This invention relates to a photographic element comprising at least one silver halide containing imaging layer and a cushioning layer below said at least one imaging layer having a compression percentage of between 5 and 25%.

This invention relates to photographic materials. In the preferred form it relates to base materials for photographic prints.
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. While the polyethylene does provide waterproofing to the paper, the melt extruded polyethylene layer used in color paper has very little dimensional strength and, as a result, cannot be used alone as a carrier of an image. 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 U.S. Pat. No. 5,244,861 high strength biaxially oriented sheets are laminated to cellulose paper with low density polyethylene. The biaxially oriented sheet utilized in U.S. Pat. No. 5,244,861 is an efficient thermal dye transfer support for the thermal dye receiving layers. The materials disclosed in U.S. Pat. No. 5,244,861 for use as thermal dye receiving layers are not sensitive to compressive forces.
In prior art photographic papers, great care needs to be taken in handling the materials prior to exposure and development of the image. Prior art silver halide photographic papers are sensitive to compressive forces. If sufficient force is applied to the photosensitive layers, an unacceptable latent site on the silver grain may be created. Compressive forces are typically applied during the slitting of sensitized rolls of photographic paper and during photographic processing of images. If this occurs, an area of the photosensitive layer may actually form dye coupler density in an area that, under normal circumstances, would not form density. It would be desirable to have a photographic base material that has a degree of compressibility, thus cushioning the pressure sensitive photographic layers. This will provide a photographic base material that has increased resistance to force that may be applied to it prior to exposure and development.
There is a continuing need for photographic base materials that have resistance to compressive load prior to exposure that results in fewer defects in the print after development.
It is an object of the invention to provide photographic elements having improved resistance to compressive loads.
It is another object to provide photographic elements with reduced yellow discoloration caused by compressive loads prior to development.
It is a further object to provide photographic elements that have improved resistance to defects caused by winding. These and other objects of the invention are accomplished by a photographic element comprising at least one silver halide containing imaging layer and a cushioning layer below said at least one imaging layer having a compression percentage of between 5 and 25%.
The invention provides photographic print elements having improved resistance to defects caused by compressive loads applied to said elements prior to their development.
The invention has numerous advantages over prior practices in the art. The invention provides photographic elements that have less yellow edge, an imperfection caused by emulsion compression during slitting of photographic paper. The elements of the invention also are resistant to defects caused by compression during winding. During winding relatively small defects and variations in thickness of photographic base will become magnified and create ridges, gauge bands, and hard spots that result in compression of silver halide layers causing defects after development. The photographic elements of the invention have a cushioning layer below the silver halide and will not suffer from these defects, as the cushioning layer will compress during winding, thereby relieving strain on silver halide grains which will result in defects. Further, the cushioned elements of the invention will rebound somewhat after compression during winding or contact with compressive forces during processing, thereby resulting in a flat surface of the photographic element even after compression has been applied in winding or processing. The flat surface will, therefore, present a pleasing image in the product without undesirable surface streaks or discoloration caused by damage to silver halide grains which results in undesirable image errors.
Because silver halide grains tend to be pressure sensitive, the selection of grain size has been limited by the compressive forces that occur during manufacturing and photographic processing. By providing a compressive layer just below the pressure sensitive emulsion, pressure sensitive silver grain designs that perform better for printing speed and image sharpness can now be utilized because the compressive forces are absorbed by the support. These and other advantages will be apparent from the detailed description below.
The terms as used herein, xe2x80x9ctopxe2x80x9d, xe2x80x9cupperxe2x80x9d, xe2x80x9cemulsion sidexe2x80x9d, and xe2x80x9cfacexe2x80x9d mean the side or towards the side of an imaging member bearing the imaging layers. The terms xe2x80x9cbottomxe2x80x9d, xe2x80x9clower sidexe2x80x9d, and xe2x80x9cbackxe2x80x9d mean the side or towards the side of the imaging member opposite from the side bearing the imaging layers or developed image. The term xe2x80x9ctie layerxe2x80x9d as used herein refers to a layer of material that is used to adhere biaxially oriented sheets to a base such as paper, polyester, fabric, or other suitable material for the viewing of images.
As used herein, the term xe2x80x9ccompression percentagexe2x80x9d is defined as the percent that a material deforms under a load referenced to when there is no compressive load applied. Compressive percentage is measured by using an xe2x80x9copticalxe2x80x9d thickness reading vs. a high pressure caliper reading. The optical thickness is obtained by slicing the photographic element with a microtome to expose the thickness dimension perpendicular to a microscope suitable for making uncompressed thickness readings. A high pressure caliper is hereby defined as a device to measure the thickness, as it is compressed between two pads with an area of 0.317 cm2 and a pressure of at least 70 newtons/cm2. The compression percentage is 1 minus the ratio of the high pressure caliper to the optical thickness.
As used herein, the term xe2x80x9crecoveryxe2x80x9d is defined as the optical thickness of a material after the compressive load is removed. The recovery of a material after being subjected to a compressive load is related to the magnitude of the compressive load, the type of material, the elastic limit of the material, the humidity and temperature of the materials, and the dynamics of the compressive load. The recovery percentage is 1 minus the ratio of the optical thickness after the compressive load has been removed to the optical thickness before the compressive load has been applied. The compressive load used to determine recovery percentage is 1.0 MPa.
The cushioning layers of the invention have levels of voiding adjusted to provide optimum compression. When a sufficiently large compressive load is applied to unexposed prior art photographic paper, an undesirable latent image can form significantly reducing the commercial value of the image. It is well known in the field of silver halide imaging that silver emulsions tend to be pressure sensitive. By providing a voided layer, the compressive forces applied to the materials of the invention are relieved by the voided layer, reducing the undesirable formation of a latent image. An imaging layer having a compression percentage of between 5 to 25% has been found to eliminate unwanted latent image formation during manufacturing and photographic processing of images. Compression percentage less than 4% does not significantly improve pressure sensitivity of silver halide imaging layers. Compression percentage greater than 30% has been found to stress fracture, creating undesirable fracture lines in the image. It has been found that after the compressive load has been removed, the cushioning layer must partially recover so that undesirable permanent depressions in the image are reduced. Suitable recovery percentage is between 50% and 100%. A recovery percentage less than 40% will result in an undesirable permanent depression in the image. The most preferred recovery percentage is greater than 90%. A 90% recovery of the cushioning layer is not perceptually significant to consumers.
