DE69109695T2 - IMPROVEMENTS IN OR RELATED TO THE COATING. - Google Patents

Description

This invention relates to the coating of supports, such as continuous webs, with liquid mass.

Carriers are coated with liquid mass using a process known as curtain coating.

US-A-3,508,947 describes an apparatus for producing photographic elements by the curtain coating process. The apparatus described therein comprises a hopper for forming a free-falling curtain of liquid photographic coating compositions. The curtain comprises a plurality of discrete, adjacent layers. The liquid flows downward over an upwardly facing inclined surface of the hopper and over an eave at the lower end of the surface to form a curtain. Alternatively, the liquid can be extruded from one or more slot-like openings in a downwardly facing portion of a hopper to form a curtain. The liquid in the curtain strikes a continuous support in the form of a web where it passes around a support roll and forms a discrete layer coating on the web.

In the aforementioned patent US-A-3,508,947 it is mentioned that problems with marbling and omissions caused by air pockets between the carrier and the coating can be eliminated by curtain coating. In addition, it is believed that the inventive method of the aforementioned patent can eliminate the problems with marbling and omissions due to the kinetic energy of the free-falling curtain causing penetration or displacement of the air cushions on the support. As mentioned in the aforementioned patent, the height of the curtain is usually between 5 and 20 cm (approximately 2 to 8 inches).

The aforementioned problem of mottle and omission, sometimes referred to as dynamic wetting failure or air entrapment, is due to the presence of air that prevents the liquid mass from contacting and wetting the substrate evenly across its entire width. This failure occurs as the speed of the web is increased. Other parameters such as the viscosity of the substrate coating mass, the total wet thickness (i.e., the total thickness of the coating on the substrate before drying), and the chemical composition and roughness of the substrate surface have an effect on the coating speed at which the failure occurs. Thus, undesirable limitations can arise in terms of coating speed, viscosity of the substrate coating mass, thickness of one or more layers or of a layer in a multi-layer coating, and/or properties of the substrate surface.

The object of the present invention is a method for coating a carrier with a liquid mass, including:

Moving the carrier through a coating zone;

Creating a moving path consisting of the liquid mass;

Positioning the web and the carrier material in such a way that the web strikes the carrier material in the coating zone at an acute angle A between 30 and 60º;

characterized in that the angle A formed between the plane of the web just before its impact and a plane running tangentially to the carrier material on the line of impact of the web is determined from the equation

l = L/[W/sin A]

is calculated, where

l the range between 0.8 and 2 of the relation lm = 0.25+K/V

given value 1, where

Va is the instantaneous viscosity in Pas,

L is the lateral distance of the wetting line to the plane of the front of the curtain,

W represents the curtain thickness shortly before it hits the web, and

K is 0.015 at high surface tension and 0.010 at low surface tension;

with the requirement that the web moves at a speed of at least 2 m/s shortly before it hits the carrier material.

If the value of Va is given in poise, the value of K is 0.15 or 0.10.

In certain embodiments, the web can be considered as a curtain, since its speed shortly before impact with the carrier is mainly due to the attraction In such embodiments, the liquid mass may have fallen freely from the drip lip of a sliding hopper. In such embodiments, the carrier is moved downwardly through the coating zone and through the plane of the curtain just before the impact is substantially vertical. In such embodiments, the plane of the carrier is inclined to the horizontal at the line of impact of the curtain on the carrier at an angle of (90-A)º. When the carrier is passed around a support roller in the coating zone, the tangent to the carrier at the line of impact is inclined to the horizontal at an angle of (90-A)º.

In other embodiments, the web of liquid processing mass is formed by extrusion by means of a device, for example an extrusion nozzle, which moves a web of liquid mass at a speed such that at least 2 m/s is reached shortly before impact with the carrier. In such embodiments, the positioning of the web and the carrier can be independent of the direction of gravity. For example, the web can be horizontal or vertical, with the liquid mass flowing upwards towards the carrier.

In the embodiments in which the web is in the form of a free-falling curtain, the height of the curtain is preferably more than 20 cm, which is the height required to achieve a speed of about 2 m/s on impact.

In cases where the carrier is guided around a support roller when the curtain strikes the carrier and where the plane of the curtain is substantially vertical just before impact, the line of impact is at an angle of (90-A)º, ie 30º to 60º behind top dead center. The Angle (90-A)º is called the angle of application. Top dead center is the line on the carrier which lies in a vertical plane, this includes the axis of the support roller around which the carrier is guided. In this context, "behind" the top dead center side means to which the carrier moves after passing through top dead center.

In all the embodiments described and illustrated in the aforementioned patent US-A-3,508,947, the curtain strikes the carrier at top dead center or on a horizontally moving carrier. The same is true of the embodiments described in US-A-3,632,374 and US-A-3,867,374 (which arose from an application which was an amendment of an application which was itself an amendment of a divisional application which resulted in US-A-3,632,374). In the latter two patents, Fig. 11 shows a schematic view illustrating the text which states that it is not necessary for the plane of the freely falling curtain to be oriented so as to intersect the axis of the carrier roll (ie, strike at top dead center) in order to satisfactorily coat a web running therearound. The aforementioned patent also states that if the web to be coated is directed toward and away from the carrier roll so that sufficient carrier area on the web remains accessible, the free-falling curtain can be directed toward or away from the axis of the roll. It is further stated that the free-falling curtain should not be so far from the axis that the direction of travel of the web at impact is so far from the horizontal that the coating process suffers. No distinction is made between the curtain positions before top dead center and the curtain positions after top dead center. Accordingly, the patents simply state that the coating point need not be at top dead center. They do not state that a coating point (the position where the curtain strikes the support) that is not at top dead center is advantageous. In particular, they do not say anything about the importance of optimizing the position of the wetting line in increasing speed and maintaining uniformity of the coating. The curtain must normally be well past top dead center in order to optimize the wetting line position. Nor do they state that increasing the curtain height to increase coating speed is most effective when the position of the wetting line is optimized. The inventors of the present invention have learned that it is advantageous to combine long curtains with application angles that are substantially in front of top dead center. These advantages would not be expected by those skilled in the art. It is indicated in the cited patents that the preferred height of the curtain is between 5 and 20 centimeters.

Another problem in the prior art is so-called puddling. When the curtain contacts the carrier, the liquid mass tends to move against the direction of travel of the carrier until it is carried along by the viscous shearing generated by the carrier. If the impact velocity is high and the flow rate is high and/or the viscosity is low, a step develops at the foot of the curtain. If sufficiently large, the step may contain a vortex which may trap air bubbles or dirt. Such trapped bubbles or dirt particles may leave streaks or lines in the coating. Furthermore, in the embodiments in which the liquid mass comprises a plurality of layers, the step may promote mixing or some mutual displacement of the materials in the various layers. If, according to the invention, When application angles of 30º or more are used to control the wetting line position, puddling is prevented. At such larger application angles, the direction of movement of the curtain at the time of impact with the substrate is closer to the direction of substrate movement, so the liquid mass is less likely to move upwards on the substrate.

EPA 0 197 493 mentions problems called "dashed line", "streaks", "glue interruption", "thicker edges" and "air flow disturbances" and suggests the use of a short curtain to overcome such problems. Thus, the patent application does not address the same problem as the present invention. A short curtain is defined as a curtain of 0.5 to 50 millimeters in length. The short curtain is intended to strike the web at an angle of 30º to 90º behind top dead center. It was found that a procedure according to the descriptions of the previously published patent application had a negative effect on the problem of air entrapment.

None of the patents US-A-3,508,947, US-A-3,632,374 or 3,867,901 and EPA 0 197 493 describes a coating process according to the present invention in which the moving web of liquid mass impinges on the support at an acute angle Aº between the web and the uncoated support, the acute angle Aº between the plane of the tangent of the support to the line of impact and the plane of the web just before the web impinges on the support and is in the range of 30º to 60º, and the height of the curtain is greater than 20 cm, or, expressed in another way, the liquid at the time of impact has a velocity higher than that determined by the free fall of over 20 cm, which corresponds to a speed of 2 m/s.

According to the state of the art, the height of the curtain should be chosen so that the free-falling curtain has sufficient momentum at the time of impact to effectively penetrate or push away the air cushion and wet the moving carrier. However, it has been found that by increasing the curtain height, a small increase in coating speed can be achieved in practice before the aforementioned problems related to trapped air between the carrier and the coating occur, unless the carrier is inclined downwards, which is achieved when the curtain impacts the carrier in a line in the range 30º to 60º behind top dead center. It has been found that, compared with the prior art short curtains and horizontal beams, higher coating speeds of 50% or more can be achieved by using the present invention by having a curtain height of at least 20 cm and a coating point of, for example, 30º to 60º behind top dead center. When the coating point is 30º behind top dead center, the beam is inclined 30º to the horizontal. Similarly, when the coating point is 60º behind top dead center, the beam is inclined 60º. The significant speed gains achievable by using the present invention before the air entrapment problems set in were not expected.

While the prior art taught that the magnitude of the kinetic energy of the liquid in the curtain at the time of impact on the carrier was an important factor in coating without air inclusions between the carrier and the coating, it has been found that high kinetic energy is only effective in avoiding problems when the wetting line lies substantially in the plane of the curtain. The wetting line is where the liquid from the curtain first contacts the substrate. In such a case, the kinetic energy of the liquid is used most effectively to exclude air.

When the curtain impacts the carrier, the fluid tends to move both with and against the direction of movement of the carrier. The motion of the carrier is transferred to the fluid by viscous shear. In particular, a viscous boundary layer starts at the wetting line and extends to the front of the curtain.

