Abstract:
A movable trough is taught for establishing a uniform wetting line on the rear face of a curtain coating hopper lip. The trough can be pivoted or moved linearly into a position such that the hopper lip resides in or proximate to the movable trough. The curtain coating apparatus is then started and the coating solution leaving the hopper lip is intercepted by the trough. The coating solution flowing over the lip fills and floods the movable trough. The flooding of the trough forces the coating solution to substantially wet (to a height on the back side of the lip significantly higher than that of natural product flow) the back side of the hopper lip. The movable trough is then retracted from its position immediately beneath the hopper lip and intercepting the coating solution exiting the hopper lip to thereby allow the free-falling curtain to form and begin impingement on the moving support web to be coated. As the curtain forms, the wetting line on the back of the hopper lip naturally retracts toward the tip of the hopper lip thereby forming a uniform wetting line and a uniform curtain.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of curtain coating and, more particularly, to methods and apparatus for establishing a uniform wetting line on the back surface of a curtain coating hopper lip. 
     BACKGROUND OF THE INVENTION 
     The technique of curtain coating is widely used in the industry of manufacturing photographic films and papers. Typically, a curtain coating apparatus comprises a feed system in the form of one or more slots fed with photographic emulsions and from which the photographic emulsions flow in the form of one or more layers which are superimposed on a slightly inclined plane. The photographic layers then flow onto a lip, where they leave the coating device to form a liquid curtain in substantially vertical free fall. The free-falling curtain is deposited on a moving support web typically while the web is supported on a driven roller. Structurally, the lip is substantially vertical and has a front face on which the layers of photographic emulsion flow, and a rear face forming, with respect to the front face, an angle which is typically around 30° to 45°. The bottom edge of the front face and the bottom edge of the rear face are separated by a bevel, the width of which varies overall between 0.1 mm and 2.5 mm. For applications of this type, the flow rates (per unit width of the lip) vary from 0.6 cm 2 /s to 6 cm 2 /s. The viscosity of the photographic layers varies from 0.005 to 3 poise. All these quantities are, of course, mentioned only by way of reference. 
     Such curtain coating systems have been the subject of numerous patents. By way of example, reference can be made to the European Patent No. EP-A-107 818; U.S. Pat. No. 4,510,882, U.S. Pat. No. 3,632,374; U.S. Pat. No. 3,867,901; and French Patent No. FR-A-2 346 057. 
     One condition that a curtain coating system can be particularly sensitive to (notably for photographic applications for which uniformity of coating is essential) is the formation of a curtain that is not uniform and homogeneous. This is because a non-uniform curtain creates streaks on the photographic product. That is, the coating is applied to the support web with variations in thickness across the width of the support web. These variations have an appreciable effect on the photographic properties of the film and consequently it is important to minimize such variations. 
     U.S. Pat. No. 5,725,666 to Baumlin, entitled “Method and Apparatus for Improving the Uniformity of a Liquid Curtain in a Curtain Coating System,” teaches a tool for creating a uniform wetting line on the rear face of the lip of a curtain coater. A perspective view of the tool is shown in FIG.  1 . The device comprises two fingers  1 ,  2  mounted on a frame  3 . Each of the fingers  1 ,  2  defines a first surface  4 ,  5  designed to be brought to bear on the front face of the lip of the coating device, and a second or rear surface  6 ,  7  designed to be applied substantially to the rear face of the lip. The first surface forms, with respect to the second surface, an angle substantially equal to the angle formed by the front and rear faces of the lip. Generally, the angle between the two surfaces varies from 30° to 45°. The height of the rear surface  6 ,  7  of each of the fingers is at least equal to the height over which it is intended that the liquid should wet the rear face of the lip. 
     During operation, an operator applies the wetting device to the lip of the coating device and slides it so as to cause it to travel at least once over substantially the whole width of the lip. Thus, the rear surface  6 ,  7  of each of the fingers is applied opposite the rear face of the lip and forces the liquid to wet the rear face of the lip over a height greater than its natural wetting height. There are some problems associated with the use of the device taught by Baumlin. Operator intervention is required. Operation of the tool is manually intensive. Operation of the tool results in generating substantial liquid waste at startup. 
     U.S. Pat. No. 5,759,633 to Baumlin et al. and entitled “Method for Improving the Uniformity of a Liquid Curtain in a Curtain Coating System,” teaches a method for improving curtain uniformity by forming a liquid curtain over the front face of a lip, progressively reducing the flow rate over the lip to a set value for a period of time so that the rear face of the lip is wet to a greater height, and increasing the flow rate to defined coating conditions. According to the teachings of Baumlin et al., there is initially a liquid composition with a high flow rate (6 cm 2 /s) and a low viscosity (6.5×10 −3 P, which typically corresponds to water at 40° C. to which surfactants are added to facilitate the formation of the curtain). The flow rate is reduced (1.5 to 2 cm 2 /s) so as to attain the flow rate level of a wettability window defining a flow rate and viscosity region within which the liquid composition wets the rear face of the lip over a height greater than the natural wetting height over which the coating composition would wet under the operating coating conditions (50×10 −2 P at a flow rate of 4 cm 2 /s). There is a progressive change from water to the photographic composition, while the flow rate is held substantially at the reduced value. The change from water to the photographic composition results in an increase in viscosity, which takes place progressively so that the process stays within the wettability window for a sufficiently long period (generally longer than 1second). The viscosity of the coating composition continues to increase outside the conditions of the wettability window. The flow rate is then increased to attain the coating rate. The wetting of the rear face of the lip remains uniform and has an average height of around 0.1 mm. 
     The location and size of the wettability window are, to a large extent, dependent on the geometry of the lip. Baumlin et al. teaches that for each type of lip there is a corresponding wettability window. 
     There are some drawbacks associated with the method taught by Baumlin et al. First, the method requires that water precede the introduction of product solutions on the slide surface of the curtain coating apparatus. Further, it is difficult to control the flow rates of the various coating layers in conjunction with the viscosity. The method relies on establishment of the wetting line to substantially wet the back of the lip uniformly across the entire width of the lip. There is also the dependence of the wettability windows on lip geometry requiring that a wettability window be established for each coating lip of different geometry. 
     Baumlin et al. also teaches a second embodiment of the method. According to this embodiment, a solution of gelatin and surfactant having a viscosity of 0.03P is used. Initially, the curtain is established with a high flow rate (around 6 cm 2 /s). The rate is then reduced to about 1.5 cm 2 /s, producing a significant wetting of the rear face of the lip. These conditions are maintained for a few seconds, and the flow rate is again increased to 6 cm 2 /s. This embodiment of Baumlin et al. has drawbacks similar to those discussed above. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and apparatus for generating a uniform wetting line on the rear face of a curtain coating hopper lip. 
     It is a further object of the present invention to provide a method and apparatus for generating a uniform wetting line on the rear face of a curtain coating hopper lip which is not dependent on wettability windows and hopper lip geometry. 
     Yet another object of the present invention is to provide method and apparatus for generating a uniform wetting line on the rear face of a curtain coating hopper lip which can be automatically actuated and minimizes waste. 
     Still another object of the present invention is to provide a method and apparatus for generating a uniform wetting line on the rear surface of a curtain coating hopper lip which does not require physical contact between the apparatus and the hopper lip. 
     Briefly stated, the foregoing and numerous other features, objects and advantages 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 using a movable trough positioned in close proximity to the hopper lip. The movable trough can be pivoted or moved linearly into a position such that the hopper lip resides proximate to the movable trough. The curtain coating apparatus is then started and the coating solution leaving the hopper lip is intercepted by the trough. The coating solution flowing over the lip fills and floods the movable trough. The flooding of the trough forces the coating solution to substantially wet (to a height on the back side of the lip significantly higher than that of natural product flow) the back side of the hopper lip. The movable trough is then retracted from its position immediately beneath the hopper lip and intercepting the coating solution exiting the hopper lip to thereby allow the free-falling curtain to form and begin impingement on the moving support web to be coated. As the curtain forms, the wetting line on the back of the hopper lip naturally retracts toward the tip of the hopper lip thereby forming a uniform wetting line and a uniform curtain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art device used to achieve a uniform wetting line on the rear surface of a hopper lip of a curtain coating apparatus. 
