Abstract:
An apparatus for dispensing a fluid onto a moving substrate is disclosed. In one embodiment, the apparatus is a die comprising a die body having a cavity therein, wherein the cavity is in fluid communication with an applicator slot. The die is then oriented such that the applicator slot is positioned so as to dispense the fluid onto the moving substrate. The fluid is introduced into the die cavity such that the fluid is dispensed onto the moving substrate through the applicator slot. At least one end of the slot includes means for preventing lateral widening of the dispensed fluid. In another embodiment, means are disposed at both ends of the applicator slot. The apparatus is particularly useful when the capillary number characteristic for dispensing the fluid onto the moving substrate is less than 0.5

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. application Ser. No. 10/760,794, filed Jan. 20, 2004, now allowed, the disclosure of which is incorporated by reference in their entirety herein. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates generally to coating apparatus and methods. More particularly, the present invention relates to coating apparatus and methods adapted for use when the capillary number characteristic of the process is low.  
       BACKGROUND  
       [0003]     Coating a fluid onto a web of material is well known. Such coating can often be conveniently done using a coating die having a cavity communicating with an applicator slot. Liquid under pressure is introduced into the cavity, and is then extruded out of the applicator slot onto a desired substrate.  
         [0004]     Depending on the exact result desired, variations on this theme are numerous, with various coating aids being known. In particular, it is known that under certain conditions, particularly when the speed of the web past the coating die is very rapid, the material dispensed from the applicator slot may neck inwards erratically. One parameter that may be predictive of whether this necking will occur is the so-called “capillary number” characteristic of the coating process.  
         [0005]     The capillary number is a dimensionless parameter defined as:  
       Ca   =       μ   ⁢           ⁢   V     σ         
 
 where Ca is the capillary number, μ is the viscosity of the material dispensed or coated at the characteristic shear rate of the coating process, V is the speed of the moving web or other substrate, and μ is the surface tension of the material. At higher capillary numbers, the necking inwards of the edges of the dispensed material is more likely to be a problem. 
 
         [0006]     Various expedients are known by those skilled in the art for controlling this tendency of the dispensed material to pull inwards. The art is replete with mechanical aids to draw the dispensed material back to a predictable width. These are often called “edge guides” in the literature. They are particularly to be seen in descriptions of slide and curtain coating.  
         [0007]     However, literature is silent about what might be considered the opposite problem. Recently, attempts to coat high value materials (substrates) in very thin dry layers at very low speeds have resulted in coating of erratic width as capillary forces draw the dispensed material laterally along the gap between the die surface and the substrate at the ends of the applicator slot. This is because the thin dry layers coated onto the high value materials are diluted in a solvent for delivery to the substrate, which reduces viscosity and increases the coating thickness of the coating and solvent mixture delivered to the substrate.  
         [0008]     Improvements are desired.  
       SUMMARY OF THE INVENTION  
       [0009]     One aspect of the present disclosure is directed to a method of applying a material to a moving substrate, including providing a die comprising a die body having a cavity therein, wherein the cavity is in fluid communication with an applicator slot. This die is then oriented such that the applicator slot is positioned so as to dispense the material onto the substrate. The material is introduced into the die cavity such that the material is dispensed onto the substrate through the applicator slot. A means is disposed for preventing the widening of the dispensed material laterally of the applicator slot at least one end of the applicator slot.  
         [0010]     Another aspect of the present disclosure is directed to a coating die for dispensing material onto a moving substrate. The coating die includes a die body having a cavity therein, wherein the cavity is in fluid communication with an applicator slot. The coating die also includes means for preventing outward lateral movement of the dispensed material at least one end of the applicator slot.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]     In the several figures of the attached drawing, like parts bear like reference numerals, and:  
         [0012]      FIG. 1  is a perspective view of an example embodiment of a system including a coating die according to the present disclosure.  
         [0013]      FIG. 2  is an exploded perspective view of the die of  FIG. 1 .  
         [0014]      FIG. 3  is a plan view of an exemplary shim according to the present disclosure.  
         [0015]      FIG. 4  is a section view of another example embodiment of a coating die according to the present disclosure.  
         [0016]      FIG. 5  is a section view of another example embodiment of a coating die according to the present disclosure.  
         [0017]      FIG. 6  is a section view of another example embodiment of a coating die according to the present disclosure.  
         [0018]      FIG. 7  is a section view of another example embodiment of a coating die according to the present disclosure. 
     
    
     DETAILED DESCRIPTION  
       [0019]     In pre-metered coating, such as die coating, it is important for the width of the coated layer to be known to a high degree of accuracy. For the coated layer to be uniform, its width has to be equal to the width of the feed slot. It is, however, common to have some widening of the coating bead past the width of the feed slot, especially at low capillary number flow, such as slow coating speeds and low liquid viscosity. The bead widening causes non-uniformity of coating edges and, sometimes, an instability. These phenomena occur at low capillary number flow, which are typically less than about 0.5, and more typically less than 0.1, and can be less than 0.005, and even 0.001.  
         [0020]     The coating bead changes its width when pressure that is generated by capillary forces at the edges of the bead do not match pressure generated in the coating bead. If pressure in the coating bead is larger than a maximum capillary pressure the edge meniscus can sustain, the bead widens; if it is lower than a minimum pressure, the bead narrows. The minimum and maximum capillary pressures depend, among other things, on conditions at the static contact line on the coating die and contact angle between liquid and substrate. The pressures also depend on the flow rate of the dispensed material.  
         [0021]     An apparatus and method for controlling the static contact line on the die is disclosed herein. The static contact line can be either pinned or it could move to keep the static contact angle between the liquid and the die constant. When the static contact line is pinned, the range of admissible capillary pressures is the greatest.  
         [0022]     Generally, the present disclosure is directed to a coating die having a slot and a pinning location at one or both ends of the slot. The coating die also includes a cavity in fluid communication with the slot. Coating material within the cavity is forced through the slot and then coated onto a substrate. As the coating material exits the slot to form a coating bead, each pinning location holds the coating bead at the pinning location. By pinning the coating bead at each end, control of the coating bead is improved.  
         [0023]     Referring to  FIG. 1 , a perspective view of a portion of an exemplary coating line  10  using a die  12  according to the present disclosure is illustrated. The die  12  is positioned over substrate  14 , which in this illustration is a web of indefinite length material moving in direction “A,” but could be any other continuous or discrete article requiring coating. The substrate  14  is supported in this motion by a coating roll or drum  16 , which is rotatably mounted on support  18 . Material  17  to be dispensed by die  12  is delivered by a material supply source  20  and dispensed in a coating  22  upon the substrate  14  through applicator slot  24 .  
         [0024]     The illustrated embodiment of the die  12  includes a first portion  26 , a second portion  28 , and a shim  30 . However, this construction is merely convenient; for example, the shim  30  and its function are optional, and die  12  could be constructed as a single element. Also, one of ordinary skill in art will appreciate that the die could also include a replaceable and interchangeable lip portion including the applicator slot. Such a replaceable and interchangeable lip portion would allow the same main die body, including the cavity, to be used with various sized applicator slots. An example of such a replaceable and interchangeable lip portion is described in U.S. Pat. No. 5,067,432, to Lippert, which is incorporated by reference herein.  
         [0025]     Referring now to  FIG. 2 , an exploded perspective view of the die  12  is illustrated. In this view it can be better seen that first die portion  26 , second die portion  28 , and shim  30  each have a pair of notches  26 N,  28 N, and  30 N, respectively, that are in alignment when die  12  is assembled. Together the notches  26 N,  28 N, and  30 N define the lateral edges  32 ,  34  of the applicator slot  24  and prevent the lateral widening of the coating  22  (in  FIG. 1 ) during operation in low capillary number regimes. Typically, a low capillary number regime exists when the capillary number is less than about 0.1; but as discussed previously, the lower capillary number flow regime can also range up to a capillary number of about 0.5.  
         [0026]     As previously discussed, the present disclosure is directed to preventing widening of the coating bead by providing a pinning location for the edge of the coating bead. In some embodiments, the pinning location can be structural, such as a geometrical step with minimal radius of curvature at the apparent corner. Alternatively, physical properties of materials, such as a rapid or step-change in wetting properties of the die materials of construction, can be used to create a pinning location to prevent lateral widening of the coating bead. Also, the pinning location should span the entire length L of the wetted part of the die in the down web direction (as illustrated in  FIG. 7 ).  
         [0027]     Referring to  FIGS. 4-6 , illustrated are other example embodiments for creating pinning locations at the edge of the die slot. Referring to  FIG. 4 , a cross-section of an example embodiment of a coating die  412  is illustrated. The die  412  includes a slot  424  from which coating material  417  is dispensed. The slot  424  includes first  432  and second  434  opposed edges. Each edge  432 ,  434  includes a corner  433 ,  435  having a small radius. The small radius acts as a pinning location and the coating material  417  is kept pinned to the corners when coating material  417  is dispensed, thereby preventing lateral widening of the coating bead. The small radius is typically smaller than about 0.050 inches (1.3 millimeters), and ideally is a discontinuity forming an angle θ of about 90 degrees. However, the angle can be more or less than 90 degrees, depending on the particular application where the die is used. Also, the main body of the die  412  should be recessed a sufficient distance R from the pinning corner  435  so that surges and pulsation of the coated material from the die does not creep outside of the pinning corner  435  due to capillary action. While the particular recessed distance R depends on the coating application, for most low capillary number flows, 0.125 inches (3.18 millimeters) is sufficient.  
         [0028]     The die of the present disclosure can also be used with a vacuum assisted coating. Referring to  FIG. 5 , the coating die  512  can also include a sealing member  519  proximate to each end  532 ,  534  of the slot  524 . The sealing members  519  allow the die  512  illustrated in  FIG. 4  to be used in a vacuum assisted coating operation. The gap between G between the pinning corner  435  and the sealing member  519  should be a sufficient distance so that surges and pulsation of the coated material from the die does bridge between the pinning corner  435  and the sealing member  519  due to capillary action. While the particular gap distance G depends on the coating application, for most low capillary number flows, 0.063 inches (1.60 millimeters) is sufficient.  
         [0029]     Referring to  FIG. 6 , an example embodiment of a coating die  812  having a slot  824  with pinning locations at each edge  832 ,  834  of the slot  824  is illustrated. Pinning is accomplished using the physical properties of the die  812  and coating material  817 . In the example embodiment shown, the die  812  includes inlays  819  at the edges  832 ,  834  of the slot  824 . The inlays  819  are formed from a poorly or non-wetting material that is one where the material used for the inlay has a larger static contact angle with the coating material than the material used for the die body. Using a material not wetted by the coating material  817  creates the pinning locations by keeping capillary forces from pulling the coating material  817  onto the inlay  819 , thereby preventing lateral widening of the coating bead. Examples of poorly or non-wetting materials are PTFE (polytetrafluoroethylene), sold under the trade designation TEFLON, and acetal polyoxymethylene, sold under the trade designation DELRIN, both available from DuPont. Other materials include release polymers, such as fluoropolymers. Examples of fluoropolymers include basic monomers, such as, tetrafluoroethylene (TFE), vinyl fluoride (VF), perfluoroalkylvinylether (PAVE), 2,2-Bistrifluoromethyl-4,5 difluoro-1,3-dioxole (PDD), vinylidene fluoride (VDF), hexafluoropropylene (HFP), and chlorotrifluoroethylene (CTFE); and polymers, such as, fluorinated ethylene propylene (surface energy of about 18-22 dynes/cm), polyvinyl fluoride (surface energy of about 28 dynes/cm), polyethylene copolymer (surface energy of about 20-24 dynes/cm), and silicones (surface energy of about 24 dynes/cm). Other exemplary materials are described in U.S. Pat. No. 5,980,992, to Kistner et al. and U.S. Pat. No. 5,998,549, to Milbourn et al., both of which are incorporated by reference herein.  
         [0030]     Alternatively, the die body can be coated with a preferentially wetting material in the wetted region, such as gold plating. The preferentially wetted material keeps the coating bead from migrating or moving laterally out of the pinning location. In another example embodiment, hydrophobic tape can be applied along the edges of the wetted area of the die when using water-based coating materials or solutions.  
         [0031]     For each of the example embodiments described, it is preferred that the pinning location spans the entire length L of the slot  924  in the machine direction (as illustrated in  FIG. 9 ). Also, while both edges  932 ,  934  of the slot  924  typically have identical pinning arrangements, any combination of the types of the pinning locations described can be used, as the particular conditions of the use of the coating die require.  
         [0032]     Various modifications and alterations of the present invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein.