EP0563308B1

EP0563308B1 - Improvements in or relating to coating

Info

Publication number: EP0563308B1
Application number: EP92904009A
Authority: EP
Inventors: Terence Desmond Blake; Kenneth J. Ruschak
Original assignee: Kodak Ltd; Eastman Kodak Co
Current assignee: Kodak Ltd; Eastman Kodak Co
Priority date: 1990-12-20
Filing date: 1991-12-18
Publication date: 1995-05-10
Anticipated expiration: 2011-12-18
Also published as: JP2619190B2; WO1992011571A1; DE69109695T2; DE69109695D1; EP0563308A1; WO1992011572A1; JPH08510679A

Abstract

Described herein is method for coating a support (14, 14'), such as a web of paper or film, with a liquid composition, such as photographic emulsion. The support is moved through a coating zone and a moving sheet (32, 134) of the composition is directed at the support. The moving sheet may be a falling curtain (32) or it may be formed by an extrusion die with the sheet directed other than vertically. The sheet and the support are positioned relative to one another so that the sheet impinges on the support in the coating zone with an acute angle between the sheet and the uncoated support and so that the angle between the plane of the sheet just prior to infringement and a plane tangential to the support at the line of impingement of the sheet on the support is A° which lies in the range of 30° to 60°. It is ensured that the liquid in the sheet has a speed just before impingement on the support of at least 200 cm per sec.

Description

Technical Field

This invention relates to coating supports, such as continuous webs or sheets, with liquid composition.

Background of the Invention

It is known to coat supports with liquid composition by what is known as the curtain coating method.
U.S. Patent No. 3,508,947, issued April 28, 1970 to Hughes, describes apparatus for production of photographic elements by the curtain coating method. The apparatus therein described includes a hopper for forming a free-falling curtain of liquid photographic coating compositions. The curtain includes a plurality of discrete contiguous layers. The liquid flows down an upwardly facing inclined surface of the hopper and off a lip, at the lower end of the surface, to form a curtain. Alternatively, the liquid may be extruded from one or more slit-like orifices in a downwardly facing portion of a hopper, to form a curtain. The liquid in the curtain impinges on a continuous support, in the form of a web, where it is trained about a backing roll, and forms a coating, comprising discrete layers, on the web.
The above-mentioned U.S. Patent no. 3,508,947 mentions that skip and mottle problems, which are caused by entrainment of air between the support and the coating, can be avoided by curtain coating. It states that it is believed that the method of the invention of that patent avoids the skip and mottle problems because of the momentum of the free-falling curtain, which causes penetration or displacement of the air barrier on the support. As stated in the aforementioned patent, the height of the curtain is usually from 5 to 20 cm (about 2 to 8 inches).
The above-mentioned problem of skip and mottle, sometimes also called skottle or dynamic wetting failure or air entrainment, is due to the presence of air which prevents the liquid composition from contacting and wetting the support uniformly across the width of the support. The imperfection is encountered as the speed of the web or sheet is increased. Other parameters, such as viscosity of the composition being coated on the support; total wet thickness (that is, the total thickness of the coating on the support prior to drying); and the chemical composition and the roughness of the surface of the support, have an effect on the coating speed at which the imperfection is encountered. Thus, undesirable constraints can be placed on coating speed, viscosity of the composition being coated onto the support, thickness of the layer or layers or of a layer in a multi-layer coating, and/or properties of the surface of the support.

Summary of the Invention

It is an object of the present invention to alleviate the aforesaid constraints and/or to allow coating speeds to be increased before the constraints are encountered, while maintaining satisfactory quality of product.
In accordance with the present invention, there is provided a method of coating a support with a liquid composition, including:
moving the support through a coating zone;
forming a moving sheet of the liquid composition;
positioning the sheet and support relative to one another so that the sheet impinges on the support in the coating zone with an acute angle A, having a value between 30° and 60°, between the sheet and the uncoated support;
characterized in that the angle A between the plane of the sheet just prior to impingement and a plane tangential to the support at the line of impingement of the sheet is derived from the relationship: $l = L/[W/sin A]$

wherein l is in the range of 0.8 to 2 of the value of l_m given by the relationship: $l_{m} {= 0.25 + K/V}_{a}$