     FIG. 2 is a perspective view of a movable trough apparatus of the present invention which can be manually positioned to intercept the coating solution exiting the hopper lip of a curtain coating apparatus, such that the hopper lip is flooded to establish a uniform wetting line on the rear surface of the hopper lip. 
     FIG. 3 is a side elevational schematic showing the position of the movable trough of the present invention in close proximity to the lip of a curtain coating hopper to thereby cause the lip to flood on the rear face thereof to an elevation higher than the operating wetting line on the rear face. 
     FIG. 4 is a perspective view of the movable trough of the present invention shown in combination with a curtain coating hopper wherein the movable trough is provided with a motorized system for positioning the trough. 
     FIG. 5 is a side elevational schematic of the movable trough and system depicted in FIG.  4 . 
     FIG. 6 is a perspective view of the movable trough of the present invention shown in combination with a curtain coating hopper wherein the movable trough is provided with a first alternative automated and powered positioning system from that depicted in FIGS. 4 and 5. 
     FIG. 7 is a side elevational schematic of the movable trough and system depicted in FIG.  6 . 
     FIG. 8 is a perspective view of the movable trough of the present invention shown in combination with a curtain coating hopper wherein the movable trough is provided with a second alternative automated and powered positioning system from that depicted in FIGS. 4 and 5. 
     FIG. 9 is a side elevational schematic of the movable trough and system depicted in FIG.  8 . 
     FIG. 10 is a side elevational schematic of the movable trough of the movable trough of the present invention shown in combination with a curtain coating hopper wherein the movable trough is provided with a third alternative automated and powered positioning system from that depicted in FIGS. 4 and 5. 
     FIG. 11 is a schematic depicting a curtain coating system in combination with the movable trough of the present invention. 
     FIG. 12 is a rear elevational view of the coating lip showing the relative elevations of the wetting line thereon when the movable trough of the present invention in close proximity to the lip of a curtain coating hopper to thereby cause the lip to flood and the operating wetting line when the movable trough has been retracted. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning to FIG. 2, there is shown a perspective view of a movable trough system  10  adapted to be used in conjunction with a coating hopper (not shown). The system  10  includes feet  12  adapted to be affixed to the floor of a coating room. Extending up from feet  12  are adjustable legs  14 . Affixed to the top of legs  14  are gussets  16 . There is a frame  18  pivotally attached to gussets  16  by means of pins  20 . An arcuate slot  22  in one or both gussets  16 , in conjunction with pins  24  extending from frame  18  into a respective arcuate slot  22 , serve as travel stops, limiting the amount of travel that frame  18  can be pivoted. Affixed to frame  18  is trough  26 . Through the pivoting movement of frame  18 , trough  26  can be positioned in close proximity to the curtain coating hopper lip Such that the hopper lip resides in or proximate to trough  26 . Positioning of trough  26  can be performed manually by an operator, or can be powered such as through the use of a stepper motor. In either case, it is preferred that the trough position be governed by hard stops to insure that there will be no physical contact between the trough  26  and the coating lip. If positioning of trough  26  is performed manually, then the trough  26  will have to be held in position manually for a short period of time while the channel floods and a uniform wetting line is established. Alternatively, a position locking mechanism can be used (such as substituting threaded bolts and nuts for pins  24  so that the position of trough  26  can be positioned by tightening the nuts on the bolts) to hold the trough  26  in position while the channel floods and the uniform order line is established. 
