US3526536A

US3526536A - Process and apparatus for bead coating a web

Info

Publication number: US3526536A
Application number: US663613A
Authority: US
Inventors: Aris C Spengos; Nelson L Fegley
Original assignee: Scott Paper Co
Current assignee: Kimberly Clark Tissue Co
Priority date: 1967-08-28
Filing date: 1967-08-28
Publication date: 1970-09-01
Anticipated expiration: 1987-09-01

Description

Sept 3, 1970 A. c. SPENGOS ET 3,525,536

PROCESS AND APPARATUS FOR BE'AD COATING A WEB Filed Aug. 28, 1967 2 Sheets-Sheet l INVENTORS.

Aris C. Spengos BY Nelson L. Fegley ATT RNEY.

Sept. 1, 1976 A. c. SPENGOS ET A 3,526,535

PROCESS AND APPARATUS FOR BE'AD COATING A WEB Filed Aug. 28. 1967 2 Sheetsv-S'neet ATT RN EY.

INVENTOR Aris C. Spengos Nelson L. Fegley United States Patent Int. Cl. B05c US. Cl. 117-120 18 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus are disclosed for applying a liquid to the surface of a web, for example, applying a light-sensitive coating to a plastic strip or film, by the dip coating and bead coating processes. Improvements are disclosed which reduce or eliminate problems experienced in bead coating in which a web to be coated is held by a guide roll in a position where it forms a bead with a liquid to be applied to the web. A moving surface element such as a rotatably mounted roll is employed generally beneath the bead and spaced from the web to be coated, the moving surface element having a direction of motion generally corresponding to the general direction of motion of the web through the liquid in the bead. This imparts motion to portions of the liquid generally beneath the bead and between the surface of the moving web and the moving surface element in a direction corresponding to the general direction of motion of the web through the liquid in the head. In this manner, the liquid beneath the bead has a more uniform velocity profile. The method and apparatus of the invention reduce or eliminate undesirable streaking of the coating on the web which very often occurs in bead coating. Also disclosed is the incorporation of control means adjacent the web and above the surface of liquid in the reservoir which, in conjunction with the moving surface element, permits coating of a web with a liquid at considerably higher speeds without streaking. The control means is adapted to elevate the meniscus on the trailing side of the head to a point where it is more remotely spaced from the surface of the liquid than is the meniscus on the leading side of the bead. By regulating the height of the meniscus at the trailing side of the bead, the radius of curvature of the meniscus at the trailing side of the bead along the line of contact between the liquid and the web is controlled and, correspondingly, the thickness of the liquid coating on the web is controlled.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to coating and, more particularly, to a method and apparatus for applying a liquid to a moving web. The method and apparatus of the invention are particularly suitable for the coating of plastic films with light-sensitive materials in liquid form in a manner which reduces or eliminates streaks in the coating even when performed at relatively high speeds.

Description of the prior art In coating a web with a liquid, it is customary to run the web about a guide roll and into contact with the liquid either by submerging or dipping the web into the liquid in some cases or by creating a coating bead between the surface of the liquid and the web, while the web is supported upon the guide roll. The latter form of coating is conventionally used for preparing light sensitive film materials in which plastic film strips are coated with lightsensitive liquid resins. In both types of coating, it is usually desirable to avoid applying coating, material to ice the back side of the plastic film, that is, the side of the film contacting the guide roll. One of the problems with dip. coating such materials, where total submergence of successive transverse portions of the film occurs, has been the inability to seal the edges of the moving strip to reliably prevent the application of fluid to the back side thereof.

One of the problems experienced with the use of the above-described processes and apparatus in coating films hasbeen the formation of coating abnormalities and imperfections in the resulting coated film. Typical imperfections" involve areas of differential coating thickness across thej-width of the coated web which appear as hills and valldys in the surface of the freshly coated web. In the case. of photographic films, when the coating on the web is dried, during which volatile solvents in the coating are evaporated, greater amounts of photosensitive material are deposited in the areas beneath the hills on the surface of the coated web. These heavier deposits of photosensitive materials appear as streaks upon subsequent development of the film. Such defects have been more prevalent when the coating has been performed at higher speeds, that is, on the order of 15 feet per minute and higher. Various attempts have been made to eliminate these problems but these have been largely unsuccessful in the past.

