Photographic developer/amplifier compositions

Abstract: An aqueous redox amplifier solution having a pH in the range 10.0 to 13.5 comprises: a colour developing agent and/ or auxiliary developing agent, hydrogen peroxide or a compound that provides hydrogen peroxide in a concentration equivalent to 0.1 to 50.0 ml/l of 30%w/w solution and a borate or other complexing compound capable of reversibly forming a complex with hydrogen peroxide, the complexing compund being in a concentration of from 1.0 to 100.0 g/l and wherein the relative amounts of complexing compound and hydrogen peroxide are such as to provide an amount of hydrogen peroxide effective for redox amplification and buffer the hydrogen peroxide and thereby reduce the decline in the rate of redox amplication. The borate may be boric acid, a metaborate or a complex borate such as a tetraborate.

Description: Field of the Invention This invention relates to photographic developer/amplifier solutions useful in redox amplification processes. Background of the Invention Redox amplification processes have been described, for example in British Specifications Nos. 1,268,126; 1,399,481; 1,403,418; and 1,560,572. In such processes colour materials are developed to produce a silver image(which may contain only small amounts of silver) and then treated with a redox amplifying solution ( or a combined developer/amplifier) to form a dye image. The developer-amplifier solution contains a colour developing agent and an oxidising agent which will oxidise the colour developing agent in the presence of the silver image which acts as a catalyst. Oxidised colour developer reacts with a colour coupler to form the dye image. The amount of dye formed depends on the time of treatment or the availability of the colour coupler and is less dependent on the amount of silver in the image than is the case in conventional colour development processes. Examples of suitable oxidising agents include peroxy compounds including hydrogen peroxide and compounds which provide hydrogen peroxide e.g. addition compounds of hydrogen peroxide, cobalt(III) complexes including cobalt hexammine complexes, and periodates. US-A-4 529 687 describes a method of forming a color image from a low silver containing photographic material in which the material is developed in the presence of nitrogen-containing heterocyclic compounds using a monobath developing intensifying solution substantially free of bromide and iodide, but containing both hydrogen peroxide or compounds which can release hydrogen peroxide and color developing agents, wherein the developing intensifying process is carried out in the presence of an anion exchange resin. US-A-4 045 225 describes a method of forming an image which comprises subjecting a photographic element comprising a support and at least one photographic layer in which there is imagewise distributed a substance having a catalytic effect for imagewise intensification in the presence of a reducing agent using a solution containing (1) at least one of hydrogen peroxide and compounds capable of releasing hydrogen peroxide or a halogenite, and (2) at least one of certain nitro-substituted nitrogen containing heterocyclic compounds. Problem to be solved by the Invention When hydrogen peroxide is used as the oxidising agent the rate of amplification depends on the level of hydrogen peroxide in the solution and this falls with time due to reaction with the colour developing agent, antioxidant (if present) and dismutation. This loss of concentration of hydrogen peroxide is a problem. For example,because in a continuous process the concentration must be made up, it introduces a risk of either over or under replenishment of the hydrogen peroxide concentration. The present invention provides a solution to this problem by addition of boric acid or a borate (hereafter referred to collectively as borate) or other complexing compound capable of reversibly complexing with hydrogen peroxide to the developer solution. The borate or other complexing compound forms a reversible complex with the hydrogen peroxide and, as the hydrogen peroxide in the solution is used up, the complex will dissociate to provide a buffering effect,tending to stabilise the level of hydrogen peroxide in the solution and thereby reducing the decline in the rate of redox amplification. Perborates which are complexes of borates and hydrogen peroxide and which readily dissociate in solution have been previously described in redox