Abstract:
A method and apparatus for drying a coated web is taught that reduces dryer induced mottle. The apparatus comprises a plurality of ducts positioned in series within an enclosure, the coated web travelling through the enclosure in a direction of travel, each duct including an arcuate portion terminating in a discharge nozzle, each discharge nozzle directed such that air exiting therefrom is flowing generally parallel to the coated web and in the direction of travel of the web; and at least one of the plurality of ducts having a baffle plate extending back therefrom toward another one of the plurality of ducts upstream thereof, the baffle plate being generally parallel to the coated web. Each duct preferably also includes a plate extension projecting from a bottom portion of the discharge nozzle in the direction of travel of the web, the plate extension being substantially parallel to the coated web.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to drying of coated films and, more particularly, to drying methods and apparatus for minimizing mottle and creating a uniform drying atmosphere. 
     BACKGROUND OF THE INVENTION 
     One of the most common defects associated with organic solvent coatings is mottle. Direct impingement air can cause mottle by disturbing the coating. Also, the heat transfer uniformity is critical. Local variations in heat transfer can show up as mottle. Even if coatings are allowed to dry without direct air impingement, the shear forces caused by the web moving through still air can cause mottle. This will limit the speed at which a product can be manufactured. The occurrence of mottle is often cited as the single greatest limitation to productivity improvement in the drying of coated webs. In order to produce acceptable coatings, web speeds are often reduced significantly from what the machine is capable of coating and drying. 
     Mottle patterns can range from random and blotchy to “liney-streaky” depending on the coating and process conditions. Typically, in photographic film and paper, mottle becomes more severe and oriented in the direction of web travel as web speed is increased. Sensitive products can be limited to web speeds of around 150 feet per minute (fpm). Coatings can be made to be more robust to mottle by increasing the viscosity of the solutions and decreasing the wet thickness of the coating (concentrating the solution) such as described in Miller, C. A. and Neogi, P.; “Interfacial Phenomena”; Marcel Decker; 1995 but, this is not always possible because of coatability or solution stability concerns. 
     When the coating solutions cannot be made to be robust to mottle, disturbances to the coating created in the coating and drying machine must be minimized in order to produce acceptable coatings. One of the most important disturbances is air. Air can directly disturb a wet coating if the pressure or shear forces are great enough (Gutoff, E. B. and Cohen, D.C.; “Modern Coating and Drying Technology”; J. Wiley and Sons; p. 289; 1995). Even if the pressure and shear forces are not great enough to blow the coating around, non-uniformities in the air velocity impinging on the coating can cause surface tension driven flow. Surface tension driven flow arises as a result of variations in concentration and temperature along the surface of the coating. Non-uniform air flow can cause local variations in heat and mass transfer rates which in turn cause concentration and temperature variations. 
     In the last several years there have been only a limited number of published reports on the reduction of mottle by controlling air flow in a solvent coating machine. U.S. Pat. No. 4,365,423 to Arter et al. describes using two-layer screens very close to the coating to protect it from air disturbances and to raise the local solvent concentrations in the gas. U.S. Pat. No. 4,999,927 discloses another apparatus and method for drying a liquid layer that has been applied to a carrier material moving through a drying zone and which contains both vaporizable solvent components and non-vaporizable components. Dying gas flows essentially parallel to and in the direction of the carrier material and is accelerated within the drying zone in the direction of flow. In this manner, laminar flow of the boundary layer of the drying gas adjacent to the liquid layer on the carrier material is maintained. By avoiding turbulent air flow, mottle is reduced. U.S. Pat. No. 5,105,562 to Hella et al. describes a ventilating and impinging air bar assembly primarily for improved conveyance. However, this design relies on direct front side air impingement which is, in general, not desirable from the standpoint of minimizing mottle. 
