Abstract:
In a method of application of a coating medium by use of a spray device onto a moving surface, which in the direct application method is the surface of a material web, specifically a paper or cardboard web, an atmosphere of back-moistening and/or moistening medium for the atomized coating medium is maintained in the area of the spray device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the invention.  
           [0002]    The present invention relates to the application of a coating medium by use of a spray device onto a moving surface, whereby in the direct application method the moving surface is the surface of a material web, specifically a paper or cardboard web, and in the indirect application method is the surface of a transfer element from which the coating medium is transferred to the material web.  
           [0003]    2. Description of the Related Art.  
           [0004]    Devices for application of a coating medium by use of a spray device onto a moving surface are already known from EP 0 670 004 B1, WO 94/11116 and EP 0 435 904 B1. Generally, the objective of this type of spray applicator device is to apply the coating medium with as little excess as possible, and in the best case scenario even with no excess at all (1:1 application) to the moving surface. In the application methods described in the above referenced publications, the coating medium is atomized by use of air and is applied to the moving surface. After the coating medium has settled there, the carrier air is suction removed from the application area through channels surrounding the spray apparatus. Even with very low application base weights, very large volumes of air must be supplied to and suction removed from the moving surface in a given time period. It has been demonstrated in practice that, from a quality point of view, only an insufficiently satisfactory coating result can be achieved with the previously known coating methods. Despite subsequent smoothing of the applied layer, the layer still displays an uneven structure as well as a rough, sandpaper-like surface.  
           [0005]    Faced with these challenges, the current invention provides a spray coating method with which coating layers having more uniform and smooth surfaces may be achieved.  
           [0006]    In the area of the spray device, an atmosphere of a back moistening and/or moistening medium or atomized coating medium is maintained. The back moistening and/or moistening medium may be the vapor of a liquid which may also act as a carrier liquid for the coating medium which, prior to spraying, is liquid or viscous, for example steam, toluol vapor, etc. In the following explanation of the current invention, only water vapor, that is, steam, will be referred to as a back moistening and/or moistening medium for reasons of simplifying the discussion. However, this does not imply that the scope of protection for the method according to the invention is limited to the usage of water vapor.  
           [0007]    The maintenance of a steam atmosphere in the area of the spray device is advantageous in several respects for achieving a high quality coating result. On the other hand, the atomized coating medium moves in an atmosphere from the spray device to the moving surface which prevents excessive escape of moisture from the coating medium droplets and/or particles, thereby preventing drying out at least the surface of these droplets and/or particles. Because of this, the droplets and/or particles maintain their liquidity and are more able to flow after coming into contact with the moving surface, which contributes to a smoother surface of the applied coating layer.  
           [0008]    On the other hand, it is not necessary to suck the steam atmosphere from the application area. It is merely necessary to compensate for steam losses which are caused possibly by precipitation of the steam as moisture onto the moving surface or onto the coating layer, by supplying fresh steam. This will largely prevent turbulence in the application area, such as is caused in the state of the art equipment by air supply and by air suction removal and will have a positive effect upon the uniformity of the layer application.  
           [0009]    The previously addressed precipitation of steam as moisture onto the coating layer has no negative effects, since the water in the coating layer is present anyway in the form of carrier liquids for the coating medium. On the contrary, the precipitated moisture improves the flowability characteristics of the coating medium in the applied layer, resulting in a smoother surface. The same also applies to all other carrier liquids commonly used in coating mediums, and their vapor, for example toluol and toluol vapor.  
           [0010]    It must be emphasized that, in the previously referred to sense, the term “drying” is to be understood as the escape of carrier liquid from the coating medium in the form of vapor, and the term “back moistening” is to be understood as the supply of carrier liquid from the steam atmosphere into the coating medium droplets and/or particles. In the best case scenario, the nature of the steam atmosphere in the area of the spray device provides a balance between drying and back moistening for the atomized coating medium.  
           [0011]    With the method of the current invention, powdery coating mediums can be converted. The steam in this instance condensates on the coating medium particles while they travel from the spray device to the moving surface, thereby moistening them, so that after making contact with the moving surface they are able to flow and produce a smooth coating layer.  
