EP0844514A1 - Photothermographic recording material having tabular grains - Google Patents

Abstract

A transparent photothermographic recording material is disclosed, wherein said material comprises a support bearing a photo-adressable thermosensitive element comprising photosensitive silver halide grains, a reducing agent for silver ions and a binder, characterised in that said silver halide grains are tabular silver halide grains having silver chloride in an amount of 50 mole % or more, more preferably in an amount of 70 mole % or more, and an average grain thickness of 0.20 µm or less and wherein said tabular grains account for at least 50 % of the total projective surface area of all silver halide grains.

Description

1. Field of the invention.

The present invention relates to a photothermographic recording material comprising photosensitive silver halide.

2. Background of the invention

Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.

In thermography three approaches are known:

1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.

2. Imagewise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element.

3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.

Thermographic materials of type 1 become photothermographic when a photosensitive agent is present which after exposure to UV, visible or IR light is capable of catalysing 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.

When coated onto transparent supports for viewing or projection, photothermographic recording materials suffer from a phenomenon, called "haze". Haze occurs when a fraction of the light which is incident on the thermographic layer is scattered rather than being transmitted. This light-scattering causes degradation in the quality of the viewed image, which increases with increasing light scattering.

Increased transparency, which in photographic materials is often related to fog or stain and in general minimum density, is desirable in photothermographic materials for use in viewing or projection.

Typically the photosensitive agent in photothermographic materials is a photosensitive silver halide. It is normally present in materials in a concentration of 0.1 to 35 mole % vs. a substantially light-insensitive organic silver salt. Silver bromo(iod)ide is normally used, which may, in principle, have any crystal habit.

US-A 4,435,499 describes a photothermographic material wherein said material comprises a support bearing in reactive association photosensitive thin tabular exemplified silver bromoiodide grains having an average grain thickness of less than 0.3 µm and a photosensitive silver halide processing agent.

According to WO96/15479 tabular grains are the least preferred of the silver halide grain types if photothermographic elements are required with reduced woodgrain interference patterns.

2. Objects of the invention.

It is a first object of the present invention to provide a photothermographic recording material with enhanced transparency.

It is a second object of the present invention to provide a photothermographic recording material with a reduced haze level.

It is a third object of the present invention to provide a method of preparing a photothermographic recording material, which can be coated from aqueous media, said material comprising a photo-addressable thermally developable element based on a substantially light-insensitive organic silver salt, an organic reducing agent for the organic silver salt and photosensitive silver halide in catalytic association with the light insensitive organic silver salt, further having in addition to a reduced haze level an improved transparency.

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

3. Summary of the invention

According to the present invention, a transparent photothermographic recording material is disclosed wherein said material comprises a support bearing a photo-adressable thermosensitive element comprising photosensitive silver halide grains, a reducing agent for silver ions and a binder, characterised in that said silver halide grains are tabular silver halide grains having silver chloride in an amount of 50 mole % or more, more preferably in an amount of 70 mole % or more, and an average grain thickness of 0.20 µm or less and wherein said tabular grains account for at least 50 % of the total projective surface area of all silver halide grains.

In a preferred embodiment said material has, prior to exposure, a density difference of less than 0.10, and preferably not more than 0.05, said density difference being measured by means of a MacBeth™ TD501 densitometer between density of the said material prior to exposure and density of the bare support of said material before coating with a photosensitive layer.

4. Detailed description of the invention.

A transparent photothermographic recording material having a low haze level is highly preferred as it contributes to good image quality.

Moreover prior to exposure a density difference, expressing transparency, measured by means of a MacBeth TD501 densitometer, of less than 0.10 and preferably of 0.05 or less is measured, wherein said density difference has been measured between the density of the material prior to exposure and the density of the bare support of the material before coating with a photosensitive layer.

Photo-addressable thermosensitive element

The photo-addressable thermosensitive element comprises photosensitive silver halide, a reducing agent for silver ions and a binder. The thermosensitive element may further comprise a substantially light-insensitive silver salt in catalytic association with the photosensitive silver halide and in thermal working relationship with the reducing agent for silver ions. The element may comprise a layer system with the silver halide in catalytic association with the substantially light-insensitive organic silver salt ingredients, spectral sensitiser optionally together with a supersensitiser in intimate sensitising association with the silver halide particles and the other ingredients active in the thermal development process or pre- or post-development stabilization of the element being in the same layer or in other layers the proviso that the organic reducing agent and the toning agent, if present, are in thermal working relationship with the substantially light-insensitive organic silver salt i.e. during the thermal development process the reducing agent and the toning agent, if present, are able to diffuse to the substantially light-insensitive organic silver salt, e.g. a silver salt of a fatty acid.

If the photosensitive silver halide grains set forth hereinbefore are employed together with a substantially light-insensitive organic silver salt it is used in a range of 0.1 to 90 mole percent of substantially light-insensitive organic silver salt, with the range from 0.2 to 50 mole % and from 0.5 to 35 mole percent even more being preferred and the range from 1 to 12 mole percent of said subtantially light-insensitive organic silver salt being particularly preferred.

Photosensitive silver halide

According to the present invention photosensitive silver halide crystals are tabular crystals having silver chloride in an amount of 50 mole % or more, more preferably in an amount of 70 mole % or more, having an average grain thickness of 0.2 µm or less, and more preferred from 0.03 µm up to 0.18 µm, an average aspect ratio of 2 or more, preferably at least 5:1 and still more preferably from 5:1 to 50:1.

Moreover according to the present invention said tabular silver halide crystals rich in silver chloride are accounting for at least 50 %, more preferably at least 70 %, and even more preferably at least 90 % of the total projected area of all grains.

According to the present invention said silver halide crystals rich in silver chloride are silver chloride, silver chloroiodide, silver chlorobromide or silver chlorobromoiodide grains.

In a further preferred embodiment said tabular silver chloroiodide grains have iodide in an amount of from 0.1 up to 3 mole %.

In a further preferred embodiment according to the present invention said tabular crystals are present in an amount of up to 2 g/m² of silver, expressed as an equivalent amount of silver nitrate, and more preferably in an amount of from 0.05 g/m² up to 1 g/m².