amplification, where they have been used as a source of hydrogen peroxide, for example in G.B. patent Nos 1,524, 438 and 1,546,739 . Sodium perborate which is usually represented by the formula NaBO 3 4H 2 O and is a complex of sodium metaborate(formula NaBO 2 ) and hydrogen peroxide has been described in GB patent No 1,546 739 as a source of hydrogen peroxide. The perborates whose use has been described have molar proportions of borate and hydrogen peroxide of approximately 1:1 and do not provide the above mentioned buffering effect. Summary of the Invention According to the present invention there is provided an aqueous redox amplifier solution having a pH in the range 10.0 to 13.5 comprising: a colour developing agent and/or auxiliary developing agent, hydrogen peroxide or a compound that provides hydrogen peroxide in a concentration equivalent to 0.1 to 50.0 ml/l of 30% w/w solution and a borate complexing compound capable of reversibly forming a complex with hydrogen peroxide the complexing compound being in a concentration of from 1.0 to 100.0 g/l and wherein the molar ratio of borate complexing compound to hydrogen peroxide is at least 2:1 such as to buffer the hydrogen peroxide and thereby reduce the decline in the rate of redox amplification. Boric acid, alternatively known as boracic acid, which is of formula H 3 BO 3 is particularly suitable for use in the present invention. Besides boric acid itself, its salts and esters and those of metaboric acid hereafter referred to as metaborates can be used, for example, sodium metaborate eg of formula NaBO 2 4H 2 O. Also complex borates containing more than one boron atom in the anion such as tetraborates, also known as pyroborates eg sodium pyroborate decahydrate of formula Na 2 B 4 O 7 10H 2 O commonly known as borax may also be used in the present invention. Advantageous Effect of the Invention By use of the present invention the sensitometric change due to the ageing of redox amplifer/developers is reduced. Further the effect of over or under replenishment in a continuous process is reduced. A further advantage is that the effect of a succession of high or low density prints passing through a low volume processor will be reduced and also the effect on sensitometry of any local depletion or build up of peroxide caused by insufficient agitation will be reduced. Detailed Description of the Invention Preferably, the complexing compound is a borate containing one boron atom in the anion. Preferably, the molar ratio of borate to hydrogen peroxide is at least 5:1 and can exceed 10:1, but preferably does not exceed 20:1. Preferably the pH of the aqueous redox amplifier solution is in the range from 10.5 to 12.0. The concentration range of the hydrogen peroxide in the redox amplifier solution is preferably equivalent to 0.5 to 10.0 ml/l of 30% w/w solution. Hydrogen peroxide is frequently supplied as a 30%w/w aqueous solution. However peroxide of a different concentration can be used to make up the solutions of the invention provided the amount of hydrogen peroxide is the same as that in the specified amount of 30%w/w solution. The concentration range of borate is preferably from 5.0 to 40.0 g/l. Unless the context clearly requires otherwise, references in this specification to the concentration of hydrogen peroxide and borate or other complexing compound are to the total amounts of these materials in the redox amplifier solution ie both free and complexed. A convenient way of measuring the rate of redox amplification for a given solution is by the well known sensitometric method which measures the colour development using a preexposed test strip and involves measuring the dye Dmax values for red, green and blue. Preferably the relative amounts of borate or other complexing compound and hydrogen peroxide in the solution are such as to provide a solution whose sensitometric values are substantially equal to those of a freshly prepared solution containing no complexing compound and a hydrogen peroxide concentration equivalent to 0.5 to 5.0 ml/l of 30% w/w solution. Any of the colour developing agents known in the art may be used in the present invention, for example,the p-phenylene diamine colour developing agents. The term auxiliary developing agent is well known in the art and includes compounds known as electron transfer agents such as Phenidone and its derivatives, p-amino phenols and certain substituted p-phenylene diamines eg N,N,N',N'-tetramethyl p-phenylene diamine. Preferred