Method of and apparatus for coating high speed traveling webs

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.

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.

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.

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.