DE69515496T2 - LASER ADDRESSABLE THERMOGRAPHIC ELEMENTS - Google Patents

Description

The present invention relates to novel thermographic imaging elements (imaging elements) and, more particularly, to thermographic imaging elements that can be directly imaged using an infrared laser diode. The present invention also relates to methods for imaging the thermographic imaging elements of the invention using an infrared laser diode.

Imaging elements which can be imagewise exposed by light or heat are well known in the imaging art. Conventional photographic and photothermographic silver halide elements are the most typical elements of the class of photosensitive materials. In both conventional photographic ("wet silver") and photothermographic ("dry silver") elements, exposure of silver halide in the photosensitive emulsion produces small clusters of silver atoms (Ag). The imagewise distribution of these clusters is known in the art as a latent image. Generally, the latent image produced is not visible by conventional means and the photosensitive emulsion must be further processed to produce a visible image. In both dry and wet silver systems, the visible image is produced by reduction of silver ions present in catalytic proximity to silver halide grains which form the clusters of silver atoms, i.e. the latent image. This results in a black and white image.

Conventional silver halide photographic elements require a wet development process to make the latent image visible. The wet chemistry used in this process requires special handling and disposal of the chemicals used. The equipment for the process is extensive and requires special installation work.

In photothermographic elements, the photographic silver halide is in catalytic proximity to a light-insensitive, reducible silver source (e.g., silver behenate) so that when silver grains are produced by exposure of the silver halide, these grains can catalyze the reduction of the reducible silver source. The latent image is revealed by applying uniform heat across the element. The thermal devices used to develop photothermographic elements resolve the Problems in conventional photographic elements due to use of a dry process. However, photothermographic elements processed using these devices may have uneven or non-uniform image density, image distortion, and/or surface abrasion defects. Defects due to non-uniform image density can occur, for example, due to surface changes on the heated element, the presence of foreign matter on the photothermographic element or the heated element, and insufficient removal of volatile materials generated during processing. Image defects can occur due to uncontrolled size changes on the support of the photothermographic element during heating and/or cooling of the photothermographic element. Surface abrasion or damage can occur due to dragging the photothermographic element across a stationary component of the heating device. For many applications, such as text or line drawings, these defects can be tolerated. Users in medical diagnostics, industrial, graphic arts, printed circuit boards and other imaging applications require consistent, high-quality images.

U.S. Patent No. 5,041,369 describes a process that takes advantage of a dry-processed photothermographic element without the need for surface contact with a heating device. The photothermographic element is imagewise exposed to a laser that splits the beam using a second matching radiation device. In this process, the element is simultaneously exposed to one wavelength of light and thermally activated by the absorption of a light/heat near infrared (NIR) dye at the second wavelength of light. Although this process also has the advantage of simultaneous exposure and heat development of the image, the equipment required is complicated and limited by laser outputs that can produce two useful separate wavelengths. In addition, the photosensitive emulsion still requires both light and heat activation to produce an image.

Photosensitive emulsions containing silver halide are known in the art to provide high minimum density (Dmin) in both the visible and ultraviolet (UV) parts of the spectrum. The high UV Dmin is due to the inherent absorption of silver halides, particularly silver bromide and silver iodide, in the short UV and the strong fog when silver halide and organic silver salts are present together. For screens in the graphic arts and for imaging unit films, a high UV Dmin is undesirable because it increases the required exposure time during contact exposure with other media such as UV sensitive printing plates, proof films and papers. A strong fog also results in a loss of image resolution when imaged photothermographic elements are used as contact films. It is also known that silver halides in photothermographic elements can result in poor light stability of the image background density, producing fog.

One class of imaging elements that does not rely on silver halide chemicals are thermographic elements. These materials are widely used for facsimile machines, labels, tickets, charts for recording data from medical or scientific monitoring equipment, and the like. In the most common form, the thermographic element comprises a support having a coating of a thermally sensitive composition comprising a color former, usually a substantially colorless electron donating dye precursor, and a color developer, usually an electron accepting compound. Heat is applied to the element imagewise, by a thermal tip, thermal style, or laser beam, and upon imagewise heating, the color former immediately reacts with the color developer to form an image. U.S. Patent No. 4,904,572 describes a thermographic element that uses leuco dyes, enhancing the developed image. A leuco dye is the reduced form of a color-bearing dye. It is generally colorless or very lightly colored. In this application, silver behenate acts as a Lewis acid that reacts with the leuco dye upon imagewise application of heat to produce a colored image. A black image is obtained by combining subtractive colors (cyan, yellow and magenta). It is known in the art that it is very difficult to obtain a high density neutral black using subtractive colors. Since the image is formed by forming colored dyes, the absorption of the image in the ultraviolet is weak and therefore it provides little utility as a UV masking film.

Conventional thermographic films typically require imaging dwell times of 1 to 5 seconds. Treatment times of this length are not practical in a laser imaging application. To provide a suitable imaging dwell time for a laser delivery system, a thermographic film construction is required that produces an image in-situ in microseconds. A laser-addressable thermal recording material is known from EP-A-0 582 144.

Each of the above-mentioned classes of imaging elements has some disadvantages. For example, conventional silver halide photographic materials have a high environmental impact due to wet chemical processing, photothermographic materials have lower image permanence, limited optical density and poor dimensional stability, silver halide-based emulsions typically use visible sensitizers that can be bleached or removed and stored in the dark or must be handled in dim light, both conventional photographic and photothermographic elements require a two-step process (exposure and development), and conventional thermographic elements require high energy to form images, relatively long thermal dwell times, and have poorer image permanence and limited UV optical density.

What is needed in the industry are imaging elements and methods that help to overcome the problems disclosed above. The present invention has been developed under these conditions.

The present invention relates to the subject matter disclosed in the claims.

In one embodiment, the present invention provides a thermographic imaging element comprising a support coated on at least one surface with a thermographic imaging system comprising at least one layer comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a binder, benzotriazole as a light stabilizer, a toner comprising barbituric acid, and a dye that absorbs radiation in the wavelength range of 750-1100 nm, wherein the at least one layer comprising the light-insensitive organic silver salt produces an image density of greater than about 1.0 when exposed to radiation of 0.10-2.0 joules/cm2 (in the wavelength range of 750-1100 nm) in 0.2-200 microseconds.

In another embodiment, the present invention provides a thermographic imaging element comprising a support coated with a thermographic imaging system, the thermographic imaging system comprising at least two contiguous layers, one of the contiguous layers comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a binder, benzotriazole as a light stabilizer, a toner comprising barbituric acid, and optionally a dye which absorbs radiation in the wavelength range of about 750-1100 nm, and the other contiguous layer consisting essentially of a dye which absorbs radiation in the wavelength range of about 750-1100 nm and a binder, the layer comprising the light-insensitive organic silver salt producing an image density of greater than about 1.0 when exposed to radiation of 0.10-2.0 joules/cm2 (having a wavelength range of about 750-1100 nm) in 0.2-200 microseconds.

In another embodiment, the present invention provides a method of producing an image comprising the step of exposing a thermographic imaging element comprising a support coated with a thermographic imaging system and comprising at least one layer comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a binder, Benzotriazole as a light stabilizer, a dye which absorbs radiation in the wavelength range of about 750-1100 nm, and a toner comprising barbituric acid, by radiation in the range of about 750-1100 nm, wherein the at least one layer comprising the light-insensitive organic silver salt produces an image density greater than about 1.0 when exposed to radiation of 0.10-2.0 joules/cm² (having a wavelength range of about 750-1100 nm) in 0.2-200 microseconds.

In yet another embodiment, the present invention provides a method of producing an image comprising the step of exposing a thermographic imaging element comprising a support coated with a thermographic imaging system, the thermographic imaging system comprising at least two adjacent layers, one of the adjacent layers comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a binder, benzotriazole as a light stabilizer, a toner comprising barbituric acid, and optionally a dye which absorbs radiation in the wavelength range of about 750-1100 nm, and the other adjacent layer consisting essentially of a binder and a dye which absorbs radiation having a wavelength range of about 750-1100 nm, to radiation having a wavelength range of about 750-1100 nm which is directed onto the thermographic imaging element through the layer comprising the light-insensitive organic silver salt, before it falls on the adjacent layer consisting essentially of a binder and a dye, such that the layer comprising the light-insensitive organic silver salt produces an image density greater than about 1.0 when exposed to radiation of 0.1-2.0 joules/cm² (having a wavelength range of about 750-1100 nm) in 0.20-200 microseconds.

In a preferred embodiment of the foregoing inventions, an image density of greater than about 2.00, more preferably greater than about 2.50, most preferably greater than about 2.75, comprising the formation of metallic silver in the layer comprising a light-insensitive organic silver salt, a reducing agent, etc., is produced upon exposure to radiation of 0.10-2.0 joules/cm2 (having a wavelength range of about 750-1100 nm) in 0.2-200 microseconds.

The layers ("thermographic silver emulsion layers") comprising light-insensitive organic silver salt, a reducing agent for silver ions, etc., in all of the above-disclosed embodiments of the present invention may have up to about 1.0% by weight of silver halide, based on the total weight of the layer, mixed in.

The silver-based thermographic imaging elements and the methods of using the thermographic imaging elements as a laser-addressable direct The writable film provided by the present invention overcomes many of the problems associated with prior art systems. Since the thermographic imaging element is thermally sensitive rather than light sensitive, it is handleable in white light and removal of a visible sensitizer is not necessary. Unlike wet silver and photothermographic elements, no post-processing steps are required to develop the image. When a high energy laser diode scans the thermographic imaging element, a black image is printed in-situ in the thermographic silver emulsion, thus enabling many useful applications such as an on-line inspection system for printed circuit board photomask fabrication. In addition, heat shrinkage of the film is greatly reduced since only the imaged portion of the emulsion is heated and the temperature of the substrate remains relatively unchanged. This is particularly important for applications where registration is critical such as image set films for color reproduction and photoassistants for printed circuit boards. In addition, the thermographic imaging element is capable of producing high resolution halftone images that are useful in color reproduction processes.

Other aspects, advantages and benefits of the present invention will be apparent from the detailed description, examples, drawings and claims.

Fig. 1 shows a schematic representation of a laser sensitometer.

Fig. 2 shows a graph of distance versus relative intensity of a laser diode, comparing the plots for theoretical to actual profile values for a flat truncated cone-shaped laser spot on the film surface.

Fig. 3a shows a graphical representation of the total incident exposure energy plotted against the distance across the laser beam in the cross-scan direction.

Fig. 3b shows a microdensitometer profile of a line imaged with an energy profile depicted in Fig. 3a on a thermographic element (Example 8, Sample N is not shown).

Figure 4 shows a plot of density versus log of exposure using the data shown in Figure 3a.

Figure 5 shows a plot of absorbance versus wavelength comparing the imaged and non-imaged areas of a thermographic imaging element. (Example 8, Sample N not shown).

The term "thermographic imaging element" as used herein refers to a support coated on at least one surface with a "thermographic imaging system".

The photoinsensitive silver salts are materials that are reduced in the presence of a reducing agent at elevated temperatures, e.g., 60º-225ºC, to produce metallic silver. These materials are preferably silver salts of long chain alkanoic acids (also known as long chain aliphatic carboxylic acids or fatty acids) containing 4 to 30 carbon atoms, more preferably 8 to 28 carbon atoms, most preferably 10 to 22 carbon atoms. The latter are also known in the art as "silver soaps."

Non-limiting examples of silver salts of aliphatic carboxylic acids include silver behenate, silver stearate, silver oleate, silver erucate, silver laurate, silver caproate, silver myristate, silver palmitate, silver maleate, silver fumarate, silver tartrate, silver linoleate, silver camphorate, and mixtures thereof. Complexes of organic or inorganic silver salts wherein the ligand has an overall stability constant between 4.0 to 10.0 may also be used. Silver salts of aromatic carboxylic acids and other carboxyl group-containing compounds include silver benzoate, a substituted silver benzoate such as silver 3,5-dihydroxybenzoate, silver o-methylbenzoate, silver m-methylbenzoate, silver p-methylbenzoate, silver 2,4-dichlorobenzoate, silver acetamidobenzoate, silver p-phenylbenzoate, etc., silver gallate, silver tannate, silver phthalate, silver terephthalate, silver salicylate, silver phenylacetate, silver pyromellitate, silver salts of 3-carboxymethyl-4-methyl-4-thiazoline-2-thiones or the like as described in U.S. Patent No. 3,785,830; and silver salts of aliphatic carboxylic acids containing a thioether group as disclosed in U.S. Patent No. 3,330,663. Silver salts of compounds containing mercapto or thione groups and derivatives thereof can also be used. Preferred examples of these compounds include silver 3-mercapto-4-phenyl-1,2,4-triazolate, silver 2-mercaptobenzimidazolate, silver 2-mercapto-5-aminothiadiazolate, silver 2-(S-ethylglycolamido)benzothiazolate, silver salts of thioglycolic acids such as silver salts of S-alkylthioglycolic acids (wherein the alkyl group has 12 to 22 carbon atoms), silver salts of dithiocarboxylic acids such as silver dithioacetate, silver thioamidate, silver 1-methyl-2-phenyl-4-thiopyridine-5-carboxylate, silver triazinethiolate, silver 2-sulfidobenzoxazole, and silver salts disclosed in U.S. Patent No. 4,123,274. In addition, silver salts of a compound containing an amino group can be used. Preferred examples of these compounds include silver salts of benzotriazoles such as silver benzotriazolate; silver salts of alkyl-substituted benzotriazoles such as silver methylbenzotriazolate, etc., silver salts of halogen-substituted benzotriazoles such as silver 5-chlorobenzotriazolate, etc., silver salts of carbimidobenzotriazoles, etc., silver salts of 1,2,4-triazoles and 1-H-tetrazoles as described in U.S. Patent No. 4,220,709, silver salts of imidazoles, and the like. The light-insensitive silver salt material should preferably contain about 5 to 60 wt.%, more preferably about 30 to 50 % by weight, based on the total weight of the thermographic silver emulsion layer.

Any reducing agent for silver ions can be used in the present invention. The reducing agents are known in the art. Examples of the reducing agents include, but are not limited to, methyl gallate, hindered phenols, catechol, pyrrogallol, hydroquinones, substituted hydroquinones, ascorbic acid, ascorbic acid derivatives, leuco dyes and the like. The most preferred reducing agents are methyl gallate, butyl gallate and propyl gallate. The reducing agent used in the present invention is preferably used in an amount of from about 5.0 to 25.0% by weight, more preferably from about 10.0 to 20.0% by weight, based on the total weight of the thermographic silver emulsion layer.