           [0012]    In a further development of the invention, the spray device is located in a chamber into which the back moistening and/or moistening medium is fed by a supply device provided for this purpose.  
           [0013]    The spray device may, for example, take the form of a single substance spray device. It is, however, also possible to utilize a two-substance spray device within the scope of the method according to the invention, which sprays the coating medium by use of steam. The steam used for atomization may, for example, alone serve in the formation of the steam atmosphere in the area of the spray device. It is, however, also possible to supply additional steam to the area of the spray device.  
           [0014]    Depending upon the boundary conditions of each individual application, for example the consistency and structure of the coating medium that is to be sprayed, two-substance spray devices working according to the internal mixing principle, as well as the external mixing principle, may be utilized.  
           [0015]    A rotary spray device may also be utilized whereby the coating medium is sprayed by a high speed rotary spray element by the effect of centrifugal forces acting upon it. The coating medium sprayed in this manner can be supplied to the moving surface, for example, by use of the steam. A ring slot-type steam outlet opening may, for example, be provided around the rotary spray element, from which the steam is emitted in the shape of a cone of a predetermined cone angle. A ring-shaped arrangement of several individual steam outlet openings may also be used.  
           [0016]    In addition, or as an alternative, electrostatic forces may also be utilized in order to bring the atomized coating medium to the moving surface. For example, the rotary spray device may be connected to a predetermined electrical potential, so that the atomized coating medium droplets and/or particles leave the spray device electrically charge. The moving surface, or an element assigned to the moving surface, can be connected to ground potential so that an attractive force is exerted upon the charged droplets and/or particles.  
           [0017]    Finally, a device may be provided prior to the area of application, viewed in direction of travel of the moving surface, which, by utilizing steam, removes an air boundary layer from the moving surface which carries it. This facilitates the coverage of the entire surface of the moving web/surface with coating medium, which in turn has a positive effect on the coverage quality of the applied coating layer.  
           [0018]    If steam is supplied to the moving surface prior to the spray device when viewed in direction of travel, and is then suction removed after the spray device when viewed in direction of travel, a movement component may be imparted to the atomized coating medium which would favor its deposit on the moving surface in a way maintaining a smooth coating layer. The steam volumes required for this, compared to the previously discussed unfavorable air volumes, are considerably less. Thus, no deterioration in the uniformity of the coating result occurs.  
           [0019]    Another aspect of the invention relates to a device for the application of a coating medium by use or a spray device onto a moving surface. Regarding the constructive details, design variations, and the advantages that are achievable with this device, reference can be made to the previous discussion regarding the coating method in accordance with the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of one invention taken in conjunction with the accompanying drawings, wherein:  
         [0021]    [0021]FIG. 1 is a schematic side view of one embodiment of an applicator device in accordance with the invention;  
         [0022]    [0022]FIG. 2 is a schematic side view of another embodiment of an applicator device in accordance with the invention;  
         [0023]    [0023]FIG. 3 is a schematic side view of yet another embodiment of an applicator device in accordance with the invention; and  
         [0024]    [0024]FIG. 4 is a schematic side view of a further embodiment of an applicator device in accordance with the invention. 
     
    
       [0025]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invenion, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    Referring now to the drawings and particularly to FIG. 1, there is shown an applicator device in accordance with the invention, generally identified with  10 . The applicator device  10  serves to apply a coating medium  12  to the surface  14   a  of a material web  14  which, in the area of the applicator unit  10  is led around a support roll  16 . As a result of rotation of the support roll  16 , the material web  14  travels past the applicator device  10 , in the direction of arrow L. The applicator device  10  includes a spray nozzle  18  which releases the coating medium  12  in atomized form, that is, in the form of coating medium droplets and/or particles  12   a,  to the material web  14 . The droplets and/or particles  12   a  making contact with the material web surface  14   a,  flow and form a coating layer  20  on the material web surface  14   a.    
