ORGANIC HALOGEN COMPOUNDS USED IN DIRECT POSITIVE EMULSIONS

Abstract: An organic halogen compound is added to a fogged direct positive silver halide photographic emulsion, and the emulsion is digested prior to coating on a support. Increased sensitometric speed and stability result. Brominated quinolines and quinoxalines are particularly effective.

Description: ORGANIC HALOGEN COMPOUNDS USED IN DIRECT POSITIVE EMULSIONS DESCRIPTION TECHNICAL FIELD This invention relates to the field of photography, and more particularly to direct positive silver halide emulsion technology. BACKGROUND ART The prior art teaches that organic halogen compounds are useful for certain purposes in photoimaging processes and compositions. For example, Illingsworth, U.S. 3,632,340, "Cored Direct Positive Silver Halide Emulsion Developed with Polyhydroxybenzene", teaches mixing l,l'-diethyl-2,2'- cyanine chloride dye with bromosuccinimide in order to increase the sensitometric response in direct positive emulsions. Other organic halogen compounds, in addition to bromosuccinimide, are disclosed in Moore, U.S. 3,495,987, "Photopolymerizable Products", as useful in photopolymerizable products to provide free radicals when initiated by a radiation-sensitive compound. Japanese Patent No. 919,049, "Direct-Reversal-Type Silver Halide Photographic Emulsions", discloses that organic halogen compounds release halogen on light exposure to increase speed in direct positive emulsions. The present invention advances beyond the prior art and is directed to a process requiring the addition of certain organic halogen compounds to direct positive silver halide photographic elements, characterized in that the addition step is followed by a digestion step. The term "digestion" is used here to denote a process in which a direct positive OMPI silver halide emulsion containing one of the aforesaid organic halogen compounds is heated for a time and at a temperature sufficient to permit the organic halogen compound to selectively oxidize silver fog centers in the emulsion. The result is an increase in the photosensitivity of the emulsion, with no adverse effect on stability. SUMMARY OF THE INVENTION A direct positive silver halide photographic emulsion of superior speed and stability is produced by a process which consists essentially of the steps of (1) preparing a silver halide precipitate in a colloid binder, (2) fogging the resulting emulsion, (3) adding an organic halogen compound, and (4) coating the emulsion on a support, characterized in that, the emulsion is digested after step (3) and before step (4) to promote thermal reaction of the organic halogen compound with the fog centers, whereby the organic halogen compound is reduced and the fog centers are partially oxidized. DETAILED DESCRIPTION OF THE INVENTION In the field of photographic science the search for higher sensitivity has been a continual one. Since direct positive emulsions are newer in the photographic field than more conventional negative working emulsions, sensitization of such emulsions has been less predictable. Also it has been found that discoveries from the negative working field do not necessarily apply to the direct positive field. While elements such as colloid binders, coagulating agents, and grain growth can apply equally well to both fields, there are frequently opposing effects when such items as general OMP sensitizers, spectral sensitizers, e.g., dyes, and antifoggants are considered. The present invention provides the means to increase the sensitometric speed response of direct positive silver halide photosensitive elements without undue sacrifice of gradient, top density, and resolution, and without the need for formulation changes. Speed can be increased by a factor of 5 without affecting Dmax or contrast. The degree of speed increase is surprising in comparison to other means of increasing sensitivity, which include: alteration of silver halide grain growth, metal doping of the grains, and the use of new sensitizing and desensitizing dye structures and dye combinations. Halogen substitutions found to be most effective are