The toners used in the thermographic silver emulsion layer(s) include barbituric acid. A combination of barbituric acid with other toners is particularly useful, the preferred combinations being phthalazinone with barbituric acid and phthalimide with barbituric acid, and most preferably succinimide with barbituric acid. The toner(s) should preferably be present in an amount ranging from about 0.2 to 10 wt%, more preferably about 1.0 to 8.0 wt%, most preferably about 2.0 to 6.0 wt%, based on the total weight of the thermographic silver emulsion layer.

Optionally, auxiliary reducing agents or development accelerators, depending on the silver source used, may be included in the thermographic silver emulsion layer. The auxiliary reducing agent preferably comprises a 3-indazolinone or urea compound as a development accelerator.

3-Indazolinone compounds used in the present invention preferably have the following structure:

wherein R is selected from a hydrogen atom; an alkyl group having 1 to 4 carbon atoms; a halogen atom; -COOH and R¹COOH, wherein R¹ is an alkyl group having 1 to 4 carbon atoms. R is preferably a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, most preferably R is a hydrogen atom.

The 3-indazolinone compounds can be synthesized by methods that are known to those skilled in synthetic organic chemistry. Alternatively, the materials are commercially available from, for example, Aldrich Chemical Company of Milwaukee, Wisconsin; Lancaster Chemical Company of Windham, New Hampshire; and K&K Laboratories of Cleveland, Ohio.

It is understood in the art that a high degree of substitution is not only tolerated but often recommended. Therefore, the term "radical" as used herein includes not only pure hydrocarbon substituents such as methyl, ethyl and the like, but also hydrocarbon substituents having substituents common in the art such as hydroxy, alkoxy, phenyl, halogen (F, Cl, Br, I), cyano, nitro, amino, etc. Urea compounds used in the present invention preferably have the following formula:

wherein R² and R³ independently represent a hydrogen atom, a C₁-C₁₀ alkyl or cycloalkyl group or a phenyl group, or R² and R³ together form a heterocyclic group having up to 6 ring atoms. R² and R³ preferably represent a hydrogen atom, a C₁ to C₅ alkyl or cycloalkyl group or a phenyl group, or R² and R³ together form a heterocyclic group having up to 5 ring atoms. The urea compounds can be easily synthesized and are commercially available. Non-limiting examples of the urea compounds include urea, 1,3-diphenylurea, 1,3-diethylurea, butylurea and 2-imidazolidone. The most preferred development accelerator is 2-imidazolidone.

The thermographic imaging elements of the present invention are light insensitive in the traditional sense and therefore do not need to contain photosensitive agents such as silver halides, photoinitiators or photobleaching agents. The thermographic silver emulsion layers can contain less than 1%, less than 0.75%, less than 0.5% or 0% by weight based on the total weight of the thermographic silver emulsion and work well. The silver halide is considered ineffective if it does not catalyze the formation of a latent image.

One light stabilizer used is benzotriazole. Light stabilizers such as phenylmercaptotetrazoles and other light stabilizers known in the art may be added to the thermographic silver emulsion. The light stabilizer should preferably be present in an amount ranging from about 0.1% to 3.0% by weight of the thermographic silver emulsion layer, more preferably from 0.3% to 2.0% by weight.

A binder is also used in the thermographic silver emulsion layer used in the present invention. Any conventional polymeric binder known in the art can be used. The binder can, for example, be selected from any of the known natural and synthetic resins such as gelatin, polyvinyl acetals, polyvinyl chloride, cellulose acetate, polyolefins, polyesters, polystyrene, polyacrylonitrile, polycarbonates and the like. Copolymers and terpolymers are of course included in these definitions and examples thereof include, but are not limited to, polyvinyl aldehydes such as polyvinyl acetals, polyvinyl butyrals, polyvinyl formals and vinyl copolymers. The binder should preferably be present in an amount in the range of 10 to 60% by weight, more preferably 15 to 50% by weight, based on the total weight of the thermographic silver emulsion layer.

The thermographic element of the present invention utilizes a dye that absorbs electromagnetic radiation having a wavelength range between about 750-1100 nm, preferably in the range of about 750-900 nm, most preferably in the range of about 750-870 nm.

The dye should be soluble in the coating solvent, preferably ketones or aromatic solvents such as methyl ethyl ketone or toluene. The dye should also be miscible with the binder and compatible with the silver salts, activators and developers used in the emulsion. For use in a UV (ultraviolet) contact film or mask application, the optical density of the dye is preferably greater than 1.0 units of optical density with a weak simultaneous absorption of less than 0.2 units of optical density in the UV region corresponding to the wavelength of the exposure apparatus for which the material is intended. as a mask (250-450 nm). The optical density is measured using a MacBeth Model TD 523 densitometer fitted with a Status 18A filter. It is also desirable, but not necessary, that the dye have a low visible background absorption.

The radiation absorbing dye can be applied in the same layer as the light-insensitive organic silver salt, a reducing agent for silver ions, a toner and a binder. Alternatively, the dye can be applied both in the preceding layer and in an adjacent layer or primarily in the adjacent layer. The radiation absorbing dye can be added directly to the thermographic silver emulsion layer or indirectly by allowing the dye to migrate from the adjacent layer containing the dye into the thermographic silver emulsion layer during the manufacturing process of the thermographic imaging element.

Suitable dyes include, but are not limited to, oxonol, squarylium, chalcogenopyrylarylidene, bis(chalcogenopyrylo)polymethine, bis(aminoaryl)polymethine, merocyanine, trinuclear cyanine, polymethine-indene bridge, oxyindolizine, iron(II) complex, quinoid, nickeldithiolene complex and cyanine dyes such as carbocyanine, azacarbocyanine, hemicyanine, styrene, diazacarbocyanine, triazacarbocyanine, diazahemicyanine, polymethinecyanine, azapolymethinecyanine, holopolar, indocyanine and diazahemicyanine dyes.

The amount of dye present in the thermographic imaging element depends on whether the dye is incorporated solely in the thermographic silver emulsion layer or also in an adjacent layer. When the dye is incorporated solely in the thermographic silver emulsion layer, the dye is present in an amount of 0.10-5.0 wt.%, preferably 0.2-3.0 wt.%, based on the total weight of the thermographic silver emulsion layer.

When the dye is present in an adjacent layer, it is present in the thermographic silver emulsion layer in an amount of 0-5.0 wt.%, preferably about 0-1.0 wt.%, based on the total weight of the thermographic silver emulsion layer. In the adjacent layer containing dye and binder, the dye is present in an amount of 1-25 wt.%, preferably 5-20 wt.%, based on the total weight of the adjacent layer.

Any suitable base or support material known in the art may be used in the present invention. The materials may be opaque, semi-transparent, or transparent. Commonly employed base or support materials used in the thermographic arts include paper, opaque or transparent polyester and polycarbonate films, and specular light-reflecting specular light-reflecting metallic substrates such as silver, gold and aluminum. The term "specular light-reflecting metallic substrates" as used herein refers to metallic substrates that reflect light at a particular angle when struck by light, as opposed to reflecting light over a range of angles.