' Another problem which has hampered any increase in the speed of coating by the above-described method and apparatus has been runback or curtaining of the coating down the film surface when it emerges from the liquid. It has been postulated that this problem could be overcome by employing some means of doctoring the liquid coating to remove excesses from the surface of the film as it emerged from the liquid. Although some degree of control has been achieved in this manner, the results have been largely unsatisfactory and speed increases have been minimal. Furthermore, the action of a conventional doctor blade or an air doctor has been damaging to the coating uniformity. None of the prior art processes or apparatus has made it possible to closely regulate and controllably vary the thickness of the coating applied to the web without destroying the coating uniformity and forming streaks.

It is an object and advantage of the present invention to provide a method and apparatus for coating which improves both the speed and the uniformity of coating.

It is a further object and advantage of the invention to provide a method and apparatus for controlling the thickness of a liquid coating applied to a moving web and which reduces or eliminates streaking.

SUMMARY OF THE INVENTION The coating apparatus of the invention includes a reservoir containing a liquid to be applied to a web and a guide roll rotatably mounted above the reservoir. The guide roll is adapted to guide and support a web to be coated with the liquid as the web moves through a position where it creates an elongate elevated bead of liquid between the web and the remainder of the liquid in the reservoir. The elevated bead of liquid is sustained by surface tension effects and extends transversely across at least a portion of the web. The apparatus includes a moving surface element submerged within the liquid and disposed generally beneath the bead. The moving surface element is adapted to impart motion to liquid between it and the web in a direction corresponding to the general direction of motion of the web through the liquid in the head. In a preferred embodiment, the moving surface element is a rotatably mounted roll but, in other embodiments, comprises other structures such as a continuous belt mounted for movement along a path beneath the bead. The most preferred embodiment of apparatus of the invention includes control means disposed adjacent the web and above the surface of the liquid, which control means are adapted to elevate the meniscus on the trailing side of the bead to where it is more remotely spaced from the surface of the liquid than is the meniscus on the leading side of the bead. The control means are movable relative to the surface of the liquid in the reservoir and to the guide roll, enabling regulation of the thickness of the coating being applied to the web. In some embodiments the moving surface element is included in an arrangement of the above-described coating apparatus in which the web is moved through a position where itis submerged in the liquid rather than creating a bead with the liquid.

In the method of the invention, an elongate head of liquid is formed between the surface of the moving web to be coated and the surface of the liquid to be applied to the web, which bead extends transversely across at least a portion of the width of the web. Motion is imparted to the liquid disposed generally beneath the bead and spaced from the surface of the moving web in a direction corresponding to the general direction of motion of the web through the liquid. This causes the liquid beneath the bead to flow with a more uniform velocity which is believed to eliminate eddies and which reduces or eliminates streaks in the resulting coating. In another embodiment of the method of the invention, the meniscus on the trailing side of the head is elevated to where it is more remotely spaced from the surface of the liquid than is the meniscus on the leading side of the bead. A more particular embodiment of the method of the invention involves controllably regulating the radius of curvature of the meniscus at the trailing side of the bead along the line of contact between the liquid and the web. This enables regulation and control of the thickness of the liquid coating applied to the web.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional elevation view of coating apparatus of the invention,

FIG. 2 is a schematic sectional elevation view of an alternative embodiment of the apparatus of the invention,

FIG. 3 is a schematic sectional elevation view of a still further embodiment of coating apparatus of the invention, incorporating control means, and

FIG. 4 is a schematic perspective view of the coating apparatus shown in FIG. 3 illustrating one manner in which the ends of the elevated meniscus of the head are blocked.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, a reservoir is shown containing a liquid 11 to be applied to a web 12. Typically, the liquid 12 may be a light-sensitive resin coating of the type used as a coating upon photographic films. A guide roll 13 is mounted for rotation above the reservoir 10 and generally above the liquid 11 contained therein. The rotational axis of the guide roll 13 is generally horizontal with the surface 19 of the liquid 11 contained in the reservoir 10. The web 12, which may be a continuous strip of sheet material, such as metal, paper, or plastic, is fed downwardly into partial wrapping engagement about guide roll 13 and withdrawn in a generally upward direction from the opposite side thereof away from the reservoir 10.

A head 14 is formed with the liquid 11 in the reservoir 10 and comprises an elongate upstanding quantity of liquid 11 held against the surface of web 12, which is supported upon guide roll 13, and elevated above the surface 19 of liquid 11 in reservoir 10 by surface tension affects creating a meniscus 15 on the leading side 16 of bead 14 and a meniscus 17 on the trailing side 18 of bead 14.