amounts of colour developing agent are from 0.5 to 10g/l, more prefereably from 1 to 7.5g/l. Certain redox amplification systems have the colour developing agent associated with the material being developed eg immobilised in a layer on the paper rather than the redox amplifier solution and employ an auxiliary developing agent in the solution. The present invention can be used with such systems by providing the auxiliary developing agent in the redox amplifier solution. The solution may also contain other additives, for example,antioxidants, sequestrants and buffers. Examples of suitable antioxidants are diethylhydroxylamine and hydroxylamine and salts thereof such as the sulphate,chloride or phosphate. Typical amounts are in the range from 0.25 to 5.5 g/l (as hydroxylamine sulphate). The phosphate buffer may be potassium hydrogen phoshate (K 2 HPO 4 ), or other phosphates, carbonates, silicates or mixtures thereof The solutions of the present invention can be prepared by dissolving the borate in water and adding hydrogen peroxide solution and the other components. Alternatively the hydrogen peroxide and borate can be added to the water in the form of a complex eg as a perborate such as sodium perborate. As stated above, sodium perborate which has been previously described for use in redox amplification contains hydrogen peroxide and sobium metaborate in 1:1 molar proportions. In order to obtain a solution having the buffering effect according to the invention it is necessary to add further borate to produce a molar ratio of at least 2:1. According to another aspect of the invention a process for the preparation of a redox amplifier solution as hereinbefore defined comprises either (i) dissolving the compounds in water in the amounts defined or alternatively (ii) providing the peroxide in the form of a complex such as a perborate and adjusting the molar ratio of borate to hydrogen peroxide to obtain the buffering effect. In the preparation of the solutions ccording to the invention the compounds may be added in any order. According to further aspect of the invention a method of redox amplification comprises subjecting an exposed photographic material to redox amplification using a solution as hereinbefore defined. The photographic material may be first subjected to a development step with a developer solution containing no peroxide before the redox amplification. Alternatively the development and amplification may be combined in a single step using a combined developer/ amplifier solution. Colour photographic material to be processed by the redox amplifier solution of the present invention may be of any type but will preferably contain low amounts of silver halide. Preferred total silver halide coverages are in the range 6 to 300, preferably 10 to 200mg/m 2 and particularly 10 to 100 mg/m 2 (as silver). The material may comprise the emulsions, sensitisers, couplers, supports, layers, additives etc described in Research Disclosure, December 1978, Item 17643, published by Kenneth Mason Publications Ltd, Dudley Annex, 12a North Street, Emsworth, Hants PO10 7DQ, UK. In a preferred embodiment the photographic material to be processed comprises a resin coated paper support and the emulsion layers comprise more than 80%, preferably more than 90% silver chloride and are more preferably composed of substantially pure silver chloride. The photographic materials can be single colour materials or multicolour materials. Multicolour materials contain dye image forming units sensitive to each of the three primary regions of the spectrum. Each unit can be comprised of a single emulsion layer of multiple emulsion layers sensitive to a given region of the spectrum. The layers of the materials, including the layers of the image-forming units can be arranged in various orders as is known in the art. A typical multicolour photographic material comprises a support bearing a yellow dye image-forming unit comprised of at least one blue sensitive silver halide emulsion layer having associated therewith at least one yellow dye-forming coupler, and magenta and cyan dye image -forming units comprising at least one green or red sensitive silver halide emulsion layer having associated therewith at least one magenta or cyan dye-forming coupler respectively. The material can contain additional layers, such as filter layers. The invention is illustrated by the following Examples of which Examples 1 and 3 are not according to the invention and are