bromine and- chlorine. Iodine compounds have not been shown to be effective, and fluorine compounds represent an unreactive class of compounds. The organic molecule to which the bromine or chlorine is attached must serve as a suitable carrier and not allow premature discharge, decomposition, or volatilization since this would negate the desired enhancement of photosensitivity. For example, new control or environmental protection facilities are often necessitated by the use of compounds of high volatility, but the compounds of the present invention have no such disadvantage. Instead, these organic halogen compounds, which may be characterized as weak oxidizing agents, derive their utility from being reacted in the emulsion during the thermal treatment, i.e., digestion, whereby the weak oxidant is reduced and therefore no further changes in properties occur upon aging. OMPI /.- IPO - It is as a result of practical limitations imposed by the direct positive 'photosyste that the activity of particularly effective organic halogen compounds must be determined expe imentally. Particularly effective compounds of the instant invention are the following: 2-chloro-4-nitrobenzyl chloride, 2,2,3,4-te rachloronaphth-l-one, 1,4-bis(trichloro ethyl) -benzene, tribromoacetic acid, tetrachlor-orthobenzoquinone, l,3-dibromo-5,5-dimethyl hydantoin, 1-chlorobenzotriazole, α-bromo-p-toluic acid, 2-trichloromethyl-4 (3H) quinazolinone, p-bromoacetophenone, m-nitrobenzyl chloride, 2-tribromomethylquinaldine, 2,6-dichlorobenzonitrile 2-tribromomethylquinoline, 2-dibromomethylquinoline, 2-tribromomethylquinoxaline, and α, α, α ',α'- tetrabromoxylene. By contrast, such compounds as bromosuccinimide, p-nitrobenzyl bromide, 2-bromo-l-indanol, 3-bromoquinoline, p-nitrobenzyl chloride, 2-amino-5-bromo pyridine and 2-bromoethano have shown no advantage over controls without these compounds. The increase in direct positive photosensitivity provided by the present invention results from the simple step of adding and digesting the organic halogen compound in the emulsion. As a result of this straightforward approach it is possible to use this technique to obtain desirable speed increase without the necessity of further modification of an existing direct positive emulsion formulation. By contrast, other techniques for spee increase usually require that other changes and modifications accompany the means for speed O increase. For example, to provide a new grain might require alteration of the steps of ripening the emulsion; of washing, sensitizing and stabilizing the emulsion; using auxiliary additives, etc. to allow coating and drying of the emulsion. A new or improved dye may not be compatible with a sensitization technique previously used. The present invention is operative with silver halide grains produced by both single jet and double jet precipitation techniques, to yield heterodisperse and monodisperse grain size distribution. Into the grains made by such known techniques metal ions may be introduced to modify the photographic response and/or to act as fogging agents. Nonmetals, and compounds thereof, may also be used to fog or precondition the grains during physical ripening, or, as an alternate fogging mechanism, the grains may be exposed to actinic radiation. In some cases it may. be desirable to wash grains which have been chemically modified and then further increase the size of the grains by precipitating a layer of silver halide over the original grains. The term "core-shell" grain has come to apply to such layered grains. In the instant invention, as in the prior art, both the type of fogging agent to be used and the degree of fogging are dependent on concentration, pH, pAg, temperature, and duration of the treatment. Both surface and internal fog centers are recognized as functional for direct positive emulsions. Suitable metals for fogging include gold, tin, lead, palladium, and iridium. Suitable nonmetallic compounds include amine borane, hydrazine, thiourea- OMPI J WIPO dioxide, formaldehyde, and allyl thiourea. Once the fogging reaction is complete the organic halogen compound may be