     Looking next at FIG. 3 there is shown a cross-sectional view of trough  26  residing proximate to the lip  27  of a curtain coating hopper  29 . Trough  26  includes a channel  28  into which the solution from the hopper lip  27  pours. Preferably, channel  28  is semicircular in cross-section. However, it is believed that a variety of cross-sectional shapes can be employed successfully including V-shaped, square, trapezoidal, rectangular, and arcuate. The rear wall  30  of the trough  26  near the front face  31  of the curtain coating hopper  29  preferably has a beveled surface  32  to reduce the tendency of the coating liquid to splash onto the front face  31  of the hopper  29 . Positioning of the trough  26  is critical. The preferred position is such that the lowest point of the tip of the lip  27  is in the same horizontal plane as the top portion of the trough  26 . Importantly, the travel stops in combination with the adjustable legs  14  prevent the trough  26  from travelling into and damaging the hopper lip. 
     The width of the trough  26  is preferably approximately two (2) inches narrower than the coating width. Thus, there should be about one (1) inch of spacing between each end of the trough  26  and the curtain edge guide equipment (not shown) although the spacing between each end of the trough  26  and the curtain edge guide equipment can be as little as 0.1 inches. Trough  26  is preferably open at each end thereof, such that the excess coating solution is able to flow out of the ends of the channel  28 . In this way, although channel  28  substantially fills with liquid excess coating solution liquid does not flow over the top surfaces of trough  26  which could result in contamination of the web and backup roller. In addition, the spacing between the ends of trough  26  and the edge guides is sufficient to prevent the edge guides from being contaminated with the coating solution. 
     The method and apparatus of the present invention is preferably used prophylactically as discussed above. Trough  26  is positioned such that the coating lip  27  resides in or at least proximate to trough  26  prior to the introduction of product solutions to the curtain coating hopper. This can be performed while a pre-product solution fluid (e.g. water) is flowing over the hopper lip, or when no solutions are flowing over the hopper lip  27 . If the trough  26  is moved into position while fluid is flowing over the lip  27 , then trough  26  should be moved at a relatively slow rate of speed (about  1  inch per sec) in order to prevent the fluid splashing onto the front face  31  of the coating hopper  29 . The product solution is introduced into the curtain coating hopper  29  by conventional methods (at coating flow rates or at specific flow rates). Trough  26  is allowed to reside in close proximity to the hopper lip  27  (i.e. breaking the liquid curtain) for approximately 5 seconds. Once product solution flow has been established throughout the entire hopper  29 , channel  28  quickly fills, thereby wetting the rear face  33  of the coating lip  27  to an elevation  35  higher than the operating wetting line  37  (see FIG.  12 ). Trough  26  is then retracted and a uniform operating wetting line  37  is established. Trough retraction rate is preferably relatively quick (on the order of magnitude of 12 inches per second). This can be accomplished by releasing the trough position locking mechanism to thereby allow the trough to fall away in an arcuate path under the force of gravity. 
     The method and apparatus of the present invention can also be used as a corrective tool if wetting line non-uniformities are observed on the back surface  33  of the coating lip  27 . When used as corrective tool, trough  26  begins in the retracted position, that is, not in contact with the liquid curtain (not shown), while the product solution is forming a free-falling curtain. Trough  26  is then moved into a position in close proximity to the coating hopper lip  27  and intercepting the free-falling curtain. The action of placing the trough  26  in close proximity to the hopper lip  27  while product solution is flowing over the lip  27  is performed slowly—at a rate of approximately 1 inch per sec, such that fluid does not splash on the front face  31  of the hopper  29 . With the free-falling curtain intercepted by trough  26 , channel  28  quickly fills, thereby wetting the rear face  33  of the coating lip  27  to an elevation higher than the operating wetting line. Trough  26  is then retracted and a uniform operating line is established. The trough  26  is allowed to reside in close proximity to the hopper lip  27  (i.e. breaking the liquid curtain) for approximately 5 seconds, then the trough  26  is retracted from its position in close proximity to the lip. Retraction of the trough  26  should again be done quickly such as by releasing trough  26  to allow to fall away in an arcuate path under the force of gravity. 