included for comparative purposes. In the Examples the following process cycle was used: develop: details of developer/amplifier and times given in Tables 1 and 6 stop: 30 seconds with a solution of sodium metabisulphite (15g/l) bleach-fix: 45 seconds in a solution of pH 6.2 containing 75 ml/l of ammonium ferric ethylene dimine tetraacetic acid (1.56M), 80ml/l of ammonium thiosulphate (58%) and 7.5g/l of anhydrous sodium sulphite, wash: 90 seconds with water and finally drying in warm air. Example 1 . A redox split developer system consisting of a first developer followed by a developer/amplifier of the composition shown in Table 1 was left to age without any replenishment. Developer Developer/amplifier sequestrant 1 0.9g/l 0.6g/l sequestrant 2 3.0ml/l 2.0ml/l K 2 HPO 4 .3H 2 O 30g/l 20g/l KBr 1mg/l 1mg/l KCl 1.0g/l 0.7g/l component 3 0.45g/l 0.3g/l component 4 3.0ml/l 2.0ml/l component 5 5.5g/l 1.5g/l pH 11.7 11.2 H 2 O 2 (30%) nil 1.3ml/l Time 15sec 45sec Temperature 32C 32C Sequestrant 1 is a 60% solution of 1-hydroxyethylidene -1,1-diphosphonic acid Sequestrant 2 is a 41% solution of the penta sodium salt of diethylene triamine penta acetic acid. Component 3 is catechol disulphonate. Component 4 is an 85% solution of diethylhydroxylamine used as an antioxidant and component 5 is 4-N-ethyl-N-(beta-methanesulphonamidoethyl)-o-toluidene sesquisulphate as a colour developing agent. The sensitometric response was measured at intervals of time by treating a preexposed low silver control strip with the solutions for the times stated and measuring the dye Dmax values which are recorded in Table 2. Dmax (times 100) with time (no boric acid) Time (hours) R G B 0 267 267 231 1.83 270 270 231 3.5 268 268 230 5.0 248 258 231 6.42 220 240 229 23 76 98 99 It can be seen from Table 2 that the red,green and blue Dmax values fall with time. In another experiment the same developer/amplifier solution was prepared except that 30g/l of boric acid was added and the pH adjusted back to 11.2. When used as a developer/amplifier it was found that a much reduced dye image was formed as compared with the freshly prepared solution containing no boric acid. This shows that when hydrogen peroxide is complexed with boric acid it is not free to act as an oxidising agent. The experiments recorded in Example 1 are included for comparative purposes only. Example 2 Four solutions of the composition given in Table 1 containing 30g/l of boric acid were prepared and to each solution a different amount of hydrogen peroxide was added and the sensitometric response measured. The amounts of hydrogen peroxide and Dmax values are recorded in Table 3. Dmax and peroxide level Peroxide level (ml/l) Dmax (times 100) R G B 1.3 125 164 198 2.7 164 217 238 4.0 206 245 235 5.0 235 260 238 It can be seen that increasing the level of hydrogen peroxide progressively restores the sensitometric response and that the latter is almost fully restored when the level of peroxide is approximately 4 times the original level in the absence of the boric acid i.e. 5.0ml/l against 1.3ml/l. This indicates that only a fraction of the hydrogen peroxide is free to act as an oxidising agent and that most of the peroxide is complexed with the boric acid. The standing stability of the solution containing 5.0 ml/l of hydrogen peroxide was measured and the results recorded in Table 4. Dmax (times 100) with time boric acid, 5ml/l peroxide Time hours R G B 0 235 260 238 2.5 237 261 239 3.5 234 258 236 5.0 232 257 238 6.42 230 256 239 23 189 226 220 These results show that the fall off in activity of the solution is much reduced as compared with the solution in Table 2 containing no boric acid and only 1.3 ml/l of hydrogen peroxide. Example 3 A developer solution of composition as detailed in Table 1 i.e. containing no boric acid was prepared and hydrogen peroxide added to a concentration of 5ml/l. The standing stability was measured by measuring the Dmax values at intervals of time and the results are given in Table 5. Dmax (times 100) with time: no boric acid. Time hours R G B 0 275 273 198 2.5 273 273 195 3.5 273 271 195 5.0 267 265 191 6.42 242 239 190 23 81 96 101 These results show that the standing stability is inferior to that recorded in Table 4 (which is for the same solution but containing boric acid). The improved stability is therefore due to the combination of boric acid and increased peroxide and not to the increased peroxide alone. Example 4 In this example boric acid is used in a redox