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Description  |
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FIELD OF
THE INVENTION
This invention relates to imaging materials. It particularly relates to base materials for photographic imaging.
BACKGROUND OF THE INVENTION
In the formation of color paper it is known that the base paper has applied thereto a layer of polymer, typically polyethylene. This layer serves to provide waterproofing to the paper, as well as providing a smooth surface on which the
photosensitive layers are formed. The formation of a suitably smooth surface is difficult requiring great care and expense to ensure proper laydown and cooling of the polyethylene layers. One defect in prior formation techniques is caused when an air
bubble is trapped between the forming roller and the polyethylene which will form the surface for casting of photosensitive materials. This air bubble will form a pit that will cause a defect in the photographic performance of photographic materials
formed on the polyethylene. It would be desirable if a more reliable and improved surface could be formed at less expense.
In color papers there is a need for providing color papers with improved resistance to curl. Present color papers will curl during development and storage. Such curl is thought to be caused by the different properties of the layers of the color
paper as it is subjected to the developing and drying processes. Humidity changes during storage of color photographs lead to curling. There are particular problems with color papers when they are subjected to extended high humidity storage such as at
greater than 50% relative humidity. Extremely low humidity of less than 20% relative humidity also will cause photographic papers to curl.
In photographic papers the polyethylene layer also serves as a carrier layer for titanium dioxide and other whitener materials as well as tint materials. It would be desirable if the colorant materials rather than being dispersed throughout the
polyethylene layer could be concentrated nearer the surface of the layer where they would be more effective photographically.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene in receiver sheets for thermal dye transfer.
In the formation of imaging elements having microvoided base materials, it becomes apparent that when subjected to somewhat extreme bending the microvoided layers are a source of failure of the substrate such that the image quality on the base is
deteriorated. There is a need to control the microvoided bases such that they will not fail and deteriorate image quality. This problem will occur both with photographic elements having microvoided bases and with other imaging systems such as thermal
transfer images on microvoided bases. The failure of the microvoided base creates markings that take the form of closely spaced ridges.
PROBLEM TO BE SOLVED BY THE INVENTION
There is a need for improved microvoided bases for imaging materials. There is a need for strong, optically desirable microvoided base materials that resist deformation and cracking upon bending.
SUMMARY OF THE INVENTION
An object of the invention is to provide a photographic element comprising a paper base with a thickness from 0.025 mm to 0.25 mm, at least one photosensitive silver halide layer, and a layer of microvoided and biaxially oriented polyolefin sheet
having a layer thickness of 0.020 mm to 0.050 mm, a density of between 78% and 100% of the solid polymer density located between said paper base and said at least one photosensitive silver halide layer, and wherein the entire photographic element is
substantially free from surface distortion or cracking when the photographic element is forced to the shape of a diameter of no less than 8.0 mm.
In another embodiment, the invention provides a laminated base for imaging comprising a paper base, a sheet of microvoided biaxially oriented polyolefin on the top of said base and a sheet of biaxially oriented film on the bottom of said paper
base, wherein said top microvoided sheet has a thickness of 0.020 mm to 0.050 mm, a density of between 78 and 100 percent of the solid polymer density and wherein said base is substantially free of distortion when forced to the shape of a diameter of no
less than 8.0 mm.
ADVANTAGEOUS EFFECT OF THE INVENTION
The invention provides the advantage that microvoided sheets may be used in the substrate for imaging materials. The microvoided materials of the invention are strong, optically desirable, and resist deformation on bending that will decrease
image quality.
DETAILED DESCRIPTION OF THE INVENTION
The invention has numerous advantages over prior microvoided sheets for use in imaging base materials. The laminated base materials of the invention provide a substrate that does not deform during routine handling of imaging materials such as
color photographic papers. The imaging materials of the invention are not subject to cracking when passing over small diameter rollers in processing machines utilized in the handling of imaging materials during their formation, processing of images, and
handling after development. Small rollers also appear in processing equipment for handling imaging materials for ink jet printing, thermal transfer printing, and electrophotography.
