US 3847612 A
Dust formation is avoided and dense black images are obtained during heat development of exposed dry silver sheet materials by employing a synergistic combination of individually ineffective toner materials.
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Description (OCR text may contain errors)
United States Patent 1 Winslow Nov. 12, 1974 1 LIGHT-SENSITIVE HEAT-DEVELOPABLE SHEET MATERIAL  Inventor: John M. Winslow, St. Paul, Minn.
 Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.
 Filed: Feb. 2, 1973  Appl. No.: 329,107
 US. Cl 96/67, 96/114.1, 96/1 14.6  Int. Cl G036 1/02  Field of Search..... 96/67, 52, 58, 114.1, 114.6;
 References Cited UNITED STATES PATENTS 3,080,254 3/1963 Grant, Jr 117/36.8
Ritter 96/67 Birkeland 96/67 Primary Examine rDavid Klein Assistant Examiner-A1fonso T. Suro Pico Attorney, Agent, or FirmAlexander, Sell, Steldt & DeLaHunt [5 7 1 ABSTRACT 6 Claims, No Drawings LIGHT-SENSITIVE HEAT-DEVELOPABLE SHEET MATERIAL I This invention relates to photosensitive heatdevelopable dry silver sheet materials and has particular reference to improved means for obtaining images of desirable density and color.
Photosensitive heat-developable dry silver sheet materials, as described for example in U.S. Pat. No. 3,457,075, contain a photosensitive silver halide catalyst-forming means in catalytic proximity with a heatsensitive combination of organic silver salt and reducing agent. Particularly where the reducing agent is a hindered phenol, it is found. desirable to include an image intensifier or toner material, of which phthalazinone has been an outstanding example.
Phthalazinone and various other materials have previously been employed as toners in light-stable heatsensitive copy-sheets based on organic silver salts and reducing agents. Phthalazinone is a preferred material in compositions shown in U.S. Pat. No. 3,080,254; other materials therein disclosed, such for example as imidazole, must be employed in much larger amounts and are generally less effective. Both phthalic acid and phthalazinone areshown in U.S. Pat. No. 3,218,166 to be effective toners for heat-sensitive compositions'containing organic silver salts and hindered phenolic reducing agents. However in dry silver sheet materials employing the hindered phenolic reducing agents and containing also a catalytic quantity of photosensitive silver halide, neither phthalic acid nor imidazole has been found effective in causing useful darkening of the image. Phthalazinone is fully effective in dry silver sheets both as an image toner and for other purposes, as noted for example in U.S. Pat. No. 3,589,903; but this compound has been found to cause slight dusting, which becomes noticeable during heat development of large numbers of the exposed sheets. The dust factor apparently is a result of sublimation of the toner material, the vapor then condensing on cooler portions of the developing apparatus. It has also been observed that dry silver sheets containing phthalazinone have a tendency to lose photosensitivity and to provide less desirable images when first subjected to prolonged storage at elevated temperature and humidity.
It has now been found possible to provide photosensitive heat-developable dry silver imaging sheets which given dense black images, retain full effectiveness dur ing prolonged storage, and do not emit dust-forming vapors during development, by incorporating with the combination of silver halide, organic silver salt and bindered phenolic reducing agent significant small amounts of each phthalic acidand imidazole.
As'previously indicated, neither of these materials is at all effective by itself as an image toner for these dry silver sheet materials; yet the two together are found to be fully as effective as phthalazinone in providing image areas of desirable cold tone and this ability is maintained during prolonged storage.
Phthalic acid may effectively be replaced in whole or in part by corresponding molar amounts of naphthalene-2,3-dicarboxylic acid or of o-phthalamic acid. Each of these compounds will be seen to be capable of condensation to form a five-membered heterocyclic ring structure.
Somewhat analogously, substituted imidazoles having the S-membered heterocyclic imidazole ring strucparts by weight of phthalic acid and about 15 parts by.
weight of imidazole in a coating prepared with about 225 parts of silver behenate half soap, 155 parts of hindered phenolic reducing agent, and 4 par-ts of resinous binders, applied at a dry coating weight, after. removal of volatile liquid vehicle, of about ten grarnsper.
square meter. In another example, 50 parts by weight of phthalic acid and 10 parts of benzimidazole. are foundto be effective in atransparent coating prepared with approximately 715 parts of silver behenate full soap, 1,000 parts of resinous binders, and 770. parts of hindered phenolic reducing agent, applied at 19 grams per sq. meter.
The coating made with the half soap, containing 14.5 percent silver, may be shown by calculation to contain 0.44 mole of phthalic acid and 0.73 mole of imidazole per mole of silver. Similarly, the transparent coating containing the full soap may be shown to contain 0.18 mole of phthalic acid and 0.057 mole. of benzimidazole per mole of silver. More broadly, it has been foundipossible to obtain distinct improvement in imageappearance with dry silver coatings based onv organic. silver. salts and hindered phenolic reducingagents by incorporating as toner materials animidazoleand;phthalic acid orequivalent in amounts within the. approximate range of 0.04 to 0.8 mole of the acid and 0.02 to 1.4 moles of the imidazole per mole of total silver. Apreferred range is about 0.15 to 0.5 mole of acid and 0.1 to 0.7 mole of imidazole per mole of silver. In these coatings it willbe recognized that-some minor amount, not more than about ten percent, of the total silvenwill be present as photosensitive silver halide, the remainder being light-stable organic silver salt.