In addition to reducing the pressure sensitivity of the silver halide image forming layers, the compression of the voided layer also reduces the impact of the lack of support thickness uniformity. A lack of uniformity can be caused by lamination ridges, protrusions, or hard spots. Prior art polyethylene coated photographic paper support does not have a voided layer; thus, a lack of uniformity causes manufacturing waste as the lack of uniformity causes unwanted latent image formation and can create unacceptable impression in the roll. By locating a voided layer just below the emulsion, the inherent lack of uniformity created during manufacturing is accommodated by the voided layer.
An example of compressive loads commonly applied to photographic paper is the load applied in converting sensitized rolls to smaller rolls used in photographic printing process. When a photographic element is subjected to a compression force, such as that created with a slitter knife with a rake angle of less than 20 degrees, there is a compressive shear force that concentrates in the emulsion layer. Such a force, acting at the edge of the cut, can then cause the silver halide grains to develop a site on the grain that, when developed, leaves an undesired visual defect. The cushioned layer, as incorporated in this invention, allows the compressive forces to be applied over a larger footprint, therefore reducing the pressure on the silver grains and significantly reducing the tendency for the grains to develop pressure sensitive imperfections. It is important that the cushioning layer undergoes an elastic deformation and has sufficient resilience to rebound or recover from the applied load.
Since the polyolefin layer contains particles to help form the voids during orientation, the layer is not solid, there are limits to the point load that can be applied, and rebounding is still present. The cushioning layer is a polymeric layer containing voids and in the preferred method is an oriented polyolefin sheet which is adhered to the photographic base by an adhesive. The preferred location of said biaxially oriented voided polyolefin sheet is under the photographic emulsion and on top of the photographic base substrate. This location is preferred because the voided oriented sheet is most effective when the cushioning layer is located between the base substrate and the emulsion. In this situation the force being applied to the emulsion can be more effectively dissipated. It is also possible to either add a second cushioning layer to the bottom side of a photographic element or to use the backside location as the sole cushion layer.
Nonuniformities in the thickness of the polyolefin layers, whether they are extrusion coated directly or laminated to a base substrate, can cause hard streaks or ridges. As there may be a thousand laps of these nonuniform layers wound on top of each other, high spots are developed and pressure is built up in localized areas. In some cases, it is possible to cause pressure sensitive imperfections in these areas. The cushioning effect of these layers can help spread the force load caused by this situation. The presence of at least one biaxially oriented voided layer is needed to provide the desired enhancement to pressure sensitivity. Additional improvements may be realized with additional layers on the same side or in combination on the top and bottom sides of the base substrate. Since the act of voiding a polymeric layer usually entails the addition of a particle or non-compatible polymer to the base polymer and then orienting in at least one direction, it may be necessary to add a skin layer to one or more side of the voided polymeric sheet. The addition of the skin layer adds strength to the voided sheet to improve its handling characteristics during said sheet orientation process. A skin layer comprising polyethylene adds additional functionality to the voided sheet structure. This greatly enhances the photographic emulsion adhesion to the biaxially oriented sheet when said polyethylene layer is located between the emulsion and the cushioned layer. The solid skin layer also provides holdout of the emulsion from the holes in the cushioning layer.
The photographic element containing a biaxially oriented voided polyolefin sheet is normally adhered to a paper base by a lamination process. The preferred embodiment of this invention uses a melt extrudable polyolefin polymer to adhere the sheet to the paper base. Melt extrudable polyolefin polymers are used because of their relative low cost, stability, chemical inertness, and general ease of handling. Depending on the end use of the photographic element, it may be desirable to use a polyester base substrate in place of paper. In this case, a laminated cushion layer is critical because the polyester base has little or no compressibility in the thickness direction, and there is a greater need to have a force reducing layer to minimize pressure induced imperfections.
The sensitivity of a photographic emulsion layer containing silver halide may be impacted by a variety of parameters such as silver grain size, the ratio of silver grains to binder, as well as the addition of chemical addenda. In the case of this invention the silver halide layer comprises an emulsion having an average grain size greater than 0.30 xcexcm. In general, the larger the grain size, the more intolerant the pressure sensitivity of the emulsion. The grain size is important to the image quality, as the probability that a grain will absorb a quantum from a given exposure is proportional to the grain volume. The granularity of the developed image is also dependent on the size of the silver halide grains.
In addition to voided biaxially oriented polymer sheets, the cushioning layer may be formed by the incorporation of solid particles or non-compatible polymer within the base resin and then oriented in at least one direction. Cushioned layer may also be formed be chemical or physical blowing agents. Typical material comprises one or more from the list of azodicarbonamide, zeolite or molecular sieves, gases such as nitrogen, carbon dioxide, or liquids that turn to gas at atmospheric pressure. Microcellular polymer may be created by saturation of the polymer with a gas such as nitrogen, carbon dioxide, or other gas to achieve a bubble density in the range of 0.05 to 5 billion/cm3. It is desirable to balance the density of foam to solid phase polymer. Excessive bubble density will alter the mechanical properties of the polymer sheet. Such properties as tensile yield, modulus, compressibility, mechanical stress cracking, and others are impacted. Annealing the sheet provides some beneficial impact to the mechanical properties. Advantages to a microcellular foamed sheet or layer in the case when it is coextruded with other solid or filled layers enhance opacity, sharpness, cushioning, and overall whiteness of the structure. The mircocellular foam layer may be coextruded with other solid layers that are either clear or filled with pigment, tinting, and optical brightening materials to achieve end photographic property. A preferred embodiment would comprise an upper surface of a solid polymer such as a polyolefin. This layer may further comprise tinting pigments or dyes to offset any color imparted to the final photographic print. Directly under this layer is another polymer layer that is most preferably a polyolefin or polyester comprising a white pigment. Suitable white pigments may be TiO2, BaSO4, clay, ZnS, talc, and others known in the art. Said pigment loading may be between 4-60% by weight of that layer. Thickness of said layer may also be varied to achieve the desired optical properties. Directly under this layer is a layer of microcellular foamed polymer. Such a layer may comprises any suitable polymer such as polyolefin and their copolymers, polyester, polystyrene, and others that has been supersaturated with a gas such that as it is heated to the optimal temperature, that microcellular foam is generated within that polymer layer. Said structure would further comprise a solid layer of polymer on the backside or side next to the support substrate. This structure may be coextruded directly onto the support substrate or may be formed, oriented, and annealed as a separate polymer sheet that is then laminated to a support utilizing an adhesive. Such a structure is able to develop good mechanical properties, excellent optical properties, as well as having excellent cushioning and compressibility properties to prevent pressure sensitivity issues such as yellow edge with photographic emulsions. Such a structure will also provide excellent cushioning for the thermal dye transfer process for generating near photographic image quality. The compressibility of the microcellular layer allows the dye donor ribbon and the image receiving layer to develop intimate contact, as well as to provide a degree of thermal insulation to assure complete transfer of dye to the desired area of the receiver. Such a compressible layer will allow the use of slightly rougher supports without loss of dye transfer. Furthermore, it enhances the speed at which the dyes may be transferred to the receiver.
Another means to achieve a good cushioning layer for pressure sensitive emulsions to prevent yellow edge is to generate pores using a chemical blowing agent. A blowing agent is any material which yields an insoluble gas in a polymer matrix under conditions for extrusion. Two of the preferred blowing agents are azodicarbonamide and sodium bicarbonate. Azodicarbonamide exothermially forms nitrogen and carbon dioxide. The microcellular foam structure is produced by the decomposition of the chemical blowing agent. The gas dissolves in the molten polymer because of the high pressure in the extruder. It is important to optimize the foam nucleation at the point of exiting the die. The drop in pressure causes the gas to become super-saturated. Once the polymer is chilled rapidly, the foam bubbles freeze into the polymer as its viscosity increases. This technique is sensitive to processing conditions within the extruder, as well as the rheological properties of the polymer. The most preferable means is to combine the chemical blowing agent within a polymer in combination with coextrusion of other layers to provide enhanced adhesion of the photographic or imaging layer to the topmost polymer layer. Furthermore, layers of polymer may also be coextruded that comprise pigment, tinting, or optical brightening materials. The addition of these materials is critical to providing an element that has optimal whiteness, sharpness, and opacity. It may also be necessary to add processing aid to enhance the foaming process, as well as the compatibility of the other polymer layers during extrusion. Materials such as antioxidants, slip agents, filler, ultraviolet screening, and others may be necessary.
Any suitable biaxially oriented voided polyolefin sheet may be used for the sheet on the top side of the laminated base used in 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 may be formed as in U.S. Pat. Nos. 4,377,616; 4,758,462; and 4,632,869.
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 xe2x80x9cpercent of solid densityxe2x80x9d is calculated as follows:                     Composite                                      xe2x80x83                                          xe2x80x83                                  Sheet                          xe2x80x83                        Density                    Polymer                          xe2x80x83                        Density              xc3x97    100    =      %              xe2x80x83            of              xe2x80x83            Solid                      xe2x80x83                          xe2x80x83                  Density
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 xcexcm, preferably from 20 to 70 xcexcm. Below 20 xcexcm, 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 thickness higher than 70 xcexcm, 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 0.85xc3x9710xe2x88x925 g/mm2/day. This allows faster emulsion hardening, as the laminated support of this invention greatly slows the rate of water vapor transmission from the emulsion layers during coating of the emulsions on the support. The transmission rate is measured by ASTM F1249.
xe2x80x9cVoidxe2x80x9d is used herein to mean devoid of added solid and liquid matter, although it is likely the xe2x80x9cvoidsxe2x80x9d contain gas. The void-initiating particles which remain in the finished packaging sheet core should be from 0.1 to 10 xcexcm 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 Arxe2x80x94C(R)xe2x95x90CH2, 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 CH2xe2x95x90C(Rxe2x80x2)xe2x80x94C(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 Rxe2x80x2 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 CH2xe2x95x90CH(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(CH2)nOH 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 void initiating cross-linked 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 nonuniformly 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 and 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, and 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 or more 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 photosensitive layers. Examples of this would be acrylic coatings for printability and 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 preferred biaxially oriented sheet of the invention where the imaging layers are coated on the polyethylene skin layer is as follows:
The sheet on the side of the base paper opposite to the emulsion layers 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 by 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 xcexcm. Below 15 xcexcm, 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 xcexcm, 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. Nos. 2,465,319 and 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 combination with a 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 backside 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 backside 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 an 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 backside 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 photosensitive 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 preferred backside biaxially oriented sheet of this invention wherein styrene butadiene methacrylate coated layer is on the bottom of the photographic element is as follows:
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 preferred 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 sheets in order to minimize curl in the resulting laminated support. For high humidity applications ( greater than 50% RH) and low humidity applications ( less than 20% RH), it is desirable to laminate both a front side and backside film to keep curl to a minimum.
The surface roughness of this invention can also be accomplished by laminating a biaxially oriented sheet to a paper base that has the desired roughness. The roughness of the paper base can be accomplished by any method known in the art such as a heated impression nip or a press felt combined with a roller nip in which the rough surface is part of the press nip. The preferred roughness of the base paper is from 35 xcexcm to 150 xcexcm. This preferred range is larger than roughness range for the imaging support because of the loss of roughness that occurs in melt extrusion lamination.
In one preferred embodiment, in order to produce photographic elements with a desirable photographic look and feel, it is preferable to use relatively thick paper supports (e.g., at least 120 mm thick, preferably from 120 to 250 mm thick) and relatively thin microvoided composite sheets (e.g., less than 50 mm thick, preferably from 20 to 50 mm thick, more preferably from 30 to 50 mm thick).