wherein
V_a is the apparent viscosity in Pas;
L is the distance of the wetting line from the plane of the downstream face of the curtain;
W is the thickness of the curtain just before impingement on the web;
and
K is 0.015 for high surface tension and 0.010 for low surface tension;
ensuring that the liquid in the sheet has a speed just before impingement on the support of at least 200 cm per sec.
If the value of V_a is in poise, the values of K are 0.15 and 0.10 respectively.
In certain embodiments, the sheet may be regarded as a curtain because its velocity just prior to impingement on the support is predominantly due to gravity. In such embodiments, the liquid composition may have free fallen from the lip of a slide hopper. In such embodiments, the support is moved downwardly through the coating zone and the plane of the curtain just prior to impingement is substantially vertical. In such embodiments, the plane of the support at the line of impingement of the curtain on the support is inclined at (90 - A)° to the horizontal. If the support is trained about a backing roller in the coating zone, it is the tangent to the support at the line of impingement which is inclined at (90 - A)° to the horizontal.
In other embodiments, the sheet of liquid processing composition is formed by extrusion from a device, for example, an extrusion die, capable of forming a sheet of the liquid composition moving at such a velocity that just prior to impingement on the support it has a speed of at least about 200 cm per sec. In such embodiments as this, the positioning of the sheet and of the support may be independent of the direction of gravity. For example, the sheet may be horizontal or vertical with the liquid composition flowing upwards towards the support.
In those embodiments in which the sheet is in the form of a free falling curtain, the height of the curtain is preferably in excess of 20 cm, this being the height necessary to give a velocity of about 200 cm per sec at impingement.
In those cases in which the support is trained about a backing roll when the curtain impinges on the support and in which the plane of the curtain just prior to impingement is substantially vertical, the line at which the impingement occurs is at (90-A)°, i.e. 30° to 60°, beyond top dead center. The angle (90-A)° is termed the application angle. Top dead center is the line on the support which is in a vertical plane which includes the axis of the backing roll about which the support is trained. In this context, "beyond" means on the side of top dead center to which the support moves after passing through top dead center.
In all of the embodiments described and illustrated in the aforesaid U.S. Patent no. 3,508,947, the curtain impinges on the support at top dead center or impinges on a horizontally moving support. The same is also true of the embodiments described in U.S. Patent no. 3,632,374 issued on January 4, 1972 to Greiller, and of U.S. Patent no. 3,867,374 issued to Greiller on February 25, 1975 (which resulted from an application which was a continuation of an application which was itself a continuation of a divisional out of the application which resulted in Patent no. 3,632,374). However, in the latter two Greiller patents, there is a Figure 11 which is a schematic view which illustrates the text which says that it is not necessary that the plane of the free-falling curtain be oriented to intersect the axis of the supporting roll (i.e. impinge at top dead center) in order to satisfactorily coat a web passing therearound. It is further stated in the aforesaid patents that, if the web to be coated is directed to and from the supporting roll so as to leave sufficient supported area of the web accessible, then the free-falling curtain can be directed on or off axis of the roll. It is further stated that the free-falling curtain should not be so far off axis that the direction of travel of the web at impingement is so far from horizontal as to detrimentally affect the coating operation. No distinction is made between curtain positions forward of top dead center and curtain positions backward of top dead center. Thus, the Greiller patents are merely saying that the coating point does not have to be at top dead center. They do not teach that there is any advantage in having a coating point (the location where the curtain impinges on the support) other than top dead center. Specifically, they do not teach the importance of optimizing the position of the wetting line in maximizing speed and maintaining the uniformity of the coating. The curtain must usually be positioned significantly beyond top dead center to optimize wetting line position. Neither do they teach that increasing curtain height to increase coating speed is most effective when the position of the wetting line is optimized. The present inventors have learned that it is advantageous to combine tall curtains with application angles substantially forward of top dead center. These advantages would not be expected by those skilled in the prior art. It is stated again in the aforesaid Greiller patents that the preferred height of the curtain is from about 5 to 20 centimeters.
Another problem encountered with the prior art is often referred to as puddling. Where the curtain strikes the support, the liquid composition has a tendency to move against the direction of travel of the support, until it is entrained by viscous shearing generated by the support. When impingement speed is high, the flowrate is high and/or viscosity is low, a "heel" develops at the foot of the curtain. If sufficiently large, the heel may contain an an eddy which can trap air bubbles or debris. Such trapped bubbles or debris can produce streaks or lines in the coating. Also, the heel may oscillate and such oscillations produce non-uniformities in the coating. Furthermore, in those embodiments in which the liquid composition includes a plurality of layers, the heel may promote mixing or a degree of mutual displacement of the materials in the different layers. When, according to the present invention, application angles of 30° or more are employed to control wetting line position, puddling is prevented. It can be appreciated that at such increased application angles, the direction of motion of the curtain at the time of impingement on the support, is closer to the direction of movement of the support, and hence there is less tendency for the liquid composition to move upstream on the support.
The specification of Published European Patent Application 0 197 493 mentions problems known as "pencil line", "streak", "interruption by splice", "thicker edges" and "disturbances by air current", and proposes the use of a short curtain for overcoming such problems. Thus, that Patent Application is not concerned with the same problem as is the present invention. A short curtain is defined as 0.5 to 50 millimeters. The short curtain is to impinge on the web at an angle of 30° to 90° past top dead center. It has been found that following the teachings of that earlier published patent application is detrimental to the problem of air entrainment.
Thus, none of U.S. Patents nos. 3,508,947, 3,632,374 or 3,867,901 and European Patent Application 0 197 493 teaches a method of coating in accordance with the present invention wherein the moving sheet of the liquid composition to be coated impinges on the support with an acute angle A° between the sheet and the uncoated support, the acute angle A° being between the plane of the tangent to the support at the line of impingement and the plane to the sheet just prior to impingement of the sheet on the support and being in the range of 30° to 60° and the height of the curtain is greater than 20 cm or, expressed another way, the liquid has at the time of impingement, a velocity in excess of that gained in freefall from a height in excess of 20 cm, which velocity is about 200 cm/sec.
The prior art has taught that the height of the curtain should be so selected that the free-falling curtain has adequate momentum at impingement to effectively penetrate or displace the air barrier and wet the moving support However, it has been found that increasing the curtain height may, in actual practice, result in little gain in coating speed before the onset of the above-mentioned problems associated with air being entrained between the support and the coating, unless the support is inclined downwardly as is achieved if the curtain impinges on the support on a line in the range of 30° to 60° beyond top dead center. It has been found that in comparison to the short curtains and horizontal supports of the prior art, coating speeds may be increased by 50% or more by adopting the present invention, namely by having a curtain height of at least 20 cm and a coating point, for example, in the range of 30° to 60° beyond top dead center. Of course, when the coating point is at 30° beyond top dead center, the support is inclined at 30° to the horizontal. Likewise the support is inclined at 60° when the coating point is at 60° beyond top dead center. The substantial speed gains before the onset of the problems of air entrainment, achievable by practice of the present invention, were not to be expected.
While the prior art taught that the magnitude of the momentum of the liquid in the curtain at the time of impingement on the support is an important factor in coating without entrainment of air between the support and the coating, it has been found that high momentum is effective at avoiding the problem only when the wetting line is substantially in the plane of the curtain. The wetting line is where the liquid from the curtain first contacts the support. In such a case, momentum of the liquid is used to its best effect in excluding the air.
When the curtain impinges on the support, the liquid has a tendency to move both with and against the direction of movement of the support. The motion of the support is transmitted to the liquid through viscous shearing. More specifically, a viscous boundary layer begins at the wetting line and extends to the downstream surface of the curtain.