         [0027]    In the embodiment of the invention according to FIG. 1, the spray nozzle  18  is located in a chamber  22 , which is enclosed by a housing  24 . The housing  24  is open toward the support roll  16  and is located from it at a small distance such that the material web  14  enters the area B of chamber  22  without making contact and also exits this area B complete with coating layer  20  without making contact. However, the distance is small enough that it is ensured at the same time that the atmosphere in chamber  22  cannot escape from this chamber  22 .  
         [0028]    Steam  28  is supplied to the chamber  22  through a supply line  26 , as is shown in FIG. 1 by arrow Z. This steam  28  creates a moist atmosphere in chamber  22  which prevents drying out of the droplets and/or particles  12   a  on their journey from the spray nozzle  18  to the material web  14 . This maintains a low viscosity of the coating medium  12 , which is desirable for the formation of the coating layer  20 , so that the droplets and/or particles  12   a  flow well after making contact with the material web surface  14   a,  thereby enabling them to form a coating layer  20  with a smooth surface. Beyond that, a part of the steam  28  precipitates on the material web  14  or the coating layer  20 , thereby further aiding the flowability of the coating medium  12 . This additional moistening of the material web  14  or the coating layer  20  can easily be tolerated, since material web  14  and/or coating layer  20  must be dried in any case following the coating process in the applicator device  10 . The drying is necessary due to residual water that is in the material web  14  from the manufacturing process and/or the water serving as carrier liquid for the coating medium  12 .  
         [0029]    An amount of steam  28  must be supplied to the chamber  22  through supply line  26  which is consistent with that which has escaped from the chamber  22 , due to condensation on the material web  14  and/or the coating layer  20  and due to the carry-along effects on the surface of the coating layer  20 . The steam atmosphere prevailing in chamber  22  is therefore relatively stable. It specifically does not affect the spray jet of coating medium  12  emitted from spray nozzle  18  through turbulence, so that, moreover, the coating layer  20  is relatively uniform. In addition, the coating medium  12  can be applied by the spray nozzle  18  without, or substanially without, excess to the material web  14  (1:1 application). Thus, only leveling, not metering, of the coating layer  20  is necessary following the applicator device  10 , viewed in flow direction L.  
         [0030]    The coating medium  12  is supplied to the spray nozzle  18  through a supply line which connects to the longitudinal ends of scray nozzle  18 , which are located in front or at the back of the plane of projection, viewed in cross direction Q of tle arrangement. In addition, the housing  24  is equipped with an outlet line  30  at its lower end through which coating medium  12  which inadvertently does not reach the material web  14  and which has deposited itself on the walls of the housing  24 , can be discharged from the chamber  22  (direction of arrow A).  
         [0031]    Spray nozzle  19  is a single substance spray nozzle which atomizes the coating medium, for example through ultra sound.  
         [0032]    [0032]FIG. 2 illustrates another variation of the applicator device, according to the invention which, particularly in its operation, is consistent with the design form in FIG. 1. Therefore, the same components in FIG. 2 are identified with the same reference numbers as in FIG. 1, but increased by 100. Further, the embodiment according to FIG. 2 will be described only in as far as it differs from the embodiment illustraed in FIG. 1, and to the description of which we expressly refer.  
         [0033]    The major difference of the applicator device  110  according to FIG. 2 compared to the applicator device  10  in FIG. 1 is that instead of a single substance spray nozzle  18 , a two-substance spray nozzle  118  is utilized for spraying the coating medium  112 . The coating medium  112  is ejected at an outlet opening  118   a  of the spray nozzle  118  where it is captured by the water steam  128  which is supplied through supply line  126 , atomized and guided to the surface  114   a  of the material web  14  which is guided around roll  116 , where it precipitates in the form of coating layer  120 .  
         [0034]    Maintenance of the moisture in the atomized coating medium droplets or particles  112   a  is ensured by the atomizing medium that is, the steam  128 , without the spray nozzle  118  being enclosed by a steam chest similar to housing  24  in FIG. 1. However, this does not preclude such a steam chest additionally being provided for the applicator device  110  illustrated in FIG. 2, especially in order to achieve a certain moisture level of material web  114  and the coating layer  120 .  
