FIELD OF THE INVENTION
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This invention relates to a process for producing thermoplastic coated photographic paper by extrusion coating, and more particularly to a process for producing thermoplastic coated paper with improved curl resistance, lower optical brightener migration, and better adhesion with the photographic emulsion.
BACKGROUND OF THE INVENTION
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Photographic paper is coated with a polyolefin layer or layers preferably by extrusion from a hot melt as is known in the art. The polyolefin can be any coatable polyolefin material known in the photographic art. Representative of these materials are low and high density polyethylene and polypropylene.
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If the polyolefin is to be applied to the side of the paper whereupon the photographic emulsion will be applied, then the polyolefin is preferably a low density polyethylene, with a density of between 0.91 and 0.93. In addition the emulsion side polymer may include a suitable optical brightener in an amount of from .001 to .25 percent by weight based on the total weight of the polyolefin coating, a suitable white pigment such as titanium dioxide in the range of from 3 to 35 percent by weight based on the total weight of the polyolefin coating, a heat stabilizer, such as magnesium stearate, dyes; and the like, as is well known to those skilled in the art. Additionally, photographic emulsions side resins can contain one or more pigments, such as the blue, violet or magenta.
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If the polyolefin is to be applied to the back side of the paper, the polyolefin is preferably often of a medium or high density polyethylene, having a density between .94 and .97.
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The paper base material employed in accordance with the invention can be any paper base material which has heretofore been considered useful for a photographic support, typically with a weight range of from 20 g/m2 to 500 g/m2.
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The polyolefin can be extruded over a wide temperature range, i.e., 150°C -350°C through a coat hanger or T-slot die into the nip formed by a metallic chill roll and an elastomeric pressure roll, where the polyolefin is forced into the interstices of the porous paper forming a mechanically interlocked polymer/paper composite.
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The coated paper is then cooled on the chill roll which is operated at a temperature of between 4°C to 32°C and conveyed to the stripping point of the chill roll where the paper is removed by tension, and wound into a roll or conveyed to the next coating operation.
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Before the photographic emulsion is applied, the polymeric layer on the emulsion side is pretreated with a corona discharge treatment process to improve adhesion between the polymer and the emulsion.
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It has been found that the optical brightener migrates out of the resin over time, forming an unsightly dust on the surface of the polymer. In addition, when the polymeric layer is subjected to the corona discharge treatment, chemicals are formed that can cause spots in the sensitized layer. Finally, the difference in density between the emulsion side polyethylene and the back side polyethylene causes the paper to curl.
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As a result, there is a need for a process which decreases optical brighener migration, improves adhesion between the polymer and the emulsion without forming spots, and decreases the propensity of photographic paper to curl.
SUMMARY OF THE INVENTION
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The invention provides a method of making a thermoplastic coated paper wherein the emulsion side thermoplastic is flame annealed after lamination. The thermoplastic coated paper is treated with a flame in such a way that the thermoplastic is heated above its' vicat softening temperature. Although this treatment will increase the adhesion between the polymer and the emulsion, the treatment has the unexpected benefits of decreasing optical brightener migration and improving curl. The thermoplastic is cooled slowly, allowing the polymeric molecules to relax, thereby decreasing the amount of free volume in the polymer. This decrease in free volume decreases the mobility of the optical brightener, as well as increasing the density and the modulus of the treated side. This increase in modulus more effectively balances the curl.