Brief Description of the Drawings

Reference will be now made to the accompanying drawings, in which:

Fig. 1 is a perspective view of a coater;
Fig. 2 is a diagrammatic representation of various features and angles in the coating zone;
Fig. 3 is a section at the coating point, on a greatly increased scale;
Figs. 4 to 11 and 13 are plots of various experimental results and of derivations therefrom;
Figs. 12 and 14 are Tables of values of various rheologies of several materials, the results of experiments which are plotted in Figs. 11 and 13; and
Fig. 15 is a diagrammatic representation of another embodiment of the present invention.

Description of the Preferred Embodiments

Reference is made to Figs. 1, 2 and 3 of the accompanying drawings, wherein there is shown part of apparatus 10 suitable for making photographic film or paper. The apparatus includes a coater including a backing roll 12 about which is trained support to be coated in the form of a web 14. Typically, the web 14 is continuous. The web may be formed of, for example, cellulose acetate, in known manner. The roll 12 has an axis 16 of rotation which is very accurately at the geometric center of its very accurate circular cylindrical surface 18. Preferably the axis 16 of rotation is horizontal. A hopper 20, of known form, has a slide surface 22 to which extend a plurality of slots, only the uppermost one of which is visible and is designated 24. As is known, liquid compositions are supplied to cavities within the hopper which communicate with the respective slots. The various liquid compositions are supplied through conduits 26 in which are located pulsation dampeners 28. The slide surface 22 is inclined so that liquids issuing from the slots flow down the slide surface forming a composite layer formed of a plurality of, in the illustrated example three, discrete layers. At its lower end, the slide surface 22 has a lip 30 from which the composite layer falls cleanly into a sheet, in the form of a curtain 32, in known manner. The curtain is guided by known vertical edge guides 34. For an understanding of the nature and role of edge guides, reference is directed to U.S. Patent Specification No. 4,830,887 issued May 16, 1989 to T.C. Reiter. The lip 30 of the hopper 20 is parallel to the axis 16 of rotation of the backing roll 12. The lip is so located that the curtain 32 falling vertically will impinge on the web 14 along a line 35 which is in a plane 36 containing the axis 16, at an angle (90-A)° in the range of 30° to 60°, in accordance with the present invention (see Fig. 2). The plane 38 includes top dead center 40. The roll 12 is rotating clockwise as seen in Figs. 1 and 2 so that the web 14 is moving upwards towards the left hand side of the roll 12 and is moving downwards through the coating point and away from the right hand side of the roll, having been coated with liquids from the curtain 32. Thus, it can be said that the coating point, i.e. the line 35, is at (90 - A)° beyond top dead center and (90 - A)° is termed the application angle. Of course, the tangent to the web is inclined at (90 - A)° to the horizontal at the coating point.
Each of the pulsation dampeners 28 comprises a chamber closed in part by a diaphragm on the other side of which is a second chamber filled with gas. The second chamber is in communication with a third chamber through a passage which presents a resistance to gas flow. The first chamber is open to the conduit 26 so that hydraulic pressure pulsations which may occur in the delivery of the coating composition are applied to the diaphragm and are absorbed by the diaphragm and the gas in the second chamber. The resistance, which is selectable, in the passage from the second to the third chamber selectively dampens the system.
Also illustrated in Fig. 1 is a baffle device 41 for applying slight suction over the side of the support which is to be coated, just before it reaches the coating point. The application of suction at this location, serves, in known fashion, to reduce the amount of air which is carried along with the fast moving support. The removal of such air assists in maintaining the curtain in its desired plane without air-motion-induced excursion out of that plane.
Reference is now made also to Fig. 3 of the accompanying drawings. In Fig. 3 there is illustrated a line 43 which is parallel to the plane of the support 14 at the line of impingement of the curtain 32 on the support. Also illustrated is a line 45 which is parallel to the plane of the curtain just prior to impingement on the support. The angle between the lines 43 and 45 is designated A. It will be observed that it is between the curtain and the uncoated support.
With continued reference to Fig. 3, the curtain 32 has an upstream surface 42 and a downstream surface 44. The line at which the liquids from the curtain actually wet the web 14, termed the wetting line, is indicated at 46 in Fig. 3. It will be seen that in the condition illustrated in Fig. 3 the wetting line 46 is just downstream of the plane of the upstream surface 42 of the curtain.
A possible explanation for why the advantages of the invention are obtained, leading to equations which are useful in the practice of the invention, will now be given. It is to be understood that the advantages of the invention are in no way dependent upon the correctness of the explanation.
Liquid in the boundary layer 47 is entrained by the web through the action of viscosity and thereby comes to move in the same direction as the web. At the downstream end of the boundary layer, which is in the plane of the downstream surface 44 of the curtain 32, all of the liquid being supplied in the curtain has been entrained by the moving support. The boundary layer is bounded approximately by a broken line 48 and, of course, by the surface of the web 14. The plane of the downstream surface 44 of the curtain 32 is shown in Fig. 3 by a broken line 50. Downstream of the downstream end of the boundary layer, the velocity profile of the boundary layer gradually relaxes to the uniform velocity of the web 14.
The length of the boundary layer, that is the distance measured along the web, which is required for the liquid to be entrained, determines the position of the wetting line relative to the position of the downstream surface of the curtain. When the boundary layer is relatively long, the wetting line is remote from the curtain and the momentum of liquid in the curtain cannot be effective at promoting dynamic wetting and excluding air. Similarly, when the boundary layer length is small, the wetting line is again not positioned to benefit to the maximum from the momentum in the curtain. The momentum in the curtain has the most effect on avoiding air entrainment when the wetting line 46 is located approximately in the plane of the upstream surface 42 of the curtain 32.
For the purposes of understanding the present invention, the term relative wetting line position quotient will now be defined and used. The relative wetting line position quotient is the ratio of the distance L between the wetting line 46 and the intersection of the plane of the downstream surface 44 of the curtain 32 with the surface of the web 14, to the distance between the planes of the upstream and downstream surfaces of the curtain just above the region where the thickness of the curtain is affected by the impingement, measured in a plane parallel to the surface of the web. In mathematical terms the relative wetting line position quotient "l" is defined as: $1 = L/[W/sin A]$

wherein L is the distance of the wetting line 46 from the plane of the downstream surface 44 of the curtain 32 measured along the surface of the support 14, and L is the length of the boundary layer, W is the thickness of the curtain 32 just above the point where the thickness of the curtain 32 is affected by impingement of the liquid in the curtain 32 on the support 14; and A is the complement of the angular displacement of the coating point 35 beyond top dead center 40.
The boundary layer length L can be measured by direct observation in some cases, but, more practically, can be estimated using boundary layer theory. For an understanding of boundary layer theory, reference is directed to Boundary-Layer Theory (seventh edition), H. Schlichting, McGraw-Hill, New York 1979, and to Boundary-Layer Behaviour on Continuous Solid Surfaces, B. C. Sakiadis, AIChE Journal, 1961, volume 7, page 26. Approximately: $L = \frac{3s [3s + 2 cos A] x RxD}{20 [s + 2 cos A]²}$

wherein: s = S/U wherein S is the speed of the web 14 and U is the speed of the curtain 32 just prior to impingement on the support; D is the total thickness of the coating just downstream of the curtain and before any drying has occurred (sometimes termed the total wet thickness); and R is the Reynolds number of the liquid in the curtain, and is defined as: $R = dq/V$

wherein d is the density of the liquid; q is the total volumetric flow rate per unit of curtain width; and V is Newtonian viscosity.
For a vertically free-falling curtain, curtain speed can be taken as: $U = √ [2 GH]$

wherein G is the acceleration due to gravity and H is curtain height, that is, the height of the lip 30 of the hopper 20 above the web 14, measured in the curtain 32. The contribution to U due to the speed of the liquid as it left the hopper lip and entered the curtain can be neglected for curtains of greater than about 5 cm height, that is, for all curtains for which the present invention is concerned. However, when the initial speed is substantial, then H is the effective height of the curtain, that is, the height of that curtain which, free-falling with zero initial vertical velocity, would create the same curtain velocity just prior to impact.
The formula given above for determining the value of L, the boundary layer length, is appropriate for a Newtonian liquid. However, coating liquids in the photographic industry contain polymers and, therefore, are usually pseudoplastic, that is, shear thinning. An apparent Newtonian viscosity V_a can be estimated from rheological data at the representative rate of shear in the boundary layer of $[S-UcosA]/D$
The value of this representative shear rate in curtain coating can be 100,000 sec⁻¹, or more. More accurate means of calculation of the value of L can, of course, be used, but the above formulae are simple and useful in the practice of the invention. Also, they are readily generalized for the case where the different coating compositions in the different layers forming the curtain have significantly different viscosities. As is known, it is generally preferable for curtain coating uniformity that the viscosities of the different layers be substantially the same and that they be relatively high.
The speed at which air entrainment problems cease, as coating speed is reduced after onset of such problems as speed was increased, is generally less than that speed at which the problems started. The term highest practical coating speed will be used herein for that speed which is just below the speed at which air entrainment problems cease as coating speed is decreased.
For a given curtain height and application angle (i.e. complement of the angle A), the highest practical coating speed depends on the total flowrate. In particular, there is a flowrate at which the highest practical coating speed is maximized, and this maximum highest practical coating speed will be referred to herein as S_m. Thus, at a given curtain height and angle A, there is one wet coating thickness which gives the maximum highest practical coating speed. If the wet coating thickness desired is greater or smaller, then the highest practical coating speed for coating that thickness may be substantially less than S_m. With the value taught for application angle and with the curtain height taught in the above-mentioned U.S. Patent no. 3,508,947, the wet coating thickness corresponding to S_m is in the neighborhood of 30 »m. Because wet thicknesses substantially greater than 30 »m are often desired, highest practical coating speeds are often much less than S_m. It has been found that as the application angle is increased (in other words, as the angle A is decreased), S_m moves to higher flowrates, and, as a result, the highest practical coating speed is increased for such thicker coatings.
It has been found that the value of the relative wetting line position quotient corresponding to the maximum highest practical coating speed, which will be denoted l_m, is independent of angle A and curtain height. Thus, this quotient is useful in selecting the angle A at which a desired wet thickness will correspond to the maximum highest practical coating speed. It has been found that l_m is in the neighborhood of unity. More specifically, it has been found to depend, for pseudoplastic coating liquid compositions, on the apparent viscosity according to the relationships:

wherein V_a is the apparent viscosity in poise. By high surface tension is meant values of surface tension in the curtain just above the point of impingement in the range of approximately 60 to 70 dynes/cm, which includes aqueous solutions without added surfactant, such as aqueous solutions of gelatin. By low surface tension is meant values of the surface tension in the curtain just above the point of impingement in the range of approximatedly 24 to 40 dynes/cm, which includes aqueous solution to which a surfactant has been added, such as aqueous solutions of gelatin containing surfactants as commonly practiced in the coating of photographic products. While it is of greatest advantage to operate at the value of l_m given by this expression, it has been found that highest practical coating speed will be at least 70% of S_m when l_m is in the range of 0.8 to 2.0 of the optimum value.
While it has been found that changing the angle A in order to operate at or near l_m, and hence S_m, is beneficial, it has also been found that S_m decreases as the application angle (90 - A)° is increased. More specifically, S_m has been found to depend on the quantity curtain speed multiplied by the cosine of the application angle (90 - A)°, i.e. U x sin A. Away from the origin, S_m depends on (U x sin A) raised to a power which is approximately 0.8. Thus, although S_m decreases as the application angle (90 - A)° is increased, curtain height can be increased to increase impingement speed U and partially or fully offset the effect due to increase in application angle (90 - A)°, i.e. decrease in angle A. It is for this reason that high curtains are preferred in the practice of coating at application angles significantly greater than 0° (i.e. values of A significantly less than 90°).
As is apparent from equations (1), (2) and (3) above, the relative wetting line position quotient is sensitive to the viscosity of the liquids being coated onto the web and to the angle of the web at the coating point. For wet coating thicknesses greater than 30 »m, as is the case in many photographic products, application angles substantially larger than zero degrees have been found to be advantageous. It has been found that the thicker the coating and the lower the viscosity of the coating liquids, the greater is the optimum application angle for highest practical coating speed.
An advantage of calculating the relative wetting line position quotient is that, for any coating thickness and curtain speed, it leads to an application angle which is close to the optimum. The method above of estimating the quotient in order to derive an application angle applies when the rheology of the coating liquids is Newtonian or pseudoplastic. For some coating compositions, it may be necessary to measure and include also other rheological effects. Some coating supports have substantial surface roughness and cannot be considered hydrodynamically flat.
Of course, the optimum application angle can be determined purely experimentally by increasing the angle incrementally until highest practical coating speed reaches a maximum.
Some examples of actual experiments will now be given.

Experiment 1.