Further it has been found that, according to a preferred embodiment of the present invention said photosensitive silver halide crystals are tabular crystals, comprising silver chloride in an amount of at least 50 mole %, more preferably at least 70 mole %, and still more preferably at least 90 mole %, wherein bromide ions and iodide ions may be present. Said iodide ions, as well as bromide ions if present, may be added as iodide salts or bromide salts respectively or as organic iodide or bromide releasing agents respectively, as those described e.g. in EP-A's 0 561 415, 0 563 701, 0 563 708, 0 649 052 and 0 651 284 and in WO 96/13759. Said inorganic salts or organic products providing iodide and/or bromide ions may be added in order to cause a homogeneous or a heterogeneous distribution of the said ions over the crystal volume of these crystals. Heterogenous distributions over the said crystal volume can be attained by addition in separate steps of solutions providing said ions, wherein said ions are incorporated in the crystal lattice by conversion.

Tabular grains suitable for use in one or more hydrophilic emulsion layer(s) of the silver halide photographic materials of the present invention have {111} or {100} major faces.

Tabular silver halide grains having {111} major faces known since 1982 to be suitable for use in photographic materials are crystals possessing two parallel major faces with a ratio between the diameter of a circle having the same area as these faces, and the thickness, being the distance between the two major faces, of at least 2.

For tabular emulsion crystals rich in silver chloride having {111} major faces the stability of said crystals requires the use during preparation of a crystal habit modifier in relatively high amounts, so that after flocculation, washing and redispersing an excess of said modifier remains strongly adsorbed on the {111} surfaces of the tabular grains rich in chloride, even after protonation. Said protonation causes release of crystal habit modifier as has been described e.g. in EP-A 0 481 133, and in US-A's 5,176,991; 5,176,992; 5,272,052 and 5,399,478. The presence of a crystal habit modifier can lead to poor reproducibility. Therefore preparation conditions of tabular grains rich in silver chloride should be carefully controlled as morphological instability of the tabular silver halide grains rich in chloride, reflected in the occurrence of a rounded edges after the disappearance of sharp, well-defined edges of hexagons and triangles, is undesirable.

Compounds that are useful as crystal habit modifier of crystals rich in silver chloride besides the most frequently used adenine, include substances disclosed in EP-A's 0 481 133 and 0 532 801 and in US-A's 5,176,991; 5,176,992; 5,178,997; 5,178,998; 5,183,732; 5,185,239; 5,217,858; 5,221,602; 5,252,452; 5,264,337; 5,272,052; 5,298,385; 5,298,387; 5,298,388; 5,399,478; 5,405,738; 5,411,852 and 5,418,125.

Tabular silver halide grains rich in chloride, bounded by {111} major faces and/or the preparation method thereof and/or materials in which said grains are incorporated have also been described in e.g. US-A's 4,399,215; 4,400,463; 4,804,621; 5,061,617; 5,275,930; 5,286,621; 5,292,632; 5,310,644; 5,320,938; 5,356,764; and in the published EP-A's 0 503 700, 0 533 189, 0 647 877 and 0 678 772.

Tabular emulsion crystals rich in silver chloride having {100} major faces can be produced with a stable crystal habit. The said {100} tabular grains rich in chloride, although having been developed later than {111} tabular grains rich in chloride, are the subject of intensive studies as no crystal habit modifier is required in their preparation and consequently better reproducibility can be attained.

Silver halide grains rich in chloride, bounded by the said {100} major faces and/or the preparation method thereof and/or materials in which said grains can be incorporated have been described in e.g. US-A's 5,024,931; 5,264,337; 5,275,930; 5,292,632; 5,310,635; 5,314,798; 5,320,938; 5,356,764 and in WO 94/022051; in the published EP-A's 0 534 395, 0 569 971, 0 584 815, 0 584 644, 0 602 878, 0 616 255, 0 617 317, 0 617 320, 0 617 321, 0 617 325, 0 618 492 and 0 653 669.

Dopants may be used in the preparation of silver halide crystals rich in silver chloride have e.g. been described (preferred elements between brackets) in US-A's 4,269,927 (Cd,Pb,Zn) ; 4,835,093 (Re-complexes); 4,981,781 (Ru,Fe,Re,Os,Mn,Mo,Cr,W); 5,024,931 (Ru,Rh,Os,Ir,Pd,Pt); 5,252,456 (Pt,Ir) and 5,360,712 (groups 8 and 9 from periods IV, V and VI): EP-A's 0 017 148 (Cd,Pb,Zn,Cu); 0 336 426 (Ru,Re,Os); 0 336 427 (Ru,Os); 0 415 481 (Rh,Ir,Os,Ru,Fe,Co); 0 658,802 (Cr); JP-A 04-009939 and WO 90/016014 (group VIb).

A survey of dopants in tabular {111} grains has been given in US-A 5,503,971. Most preferred are ruthenium, rhodium and iridium. Combinations of one or more dopant(s) may be added, in the same or different preparation steps of silver halide crystals. As with halide ions the said dopants can be divided homogeneously or heterogeneously over the total crystal volume. So in the core or in the shell or even at the crystal surface, as is e.g. the case when conversion techniques are applied, the said halide ions and/or the said dopants may be concentrated in particular regions of the crystals.

Mixtures of emulsions having the same or different halide composition, crystal habit, crystal size, crystal size distribution and/or dopant(s) may be used in order to obtain improved sensitometry and image quality.

The silver halide used in the present invention may be employed without modification. However, it may be chemically sensitised with a chemical sensitising 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. 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. If mixtures of silver halide emulsion crystals are used chemical sensitisation is preferably performed before mixing them in order to preserve an optimised chemical sensitisation for each silver halide tabular grain present.

As thermographic materials of type 1, as discussed in the background of the invention, become photothermographic when a photosensitive agent. In order to enhance the photosensitivity to visible or IR light, a suitable spectral sensitising dye should be added in order to make light-sensitive silver halide sensitive to the practically used irradiation. So the silver halide may be spectrally sensitized with various known dyes including cyanine, merocyanine, styryl, hemicyanine, oxonol, hemioxonol and xanthene dyes optionally, particularly in the case of sensitization to infra-red radiation, in the presence of a so-called supersensitiser. Useful cyanine dyes include those having a basic nucleus, such as a thiazoline nucleus, an oxazoline nucleus, a pyrroline nucleus, a pyridine nucleus, an oxazole nucleus, a thiazole nucleus, a selenazole nucleus and an imidazole nucleus. Useful merocyanine dyes which are preferred include those having not only the above described basic nuclei but also acid nuclei, such as a thiohydantoin nucleus, a rhodanine nucleus, an oxazolidinedione nucleus, a thiazolidinedione nucleus, a barbituric acid nucleus, a thiazolinone nucleus, a malononitrile nucleus and a pyrazolone nucleus. In the above described cyanine and merocyanine dyes, those having imino groups or carboxyl groups are particularly effective. Suitable sensitizers of silver halide to infra-red radiation include those disclosed in the EP-A's 0 465 078, 0 559 101, 0 616 014, 0 635 756 and 0 638 841, the JP-A's 03-080251, 03-163440, 05-019432, 05-072662 and 06-003763 and the US-A's 4,515,888, 4,639,414, 4,713,316, 5,258,282 and 5,441,866. Suitable supersensitizers for use with infra-red spectral sensitizers are disclosed in EP-A's 559 228 and 587 338 and in the US-P's 3,877,943 and 4,873,184. As has been shown in unpublished EP-Application No. 96202100, filed July 24, 1996, the suitable infrared spectral sensitizers are preferably applied without requiring the presence of a supersensitiser.