added to the emulsion, and the oxidation-reduction reaction is carried out by holding the system at an elevated temperature to allow it to proceed towards completion. The organic halogen compounds of this invention will function in the presence of dyes known to be useful in direct positive emulsions, including those disclosed, for example, in U.S. 3,615,643; U.S. 3,583,870; U.S. 3,867,149; U.S. 3,898,216; and U.S. 3,747,986. These dyes are useful as desensitizers which increase the response of direct positive silver halide emulsions. While useful with these desensitizing dyes, the present invention also functions in their absence. Specially sensitizing dyes that are not electron accepting may also be present in the emulsion such as single and complex cyanines, merocyanines, styryls, and hemicyanines. The silver halide constituent of the direct positive silver halide emulsions described herein may consist of pure or mixed silver chloride, bromide, or iodide, and the grains may be regular or irregular in shape, e.g., cubic, octahedral, rhombohedral, etc. ■ The novel emulsions described in copending application Serial No. 972,972 filed December 26, 1978, "Novel Silver Halide Crystals with Two Surface Types", are also useful in the practice of the present invention. As a binding agent and peptizing media for these emulsions it is normal to employ gelatin. However, gelatin may be partially or wholly replaced by other natural or synthetic protective colloids known in the art. Other useful additives OM WIP include speed-increasing compounds such as polyalkylene glycols; surface active agents which are useful as coating aids; antifoggants; and stabilizers, including indazoles, imidazoles, azaindenes, mercury, and polyhydroxy benzene compounds. Other useful ingredients for these direct positive elements include hardeners, antistatics, matting agents, plasticizers, brighteners, and natural and synthetic wetting agents. All these useful ingredients may be combined to yield formulations capable of being coated on suitable supports such as cellulose nitrate film, cellulose ester film, polyvinyl acetal film, polystyrene film, poly(ethylene terephthalate) film, and related films or resinous materials, as well as glass, paper, metal and the like. The invention is illustrated by the following Examples. EXAMPLE 1 A rhodium-doped onodisperse cubic Ag(I)Br emulsion containing 1.0% iodide and 0.01% rhodium was prepared by the balanced double jet precipitation technique, maintaining a pAg of 8.2. After completion of physical ripening, the emulsion was coagulated with polyvinyl alcohol o-sulfobenzaldehyde acetal and the excess halide salts removed by decanting. The washed emulsion was redispersed in gelatin; the pH was adjusted to 7.5, and the pAg to 8.0. The emulsion was fogged by adding, per mole of silver halide, 2.4 x 10 mole of cesium —6 thiodecaborane and 3.1 x 10 mole of gold chloride stabilized with hydrochloric acid. After digesting OMPI at 72°C for 2 hours the emulsion was cooled to 35°C and cetyl betaine was added as a coating aid. The emulsion was split into portions and additions made as in Table 1 below. TABLE 1 Grams Grams of Cya- Organic nine Halogen Dye*/ Compound/ Mole Organic Mole of Halogen Silver Silver Sample Compound Halide Halide I 2-tribromo- methyl- quinoline 0.122 II 2-tribromo- methyl- quinoline 0.122 0.97 III α , α , α ' α '- tetra- ** bromoxylene 0.122 0.97 IV 0.97 V — — ——— *4- j[l,3-dimethylimidazo 4,5-b quinoxaline-2(3H)- ylidene] ethylidenej -2-methyl-3-phenyl-2- isoxazoline-5-one. The samples were coated on photographic 2 grade polyester .base at 35 mg/dm silver bromide. The samples with dye were exposed at 10 " sec. with a 19 line compensator on an EGG sensitometer through a \U wedge. The nondyed samples were exposed using a tungsten lamp (G.E. 2F Photo Flood-750 watts) OM operated at 35 volts, for 15 sec. The samples were developed in a standard metol-hydroquinone developer for one minute, dipped in an acetic acid stop for 30 sec, fixed in a standard thiosulfate fixer for one minute, and dried. The samples showed the sensitometric response reported in