, of the
 prints given in table 2 were measured through visible or blue filters with
 a MACBETH.TM. TR924 densitometer in the grey scale step corresponding to
 data levels of 64 and 0 respectively and are given in table 2.
 Light Box Test
 The stability of the image background of the prints made with the
 substantially light-insensitive thermographic recording materials of
 INVENTION EXAMPLES 1 to 3 was evaluated on the basis of the change in
 minimum (background) density measured through a blue filter using a
 MACBETH.TM. TR924 densitometer upon exposure on top of the white PVC
 window of a specially constructed light-box placed for 3 days in a VOTSCH
 conditioning cupboard set at 30.degree. C. and a relative humidity (RH) 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.TM. 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 results are summarized in table 2.
 The results of the thermographic evaluation of the thermographic recording
 material of INVENTION EXAMPLES 1 to 3 show no significant
 photo-instability in the light-box test indicating that up to a chloride
 ion concentration of 201 ppm with respect to the gelatin present there is
 no adverse effect of the chloride ion content upon the light stability of
 thermographic recording materials with the very stable THERMOGRAPHIC
 COMPOSITION I used.
 TABLE 2
 concentrations of ions with Light
 box:
 Invention AgBeh respect to gelatin present fresh
 .DELTA.D.sub.max /.DELTA.D.sub.min blue
 example coverage [Cl.sup.- ] [Na.sup.- ] D.sub.max D.sub.min
 after 3 days at
 number [g/m.sup.2 ] gelatin [ppm] [ppm] blue blue 30.degree. C./85%
 RH
 1 3.4 GEL05 201 44 2.26 0.05
 +0.14/+0.01
 2 3.7 GEL05 167 22 2.63 0.05
 -0.07/+0.01
 3 3.7 GEL05 140 4 2.43 0.05 +0.22/+0.02
 COMATIVE EXAMPLES 1 & 2 AND INVENTION EXAMPLES 4 to 6
 Thermographic Composition II
 Preparation of a Tone Modifier Dispersion
 The tone modifier dispersion was prepared by first dissolving 8.8 g of
 GEL05 in 71.4 g of deionized water by first adding the gelatin, then
 allowing the gelatin to swell for 30 minutes and finally heating to
 50.degree. C. 20 g of T01 was added with ULTRA-TURRAX.TM. stirring to this
 gelatin solution at 50.degree. C., and the stirring continued for a
 further 5 minutes. Finally the resulting dispersion was pumped through a
 DYNOMILL.TM. for 2 hours to produce the final tone modifier dispersion
 containing: 20% of T01 and 8.8% of GEL05.
 Preparation of Thermographic Recording Materials
 Aqueous silver behenate dispersion was first prepared as described for
 INVENTION EXAMPLES 1 to 3 except that the surfactant used was Surfactant
 Nr. S03 and was present at a concentration of 0.1 g/g silver behenate and
 the silver behenate concentration was 16.9%.
 The coating dispersion for the thermosensitive element was produced by
 first adding 2.059 g of gelatin (for the type see table 3) to 7.64 g of
 deionized water or in the case of COMATIVE EXAMPLE 1 2.059 g of gelatin
 (for the type see table 3) together with 1.949 g of GEL02 to 13.11 g of
 deionized water, allowing the gelatin to swell for 30 minutes and then
 heating the mixture to 36.degree. C. then adding the following solutions
 and dispersions with stirring while maintaining a temperature of
 36.degree. C.: 6.93 g of the toner modifier dispersion as flakes (contains
 GEL05), then for COMATIVE EXAMPLE 2 and INVENTION EXAMPLES 4 to 6:
 7.430 g of a 26.2% dispersion of LATEX 02, then 30.72 g of the aqueous
 silver behenate dispersion followed by stirring, then 12.35 g of an
 aqueous solution containing 2.78% of boric acid, 8.17% of R01 and 15.23%
 of ethanol and finally 2.88 g of a 3.7% aqueous solution of formaldehyde.
 The chloride and sodium ions present in the dispersion only arise from the
 gelatin used.
 The coating dispersion was doctor-blade coated at a pH of ca. 5.4 onto a
 175 .mu.m thick subbed polyethylene terephthalate support to provide,
 after drying in a drying cupboard at 50.degree. C., the thermographic
 recording materials of COMATIVE EXAMPLES 1 & 2 and INVENTION EXAMPLES 4
 to 6 with the silver behenate coating weights given in table 3 below.
