Abstract:
A method and apparatus for continuously coating moving web and splices with a coating fluid. The system includes a slide coating die having a slide surface with at least one feed slot for extruding the coating fluid onto the moving web. The slide coating die defines a coating gap with the moving web. The coating gap is adjustable between a coating position and a splice coating position. A web guide is positioned to guide the moving web in a first direction past the slide coating die such that a coating bead of the coating fluid can be formed in the coating gap. A vacuum system is positioned to generate a reduced pressure condition along a lower surface of the slide coating die. The vacuum system defines a vacuum gap with the moving web. The vacuum gap is adjustable independent of the coating gap between a coating position and a splice coating position. A detector signals an increase in web thickness. A controller is functionally connected to the detector. The controller adjusts the coating gap and the vacuum gap to the splice coating position in response to an increase in web thickness in excess of a predetermined magnitude while maintaining a stable coating bead.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a web coating method and apparatus for maintaining a stable coating bead while coating over splices. 
     BACKGROUND OF THE INVENTION 
     The production of high quality articles, particularly photographic, photothermographic, and thermographic articles, consists of applying a thin film of a coating solution onto a continuously moving substrate or web. Thin films can be applied using a variety of techniques including: dip coating, forward and reverse roll coating, wire wound rod coating, blade coating, slot coating, slide coating, and curtain coating. Coatings can be applied as a single layer or as two or more superimposed layers. Although it is usually most convenient for the substrate to be in the form of a continuous web, it may also be formed of a succession of discrete sheets. 
     Slide coaters have been used extensively since the 1950s in the photographic and related industries for coating aqueous photographic emulsions with relatively low viscosity (less than 100 cP). In slide coating, it is well known to start and stop coating of a moving web by means known as “pick-up.” In the pick-up phase, the flow of the coating liquid is established with the coater die retracted from the web. The coating liquid drains over the die edge into a vacuum box and drain. Once the flows of all the coating liquids are stabilized from all the feed slots of the slide coating die, the die and vacuum box are moved into the coating position in a rapid manner with the web moving at the desired coating speed. 
     Mechanical disturbances such as nicks in the die edge can cause streak-type defects to be formed in the coated article. Contamination disturbances that may cause streaking include dirt particles lodged near the coating bead, dried or semi-dried particles of coating compound, and non-uniform wetting of the contact line of the coating liquid on the coating die edge. Non-uniform wetting on the die edge, especially after pick-up, appears to be an important factor when coating fluids containing volatile solvents. For example, contamination may adhere to the front face and/or die edge of the slide coating die. That contamination may lead to a non-uniform wetting line and possible streaking of the coating compound. 
     The coating gap between the moving web and the coating die is typically less than about 4 millimeters (0.157 inch). Web splices, debris on, or defects in, the web in excess of the coating gap can cause serious damage to the coating die. It is common practice to retract the coating die, and break the coating bead, to permit web splices to pass through the coating gap. After the web splice passes the coating gap, the pick-up cycle must be repeated to reestablish the coating bead. 
     Another problem related to slide coating is contamination of vacuum ports and drains in the vacuum box when the die is retracted from the moving web (i.e., no coating bead is present) and the coating liquid is flowing freely. Contamination of the vacuum ports and drains can lead to unstable vacuum operation causing defects and eventually requiring cessation of the coating operation to clean the vacuum box and ports. This problem is exacerbated with high viscosity fluids (about 100-10,000 centipoise or greater) that contain volatile solvents that dry much faster than water (such as methyl ethyl ketone, tetrahydrofuran, or methanol). 
     FIG. 1 is a schematic illustration of the interface between a coating fluid  20  traversing a top surface  22  of the coating bar  24  and a moving web  26 . Front face  28  of the coating bar  24  may include a durable, low surface energy portion. The low energy portion is intended to provide the desired surface energy properties to specific locations to prevent build-up of dried material. Details regarding the process of making such durable, low surface energy portions are disclosed in commonly assigned U.S. patent application Ser. No. 08/659,053 filed May 31, 1996. 
