Edge removal apparatus for curtain coating

The present invention is an edge blade (20) attached to an edge guide (12) for removing an edge of a falling curtain (10). The edge of the falling curtain is intercepted by the edge blade (20) and is vacuumed away by a vacuum tube (21) disposed near the edge blade. The present invention provides a uniform coated edge with maximum coating speeds at the edge being equal to maximum speed possible for the coating system far from the edge.

FIELD OF THE INVENTION 
The present invention relates to a method and apparatus for coating objects 
or moving supports, advancing continuously past a coating station, by the 
curtain coating method. More particularly, it relates to an improved 
curtain coating method and apparatus for the manufacture of photographic 
materials such as photographic film and paper. 
BACKGROUND OF THE INVENTION 
In coating apparatus of the curtain coating type, the moving support is 
coated by causing a free falling curtain of coating liquid to impinge onto 
the moving support to form a layer on said support. An apparatus is 
described and used in U.S. Pat. No. 3,508,947 wherein a multilayer 
composite of a plurality of distinct layers is formed on a slide hopper 
and caused to impinge onto an object or moving support to form a coated 
layer thereon. U.S. Pat. No. 3,508,947 particularly relates to the 
manufacture of multilayer photographic materials such as photographic film 
and paper. 
In the coating of photographic products it is necessary to constrain the 
edges of the curtain to eliminate narrowing of the curtain and a reduction 
in coating width. It is desirable to have the edges of the curtain be 
internal to the edges of the film or paper base, henceforth this will be 
referred to as internal edging. Internal edging is preferable to the 
practice of maintaining a curtain wider than the base and coating over the 
edges of the base. However, the edge guides are solid surfaces which slow 
the coating liquids because of drag they produce. This reduction in 
velocity results in a significant penalty in the maximum coating speed 
attainable near the edge. The prior art teaches introducing a lubricating 
layer of water, or another low viscosity liquid, along the edge guide to 
reduce the drag and increase the velocity of the coating solutions in the 
curtain. This water layer or low viscosity liquid layer must, however, be 
removed in order to maintain acceptable coating latitude and quality and 
to avoid any penalty in speed for drying the edges. In the removal of the 
lubricating layer the velocity of the coating liquids must not be reduced 
in the vicinity of the edge if high speed coating is desired. The prior 
art teaches the use of a vertical slit connected to a vacuum source at the 
bottom of the edge guide as the means by which the lubricating water is 
removed. This is described in U.S. Pat. No. 4,830,887 which is 
incorporated by reference herein. This technique tends to slow down the 
coating liquids as the lubricating layer is being removed, hence reducing 
the maximum attainable coating speed at the edge. Also, some lubricating 
liquid may flow beyond the slit and not be captured. 
Therefore, it is desirable to remove the lubricating liquid layer very 
abruptly giving the coating liquids near the edge guide very little 
opportunity to slow down. This maximizes the momentum of the coating 
liquids in the falling curtain and therefore, maximizes the attainable 
coating speed for the specific layer viscosities and flow rates being 
used. It is also desirable to ensure complete removal of the lubricating 
liquid. The present invention describes a method and apparatus in which 
the lubricating liquid layer is removed completely and very abruptly. This 
allows the coating speed of the curtain coating process to be maximized. 
SUMMARY OF THE INVENTION 
The present invention is a method and apparatus by which the lubricating 
layer of liquid and/or edge of the curtain in a curtain coating operation 
are removed very abruptly and efficiently. This is achieved by having the 
lubricating liquid and optionally, an adjacent narrow section of the 
curtain fall onto a thin solid blade. The lubricating liquid and curtain 
which impinge on the blade are then vacuumed away. This allows the 
remaining curtain to coat with little or no reduction in velocity due to 
the removal of the edge band of the falling curtain. The key element in 
the invention is the abruptness by which the lubricating layer is removed 
by the blade/vacuum combination of the present invention.

Detailed Description of the Preferred Embodiment 
FIG. 1 illustrates how momentum at the edge of a curtain is lost in a prior 
art curtain coater when a lubricating liquid is removed. FIG. 1 shows 
curtain 10 and two prior art edge guides 12 guiding the curtain from the 
hopper lip 13 to the support 16 to be coated. As the curtain 10 leaves the 
hopper lip 13 it falls until it impinges upon moving support 16. The 
curtain is guided at its edges by edge guides 12. The edge guides include 
a lubricating liquid introduction point 14 for introducing a lubricating 
liquid layer or band 14a at the edge of the curtain. However, this 
lubricating band is preferably removed prior to coating the support. 
