Thermographic recording material with improved image tone

Details Thermographic recording material with improved image tone Thermographic recording material with improved image tone

2001-11-29

2003-12-16

Hess, B. Hamilton (Department: 1774)

Record receiver having plural interactive leaves or a colorless

Having a colorless color-former, developer therefor, or...

Method of use, kit, or combined with marking instrument or...

C503S209000, C503S212000

Reexamination Certificate

active

06664211

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to thermographic recording materials whose prints have improved shelf-life image tone stability.
BACKGROUND OF THE INVENTION
Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material.
U.S. Pat. No. 3,218,166 discloses a heat-sensitive copy sheet product capable of providing high contrast dense black images on a white background when briefly heated at image areas to conversion temperature within the approximate range of 90-150° C., said product containing, uniformly applied over a paperlike carrier and in position for heat-induced inter-reaction, reactants comprising a silver soap of an organic acid, a toner for the silver image, a di-tertiaryalkyl substituted phenol first reducing agent for the silver ion in an amount of at least about 0.01 mol per mol of silver soap and insufficient to form with said silver soap a dense image when briefly heated therewith, and a photosensitive different active-hydrogen aromatic organic second reducing agent for said silver ion in an amount no greater than that of said first reducing agent, said first reducing agent being further characterized as forming with an equal weight of silver behenate and one-fifth said weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at 100° C., before rapidly darkening at that temperature, and said second reducing agent on exposure to actinic radiation being rendered incapable of reducing silver ion on briefly heating with silver soap at 150° C.
U.S. Pat. No. 3,547,648 discloses sheet material in copying processes, including a reactant layer comprising a silver soap of an organic acid, a tertiary-alkyl-substituted monophenol, and a tertiaryalkyl-substituted bisphenol, said substituted phenols each being a reducing agent for the silver ion and being characterized as forming with an equal weight of silver behenate and one fifth said weight of phthalazinone a uniformly blended trace deposit requiring at least six seconds preheat at 100° C. before rapidly darkening at that temperature.
WO 97/34196 discloses a black and white thermographic element comprising a support bearing at least one, image-forming, thermographic emulsion layer comprising: (a) a non-photosensitive, reducible silver source; (b) a reducing agent system for silver ion; and (c) a binder; wherein said reducing agent system comprises: (i) at least one hindered phenol; and (ii) at least one co-developer of the formula
wherein: R represents an aromatic group or an electron withdrawing group.
A thermographic element comprising a support bearing a support bearing at least one image-forming, thermographic emulsion layer comprising: (a) a non-photosensitive, reducible silver source; (b) a reducing agent system for said non-photosensitive, reducible silver source; and (c) a binder; wherein said reducing agent system comprises: (i) at least one hindered phenol; (ii) at least one co-developer of the formula R
1
—(C═O)—NHNH—R
2
wherein R
1
represents hydrogen and R
2
represents an aryl group; or, R
1
represents hydrogen, alkyl or alkenyl of 1 to 20 carbon atoms; aryl, alkaryl, or aralkyl groups of up to 20 carbon atoms; aryloxy, thioaryloxy, or anilino groups of up to 20 carbon atoms; aliphatic or aromatic heterocyclic ring groups containing up to 6 ring atoms; carboxylic ring groups comprising up to 6 ring carbon atoms; or fused ring or bridging groups comprising up to 14 ring atoms; and R
2
represents a trityl group; and (iii) at least one hydrogen atom donor compound of the formula: R
3
—H wherein R
3
—H represents an organic compound, the corresponding radical of which, R
3•
, has a stability of from 350 to 6000 times that of a primary alkyl radical.
In printing with thermographic materials for medical applications for viewing with a light box, optimum diagnosis requires a blue-black image tone so that the higher ability of the human eye to distinguish detail with such image tone can be exploited, thereby improving the diagnostic value of such prints. Such image tone should be independent of the shelf-life of the thermographic recording material prior to printing and also of archival time after printing. Image tone can be assessed on the basis of the L*, a* and b* CIELAB-values as determined by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90. Ingredients are required for substantially light-insensitive thermographic recording materials, which provide a balance of image tone stabilizing properties, enabling prints to be obtained with image tone which after pretempering is less dependent upon storage time prior to printing and to archival time after printing, while exhibiting image tone acceptable for radiologists viewing images in transmission on a light box.
It is therefore an aspect of the present invention to provide substantially light-insensitive black and white thermographic recording materials with improved image tone stability not only capable of producing prints with image tone which is less dependent upon storage time prior to printing.
Further objects and advantages of the invention will become apparent from the description hereinafter.
BRIEF SUMMARY OF THE INVENTION
It has been surprisingly found that substantially light-insensitive thermographic recording materials comprising a thermosensitive element containing sterically hindered monophenols are capable of providing prints whose image tone is less dependent upon storage time prior to printing than prior art materials substantially light-insensitive thermographic recording materials.
Aspects of the present invention are provided by a substantially light-insensitive black and white thermographic recording material comprising a support and a thermosensitive element exclusive of bisphenol compounds, 4-substituted isoxazole compounds of the formula
wherein R represents an aromatic group or an electron withdrawing group, organic compounds represented by R
3
—H, the corresponding radical of which, R
3•
, has a stability of from 350 to 6000 times that of a primary alkyl radical, and photosensitive active-hydrogen aromatic organic reducing agents, the thermosensitive element containing at least one substantially light-insensitive organic silver salt, an organic reducing agent therefor in thermal working relationship therewith, an image tone stabilizer and a binder, characterized in that the image tone stabilizer is a sterically hindered monophenol.
Aspects of the present invention are also provided by a recording process comprising the steps of: (i) bringing an outermost layer of a thermographic recording material as described above into proximity with a heat source; (ii) applying heat from the heat source imagewise to the thermographic recording material in a substantially water-free condition while maintaining proximity to the heat source to produce an image; and (iii) removing the thermographic recording material from the heat source.
Use of a sterically hindered monophenol in a substantially light-insensitive thermographic recording material as an image tone stabilizer is also provided by the present invention.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
According to a first aspect of the thermographic recording process, according to the present invention, the heat source is a thermal head.
According to a second aspect of the thermographic recording process, according to the present invention, the heat source is a thin film thermal head.
The term alkyl means all variants possible for each number of carbon atoms in the alkyl group i.e. for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-but

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