A moving surface element which is shown as a rotatably mounted roll 20 in FIG. 1, is disposed generally beneath the bead 14, the roll 20- being driven by drive means (not shown). The direction of rotation of roll 20 is shown by the arrow 21 while the direction of rotation of the guide roll 13 is shown by the arrow 22. The directions of motion of successive transverse portions of their respective surfaces momentarily facing each other correspond so that the direction of motion of the lowermost portion of web 12 carried upon guide roll 13 corresponds generally to the direction of motion of that portion of the surface of the roll 20 momentarily closest to web 12.

It has been found that upon rotation, roll 20 imparts motion to the liquid 11 disposed generally beneath the bead 14 and imparts a more uniform velocity to liquid beneath the bead 14. It has further been found that this reduces or eliminates streaks in the coating applied to the film or web 12 believed to be caused by eddies or other disturbances in the liquid 11 resulting from movement of the surface of the web 12 to be coated through the coating liquid 11 which was previously somewhat stationary. Roll 20 is driven by means (not shown) at a predetermined rotational velocity where its surface speed V bears a predetermined relationship with the speed V of the surface of guide roll 13 which is substantially equal to the speed of the exposed surface of web 12 to be coated. That is, the ratio of V to V is preferably within a predetermined range within which good results are obtained. The diameter D of the guide roll 13 may vary within wide limits as may also the diameter D of the roll 20.

The roll 20 is generally disposed beneath the head 14 and spaced fro-m the lowermost transverse portion of the guide roll 13 by a distance s generally greater than 0.1 inch. In practice, with a roll 20 having a D of about 3 inches, an s of about .5 inch has given the best results. In that instance, a guide roll 13 having a D of about 3 inches was used creating a bead 14 having a w of about .75 inch at a coating speed of 2-0 feet per minute and a w of .6 inch at a coating speed of 15 feet per minute.

FIG. .2 illustrates an alternative embodiment of apparatus of the invention in which the moving surface element is in the form of a continuous belt 23 mounted for movement along a path about rolls 24 beneath the bead 14. The outwardly-facing surface of belt 23 functions in a similar manner as the surface of the rotatably mounted roll 20 functions and generaly increases the velocity of the fluid 11 between the head 14 and the surface of the belt 23, thereby promoting flow uniformity and reducing or eliminating streaks in the resulting coating.

Several factors have been found to alter the effect of the moving surface element upon the uniformity of coating. For example, it has been found desirable to maintain the velocity of motion V of the moving surface element at from about 20% to about of the surface velocity V of the guide roll 13 and of the web being coated. There are several factors which affect the precise nature of these limits to some degree, such as relative size of the moving surface elements, viscosity of the liquid, for example, which will be apparent to those skilled in the art. Similarly, the spacing of the moving surface element relative to the web 12 at its closet position should be at least as great as 0.1 inch and preferably is greater than at least 0.3 inch. At positions closer than 0.1 inch, eddies or other flow distortions in the liquid 11 appear to be created causing considerable nonuniformity in the coating. At spacing greater than 0.3 inch, that is up to 0.5 inch and even 0.75 inch, excellent results have been obtained under commercial coating conditions.

It is desirable to utilize generally smooth surfaces on the moving surface elements to promote uniform flow. Surfaces ranging from a high polish down to a roughness approaching various grades of abrasive paper such as sandpaper have been utilized satisfactorily. The degree of roughness desired depends to some extent upon the viscosity of the liquid being coated as well as the closest spacing between the surface of the web being coated and the moving surface element. The use of moving surface elements having extremely rough surfaces of a coarse nature results in the creation of eddies in the coating liquid and accentuates streaking effects and other nonuniformities in the coating.

The above-described apparatus can be used for performing one embodiment of the method of the invention in which the elongated bead 14 of liquid 11 is formed between the surface of the moving web 12 and the surface of the liquid 11 to be applied to the web. The bead 14 extends transversely across the width of the web 12. The method also involves imparting motion to the liquid 11 disposed generally beneath the bead 14 and spaced from a surface of the moving web 12 in a direction corresponding to the general direction of motion of the web through the liquid in the bead at the point where the web is closest to the moving surface element. This causes the liquid beneath the bead to flow with a more uniform velocity. In other words, motion is imparted to the liquid 11 in a manner which minimizes the creation of eddies and other disturbances in the liquid 11 adjacent the web 12 and within the bead 14 so that a substantially uniform coating is obtained on web 12 free from streaks or other nonuniformities. Other forms of apparatus might also be utilized to perform the method of the invention which include other means for causing portions of the liquid 11 beneath and within the head to move in a direction and at a velocity more closely approaching the direction and velocity of the web being contacted by the bead 14 and coated with liquid 11.

