EP0266410B1

EP0266410B1 - Imaging elements having hydrophilic layers containing hydrophobes in polymer particles and method for manufacture thereof

Info

Publication number: EP0266410B1
Application number: EP87903494A
Authority: EP
Inventors: Robert H. Fehnel; Paul Mitacek
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1986-05-01
Filing date: 1987-04-17
Publication date: 1990-09-19
Anticipated expiration: 2007-04-17
Also published as: WO1987006723A3; JPS63503486A; WO1987006723A2; EP0266410A1

Abstract

Imaging elements, including radiation-sensitive elements such as photographic color paper products, are comprised of a support bearing a substantially crystal- and agglomeration-free hydrophilic layer. This layer comprises a hydrophilic composition containing a hydrophilic binder and water-insoluble polymer particles dispersed therein. These particles have recurring units derived from a monomeric composition composed of one or more ethylenically unsaturated polymerizable monomers with more than about 5 percent and up to 100 percent by weight of the monomeric composition consisting of at least one ethylenically unsaturated polymerizable monomer having a crosslinkable moiety, and comprise at least about 0.5 percent by weight of a hydrophobe uniformly distributed throughout.

Description

Field of the Invention

This invention relates to imaging elements, including radiation-sensitive elements such as photographic color paper products. In particular, it relates to such elements having a hydrophilic layer which contains a hydrophobic compound (e.g. optical brightener) uniformly distributed in polymeric particles. This invention also relates to a method of making such elements.

Background of the Invention

Several techniques have been used heretofore to distribute hydrophobic compounds (hereinafter "hydrophobes"), particularly non-polymeric compounds such as color-forming couplers, ultraviolet light absorbing materials, optical brighteners, etc., uniformly throughout layers of gelatin or other hydrophilic binder materials in the manufacture of radiation-sensitive products. One of the simplest of these techniques involves mechanically dispersing the hydrophobe in solid or liquid form in the binder material by passing a blend of the hydrophobe and binder material several times through a high energy mill. This technique, however, generally produces unsuitable dispersions which are often unstable.
Another technique is described in U. S. Patent 4,203,716 (issued May 20, 1980 to Chen). That technique involves "loading" polymeric latex particles with the hydrophobe using an organic solvent. Generally, "loading" involves (1) dissolving the hydrophobe in a suitable water-miscible organic solvent; (2) mixing the resulting solution with polymeric latex particles; and (3) removing residual solvent as desired, particularly if necessary to drive the "loading" process to completion, or to provide material sufficiently "loaded" with the hydrophobe. The "loaded" latex is then usually dispersed in a hydrophilic binder in preparation for coating.
Depending upon the hydrophobe, sometimes only a limited amount of hydrophobe can be successfully "loaded" into latex particles, and any residual hydrophobe must be removed to prevent deleterious image effects. Often some of the hydrophobe "leaches" out of the latex particles and forms "crystals". Such crystals deleteriously affect image quality (e.g. reduce sharpness) and, when clumped together, reduce layer smoothness which is important for very thin coatings. This leached-out hydrophobe can also wander into adjacent layers, causing additional problems.
U. S. Patent 3,418,127 (issued December 24, 1968 to Millikan) discloses a method of finely dispersing fluorescent compounds in latex particles by mixing the fluors in polymerizable monomers and emulsion polymerizing the monomers having the fluors therein. The resulting latex purportedly can be coated and dried to form a thin film, preferably over the radiation-sensitive layers of a photographic element. Similarly, West German Patent 2,509,342 (published September 11, 1975) teaches the incorporation of optical brighteners into polymeric particles by dissolving the optical brighteners in polymerizable monomers and emulsion polymerizing the monomers. Emulsion polymerization proceeds in micelles formed by water-soluble surfactant. Additional monomer and hydrophobe migrate from monomer droplets through the water phase and into the micelles prior to polymerization. The resulting latex is purportedly mixed with a compatible colloid (e.g. gelatin) and coated either with a photographic emulsion or in a separate layer in a photographic element.
However, attempts to prepare substantially crystal- and agglomeration-free hydrophilic coating compositions according to the teaching of West German Patent 2,509,342 have been unsuccessful. This is demonstrated by Remley, U. S. Patent 4,584,255, issued April 22,1986, and especially by Example 1 thereof which shows that polymer particles of an emulsion polymerized latex having an optical brightener dissolved therein tend to agglomerate during polymerization. This tendency to agglomerate appears to increase with time and the resulting polymeric mass cannot be coated to form a thin film. The disclosure of U. S. Patent 4,584,255 is incorporated herein by reference in its entirety.
The aforesaid U. S. Patent 4,584,255, which is directed to the objective of providing hydrophobe-containing compositions which can be coated to provide substantially crystal- and agglomeration-free hydrophilic layers, provides an important advance in this art. In accordance with the invention described therein, the monomers containing the hydrophobe are dispersed in an aqueous medium and allowed to polymerize as a suspension of fine monomer droplets. This technique avoids the problems encountered with emulsion polymerization of hydrophobe-containing monomers. For example, in an emulsion polymerization process, monomer having hydrophobe dissolved therein must migrate through water and cross the interface of latex micelles. During such migration, the monomer tends to migrate faster than the hydrophobe. As a result, the concentration of the hydrophobe in the monomer droplets outside the micelles increases to a point where the hydrophobe concentration reaches a saturation point in those droplets, and hydrophobe crystallization then occurs. The suspension polymerization process of the aforesaid U. S. Patent 4,584,255 does not involve micelle formation, and thereby avoids the problem.
It is toward the objective of providing a further improvement in this art, whereby the stability, and especially the storage stability, of the coating compositions of U. S. Patent 4,584,255 is enhanced, that the present invention is directed.

Summary of the Invention

In accordance with this invention, an element comprises a support having thereon a substantially crystal- and agglomeration-free hydrophilic layer. This layer comprises a hydrophilic composition containing a hydrophilic binder and water-insoluble polymer particles dispersed therein. These particles have recurring units derived from a monomeric composition composed of one or more ethylenically unsaturated polymerizable monomers with more than about 5 percent and up to 100 percent by weight of the monomeric composition consisting of at least one ethylenically unsaturated polymerizable monomer having a crosslinkable moiety, and comprise at least about 0.5 percent by weight of a hydrophobe uniformly distributed throughout.
The present invention differs in a critical manner from the invention of the aforesaid U. S. Patent 4,584,255 in that it requires the use of from more than about 5 up to 100 weight percent of an ethylenically unsaturated polymerizable monomer having a crosslinkable moiety. In contrast with the present invention, in the invention of the aforesaid U. S. Patent 4,584,255 the use of an ethylenically unsaturated monomer having a crosslinkable moiety is optional and, when such monomer is employed, it is used in amounts of less than about 5 weight percent, as compared with the more than about 5 up to 100 weight percent required in the present invention.
In a preferred embodiment, the elements of this invention are radiation-sensitive elements (e.g., color photographic paper products) which have the hydrophilic layer described above located between the support and the radiation-sensitive layer(s). The hydrophobe in this embodiment is usually an optical brightener.
This invention also comprises a method of making the element described hereinabove. The steps of this method comprise:

(a) dissolving at least about 0.5 percent, based on total monomer weight, of a hydrophobe in a monomeric composition composed of one or more ethylenically unsaturated polymerizable monomers with more than about 5 percent and up to 100 percent by weight of the monomeric composition consisting of at least one ethylenically unsaturated polymerizable monomer having a crosslinkable moiety;
(b) dispersing the solution formed in (a) as fine droplets in water under conditions sufficient to promote polymerization of the monomers in the suspended droplets and to form polymeric particles having the hydrophobe uniformly distributed throughout the particles;
(c) dispersing the polymeric particles in a hydrophilic binder to form a hydrophilic composition; and
(d) applying the hydrophilic composition to a support to form a substantially crystal- and agglomeration-free hydrophilic layer.

In a preferred embodiment of this invention, a radiation-sensitive composition is applied over the hydrophilic layer formed in step (d).

Detailed Description of the Invention

The hydrophobe useful in the practice of this invention is a compound which is essentially insoluble in distilled water at 25°C. Preferably, the dissolved concentration of hydrophobe in water under these conditions is less than about 0.5 weight percent, based on the weight of the water. Any such hydrophobe can be used in the practice of this invention as long as it can be dissolved or uniformly dispersed in the monomeric composition to be used in making the polymer particles described hereinbelow. Preferably, the hydrophobe is soluble in the monomeric composition at a concentration of at least about 8 weight percent, based on the total monomer weight.
Examples of useful functional classes of hydrophobes include, but are not limited to, photographic dyes; photographic dye-forming couplers; photographic developing agents or other photographic addenda; optical brighteners; ultraviolet light absorbing compounds; and others known to one skilled in the photographic art. Specific photographic addenda which can act as hydrophobes include those compounds used to perform coupling, silver halide development, oxidized developer scavenging, absorbtion of light of certain wavelengths, spectral sensitizing or desensitizing, or diffusion transfer dye- image-forming. Examples of such hydrophobes are listed in considerable detail in U. S. Patent 4,203,716 (noted hereinabove), the disclosure of which is incorporated herein by reference in its entirety; and in Research Disclosure, publications 15162 (November, 1976) and 17643 (December, 1978), paragraphs III, IV, VI, VII and VIII (Research Disclosure is published by Kenneth Mason Publications Limited, Emsworth, Hampshire, P010 7DD, United Kingdom). Mixtures of hydrophobes can be used if desired.
Hydrophobes of particular usefulness in the practice of this invention are optical brighteners. In general, useful optical brighteners include such classes of compounds as: oxazoles; oxadiazoles, including benzoxazoles; imidazoles, including benzimidazoles; pyrazolines; coumarins; stilbenes; triazines; imidazolones; naphthotriazoles; acetylenes; vinylene compounds; and others known to a skilled worker in the art. Specific examples of such optical brighteners are described in Research Disclosure, publication 17643, paragraph V, noted hereinabove, U. S. Patent 3,666,680 (issued May 30, 1972 to Briggs) and W. German OLS 2,509,342, noted hereinabove, the disclosures of which are incorporated herein by reference to illustrate optical brighteners useful in this invention.
Of the many classes of optical brighteners which can be used in the practice of this invention, the stilbene and naphthotriazole compounds are preferred, with such brighteners as Uvitex OB^T^", Tinopal PCRTM and Tinopal SFG ^T" being particularly useful. These optical brighteners are commercially available from Ciba-Geigy, Ardsley, New York.
The amount of hydrophobe in the polymer particles can be varied widely depending upon intended use, but it must be at least about 0.5 weight percent and is preferably from about 0.5 to about 10 weight percent, and more preferably from about 5 to about 10 weight percent.
As explained hereinabove, the polymer particles useful in the practice of this invention are formed from a monomeric composition composed of one or more ethylenically unsaturated polymerizable monomers, at least one of which must have a crosslinkable moiety. The lower limit for the amount of ethylenically unsaturated polymerizable monomer having a crosslinkable moiety is more than about 5 percent by weight, and preferably it is at least about 10 percent by weight. The upper limit is 100 percent by weight.
More preferably, the monomeric composition comprises about 50 to about 90 weight percent of at least one ethylenically unsaturated polymerizable monomer, and about 10 to about 50 weight percent of at least one ethylenically unsaturated polymerizable monomer having a crosslinkable moiety. Most preferably, the monomeric composition comprises about 75 to about 90 weight percent of at least one ethylenically unsaturated polymerizable monomer, and about 10 to about 25 weight percent of at least one ethylenically unsaturated polymerizable monomer having a crosslinkable moiety.
Use of more than about 5 and up to 100 percent by weight of polymerizable monomer having a crosslinkable moiety is a key feature of the present invention. As compared with the invention of the aforesaid U. S. Patent 4,584,255, it provides highly beneficial advantages. For example, use of the crosslinkable monomer in these amounts greatly facilitates incorporation into the polymer of high concentrations of the hydrophobe. Moreover, it provides substantially improved shelf life at all levels of concentration of the hydrophobe. With aqueous coating compositions, prepared in accordance with the teachings of U. S. Patent 4,584,255 by utilizing less than about 5 weight percent of monomer having a crosslinkable moiety, after only a few weeks of storage, significant quantities of the hydrophobe can be found to have migrated out of the polymer and crystallized in the surrounding water phase. It is believed that this can be explained by the relatively slow reaction rate of the monomer system allowing hydrophobe molecules to move out of the way of the multiplying polymer chains. As a result, there is a high concentration of hydrophobe near the surface of the polymer particle, and, with time, some of this hydrophobe migrates into the water phase.
While applicants do not wish to be bound by any theoretical explanation of the manner in which their invention functions, it is believed that their use of higher amounts of monomer having a crosslinkable moiety results in the formation of a crosslinked network at a low level of monomer conversion - for example, a degree of conversion of as low as 10 percent-as a consequence of the well-known "gel effect" which occurs in the polymerization of crosslinkable monomer systems. Once the crosslinked network is formed, the hydrophobe is immobilized and thereafter will not migrate out of the polymer particles even upon prolonged storage of the coating composition. Thus, the key to the success of the present invention is believed to be gelation at a low degree of monomer conversion, so that the hydrophobe is "trapped" at a stage in the polymerization process where there is still ample unpolymerized monomer to keep the hydrophobe in solution. The result is a highly uniform distribution of the hydrophobe throughout the polymer particle and little or no tendency for the hydrophobe to migrate.
Preferably, the water-insoluble polymeric particles useful in this invention comprise polymers characterized by the structure:
where -A- represents randomly recurring units in the polymer chain derived from one or more vinyl aromatics; vinyl esters; olefins, diolefins; or esters of a, (3-unsaturated polymerizable carboxylic acids. Examples of useful vinyl aromatics include styrene, a-methylstyrene, p-bromostyrene, o-chlorostyrene, 2- vinylmesitylene, 1-vinylnaphthalene, m- and p-vinyltoluene, 3,4-dichlorostyrene and the like. Useful vinyl esters include, for example, vinyl acetate, vinyl propionate, vinyl butyrate and the like. Examples of useful esters of a, β-unsaturated polymerizable carboxylic acids include methyl acrylate, methyl methacrylate, n-butyl acrylate, n-butyl methacrylate, t-butyl methacrylate, benzyl methacrylate, methyl a-bromoacrylate, 4-chlorobutyl acrylate, cyclohexyl acrylate, 2-norbornylmethyl acrylate, 2-ethylhexyl acrylate, lauryl methacrylate, tetrahydrofurfuryl methacrylate, 2-ethoxyethyl methacrylate, 3-chloropropyl acrylate, 2-2-dimethylbutyl acrylate, and the like. Useful olefins and diolefins include, for example, ethylene, propylene, 1,3-butadiene, isoprene, chloroprene, cyclopentadiene, 5-methyl-1,3,6-heptatriene, and the like.
Preferably -A- represents randomly recurring units derived from one or more vinyl aromatics, e.g., styrene, or esters of α-β-unsaturated polymerizable carboxylic acids, e.g., methyl methacrylate, butyl acrylate and tetrahydrofurfuryl methacrylate. More preferably, -A- is derived from styrene or methyl methacrylate or both.
In the above-identified structure, -B- represents randomly recurring units in the polymer chain derived from one or more ethylenically unsaturated polymerizable monomers having one or more anionic moieties, e.g., sulfo, phosphono or carboxy moieties (including alkali metal or ammonium salts thereof). These recurring units contribute to the dispersibility of the polymer particles in hydrophilic binders. Examples of useful monomers having such anionic moieties include 4-acryloyloxybutane-1-sulfonic acid, sodium salt; 3-acryloyloxy-1-methylpropane-1-sulfonic acid, sodium salt; acrylic and metbacrylic acids and alkali metal salts thereof; m- and p-styrenesulfonic acid and alkali metal salts thereof; 3-methacryloyloxypropane-1-sulfonic acid, sodium salt; lithium methacrylate, N-[3-(N-phenylsulfonyl-N-sodiosulfamoyl)phenyl]acrylamide, N-[2-(N-methylsulfonyl-N-potassiosulfamoyl)ethyl]methacrylamide, ammonium p-styrenesulfonate, 2-acrylamido-2-methylpropanesulfonic acid, sodium salt, and the like.
Preferably, -B- represents randomly recurring units derived from one or more monomers having sulfo or carboxy moieties, such as styrenesulfonic acids or alkali metal salts thereof, acrylic acid, methacrylic acid and 2-acrylamido-2-methylpropanesulfonic acid. More preferably, -B- is derived from styrenesulfonic acids or salts thereof.
Also, in the above-identified structure, -C- represents randomly recurring units in the polymer chain derived from one or more ethylenically unsaturated polymerizable monomers having crosslinkable moieties. Such units contribute to the water-insolubility of the resulting polymer. They also make the polymer less soluble in organic solvents generally used in coating operations and thereby reduce the tendency of the hydrophobe to wander.
These monomers have two or more ethylenically unsaturated moieties which crosslink during polymerization (e.g., diacrylates, divinylbenzene, etc.). Alternatively, they can also have moieties which do not react to provide crosslinking during polymerization, but provide a post-polymerization crosslinking because of reaction with a hardener or with another moiety on a different monomer. Such monomers include, for example, 2-acetoacetoxyethyl methacrylate, N-(2-acetoacetoxyethyl)acrylamide, N-(2-acetoacetamidoethyl)acrylamide and 2-aminoethyl methacrylate hydrochloride. Monomers having two or more ethylenically unsaturated sites available for reaction include, for example, diacrylates; dimethacrylates; triacrylates; trimethacrylates; divinyl compounds; and the like. Examples of such monomers include divinylbenzene, ethylene dimethacrylate, 2,2-dimethyl-1,3-propylene diacrylate, propylidene dimethacrylate, 1,6-hexamethylene diacrylate, phenylethylene dimethacrylate, tetramethylene dimethacrylate, 2,2,2-trichloroethylidene dimethacrylate, ethylenebis(oxyethylene)diacrylate, oxydiethylene diacrylate, ethylidyne trimethacrylate, allyl acrylate, vinyl allyloxyacetate, 1-vinyloxy-2- allyloxyethane, 2-crotonoyloxyethyl methacrylate, diallyl phthalate, triallyl cyanurate, 2-(5-phenyl-2,4-pentadienoyloxy)ethyl methacrylate, N,N'-methylenebisacrylamide, N,N'-bis(methacryloyl)urea, and the like.
Preferably, -C- represents randomly recurring units derived from one or more diacrylates or dimethacrylates, e.g., ethylene diacrylate or ethylene dimethacrylate or both, or from divinyl compounds, e.g., divinylbenzene.
The polymers useful in the practice of this invention can also comprise minor amounts (typically less than about 5 weight percent) of randomly recurring units in the polymer chain derived from one or more ethylenically unsaturated polymerizable monomers other than those described for -A-, -B- or -C-hereinabove. Generally, these units are present in very small amounts in the polymer chain so as not to deleteriously affect polymer water insolubility or other desirable polymer properties. For example, they can be derived from vinyl amides (e.g., acrylamide, methacrylamide, N-isopropylmethacrylamide, methacrylamide, N-isopropylmethacrylamide, N-isopropylacrylamide, N-(3,6-dithiaheptyl)-acrylamide, etc.); vinyl nitriles (e.g., acrylonitrile, methacrylonitrile, 3-butenenitrile, etc.); vinyl ketones (e.g., methyl vinyl ketone, diacetone acrylamide, etc.); vinyl halides (e.g., vinyl chloride, vinyl bromide, vinylidene chloride, etc.); vinyl ethers (e.g., allyl methyl ether, allyl phenyl ether, 2-chlorovinyl methyl ether, etc.); N-vinylsuccinamide; N-vinylphthalimide; N-vinylpyrazolidinone; and others known to one skilled in the polymer chemistry art.
Preferably, the proportions of the various units of the polymer structure defined herein are as follows: w represents a weight percent of from 0 to about 90, x represents a weight percent of from 0 to about 20, and y represents a weight percent of from about 10 to 100. All weight percentages are based on total monomer weight.
More preferably, w represents a weight percent of from about 50 to about 90, x represents a weight percent of from 0 to about 20, and y represents a weight percent of from about 10 to about 50. Most preferably, w represents a weight percent of from about 75 to about 90, x represents a weight percent of from 0 to about 20, and y represents a weight percent of from about 10 to about 25.
In a particularly advantageous embodiment of this invention wherein the hydrophobes are optical brighteners, w is from 75 to 85 weight percent, x is from 0 to 5 weight percent and y is from 15 to 25 weight percent.
Although the glass transition temperature (Tg) of the polymers useful in the practice of this invention can be varied widely, they generally have a glass transition temperature (Tg) greater than about 70°C, and preferably in the range of from about 90 to about 120°C, in order to prevent diffusion of hydrophobe into the coated layers during drying and storage and to improve compatibility with coating addenda. The glass transition temperature can be determined by any convenient method suitable for this purpose. For example, one such method is differential scanning calorimetry as described in Techniques and Methods of Polymer Evaluation. Volume 2, Marcel Dekker, Inc., New York, N. Y., 1970.
Examples of polymers useful in the practice of this invention include:

poly(styrene-co-divinylbenzene) (80:20 weight ratio)
poly(a-methylstyrene-co-divinylbenzene) (80:20 weight ratio)
poly(methylmethacrylate-co-ethylene dimethacrylate) (85:15 weight ratio)
poly(methylmethacrylate-co-styrene-co-divinylbenzene) (40:40:20 weight ratio)
and poly(methylmethacrylate-co-styrene-co-sodium styrenesulfonate-co-divinylbenzene) (40:40:5:15 weight ratio).

The polymer particles useful in the practice of this invention are generally prepared by addition polymerization of the monomers in an aqueous suspension. This is commonly known as "suspension polymerization". It can be carried out in batch, semi-continuous or continuous operations, as is well known in the art.
Particularly preferred polymers for the purpose of this invention are copolymers of styrene and divinylbenzene. For detailed information with respect to the kinetics of the free radical crosslinking copolymerization of a monovinylic monomer such as styrene and a bi-unsaturated comonomer such as divinylbenzene, attention is directed to Hild and Okasha, "Kinetic Investigation Of The Free Radical Crosslinking Copolymerization In The Pre-gel State", Makromol. Chem. 186 93-110 (1985).
Generally, the method of this invention includes dissolving the hydrophobe(s) in solution with the ethylenically unsaturated polymerizable monomers. The monomer solution is then dispersed as fine droplets in water and subjected to conditions sufficient to promote suspension polymerization of the monomers. Although it is not always required, it is advantageous to use one or more polymerization initiators to initiate polymerization and promote its completion. At least one of the initiators, if used, is oleophilic and is dissolved in the monomers along with the hydrophobe. Useful oleophilic initiators include azo compounds, such as the VAZO" initiators commercially available from DuPont, Wilmington, Delaware, e.g., VAZO-64^T"' which is 2,2'-azobis(2-methylpropionitrile), VAZO-52^T"' which is 2,2'-azobis(2,4-dimethylvaleronitrile), VAZO-33^T"' which is 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile) and VAZO-67TM which is 2,2'-azobis(2-methylbutanenitrile); peroxides, such as lauroyl peroxide and benzoyl peroxide; and others known to one skilled in the art. Water-soluble polymerization initiators can be used in addition to oleophilic initiators as long as there is sufficient oleophilic initiator to initiate the polymerization of the suspended monomer droplets and an insubstantial amount of emulsion polymerization occurs.
One or more surface active agents (i.e., surfactants) are also often employed in suspension polymerization to aid in keeping the dispersed monomer droplets from clumping together in the aqueous medium. At least one of the surfactants, if used, is oleophilic and is incorporated into the reaction mixture by dissolving it in the monomer(s) along with the hydrophobe.
It is often desirable to agitate the monomers in a suitable manner while the hydrophobe, initiator or surfactant is added and dissolved therein. Also, it may be advantageous to heat the monomers prior to and during such addition to facilitate dissolution. Normally, if this is done, the temperature of the monomers is maintained at greater than room temperature (20-25°C), but less than the temperature at which the monomers undergo spontaneous polymerization (this varies with the monomer(s) and initiators used). Generally, the temperature used to this purpose is in the range of from about 30 to about 45°C.
Once the hydrophobe is dissolved within the monomers, the resulting solution is dispersed in water as fine droplets and subjected to pressure and temperature conditions suitable for polymerization of the monomers in the suspended droplets and formation of small, suspended polymer particles. The monomer solution is generally present in droplet form in this dispersion in a range of from about 20 to about 50 percent, based on total dispersion weight. The pressure employed in the polymerization is generally only that needed to maintain the reaction mixture in liquid form, and is usually atmospheric pressure. The polymerization temperature is subject to wide variation as it depends upon several variables including the monomer, initiator and weight percent of monomers in the dispersion. However, generally the temperature is in the range of from about 20°C to about 120°C, and preferably from about 50 to about 70°C. The temperature can vary during the polymerization reaction because of the evolution of heat from the reaction itself.
The monomer solution can be dispersed in the aqueous medium prior to polymerization in any suitable manner which may depend upon the polymerization technique (batch, continuous or semi-continuous) employed. Generally, the solution is dispersed in the aqueous phase by any means which produces high shear sufficient to form very fine droplets containing monomer, hydrophobe and preferably, oleophilic initiator and surfactant. For example, such dispersing can be accomplished by mechanical means such as high-speed stirring or vigorous agitation of some manner, or by pumping a monomer-water mixture through a small orifice or high shear mill into a reactor vessel.