         [0035]    As a further measure in improving the coating result in the arrangement illustrated in FIG. 2, specifically when covering the material web  114  with the coating medium  112 , a steam blower  132  is provided prior to the spray nozzle  118 , viewed in flow direction L. Steam blower  132  is set at an acute angle against the surface  114   a  of material web  114 , in opposite direction to the direction of rotation L of roll  116 , similar to how a blade would be. The steam blower  132  blasts a steam jet  134 , which “scrapes off” an air boundary layer  136  from the material web surface  114   a  which otherwise may impede coating of the material web  114  with the coating medium  112 . Naturally, such a steam blower  132  may also be provided in the arrangement according to FIG. 1.  
         [0036]    Although according to FIG. 2 the spray nozzle  18  illustrated in a rough schematic as a two-substance spray nozzle operating according to the external mixlng principle, two-substance spray nozzles operating according to the internal mixing principle may also be utilized according to the invention, as shown by spray nozzle  138  (FIG. 4).  
         [0037]    [0037]FIG. 3 illustrates a further embodiment of the applicator according to the invention, which essentially is consistent with the embodiment in FIG. 1. Therefore, the same reference numbers are used in FIG. 3 for the same components as in FIG. 2, but are increased by an increment of 100, that is, compared with FIG. 2, increased by 200. Also, the embodiment according to FIG. 3 will only be described in as far as it differs from the embodiment illustrated in FIG. 2, the description of which we otherwise expressly refer to.  
         [0038]    In the applicator device  210  illustrated in FIG. 3 a rotary spray nozzle  218  is utilized for coating of surface  214   a  of the material web  214  which is guided around the roll  216 , instead of the two-substance spray nozzle which operates with steam atomization. The rotary spray nozzle  218  includes a spray disk  240  which is driven by motor  242  and rotates at high speed. The rotary spray element  240  is supplied via a supply line  244  with coating medium  212  which is being thrown radially toward the outside due to centrifugal forces, thereby being atomized. After the coating medium droplets  212   a  have left the spray element  240 , they are picked up by a steam jet which is emitted in ring form around the spray element  240  and are diverted to the material web surface  214   a  where they deposit themselves in the coating layer  220 .  
         [0039]    Diversion of the radially thrown droplets and/or particles  212   a  may also be achieved with the assistance of electrostatic forces. In FIG. 3 this is suggested in that the spray nozzle  218  is connected with a high voltage source via a connection  246 , while the roll  216  is connected to ground potential. This enables the droplets and/or particles  212   a  to be electrically charged during spraying and to be drawn to the material web surface  214   a  by the potential differential between nozzle  218  and roll  216 .  
         [0040]    Regarding the avoidance of drying out of the droplets  212   a  ard/or their back moistening, reference is made to the relevant explanations, in their entirety, relating to the arrangement illustrated in FIG. 2.  
         [0041]    Furthermore, the spray nozzle  218  in FIG. 3 may also be located in a steam box, similarly to FIG. 1. Finally, it is also feasible to remove the air boundary layer that is carried along by the material web  214  from the material web surface  214   a,  prior to the rotary spray nozzle  218 , when viewed in flow direction L.  
         [0042]    The fact that the coating medium according to the invention is supplied to the material web in a back-moistening and/or moistening atmosphere, thereby maintaining its flowablity characteristics, has the further advantage of reducing the risk of caking of coating medium at the nozzle opening and the chamber walls. The device of the current invention, therefore, may be operated for long periods between subsequent maintenances and the down times required to carry out such maintenance are short.  
         [0043]    With the coaing method and/or the applicator device of the current invention, solid, powdery coating mediums can also be converted. It is the function of the steam to supply sufficient moisture to the atomized powder granules and the material web surface that the coating medium can flow to the desired extent after making contact with the material web.  
         [0044]    Although the coating medium is applied directly to the material web in all three previously discussed design forms, it is also feasible for the coating medium to be applied to the surface of a transfer roll which then transfers the coating layer to the material web.  
         [0045]    In addition to water, other mediums, for example toluol, are also suitable for the creation of a back-moistening and/or moistening atmosphere.  
         [0046]    While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which all within the limits of the appended claims.