Substantially light-insensitive organic silver salts

Preferred substantially light-insensitive organic silver salts are silver salts of organic carboxylic acids in particular 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": silver dodecyl sulphonate described in US-A 4,504,575; and silver di-(2-ethylhexyl)-sulfosuccinate described in EP-A 0 227 141. Modified aliphatic carboxylic acids with thioether group as described e.g. in GB-P 1,111,492 and other organic silver salts as described in GB-P 1,439,478, e.g. silver benzoate and silver phthalazinone, may be used likewise to produce a thermally developable silver image. Further are mentioned silver imidazolates and the substantially light-insensitive inorganic or organic silver salt complexes described in US-A 4,260,677.

Emulsion of organic silver salt and photosensitive silver halide

A suspension of particles containing a substantially light-insensitive organic silver salt may be obtained by using a process, comprising simultaneous metered addition of an aqueous solution or suspension of an organic carboxylic acid or its salt; and an aqueous solution of a silver salt to an aqueous liquid, as described in EP-A 0 754 969.

The silver halide emulsion grains described hereinbefore may be added to the photo-addressable thermally developable element in any fashion which places it in catalytic proximity to the substantially light-insensitive organic silver salt. Silver halide and the substantially light-insensitive organic silver salt which are separately formed, i.e. ex-situ or "preformed", in a binder can be mixed prior to use to prepare a coating solution, but it is also effective to blend both of them for a long period of time in order to get an intimate contact at the large specific surface of the tabular grains accounting for at least 50 % (and preferably more) of the total projective surface area. Furthermore, it is effective to use a process which comprises adding a halogen-containing compound to the organic silver salt to partially convert the substantially light-insensitive organic silver salt to silver halide as disclosed in US-A 3,457,075.

A particularly preferred mode of preparing the emulsion of organic silver salt and photosensitive silver halide for coating of the photo-addressable thermally developable element from solvent media, according to the present invention is that disclosed in US-A 3,839,049, but other methods such as those described in Research Disclosure, June 1978, item 17029 and US-A 3,700,458 may also be used for producing the emulsion.

A particularly preferred mode of preparing the emulsion of organic silver salt and photosensitive silver halide for coating of the photo-addressable thermally developable element from aqueous media, according to the present invention is that disclosed in unpublished Application PCT/EP 96/02579, filed June 13, 1996, which discloses a production method for a photothermographic recording material comprising the steps of:

(i) providing a support; (ii) coating the support with a photo-addressable thermally developable element comprising a substantially light-insensitive organic silver salt, photosensitive silver halide in catalytic association with the substantially light-insensitive organic silver salt, a reducing agent in thermal working relationship with the substantially light-insensitive organic silver salt and a binder, characterised in that the photosensitive silver halide is formed by reacting an aqueous emulsion of particles of the substantially light-insensitive organic silver salt with at least one onium salt with halide or polyhalide anion(s) and that the photo-addressable thermally developable element is coated from an aqueous dispersion medium.

In a preferred embodiment molar ratios of light-sensitive silver halide and substantially light-insensitive organic silver salt are from 0.1 up to 90 mole %, more preferably from 0.2 up to 50 mole %, even more preferably from 0.5 up to 35 mole % and especially preferred from 1 to 12 mole % of the said substantially light-insensitive organic silver salt.

Organic reducing agent for photo-addressable thermosensitive elements

Suitable organic reducing agents for the reduction of the substantially light-insensitive organic silver salts in photo-addressable thermosensitive element are organic compounds containing at least one active hydrogen atom linked to O, N or C, such as is the case with, mono-, bis-, tris- or tetrakis-phenols; mono- or bis-naphthols; di- or polyhydroxy-naphthalenes; di- or polyhydroxybenzenes; hydroxymonoethers such as alkoxynaphthols, e.g. 4-methoxy-1-naphthol described in US-A 3,094,41; pyrazolidin-3-one type reducing agents, e.g. PHENIDONE (tradename); pyrazolin-5-ones; indan-1,3-dione derivatives; hydroxytetrone acids; hydroxytetronimides; 3-pyrazolines; pyrazolones; reducing saccharides; aminophenols e.g. METOL (tradename); p-phenylenediamines, hydroxylamine derivatives such as for example described in US-A 4,082,901; reductones e.g. ascorbic acids; hydroxamic acids; hydrazine derivatives; amidoximes; n-hydroxyureas; and the like, see also US-A 3,074,809, 3,080,254, 3,094,417 and 3,887,378.

Polyphenols such as the bisphenols used in the 3M Dry Silver™ materials, sulfonamide phenols such as used in the Kodak Dacomatic™ materials, and naphthols are particularly preferred for photothermographic recording materials with photo-addressable thermally developable elements on the basis of photosensitive silver halide/organic silver salt/reducing agent.

During the thermal development process the reducing agent must be present in such a way that it is able to diffuse to the photosensitive silver halide and, if present, the substantially light-insensitive organic silver salt particles so that reduction thereof can take place.

Auxiliary reducing agents

The above mentioned reducing agents, regarded as primary or main reducing agents, may be used in conjunction with so-called auxiliary reducing agents. Auxiliary reducing agents that may be used in conjunction with the above mentioned primary reducing agents are sulfonyl hydrazide reducing agents such as disclosed in US-A 5,464,738, trityl hydrazides and formyl-phenyl-hydrazides such as disclosed in US-A 5,496,695 and organic reducing metal salts, e.g. stannous stearate described in US-A's 3,460,946 and 3,547,648.