Some improvement in image appearance may be achieved in accordance with the principles of the invention when using other hindered phenolic reducing agents, such for example as those listed in the afore mentioned U.S. Pat. No. 3,218,166; but the best image appearance has been realized when using as the reducing agent the compound 2,6-methy1ene?bis(2-hydroxy- 3-tertiarybuty1-5-methy1phenyl)-4-methylphenol, a commercial form of which is also known as Plastanox 80 antioxidant; and this substance is presently greatly preferred in the sheet materials of this invention.
For use on paper or other non-transparent backings it is found convenient to use silver half-soaps, of which an equimolar. blend of silver behenate and behenic acid, prepared by precipitation from aqueous solution of the sodium salt of commercial behenic acid andanalyzing about 14.5 percent silver, represents a preferred example. Transparentsheet materials .rnade ontransparent film backings require atransparentcoatingand for this purpose the silver behenate full soap, containing not more than about fouror five percent of free behenic acid and analyzing about 25 .2=percent silver, may be used. Other components, suchfor example as colorings, opacifiers, extenders, spectral sensitizing-dyesetc. may be incorporated as required for various specific purposes.
The following examples are offered for further illustration, but not as limitation, of the principles of the invention'. Proportions are in parts by weight unless otherwise noted.
EXAMPLE 1 Silver behenate half soap is homogenized in acetone and then diluted with further acetone and toluene to the formula half soap 10.83 parts acetone 58.94 toluene 30.23 v
' To 80.85 grams of the homogenate is added, under darkroom conditions,
acetone 6.37 grams polyvinyl butyral (B-76 resin) 10.40 gm. 3% solution of mercuric acetate in methanol 1 ml. 5% solution of calcium bromide in methanol 1 ml.
6 grams hindered phenolic reducing agent (Plastanox 80) i and the mixture is coated on paper pre-treated with polyvinyl alcohol. The coating weight after drying is seven grams per square meter.
A second composition is prepared by first mixing together acetone 339.9 grams methanol 1 18.5 cellulose acetate 25.0 colloidal silica 3.5
To 100 grams of the mixture is then added 0.25% Victoria Blue dye in methanol 0 1 ml. 0.05% spectral sensitizer in methanol 1.5 ml. phthalic acid 0.6 gm. imidazole 0 4 gm.
The mixture is applied over the previous layer and dried. The dry coating weight is 2.7 grams/sq.m. The spectral sensitizer is l-methyl-3-allyl-5[ (3-ethyl-2- benzoxazolinylidene) ethylidine]-2-thiohydantoin. The molar ratios of phthalic acid and imidazole to total silver are 0.37 and 0.71 respectively.
A sample is exposed sensitometrically and is developed by heating for three seconds at 260F. with results as follows:
D D,,,,, gamma speed image .19 1.28 1.35 1.0 black 1),, D g, gamma speed image color .13 1.30 1.20 1.20 blaclt The test conditions, which are sufficiently severe to cause significant deterioration of properties in sheets prepared with phthalazinone, are seen to have very little if any adverse effect on the .productof the present example.
EXAMPLE 2 A homogenate is prepared of silver behenate full soap gm. methyl isobutyl ketone 54 I toluene methyl ethyl ketone To 72 grams of the homogenate is added acetone acetone mercuric bromide (as .5% soln. in methanol) .0045
mercuric acetate (as 2.5% soln. in methanol) .216. polyvinyl butyral 92 vinyl resin (VAGHU 1.8
Sudan yellow (as .05% soln. in methanol .015.
The vinyl resin is a copolymer, of vinyl acetate and vinyl chloride, containing hydroxyl radicals.
The Sudan yellow dye is optional The mixture is prepared under darkroom conditions.
It is coated on transparent polyester film to a coating weight, after drying, of 16.2 gm/sq.m. Y
A second composition is prepared using toluene 34 gm. methyl ethyl ketone 32 methanol 22 vinyl chloride-vinyl acetate copolymer 6 tetrachlorophthalic anhydride .43 phthalic acid 2.2 benzimidazole .43 phenolic reducing agent of Example 1 3.42 antistatic agent (optional) ("Catanac SN") .43
and is applied over the first coat at a coating weight, after drying, of 2.7 grams/sqm. The molar ratios of phthalic acid and benzimidazole to total silver are 0.167 and 0.046 respectively.
The film is exposed through a 0-4 continuous wedge to illumination from a carbon arc and developed by holding for 30 seconds in a bath of inert fluorocarbon liquid maintained at 260F. The sensitometric values attained are:.
D, D gamma image color .08 2.60 2.0 blue-black What is claimed is as follows:
1. A photosensitive heat-developable sheet material having an image-forming stratum comprising (1) photo-sensitive silver halide catalyst-forming means and (2) heat-sensitive reactant image-forming means including an organic silver salt oxidizing agent and a hindered phenolic reducing agent for silver ion, the oxidation-reduction reaction of which to produce a visible change is accelerated by said catalyst, and (3) as sifier material consists of phthalic acid and imidazole.
5. Sheet material of claim 3 wherein said image intensifier material consists of phthalic acid and benzimidazole. Y
6. Sheet material of claim 1 wherein said hindered phenolic reducing agent is 2,6-methylene-bis(2- hydroxy-3-tertiarybutyl-5-methylphenyl)-4- methylphenol and said image intensifier material consists of 0. l 5 to 0.5 mole of phthalic acid and 0.04 to 0.7 mole of imidazole or benzimidazole per mole of total silver.