An aqueous solution of gelatin, 15% gelatin by weight, and having a viscosity at low rates of shear of 0.063 Pas [63 centipoise], was coated at a wide range of flowrates onto polyethylene terephthalate support web having a gelatin subbing layer. At each flowrate the speed of the web was increased until air was entrained between the coating and the support. After that condition was reached, the web speed was gradually reduced until the air entrainment ceased, i.e. until the highest practical coating speed was achieved. The highest practical coating speed, as defined above, is considered, according to good practice, to be the practical speed limit and is the speed recorded and plotted.
The experiment was carried out at an application angle of 0° (i.e. an angle A of 90°) for curtain heights (c.h.) of 2, 6, 10 and 25 centimeters and at an application angle of 45° (i.e. an angle A of 45°) for curtain heights of 10 and 25 centimeters.
The data for the 0° application angle experiments are shown in Fig. 4, wherein coating speed in cm/sec is plotted against flowrate of the coating liquid in cc/sec per centimeter of coated width. The curves for the 10 cm and 25 cm heights have been truncated at approximately 5 cu cm per sec per cm of width above which the highest practical coating speed rapidly falls. Also included in the graph are straight lines passing through the origin which represent wet coating thicknesses of 25, 50, 100 and 150 microns, respectively.
It will be observed from the four plots that, within the bounds of the experiment, the higher the curtain the higher is the highest practical coating speed. It will also be observed that, within the bounds of the experiment, the highest speeds are achieved with thin coatings, and this represents a major limitation on manufacturing rates with thicker coatings in accordance with the prior art. Further, it will be observed that, at a wet coating thickness of 50 »m, coating speed is limited to about 500 cm/sec, and there is little benefit to be gained from increasing the curtain height from 6 to 25 cm. At 100 »m wet coating thickness, speed is limited to only about 375 cm/sec at the 25 cm curtain height.
Fig. 5 is a plot of the experimental results for the 10 and 25 cm curtain heights at an application angle of 45° (A = 45°). It is readily apparent that with such an application angle and a high curtain, higher wet coating thicknesses can be coated at substantially increased speed. It will be observed that with a curtain height of 25 cm., a coating of 50 »m thickness can be made at 700 cm/sec. Also, a coating of 100 microns thickness can be made at 625 cm/sec. These two speeds represent increases of 40% and 67%, respectively, over the speeds attainable, with the same coating thicknesses, at an application angle of 0° (i.e. an angle A of 90°).
The data may also be used in the calculation of the relative wetting line position quotient. Reference is now made also to Fig. 6 which shows plots of relative wetting line position as a function of the ratio of actual highest practical coating speed S to the maximum highest practical coating speed S_m. Each of the six curves in Figs. 3 and 4 has a maximum highest practical coating speed, S_m. For each curve, the highest practical coating speed for each flow rate is normalized by dividing the speed for each data point on the curve by the value of S_m for that curve. The relative wetting line position quotient is calculated for each data point. The viscosity of the gelatin solution is measured as a function of shear, on a rheometer. The data were fit to the Carreau model of a pseudoplastic liquid with parameters: power law index 0.85 and relaxation time 0.00027 seconds. An understanding of a Carreau model of a pseudoplastic liquid may be gained from Dynamics of Polymeric Liquids, Volume 1, Fluid Mechanics, Second Edition, by R. B. Bird, R. C. Armstrong and O. Hassager, published by Wiley-Interscience, New York, 1987, pages 171-172. The six curves thus produced are plotted in Fig. 6.
The six plots define a master curve, within scatter which is reasonable for this type of measurement. The relative wetting line position quotients corresponding to the greatest speed for each curve are plotted in Fig. 7 against the maximum highest practical coating speed for that curve. It is apparent that the relative wetting line position quotient corresponding to the maximum highest practical coating speed is about 0.63 in each case. The maximum highest practical coating speed varies with the component (U sin A) of curtain impingement speed U, perpendicular to the web, as shown in Fig. 8. The formula for the fitted curve line through the data points is $S_{m} {= 20 (U x sin A)}^{0.7}$

the units of speed being cm per sec. Although an exponent of 0.7 is obtained from these few data points, a much larger set of data for gelatin coated at different concentrations, curtain heights and application angles yields an exponent of approximately 0.8.
Using the expressions for S_m (7) and the optimum relative wetting line position quotient of 0.63, the optimum application angle (90-A)° can be estimated for particular coating conditions. For example, in Fig. 9, the optimum coating thickness and corresponding maximum highest practical coating speed are both plotted against the application angle for the curtain height of 25 cm. It will be observed that above about 30 »m wet coverage thickness the optimum application angle is substantially different from 0° and that the optimum application angle increases with coating thickness. For 150 »m thickness, which is within the range of practical interest in the photographic industry, the optimum application angle is about 60° (i.e. the optimum value of angle A is 30°). Although the highest practical coating speed falls as the application angle is increased at a fixed curtain height, this can be offset, at least partially, by increasing the curtain height. Thus, for many practical coating thicknesses, high curtains and steep web inclinations are preferred, contrary to the expectation of those skilled in the art.

Experiment 2

An aqueous solution of gelatin having a viscosity of 0.020 Pas [20 centipoise], was curtain coated at a range of speeds and flowrates as in Experiment 1. A suitable surfactant was added to reduce the surface tension to an estimated 31 mN/m. Surfactants are often present in photographic coating compositions and are known to be useful in the practice of curtain coating.
Fig. 10 is a plot of flowrate per centimeter of curtain width against highest practical coating speed. Curtain heights were 12 and 25 cm at application angles (90 - A)° of 0 and 45°. The advantages of the present invention are apparent for coating thicknesses exceeding about 25 »m. At the thickness of 50 »m, for example, at a curtain height of 12 centimeters and an application angle of 0° (i.e. A = 90°), as taught in the prior art, highest practical speed is limited to about 440 cm/sec. Increasing curtain height to 25 centimeters, or increasing application angle to 45° at the 12 centimeter height, gives only slightly higher speeds. However, the combination of a 45° application angle (i.e. A = 45°) and a 25 centimeter height gives a highest practical speed of about 690 cm/sec., an increase of 57%.