     When the coating bar  24  is moved into the coating position for pick-up, as illustrated in FIG. 1, a stable coating bead  30  is formed in coating gap  32  between die edge  34  and the moving web  26 . The coating gap  32  is typically between 0.0254 mm and 3.81 mm. The coating bead  30  has a static wetting line  36  along the front face  28  and a dynamic wetting line  38  on the moving web  26 . The pressure just under lower meniscus  40  is preferably maintained below atmospheric pressure by a vacuum box (not shown) to stabilize the coating bead  30 . 
     If the coating process needs to be interrupted, such as when a web splice passes the coating gap  32 , the coating bar  24  and vacuum box assembly can be retracted from the web  26  until resumption of the coating is desired. Retracting the coating bar  24  increases the coating gap  32 . The movement of the coating bar  24 , disruption of the vacuum force on the coating bead  30  and/or the increase in the coating gap  32  typically destabilizes or breaks the coating bead  30 . A significant amount of web  26  may need to be advanced before a stable coating bead  30  is reestablished, resulting in wasted coating fluid  20  and web  26 . 
     In slide coating, it is known to deckle the coating width for various reasons such as for different products and formats. Deckling often results in unwanted leakage of air into the vacuum box because the coating bead bridging the gap between the web and the front of the coating bar is typically narrower than the width of the coating bar. Leakage is more pronounced in modern die lip designs, such as square lips, that offer little resistance to air flow. Vacuum leakage into the vacuum box is particularly troublesome because it becomes difficult to maintain an adequate level of vacuum and because the excessive volume of air flow can destabilize the coating bead. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a web coating method and apparatus for continuously coating over splices with a coating fluid. The present method and apparatus permit coating over splices with minimal splice generated waste by eliminating the retraction and pick-up cycle. 
     The apparatus includes a coating die defining a coating gap with the moving web. The coating gap is adjustable between a coating position and a splice coating position. A web guide is positioned to guide the moving web in a first direction past the coating die such that a coating bead of the coating fluid can be formed in the coating gap. A vacuum system is positioned to generate a reduced pressure condition along a lower surface of the coating die. The vacuum system defines a vacuum gap with the moving web. The vacuum gap is adjustable independent of the coating gap between a coating position and a splice coating position. A detector signals an increase in web thickness. A controller is functionally connected to the detector. The controller adjusts the coating gap and the vacuum gap to the splice coating position in response to an increase in web thickness in excess of a predetermined magnitude while maintaining a stable coating bead. In one embodiment, the coating die is a slide coating die. 
     In one embodiment, the vacuum system includes a vacuum box with a front seal opposite the moving web upstream of the coating gap. The front seal rotates away from the moving web in the splice coating position. In the illustrated embodiment, the web guide is a support roll. The support roll moves horizontally away from the coating gap in the splice coating position. 
     In one embodiment, the controller is capable of adjusting a magnitude of the reduced pressure condition in response to the detector signaling an increase in web thickness. The change in the magnitude of the reduced pressure condition preferably corresponds to the increase in web thickness reaching the coating gap. In another embodiment, the slide coating die has a die edge with a centrally located coating portion interposed between a pair of coating gap seals. The coating gap seals comprise vacuum seal land areas having a contour corresponding to a contour of the web guide. 
     The invention is also directed to a method for continuous coating of a moving web and splices with a coating fluid. A coating die is located opposite the moving web. The coating die defines a coating gap with the moving web in a coating position. The moving web is guided in a first direction past the coating die such that a coating bead of the coating fluid is formed in the coating gap. A reduced pressure condition is generated along a lower surface of the coating bead. An increase in web thickness is signaled to a controller. A vacuum gap is adjusted to the splice coating position in response to an increase in web thickness. The coating gap is adjusted to the splice coating position independently of the vacuum gap in response to an increase in web thickness in excess of a predetermined magnitude while maintaining a stable coating bead. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic illustration of an interface of a slide coating die with a moving web as is known in the art. 
     FIG. 2 is a perspective view of an exemplary slide coater assembly. 
     FIG. 3 is a side sectional view of the slide coating assembly of FIG. 2 in a coating configuration. 
     FIG. 4 is a side sectional view of the slide coating assembly of FIG. 2 in a splice coating configuration. 