In the prior art as described in U.S. Pat. No. 4,830,387, the lubricating 
liquid is removed by a vertical vacuum slot in fluid communication with 
the vacuum inlet 15 which can act to reduce velocity at the edge of the 
curtain. As the removal of the lubricating layer occurs, the curtain 
liquids tend to move into closer proximity to the edge guide and hence are 
affected to a greater degree by drag. This can create nonuniformities at 
the edges of the support 16 which has been coated by the curtain 10 as the 
momentum of the coating solutions near the edge are reduced. For example, 
the curtain can fail to wet the support completely. The edge of the 
coating may then be ragged and may contain air bubbles. Drops of coating 
composition can also be created at the point where the edge of the curtain 
strikes the support. These may create contamination, streaks in the 
coating, and other process problems. 
Therefore, it is desirable to remove the lubricating water layer very 
abruptly, giving the coating liquids very little time to slow down. This 
maximizes the momentum of the coating liquids and maximizes the attainable 
coating speed for the specific layer viscosities and flow rates being 
used. 
Shown in FIG. 2 is a perspective view of the device of the present 
invention from behind the curtain 10. FIG. 3 shows a front perspective 
view of the present invention. FIG. 2 shows a blade 20 attached to the 
edge guide 12 described in U.S. Pat. No. 5,328,726. Blade 20 extends 
inward and intercepts the curtain 10. Attached to the blade is a vacuum 
tube 21 which removes all of the liquids intercepted by the blade 20. The 
liquids which impinge on the blade are drawn away through the vacuum tube 
21 through vacuum slot 22. The liquids which do not contact the blade 
continue with very little velocity reduction until they impinge on the 
support 16. The width W of the curtain liquids which impinge on the blade 
20 can be varied by adjusting the position of the device with respect to 
the curtain. The width W is adjusted by the position of pin 23 which 
positions the edge guide 12, although other methods are possible. A 
similar pin at the top of the edge guide also has to be adjusted for the 
edge guide to remain vertical. It is preferable that at least some of the 
coating liquids be removed to insure that all of the lubricating layer is 
removed. Edge non-uniformities in the coating liquids originating in the 
hopper and on the hopper slide can also be removed in this manner. Below 
blade 20, the free edge of the curtain makes an angle .alpha. (not shown) 
with respect to the vertical. 
The angle, .alpha., which the free edge of the curtain makes with the 
vertical is given by the following equation: 
EQU sin (.alpha.)=(2.gamma./dqv).sup.1/2 
wherein; 
.alpha. is the angle; 
.gamma. is the surface tension of the liquid; 
d is the density of the liquid; 
q is the volumetric flow rate of the liquid per unit width; and 
v is the velocity of the liquid (See Journal of Colloid and Interface 
Science, Vol 77, No. 2, October, 1980, pp 583-585). Therefore, as the gap 
D (See FIG. 4) between the support 16 and blade 20 is widened the coating 
width will become narrower, the edge coverage will increase with respect 
to the middle. This makes it desirable to minimize the gap between the 
blade and the support. Typical gap values are on the order of one mm. FIG. 
4 shows a side view of the vacuum removal device of the present invention. 
The blade 20 is oriented at an angle .theta. sloping downward from back to 
front. The angle .theta. is equal to the slope of the base with respect to 
the horizontal i.e., the forward application angle, so that the curtain 
impingement point may be placed as close to the support 16 as desired 
without interference and is shown in FIG. 4. Preferably, the edge of the 
blade from which the curtain breaks is parallel to the tangent of the 
coating roll. Also shown in FIGS. 2 and 4 are the vacuum tube 21 and a 
vacuum slot 22. The curtain edge liquids intercepted by blade 20, flow 
into the vacuum slot 22 and are suctioned away through the vacuum tube 21. 