     Although the trough  26  depicted in FIG. 2 is described herein as being manually positioned, it should be apparent to those skilled in the art that an automated driving mechanism can be employed to position trough  26  proximate to a coating lip  27 . A variety of different rotational and or linear driving mechanisms can be used to position trough  26 . Looking next at FIGS. 4 and 5, there is shown an alternative embodiment of the present invention wherein the means for positioning of a trough  40  is through a powered mechanism. Trough  40  is substantially identical to trough  26 . A motor  42  having a drive shaft  44  extending therefrom is used to drive the movement of trough  40  in an arcuate path. There are bearings  46  providing rotational support for drive shaft  44 . Motor  42  and bearings  46  are supported by a support frame (not shown). Affixed to drive shaft  44  arc arms  48  which support trough  40 . Motor  42  drives rotation of drive shaft  44  to cause trough  40  to be moved in an arcuate path either into close proximity with the coating lip  50  of coating hopper  52  to thereby be in position to intercept the free-falling curtain, or away from coating lip  50  such that the freefalling curtain is not intercepted by trough  40 . In such manner, trough  40  can be used prophylactically or as a corrective tool as described above with reference to trough  26  to establish a uniform wetting line on the back surface of coating lip  50 . 
     Turning next to FIGS. 6 and 7, there is shown yet another alternative embodiment of the present invention similar to that shown in FIG. 4 and 5. The positioning of trough  60  is driven by a linear actuator  62 . The piston  64  of linear actuator  62  has pivotally attached thereto an arm  66 . Attached to the opposite end of arm  66  is shaft  68 . Shaft  68  is supported for rotational movement by bearings  70 . Bearings  70  arc supported by a frame (not shown). Affixed to shaft  68  are arms  72  which support trough  60 . Linear actuator  62  drives rotation of shaft  68  to cause trough  60  to be moved in an arcuate path either into close proximity with the coating lip  74  of coating hopper  76  to thereby be in position to intercept the free-falling curtain, or away from coating lip  74  such that the free-falling curtain is not intercepted by trough  60 . In such manner, trough  60  can be used prophylactically or as a corrective tool as described above with reference to trough  26  to establish a uniform wetting line on the back surface of coating lip  74 . 
     FIGS. 8 and 9 schematically depict yet another alternative embodiment for driving the movable trough of the present invention. The trough  80  (which is substantially identical to trough  26 ) is mounted on a support frame  82 . Support frame  82  is in turn affixed to a pair of vertical guide bars  84 . Support frame  82  includes a cantilevered section  86 . A linear actuator  88  is provided wherein the piston  90  thereof engages a cantilevered section  86 . In such manner, linear actuator  88  can raise and lower support frame  82  with vertical guide bars  84  sliding in bearings  92 . Bearings  92  are supported by means not shown. It should be noted that the curtain coating hopper  94  moves between a coating position  96  in a preparation position  98 . When the hopper  94  is in the coating position  96 , the free falling curtain exiting lip  99  will impinge upon a moving web supported on a coating roller (not shown). Thus, in order to establish a uniform wetting line on the back surface of lip  99 , the coating hopper  94  is moved into the preparation position  98 . With the coating hopper  94  in the preparation position  98 , linear actuator  88  is used to drive frame  82  vertically upward to thereby position trough  80  proximate to lip  99 . In such manner, coating liquid or the startup liquid flowing over lip  99  floods the channel of trough  80  thereby establishing a wetting line on the back surface of lip  99  which is higher than the operating wetting line on the back surface of lip  99 . Linear actuator  88  than lowers frame  82  and trough  80  away from lip  99 . Then, with liquid still flowing from hopper  94  over lip  99 , hopper  94  it is retracted to the operating position  96  and curtain coating of the moving web is begun. 