     U.S. Pat. No. 4,894,927 describes a process for drying a moving web coated with a coating composition containing a flammable organic solvent. The web is passed through a closed-type oven filled with an inert gas and planar heaters on top and bottom of the web. The flow of drying gas is parallel but counter-current to the direction of web movement. The coating surface is reported to be barely affected by movement of the inert drying gases due to the small amounts of gas required. No discussion of the criticality of the gas flow system or of the need to prevent mottle is given. 
     Generally the drying of coated webs is accomplished by direct impingement of air from a nozzle wherein the air is supplied perpendicular to the place of the coated web. Using this technique, mottle occurs in the coating. 
     U.S. Pat. No. 1,776,609 to Andrews discloses a web drying apparatus that consists of nozzles which discharge heated air onto a deflector member. The air is discharged in the direction of the web and the discharge velocity is high to provide a large heat transfer. There is no mention of mottle control or matching of air velocity to web velocity. 
     U.S. Pat. No. 5,105,562 to Hella et al discloses a web drying apparatus which consists of a direct impingement air bar discharging air against the coated surface and a dilution air bar mounted on both sides of the impingement bar. This configuration provides both parallel (to the web travel) air flow and counter (to the web travel) air flow. The direct impingement and dilution bars are supplied air independently of each other. There is no mention of trying to match the air velocity to web velocity to control coating mottle. 
     U.S. Pat. No. 6,018,886 to Bell et al. teaches the use of curved nozzles to deliver air uniformly across the web width in the drying section. The air is delivered substantially parallel to the substrate surface and in the direction of the substrate movement. The goal was to deliver air at approximately the same velocity as the substrate speed. Although the average air velocity through the length of the drying section is close to the speed of the coated substrate through the drying section, the local air velocity can vary considerably along the length thereof. Even localized variability in the air velocity can lead to mottle. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an apparatus and a method for drying coated webs without causing mottle. 
     It is a further object of the invention to substantially eliminate the shear effects that can be created by the coated web as it passes through the air in a dryer. 
     Briefly stated, the foregoing and other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing an improved configuration of the nozzles with the dryer over that taught in Bell et al. A minimum distance between the nozzle and the substrate surface is required to prevent periodic low and high regions of air velocity. An attachment plate is required to capture the air from the preceding nozzle and to further prevent the regions of low air velocity. This attachment plate is substantially parallel to the substrate surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational schematic of a prior art drier nozzle configuration. 
     FIG. 2 is a side elevational schematic of a drier nozzle configuration of the present invention including a baffle extending therefrom. 
     FIG. 3 is a schematic cross-sectional view of the dryer apparatus showing the nozzle arrangement therein. 
     FIG. 4 is a schematic diagram process used in conjunction with the dryer apparatus shown in FIG.  3 . 
     FIG. 5 is a graph plotting the air velocity versus distance from one nozzle exit for the configuration of the present invention and the prior art configuration, which demonstrates the ability of the present invention to deliver an improved air velocity profile through the dryer section as compared to the prior art apparatus. 
     FIG. 6 is a graph plotting the air velocity versus distance from the nozzle exit using the dryer and nozzle configuration of the present invention showing the effect of increasing the distances between the bottom of the discharge nozzle and the surface of the web. 
     FIG. 7 is a graph plotting the air velocity versus distance from the nozzle exit using the dryer and nozzle configuration of the present invention and the prior art configuration. 
     FIG. 8 is a graph plotting the air velocity versus distance from the nozzle exit using the dryer and nozzle configuration of the present invention for baffle plates that are parallel to the web, and baffle plates that are angled upward at angles of 7°, 20°, and 40° with respect to the plane of the web. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the practice of the method of the present invention, a coating applied to the top side thereof is dried in a manner to reduce mottle. The substrate can be one of a variety of webs including, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), acetate, paper or metal. The coating is generally a solvent coating but the method of the present invention can be used in conjunction with aqueous based coatings as well. The present invention is particularly useful in reducing or eliminating mottle in coating compositions such as used for subbing layers for light sensitive emulsions, graphic arts films, and photographic films. Exemplary coating compositions include polymers (e.g.—polyvinyl butyral resin) and cellulose acetate, cellulose acetate butyrate, polymethyl methacrylate, and solvents such as methylene chloride, ketones (e.g.—methyl ethyl ketone), alcohols (e.g.—ethanol), toluene, and water. The coating compositions may also include dyes and surfactants. 
     Turning first to FIG. 1, there is shown a prior art nozzle configuration that typically results in an air velocity that varies strongly along the dryer path. Air flow is conducted through a plurality of conduits or ducts  10  in a dryer section through which the web or substrate  12  is conducted after a coating has been applied thereto. Each conduit or duct  10  includes a generally vertical section  13  and an arcuate section  14  terminating in a discharge nozzle  16 . Arrows  18  are used to indicate air flowing from the discharge nozzles  16 . As the air flow from one discharge nozzle  16  approaches the back side of the next conduit or duct  10  some of the air flow is diverted (as indicated by arrows  20 ) upwardly along the back side of the vertical section  13 . Only a portion of the air flow (as indicated by arrows  22 ) can pass under the arcuate section  14  and nozzle  16  of that next conduit or duct  10 . As a result, a region of low air velocity  24  occurs just at the back of the next conduit or duct  10 . Further, when the arcuate section  14  is located too close to the substrate  12 , a region of higher air velocity  26  occurs just down stream of the next discharge nozzle  16 . Thus, although the average air velocity through dryer section using this prior art configuration approximate the velocity of the web, it can be seen that localized variability in the air velocity can still be present. 
     Turning next to FIG. 2, there is shown one embodiment of the improved baffle arrangement of the present invention. Air flow is conducted through a plurality of conduits or ducts  30  in the a dryer section through which the web or substrate  32  is conducted after a coating has been applied thereto. Each conduit or duct  30  includes a generally vertical section  33  and an arcuate section  34  terminating in a discharge nozzle  36 . Air flow (as indicated by arrow  38 ) from each discharge nozzle  36  is essentially parallel to the surface of the web or substrate  32 . The discharge nozzle  36  includes a bottom lip plate extension  40  that extends beyond the top lip  41  of the nozzle  36 . The bottom lip plate extension  40  is substantially parallel to the substrate  32  and aids in directing the air flow from the discharge nozzle  36  to be substantially parallel to the substrate  32  thereby minimizing the impingement of air onto the substrate  32 . A plate baffle  42  extends from the arcuate section  34  of the conduit or duct  30 . Baffle  42  captures a significant portion of the air delivered by the prior discharge nozzle  36  which is thereby caused to flow under the next conduit or duct  30 . Baffle  42  is positioned to be generally parallel to the substrate  32 . The lower portion of the arcuate section  34  of conduit  30  is spaced above the substrate  32  at a sufficient height to avoid the creation of a substantial resistance to flow. The height can be determined empirically. The bottom of discharge nozzle  36  is also placed at a height above the web  32  sufficient to minimize the impact of the impingement of air exiting from the discharge nozzle  36  on the surface of the substrate  32 . The preferred spacing between the nozzle exit and the substrate is 1.5 to 6 inches, or more preferably 2.0 to 3.0 inches with air velocities in the range of from about 400 to about 1000 fpm. The air velocity should be set to minimize the difference between the air velocity and the velocity of the moving web. 