DESCRIPTION OF PREFERRED EMBODIMENTS
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In the preparation of a thermoplastic coated paper for photographic paper base in accordance with this invention, a thermoplastic resin is prepared from any coatable polyolefin material known in the photographic art. Representative of these materials are polyethylene, polypropylene, polystyrene, polybutylene, and copolymers thereof. The polyolefin can be copolymerized with one or more copolymers including polyesters, such as, polyethylene terephthalate, polysulfones, polyurethane's, polyvinyls, polycarbonates, cellulose esters, such as cellulose acetate and cellulose propionate, and polyacrylates. Specific examples of copolymerizable monomers include vinyl stearate, vinyl acetate, acrylic acid, methylacrylate, ethylacrylate, acrylamide, methacrylic acid, methylmethacrylate, ethylmethacrylate, methacrylamide, butadiene, isoprene, and vinyl chloride. Preferred polyolefins are film forming and adhesive to paper. For the emulsion side resin, Polyethylene of low density, between 0.91 g/cm3 and 0.94 g/cm3 is preferred. Polyethylene having a density in the range of from 0.94 grams/cm3 to 0.98 grams/cm3 is most preferred for the back side layer. The polyolefin to be applied to the side of the paper whereupon the photographic emulsion will be applied includes a suitable optical brightener such as those described in Research Disclosure Issue N. 308, December 1989, Publication 308119, Paragraph V, Page 998, in an amount of from .001 to .25 percent by weight based on the total weight of the polyolefin coating, including any white pigment present, with .01 to .1 percent being the most preferred. Any suitable white pigment may be incorporated in the polyolefin layer, such as, for example, titanium dioxide, zinc oxide, zinc sulfide, zirconium dioxide, white lead, lead sulfate, lead chloride, lead aluminate, lead phthalate, antimony trioxide, white bismuth, tin oxide, white manganese, white tungsten, and combinations thereof. The pigment is used in any form that is conveniently dispersed within the polyolefin. The preferred pigment is titanium dioxide in the anatase crystalline form. Preferably, the white pigment should be employed in the range of from 3 to 35 percent by weight, based on the total weight of the polyolefin coating. Anatase titanium dioxide at from 5 to 20 percent is most preferred.
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In addition to the brightener mixture and the white pigment, the polyolefin coating can contain, if desired, a variety of additives including antioxidants such as 4,4'-butylidene-bis(6-tert-butyl-meta-cresol), di-lauryl-3,3'-thiodipropionate, N-butylated-p-aminophenol, 2,6-di-tert-butyl-p-cresol, 2,2-di-tert-butyl-4-methyl-phenol, N,N-disalicylidene-1,2-diaminopropane, tetra(2,4-tert-butylphenyl)-4,4'-diphenyl diphosphonite, octadecyl 3-(3',5'-di-tert-butyl-4'-hydroxyphenyl propionate), combinations of the above, and the like; heat stabilizers, such as higher aliphatic acid metal salts such as magnesium stearate, calcium stearate, zinc stearate, aluminum stearate, calcium palmitate, sodium palmitate, zironium octylate, sodium laurate, and salts of benzoic acid such as sodium benzoate, calcium benzoate, magnesium benzoate and zinc benzoate; calcium stearate of concentrations between .1 and 1.0% with .4-.6% being most preferred. Addition of antistatic agents; lubricants; dyes; and the like, is well known to those skilled in the art. Additionally, emulsion side resins can contain one or more pigments, such as the blue, violet or magenta pigments described in U.S. Pat. No. 3,501,298, or pigments such as barium sulfate, colloidal silica, calcium carbonate and the like, with the preferred colorant combination consisting of cobalt aluminate and quinacridone, present in concentrations of between 0.02 to 0.5% and 0.0005 to 0.05% respectively, with the most preferred concentrations being from 0.1 to 0.2% and .001 to 0.003 % respectively.
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The paper base material employed in accordance with the invention can be any paper base material which has heretofore been considered useful for a photographic support. The weight and thickness of the support can be varied depending on the intended use. A preferred weight range is from 20 g/m2 to 500 g/m2, with 100-200 g/m2 being the most preferred. Preferred thickness (those corresponding to commercial grade photographic paper) are from 20 µm to 500µm with the most preferred thickness being from 100-200 µm. It is preferred to use a paper base material calendered to a smooth surface. The paper base material can be made from any suitable paper stock preferably comprising hard or softwood. Either bleached or unbleached pulp can be utilized as desired. The paper base material can also be prepared from partially esterified cellulose fibers or from a blend of wood cellulose and a suitable synthetic fiber such as a blend of wood cellulose and polyethylene fiber.
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As is known to those skilled in the art, the paper base material can contain, if desired, agents to increase the strength of the paper such as wet strength resins, e.g., the amino-aldehyde or polyamide-epichlorohydrin resins, and dry strength agents, e.g., starches, including both ordinary starch and cationic starch, or polyacrylamide resins. In a preferred embodiment of this invention, the amino-aldehyde or polyamide-epichlorohydrin and polyacrylamide resins are used in combination as described in U.S. Pat. No. 3,592,731. Other conventional additives include water soluble gums, e.g., cellulose ethers such as carboxymethyl cellulose, sizing agents, e.g., aldyl ketene dimers, sodium stearate which is precipitated on the pulp fibers with a polyvalent metal salt such as alum, aluminum chloride or aluminum salts.
