US3749053A

US3749053A - Coating apparatus

Info

Publication number: US3749053A
Application number: US00194669A
Authority: US
Inventors: W Timson
Original assignee: Polaroid Corp
Current assignee: Polaroid Corp
Priority date: 1971-11-01
Filing date: 1971-11-01
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31

Abstract

Multiple layers of superposed coating solutions cascade onto the surface of a moving web which is maintained substantially flat in the region of application of the coating solutions. The web is passed through a vacuum chamber just prior to the coating region to provide a vacuum assist in drawing the coating solutions onto the web. Prior to entry into the vacuum chamber, the web may be sprayed with a coolant which is evaporated and drawn off by the vacuum so that gelling of the coating solutions will be hastened by the reduced temperature of the web.

Description

United States Patent 11 1 Timson 51 July 31, 1973 COATING APPARATUS [75] Inventor: William J. Timson, Belmont, Mass.

[73] Assignee: Polaroid Corporation, Cambridge,

Mass.

22 Filed: Nov. 1, 1971 21 Appl. No.: 194,669

[52]v US. Cl 118/50, 118/412, 117/34 [51] Int. Cl. 305C 7/02, B050 1 1/06 [58] Field of Search 118/50, 410-412; 117/34, 120

[56] References Cited UNITED STATES PATENTS 3,531,319 9/1970 Martorana 118/50 X 3,627,564 12/1971 Mercier 118/412 X 3,413,143 11/1968 Cameron et a1. 118/410 X 3,496,005 2/1970 lshiwata et a1 1 17/34 3/1970 lshiwata et al 118/50 X 1/1972 Herzhoffet a1 118/50 Primary Examiner-James Kee Chi AttarneyCharles Mikulka et al.

[57] ABSTRACT Multiple layers of superposed coating solutions cascade onto the surface of a moving web which is maintained substantially flat in the region of application of the coating solutions. The web is passed through a vacuum chamber just prior to the coating region to provide a vacuum assist in drawing the coating solutions onto the web. Prior to entry into the vacuum chamber, the web may be sprayed with a coolant which is evaporated and drawn off by the vacuum so that gelling of the coating solutions will be hastened by the reduced temperature of the web.

11 Claims, 6 Drawing Figures VPATENTEDJULWW 3,749,053 7 SBEEIiUFB lealam W m ATTORNEYS PAIENIED JUL 3 1 ms SHEET 2 [IF 3 I N V EN TOR. WILLIAM J. TIMSON amd AT TORNEYS PATENTED JULBI ma SHEET 3 [IF 3 INVENTOR. WILLIAM J. TIMSON l lwwoz and am nrronusvs 1 COATING APPARATUS BACKGROUND OF THE INVENTION The present invention relates to novel coating apparatus for applying multiple superposed coatings to the surface of a moving web, and more particularly to coating apparatus in which layers of coating solutions cascade onto a moving web and are thereafter set or gelled.

Known cascade coating apparatus is capable of applying a plurality of separate coatings onto the surface of a moving web. The coatings are deposited onto the web from downwardly flowing coating solution lamina, superposed one upon another in a layer relationship, and thereafter gelled, or set, to produce an article having a plurality of substantially discrete superposed layers on the web base. Conventionally, the coating solutions have been applied to the web as the web is being supported by a roller, i.e., the web is curved in the direction of travel at the time of application of the coating solutions. The use of rotating rolls as web supports in combination with a stationary coating head apparatus has met with success; however, there are a number of disadvantages associated with such structures. For example, when attempting to. coat at relatively high web speeds, particularly with high viscosity coating fluids, there have been such problems as air bubbles being entrapped under the coatings hereby providing nonuniformity of the coatings. Such problems have been minimized by applying a vacuum to the side of the web which is to be coated as the web passes around the rotating support roll. The use of a vacuum box in this position has to some extent stabilized the bead" of the coating solution which forms at the point of contact of the coating solution with the web. However, this approach has not been fully satisfactory due to, for example, the rotating backing roll, variations in flatness of the coating head facing the roll, defects in the roll surface, film base flatness variations, etc. Because of the denoted problems, very small coating gaps are difficult to obtain on production machines without scraping at some point in the operation. Defects caused by the above cannot be tolerated in products such as photographic films in which substantially uniform coatings are necessary in order to achieve consistently good quality image reporduction.

Typical prior art patents relating to known structures of the type mentioned above are U. S. Pat. Nos. 2,761,419; 3,206,323; and 3,220,877.

SUMMARY OF THE INVENTION A primary object of the present invention is to provide a novel cascade coating apparatus which is capable of applying a plurality of superposed coating solutions onto a web at high throughput with a high degree of uniformity. Another primary object of the invention is to provide a cascade coating apparatus having one or more internal vacuum boxes for applying vacuum to both sides of a web immediately before it is contacted by the coating solution so that more uniform coatings will be obtained. Another object of the invention is to coat a web while it is travelling in a substantially straight path as distinguished from an arcuate path of web travel around a support roller.

In order to improve film coating stability, particularly at high web throughput speeds-and to obtain more uniform coatings on the web, a web is passed through one or more vacuum chambers and generally forms a partition within such. These vacuum chambers are located within the coating apparatus immediately upstream of the point of application of the coating solutions to the web. The negative pressures applied above and below the web are such that the pressures are either equal or the pressure above the web is slightly less than that below the web. In the latter case, there is a slight pressure differential urging the web upwardly against the upper lip of the coating head to reduce the gap between the coating head and the web. In either case the web must not experience vertical force in either direction within the vacuum chambers which might impart a deviation from substantial flatness. It will be appreciated, however, that the application of negative pressure at a point prior to the solution deposition point will cause the coating solution bead to experience a significant inward pull. The vacuums in the chambers may be higher than that employed in the prior art single vacuum boxes operating on only one side of the web since there is little net pressure differential on the web within the vacuum chambers. In this fashion the higher vacuums provide a more stable coating bead at high coating speeds, and bubble entrainment, which has been a problem in the prior art, is prevented or minimized.

In the event equal pressures are desired on both sides of the web passing through the chamber there is no necessity for providing more than a single exhaust port In such an embodiment the web would form only a partial partition within the chamber. For uniformity of convention, it will be assumed herein that separate vacuums are drawn on each side of the web.

The relatively balanced vacuums applied to the web, which may exert a small net pressure differential upwardly on the sheet, allow the sheet to pass through the chambers with low frictional resistance and facilitate a smooth coating operation. It is also contemplated that, in lieu of allowing the web to pass through the vacuum chamber without any restraint, the web may pass between two perforated or porous guide plates in order to keep it from fluctuating too much either upwardly or downwardly during its passage through the chamber and assure web flatness.

The application of the coating solutions to the web while the web is running in a substantially straight path, in contradistinction to travelling around a support roll, has the advantage that after the coating solutions are deposited upon the web, they are not subjected to immediate bending as is the case with the prior art support rollers where stress is applied to the layers of coating solution due to the arcuate path of movement of the web. It is theorized that the bending of the web carrying the coatings tends to result in nonuniformities in the coatings. For example, upon unbending of the web, the upper surface of each coating will be contracted or compressed relative to the lower surface of the coating, which may result in some of the irregularities found in prior art laminated products.

In order to increase the rate at which the coatings gell upon contact with the web, the web is preferably precooled by spraying the web with a coolant upstream of the vacuum chamber(s). The vacuum chamber(s) of the present invention performs the additional function of evaporating residual coolant from the web surface. To insure substantially completely removal of the coolant, it may be desirable to utilize at least two vacuum chambers in series.

In order to accommodate the passage of a splice in the web through the coating gap if the splice is too thick for the gap, it is contemplated that the lower lip of the coating head defining the bottom of the gap may be a hinged member which is either spring biased to its normal operating position, or may be slightly rotated from the normal operation position by an operator or by automatic equipment. In the event that a splice which is too thick for the gap is encountered, the lower lip will be momentarily pivoted downwardly so that the splice may pass through without resulting in breaking the web.

In accordance with another aspect of the invention, a pressure chamber may be provided adjacent the lower portion of the coating head at the point of application of the coatings to the web. The positive pressure provided by this chamber tends to press the web upwardly, thereby reducing the magnitude of any coating gap in order to achieve uniform coatings.

Furthermore, to facilitate the flow of the cascading coating solutions onto the moving web base in a stable manner, a small cavity or reservoir for the coating bead may be provided in the lower surface of the upper coating lip in order to reduce the possibility of cavitation.

The above and other objects, features and advantages of the invention will become more apparent as this description proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view taken primarily in vertical longitudinal section of a presently preferred embodiment of the invention;

FIG. 2 is a view of another embodiment of the invention;

FIG. 3 is a view on an enlarged scale showing details of the coating head lips employed in either of the embodiments of FIGS. 1 and 2;

FIG. 4 is a view similar to FIG. 3 showing a different arrangement for supplying a carrier fluid to the web along a wall portion of the upper vacuum chamber; and

FIGS. 5 and 6 are views similar to FIG. 4 showing different configurations of the surface of the upper coating lip in order to prevent fluid cavitation.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings wherein like reference numerals are used in the several views to designate like parts, and more particularly to FIG. 1, reference number 10 generally designates one embodiment of cascade coating apparatus in accordance with the present invention. A web 12 which may be, for example, a film such as a cellulose acetate, polyethylene terephthalate, etc., initially has its upper surface sprayed with a coolant such as water or preferably a volatile alcohol such as methanol or ethanol from a spray head 14. Excess coolant falling off of the web is collected in a tray 16 for recycle to the coating head. The web is precooled to a temperature which will expedite the gelling or setting of the coating solutions upon contact with the web in the coating head. The web with residual coolant on its upper surface is introduced into the coating apparatus through slot 28 into a chamber which the web may partition into an upper vacuum area 30 and a lower vacuum area 32. These vacuum areas have separate evacuation conduits 34 and 36. Coolant is evaporated from the web and exhausted through evacuation conduit 34 (and possibly 36 if the chamber is not fully partitioned by the web). While one vacuum chamber has been illustrated, it is contemplated that if desired, additional vacuum chambers arranged in series may be utilized to insure complete removal of the coolant. For most coating systems a single pair of vacuum areas (30 and 32) will suffice. It will be appreciated that, in addition to spraying coolant, various surfactants and other coating aids may be deposited on the web prior to its being admitted to the-vacuum environment.

Upon leaving

vacuum areas

30 and 32, the web passes through a slot in the coating head defined by an upper coating lip 38 and a lower coating lip 40 which is generally of a smaller width than the upper lip 38, i.e., a smaller dimension in the direction of web movement. A multiplicity of coating solutions represented in the FIG. 1 embodiment by two superposed layers or strata of

coating solutions

42 and 44 flow down the inclined plane outer or downstream surface 46 of the upper lip 38 onto the web 12 where the coating solutions quickly set to form a laminated article. The coating solutions may be, e.g., a silver halide emulsion and a protective gelatin coating which are commonly employed in the manufacture of photographic film. As is well known in the art, the thicknesses of the resulting layers in the laminated article do not necessarily correspond to the thicknesses of the cascading coating solutions. The coating solutions are supplied to the coating head from reservoirs through conduits 48 and 50.

A passageway 56 through the upper coating lip 38, upstream of the inclined plane surface, may be provided to discharge a surfactant solution or another coating aid solution onto the web just before its contact with a coating bead from the

coating solutions

42 and 44. The passageway 56 might also be utilized to supply the bottom layer of coating solution to the web. The surfactant solution or other solution from passageway 56 may be utilized to facilitate the wetting of the main coating solution onto the web 12 and also to act as a vacuum seal.

It is presently preferred that the vacuum in area 30 be greater, i.e., the pressure is less, than in vacuum area 32 by a small amount. In this case there will be a small net upward pressure on the web section between the

vacuum areas

30 and 32. Accordingly, the moving web will tend to be pressed upwardly against the under surface of the upper coating lip 38 to minimize the gap between the web and the lip. By way of example, a substantially uniform normally less than 0.001 inch gap above the web may be maintained. With such a small gap, even a relatively high vacuum maintained in the

areas

30 and 32 will not pull the coating solutions through the gap but will function to assure the maintenance of a stable bead of coating solution even at high coating speeds, and the possibility of bubble entrainment is reduced.

As shown in FIG. I, the width of the lower coating lip 40 is less than that of the upper coating lip 38 with the upper coating lip projecting beyond the lower lip. Atmospheric pressure beneath the web which has passed the lower lip 40 functions in conjunction with the above discussed vacuums maintained in

areas

30 and 32 to urge the web upwardly minimizing the gap between the upper surface of the web and the bottom of upper coating lip 38.

Referring to FIG. 3 which shown the position of the

coating lips

38 and 40 and their configuration on an enlarged scale; it will be seen that the upper lip 38 has a rounded cut-away surface 58 in the region of the coating solution bead. The lip 38 is cut away in this region to enable a larger bead of coating solution to form and to facilitate the flow of the cascading coating solutions onto the web 12. This provision of a cut-away section which may be considered as a small cavity reduces the fluid sheer-induced vacuum which might be formed by the moving web and minimizes the likelihood of fluid cavitation. Similarly, a cavity 60 may be provided in the lip 38 at the discharge end of passageway 56 in order to prevent cavitation of the solution discharging onto a web from passageway 56 It is a significant feature of the invention that the web 12 is substantially flat as it passes under the surface 58 and the coating solutions are deposited thereon. The now coated web is then drawn off in a substantially straight line so that the coatings may gell without being subjected to laminar problems which might be the case if the web was moving on a path around a roller as in the prior art. Increased processing speeds are also obtainable by the web arrangement of the invention.

As shown in FIG. 1, the lower coating lip 40 is pivotally mounted about a pivot 61 and may be pivoted from its normal operative position to accommodate splices. In the event that an irregularity in the web 12 passes through the apparatus, such as a splice in the web, the lower lip 40 is pivoted downwardly about pivot 61 to allow the splice to pass through. The lip thereafter returns to its normal position. In the absence of the pivotal mounting of the lip 40, a thick splice might jam in the gap between the lips and result in breakage of the web which would necessitate that the apparatus be shut down.

FIG. 4 is a view similar to FIG. 3 of a modified coating head upper lip structure in which in lieu of the passageway 56 extending to the cavity 60 approximately midway in the width of the upper lip 38, a passageway 63 for a fluid such as a surfactant, another coating assistant, or the bottom layer of the eventual product is provided to discharge a fluid 64 down along a portion of the wall of the upper vacuum area 30 to contact the web 12 just as it reaches the gap between the upper and lower coating lips.

FIG. 5 illustrates another embodiment of the invention which is the same as that of FIG. 4 with the exception that in lieu of the curved surface 58 in the vicinity of the coating bead, the FIG. 5 embodiment has a slanted surface 66 so that a substantially triangular cavity is formed to accommodate the coating bead and minimize cavitation.

Still another arrangement for minimizing cavitation and providing for an enlarged coating bead is illustrated in FIG. 6 in which the upper lip 38 has a concave recess 68 in lieu of the slanted surface 66 of the FIG. 5 embodiment.

Referring to FIG. 2, this is a view similar to FIG. 1 but of another embodiment of the invention. In this embodiment in lieu of the thin lower coating lip 40 of the FIG. 1 embodiment, there are a pair of longitudinally spaced lower

coating lip sections

70 and 72 with a pressure chamber 74 intermediate these sections. A pressure fluid which may be air enters chamber 74 through inlet 76 and acts upon the undersurface of the web 12 to elevate the web towards the upper coating lip resulting in a very small coating gap as is the case with the FIG. 1 embodiment. Since there is a positive, greater than atmospheric force elevating the web 12, the dis tance between the upper and lower coating lips may be sufficient to allow a splice to pass through without jamming. Alternatively, the coating lip sections and 72 might be pivotally mounted as explained in connection with the lower coating lip 40 of the FIG. I embodiment.

In addition the embodiment of FIG. 2 illustrates the utilization of multiple pairs of vacuum areas to facilitate coolant evaporation. The web with coolant on its upper surface is introduced into the apparatus through inlet 18 into a chamber which is divided by the web into an upper vacuum chamber 20 and a lower vacuum chamber 22, which chambers, it will be appreciated, need not be totally partitioned by the web. These vacuum chambers are separately connected to suitable evacuating means through

evacuation conduits

24 and 26, respectively. The vacuum in both chambers is maintained substantially the same so that there is little or no net pressure acting on the opposite sides of the web 12. The residual coolant on the web is evaporated within the vacuum chamber 20 and withdrawn through evacuation pipe 24 (and 26 if the chamber is not fully partitioned by the web). If desired, the coolant may be recovered and recycled to the spray head 14. The vacuum in the lower vacuum area 32 of the second vacuum chamber is preferably less than that of the upper vacuum area 30 to assist the action of the pressure chamber 74 in urging the web upwardly against the upper coating lip 38. The provision of the pressure chamber 74 to insure that the web is maintained substantially in contact with the upper coating which is also advantageous in installations in which it is intended that the coated web be drawn off in a straight path which is at an inclination with regard to the path of the web through the coating apparatus.

In each pair of vacuum areas there are permeable upper and lower web guide members 73 and which are spaced apart by a distance greater than the thickness of the web 12. Since these guide members are permeable and are comprised of screens or the like, the vacuum in the various areas can still act upon the web through the guide members. The guide members service to maintain alignment of the web and prevent undesirable material vertical fluctuations in the'web as it passes through the vacuum areas. Similar guide members may also be utilized in the vacuum areas of the previously described embodiments of the invention.

While presently preferred embodiment of the invention have been shown and described with particularity, it will be appreciated that various changes and modifications may readily suggest themselves to those of ordinary skill in the art on being apprised of the present invention. It is intended to encompass all such changes and modifications as fall within the scope and spirit of the appended claims.

For purposes of the present application, the term "atmospheric defines the ambient air pressure in the environment in which the web coating apparatus defined in the claims hereof is employed.

What is claimed is:

1. Web coating apparatus comprising first and second opposed members, each of said opposed members including an upstream and a downstream section, said members defining, in combination:

chamber means adapted to accommodate passage of a substantially planar web therethrough;

said upstream sections forming in combination an inlet in communication with said chamber means through which a substantially planar web may enter into said chamber means;

said downstream sections forming in combination an outlet in communication with said chamber means whereby a substantially planar web may exit from said chamber means;

means for reducing the air pressure in said chamber means to a pressure below atmospheric pressure; and

coating composition applicator means associated with at least one of said downstream sections for providing a continuous layer of liquid coating composition to a planar web passing from said chamber means through said outlet; whereby reduced air pressure provided by said means for reducing air pressure in said chamber acts only on the upstream side of said liquid coating composition layer extending from said applicator means to said web substantially at the point of deposition of such liquid coating composition upon said web.

2. The apparatus as defined in claim 1 wherein said means for depositing a liquid coating composition onto a substantially planar web comprises means for continuously providing to said planar web a bead of liquid coating composition.

3. The apparatus as defined in claim 2 wherein the air pressure at the surface of the bead next adjacent said chamber means is less than the air pressure at the surface of the bead opposed thereto.

4. The apparatus as defined in claim 3 wherein said downstream sections each comprise a lip member and said outlet comprises said lip members in substantially parallel opposition.

5. The apparatus as defined in claim 4 wherein said means for depositing a liquid coating composition onto a substantially planar web comprises an inclined plane slide surface associated with at least one of said lip members.

6. The apparatus as defined in claim 1 wherein said chamber means comprises means for independently regulating the air pressure on either side of a substantially planar web passing therethrough.

7. The apparatus as defined in claim 6 wherein air permeable web guide means are disposed within said chamber means.

8. The apparatus as defined in claim 4 wherein at least one lip member associated with means for depositing a liquid coating composition onto a substantially planar web comprises a configuration capable of accumulating a bead of liquid coating composition.

9. The apparatus as defined in claim 1 further comprising liquid supply means adapted to deposit a vaporizable liquid onto a substantially planar web at a point upstream of said chamber means.

10. The apparatus as defined in claim 4 wherein said means for depositing a liquid coating composition onto a substantially planar web is associated with only one of said lip members.

11. The apparatus of claim 10 wherein an air pressure chamber in which is maintained a pressure higher than atmospheric pressure is associated with said lip member opposed to said lip member associated with said means for depositing a liquid coating composition onto a substantially planar web and adapted to urge a substantially planar web passing from said outlet toward said lip member associated with said liquid coating composition depositing means. 1 1! IF I

Claims

1. Web coating apparatus comprising first and second opposed members, each of said opposed members including an upstream and a downstream section, said members defining, in combination: chamber means adapted to accommodate passage of a substantially planar web therethrough; said upstream sections forming in combination an inlet in communication with said chamber means through which a substantially planar web may enter into said chamber means; said downstream sections forming in combination an outlet in communication with said chamber means whereby a substantially planar web may exit from said chamber means; means for reducing the air pressure in said chamber means to a pressure below atmospheric pressure; and coating composition applicator means associated with at least one of said downstream sections for providing a continuous layer of liquid coating composition to a planar web passing from said chamber means through said outlet; whereby reduced air pressure provided by said means for reducing air pressure in said chamber acts only on the upstream side of said liquid coating composition layer extending from said applicator means to said web substantially at the point of deposition of such liquid coating composition upon said web.

11. The apparatus of claim 10 wherein an air pressure chamber in which is maintained a pressure higher than atmospheric pressure is associated with said lip member opposed to said lip member associated with said means for depositing a liquid coating composition onto a substantially planar web and adapted to urge a substantially planar web passing from said outlet toward said lip member associated with said liquid coating composition depositing means.