The laminated base sheets of the instant invention are designed such that the microvoided area does not fail on bending to an 8 mm radius. The microvoided materials if containing voids at an amount such that percent solid density is less than
70% may fail if bent to a 10 mm radius. It is preferred that the percent solid density between 75 and 80% as this gives sheet materials that are strong, light in weight, and economical. Further the preferred material has desirable optical properties of
whiteness and opacity.
The invention provides a photographic element that has much less tendency to curl when exposed to extremes of humidity. Further, the invention provides a photographic paper that is much lower in cost as the criticalities of the formation of the
polyethylene are removed. There is no need for the difficult and expensive casting and cooling in forming a surface on the polyethylene layer as the biaxially oriented polymer sheet of the invention provides a high quality surface for casting of
photosensitive layers. The optical properties of the photographic elements in accordance with the invention are improved as the color materials may be concentrated at the surface of the biaxially oriented sheet for most effective use with little waste
of the colorant materials. Photographic materials utilizing microvoided sheets of the invention have improved resistance to tearing. The photographic materials of the invention are lower in cost to produce as the microvoided sheet may be scanned for
quality prior to assembly into the photographic member. With present polyethylene layers the quality of the layer cannot be assessed until after complete formation of the base paper with the polyethylene waterproofing layer attached. Therefore, any
defects result in expensive discard of expensive product. The invention allows faster hardening of photographic paper emulsion, as water vapor is not substantially transmitted from the emulsion through the biaxially oriented sheets.
Another advantage of the microvoided sheets of the invention is that they are more opaque than titanium dioxide loaded polyethylene of present products. They achieve this opacity partly by the use of the voids as well as the improved
concentration of titanium dioxide at the surface of the sheet. The photographic elements of this invention are more scratch resistant as the oriented polymer sheet on the back of the photographic element resists scratching and other damage more readily
than polyethylene. These and other advantages will be apparent from the detailed description below.
The biaxially oriented microvoided sheets which form the laminated base structure are provided with a skin material on each side. This skin material provides strength, smoothness, and a suitable surface for coating with imaging materials.
The overall density of the microvoided sheet will vary depending on the thickness of the skin layers, the polymer composition, and the amount of microvoiding. For the preferred polypropylene materials of the invention, the overall percent solid
density of the microvoided sheet is preferably between about 78% and 100% for best strength and resistance to cracking during bending.
The paper base forming part of the laminated substrate generally has a thickness between about 0.025 mm and 0.25 mm for adequate strength.
The test to determine whether the sheet creases or cracks is generally carried out by wrapping the imaging element around a rod of an 8 mm radius, rolling the rod back and forth while the element is wrapped on the rod, and then examining for
visual defects.
The terms as used herein, "top", "upper", "emulsion side", and "face" mean the side of a photographic member bearing the imaging layers. The terms "bottom", "lower side", and "back" mean the side of the imaging member opposite from the side
bearing the photosensitive imaging layers, developed image, or printed image if used for thermal or ink jet imaging.
Any suitable biaxially oriented polyolefin sheet may be used for the sheet on the top side of the laminated base of the invention. Microvoided composite biaxially oriented sheets are preferred and are conveniently manufactured by coextrusion of
the core and surface layers, followed by biaxial orientation, whereby voids are formed around void-initiating material contained in the core layer. Such composite sheets are disclosed in, for example, U.S. Pat. Nos. 4,377,616; 4,758,462 and
4,632,869, the disclosure of which is incorporated for reference.
The core of the preferred composite sheet should be from 15 to 95% of the total thickness of the sheet, preferably from 30 to 85% of the total thickness. The nonvoided skin(s) should thus be from 5 to 85% of the sheet, preferably from 15 to 70%
of the thickness.
The density (specific gravity) of the composite sheet, expressed in terms of "percent of solid density" is calculated as follows:
Percent solid density should be between 45% and 100%, preferably between 67% and 100%. As the percent solid density becomes less than 67%, the composite sheet becomes less manufacturable due to a drop in tensile strength and it becomes more
susceptible to physical damage.
The total thickness of the composite sheet can range from 12 to 100 microns, preferably from 20 to 70 microns. Below 20 microns, the microvoided sheets may not be thick enough to minimize any inherent non-planarity in the support and would be
more difficult to manufacture. At thicknesses higher than 70 microns, little improvement in either surface smoothness or mechanical properties are seen, and so there is little justification for the further increase in cost for extra materials.