In laboratory tests using a guide roll having a diameter of 3 inches, variations in the radius of curvature of the meniscus at the trailing side of the bead were observed at a series of locations along the bead where the roll surface was withdrawn from the liquid in the bead. The variations in the radius of curvature of the meniscus along the bead resulted in the formation of arcuate ridges which were more noticeable at higher speeds, that is, more noticeable as the velocity of the surface of the guide roll was increased from about feet per minute to about 50 feet per minute and higher. It was observed that the spacing between the arcuate ridges was unequal and the average spacing seemed to decrease with an increase in speed. The arcuate ridges appeared to have a weaving motion so that their locations along the bead continuously changed.

Correlation was established between the formation of arcuate ridges in the meniscus and the presence of streaks in the film coated under those conditions. Apparatus adapted to produce local changes in the radius of curvature on the meniscus on the trailing side of the bead was used during the coating of samples of photographic film with a light-sensitive material. Well-defined coating streaks were found on the film at locations corresponding to the arcuate ridges formed by the above apparatus.

It has been hypothesized that such arcuate ridges are an organized flow instability produced by a particular type of flow pattern beneath the meniscus surface. For such analysis directed to systems similar to meniscus or bead coating see Pearson, I. R. A., The Instability of Uniform Viscous Flow Under Rollers and Spreaders, J. Fluid Mechanics, 7, 1960, pp. 481-500; and Pitts, E., and Greiller, J., The Flow of Thin Liquid Films Between Rollers, I. Fluid Mechanics, 11, 1961, pp. 33-50. The common element in each system is the fluid pressure decreasing toward the underside of the meniscus. A disturbance in the meniscus surface is fed by fluid moving into it in response to the pressure gradient. The liquid has a stagnation point at the meniscus surface and in each case there is flow impinging on the underside of the meniscus which forces fluid into a disturbance in the surface. The growth of the disturbance is opposed by surface tension forces, and if the conditions required for an organized instability exist, arcuate ridges will be formed as described above.

In arriving at the method and apparatus of the invention, relation of the flow pattern under the meniscus to the arcuate ridges and of the arcuate ridges to coating streaks was attempted. Apparatus was devised which was effective in improving coating uniformity by changing the flow pattern under the meniscus so that conditions producing arcuate ridges were eliminated. The tendency for a disturbance in the meniscus to be fed and amplified by the flow under the meniscus was reduced or eliminated. It was found that such action could be achieved by a device which reduced the severity of the velocity gradient in the coating fluid directly under the bead. If the coating speed was to be kept constant, the device had to set a portion of the liquid in the reservoir in motion with a velocity in the general direction of web travel within the bead. Although the exact manner in which the apparatus of the invention alters the flow pattern is not known, it is believed that the flow is altered in one or more of the following ways:

1) The stagnation point is moved down along the meniscus toward the surface of the liquidin the reservoir.

(2) The stagnation point is moved into the bead from the meniscus surface, and flow at all parts of the curved meniscus is in an upward direction.

(3) The conditions bringing about the movement of the stagnation point into the bead, initially or simultaneously, cause a decrease in the stagnation pressure at the stagnation point.

A modified embodiment of the method of the invention includes elevating the meniscus 17 on the trailing side 18 of the bead 14 to where it is more remotely spaced from the suface 19 of the liquid 11 than is the meniscus 15 on the leading side 16 of the bead 14, while motion is imparted to the liquid 11 disposed generally beneath the bead 14 and spaced from the surface of the moving web 12 in a direction and in a manner described above. In employing the above method, it has been found possible to control the thickness of the liquid coating applied to the web 12. This is done by controlling the radius of curvature of the meniscus 17 at the trailing side 18 of the bead 14 along the line of contact between the liquid 11 and the web 12. It is believed that the radius of curvature of the meniscus 17 controls or has an effect upon the surface tension force exerted upon the layer of liquid 11 being applied to the web 12 and, correspondingly, governs the thickness of this coating. The radius of curvature of the meniscus 17 may be changed by varying the height of the meniscus 17 above the surface 25 at the trailing side 18 of the bead 14.