Table 2 where the speed was measured at 0.1 optical density over Dmin and the gradient is measured over the straight line curve portion. TABLE 2 Relative Sample Speed Gradient Dmax I 4- 2.36 2.56 II 350 2.80 2.37 III 140 2.80 2.50 IV 100 2.70 2.39 V 1 2.40 2.69 Table 2 shows that similar speed increases can be obtained with and without the presence of the direct positive dye. The results also show that the effects of dye and halogen-donating compound are additive. EXAMPLE 2 An emulsion prepared and fogged in the same manner as that in Example 1 was split into two equal portions. To Part VI, 48.5 mg of 2-tribromomethylquinoline and 1 gm of the cyanine dye reported in Example 1 were added per mole of silver halide. To part VII, 18.7 mg of 2-tribromomethylquinoline/mole silver halide were added but the emulsion was heated to 54°C for 30 minutes. On cooling, 0.97 gms/mole of the direct positive dye were added. The samples were coated at OMPI Λ. wipo - 35 mg AgBr/dm . Samples of both were aged in the ' oven at 50°C/50% relative humidity for one week. The samples were exposed at 10 seconds with a 19 line compensator on the EGG. sensitometer and processed as reported earlier. The following results were obtained. TABLE 3 Control After Oven % Sample Dmax Aging Dmax Dmax Loss VI 3.0 2.01 33 VII 3.44 3.21 7 The results show that heating the sample after addition of the organic halogen compound allowed the use of smaller quantities and provided increased stability compared to using the compound as a simple afteraddition. The drop in Dmax for sample VI was also accompanied by an increase in speed, yielding poorly reproducible sensitometry. EXAMPLE 3 An emulsion prepared as in Example 1 was split into three equal portions. To Part VIII, 1.1 gms of 2-dibromomethylquinoline/mole of silver halide was added. To Part IX 0.12 gms of 2-tribromomethyl- quinoline/mole silver halide were added. Part X was kept as a control. The direct positive dye was added and samples coated at a coating weight of 40 mg AgBr/dm . The samples were exposed for one minute through a V " 2 wedge in a contact mode using a K&Ξ Tungsten source at maximum output with a 0.6 neutral density filter. They were then processed in a standard metolhydroquinone developer, put in an acid stop for 30 seconds, and fixed in a thiosulfate fixer for one minute.. The results are listed in Table 4. O WI TABLE 4 Relative Sample Speed Gradient Dmax VIII 150 2.2 1.0 IX 150 2.2 0.9 X 100 2.3 1.3 The results show that the dibrorao compound will give similar results compared to the tribromo species but ^10 more is needed to give the effect. EXAMPLE 4 Emulsion samples were prepared as in Example 1 and split into three portions. To sample XI was added 0.122 mg of 2-tribromomethyl- quinoxaline/mole of silver halide, to sample XII 0.122 mg of 2-tribromomethylquinoline/mole silver halide, and sample XIII was the control. The samples were coated without the direct positive dyes and processed as in Example 1. The results are reported in Table 5 * and they show that under similar processing conditions the 2-tribromomethylquinoline and 2-tribromomethyl-quinoxaline compounds behave identically. TABLE 5 Relative Sample Speed Dmax XI 150 2.77 XII 150 2.60 XIII 100 2.66 EXAMPLE 5 Five organic halogen compounds were evaluated as oxidants to increase direct positive speed. These were tested in comparison to 2-tribromoraethylquinoline in the emulsion of Example 1. Each experimental stage contained 0.97 g/mole of silver halide of the direct positive dye and was held for 10 min. at 54°C after the addition of the organic halogen compound. The control sample XIV contained only 0.97 g/mole of silver halide of the direct positive dye, but was also digested for 10 min. at 54°C to insure that the superior results of this invention require the presence of the organic halogen compound. TABLE 6 Grams of Organic Halogen Compound/ Organic Mole of Halogen Silver Relative Sample. Compound Halide Speed XIV 100 XV 2-tribromo- meth l- . quinoline .140 500 XVI 3-bromo- quinoline .140 100 XVII N-bromosuc- cinimide .140 100 XVIII 2,2,2-tribromo- ethanol .140 130 XIX hexachloro-p- xylene .140 100 XX 2-bromoethanol .140 100 These results show that by increasing the amount of 2-tribromomethylquinoline relative to Example 1, and using a shorter heat treatment than in Example 2, it was possible to obtain a 5x speed increase relative to a control emulsion. In that particular test, however, only one of the organic halogen compounds, (2,2,2-tribromoethanol) showed any indication of increasing speed. This is illustrative of the unobvious nature of the present invention, since it is necessary to experimentally determine which compounds actually work in this process. Sample XV represents the best mode contemplated for carrying out the invention. EXAMPLE 6 An emulsion similar to that of Example 1 but fogged with 2.3 x 10 mole of cesium thiodecaborane and 1.9 x 10~ ' mole of gold chloride/mole of silver halide and without direct positive dye, was used for further comparisons. Saponin was used as a coating aid in place of cetyl betaine. Table 7 gives comparative results. TABLE 7 Grams Organic Halogen Compσund/- Organic Mole Halogen Silver Relative Sample Compound Halide Speed XXI 100 XXII 2-tribromomethyl- quinoline .122 200 XXIII 2-tribromomethyl- quinoxaline .122 200 XXIV α,α,α' ,α'-tetra bromoxylene .122 130 Those results show that the quinoline and quinoxaline compounds with a tribrominated group are capable of giving speed increases similar to those in Example 4 while maintaining an advantage over α,α,α ' ,α '-tetrabromoxylene as in Example 1. EXAMPLE 7 A monodisperse cubic trihalide emulsion (Br-90/Cl-9.5/I-0.5) was prepared by a balanced double jet precipitation as described in Example 1 of S.N. 972,972. Unlike the iodobromide emulsion of Example 1, no rhodium was included during precipitation. The emulsion was washed and redispersed as in Example 1. The emulsion was fogged by adding thereto 7.3 x 10 —8 moles of cesium thiadecaborane and 1.6 x 10 -4 mole of gold chloride, stabilized with HC1, per mole of silver halide, followed by digesting at 72°C for 100 minutes. The emulsion was cooled at 35°C and the direct positive dye and coating aid of Example 1 were added. The emulsion was then split into 5 portions. An organic halogen compound was added to 4 of these portions, with the 5th serving as control. Table 8 gives comparative results obtained with the splits of this emulsion with and without the digestion step of the invention. Samples were coated at 35 mg/dm and exposed and processed as in Example 1. O TABLE 8 Grams Organic Halogen Organic Compound/ Minutes Rela¬ Halogen Mole Silver Digestion tive Sample Compound Halide at 54°C Speed XXV —— —- 100 XXVI 2,2,3,4- tetrachlor- naphth-1-one .035 — 250 XXVII 2-tribromo- methylquinoline .140 -?— 250 XXVIII 2,2,3,4-tetra- chloronaphth- 1-one .035 10 " 450 XXVIX 2-t ibromo- methylquinoline .140 30 450 When these same coatings were tested after one month normal aging it was determined that only the control XXV and the two digested emulsions (XXVIII and XXVIX) showed speed values within 10% of those in Table 8. Since a 10% variation is within experimental error, this demonstrates that the digested samples were remarkably storage-stable. EXAMPLE 8 Emulsion splits were prepared and fogged as in Example 7. Table 9 gives comparative results obtained when varied amounts of an organic halogen compound were added with and without the digestion step. OMPI " TABLE 9 Grams Organic Halogen Compound/ Organic Mole Minutes Rela- Halogen Silver Digestion tive Sample Compound Halide at 54°C Speed XXX — — 100 XXXI 1-4-bis(tri- chloromethyl)- benzene .419 100 XXXII 1-4-bis(tri- chloromethyl)- benzene .838 100 XXXIII 1-4-bis (tri- chloromethyl)- benzene 1.676 100 XXXIV 1-4-bis(tri- chloromethyl)- benzene .042 90 300 XXXV 1-4-bi (tri- chlororaethyl)- benzene .419 90 400 Unlike other organic halogen compounds which show an effect on sensitivity even without digestion, this compound requires a longer digestion period than more active halogen oxidizing agents used in previous examples. EXAMPLE 9 An emulsion was prepared as in Example 1 except that the fogging reaction was carried out with 2.9 x 10 mole of cesium thiodecaborane