As used herein, the phrase xe2x80x9cphotographic elementxe2x80x9d is a material that utilizes photosensitive silver halide in the formation of images. The photographic elements can be black-and-white, single color elements, or multicolor elements. Multicolor elements contain image dye-forming units sensitive to each of the three primary regions of the spectrum. Each unit can comprise a single emulsion layer or multiple emulsion layers sensitive to a given region of the spectrum. The layers of the element, including the layers of the image-forming units, can be arranged in various orders as known in the art. In an alternative format, the emulsions sensitive to each of the three primary regions of the spectrum can be disposed as a single segmented layer.
The photographic emulsions useful for this invention are generally prepared by precipitating silver halide crystals in a colloidal matrix by methods conventional in the art. The colloid is typically a hydrophilic film forming agent such as gelatin, alginic acid, or derivatives thereof.
The crystals formed in the precipitation step are washed and then chemically and spectrally sensitized by adding spectral sensitizing dyes and chemical sensitizers, and by providing a heating step during which the emulsion temperature is raised, typically from 40xc2x0 C. to 70xc2x0 C., and maintained for a period of time. The precipitation and spectral and chemical sensitization methods utilized in preparing the emulsions employed in the invention can be those methods known in the art.
Chemical sensitization of the emulsion typically employs sensitizers such as sulfur-containing compounds, e.g., allyl isothiocyanate, sodium thiosulfate and allyl thiourea; reducing agents, e.g., polyamines and stannous salts; noble metal compounds, e.g., gold, platinum; and polymeric agents, e.g., polyalkylene oxides. As described, heat treatment is employed to complete chemical sensitization. Spectral sensitization is effected with a combination of dyes, which are designed for the wavelength range of interest within the visible or infrared spectrum. It is known to add such dyes both before and after heat treatment.
After spectral sensitization, the emulsion is coated on a support. Various coating techniques include dip coating, air knife coating, curtain coating, and extrusion coating.
The silver halide emulsions utilized in this invention may be comprised of any halide distribution. Thus, they may be comprised of silver chloride, silver bromide, silver bromochloride, silver chlorobromide, silver iodochloride, silver iodobromide, silver bromoiodochloride, silver chloroiodobromide, silver iodobromochloride, and silver iodochlorobromide emulsions. It is preferred, however, that the emulsions be predominantly silver chloride emulsions. By predominantly silver chloride, it is meant that the grains of the emulsion are greater than about 50 mole percent silver chloride. Preferably, they are greater than about 90 mole percent silver chloride; and optimally greater than about 95 mole percent silver chloride.
The silver halide emulsions can contain grains of any size and morphology. Thus, the grains may take the form of cubes, octahedrons, cubo-octahedrons, or any of the other naturally occurring morphologies of cubic lattice type silver halide grains. Further, the grains may be irregular such as spherical grains or tabular grains. Grains having a tabular or cubic morphology are preferred.
The photographic elements of the invention may utilize emulsions as described in The Theory of the Photographic Process, Fourth Edition, T. H. James, Macmillan Publishing Company, Inc., 1977, pages 151-152. Reduction sensitization has been known to improve the photographic sensitivity of silver halide emulsions. While reduction sensitized silver halide emulsions generally exhibit good photographic speed, they often suffer from undesirable fog and poor storage stability.
Reduction sensitization can be performed intentionally by adding reduction sensitizers, chemicals which reduce silver ions to form metallic silver atoms, or by providing a reducing environment such as high pH (excess hydroxide ion) and/or low pAg (excess silver ion). During precipitation of a silver halide emulsion, unintentional reduction sensitization can occur when, for example, silver nitrate or alkali solutions are added rapidly or with poor mixing to form emulsion grains. Also, precipitation of silver halide emulsions in the presence of ripeners (grain growth modifiers) such as thioethers, selenoethers, thioureas, or ammonia tends to facilitate reduction sensitization.
Examples of reduction sensitizers and environments which may be used during precipitation or spectral/chemical sensitization to reduction sensitize an emulsion include ascorbic acid derivatives; tin compounds; polyamine compounds; and thiourea dioxide-based compounds described in U.S. Pat. Nos. 2,487,850; 2,512,925; and British Patent 789,823. Specific examples of reduction sensitizers or conditions, such as dimethylamineborane, stannous chloride, hydrazine, high pH (pH 8-11) and low pAg (pAg 1-7) ripening are discussed by S. Collier in Photographic Science and Engineering, 23, 113 (1979). Examples of processes for preparing intentionally reduction sensitized silver halide emulsions are described in EP 0 348 934 A1 (Yamashita), EP 0 369 491 (Yamashita), EP 0 371 388 (Ohashi), EP 0 396 424 A1 (Takada), EP 0 404 142 A1 (Yamada), and EP 0 435 355 A1 (Makino).
The photographic elements of this invention may use emulsions doped with Group VIII metals such as iridium, rhodium, osmium, and iron as described in Research Disclosure, Sep. 1994, Item 36544, Section I, published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire PO10 7DQ, ENGLAND. Additionally, a general summary of the use of iridium in the sensitization of silver halide emulsions is contained in Carroll, xe2x80x9cIridium Sensitization: A Literature Review,xe2x80x9d Photographic Science and Engineering, Vol. 24, No. 6, 1980. A method of manufacturing a silver halide emulsion by chemically sensitizing the emulsion in the presence of an iridium salt and a photographic spectral sensitizing dye is described in U.S. Pat. No. 4,693,965. In some cases, when such dopants are incorporated, emulsions show an increased fresh fog and a lower contrast sensitometric curve when processed in the color reversal E-6 process as described in The British Journal of Photography Annual, 1982, pages 201-203.
A typical multicolor photographic element of the invention comprises the invention laminated support bearing a cyan dye image-forming unit comprising at least one red-sensitive silver halide emulsion layer having associated therewith at least one cyan dye-forming coupler; a magenta image-forming unit comprising at least one green-sensitive silver halide emulsion layer having associated therewith at least one magenta dye-forming coupler; and a yellow dye image-forming unit comprising at least one blue-sensitive silver halide emulsion layer having associated therewith at least one yellow dye-forming coupler. The element may contain additional layers, such as filter layers, interlayers, overcoat layers, subbing layers, and the like. The support of the invention may also be utilized for black-and-white photographic print elements.
