Abstract:
A system prevents gas currents from impacting a coating process for a multi-slot slide bead coating apparatus. The system includes a multi-layer slide coating apparatus for forming a multilayer composite including a carrier layer and a slide surface; and a web for coating by the multi-slot slide bead coating apparatus. Additionally, a proximity shield is placed in close proximity to both the web and the slide surface of the multi-slot slide bead coating apparatus such that gas currents do not disturb the multilayer composite on the slide surface.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to the field of coating a moving web, and in particular to slide bead coating. More specifically, the invention relates to a proximity shield to prevent gas currents from disturbing the coating layers as they are applied using slide bead coating.  
       BACKGROUND OF THE INVENTION  
       [0002]     Bead coating is well known in the prior art as described, for example, in U.S. Pat. No. 2,761,791. One of ordinary skill in the art uses bead coating to apply multiple layers of liquid to a moving web. In the method typically referred to as slide bead coating, a multilayer composite comprised of superimposed individual coating layers is delivered to the moving web through the use of a coating die. At the end of the coating die, the layers form a continuous liquid bridge or coating bead between the die and the moving web. The slide bead coating method is useful for making thin, highly uniform, composite elements suitable for numerous applications including photographic, thermographic, x-ray, and photoelectric films, among others.  
         [0003]     U.S. Pat. No. 6,579,569 teaches the use of a carrier slide coating method where the viscosity of the lowermost layer or carrier layer is less than 1 cp and the wet thickness of the carrier layer is less than 5 microns. The carrier layer is comprised of a single organic solvent or a blend of organic solvents. Additional coating layers with higher viscosity are applied to the web on top of the carrier layer. This method allows for application of the coatings at high web speeds and with reduced coating artifacts caused by contamination of the slide surface.  
         [0004]     Previous attempts to eliminate the disturbance of flow of photographic coating compositions caused by impact of gas surrounding a slide coating apparatus have not been entirely successful. In some coating rooms, peak gas velocities of 200 feet per minute have been measured. The protective enclosures described in U.S. Pat. No. 4,287,240 have been found to reduce gas flow around the coating station. The enclosures are formed of a foraminous material and are effective in deflecting, diffusing and decelerating ambient forced gas currents. Such forced gas currents are frequently generated by the ventilating and exhausting equipment in the vicinity of the coating apparatus, or by the opening and closing of doors to the coating room, or by movement of personnel in the vicinity of the coating apparatus. The foraminous enclosure is designed to enclose the entire slide coating apparatus and the coating zone, and is not closely spaced to the slide surface of the coating apparatus. Indeed, in U.S. Pat. No. 4,287,240 it is stated that the enclosure should be spaced in the range of about 5 to about 60 cm from the coating composition. Optimum results have been achieved with enclosures formed of a plurality of spaced wall members, each of which is composed of a foraminous material. The best enclosures reduce peak velocities of gas flow to approximately 13 cm/sec. However, even such velocities have been shown to cause disturbances in the coating compositions on the slide which often appear as broad longitudinal streaks in the resulting coating. In most products these streaks are objectionable.  
         [0005]     WO Patent No. 90/01178 describes the use of a close proximity shield to protect liquid flowing down the inclined slide surface from adverse effects of convection gas currents. The temperature of the proximity shield was described to be kept at the same temperature as the coating fluid to prevent condensation of evaporated water. The proximity shield was required to be uniformly spaced 6 to 10 mm from the liquid surface. The proximity shield was described to extend over substantially all the inclined slide surfaces of the coating apparatus. The precise position of the end of the shield was not specified, however it was described to be far enough from the coating backing roller that it allows the coating bead to be viewed by the operators. At least 13 mm spacing would be required for the operator to view the coating bead. The convection gas flow between the solid surface of the coating apparatus and the shield was minimized by closing the space between the shield and the backland area above the uppermost metering slot. Although this shield may work well for the coating composition and thickness described therein, the shield-to-web gap described, therein, is not adequate for carrier slide composition as described in U.S. Pat. No. 6,579,569. Unwanted bands of non-uniform density, or longitudinal streaks, occur when the proximity shield is spaced far enough from the coating backing roller for the operator to view the coating bead.  