Binder

The film-forming binder for the photo-addressable thermosensitive element according to the present invention may be coatable from a solvent or aqueous dispersion medium. The film-forming binder for the photo-addressable thermosensitive element according to the present invention may be coatable from a solvent dispersion medium, according to the present invention, may be all kinds of natural, modified natural or synthetic resins or mixtures of such resins, wherein the organic silver salt can be dispersed homogeneously: e.g. 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 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, polyacrylic acid esters, polymethacrylic acid esters, polystyrene and polyethylene or mixtures thereof. A particularly suitable polyvinyl butyral containing a minor amount of vinyl alcohol units is marketed by MONSANTO USA under the trade names BUTVAR™ B76 and BUTVAR™ B79 and provides a good adhesion to paper and properly subbed polyester supports. The film-forming binder for the photo-addressable thermosensitive developable element coatable from an aqueous dispersion medium, according to the present invention, may be all kinds of transparent or translucent water-dispersible or water soluble natural, modified natural or synthetic resins or mixtures of such resins, wherein the organic silver salt can be dispersed homogeneously for example pro-teins, such as gelatin and gelatin derivatives (e.g. phthaloyl gela-tin), cellulose derivatives, such as carboxymethylcellulose, poly-saccharides, such as dextran, starch ethers, galactomannan, polyvi-nyl alcohol, polyvinylpyrrolidone acrylamide polymers, homo- or co-polymerized acrylic or methacrylic acid, latexes of water disper-sible polymers, with or without hydrophilic groups, or mixtures thereof. Polymers with hydrophilic functionality for forming an aqueous polymer dispersion (latex) are described in US-A 5,006,451, but serve therein for forming a barrier layer preventing unwanted diffusion of vanadium pentoxide present as an antistatic agent.

Weight ratio of binder to organic silver salt

The binder to organic silver salt weight ratio is preferably in the range of 0.2 to 6 µm, whereas the thickness of the photo-addressable thermally developable element is preferably in the range of 5 to 50 µm.

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.

By the term "heat solvent" in this invention is meant a non-hydrolyzable organic material which is in solid state in the recording layer at temperatures below 50°C but becomes a plasticizer for the recording layer in the heated region and/or liquid solvent for at least one of the redox-reactants, e.g. the reducing agent for the organic silver salt, at a temperature above 60°C. Useful for that purpose are a polyethylene glycol having a mean molecular weight in the range of 1,500 to 20,000 described in US-A 3,347,675. Further are mentioned compounds such as urea, methyl sulfonamide and ethylene carbonate being heat solvents described in US-A 3,667,959, and compounds such as tetrahydro-thiophene-1,1-dioxide, methyl anisate and 1,10-decanediol being described as heat solvents in Research Disclosure, December 1976, (item 15027) pages 26-28. Still other examples of heat solvents have been described in US-A 3,438,776, and 4,740,446, and in published EP-A 0 119 615 and 0 122 512 and DE-A 3 339 810.

Toning agent

In order to obtain a neutral black image tone in the higher densities and neutral grey in the lower densities the photo-addressable thermosensitve element contains preferably in admixture with the organic heavy metal salts and reducing agents a so-called toning agent known from thermography or photothermography.

Suitable toning agents are succinimide and the phthalimides and phthalazinones within the scope of the general formulae described in US-A 4,082,901. Further reference is made to the toning agents described in US-A 3,074,809, 3,446,648 and 3,844,797. Other particularly useful toning agents are the heterocyclic toner compounds of the benzoxazine dione or naphthoxazine dione type within the scope of following general formula :

in which: X represents O or N-alkyl; each of R¹, R², R³ and R⁴ (same or different) represents hydrogen, alkyl, e.g. C1-C20 alkyl, preferably C1-C4 alkyl, cycloalkyl, e.g. cyclopentyl or cyclohexyl. alkoxy, preferably methoxy or ethoxy, alkylthio with preferably up to 2 carbon atoms, hydroxy, dialkylamino of which the alkyl groups have preferably up to 2 carbon atoms or halogen, preferably chlorine or bromine; or R¹ and R² or R² and R³ represent the ring members required to complete a fused aromatic ring, preferably a benzene ring, or R³ and R⁴ represent the ring members required to complete a fused aromatic aromatic or cyclohexane ring. Toners within the scope of the general formula are described in GB-P 1,439,478 and US-A 3,951,660.

A toner compound particularly suited for use in combination with polyhydroxy benzene reducing agents is 3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine described in US-A 3,951,660.

Alternatively in a transparent photothermographic recording material comprising tabular silver halide grains according to the present invention use in photosensitive silver halide emulsion layers comprising said tabular grains of blue coloured polymeric matting particles as disclosed in EP-Application 96203262, filed November 21, 1996, is recommended in favour of image tone.

Stabilisers and antifoggants

In order to obtain improved shelf-life and reduced fogging, stabilizers and antifoggants may be incorporated into the photothermographic materials of the present invention. Examples of suitable stabilizers and antifoggants and their precursors, which can be used alone or in combination, include the thiazolium salts described in US-A 2,131,038 and 2,694,716; the azaindenes described in US-A 2,886,437 and 2,444,605; the urazoles described in US-A 3,287,135; the sulfocatechols described in US-A 3,235,652; the oximes described in GB-P 623,448; the thiuronium salts described in US-A 3,220,839; the palladium, platinum and gold salts described in US-A 2,566,263 and 2,597,915; the tetrazolyl-thio-compounds described in US-A 3,700,457; the mesoionic 1,2,4-triazolium-3-thiolate stablizer precursors described in US-A 4,404,390 and 4,351,896; the tribromomethyl ketone compounds described in EP-A 0 600 587; the combination of isocyanate and halogenated compounds described in EP-A 0 600 586; the vinyl sulfone and β-halo sulfone compounds described in EP-A 0 600 589; and those compounds mentioned in this context in Chapter 9 of "Imaging Processes and Materials, Neblette's 8th edition", by D. Kloosterboer, edited by J. Sturge, V. Walworth and A. Shepp, page 279, Van Nostrand (1989); in Research Disclosure 17029 published in June 1978; and in the references cited in all these documents.

Surfactants for photoadressable thermosensitive elements coated from aqueous media

Non-ionic, cationic or anionic surfactants may be used, according to the present invention, to produce dispersions of particles of the substantially light-insensitive organic silver salt in aqueous media and to disperse water-dispersible binders, such as polymer latexes, in aqueous media. A mixture of non-ionic and anionic surfactacts, of non-ionic and cationic surfactants, of cationic and anionic surfactants or of non-ionic, cationic and anionic surfactants may also be used, according to the present invention.