The photographic elements may also contain a transparent magnetic recording layer such as a layer containing magnetic particles on the underside of a transparent support, as in U.S. Pat. No. 4,279,945 and 4,302,523. Typically, the element will have a total thickness (excluding the support) of from about 5 to bout 30 xcexcm.
The invention may be utilized with the materials disclosed in Research Disclosure, 40145 of September 1997. The invention is particularly suitable for use with the materials of the color paper examples of sections XVI and XVII. The couplers of section II are also particularly suitable. The Magenta I couplers of section II, particularly M-7, M-10, M-11, and M-18 set forth below are particularly desirable.
In order to successfully transport display materials of the invention, the reduction of static caused by web transport through manufacturing and image processing is desirable. Since the light sensitive imaging layers of this invention can be fogged by light from a static discharge accumulated by the web as it moves over conveyance equipment such as rollers and drive nips, the reduction of static is necessary to avoid undesirable static fog. The polymer materials of this invention have a marked tendency to accumulate static charge as they contact machine components during transport. The use of an antistatic material to reduce the accumulated charge on the web materials of this invention is desirable. Antistatic materials may be coated on the web materials of this invention and may contain any known materials in the art which can be coated on photographic web materials to reduce static during the transport of photographic paper. Examples of antistatic coatings include conductive salts and colloidal silica. Desirable antistatic properties of the support materials of this invention may also be accomplished by antistatic additives which are an integral part of the polymer layer. Incorporation of additives that migrate to the surface of the polymer to improve electrical conductivity include fatty quaternary ammonium compounds, fatty amines, and phosphate esters. Other types of antistatic additives are hygroscopic compounds such as polyethylene glycols and hydrophobic slip additives that reduce the coefficient of friction of the web materials. An antistatic coating applied to the opposite side of the image layer or incorporated into the backside polymer layer is preferred. The backside is preferred because the majority of the web contact during conveyance in manufacturing and photoprocessing is on the backside. The preferred surface resistivity of the antistat coat at 50% RH is less than 1013 ohm/square. A surface resistivity of the antistat coat at 50% RH is less than 1013 ohm/square and has been shown to sufficiently reduce static fog in manufacturing and during photoprocessing of the image layers.
In the following Table, reference will be made to (1) Research Disclosure, December 1978, Item 17643, (2) Research Disclosure, December 1989, Item 308119, and (3) Research Disclosure, September 1996, Item 38957, all published by Kenneth Mason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth, Hampshire PO10 7DQ, ENGLAND. The Table and the references cited in the Table are to be read as describing particular components suitable for use in the elements of the invention. The Table and its cited references also describe suitable ways of preparing, exposing, processing and manipulating the elements, and the images contained therein.
The photographic elements can be exposed with various forms of energy which encompass the ulrtaviolet, visible, and infrared regions of the electromagnetic spectrum, as well as with electron beam, beta radiation, gamma radiation, X ray, alpha particle, neutron radiation, and other forms of corpuscular and wavelike radiant energy in either noncoherent (random phase) forms or coherent (in phase) forms, as produced by lasers. When the photographic elements are intended to be exposed by X rays, they can include features found in conventional radiographic elements.
The photographic elements are preferably exposed to actinic radiation, typically in the visible region of the spectrum, to form a latent image, and then processed to form a visible image, preferably by other than heat treatment. Processing is preferably carried out in the known RA-4(trademark)(Eastman Kodak Company) Process or other processing systems suitable for developing high chloride emulsions.

Photographic element with voided cushioning layer


In a process for sterilizing objects, the surfaces of the objects are dampened by condensating a steam compound of water and hydrogen peroxide. The steam compound reaches hereby the objects to be sterilized without any additional transport gas flow. Subsequent drying occurs by means of evacuation at a pressure below the boiling points of water and hydrogen peroxide.

This application claims the priority of German patent application 100 42 416.3, filed in Germany, Aug. 30, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for sterilizing objects on their surfaces, which are dampened by means of condensing a steam compound of water and hydrogen peroxide, whereby the condensation fluid is subsequently removed from the surfaces by means of drying.
In sterilization using hydrogen peroxide, which is always present in the form of a watery solution, a purely chemical sterilizing process takes place effected by the "activated" hydrogen peroxide. The term "to activate" used here is undefined, yet a chemical and/or physical reaction takes place as a result of the addition of heat to the hydrogen peroxide, which then effects the sterilization. In a cool state, namely at room temperature or slightly above, hydrogen peroxide has practically no technically applicable sterilizing effect.
In practice, various procedures are known for "activating" hydrogen peroxide for the purpose of sterilizing.
In a known process, European published patent 0 243 003 (corresponding U.S. Pat. No. 4,797,255), a solution comprising water and hydrogen peroxide vaporizes and is fed by a hot transporting air flow to the cooler surfaces, where atmospheric pressure prevails, which cooler surfaces are to be dampened by the condensation of the steam compound. The subsequent drying process takes place by a separate hot air flow.
In the known process, the "activating" of the hydrogen peroxide takes place exactly then when it is required for sterilization, that is, during condensation. Due to the transporting air flow, the concentration of the hydrogen peroxide is, unfortunately, reduced. In addition to this, it cannot be guaranteed that the fed transporting air flow actually reaches all the surfaces to be sterilized. Finally, the subsequent drying process is time-consuming due to the renewed feeding of hot air.
It is an object of the present invention to improve the sterilization effect and to reduce the duration of the sterilizing process.
This object has been achieved in accordance with the present invention in that dampening by means of condensation of the steam compound is carried out without an additional transport gas flow, and in that the drying process is carried out by means of evacuation at a pressure below the boiling points for water and hydrogen peroxide.
In the process according to the present invention, the "activating" of the hydrogen peroxide takes place as in prior art exactly then, when it is required for sterilization, namely during condensation. But because the steam compound reaches the objects to be sterilized without additional transport gas flow, the concentration of hydrogen peroxide during sterilization is significantly higher. The removal of hydrogen peroxide traces is not carried out by subsequent heating, but rather by simple evacuation at a pressure below the boiling points of water and hydrogen peroxide, which process is executed in seconds and reliably removes all traces of hydrogen peroxide.