The following equation, taken from a reference by V. G. Levich, entitled Physicochemical Hydrodynamics, published in 19 62 by Prentice-Hall, relates the coating thickness to other variables of a meniscus or bead coating system:

where h =coating thickness 1 =dynamic viscosity a=surface tension v=coating speed =coating fluid density g=gravitational constant alpha=angle of withdrawal of coated surface.

For the geometry of the meniscus bead coating system of the invention, a static force balance across the meniscus produces the following relationship:

where R=the radius of curvature of the menicus where the coating film leaves the bead.

Substitution of the above relationship into the coating thickness equation presented above yields:

2 h=1.32R l3 From the above, it can be seen that the coating thickness therefore, is determined by the meniscus curvature, the coating speed, and the viscosity and surface tension of the solution. From the above, it was also reasoned and subsequently established by test that if the elevation of the menicus 17 at the trailing side 18 of the bead 14 is controllably regulated above the surface 19 of liquid 11 in reservoir 10, the angle of withdrawal of the surface of web 12 being coated from the liquid in bead 14 is controllably regulated. This results in controlled regulation of the radius of curvature of the meniscus and, correspondingly, controlled regulation of the thickness of the coating applied to web 12. In the above instance, the viscosity and surface tension of the liquid 11 were held constant.

FIGS. 3 and 4 illustrate an alternative embodiment of the apparatus of the invention capable of carrying out the method of the invention described above. The apparatus includes control means in the form of a blocking member 25 which is disposed adjacent to the web 12 and extends from beneath the surface of the liquid 11 to a point above the surface of the liquid 11. The blocking member 25 is part of an alternative embodiment of the coating apparatus described with reference to FIG. 1, and like parts have been designated with like reference numerals. The blocking member 25 is adapted to elevate the meniscus 17 on the trailing side 18 of the bead 14 to a point where it is more remotely spaced from the surface 19 of the liquid 11 in the reservoir 10 than is the meniscus 15 on the leading side 16. That is, when the bead 14 is formed, the space between the blocking member 25 and the bead 14 is filled with liquid 11. Surface tension between the liquid 11 and the blocking member 25 as well as between the surface of the web 12 and the liquid 11 holds the meniscus 17 on the trailing side 18 of the bead 14 above the surface 19 of the remainder of the liquid 11 in the reservoir 1'0. It has been found that when this arrangement of apparatus is utilized for the coating of a web 12, much higher speeds can be achieved without the creation of undesirable streaks or other uniformities in the web coating.

FIG. 4 illustrates the manner in which the bead 14 extending above the surface of the liquid 11 in the reservoir 10 is retained at each end of the blocking member 25 and at each side of the Web 12 being coated. End plates 26 are disposed perpendicularly to the blocking member 25 and extend from the blocking member 25 into sealing contact with each edge of the web 12 wrapping guide roll 13 and passing through the liquid 11. The end plates 26 contact the web 12 from a point below the meniscus 17 at the trailing side 18 of the head 14 and extend above the upper surface of the elevated portion of the bead 14 forming meniscus 17. The end plates 26 are preferably constructed of a material which minimizes friction between the web 12 and the end plates 26 and are preferably resiliently mounted to urge them under light force against the web 12 being coated.

By moving the control means upward or downward while maintaining a predetermined spacing of the portion of the control means adjacent the web being coated, it is possible to control the height of the meniscus at the trailing side of the bead and, correspondingly, its radius of curvature which enables control of the thickness of the coating being applied.

What is claimed is:

1. In coating apparatus, including a reservoir containing a liquid to be applied to a web, and a guide roll rotatably mounted above said reservoir, where said guide roll is adapted to guide and support the web to be coated with said liquid as the web moves through a position where it is submerged in said liquid, said web creating a meniscus with said liquid both as it enters and as it emerges therefrom, the improvement comprising:

a moving surface element submerged within said liquid and disposed generally beneath the submerged portion of said web, said moving surface element being adapted to impart motion to liquid between it and said web in a direction corresponding to the general direction of motion of said web through said liquid.

2. The improvement in coating apparatus according to claim 1, including control means disposed adjacent said web and above the surface of said liquid, said control means being adapted to elevate the meniscus created between the liquid and the emerging surface of the web to a point more remotely spaced from the surface of said liquid than the meniscus created between the liquid and the entering surface of the web.