Optionally, a protective or non-adhesive layer coated as a topcoat of the thermographic imaging element may be used. Any conventional non-adhesive material may be used in the present invention. Examples of the non-adhesive materials include, but are not limited to, waxes, silicate particles, styrene-containing elastomeric block copolymers such as styrene-butadiene-styrene, styrene-isoprene-styrene, and blends thereof with materials such as cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, and poly(vinyl butyral).

Additional layers, such as a primer layer or an antistatic layer, may be applied to the thermographic elements of the invention. Additionally, an antistatic or non-adhesive layer may optionally be applied to the back of the support. Materials for these purposes are known in the art.

The thermographic imaging system, non-adhesive, infrared or near infrared dye absorbing and antistatic layers employed in the present invention can be coated by any conventional method such as knife coating, roll coating, dip coating, curtain coating, bulk coating, etc. If desired, two or more layers can be coated simultaneously by the methods described in U.S. Patent No. 2,761,791 and British Patent No. 837,095.

The thermographic imaging elements of the present invention are imaged by exposure to infrared or short infrared laser radiation, typically from an infrared or short infrared laser diode. As is known in the thermal imaging art, infrared or short infrared laser diodes can be advantageously arranged in series to increase imaging sensitivity. Lasers that can be used to provide infrared or short infrared radiation include essentially any laser capable of producing light in the infrared and short infrared region of the electromagnetic spectrum from about 750 to 1100 nm, including dye lasers, solid state diode lasers such as aluminum gallium arsenide diode lasers emitting in the range of 780 to 870 nm, and diode pumped solid state lasers such as Nd:YAG, Nd:YLF or Nd:glass.

Some embodiments and preferred embodiments of the present invention are summarized in the following points.

1. A thermographic imaging element comprising a support coated on at least one surface with a thermographic imaging system comprising at least one layer comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a binder, benzotriazole as a light stabilizer, a toner comprising barbituric acid, and a dye which absorbs radiation in the wavelength range of 750-1100 nm, wherein the at least one layer comprising the light-insensitive organic silver salt produces an image density of greater than about 1.0 when exposed to radiation of 0.10-2.0 joules/cm² in 0.20 to 200 microseconds.

2. A thermographic imaging element comprising a support coated with a thermographic imaging system, the thermographic imaging system comprising at least two adjacent layers, one of the adjacent layers comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a toner comprising barbituric acid, a binder, benzotriazole as a light stabilizer, and optionally a dye which absorbs radiation in the wavelength range of about 750-1100 nm, and the other adjacent layer essentially comprising a binder and the dye which absorbs radiation in the wavelength range of about 750-1100 nm, the layer comprising the light-insensitive organic silver salt producing an image density of greater than about 1.0 when exposed to radiation of 0.10-2.0 joules/cm2 in 0.20 to 200 microseconds.

3. The element according to item 1 or 2, wherein the light-insensitive organic silver salt is a silver salt of a carboxylic acid having 10-30 carbon atoms.

4. The element according to item 1 or 2, wherein the silver salt of a carboxylic acid is silver behenate or silver laurate.

5. The element of item 1 or 2, wherein the thermographic imaging system further comprises a development accelerator.

6. Element according to item 5, wherein the development accelerator is selected from:

(i) a 3-indazolinone compound of the formula:

wherein R is selected from a hydrogen atom, an alkyl radical having 1 to 4 carbon atoms, a halogen atom, -COOH and R¹ COOH, wherein R¹ is an alkyl radical having 1 to 4 carbon atoms; and

(ii) a urea compound of the formula:

where R² and R³ independently represent a hydrogen atom, a C₁-C₁₀ alkyl or cycloalkyl radical or a phenyl group or R² and R³ together form a heterocyclic radical having up to 6 ring atoms.

7. Element according to item 6, wherein R represents a hydrogen atom; R² and R³ independently represent a C₁ to C₅ alkyl or cycloalkyl radical or a phenyl group or R² and R³ together form a heterocyclic radical having up to 5 ring atoms.

8. The element of item 1 or 2, wherein the dye absorbs radiation having a wavelength range of about 750 to 900 nm.

9. The element of item 1 or 2, wherein the image density is greater than about 2.00, more preferably greater than about 2.50, most preferably greater than about 2.75, and comprises metallic silver.

10. The element of item 1 or 2, wherein the toner further comprises at least one selected from phthalazinone, succinimide and phthalimide.

11. A method of producing an image comprising the step of exposing a thermographic imaging element comprising a support coated with a thermographic imaging system, the thermographic imaging system comprising at least one layer comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a dye which absorbs radiation in the wavelength range of about 750-1100 nm, a toner comprising barbituric acid, benzotriazole as a light stabilizer and a binder, wherein the exposure is with radiation having a wavelength range of about 750-1100 nm, wherein the at least one layer comprising the light-insensitive organic silver salt has an image density greater than about 1.0 when exposed to radiation of 0.10-2.0 joules/cm² in 0.20-200 microseconds.

12. The method of item 11, wherein the image density is greater than about 2.75 and comprises metallic silver.

13. A method of producing an image comprising the step of exposing a thermographic imaging element comprising a support coated with a thermographic imaging system, the thermographic imaging system comprising at least two adjacent layers, one of the adjacent layers comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a toner comprising barbituric acid, a binder, benzotriazole as a light stabilizer, and optionally a dye which absorbs radiation having a wavelength range of about 750-1100 nm, and the other adjacent layer consisting essentially of a binder and the dye which absorbs radiation having a wavelength range of about 750-1100 nm, the exposure being to radiation having a wavelength range of about 750-1100 nm directed onto the thermographic imaging element through the layer comprising the light-insensitive organic silver salt before being applied to the adjacent layer consisting essentially of binder and the dye which absorbs the radiation such that the layer comprising the light-insensitive organic silver salt produces an image density of greater than about 1.0 when exposed to radiation of 0.10-2.0 joules/cm² in 0.20 to 200 microseconds.

14. The process according to item 11 or 13, wherein the light-insensitive organic silver salt is a silver salt of a carboxylic acid having 10-30 carbon atoms.

15. A process according to item 11 or 13, wherein the silver salt of a carboxylic acid is silver behenate or silver laurate.

16. The method of item 11 or 13, wherein the thermographic imaging system further comprises a development accelerator.

17. Method according to item 16, wherein the development accelerator is selected from

(i) a 3-indazolinone compound of the formula:

wherein R is selected from a hydrogen atom, an alkyl radical having 1 to 4 carbon atoms, a halogen atom, -COOH and R¹ COOH, wherein R¹ is an alkyl radical having 1 to 4 carbon atoms; and

(ii) a urea compound of the formula:

where R² and R³ independently represent a hydrogen atom, a C₁-C₁₀ alkyl or cycloalkyl radical or a phenyl group or R² and R³ together form a heterocyclic radical having up to 6 ring atoms.

18. Process according to item 17, wherein R is a hydrogen atom, R² and R³ independently represent a C₁-C₅ alkyl or cycloalkyl radical or a phenyl group, or R² and R³ together form a heterocyclic radical having up to 5 ring atoms.

19. The method according to item 11 or 13, wherein the dye absorbs radiation having a wavelength range of about 750 to 900 nm.

20. The method of item 11 or 13, wherein the image density is greater than about 2.00, more preferably greater than about 2.50, and comprises metallic silver.

