Claims
- 1. A method of rectifying the problems of an appearance of machine direction streakiness and reduced surface smoothness due to cross direction caliper variations in a coating applied to a moving web of paper by short dwell time type application means when operated at web speeds of 3,000 and more feet per minute, wherein the method includes the steps of
- moving the web in a given direction at a speed of at least 3,000 feet per minute,
- supporting the moving web through a portion of its path of movement,
- forming on the supported portion of the web a short dwell coating liquid application zone having a limited dimension in the direction of web movement and having rear and side edges effectively engageable with the supported web and a front edge spaced from the supported web and defining a gap between itself and the web at the upstream front edge of the application zone,
- forming and maintaining a reservoir of coating liquid in a turbulent state on the supported web within the application zone by substantially sealing the side edges of the application zone to the web and by forming a liquid seal between the turbulent liquid in the application zone and the surface of the web in the vicinity of the gap between the web and the front edge of the zone,
- continuously flowing an excess of coating liquid through the application zone for purging the zone, for forming and maintaining the liquid seal, and for excluding air and foreign matter from the coating liquid adjacent the rear edge of the zone,
- distributing coating liquid in a turbulent state over the surface of the supported web through the limited application zone,
- subjecting the coating liquid on the supported web to a primary doctoring immediately at the rear edge of the application zone, within about 0.0004 to about 0.0100 second of its turbulent application to the web within the zone, by means of a primary doctor blade biased under a relatively low doctoring pressure against the coated web to form on the traveling web a generally uniform and relatively quiescent layer of coating liquid having a wet film thickness sufficiently in excess of the final wet film thickness to accommodate a subsequent final wet film doctoring of the coating liquid on the web, and
- at a location spaced downstream from the rear edge of the application zone and physically and hydrodynamically isolated from the turbulent coating liquid in the application zone, but within about 0.003 to about 0.040 second following the primary doctoring step, subjecting the generally uniform and relatively quiescent layer of coating liquid on the supported web to a final doctoring by means of a final doctor blade biased under final doctoring pressure against the coated web to remove the excess coating from the web and to level and smooth the coating retained on the web to final wet film thickness and smoothness,
- the wet film thickness of the generally uniform and quiescent layer of coating liquid formed on the web by the primary doctor blade having a lower limit sufficiently in excess of the final wet film thickness to provide at the final doctor blade an excess amount of coating liquid adequate to cause a substantially continuous run-off of excess coating liquid from the final blade to purge and flush the final doctor blade, and having an upper limit preventing hydrodynamic overloading of the final doctor blade and minimizing the work required of the final doctor blade,
- the dwell time of the coating liquid on the web between the primary and final doctor blades enabling the boundary layer of coating liquid next to the web to become substantially immobilized for supporting the final doctor blade, whereby the final leveling and smoothing of the coating takes place where the coating is substantially stable,
- the final doctor blade being isolated from the turbulence of the coating liquid in the application zone and being biased under pressure substantially uniformly across the web against a substantially uniform, quiescent and stable layer of coating liquid of minimal excess wet film thickness for leveling and smoothing the coating liquid retained on the web to a uniform final wet film thickness, an improved surface smoothness substantially free of cross direction caliper variations, and an appearance free of machine direction streakiness.
- 2. A method as set forth in claim 1, including the step of doctoring the coating liquid on the web at the rear edge of the application zone to a wet film thickness that exceeds the final wet film thickness by from about 0.001 to about 0.004 inch.
- 3. A method as set forth in claim 1, including the step of doctoring the coating liquid on the web at the rear edge of the application zone to doctor onto the web an amount of excess coating to be removed from the web in the final doctoring step at a rate of from about 0.25 to about 0.75 gallons per minute per inch of web width.
- 4. A method as set forth in claim 1, including the steps of doctoring onto the web at the rear edge of the application zone a layer of coating that is the equivalent of a bone dry coat weight of from about 25 to about 85 bone dry pounds per 3300 square foot ream, and subsequently doctoring the layer of coating liquid on the web to a layer of coating that is the equivalent of a bone dry coat weight of from about 5 to about 15 bone dry pounds per 3300 square foot ream.
- 5. A method as set forth in claim 1, wherein the coating doctored to the web at the rear edge of the application zone is from about 2 to about 10 times the amount of coating retained on the web after final doctoring.
- 6. A method as set forth in claim 1, wherein the doctoring at the rear edge of the application zone is performed at a primary doctor blade doctoring pressure in the order of from about 1 to about 41/2 pounds per lineal inch of web width, and the final doctoring is performed at a final doctor blade doctoring pressure in the order of from about 2 to about 9 pounds per lineal inch of web width.
- 7. A method as set forth in claim 1, including the step of performing the final doctoring within about 4 to about 24 inches of web travel following doctoring at the rear edge of the application zone.
- 8. A method as set forth in claim 1, including the step of applying coating liquid to the web upstream from the front edge of the application zone immediately prior to distributing coating liquid over the web in the application zone.
- 9. A method as set forth in claim 1, including the steps of applying coating liquid to the web upstream from the front edge of the application zone and subjecting the coating liquid to a zone of shear substantially immediately prior to distributing coating liquid over the web in the application zone.
- 10. A method of rectifying the problems of an appearance of machine direction streakiness and reduced surface smoothness due to cross direction caliper variations in a coating applied to a moving web of paper by short dwell time application means when operated at web speeds of 3,000 and more feet per minute, wherein the method includes the steps of
- moving the web in a given direction at a speed of at least 3,000 feet per minute,
- supporting the moving web through a portion of its path of movement,
- forming on the supported portion of the web a short dwell coating liquid application zone having a limited dimension in the direction of web movement and having rear and side edges effectively engageable with the supported web and a front edge spaced from the supported web and defining a gap between itself and the web at the upstream front edge of the application zone,
- forming and maintaining a reservoir of coating liquid under pressure and in a turbulent state on the supported web within the application zone by substantially sealing the side edges of the application zone to the web and forming a liquid seal between the liquid in the application zone and the surface of the web in the vicinity of the gap between the web and the front edge of the zone,
- continuously flowing an excess of coating liquid under a pressure of from about 7 to about 100 inches of water into and through the application zone and outwardly through the gap at the front edge of the zone reversely of the direction of web movement for maintaining the reservoir of liquid in the zone under pressure, for continuously purging the application zone, for forming and maintaining the liquid seal, and for excluding air and foreign matter from the coating liquid adjacent the rear edge of the zone,
- distributing coating liquid in a turbulent state and under said pressure over the surface of the supported web through the limited application zone,
- subjecting the coating liquid on the supported web to a primary doctoring immediately at the rear edge of the application zone, while the coating liquid is maintained under pressure and within about 0.0004 to about 0.0100 second of its pressure application to the web within the zone, by means of a primary doctor blade biased under a relatively low doctoring pressure against the coated web to form on the traveling web a generally uniform and relatively quiescent layer of coating liquid having a wet film thickness that exceeds the amount of coating required for the final wet film thickness by an amount in the order of from about 0.25 to about 0.75 gallons per minute per inch of web width, and
- at a location spaced downstream from the rear edge of the application zone and physically and hydrodynamically isolated from the turbulent coating liquid in the application zone, but within about 0.003 to about 0.040 second following the primary doctoring step, subjecting the generally uniform and relatively quiescent layer of coating liquid on the supported web to a final doctoring by means of a final doctoring blade biased under final doctoring pressure against the coated web to remove the excess coating from the web and to level and smooth the coating retained on the web to final wet film thickness and smoothness,
- the wet film thickness of the generally uniform and quiescent layer of coating liquid formed on the web by the primary doctor blade having a lower limit sufficiently in excess of the final wet film thickness to provide at the final doctor blade an excess amount of coating liquid adequate to cause a substantially continuous run-off of excess coating liquid from the final blade to purge and flush the final doctor blade, and having an upper limit preventing hydrodynamic overloading of the final doctor blade and minimizing the work required of the final doctor blade,
- the dwell time of the coating liquid on the web between the primary and final doctor blades enabling the boundary layer of coating liquid next to the web to become substantially immobilized for supporting the final doctor blade, whereby the final leveling and smoothing of the coating takes place where the coating is substantially stable,
- the final doctor blade being isolated from the turbulence of the coating liquid in the application zone and being biased under pressure substantially uniformly across the web against a substantially uniform, quiescent and stable layer of coating liquid of minimal excess wet film thickness for leveling and smoothing the coating liquid retained on the web to a uniform final wet film thickness, an improved surface smoothness substantially free of cross direction caliper variations, and an appearance free of machine direction streakiness.
- 11. A method as set forth in claim 10, including the steps of establishing a gap of about 1/8 to 3/8 inch between the web and the front edge of said application zone, and
- continuously flowing coating liquid under pressure reversely of the direction of web travel through the gap in an amount of from about 0.75 to about 2 gallons per minute per inch of web width for forming said liquid seal, for excluding air and foreign matter from the vicinity of the primary doctor blade, and for continuously purging the application zone.
- 12. A method as set forth in claim 10, including the steps of biasing the primary doctor blade against the coated web at a blade pressure of from about 1 to about 41/2 pounds per lineal inch of blade width, and biasing the final doctor blade against the coated web at a blade pressure of from about 2 to about 9 pounds per lineal inch of blade width.
- 13. A method as set forth in claim 10, including the step of applying an initial coating to the web upstream from the front edge of the application zone substantially immediately prior to applying coating liquid to the web in the application zone.
- 14. A method as set forth in claim 10, including the steps of applying an initial coating to the web upstream from the front edge of the application zone and subjecting the initial coating to a zone of shear substantially immediately prior to applying coating liquid to the web in the application zone.
- 15. Apparatus for applying a coating essentially free of machine direction streakiness and cross direction caliper variations to a web of paper by short dwell time type applicator means when operated at a web speed of 3,000 and more feet per minute, comprising
- means for moving the web in a given direction at a speed of at least 3,000 feet per minute.
- means for supporting the moving web,
- means forming a coating liquid receiving chamber extending substantially across and parallel to the supported web, said chamber defining a narrow, short dwell application zone having front, rear and side edges and facing toward and extending across the supported web,
- the front edge of said application zone being spaced from the supported web and defining a gap between itself and the web at the upstream front edge of said zone,
- a first doctor blade located at the rear edge of said application zone,
- means for delivering coating liquid to said chamber and into said application zone for distribution over the supported web, said means delivering coating liquid into and through said gap for forming a liquid seal between the coating liquid and the web within said zone in the vicinity of the front edge thereof for excluding air an foreign matter from said first doctor blade, said means delivering an excess of coating liquid to said chamber for continuously purging said application zone and said first doctor blade,
- means for biasing said first doctor blade against the coated supported web at a relatively low doctoring pressure to doctor to the web a generally uniform and relatively quiescent layer of coating liquid sufficiently thicker than the wet film thickness of the coating ultimately to be retained on the web to accommodate a subsequent wet film blading of such thicker layer,
- said first doctor blade being spaced from about 3/4 to about 3 inches downstream from the front edge of said zone for doctoring the coating liquid on the web within about 0.0004 to about 0.0100 seconds following distribution of the coating liquid over the web within said zone,
- a second doctor blade spaced from about 4 to about 24 inches downstream from the first named doctor blade for engagement with the coated supported web at a location that is physically and hydrodynamically isolated from said application zone,
- the spacing between said first and second doctor blades providing a dwell time of from about 0.003 to about 0.040 second of the quiescent layer of coating liquid on the web for enabling the boundary layer of coating next to the web to become substantially stabilized on the web, and
- means for biasing said second doctor blade against the generally uniform, stable and quiescent layer of coating liquid on the supported web at a finishing doctoring pressure for removing excess coating liquid from the web and leveling and smoothing the retained coating to a uniform final wet film thickness, an improved surface smoothness substantially free of cross direction caliper variations, and an appearance free of machine direction streakiness.
- 16. Apparatus for applying a coating essentially free of machine direction streakiness and cross direction caliper variations to a web of paper by short dwell time type applicator means when operated at a web speed of 3,000 and more feet per minute, comprising
- means for moving the web in a given direction at a speed of at least 3,000 feet per minute,
- means for supporting the moving web,
- means forming a coating liquid receiving chamber extending substantially across and parallel to the supported web, said chamber defining a narrow, short dwell application zone having front, rear and side edges and facing toward and extending across the supported web,
- the front edge of said application zone being spaced from the supported web and defining a gap between itself and the web at the upstream front edge of said zone,
- a first doctor blade located at the rear edge of said application zone,
- means for delivering coating liquid to said chamber and into said application zone for distribution over the supported web, said means delivering coating liquid into and through said gap for forming a liquid seal between the coating liquid and the web in the vicinity of the front edge of said zone for excluding air and foreign matter from said first doctor blade, said means delivering an excess of coating liquid to said chamber for continuously purging said application zone and said first doctor blade,
- means for biasing said first doctor blade against the coated supported web at a relatively low doctoring pressure to doctor to the web a generally uniform and relatively quiescent layer of coating liquid sufficiently thicker than the wet film thickness of the coating ultimately to be retained on the web to accommodate a subsequent wet film blading of such thicker layer,
- said first doctor blade being spaced no more than about 4 inches downstream from the front edge of said zone for doctoring the coating liquid on the web within no more than about 0.0100 seconds following distribution of the coating liquid over the web within said zone,
- a second doctor blade spaced no more than about 24 inches downstream from the first named doctor blade for engagement with the coated supported web at a location that is physically and hydrodynamically isolated from said application zone,
- the spacing between said first and second doctor blades providing a dwell time of no more than about 0.040 second of the quiescent layer of coating liquid on the web for enabling the boundary layer of coating next to the web to become substantially stabilized on the web, and
- means for biasing said second doctor blade against the generally uniform, stable and quiescent layer of coating liquid on the supported web at a finishing doctoring pressure for removing excess coating liquid from the web and leveling and smoothing the retained coating to a uniform final wet film thickness, an improved surface smoothness substantially free of cross direction caliper variations, and an appearance free of machine direction streakiness.
- 17. Apparatus as set forth in claim 16, wherein said first blade is spaced from about 1/2 to about 11/2 inches downstream from the front edge of said zone.
- 18. Apparatus as set forth in claim 16, wherein the first biasing means applies a doctoring pressure on said first blade in the order of about 1 to about 41/2 pounds per lineal inch of web width and causes said blade to doctor onto the web a layer of coating that is about 0.0010 to about 0.0040 of an inch thicker than the wet film thickness of the coating ultimately to be retained on the web.
- 19. Apparatus as set forth in claim 16, wherein the second biasing means applies a doctoring pressure on said second blade in the order of about 2 to about 9 pounds per lineal inch of web width and causes said second blade to doctor onto the web a layer of coating providing a final bone dry coat weight in the order of from about 5 to about 15 pounds per 3,300 square foot ream.
- 20. Apparatus as set forth in claim 18, wherein the second biasing means applies a doctoring pressure on said second blade in the order of about 2 to about 9 pounds per lineal inch of web width and causes said second blade to doctor onto the web a layer of coating providing a final bone dry coat weight in the order of from about 5 to about 15 pounds per 3,300 square foot ream.
- 21. Apparatus as set forth in claim 16, including an applicator engaging the supported web substantially immediately upstream but spaced from the front edge of said chamber for applying coating liquid to the web substantially immediately prior to the distribution of coating liquid over the web within the said application zone.
- 22. Apparatus as set forth in claim 21, including means defining a coating liquid shear zone between said applicator and said application zone for subjecting the coating applied to the web by said applicator to shear immediately prior to the distribution of coating liquid over the web within said application zone.
- 23. Apparatus as set forth in claim 16, wherein said means for supporting the moving web comprises a backing roll having ah upwardly moving outgoing side including a lower quadrant of the roll surface, the web is supported on at least said upwardly moving lower quadrant of the roll surface during its passage about the roll, and said apparatus engages the roll supported web within said quadrant.
- 24. Apparatus as set forth in claim 23, wherein said second blade engages the roll supported web in the vicinity of the horizontal center line of the roll and said application zone and said first blade engage the roll supported web within said quadrant below said second blade.
- 25. Apparatus as set forth in claim 24, including a preliminary coating liquid applicator engaging the roll supported web in proximity to the lower end of said quadrant below said application zone.
- 26. Apparatus as set forth in claim 25, wherein said preliminary applicator is a dip roll applicator having a dip roll engageable with the roll supported web proximate to the bottom dead center position of said roll.
- 27. Apparatus as set forth in claim 25, wherein said preliminary applicator comprises means forming a coating liquid receiving chamber extending substantially across and parallel to the supported web, said chamber defining a narrow coating liquid outlet slot having front, rear, and side edges facing toward and extending across the supported web, and
- means for delivering coating liquid under pressure to said chamber and through said outlet slot for pressurized application onto the web.
- 28. Apparatus as set forth in claim 27, including a doctor blade located at the rear edge of said outlet slot for doctoring coating liquid applied to the supported web while the coating liquid is held under pressure.
- 29. Apparatus as set forth in claim 25, including means defining a coating liquid shear zone on the roll supported coated web intermediate said preliminary applicator and said application zone for subjecting the coating liquid on the web to shear prior to distribution of the coating liquid over the web within said application zone.
- 30. Apparatus for coating a high speed moving web of paper comprising
- a backing roll for supporting the moving web,
- a short dwell time coating device having a limited application zone facing the roll supported web and extending transversely across the web parallel thereto, said zone having front, rear and side edges,
- a first doctor blade carried by said device at the rear edge of said zone,
- first mounting means mounting said device and said first doctor blade for movement toward and away from the roll supported web,
- second mounting means pivotally mounting said first doctor blade on said first mounting means for pivotal movement about an axis substantially coincident with the tip of said first doctor blade,
- a second doctor blade spaced downstream from said first doctor blade in the direction of web travel,
- third mounting means movably mounting said second doctor blade on said first mounting means for movement with said first mounting means and for independent movement relative to said first mounting means, and
- fourth mounting means pivotally mounting said second doctor blade on said third mounting means for pivotal movement about an axis substantially coincident with the tip of said second doctor blade,
- said first and third mounting means mounting said device and said first and second doctor blades for conjoint movement toward and away from the roll supported web, said third mounting means mounting said second doctor blade for separate movement toward and away from the web,
- said second and fourth mounting means respectively mounting said first doctor blade and said second doctor blade for respective independent adjustment of the angles of the first and second blades relative to the web.
- 31. Apparatus as set forth in claim 30, including independent actuator means for each of said mounting means for effecting independent adjustment of each of said device and said first and second doctor blades relative to the roll supported web.
SUMMARY OF THE INVENTION
This is a continuation-in-part of copending application Ser. No. 07/375,241 filed on 7/3/89 now abandoned.
1. Technical Field
This invention relates to the technical field of applying a liquid coating composition to a moving web of paper, and more particularly to a coating apparatus and method involving new and improved applications of apparatus of the inverted trailing blade type. The invention is principally concerned with the application of heavier weight coatings, e.g., 51/2 and more pounds per side per ream, to paper webs traveling at ultra-high speeds of 3,000, 4,000 and more feet per minute.
2. Background Art
U.S. Pat. No. 4,250,211 discloses a novel inverted blade type of paper coating method and apparatus that has come to be known as the "short dwell time application" or "SDTA" method and apparatus. The SDTA coater has essentially revolutionized the paper coating art.
The present invention provides a new and improved coating apparatus and method which utilizes, in a specific non-conventional interrelationship, modifications of and improvements upon SDTA and other web coating technologies.
A conventional coater of the trailing blade type includes means for applying a liquid coating composition to a moving web of paper, usually while the web is supported and carried by a resilient backing roll, together with a doctor blade located on the trailing side of the applicator and bearing under pressure against the roll supported coated web to level the applied coating. In general, an excess of coating material is applied to the web, and the trailing blade then meters or removes the excess while uniformly spreading the retained coating onto the web surface.
A first generation of blade coating apparatus, known as the "pond" or "puddle" coater, is comprised essentially of a blade angled downwardly toward and contacting the backing roll on the downwardly moving, incoming side of the roll and forming therewith a reservoir for coating material. The web is moved on the backing roll continuously through the reservoir and the "pond" or "puddle" of coating material therein, whereupon the exposed surface of the web picks up coating material which is struck off and leveled to the desired final thickness or coat weight as a consequence of passage of the web through the nip defined between the blade and the backing roll. Examples of this type of coater are shown in Pulp & Paper, Apr. 29, 1963, pp. 56-58, Paper Trade Journal, Oct. 27, 1969, pp. 58-62 and Paper Trade Journal, Feb. 22, 1971, p. 56.
A variant on the pond type coater, publicized as the Kohler Coater, eliminates the backing roll, disposes the pond or puddle in the horizontal plane, moves the web across the surface of the pond, and utilizes a variable pressure air knife to press the paper web against the blade at the web outlet end of the pond. The Kohler Coater, which is not known to have gained commercial acceptance, is disclosed in Kohler patent 3,113,884, Colgan patent 3,083,685, and articles appearing in the June 1959 issue of The Paper Industry, p. 232; the June 8, 1959 issue of Paper Trade Journal, pp. 31-32; the February 1960 issue of Tappi, pp. 183-187; Pulp and Paper, Second Edition, Vol. III, Interscience Publishers, pp. 1565-1566; and Pulp and Paper Manufacture, Second Edition, Vol. II, 1969, McGraw Hill Book Company, pp. 510-511.
A second generation of blade coating apparatus is comprised of a dip roll applicator, which usually bears against the roll supported web at or adjacent the bottom dead center position of the roll, and a blade spaced downstream from the dip roll and converging toward and contacting the roll supported web, usually on the upwardly moving, outgoing side of the roll. Since this results in the blade converging upwardly into engagement with the roll supported web, the blade is known as an inverted trailing blade. As the web moves with the backing roll, the dip roll is rotated through a reservoir of coating liquid and picks up and transfers to the web an excess of coating liquid. The web then travels to the inverted blade where the excess coating liquid is removed from the web and the retained coating is leveled to the desired final coat weight thickness. Examples of the dip roll applicator with inverted blade (known by the acronym "drib") are disclosed in Rush patent 2,746,877, Dickerman et al. patent 2,949,382, Brezinski patent 3,202,536, the Apr. 29, 1963 issue of Pulp & Paper, p. 57, and the Oct. 27, 1969 issue of Paper Trade Journal, pp. 60-61. In installations wherein a pool of coating liquid is accumulated at the nip between the two rolls, the coater may also be known as a "flooded nip" coater. Another version, involving the use of several applicator rolls in sequence, called the Champflex Coater, is disclosed at pages 56-57 of the Apr. 29, 1963 issue of Pulp & Paper. Also, dip roll applicators may be used in combination with other coaters for precoating or prewetting the web, as is shown for example in the illustration of the Kohler Coater in Pulp and Paper Manufacture, p. 511, and also in Damrau et al patent 4,250,211 and Damrau patent 4,310,573.
A major shortcoming of dip roll coaters is the development of film split pattern in the final coated web, i.e., the appearance in the coating of substantially continuous longitudinal stripes or lines, as web coating speeds are increased upwardly above 2,500 feet per minute and coatweights exceed about 51/2 bone dry pounds per side per 3,300 square foot ream.
A third generation of blade coater, called the flexible blade or "Flexiblade" Coater, is comprised of a closed, pressurized, coating application chamber which sealingly engages the roll supported web, usually near the bottom of the backing roll, and has a back, rear or outgoing wall comprised of a flexible blade for spreading the coating material uniformly on the web surface. The "Flexiblade" Coater made by The Black-Clawson Company is disclosed in Jacobs et al. patent 3,079,889 and in an article appearing in the Apr. 8, 1963 issue of Paper Trade Journal. It is also briefly described at p. 57 of the Apr. 29, 1963 issue of Pulp & Paper as well as other trade periodicals, both U.S. and foreign.
Other flexible blade coaters employing a closed or sealed, pressurized application chamber are described in patent 2,796,846 to Trist and patent 3,273,535 to Krikorian.
In another variant of the sealed chamber type of coater, coating liquid under pressure is extruded onto the web in the closed application chamber and an excess of coating is metered onto the traveling web by a metering bar at the rear or outgoing end of the chamber and the excess is then removed and the coating leveled to its final coat weight thickness by an inverted trailing blade engaging the web downstream from the metering bar. Patents describing coaters of this type include Galer 3,192,895, Hunger 3,486,482 and Nagler 3,518,964. Of the three, the patent to Hunger 3,486,482 is the most representative.
The closed chamber type of coaters suffered the problem of excessive web breaks due to engagement of the traveling paper web with the mechanical sealing means required at the incoming, front or upstream end of the closed application chamber. Efforts to alleviate the problem, for example, by the use of flexible blade seals, such as those of Trist, or by spacing the Jacobs et al. seal member slightly from the web as suggested in the literature, failed to cure the problem. As a consequence, closed chamber coaters, including the Black-Clawson "Flexiblade" Coater, have been substantially if not entirely replaced by subsequent developments in paper coating technology. The above described variant thereof, as represented by the patent to Hunger, is not known to have been used commercially at all.
A fourth generation of blade coater, which was introduced by Black-Clawson as a replacement for the "Flexiblade" Coater, is characterized by an inverted trailing blade preceded by a fountain applicator which, like a dip roll, applies an excess of coating liquid to the web, which excess is subsequently removed and the coating leveled to its desired thickness by the trailing blade. Apparatus of this type, which are called Fountain Blade Coaters, are described in the Mar. 13, 1967 and May 13, 1968 issues of Paper Trade Journal (at pp. 52-53 and 64-67, respectively) and in a paper presented by Black-Clawson at a Tappi conference in 1978, and are disclosed in detail in the patents to Phelps et al. 3,418,970, Penkala et al. 3,453,137 and Coghill 3,521,602. A competitive apparatus, employing a jet applicator rather than a fountain applicator, is described in the German periodical Das Papier, No. 7, 1972, pp. 332-338, at page 334. Similar disclosures appear in an article by Ing. Josef Geistbeck, appearing in the German publication Walzen Und Glattschaberstreichanlagen, and in German Auslegeschrift No. 2359413.
With these prior art fountain and jet applicators, the amount of excess coating that is delivered to the trailing blade is purportedly metered onto the web by a metering or overflow strip which is located at the downstream edge of the applicator and adjustably spaced from the surface of the web to accomodate the escape of coating liquid between the web and the overflow strip. In use, these coaters encounter difficulties when running at high speed because the web catches on the metering bar and tears, thereby producing web breaks and causing machine down time and loss of production.
Some prior art coaters inherently employ a relatively long coating liquid dwell or soak time on the web, i.e., the time interval between the initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as the water soluble or dispersable materials contained therein, migrate into the moving web at a more rapid rate than the pigment and eventually cause an undesirable imbalance in the coating constituents and their rheological properties. Long soak periods are also incompatible with the application of successive wet coats without intervening drying, i.e., wet on wet coatings, because the successive coat tends to migrate into and contaminate the previous coat.
In an effort to control soak time, Black-Clawson introduced a variation of its fountain blade coater wherein the fountain applicator and the doctor blade are separate assemblies and are relatively adjustable toward and away from one another in order to vary the dwell time of the coating on the web between application and doctoring. This coater, called the Vari-Dwell Coater, is described in the proceedings of the Tappi 1986 Blade Coating Conference, pages 109-113, and the Tappi 1987 Coating Conference, pages 141-149.
The problems associated with long dwell times are discussed in Neubauer U.S. Pat. No. 3,348,562, who discloses a coater wherein a narrow stream of viscous coating is extruded onto an inverted trailing blade that defines a nip region with the roll supported web. Since the coating is bladed immediately after application, soak times are purportedly kept to a minimum. However, the coating application is such that the coating material is unpressurized after leaving the orifice and is supported on the blade or trailing side only, with the leading side of the stream being unsupported and exposed to the environs in the zone of application. Consequently, the coating material is not properly o uniformly applied to the web. Disclosures of a related nature are contained in patent 3,484,279 (FIG. 3) to Clark et al. and patent 3,070,066 to Faeber.
The fifth generation of blade coater comprises the short dwell time application coater or "SDTA" coater which is rapidly replacing the prior art blade coaters. In essence, the closed chamber, flexible blade, fountain blade and jet applicator coaters have been rendered obsolete, and the puddle and roll type coaters are being relegated to web precoating or prewetting functions in wet on wet coating systems. The short dwell time or "SDTA" coater is disclosed in detail in patent 4,250,211, and its advantages are discussed in the May 1984 issue of Pulp & Paper, pages 102-104.
The "SDTA" coater is characterized by a coating application chamber having a very small dimension in the direction of web travel, a doctor blade pressure loaded against the coated web at and defining the downstream or web outlet end of the chamber, a novel liquid seal formed within a fairly generous gap defined between the applicator and the web at the upstream or web inlet end of the chamber, and means for supplying coating liquid to the chamber under pressure and in such copiously excess quantities as to cause a continuous high volume flow of coating liquid through the gap out of the upstream or front end of the chamber reversely of the direction of web travel, thereby to form and maintain a liquid seal within the gap and to maintain the coating liquid under pressure in the chamber and as it is applied to and doctored off the web; the doctoring occuring immediately at the downstream end of the application zone while the coating liquid is maintained under pressure. The flow of excess coating liquid through the gap defined between the web and the front edge of the application zone, in the direction reverse to the direction of movement of the web, is such that the gap is continuously and completely filled with reversely flowing coating liquid in quantity sufficient to: (a) close and seal off the gap at the front edge of the zone to maintain the pressure application of the coating liquid to the web within the application zone; (b) strip air off the web as it approaches and enters the application zone, thereby to eliminate air induced skips and voids in the layer of coating applied to the web and insure uniform overall coating of the web; (c) prevent entrainment of air in the coating liquid in the application zone and in the coating liquid that is applied to the web, thereby to eliminate coating imperfections due to the presence of air bubbles in the coating on the web; (d) prevent entry of foreign matter through the gap into the application zone and the coating liquid therein; and (e) continuously clean and purge the application chamber and application zone to insure the integrity, homogeneity and uniform distribution of a continuously fresh supply of coating liquid within the application zone, and to ensure that no foreign matter or impurities, e.g., lumps or coagulated coating, reach the doctor blade where they could cause scratching of the coating or create other problems deleterious to the coating process, or result in web breaks.
Due to the facts that the moving web of paper is pressed firmly, continuously and tightly against the surface of the backing roll by the reversely flowing liquid seal at the front or web entry end of the application zone, by the pressure of the coating liquid within the application zone, and by the pressure loaded doctor blade at the rear or web exit end of the zone, the web cannot catch or snag on coater components and the web breaking and other disadvantages of prior art coaters are eliminated. Consequently, coating compositions can be applied to the web under pressure within a short dwell time, free of skips and voids even at very high web speeds. The SDTA coater has proven itself in use at speeds up to 4000 feet per minute ("fpm") and beyond to apply a more uniform layer of coating onto a web than any prior art coater.
Characteristics of the applied coating can be varied or enhanced by precoating the web, e.g., by a roll applicator as shown in patent 4,250,211 and improvement patent 4,310,573, or by use of an internal leveling blade as disclosed in improvement patent 4,369,731, or by use of a second, internal liquid seal as disclosed in improvement patent 4,452,833, or by use of other improvements of note such as disclosed in patents 4,396,648, 4,440,105, and 4,503,804.
A proposed variation on the SDTA coater, one version of which is disclosed in FIG. 3 of Wohlfeil patent 4,706,603, involves essentially closing off the gap between the coater and the web at the upstream or web inlet end of the coating application chamber and draining excess coating from the chamber via drain holes in the upstream or front wall of the application chamber; the rate of drainage being such as to maintain the coating liquid in the chamber under pressure and to insure a sealed relationship between the web and the coater at the web inlet end of the application zone.
Another variant, a version of which is disclosed in Michael A. Mayer et al. U.S. Pat. No. 4,963,397, involved utilization of a short dwell type of apparatus to rework a previously applied excess layer of coating liquid, e.g., a dip roll applied excess layer, to distribute over the web a more uniform layer of the coating; specifically, a layer of coating that is free of the film split pattern of dip rolls when operated at speeds above about 2,500 fpm; the blade of the short dwell coater being used to remove excess coating from the web and to smooth and level the coating to the desired wet film thickness and coat weight; the excess coating removed by the blade being drained away via the SDTA, e.g., in a manner such as disclosed in Wohlfeil. For another variant, see also Wayne A. Damrau U.S. Pat. No. 4,859,507.
While the SDTA, including the above-described variation and variants thereof, has significantly advanced the state of the art, it has not provided a final solution to all the expectations of the paper coating industry. As the industry presses forward to attain even greater capacity, efficiency and economy in the production of coated papers, even the SDTA coater has on occasion produced coated papers that would not satisfy the increased demand for high quality coatings at higher web speeds. In particular, when applying heavier weight coatings, for example, 51/2 and more pounds per side per 3,300 square foot ream, to the higher grades of paper webs, e.g., groundwood free merchant grades, at ever increasing production speeds, SDTA coatings, though of substantially uniform thickness and free of skips and voids, have exhibited decreased surface smoothness and streakiness in the direction of web travel through the coater, i.e., so-called machine direction or "MD" streakiness. Precoating or prewetting the web or reworking a previously applied excess coating on the web will not eliminate these problems. Dip roll applicators in particular encounter their own inherent limitations at web speeds in the order of 2,800 fpm due to splitting of the film of coating liquid being applied by the roll, resulting in a nonuniform coating having a longitudinally streaked or striped appearance, i.e., film split pattern.
While the SDTA coater and the above-described variants thereof can in most instances eliminate the film split pattern of the dip roll coating, MD streakiness and/or unacceptably diminished surface smoothness, i.e., surface roughness, may still result. Thus, whether used alone or in combination with a dip roll applicator, existing apparatus and methods, when operated at higher speeds to apply heavier weight coatings, may not in all cases produce a coated paper that will satisfy the exacting demands of the high quality printing, graphic arts and publishing trades.
The present invention comprises an improved paper coating apparatus and method capable of extremely high speed production of coated papers fullfilling the exacting demands of the trade, and specifically eliminating both film split pattern and MD streakiness in heavier weight coatings produced at high web coating speeds. The invention provides an improved coater and coating method making non-conventional use of SDTA type applicator apparatus for distributing excess coating liquid in a highly turbulent state over the surface of the web, and utilizing primary and secondary trailing blades for effecting precisely controlled sequential doctoring of the excess to the final wet film thickness of coating desired on the web; the primary blade being located at the downstream or web outlet end of the distribution zone of the apparatus and doctoring onto the web a substantially uniform layer of coating having a limited or controlled thickness which is in excess of the desired final wet film thickness (and significantly in excess of that conventionally applied by an SDTA coater); the secondary blade being spaced downstream from the primary blade and being physically and hydrodynamically isolated from the coating application zone; the secondary blade doctoring the primary blade's limited excess of coating off the web and leveling and smoothing the retained coating to the final wet film thickness desired.
As used in accordance with the present invention, the SDTA type of apparatus is effective to distribute over the entire surface of the high speed traveling web, within a limited application zone, an excess of coating that is entirely free of skips, voids, film split pattern and other imperfections, except MD streakiness and surface roughness. Due to the liquid turbulences, eddy currents and other hydrodynamic disturbances that are generated in the coating liquid in the application zone of the apparatus at very high web speeds, the coating medium in the zone exhibits extreme hydrodynamic impulse variations and fluctuations across the width of the web which cause transversely shifting variations across the width of the web in the thickness or caliper of the coating liquid being applied to the web, i.e., cross direction or "CD" caliper variations, which result in overall MD streakiness, diminished surface smoothness, and other imperfections in the final coated web.
According to the present invention, the primary blade is utilized to contain and isolate the hydrodynamic pressure fluctuations and impulse forces, and to gain a preliminary degree of control over the coating to be retained on the web, but without overwhelming the primary blade. First, the primary blade is utilized to isolate the hydrodynamic eddy currents and turbulences with the application zone and to confine the same therein. Second, the primary blade is employed to doctor onto the moving web an excess of coating liquid in the form of a relatively quiescent layer having an overall high degree of uniformity, except for small but nevertheless unacceptable variations in CD caliper profile. Third, the primary blade effects a controlled doctoring of this quiescent layer to a limited thickness just sufficiently in excess of the desired final wet film thickness to accomodate a subsequent final wet film doctoring of the liquid on the web under optimum blading or doctoring conditions.
Even with a relatively light mechanical loading thereon, the primary blade in the coater of the invention results in transport to the secondary blade, on the high speed traveling web, of a generally uniform, relatively quiescent layer of coating liquid of precisely controlled and limited excess thickness that is free of skips, voids and other anomalies or abberations, other than the unacceptable variations in CD caliper profile.
The secondary blade of the coater of the invention is spaced downstream from the primary blade and is thereby isolated from the turbulences and hydrodynamic impulses generated in the application or distribution zone. Because the secondary blade is isolated from such forces and disturbances, and because the primary blade applies a carefully controlled and uniform though potentially imperfect layer of excess coating onto the web, and because the caliper variations in the layer of coating on the web are instable and continuously shift back and forth transversely of the web, the hydrodynamic pressure exerted by the coating medium on the secondary blade is extremely uniform and constant across the entire width of the blade. The secondary blade can therefore exert a constant doctoring pressure or force on the coated web substantially uniformly across the width of the web, thereby to produce an extremely uniform coating lay on the web, free of film split pattern, CD caliper variations and MD streakiness.
In addition, the surface of the final coating on the web exhibits increased smoothness over conventionally applied coatings, and as well, a significant decrease in blade scratches. The decrease in scratches may be attributed to the fact that the primary blade is continuously flushed with the excess coating liquid in the application zone so that any debris in the coating liquid supply is quickly flushed away from the primary blade and does not by-pass the primary blade to interfere with optimum operation of the secondary blade. Thus, use in accordance with the invention of two spaced blades working sequentially on the same coating results in a coating lay that is very smooth surfaced and substantially scratch free.
The dwell time of the relatively quiescent layer of coating liquid on the web, occasioned by the spacing between the primary and secondary blades, is beneficial in that it enables the boundary layer of coating next to the web to become somewhat immobilized, which immobilized coating uniformly supports the tip of the secondary blade so that the final leveling and smoothing of the applied coating takes place where the coating is quite stable, thereby to provide a very uniform coating entirely free of MD streakiness, and exhibiting smoothness and other quality improvements over conventionally applied coatings.
The invention further resides in preferred time intervals between the two blading operations and preferred minimum and maximum rates of delivery of excess coating liquid from the primary blade to the secondary blade to insure proper performance of the final blading operation. The invention also includes various precoating and/or web preconditioning techniques useful in producing extremely high quality coatings at very high production speeds.
The invention thus engenders a further step forward in the art of blade coating, and envisions improved multi-stage wet on wet coating methods.
Other objects and advantages of the invention will become apparent from the following detailed description, considered in conjunction with the accompanying drawings.
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| Number |
Date |
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| 1942351 |
Apr 1971 |
DEX |
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Feb 1987 |
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Oct 1974 |
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| Entry |
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| Pulp & Paper, Apr. 29, 1963, "What's New in Coating", M. R. Castagne, pp. 51-58, esp. pp. 56-58. |
| Paper Trade Journal, Oct. 27, 1969, "Blade Coaters Newest, Fastest Growing Coating Process", G. L. Booth, pp. 58-62. |
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| Tappi, Feb. 1960, vol. 43, No. 2, "The Kohler Method of Coating", J. R. Gunning & J. B. Kohler, pp. 183-187. |
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Continuation in Parts (1)
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Number |
Date |
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| Parent |
375241 |
Jul 1989 |
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Patent Grant
5112653