Once polymerization has begun, it is continued until substantially all monomer has reacted. This may take up to 24 hours, depending upon the polymerization conditions employed.
Specific details of polymerization of the monomers having the hydrophobe dissolved therein are illustrated in the example presented hereinbelow.
The resulting polymer is in the form of small particles, the size of which can be varied by changing the dispersing conditions or amount of surfactant. The average particle size is generally in the range of from about 0.1 to about 20 microns, with polymer particles in the range of from about 0.4 to about 1 microns being particularly useful in the preferred embodiment of this invention utilizing optical brighteners as the hydrophobe.
The resulting aqueous suspension of polymeric particles can be used directly after polymerization. Water may be removed, if desired, to increase the percent solids of the suspension.
The polymeric suspension is then uniformly dispersed in one or more hydrophilic binder materials, or "vehicles" as they are often called in the art, to form a hydrophilic composition. Such binders act as peptizers for the polymeric particles to reduce their tendency to settle. Suitable hydrophilic binders include both naturally-occurring substances, such as proteins (e.g., gelatin, gelatin derivatives, cellulose derivatives), polysaccharides (e.g., dextran), gum arabic, etc.; and synthetic polymeric substances such as water-soluble polymers (e.g., poly(vinyl alcohol), acrylamide polymers, poly(vinyl pyrrolidones), etc.), and others known to one skilled in the art, as described, for example, in Research Disclosure, publication 17643, noted hereinabove, paragraph IX. Gelatin is a preferred binder in the practice of this invention.
Generally, the polymeric particles are present within a binder in an amount of at least about 15, and preferably from about 20 to about 70 percent, based on total dry weight of hydrophilic composition. This corresponds to a coating coverage of polymeric particles of at least about 20 mg/m² of coated surface area. Particles of different polymers containing the same or different hydrophobes can be used in the same hydrophilic composition, if desired.
Once the particles are blended in the binder, the resulting hydrophilic composition can be purified, if desired, in any suitable manner to remove any unwanted addenda.
The described hydrophilic composition can be applied to a suitable substrate, such as a conventional support, using conventional techniques to provide an element having a hydrophilic layer. Additional compositions can be applied simultaneously or subsequently to form additional layers over or under the hydrophilic layer. It is specifically contemplated to apply these compositions to a support using coating hoppers or other coating apparatus conventionally employed in preparing single or multiple layer radiation-sensitive elements. Useful coating and drying techniques and supports (e.g., paper, polymeric films, glass, etc.) are described, for example, in Research Disclosure, publication 17643, noted hereinabove, paragraphs XV and XVII.
The hydrophilic compositions described herein can be used in radiation-sensitive elements of various types. Generally, the coating coverage of the hydrophilic composition depends upon its use and the type of element it is incorporated into. Radiation-sensitive elements of this invention include, for example, image transfer materials, lithographic materials, physical development materials, radiographic materials, dry development materials, negative- and positive-working color-forming materials (including color films and color photographic papers), black-and-white films and papers, and the like. The details of such materials are well known in the art and are described, for example, in Research Disclosure, publication 17643, noted hereinabove.
In a preferred embodiment of this invention, the described hydrophilic compositions are useful in multilayer color photographic paper products having a resin-coated photographic paper support and a plurality of color-forming silver halide emulsion layers coated thereon.
The hydrophilic compositions can be used in any location in the radiation-sensitive elements of this invention, including within the radiation-sensitive layers themselves. Preferably, however, they are coated as individual hydrophilic layers, above, below or in between radiation-sensitive layers. In a preferred embodiment, the hydrophilic composition contains an optical brightener as the hydrophobe and is incorporated between the support and the radiation-sensitive layer(s) to provide an optical brightener layer.
The most preferred embodiment of the present invention is photographic color paper comprised of a paper support having a polyethylene layer on each side thereof and a hydrophilic layer of this invention between the polyethylene layer and the overlying silver halide emulsion layers. The hydrophilic layer serves as a reflective layer which is commonly referred to as a "white rug". Advantageously, the hydrophilic layer, which functions to provide improved sharpness and reduced Dmi,,, comprises the following ingredients:

(1) gelatin or other hydrophilic colloid in an amount sufficient to serve as a binder,
(2) polymer particles as described herein containing, as the hydrophobe, an optical brightener,
(3) hollow polymeric beads which function to scatter incident light, and
(4) an inorganic pigment such as titanium dioxide, zinc oxide, barium sulfate, lead carbonate, antimony oxide, zinc sulfide, zinc phosphate, calcium carbonate, and the like.

Hollow polymeric beads of any type may be employed as long as they perform the desired function. Particularly useful hollow polymeric beads are core/sheath polymeric particles adapted to serve as an extender for inorganic pigments. These are well known materials and are described, for example, in United States Patents 4,427,836, 4,468,498, and 4,469,825. A preferred material is manufactured by Rohm and Haas Company, and sold under the trademark ROPAQUE OP-84. This material is described as being hollow spheres of an aqueous acrylic copolymer emulsion with an outside diameter of approximately 0.5 - 0.6 microns and an inner void-diameter of approximately 0.3 microns. As supplied, the beads appear as a milky white liquid, are approximately 40% solids by weight, and have a pH of 9 - 10.
For preparation of the "white rug" layer, the preferred inorganic pigment is titanium dioxide and the preferred optical brightener is 2,5-bis(6-butyl-2-benzoxazolyl)thiophene.
In using a hydrophilic layer prepared in accordance with the principles of this invention as a so-called "white rug" in a photographic color paper, it is important that the optical brightener be incorporated in the polymer at as high a concentration as is feasible. The polymer merely serves as a carrier for the optical brightener so that high concentrations of optical brightener mean less of the carrier is required, which results in a thinner coated layer and consequent improvement in the sharpness of the color paper.
In the present invention, the optical brightener can be incorporated at a high concentration level, e.g., 8 to 10 percent by weight, yet it remains in the amorphous state in the polymer particles. This is important since, in its crystallized form it is not effective for the purposes for which it is incorporated in photographic reflection print materials, that is, for the purposes of providing improved sharpness and reduced D_min.
The invention is further illustrated by the following example of its practice.
Styrene (14.4 kg) and divinylbenzene (6.2 kg) were added to a water-jacketed 75-liter holding tank and warmed to 30°C. The divinylbenzene was commercial grade material containing about 55% divinylbenzene, i.e., a total of about 3.4 kg of pure divinylbenzene, with the remainder being primarily ethyl vinyl benzene. Thus, the pure divinylbenzene constituted 16.5% by weight of the monomeric composition. Uvitex OB^@ (1.37 kg), an optical brightener available commercially from Ciba-Geigy, Aerosol OT 100° (405 g), an oleophilic surfactant available commercially from American Cyanamid, and VAZO-64^@ (202 g), a polymerization initiator available commercially from DuPont, were added to and dissolved in the mixture of styrene and divinylbenzene. Demineralized water (13.48 kg) and Triton X-100^@ (52 g), a surfactant available commercially from Rohm and Haas, were then added to the holding tank.
Demineralized water (33.75 kg) and Triton X-1 000 (52 g), were added to a 75-liter reactor equipped with a temperature control system and heated to a temperature of 70°C.
The holding tank was pressurized with nitrogen and the contents were forced through an APV Gaulin Model 2F colloid mill into the reactor over a twenty-minute period. The reaction was allowed to proceed for four hours at 70°C and the reaction mixture was then heated to 80°C and reacted for an additional five hours, whereupon it was cooled to 25°C. The product was filtered through a three micron filter.
The storage stability of the polymer suspension was evaluated over a period of twenty-four weeks, and compared with the stability of an otherwise identical suspension which had been prepared using 2% by weight of divinylbenzene rather than the 16.5% by weight used in the above example. Stability was determined by viewing the water phase of the suspension under a microscope and counting the crystals of optical brightener present therein. The results obtained are summarized in the table below in which each of the values reported represents the average of several measurements.
As indicated by the data in the above table, the number of crystals formed in the suspension prepared from the 16.5% by weight divinylbenzene composition is much less than the number formed in the suspension prepared from the 2% by weight divinylbenzene composition.
To form a coating composition useful for forming a reflective layer in a photographic color print material, the suspension prepared from the 16.5% by weight divinylbenzene composition was combined with gelatin, titanium dioxide, and a core/sheath polymer of the type described in United States Patent 4,427,836. This composition provided excellent results when used to form a reflective layer, positioned between the polyethylene-coated paper support and the emulsion layers of a photographic reflection print material of the type described, for example, in Research Disclosure, publication 17643, noted hereinabove.
The invention has been described in detail with reference to preferred embodiments thereof; but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

1. An element comprising a support having thereon a substantially crystal- and agglomeration-free hydrophilic layer, said layer comprising a hydrophilic composition containing a hydrophilic binder and water-insoluble polymer particles dispersed therein, said polymer particles (1) having recurring units derived from a monomeric composition composed of one or more ethylenically unsaturated polymerizable monomers and (2) comprising at least 0.5 percent by weight of a hydrophobe uniformly distributed throughout, said monomeric composition being polymerized in the presence of said hydrophobe,

characterized in that more than 5 percent and up to 100 percent by weight of said monomeric composition consists of at least one ethylenically unsaturated polymerizable monomer having a crosslinkable moiety, said monomer being crosslinked during polymerization.

2. The element of claim 1 further comprising one or more radiation-sensitive layers.

3. The element of claim 1 or claim 2 wherein said hydrophilic layer is interposed between said support and said radiation-sensitive layer(s).