       PROBLEM TO BE SOLVED BY THE INVENTION  
       [0006]     Longitudinal streaks appear as a result of slide bead coating a coating composition that includes higher viscosity layers and a bottom most layer having a viscosity of less than 1 cp. Accordingly, elimination of these streaks and bands is paramount for a high quality coating process.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is directed to overcoming one or more of the problems set forth above. One aspect of the present invention provides a system for preventing gas currents from impacting a coating process for a multi-layer slide coating apparatus, the system includes a multi-layer slide coating apparatus for forming a multilayer composite including a carrier layer and an inclined slide surface; and a web for coating by the multi-layer slide coating apparatus. Additionally, a proximity shield is placed in close proximity to both the web and the inclined slide surface of the multi-layer slide coating apparatus such that gas currents do not disturb the multilayer composite on the inclined slide surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The above and other features and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein:  
         [0009]      FIG. 1  is a schematic of an exemplary multi-slot slide bead coating apparatus including a proximity shield which may be used in the practice of the method of the present invention;  
         [0010]      FIG. 2  shows the proximity shield in profile;  
         [0011]      FIG. 3  is a close-up drawing of different embodiments of the shield lip area; and  
         [0012]      FIG. 4  is a drawing of the edge of the shield and how it integrates with the edge guide. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]     Referring to  FIG. 1 , a schematic shows an exemplary multi-slot slidebead coating apparatus  10  suitable for practicing the method of the present invention. The multi-slot slide bead coating apparatus  10  is typically used to deliver and coat multiple coating compositions simultaneously as a stacked composite of layers. Multi-slot slide bead coating apparatus  10  is shown as having only four slots but multiple slot coating apparatuses may have fewer than four slots and are also known to deliver a composite layer comprised of five or six (or even more) coating composition layers.  
         [0014]     Multi-slot slide bead coating apparatus  10 , shown in a side elevation cross-section in  FIG. 1 , includes a front section  15 , a second section  20 , a third section  25 , a fourth section  30 , and a back plate  35 . There is an inlet  40  into second section  20  for supplying coating liquid to first metering slot  45  via pump  50  to thereby form a lowermost layer or carrier layer  55 . There is an inlet  60  into third section  25  for supplying coating liquid to second metering slot  65  via pump  70  to form layer  75 . There is an inlet  80  into fourth section  30  for supplying coating liquid to third metering slot  85  via pump  90  to form layer  95 . There is an inlet  100  into back plate  35  for supplying coating liquid to fourth metering slot  105  via pump  110  to form layer  115 . Each metering slot  45 ,  65 ,  85 , and  105  includes a transverse distribution cavity. Front section  15  includes a first inclined slide surface  120 , and a coating lip  125 . There is a second inclined slide surface  130  at the top of second section  20 . There is a third inclined slide surface  135  at the top of third section  25 . There is a fourth inclined slide surface  140  at the top of fourth section  30 . Back plate  35  extends above the fourth inclined slide surface  140  to form a back land surface  145 .  
         [0015]     Residing adjacent to the multi-slot slide bead coating apparatus  10  is a coating backing roller  150  about which a web  155  is conveyed. Typically, the multi-slot slide bead coating apparatus  10  is movable from a non-coating position toward the coating backing roller  150  and into a coating position.  
         [0016]     Still referring to  FIG. 1 , the method of the present invention has a proximity shield  160  (also shown in greater detail in  FIG. 2 ) placed a certain distance from the first inclined slide surface  120 , forming a shield-to-slide surface gap  165  between the proximity shield  160  and the first inclined slide surface  120 . The shield-to-slide surface gap  165  is defined as the closest distance between the proximity shield  160  and the first inclined slide surface  120 . The proximity shield is positioned to be substantially parallel to the first inclined slide surface  120 . The proximity shield  160  is also placed or designed in such a manner that there are shield-to-liquid gaps  170 ,  175 ,  180 , and  185  between the proximity shield  160  and the liquid layer  115 . The proximity shield  160  is positioned so that there is a specific shield-to-web gap  190  between the shield lip  195  and the web  155 . A seal  200  is made between the shield back  205  and the back land surface  145 . The shield-to-slide surface gap  165  can range from 4 mm to 13 mm. The more preferred range is 5 mm to 8 mm, with the most preferred value equal to 6 mm.  