21. The method of item 13, wherein the image density is greater than about 2.75.

22. The method according to item 11 or 13, wherein the toner further comprises at least one selected from phthalazinone, succinimide and phthalimide.

23. The method according to item 13, wherein the dye which absorbs radiation having a wavelength of about 750-1100 nm is present in both adjacent layers.

The following non-limiting examples are intended to further illustrate the present invention.

Examples

The materials used in the examples below are available from common commercial sources such as Aldrich Chemical Co. (Milwaukee, WI), unless otherwise indicated.

Silver behenate and silver laureate homogenates can be prepared as disclosed in U.S. Patent No. 4,210,717 (column 2, lines 55-57) or U.S. Patent No. 3,457,075 (column 4, lines 23-45 and column 6, lines 37-44).

The following dyes were used in the various examples below:

Preparation of Dye 1: 5-Sulfamoyl-2,3,3-trimethylindolenine was prepared according to the procedure described in U.S. Patent No. 4,062,682. A mixture of 37.0 g of 5-sulfamoyl-2,3,3-trimethylindolenine, 16.7 mL of 2-chloroethanesulfonyl chloride, and 200 mL of acetonitrile was heated to reflux for 6 hours. After addition of 18.5 mL of water, the mixture was stirred overnight. The separated solid was filtered, washed with acetonitrile, and dried to give 11.0 g of a 1-sulfoalkylated quaternary salt as an intermediate.

A mixture of 6.5 g of chlorocyclopentenedialdehyde, 26 g of the intermediate quaternary salt prepared above, 108 ml of acetic anhydride and 72 ml of acetic acid was stirred at room temperature for 10 minutes. After addition of 12.8 ml of diisopropylethylamine, the mixture was stirred overnight. The separated solid was filtered, washed with the solvent mixture and dried to give 20.0 g of Dye 1.

Preparation of Dye 2: In a 3 L flask was added 385 g of trimethylsulfonamidoindolenine with 250 mL of butyronitrile. To the mixture was added, without exotherm, 225 mL (364 g) of butyl iodide and then 750 mL of additional butyronitrile. The mixture was heated to reflux with thorough overhead stirring for 22.5 hours. The heat was removed and the mixture was allowed to cool to about 40°C. Stirring was continued for 1 hour after addition of 1 L of ethyl acetate. The solid was filtered off, washed with ethyl acetate and dried to give 595.6 g of N-butyl-2,3,3-trimethyl-5-sulfonamidoindoleninium iodide.

To a solution of 370 mL of methylene chloride and 558 mL of dimethylformamide cooled to 5°C was added 277 mL of phosphorus oxychloride dropwise over 1 hour at a rate such that the temperature did not rise above 5°C. After the addition was complete, external cooling was removed and the mixture was stirred for 1 hour. Over 30 minutes, 75 mL of cyclopentanone was added in two portions. After the first addition, a slow increase in temperature to about 35°C and an increase in color was observed, then the second portion was added, resulting in a strong exothermic reaction. After the exothermic reaction had subsided, the mixture was heated to reflux for 4 hours. The mixture was distilled under a slight vacuum after the addition of 1 L of ethyl acetate. About 250 mL of liquid was collected to which 700 mL of ethyl acetate was added, then a precipitate began to form. The mixture was stirred overnight. The solid was filtered off, washed with 1 L of ethyl acetate, then with heptane and dried in vacuo at 35°C for 4 hours to give 115.8 g of crude chlorocyclopentenedialdehyde.

The crude chlorocyclopentenedialdehyde was dissolved in 1250 mL of water. After about 1 hour, crystallization occurred. The mixture was left to stand over the weekend. The brownish solid was filtered, washed with water and dried under vacuum at 35°C for 7 hours to yield 61.0 g of chlorocyclopentenedialdehyde.

To a solution of 450 mL of acetic acid and 450 mL of acetic anhydride were added 278.7 g of N-butyl-2,3,3-trimethyl-5-sulfonamidoindoleninium iodide and 47.6 g of chlorocyclopentenedialdehyde. To the mixture was added dropwise 90 mL of triethylamine over 5 minutes at 60-65°C with stirring. No strong exotherm was observed. The mixture was heated for an additional 30 minutes, then the heat source was removed and the reaction mixture was cooled to 15°C. The resulting golden brown solid was filtered and washed with a 1:1 mixture of acetic acid:acetic anhydride until the wash liquor was green rather than purple. Residual acetic acid and acetic anhydride were removed by suspending the solid in 1 L of ethyl acetate followed by stirring for 90 minutes. The solid was filtered and washed with ethyl acetate The filtrate was pink in color. The solid was dried overnight at 45 °C in vacuo to give 250.0 g of dye 2.

Examples 1-3

The following coating solutions were used in the preparation of Examples 1-3 (Examples 1 and 2 are not covered by the claims):

Silver emulsion:

Silver behenate homogenate (10 wt.% in methyl ethyl ketone) 160 g

Butvar® B76 Poly(vinyl butyral), available from Monsanto Co. 20 g

Thermographic coating solutions:

The thermographic coating solutions for Examples 1-3 were prepared by mixing the following ingredients with 20 g of the silver emulsion described above:

Each solution was coated onto a 0.08 mm (3 mils) polyester support at a 0.1 mm (4 mils) wet thickness and air dried at 60ºC for 3 minutes.

An infrared absorbing overcoat solution was prepared by mixing 0.08 g of Dye 1, 1.0 g of CA398-6 cellulose acetate resin, and 20.0 g of MEK. The overcoat solution was coated onto the thermographic layer at a wet thickness of 0.05 mm (2 mils) and air dried at 60ºC for 3 minutes.

Examples 4-6

The following coating solutions were used to prepare Examples 4-6 (Examples 4 and 5 are not covered by the claims):

Silver emulsion:

Silver laurate whole soap homogenate (10 wt.% in methyl ethyl ketone) 160 g

BX-1 Poly(vinyl butyral), available from Sekisui Chemical Co. 10 g

Thermographic coating solutions:

The thermographic coating solutions for Examples 4-6 were prepared by mixing the following ingredients with 20 g of the silver emulsion described above:

Each solution was coated onto a 0.08 mm (3 mils) polyester support at a 0.1 mm (4 mils) wet thickness and air dried at 60ºC for 3 minutes.

An infrared absorbing overcoat solution was prepared by mixing 0.08 g of Dye 1, 1.0 g of CA398-6 cellulose acetate resin, and 20.0 g of MEK. The overcoat solution was coated onto the thermographic layer at a wet thickness of 0.05 mm (2 mils) and air dried at 60ºC for 3 minutes.

Table 1 summarizes the results of exposing the materials of Examples 1-6 to an 810 nanometer laser diode (available from Spectra Diode Labs of San Jose, CA) at 1.75 J/cm2 directed at the film surface with a spot size of 7 µm. Visible optical densities were measured using a Perkin Elmer PDS 1010M microdensitometer and UV optical densities were measured using a MacBeth TD523 densitometer equipped with a Status 18A filter. UV light stability was determined by leaving the sample in a fluorescent light housing (1000 foot candles, 90ºF) for 24 hours. Table 1

Examples 3 and 6 clearly show a significant improvement in UV Dmin light stability when benzotriazole is added to the thermographic silver emulsion.