Other additives

In addition to the ingredients the photo-addressable thermosensitive element may contain other additives such as free fatty acids, surface-active agents, antistatic agents, e.g. non-ionic antistatic agents including a fluorocarbon group as e.g. in F₃C(CF₂)₆CONH(CH₂CH₂O)-H, silicone oil, e.g. BAYSILONE Ö1 A (tradename of BAYER AG - GERMANY), ultraviolet light absorbing compounds, white light reflecting and/or ultraviolet radiation reflecting pigments, silica, colloidal silica, fine polymeric particles [e.g. of poly(methylmethacrylate)] and/or optical brightening agents.

Antihalation dyes

According to a preferred embodiment of the present invention, the photothermographic recording material further comprises an antihalation or acutance dye which absorbs light which has passed through the photosensitive layer, thereby preventing its reflection. Such dyes may be incorporated into the photo-addressable thermosensitive element or in any other layer comprising the photothermographic recording material of the present invention. The antihalation dye may also be bleached either thermally during the thermal development process, as disclosed in the US-A's 4,033,948, 4,088,497, 4,153,463, 4,196,002, 4,201,590, 4,271,263, 4,283,487, 4,308,379, 4,316,984, 4,336,323, 4,373,020, 4,548,896, 4,594,312, 4,977,070, 5,258,274, 5,314,795 and 5,312,721, or photo-bleached after removable after the thermal development process, as disclosed in the US-A's 3,984,248, 3,988,154, 3,988,156, 4,111,699 and 4,359,524. Furthermore the antihalation dye may be contained in a layer which can be removed subsequent to the exposure process, as disclosed in US-A 4,477,562 and EP-A 0 491 457. Suitable antihalation dyes for use with infra-red light are described in the EP-A's 377 961 and 652 473, the EP-B's 101 646 and 102 781 and the US-P's 4,581,325 and 5,380,635.

Support

The support for the photothermographic recording material according to the present invention is a transparent resin film, e.g. made of a cellulose ester, e.g. cellulose triacetate, corona and flame treated polypropylene, polystyrene, polymethacrylic acid ester, polycarbonate or polyester, e.g. polyethylene terephthalate or polyethylene naphthalate as disclosed in GB 1,293,676, GB 1,441,304 and GB 1,454,956.

The support may be in sheet, ribbon or web form and subbed if need be to improve the adherence to the thereon coated thermosensitive recording layer. The support may be made of an opacified resin composition, e.g. polyethylene terephthalate opacified by means of pigments and/or micro-voids and/or coated with an opaque pigment-binder layer, and may be called synthetic paper, or paperlike film; information about such supports can be found in EP-A's 0 194 106 and 0 234 563 and US-A's 3,944,699, 4,187,113, 4,780,402 and 5,059,579. Should a transparent base be used, the base may be colourless or coloured, e.g. having a blue colour.

One or more backing layers may be provided to control physical properties such as curl or static.

Protective layer

According to a preferred embodiment of the photothermographic recording material of the present invention, the photo-addressable thermosensitive element is provided with a protective layer to avoid local deformation of the photo-addressable thermosensitive element, to improve its resistance against abrasion and to prevent its direct contact with components of the apparatus used for thermal development.

This protective layer may have the same composition as an anti-sticking coating or slipping layer which is applied in thermal dye transfer materials at the rear side of the dye donor material or protective layers used in materials for direct thermal recording.

The protective layer preferably comprises a binder, which may be solvent soluble (hydrophobic), solvent dispersible, water soluble (hydrophilic) or water dispersible. Among the hydrophobic binders cellulose acetate butyrate, polymethylmethacrylate and polycarbonates, for example as described in EP-A 0 614 769, are particularly preferred. Suitable hydrophilic binders are, e.g., gelatin, polyvinylalcohol, cellulose derivatives or other polysaccharides, hydroxyethylcellulose, hydroxypropylcellulose etc., with hardenable binders being preferred and polyvinylalcohol being particularly preferred.

A protective layer of the photothermographic recording material, according to the present invention, may be crosslinked. Crosslinking can be achieved by using crosslinking agents such as described in WO 95/12495 for protective layers, e.g. tetra-alkoxysilanes, polyisocyanates, zirconates, titanates, melamine resins etc., with tetraalkoxysilanes such as tetramethylorthosilicate and tetraethylorthosilicate being preferred.

A protective layer according to the present invention may comprise in addition at least one solid lubricant having a melting point below 150°C and at least one liquid lubricant in a binder, wherein at least one of the lubricants is a phosphoric acid derivative, further dissolved lubricating material and/or particulate material, e.g. talc particles, optionally protruding from the outermost layer. Examples of suitable lubricating materials are surface active agents, liquid lubricants, solid lubricants which do not melt during thermal development of the recording material, solid lubricants which melt (thermomeltable) during thermal development of the recording material or mixtures thereof. The lubricant may be applied with of without a polymeric binder. The surface active agents may be any agents known in the art such as carboxylates, sulfonates, aliphatic amine salts, aliphatic quaternary ammonium salts, polyoxyethylene alkyl ethers, polyethylene glycol fatty acid esters, fluoroalkyl C₂-C₂₀ aliphatic acids. Examples of liquid lubricants include silicone oils, synthetic oils, saturated hydrocarbons and glycols. Examples of solid lubricants include various higher alcohols such as stearyl alcohol and fatty acids. Suitable slipping layer compositions are described in e.g. EP-A 0 138 483, EP-A 0 227 090, US-A 4,567,113; 4,572,860 and 4,717,711 and in EP-A 0 311 841.

A suitable slipping layer being a layer comprising as binder a styrene-acrylonitrile copolymer or a styrene-acrylonitrile-butadiene copolymer or a mixture hereof and as lubricant in an amount of 0.1 to 10 % by weight of the binder (mixture) a polysiloxane-polyether copolymer or polytetrafluoroethylene or a mixture hereof.

Other suitable protective layer compositions that may be applied as slipping (anti-stick) coating are described e.g. in EP-A's 0 501 072 and 0 492 411.

Such protective layers may also comprise particulate material, e.g. talc particles, optionally protruding from the protective outermost layer as described in WO 94/11198. Other additives can also be incorporated in the protective layer e.g. colloidal particles such as colloidal silica.

Antistatic layer

In a preferred embodiment the recording material of the present invention an antistatic layer is applied to the outermost layer on the side of the support not coated with the photo-addressable thermosensitive element.

Suitable antistatic layers therefore are described in EP-A's 0 444 326, 0 534 006 and 0 644 456, US-A's 5,364,752 and 5,472,832 and DE-OS 4 125 758.

Coating

The coating of any layer of the photothermographic recording material 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, NY 10010, U.S.A.