Experiment 3

Fig. 11 is a plot of 1_m, the best relative wetting line position quotient attainable, against 1/V_a, the reciprocal of the apparent Newtonian viscosity, for aqueous solutions of polymers to which no surfactants have been added (high-surface-tension case). The values are shown for fourteen different pseudoplastic materials, whose characteristics and coating parameters are given in the Table which is Fig. 12. Fig. 13 is a similar plot for aqueous gelatin solutions to which surfactant has been added to lower the surface tension in the curtain (low-surface-tension case). Material properties and coating parameters are listed in the Table which is Fig. 14. In each case the base is polyethylene terephthalate, gelatin subbed unless otherwise indicated. The dependency of 1_m on 1/V_a is evidently linear with a slope which changes with the level of surface tension; otherwise, the relationship appears to be independent of material properties other than the rheological parameters which determine apparent viscosity. Fitted straight lines through the two data sets are given by relationship (6) above. The optimum relative wetting line position has been found to depend on the apparent viscosity in the boundary layer.

Experiment 4

Two gelatin layers were curtain coated simultaneously at a speed of 200 cm/sec and with a curtain height of 25 centimeters. The top layer had a wet thickness of 60 »m and a viscosity of 0.035 Pas [35 centipoise], while the bottom layer had a wet thickness of 40 »m and a viscosity of 0.003 Pas [3 centipoise]. Although viscosities considerably higher than 0.003 Pas [3 centipoise] are preferable in practice, this is not always possible in light of, for example, solubility constraints on components, or the rate at which a crosslinking agent added to the composition reacts with gelatin. A suitable surfactant was added to the layers to reduce their surface tensions to an estimated 31 m/Nm. Curtain height was 25 centimeters.
At an application angle of 0°, coating uniformity was not acceptable because of puddling. At an application angle of 45°, acceptable coating quality was obtained, and furthermore coating speed was increased to 650 cm/sec while maintaining coating quality.

Experiment 5

Three layers were curtain coated simultaneously at 900 cm/sec using a curtain height of 25 centimeters and an application angle of 45° (i.e. A = 45°). The top and middle layers comprised aqueous gelatin solutions having viscosities of 0.063 Pas [63 centipoise] and 0.067 Pas [67 centipoise], respectively, and a combined total wet thickness of 100 »m. The bottom layer was demineralized water at 42.5°C with viscosity 0.00062 Pas [0.62 centipoise] and wet thickness 3.5 »m. Such a water layer may be used, for example, to obtain increased coating speed without air entrainment, or to deliver a hardening agent or other chemical which reacts with gelatin. The top and bottom layers contained suitable surfactants to promote spreading on the middle layer, and the resulting surface tensions were 24.4 mN/m for the top layer, 46.3 mN/m for the middle layer, and 19.3 mN/m for the bottom layer. Because a relatively thin and low viscosity layer is difficult to deliver as a bottom layer on a slide surface without waves and other instabilities developing in the layers, the arrangement of the hopper relative to direction of web movement was such that the bottom layer (in the sense of the layer which contacts the web) was the top layer on the slide surface, an arrangement which gives more latitude for slide instabilities. Although it is preferable that the layers have similar high viscosities to promote uniform flow on the slide and overall coating quality, this is not always consistent with other objectives.
At the speed of 900 cmlsec, the water layer flow was reduced to 0.8 microns, and air entrainment occurred. When the water flow was restored to its original value, air entrainment cleared, indicating that this is a practical speed under the stated conditions.
While the invention has been described in embodiments in which a curtain is created by liquid falling from the lip of a slide hopper, it is to be understood that the curtain may be created in other ways. For example, in some embodiments of the invention, an extrusion hopper with its orifice facing downwardly may extrude a curtain of liquid. In such embodiments, the curtain may have a starting velocity substantially other than zero at the top of the curtain, but does not have to. It is for such a reason that reference is made herein to the curtain having a velocity equal to that attained by a curtain free-falling from a specified height and in such an expression the curtain is assumed to have a starting velocity of substantially zero. Thus, in the case of the liquid being extruded with a substantial velocity from an extrusion hopper, the curtain height need be less than that of a curtain created by liquid falling from the lip of a slide hopper, in order for the velocity at impact to be the same in the two cases.
The angle A has been considered above, it being described as the angle included between the plane of the sheet of liquid composition just prior to impingement on the support and the tangent to the support at the line of impingement, the angle being measured at the side of the sheet facing the uncoated support. It has also been pointed out, with reference to Figure 2, that (90 - A)° is the angle of inclination of the plane 36 to the plane 38 and has been termed the application angle in those embodiments which include a curtain and a backing roll.
In the embodiments particularly described above, the liquid composition is given most of its speed just prior to impingement on the support, by gravity. This is because the liquid composition, when it falls off the lip of the slide hopper, has only a small speed. The present invention may be embodied in systems in which the speed of the liquid composition just prior to impingement on the support is entirely or very largely due to the velocity it is given in exiting an extrusion die. Thus, in some embodiments of the present invention the liquid composition may be in the form of a sheet which is directed horizontally or even vertically upwards or at other inclinations to the vertical. The liquid composition in such embodiments, when moving through space towards the support, should be termed a sheet rather than a curtain. However, the term sheet may be regarded as including the more specific term curtain. In such embodiments wherein the speed of the sheet just prior to impingement on the support is due primarily to apparatus rather than to gravity, the distance between the apparatus and the support may be quite short or, indeed, long, because the distance is not determinative of the speed. However, gravity does affect velocity during the flight of the liquid composition between the apparatus projecting it and the support, and such effect on both the direction and speed aspects of the velocity should be taken into account.
Fig. 15 diagrammatically represents a backing roll 12′ having an axis of rotation 16′. A support, in the form of a web 14′ is trained about the backing roll 12′ which is rotating counterclockwise as seen in Fig. 15 and as is indicated by the arrow. An extrusion hopper 130 has a slot 132 from which is directed a sheet 134 of liquid composition at a velocity in excess of 200 cm per sec. Typically, the extrusion slot 132 is parallel to the axis of rotation 16′ of the backing roll 12′. The distance between the mouth of the extrusion hopper slot and the support may be quite small, of the order of 1 cm or less. The plane of the sheet 134 just prior to impingement of the liquid composition on the support is indicated by the line 45′. A plane tangential to the support at the line of impingement of the liquid composition on the support is indicated by the line 43′ in Fig. 15. The above-discussed angle A is, again, the angle between the planes 43′ and 45′, just as it is the angle between the planes 43 and 45 in Fig. 3. It will be recognized that in the immediately preceding description relating to Fig. 15, there is no implicit reference to the direction of gravity.
It is to be understood that the term "liquid composition" as used herein is to be understood as including a plurality of compositions contained in a plurality of layers. The viscosities of such multiple layers may be the same or different.
While the invention has been described in embodiments for the coating of photographic compositions onto continuous webs, it is to be understood that the invention is also advantageous in other industries.
As has become apparent in the foregoing description, liquid coated in accordance with the present invention may be Newtonian or non-Newtonian, with non-Newtonian including, but not limited to, pseudoplastic liquids.