         [0017]      FIGS. 2 &amp; 3  show the proximity shield in more detail.  FIG. 2  shows a shield lip  195  and a front face  210 . Different embodiments of the shield lip  195  and front face  210  are shown in  FIG. 3 . The shield lip  195  can be a sharp point as shown in configuration  3 A. The shield lip  195  can also be rounded as shown in configuration  3 B. The radius of curvature of the shield lip  195  can range from 1 micron to 10 mm. In the extreme, the radius can be infinite corresponding to the flat surface shown in configuration  3 C. In this embodiment, there is no shield lip  195 , only a front face  210 . In configurations  3 A and  3 B, the front face  210  is cut away forming an angle  215  so that the shield lip  195  is the closest point to the moving web  155 . This angle  215  can be between 10 and 80 degrees. For the exemplary embodiment shown in  FIG. 2 , the angle is 56 degrees. The shield-to-web gap  190  is defined as the closest distance between the proximity shield  160  and the web  155 . For configurations  3 A and  3 B the closest point of the proximity shield  160  would typically be the shield lip  195 . For configuration  3 C, this would depend on location of the coating lip  125  in relation to the coating backing roller  150 , as well as the angle of the first inclined slide surface  120 .  
         [0018]     Configuration  3 D is an alternative embodiment where the front face  210  is curved to match the curvature of the coating backing roller  150 . In this case the entire front face  210  is substantially the same distance from the web  155 . There is no shield lip  195  to define for configuration  3 D.  
         [0019]      FIG. 2  demonstrates a step cutback angle  265 . For example, the proximity shield  160  may be angularly cut from 0-65°. This portion of the proximity shield  160  is cut back in order to maintain the shield-to-liquid gaps  175 ,  180 , and  185 . If the combination of total coating layer thickness and any difference in height between the inclined slide surfaces  120 ,  130 ,  135 ,  140  (not shown in  FIG. 1 ) would cause the coating liquid to close the shield-to-liquid gaps  175 ,  180 ,  185 , then the proximity shield  160  can be cut back to avoid this. For some fluid and coating apparatuses, it may not be necessary to have a step cutback angle  265 .  
         [0020]      FIG. 4  shows a nexus between the proximity shield  160  and an edge guide  220 . The edge guide  220  contains the fluid on the inclined slide surfaces  120 ,  130 ,  135 , and  140  (shown in  FIG. 1 ) and defines the coating width (not shown). An edge guide holder  225  is used to hold the edge guide  220  to the inclined slide surfaces  120 ,  130 ,  135 , and  140 . A pin  230  is attached to the edge guide holder  225 . The edge guide  220  has an overhang portion  235  which extends over the coating layer  115 . The proximity shield  160  has a cut out area  240  which mates with the overhang portion  235  of the edge guide  220 . This mating forms an effective seal to prevent gas from leaking into or out of a gas space  245  located under the proximity shield  160 . The proximity shield  160  also has a bracket  250 , which has a hole (not shown) that fits over the pin  230 . The connection between the bracket  250  and the proximity shield  160  is adjustable so that the pin  230  maintains the desired shield-to-web gap  190 .  
         [0021]     The gas contained within the gas space  245  may be air. It may also be an inert gas such as Nitrogen or Carbon Dioxide. The inert gas could also have added solvent vapors to retard drying of the coating fluids on the edge guide  220  or the back land surface  145 .  
         [0022]     In addition to the embodiment shown in  FIG. 4 , alternate arrangements for positioning the shield are possible. The pin  230  could be replaced with a notch or hook or screw. Instead of a pin  230 , the edge guide  220  could have a ledge on which the bracket  250  rests. Other arrangements are envisions that set both the shield-to-slide surface gap  165  and the shield-to-web gap  190 . The proximity shield  160  could also not include a cut out area  240 , in which case the proximity shield  160  would sit directly on top of the overhang portion  235 .  
         [0023]     In one embodiment of the present invention, the lowermost or carrier layer  55  (shown in  FIG. 1 ) is an organic solvent or blend of organic solvents that is substantially free of other constituents. The term “substantially free of other constituents” as used herein is intended to mean that the organic solvent or blend of organic solvents have a purity level of at least about 98% and that any contaminants or additives present do not affect the viscosity of the carrier layer  55 . Examples of suitable organic solvents include methanol, acetone, methylethyl ketone, methyl isobutyl ketone, methylene chloride, toluene, methyl acetate, ethyl acetate, isopropyl acetate, and n-propyl acetate. In another embodiment, the carrier layer  55  may also be a diluted version of the upper liquid layer  75 . The carrier layer  55  may also contain other addendum such as polymers or dyes as long as they do not significantly affect the viscosity of the carrier layer  55 .  
         [0024]     The second liquid layer  75  which is metered through a second metering slot  65 , moves down the second inclined slide surface  130 , and is accelerated by the carrier layer  55  down the first inclined slide surface  120  to the coating bead  255 . The second liquid layer  75  should preferably be totally miscible with lowermost layer  55  and is therefore preferably organic, but may also contain water. As layers  95  and  115  in  FIG. 1  are shown, additional upper layers may also be applied using the multi-slot slide bead coating apparatus  10 . These additional upper layers may be of a distinct composition relative to the second liquid layer  75  or of the same composition. Similarly, the number of upper layers may also be further increased by extension of the number of metering slots (not explicitly shown in  FIG. 1 ).  
         [0025]     Because the method of the present invention may involve application of highly volatile organic solvents, the temperature at which coating is performed is preferably less than or equal to 25° C. to avoid non-uniformities due to streaks and mottle. Methylene chloride, acetone, methyl acetate and methanol are examples of highly volatile organic solvents having a vapor pressure above 100 mm Hg at 25° C. The proximity shield  160  is typically maintained at the same temperature as coating fluids in order to avoid thermal gradients within the gas space  245 .  
         [0026]     The carrier slide coating method, as described in U.S. Pat. No. 6,579,569, is extremely sensitive to stray gas currents as well as gas currents induced by the coating method itself. This is especially true when the coating layers are very thin (&lt;5 microns for the carrier layer  55  and &lt;10 microns for the sum of the subsequent layers  75 ,  95 , and  115 ). Conventional slide coating typically uses layers that are much greater in thickness. This sensitive nature of the coating layers results in very precise requirements for the placement of a proximity shield  160 . Conventional methods teach that the shield-to-web gap  190  can be large enough that an operator can view the coating bead  255 . For carrier slide coating with coating construction described herein, if the shield-to-web gap  190  were allowed to be this large, the subsequent coating quality would be very poor. This is because the coating bead  255  would be disturbed by gas currents and longitudinal streaks would occur.  
         [0027]     When the coating solutions contain volatile organic solvents, the drying at the static contact lines can be substantial. In order to prevent this drying, a clam shell must be created wherein the shield edges  260  and the shield back  205  are sealed. This clamshell can be either passive or solvent laden gas can be supplied. If the proximity shield  160  is sealed at the shield edges  260  outside the edge guides  220 , there will be a greater region of atmosphere requiring saturation as well as the risk of stray gas currents occurring at the edges. In order to prevent these problems, the proximity shield  160  is integrated with the edge guide  220  as shown in the  FIG. 4 . This integration effectively creates the enclosure. Referring to  FIG. 4 , the overhang portion  235  of the edge guide  220  serves as a means for creating a seal, as a means for holding the proximity shield  160  in place; as a means for setting the shield-to-slide surface gap  165 , as a means for maintaining a parallelism between the proximity shield  160  and the first inclined slide surface  120 ; and as a means for creating a partially saturated environment at the edges when the proximity shield  160  is not yet in place.  
         [0028]     The proximity shield  160  can be sealed in the back in a number of ways. A gasket material, such as rubber, can be used to create a seal  200 . Alternatively, the proximity shield  160  can rest on the back land surface  145  of the multi-slot slide bead coating apparatus  10 . The proximity shield  160  can either be placed directly on the edge guides  220  and seal  200  or else a movable and/or hinged design could be envisioned. Another embodiment is to have no back seal  200  where there is an opening between the shield back  205  and the back land surface  145 .  
         [0029]     When the proximity shield  160  is completely sealed, the only place for gas exchange between the outside and the gas space  245  under the proximity shield  160  is through the shield-to-web gap  190 . The placement of the proximity shield  160  relative to the web  155 , i.e. the shield-to-web gap  190 , was found to be instrumental to forming a coating without objectionable defects, such as longitudinal streaks.  
         [0030]     The proximity shield  160  can be constructed from a variety of materials, such as plastic, glass, metal, metal alloys, wood, or paper. The proximity shield  160  can also be made from a combination of these materials. Example plastic materials are polyethylene, Teflon, and polycarbonate. The proximity shield  160  can be made from a transparent material in order to enable the operator to see the fluid underneath. A transparent plastic material, such as polycarbonate, could be coated with a protective layer. Some of the purposes for this protective layer are to provide static dissipation properties and to protect the material from attack by the organic solvents. Hence, a semi-transparent metal may coat the transparent plastic.  
       COMPARATIVE EXAMPLE 1  
       [0031]     The multi-slot slide bead coating apparatus  10  illustrated in  FIG. 1  was used to apply two organic layers to a moving web  155  of untreated polyethylene terephthalate (PET). The carrier layer  55  consisted of a mixture of solvents, having a viscosity of 0.9 cp and a wet thickness of 3.23 μm on the web  155 . The second layer was a mixture of polymer, dye and organic solvents. The second layer was delivered through the second, third and fourth metering slots  65 ,  85 ,  105 , respectively, and had a viscosity of 750 cp and a combined final wet thickness of 3.49 μm on the web  155 . Coatings were applied at a temperature of 23.9° C. The gap between the coating lip  125  and the moving web  155  was 200 μm. The pressure differential across the coating bead  255  was 1.8 cm H 2 O. The web speed was 190 m/min.  
         [0032]     When the proximity shield  160  was used, the shield-to-slide surface gap  165  was set to 6 mm and the shield-to-web gap  190  was set to 3.18 mm. Table A demonstrates the effectiveness of the proximity shield  160  for preventing density bands (or longitudinal streaks).  
                   TABLE A                       Proximity Shield 160   Resulting Coating Quality                   Off   Severe wide variable bands       On   No bands or streaks                  
 
       COMPARATIVE EXAMPLE 2  
       [0033]     The same coating compositions were used as described in comparative example 1. In this case the shield-to-web gap  190  was varied according to Table B. There is an optimum value for the shield-to-web gap  190 . When the distance is too small, short narrow wavy bands occur. When the distance is too large, severe bands occur similar to that seen when there is no proximity shield  160  in place. The shield-to-web gap  190  that would allow the operator to see the coating bead  255  is the last value in Table B, 13 mm. At this distance, the bands are severe. The available range for shield-to-web gap  190  is between 2.5 and 4.5 mm. The most preferred shield-to-web gap  190  is 3.18 mm.  
                             TABLE B                       Shield-to-Web Gap 90 (mm)   Resulting Coating Quality                                1.27   Short narrow wavy bands       1.91   Narrow bands or streaks       2.54   Narrow streaks that move       3.18   No bands or streaks       4.45   Wide variable bands       6.35   Severe wide variable bands       13.0   Severe wide variable bands                  
 
         [0034]     The invention has been described with reference to one or more embodiments. However, it will be appreciated that a person of ordinary skill in the art can effect variations and modifications without departing from the scope of the invention.  
       PARTS LIST  
       [0000]    
       
           10  Multi-slot slide bead coating apparatus  
           15  Front section  
           20  Second section  
           25  Third section  
           30  Fourth section  
           35  Back plate  
           40  Inlet  
           45  First metering slot  
           50  Pump  
           55  Carrier layer  
           60  Inlet  
           65  Second metering slot  
           70  Pump  
           75  Layer  
           80  Inlet  
           85  Third metering slot  
           90  Pump  
           95  Layer  
           100  Inlet  
           105  Fourth metering slot  
           110  Pump  
           115  Layer  
           120  First inclined slide surface  
           125  Coating lip  
           130  Second inclined slide surface  
           135  Third inclined slide surface  
           140  Fourth inclined slide surface  
           145  Back land surface  
           150  Coating backing roller  
           155  web  
           160  Proximity shield  
           165  Shield-to-slide surface gap  
           170  Shield-to-liquid gap  
           175  Shield-to-liquid gap  
           180  Shield-to-liquid gap  
           185  Shield-to-liquid gap  
           190  Shield-to-web gap  
           195  Shield lip  
           200  Seal  
           205  Shield back  
           210  front face  
           215  Angle  
           220  Edge guide  
           225  Edge guide holder  
           230  Pin  
           235  Overhang portion  
           240  Cut out area  
           245  Gas space  
           250  Bracket  
           255  Coating bead  
           260  Shield edge  
           265  Step cutback angle