     The preferred ratio of the length of the attached baffle plate to the spacing between the nozzles is between 10 and 50 percent, or more preferred between 15 and 35%, or even more preferred between 20 and 30%. For example, the preferred length of the attached plate baffle  42 , defined as the distance from where the plate baffle  42  diverges from the curve of the conduit  30  to the end  43  of plate baffle  42  is 4.0 to 12.0 inches. More preferably, the distance from where the plate baffle  42  diverges from the curve of the conduit  30  to the end  43  thereof is 6.0 to 8.0 inches. 
     As stated above, baffle plate  42  is positioned to be generally parallel to substrate  32  moving through the dryer section. The term “generally parallel” as used herein is intended to include the situation where the angle of the baffle plate  42  relative to the substrate  32  is ±5° from parallel. Positive angles may be defined as having the cantilevered end of the baffle plate  42  being further above the substrate  32  than the connection between the baffle plate and conduit  30 . Preferably, the angle of the baffle plate  42  relative to the substrate  32  is ±2°. Most preferably, baffle plate  42  and substrate  32  are parallel. Further, although depicted as being planar, baffle plate  42  can also have a curved shape where the distance between the substrate  32  and baffle plate  42  increases as the further back the baffle plate  42  extends from the conduit  30 . 
     The preferred spacing between nozzles from bottom lip plate extension  40  to bottom lip plate extension  40  is 1.5 to 4.0 feet, and more preferably 2.0 to 3.0 feet. The bottom lip plate extension  40  of each discharge nozzle  36  can be approximately the same length. As stated above, the bottom lip plate extension  40  extends beyond the top lip  41 . Preferably, the ratio (l/h) of the length (l) of the bottom lip plate extension  40  to height (h) of discharge nozzle  36  is between 2.0 and 5.0 and, most preferably between 2.0 and 4.0. 
     Looking next at FIG. 3 there is depicted a schematic cross sectional view of a dryer enclosure  50 . The conveyance system used to drive the coated web  52  therethrough is not shown, although it is preferred that the coated web be moving at a line speed above 400 fpm. The coated web  52  passes through the dryer enclosure  50  under the discharge nozzles  56  of conduits or ducts  58  (identical to ducts  30  of FIG.  2 ). Air is supplied to the discharge nozzles  56  via an air from primary supply air duct  60  which delivers air to a supply plenum  62 . The conduits or ducts  58  are connected to the supply plenum  62 . A perforated distribution plate  64  is used to ensure uniform air flow from the downstream discharge nozzles  56 . The air pressure can be controlled by the pivoting air damper  66  in the primary supply air duct  60 . This allows the same machine to coat a variety of products without sensitivity to dry point location. 
     FIG. 4 illustrates the preferred process flow. Air is supplied by the supply air fan  70  which is obtained from an exhaust air fan  72  through a recirculate damper  74  assisted by a make-up air damper  76  and conditioned by either the cooling  78  or heating  80  coils, and then cleaned by the filters  82 . It is often preferred to supply the air at temperatures between 2° C. and 150° C. The air pressure is controlled by the supply air damper  66 . The air pressure is determined by the desired heat transfer rate and product sensitivity to coating mottle. Another factor in determining the air pressure is maintaining the solvent level in the enclosure  50  below the explosive limit. The supply air duct  60  delivers the air to supply air plenum  62 . The air then passes through the perforated distribution plate  64  as shown in FIG. 3 to ensure uniform discharge velocities from the exit of the discharge nozzles  56  via ducts  58 . 
     EXAMPLES 
     Looking next at FIG. 5 a graph is presented which demonstrates the ability of the present invention to deliver an improved air velocity profile through the dryer section as compared to the prior art apparatus of FIG.  1 . The different configurations are described in Table 1. The prior art data was generated with a configuration that included a bottom lip plate extension. The present invention delivers a significantly smoother air velocity profile as compared to the prior art configuration. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Prior Art with Bottom 
               
