Patent Application
20050208223
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.
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.
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.
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.
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.
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.
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.
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:
Referring to
Multi-slot slide bead coating apparatus 10, shown in a side elevation cross-section in
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.
Still referring to
Configuration 3D 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 3D.
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.
In addition to the embodiment shown in
In one embodiment of the present invention, the lowermost or carrier layer 55 (shown in
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
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.
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 (<5 microns for the carrier layer 55 and <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.
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
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.
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.
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.
The multi-slot slide bead coating apparatus 10 illustrated in
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).
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.
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.