The biaxially oriented sheets of the invention preferably have a water vapor permeability that is less than 1.55.times.10.sup.-4 g/mm.sup.2 /day/atm. This allows faster emulsion hardening during formation, as the laminated invention support does
not transmit water vapor from the emulsion layers during coating of the emulsions on the support. The transmission rate is measured by ASTM F1249.
"Void" is used herein to mean devoid of added solid and liquid matter, although it is likely the "voids" contain gas. The void-initiating particles which remain in the finished packaging sheet core should be from 0.1 to 10 microns in diameter,
preferably round in shape, to produce voids of the desired shape and size. The size of the void is also dependent on the degree of orientation in the machine and transverse directions. Ideally, the void would assume a shape which is defined by two
opposed and edge contacting concave disks. In other words, the voids tend to have a lens-like or biconvex shape. The voids are oriented so that the two major dimensions are aligned with the machine and transverse directions of the sheet. The
Z-direction axis is a minor dimension and is roughly the size of the cross diameter of the voiding particle. The voids generally tend to be closed cells, and thus there is virtually no path open from one side of the voided-core to the other side through
which gas or liquid can traverse.
The void-initiating material may be selected from a variety of materials, and should be present in an amount of about 5 to 50% by weight based on the weight of the core matrix polymer. Preferably, the void-initiating material comprises a
polymeric material. When a polymeric material is used, it may be a polymer that can be melt-mixed with the polymer from which the core matrix is made and be able to form dispersed spherical particles as the suspension is cooled down. Examples of this
would include nylon dispersed in polypropylene, polybutylene terephthalate in polypropylene, or polypropylene dispersed in polyethylene terephthalate. If the polymer is preshaped and blended into the matrix polymer, the important characteristic is the
size and shape of the particles. Spheres are preferred and they can be hollow or solid. These spheres may be made from cross-linked polymers which are members selected from the group consisting of an alkenyl aromatic compound having the general formula
Ar--C(R).dbd.CH.sub.2, wherein Ar represents an aromatic hydrocarbon radical, or an aromatic halohydrocarbon radical of the benzene series and R is hydrogen or the methyl radical; acrylate-type monomers include monomers of the formula CH.sub.2
.dbd.C(R')--C(O)(OR) wherein R is selected from the group consisting of hydrogen and an alkyl radical containing from about 1 to 12 carbon atoms and R' is selected from the group consisting of hydrogen and methyl; copolymers of vinyl chloride and
vinylidene chloride, acrylonitrile and vinyl chloride, vinyl bromide, vinyl esters having formula CH.sub.2 .dbd.CH(O)COR, wherein R is an alkyl radical containing from 2 to 18 carbon atoms; acrylic acid, methacrylic acid, itaconic acid, citraconic acid,
maleic acid, fumaric acid, oleic acid, vinylbenzoic acid; the synthetic polyester resins which are prepared by reacting terephthalic acid and dialkyl terephthalics or ester-forming derivatives thereof, with a glycol of the series HO(CH.sub.2).sub.n OH
wherein n is a whole number within the range of 2-10 and having reactive olefinic linkages within the polymer molecule, the above described polyesters which include copolymerized therein up to 20 percent by weight of a second acid or ester thereof having
reactive olefinic unsaturation and mixtures thereof, and a cross-linking agent selected from the group consisting of divinylbenzene, diethylene glycol dimethacrylate, diallyl fumarate, diallyl phthalate and mixtures thereof.
Examples of typical monomers for making the crosslinked polymer include styrene, butyl acrylate, acrylamide, acrylonitrile, methyl methacrylate, ethylene glycol dimethacrylate, vinyl pyridine, vinyl acetate, methyl acrylate, vinylbenzyl chloride,
vinylidene chloride, acrylic acid, divinylbenzene, acrylamidomethyl-propane sulfonic acid, vinyl toluene, etc. Preferably, the cross-linked polymer is polystyrene or poly(methyl methacrylate). Most preferably, it is polystyrene and the cross-linking
agent is divinylbenzene.
Processes well known in the art yield non-uniformly sized particles, characterized by broad particle size distributions. The resulting beads can be classified by screening the beads spanning the range of the original distribution of sizes.
