Abstract:
Applicators for applying a film of coating material on a moving web of paper carried through an application zone include a chamber which receives coating material under pressure and directs the material through a narrow outlet orifice into the application zone, and a doctor blade which extends from a trailing side of the zone in contact with the web. The upper edge of a front wall of the chamber is closely spaced from the web to form, in conjunction with the pressurized coating material flowing from the orifice, a liquid seal at a forward end of the application zone, and seals are established at opposite side ends of the zone to allow development of a positive liquid pressure in the zone. In the various embodiments, rotating rolls located in the chamber, in the orifice, in the space between the upper edge of the front wall and the web and in both the space and the orifice serve to meter, homogenize and control the pressure of coating material in the application zone.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for applying coating material on a moving web of paper, and in particular to coating apparatus of the trailing blade type wherein accurate control over flow of material to and pressure of material in an application zone, and treatment of the material to homogenize the same, is accomplished in novel and improved manners. 
     Conventional coaters of the trailing blade type include means for applying coating material to a paper web that is usually supported and carried by a resilient backing roll, together with a flexible coater or doctor blade located on the trailing side of the applicator, which serves to doctor or level the applied coating. In general, an excess of coating material is applied on the web and the coater blade then meters or removes the excess while uniformly spreading the coating onto the web surface. 
     In recent years it has become desirable to produce papers having a minimum amount of coating. To achieve low coat weights with conventional trailing blade equipment it is necessary to increase the pressure of the coater blade against the web, which results in a high rate of wear of the blade the necessitates more frequent blade replacement. High blade pressure also increases the possibility of web breaks and streaking caused by foreign particles caught between the blade and web. 
     Conventional coaters employ a relatively long dwell or soak time, which is the time interval between initial application and final blading of the coating. As a result, the water portion of the coating composition, as well as some of the water soluble or dispersible 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 web coats without intervening drying, because the successive coats tend to migrate into and contaminate the previous coat. 
     To overcome the disadvantages of prior applicators and to apply lightweight coatings on paper, there has been developed a short dwell time applicator as disclosed in U.S. Pat. No. 4,250,211, issued to Damrau et al. and assigned to the assignee of the present invention. In that applicator, coating material is introduced in excess into a relatively narrow application zone for being applied on a web of paper carried therethrough. A forward wall of the applicator defines a relatively narrow gap with the web at the upstream end of the zone, and excess material in the zone overflows through the gap and forms therein a liquid seal, so that coating material in the zone and as applied to the web is maintained under pressure. The speed of the web is adjusted for a relatively short dwell time, and a flexible coater blade doctors the web at the downstream end of the zone, thereby removing excess material from the web and at the same time uniformly spreading the material on the web. In consequence of the short dwell time of the pressurized application of coating material on the web, an appropriate yet lightweight amount of coating may be applied without need for high blade pressures. 
     The coating material which flows through the gap between the forward wall and the web to form the liquid seal is returned to the supply for recirculation, which prevents stagnation and hardening of the material in the application zone. However, despite recirculation and although new coating material is added to the supply to replace that applied on the web, with the passage of time the viscosity of the material changes and interferes with application of a uniform coating on the web. 
     In addition to the viscosity of the coating material affecting the uniformity of coating applied on the web, also of importance to the quality and quantity of coating is the pressure of the material within the application zone. The pressure in the zone is influenced by the pressure of the material within the chamber, along with the size of the metering slot between the chamber and zone and of the orifice between the front wall and the web. Therefore, the sizes of the metering slot and orifice are adjustable and the pressure of coating material introduced into the chamber is controllable to control the pressure in the zone. However, while the sizes of the orifice and metering slot are not ordinarily subject to to change once adjusted, it often happens that changes occur in the pressure of coating material introduced into the chamber, and slight variations in the pressure cause variations in the pressure in the application zone and affect the coating rate, either by causing too much or too little coating material to be applied on the paper. 
     SUMMARY OF THE INVENTION 
     The short dwell time applicators or coaters of the present invention constitute improvements over the aforesaid prior applicators, in that an enclosed pressure reservoir or coating material application zone is established between the coating applicator, the blade and the supported web, with a liquid seal being formed at the forward end of the zone and rotating rolls being used to meter the amount of coating material flowed into the zone to control the pressure of material therein and to homogenize the material to improve the viscosity thereof. 
     The applicators may be used with a backing roll carrying a web of paper, or a pair of applicators may be arranged on opposite sides of the web so that a web supporting roll is not needed. The applicators include a chamber which receives a supply of coating material under pressure and directs the material through a narrow metering slot into the application zone, and a doctor blade at the trailing end of the zone extending into contact with the web. The upper edge of the chamber front wall is closely spaced from the supported web and doctor blade so as to form, in conjunction with the pressurized liquid flowing from the metering slot, a liquid seal with the web. Means are provided for sealing the sides of the application zone to the web to allow establishment of positive liquid pressure in the zone. The applicator forms an enclosed pressure reservoir of coating material with the web to apply a continuous narrow strip or band of pressurized coating material on the web, which enables application of low coat weights. 
     Excess material overflowing through the forward gap to form a liquid seal is collected for return to the supply of coating material and recirculation. Although recirculation agitates the material to prevent stagnation and hardening of the material in the application zone, and while fresh material is added to the supply to replace that applied on the web, some changes nevertheless occurs in the viscosity and uniformity of the material. At the same time, although attempts are made to maintain as uniform as possible the pressure of coating material introduced into the chamber, slight changes in pressure occasionally occur and affect the pressure of the material in the application zone. 
     To overcome the foregoing disadvantages, in accordance with one embodiment of the invention a rotating pump roll in and extending coextensively with the chamber is sealed on one side with the chamber and rotates in close proximity to a concave chamber wall portion on an opposite side. Coating material introduced at an inlet to the chamber on one side of the roll flows in a narrow passage defined between the roll and the concave wall portion to the application zone. Rotation of the roll homogenizes and improves the viscosity of the material as it moves therepast, and the rate of rotation influences the volume flow of material to and therefore the pressure of material in the application zone. 
     In another embodiment, a rotating roll carried in the front wall of the chamber extends partially into the chamber to define between its exposed surface and the chamber rear wall the metering slot between the chamber and application zone. The roll may be rotated at variable speeds in either direction, thereby to homogenize coating material moving therepast and to control the amount of material introduced into, and therefore the pressure of material in, the application zone. 
     In a further embodiment, a rotating roll extending transversely of the chamber is carried by the upper end of the chamber front wall, and defines between its exposed surface and the web the orifice through which coating material flows to form the liquid seal. Rotation of the roll homogenizes coating material flowing through the orifice, with the rate and direction of rotation influencing the volume flow rate of material out of, and therefore the pressure of material in, the application zone. 
     The invention also contemplates combining the features of the two immediately aforementioned embodiments, thereby to expose the surface of a single rotating roll to coating material in the metering slot, orifice and application zone. The roll is supported at the upper end of the chamber front wall, such that an exposed surface thereof forms one side of the metering slot and one side of the orifice. A member on the chamber rear wall opposite from the roll engages the roll with a surface thereof to hold the roll on the front wall, and a plurality of spaced channels in the surface extend between the chamber and application zone and define metering slots for flow of coating material into the zone. Rotation of the roll thus agitates or homogenizes coating material in the metering slots, application zone and orifice, with the direction and rate of rotation controlling the volume flow rate of material to and therefore the pressure of material in the zone. 
     The foregoing and other advantages and features of the invention will become apparent upon a consideration of the following detailed description, when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a short dwell time applicator for applying a coating to a moving web of paper, wherein in accordance with one embodiment of the invention a rotating roll mounted in a front wall of a coating material supply chamber defines with a rear wall of the chamber a narrow metering slot and homogenizes and meters coating material flowing through the slot into an application zone; 
     FIG. 2 illustrates an applicator, which in accordance with another embodiment has a rotating roll carried by the upper end of the chamber front wall to define with the web an overflow orifice in which a liquid seal is formed and to homogenize and meter coating material overflowing from the application zone; 
     FIG. 3 shows an applicator in accordance with a third embodiment of the invention, wherein a rotating roll carried at the upper end of the chamber front wall defines both the metering slot and the orifice to homogenize and meter coating material both as it flows into and as it flows out of the application zone; 
     FIG. 4 is a cross sectional view taken substantially along the lines 4--4 of FIG. 3; and 
     FIG. 5 illustrates an applicator which has a rotating roll in the chamber thereof for pumping, metering and homogenizing coating material introduced into the application zone. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown an applicator or coater portion of a paper coating machine configured in accordance with the teachings of one embodiment of the invention. The applicator comprises a main beam, indicated generally at 20, extending parallel to and coextensively with a movable support or backing roll 22 which rotates in the direction shown by an arrow 24 and supports a web of paper 26 during its travel through an application zone. The beam has rear and front walls 28 and 30 forming a chamber 32 therebetween for reception of liquid coating material under pressure from a source of material (not shown), and the walls converge upwardly toward one another and define a metering slot 34 which extends upwardly adjacent to and facing the web support surface of the roll 22. Although not shown, the front wall is pivotally mounted relative to the rear wall to permit the chamber 32 to be opened for cleaning and also to adjust the width of the metering slot 34. 
     A flexible doctor or coater blade 36 at the downstream end of the application zone is held against a rearward surface of the wall 28 by a pneumatic tube 38 which is expandable by the introduction of fluid under pressure therein to press against the blade. The coater blade extends beyond the metering slot 34 and is urged into engagement with the web supported on the roll by a pneumatic tube 39 to meter and level the coating applied on the surface of the web. An orifice plate 40 is mounted and vertically adjustable on the front wall 30, and converges toward the roll supported web and the coater blade. The orifice plate has a free edge 42 which is juxtaposed to but spaced slightly from the web, such that an orifice, gap or space 44 between the edge and the web is relatively small and less than one inch. 
     At the two ends of the coater, the spaces between the coater blade 36 and the orifice plate 40 are sealed off in a manner known in the art by flexible edge dams (not shown), which seal with the upper edges of the walls 28 and 30, the blade 36, the orifice plate 40 and the roll supported web 26, thereby to define a coating material application zone 46 downstream from the chamber 32 and the metering slot 34. 
     In operation of the applicator thus far described, coating liquid is introduced into the chamber 32 under sufficient pressure and in sufficient quantity to completely fill the chamber, the metering slot 34 and the application zone 46 defined by the doctor blade 36, the orifice plate 40 and the end dams to cause a continuous, copious flow of coating material reversely of the direction of web travel through the narrow orifice 44. This forms a liquid seal at the forward end of the application zone and causes the coating liquid to be applied to the web in a very narrow transverse band under a constant positive pressure. The copious excess of coating liquid that flows through the orifice reversely of the direction of web travel forms a non-abrasive liquid seal with the web at the upstream or forward edge of the coating application zone; causes the coating liquid in the zone to be maintained under pressure and to be applied to the web under pressure; seals off the forward edge of the zone against entry of air and foreign matter; strips air from the high speed web and prevents such air from causing streaks or skips in the coating on the web; and causes the downstream coater blade 36 to doctor the coating liquid while the liquid is held under pressure. 
     The applicator is generally referred to as a short dwell time applicator. That is, to avoid saturation of the web with coating material, thereby to prevent the water portion of the coating composition as well as the water solution or dispersible materials contained therein from migrating into the web at a more rapid rate than the pigment, the web is exposed to the coating material in the application zone for only a relatively short time. To this end, the width of the application zone in the direction of web travel, as well as the speed of travel of the web through the zone, are controlled to provide a relatively short dwell time of the web within the zone. 
     The coating applicator thus far described is of the type disclosed in detail in aforementioned U.S. Pat. No. 4,250,211, assigned to the assignee of the present invention, and the teachings of which are specifically incorporated herein by reference. For a more specific description of the applicator, reference is made to said patent. 
     The sizes of the metering slot 34 and of the orifice 44 are critical to operation of the coating applicator, since in combination with the pressure of coating material introduced into the chamber 32 they strongly influence the pressure of the material within the application zone 46, and therefore the quality and quantity of coating applied onto the paper web. As previously described, the wall 30 is movable toward and away from the wall 28 to control the size of the metering slot, and the orifice plate 40 is vertically adjustable on the wall 30 to control the size of the orifice. The orifice is ordinarily made to be relatively small, so that coating material flowing therethrough from the application zone forms a fluid seal for the zone, whereby material in the zone is maintained under pressure for application on the web. The flow of material through the orifice prevents stagnation and hardening of the material in the application zone, and is returned to the supply for recirculation. 
     Once adjusted, the sizes of the metering slot 34 and orifice 44 ordinarily remain constant. However, it is difficult to maintain a constant pressure of the coating material delivered to the chamber 32, and even slight variations in the pressure affect the coating rate, either by causing too much or too little material to be applied to the paper. Accordingly, the embodiment of invention shown in FIG. 1 contemplates a metering roll 48 rotatably mounted in an opening formed in the upper end of the wall 30, such that a portion of a surface 50 of the roll extends inwardly of the wall. The roll, which may be on the order of 1/2 inch to 11/2 inches in diameter, extends parallel to and coextensively with the chamber, and with the rear wall 28 defines the metering slot 34. Although not shown, a conventional motor means rotates the roll at a controlled rate of from about 30-300 rpm in either direction to assist or impede a flow of coating material through the metering slot. That is, since the surface 50 of the roll is exposed to the flow of coating material in the metering slot, rotation of the roll in a counterclockwise direction assists, and in the clockwise direction impedes, the flow of material through the slot, with the rate of rotation determining the degree to which the flow is assisted or impeded. 
     Consequently, the roll provides an additional measure of control over the rate at which coating material is delivered to, and therefore the pressure of material within, the application zone 46. At the same time, the rotating surface 50 of the roll works or shears the coating material immediately prior to its introduction into the application zone, which homogenizes the material and improves its viscosity so that improvements in the quality of coating applied on the web are obtained. 
     In the embodiment of applicator shown in FIG. 2, a modified orifice plate 52 is vertically adjustable on the front wall 30, and rotatably carries a metering roll 54 in an opening in an upper end thereof. The roll extends parallel to and coextensively with the application zone 46, and a portion of a surface 56 of the roll extends from the upper surface of the orifice plate and is exposed to coating material in the application zone 46 and to material flowing from the zone through a relatively narrow orifice 58 defined between the roll and the paper web. A liquid seal is formed in the orifice, and although not shown, a conventional motor means rotates the rod in either direction at a controlled rate of rotation. 
     Coating material in the application zone 46 and orifice 58 is controlled by the metering roll 54 to provide a uniform flow of material across the width of the paper web. It has been found that the roll provides a smooth and controlled return flow of coating material, and that rotation of the roll agitates and homogenizes the material to improve its viscosity and assists in preventing air from entering the application zone. Also, the direction and rate of rotation of the roll may be controlled to either assist or impede a flow of coating material from the zone through the orifice, thereby to provide a measure of control over the pressure of material in the zone, with rotation of the rod in a clockwise direction impeding a flow to increase the pressure and in a counterclockwise direction assisting a flow to decrease the pressure. 
     The embodiment of applicator in FIGS. 3 and 4 combines the features of the above two embodiments, and includes a rotatable metering cylinder or roll 60 having a surface 62 which is exposed both to the inlet and outlet sides of the application zone 46, as well as to the zone itself. The roll is rotatably supported in a concave holder 64 at the upper end of the front wall 30, and at the outlet from the zone the surface 62 forms with the web a narrow orifice 66 in which a liquid seal is formed. The cylinder is maintained in the holder by a curved surface 68 of a member 70 mounted on the wall 28, and the surface is provided with spaced channels 72 which extend between the chamber 32 and the application zone 46, are approximately 1/4 inch wide and provide about a 50 percent open area through the member. The channels define metering slots between the chamber and the application zone, and rotation of the roll in either direction at a controlled rate by any suitable means homogenizes and shears the coating material in the metering slots, orifice and zone, thereby to improve the viscosity of the material and the uniformity of its application onto the web. 
     The metering roll 60 also provides two additional advantages. In a first instance, the roll itself defines, with the paper web, the outlet orifice from the application zone, thus controlling overflow from and providing a fluid seal to the zone. It has been found that use of a rotating cylinder for an outlet orifice gives a particularly smooth and controlled character to the excess coating material returned to the main supply. A second advantage resides in the fact that the speed and direction of rotation of the roll affords a degree of control over the rate of delivery of coating material from the chamber 32 to the application zone, and from the zone through the orifice 66, and therefore over the pressure of material in the zone. Rotation of the cylinder in a counterclockwise direction assists a flow of material into and from the zone, and in the counterclockwise direction impedes the flow, with the rate of rotation determining the degree to which the flow is assisted or impeded. 
     In the applicator illustrates in FIG. 5, a pump or metering roll 76 is rotatably mounted in the chamber 32 intermediate a material inlet 78 to the chamber and the outlet therefrom. The roll extends parallel to and coextensively with the chamber, and is rotatable at a controlled rate in the direction shown by any suitable means. The roll is sealed at its upper end to one side of the chamber by a pair of seals 80 and is slightly spaced from a concave surface 82 of the wall 28 to define therewith a narrow passage 84 between the inlet to and the outlet from the chamber. 
     The arrangement is such that the roll, upon rotation in the direction shown, pumps coating material from the chamber inlet to the outlet, and works and shears the material to homogenize it immediately prior to its introduction into the application zone. The roll supplies a uniform, mixed and metered supply of coating material into and across the entire width of the application zone. As a result, there are no nonuniformities in the supply of material to the zone, which otherwise may occur with conventional applicators wherein a manifold is provided in the chamber in an attempt to evenly distribute coating material introduced therein. The flow rate of material into the application zone is controlled by the clearance between the roll and the wall surface 82, as well as by the rotational rate of the roll and the characteristics of the coating material. 
     The invention thus provides improved means for treating or homogenizing coating material for a paper coater and for controlling the rate of flow of material into and out of an application zone, thereby to control the pressure of material in the zone for very accurate and uniform pressure application of coating material on a web. Although not specifically illustrated, it is understood that in use of the various embodiments of invention a pressure transducer would be positioned in the application zone to sense the pressure of coating material therein, with the output from the transducer being used to control the motor means for rotating the rolls, thereby to control the direction and speed of rotation of the rolls in a manner to maintain the pressure at a selected and constant value. 
     While embodiments of the invention have been described in detail, various modifications and other embodiments thereof may be devised by one skilled in the art without departing from the spirit and scope of the invention, as defined in the appended claims.