Coating processes – With pretreatment of the base – Preapplied reactant or reaction promoter or hardener
Reexamination Certificate
1999-12-17
2002-07-16
Beck, Shrive P. (Department: 1762)
Coating processes
With pretreatment of the base
Preapplied reactant or reaction promoter or hardener
C427S301000, C427S333000, C427S393500, C427S412500, C427S419200, C427S419700
Reexamination Certificate
active
06419987
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for coating a continuous web for use in making imaging or printing media, including, for example, photographic film, photothermographic film, or ink-jet media. In particular, the method is directed to controlling the viscosity of the coating during the coating process. The invention is also directed to an imaging or printing media comprising a base material made from a continuous web over which extends a coated layer made by the present method.
BACKGROUND OF THE INVENTION
In general, to form a film or coating on a flexible support, a solution containing the desired film materials is coated onto the support and dried. For high productivity and lower costs, these coatings are applied to continuous webs at high speeds and dried in an oven. Because of air impingement during drying and artifacts from the actual coating application method, coating defects may occur, for example non-uniformity in thickness and streaks. For applications that require a high degree of coating uniformity, such as high-quality photographic media, photothermographic media, or ink-jet media, this problem may be solved by using coating solutions that contain a thermoreversible gelling material such as gelatin. At high temperatures, these solutions have a low viscosity, which enables good coatability. After applying the thermoreversible gelling solution to the web, the coating is then cooled to thicken or gel the coating.
Very few materials are available that undergo a thermoreversible gelling behavior. Therefore, it would be desirable to provide a method that allows the coating of a solution containing a non-thermoreversible gelling material which has a low viscosity during the coating process and then rapidly thickens or gels once on the web. One such method is to add a thickening material to the coating solution by traditional methods, such as mix melting or simultaneous slide coating. This creates additional problems, however, especially with fast-acting thickeners. These problems include limited solution stability, delivery problems for high viscosity fluids, and a greater propensity for coating streaks due to slug formation.
Another solution to the problem of coating a web support is to use shear-thinning solutions. These solutions have a low viscosity at high shear rates (as generated during the coating process) and a high viscosity at zero/low shear rates (as encountered on the web after the coating has been applied). Because of the high viscosity at low shear rates, however, it is often difficult preparing and delivering these solutions to the coating, sometimes requiring additional manufacturing expense.
GB 2132784 to Fuji describes the use of an overcoat for heat-sensitive recording paper that comprises a mixture of poly(vinyl alcohol) and boric acid which is applied to a heat-sensitive color-forming layer. This layer contains an inorganic pigment and has a surface pH between 6 and 9. This patent does not disclose a process for modifying the viscosity of a coating solution by separate application of the poly(vinyl alcohol) and boric acid.
Thus a need exists for an improved method for manufacturing and coating imaging or printing media, wherein coating defects are reduced or eliminated in the coated film and higher coating rates are facilitated.
SUMMARY OF THE INVENTION
This invention is useful for providing coated webs with minimal or no defects, especially at higher coating speeds. Two interacting components, a first component and a second component, are selected such that when in solution together they interact with each other to increase the viscosity or to gel/crosslink the solution. The first component, a viscosity-increasing agent, is preapplied in a first solution onto the web through a coating and drying process. Then, a second solution containing the second component, a film-forming polymer used in an imaging layer or image-receiving layer, is coated on the web, after which the viscosity-increasing agent is solubilized into, and diffuses through, the applied second solution. As the above-defined first and second components interact with each other during this diffusion process, the viscosity of the solution increases. The change in viscosity can be controlled, for example, by varying the concentrations of the interacting components, by adding coating addenda such as low molecular weight diluents, or by adjusting the pH of the second solution.
An advantage of this process is the ability to coat solutions at high speeds, since the solution can be applied at a relatively low viscosity to the coating and then quickly thickened on the web. Another advantage is the ability to provide gelling or crosslinking materials to a layer without solution stability concerns.
The invention is also directed to an imaging or recording element comprising a base material made from a continuous web over which extends a coated imaging layer or image-receiving layer comprising a film-forming polymer and an amount of a viscosity-increasing agent that is higher in concentration than existed in the coating when first applied to the base material.
DETAILED DESCRIPTION OF THE INVENTION
This invention provides a method for making a coated web having uniform properties and reduced defects, even when coated at high speeds and exposed to air drying.
As used herein, the term “web,” “support,” or “sheet” refers to a continuous planar polymeric and/or paper material or discrete sections thereof.
The term “polyvinyl alcohol” referred to herein means a polymer having a monomer unit of vinyl alcohol as a main component.
As used herein, the term “image-functional layer” refers to a coating that produces or receives an image or is otherwise primarily and directly involved in the image formation, for example, a photosensitive or thermosensitive silver-halide emulsion or a layer that receives an image from an ink-jet printer or a layer that receives a component of the ink-jet fluid.
As used herein, the term “viscosity-increasing agent” refers to a diffusable compound that is capable of increasing the viscosity of a polymer-containing solution through the interaction of the agent with the polymer. In the case of a web used in making a photographic element, for example, the base/support preferably comprises polyester and can also comprise a conductive oxide, a lubricating agent, or a magnetic recording layer. In the case of a web used to make an ink-jet recording element, the support typically comprises on at least one surface thereof an ink-receiving (image-recording layer), and includes those intended for reflection viewing, which have an opaque support, and those intended for viewing by transmitted light, which have a transparent support.
As indicated above, two interacting components, a first component and a second component, are selected such that when in solution together they interact with each other to increase the viscosity or gel/crosslink the solution. The first component, a viscosity-increasing agent, is preapplied in a first solution onto the web through a coating and drying process. When a second solution containing the second component, a film-forming polymer used in forming an imaging or image-receiving layer, is then coated on the web, the viscosity-increasing agent is solubilized into, and diffuses through, the applied second coating solution. As the above-defined first and second components interact with each other during this diffusion process, the viscosity of the solution increases. The change in viscosity can be controlled, for example, by varying the concentrations of the interacting components, by adding coating addenda such as low molecular weight diluents, or by adjusting the pH of the second solution. At high levels, the solution can be gelled or crosslinked with this process.
The change in viscosity can be controlled, for example, by varying the concentrations of the interacting components, by adding coating addenda such as low molecular weight diluents, or by adjusting the pH of the second solution. At high levels, the solution can be gelled or cro
Bauer Charles L.
Reczek Joseph
Shaw-Klein Lori
Beck Shrive P.
Crockford Kirsten A.
Konkol Chris P.
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