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Prior to the polyolefin extrusion step, the paper is preferably treated with a corona discharge to improve the adhesion of the polyolefin to the paper support as described in U.S. Pat. No. 3,411,908.
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The emulsion side polymer is melted and laminated onto the paper. It is then stripped off the chill roll at the stripping roll. After leaving the chill roll, the emulsion side is flame treated. The amount of flame treatement must be sufficient to heat the resin above the vicat softening temperature to allow the plastic to flow. For low density polyethylene, this temperature is 99°C. The preferred temperature range is above 200°C. After flame treating, the polymer must be allowed to cool below the vicat softening point in ambient air without contacting additional rollers for between .1 and 10 seconds, with the preferred time being 1 to 10 seconds.
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The invention will be further illustrated by the following examples.
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In the examples, the diffusion of optical brightener is measured in the following way:
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Samples were incubated in a 60°C oven for 7 days and qualitatively judged for optical brightener migration by observing the amount of OB which could be wiped off the sample by a white glove. The ratings were from 0 (no migration) to 4 (severe migration).
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In the example, curl was measured using the ANSI curl test at humidities of 5%, 20%, 50%, 70%, and 85%.
Example 1 (Control)
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The emulsion side resin, consisting of 85.68% polyethylene of density .925 g/cc, 12.5% anatose TiO2, 3.0% ZnO, 5% calcium stearate, .1%, 4,4'-butadiene-bis(6-tert-butyl-meta-cresol), and .05% bis(benzoxazolyl)-stilbene, is melted in a single screw extruder and is force through a coat hanger die at a temperature of 316°C, and laminated with photographic grade paper support where the thickness of the paper is 165 µm, and the thickness of the polymer layer is 25 µm. The backside resin thickness is 26 µm, consisting of polyethylene at a ensity of .945 g/cc. After 7 days, optical brightener migration was level 4 (severe), and the humidity curl was -13 at 5% RH, -13 at 20% RH, -11 at 50% RH, -27 at 70% RH, and -44 at 85% RH. This formulation requires pretreatment with corona discharge immediately before sensitizing and small red spots are occasionally observed.
Example 2
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Same as Example 1, except the emulsion side polyethylene is post treated with flame at a rate of 2.3 joules/square meter, and allowed to cool in ambient air for 3 seconds. Optical brightener migration after one week in "1" (mild), and the humidity curl was -9 at 5% RH, -7 at 20% RH, -9 at 50% RH, -18 at 70% RH, and - 38 at 85%RH. This support adequately adheres to the emulsion without secondary corona discharge treatment, and no spots are detected after sensitizing.
Example 3
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Silver halide emulsions were coated on the paper of Example 2. The emulsions were chemically and spectrally sensitized as described below.
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Blue Sensitive Emulsion (Blue EM-1, prepared similarly to that described in U.S. 5,252,451, column 8, lines 55-68): A high chloride silver halide emulsion was precipitated by adding approximately equimolar silver nitrate and sodium chloride solutions into a well-stirred reactor containing gelatin peptizer and thioether ripener. Cs2Os(NO)Cl5 dopant was added during the silver halide grain formation for most of the precipitation, followed by a shelling without dopant. The resultant emulsion contained cubic shaped grains of 0.76 µm in edgelength size. This emulsion was optimally sensitized by the addition of a colloidal suspension of aurous sulfide and heat ramped up to 60 °C during which time blue sensitizing dye BSD-1, 1-(3-acetamidophenyl)-5-mercaptotetrazole and potassium bromide were added. In addition, iridium dopant was added during the sensitization process.
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Green Sensitive Emulsion (Green EM-1): A high chloride silver halide emulsion was precipitated by adding approximately equimolar silver nitrate and sodium chloride solutions into a well-stirred reactor containing gelatin peptizer and thioether ripener. Cs2Os(NO)Cl5 dopant was added during the silver halide grain formation for most of the precipitation, followed by a shelling without dopant. Iridium dopant was added during the late stage of grain formation. The resultant emulsion contained cubic shaped grains of 0.30 µm in edgelength size. This emulsion was optimally sensitized by addition of green sensitizing dye GSD-1, a colloidal suspension of aurous sulfide, heat digestion followed by the addition of 1-(3-acetamidophenyl)-5-mercaptotetrazole and potassium bromide.
