Abstract:
A method is taught for preparing a coating hopper prior to initiation of coating a liquid coating composition at a predetermined coating flow rate onto a moving substrate wherein the coating hopper includes at least one internal flow path therethrough. The internal flow path(s) of the coating hopper are drained. The liquid coating composition is introduced into internal flow path(s) at a purge flow rate which may be determined empirically. The internal flow path(s) are filled with the liquid coating composition and with the liquid coating composition then discharging from the internal flow path(s) onto a slide surface of the coating hopper, the liquid coating composition flowing down the slide surface and over a lip of the coating hopper at the purge rate to a drain. The flow of the liquid coating composition through the internal flow path(s) is maintained at the purge rate until air within the internal flow path(s) has been displaced from the coating hopper. Preferably, the internal flow path(s) are flushed with water prior to draining.

Description:
FIELD OF THE INVENTION 
     The invention relates to delivery of a liquid composition to a substrate surface to form a coated layer thereupon, more particularly to a method and apparatus for preparing a coating hopper prior to initiation of delivery of a composition to a substrate surface, and most particularly to such a method and apparatus wherein composition is introduced into an empty coating hopper. 
     BACKGROUND OF THE INVENTION 
     In forming a flowing sheet of a liquid composition for coating onto a substrate surface, the shape of flowing liquid composition is reconfigured from flow through a typically cylindrical conduit to flow though any of a variety of apparatus that create a sheet flow. These apparatus for creating a sheet flow are well known in the art and include, for example, a die, a distributor, an extruder, a weir, a slide surface, and a hopper. As used herein, all such types of apparatus are referred to collectively as hoppers. A hopper may comprise one or more parallel longitudinal members (typically referred to as hopper bars in the art) which are oriented transverse to the direction of liquid flow, which members may be bolted together or otherwise attached to form a hopper unit. A primary member may be referred to as a “hopper body,” and one or more secondary members as “hopper bars.” Typically, hopper bars are configured on their mating surfaces in such a way that internal flow passages for the composition are formed within the hopper when the bars are assembled together. Within a hopper, a flow path for liquid composition typically includes (in flow sequence) an inlet, one or more transverse distribution voids known as cavities, and a slotted exit from each cavity communicating with either a successive cavity or the exterior of the hopper. The last such slot is commonly known as an exit slot. Alternatively, a hopper distribution apparatus may include a distribution chamber open at the top and having a wall forming a weir for overflow cascade or curtain coating therefrom, the wall and weir being within the scope of the current invention. 
     In an extrusion hopper, the downstream end of the exit slot typically defines a coating lip from which the extruded sheet of composition is transferred directly to the passing substrate. In slide hoppers, as are used typically in the manufacture of photographic films and papers, composition is extruded from the exit slot onto an inclined slide surface terminating at a lower edge in a coating lip. The extruded sheet flows down the slide surface under gravity and is transferred to the passing substrate either through a dynamic bead, as in bead coating, or a falling curtain, as in curtain coating. 
     It is well known in the art that bubbles or particulate debris may be carried into or formed within a hopper and can become lodged in the composition flow path at any of numerous locations in or on the hopper. These bubbles and/or particulates can subsequently cause flow disturbances during coating resulting in unacceptable and continuous thickness variations in the coating as applied to the substrate. Further, such bubbles and debris may become dislodged during coating and be transferred to the substrate, resulting in unacceptable discontinuous thickness variations. Thus, it becomes very important that all debris and bubbles be eliminated from a hopper prior to commencing a coating operation. A strategy in the known art for accomplishing this is to conduct any of various rigorous hopper cleaning protocols using flush water. Typically, these protocols involve supplying particle-free and bubble-free flushing water through a coating composition/water manifold and valve arrangement positioned proximate to the coating hopper. The particle-free and bubble-free flushing water is pumped continuously through the hopper to a drain, the hopper being out of coating position. Mechanical devices such as plastic picks may be inserted into the hopper and agitated to assist in dislodging bubbles and composition residues from prior coatings into the flush water. Typically, such cleaning of a coating hopper used to coat radiation-sensitive coatings is conducted under white lights during a delay or product change in the coating operation. Flush cleaning may proceed for several minutes or more, until an operator is satisfied that no further composition or bubbles are exiting the hopper, and that the hopper is ready for introduction of composition. 
     One common method of coating hopper preparation used in the photographic coating art is to flush the hopper with water to displace air from the hopper much as described above. Once the flushing with water step is completed then coating composition is used to displace the water from the hopper, resulting in a hopper that is apparently ready for coating. Ideally, after all air and particulates have been displaced from the hopper, liquid coating composition is introduced into the hopper through the coating composition/water manifold and valve arrangement mentioned above. Specifically, the flow of flush water is stopped and the flow of liquid coating composition is begun. The liquid coating composition then begins to displace the water. Because the coating composition typically is an aqueous gelatin solution or emulsion, and because flow is not laminar through much of the flow path, the displacement typically does not occur as plug flow but rather there is substantial mixing of the coating composition with the residual water in the hopper. In good practice, the hopper may not be moved into coating position and coating may not commence until substantially all the residual water is eliminated from the hopper, and the slide surfaces and coating lip are observed to be conveying composition with absolute visual uniformity and no dilution. 
     For multiple-slot hoppers that simultaneously deliver a plurality of superimposed layers of coating compositions to form a multiple-layer composite coating, it is typical that the individual slots in the coating hopper are changed over from flush water to coating composition sequentially. It is also typical that the lower-most hopper slot is changed over first and the higher coating slots are changed over in sequence moving up the slide hopper surface. However, because in some facilities and coating applications, the top layer is more critical than the lower layers, it is sometimes preferable to purge the upper-most hopper slot first and purge sequentially down the hopper to purge the lower slots. The quality of hopper preparation for each slot is confirmed before the next slot is changed over. Thus the lower compositions may be flowed to drain for an extended period of time before the hopper preparation is complete. 
     In the practice of the method of the present invention purging of all of the hopper slots can also be performed simultaneously. To purge simultaneously the coating compositions flowing to each slot should reach the slide surface of the slide hopper at about the same time. Purge flow rates, coating composition delivery line lengths and head losses, and coating composition viscosity and density will all have to be considered if a simultaneous purge of all of the coating slots is contemplated. 
     Further, each composition delivery system typically is changed over from flush water to composition flow at a purge flow rate (which is significantly higher than the actual coating composition flow rate) to displace water more rapidly. The length of time required for the purge flow step and the actual flow rate during the purge flow step is typically empirically determined for each coating hopper and the various coating compositions used therein. However, in all cases, a large amount of coating composition may be wasted in displacing water from the hopper. Therefore, the known art hopper preparation method is costly, both in terms of lost machine time and in terms of coating composition waste. 
     What is needed is an improved method whereby a clean hopper can be filled with coating composition and prepared for coating initiation in a shorter time and with reduced composition waste as compared with the known art method. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to reduce the amount of coating composition wasted in preparing a coating hopper prior to initiation of coating. 
     It is a further object of the present invention to reduce the time required to prepare a coating hopper prior to initiation of coating. 
     Yet another object of the present invention is to prevent the starting of a coating with composition which is diluted with water due to incomplete purging of residual flush water. 
     Briefly stated, the foregoing and numerous 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 preferably by flowing flush water through the coating hopper (which may be a slide hopper or an extrusion hopper), the flush water filling the internal passages and exiting an exit slot or die, draining the flush water from the coating hopper, and flowing a coating composition at a purge flow rate through the coating hopper and across the external surfaces of the hopper normally wetted by coating composition. The purge flow rate may be greater than, equal to, or less than an actual or predetermined coating flow rate depending on a number of factors discussed hereinafter. With a slide hopper coating operation the flush water also covers the slide surface of the coating hopper. Alternatively, the method of the present invention may be practiced by flowing a coating composition through the coating hopper at a purge flow rate which is greater than an actual or predetermined coating flow rate while not performing a precursor water flush step at all. 
     The purge flow rate for purging with coating composition will vary from system to system. Although the purge flow rate will generally be greater than the actual or predetermined coating flow rate, the purge flow rate is actually dependent upon a number of factors. These factors include the viscosity and density of the coating composition, whether or not the coating composition is Newtonian in nature, whether or not the coating composition contains a surfactant, and the internal geometry of the coating hopper. A higher viscosity will generally allow for purging to take place at a lower purge flow rate. Similarly, a higher density will generally allow for purging to take place at a lower purge flow rate. As to whether or not the coating composition is Newtonian, Newtonian fluids are generally better for purging air. 
     Further, it should be appreciated that if the components of the coating delivery system are more difficult to purge of air than the coating hopper itself then such components should be positioned upstream of the coating hopper drain valve. Also, that portion of the coating supply line from hopper drain valve up to the hopper should be oriented to have at least some vertical slope component such that the drain valve is at the lowest position and such that there is no horizontal or sagging section in the supply line between the drain valve and the hopper. Horizontal lines and lines with sags are an obstacle to the air purging process. 
     The method of the present invention is useful in providing uniform coatings of liquid compositions to moving webs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a single-slot hopper for coating a single layer of a liquid composition. 
     FIG. 2 is a schematic diagram similar to FIG. 1 showing a multiple-slot coating hopper for simultaneously coating a plurality of liquid compositions as a composite layer; and 
     FIG. 3 is a schematic diagram of the same multiple-slot coating hopper shown in FIG. 2 modified with valving and drain lines to allow for the practice of the method of the present invention for preparing a hopper for coating. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning first to FIG. 1, there is depicted a schematic of a single-slot coating hopper  10 . Hopper  10  is formed as is well known in the coating art and is shown as a dual-cavity single-slide extrusion hopper, although other well-known types of hoppers, extruders, and dies as described above may also benefit from use of the present invention. Hopper  10 , shown in elevational cross-section, includes a front section  12  having an inlet  14 , a primary transverse distribution cavity  16 , an inner slot  18 , a secondary transverse distribution cavity  20 , an outer or metering slot  22 , an inclined slide surface  24 , and a coating lip  26 . Hopper  10  further includes a back plate  28  which extends above inclined slide surface  24  to form a back land surface  30  having an upper corner  31 . The inlet  14 , slots  18 ,  22 , and distribution cavities  16 ,  20  comprise internal passages of the hopper  10 , and the back land surface  30 , slide surface  24 , and lip  26  comprise the external surfaces of the hopper  10 . 
     Residing adjacent the hopper  10  is a coating backer roller  32  about which a web  34  is conveyed. Typically, the hopper  10  is movable from a non-coating position toward the coating backer roller  32  and into a coating position. 
     There is a vessel  36  in which a liquid coating composition  38  is stored. The liquid coating composition  38  in vessel  36  is pumped via a composition delivery system  40  through delivery line  42  to inlet  14  of hopper  10 . Such a delivery system  40  is well known in the art, comprising apparatus and controls for metering flow, maintaining temperature, eliminating bubbles, and like functions, shown as unit  44  within delivery system  40 , and need not be further described herein. 
     In delivery system  40 , there is a composition control valve  46  for starting and stopping the flow of composition from vessel  36  through delivery line  42  to hopper inlet  14 . There is also a flush water control valve  48  for starting and stopping the flow of flush water from a source  50  through delivery line  42  to the inlet  14  of hopper  10 . With hopper  10  residing in a non-coating position, water from source  50 , or coating composition  38  from vessel  36 , or a combination of both water and coating composition  38  may be delivered to inlet  14  of hopper  10 . The liquid fills transverse cavities  16 ,  20  and is transmitted therethrough to slots  18 ,  22 . Upon exiting slot  22  the liquid forms a layer  51  flowing down slide surface  24  and over lip  26  to form a free-falling liquid curtain  52 . In the non-coating position the free-falling curtain  52  is captured in drain  54 . 
     In operation in accordance with a prior art method of preparing a coating hopper  10 , for coating, flush water control valve  48  is opened to pass flush water through the inlet  14 , transverse cavities  16 ,  20 , slots  18 ,  22 . The flush water then flows down the slide surface  24 , and off the lip  26  into drain  54 . Flushing is continued until all air is removed from the internal passages and no bubbles or particles are detected clinging to the hopper  10  anywhere in the composition flow path. The flush water control valve  48  then is gradually closed and, simultaneously, composition control valve  46  is gradually opened, and unit  44  begins delivering composition  38  at a pre-determined flow rate through delivery line  42  to hopper  10  to displace residual flush water from the hopper  10  to the drain  54 . As already noted above, such displacement may require up to several minutes of composition flow, at a significant waste in composition and coating machine time. 
     Referring next to FIG. 2, there is schematically depicted a well known multi-slot coating hopper  60  which may be used to deliver and coat multiple coating compositions simultaneously as a stacked composite of layers. Coating hopper  60  is shown as having only two slots for purposes of simplicity but multiple slot hoppers are known which can deliver a composite layer comprised of five or six (or even more) coating composition layers. Operationally, hopper  60  and the accompanying supply system is similar to that discussed above with reference to hopper  10 . 
     Hopper  60 , shown in elevational cross-section, includes a front section  62  having an inlet  64 , a middle section  63 , and a back plate  65 . There is a primary transverse distribution cavity  66 , an inner slot  68 , a secondary transverse distribution cavity  70 , and an outer or metering slot  72  between front section  62  and middle section  63 . Front section  62  includes an inclined slide surface  74 , and a coating lip  76 . There is an inlet  77 , a primary transverse distribution cavity  78 , an inner slot  80 , a secondary transverse distribution cavity  82 , and an outer or metering slot  84  between middle section  63  and back plate  65 . There is an inclined slide surface  86  at the top of middle section  63 . Back plate  65  extends above inclined slide surface  86  to form a back land surface  90  having an upper corner  91 . The inlets  64 ,  77 , inner slots  68 ,  80 , outer slots  72 ,  84 , and distribution cavities  66 ,  70 ,  78 ,  82  comprise internal passages of the hopper  60 , and the back land surface  90 , slide surface  74 ,  86  and lip  76  comprise the external surfaces of the hopper  60 . 
     Residing adjacent the hopper  60  is a coating backing roller  92  about which a web  94  is conveyed. Typically, the hopper  60  is movable from a non-coating position toward the coating backing roller  92  and into a coating position. 
     There is a vessel  96  in which a first liquid coating composition  98  is stored. The liquid coating composition  98  in vessel  96  is pumped via a composition delivery system  100  through delivery line  102  to inlet  64  of hopper  60 . Such a delivery system  100  is well known in the art, comprising apparatus and controls for metering flow, maintaining temperature, eliminating bubbles, and like functions, shown as unit  104  within delivery system  100 , and need not be further described herein. 
     In delivery system  100 , there is a composition control valve  106  for starting and stopping the flow of composition from vessel  96  through delivery line  102  to hopper inlet  64 . There is also a flush water control valve  108  for starting and stopping the flow of flush water from a source  110  through delivery line  102  to the hopper inlet  64  of hopper  60 . With hopper  60  residing in a non-coating position water from source  110 , or coating composition  98  from vessel  96 , or a combination of both water and coating composition  98  may be delivered to inlet  64  of hopper  60 . The liquid fills transverse distribution cavities  66 ,  70  and is transmitted therethrough to outlet slot  72 . Upon exiting slot  72  the liquid forms a layer  120  flowing down slide surface  74  and over lip  76  to form a free-falling liquid curtain  112 . In the non-coating position the free-falling curtain  112  is captured in drain  116 . 
     There is a vessel  126  in which a second liquid coating composition  128  is stored. The liquid coating composition  128  in vessel  126  is pumped via a composition delivery system  130  through delivery line  137  to inlet  77  of hopper  60 . Delivery system  130  is identical to delivery system  100 . 
     In delivery system  130 , there is a composition control valve  136  for starting and stopping the flow of composition from vessel  126  through delivery line  137  to hopper inlet  77 . There is also a flush water control valve  138  for starting and stopping the flow of flush water from a source  110  through delivery line  137  to inlet  77  of hopper  60 . With hopper  60  residing in a non-coating position water from source  110 , or coating composition  128  from vessel  126 , or a combination of both water and coating composition  128  may be delivered to inlet  77  of hopper  60 . The liquid fills transverse distribution cavities  78 ,  82  and is transmitted therethrough to outer slot  84 . Upon exiting slot  84  the liquid forms a layer  140  flowing down slide surface  86 , over layer  120  on slide surface  74 , and over lip  76  to form a free-falling liquid curtain  112 . In the non-coating position the free-falling curtain  112  is captured in drain  116 . 
     The prior art method of preparation of hopper  60  is substantially the same as the preparation of hopper  10  as described above. Flushing of the internal flow paths is preferably performed sequentially. Flush water control valve  108  is opened to pass flush water through the inlet  64 , transverse cavities  66 ,  70 , and slots  68 ,  72 . The flush water then flows down the slide surface  74 , and off the lip  76  into drain  116 . Flushing is continued until all air is removed from the internal passages of that portion of hopper  60  and no bubbles or particles are detected clinging to the hopper  60  anywhere in the composition flow path. Flush water control valve  138  is then opened to pass flush water through the inlet  77 , transverse cavities  78 ,  82 , and slots  80 ,  84 . The flush water then forms a layer  140  that flows down the inclined slide surface  141 , and off the lip  76  into drain  116 . Flushing is continued until all air is removed from the internal passages of that portion of hopper  60  and no bubbles or particles are detected clinging to the hopper  60  anywhere in the composition flow path. The flush water control valves  108 ,  138  are then gradually closed sequentially and, simultaneously, composition control valves  106 ,  136  are gradually opened sequentially, and units  104 ,  134  begin delivering compositions  98 ,  128  at a pre-determined flow rate through lines  102 ,  137  to hopper  60  to displace residual flush water from the hopper  60  to the drain  116 . As already noted above, such displacement may require up to several minutes of composition flow, at a significant waste in composition and coating machine time. 
     It has now been surprisingly found that a hopper can be prepared much more efficiently in terms of composition waste and machine down time, and with at least equal reliability in terms of purging of hopper air, by filling the empty hopper directly with the liquid coating composition rather than using the prior art strategy of first purging all hopper air with water and then purging water with the liquid coating composition. Preferably the method of the present invention is practiced by first flushing the hopper with water and then draining the hopper prior to purging the hopper with the liquid coating composition(s). 
     The method of the present invention will be discussed with reference to FIG. 3 which is identical to FIG. 2 with the exceptions of a drain valve  150  and drain leg  152  in delivery line  102 , and drain valve  154  and drain leg  156  in conduit  137 . For simplicity, all other elements in FIG. 3 are numbered identically to their counterparts in FIG.  2 . 
     In the practice of the method the present invention, valves  106 ,  136  are opened to introduce composition  98 ,  128  into delivery lines  102 ,  137  to flow into hopper  60 . Preferably, an initial purge flow rate is established for each composition  98 ,  128  which typically is significantly higher than the eventual coating flow rate. It is more efficient in terms of liquid waste to deliver composition at a high flow rate for a short period of time than at a lower flow rate for a longer period of time. After an internal flow path (e.g. inlet  64 , transverse distribution channels  66 ,  70 , inner slot  68  and outer slot  72 ) has been purged of air, the flow rate is reduced to the coating flow rate, and once all flow paths through the hopper  60  are purged, coating can commence. 
     Preferably, in a multiple layer delivery, composition flows in the multiple delivery systems are timed and sequenced, based on the length of delivery lines  102 ,  137 , such that all compositions arrive at their respective hopper slide surfaces  74 ,  141  simultaneously, as this promotes the most uniform wetting of the slide surfaces and hopper lip  76 . To ensure, however, that upper layer(s)  140  do not inadvertently arrive first and flow down into the empty slots of lower layers, the timing of the lower layers may be sequence-biased such that the lowermost composition arrives first. 
     In practice, the hopper  60  is preferably cleaned and flushed with water as described above in the prior art method, either after installation of the hopper  60  or at the conclusion of a coating event to prepare for the next coating event. Thus, to facilitate hopper preparation in accordance with a method of the present invention, means are preferably provided for easy and automatic draining of flush water from the hopper prior to introduction of composition. Draining of the flush water is accomplished with drain valve  150  and drain leg  152  in delivery line  102 , and drain valve  154  and drain leg  156  in delivery line  137  which are installed at the lowest point of each delivery line  102 ,  137 . In practice, preferably, the drain valves  150 ,  154  are disposed at elevations lower than inlets  64 ,  77  so that delivery lines  102 ,  137  are routed upwards to hopper  60  to aid in displacement of air by coating composition. Preferably, each drain valve  150 ,  154  may be opened and closed either automatically as part of a pre-determined hopper preparation procedure, or manually by an operator as desired. Preferably, each drain leg  152 ,  156  is also provided with a conductivity sensor and alarm (not shown) to prevent coating of incorrect composition thickness in the event the drain valve is not fully closed or leaks during coating. In operation, each drain valve  150 ,  154  is opened for a pre-determined length of time at the conclusion of flush cleaning to allow for substantially all water to drain from the composition flow portions of the hopper  60 . The drain valves  150 ,  154  are then closed prior to introduction of composition  98 ,  128  into the hopper  60 . 
     It is not necessary that the internal surfaces of the hopper  60  be completely dry when compositions  98 ,  128  are introduced into the hopper  60 . However, to promote uniform distribution of compositions  98 ,  128  on the hopper slide surfaces  74 ,  141 , it is advantageous to precondition the hopper slide surface such that it is wettable. This may be readily achieved by stopping the flow of flush water, applying a surfactant to the slide surface and hopper lip, and then resuming flow of flush water for a short time to rinse away excess surfactant. Exemplary surfactants that may be used include SPO (2-(2-(4-(1,1,3,3-tetramethylbutyl)phenoxy)ethoxy)-ethanesulfonic acid, sodium salt in water), and Alkanox XC (Naphthalenesulfonic acid, sodium salt isopropylated in water). 
     It is highly desirable that both back land  90  and lip  76  be scrupulously clean and dry to achieve a uniform transverse wetting line of composition at the back land  90  and on the backside of the lip  76 . Thus, when water flow is subsequently stopped and draining has commenced, the back land, slides, edging, and hopper lip preferably are manually dried with a lint-free fabric. 
     Using the prior art protocol, typically between 2 and 6 minutes of composition flow at between 2 and 5 liters per minute are required to completely purge water from a hopper having a 50 to 60 inch coating width, thus consuming between 2 and 30 liters of composition, and typically at least 6 liters. 
     In contrast, using the method of the present invention, the same hopper can be purged in less than 1 minute, and in many cases in only 10 to 20 seconds. Thus, typical composition waste is less than 2 liters, and in most cases is less than 1 liter, at a savings of at least 5 minutes of machine time and removal of concern for accidental dilution of composition at the start of a coating by incompletely-purged residual flush water as can happen with the prior art method. 
     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 an illuminating sense. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                                                PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 single slot coating hopper 
               
               
                 12 
                 front section 
               
               
                 14 
                 inlet 
               
               
                 16 
                 primary transverse distribution cavity 
               
               
                 18 
                 inner slot 
               
               
                 20 
                 secondary transverse distribution cavity 
               
               
                 22 
                 outer or metering slot 
               
               
                 24 
                 inclined slide surface 
               
               
                 26 
                 coating lip 
               
               
                 28 
                 back plate 
               
               
                 30 
                 back land surface 
               
               
                 31 
                 upper corner 
               
               
                 32 
                 coating backing roller 
               
               
                 34 
                 web 
               
               
                 36 
                 vessel 
               
               
                 38 
                 liquid coating composition 
               
               
                 40 
                 delivery system 
               
               
                 42 
                 delivery line 
               
               
                 44 
                 unit comprising apparatus and controls 
               
               
                 46 
                 composition control valve 
               
               
                 48 
                 flush water control valve 
               
               
                 50 
                 flush water source 
               
               
                 51 
                 layer 
               
               
                 52 
                 liquid curtain 
               
               
                 54 
                 drain 
               
               
                 60 
                 multi-slot coating hopper 
               
               
                 62 
                 front section 
               
               
                 63 
                 middle section 
               
               
                 64 
                 inlet 
               
               
                 65 
                 back plate 
               
               
                 66 
                 primary transverse distribution cavity 
               
               
                 68 
                 inner slot 
               
               
                 70 
                 secondary transverse distribution cavity 
               
               
                 72 
                 outer or metering slot 
               
               
                 74 
                 inclined slide surface 
               
               
                 76 
                 coating lip 
               
               
                 78 
                 primary transverse distribution cavity 
               
               
                 80 
                 inner slot 
               
               
                 82 
                 secondary transverse distribution cavity 
               
               
                 84 
                 outer or metering slot 
               
               
                 86 
                 inclined slide surface 
               
               
                 90 
                 back land surface 
               
               
                 91 
                 upper corner 
               
               
                 92 
                 coating backing roller 
               
               
                 94 
                 web 
               
               
                 96 
                 vessel 
               
               
                 98 
                 first liquid coating composition 
               
               
                 100 
                 delivery system 
               
               
                 102 
                 delivery line 
               
               
                 104 
                 unit comprising apparatus and controls 
               
               
                 106 
                 composition control valve 
               
               
                 108 
                 flush water control valve 
               
               
                 110 
                 flush water source 
               
               
                 112 
                 liquid curtain 
               
               
                 116 
                 drain 
               
               
                 120 
                 layer 
               
               
                 126 
                 vessel 
               
               
                 128 
                 second liquid coating composition 
               
               
                 130 
                 delivery system 
               
               
                 134 
                 unit comprising apparatus and controls 
               
               
                 136 
                 composition control valve 
               
               
                 137 
                 delivery line 
               
               
                 138 
                 flush water control valve 
               
               
                 140 
                 layer 
               
               
                 141 
                 inclined slide surface 
               
               
                 150 
                 drain valve 
               
               
                 152 
                 drain leg 
               
               
                 154 
                 drain valve 
               
               
                 156 
                 drain leg