US3474758A

US3474758A - Multiple coating apparatus

Info

Publication number: US3474758A
Application number: US680672A
Authority: US
Inventors: Theodore A Russell
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1967-11-06
Filing date: 1967-11-06
Publication date: 1969-10-28
Anticipated expiration: 1986-10-28
Also published as: GB1256515A; FR1590857A; DE1806887A1

Description

, Od. 28, `1969 TLA. RUSSELL.-

MULTIPLE COATING APPARATUS `Fina@ Nov. e. 1967 INVENTOR THEODORE A. RUSSELL United States Patent O 3,474,758 MULTIPLE COATING APPARATUS Theodore A. Russell, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Nov. 6, 1967, Ser. No. 680,672 Int. Cl. BOSe 3/132, 9/06 U.S. Cl. 118-412 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention The present invention relates to coating apparatus and particularly to an improved multi-slide coating hopper.

Description of the prior art A multi-slide hopper is an apparatus that will simultaneously coat two or more liquids onto a solid support in such a way that the layers are not mixed and are individually of uniform thickness. Such a hopper is disclosed in U.S. Patent 2,761,419 and the present invention relates to an improvement in such hoppers which overcomes a decided shortcoming of known hoppers of this type.

The conventional slide hopper performs its coating operation by metering a first coating liquid from a supply through a narrow distributing slot which distributes the liquid uniformly across the top of a downwardly inclined slide surface. This layer of liquid moves down the slide surface by gravity so as to supply an evened out and steady supply to a coating bead across and in contact with which the web to be coated is moved to pick up a layer of liquid therefrom. If the simultaneous coating of two liquids is desired, a second liquid is supplied to, and distributed by, a second distributing slot which in turn directs a uniform layer of the liquid onto the top of a second slide surface so as to flow down to the coating bead, first alone on its own slide surface, and then onto the top of the layer of liquid issuing from the first distributing slot and then down to the coating bead in superposed relation with the layer of first coating liquid. Subsequent liquids may be coated simultaneously by equipping the hopper with the appropriate number of distributing slots and slide surfaces.

It has been found that the conventional multi-slide hopper often tends to produce a defect in the final coated product which appears as a long line or lines running parallel to the direction of coating. These defects are not always visible in the product as coated and very often they become visible only after the product is dried and/ or processed (if the coated web is a photographic product) and then is visibly checked, Experimentation has shown that these lines, which will hereinafter be referred to as pencil lines, are the result of long lines which are locally Vdeficient in the coating liquid issuing from the lowermost slot, where a two-slot hopper is used, and which are thicker in the second coating liquid, the total thickness of the two liquids being the same as the unaffected parts of the coatings.

As described in U.S. Patent 3,005,440, these pencile lines can be caused by a blockage in the lowermost of two distributing slots, or in the slot where two layers of liquid ice come together in moving down the slides to the coating bead. These pencil lines can be avoided either by removing the cause of the blockage in the distributing slots, i.e., taking out gelatin slugs or unusually large silver grains when silver halide solutions are being coated, etc., or by redesigning a part of the hopper so that the interruption of flow due to such a blockage does not result in a pencil lne.

The invention in U.S. Patent 3,005,440 attacked the problem of pencil lines by widening out the exit end of the discharge slots where two liquids come into contact in such a way as to produce a turbulence in the stream of liquid passing through the slot by reason of a sudden change in velocity and before it leaves the slot. This turbulence caused any pencil line formed in the stream by -reason of a blockage in the narrow part of the slot to heal itself before the stream issued from the slot and onto its slide surface where it is immediately contacted by a second layer of liquid moving down upon it from a higher slide surface.

SUMMARY OF THE INVENTION The present invention is directed to an improved arrangement of the discharge slots in a multi-slide coating hopper to eliminate a newly discovered source of pencil lines when using such a coating hopper.

When hunting for particles that were likely to form pencil lines, it was observed that these particles, and sometimes bubbles, frequently lodged on the first 1/s of an inch of the slide below the lowermost of two adjacent slides. It was easy to remove such a particle, or bubble once it was detected, and however gently it was shifted, it would always continue on its way down the remainder of the slide. These observances suggested that there was a stagnant area of liquid on the first part of the slide and that once the particle had settled there it would be unlikely to be swept away by flowing uid and it would quite probably be large enough to influence the flow pattern in its vicinity. When multiple layers are applied with hoppers having slots arranged as shown in FIGS. 9 and l0 in U.S. Patent 2,761,791, the materials flowing vertically upward must stop their upward motion before they start down the slides. Particles sometimes stop where they should make this turn and are held by the noted stagnant condition existing at this point. In order to prevent particles and/or bubbles from stopping at or near the exit of a discharge slot where two layers of coating come together, and thus produce pencil lines in the final coating, the exit end of the slot(s) is designed or arranged to direct the flow of liquid therefrom downwardly of the slide at full, or near full, flow velocity. To this end the slots are arranged so that their exit ends are inclined in a range of to 145 inclination in relation to the slide surface onto which they exit. This configuration, which may also be used in conjunction with widened slot ends, such as described in U.S. Patent 3,005, 440, provides a desirable solution to the prior a-rt problem of pencil lines described above.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its details of construction and its methods of operation, together with additional objects and advantages thereof, will best be understood from the following description when read in connection with the vaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a sectional detail of a dual hopper constructed in accordance with a preferred embodiment of the present invention;

FIG. 2 is an enlarged sectional detail showing the exit end of the lowermost distributing slot enlarged on the down side of the slot in accordance with one embodiment of my invention;

FIG. 3 is an enlarged sectional detail showing another embodiment wherein the upper end of the lowermost discharge slot is curved so that its exit end directs the flow of liquid therefrom downwardly of its associated slide at full or near full flow velocity;

FIG. 4 is an enlarged transverse sectional detail of a web coated with a conventional dual slide hopper and illustrating, in an exaggerated way, how undesired pencil lines might appear in the final product.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 my invention is shown on a conventional dual slide hopper of the type disclosed in U.S. Patent 2,761,419. With this hopper, one fluid coating composition C1 is continuously pumped by a constant discharge metering pump P into a cavity at a given rate through inlet 11 and from which it is forced through a narrow distributing slot 12 in the form of a ribbon and out onto a downwardly inclined slide surface 13 down which it flows by gravity in the form of a layer 14 to a point where it forms a coating bead B between the lip 1,5 of the slide surface 13 and a web W moved upwardly across and in contact with the bead by a supporting roll 16.

The second coating composition C2 is continuously pumped into a second cavity 17 at a given rate by another constant discharge metering pump P through inlet 18 and from which it is forced through a narrow distributing slot 19 in the form of a ribbon and out onto a second downwardly inclined slide surface 20. This ribbon of the second coating composition, in owing down the slide surface 20 under the influence of gravity, forms a smooth and uniformly thick layer 21. The two

slide surfaces

13 and 20 are coplanar, or substantially so, so that as the layer 21 of the second coating composition reaches the ribbon of the first coating composition issuing from slot 12, it flows up on top of the same and the two then flow together down slide surface 13 and into the coating bead B. The relative thickness of the two layers of coating materials will depend upon the rate at which the individual coating uids are pumped to the cavities 10 and 17 by pumps P and P. The slide hopper and the method of coating carried out by the use thereof described up to this point is shown in the above noted U.S. Patent 2,761,419, and the improvement therein which constitutes the present invention will now be described.

The purpose of the distributing

slots

12 and 19 is to cause the issuance or extrusion of the liquid coating compositions in a uniform ribbon across the entire width of the hopper or the length of the distributing slots. Since the hopper may be as wide as 48 to 75 and the cavities 10 and 17 for the coating compositions do not extend the full length of the slots, but only over a limited length 0f the center portion thereof, these distributing slots must be quite narrow in order to do a satisfactory job of distributing the coating compositions throughout the full length of the slots. The length and depth of these distributing slots may vary with the solutions to be coated, the uniformity in thickness required over the full length of the slot, and the flow rates to be used. At best they are very narrow, e,g. .009 or .010 inch. As pointed out above, pencil lines often appeared in the coated product made with slide hoppers.

In FIG. 4 an attempt has been made to illustrate such pencil line imperfections and their character. Looking at FIG. 4, which is a partial and greatly enlarged transverse cross-section of ya web coated with a dual slide hopper, it will be seen that the layer 14 of coating composition C1 directly engaging a web W has two indentation 30 running lengthwise of the web which are filled with the second coating solution for-ming layer 21. It will thus be appreciated that at the indentation 30 the thickness of

layers

14 and 21 will be relatively different than at other portions of the web notwithstanding the fact that the overall physical thickness of the two layers at these particular points may be the same as at all other points across the web. Obviously then these pencil lines cannot be readily physically measured but will be visually apparent after drying or processing if one of the layers is a silver halide solution and the other is a clear gelatin solution as found in photographic film. As pointed out above, these pencil lines may occur by reason of a small obstruction in the distributing slot 12, e.g., a grain of silver in the silver halide solution or a slug of gelatin in a photographic emulsion, which causes an interruption in the flow pattern -moving out of slot 12 and onto the slide surface 13 or as the result of a particle or air bubble being caught and held at the edge of the exit of the slot, or in the first one-eighth inch of the associated slide, by reason of a stagnant condition resulting from the abrupt change in flow of the uid as it imerges from the slot and then moves down the slide under the force of gravity. In a single layer exposed to the air this is self-healing due to surface forces tending to reduce the total exposed surface to a minimum. However, when the layer 21 of the second coating composition C2 is run onto the first as it issues from slot 12, then the deficiency, or indentations 30, in layer 14 is filled up with the second coating composition, and providing the interfacial tension between coating composition C1 and composition C2 is small or zero, then a pencil line is formed, as shown at 30 in FIG. 4, because there is no healing force at their interface. The top surface of layer 21 soon heals due to its surface tension.

Those pencil lines caused by obstructions within the length of the slots can be removed by redesigning a part of the hopper as taught by U.S. Patent 3,005,440 so that an interruption of flow of this nature does not result in pencil lines. This approach does not, however, eliminate pencil lines caused by particles and/ or air bubbles which are trapped on the edge of the exit end of the discharge slot, or on the slide near the exit of the slot, by reason of the stagnant condition resulting from the abrupt change in direction in fiow of the coating fluid as it leaves the slot and moves down the slide surface.

The present invention relates to a modification in the hopper design, where it has two or more slides, so as to enable coatings to be made free of pencil lines which result from the trapping of particles and/or bubbles at, or on the slide surface near the edge of the exilt end of the slide due to the abrupt change in direction of flow of the coating solution.

In the dual hopper shown in FIG. 1 this modification in design consists in directing the exit end of the discharge slot 12 downwardly of its associated slide surface toward the coating bead rather than having it extend vertically and enter its slide surface at an acute angle as shown in the noted prior art. The angle 0 should preferably be from to 145. With this configuration, a component of velocity of the flow emerging from the slot will be directed parallel to the slide surface. This cornponent of velocity will tend to dislodge particles and bubbles which would otherwise be trapped at, or on the slide surface near, the exit end of the discharge slot.

To also overcome pencil lines which might result from a blockage within the slot proper the inclined slot may also be modified according to the description of U.S. Patent 3,005,440. Thus, the discharge slots may have their ends enlarged as shown at 23 in FIGURE 2. For further details of this modification reference should be made to U.S. Patent 3,005,440.

In another embodiment, as shown in FIG. 3, the leaving end'of the slot 12' is curved, instead of straight, so that the ow of coating fluid emerges therefrom and onto the slide surface 13 at full, or near full, flow velocity. This will eliminate any stagnant condition which might result from an abrupt change of direction in the ow of fluid onto the slide surface and insure that any and all particles and/or bubbles emerging from the slot will pass on down the slide surface rather than being caught at this point and produce pencil lines in the nal coating.

While I have disclosed my invention as applied to a dual slide hopper, it follows that it could also be used on a hopper having three, four, or more slides. In these cases, any or all distribution slots below any slide surface, or where two layers are brought into contact, will be angled, or redirected downwardly of its associated slide surface, to prevent the formation of pencil lines.

I claim:

1. In a multiple-layer hopper for feeding a plurality of fluid coating solutions in superposed and distinct layer relationship to a coating bead from which they are simultaneously deposited in such strata relationship onto the surface of a web moving across and in contact with said coating bead, and comprising a multiple-layer slide hopper, a slide surface on said hopper inclined downwardly and terminating in a lip adjacent said coating bead; a plurality of narrow distributing slots, one for each coating composition, extending from the interior of said hopper and opening into said slide surface and through each of which a thin layer of coating composition is adapted to be discharged onto said slide surface, the lowermost of said distributing slots spaced from said lip and the remaining distributing slots spaced above the lowermost slot and each other to provide an uninterrupted slide portion below each slot over which the layer of coating solution discharged therefrom flows by gravity to form a smooth layer of uniform thickness before, in the case of all but the lowermost slot, it reaches the next lowest distributing slot and ows onto the top of, and along with, the layer of coating solution discharged therefrom, and, in the case of the lowermost discharge slot, before it reaches said lip; the improvement consisting of, arranging at least one of said lowermost slots so that its discharge end enters onto its associated slide portion at an obtuse angle relative thereto and directs the layer of coating solution emerging therefrom downwardly of said slide portion so that a component of velocity of the solution emerging from said slot is directed down said slide portion.

2. A multiple-layer hopper according to claim 1, characterized by the fact that the portion of at least one of said lowermost of said slots approaching the slide surface is curved downwardly toward said lip and the discharge end thereof exits upon the slide portion immediately below it in a downward direction.

3. In a multiple-layer hopper for feeding a plurality of fluid coating solutions in superposed and distinct layer relationship to a coating bead from which they are simultaneously deposited in such strata relationship onto the surface of a web moving across and in contact with said coating bead, and comprising a multiple-layer slide hopper, a slide surface on said hopper inclined downwardly and terminating in a lip adjacent said coating bead; a plurality of narrow distributing slots, one for each coating composition, extending from the interior of said hopper and opening into said slide surface and through each of which a thin layer of coating composition is adapted to be discharged onto said slide surface, the lowermost of said distributing slots spaced from said lip and the remainingdistributing slots spaced above the lowermost slot and each other to provide an uninterrupted slide portion below each slot over which the layer of coating solution discharged therefrom flows by gravity to form a smooth layer of uniform thickness before, in the case of all but the lowermost slot, it reaches the next lowest distributing slot and flows onto the top of, and along with, the layer of coating solution discharged therefrom, and, in the case of the lowermost discharge slot, before it reaches said lip; the improvement consisting of, arranging at least one of said lowermost slots so that it is inclined downwardly in relation to the slide portion below it at an angle from approximately to 145 so that a component of velocity of the. layer of solution discharged therefrom is directed down said slide portion.

4. A multiple-layer hopper according to claim 3, characterized by thte fact that the discharge end of at least one of said lowermost of said slots is wider than the major portion of the slot and the enlargement in the slot being sudden and sufficiently great so as to produce a turbulence in the coating solution through the wider portion and thereby heal any interruption of ow that may occur as the result of a blockage in the narrow portion of the slot.

References Cited UNITED STATES PATENTS 2,761,791 9/1956 Russell 118-410 X 3,005,440 10/1961 Padday 118-410 WALTER A. SCHEEL, Primary Examiner JOHN P. MCINTOSH, Assistant Examiner