Other processes such as suspension polymerization, limited coalescence, directly yield very uniformly sized particles.
The void-initiating materials may be coated with agents to facilitate voiding. Suitable agents or lubricants include colloidal silica, colloidal alumina, and metal oxides such as tin oxide and aluminum oxide. The preferred agents are colloidal
silica and alumina, most preferably, silica. The cross-linked polymer having a coating of an agent may be prepared by procedures well known in the art. For example, conventional suspension polymerization processes wherein the agent is added to the
suspension is preferred. As the agent, colloidal silica is preferred.
The void-initiating particles can also be inorganic spheres, including solid or hollow glass spheres, metal or ceramic beads or inorganic particles such as clay, talc, barium sulfate, calcium carbonate. The important thing is that the material
does not chemically react with the core matrix polymer to cause one or more of the following problems: (a) alteration of the crystallization kinetics of the matrix polymer, making it difficult to orient, (b) destruction of the core matrix polymer, (c)
destruction of the void-initiating particles, (d) adhesion of the void-initiating particles to the matrix polymer, or (e) generation of undesirable reaction products, such as toxic or high color moieties. The void-initiating material should not be
photographically active or degrade the performance of the photographic element in which the biaxially oriented polyolefin sheet is utilized.
For the biaxially oriented sheet on the top side toward the emulsion, suitable classes of thermoplastic polymers for the biaxially oriented sheet and the core matrix-polymer of the preferred composite sheet comprise polyolefins.
Suitable polyolefins include polypropylene, polyethylene, polymethylpentene, polystyrene, polybutylene and mixtures thereof. Polyolefin copolymers, including copolymers of propylene and ethylene such as hexene, butene, and octene are also
useful. Polypropylene is preferred, as it is low in cost and has desirable strength properties.
The nonvoided skin layers of the composite sheet can be made of the same polymeric materials as listed above for the core matrix. The composite sheet can be made with skin(s) of the same polymeric material as the core matrix, or it can be made
with skin(s) of different polymeric composition than the core matrix. For compatibility, an auxiliary layer can be used to promote adhesion of the skin layer to the core.
Addenda may be added to the core matrix and/or to the skins to improve the whiteness of these sheets. This would include any process which is known in the art including adding a white pigment, such as titanium dioxide, barium sulfate, clay, or
calcium carbonate. This would also include adding fluorescing agents which absorb energy in the UV region and emit light largely in the blue region, or other additives which would improve the physical properties of the sheet or the manufacturability of
the sheet. For photographic use, a white base with a slight bluish tint is preferred.
The coextrusion, quenching, orienting, and heat setting of these composite sheets may be effected by any process which is known in the art for producing oriented sheet, such as by a flat sheet process or a bubble or tubular process. The flat
sheet process involves extruding the blend through a slit die and rapidly quenching the extruded web upon a chilled casting drum so that the core matrix polymer component of the sheet and the skin components(s) are quenched below their glass
solidification temperature. The quenched sheet is then biaxially oriented by stretching in mutually perpendicular directions at a temperature above the glass transition temperature, below the melting temperature of the matrix polymers. The sheet may be
stretched in one direction and then in a second direction or may be simultaneously stretched in both directions. After the sheet has been stretched, it is heat set by heating to a temperature sufficient to crystallize or anneal the polymers while
restraining to some degree the sheet against retraction in both directions of stretching.
The composite sheet, while described as having preferably at least three layers of a microvoided core and a skin layer on each side, may also be provided with additional layers that may serve to change the properties of the biaxially oriented
sheet. A different effect may be achieved by additional layers. Such layers might contain tints, antistatic materials, or different void-making materials to produce sheets of unique properties. Biaxially oriented sheets could be formed with surface
layers that would provide an improved adhesion, or look to the support and photographic element. The biaxially oriented extrusion could be carried out with as many as 10 layers if desired to achieve some particular desired property.
These composite sheets may be coated or treated after the coextrusion and orienting process or between casting and full orientation with any number of coatings which may be used to improve the properties of the sheets including printability, to
provide a vapor barrier, to make them heat sealable, or to improve the adhesion to the support or to the photo sensitive layers. Examples of this would be acrylic coatings for printability, coating polyvinylidene chloride for heat seal properties.
