Method and apparatus for coating a surface

Sept. 19, 1961 j. _F. greiller 3,000,760

METHOD AND APPARATUS FOR COATING A SURFACE

Filed May 27, 1958 2 Sheets-Sheet 1

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Sept. 19, 1961 j. _F. greiller 3,000,760

METHOD AND APPARATUS FOR COATING A SURFACE

Filed May 27, 1958 _{2 Sheets}__Sheet 2

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Jack F. (Lceillcr

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United States Patent Office

3,090,760

Patented Sept. 19, 1961

3,000,760

METHOD AND APPARATUS FOR COATING A SURFACE

Jack F. Greiller, Wealdstone, England, assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey

Filed May 27,1958, Ser. No. 738,120

Claims priority, application Great Britain June 28, 1957 10 Claims. (Cl. 117—111)

This invention relates to a method and apparatus for transferring liquid from one moving surface to another, and especially to the control of the thickness and uniformity of the layer of liquid which is transferred.

In some known methods of coating the thickness of a layer of liquid composition is regulated by means of a doctor blade which usually functions by shearing off the top, unwanted layer of composition or by squeezing the composition down to the required thickness.

In one system of coating liquid compositions, known as transfer roll coating, liquid coated on a roller is transported in a layer to the nip (or point of least separation) between that roller and another by rotation of the first roller. At, or just before, the nip, the layer of liquid makes contact with the second roller and on leaving the nip the layer is ruptured so that some of the liquid remains on the first roller and some is taken away by the second roller.

With any given system of rollers and with a given liquid there are usually speeds of rotation of the rollers at which the liquid transfers to the second roller with a smooth, even surface and with a uniform thickness. However, if one or both rollers are made to rotate faster, a speed of rotation occurs at which rupturing of the layer of liquid between the two rollers is no longer smooth with the result that the surface of the liquid transferred to the second roller becomes undulated and the thickness of the liquid uneven. At still faster speeds of rotation, it is not uncommon for some of the liquid entering the nip between the rollers not to follow either roller, but to leave the surface of the liquid in the form of a spray or mist in the surrounding medium just after the nip.

The composition is picked up by the second roller and transferred to the web to be coated therewith at some other point, or is picked up directly on the web by passing the web around the second roller and through the nip between the two rollers. The thickness of the layer transferred by this means can be adjusted to some extent by adjusting the speed of one roller relative to the other, or in some cases by adjusting the separation between them.

The coating device and method provided by the present invention enables a layer of liquid with a smooth even surface and with controlled thickness to be transferred from one moving surface carrying the liquid to another, such as from one roller or web to another roller or web, at speeds greater than could be used previously with any given system of the above nature.

According to the present invention, there is provided a method of transferring a liquid from one moving surface to another disposed in liquid transfer relationship thereto, in which that part of the liquid layer moving in contact with both surfaces is sharply separated layerwise into two layers before the layer ruptures.

According to the present invention, there is also provided a liquid transfer apparatus which comprises two movable surfaces in liquid transfer relationship and a blade having an edge disposed along and at or close to the nip between said surfaces and directed against the motion of the surface to which the liquid is transferred.

By transfer relationship we mean that the two rollers are positioned in relation to one another so that some or all of the liquid carried on one roller to the nip is carried away by the other roller.

In the method of the present invention a layer of a liquid is carried on a moving surface to the nip between 5 the surfaces disposed in liquid transfer relationship and as the layer passes through the nip but before or at the point where the layer is ruptured layerwise due to the action of the two surfaces, the layer is cleanly divided layerwise by a blade with an edge disposed along and at 10 or close to the nip so that a layer of the composition is carried from the nip on the one surface, the remaining layer being carried away on the other surface.

The present invention can be applied to transfer from one roller to another, from a pliable belt around a roller 15 to a roller, or to another belt around a roller, or to and from belts not around rollers.

In the method and apparatus of the present invention the blade employed extends from its edge a distance back sufficient to ensure that the separated layers of liquid com20 position cannot reunite, that is, sufficient to ensure that the original composition does not merely flow around the blade to be split again by the counteraction of the two surfaces, e.g., rollers, webs or roller and web. The blade may have any desired cross-section such as rec25 tangular or circular (in this case the edge of the blade is round), but the preferred shape is a wedge shape. In constructing a blade of the wedge type the length of the wedge (from shoulder to point) is such that at the shoulders there is sufficient thickness to support the wedge ³⁰ after shoulders have been formed. With this condition the wedge may extend along the roller surface from the nip to any convenient point. A blade giving very satisfactory results has surfaces at least approximately complementary to the rollers, webs or roller and web, or other ³⁵ moving surfaces. In the first case, the blade has two concave surfaces each of substantially the same center of curvature as the roller which it faces and in the second case one surface is concave matching the roller and the other is flat, matching the web.

⁴⁰ The surfaces of the blade are not, however, necessarily complementary to the surfaces of the rollers or webs, as the case may be, which they face. They may be such that the distance of a shoulder of the wedge from the facing roller or web is different from the distance of the edge 45 from that surface. If different, it is preferred that the distance of the shoulder from the roller or web is less than that of the edge. In the case where the blade is wedge shaped the shoulders of the wedge are preferably of 90° angle in cross section but they may have another 50 angle, for example, 70° or 110°. The edge of the blade may be sharp, flat, radiused or otherwise shaped.

The blade may be fixed but is preferably adjustable in relation to the nip and to its distance from either roller or web or from the roller or web. The blade may be ad55 justable in a fixed support or may be fixed in the support which is itself adjustable. In either case, the distance of the blade from the second roller or web can be finely adjusted by mounting the blade or support on micrometer screws to give very close control over the thickness of 60 the transferred or coated layer and to closely regulate the distance of the edge from the nip.

The gap between the blade and the coating roller is the most important from the point of view of width, and taper, if any.

In the present invention this gap may be formed by having the surfaces complementary or by providing that this gap narrows in the direction away from the nip. The gap may narrow by about 10 to 20%, for example. That is to say, the gap becomes 10 or 20% less in width from the edge of the blade to the shoulder of the blade.

The blade may be of any suitable material such as metal, e.g., brass or stainless steel, or of synthetic material

3,000,760 such as a thermoset resin. The most suitable materials have good rigidity and are readily wetted by the coating liquid. The blade may also be substituted by an air or fluid knife.

In carrying out the method of the invention the roller or web or other moving surface bringing the liquid to the nip may move in the same direction or counter to the other roller or web or moving surface at the nip provided that the edge of the blade is directed against the motion of the liquid on the latter surface.

The rollers used in the present invention may be made of any suitable material such as metal, rubber or other synthetic material.

In one embodiment of the present invention one of the webs is coated with the composition, for example, a film base is coated with a photographic emulsion from a roller or from a belt passing around a roller, the coating being effected with the use of a blade by the method of the invention.

The accompanying drawings illustrate embodiments of the method and apparatus of the present invention. In these drawings:

FIG. 1 is an end view, partly in section, of an arrangement of a web and a roller between which a coating composition is being transferred using a blade in accordance with the invention, the web being supported on another roller.

FIG. 2 is an end view, partly in section, of an arrangement of a roller and a web between which a coating composition is being transferred using a blade in accordance with the invention, the web being flat.

FIGS. 3 to 5 are vertical sections through the nips of apparatuses of the invention having blades of various cross-sections.

FIG. 6 is an end view, partly in section, of an apparatus comprising two rollers and a fluid knife.

FIG. 7 is a diagrammatic view of an arrangement of two rollers and a blade in accordance with the invention, the blade being adjustable.

Referring to FIG. 1, a belt 2 passes under roller 1 and below this is a roller 3. A wedge 4 having concave surfaces 6 and 7 concentric with the outer surface of the belt 2 and the surface of the roller 3 which they respectively face is disposed with its edge 8 very close to the nip between the belt 2 and the roller 3. The edge 8 of the wedge is parallel to the axes of the rollers 1 and 3. The wedge 4 is mounted on a fixed support (not shown) and the position of the edge 8 is adjustable in relation to the nip and the rollers 1 and 3 by means of micrometer screws (not shown). The shoulder 13 of the wedge 4 is square.

In operating this embodiment of the apparatus of the invention, roller 3 is rotated and made to pick up a layer of a liquid coating composition 9 of thickness b, such as by rotation with a lower portion thereof (not shown) in a bath of the composition. The roller 3 carries the composition 9 to the nip where belt 2, being arranged in liquid transfer relationship to roller 3, i.e., such that “a” is less than “b,” picks up composition from roller 3.

The point 10 at which belt 2 picks up composition 9 may be varied from at the nip outwards against the direction of roller 3.

At the nip, the layer of composition 9 meets the edge 8 of wedge 4 and thereafter divides into two layers 11 and 12, one, 11, carried on belt 2 and the other, 12, carried on roller 3.

The thickness of the layers 11 and 12 can be controlled by moving the wedge 4, by adjusting means (not shown), toward or from the belt 2. The position of the edge 8 can also be displaced from the nip. The wedge 4 may be adjusted by first setting the edge 8. beyond the nip and then withdrawing the wedge until on operating the apparatus a layer of coating 11 of satisfactory thickness is obtained on belt 2.

It will be seen that if edge. 8 is at the nip, the shoulder and the edge 8 are at the same distance from belt 2 but if the edge 8 is beyond the nip its distance from belt 2 will be greater than that of the shoulder 13, from belt 2 and, conversely, if the edge 8 is not up to the nip it will be nearer to belt 2 than shoulder 13. In the preferred method and apparatus of the invention wedge 4 is adjusted so that the shoulder 13 is nearer to belt 2 than the edge 8. Alternatively, the wedge can be shaped to provide this feature in the apparatus with the edge 8 at the nip.

In an example of the method of the invention using two rollers, a belt and a wedge as illustrated in FIG. 1, roller 3 was made to pick up a layer of a 7% aqueous gelatin composition by rotation partly in a bath of the composition. A satisfactory layer of composition was picked up on the belt at the rate of 600 feet per minute. Without the blade of the invention it was found that an uneven layer showing longitudinal lines was formed on the belt at a rate of 40 feet per minute.

In another example, a 20% aqueous gelatin composition was coated on the belt from the lower roller at the rate of 350 feet per minute. Without the blade the coating showed longitudinal lines at the rate of 20 feet per minute.

Referring now to FIG. 2, a roller 14 is supported beneath a flat belt 15 and disposed between them is a wedge 16 with its edge 17 at the nip between the roller 14 and the belt 15. The surface 18 of wedge 16 which faces the belt 15 is flat and the surface 19 which faces roller 14 is concentric with the roller 14. The edge 17 of wedge 16 is parallel to the axis of roller 14 and the plane of the belt 15. The shoulders 20 and 21 of wedge 16 are square. Wedge 16 is supported by a framework, not shown, by means of which the position of wedge 16 in relation to the belt 15 and roller 14 and to the nip can be adjusted.

In operating this apparatus in accordance with the invention, roller 14 is made to pick up a layer 22 of a coating composition such as by rotation partly in a bath (not shown) thereof or from another belt or roller (not shown) and carries the layer 22 from either direction to the nip between the roller 14 and belt 15, which are disposed in transfer relationship, where the belt 15, moving either with or against the direction of the roller 14 at the nip, but toward the edge 17 of the blade 16 picks up composition from layer 22.

In the arrangement shown in FIG. 2 the roller 14 is moving in the same direction as the belt 15 at the nip. The layer 22 contacts the belt 15 just before the nip. At the nip, the layer of composition between the roller 14 and belt 15 meets the edge 17 of wedge 16 about which it divides into two layers 23 and 24, layer 23 being carried by the belt 15 and layer 24 by the roller 14.

Further embodiments of the apparatus according to the present invention are illustrated in FIGS. 3 to 5 in which 25 and 26, 27 and 28, 29 and 30 represent pairs of rollers disposed in transfer relationship. Rollers 26, 28 and 30 each carry a layer of a coating composition 31, 32 and 33, respectively, which contacts the second roller 25, 27 or 29, respectively, at or near the nip. After passing through the nip as shown in FIG. 3 or at the nip as shown in FIGS. 4 and 5 the layer 31, 32 or 33 is divided into two by the blade 34, 35 or 36, respectively. As shown, blade 34 is of square cross section, blade 35 is of rectangular cross section and blade 36 is of circular cross section. These blades are mounted on supports (not shown) so as to be adjustable in relation to their distances from the rollers and from the nips. In the case of blades 34 and 35 one edge thereof is set against the movement of the layer carried on rollers 26 and 28. In examples employing the embodiments of the apparatus of the invention illustrated in FIGS. 3 to 5 aqueous gelatin was coated satisfactorily at 200 feet per minute.

Referring now to FIG. 6, 37 and 38 are partial cross sections of rollers between which a belt 39 passes in contact with the upper roller 38, the lower roller 37 and the

3.000.760 . . ⁵ belt 39 being in transfer relationship. Roller 37 picks up in a known manner (not shown) a layer 40 of a coating composition which contacts belt 39 just before the nip between the belt 39 and the roller 37, belt 39 moving in the same direction as roller 37 at the nip.

In accordance with the present invention means to separate this layer 40 layerwise before it ruptures between the diverging belt 39 and roller 37 is provided; in this embodiment the means is a fluid knife 41. In using this knife fluid passes through pipe 42 and out through a knife jet 43 disposed parallel to the axes of rollers 37 and 38, the force of the jet of fluid being sufficient to separate the coating composition layerwise at the nip. Means not shown may be provided to remove the jet fluid from the nip, e.g., a suction device.

Referring now to FIG. 7, rollers 44 and 45 are mounted on a support 46 so that a coating composition is picked up from a bath 47 by roller 45 rotating in the direction indicated and is transferred at the nip between rollers 44 and 45 to roller 44. Blade 48 is supported parallel to the axes of rollers 44 and 45 by an extension 49 to support 46 and is adjustable in the support 49 by means of springs 50, 51 and 52 attached to the support 49 and micrometer screws 53, 54, 55, 56 and 57 passing through support 49. Springs 50 and 51 press the blade 48 upward against screws 53, 54 and 57 and spring 52 pulls the blade 48 away from the nip against the screws 55 and 56.

In employing the apparatus of FIG. 7 in the method of the present invention roller 45 dips into the bath 47 of coating composition and carriers a layer of the composition to the nip between rollers 44 and 45 where it makes contact with roller 44. As it passes through the nip this layer divides layerwise around blade 48, one layer adhering to roller 44 and the other to roller 45. The thickness of the layer adhering to roller 44 can be reduced by slackening screws 53, 54 and 57 allowing the blade 48 to move nearer to roller 44 under the influence of springs 50 and 51 and the point of division of the layer of composition can be adjusted in and out of the nip by adjusting screws 55 and 56. The percentage taper of the gap between the blade 48 and roller 44 can be adjusted by tightening or slacking screw 57 relative to screws 53 and 54.

In the liquid transfer system using rollers or belts the method and device of the present invention enables the speed of liquid transfer to be increased, on occasions by many times and also enables the amount of liquid transferred to be controlled.

By means of the present invention the thickness of the film of liquid transferred can be controlled so as to enable more liquid to be transferred at one point along the rollers or webs than at another. In this case the blade is tilted along its length with respect to the axes of the rollers or is stepped along its length.

It is believed that the device of the present invention inserted in the transferring liquid at or close to the point at which the film of liquid ruptures as some follows the contour of one belt or roller and some follows the contour of the other belt or roller may remove the tendency to cavitation in the liquid at this point and may anchor the meniscus in the liquid more firmly at this point.

This invention is applicable to any liquid or suspension or other composition used in forming coatings or coverings, such as aqueous gelatin compositions, suspensions, e.g., of baryta, paint, printing inks, solutions of polymers or casting substances such as cellulose acetate or polymers.

The present invention has also been applied to coating baryta suspensions in water (BaSO₄) of strength 37, 50 and 60 grams BaSO₄/100 grams suspension at 200 and more feet per minute.

The present invention can be used in the printing industry in transferring printing inks from one roller to another. The present invention is also applicable to coating webs or strips of all kinds using the transfer system. It is, for example, useful in coating aqueous gelatin composition and photographic emulsions on film base or paper. Thus a gelatin composition can be transferred directly to a film base from a roller, using the device of the invention to ensure even coating.

The present invention thus provides a means and method of transferring a coating liquid from one moving surface to another such that the layer of transferred liquid has a smooth even surface and such that the thickness of the transferred layer is controlled.