REVERSE GRAVURE KISS COATING SYSTEM WITH OUTPUT ROLLER

Abstract: A system of reverse gravure kiss coating of a fluid coating material (10) on a web (18) of material includes contacting a surface of the web with a gravure roller (14) while the web is unsupported on the rear surface (30). After the coating material is applied, the rear surface (30) is contacted with an output roller (32) downstream of the gravure roller (14). The output roller (32) can be driven and the web (18) wraps around the output roller (32). The tension at a portion of the web (18) that contacts the gravure roller (14) can be isolated from the tension at a portion of the web (18) that is upstream of the gravure roller (14) using a nip roller (34) to nip the web to the output roller (32) upstream of the gravure roller (14).

Description: REVERSE GRAVURE KISS COATING SYSTEM WITH OUTPUT ROLLER TECHNICAL FIELD The present invention relates to gravure coating. More particularly, the present invention relates to reverse gravure kiss coating with improved performance. BACKGROUND OF THE INVENTION Gravure coating, used for producing continuous coatings, uses a gravure roller with depressions or recesses on the surface which control the thickness and uniformity of the coated layer. Ideally the fluid is uniformly "picked out" of the recesses and transferred to the substrate. Typically these recesses are in a regular pattern in the region where a continuous coating is desired. Several styles of gravure coating may be used and designed to enhance individual recess or groove pickout, to transfer coating uniformly, and to optimize other coating responses such as roller surface life, scratching of the substrate, and pattern attenuation. Figure 1 shows a direct gravure coater. The coating fluid 10 is supplied from a pan 12 or other supply such as an extruder-type flow bar to the gravure roller 14. This coating fluid 10 is metered with a doctor blade 16 or roller, and is transferred to a substrate such as a web 18 at a nip point by contacting the web 18 with the fluid in the recesses using a resilient backup roller 20. The gravure roller 14 has etched, machined, or knurled recesses on its surface which can be any shape or size, discontinuous, or continuous over the roller surface. The volume of these recesses controls the average coating thickness, and the specific geometry can be designed to enhance the stability of the pickout from the recesses. The ability to accurately control the volume and shape of these recesses together with the stability of the pickout of a consistent fraction of the fluid in these recesses improves coating thickness uniformity in the downweb and crossweb directions over other coating techniques. Typically the physical characteristics of interest are the average thickness, its uniformity, the presence of discontinuities (such as pinholes and streaks) , and the relative smoothness or initial leveling of the gravure roller pattern upon transfer to the web. The removal of a fraction of the fluid from a recess is the pickout. There are three styles of stable pickout that can occur with transitions between them. Open coating is the transfer of fluid from the recesses individually with no fluid transferred in the region between them. Merged pickout occurs when the material in the recesses merges with the fluid in adjacent recesses to form a continuous coating at the time of transfer. Combined pickout is when the recesses combine together at the time of transfer to form a pattern that is a multiple or fraction of the recess pattern. There are several controlling variables for pickout quantity and style in various gravure coating systems. The volume factor is the internal recess volume per unit area of the gravure roller surface. This controls the average wet coating thickness as a generally constant fraction of the available volume is picked out under similar operating conditions. A flooded inlet is an excess of fluid at the convergence of the web and gravure roller held there by their motion. Whenever a film split occurs in a diverging channel as at the roller exit on a forward roller coater, a natural liquid instability creates machine direction ribs in the coating. When imposing a gravure pattern other than the machine directional ribbing. enhanced individual groove pickout stability can be achieved by matching the frequency of the gravure pattern to the natural liquid ribbing instability frequency in the machine direction. The land/volume factor ratio influences the pickout style. A low land/volume factor ratio encourages merged pickout, and a high ratio promotes open coating. The helix angle is the angle that the grooves in a trihelical pattern make with the gravure roller shaft. Commercially available gravure roller patterns typically have a helix angle of 45°. Smaller helix angles promote pickout instability in forward gravure, and larger angles reduce the width of the pickout transition regions. The internal tooth angle is the angle between the opposing recess walls. Truncation is the flattening of the recess bottom. Web thickness, web tensile modulus, backup roller hardness, and backup roller pressure all influence the average coating thickness because the effective volume factor of the gravure roller is reduced by any penetration of the web into the gravure roller recesses. Increased land widths improve gravure roller wear life. Decreasing land width and increasing the number of recesses enhances recess pattern attenuation at the transfer of the fluid to the web, promoting leveling of the fluid surface. The speed ratio is the ratio of the surface speed of the gravure roller to that of the web. The coating thickness may be changed by changing the speed ratio. The design of the recess pattern for gravure coating influences the stability of the pickout of the fluid from the recesses. Seemingly minor changes in the discussed variables can change the maximum coating speed for stable pickout by hundreds of feet per minute and can affect the ability to achieve significantly thinner wet coatings. As coating speeds are increased, stable, merged pickout becomes more difficult to achieve. An instability in the film split, at the web separation from the gravure roller, commonly referred to as misting (droplets of fluid spitting from the film split) generally occurs as coating speeds increase. One way to suppress misting is to reduce the gravure roller speed relative to the web and operate in a differential speed condition. The shear applied to the fluid as the film split occurs increases the operational coating speed before misting begins. Figure 2 shows a differential speed gravure coater. The web 18 is brought into a controlled engagement with the gravure roller 14 through relatively slight deformation of the backup roller 20 as compared to the high pressure no slip condition with direct gravure coating. The gravure roller 14 and resilient backup roller 20 are driven independently. There is a great deal of interaction between the variables in differential speed gravure coating. Successful operation at elevated speeds with merged pickout may be easily disturbed by minor changes in knurl design, coater head variables, or dispersion rheology. Changes in doctoring and speed ratio will alter the coating thickness. The changes may reverse as the result of interactions with other variables. Figure 3 shows a reverse gravure coater. The operation of this gravure coater is similar to that of a differential speed coater but the gravure roller surface moves in the opposite direction to the web. Again, the web 18 is brought into a controlled engagement with the gravure roller 14 through position control of the resilient backup roller 20. The gravure roller 14 and backup roller 20 are driven independently. The surface speed of the gravure roller 14 may be above or below web speed. Reverse gravure generally exhibits a larger operating window for acceptable coating than do other gravure coaters. ■ A- Figure 4 shows a gravure kiss coater. As the gravure roller 14 may be rotated in either direction, fluid supply and doctoring are not displayed. The operation of this gravure coater is similar to that of a reverse gravure coater but the web 18 is brought into contact with the gravure roller 14 by positioning an input idler roller 22 and output idler roller 24 to create a slight wrap over the gravure roller 14. The surface speed of the gravure roller 14 may be above or below the speed of the web 18. With proper recess design and fluid rheology, fluids which do not totally attenuate the recess pattern upon transfer with other gravure styles may do so with this technique. In reverse gravure kiss coating, the web 18 passes between the idler rollers 22, 24 in a free span. There is a relatively long span between the idler rollers 22, 24 adjacent the gravure roller 14. This span can be 30 cm, with the gravure roller 14 centered between the idler rollers 22, 24. This relatively long span can permit high frequency web fluctuations at the kiss transfer point, where the coating is applied to the web. Also, tensioning of the web can cause buckling in the crossweb direction, resulting in crossweb coating thickness deviations which manifest themselves in downweb marks. Figure 5 shows an offset gravure coater. An intermediate offset roller 26 is used between the resilient backup roller 20 and the gravure roller 14. The gravure and offset rollers may rotate in either direction, creating transfers in the forward or reverse mode. Typically this coater is used for very thin coatings. The ratio between the web 18 and the gravure roller 14 speed can be as high as 10:1. The offset roller 26 must be made of a resilient material so it can deform into the recesses of the gravure roller and pickout the fluid. The effect of the variables at the transfer point between the gravure roller 14 and the offset roller 26 is similar to that of other gravure coaters. Generally the transfer to the web is made at a speed ratio close to 1:1 and the transfer from the offset roller 26 greatly attenuates the gravure roller recess pattern. SUMMARY OF THE INVENTION The invention is an apparatus and method of coating a fluid coating material on a substrate of material having a coating surface and an opposing, rear surface. The coating surface of the substrate is contacted with a gravure roller which applies fluid coating material to the coating surface while the substrate is unsupported on the rear surface. After the coating material is applied, the rear surface of the substrate contacts an output roller less than 1.0 cm downstream of the gravure roller. The output roller can be located above and spaced from the gravure roller to create a gap, causing the substrate to move between the gravure roller and the output roller at an upward angle. The output roller can be located downstream from the gravure roller by a distance of less than 0.075 cm. The gravure roller can be rotated in a direction opposite to the direction of the substrate to perform reverse gravure coating. The rotation of the output roller can be driven independent of substrate contact, in the same direction as that of the substrate. The substrate contacts the gravure roller by kissing the gravure roller and the substrate contacts the output roller by wrapping around the output roller. The system can also include an idler roller around with the substrate passes upstream of the gravure roller. The center of the idler roller can be located above the center of the gravure roller and below the center of the output roller. In one modification, tension isolation can be accomplished. The tension at a portion of the substrate that contacts the gravure roller can be isolated from the tension at a portion of the substrate that is upstream of the gravure roller. Higher or lower tension can be located upstream of the gravure roller. Tension isolation can be accomplished by using a nip roller to nip the substrate to the output roller upstream of the gravure roller. This nipping can be performed upstream of the idler roller if one is used. BRIEF DESCRIPTION OF THE DRAWINGS Figure l is a schematic view of a direct gravure coater. Figure 2 is a schematic view of a differential speed gravure coater. Figure 3 is a schematic view of a reverse gravure coater. Figure 4 is a schematic view of a gravure kiss coater. Figure 5 is a schematic view of an offset gravure coater. Figure 6 is a schematic view of a reverse gravure kiss coater of the invention. Figure 7 is a schematic view of another embodiment of the reverse gravure kiss coater of the invention. DETAILED DESCRIPTION The reverse gravure kiss coating system of the present invention coats a fluid coating material 10 on a substrate such as a web 18. The web has a coating surface 28, on which the coating material 10 is coated, and an opposing rear surface 30. Any substrates that can be coated using kiss coating can be coated using this system. One product for which this coating system works well is magnetic media, in which a wet coating of magnetizable particles in a binder is coated on a web. The coating system includes rollers or other devices (not shown) for transporting the web to and from the coating station at which the web is coated. Referring to Figure 6, the web first passes an input idler roller 22 after which it travels to the gravure roller 14. The idler roller 22 can be made rubber, carbon fiber composites, steel, aluminum or other materials. The gravure roller 14 rotates in a direction opposite to the direction of web travel and deposits fluid coating material 10, stored in the knurls of the gravure pattern, onto the coating surface 28 of the web 18. The coating material 10 can be deposited in the knurls in any manner, such as by rotating the gravure roller 14 through a pan 12 of coating material 10. The gravure roller 14 contacts the web 18 in a free span without any support on the rear surface 30 of the web 18 opposite the line of contact between the gravure roller 14 and the web 18. As shown, the center of the idler roller 22 is above the center of the gravure roller 14. A driven, large, accurate output roller 32, which can be a polished steel roller, is located downstream of the gravure roller 14. The output roller 32 is driven in the same direction as the web 18, for example by a motor, independent of the web movement past it. As shown, the center of the idler roller 22 is below the center of the output roller 32. The output roller 32 can have a diameter of about 12-16 cm, significantly larger than that of the idler roller 22. After the web 18 kisses the gravure roller 14 and is coated with the coating material 10, it wraps around the output roller 32, which serves as a take out roller. There is a short span between the kiss contact between the gravure roller 14 and the web 18 and the wrapping contact of the web 18 around the output roller 32. This distance can be less than 1 cm and in some cases can be less than 0.075 cm. The dynamic wetting line occurs in the short span between the gravure roller 14 and the output roller 32. This assures greater web stability during the transfer of the coating to the web, keeping the web motionless in the non-downweb directions. This eliminates web flutter and bag and prevents the wetting line from pulsing and causing caliper deviations known as chatter. This, in turn, yields superior downweb and crossweb uniformity when compared to conventional kiss coating using gravure rollers. As shown, the output roller 32 can be located above the gravure roller 14. The web 18 moves between the gravure roller 14 and the output roller 32 at an upward angle. Gravity has no effect and the entire system can be oriented with the web moving in any direction, even upside down. The coating system has a reduced web span between the gravure roller 14 and the closest upstream idler roller 22 and a greatly reduced web span between the gravure roller 14 and the output roller 32, as compared to the common kiss coating systems. Because the kiss transfer point is less than 2.5 cm from the point of contact between the web 18 and the output roller 32, the amplitude of web fluctuations is small. Because the output roller 32 has stable supports and is accurately constructed, no web flutter is observed. This produces virtually uniform coating in the downweb direction. In the production of magnetic diskettes, this decreases diskette modulation and increases the performance of on-line image detection systems. Similarly, coating uniformity is important in other applications, such as those where optical clarity (whether measured objectively or subjectively using the human eye) is important. In the embodiment of Figure 7, the tension in the web upstream of the coating system (the input web) can be isolated from the tension downstream of the coating system (the output web). A nip roller 34, which can be upstream of the idler roller 22, nips the incoming web 18 between the output roller 32 and itself. This nipping allows a tension differential before and after the nipped contact with the output roller 32. A higher tension can be located either upstream of the nip roller 34 or downstream of the nip roller 34.

Claims: CLAIMS 1. A method of coating a fluid coating material on a substrate 18 of material having a coating surface 28 and an opposing, rear surface 30, wherein the method comprises the steps of: contacting the coating surface 28 of the substrate with a gravure roller 14 having fluid coating material 10 to apply the fluid coating material to the coating surface of the substrate 18 in a free span while the substrate is unsupported on the rear surface 30; and contacting the rear surface 30 of the substrate 18 with an output roller 32 at a point on the substrate less than 1.0 cm downstream of the gravure roller 14. 2. The method of claim 1 further comprising: locating the output roller 32 above and spaced from the gravure roller 14 to create a gap; and moving the substrate 18 between the gravure roller 14 and the output roller 32 at an upward angle. 3. The method of any of claims 1 and 2 further comprising at least one of the steps of rotating the gravure roller 14 in a direction opposite to the direction of the substrate 18 to perform reverse gravure coating; and driving, independent of substrate 18 contact, the rotation of the output roller 32 in a direction that is the same as the direction of the substrate. 4. The method of any of claims 1, 2, and 3 further comprising the step of passing the substrate 18 around an idler roller 22 upstream of the gravure roller 14. 5. The method of any of claims 1, 2, 3, and 4 wherein the contacting the coating surface 28 step comprises kissing the coating surface of the substrate 18 with the gravure roller 14 and wherein the contacting the rear surface step 30 comprises wrapping the substrate around the output roller 32. 6. The method of any of claims 1, 2, 3, 4, and 5 further comprising the step of isolating tension between a portion of the substrate 18 that contacts the gravure roller 14 and a portion of the substrate that is upstream of the gravure roller. 7. The method of claim 6 further comprising the step of passing the substrate 18 around an idler roller 22 upstream of the gravure roller 14 and wherein the isolating tension step comprises nipping the substrate to the output roller 32 upstream of the idler roller and upstream of the gravure roller. 8. An apparatus for coating a fluid coating material 10 on a substrate 18 of material having a coating surface 28 and an opposing, rear surface 30, wherein the apparatus comprises: a gravure roller 14 which contacts the coating surface 28 of the substrate 18 with a fluid coating material 10 to apply the fluid coating material to the coating surface of the substrate in a free span while the substrate is unsupported on the rear surface 30; and means for contacting the rear surface 30 of the substrate 18 at a point on the substrate less than 1.0 cm downstream of the gravure roller 14 comprising an output roller 32. 9. The apparatus of claim 8 further comprising at least one of means for rotating the gravure roller 14 in a direction opposite to the direction of the substrate 18 to perform reverse gravure coating; and means for rotating, independent of substrate 18 contact, the output roller 32 in a direction that is the same as the direction of the substrate. 10. The apparatus of any of claims 8 and 9 further comprising an idler roller 22 around which the substrate 18 passes upstream of the gravure roller 14. 11. The apparatus of any of claims 8, 9, and 10 further comprising means for isolating tension between a portion of the substrate 18 that contacts the gravure roller 14 and a portion of the substrate that is upstream of the gravure roller. 12. The apparatus of claim 11 further comprising an idler roller 22 around which the substrate 18 passes upstream of the gravure roller 14 and wherein the isolating tension means comprises a nip roller 34 located upstream of the idler roller for nipping the substrate to the output roller. 13. The apparatus of any of claims 8, 9, 10, and 11 wherein the output roller 32 is a steel roller.

Inventors: IAFRATE GARY J, MUNTER JOHN D, IAFRATE, GARY, J., MUNTER, JOHN, D.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1996
Main classification: B05C1
IPC codes: B05C1, B05D1