     FIG. 5 is a schematic illustration of a splice detector in accordance with the present invention. 
     FIG. 6 is a front view of one embodiment of the die edge of a slide coating die in accordance with the present invention. 
     FIG. 7 is an end view of the die edge of a slide coating die of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a web coating method and apparatus for maintaining a stable coating bead while coating over splices. An unstable coating bead is subject to fluctuations and non-uniformity of the wetting lines, such as movement of the static wetting line along the die edge, movement of the dynamic wetting line on the moving web, and necking of the coating bead along the edges. A stable coating bead refers to generally laminar flow of the coating fluid, and dynamic and static wetting lines that exhibit minimal movement along the moving web and die edge, respectively. 
     FIGS. 2 through 4 are schematic illustrations of a slide coater assembly  50  for maintaining a stable coating bead while coating over splices  100  on a moving web  60 . A series of slide coating bars  52 ,  54 ,  56 ,  58  are positioned in a downward sloping configuration at an angle a (see FIG.  3 ). One or more coating fluids V 1 , V 2 , V 3 , V 4  are extruded through a series of feed slots and are permitted to flow under the force of gravity towards a die edge  62 . In the coating position illustrated in FIG. 3, the coating fluids V 1 , V 2 , V 3 , V 4  form a coating bead  72  in coating gap  71  which is picked up by the moving web  60  to form the coated article  60 ′. Formation of the coating bead  72  is typically referred to as “pick up” of the coating fluid. 
     The die edge  62  is located immediately above a vacuum box  80 . A plurality of vacuum ports  67  are located across the width of the vacuum box  80  to minimize air flow resistance and generate a generally uniform vacuum pressure across the width of the coating bead  72 . The vacuum box  80  preferably has a front seal  82  that engages with the web  60  upstream from the die edge  62 . As best illustrated in FIG. 2, a pair of side seals  84 ,  86  are located along the sides of the vacuum box  80 . In the illustrated embodiment, outer plates  87 ,  89  surround the side seals  84 ,  86 . The side seals  84 ,  86  and front seal  82  are pivotally attached to the vacuum box  80  at locations  66 , as will be discussed below. The side seals  84 ,  86  preferably have a radius that corresponds to the radius of supporting roll  64  (or web  60  traversing the support roll  64 ). Slots may be formed in the edge of the side seals  84 ,  86  that engage with the supporting roll  64  and/or web  60  so as to enhance the sealing capabilities thereof The coating bead  72  completes the seal between the vacuum box  80  and the moving web  60 . A drain (not shown) is located at the bottom of the vacuum box  80  so that excess coating fluid collected in drain chamber  92  can be effectively collected. 
     FIG. 4 illustrates the splice coating gap  71 ′ between the die edge  62  and the backup roll  64  greater than the coating gap  71 . In the preferred embodiment, backup roll  64  is moved to a splice coating position  61  by a hydraulic piston with a check valve arrangement in an air over oil type actuation system, stepper motors, piezoelectric stacks on the mechanical stops, or a variety of other methods known to those of skill in the art. The vacuum gap  81  between the seals  82 ,  84 ,  86  and the backup roll  64  is increased to the splice clearance gap  81 ′. In the illustrated embodiment, increasing the coating gap  71  by a distance “x” does not increase vacuum gap  81  between the front seal  82  and the web  60  by a corresponding distance because the front seal  82  is located around the circumference of the support roll  64 . Consequently, the front seal  82  and side seals  84 ,  86  are rotated clockwise around a pivot point  66  to the splice coating position  85  by the actuator  116 , independent of the movement of the backup roll  64  along the axis “B”. 
     The actuator  116  may be located along a bottom edge of the front seal  82  to simultaneously rotate the front seal  82  and side seals  84 ,  86  to the splice coating position  85  independently of the movement of the backup roll  64 . The precise location of the backup roll  64 , the front seal  82  and the side seals  84 ,  86  in both the coating position and the splice coating position is preferably determined by mechanical stops. In an alternate embodiment, the entire vacuum box  80  could rotate away from the web  60  to a splice coating position  85 . 
     Increasing the coating gap  71  to the splice coating gap  71 ′ by moving the support roll  64  along the axis “B” permits the slide coating bars  52 - 58  to remain substantially fixed and stable during passage of the web splice  100  through the splice coating gap  71 ′. Additional structural support can be provided to the slide coating bars  52 - 58  to increase stability and reduce vibration. Retaining the slide coating bars  52 - 58  in a fixed and stable position permits a greater splice coating gap  71 ′ without destabilizing or breaking the coating bead  72 . In an alternate embodiment, the slide coater assembly  50  can be retracted along an axis “A” from the backup roll  64  to form the splice coating gap  71 ′. In yet another embodiment, both the backup roll  64  and the slide coater assembly  50  can be retracted to form the slide coating positions  61 . 
     In the illustrated embodiment, the coating configuration defines a coating gap  71  between the die edge  62  and the web  60  of about 0.203 millimeters to about 0.381 millimeters (0.008 to 0.015 inch). The front seal  82  forms a coating gap  81  of about 0.178 millimeters (0.007 inch) with the moving web  60 . In the splice coating position, the splice coating gap  71 ′ is increased by about 0.635 millimeters (0.025 inch) without destabilizing the coating bead. In the splice coating position  85 , the seals  82 ,  84 ,  86  are rotated around the pivot point  66  so that the splice clearance gap  81 ′ is about 0.813 millimeters (0.032 inch). Measurements are within about 0.0127 millimeters (±0.0005 inch). 
     The maximum attainable splice coating gap  71 ′ is dependent upon the viscosity and other properties of the coating fluid, speed of the moving web  60 , vacuum, and a variety of other factors. The maximum splice coating gap  71 ′ must be less than the gap at which the coating bead  72  destabilizes, typically less than 3.81 millimeters (0.150 inch) and more typically less than 1.78 millimeters (0.070 inch). The maximum splice coating gap  71 ′ for water based emulsions is typically less. Larger gaps forming a meta-stable coating bead can be used where the splice coating operation is on the order of a few seconds (usually less than 10 seconds). 
     A web thickness detector  102  illustrated in FIG. 5 is located after the unwinder/splicer (not shown) and before the vacuum box  80 . In the illustrated embodiment, the detector  102  is designed as a straight tube trip bar  104  adjacent an idler roll  110  suspended by a leaf spring  106  attached to an electrical switch  108 . A gap  109  is preferably maintained between the web  60  and the trip bar  104  when the web  60  is not moving. The gap  109  is typically about 0.0254 millimeters to about 0.381 millimeters (0.001 to 0.015 inch). 
     If a splice  100  or other defect in the web  60  is sensed by the detector  102 , a signal is sent to a controller  112 . The controller  112  increases the coating gap  71  to a splice coating gap  71 ′, typically by moving the support roll  64  along the axis “B” to splice coating position  61 , illustrated in FIG.  4 . At about the same time, the controller  112  rotates the seals  82 ,  84 ,  86  around the pivot point  66  a predetermined distance to splice coating position  85 . In the illustrated embodiment, the controller  112  uses the speed of the web  60  and distance from the detector  102  to the die edge  62  to calculate when the splice  100  will reach the die edge  62  and when to adjust the gaps  71 ,  81  to the coating gaps  71 ′,  81 ′. Alternatively, a webline controller signals the controller  112  when a splice is made. If the controller  112  detects a splice or other defect in the web  60  in excess of the splice coating gaps  71 ′,  81 ′ (uncoatable splice), the backup roll  64  and seals  82 ,  84 ,  86  can be moved to their fully retracted positions. The fully retracted position refers to a coating gap  71  at least large enough to break the coating bead  72 . In an alternate embodiment, two thickness detectors  102  could be used. The first is positioned to trigger when a coatable splice passes so that the coater  50  is configured to the splice coating positions  61 ,  85 . The second detector is positioned to trigger when an uncoatable splice passes so that the coater  50  is moved to the fully retracted position. 
     In an alternate embodiment, the gap  109  and/or sensitivity of the switch  108  can be configured so that only a splice  100  in excess of a predetermined thickness activates the switch  108 . In this embodiment, some splices pass the detector  102  without triggering the switch  108 . Consequently, the support roll  64  and the seals  82 ,  84 ,  86  are not moved to the splice coating positions  61 ,  85  unless the splice  100  exceeds the predetermined thickness. In yet another embodiment, the switch  108  is a measuring device capable of measuring absolute or incremental increases in web thickness. Absolute or incremental thickness data permits the controller  112  to anticipate an increase in web thickness in excess of the predetermined limit or to alert the operator to possible malfunctions. 
     In die coating, it is important to keep leaks in the vacuum system to a minimum since excess air flow can destabilize the coating bead. Increasing the coating gap  71  to the splice coating gap  71 ′ allows air to be drawn along the edges of the coating bead  72 . Where the die edge  62  is square, there is essentially no resistance to air flow so Bernoulli&#39;s equation applies. For example, assuming the height of the die edge  62  is negligible and the initial air velocity is zero, a typical vacuum of 249 Pascals (1 inch column water) in vacuum box  80  will draw air through a 0.254 millimeter (0.010 inch) coating gap along the edges of coating bead  72  at a rate of about 1230 meters/minute (4000 feet/minute) or 0.458 meter 3 /minute (3.33 ft 3 /minute) for each 30.48 centimeters (12 inches) of coating gap length. 
     In another embodiment of the present invention, the die edge  62  is deckled to minimize vacuum leaks along the splice coating gap  71 ′ that could destabilize the coating bead  72  and adversely affect the coating process. As illustrated in FIGS. 6 and 7, a conventional die edge geometry, such as a square lip, small flat or “ski-jump” design, can be maintained across the coating width  134  of the coating portion  136  of the coating bar  62 ′. Slide coating bar  62 ′ is constructed with seals  130 ,  132  that provide vacuum seal lands  130   a ,  132   a  at the edge of the coating width  134 . The vacuum seal lands  130   a ,  132   a  preferably have the same radius as the support roll  64  and the web  60 . The sealing lands  130   a ,  132   a  provide a vacuum seal to minimize the air flow through the coating gap  71  and slide coating gap  71 ′ into the vacuum box that can adversely affect coating performance. The tortuosity of seal gap  73  increases resistance to air flow that could destabilize the coating bead  72 . 
     In the embodiment illustrated in FIGS. 6 and 7, the seals  130 ,  132  and the coating portion  136  are retained to the slide coating bar  62 ′ by fasteners  138 , such as screws, so that they are easily changed in the event of damage that might cause streaking or to adjust for different coating widths. The members  130 ,  132 ,  136  are typically manufactured from a material such as titanium or stainless steel. 
     In one embodiment, the distance from sealing lands  130   a ,  132   a  to the web  60  defining the seal gap  73  is about the same as the coating gap  71 . The vacuum sealing lands  130   a ,  132   a  preferably have a surface area of about 6.45 millimeters 2  to about 645 millimeters 2  (0.1 inch 2  to about 1.0 inch 2 ) for each 2.54 centimeters (1 inch) of die edge length. The relatively large surface area of the seal lands  130   a ,  132   a  sufficiently restricts the flow of air through the seal gap  73  into the vacuum box  80  to minimize disruption of the coating bead. For example, in a coating configuration with seal lands 19.05 millimeters (0.75 inch) in length, a coating gap of 0.254 millimeter (0.010 inch) and a vacuum of 249 Pascals (1 inch column of water), air is drawn through the coating gap at a rate of 0.86 meter 3 /minute (0.635 ft 3 /minute) for each 30.48 centimeters (12 inches)of coating gap length. 
     The vacuum system  114  is designed to keep a generally uniform vacuum level, regardless of the gaps  71 ,  81  or splice gaps  71 ′,  81 ′, by utilizing a large capacity blower fan as the vacuum source that can compensate for the leakage. The vacuum system  114  preferably maintains the vacuum box  80  at the lowest possible vacuum, while still maintaining a stable coating bead  72 . In the illustrated embodiment, the vacuum system  114  maintains the vacuum box  80  at about 99.6 Pa (0.4 inch water column) to about 747 Pa (3.0 inches water column) during normal coating and splice coating. In one embodiment, the controller  112  signals the vacuum system  114  to increase the flow rate in anticipation of a web splice  100  and the resulting leakage around the vacuum box  80  so as to maintain a generally stable pressure in the vacuum box  80 . A method for adjusting flow rates in a vacuum system is discussed in U.S. Pat. No. 5,154,951 (Finnicum et al.). Alternatively, a solenoid operated valve could be positioned to vent the vacuum line to the vacuum box, thereby reducing the vacuum during coating. The valve would be in the open position during normal coating. The valve would be closed during splice coating to increase the vacuum to compensate for leakage around the vacuum box  80 . An adjustable valve could be placed in the venting line so that the leak to the vacuum system through the solenoid valve during normal coating corresponds to the leakage around the vacuum box in the splice coating position. 
     The internal volume of the duct work for the vacuum system  114  is preferably extremely large (by a factor of 5 or more) in relation to the volume of the vacuum chamber  92 . The large volume of the duct work tends to dampen or attenuate changes in vacuum caused by the splice gaps  71 ′,  81 ′. To a certain extent, the duct work volume acts like a reservoir of vacuum. The vacuum connection from the vacuum system  114  is well distributed across the front edge of the vacuum box  80  by vacuum ports  67  to provide uniformity of vacuum across the width of the coating bead  72 . Arranging the vacuum ports  67  near the front seal  82  also permits major leaks along the front seal  82  to be pulled out to the vacuum system  114  before entering the main vacuum chamber  92 . In the illustrated embodiment, the vacuum blower is a standard industrial blower available from New York Blower located in Willowbrook, Ill. under model number 1404. The blower is preferably operated at a small fraction of its rated capacity so that its suction pressure is nearly independent of the volume of air flowing through the blower. The speed of the blower is controlled by a DC drive system for accurate pressure control. 
     Various methods of coating a plurality of fluid layers onto a substrate are disclosed in commonly assigned U.S. Pat. Nos. 5,861,195; 5,843,530; and 5,849,363. Additional disclosure relating to a slide coater assembly is set forth in commonly assigned U.S. patent application Ser. No. 08/177,288 entitled “Coater Die Enclosure System, filed Jan. 4, 1995, and U.S. Pat. No. 5,725,665. 
     Any coated material, such as graphic arts materials, non-imaging materials such as adhesives and data storage media, and imaging materials such as photographic, photothermographic, thermographic, photoresists and photopolymers, can be coated using the method and apparatus of the present invention. Materials particularly suited for coating using the present method and apparatus include photothermographic imaging constructions (e.g., silver halide-containing photo sensitive articles which are developed with heat rather than with a processing liquid). Photothermographic constructions or articles are also known as “dry silver” compositions or emulsions and generally comprise a substrate or support (such as paper, plastics, metals, glass, and the like) having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example, for the silver ion in the non-photosensitive, reducible silver source; and (d) a binder. 
     Thermographic imaging constructions (e.g., heat-developable articles) can also be coated using the method and apparatus of the present invention. These articles generally comprise a substrate (such as paper, plastics, metals, glass, and the like) having coated thereon: (a) a thermally-sensitive, reducible silver source; (b) a reducing agent for the thermally-sensitive, reducible silver source (i.e., a developer); and (c) a binder. 
     Photothermographic, thermographic, and photographic emulsions used in the present invention can be coated on a wide variety of substrates. The substrate (also known as a web or support)  60  can be selected from a wide range of materials depending on the imaging requirement. Substrates may be transparent, translucent, or opaque. Typical substrates include polyester film (e.g., polyethylene terephthalate or polyethylene naphthalate), cellulose acetate film, cellulose ester film, polyvinyl acetal film, polyolefinic film (e.g., polyethylene or polypropylene or blends thereof), polycarbonate film, and related or resinous materials, as well as aluminum, glass, paper, and the like. 
     All patents and patent applications cited above are hereby incorporated by reference. The present invention has now been described with reference to several embodiments described herein. It will be apparent to those skilled in the art that many changes can be made in the embodiments without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures or methods described herein, but only to structures and methods described by the language of the claims and the equivalents thereto.