According to the present invention a method of preparing a photothermographic material is further disclosed, wherein said method comprises the steps of coating on a support as adjacent layers a photosensitive layer comprising silver halide crystals, i.a. the tabular crystals rich in silver chloride as set forth hereinbefore and a protective layer coated thereover.

Photothermographic recording process

Photothermographic 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 focussed 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 780nm, 830nm or 850nm; or a light emitting diode, for example one emitting at 659nm; or by direct exposure to the object itself or an image therefrom with appropriate illumination e.g. with UV, 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..

According to the present invention a photothermographic recording process is also provided comprising the steps of:

(i) image-wise exposing a photothermographic recording material, as referred to above, with actinic radiation to which the photothermographic recording material is sensitive; and

(ii) thermally developing the image-wise exposed photothermographic recording material.

In praxis said thermally developing step consist of heating said material according to the the present invention to a temperature within a range of 90°C to 180°C until a density of at least 3.0 is reached.

Applications

The photothermographic recording materials of the present invention can be used for both the production of transparencies and reflection type prints. This means that the support will be transparent or opaque, e.g. having a white light reflecting aspect. For example, a paper base substrate is present which may contain white reflecting pigments, optionally also applied in an interlayer between the recording material and the paper base substrate. Should a transparent base be used, the base may be colourless or coloured, e.g. has a blue colour. In order to get a suitable image tone it may be advised to add blue coloured polymeric matting particles, e.g. polymethylmethacrylate particles, to the photosensitive layer comprising tabular grains as has been described in unpublished EP-Application No. 96203262, filed November 21, 1996. In another embodiment a blue dye, preferably an indanthron dye, may be added a has been described in unpublished EP-Application No. 96200417, filed February 19, 1996.

In the hard copy field photothermographic recording materials on a white opaque base are used, whereas in the medical diagnostic field black-imaged transparencies are widely used in inspection techniques operating with a light box.

While the present invention will hereinafter be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included in the spirit and scope of the invention as defined by the appending claims.

EXAMPLES

Support used in the experiments described below:

A polyethyleneterephthalate (PET) foil was first coated on both sides with a subbing layer consisting of a terpolymer latex of vinylidene chloride-methyl acrylate-itaconic acid (88/10/2) in admixture with colloidal silica (surface area 100 m²/g). After stretching the foil in the transverse direction the foil had a thickness of 175 µm with coverages of the terpolymer and of the silica in the subbing layers of 170 mg/m² and 40 mg/m² respectively on each side of the PET-foil. The specular density measured from non-diffusing white light incident perpendicularly onto the bare support measured and passing through the said support was 0.180.

MATERIALS Nos. 1 to 14

The following silver halide emulsions (Nos. 1-5) were prepared.

Emulsion No. 1: thin tabular AgBr(I) emulsion
For the preparation of a tabular silver bromoiodide emulsion containing 1 mole% of silver iodide the following solutions were prepared:

• 3 l of a dispersion medium (C) containing 0.026 moles of potassium bromide, 7.5 g of oxidised gelatin, containing less than 30 µmoles of methionine per gram of gelatin (see e.g. US-A 4,713,323); the temperature was established at 45 °C and pH was adjusted to a value of 1.8;
• a 1.2 molar silver nitrate solution (A);
• a 1.2 molar potassium bromide solution (B1);
• an aqueous solution containing 341 grams of potassium bromide and 7.5 grams of potassium iodide (B2);

Preparation steps:
A nucleation step was performed by introducing solution A and solution B1 simultaneously in dispersion medium C both at a flow rate of 80 ml/min during 9 seconds. After one minute the reaction temperature of this mixture was raised to 70 °C in 25 minutes and a solution of 50 g of oxidised gelatin, containing less than 30 µmoles of methionine per gram, and 500 ml of distilled water were added. The pH was adjusted to a value of 6.0 and after 6 minutes a neutralisation step was started.

Neutralisation step:
Solution A was added to the reaction vessel during 7 minutes at a rate of 7.5 ml per minute in order to reach a pAg of 8.89, whereafter the first growth step was started.

First growth step:
A double jet precipitation was started using solutions A and B1, which continued for 44 minutes 36 seconds. During this precipitation, the pAg value was kept at 8.89. The flowing rate of solution A was 7.5 ml at the start, linearly increasing to 30 ml per minute at the end of the precipitation. At the end of the precipitation, the mixture was physically ripened during 5 minutes, whereafter a second neutralisation step was started.

Second neutralisation step:
73 ml of solution A was injected into the reaction vessel at a constant rate of 7.5 ml per minute. The pAg was kept constant 8.89 using solution B2. Thereafter the second growth step was started

Second growth step:
1593 ml of solution A was injected into the reaction vessel at a rate of 7.5 ml per minute at the start, linearly increasing to 40 ml per minute at the end of the precipitation. The pAg was kept constant at 8.89 using solution B2 for 63 minutes and 57 seconds.
After cooling to about 40 °C the pH value of the said dispersing medium was adjusted to a value of 3.0 with sulphuric acid, and after the addition of polystyrene sulphonic acid, the obtained flocculate was decanted and washed three times with an amount of 4 l of demineralised water in order to remove the soluble salts present. The thus obtained silver bromide emulsion having {111} tabular crystals showed the following grain characteristics: an average equivalent circular diameter, ECD, of 1.7 µm, an average thickness T of 0.09 µm and an average aspect ratio, AR, of 19 were obtained from electron microscopic photographs. The equivalent circular diameter of the grain was defined as the diameter of the circle having an area equal to the projected area of the grain as viewed in the said photographs.

Emulsion No. 2: thin tabular AgBr(I) emulsion
Emulsion No. 2 was prepared in the same way as Emulsion No. 1, except for the following changes. The pAg in neutralisation step 1, growth step 1, neutralisation step 2 and growth step 2 was taken equal to a value of 8.58. Moreover, inert gelatine instead of oxidised gelatine was added after physical ripening.
The thus obtained emulsion having {111} tabular silver bromide grains showed the following grain characteristics: an average equivalent circular diameter, ECD, of 1.2 µm, an average thickness, T, of 0.11 µm and an average aspect ratio, AR, of 11.

Emulsion No. 3: thick tabular AgBr(I) emulsion
Emulsion No. 3 was prepared in the same way as Emulsion No. 1, except for the following differences. The pAg in the neutralisation step 1, the growth step 1, the neutralisation step 2 and the growth step 2 was taken equal to 7.8. Moreover inert gelatin instead of oxidised gelatin was added after physical ripening. The thus obtained emulsion having {111} tabular silver bromide grains showed the following grain characteristics: an average diameter, ECD, of 0.84 µm, an average thickness, T, of 0.23 µm and n average aspect ratio, AR, of 4.