The invention is explained in more detail below using an embodiment shown in the drawing. It shows

Fig. 1 is a perspective view of a coater;

Fig. 2 is a schematic representation of the various features and angles of the coating zone;

Fig. 3 a sectional view at the coating point in high magnification;

Fig. 4 to 11 and 13 curves of various test results and resulting derivatives;

Fig. 12 and 14 Value tables of various flow properties of different materials, the test results of which are shown in the curves in Fig. 11 and 13; and

Fig. 15 is a schematic representation of another embodiment of the present invention.

Reference is made to Figures 1, 2 and 3 of the accompanying drawings, in which parts of the apparatus 10 for producing photographic film or paper are shown. The apparatus comprises a coater having a support roller 12 around which a web-shaped support 14 to be coated is guided. Typically the web 14 is continuous. The web may be formed, for example, from cellulose acetate in a known manner. The roller 12 has an axis of rotation 16 which lies very precisely at the geometric center of its very precisely circular cylindrical surface 18. Preferably the axis of rotation 16 is horizontal. A hopper 20 of known shape has a sliding surface 22 into which extend a plurality of slots, only the uppermost of which is visible and designated by the reference numeral 24. As is known, liquid masses are fed into cavities within the hopper which communicate with the respective slots. The various liquid masses are guided over guides 26 in which pulsation dampers 28 are located. The sliding surface 22 is inclined so that the liquids emerging from the slots flow down the sliding surface and form a layer of material made up of a large number of individual layers, whereby in the example shown here three layers can be seen. At the lower end, the sliding surface 22 has an eaves edge 30 from which the layer of mass falls cleanly onto the web in the form of a curtain 32 in a known manner. The curtain is guided by known vertical edge guides 34. In order to understand the nature and purpose of the edge guides, reference is made to US-A-4,830,887. The eaves edge 30 of the funnel 20 is parallel to the axis of rotation 16 of the support roller 12. The eaves edge is positioned so that the vertically falling curtain 32 strikes the web 14 along a line 35 lying in a plane 36 containing the axis 16, according to the invention at an angle of (90-A)º in the range 30º to 60º (see Fig. 2). The plane 38 includes a top dead center 40. The roll 12 rotates clockwise as shown in Figs. 1 and 2 so that the web 14 moves up to the left side of the roll 12 and down through the coating point and away from the right side of the roll after being coated with liquids from the curtain 32. Thus, the coating point, i.e. line 35, can be said to lie at an angle of (90-A)º past top dead center, and that the angle of (90-A)º is the application angle. Of course, the tangent to the track is inclined at an angle of (90- A)º to the horizontal at the coating point.

Each of the pulsation dampers 28 comprises a chamber which is partially separated from a second gas-filled chamber by a diaphragm on the other side. The second chamber communicates with a third chamber by a passageway which provides resistance to the flow of gas. The first chamber is open to the feedthrough 26 so that hydraulic pressure pulsations which may occur during the delivery of the coating mass act on the diaphragm and are absorbed by the diaphragm and the gas in the second chamber. The selectable resistance in the feedthrough from the second to the third chamber selectively dampens the system.

Also shown in Fig. 1 is a deflection device 41 for applying a slight suction effect to the carrier to be coated just before it reaches the coating point. The application of the suction effect at this point serves in a known manner to reduce the amount of air that is carried along with the rapidly moving carrier. The removal This air helps to keep the curtain in the desired plane without air movement induced deviation from that plane.

Reference is now made to Fig. 3 of the accompanying drawings. In Fig. 3, a line 43 is shown which runs parallel to the plane of the support 14 at the line of impact of the curtain 32 on the support. In addition, a line 45 is shown which runs parallel to the plane of the curtain just before impact with the support. The angle between lines 43 and 45 is designated A. It can be seen that it lies between the curtain and the uncoated support.

With further reference to Fig. 3, the curtain 32 has a rear side 42 and a front side 44. The line at which the liquids from the curtain actually wet the web 14 is called the wetting line and is indicated in Fig. 3 by the reference numeral 46. It can be seen that in the condition shown in Fig. 3, the wetting line 46 is located just in front of the plane of the rear side 42 of the curtain.

What follows is a possible explanation of why the advantages of the invention are achieved, including equations that are useful for exploiting the invention. However, the advantages of the invention are in no way dependent on the correctness of the explanation.

Liquid from the boundary layer 47 is entrained by the web due to viscosity and therefore moves in the same direction as the web. At the front end of the boundary layer, which is the plane of the front face 44 of the curtain 32, all the liquid provided in the curtain is entrained by the moving carrier. The expansion the boundary layer is approximately represented by a dashed line 48 and, of course, by the surface of the web 14. The plane of the front side 44 of the curtain 32 is shown in Fig. 3 by a dashed line 50. Before the front end of the boundary layer, the velocity profile of the boundary layer gradually approximates the uniform velocity of the web 14.

The length of the boundary layer, i.e. the distance measured along the web that is required to accommodate the liquid, determines the position of the wetting lines in relation to the position of the front of the curtain. If the boundary layer is relatively long, the wetting line is away from the curtain and the kinetic energy of the liquid in the curtain cannot effectively carry out the dynamic wetting and air exclusion. Even if the boundary layer is short, the wetting line is also not located in such a way as to be able to benefit maximally from the kinetic energy in the curtain. The kinetic energy in the curtain has the greatest effect in preventing air entrapment when the wetting line 46 is located approximately in the plane of the rear side 42 of the curtain 32.

To understand 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 front face 44 of the curtain 32 with the surface of the web 14 to the distance between the planes of the rear and front faces of the curtain immediately above the area where the thickness of the curtain is affected by the impact, measured in a plane parallel to the surface of the web. In mathematical terms In terms of terms, the relative wetting line position quotient "1" is defined as follows:

1 = L/[W/sin A] (1)

where L is the distance of the wetting line 46 from the plane of the front face 44 of the curtain 32 measured along the surface of the support 14, and L is the length of the interface layer; W is the thickness of the curtain 32 just above the point at which the thickness of the curtain 32 is affected by impingement of liquid in the curtain 32 on the support 14; and A is the complement of the angular displacement of the coating point 35 past top dead center 40.

The boundary layer length L can be measured directly by observation in some cases, but is more conveniently estimated from boundary layer theory. To understand boundary layer theory, reference is made to Boundary-Layer Theory (seventh edition), H. Schlichting, McGraw-Hill, New York 1979, and to Boundary-Layer Behaviour on Continuous Solid Surfaces, BC Sakiadis, AIChE Journal, 1961, Volume 7, page 26.

where: s = S/U and S is the speed of the web 14, U is the speed of the curtain 32 just before impacting the substrate; D is the total thickness of the coating just before the curtain and before drying has begun (sometimes referred to as the total wet thickness); R is the Reynolds number of the liquid in the curtain, as defined below:

R = dq/V (3)

where d is the density of the liquid; q is the total volumetric flow rate per unit curtain width; and V is the Newtonian viscosity.

For a vertically free-falling curtain, the curtain speed is calculated as follows:

U = [2 GH] (4)

where G is the acceleration due to gravity and H is the curtain height, i.e. the height of the eaves 30 of the funnel 20 above the web 14' measured in the curtain 32. The contribution of U due to the velocity of the liquid as it exits the eaves and enters the curtain can be neglected for curtains higher than about 5 cm, i.e. for all curtains to which the present invention relates. However, if the initial velocity is important, then H is the effective height of the curtain, i.e. the height of the curtain falling freely with an initial velocity of zero would produce the same curtain velocity just before impact.

The formula mentioned above for determining the value of L, the boundary layer length, is suitable for a Newtonian fluid. However, coating fluids in the photographic industry contain polymers and are therefore normally pseudoplastic, i.e. difficult to dilute. An instantaneous Newtonian viscosity Va can be determined from rheological data at the representative shear rate in the boundary layer of

[S-UcosA]/D (5)

to be appreciated.

The value of this representative shear rate in the curtain coating may be 100,000 s-1 or greater. More precise means of calculating the value of L may, of course, be used, but the formulae given above are simple and useful in the practice of the invention. They are also easily generalized to the case where the various coating compositions in the various layers forming the curtain have significantly different viscosities. As is known, it is normally preferable for the uniformity of the curtain coating that the viscosities of the various layers be substantially equal and relatively high.

The rate at which air entrapment problems disappear when the coating speed is reduced after the occurrence of these speed-increasing problems is generally less than the rate at which the problems began. The term maximum practicable coating speed is used here to refer to the rate just below the rate at which air entrapment problems disappear when the coating speed is reduced.

For a given curtain height and application angle (i.e. complement of angle A), the maximum practicable coating speed depends on the total flow rate. In particular, there is a flow rate at which the maximum practicable coating speed is maximized, and this maximum, maximum practicable coating speed is referred to herein as Sm. At For a given curtain height and angle A, there is a wet coating thickness which will give the maximum practicable coating speed. If the desired wet coating thickness is greater or less, then the maximum practicable coating speed for coating that thickness may be substantially less than 5m. For the value of the application angle taught and for the value of the curtain height given in the aforementioned US-A-3,508,947, the wet coating thickness corresponding to Sm is in the range of 30 µm. Since wet thicknesses much greater than 30 µm are often desired, the maximum practicable coating speeds are often much less than Sm. It has been found that as the application angle is increased (in other words, as the angle A is decreased), Sm moves toward higher flow rates and, consequently, the maximum practicable coating speed for such thicker coatings is increased.

It has been found that the value of the relative wetting line position quotient corresponding to the maximum, most practicable coating speed, denoted lm, depends on the angle A of the curtain height. Thus, this quotient is useful in selecting the angle A at which a desired wet thickness corresponds to the maximum, most practicable coating speed. It has been found that lm is in the range of one. In particular, it has been found that for pseudoplastic coating fluid masses it depends on the instantaneous viscosity according to the following relationship:

lm = 0.25 + 0.15/Va (high surface tension) (6)

lm = 0.25 + 0.10/Va (low surface tension)

where Va is the instantaneous viscosity in poise. By high surface tension is meant values for the surface tension in the curtain just above the point of impact that are in the range of about 60 to 70 dynes/cm, which includes aqueous solutions without surfactants, such as aqueous gelatin solutions. By low surface tension is meant values for the surface tension in the curtain just above the point of impact that are in the range of about 24 to 40 dynes/cm, which includes aqueous solutions with added surfactants, such as aqueous gelatin solutions with surfactants, as is common in the coating of photographic products. Although it is very advantageous to work with the value of lm determined in this expression, it has been found that the highest practicable coating speed is at least 70% of Sm when lm is in the range of 0.8 to 2.0 of the optimum value.

