Coating fluid for imaging element comprising solubilized...

Radiation imagery chemistry: process – composition – or product th – Nonradiation sensitive image processing compositions or...

Reexamination Certificate

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C430S466000, C430S537000, C430S539000, C430S622000, C430S621000, C430S642000, C106S160100, C530S354000, C530S355000

Reexamination Certificate

active

06706467

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to aqueous coating solutions comprising gelatin which is prepared by the hydrolysis of ossein using sodium or potassium hydroxide, where the coating solution contains a colloidal particle dispersed material phase at a volume fraction of at least 0.01.
BACKGROUND OF THE INVENTION
Imaging elements, particularly photographic silver halide imaging elements, commonly use a hydrophilic colloid as a film forming binder for layers thereof, most commonly ossein. The layers of such imaging elements are typically coated employing multilayer slide bead coating processes such as described in U.S. Pat. No. 2,716,419 and multilayer slide curtain coating processes such as described in U.S. Pat. No. 3,508,947. The binder of choice in most cases is gelatin, prepared from various sources of collagen (see, e.g., P. I. Rose, The Theory of Photographic Process, 4th Edition, edited by T. H. James (Macmillan Publishing Company, New York, 1977) p. 51-65). The binder is expected to provide several functions, primarily to provide an element with some level of mechanical integrity and contain all the materials within the imaging element, which are required to provide an image.
The various layers of imaging elements comprising gelatin are typically coated from aqueous coating solutions. In addition to serving as a binder, gelatin also functions as a stabilizer to dispersed aqueous insoluble materials of colloidal dimensions which may also be present in the aqueous coating solutions. Such materials can include photographically-useful materials such as coupler drops, UV-absorbers, scavengers of oxidized developer, silver halide grains, dye particles or materials needed for other functions, such as polymer latexes and silica particles. A colloidal dispersed material particle has at least one dimension in the range 1 nm to 1 &mgr;m. The viscosity of fluids containing gelatin and such colloidal materials is an important parameter affecting the efficiency of the manufacture of imaging materials such as photographic products. The most important impact of viscosity is on the coating process. If the viscosity is too high, then the fluid cannot be pumped sufficiently fast. If the viscosity is too low then defects may arise due to ripple, flow on the web after coating and failure of the multilayer pack to gel thermally (chill set). Coating fluid viscosity increases with gelatin and dispersed phase concentrations, as the mean molecular weight of the gelatin increases, and as the size of dispersed colloidal particles decreases. One of the most expensive processes in manufacturing of multilayer photographic products is drying of water after coating. If the concentration of solids within the coating fluid can be increased, then less water is coated and less drying is required at a given coating speed (or the coating speed can be increased without increasing the throughput capacity of the dryers). However, as the concentration increases, the viscosity of the coating fluid may become too high to pump easily and the coating fluids may exhibit too much shear thinning (viscosity decreasing as shear rate increases) to give uniform laydown across the web.
It is well recognised that the presence of sub-micron colloidal particles increases the viscosity of gelatin solutions. For a given volume fraction of colloidal material, the viscosity increases as the particle size of the colloid is reduced. The affect arises through adsorption of gelatin to the surfaces of the dispersed particles leading to an increase in the effective volume occupied by the colloid. Examples are given, e.g., in the following references: Howe A M, Clarke A and Whitesides T H, “Viscosity of Emulsions of Polydisperse Droplets with a Thick Adsorbed Layer” Langmuir 13:2617-2626 (1997); Dreja M, Heine K, Tieke B and Junkers G, “Effects of functionalized latex particles and anionic surfactants on the flow behavior of aqueous gelatin dispersions” J. Colloid Interface Sci. 191(1):131-140 (1997); Vaynberg K A, Wagner N J, Sharma R and Martic P, “Structure and extent of adsorbed gelatin on acrylic latex and polystyrene colloidal particles” J. Colloid Interface Sci., 205:131-140 (1998); Hone J H E, Howe A M and Whitesides T H, “Rheology of polystyrene latexes with adsorbed and free gelatin” Colloids Surfaces 161:283-306 (2000); Vaynberg K A and Wagner N J, “Rheology of polyampholyte (gelatin)-stabilised colloidal dispersions: The tertiary electroviscous effect” J. Rheology 45(2):451-466 (2001).
For various applications, it would be desirable to be able to increase the concentration of a coating fluid containing gelatin and dispersed sub-micron colloidal materials, reduce the size of the sub-micron colloidal materials in such a coating fluid, and/or enable the inclusion of higher molecular weight gelatin in such a coating fluid without detrimentally increasing the viscosity. For other applications, it would be desirable to be able to reduce the viscosity of an aqueous coating fluid containing gelatin and dispersed insoluble colloidal material without needing to reduce the concentration of gelatin or colloidal materials, increase the size of the sub-micron colloidal materials, and/or reduce the molecular weight of the gelatin. It would further be desirable to be able to make such changes, either singly or in combinations thereof, without fundamentally changing the composition of the materials in the coating fluid, or otherwise having to use undesirable conditions with respect to temperature (viscosity typically decreases with increasing T), pH (viscosity typically decreases with reducing pH) or ionic strength (adding salts typically causes viscosity to decrease).
High purity gelatins are generally required for imaging applications. Currently the most commonly employed manufacturing process for obtaining high purity gelatins involves demineralization of a collagen containing material, typically cattle bone (ossein), followed by extended alkaline treatment (liming) and finally gelatin extractions with water of increasing temperature as described in U.S. Pat. Nos. 3,514,518 and 4,824,939. The gelatin produced by this process, commonly referred to as lime processed ossein gelatin, has existed with various modifications throughout the gelatin industry for a number of years. The liming step of this process requires up to 60 days or more, the longest step in the approximately 3 month process of producing gelatin. The hydrolyzed collagen is extracted in a series of steps to obtain several gelatin fractions with varying molecular weights. In order to obtain gelatin of desired molecular weight to provide suitable coating solution viscosities, these fractions can be further hydrolyzed by high temperature hydrolysis. The fractions are then blended to obtain the appropriate molecular weight for photographic use. U.S. Pat. No. 5,908,921 describes a relatively new process for the preparation of photographic grade gelatin, where the agent for hydrolysis is a strong alkali, such as sodium or potassium hydroxide. The reaction rate is disclosed to be from 10 to 120 hours (substantially faster than the prior lime processes), after which a single extraction step yields a single batch of gelatin, which is then purified and deionized. The characteristics of the gelatin produced are that it has a high gel strength and narrow molecular weight distribution compared to gelatins produced by the conventional process where lime is used as the agent for hydrolysis. There is no disclosure in U.S. Pat. No. 5,908,921, however, regarding any possible impact use of the gelatin produced by such process may have on aqueous coating fluids containing such gelatin and dispersed colloidal material.
SUMMARY OF THE INVENTION
In accordance with the invention, an aqueous coating fluid is described comprising gelatin at a concentration of at least 1 wt % and a colloidal particle dispersed material phase at a volume fraction of at least 0.01, wherein at least 20% of the gelatin comprises a gelatin prepared from hydrolysis of ossein using sodium or potassium hydroxide.
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