     Looking next at FIG. 10, still another alternative embodiment for driving the movement of the movable trough is depicted. In this embodiment the position of trough  100  is driven by the movement of the coating hopper  102 . There is a bracket  104  mounted to the coating hopper  102 . Bracket  104  includes a curved engaging surface  108 . Trough  100  is mounted on beams  106  which extend from an axle not shown. The axle is rotatably supported in bearings  110  which are in turn supported by means not shown. Extending from each end of the axle are struts  112 . In operation, when hopper  102  is moved from the operating position  114  to a preparation position  116  the curved engaging surfaces  108  of brackets  104  engage struts  112  for driving struts  112  to an upright position thereby causing beams  106  to be pivoted upwards. In such manner, trough  100  is raised to be positioned proximate to lip  118 . Once the flow of liquid from lip  118  floods the channel in trough  100  thereby establishing a wetting line on the back surface of lip  118 , hopper  102  is retracted to the operating position  114  so that coating of the moving web can be performed. 
     Generally, the movable trough  26 ,  40 ,  60 ,  80 ,  100  of the present invention is used when the coating hopper is in a preparation position as discussed above with reference to FIGS. 1 through 10. Looking at FIG. 11, there is schematically depicted a curtain coating system with the movable trough of the present invention  26 ,  40 ,  60 ,  80 ,  100 . When the coating hopper  120  is in the preparation position  122 , fluid flowing over the coating lip  124  will be collected in a preparation trough  126  or drain collection trough  128 . Therefore, fluid exiting channel  28  at the ends thereof will also be captured in the preparation trough  126  or drain collection trough  128 . Once a wetting line has been established on the back face of lip  124 , the movable trough ( 26 ,  40 ,  60 ,  80 ,  100 ) is retracted, and the coating hopper  120  is retracted to an operating position  130 . In the operating position, the coating lip  124  is positioned above a moving web  132 , which is supported on a coating roll  134 . The solution is captured by the start/finish pan  136  until the coating is ready to begin. 
     Those skilled in the art will recognize that, typically, coating hoppers in a curtain coating operation are used to coat the moving web with a composite layer. The composite layer is comprised of a plurality of superimposed individual layers. In the practice of the method of the present invention it is generally preferred to position the movable trough in close proximity to the lip prior to the introduction of the product solution. The trough is preferably not moved away from the lip until all product coating layers have been fully established through the coating hopper. 
     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. 
     PARTS LIST 
       1  finger 
       2  finger 
       3  frame 
       4  first surface 
       5  first surface 
       6  second or rear surface 
       7  second or rear surface 
       10  movable trough system 
       12  feet 
       14  adjustable legs 
       16  gussets 
       18  frame 
       20  pins 
       22  arcuate slot 
       24  pins 
       26  trough 
       27  hopper/coating lip 
       28  channel 
       29  curtain coating hopper 
       30  rear wall 
       31  front face 
       32  beveled surface 
       33  rear face 
       35  elevation 
       37  operating wetting line 
       40  trough 
       42  motor 
       44  drive shaft 
       46  bearings 
       48  arms 
       50  coating lip 
       52  coating hopper 
       60  trough 
       62  linear actuator 
       64  piston 
       66  arm 
       68  shaft 
       70  bearings 
       72  arms 
       74  coating lip 
       76  coating hopper 
       80  trough 
       82  support frame 
       84  vertical guide bars 
       86  cantilevered section 
       88  linear actuator 
       90  piston 
       92  bearings 
       94  curtain coating hopper 
       96  coating position 
       98  preparation position 
       99  exiting lip 
       100  trough 
       102  coating hopper 
       104  bracket 
       106  beams 
       108  curved engaging surface 
       110  bearings 
       112  struts 
       114  operation position 
       116  preparation position 
       118  coating lip 
       120  coating hopper 
       122  preparation position 
       124  coating lip 
       126  preparation trough 
       128  drain collection trough 
       130  operating position 
       132  moving web 
       134  coating roll 
       136  start/finish pan