3. In coating apparatus, including a reservoir containing a liquid to be applied to a web, and a guide roll rotatably mounted above said reservoir, where said guide roll is adapted to guide and support the web to be coated with said liquid as the web moves through a position where it creates an elongate elevated bead of liquid with the remainder of said liquid in said reservoir, said elevated bead of liquid extending transversely across at least a portion of said web, the improvement comprising:

a moving surface element submerged within said liquid and disposed generally beneath said head, said moving surface element being adapted to impart motion to liquid between it and said Web in a direction corresponding to the general direction of motion of said web through said liquid in the bead.

4. The improvement in coating apparatus according to claim 3, wherein said moving surface element comprises a rotatably mounted roll.

5. The improvement in coating apparatus according to claim 4, wherein the rotational axis of said rotatably mounted roll is substantially parallel to the rotational axis of said guide roll.

6. The improvement in coating apparatus according to claim 3, wherein said moving surface element comprises a continuous belt mounted for movement along a path beneath said bead.

7. The improvement in coating apparatus according to claim 3, wherein said moving surface element is spaced from said web at its closest position by at least 0.1 inch.

8. The improvement in coating apparatus according to claim 3, wherein said moving surface element is spaced from said web at its closest position by at least 0.3 inch.

9. The improvement in coating apparatus according to claim 3, wherein said bead has a leading side and a trailing side, the leading side and the trailing side of said bead each terminating the said web in a meniscus, and wherein said apparatus includes control means disposed adjacent said web and above the surface of said liquid, said control means being adapted to elevate the meniscus on said trailing side to where it is more remotely spaced from the surface of said liquid than is the meniscus on said leading side.

10. Coating apparatus according to claim 9, wherein said control means include means for retaining liquid at such end of said head in a position above the surface of the remainder of said liquid in said reservoir in a manner preventing flow of liquid from said bead onto the back side of said Web.

11. Coating apparatus comprising a reservoir containing a liquid to be applied to a web,

a guide roll rotatably mounted above said reservoir, said guide roll being adapted to support a Web to be coated with said liquid in a position where said web creates an elongate elevated bead of liquid with the remainder of said liquid in said reservoir, said bead extending transversely across at least a portion of the width of said web and having a leading side and a trailing side, the leading side and the trailing side of said bead each terminating at said web in a meniscus, and

control means disposed adjacent said web and above the surface of said liquid, said control means being adapted to elevate the meniscus on said trailing side to where it is more remotely spaced from the surface of said liquid than is the meniscus on said leading side.

12. A method for coating a web with a liquid comprising:

forming an elongate head of liquid between the surface of a moving web and the surface of a liquid to be applied to said web, said bead extending transversely across at least a portion of the width of said web, and

imparting motion to said liquid disposed generally beneath said bead and spaced from the surface of said moving web in a direction corresponding to the general direction of motion of said web through said liquid, whereby the liquid beneath said bead flows with a more uniform velocity.

13. A method according to claim 12, wherein motion is imparted to said liquid in a manner which minimizes the creation of eddy currents in the liquid adjacent said web and within said bead, whereby a substantially uniform coating is obtained.

14. A method according to claim 12, wherein motion is imparted to said liquid in a manner resulting in more uniform flow velocity along a line extending downwardly from said web through said head.

15. A method according to claim 12, wherein said bead has a leading side and a trailing side, the leading side and the trailing side of said bead each terminating at said web in a meniscus, and including the step of:

elevating the meniscus on said trailing side to where it is more remotely spaced from the surface of said liquid than is the meniscus on said leading side.

16. A method according to claim 12, wherein said head has a leading side and a trailing side, the leading side and the trailing side of said head each terminating at said web in a meniscus, and including the step of:

controlling the radius of curvature of the meniscus at the trailing side of said bead along the line of contact between the liquid and the web so as to control the thickness of the liquid coating applied to said web.

17. A method according to claim 16, wherein said radius of curvature is controlled by changing the height of the meniscus at the trailing side of said bead above the surface of said liquid in said reservoir.

18. A method according to claim 12, wherein said motion is imparted to the liquid by a moving surface element submerged within said liquid and disposed generally beneath said bead, and wherein said method includes moving said surface element at a velocity of from about 20% to about of the surface velocity of said web.

References Cited UNITED STATES PATENTS 1,313,686 8/1919 Hampson 118123 2,168,997 8/1939 Lankes et al. 118407 X 2,855,894 10/1958 White et a1. 118-258 3,335,026 8/1967 De Geest et al.

RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R.