Claims

A method of coating a support with a liquid composition, including:
moving the support through a coating zone;
forming a moving sheet of the liquid composition;
positioning the sheet and support relative to one another so that the sheet impinges on the support in the coating zone with an acute angle A, having a value between 30° and 60°, between the sheet and the uncoated support;
characterized in that the angle A between the plane of the sheet just prior to impingement and a plane tangential to the support at the line of impingement of the sheet is derived from the relationship: $l = L/[W/sin A]$
wherein l is in the range of 0.8 to 2 of the value of l_m given by the relationship: $l_{m} {= 0.25 + K/V}_{a}$
wherein
V_a is the apparent viscosity in Pas;
L is the distance of the wetting line from the plane of the downstream face of the curtain;
W is the thickness of the curtain just before impingement on the web;
and
K is 0.015 for high surface tension and 0.010 for low surface tension;
ensuring that the liquid in the sheet has a speed just before impingement on the support of at least 200 cm per sec.
A method as claimed in claim 1, wherein:
the support is moved downwardly through the coating zone; and
the step of forming a moving sheet of the liquid composition is performed by forming the liquid composition into a free-falling curtain;
whereby the angle between plane of the sheet just prior to impingement and the plane tangential to the support at the line of impingement is said acute angle A.
A method as claimed in claim 2, wherein said support is a continuous web.
A method as claimed in claim 3, wherein:
the web is trained about a backing roll mounted for rotation about a horizontal axis;
the curtain is disposed so that its plane is parallel to the axis of rotation of the support roll; and
the curtain is oriented relative to the backing roll so that it impinges on a web on the backing roll along a line disposed in a plane containing the axis of the backing roll which plane is at an angle of (90-A)° to the vertical plane containing the axis of rotation, the line being located at the side of the vertical plane to which the web moves through the vertical plane.
A method as claimed in claim 4, including:
forming the curtain by passing the liquid composition through a coating hopper so that the liquid flows downwards over the inclined slide surface of the hopper and then falls off the lip of the hopper.
A method as claimed in claim 5, including:
providing a coating hopper having a plurality of slots; and
passing liquid composition through each of said plurality of slots whereby the curtain is formed of multiple layers.
A method as claimed in any one of the preceding claims, wherein the liquid composition is appropriate for forming a layer or layers in photographic film or paper.
A method as claimed in claim 4, including:
providing an extrusion hopper and so positioning it that its extrusion slot is parallel to the axis of rotation of the backing roll; and
passing the liquid through the extrusion hopper so that it is extruded from the extrusion slot of the extrusion hopper with a speed such that the speed of the sheet is greater than 200 cm per sec just prior to impingement on the support.
A method as claimed in claim 8, wherein the plane of the sheet just prior to impingement on the web, is vertical.
A method as claimed in claim 1, wherein the dimension L is derived from observation.
A method as claimed in claim 10 wherein, for a pseudoplastic or Newtonian liquid, the dimension L is derived from the relationship wherein: $L = \frac{3s [3s + 2 cos A] x RxD}{20 [s + 2 cos A]²}$
s = S/U wherein S is the speed of the web;
U is the speed of the curtain just prior to impingement;
D is the total wet thickness of the coating just downstream of the curtain;
R is the Reynolds number of the liquid in the curtain and is defined as R = dq/V
wherein:
d is the density of the liquid;
q is the total volumetric flow rate per unit of curtain width; and
V is the Newtonian viscosity or apparent viscosity V_a in the case of a pseudoplastic liquid at a rate of shearing given by [S-U cosA]/D.
A method as claimed in claim 1 wherein, the support is moved downwardly through the coating zone and the plane of the sheet is vertical.
A method of coating as claimed in claim 1 wherein the liquid composition is appropriate for forming one or more layers in photographic film or paper.