It is preferred that the blade 20 of the edge removal device is tilted 
downwards toward the coating roll, in the direction of the center of the 
curtain and in the plane of the curtain. This is shown in FIG. 3. In this 
preferred orientation drips cannot occur from the bottom surface of the 
blade as the liquid cannot run back beneath the blade. If the blade has no 
inclination in the plane of the curtain as shown in FIG. 2, or if it is 
inclined upwards away from the coating roll in the direction of the main 
body of the curtain and in the plane of the curtain, no vacuum removal 
means is required, as the edge liquids intercepted by the blade will flow 
down the blade by gravity, away from the edge of the coating, and can be 
collected. However, to prevent drips from the bottom surface of the blade, 
and to minimize excess coating thickness at the edge, a blade tilted 
toward the coating roll at approximately 30 degrees in the direction of 
the main body of the curtain and in the plane of the curtain and a suction 
removal means is preferred. 
FIG. 5 shows the evacuation path of the solutions that contact the blade 
20. The curtain 10 falls just behind a vacuum slot 22, i.e. approximately 
1 mm from the slot 22, which is open on the two perpendicular faces of the 
vacuum body. The liquids enter the vacuum body through the vacuum slot 22 
and then are channelled through an internal vacuum slot 32, vacuum cavity 
33 and vacuum tube 21 which is connected to the vacuum source. The 
internal vacuum slot 32 and cavity 33 are designed to obtain uniform 
vacuum potential along the vacuum slot 22. 
For strength, the blade is preferably corrosion resistant metal, such as 
stainless steel or titanium alloy. In the example the blade and vacuum 
body were made of titanium. The blade is preferably thin to help minimize 
the distance between the top surface of the blade and the support at the 
blade's edge (the point of curtain breakoff), and to minimize drag on the 
edge of the curtain formed at the blade's edge. Metal blades 0.1-0.25 mm 
thick perform well and have adequate mechanical integrity. The top surface 
of the blade is preferably smooth enough that flow of liquids is not 
impeded. The bottom surface of the blade is preferably polished to 
minimize wetting of the bottom surface. However, blade finish is not 
crucial to operation of the invention. The working edge of the blade is 
machined square, no attempt is made to sharpen the blade for safety 
reasons. 
The width and height of the vacuum slot are chosen, along with vacuum level 
and capacity to insure that all of the intercepted edge band is removed. 
Preferably, the width of the vacuum slot is comparable to the width of the 
intercepted edge band. The slot width can be as small as 50% of the 
curtain removal width. As the slot width becomes much greater than the 
curtain width a higher vacuum level is required to handle the extra volume 
of air drawn into the slot which extends outboard of the curtain edge. A 
slot height of about 0.5 mm has been found to provide sufficiently uniform 
vacuum potential around the slot without unduly high resistance to flow. 
The advantage of this invention when compared to prior art edge guides is 
that it is possible to coat a very straight edge with better thickness 
uniformity near the edge than was possible with the prior art. It is also 
possible to coat products at higher speeds while incurring less waste due 
to the curtain edging apparatus of the present invention. 
EXAMPLE 
A single layer application of 50 centipoise aqueous gelatin solution with 
surfactant and 1.65 cc/cm/sec total flow rate was curtain coated. The 
application angle .theta. used was +30 degrees and the support coated was 
gelatin-subbed polyethylene terephthalate. The speed was gradually 
increased until wetting failure occurred at the edge. Two different edge 
guide configurations were used: a slotted tube (prior art); and an edge 
guide with a blade removing 4 mm of curtain solutions and all the 
lubricating liquid, which was water. 
For the prior art edge guide the maximum attainable speed was 480 cm/sec. 
At this point the edge entered wetting failure and the speed could no 
longer be increased. For the edge guide with blade/vacuum assembly 
removing 4 mm of curtain solutions and all the lubricating liquid the 
maximum attainable speed was 660 cm/sec. At this point the edge entered 
wetting failure. The maximum attainable coating speed for this curtain 
sufficiently far from the edge as to be unaffected by the edge guide was 
700 cm/sec. 
Therefore, the invention provided a 37.5% increase in maximum coating speed 
attainable in the center of the curtain. An increase in coating speed in 
manufacturing of this magnitude would greatly improve efficiency of the 
operation and increase the capacity of the plant. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes, alterations and modifications may 
be made therein without departing from the scope of the invention as 
defined by the appended claims.