MAGNETIC DISPERSION CONDITIONING PROCESS

Abstract: A process for conditioning a magnetic pigment dispersion prior to coating comprising the steps of: a) combining the dispersion components to form a semi-dispersed mixture; b) pressurizing the mixture; c) forcing the pressurized mixture through at least one impingement chamber assembly, wherein the mixture is divided into at least two streams, each stream is forced through an orifice assembly, and on exit from the orifice assembly the streams impinge upon each other; d) optionally, filtering the impinged mixture; and e) immediately coating the fully processed dispersion onto a substrate.

Description: MAGNETIC DISPERSION CONDITIONING PROCESS FIELD OF THE INVENTION This invention relates to a process and an apparatus for conditioning dispersions of hard, non-compliant particles, especially magnetic pigment dispersions, prior to coating these dispersions during production of coated products, especially magnetic media products. BACKGROUND OF THE INVENTION In manufacture of magnetic recording elements, such as audio or video tapes, computer diskettes, etc., a dispersion of magnetic pigment particles, polymeric binder, and various other additives in a solvent or combination of solvents is typically coated onto a flexible or rigid substrate Coating quality can affect the performance of the magnetic recording element Therefore, various techniques have been employed in the preparation of the coating dispersion U.S. Patent 4,728,405, for example, discloses a process of continuously recirculating a dispersion through an ultrasonic field before filtration and coating The patent teaches that this increases filter life and provides improvements in coating surface smoothness and squareness ratio Japanese Ko ai Publication 62-006612 teaches reducing dispersion viscosity by adding more solvent followed by processing the high solvent content dispersion through a homogenizer SUMMARY OF THE INVENTION The present invention provides a novel method of preconditioning dispersions comprising hard, non-compliant particles. The method is especially effective in preparing magnetic coating dispersion that is very effective in both removing large particulates and contaminants, which may cause errors during formatting or recording of the magnetic recording element, and also reducing the viscosity of the coating dispersion to facilitate orientation of the magnetic pigment particles. The inventive process comprises the steps of: a) combining the dispersion components to form a semi- dispersed mixture; b) pressurizing the mixture; c) forcing the pressurized mixture through at least one impingement chamber assembly, wherein the mixture is divided into at least two streams, each stream is forced through an orifice assembly, and on exit from the orifice assembly the streams impinge upon each other; d) optionally, filtering the impinged mixture; and e) immediately coating the fully processed dispersion onto a substrate. To avoid stagnation of the processed dispersion during the coating step, according to one embodiment of this invention, the dispersion is recirculated from the coating apparatus through the impingement and filtering steps. The process may also be used advantageously in the production of backcoat dispersions for magnetic recording elements. These backcoat dispersions comprise carbon black or another pigment in a polymeric binder. When used in preparation of a backcoat dispersion, the method prevents clumps of pigment which may cause a rougher backcoat surface and indentations on the magnetic layer when magnetic tape is rolled up. These indentations can add to errors during reading or writing of information on the magnetic layer. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of the process of the invention including mixing vessel, high pressure pump, series of impingement assemblies, series of ultra-restrictive filters and coating element. Figures 2 is a schematic view of an individual impingement chamber assembly. Figures 3-5 are cross-sectional side views of variations of impingement orifice assemblies. Figure 6 is a cross-sectional view of an alternative input manifold. DETAILED DESCRIPTION OF THE INVENTION With reference to Figure 1, the process comprises adding hard, noncompliant particles, a solvent, a polymeric binder and any other desired additive, such as surfactants, wetting agents, cross-linking agents, etc., to a vessel 20. The components are mixed using a mixing apparatus 21, such as a high speed, dissolver type mixer, sand mill, ball mill, etc., into a semi-dispersed premix. The process mix preferably contains up to 50% by weight solids, preferably 20- 50% by weight solids. The percent solids by volume may be up to about 20%, preferably 5-20%, more preferably 10-17%. The particle may be any hard, substantially non-compliant particle such as magnetic pigments, such as iron oxides, barrium ferrite, metal particles, and chromium dioxide; carbon black; many color pigments, such as, for example phthalocyanines, such as copper phthalocyanine, nigrosine dye, Aniline Blue, Chrome Yellow, Dupont Oil Red (from DuPont), Monoline Yellow, Sunfast Blue, Sun Yellow, Sun Red and other pigments available from Sun Chemical Co., Harmon Quindo Red, Regal 300, Fluorol Yellow 088, Fluorol Green Gold 084, Lumogen Yellow S 0790, Ultramarine Blue, Ultramarine Violet, Ferric Ferrocyanide, and other pigments available from BASF, Malachite Green Oxalate, lamp black, Rose Bengal, and Malastral Red; titanium dioxide; abrasive materials, such as aluminum oxide, silicon carbide, alumina, cerium oxide, zirconia, silica, boron carbide, and gamet; etc. According to one preferred embodiment the pigment is a magnetic pigment such as a magnetic iron oxide, magnetic metal particles, barium ferrite, etc. These pigments are coated with a polymeric binder onto a substrate to form a magnetic recording element. According to a second preferred embodiment the pigment is carbon black and the coating dispersion is coated onto the backside of a magnetic recording element. The premix is then fed to a high pressure pump 23, preferably via a low pressure pump 22 capable of generating approximately 50 to 150 psi (300-1000 kPa) of liquid head pressure. The pressure of the process stream is raised to greater than about 2000 psi (14 MPa) by the high pressure pump 23. The system fiinctions best when abrasive resistant check valves 24, which prevent backflow of the process stream, are located both before and after the high pressure pump. The inventors have found that having a larger, seat for the check wedge or ball are effective in preventing particles from bridging between the seat and the check wedge or ball thereby causing backflow of the process material. Suitable check valves are disclosed for example in copending U.S. Application Serial No. 08/339,027. The pressurized process stream then enters impingement zones 1. The minimum number of individual jet impingement chamber assemblies la, etc. is one, but there are preferably multiple individual jet impingement chamber assemblies in series. The jet impingement process effectively reduces the viscosity of the magnetic dispersion. The reduced viscosity is beneficial in coating the magnetic dispersion onto a substrate as it may facilitate orientation of the magnetic pigment particles. According to one embodiment the processed magnetic dispersion may be coated onto a substrate immediately after passing through the jet impingement chambers or immediately after exiting any subsequent filters which are in the process stream. Immediately in this situation means that the dispersion is coated before reagglomeration or flocculation of the particles occurs and before viscosity reduction effects are lost. The precise timing depends upon the dispersion used Preferably the dispersion is coated not more than 10 minutes after processing, more preferably not more than 5 minutes after processing, more preferably still not more than 1 minute after processing, and most preferably within 30 seconds of processing. However, conveyance of the dispersion to the coating apparatus in less than about 1-2 seconds has been found to work well The impinged dispersion after exiting the jet impingement chambers, optionally, passes through a heat exchanger 40 to reduce the temperature of the dispersion. The heat exchanger may be helpful in preventing volatilization of the solvent and reaction of any reactive components which may be present. A Brazepak™ ITT Standard heat exchanger has been found to work well. After cooling, or after exiting the last impingement zone if the cooling step is omitted, the dispersion is preferably filtered. Any filtering system typically used in a magnetic dispersion coating process may be used. However, the dispersion is preferably passed through ultrarestrictive filters 42. Ultrarestrictive filters, for the purpose of this invention, are filters capable of removing 0.8 μm particles at about 99% efficiency. Preferably, the filters used in this process remove about 99% of 0.6 μm particles, more preferably about 99% of about 0.5 μm particles. Use of such ultrarestrictive filters with dispersions of relatively high solids content, 20- 50% by weight, is feasible without need for unduly frequent changes of filters due to the decrease in viscosity and the breakdown of particle agglomerations attained in the jet impingement portion of the process. Commercially available examples of suitable ultrarestrictive filters include Nippon Roki HT-04, HT-05, HT-06 and HT-08. Use of a series of these filters may be desirable, and four of these filters, having increasing restrictivity, in series has been found to work well. After passing through the ultrarestrictive filters, the dispersion proceeds immediately to the coating apparatus 44. Again, coating should occur before reagglomeration or loss of viscosity reduction effects. Any known method and apparatus for coating magnetic dispersions may be used. Suitable examples are disclosed in U.S. Patents 4,327,130, 4,345,543, 4,357,899, 4,387,124, 4,442,144, and 4,475,478, incorporated herein by reference. To avoid stagnation of the processed dispersion, it may be desirable when using certain coating methods to recirculate the dispersion from the coating apparatus back through the impingement and filtering steps. The recycle stream 30 is preferably combined with the feed stream 32 before the dispersion is pressurized for impingement processing. Referring to Figure 2, the individual jet impingement chamber assemblies include an input manifold 2 in which the process stream is split into two or more individual streams, an output manifold 6 which contains the impingement chamber in which the individual streams are recombined, and a passage 3 directing the individual streams into the impingement chamber. Figure 2 shows a preferred construction of the jet impingement chamber assembly where the process stream is divided into two independent streams. The input manifold 2 and the output manifold 6 are connected to high pressure tubing 3 by means of gland nuts 4 and 5. The output manifold 6 itself is preferably capable of disassembly so that the orifice cones 8 and extension tubes 9 may be replaced if different parameters are desired or if the parts are worn or plugged. The high pressure tubing 3 is optionally equipped with thermocouples and pressure sensing devices which enable the operator of the system to detect flow irregularities such as plugging. In the impingement chamber the streams are recombined by directing the flow of each stream toward at least one other stream. In other words, if two streams are used the outlets must be in the same plane but may be at various angles from each other. For example, the two streams could be at 60, 90, 120, or 180 degree angles from each other, although any angle may be used. If four streams are used, two of the streams could be combined at the top of the impingement chamber and two more combined midway down the impingement channel 10 or all four streams could be combined at the top of the impingement chamber. While it is preferred that the orifice cone and extension tubes be perpendicular to the impingement channel, that is not required Figures 3 through 5 show a variety of embodiments of the jet impingement orifice assembly 7. The orifice cone 8 and the extension tube 9 may be separate components but are generally combined into one component The orifice 31 is secured within the orifice cone 8. The orifice should be constructed of a hard and durable material. Suitable materials include sapphire, tungsten carbide, stainless steel, etc. The shape of the orifice can be circular, hexagonal, oval, square, etc. However, circular orifices may be desirable due to relative ease of manufacture. The inner diameter of the orifice is determined in part by the size of the individual particulates being processed. For preparation of a magnetic pigment dispersion preferred orifice diameters range from 0.005 through 0.05 inches (0.1-1 mm). If more than one jet impingement assembly is used in series, it is preferable that the orifice inner diameter in each succeeding impingement chamber is the same size or smaller than the orifice inner diameter in the preceding impingement chamber. The length of the orifice may be increased if desired to maintain a higher velocity for the process stream for a longer period of time. The velocity of the stream when passing through the orifice is generally greater than 1000 ft/sec (300 m s). The extension tube 9 maintains the velocity of the jet until immediately prior to the point where the individual streams impinge each other. The inner portion of the extension tube may be of the same or different material than the orifice and may be of the same or slightly different diameter than the orifice. The length of the extension tube and the distance from the exit of the extension tube to the center of the impingement chamber has an effect on the degree of dispersion obtained. For magnetic pigment dispersions the distance from the exit of the extension tube to the center of the impingement zone is preferably no greater than 0.3 inches (7.6 mm), more preferably no greater than 0.1 inches (2.54 mm), and most preferably no greater than 0.025 inches (0.6 mm). The inventors have found that, although not necessary, it may be beneficial to provide a filter upstream from the initial impingement chamber assembly. The purpose of this filter is primarily to remove relatively large (i.e., greater than 100 μm) contaminants without removing pigment particles. As an alternative to this, the inventors have developed a modified input manifold 2 as shown in Figure 6 which comprises a filter. This input manifold 2 is comprises a removable cover means 26 which allows for removal and replacement of the filter element 29 which is held in the housing 29. Examples Example 1 A magnetic recording tape was produced for comparative purposes using standard processing procedures, i.e. combining and sand milling coating solution ingredients, followed by coating onto a polyethylene terephthalate (PET) substrate. The dispersion ingredients included: HM31™ (metal pigment from Dowa); vinyl binder; a urethane type binder; and MEK/xylene solvents. The weight percent solids was about 33%. A second magnetic recording tape was prepared. The dispersion ingredients were the same as above. This dispersion was initially sandmilled. The dispersion was then pressurized to about 5000 psi (34.5 MPa), for Example A, and 10,000 psi (68.9 MPa), for Example B, and forced through a series of 2 impingement chambers having orifices with diameters of 0.022 in (0.056 cm) and 0.018 in (0.046), respectively. The impinged dispersion was cooled in a heat exchanger to 20°C. The viscosity was measured on an ICI viscometer at 10,000 s * \ The orientation ratio was measured on a VSM™ vibrating sample magnetometer. The smoothness was measured on a Wyko Inferometer. The results are shown below. The inventive process clearly provides viscosity advantages and produces a tape with better pigment orientation than does the comparative process SAMPLE VISCOSITY ORIENTATION RATIO SMOOTHNESS (cP) (nm) Comparative 23 1.37 9 1 Example A 11 1.67 7 0 Example B 12 1 69 6 0 Example 2 Tapes C were produced using a dispersion of TOD A B-3 metal pigment, with a 60/40 vinyl/urethane binder system, 3% by weight lubricant, and 5% head cleaning agent based on metal pigment weight in a MEK/toluene 90/10 solvent blend The dispersion was preprocessed in a sandmill The dispersion was then pressurized to 7000 psi and passed through a series of two impingement chambers having orifice diameters of 0 022 in (0 056 cm) and 0 018 in (0 046), respectively The dispersion was cooled in a heat exchanger and then passed through a series of Nippon Roki filters, with the last filter being a Nippon Roki HT-05 The dispersion was coated onto polyethylene terephthalate substrate These tapes had from about 0.8xl0 7 to about 1.5xl0 7 bits between errors Tapes D were produced using a dispersion of TOD A B-3 metal pigment, a self-wetting vinyl/urethane binder system, and 5% head cleaning agent based on metal pigment weight. This dispersion was processed in the same manner as dispersion C. However, the last filter was the more restrictive Nippon Roki HT- 04. Tapes produced using this dispersion had from 2.5xl0 7 to about 4.0xl0 7 bits between errors. Example 3 The following backcoat formulation was prepared (the amounts are in parts by weight): Carbon black 34 Al 2 O 3 3.2 wetting agents 1.7 vinyl binder 19.8 urethane binder 29.8 CB601 crosslinking agent 11.5 The ingredients were combined at 12-15 weight % solids in an 85/8.5/5/1.5 blend oftetrahydrofuran/methyl ethyl ketone/toluene/propyleneglycol monomethyl ether acetate. Two dispersions were prepared by sand milling. One of the dispersions was processed through the sand mill for extra passes to attain a smoother dispersion. These dispersions were then coated onto the backside of a magnetic recording tape. Each of the dispersions was coated with and without passing through a jet impingement system before coating. The samples may be identified as follows: Sample milling jet impingement 3A standard no 3B standard yes 3C extra no 3D extra yes The coated tape was dried and wound. The tape was ten unwound and tested for air bleed time (ABT), magnetic side roughness on an interferometer (Mag. RQ), backside roughness on an interferometer (BS RQ), and backside gloss at 60° and 70°. The magnetic side roughness after winding can be indicative of any impressions created on the magnetic layer by the roughness of the backside. The results are shown below: Sample ABT (sec) Mag. RQ (nm) BS RQ (μm) 60° ' gglloossss 70° gloss 3A* 564 11.8 0.35 32 76.5 3B 419 11.2 0.28 45 86 3C 670 9.3 0.20 75 98 3D 731 9.0 0.19 80 100.5 * Average for two separately coated samples. Example 4 The following backcoat formulation was prepared (the amounts are in parts by weight): Carbon black 38 Al 2 O 3 1.6 wetting agents 0.04 TiO 2 9.5 lecithin 1.5 nitrocellulose 14.7 urethane binder 29.8 Amical 48 fungicide 0.2 NR320 crosslinking agent 11.5 The ingredients were combined at 1 1-13 weight % solids in a 68/16/2/12.5/1.5 blend oftetrahydrofuran/methyl ethyl ketone/toluene/cyclohexanone/propylene-glycol monomethyl ether acetate. Two dispersions were prepared by sand milling. One of the dispersions was processed through the sand mill for extra passes to attain a smoother dispersion. These dispersions were then coated onto the backside of a magnetic recording tape. Each of the dispersions was coated with and without passing through a jet impingement system before coating. The samples may be identified as follows: Sample milling iet impingement 4A standard no 4B standard yes 4C extra no 4D extra yes The coated tape was dried and wound. The tape was ten unwound and tested for air bleed time (ABT), magnetic side roughness on an interferometer (Mag. RQ), backside roughness on an interferometer (BS RQ), and backside gloss at 60° and 70°. The magnetic side roughness after winding can be indicative of any impressions created on the magnetic layer by the roughness of the backside. The results are shown below: Sample ABT (sec) Mag. RQ (n ) BS RQ (μm) 60° gloss 70° gloss 4A* 424 13.2 0.45 34 81 4B 525 11.6 0.41 36.5 82.5 4C 694 11.3 0 31 62 5 93 5 4D 738 10.4 0.24 78 99 Average for two separately coated samples

Claims: What is claimed is: 1. A process for coating a dispersion comprising hard, non-compliant particle, polymeric binder, and solvent wherein the process comprises the steps of: a) combining the dispersion components; b) mixing the combined components to form a dispersion; c) pressurizing the dispersion; d) forcing the pressurized mixture through at least one impingement chamber assembly, wherein the dispersion is divided into at least two streams, each stream is forced through an orifice assembly, and on exit from the orifice assembly the streams impinge upon each other; e) coating the fully processed dispersion onto a substrate. 2. The process of claim 1 further comprising the step of filtering the dispersion prior to coating. 3. The process of claim 1 further comprising cooling the dispersion in a heat exchanger after forcing the dispersion through the impingement chamber. 4. The process of claim 1 further comprising recycling at least a portion of the dispersion from the coating step through pressurizing step and the series of impingement chamber assemblies. 5. The process of claim 2 further comprising cooling the dispersion in a heat exchanger after forcing the dispersion through the impingement chamber. 6. The process of claim 5 further comprising recycling at least a portion of the dispersion from the coating step through pressurizing step, the series of impingement chamber assemblies, the cooling step and the filtering step. 7. The process of claim 1 wherein the forcing step comprises dividing the dispersion into two independent streams that impinge upon each other at an angle of 180 degrees from each other. 8. The process of claim 2 in which the filtering step removes 0.8 μm particles at 99% efficiency. 9. The process of claim 2 in which the filtering step removes 0.6 μm particles at 99% efficiency. 10. The process of claim 1 in which the forcing step comprises forcing the streams through an orifice assembly which comprises an orifice cone holding a small orifice. 11. The process of claim 10 in which the forcing step comprises forcing the streams through orifice assemblies which further comprise an extension tube. 12. The process of claim 10 in which the forcing step comprises forcing the streams through orifices which have diameters ranging from 0.1 through 1.0 mm. 13. The process of claim 1 in which the forcing step comprises impinging the streams upon each other no further than 7.6 mm from the exit of the orifice assemblies. 14. The process of claim 11 in which the forcing step comprises forcing the streams through orifice assemblies having extension tubes with inner diameters in the range from 0.1 through 1.0 mm. 15. The process of claim 1 in which the forcing step comprises forcing the dispersion through multiple impingement chamber assemblies. 16. The process of claim 15 in which the forcing step comprises forcing the pressurized mixture through impingement chamber assemblies in which the inner diameters of the orifices are the same size or smaller than the inner diameters of the orifices in the preceding impingement chamber assembly. 17. The process of claim 1 in which the hard, noncompliant particle is a magnetic pigment. 18. The process of claim 1 in which the hard, noncompliant particle is carbon black. 19. The process of claim 1 in which the dispersion is coated onto the substrate no more than 10 minutes after processing. 20. The process of claim 2 in which the dispersion is coated onto the substrate no more than 10 minutes after processing. 21. A process for reducing the viscosity of a magnetic pigment dispersion comprising the steps of: a) combining the dispersion components; b) mixing the combined components to form a dispersion; c) pressurizing the dispersion; and d) forcing the pressurized mixture through at least one impingement chamber assembly, wherein the dispersion is divided into at least two streams, each stream is forced through an orifice assembly, and on exit from the orifice assembly the streams impinge upon each other. 23. The process of claim 21 in which the forcing step comprises forcing the streams through an orifice assembly which comprises an orifice cone holding a small orifice. 24. The process of claim 23 in which the forcing step comprises forcing the streams through orifice assemblies which further comprise an extension tube. 25. The process of claim 24 in which the forcing step comprises forcing the streams through orifices which have diameters ranging from 0.1 through 1.0 mm. 26. The process of claim 21 in which the forcing step comprises impinging the streams upon each other no further than 7.6 mm from the exit of the orifice assemblies. 27. The process of claim 24 in which the forcing step comprises forcing the streams through orifice assemblies having extension tubes with inner diameters in the range from 0.1 through 1.0 mm. 28. The process of claim 21 in which the forcing step comprises forcing the dispersion through multiple impingement chamber assemblies. 29. The process of claim 28 in which the forcing step comprises forcing the pressurized mixture through impingement chamber assemblies in which the inner diameters of the orifices are the same size or smaller than the inner diameters of the orifices in the preceding impingement chamber assembly. 30. An apparatus comprising a) a high pressure pump which feeds a dispersion to b) at least one impingement chamber assembly comprising (i) an input manifold where the pressurized stream is divided into two or more independent streams, (ii) an output manifold where the independent streams are impingingly recombined in an impingement chamber after passing through orifice assemblies, and (iii) at least two connecting passages between the input manifold and the output manifold; c) a filtering means following the impingement chamber assembly; and d) a means for coating the processed dispersion onto a substrate. 31. The apparatus of claim 30 comprising multiple impingement chamber assemblies in series. 32. The apparatus of claim 30 further comprising a heat exchanger between the impingement chamber assembly and the filtering means. 33. The apparatus of claim 31 further comprising pressure sensors between impingement chamber assemblies. 34. The apparatus of claim 30 wherein the filtering means is capable of removing 99% of 0.8 μm particles. 35. The apparatus of claim 30 in which the orifice assemblies comprise an orifice cone holding an orifice. 36. The apparatus of claim 35 in which the orifice is constructed from materials selected from the following list: synthetic sapphire, tungsten carbide, diamond, stainless steel, and ceramic material. 37. The apparatus of claim 35 wherein the orifice is cylindrical and has an inner diameter in the range from 0.1 to 1.0 mm. 38. The apparatus of claim 35 wherein the orifice assembly further comprises an extension tube. 39. The apparatus of claim 38 wherein the inner diameter of the extension tube is in the range from 0.1 to 1.0 mm. 40. The apparatus of claim 30 in which the free distance from the exit of the orifice assembly to the center of the impingement channel is less than 7.6 mm. 41. A process for treating a dispersion comprising a pigment, polymeric binder, and a solvent comprising the steps of a) combining the dispersion components; b) mixing the combined components to form a dispersion; c) pressurizing the dispersion; d) forcing the pressurized mixture through at least one impingement chamber assembly, wherein the dispersion is divided into at least two streams, each stream is forced through an orifice assembly, and on exit from the orifice assembly the streams impinge upon each other; and e) filtering the impinged mixture through ultrarestrictive filters.

Inventors: BRANDT DANIEL W, EBNER STEPHEN R, GUNTHER JOHN R, LAM JOSEPH H, ROTTO NELSON T, SERAFIN MARK, BRANDT, DANIEL, W., EBNER, STEPHEN, R., GUNTHER, JOHN, R., LAM, JOSEPH, H., ROTTO, NELSON, T., SERAFIN, MARK
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1995
Main classification: B05D1
IPC codes: B05B1, B05C11, B05D1, B05D5, B05D7, G11B5