The steam compound of water and hydrogen peroxide necessary for sterilization is generated in an evaporator or gasifier, the design of which is irrelevant for the purpose of the present invention, as long as it is capable of generating a high enough concentration of hydrogen peroxide steam in a steam compound. The actual "activating" of the hydrogen peroxide in the steam compound takes place during condensation on the surfaces to be sterilized. The steam compound is caused to dampen the surfaces with as thin and homogenous a film of fluid as possible. A microscopically thin condensation layer, barely visible to the naked eye, is sufficient for the purpose of the present invention in order to achieve a sterilizing effect in the shortest time possible. Because of the thinness of the layer, the undesirable formation of ice during subsequent drying by means of evacuation does not arise.
The steam compound to be fed into the treatment chamber for sterilizing is determined by the total condensation surface area available. The volume of the sterilization chamber itself is not an important factor, as it is only on the surfaces that condensation occurs.
The applicant supposes that, in connection with the "activating" of the hydrogen peroxide, the following circumstances could be of significance.
The boiling point of hydrogen peroxide is higher than that of water. This means that, when cooling, the hydrogen peroxide condensates first, followed then by the water. It is therefore supposed that a condensation layer of mostly pure fluid hydrogen peroxide arises, on which lies a layer of water condensation. This results particularly on the surfaces to be sterilized, exactly at the right moment, seemingly to an additional increase in the concentration of the hydrogen peroxide in the steam compound.
Tests have shown that a steam compound having at least 25% hydrogen peroxide should be generated and subsequently brought into contact with the surfaces to be sterilized. It has been shown that the required time for sterilizing decreases with the increase in concentration of the hydrogen peroxide in the condensation fluid. The watery solution used for evaporating or gasifying should preferably contain 35% to 60% hydrogen peroxide. It is, however, to ensure a reliable process, helpful to introduce a predetermined amount of hydrogen peroxide solution into the evaporator or gasifier, whereby this amount is completely evaporated, so that the hydrogen peroxide concentration in the steam compound is equal to the concentration of the introduced watery solution.
The process according to the present invention can be carried out in a large variety of ways, depending on the application or on the objects to be sterilized.
In one process according to the present invention, for example, the steam compound is generated in an evaporator, which serves at the same time as a sterilization chamber, through which the dampened objects are guided. This process is in particular for the sterilization of sheet material, from which, for example, plastic cups are subsequently punched. During the passage of the sheet material through the evaporator, the condensation fluid is deposited on the colder sheet material and causes a sterilizing effect. The condensation fluid can then be removed in a downstream drying chamber by means of vacuum pumping. In this process, the pressure of the evaporator is the same as that of the sterilization chamber, which is a part of the evaporator. Thus it is ensured that the steam compound reaches all surfaces to be sterilized.
In another process according to the present invention it is provided that the steam compound is generated in an evaporator and subsequently, for the purpose of dampening the objects by means of condensation, is guided into a separate sterilization chamber, whose pressure is significantly lower than the steam pressure of the steam compound. This latter named process is thus a low pressure or vacuum process, whereby it is guaranteed that the steam compound reaches all the surfaces to be sterilized.
The pressure area to be considered is determined by the following: the temperature, at which hydrogen peroxide begins to thermally dissolve, is 140° C. This temperature should not therefore be overly exceeded during evaporation. Because of the lower boiling point of water, the steam pressure of more highly concentrated solutions at the same temperature is significantly lower than the steam pressure of water. In the case of the preferred 50% water, 50% hydrogen peroxide solution , the steam pressure measures at, for example, 120° C., approximately 1100 mb. In order that the steam compound of hydrogen peroxide steam and water steam can flow without being diluted by the residual air in the sterilization chamber, the chamber pressure must be significantly lower than the above mentioned steam pressure, in particular when the volume of the sterilization chamber is significantly larger than the volume of the steam compound available in the evaporator. In order for condensation to take place in the sterilization chamber, the chamber pressure must be able to increase significantly as a result of the in-flow of the steam compound. The chamber pressure before the in-flow of the steam compound should be in the range of between 5 and 0.1 mb, for example approximately 1 mb, when the chamber volume is, for example, forty times that of the evaporator volume. If the ratio between chamber volume and evaporator volume is smaller, the chamber pressure can be accordingly higher before the in-flow of the steam compound.
It should be mentioned here that, as a result of the higher temperature together with a very high concentration of the steam compound flowing out of the evaporator, a side effect is that a heated conduit to the sterilization chamber is at the same time sterilized, even without condensation. This is of great importance for fully aseptic applications.
The feeding of the steam compound into the sterilization chamber without additional transport gas flow can take place in two ways, either by means of adiabatic expansion or by means of so-called continuous oversaturation. The transition from adiabatic expansion to continuous oversaturation is to a large extent simple, when adiabatic expansion occurs periodically and the intervals between two expansions are used to return to zero.
In the case of adiabatic expansion, a predetermined amount of the steam compound flows abruptly from the evaporator or gasifier into the sterilization chamber. Due to the speed of the action, the over-all energy contained in the steam compound cannot alter, it remains constant, so that the state alteration is adiabatic. However, during the adiabatic expansion, the volume taken up by the steam compound increases greatly, as the steam compound must now, in addition to the evaporator volume, also fill out the volume of the sterilization chamber. As a result the temperature is drastically reduced and thus falls very far below the dew point, whereby an extremely oversaturated steam compound arises, so that almost the entire mass contained in the steam compound condensates in the shortest possible time on all the surfaces exposed thereto.
In order to achieve the abrupt expansion, the chamber pressure must be much lower than the pressure in the evaporator. For example, the expansion of a steam compound from 90° C. to double the volume results in a temperature of approximately 10° C., the expansion to ten times the volume results in a temperature of −110° C. The advantage of adiabatic expansion lies above all in the extremely short time required for the generation of a condensation layer. It is therefore preferred in such applications where a particularly short sterilization time is desirable, for example in the case of sterilization of PET bottles.
In the case of another variation, wherein the steam compound flows out of the evaporator into the sterilization chamber due to continuous oversaturation, the steam compound also reaches the sterilization chamber from the evaporator due to the applied pressure drop, whereby, however, the process takes longer. Over a certain time span, a watery solution is continuously evaporated, whereby overpressure in the evaporator is maintained and thus a steam compound can be continuously fed into the sterilization chamber. As the temperature, and thus the pressure, is higher in the evaporator, the steam compound expands in the sterilization chamber, which in turn leads to a cooling down. In addition hereto, however, the pressure in the sterilization chamber is increased due to the continuous follow-on flow of the steam compound. Cooling and pressure increase result in an oversaturation of the steam compound accumulating in the sterilization chamber, as both processes force the state of the steam compound below the dew point. As long as the steam compound is fed in, condensation occurs on all accessible surfaces.
In the continuous oversaturation method, a very large amount of condensation fluid can be generated and precipitated using a relatively small evaporator volume, whereby, however, the process takes up a correspondingly longer amount of time. The chamber pressure of the sterilization chamber need only lie slightly under the pressure of the evaporator, just the amount required for the necessary amount of steam compound to flow in. In the evaporator, the temperature must thus be always high enough to permit the steam pressure to continuously exceed the chamber pressure. Continuous oversaturation is suitable for those applications in which the duration of the sterilization process is not of foremost importance. This latter method offers an additional advantage in the form of the significantly higher temperature of the steam compound, which is not greatly lowered by abrupt, extreme expansion, as well as the capability to condensate on relatively warm surfaces.
In particular in sterilizing larger and more complicated surfaces, it can be advantageous when the dampening by means of condensation, as well as the drying of the condensation, is repeated at least once. Repeated cycles, consisting of condensating with subsequent pumping out of the condensation layer, give better results in certain circumstances than one single extended condensation process lasting for the same duration. In some cases, repeated condensation, without pumping out the condensation layer in between, can delivery improved results.
Although the repeated cycle of condensation and evacuation in a non-generic sterilization process (German published patent application 198 18 224) is generally known, what is involved in the known process is not a chemical sterilization by means of hydrogen peroxide, but rather a purely thermal sterilization process using water saturated steam at a temperature of at least 120° C.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

This patent is subject to a terminal disclaimer.

Process and apparatus for sterilizing objects


An improved multi-chip module includes a main circuit board having an array of electrical interconnection pads to which are mounted a plurality of IC package units. Each IC package unit includes a pair of IC packages, both of which are mounted on opposite sides of a package carrier. The package units may be mounted on one or both sides of the main circuit board. A first primary embodiment of the invention employs a laminar package carrier having a pair of major planar surfaces. Each planar surface incorporates electrical contact pads. One IC package is surface mounted on each major planar surface, by interconnecting the leads of the package with the contact pads on the planar surface, to form the IC package unit. A second primary embodiment of the invention utilizes a carrier substrate which has a pair of recesses for back-to-back surface mounting of the IC package pair. The two IC packages may be in contact with opposite sides of a heat sink layer embedded within the carrier substrate. Each resulting IC package unit is surface mounted to the main circuit board. A third primary embodiment of the invention incorporates features of both the first and second primary embodiments. One of the packages is mounted on a planar surface of the carrier rightside up, while the other package is mounted on the carrier in a recess upside down. Several variants of this embodiment are possible. Either the IC package that is mounted on the planar surface of the carrier, or the IC package that is mounted within the recess, may be mounted adjacent to the main circuit board. In the former case, the adjacent package of the package unit fits within a recess on the main circuit board. In the latter case, the adjacent package of the package unit mounts on a planar surface of the main circuit board. For any of the three primary main embodiments, the carrier may be equipped with its own set of interconnection leads which interface with the interconnection pads on the main circuit board or connection may be made directly between the leads of one package and the interconnection pads of the circuit board.

This invention relates to the production of multi-chip electronic modules, and more particularly to a method and apparatus for attaching multiple integrated circuit packages to printed circuit boards. It also relates to high-density memory modules having three-dimensional arrangements of integrated circuit packages.
Demand for semiconductor memory is highly elastic. On one hand, when such memory is relatively inexpensive compared to the overall cost of a computer system, an almost unsatiable demand results, with computer manufacturers tending to install an amount of main memory in each system that greatly exceeds the amount required for average program use. On the other hand, when it is costly, manufacturers typically install an amount in each system that only marginally fulfills the requirement of the average program. Although the sales prices of computers may, thus, be maintained at low levels, the end user may soon find that he must upgrade his computer""s main memory.
The ever increasing demand for large random access computer memories, and the growing demand for increasingly compact computers, coupled with an incentive on the part of the semiconductor manufacturers to reduce the cost per bit, has lead to not only a quadrupling of circuit density approximately every three years, but to increasingly efficient techniques for packaging and mounting the circuit chips. Up until the late 1980""s, semiconductor memory chips were usually packaged as dual in-line pin packages (DIPPs). The pins of these DIPP packages were generally soldered directly within through-holes in a main circuit board (e.g., the motherboard), or they were inserted in sockets which were, in turn, soldered within through-holes in the main circuit board. With the advent of surface mount technology, conventional plated through-holes on printed circuit boards have been replaced with conductive mounting pads. Small Outline J-lead (SOJ) packages have lead to Thin Small Outline Packages (TSOPs). Because the pitch or spacing between centers of adjacent surface mount pins is significantly less than the conventional 0.10-inch spacing for conventional through-hole components, surface mount chips tend to be considerably smaller than corresponding conventional chips, thus taking up less space on a printed circuit board. Additionally, as through holes are no longer needed, surface mount technology lends itself to the mounting of components on both sides of a printed circuit board. Memory modules utilizing surface-mount packages on both sides have become the standard. Both the earlier single in-line memory modules (SIMMs) and the currently used dual in-line memory modules (DIMMs) are inserted into sockets on the motherboard.
Packaging density may be increased rather dramatically fabricating modules in which a plurality of integrated circuit (IC) chips, such as memory chips, are stacked in a three dimensional arrangement. As a general rule, the three-dimensional stacking of chips requires complex, non-standard packaging methods.
One example of a vertical stack of IC chips is provided by U.S. Pat. No. 4,956,694 to Floyd Eide, titled INTEGRATED CIRCUIT CHIP STACKING. A plurality of integrated circuits are packaged within package carriers and stacked, one on top of the other, on a printed circuit board. Except for the chip select terminal, all other like terminals on the chips are connected in parallel.
Another example of chip stacking is given in U.S. Pat. No. 5,128,831 to Fox, et al. titled HIGH-DENSITY ELECTRONIC PACKAGE COMPRISING STACKED SUB-MODULES WHICH ARE ELECTRICALLY INTERCONNECTED BY SOLDER-FILLED VIAS. The package is assembled from individually testable sub-modules, each of which has a single chip bonded thereto. The sub-modules are interleaved with frame-like spacers. Both the sub-modules and the spacers have alignable vias which provide interconnection between the various sub-modules.
U.S. Pat. No. 5,313,096, also issued to Floyd Eide and titled IC CHIP PACKAGE HAVING CHIP ATTACHED TO AND WIRE BONDED WITHIN AN OVERLYING SUBSTRATE, is another example. Such a package includes a chip having an upper active surface bonded to the lower surface of a lower substrate layer having conductive traces on its upper surface which terminate in conductive pads on its periphery. Connection between terminals on the active surface and the traces is made with wire bonds through apertures within the lower substrate layer. An upper substrate layer, which is bonded to the lower substrate layer, has apertures which coincide with those of the lower substrate layer and provide space in which the wire bonding may occur. After wire bonding has occurred, the apertures are filled with epoxy to form an individually testable sub-module. Multiple sub-modules can be stacked and interconnected with metal strips attached to their edges.
A final example of a stacked-chip module is disclosed in U.S. Pat. No. 5,869,353 to A. U. Levy, et al. titled MODULAR PANEL STACKING PROCESS. A plurality of panels are fabricated having apertures therein, an array of chip-mounting pads at the bottom of the apertures, and interfacing conductive pads. Both the chip-mounting pads and the interfacing conductive pads are coated with solder paste. Plastic-encapsulated surface-mount IC chips are positioned on the paste-covered mounting pads, multiple panels are stacked in a layered arrangement and the stack is heated to solder the chip leads to the mounting pads and the interfacing pads of adjacent panels together. Individual chip package stacks are then separated from the panel stack by a cutting and cleaving operation.
As can be seen by the foregoing examples, increased chip density is achieved through the use of complicated packaging and stacking arrangements, which must necessarily be reflected in a higher cost per bit of storage.
The present invention provides for increased circuit density on printed circuit boards. The invention is particularly useful for increasing the density of memory chips on memory modules used for computer systems. The invention includes a package carrier that is designed to mount on a printed circuit board (PCB) on top of a first integrated circuit (IC) package that is also mounted on the PCB. The carrier has an upper major surface having a pad array on which a second IC package is mountable. When mounted on top of the first IC package, the carrier may be thought of as a canopy, on top of which the second. IC package is mounted. The carrier has a plurality of leads by means of which the carrier is surface mounted to the PCB. Each carrier lead is also electrically connected to a single pad of the pad array on the upper surface. The invention also includes a multi-chip module assembled using at least one PCB, at least one package carrier and at least two IC packages for each carrier. For multi-chip modules where the IC package beneath the carrier shares all or most connections in common with the IC package mounted thereupon, a single lead of the carrier and a single lead of the package beneath the carrier may share a mounting/connection pad on the PCB. When separate connections must be made by similarly positioned leads on the carrier and the package beneath the carrier, the corresponding pad on the PCB may be split so that each lead has a unique connection.
A first embodiment of the carrier includes a body having a first pad array, arranged as two parallel linear rows of pads, and adhered to an upper major surface thereof. The leads of an IC package may be conductively bonded to the pads of the first pad array. The body also has a second pad array, arranged as two parallel linear rows of pads positioned along the longitudinal edges, and adhered to the lower major surface thereof. Pads of the first and second arrays are interconnected with conductively-plated vias, or through holes. The carrier leads are conductively bonded to the pads of the second array. The carrier incorporates a heat sink feature. The end leads on a first side of the carrier are both power leads. These two power leads are interconnected by a first laminar sheet which is continuous with and extends between those two leads, and which may extend the entire length of the carrier. An end portion of the first laminar sheet may be exposed at each end of the carrier to facilitate the transfer of heat to the ambient air. The end leads on a second side of the carrier are both ground leads. These two ground leads are interconnected by a second laminar sheet which is continuous with and extends between these two leads, and which may extend the entire length of the carrier. An end portion of the second laminar sheet may be exposed at each end of the carrier to facilitate the transfer of heat to the ambient air. Each laminar sheet is spaced apart from the intervening leads of the same row. The first and second laminar sheets are spaced apart from one another along the center of the carrier. Each IC package includes a dielectric body, an IC chip embedded within the body, and a plurality of leads, an end of each of which is also embedded within the body and electrically conductively coupled to a connection terminal on the IC chip. For a preferred embodiment of the multi-chip module, an upper surface of the body of the lower IC package is either in intimate contact with both laminar sheets, or thermally coupled thereto via a thermally-conductive compound, or in close proximity thereto to facilitate heat transfer from the package body to the laminar sheets.
A second embodiment of the carrier includes modified leads, each of which functions as a heat sink. A center portion of each lead is bonded to a pad of the second pad array on lower surface of the carrier body. An outer portion of each lead is shaped for surface mounting to a mounting/connection pad on a PCB. An inner portion of each lead extends toward the center of the body. For a preferred embodiment of the multi-chip module, an upper surface of the body of the lower IC package is either in intimate contact with the inner portion of each lead, or thermally coupled thereto via a thermally-conductive compound, or in close proximity thereto to facilitate heat transfer from the package body to the leads.

Electronic module having a three dimensional array of carrier-mounted integrated circuit packages