Example 7

The following coating solutions were used in the preparation of Example 7:

Silver emulsion:

Silver behenate homogenate (10 wt.% in methyl ethyl ketone 160 g

BX-1 Poly(vinyl butyral), available from Sekisui Chemical Co. 10 g

Thermographic coating solution:

The thermographic coating solution was prepared by adding 20 g of the silver emulsion to 0.6 g methyl gallate, 0.1 g succinimide, 0.1 g phthalimide, 0.1 g tetrachlorophthalic anhydride, 0.02 g benzotriazole, 0.05 g barbituric acid with 4 ml methanol and 1 ml MEK. The solution was coated on a 0.08 mm (3 mils) polyester support to a 0.08 mm (3 mils) wet thickness and air dried at 60ºC for 3 minutes.

The infrared absorbing overcoat solution was prepared by mixing 0.08 g of Dye 1, 0.5 g Sekisui BX-1 poly(vinyl butyral), and 20.0 g MEK. The overcoat solution was coated onto the thermographic layer at a wet thickness of 0.05 mm (2 mils) and air dried at 60ºC for 3 minutes.

Example 7 was exposed with an 810 nanometer laser diode (available from Spectra Diode Labs of San Jose, CA) at 1.75 J/cm2 directed at the film area with a spot size of 7 µm. The imaged film provided a visible Dmax of 3.4, a visible Dmin of 0.08, a UV Dmax of 3.6, and a UV Dmin of 0.17. Visible optical densities were measured using a Perkin Elmer Microdensitometer PDS 1010M. UV optical densities were measured using a MacBeth Model TD523 densitometer equipped with a Status 18A filter.

The radiation absorbing dye should be incorporated primarily into the thermographic silver emulsion layer. It is believed that heating the thermographic silver emulsion layer above its glass transition temperature allows the silver ion reducing agent to migrate to the light-insensitive organic silver salt (e.g., silver behenate) in the layer. The silver behenate is reduced to elemental silver by the reducing agent, producing a brown-black image. Toners are incorporated into the formulation, producing a more neutral black color. The formation of elemental silver in the imaged area not only provides UV opacity of the image in the finished element, but it is also an infrared absorber, accelerating image formation. The intensity of the infrared laser beam decreases exponentially with penetration into the thermographic silver emulsion layer. The thickness of the thermographic silver emulsion layer and the concentration of the infrared dye affect the sharpness of the image because of the decreasing intensity of the laser beam as a function of the path through the layer. The thickness of the thermographic silver emulsion layer is preferably between about 1 and 10 µm, more preferably between about 2 and 6 µm. The concentration of the infrared dye and the thickness of the layer are adjusted so that the IR absorption of the layer is generally between 20% and 99%, preferably 50-90%, more preferably 60-85%.

In high resolution imaging conditions where the image dwell time is short and the laser peak intensity is high, ablation can occur if the infrared dye is introduced only into the thermographic emulsion layer of the design. The heating rate is greater at the surface where the laser beam enters the thermographic silver emulsion layer. As the elemental silver is generated, the absorption of the laser beam increases. This can cause overheating of the thermographic silver emulsion layer, causing smoke, damage or ablation.

By excluding the infrared dye or by reducing its concentration in the thermographic silver emulsion layer and adding infrared dye into a layer adjacent to the thermographic silver emulsion layer, the penetration of the laser beam into the thermographic silver emulsion layer can be increased. The thermographic imaging element is exposed by directing the laser beam through the thermographic silver emulsion layer before impinging on the adjacent layer containing an infrared dye. The infrared absorbing layer can be placed either above or below the thermographic silver emulsion layer relative to the support to which the adjacent layers are coated. The concentration of infrared dye in the infrared absorbing layer is selected so that the highest heating rate occurs at the interface between the infrared absorbing layer and the thermographic silver emulsion layer. The concentration of infrared dye depends on the thickness of the thermographic layer and the physical properties of the dye. The concentration of infrared dye in a 1 µm thick thermographic layer, for example, is adjusted to achieve an absorption of preferably about 90% or higher.

During an imaging laser pulse, elemental silver is generated at this interface. The generated elemental silver increases the infrared absorption in this area of the thermographic silver emulsion layer and acts as a heat source for the image area within the thermographic silver emulsion layer. As the density of elemental silver builds up adjacent to the infrared absorbing layer, the intensity is attenuated near the opposite surface of the thermographic silver emulsion layer, thus reducing overheating in this area. The profile of the image resembles an hourglass shape and therefore provides a sharper image.

Example 8

The example shows the influence of the thickness of the thermographic silver emulsion layer, the resin/silver ratio, the concentration of the infrared dye and the type of the cover layer on the imaging properties of the thermographic imaging element according to the invention (Example 8 is not covered by the claims):

The following coating solutions were used in the preparation of samples A-P. X are variables given in Table 2.

Silver emulsion:

Silver behenate homogenate (10 wt.% in methyl ethyl ketone)160 g

Butvar® B-76 Poly(vinylbutyral) X g

Thermographic coating solution:

The thermographic coating solution was prepared by adding 15 g of the silver emulsion to 0.8 g of methyl gallate, 0.2 g of succinimide, 0.1 g of phthalazinone, 0.1 g of 2-imidazolidone in 4 ml of methanol and 1 ml of methyl ethyl ketone. Prior to coating, X g of Dye 2 was added to the solution. The solutions were coated onto a 0.08 mm (3 mil) polyester support at X wet thickness and air dried at 70°C for 3 minutes.

A topcoat solution comprising a 2.4 wt% solution of CA398-6 cellulose acetate, Scripset® 540 styrene-maleic anhydride copolymer available from Monsanto Company, Tyril® 880B styrene-acetonitrile resin available from Dow Chemical Company, or poly(vinyl alcohol) (PVA), Airvol® 523 available from Air Products and Chemicals, Inc., Allentown, PA, and X% Dye 2 was coated onto the thermographic layer at X wet thickness and air dried at 50°C for 3 minutes.

The AP samples were scanned with a laser sensitometer at various scanning speeds from 20 to 60 cm/sec. The density profiles of these lines at 415 nanometers were measured using a Perkin-Elmer microdensitometer PDS 1010M. Optical density measurements were made at 826 nanometers (laser diode wavelength) and at 415 nanometers for the non-imaged elements using a Shimadzu spectrophotometer MPC-3100/UV3101PC. Table 2

A laser sensitometer (1), shown in Figure 1, was used to evaluate the thermographic imaging elements in Example 8. A 700 milliwatt beam (2) emitted from a 2361-P2 fiber-coupled laser diode (3) (available from Spectra Diode Labs) was directed onto a rotating drum (4). The core diameter of the fiber (5) was 100 micrometers and the wavelength of the laser diode (3) was 826 nanometers. The power of the rotating drum (4) was 210 milliwatts and the spot shape was that of a flat-truncated cone with a spot size of 45 µm at half full width maximum (FWHM). The flat-truncated cone profile is characterized by r0, the radius of the peak intensity of the cone, and r1, the outer radius of the cone where the intensity is near zero. A scanning slit beam profiler was used to measure the intensity profile of the laser spot. Since the profiler integrates the intensity in the direction perpendicular to the slit motion, the actual spot profile was derived from the profiler values. Fig. 2 shows a comparison of the profiler values (6) and the calculated profile values (7) expected for an intensity profile for a flat truncated cone with r0 of 10 microns and r1 of 36 microns. The curve was calculated by integrating the intensity profile of the flat truncated cone model in one direction and re-spacing.

Since the intensity profile is scanned across the film, the dots below the dot profile receive a limited exposure energy. The exposure energy depends on both the position of the dot with respect to the scanned dot and the dot scanning speed. Fig. 3a shows the total incident exposure energy plotted against the distance across the beam in the cross-scan direction. The curve was calculated for the fiber-populated beamform sensitometer model assuming a scanning speed of 40 cm/sec. In Fig. 3b, a microdensitometer profile of a line imaged with the energy profile shown in Fig. 3a on the thermographic element is shown (Example 8, Sample N not shown). The density values were collected using a narrow band filter at 415 nanometers. The density edges in Fig. 3b show gradients that are larger than those of the incident exposure profile shown in Fig. 3a, indicating that the thermographic element (Example 8, Sample N not shown) has high contrast.

The contrast of the thermographic element can be quantitatively examined using a D-logE curve. A D-logE curve is a plot of the density of the imaged film versus the logarithm of the incident exposure energy. The theoretical form of this curve is given by D = γ log (EEF/Eo), where γ is the slope of the D-logE curve, E is the incident exposure energy, EF is the effective energy of the fog or background, Eo is the minimum energy required to begin developing the image, and D is the optical density of the element when exposed to exposure energy E. The background density is equal to γ logEF. Using the data shown in Fig. 3, a D-logE curve was calculated and shown in Fig. 4. From the curve model described by the optical density equation, gamma or the contrast of the element corresponds to the slope of the D-logE curve. The gamma value for the D-logE curve in Fig. 4 is 34. A typical readily available wet processed silver halide film has, for a relative comparison, a gamma of about 10. The higher contrast is advantageous for graphic applications since high contrast halftone dots are desired for smooth tone curve control and also for new stochastic screening techniques. Similar advantages also apply to printed panel light applications.

The D-logE curve in Fig. 4 shows that density development begins at about 0.9 J/cm2 and that density saturation occurs at the maximum density (Dmax) at 1.2 J/cm2. It is understandable that the optimal imaging speed and the screen exposure conditions are unique for each screen used to image the thermographic element.

Each sample described in Table 2 was scanned with the laser sensitometer (1) of Figure 1 at several different scanning speeds ranging from 20 to 60 cm/sec. The D-logE curves were calculated with a Perkin-Elmer microdensitometer using the data from the density profiles of the lines at 415 nanometers. The model parameters in the optical density equation described above were determined from the D-logE curves and summarized below in Table 3. Table 3

mD Eo, 50 = Eo value from the D-logE curve, scanning speed at 20 cm/sec

mD Esat, 50 = Esat value from the D-logE curve, scanning speed at 20 cm/sec

mD Dmax, 50 = density maximum at 20 cm/sec

mD Dmin, 50 = density minimum at 20 cm/sec

Gamma, 50 = Gamma value when scanning at 20 cm/sec

mD Eo', 100 = Eo value from the D-logE curve, scanning speed at 40 cm/sec

mD Esat, 100 = Esat value from the D-logE curve, scanning speed at 40 cm/sec

mD Dmax, 100 = density maximum at 40 cm/sec

mD Dmin, 100 = density minimum at 40 cm/sec

Gamma, 100 = gamma value when scanning at 40 cm/sec

Gamma 100/Gamma 50 = (Gamma at 40 cm/sec) divided by (Gamma at 20 cm/sec)

The average Eo for samples A to L is 0.8 ± 0.2 Joule/cm² at a scanning speed of 20 cm/sec and 0.9 ± 0.2 Joule/cm² at a scanning speed of 40 cm/sec. The minimum exposure energy required to start density development is relatively independent of the scanning speed. The Esat values are also independent of the speed. The average Esat is 1.3 ± 0.2 Joule/cm² at a scanning speed of 20 cm/sec and 1.2 ± 0.1 Joule/cm² at a scanning speed of 40 cm/sec. The gamma values show Ab Differences in image performance at the two speeds. The average gamma value for samples A through L at a scanning speed of 20 cm/sec is 12 ± 4, while the average gamma value at 40 cm/sec is 24 ± 6. Accordingly, the gamma values appear to increase substantially as the scanning speed is increased. It is possible that at the slower scanning speeds, heat diffusion is greater, causing a loss of edge sharpness and therefore reducing the gamma of the image. Unlike photothermographic silver media, thermographic silver elements show a more pronounced effect of exposure conditions.

Samples C, I and K were coated with various concentrations of infrared dye. Samples C and I have 80% absorption at the laser diode wavelength of 826 nanometers. Sample K was coated at the same thickness but loaded with a larger amount of infrared dye so that it absorbs 96% at 826 nanometers. The average Eo and Esat values for C and I are 0.93 joules/cm² and 1.21 joules/cm², respectively, at a scanning speed of 40 cm/sec. The Eo and Esat values for sample K are 0.66 joules/cm² and 1.0 joules/cm², respectively. The film speed appears to be somewhat improved by the 16% increase in the absorption of the coating.

The effect is more pronounced with thinner coatings. Samples D, J and L were coated at half the thickness compared to C, I and K. Samples D and J absorb only about 50% of the incident laser radiation and do not image at the scanning speed of 40 cm/sec. Sample L absorbs 85% of the incident laser radiation. The average Eo and Esat values for D and J are 1.0 Joule/cm2 and 1.9 Joule/cm2 at 20 cm/sec, respectively, while the Eo and Esat values for sample L are 0.35 Joule/cm2 and 1.0 Joule/cm2, respectively. The exposure energy values for L are lower than those of D and J. The sensitivity in the thermographic silver emulsion layer is increased with increasing laser absorption or with higher concentration of the infrared dye. The edges of the imaged lines scanned at 40 cm/sec for samples K and L were smoother than those of the other single infrared layer samples. Line edge sharpness can be improved by increasing the concentration of infrared dye in the layer.

A comparison of samples of different thicknesses but similar percentages of absorption shows that a thinner layer with a higher concentration of the infrared dye is more sensitive than a thicker coating. The Eo and Esat values for K are 0.56 and 1.3 Joules/cm² at 20 cm/sec, respectively, while the Eo and Esat values for L are 0.35 and 1.0 Joules/cm², respectively. Sample L is half as thick as sample K, but it absorbs 85% of the laser radiation, which is roughly comparable to K. The Increasing the concentration of the infrared dye may cause erosion due to increased peak temperatures in the thermographic layer. The sensitivity of a single thermographic silver emulsion layer containing the infrared dye can be maximized by applying the thermographic silver emulsion layer as thin as possible with the highest achievable infrared dye concentration while maintaining the desired density maximum.

The quality of the imaged line is affected by the resin to silver ratio. The exposure energy values and gamma values are not significantly affected by changes in the resin to silver ratio, as shown in Table 5 for Samples A, C and E. However, the graphic micrographs of the samples show that the resin to silver ratio affects the image quality of the lines. As the resin to silver ratio decreases, the edges of the lines become blurred and jagged and the uniformity of density across and along the imaged line decreases. The decrease in resin concentration should increase the sensitivity of the material to heat because of the lower mass of material. However, the jagged edges show that this advantage is eliminated. The resin to silver ratio is preferably between about 25-50 wt.%.

Another embodiment of the present invention involves the addition of infrared dye in a layer adjacent to the thermographic silver emulsion layer. Samples N, O, G, H and P in Example 8 evaluate the addition of an infrared dye in the topcoat layer of a thermographic element. For unknown reasons, Samples M, G and H image poorly and are therefore not included in Table 2. Samples N, O and P showed improved line quality compared to samples containing the infrared dye in the thermographic layer only. A 0.05 mm layer of Scripset® resin containing a high concentration of infrared dye was coated onto the thermographic silver layers of Samples N, O and P. A piece of pressure sensitive adhesive tape was used to separate the topcoat layer from the thermographic silver emulsion layer to verify that the two layers had not mixed together. Samples N and O have gamma values higher than 34 and 30, respectively, at a scan speed of 40 cm/sec. These gamma values are comparable to a standard silver halide duplicating film. A typical rapid access silver halide film, for comparison, has a gamma of about 10. The quality of sample P is similar to that of N and O, although no D-logE curve was calculated for this sample. Both N and O show sharp smooth line edges with an edge roughness of about 1 µm. The samples containing infrared dye only in the thermographic silver layer had rougher edges than samples N and O. The density uniformity of samples N, O and P is within a range of 1 µm. range of ± 5%. The sensitivity of samples N and O is comparable to the samples which do not have a top layer containing infrared dye. No ablation was observed in samples N, O and P. Improved edge contrast, edge sharpness and density uniformity can be achieved by adding infrared dye in a layer adjacent to the thermographic silver layer. In this construction, susceptibility to ablation is also reduced. The infrared dye in the thermographic silver layer has a concentration such that the absorption of laser radiation in the thermographic layer is preferably less than or equal to 40%, more preferably less than or equal to 35%.

Fig. 3 shows the pictured (8) and not pictured (background) (9) transmission spectrum for Example 8, Sample N. The increased absorption peak of the infrared dye at 820 nanometers is clearly noticeable. The density at the laser diode wavelength of 826 nanometers increases from 0.84 (14.5% transmission) to 1.26 (5.5% transmission), with the density at 415 nanometers increasing from 0.355 (44.2% transmission) to 5.0 (almost 0% transmission). The elemental silver generated in the thermographic layer during exposure provides an increased absorption difference in the ultraviolet (UV), which is beneficial in UV mask applications. In Table 3, Dmax was 3.7 as measured by the microdensitometer, which is smaller than the value obtained from the spectrophotometer. Apparently, the optical density maximum that can be measured by the microdensitometer is limited to about 3.7. This implies that many gamma values calculated in Table 3 are lower than the actual values and should therefore be treated as conservative estimates.

To compare the imaging characteristics of thermographic elements showing little or no effects of heat dissipation, Examples 1, 2, 3, 4, 5, 6, and 8 (Sample N) were imaged using a 150 milliwatt (110 milliwatts on the image area) laser diode (SDL-5422, available from Spectra Diode Labs) radiating at 811 nanometers. The beam was focused to an 8 micrometer spot size (full width at 1/e² level) and scanned at 213 cm/sec with a 4.5 micrometer transverse raster line spacing. Table 4 summarizes the results of this evaluation. Table 4

The data show that higher contrast (Dmax-Dmin) films are obtained when silver laurate is used in combination with barbituric acid in the thermographic silver emulsion layer (Examples 5 and 6) or a higher concentration of methyl gallate is used in the silver behenate formulation (Example 8, Sample N). To provide workable UV contrast applications, the contrast is preferably greater than about 2.50. The data also show that the addition of benzotriazole limits the speed of the film, but the decrease in speed is minimized when silver laurate is used for the silver soap. Although a small decrease in speed may be observed, the improved light stability, as shown in Table 1, provides an advantage to including benzotriazole in the thermographic silver emulsion.

Claims (11)

1. A thermographic imaging element comprising a support coated on at least one surface with a thermographic imaging system comprising at least one layer comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a binder, benzotriazole as a light stabilizer, a toner comprising barbituric acid, and a dye that absorbs radiation in the wavelength range of 750 to 1100 nm, wherein the at least one layer comprising the light-insensitive organic silver salt produces an image density of greater than about 1.0 when exposed to radiation of 0.10 to 2.0 joules/cm² in 0.20 to 200 microseconds. 2. A thermographic imaging element comprising a support coated with a thermographic imaging system, the thermographic imaging system comprising at least two contiguous layers, one of the contiguous layers comprising a light-insensitive organic silver salt, a reducing agent for silver ions, a toner comprising barbituric acid, a binder, benzotriazole as a light stabilizer, and optionally a dye which absorbs radiation in the wavelength range of about 750 to 1100 nm, and the other contiguous layer consisting essentially of a binder and the dye which absorbs radiation in the wavelength range of about 750 to 1100 nm, the layer comprising the light-insensitive organic silver salt producing an image density of greater than about 1.0 when exposed to radiation of 0.10 to 2.0 joules/cm2 in 0.20 to 200 microseconds. 3. The element of claim 1 or 2, wherein the light-insensitive organic silver salt is a silver salt of a carboxylic acid having 10 to 30 carbon atoms. 4. The element of claim 1 or 2, wherein the thermographic imaging system further comprises a development accelerator. 5. The element of claim 4, wherein the development accelerator is selected from (i) a 3-indazolinone compound of the formula: wherein R is selected from a hydrogen atom, an alkyl radical having 1 to 4 carbon atoms, a halogen atom, -COOH and R¹ COOH, wherein R¹ is an alkyl radical having 1 to 4 carbon atoms, and (ii) a urea compound of the formula: where R² and R³ independently represent a hydrogen atom, a C₁-C₁₀ alkyl or cycloalkyl radical or a phenyl group or R² and R³ together form a heterocyclic radical having up to 6 ring atoms. 6. The element of claim 5 wherein R is hydrogen, R² and R³ independently represent a C₁-C₅ alkyl or cycloalkyl radical or a phenyl group, or R² and R³ together form a heterocyclic radical containing up to 5 ring atoms. 7. The element of claim 1 or 2 wherein the image density is greater than about 2.00 and contains metallic silver. 8. The element of claim 1 or 2, wherein the toner further comprises at least one selected from phthalazinone, succinimide and phthalimide. 9. A method of forming an image comprising the step of exposing a thermographic imaging element according to any one of claims 1 and 3 to 8 to radiation in the wavelength range of about 750 to 1100 nm, wherein the at least one layer comprising the light-insensitive organic silver salt produces an image density of greater than about 1.0 when exposed to radiation of 0.10 to 2.0 joules/cm² in 0.20 to 200 microseconds. 10. A method of producing an image comprising the step of exposing a thermographic imaging element according to any one of claims 2 to 8 to radiation in the wavelength range of about 750 to 1100 nm which is directed onto the thermographic imaging element through the layer which comprises the illuminant. sensitive organic silver salt before it encounters the adjacent layer consisting essentially of binder and the dye which absorbs radiation such that the layer comprising the light-insensitive organic silver salt produces an image density of greater than about 1.0 when exposed to radiation of 0.10 to 2.0 joules/cm² in 0.20 to 200 microseconds. 11. The method of claim 10, wherein the dye which absorbs radiation in the wavelength range of about 750 to 1100 nm is present in both adjacent layers.