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Red Sensitive Emulsion (Red EM-1): A high chloride silver halide emulsion was precipitated by adding approximately equimolar silver nitrate and sodium chloride solutions into a well-stirred reactor containing gelatin peptizer and thioether ripener. The resultant emulsion contained cubic shaped grains of 0.40 µm in edgelength size. This emulsion was optimally sensitized by the addition of a colloidal suspension of aurous sulfide followed by a heat ramp, and further additions of 1-(3-acetamidophenyl)-5-mercaptotetrazole, potassium bromide and red sensitizing dye RSD-1. In addition, iridium dopant was added during the sensitization process.
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Coupler dispersions were emulsified by methods well known to the art, and the following layers were coated on a polyethlene resin coated paper support, that was sized as described in U.S. Patent 4,994,147 and pH adjusted as described in U.S. Patent 4,917,994. The polyethylene layer coated on the emulsion side of the support contained a mixture of 0.1 % (4,4'-bis(5-methyl-2-benzoxazolyl) stilbene and 4,4'-bis(2-benzoxazolyl) stilbene, 12.5 % TiO
2, and 3 % ZnO white pigment. The layers were hardened with bis(vinylsulfonyl methyl) ether at 1.95 % of the total gelatin weight.
| Layer 1: Blue Sensitive Layer |
| Gelatin | 1.530 g/m2 |
| Blue Sensitive Silver (Blue EM-1) | 0.280 g Ag/m2 |
| Y-1 | 1.080 g/m2 |
| Dibutyl phthalate | 0.260 g/m2 |
| 2-(2-butoxyethoxy)ethyl acetate | 0.260 g/m2 |
| 2,5-Dihydroxy-5-methyl-3-(1-piperidinyl)-2-cyclopenten-1-one | 0.002 g/m2 |
| ST-16 | 0.009 g/m2 |
| Layer 2: Interlayer |
| Gelatin | 0.753 g/m2 |
| Dioctyl hydroquinone | 0.094 g/m2 |
| Dibutyl phthalate | 0.282 g/m2 |
| Disodium 4,5 Dihydroxy-m-benzenedisulfonate | 0.065 g/m2 |
| SF-1 | 0.002 g/m2 |
| Layer 3: Green Sensitive Layer |
| Gelatin | 1.270 g/m2 |
| Green Sensitive Silver (Green EM-1) | 0.263 g Ag/m2 |
| M-1 | 0.389 g/m2 |
| Dibutyl phthalate | 0.195 g/m2 |
| 2-(2-butoxyethoxy)ethyl acetate | 0.058 g/m2 |
| ST-2 | 0.166 g/m2 |
| Dioctyl hydroquinone | 0.039 g/m2 |
| Phenylmercaptotetrazole | 0.001 g/m2 |
| Layer 4: UV Interlayer |
| Gelatin | 0.484 g/m2 |
| UV-1 | 0.028 g/m2 |
| UV-2 | 0.159 g/m2 |
| Dioctyl hydroquinone | 0.038 g/m2 |
| 1,4-Cyclohexylenedimethylene bis(2-ethylhexanoate) | 0.062 g/m2 |
| Layer 5: Red Sensitive Layer |
| Gelatin | 1.389 g/m2 |
| Red Sensitive Silver (Red EM-1) | 0.187 g Ag/m2 |
| C-3 | 0.424 g/m2 |
| Dibutyl phthalate | 0.414 g/m2 |
| UV-2 | 0.272 g/m2 |
| 2-(2-butoxyethoxy)ethyl acetate | 0.035 g/m2 |
| Dioctyl hydroquinone | 0.004 g/m2 |
| Potassium tolylthiosulfonate | 0.003 g/m2 |
| Potassium tolylsulfinate | 0.0003 g/m2 |
| Layer 6: UV Overcoat |
| Gelatin | 0.484 g/m2 |
| UV-1 | 0.028 g/m2 |
| UV-2 | 0.159 g/m2 |
| Dioctyl hydroquinone | 0.038 g/m2 |
| 1,4-Cyclohexylenedimethylene bis(2-ethylhexanoate) | 0.062 g/m2 |
| Layer 7: SOC |
| Gelatin | 1.076 g/m2 |
| Polydimethylsiloxane | 0.027 g/m2 |
| SF-1 | 0.009 g/m2 |
| SF-2 | 0.004 g/m2 |
| Tergitol 15-S-5™ | 0.003 g/m2 |
| DYE-1 | 0.018 g/m2 |
| DYE-2 | 0.009 g/m2 |
| DYE-3 | 0.007 g/m2 |
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The coated paper of this example performed as expected.
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The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.