US 3181949 A
Description (OCR text may contain errors)
United States Patent 3,181,949 LIGHT SENSITIVE ELEMENTS HAVING OPTI- CAL BLEACHING COMPOSITIONS COATED THEREON Willem Karel Koerber and Ferdinand Leonard Schouteden, Wilrijh-Antwerp, Belgium, assignors to Gevaert Photo-Producten N.V., Mortsel, Belgium, a company of Belgium No Drawing. Original application June 2, 1958, Ser. No. 739,211, now Patent No. 3,047,390, dated July 31, 1962. Divided and this application June 11, 1962, Ser. No. 201,299 Claims priority, application Netherlands, June 6, 1957,
4 Claims. ci. 96-82) This case is a division of US. application Serial Number 739,211, filed June 2, 1958, now US. Patent 3,047,390.
The present invention relates to a process for whitening, i.e., optical brightening, the surface color of paper, especially photographic paper, provided with one or more surface coatings composed of constituents which are not capable of strongly binding the usual optical bleaching agents, and more particularly relates to a process for brightening the white areas in the photographic image of a photographic layer in such a way that the diffusion of optical bleaching agents out of the photographic layer is inhibited.
It is generally known that the reflecting power of a paper surface can be considerably increased by applying thereto a surface coating containing finely divided fillers, which are usually pigments, having a high degree of whiteness. Examples of fillers used for this purpose are titanous dioxide, barium, sulfate, zinc sulfide, calcium carbonate and silicium dioxide, among others. In this way, the surface color of the paper can be whitened to a certain extent, but not completely. If, however, these pigments are used in conjunction with substances which are slightly colored or acquire coloration in the course of further treatment or after storage, as for example where such substances either are included in the same surface coating with the pigments or are present in one or more additional surface layers, the brightening effect attributable to the pigment is nullified wholly or partially. Thus, it is seen, for instance, that when a baryta-coated paper support is coated with a photographic emulsion, the surface color of the photographic paper is not completely white and even the whitest areas in a photographic image obtained upon such a support will still absorb a considerable amoutn of light. This coloration depends on various factors, such as the reflecting power of the paper used, the quality of the gelatin applied as binding agent for the photographic layers, and the kind and the amount of other compounds present such as e.g., sensitizing dyestuffs added to the photographic layers during the manufacture of the light-sensitive element. This coloration can also be caused by colored decomposition products fonned in the photographic layers as they are subjected to development and/ or other treatment or formed in the treatment baths and absorbed by these layers as they are immersed therein. Since the cellulose fibers from which the paper is composed, as well as the gelatin used as binding agent for the photographic layers and the decomposition products which are formed during processing, absorb light particularly in the range of the short wave-lengths, the photographic paper shows in most cases a slight yellow tinge.
It has been proposed to eliminate this defect by adding to the paper support, to a surface coating and/ or to another superposed layer, dyestuffs or pigments which absorb in the range of the long wave-lengths, so that the special character of the reflected visible light is equal to that of the incident visible light and the paper, consequently, shows a neutral tone. Thus, it is known to add organic, light fast, blue dyestuffs to a baryta-coating which is applied to a paper support destined to receive a photographic emulsion layer.
As an inherent result of the latter expedient, the incident visible light is not completely reflected but partly absorbed by the yellow paper and partly by the blue dyestuff or the blue pigment. Even under the most favorable circumstances where only an extremely small amount of dyestuff or pigment is required, such as for instance, in the case of a photographic paper manufactured from a baryta-coated base stock of good quality and containing a little colored gelatin and one or more sensitizing dyestuffs of the type which du1ing the processing can easily be removed from photographic layers, the whitest areas in the photographic image will not completely reflect the visible light.
According to a more recent proposal, substantially colorless, blue-fluorescing, ultraviolet-absorbing compounds are added during the manufacture to the paper or to one OI'I11OT6 surface coatings. These compounds absorb ultraviolet radiation of between about 3000 Angstrom units and 4000 Angstrom units and emit fluorescent light of between 4000 Angstrom units and 5500 Angstrom units and are commonly referred to in the art as optical bleaching agents. They can be added during the manufacture of a photographic paper, either to the paper support, to the layer wherein the photographic image is formed (for instance, the silver halide emulsion layer) or to a layer therebetween (for instance, a barytacoating). The photographic element can also be treated with a solution of such compounds during processing.
The spectral partition of the reflected visible light is equalized by the use of these optical bleaching agents to that of the incident 'visible light by conversion of a part of the incident ultra-violet rays to which the human eye is not sensitive into the blue light absorbed by the paper. The reflected light is now completely neutral, without loss of any part of the visible incident light. By applying this concept to the manufacture of photographic paper, not only is it possible to obtain a netural image tone together with a strong reflecting power of the white areas in the photographic image, but photographic images to be viewed by reflection can even be manufactured which reflect more visible light than they actually receive. The latter result can be achieved by the use of an excess of optical bleaching agent, so that more ultra-violet light is converted into blue light than is necessary to correct the yellowness of the photographic paper. In this way, one can change the image-tone to a colder, i.e., more bluish, tone.
In connection with the process just described, it is to be noted that if the optical bleaching agent is added to the paper alone, insufficient brightening of the images is obtained, since the whitening effect is annulled in large measure by the superposed baryta-coating which is present in most of the photographic papers.
It is evident that the degree of success obtained in the practice of the above-mentioned process depends heavily on the amount of optical bleaching agent which still remains in the completely finished paper. Consequently, eflorts have been made in the past to prevent the removal of optical bleaching agents, either in whole or in part, from the layers of the photographic element wherein they appear, as can happen when the element is treated in the usual treating baths.
None of the various suggestions for combating the tendency of the optical bleaching agents to wander, i.e., difluse, out of the emulsion layer and/ or the baryta-coating has hitherto been found wholly effective.
Summarizing, the great difiiculty in finding an effective solution to the problem of loss of optical bleaching agents 3 from photographic elements during processing is attributable mainly to the fact that optical bleaching agents are not sufficiently adsorbed on the materials, such as gelatin and baryta, of which the usual surface-coatings applied to photographic paper are composed.
It is therefore an object of the present invention to provide a method for increasing the whiteness of the surface of a paper provided with an optical bleaching agent and at least one surface coating whereby the wandering of the optical bleaching agent out if its original locus is inhibited. It is a further object of the present invention to provide a method for increasing the whiteness of photographic papers. Still further objects and advantages will appear in the following description and examples.
In accordance with the present invention, it has now been found that the wandering or diffusion of optical bleaching agents out of the layers in which they were initially incorporated can be inhibited at least substantially completely by treating certain finely divided water-insoluble macromolecular products with a solution or a dispersion of an optical bleaching agent having a pronounced substantivity for the macromolecular product used and then adding the fiuorescing mass thus obtained to one or more surface coatings for the paper. In the manufacture of photographic paper, for example, the fluorescing mass can be incorporated into a layer between the paper support and the layer wherein the photographic image is formed or into the latter layer itself.
Our parent application, Serial No. 739,211, filed June 2, 1958 (now US. Patent No. 3,047,390) originally disclosed as suitable macromolecular products for the practice of the invention, certain cellulosic, polyester, polyamide and urea formaldehyde resins but was eventually restricted to the polyester subject matter. This application is directed to the polyamide subject-matter divided out of said parent application while applications Serial No. 201,290 and No. 201,300, filed concurrently herewith, are directed, respectively, to the cellulosic and ureaformaldehyde subject-matter thereof.
In contrast to most practical applications for polyamide resins, which require polyamide compounds with a high molecular weight for example, the so-called superpolyamides having a molecular weight of at least about 30,000, it is preferred according to the present invention to employ solid linear polyamides having an average molecular weight from 1,000 to 20,000. Such materials lend themselves more readily to subdivision of a very fine extent and, in this condition, are adapted to absorb or bind a relatively larger amount of the optical bleaching agent. As employed herein, the term low degree of polymerization is intended to embrace polymers having an average DR of at least about 5,000 and not more than about 10,000.
Insofar as we have been able to determine the method of preparing the linear polyamide of the invention is not critical and any of the standard methods can be followed with more or less useful results. For example, the reaction may involve the homocondensation of aminocarboxylic acids, such as epsilon-aminocaproic acid and l 1- aminoundecanoic acid, or their progenitors, i.e., epsiloncaprolactam. Alternatively, the reaction may be between a diamine and a dibasic acid. As the diamine, there are most frequently used the alkylene diamines having a carbon chain containing about 6-10 carbon atoms, such as hexamethylene diamine, and the alkane dicarboxylic acids also containing, 610 carbon atoms, such as adipic acid. The reaction may be controlled to give the desired degree of polymerization by any of the usual techniques, such as by adjusting the severity of reaction conditions, e.g., temperature and vacuum, and the rate and extent of water removal in the case of reactants which release water to achieve the requisite equilibrium condition. Low molecular Weight aliphatic acids, such as acetic acid, may be included to act as a chain stopper. If preferred, the reactants may be formed initially into the ammonium salt, i.e., nylon salt, which can then be condensed into the polyamide by the removal of water. Co-polymeric mixtures can be substituted for single polymers.
Of the various possibilities, the homocondensation of epsilon-caprolactam is preferred since this reaction can be regulated by controlling temperature and time. In this case, the monomer is heated in the presence of water for the desired combination of time and temperature. The water serves to hydrolyze enough of the caprolactam to initiate the reaction. Other initiators and/or polymerization catalysts are known and may be utilized.
The reaction product of any of the polymerization methods can be extruded into sheets or slabs and solidified by cooling. The solid masses may then be converted into chips or flakes which are well adapted for further subdivision according to the invention.
Before use, the polyamide resin is reduced to finely divided form; consequently, the condition in which it is procured is immaterial. The subdividing operation may be carried out in any of the types of equipment useful for grinding or cutting hard resin fragments into fine particles. The extent of subdivision is determined primarily by practical considerations as is necessary to be compatible with the intended purpose of the paper. Thus, in the photographic field, the major application of the invention, the particle size should be such as to permit uniform incorporation in the formulation to be coated on the paper without disrupting or otherwise disturbing the continuity or the surface quality of the finished coating. For example, particles of a size causing the coating to have a perceptibly roughened or irregular surface would be poorly adapted, if at all, for photographic materials. In general, a particle size of about to 1 micron will be acceptable with a size in the same order as that of the usual pigments mentioned above, say about /2 micron being preferred.
The following are the structural formulae of the known optical bleaching agents that have been found useful in the practice of the invention, although it will be appreciated that others having comparable chemical properties may well exist and could be substituted with generally similar effect:
Methods of preparing compounds having the above formulae are described in the following publications, the order of numbering corresponding to the list above:
(1) Chem. Abs. B44 (1950), p. 2251.
(2) British patent specification 495,479, p. 3, No. 13; German patent specification 752,677, p. 8; Danish patent specification No. 63,688 (1945); German patent specification 972,067.
(3) US. patent specification 2,875,089, cols. 6-7.
For carrying out the process according to the present invention, the desired amount of the finely divided polyamide resin is dispersed in an excess of water. To this suspension is added a solution of the optical bleaching agent involved. If the optical bleaching agent is not sufficiently soluble, it can likewise be added in dispersed form to the suspension. Absorption of the optical bleaching agent is preferably facilitated by heating the suspension to a temperature between about 50 and 100 C., the treatment being continued until no more fluorescing substance is bound. The time required to achieve this condition will ordinarily range between about 30-60 minutes depending upon the temperature. The mixture is then filtered under suction or centrifugation, leaving a strongly fluorescing paste which strongly retains the fiuorescing substance and which can be added to the coating composition to be applied to a paper support. Thus, the fluorescing mass can, for instance, be added to a baryta-composition which is spread upon a .paper prior to applying the photographic emulsion thereto. Alternatively, paste can often be incorporated in a photographic emulsion itself. While preliminary treatment of the finely divided polyamide with the optical bleaching agent is usually best, where experience has indicated the amount of a particular agent that can be expected to be bound to a given amount of the polyamide, the treatment can often be effected in situ by admixing the amount of agent and amount of polyamide directly to the coating formulation. In this case, the heat applied to dry the coating will serve to promote the combination of the agent and polyamide.
The proportion of the treated polyamide resin needed in the coating formulation to achieve the advantages of the invention can be adjusted to meet the requirements of any particular paper and/or surface coatings. Obviously, any amount will contribute a proportionate effect and will improve the performance of the paper to that extent. As a general rule, about 5 to 25% by weight of the coating in which the resin is employed, the baryta coating, for example, will suflice, although larger or smaller amounts can be employed.
Our experiments to date have not clearly revealed the exact mechanism by which the optical bleaching agent becomes bound to the polyamide resin. Presumably, physical or chemical forces or a combination of the two are involved but the invention is not to be restricted to any particular theory.
The wandering of the optical bleaching agent out of the layer in which it was applied is practically completely precluded by the method of the present invention, so that the exposed photographic element after complete processing, which even may include a protracted rinsing step, yields a photographic image characterized by strongly brightened white areas.
The method of the present invention can be utilized in brightening the white areas in the developed image of the usual black-and-white and colored photographic papers as well as for whitening the surface color of any other surface-coated paper such as, for instance, papers used in duplication processes as, for example, where a positive photographic image is formed by a silver halide diffusion transfer process, or an image is printed by means of an hectographic process.
The present invention is further illustrated in the following examples without limiting, however, the scope thereof.
In the examples, the formula number given in parenthesis corresponds to the number in the list of structural formulae set forth previously.
EXAMPLE A. Polymer preparation One polyamide suitable for use in the method of the invention can be prepared in the following manner: gm. of epsilon-caprolactam and 0.05 gm. of carbonatefree sodium hydroxide are heated for about 10 minutes 20 gm. of epsilon-caprolactam and 0.05 gm. of carbonateat 95 C. while pure nitrogen gas is bubbled through the reaction mixture. Thereafter, the temperature is raised to 230 C. and heating is continued for an additional 20 minutes. Next the reaction mixture is poured with strong stirring into 1.5 liters of water, filtered under suction and thoroughly Washed with warm water. The resultant product is a pasty mass of polyamide particles of a size of about 3 microns having an average molecular weight of about 6,000.
B. Method of invention 1 kg. of the paste obtained according to preparation A above is suspended in 1500 cc. of a 5% aqueous solution of sodium sulfate together with 10 gm. of 4,4-bis(2-hydroxy-4-phenylamino-1,3,5-triazyl 6) diaminostilbene- 2,2disulfonic acid sodium salt (Formula 2). The mixture is kept in suspension by means of strong stirring for 30 minutes at a temperature of 90 C. The treated resin particles are recovered by filtration under suction or centrifugation and added to a conventional baryta coating composition which is then spread on a paper support. A silver halide photographic emulsion is next spread on the support on top of the baryta coating. After exposure and development of the photographic element, it is found that the whiteness of the element is not lost on rinsing and that the white areas of the photographic element are strongly brightened.
A similar result is attained when method B is repeated using as the brightening agent instead of 4,4'-bis(2-hydroxy-4-phenylamino-1,3,5-triazyl 6) diaminostilbene- 2,2'-disulfonic acid sodium salt; 1,2-di(5-methylbenzimidazyl-2)-ethylene (Formula 1) or 1,2-di(5-methylbenzoxazolyl-2)ethylene (Formula 3).
1. A light-sensitive photographic paper comprising a paper support, a white pigmented coating on a surface of said support, and a superposed light-sensitive silver halide emulsion layer, said white pigmented coating containing discrete fluorescent particles obtained by heating an aqueous suspension of finely divided polyamide resin through which suspension is uniformly distributed an optical brightening agent selected from the group consisting of:
( 1 1,2-di(S-methylbenzimidazolyl-Z)-ethylene (2) 2,2-di-sulfo 4,4'-bis [2(4-hydroxy-6-anilino)-s-t1'iaziny11stilbene disodium salt.
(3) 1,2-di 5-methylbenzoxazolyl-2 -ethylene.
2. A paper according to claim 1 wherein said fluorescent mass constituted about 5 to 25% by weight of said pigmented coating.
3. A paper according to claim 1 wherein said polyamide resin has an average molecular Weight of about 1,000 to 20,000.
4. A paper as in claim 1 wherein said polyamide resin has a particle size not greater than about 5 microns.
References Cited by the Examiner UNITED STATES PATENTS 2,332,038 10/43 Wynd et al. 96-82 2,639,990 5/53 Kendall et al. 9682 2,723,197 11/55 Libby et al. 9694 2,822,288 2/58 Harrey et al. 11733.5 2,846,397 8/58 Ackermann 260-2495 3,031,326 4/62 Fleck 117-335 3,043,710 7/62 Patten et al 11733.5 3,047,390 7/62 Koerber et al 9682 FOREIGN PATENTS 878,823 2/43 France. 573,462 4/59 Canada.
NORMAN G. TORCHIN, Primary Examiner.