Further examples include flame, plasma or corona discharge treatment to improve printability or adhesion.
By having at least one nonvoided skin on the microvoided core, the tensile strength of the sheet is increased and makes it more manufacturable. It allows the sheets to be made at wider widths and higher draw ratios than when sheets are made with
all layers voided. Coextruding the layers further simplifies the manufacturing process.
The structure of a typical biaxially oriented, microvoided sheet of the invention is as follows:
solid skin layer
microvoided core layer
solid skin layer
The sheet on the back side of the base paper opposite to the emulsion layers or image side may be any suitable sheet. The sheet may or may not be microvoided. It may have the same composition as the sheet on the top side of the paper backing
material. Biaxially oriented sheets are conveniently manufactured by coextrusion of the sheet, which may contain several layers, followed by biaxial orientation. Such biaxially oriented sheets are disclosed in, for example, U.S. Pat. No. 4,764,425,
the disclosure of which is incorporated for reference.
The preferred biaxially oriented sheet is a biaxially oriented polyolefin sheet, most preferably a sheet of polyethylene or polypropylene. The thickness of the biaxially oriented sheet should be from 10 to 150 microns. Below 15 microns, the
sheets may not be thick enough to minimize any inherent non-planarity in the support and would be more difficult to manufacture. At thicknesses higher than 70 microns, little improvement in either surface smoothness or mechanical properties are seen,
and so there is little justification for the further increase in cost for extra materials.
Suitable classes of thermoplastic polymers for the biaxially oriented sheet include polyolefins, polyesters, polyamides, polycarbonates, cellulosic esters, polystyrene, polyvinyl resins, polysulfonamides, polyethers, polyimides, polyvinylidene
fluoride, polyurethanes, polyphenylenesulfides, polytetrafluoroethylene, polyacetals, polysulfonates, polyester ionomers, and polyolefin ionomers. Copolymers and/or mixtures of these polymers can be used.
Suitable polyolefins include polypropylene, polyethylene, polymethylpentene, and mixtures thereof. Polyolefin copolymers, including copolymers of propylene and ethylene such as hexene, butene and octene are also useful. Polypropylenes are
preferred because they are low in cost and have good strength and surface properties.
Suitable polyesters include those produced from aromatic, aliphatic or cycloaliphatic dicarboxylic acids of 4-20 carbon atoms and aliphatic or alicyclic glycols having from 2-24 carbon atoms. Examples of suitable dicarboxylic acids include
terephthalic, isophthalic, phthalic, naphthalene dicarboxylic acid, succinic, glutaric, adipic, azelaic, sebacic, fumaric, maleic, itaconic, 1,4-cyclohexanedicarboxylic, sodiosulfoisophthalic and mixtures thereof. Examples of suitable glycols include
ethylene glycol, propylene glycol, butanediol, pentanediol, hexanediol, 1,4-cyclohexanedimethanol, diethylene glycol, other polyethylene glycols and mixtures thereof. Such polyesters are well known in the art and may be produced by well known
techniques, e.g., those described in U.S. Pat. No. 2,465,319 and U.S. Pat. No. 2,901,466. Preferred continuous matrix polyesters are those having repeat units from terephthalic acid or naphthalene dicarboxylic acid and at least one glycol selected
from ethylene glycol, 1,4-butanediol and 1,4-cyclohexanedimethanol. Poly(ethylene terephthalate), which may be modified by small amounts of other monomers, is especially preferred. Other suitable polyesters include liquid crystal copolyesters formed by
the inclusion of suitable amount of a co-acid component such as stilbene dicarboxylic acid. Examples of such liquid crystal copolyesters are those disclosed in U.S. Pat. Nos. 4,420,607, 4,459,402 and 4,468,510.
Useful polyamides include nylon 6, nylon 66, and mixtures thereof. Copolymers of polyamides are also suitable continuous phase polymers. An example of a useful polycarbonate is bisphenol-A polycarbonate. Cellulosic esters suitable for use as
the continuous phase polymer of the composite sheets include cellulose nitrate, cellulose triacetate, cellulose diacetate, cellulose acetate propionate, cellulose acetate butyrate, and mixtures or copolymers thereof. Useful polyvinyl resins include
polyvinyl chloride, poly(vinyl acetal), and mixtures thereof. Copolymers of vinyl resins can also be utilized.