process which consists of a single developer amplifier (SDA) solution unlike the previous examples which used developer followed by developer-amplifier. The composition of the developer amplifier is given in Table 6 below. SDA 1 SDA 2 SDA 3 sequestrant 1 0.6g/l 0.6g/l 0.6g/l sequestrant 2 2.0ml/l 2.0ml/l 2.0ml/l K 2 HPO 4 .3H 2 O 20.0g/l 20.0g/l 20.0g/l KBr 1.0mg/l 1.0mg/l 1.0mg/l KCl 0.7g/l 0.7g/l 0.7g/l component 3 0.3g/l 0.3g/l 0.3g/l component 4 2.0g/l 2.0g/l 2.0g/l component 5 3.5g/l 3.5g/l 3.5g/l boric acid 0 15g/l 30.0g/l pH 11.5 11.5 11.5 H 2 O 2 1.5ml/l 2.5ml/l 2.5ml/l Time 45 seconds 45 seconds 45 seconds Temperature 32degrees C 32degrees C 32degrees C Components 3,4 and 5 were as previously identified in Table 1. SDA 1 was the control developer-amplifier and the other developer-amplifier solutions were made to demonstrate the invention. The sensitometric response of a three colour multilayer was monitored over a period of time and the results for Dmax as a function of the age of the developer-amplifier solutions are shown in Table 7. After 24 hours an additional 1.5ml/l of hydrogen peroxide (30%) was added to each developer-amplifier and the monitoring was continued for another 6 hours. Dmax times 100 Time hours SDA 1 SDA 2 SDA 3 R G B R G B R G B 0 269 265 199 254 264 249 181 217 224 1 270 267 197 254 265 251 179 218 225 3 269 266 200 245 259 242 174 213 220 5 268 266 215 233 252 239 169 210 221 6 259 263 221 237 256 246 168 208 221 24 073 097 102 140 172 197 144 181 199 peroxide added 268 261 157 259 263 221 221 248 230 25 241 238 141 251 259 216 212 242 222 27 132 142 114 220 241 203 208 240 226 29 084 104 102 179 208 181 194 229 220 30 078 098 097 166 192 175 195 228 224 It can be seen from this data that the solutions containing boric acid maintain D max values better than the control developer-amplifier (SDA 1) which essentially has no amplification after 24 hours. On the second day after the addition of 1.5ml/l of hydrogen peroxide (30%) to all the developer-amplifier solutions, SDA 2 and SDA 3 are even more beneficial.

Claims: An aqueous redox amplifier solution having a pH in the range 10.0 to 13.5 comprising: a colour developing agent and/or auxiliary developing agent, hydrogen peroxide or a compound that provides hydrogen peroxide in a concentration equivalent to 0.1 to 50.0 ml/l of 30%w/w solution and a borate complexing compound capable of reversibly forming a complex with hydrogen peroxide, the complexing compound being in a concentration of from 1.0 to 100.0 g/l and wherein the molar ratio of borate complexing compound to hydrogen peroxide is at least 2:1 such as to buffer the hydrogen peroxide and thereby reduce the decline in the rate of redox amplification. An aqueous redox amplifier solution as claimed in claim 1 wherein the molar ratio of borate to hydrogen peroxide is at least 5:1. An aqueous redox amplifier solution as claimed in claim 1 or claim 2 wherein when the complexing compound is a borate containing one boron atom in the anion. An aqueous redox amplifier solution as claimed in any one of the preceding claims wherein the borate is boric acid. An aqueous redox amplifier solution as claimed in any one of claims 1 to 3 wherein the borate is a metaborate. An aqueous redox amplifier solution as claimed in claim 1 wherein the borate is a complex borate containing more than one boron atom in the anion. An aqueous redox amplifier solution as claimed in claim 6 wherein the borate is a tetraborate. An aqueous redox amplifier solution as claimed in any one of the preceding claims wherein the concentration of borate is from 5.0 to 40.0 g/l. An aqueous redox amplifier solution as claimed in any one of the preceding claims wherein the concentration of hydrogen peroxide is in the range equivalent to 0.5 to 10.0 ml/l of 30%w/w solution. A process for the preparation of a redox amplifier solution as claimed in any one of the preceding claims which process comprises either (i) dissolving the compounds in water in the amounts defined or (ii) providing the hydrogen peroxide in the form of a reversible complex and adjusting the molar ratio of borate to hydrogen peroxide to obtain the buffering effect.

Inventors: TWIST PETER J, WINSCOM CHRISTOPHER J, TWIST, PETER J., WINSCOM, CHRISTOPHER J., Twist, Peter J., c/o Kodak Ltd., Winscom, Christopher J., c/o Kodak Ltd.
Applicants: EASTMAN KODAK CO, KODAK LTD, EASTMAN KODAK COMPANY, KODAK LIMITED
Application year: 1995
Main classification: G03C7
IPC codes: G03C7