               
                 Attribute 
                 Preferred Embodiment 
                 Lip plate extension 
               
               
                   
               
             
             
               
                 Nozzle-to-Substrate 
                 3   
                 1   
               
               
                 Spacing (inches) 
               
               
                 Plate Attachment 
                 7   
                 (none) 
               
               
                 Length (in) 
               
               
                 Plate Angle (relative 
                 0° 
                 (none) 
               
               
                 to substrate) 
               
               
                 Spacing between 
                 3.0 
                 2.0 
               
               
                 Nozzles (ft) 
               
               
                 Bottom Lip plate 
                 2.0 
                 2.0 
               
               
                 extension (in) 
               
               
                 Nozzle Slot Opening 
                 1.0 
                 1.0 
               
               
                 (in) 
               
               
                   
               
             
          
         
       
     
     FIG. 6 illustrates the effect of increasing the distances between the lip plate extension  40  and the surface of the substrate  32 . The discharge nozzles  36  were spaced at a distance (d) (see FIG. 3) of 2 feet with no baffle plate  42  used. The air velocity measurements are shown at the distance from the exit if the lip plate extension  40 . The negative values indicate positions upstream of the lip plate extension  40  of a particular discharge nozzle  36  while positive values indicate positions downstream of the lip  40  of that particular discharge nozzle  36 . The width (w) of conduit  30  from the rear surface of vertical section  33  to the bottom lip plate extension  40  was 5 inches. At a 1 inch spacing between the bottom of each discharge nozzle  36  and the substrate  32  there is a significant drop in the air velocity at approximately −6 inches, which is near the back end of the nozzle. When the substrate-to-nozzle spacing is increased to 2 inches and 3 inches, the minimum is suppressed. The difference between the 2 inch and 3 inch spacing is small compared to the benefit seen from increasing from 1 inches to 2 inches. 
     FIG. 7 illustrates the advantageous effect of the baffle plate  42 . The discharge nozzles  36  were placed 2 feet apart and at a 2 inch spacing from the substrate  32 . The baffle plate  42  was 6 inches long. The width (w) was 5 inches (see FIG.  2 ). The baffle plate  42  was oriented parallel to the substrate  32 . The baffle plate  42  acts to suppress the minimum in the air velocity profile under the arcuate portion  34  of the duct  30 . 
     FIG. 8 illustrates the effect of the angle of the baffle plate  42  relative to the substrate  32 . The height of the discharge nozzles  36  was 1 inch off the surface of the substrate  32  and the discharge nozzles  36  were spaced 2 feet apart. The baffle plate  42  used was 4 inches long. When the baffle plate  42  was parallel to the substrate  32 , the minimum in air velocity was suppressed. When the baffle plate  42  was set at angles 7 to 40 degrees relative to the substrate  32 , the minimum in the air velocity profile was considerably larger. 
     From the foregoing, it will be seen that this invention is one well adapted to obtain all of the ends and objects hereinabove set forth together with other advantages which are apparent and which are inherent to the apparatus. 
     It will be understood that certain features and subcombinations are of utility and may be employed with reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
     As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 10 
                 ducts 
               
               
                 12 
                 web or substrate 
               
               
                 13 
                 vertical section 
               
               
                 14 
                 arcuate section 
               
               
                 16 
                 discharge nozzle 
               
               
                 18 
                 arrow 
               
               
                 20 
                 arrow 
               
               
                 22 
                 arrow 
               
               
                 24 
                 lower air velocity 
               
               
                 26 
                 higher air velocity 
               
               
                 30 
                 conduit or ducts 
               
               
                 32 
                 web or substrate 
               
               
                 33 
                 vertical section 
               
               
                 34 
                 arcuate section 
               
               
                 36 
                 discharge nozzle 
               
               
                 38 
                 arrow 
               
               
                 40 
                 bottom lip plate extension 
               
               
                 41 
                 top lip 
               
               
                 42 
                 baffle place 
               
               
                 43 
                 end 
               
               
                 50 
                 dryer enclosure 
               
               
                 52 
                 coated web 
               
               
                 56 
                 discharge nozzle 
               
               
                 58 
                 conduits or ducts 
               
               
                 60 
                 air duct 
               
               
                 62 
                 supply plenum 
               
               
                 64 
                 perforated distribution plate 
               
               
                 66 
                 pivoting air damper 
               
               
                 70 
                 supply air fan 
               
               
                 72 
                 exhaust air fan 
               
               
                 74 
                 recirculate damper 
               
               
                 76 
                 make up damper 
               
               
                 78 
                 cooling coils 
               
               
                 80 
                 heating coils 
               
               
                 82 
                 filters