The biaxially oriented sheet on the back side of the laminated base can be made with layers of the same polymeric material, or it can be made with layers of different polymeric composition. For compatibility, an auxiliary layer can be used to
promote adhesion of multiple layers.
Addenda may be added to the biaxially oriented sheet to improve the whiteness of these sheets. This would include any process which is known in the art including adding a white pigment, such as titanium dioxide, barium sulfate, clay, or calcium
carbonate. This would also include adding fluorescing agents which absorb energy in the UV region and emit light largely in the blue region, or other additives which would improve the physical properties of the sheet or the manufacturability of the
sheet.
The coextrusion, quenching, orienting, and heat setting of these biaxially oriented sheets may be effected by any process which is known in the art for producing oriented sheet, such as by a flat sheet process or a bubble or tubular process. The
flat sheet process involves extruding or coextruding the blend through a slit die and rapidly quenching the extruded or coextruded web upon a chilled casting drum so that the polymer component(s) of the sheet are quenched below their solidification
temperature. The quenched sheet is then biaxially oriented by stretching in mutually perpendicular directions at a temperature above the glass transition temperature of the polymer(s). The sheet may be stretched in one direction and then in a second
direction or may be simultaneously stretched in both directions. After the sheet has been stretched, it is heat set by heating to a temperature sufficient to crystallize the polymers while restraining to some degree the sheet against retraction in both
directions of stretching.
The biaxially oriented sheet on the back side of the laminated base, while described as having preferably at least one layer, may also be provided with additional layers that may serve to change the properties of the biaxially oriented sheet. A
different effect may be achieved by additional layers. Such layers might contain tints, antistatic materials, or slip agents to produce sheets of unique properties. Biaxially oriented sheets could be formed with surface layers that would provide an
improved adhesion, or look to the support and photographic element. The biaxially oriented extrusion could be carried out with as many as 10 layers if desired to achieve some particular desired property.
These biaxially oriented sheets may be coated or treated after the coextrusion and orienting process or between casting and full orientation with any number of coatings which may be used to improve the properties of the sheets including
printability, to provide a vapor barrier, to make them heat sealable, or to improve the adhesion to the support or to the photo sensitive layers. Examples of this would be acrylic coatings for printability, coating polyvinylidene chloride for heat seal
properties. Further examples include flame, plasma or corona discharge treatment to improve printability or adhesion.
The structure of a typical biaxially oriented sheet of the invention is as follows:
treated skin layer
solid core layer
The support to which the microvoided composite sheets and biaxially oriented sheets are laminated for the laminated support of the photosensitive silver halide layer may be a polymeric, a synthetic paper, cloth, woven polymer fibers, or a
cellulose fiber paper support, or laminates thereof. The base also may be a microvoided polyethylene terephalate such as disclosed in U.S. Pat. Nos. 4,912,333; 4,994,312 and 5,055,371, the disclosure of which is incorporated for reference.
The prefered support is a photographic grade cellulose fiber paper. When using a cellulose fiber paper support, it is preferable to extrusion laminate the microvoided composite sheets to the base paper using a polyolefin resin. Extrusion
laminating is carried out by bringing together the biaxially oriented sheets of the invention and the base paper with application of an adhesive between them followed by their being pressed in a nip such as between two rollers. The adhesive may be
applied to either the biaxially oriented sheets or the base paper prior to their being brought into the nip. In a preferred form the adhesive is applied into the nip simultaneously with the biaxially oriented sheets and the base paper. The adhesive may
be any suitable material that does not have a harmful effect upon the photographic element. A preferred material is polyethylene that is melted at the time it is placed into the nip between the paper and the biaxially oriented sheet.
During the lamination process, it is desirable to maintain control of the tension of the biaxially oriented sheet(s) in order to minimize curl in the resulting laminated support. For high humidity applications (>50% RH) and low humidity
applications (<20% RH), it is desirable to laminate both a front side and back side film to keep curl to a minimum.
In one preferred embodiment, in order to produce laminated base with a desirable photographic look and fee | | |
