GB1574241A

GB1574241A - Method and apparatus for coating a moving web

Info

Publication number: GB1574241A
Application number: GB1209177A
Authority: GB
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1976-03-23
Filing date: 1977-03-22
Publication date: 1980-09-03
Also published as: DE2712259A1; JPS52114636A; FR2345228A1; DE2712259B2; DE2712259C3; FR2345228B1

Description

(54) METHOD AND APPARATUS FOR COATING A MOVING WEB (71) We, E.I. DU PONT DE NEMOURS AND COMPANY, a corporation organized and existing under the laws of the State of Delaware, located at Wilmington, State of Delaware, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns the coating of liquid layers on a moving web. More particularly, the invention relates to an improved coating method and apparatus useful in coating at least one, and preferably, a plurality of high quality layers of photosensitive and associated compositions.

U.S. Patent 3.508,947 traces the development of various coating techniques in the photographic industry and the problems incident to them. In an effort to resolve those problems, and more particularly to develop methods of coating at higher speeds, U.S. Patent 3,632,374, suggests the use of certain curtain coating techniques for the preparation of photographic film. Curtain coating involves forming a fluid layer under precisely controlled conditions without imparting significant flow velocity to the layer and permitting it to accelerate primarily during free fall until impingement on a web to be coated. The free fall permits the fluid layer to attain sufficient velocity to penetrate the air layer entrained over the web. Such curtain coating obviates many problems incident to the older bead coating and permits high coating speeds; however, the falling liquid curtain is vulnerable to disturbances due to air currents. Reducing the length of the curtain fall minimizes the air current problem, but such reduction is not always feasible since the drop must be sufficient to provide the required momentum to the liquid layers to penetrate the air barrier entrained over the web surface if high quality coatings are to be attained. One solution proposed without complete success has been the use of baffles behind the curtain to minimize the effect of stray air currents.

An alternate approach which avoids the problem of curtain instability is described in U.S. Patent 3,749,053. This approach includes a cascading coating apparatus, positioned closely to the web, used together with an integral vacuum box to deposit multiple photographic layers on a horizontally moving web. Some little difficulty is encountered with this approach since provision must be made for enlarging a gap provided in the vacuum box through the use of a spring loaded hinged member to allow passage of web splices.

An improvement over this approach is described beginning on page 203 of a text entitled "Coating Equipment and Processes", by George L. Booth, 1970 edition, Lockwood Publishing Co., Inc. Booth teaches that the fluid layer be subjected to constant acceleration down a planar slide surface prior to free fall. This, however, has the disadvantages of subjecting the layer to excessive acceleration and to an abrupt change of flow direction as it enters free fall resulting in thickness and other aberations in the coating.

According to one aspect of the present invention there is provided a method of coating a support surface with a least one layer of fluid coating composition which comprises flowing said layer or layers down a slide surface constructed and arranged such that said layer or layers experience a continuously increasing gravitational acceleration until flow of said layer or layers is in a generally vertical direction as herein defined and thereupon immediately releasing said layer or layers from said slide surface whereby said layer or layers fall freely and impinge upong said support surface whilst said support surface is moving relative to and below said slide surface; the point of release of said layer or layers from said slide surface being located within one inch above said support surface.

According to a further aspect of the present invention there is provided an apparatus for coating a support surface with at least one layer of fluid coating composition comprising an inclined slide surface having a continuously increasing slope ending in an edge (a tangent to said slide surface at said edge being generally vertical as herein defined), means for forming the layer or layers on the slide surface, a source of fluid coating composition and means for moving the support surface relative to and below the slide surface, the apparatus being arranged and constructed such that in use said edge is located within one inch above said support surface and the layer or layers of fluid coating composition are formed on the slide surface and flow down the slide surface at a continuously increasing rate of acceleration until flow is in a substantially vertical direction (as herein defined) whereupon the layer or layers are permitted to fall freely and impinge upon the support surface whilst said support surface is moving relative to and below the slide surface.

The phrase "generally vertical direction" is used herein to define a direction of flow such that the angle cp in Fig. 1 is from 80 to 1200.

The apparatus of the invention utilizes a slide surface having a continuously increasing slope to provide the controlled acceleration desired, i.e., one having a continuously increasing rate of acceleration. By thus controlling the acceleration up to the point of free fall and causing a greater portion of the acceleration to the ultimate impingement velocity to occur under conditions of controlled acceleration, there is less danger of losing continuity of the layer than under pure free fall conditions.

The larger clearance relative to the splice thickness allows the free passage of splices with minimum disruption of the coating operation. The problem of curtain instability due to air currents is alleviated by the extremely short drop. The controlled acceleration of the fluid to a release velocity which when combined with the free fall velocity increase is sufficient to penetrate the air barrier over the substrate surface, reduces the need for accessory vacuum systems or baffles when coating at higher speeds and results in a simpler overall system.

Figure 1 is a vertical cross section of one embodiment of a coating apparatus useful in the practice of this invention; Figure 2 is a vertical cross section of another embodiment of a coating apparatus according to this invention showing the passage of a film splice through the coating area; and Figure 3 is a fragmentary cross section of an alternative embodiment of the lip portion of the slide support used in the embodiment of FIG. 1.

The method and apparatus that form the subject matter of this invention can best be understood by reference to the accompanying figures. Thus, FIG. 1 shows a preferred embodiment incorporating all the necessary elements to produce multiple superposed flowing layers, accelerate them in a controlled manner to a desired velocity and deposit them on a moving web in distinct layers.

More specifically, a support surface or web 11 which is suitable to receive a two-layer coating typically comprising a silver halide emulsion layer 24 and an antiabrasion layer 23, of the type described in the examples in this specification, is trained over a support roller 12 positioned substantially under the coating device of this invention.

The coating device of Figure 1 resembles in part the coating devices known in the art as a sliding type hopper used to practice bead coating. The body of the hopper comprises four sections. Section 15 and section 16 define a first feed slot 17 and a first reservoir 19 through which a fluid is pumped via pump 26 and tube 27. The upper part of section 15 is flat and polished and forms an inclined slide surface 20. Similarly, section 14 and section 15 define a second slot 18 and second reservoir 30 through which fluid may be pumped via pump 29 and tube 28 onto the upper surface of section 14. The upper surface of section 14 is at a level and in a plane located slightly lower than the plane of the slide surface 20. The upper surface of section 14 is also flat and polished, and forms a second slide surface 21 whose angle with the horizontal is substantially identical to that of slide surface 20. The amount by which the plane of the second slide surface 21 is recessed under the plane of the first slide surface 20 is at least equal to the desired thickness of the layer of the fluid pumped through slot 18 and flowing over slide surface 21.

Thus far described, the multiple fluent layer forming device is conventional.

According to this invention a section 13 defining a convex curved upper surface 22 is attached to the end of section 14, opposite to the side adjacent section 15. In accordance with a preferred embodiment of this invention, the curved convex slide surface 22, starts from the end of the planar slide surface 21 and ends with its bottom edge 60 at a point tangent to the vertical and located at a distance "d" from the web 11 The distance "d" preferably is varied to permit free passage of a splice in the web and may be as small as 0.03125 inches (approximately .08 cm) or as large as 0.75 inches (approximately 1.9 cm) or in the case of high web speeds can be as much as 1 inch (approximately 2.5 cm).

Preferably the distance "d" should be kept as small as possible to avoid the deleterious effects of air turbulence on the fluid layer.

The choice of the particular shape of the convex curve of slide surface 22 along the direction of flow may vary, depending on the fluid properties of the coating solution, coating weight desired and speed of the web 11. It has been found that a surface approximating a conic section, i.e., having a generally parabolic shape along the direction of flow of the type defined by the equation y= -x2/k, where x is the abeissa and y the ordinate of a plot defined by such equation, gives good results in that the rate of change of acceleration of the coating solution as it moves along the slide surface is constant. Other convex shapes are acceptable, however, and may find use as required to satisfy the parameters noted. The surface transverse to the direction of flow defines a straight line. In any event, it is preferred that the proportionate increase in velocity imparted to the coating solution under the controlled acceleration offered by the slide surface relative to the proportionate increase in velocity imparted to the coating solution during free fall be increased insofar as possible. Greater proportionate velocity increases under controlled conditions can be attained with lower viscosity fluids.

The discharge edge 60 is depicted as being squared in cross-section to provide a well defined point of departure for the fluid and to prevent undue lip wetting and consequent streaking in the deposited coating. Alternatively the discharge edge may have a rounded cross-section as depicted in FIG. 3 or a knife edge as depicted in FIG. 2. The essential requirement is that the discharge edge should not interfere with fluid flow which is preferably at the desired impingement velocity when it reaches this point.

The discharge end of the slide surface preferably is vertically oriented, i.e., a = 900, but the angle a may vary from 80" to 1200 and may, for example, be varied according to the desired impingement velocity and fluid parameters. Angles less than 90" are notpreferred because the fluid has a tendency to break away from the slide surface prematurely. Similarly angles significantly greater than 90" are not preferred because they tend to cause an abrupt change in the rate of acceleration of the fluid as it leaves the edge 60. This is contrary to the objective of the present invention which is to provide a transitory zone of flow so that the fluent layers enter free fall with minimal change in the direction of flow and minimam change in the rate of acceleration.

Figure 2 shows an alternative embodiment of this invention. In this alternative, an arc of a circle is used to define the slide surface 32 in section 33 of the coating device. The web 11 of FIG. 1 is replaced by two webs, 44 and 42, to illustrate how a splice goes through the coating station. The device of FIG. 2 is depicted as adapted to coat a single layer and the body of the coating device is comprised of two sections, 33 and 46, which define a feed slot 38 and a reservoir 39. A pump 36 supplies a fluid to the upper part of the body section 33 through a tube 37. The upper part of the body section 33 comprises a flat inclined slide section 31 which extends to a point A beyond which it becomes curved, forming a portion of a circle of radius R terminating at a point B, tangent to the vertical, and located at a distance "d" from the moving web 42. The web 42 is trained over a guide roller 12. In FIG. 2 a web splice is shown as it passes through the coating station below the discharge edge 62. The web 42 has its lagging end 40 resting over a section of the leading end 43 of web 44. Extending over the overlapping section of the two webs 44 and 42 is a strip of adhesive tape 41 which usually extends the full width of the web. The verticality of the slide surface may be varied as described in conjunction with FIG. 1 as may the distance "d" to accomodate different coating composition, web speeds and desired impingement velocities.

It is to be understood that other known hoppers such as an extrusion type hopper utilized to generate superposed discrete layers prior to accelerating them over a convex slide surface, may also be used in accordance with the invention.

While the vertical end of the convex slide surface is shown located over the center line of the guide roll 12 in FIG. 1 and off-center in FIG. 2, it should be recognized that its position may vary and that it can be located at any point over the web where such web preferably is horizontal or nearly so. It is preferred, however, that the web be supported by a roll at the coating location to minimize disturbances due to flutter. While it is preferred that the web be horizontal at the point of impingement of the fluid layer, it also can impinge at any point on the curved portion of the web 11 wrapped around the roll 12 (FIG.

1) from approximately 9 o'clock. Again, the web need not be supported at the point of impingement, although a support is preferred.

Referring back now to FIG. 1, the method of this invention will be described. Web 11 is trained over the roller 12 and set in motion in the direction of the arrows at the desired coating speed by suitable apparatus (not shown). From a reservoir (not shown in the figures) a fluid, which may be a silver halide emulsion of the type described in the examples herein, or any other similar solution conventionally used in the photographic or other coating arts, is pumped through the pump 29 and the tube 28 to fill the reservoir 30 which serves as a surge tank or pressure equalizer. The fluid passes through a slot 18 onto the lower slide surface 21. Preferably, the slot 18 extends over the full width of slide surface 21. When the reservoir 30 is full, the fluid rises into the slot 18 and emerges onto surface 21 where it starts to flow in a downward direction due to gravity. The rate of feed of the flow is such that the fluid forms a uniformly flowing layer 24 covering the full width of slide surface 21 which typically may be positioned to provide about a 23 slope.Actually the slope of the sliding surfaces may be varied from about 0 (with an extrusion hopper) to about 45" (using either an extrusion or slide hopper) according, for example, to the compositions used. Typically, the slope is selected to permit formation of the fluid layers and to permit them to move down to the convex slide surface region.

From a second reservoir, not shown in the figure, the pump 26 supplies a second layer which may be an antiabrasion layer comprising an aqueous solution of 6% gelatin and the usual antistatic and hardening agents used in the art. This second fluid is directed through the tube 27 to fill the reservoir 19 and thence through the slot 17 to form a uniform flowing layer 23 over the upper slide surface 20. As layer 23 flows downward it eventually reaches the exit of slot 18 and continues its downward flow over the layer 24, both layers now flowing in distinct superposed strata.

As the layers reach the end of the planar inclined surface 21 and enter the convex surface 22, their speed increases because of the effect of gravity at a rate controlled by the rate of change of the curvature of the curvature of the slide surface. When the layers reach the end of the convex slide surface 22, they have been accelerated, by selection of the appropriate curve, to a velocity which when combined with the increase in velocity during the short free fall drop will be sufficient to penetrate the web's entrained air barrier and become deposited onto the moving web 11. The two layers 23 and 24 bridge the gap between the edge 60 of slide 22 and web 11 by dropping freely through distance "d" in a continuousely flowing fluid drop 25.

Since the fluent layers enter free fall at a velocity that is substantially higher than the velocities taught in curtain coating, the need for large free fall distances to achieve impact velocity sufficient to overcome air drawn under problems has been reduced, and substantially shorter drops may be used. Distance "d" therefore may be kept at a minimum.

When a splice reaches the coating station, as shown in FIG. 2, the flow of the coating liquid is not disturbed provided distance "d" is such that the splice can pass through with out breaking the fluid drop 35.

The following examples serve to illustrate the invention without limiting it.

Example No. 1 A coating device of the type shown in FIG.

1 was used to coat a conventionally prepared gelatino-silver halide emulsion consisting of silver halides suspended in an aqueous solution of gelatin on a moving support. The support consisted of polyethylene terephthalate film base, of the type used in photographic film manufacturing prepared substantially as taught in U.S. Patent 2,779,784.

The support thickness was 0.007 inches (0.01778 cm).

Only one layer was applied in this instance, therefore, the pump 26 was inoperative during this experiment, and the fluid was supplied to the slide surface through the pump 29 and the slot 18. The convex slide surface was parabolic, satisfyin the equation y= - x2 and starting at a point tangent to the slide surface making a 23 angle with the horizontal. The total length in the direction of the fluid flow was 5 inches (12.7 cm). The equipment was capable of applying a 4.625 inches (11.75 cm) wide layer on the moving web. The emulsion viscosity was maintained at 9.7 centipoises at 100"F and the fluid was supplied to the coater at a rate of 720 cc/min.

The distance "d" was varied between 0.125 inches (0.3175 cm) and 0.5 inches (1.27 cm).

Satisfactory continuous coatings were obtained with the web running at 150 ft/min (76.2 cm/sec) at all settings. Dry coating weight was calculated at 149 mgs/dm2.

Example No. 2 Using the apparatus of Example 1, but with a generally parabolic convex slide surface 7 inches (17.78 cm) long this time, a single layer of a sensitized silver halide emulsion containing silver halides suspended in an aqueous solution of gelatin was deposited as a 4 inch (10.16 cm) wide layer on a moving web of the type described in Example 1, at the rate of 440 ft/min (223.52 cm/sec). The solution flow rate was 720 cc/min and its viscosity was measured at 950F as 11,4 centipoise. The drop distance "d" was varied between 0.1875 inches (0.476 cm) and 0.25 inches (0.635 cm), Dry coating weight measured 75.5 mgs/dm2. All coatings were considered satisfactory.

While the invention has been described in terms of a number of specific embodiments, these are by no means limiting. For example, hopper or overflow weirs can be used as a source of laminarly flowing liquid layers.

Also a combination extrusion die and slide hopper can be used to form multiple layers.

In addition, the use of coating edge guides on the slide surface and spanning drop distance "d" may be employed as shown in U.S.

Patent 3,632,374 to maintain full coating width.

Claims

WHAT WE CLAIM IS:

1. A method of coating a support surface with at least one layer of a fluid coating composition which comprises flowing said layer or layers down a slide surface constructed and arranged such that said layer or layers experience a continuously increasing gravitational acceleration until flow of said layer or layers is in a generally vertical direction as herein defined and thereupon immediately releasing said layer or layers from said surface whereby said layer or layers fall freely and impinge upon said support surface whilst said support surface is moving relative to and below said slide surface; the point of release of said layer or layers from said slide surface being located within one inch above said support surface.

2. A method as claimed in Claim 1 wherein the gravitational acceleration increases at a constant rate.

3. A method as claimed in any of the preceding claims wherein the slide surface defines a conic section.

4. A method as claimed in claim 3 wherein the slide surface defines a parabola.

5. A method as claimed in claim 1 or claim 2 wherein the slide surface defines substantially an arc of a circle.

6. A method as claimed in any of the preceding claims wherein the layer or layers are gravitationally accelerated until flow is in a direction at an angle of about 90" to the horizontal.

7. A method as claimed in any of the preceding claims wherein the velocity of the layer or layers at the point of departure from the slide surface approaches the impingement velocity on the support surface.

8. A method as claimed in any of the preceding claims wherein, the distance through which the layer or layers are allowed to fall freely is from 0.03125 to 1 inch

9. A method as claimed in claim 8 wherein the distance through which the layer or layers are allowed to fall freely is from 0.03125 to 0.75 inch.

10. A method as claimed in any of the preceding claims wherein the point of departure for the layer or layers from the slide surface is formed by a discharge edge having a squared or rounded cross-section or being in the form of a knife edge.

11. A method as claimed in any of the preceding claims wherein the support surface is supported by a roll at the point of impingement of the layer or layers.

12. A method as claimed in any of the preceding claims wherein the support surface is moving substantially horizontally at the point of impingement of the layer or layers.

13. A method as claimed in claim 1 wherein the increase in velocity of the layer or layers whilst flowing down the slide surface is greater than the increase in velocity whilst falling freely.

14. A method of coating a support surface as claimed in claim 1 substantially as herein described.

15. A method of coating a support surface substantially as herein described in Example 1 or Example 2.

16. A support surface whenever coated by a method as claimed in any of claims 1 to 15.

17. An apparatus for coating a support surface with at least one layer of fluid coating composition comprising an inclined slide surface having a continuously increasing slope ending in an edge, a tangent to said slide surface at said edge being generally vertical as herein defined means for forming the layer or layers on the slide surface, a source of fluid coating composition and means for moving the support surface relative to and below the slide surface, the apparatus being arranged and constructed such that in use said edge is located within one inch above said support surface and the layer or layers of fluid coating composition are formed on the slide surface and flow down the slide surface at a continuously increasing rate of acceleration until flow is in a substantially vertical direction (as herein defined) whereupon the layer or layers are permitted to fall freely and impinge upon the support surface whilst said support surface is moving relative to and below the slide surface.

18. An apparatus as claimed in claim 17 wherein the slide surface defines a substantially conic section.

19. An apparatus as claimed in claim 18 wherein the slide surface defines a parabola.

20. An apparatus as claimed in claim 17 wherein the slide surface defines substantially an arc of a circle.

21. An apparatus as claimed in claim 17 substantially as herein described with reference to any of Figs. 1 to 3.

22. An apparatus substantially as herein described in Example 1 or Example 2.