 TABLE 3
 total
 Light box:
 AgBeh binder GELATIN [Cl.sup.- ] vs fresh
 .DELTA.D.sub.max /.DELTA.D.sub.min blue
 coverage % as [Cl.sup.- ] gelatin
 D.sub.max D.sub.min after 3 days at
 [g/m.sup.2 ] LATEX 02 type [ppm] [ppm] blue blue
 30.degree. 0./85% RH
 Comparative
 example
 number
 1 4.35 0 (38%) GEL02 2900 3153 3.57 0.10
 +C.52/+0.07
 (62%) GEL05 &lt;40
 2 4.21 38 GEL03 1270 1707 3.26 0.10
 +0.19/+0.10
 Invention
 example
 number
 4 4.24 38 (76%) GEL04 17 454 3.15 0.10
 +0.24/+0.01
 (24%) GEL05 &lt;40
 5 4.11 38 GEL05 &lt;40 437 3.38 0.10
 +0.03/+0.01
 6 4.35 38 (76%) GEL06 &lt;40 437 3.18 0.10
 +0.17/+0.02
 (24%) GEL05 &lt;40
 The results of the thermographic evaluation of the thermographic recording
 materials of COMATIVE EXAMPLES 1 & 2 show a significant increase in
 D.sub.min i.e. 0.07 and 0.10 respectively after the light box test as can
 be seen from table 3, whereas the thermographic recording materials of
 INVENTION EXAMPLES 4 to 6 show increases in D.sub.min of 0.02 or less
 after the light box test indicating that for chloride concentrations above
 1500 ppm with respect to gelatin, thermographic recording materials of
 THERMOGRAPHIC COMPOSITION II exhibit significant photo-instability in the
 light-box test, whereas at chloride ion concentrations of 500 ppm or less
 with respect to gelatin, there is no significant photo-instability during
 this test.
 COMATIVE EXAMPLE 3 AND INVENTION EXAMPLE 7
 Thermographic Composion III
 Aqueous silver behenate dispersions were prepared as described for
 INVENTION EXAMPLES 1 to 3 except that the surfactant used was that given
 in table 3 and was present at a concentration of 0.1 g/g silver behenate
 and the silver behenate concentration was 21%.
 The coating dispersion for the thermosensitive element was produced by
 first adding 0.31 g of boric acid and 3.942 g of gelatin (for the type see
 table 4) to 19.46 g of deionized water, allowing the gelatin to swell for
 30 minutes and then heating the mixture to 36.degree. C. then adding the
 following solutions and dispersions with stirring while maintaining a
 temperature of 36.degree. C.: 4.93 g of the toner modifier dispersion as
 flakes, then a solution of 1 g of R01 in 3 g of deionized water and 1 g of
 ethanol at 50.degree. C. then 1.98 g of deionized water and finally by
 25.36 of a 21% dispersion of silver behenate with 0.1 g of surfactant/g
 silver behenate. The chloride and sodium ions present in the dispersion
 only arise from the gelatin used.
 The coating dispersion was doc-or-blade coated at a pH of ca. 5.0 onto a
 175 .mu.m thick subbed polyethylene terephthalate support to provide,
 after drying in a drying cupboard at 50.degree. C., the thermographic
 recording materials of COMATIVE EXAMPLE 3 and INVENTION EXAMPLES 7 with
 the silver behenate coating weights given below.
 Thermographic Evaluation
 Thermographic evaluation was carried out as described for INVENTION
 EXAMPLES 1 to 3 and the results are given in table 4 below. The results of
 INVENTION EXAMPLE 5 cannot be directly compared with those of INVENTION
 EXAMPLE 7, because THERMOGRAPHIC COMPOSITION II of INVENTION EXAMPLES 4 to
 6 and COMATIVE EXAMPLES 1 & 2 is more stable than THERMOGRAPHIC
 COMPOSITION III of INVENTION EXAMPLE 7 and COMATIVE EXAMPLE 3. However,
 the trend observed for the results with THERMOGRAPHIC COMPOSITION II is
 also to be found in the results obtained with THERMOGRAPHIC COMPOSITION
 III i.e. that the thermographic recording material of COMATIVE EXAMPLE
 3 with a chloride ion concentration greater than 1500 ppm exhibited
 significant photo-instability in the light-box test, whereas the
 thermographic recording material of INVENTION EXAMPLE 7 with less than 500
 ppm of chloride ions with respect to the gelatin exhibited no significant
 photo-instability in the light-box test in the context of the lower
 general stability of THERMOGRAPHIC COMPOSITION III.
 TABLE 4
 total
 Light box:
 AgBeh binder GELATIN [Cl.sup.- ] vs fresh
 .DELTA.D.sub.max /.DELTA.D.sub.min blue
 coverage % as [Cl.sup.- ] gelatin
 D.sub.max D.sub.min after 3 days at
 [g/m.sup.2 ] LATEX 01 type [ppm] [ppm] blue blue
 30.degree. C./85% RH
 Comparative 4.53 S03 (89%) GEL01 5300 5544 2.66 0.10
 +0.17/+0.63
 example (11%) GEL05 &lt;40
 number 3
 Invention 4.40 S02 GEL05 &lt;40 244 2.95 0.10
 +0.02/+0.04
 example
 number 7
 INVENTION EXAMPLES 8 AND 9
 Thermographic Composition IV
 Preparation of Subbing Layers
 Subbing Layer Number 01:
 A 0.34 mm thick polyethylene terephthalate sheet was coated to a thickness
 of 0.1 mm with a composition which after drying and longitudinal and
 transverse stretching produced a 175 mm thick support coated with the
 following subbing-layer composition expressed as the coating weights of
 the ingredients present:
 #terpoolyrer latex of vinylidene chloride/methyl acrylate/itaconic acid
 (88/10./2): 162 mg/m.sup.2
 #colloidal silica (KIESELSOL.TM. 100F from BAYER) 38 mg/m.sup.2
 #alkyl sulfonate surfactant (Surfactant Nr. 2): 0.6 mg /m.sup.2
 aryl sulfonate surfactant (Surfactant Nr. 3): 4 mg/m.sup.2
 Subbing Layer Number 02:
 A 0.34 mm thick polyethylene terephthalate sheet was coated to a thickness
 of 0.1 mm with a composition which after drying and longitudinal and
 transverse stretching produced a 175 mm thick support coated on with the
 following subbing-layer composition of subbing layer number 01 expressed
 as the coating weights of the ingredients present:

# copolymer of terephthalic acid/isophthalic acid/ 37.0 mg/m.sup.2
 sulfo-isophthalic acid/ethylene glycol 26.5/20/3.5/50):
 # copolymer latex of ethyl acrylate/methacrylic 3.0 mg/m.sup.2
 acid (80/20):
 # HORDAMER .TM. PE02: 1.0 mg/m.sup.2
 # EZ RESIN .TM. 707: 7.0 mg/m.sup.2
 Quantity of Leachable Non-fluoro-halide Ions Per Unit Surface of Subbing
 Layers
 The chloride-ion content leachable during overcoating with an aqueous
 dispersion or solution was simulated by placing a 10.times.5 cm.sup.2
 piece of subbing layer-coated polyethylene terephthalate in 25 mL of
 deionized water for a period of 2 hours and determining the quantity of
 chloride ions leached out by injecting samples of the leaching water
 directly into a DIONEX QIK ANALYSER ion chromatograph The detection limit
 with these measurements was limited to 0.1 ppm by the deionized water used
 in the leaching experiments, which had a chloride ion concentration of
 0.02 to 0.06 ppm. The results obtained are given below in table 1:
 Wavelength dispersive X-ray fluorescence (WDXRF) measurements were carried
 out on some of the supports to obtain a qualitative estimate of the total
 chlorine constant of the supports i.e. both covalently bound chlorine and
 chloride ions. These showed no detectable chlorine in an uncoated support,
 a very small quantity in subbing layer number 02 and a small quantity in
 subbing layer 01. The quantity of leachable chloride ions in the different
 subbing layers obtained from these measurements are summarized in table 5:
 TABLE 5
 Subbing Quantity of leachable chloride
 layer number ions [mg/m.sup.2 surface]
 01 0.65
 02 0.3
 Preparation of the Silver Behenate Dispersion
 The silver behenate dispersion was produced as follows: dispersing 25 kg
 (73.5M) behenic acid was dispersed with stirring at 80.degree. C. in 100 L
 of a 10% solution of Surfactant Nr 5/g behenic acid made up to 250 L with
 deionized water at a temperature of 80.degree. C.; then 36.75 L of a 2M
 aqueous solution of sodium hydroxide was added over a period of 10 to 20
 minutes to give a clear solution substantially containing sodium behenate;
 then 25 L of a 2.94M aqueous solution of silver nitrate was added with
 stirring at a rate of 0.163 moles/moles silver behenate.multidot.min to
 convert the sodium behenate completely into silver behenate; and finally
 ultrafiltration was carried out with a 500000 MW polysulfone cartridge
 filter at room temperature to concentrate the resulting silver behenate
 dispersion, the final AgBeh-concentration was 16.7% with 0.07 g of
 Surfactant Mr 5/g AgBeh, the residual conductivity was 1.0 mS/cm.
 Preparation of the Thermosensitive Element
 175 .mu.m thick blue pigmented polyethylene terephthalate supports coated
 with subbing layer numbers 01 & 02 were coated with an aqueous coating
 composition and the following ingredients so to obtain thereon after
 drying, a thermosensitive element containing:

GEL08 1266 mg/m.sup.2
 GEL09 100 mg/m.sup.2
 GEL10 130 mg/m.sup.2
 Surfactant Nr S09 &lt;5 mg/m.sup.2
 Surfactant Nr S10 80 mg/m.sup.2
 Surfactant Nr S11 3 mg/m.sup.2
 anti-bacterial agent 50 mg/m.sup.2
 LATEX04 100 mg/m.sup.2
 PLEXIGUM .TM. M345 50 mg/m.sup.2
 dioctadecyl phthalate 5 mg/m.sup.2
 formaldehyde 106 mg/m.sup.2
 sodium sulphate 1 mg/m.sup.2
 Thermosensitive Element
 A 175 .mu.m thick polyethylene terephthalate support with an uncoated
 subbing layer 01 on one side and backing layer B01 on the other was used
 for the thermographic recording material of INVENTION EXAMPLE 14 and a 175
 .mu.m thick polyethylene terephthalate support with uncoated subbing layer
 01 on one side and backing layer B02 on the other was used for the
 thermographic recording material of INVENTION EXAMPLE 15.
 A thermosensitive element of the following composition was applied in each
 case to the side coated with subbing layer 01: