Patent Publication Number: US-5152872-A

Title: Apparatus for the wet end coating of paper

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to apparatuses for coating paper and, more particularly, to such an apparatus for coating the web at the wet end of the papermachine. 
     2. Description of the Prior Art 
     Originally, practically all paper coating was carried out off-machine, that is as a completely separate operation which is not effected on the papermachine as opposed to on-machine coating which constitutes a step executed directly on the paper-machine. Such off-machine paper coating apparatuses obviously resulted in a costly operation requiring expensive equipment. 
     Later, on-machine coating proved feasible although it was performed on the web after the web had entered the dryer section and had been partially dried thereby. Probably the least costly on-machine coating method was based on size press coating performed on a papermachine having a size press located well within the dryer section. However, using such a size press for coating meant that water was being added to the web after most of the water had already been driven off by the dryer itself. Furthermore, the press itself limited the range of coating weights and materials. While a few wet end coating methods have been proposed, none have found commercial success as they did not perform well or were to costly. 
     Moreover, coating was generally reserved for paper products that could command a fairly high market price. Newsprint was not such a product. In addition, public demand that more recycled fibers be used in the manufacture of newsprint increased. In response, the publishers recently passed this demand onto the newsprint producers. Unfortunately, the use of recycled fibers tends to lead to increased linting. An inexpensive newsprint coating method would not only solve the linting problem but would also improve various paper properties as well as the overall quality of the printing. One such property would reside in opacity which in turn would allow for the use of less expensive fiber furnishes. 
     SUMMARY OF THE INVENTION 
     It is therefore an aim of the present invention to provide an apparatus for wet end on-machine paper coating. 
     It is also an aim of the present invention to provide such a coating apparatus adapted to apply to the surface of the web a wide variety of substances. 
     A construction in accordance with the present invention comprises an apparatus on a papermachine for applying a substance to a wet paper web. The apparatus comprises a first web conveying porous fabric means on which the web is deposited. The first fabric means passes around a first couch roll means provided with a first suction box means. A first applicator means is located on a first side of the web opposite a second side thereof contacting the first fabric means for applying the substance to the web. The first suction box means is located relative to the first applicator means in order to draw the substance at least onto the web. Therefore, the web emerges from the apparatus with at least some of the substance that was applied thereto. 
     A method in accordance with the present invention for applying a substance on a wet paper web on a papermachine comprises the following steps: 
     a) depositing the web on a first web conveying porous fabric means which passes around a first couch roll means provided with a first suction box means; 
     b) conveying the web between the first couch roll means and a first applicator means located on a first side of the web opposite a second side thereof contacting the first fabric means; 
     c) applying on the web the substance by way of the first applicator means with the first suction box means drawing the substance at least onto the web; and 
     d) conveying the web away from the first couch roll means with the web retaining at least some of the substance that was applied thereto. 
     While a primary object of the present invention resides in applying somewhat conventional coating colors to the wet web during the stages when liquid water can still be removed from the web by non-evaporative means, the present invention is not limited to the coating or to the application of coating color to the web. Indeed, the nature of the invention allows for the coating and/or application to the surface of the web of a wide variety of substances, which are drawn onto and into the web by suction and/or pressing. For instance, bonding agents, bleaching agents and various other substances that can enhance the various properties of the paper or paperboard may be applied by way of the present invention to the web, whereby it is considered that the present invention is very versatile. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying single drawing showing by way of illustration a preferred embodiment thereof and, more particularly, a cross-sectional side elevation illustrating an apparatus in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawing, a web generally indicated by W is fed throughout an apparatus A embodying the present invention. Throughout the apparatus A, the web W evolves from a first state to a second state and finally to a third state respectively indicated by the numerals 10, 12 and 14. 
     The first web 10 is the result of conventional methods of and apparatuses for depositing a suspension of fibers and water which are well known and thus need not to be herein described in details. Generally, in the older papermachines, the suspension is deposited on a single, porous endless fabric. In more recent papermachines, the suspension is deposited on and between two such porous fabrics with one of them separating from the other once the web has stabilized and carrying the formed wet web around a part of the periphery of the couch roll. This latter method is commonly referred to as the &#34;twin-wire&#34; method for the forming of paper. 
     Referring to the enclosed drawing, the first web section 10 represents a formed wet web resulting from such a twin-wire forming of paper and being carried by a single or final porous endless fabric 16, which will be hereinafter referred to as the first endless fabric 16, towards the apparatus A along the direction shown by arrow 18. 
     The first endless fabric 16 passes around a first couch roll 20 and a first return roll 22 along the direction shown by arrow 24 as the first couch and return rolls 20 and 22 rotate in the directions shown respectively by arrows 26 and 28. (The first return roll 22 can also be in some machine a fabric drive roll or turning roll. Complementary guide, wash, stretch, tension and/or breast rolls are not shown in the drawing to avoid complicating the illustration.) 
     The first couch roll 20 includes a cylindrical wall 30 defining a series of apertures 32 and a first suction box 34 disposed between the cylindrical wall 30 and a shaft 36 of the first couch roll 20. 
     The first web section 10 and the first endless fabric 16 pass between the first suction box 34 and a first coating applicator which is shown in the present embodiment as a coating roll 38 resting on the top side of the wet first web section 10 at a location with respect to the first suction box 34 that will ensure that, for a particular coating color, the right amount of color will be filtered out by the first web section 10 as it is drawn onto and through the web by and into the first suction box 34 because of the suction created thereby. Beside transferring the coating color to the first web section 10, the first coating applicator 38 can also serve to press down on the web and decrease its water content following the first suction box 34 drawing away any excess water pressed out by the first coating roll 38. 
     First coating metering and distributing rolls 40 and 42 receive, meter and distribute the coating color to the first coating roll 38. Where desired, the first metering and distributing rolls 40 and 42 can be replaced by other types of rolls, such as pick-up rolls and associated color pan and any other conventional device or method to meter and distribute the coating color to the first coating roll 38 (which is also commonly referred to as the &#34;coating head&#34;). 
     Where heavier colors are to be used, various types of gate rolls, blade and brush coaters, air doctors, print (engraven) roll coaters, cast waters, etc. can be selected to apply, meter and distribute the colors directly to the surface of the wet web. For example, depending on the nature of the web and the papermachine, a different type of coater could be used for each of the two locations such as a roll applicator in one location and a blade or roll coater in the other. High speed machines may require the use of such coaters and, in particular, the print roll coater has been found to be effective. 
     Therefore, the coating color is supplied between the first metering and distributing rolls 40 and 42 as indicated by arrow 44, with the first metering and distributing rolls 40 and 42 rotating in the directions shown respectively by arrows 46 and 48, whereas the first coating roll 38 rotates along arrow 50. From the above, it is easily understood that the coating color will be applied on the top surface of the first web section 10 by the first coating roll 38. 
     In the case where it is desired to re-use any coating color that is recovered by the first suction box 34, a first coating color recovery system 52 is provided. The first recovery system 52 includes a first make-up tank 54, a first make-up supply (not shown) and a first return pump 56 which function as follows. The excess of coating color is recovered by the first suction box 34 and is transported therefrom to the first make-up tank 54 as indicated by arrow 58. A system of pipes (not shown) could be used to convey the excess coating color from the first suction box 34 to the first make-up tank 54. In order that the level of the coating color in the first make-up tank 54 remains constant, a supply of fresh coating color is supplied to the first make-up tank as indicated by arrow 60. The first return pump 56 can thus convey the coating color contained in the first make-up tank 54 to the first metering and distributing rolls 40 and 42 as indicated by arrow 62. 
     Alternatively, the coating color recovered by the first suction box 34 can be discarded and completely fresh coating color may be supplied to the first metering and distributing rolls 40 and 42. In fact, the first recovery system 52 may consist of several associated systems depending on the grade of paper or paper board, the coating color and the applicator. For example, where the white water volume or the percentage of fibers in the coating color recovered in the first suction box 34 is large, it may be necessary to set up a separate associated system to separate as much color as possible from the fibers in order that the recovered coating color can be made-up and returned to the first metering and distributing rolls 40 and 42. Similarly, the separated recovered fibers can be returned to the conventional white water and fiber recovery system of the papermachine. Prior art systems for such recycling are well known or can be easily developed. The system which will be used depends on many factors with the type and cost of the coating color being such prime factors. 
     Once the web W has passed the first suction box 34, it enters its second state and is then referred to as the second web section 12. The second web section 12 is picked-up at its top side by a second endless porous fabric 64 which passes between a pick-up roll 66 which rotates in the direction shown by arrow 68 and the web W. The pick-up roll 66 allows for the transfer of the top side coated wet web section 12 to the second endless fabric 64 so as to allow the subsequent coating of the underside of the web W as described hereinafter. The second endless fabric 64 further passes around a second couch roll 70 and a pair of second return or guide rolls 72 which rotate in the directions shown respectively by arrows 74 and 76. 
     The second couch roll 70 is similar to the first couch roll 20 and includes a cylindrical wall 78 defining a plurality of apertures 80 with a second suction box 82 being disposed within the second couch roll 70 between the cylindrical wall 78 and a shaft 84 thereof. 
     The second web section 12 is conveyed by the second endless fabric 64 as indicated by arrow 86 and passes between the second couch roll 70 and a second coating applicator which comprises a second coating roll 88 rotating in the direction shown by arrow 90. The second coating roll 88 is in contact with second metering and distributing rolls 92 and 94 which rotate in the directions shown respectively by arrows 96 and 98. 
     A second coating color recovery system 100 comprises a second make-up tank 102, a make-up supply (not shown) and a second return pump 104. It is noted that the second couch roll 70, the second suction box 82, the second coating roll 88, the second metering and distributing rolls 92 and 94 and the second recovery system 100 are respectively similar to the first couch roll 20, the first suction box 34, the first coating roll 38, the first metering and distributing rolls 40 and 42 and the first recovery system 52. 
     Again, coating color is supplied to the second metering and distributing rolls 92 and 94 along arrow 106 by way of the second return pump 104, with the coating color recovered by the second suction box 82 being conveyed along arrow 108 into the second make-up tank 102 which has a constant level due to the coating color make-up being further supplied therein from a remote source along arrow 110. 
     After the wet web W has been coated on both the top and bottom sides thereof, it is transferred on a third endless porous fabric 112 which displaces along arrow 114 and passes around a pick-up roll 116 which rotates in the direction shown by arrow 118. The third web section 14 thus displaces on the third endless fabric 112 along the direction shown by arrow 120. 
     The third web section 14 is then transferred to the press section by a further pick-up roll and a press felt, all of which are not shown. Following the press section, the coated web section 14 is conveyed through the dryer and the remaining sections of the papermachine. 
     It is noted that various devices for cleaning the surface of the coating rolls, prior to the application of the color, such as doctor blades, are not shown in the enclosed drawing. 
     The grade of paper to be manufactured and the type of papermachine being used will dictate to a large extent the type of coating applicator (i.e. coating rolls) and coating color that will be used. As mentioned hereinabove, for very high speed machines, it may be necessary to use some type of blade coaters. While there are numerous coating materials to choose from when coating a wet web of newsprint, the least expensive ones would be normally more desirable, e.g. clay, starch, calcium carbonate, and, if necessary, inexpensive adhesives, additives and dispersing agents would also be used. The type of coating color used, the paper being manufactured and its basis weight are some of the factors which could determine the location of the coating applicator with respect to the suction box in order that the right amount of coating color would be drawn on and into the web. Where necessary, the degree of vacuum in the suction box could also be controlled. 
     While the coating substance will generally be in a liquid form or state or in suspension in a liquid (e.g. a slurry), methods are known for applying substantially dry substances to a wet web, on the wire section before the first couch roll. One such method and coating apparatus involves an electrostatic technique. Where the wet web still has a relatively high water content such methods can be used in the present invention, especially at the first couch roll. In certain cases, some of the substance may even be drawn into the first suction box and recuperated in the white water. The material recovered may be used in suspension in a further application at the first or second couch roll or it may be dried and reused in the dry state. 
     Since most first fabrics are very porous, there would normally not be any need to change that fabric. On the other hand, where a pick-up felt was already in use on an existing machine, it would have to be changed to a more porous fabric to serve as the second fabric or a space should be provided to install the second fabric and the couch roll. 
     As mentioned hereinbefore, the versatility of the present invention allows one to apply any one or more of a large number of substances to the web for conventional coating and/or for other purposes. For example, bonding agents could be applied to give greater strength to the paper, bleaching agents could be applied to whiten the paper, various types of latexes, special chemical products (e.g. Aquapel, Aquamol, PVC, etc.) could be applied for known purposes. 
     While newsprint is highlighted above as one of the paper products suited for this invention, numerous other grades or products manufactured on papermachines can benefit from the present invention such as various specialty papers and paperboards, such as linerboards, etc. 
     Beside the energy saved due to the fact that it is not necessary in the present invention to evaporate the water before coating the web as required by other coating methods, that is those performed after water has been extracted as a liquid from the web (such as size press coating), other advantages accruing from the present invention will be obvious such as greater accessability to the coating apparatus, absence of the rejection phenomena, ease of addition to existing machines, low cost, etc.