While it has been found that changing the angle A is advantageous for operation at or near lm and hence Sm, it has also been found that Sm decreases as the application angle (90-A)º is increased. In particular, it has been found that Sm depends on the magnitude of the curtain velocity multiplied by the cosine of the application angle (90-A)º, i.e. U x sin A. Away from the origin, Sm depends on (U x sin A) raised to the power of about 0.8. Although Sm decreases as the application angle (90-A)º increases, the curtain height can be increased to increase the impact velocity U and to partially or completely offset the effect of increasing the application angle (90-A)º, i.e. to decrease A. For this reason, high curtains are preferred in coating practice at coating angles which are significantly greater than 0º (ie values of A are significantly less than 90º).

As can be seen from the above equations (1), (2) and (3), the relative wetting line position quotient is related to the viscosity of the liquids used to coat the web and to the angle of the web at the coating point. For wet coating thicknesses in excess of 30 µm, used in many photographic products, it has been found that application angles substantially greater than zero degrees are advantageous. It has been found that the thicker the coating and the lower the viscosity of the coating liquids, the greater the optimum application angle for the highest practicable coating speed.

An advantage of calculating the relative wetting line position coefficient is that a nearly optimal application angle can be obtained for any coating thickness or curtain speed. The above method of estimating the quotient to obtain an application angle applies to Newtonian or pseudoplastic rheology of the coating fluids. For some coating compositions, it may be necessary to measure and include other rheological effects as well. Some coating substrates have significant surface roughness and cannot be considered hydrodynamically flat.

Of course, the optimum application angle can only be determined by experimentation, by gradually increasing the angle until the highest practicable coating speed reaches a maximum.

Some examples of experiments are given below.

Attempt 1

An aqueous gelatin solution containing 15% by weight gelatin and having a low shear viscosity of 0.063 Pas (63 centipoise) was coated onto a polyethylene terephthalate carrier web having a gelatin undercoat at a variety of flow rates. At each flow rate, the speed of the web was increased until air was trapped between the coating and the carrier. Once this condition was reached, the speed was slowly reduced until air entrapment was no longer present, i.e., until the highest practicable coating speed was reached. The highest practicable coating speed, as previously defined, is the practicable speed limit in common practice, and this speed is recorded and plotted on a graph.

The test was carried out at an application angle of 0º (i.e. an angle A of 90º) for curtain heights of 2, 6, 10 and 25 cm, and at an application angle of 45º (i.e. an angle A of 45º) for curtain heights of 10 and 25 cm.

The data for the tests with the application angle 0º are shown in Fig. 4, where the coating speed in cm/s is plotted against the flow rate of the coating liquid in cm³/s per centimeter of coating width. The curves for the heights of 10 cm and 25 cm were cut off at about 5 cm³/s per cm of width, since above this value the highest practicable coating speed drops sharply. The figure also shows straight lines passing through the origin, representing the coating thicknesses of 25, 50, 100 and 150 µm, respectively.

From the four curves it can be seen that, within the limits of this experiment, the highest practicable coating speed is higher the higher the curtain. Also, within the limits of this experiment, it can be seen that the highest speeds are achieved with thin coatings, which significantly limits the production speed with thicker coatings in the state of the art. It can also be seen that at a wet coating thickness of 50 µm, the coating speed is limited to 500 cm/s, and that little advantage can be achieved by increasing the curtain height from 6 to 25 cm. At 100 µm wet coating thickness, the speed limit is only about 375 cm/s at a curtain height of 25 cm.

Fig. 5 shows curves according to the test results for the 10 and 25 cm high curtains at an application angle of 45º (A=45º). It is clearly visible that with such an application angle and a high curtain, higher wet coating thicknesses can be applied at a much higher speed. It can be seen that with a curtain height of 25 cm, a coating of 50 µm thickness can be applied at 700 cm/s. Likewise, a thickness of 100 µm can be achieved at 625 cm/s. These two speeds represent an increase of 40% and 67% respectively over the speeds achievable with the same coating thickness at an application angle of 0º (i.e. an angle A of 90º).

The data can also be used in calculating the relative wetting line position quotient. Referring to Figure 6, relative wetting line positions are shown as a function of the ratio of the actual maximum practicable coating speed, S, and the maximum maximum practicable coating speed, Sm. Each of the six curves in Figures 3 and 4 has a maximum maximum practicable coating speed, Sm. For each curve, the maximum practicable coating speed for each flow rate is normalized by dividing the speed for each data point on the curve by the value of Sm of 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 applied to the Carreau model of a pseudoplastic solution with the following parameters: power law index 0.85 and relaxation time 0.00027 s. For the understanding of the Carreau model of a pseudoplastic fluid, one can consult 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 to 172. The six curves thus generated are shown in Fig. 6.

The six plots define a master curve which, within certain deviations, is meaningful for this type of measurement. The relative wetting line position quotients corresponding to the highest speed for each curve are plotted in Fig. 7 against the maximum, most practical coating speed for that curve. It can be seen that the relative wetting line position quotient corresponding to the maximum, most practical coating speed is in each case approximately 0.63. The maximum, most practical coating speed varies with the component (U sin A) of the curtain impact speed U, perpendicular to the path, as shown in Fig. 8. The formula for the fitted curve passing through the data points is

Sm = 20 (U x sin A)0.7 (7)

where the speed units are cm per second. Although an exponent of 0.7 is determined from these few data points, a much larger set of data for gelatin coating at various concentrations, curtain heights and application angles yields an exponent of approximately 0.8.

Using the expressions for Sm (7) and the optimum relative wetting line position coefficient of 0.63, the optimum application angle (90-A)° can be estimated for certain coating conditions. For example, in Fig. 9, the optimum coating thickness and the corresponding maximum, most practical coating speed are plotted against the application angle for the curtain height of 25 cm. Above a wet coating thickness of 30 µm, the optimum application angle deviates substantially from 0º, and the optimum application angle increases with coating thickness. For a thickness of 150 µm, which is within the range of practical interest of the photographic industry, the optimum application angle is approximately 60º (i.e., the optimum value of angle A is 30º). Although the most practical coating speed decreases as the application angle is increased for a fixed curtain height, this can be at least partially compensated for by increasing the curtain height. Thus, for many practical coating thicknesses, high curtains and steep track inclines are preferred, quite contrary to the expectations of the experts.

Attempt 2

An aqueous gelatin solution with a viscosity of 0.020 Pas (20 centipoise) was used for curtain coating at the speeds and flow rates used in Experiment 1. Suitable surfactants were added to reduce the surface tension to an estimated value of 31 mN/m. Surfactants are commonly found in photographic coating materials and are used to advantage in curtain coating.

Fig. 10 shows a graph of flow rate per centimeter of curtain width versus the highest practical coating speed. The curtain heights were 12 and 25 cm at application angles (90-A)° of 0 and 45°. The advantages of the present invention are evident for coating thicknesses greater than 25 µm. For example, at the thickness of 50 µm and with a curtain height of 12 cm and an application angle of 0° (i.e. A=90º), as taught in the prior art, the highest practical coating speed is limited to about 440 cm/s. Increasing the curtain height to 25 cm or increasing the application angle to 45° at the curtain height of 12 cm allows only slightly higher speeds. The combination of an application angle of 45º (i.e. A=45º) and a height of 25 cm results in a maximum practicable coating speed of approximately 690 cm/s, which corresponds to an increase of 57%.

Attempt 3

Figure 11 shows a plot of lm, the best achievable relative wetting line position quotient, against 1/Va, the reciprocal of the instantaneous Newtonian viscosity, for aqueous polymer solutions without the addition of surfactants (high surface tension). The values are shown for fourteen different pseudoplastic materials, the properties and coating parameters of which are listed in the table in Figure 12. Figure 13 shows a similar plot for aqueous gelatin solutions to which surfactants have been added to reduce the surface tension in the curtain (low surface tension). The material properties and coating parameters are listed in the table in Figure 14. In each case, the base is a polyethylene terephthalate with a gelatin underlayer, unless otherwise indicated. The dependence of 1m on 1/Va is obviously linear with a slope that varies with the level of surface tension; on the other hand, the relationship appears to be independent of material properties, with the exception of the rheological parameters that affect the instantaneous viscosity. The fitted straight lines through the two sets of data are given by the previously mentioned relationship (6). It was found that the optimal relative wetting line position depends on the instantaneous viscosity in the boundary layer.

Attempt 4

Two gelatin layers were curtain coated simultaneously at a speed of 20º cm/s and at a curtain height of 25 cm. The upper layer had a wet thickness of 60 µm and a viscosity of 0.035 Pas (35 centipoise), while the lower layer had a wet thickness of 40 µm and a Viscosity of 0.003 Pas (3 centipoise). Although in practice viscosities much higher than 0.003 Pas (3 centipoise) are preferred, this is not always possible in view of, for example, the solubility limitations of components, or the rapidity with which a cross-linking agent added to the mass reacts with gelatin. A suitable surfactant was added to the layers to reduce their surface tension to an estimated 31 m/Nm. The curtain height was 25 cm.

At an application angle of 0º, the coating uniformity was unacceptable due to puddling. At an application angle of 45º, an acceptable coating quality was achieved and a further increase in the coating speed to 650 cm/s was achieved while maintaining the coating quality.

Attempt 5

Three layers were curtain coated simultaneously at 900 cm/sec at a curtain height of 25 cm and an application angle of 45º (ie A=45º). The upper and middle layers comprised aqueous gelatin solutions with viscosities of 0.063 Pas (63 centipoise) and 0.067 Pas (67 centipoise) respectively and a total wet thickness of 100 µm. The lower layer was deionized water at 42.5ºC with a viscosity of 0.00062 Pas (0.62 centipoise) and a wet thickness of 3.5 µm. Such a water layer can be used, for example, to achieve an increased coating speed without entrapping air, or to add a hardener or other chemicals that react with the gelatin. The upper and lower layers contained suitable surfactants to facilitate the spreading of 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, low viscosity layer is difficult to apply as a bottom layer on a sliding surface without waves and other instabilities developing in the layers, the hopper was arranged with respect to the direction of web movement so that the bottom layer (that is, the layer contacting the web) was the top layer on the sliding surface, an arrangement which allows more room for sliding instabilities. Although it is preferable that the layers have similarly high viscosities to promote uniform flow on the sliding surface and overall coating quality, this consideration is not always consistent with other objectives.

At the speed of 900 cm/s, the water layer flow was reduced to 0.8 µm and air entrapment occurred. When the water flow was returned to its original value, air entrapment no longer occurred, indicating that this is a practicable speed under the conditions mentioned.

Although the invention has been described in embodiments in which a curtain is formed by liquid falling from an eave of a sliding hopper, it is to be understood that the curtain may be formed in other ways. For example, in some embodiments of the invention, an extrusion hopper with its mouth facing downwards may extrude a curtain of liquid. In such embodiments, the curtain may have an initial velocity which may, but need not, deviate substantially from the zero value at the top of the curtain. For this reason, reference is hereby made to the curtain having a velocity equal to that achieved by a curtain falling freely from a given height, and in such an expression it is assumed that the curtain has a starting velocity of substantially zero. In the case where the liquid is extruded from an extrusion hopper at a significant velocity, the curtain height must be less than that of the curtain formed by liquid falling from the eaves of a sliding hopper so that the impact velocity is the same in both cases.

The angle A was previously discussed and described as the angle included between the plane of the liquid web just before impact with the carrier and the tangent to the carrier at the line of impact, the angle being measured on the side of the web facing the uncoated carrier. It was further pointed out with reference to Figure 2 that (90-A)° is the angle of inclination of plane 36 to plane 38 and is referred to as the angle of application in the embodiments comprising a curtain and a support roll.

In the embodiments particularly described above, the liquid mass acquires most of its velocity due to gravity shortly before it strikes the carrier. This is because the liquid mass has only a low velocity when it falls from the eaves of the sliding hopper. The present invention can be carried out in systems in which the velocity of the liquid mass shortly before it strikes the carrier is due in whole or in large part to the velocity it acquires when it emerges from an extruder nozzle. In some embodiments of the present invention, the liquid mass can take the form of a have a path that is oriented horizontally or even vertically upwards or at other inclinations to the vertical. The liquid mass in such embodiments should be better referred to as a path rather than a curtain as it moves through space towards the carrier. However, the term path may be understood to include the sub-term curtain. In those embodiments in which the speed of the path just before it hits the carrier depends primarily on the device and not on gravity, the distance between the device and the carrier may be quite small, or it may be long, since the distance does not determine the speed. However, gravity influences the speed during the flight of the liquid mass between the device ejecting it and the carrier, so that this effect on both direction and speed should be taken into account.

Fig. 15 shows a schematic representation of the support roller 12' with an axis of rotation 16'. A carrier in the form of a web 14' is guided around the support roller 12' which rotates counterclockwise as shown in Fig. 15 and as indicated by the arrow. An extrusion hopper 130 has a slot 132 from which a web 134 of liquid mass is fed out at a speed of over 200 cm/s. Typically, the extrusion slot 132 is parallel to the axis of rotation 16' of the support roller 12'. The distance between the mouth of the extrusion hopper slot and the carrier can be relatively small, on the order of 1 cm or less. The plane of the web 134 just before the liquid mass impacts the carrier is indicated by the line 45'. A plane which is tangential to the carrier at the line of impact of the liquid mass on the carrier is indicated by the line 43' in Fig. 15. The previously discussed angle A is also here the angle between the planes 43'and 45, as is the angle between planes 43 and 45 in Fig. 3. It is obvious that in the immediately preceding description with reference to Fig. 15 no implicit reference is made to the direction of gravity.

It is assumed that the term "liquid mass" as used herein is understood to encompass a variety of compositions in a variety of layers. The viscosities of these layers may be the same or different.

Although the invention has been described in embodiments for coating photographic compositions on continuous webs, it is anticipated that the invention will also be beneficial to other industries.

As is apparent from the foregoing description, a fluid applied in accordance with the invention may be a Newtonian or non-Newtonian fluid, where non-Newtonian fluids may include, but are not limited to, pseudoplastic fluids.