Emulsion No. 4: AgBr(I)-emulsion having cubic crystals.
For the preparation of a cubic silver bromoiodide emulsion containing 1 mole % of silver iodide following solutions were prepared:

• 1 l of a dispersion medium (C) containing 50 g of phthalated gelatin and 15 g of methionine; temperature was established at 60°C, pH at a value of 5.8;
• a 2.94 molar silver nitrate solution (A);
• an aqueous solution containing 346.5 grams of potassium bromide and 10 grams of potassium iodide (B1);
• a 2.94 molar potassium bromide solution (B2)

-Neutralisation:
First 0.05 ml of solution A, 9.95 ml of distilled water and 0.55 ml of solution B2 were added to the reaction vessel. Then 28.1 ml of solution A was added at a rate of 5.7 ml per minute. During this precipitation, the pAg was kept constant at a value of 7.8.

-Growth:
A double jet precipitation was started using solutions A and B1. Said precipitation continued for 72 minutes 46 seconds. During this precipitation, the pAg was kept constant a value of 7.8. The flow rate of solution A was 5.7 ml per minute at the start, linearly increasing up to 21 ml per minute. At the end of the precipitation, 20 grams of inert gelatin and 180 ml of distilled water were added.

The thus obtained silver bromide emulsion contained crystals having a cubic habit, with an average sphere equivalent diameter (VD) of 0.71 µm. The sphere equivalent diameter of the grain (VD) is herein defined as the diameter of a hypothetical spherical grain with the same volume as the corresponding grain.

Emulsion No. 5: thin tabular AgCl(I) emulsion.
In preparing a tabular silver chloroiodide emulsion containing 1.3 mole % of silver iodide the following solutions were prepared ;

• 3 l of a dispersion medium (C) containing 0.238 moles of sodium chloride, 75 g of inert gelatin and 360 mg of adenine; temperature was established at 45°C and pH was adjusted to a value of 6.0;
• a 2.94 molar silver nitrate solution (A);
• a solution containing 2.93 moles of sodium chloride, 15 mmoles of potassium iodide and 420 mg of adenine (B1);

A nucleation step was performed by introducing solution A and solution B1 simultaneously in dispersion medium C both at a flow rate of 45 ml/min during 40 seconds. The mixture was ripened for 20 min, during which the temperature was raised to 70°C. Then a first growth step was performed by introducing by a double jet precipitation step during 28 minutes and 50 seconds solution A starting at a flow rate of 5 ml/min and linearly increasing the flow rate to an end value of 13.7 ml/min, and solution B1 at an increasing flow rate in order to maintain a constant mV-value of + 115 mV, said value measured by a silver electrode versus an Ag/AgCl Ingold electrode. used as a reference electrode. Afterwards a second growth step was performed, thereby introducing solution A by double jet during 49 minutes and 33 seconds, starting at a flow rate of 5 ml per minute and linearly increasing the flow rate to an end value of 20 ml per minute. Simultaneously, solution B1 was introduced in order to maintain a constant mV-value of 135mV, measured by a silver electrode versus an Ag/AgCl Ingold electrode, used as a reference electrode. At the end of the double jet precipitation an amount of 35 mmoles of potassium iodide was added.The thus obtained {111} silver chloride tabular emulsion showed the following grain characteristics: an average equivalent circular diameter, ECD, of 1.4 µm, an average thickness T of 0.14 µm and an average aspect ratio AR of 10.

INVENTION EXAMPLE 1 Antihalation/antistatic layer

The antihalation/antistatic layers of the photothermographic recording materials of the EXAMPLES were prepared as described in unpublished EP-Application No. 96202108, filed July 24, 1996.

Silver behenate/silver halide emulsion

The silver behenate/silver halide emulsion was prepared by adding a solution of 6.8 kg of behenic acid in 67 litre of 2-propanol at 65°C to a 400 litre vessel heated in order to maintain the temperature of the contents at 65°C, converting 96 % of the behenic acid to sodium behenate by adding with stirring 76.8 litre of 0.25 M sodium hydroxide in deionised water, then adding with stirring 10.5 kg of the above-described silver halide emulsion at 40°C and finally adding with stirring 48 litre of a 0.4 M solution of silver nitrate in deionised water. Upon completion of the addition of silver nitrate the contents of the vessel were allowed to cool and the precipitate filtered off, washed, slurried with water, filtered again and finally dried at 40°C for 72 hours. 7 kg of the dried powder containing 9 mole % of silver halide and 4 mole % of behenic acid with respect to silver behenate were then dispersed in a solution of 700 g of Butvar™ B76 (polyvinyl butyral resin available from MONSANTO Co., St. Louis, Mo., USA) in 15.6 kg of 2-butanone using conventional dispersion techniques yielding a 33 % by weight dispersion. 7.4 kg of 2-butanone were then added and the resulting dispersion homogenised in a microfluidiser. Finally 2.8 kg of Butvar™ B76 were added with stirring to produce a dispersion with 31 % by weight of solids in order to get a good image quality.

Coating and drying of silver behenate/silver halide emulsion layer

The emulsion layer coating compositions for the photothermographic recording materials were prepared by adding the following solutions or liquids to 40.86 g of the above-mentioned silver behenate/silver halide emulsion in the following sequence with stirring: 10.87 g of 2-butanone, 0.75 g of a 9 % solution of TMABP (tetramethyl ammonium bromide perbromide) in methanol followed by 2 hours stirring, 1.3 g of 2-butanone, 0.2 g of a 11 % solution of calcium bromide in methanol and 1.3 g of 2-butanone followed by 30 minutes stirring and a solution consisting of 0.21 g of LOWINOX™ 22IB46 [(2-propyl-bis-(2-hydroxy-3,5-dimethylphenyl)methane, a trademarked product from LOWI GmbH, Germany], 0.5 g of tribromomethylbenzene sulfinate and 9.24 g of 2-butanone followed by 10 minutes stirring. An IR-sensitising dye solution as in unpublished EP-Application No. 96202100, filed July 24, 1996, was then added followed by 30 minutes stirring. Finally 4.35 g of Butvar™ B76 were added followed by 45 minutes of stirring and then 2-butanone to make the total weight up to 76 g.