and 4.6 x O 10~ mole of gold chloride per mole of silver halide. Table 10 gives comparative results obtained when the same amount of an organic halogen compound was digested for different times in comparison to a control containing no addition. TABLE 10 Grams Organic Halogen Compound/ Organic Mole Minutes Rela¬ Halogen Silver Digestion tive Sample Compound Halide a 54°C Speed XXXVI — _- __ 100 XXXVII 2-tribromo- methylquino- line 0.125 — 100 XXXVIII 2-tribromo- methylquino- line 0.125 15 200 XXXIX 2-tribromo- methylquino- line 0.125 30 280 XL 2-tribromo- methylquino- line 0.125 45 280 This illustrates that after 30 minutes, further digestion has produced no speed change, which indicates that the oxidant has been consumed. It is a teaching of the instant invention that it is important to digest the emulsion at an elevated temperature after the addition of the organic halogen compound. The emulsion was held at OMPI J WIPO 54°C for 30 minutes in Example 2 and for 10 minutes in Example 5. These should not be considered limiting since they were chosen as a result of experimentation for convenience of operating conditions, and as being suitable without adverse effect on the emulsion used. Due to variations in the organic halogen compound used, the amount added, and the unique characteristics of the emulsion to which the addition is made, it should be obvious to one skilled in the art to vary the time and temperature of the digestion period to obtain optimum results. In this regard, the following information is considered helpful: Temperatures of from 40°C to 80°C are useful for the digestion reaction without adversely affecting the emulsion. Times as short as 5 minutes and up to as long as several hours can be used in order to accommodate the reactivity of the particular organic halogen compound being used to react with the fogged centers. Per mole of silver halide emulsion, the effective amount of Organic halogen compound may vary from as little as 0.01 g to as much as 1.00 g. It- has been found convenient to operate with temperature of from 45°C to 65°C for periods of from 10 to 120 minutes using from 0.015 g to 0.50 g of the organic halogen compounds. OMPI IPO

Claims: CLAIMS 1. A process for the production of a direct positive silver halide photographic emulsion which consists essentially of the steps of (1) preparing a silver halide precipitate in a colloid binder, (2) fogging the resulting emulsion, (3) adding an organic halogen compound, and (4) coating the emulsion on a support, characterized in that the emulsion is digested after step (3) and before step (4) to promote thermal reaction of the organic halogen compound with the fog centers, whereby the organic halogen compound is reduced and the fog centers are partially oxidized, resulting in an emulsion of superior speed and stability. 2. The process of claim 1 wherein the organic halogen compound is a bromine- or chlorine-containing compound. 3. The process of claim 1 wherein the organic halogen compound is a member selected from the group consisting of 2-chloro-4-nitrobenzyl chloride, 2,2,3,4-tetrachloronaphth-l-one, 1,4-bis(trichloromethyl)-benzene, tribromoacetic acid, tetrachlor-orthobenzoquinone, l,3-dibromo-5,5-dimethyl hydantoin, 1-chlorobenzotriazole, α-bromo-p-toluic acid, 2-trichloromethyl-4 (3H) quinazolinone, p-bromoacetophenone, m-nitrobenzyl chloride, 2-tribroraoraethylquinaldine, 2,6-dichlorobenzonitrile, 2-tribromomethylquinoline, 2-dibroraomethylquinoline, 2-tribromomethylquinoxaline, and , , α', '- tetrabromoxylene. 4. The process of claim 1 wherein-the OMPI A. ~ WIPO _ - organic halogen compound is added to the fogged emulsion in an amount of 0.015 - 0.50 g per mole of silver halide in the emulsion. 5. The process of claims 1 and 4 wherein the digestion is performed by heating the fogged emulsion containing the organic halogen compound at a temperature of 45° to 65°C for a time of 10 to 120 minutes. 6. A product prepared by the process of any of foregoing claims 1-5. OM Λ. WIP

Inventors: PRADOS R
Applicants: DU PONT, PRADOS R
Application year: 1979
Main classification: G03C1
IPC codes: G03C1