Drawing labeling Fig.4:

a) cm³/s per cm width

b) cm/s

Fig.5:

a) cm³/s per cm width

b) cm/s

Fig.8/9:

c) optimal wet wetting

d) max. coating speed

e) Speed

f) Wetting

Fig. 12:

g) Symbol in the curve display

h) Materials

i) Viscosity at low shear rate in the Carreau model, Pas

j) Relaxation time in the Carreau model, s

k) Power law index in the Carreau model

l) Application angle, degrees

m) Curtain height, cm

1 gelatin

2 gelatin

3 Gelatin

4 Gelatin

5 Gelatin, carrier without underlayer

6 Gelatin, carrier without underlayer

7 High viscosity gelatin

8 Gelatin plus thickener

9 Gelatin, carrier without underlayer

10 Polyvinylpyrrolidone

11 Polyvinylpyrrolidone

12 Polyvinylpyrroldone

13 Polyvinylpyrrolidone

14 Dextran

15 Polyvinyl alcohol

Fig. 13:

n) Gelatin and surfactant

Fig. 14:

o) Optimal relative wetting line position of aqueous gelatin with addition of surfactants

p) Column

q) Concentration of surfactant, percent by weight

r) Low shear viscosity, Pas

s Curtain height, cm

t) Application angle, degrees

u) Instantaneous viscosity, Pas

v) Reciprocal instantaneous viscosity

w) Volumetric flow rate per unit width, cm³/s/cm

x) Air inclusion velocity, cm/s

y) Optimal relative wetting line position, I

Claims (13)

1. Method for coating a carrier material with a liquid comprising the following steps: - Moving the carrier through a coating zone; - Creating a moving path consisting of the liquid mass; - positioning the web and the carrier material in such a way that the web hits the carrier material in the coating zone at an acute angle A of between 30 and 60º; characterized in that the angle A formed between the plane of the web shortly before its impact and a plane running tangentially to the carrier material on the line of impact of the web is determined from the equation l = L/[W/sin A] is calculated, where l the range between 0.8 and 2 of the value given by the relationship l = 0.25 + K/Va given value 1, where Va is the instantaneous viscosity in Pas, L is the lateral distance of the wetting line to the plane of the web front, W represents the web thickness shortly before it hits the support material, and K is 0.015 at high surface tension and 0.010 at low surface tension, with the requirement that the web moves at a speed of at least 2 m/s shortly before it hits the carrier material. 2. A method according to claim 1, characterized in that the carrier material is moved downwards through the coating zone, and in that the step of producing a moving web is carried out by forming a free-falling curtain consisting of the liquid mass, the angle formed between the plane of the web shortly before its impact and a plane running tangent to the carrier material at the line of impact of the web being the acute angle A. 3. Method according to claim 2, characterized in that the carrier material has a web shape. 4. Method according to claim 3, characterized in that the carrier web is guided around a support roller arranged to rotate about a horizontal axis, the curtain is aligned so that its plane runs parallel to the axis of rotation of the support roller and strikes the carrier web section located on the support roller along a line that lies in a plane enclosing the axis of the support roller, which extends at an angle of (90 - A)º to the vertical plane enclosing the axis of rotation of the support roller, the line running on the side of the vertical plane towards which the carrier web moves after it has passed through the vertical plane. 5. A method according to claim 4, characterized in that the curtain is formed by moving the liquid mass through a coating funnel so that it flows downwards in a path over the inclined sliding surface of the funnel and then falls from its eaves edge. 6. A method according to claim 5, characterized in that the coating hopper has a plurality of slots and the liquid mass is moved through each of the slots, thereby forming a multi-layered curtain. 7. Process according to one of the preceding claims, characterized in that the liquid mass is suitable for forming one or more layers for photographic films or papers. 8. Method according to claim 4, characterized by a pour hopper oriented in such a way that its pouring slot runs parallel to the axis of rotation of the support rollers, whereby the mass is moved through the pour hopper in such a way that it emerges from the pouring slot at a speed that is at least 2 m/s shortly before the web hits the support material. 9. Method according to claim 8, characterized in that the plane of the web runs vertically until shortly before it hits the support material. 10. Method according to claim 1, characterized in that the value of L is based on observations. 11. Method according to claim 1, characterized in that in the case of a pseudoplastic or Newtonian fluid, the value L is determined from the equation is calculated in the s = S/U, where S is the speed of the carrier material, U the speed of the curtain shortly before it hits D is the wetting thickness of the coating immediately following the curtain, and R represents the Reynolds number of the liquid of the curtain, defined as R = dq/V, where d is the liquid density, q is the total volumetric flow rate per unit of curtain width, and V is the Newtonian viscosity or instantaneous viscosity Va in the case of a pseudoplastic fluid at a shear coefficient given by [S-U cosA]/D. 12. Process according to claim 1, characterized in that the carrier material is moved downwards through the coating zone and the plane of the emulsion layer is vertical. 13. Process according to claim 1, characterized in that the liquid mass is suitable for forming one or more layers for photographic films or papers.