The PET-foil (polyethylene terephthalate foil) subbed and coated with an antistatic layer as described above was then doctor blade-coated at a blade setting of 150 µm on the side of the foil not coated with an antistatic layer with the coating composition to a wet layer thickness of 80 µm, which was dried for 5 minutes at 80°C on an aluminium plate in a drying cupboard.

Protective layer

A protective layer coating composition for the photo-thermographic recording materials was prepared by dissolving 4.08 g of CAB (cellulose acetate butyrate resin, available from Eastman Kodak Co.) and 0.16g of PMMA (polymethyl methacrylate beads) in 36.3 g of 2-butanone and 4.16 g of methanol adding the following solids or solution with stirring in the following sequence: 0.5 g of phthalazine, 0.2 g of 4-methyl-phthalic acid, 0.1 g of tetrachlorophthalic acid, 0.2 g of tetrachlorophthalic acid anhydride and a solution consisting of 2.55 g of LOWINOX™ 22IB46 (see above) and 5.95 g 2-butanone.

The emulsion layer was then doctor blade-coated at a blade setting of 100 µm with the protective layer coating composition to a wet layer thickness of 57 µm, which after drying for 8 minutes at 80°C on an aluminium plate in a drying cupboard produced a layer with the same composition as in unpublished EP-Application No. 96202108, filed July 24, 1996.

Emulsions Nos. 1 to 5 were then coated at different coating weights onto the support described above. The coating weights expressed per square meter as the weight of an equivalent amount of silver nitrate can be found in Table I under the heading "coated".

Optical densities of the images were measured in transmission with a MacBeth™ TR924 densitometer through a visible filter. Values of differences in D_min calculated from the differences in minimum densities measured for the coated materials and for the bare support, before coating emulsion layer and protective layer were determined and were summarised in Table 1 as "δD" values. The average equivalent circular diameter (ECD), the sphere equivalent volume diameter (VD), defined hereinbefore and the average thickness (T) as determined from electron microscopic photographs according to the method described in US-A 4,414,304 are also listed.

Material No.	Composition AgX	Coated (g/m2)	ECD (µm)	T (µm)	VD (µm)	δD	Haze
1 (comp.)	AgBr(I) tab	3.8	1.7	0.09	0.88	0.28	94%
2 (comp)	AgBr(I) tab	3.9	1.2	0.11	0.65	0.29
3 (comp)	AgBr(I) tab	4.3	0.84	0.23	0.60	0.35	84%
4 (comp)	AgBr(I) cub	3.7			0.71	0.37
5 (inv.)	AgCl(I) tab	3.7	1.4	0.14	0.90	0.13	87%
6 (comp.)	AgBr(I) tab	1.1	1.7	0.09	0.88	0.10	60%
7 (comp.)	AgBr(I) tab	1.1	1.2	0.11	0.65	0.10	60%
8 (comp.)	AgBr(I) tab	1.1	0.84	0.23	0.60	0.13	75%
9 (comp.)	AgBr(I) cub	1.1			0.71	0.16
10 (inv.)	AgCl(I) tab	1.0	1.4	0.14	0.90	0.05	50%
11(comp.)	AgBr(I) tab	0.6	1.7	0.09	0.88	0.05	38%
12(comp.)	AgBr(I) tab	0.6	0.84	0.23	0.60	0.08	51%
13(comp.)	AgBr(I) cub	0.6			0.71	0.09
14 (inv.)	AgCl(I) tab	0.6	1.4	0.14	0.90	0.03	30%

From Table 1 it can be concluded that a transparency as calculated from differences of minimum density in the photothermographic material prior to exposure and the minimum density of the bare support of less than 0.05 is only achieved for thin tabular silver halide grains at low coating weights. Moreover, it is more easily attained for tabular grains rich in silver chloride than for grains rich in silver bromide. Besides the morphology of the silver halide crystals, the halide composition and the thickness of the tabular grains, the coating weight of the said tabular grains (1.0 g or less) is also important, the transparency increasing with decreasing coating weight of tabular grains.

For selected materials, the "haze" was determined at a wavelength of 660 nm. Measurements were performed by means of a Gardner Haze Meter XL-211 Model 8011 according to ASTM D1003, haze being the percentage diffuse transmitted light in the transmitted light.

The haze values obtained with the different emulsions at different coating weights of silver halide, expressed per square meter as the corresponding equivalent weight of silver nitrate are listed in Table 2 hereinafter. It can be seen that the haze decreases with coating weight and that materials with thin tabular {111} grains rich in silver chloride exhibit significantly lower haze values than materials with thin tabular {111} grains rich in silver bromide.

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.

Claims (10)

1. A photothermographic recording material comprising a support bearing a photo-adressable thermosensitive element comprising photosensitive silver halide grains, a reducing agent for silver ions and a binder, characterised in that said silver halide grains are tabular silver halide grains having silver chloride in an amount of 70 mole % or more and an average grain thickness of 0.20 µm or less and wherein said tabular grains account for at least 50 % of the total projective surface area of all silver halide grains.
2. Material according to claim 1, wherein said silver halide grains have {111} major faces.
3. Material according to claim 1 or 2, wherein said silver halide grains have an average grain thickness of from 0.03 up to 0.18 µm.
4. Material according to any of claims 1 to 3, wherein said silver halide grains have an average aspect ratio of from 5:1 to 50:1.
5. Material according to any of claims 1 to 4, wherein said tabular silver halide grains are silver chloride, silver chloroiodide grains, silver chlorobromide or silver chlorobromoiodide.
6. Material according to claim 5, wherein said tabular silver chloroiodide grains have iodide in an amount of from 0.1 up to 3 mole %.
7. Material according to any of claims 1 to 6, wherein said tabular grains are present in an amount of up to 2 g/m² of silver, expressed as an equivalent amount of silver nitrate.
8. Material according to any of claims 1 to 7, said material having prior to exposure a density difference of less than 0.10, measured by means of a MacBeth™ TD501 densitometer between density of the said material prior to exposure and density of the bare support of the said material before coating thereupon a photosensitive layer.
9. Material according to any of claims 1 to 7, said material having prior to exposure a density difference of 0.05 or less, measured by means of a MacBeth™ TD501 densitometer between density of the said material prior to exposure and density of the bare support of the said material before coating thereupon a photosensitive layer.
10. Material according to any of claims 1 to 9, wherein said photo-adressable thermosensitive element further comprises a substantially light-insensitive organic silver salt in molar ratio amounts of light-sensitive silver halide to substantially light-insensitive organic silver salt of from 0.1 up to 90 mole % of said substantially light-insensitive organic silver salt.