4. The element of claim 1 to 3 which is a multilayer color photographic element comprising a paper support having thereon a plurality of photographic color-forming silver halide emulsion layers.

5. The element of claims 1 to 4 wherein said polymer particles are characterized by the structure:

wherein -A- represents randomly recurring units derived from one or more vinyl aromatics, olefins, diolefins, vinyl esters or esters of a, (3-unsaturated polymerizable carboxylic acids; -B- represents randomly recurring units derived from one or more ethylenically unsaturated polymerizable monomers having one or more anionic moieties; -C- represents randomly recurring units derived from one or more ethylenically unsaturated polymerizable monomers having a crosslinkable moiety; w represents a weight percent of from 0 to 90; x represents a weight percent of from 0 to 20; and y represents a weight percent of from 10 to 100, all based on total monomer weight.

6. The element of claim 5 wherein w represents a weight percent of from 50 to 90, x represents a weight percent of from 0 to 20, and y represents a weight percent of from 10 to 50.

7. The element of claim 5 wherein w represents a weight percent of from 75 to 90, x represents a weight percent of from 0 to 20, and y represents a weight percent of from 10 to 25.

8. The element of claim 5 wherein -A- represents randomly recurring units derived from one or more vinyl aromatics or esters of a, a-unsaturated polymerizable carboxylic acids; -B- represents randomly recurring units derived from one or more monomers having one or more sulfo or carboxy moieties; and -C-represents randomly recurring units derived from one or more diacrylates or dimethacrylates.

9. The element of claim 5 wherein said polymer particles are composed of poly(methyl methacrylate- co-styrene-co-ethylene dimethacrylate).

10. The element of claim 5 wherein said polymer particles are composed of poly(styrene-co-divinylbenzene).

11. The element of claim 10 wherein said polymer particles have recurring units derived from a monomeric composition comprising about 75 to about 90 weight percent of styrene and about 10 to about 25 weight percent of divinylbenzene.

12. The element of claims 1 to 11 wherein said hydrophobe is a photographic dye; photographic dye-forming coupler; photographic developing agent; optical brightener; or ultraviolet light absorbing compound.

13. The element of claim 12 wherein said hydrophobe is an oxazole, oxadiazole, imidazole, pyrazoline, coumarin, stilbene, triazine, imidazolone, naphthotriazole, acetylene or vinylene optical brightener.

14. The element of claims 1 to 13 wherein said hydrophobe is present in said polymer particles in an amount of from 0.5 to 10 percent by weight.

15. The element of claims 1 to 14 wherein said polymer particles are dispersed in said binder in an amount of at least 15 percent based on total dry composition weight.

16. The element of claim 15 wherein said binder is gelatin.

17. The element of claims 4 to 16 wherein said hydrophilic layer additionally comprises hollow polymeric beads and titanium dioxide.

18. The element of claim 4 to 17 wherein said hydrophilic layer contains gelatin, hollow polymeric beads, titanium dioxide, and particles of a polymer having recurring units derived from a monomeric composition comprising 75 to 90 weight percent of styrene and 10 to 25 weight percent of divinylbenzene, said particles having uniformly distributed therein from 5 to 10 percent by weight of 2,5-bis(6-butyl-2-benzoxazolyl)thiophene.

19. A method of making an element comprising a support having thereon a substantially crystal- and agglomeration-free hydrophilic layer, said layer comprising a hydrophilic composition containing a hydrophilic binder and water-insoluble polymer particles dispersed therein, said method comprising the steps of:

(a) dissolving at least 0.5 percent, based on total monomer weight, of a hydrophobe in solution with a monomeric composition composed of one or more ethylenically unsaturated polymerizable monomers;

(b) dispersing said solution in water as fine droplets under conditions sufficient to promote polymerization of said monomers in said droplets and to form polymeric particles having said hydrophobe uniformly distributed throughout said particles;

(c) dispersing said polymeric particles in a hydrophilic binder to form a hydrophilic composition; and

(d) applying said hydrophilic composition to a support to form a substantially crystal- and agglomeration-free hydrophilic layer,
characterized in that more than 5 percent and up to 100 percent by weight of said monomeric composition consists of at least one ethylenically unsaturated polymerizable monomer having a crosslinkable moiety, said monomer being crosslinked during polymerization.

20. The method of claim 19 wherein a radiation-sensitive composition is applied over said hydrophilic layer.

21. The method of claim 19 wherein said hydrophobe is an optical brightener.

22. The method of claim 19 wherein step (a) is carried out at a temperature greater than room temperature but less than the temperature at which spontaneous polymerization of said monomers occurs.

23. The method of claim 19 wherein said hydrophilic binder is gelatin.

24. The method of claim 19 wherein an oleophilic surface active agent and an oleophilic polymerization initiator are dissolved in said monomers prior to polymerization.

25. The method of claims 19 to 24 wherein said ethylenically unsaturated polymerizable monomer is styrene, said ethylenically unsaturated polymerizable monomer having a crosslinkable moiety is divinylbenzene, said hydrophilic binder is gelatin, and said hydrophobe is 2,5-bis(6-butyl)-2-benzoxazolyl)thiophene.