Coating apparatus having a stagnation prevention device

The present invention is a device which is inserted into a two roll coating apparatus. The device includes two elongated bars with an arcuate surface running the length of the application roller and spaced therefrom. The bars are attached to the cross pieces on each end and fill in the area between the end of the roll and the end of the open hopper. The device eliminates turbulent eddies that occur in an open mouth hopper and thereby reduces coating defects.

FIELD OF THE INVENTION 
The present invention relates to a coating apparatus wherein the coating 
solution is fed to the coating point by an application roller rotatably 
mounted in a hopper and partially submerged in a coating solution. More 
particularly, the present invention reduces turbulence around the 
application roller as it picks up the coating solution. 
BACKGROUND OF THE INVENTION 
Various roller coating methods and apparatus exist to apply solutions to 
moving flexible webs. One such method includes a two-roll coating process 
where an application roller is partially immersed and rotated in a hopper 
containing coating solution. A portion of the application roller rising 
out of the hopper containing coating solution is brought into controlled 
close contact with a substrate which is wrapped around another roller 
placed above the application roller. The space between the application 
roller and the substrate is typically between 0.003 inches and 0.015 
inches. A bead of coating solution is formed in this space or gap, a 
portion of which is deposited on the moving substrate. The remainder of 
the coating solution remains on the roll as a thin liquid coating. The 
coating solution is supplied to the hopper through an inlet and the level 
of the solution in the hopper is controlled by a suitable control device. 
Flow visualization studies have shown that the solution in the hopper 
stagnates due to turbulent eddy currents set up in the hopper by a 
combination of the rotating application roller and the flow of coating 
solution into the hopper. As a result of this stagnation, particulates 
that enter the hopper remain in the hopper and are sporadically deposited 
on the moving web causing unacceptable defects and waste. Particulates can 
enter the hopper through various vectors. These studies also show that 
when the solution is drained from the opposite end of the hopper, the eddy 
currents do not change, particulates are not purged and the flow is 
stratified with the particles remaining at the liquid/application roller 
interface continuing to generate coating defects. 
Thus, there exists a need to reduce these turbulent eddies within the 
liquid coating hopper. The present invention minimizes eddies within a 
hopper containing coating solution and an application roller thereby 
improving coating quality and minimizing waste. 
SUMMARY OF THE INVENTION 
The present invention is a coating apparatus which includes an open mouth 
hopper which has a first end and a second end. The hopper contains a 
coating solution and includes an inlet for supplying the coating solution. 
An application roller which has a first and a second end and also has a 
length is rotatably mounted in the hopper with a portion of the outer 
surface of the roller dipping into the coating solution. A means for 
rotating the roller is provided. The stagnation device is immersed in the 
open mouth hopper and includes a first elongated bar extending the length 
of the roller, the first bar having an arcuate surface spaced a distance 
from the outer surface of the application roller. A second elongated bar 
extending the length of the roller and having an arcuate surface spaced a 
second distance from the outer surface of the roller is provided. A first 
cross piece connects a first end of the first hopper bar and the first end 
of the second hopper bar and the first cross piece extends between the end 
of the hopper and the first end of the roller. A second cross piece 
connects the second end of the first bar and the second end of the second 
bar and the second cross piece extends between the second end of the 
hopper and the second end of the roller. This stagnation prevention device 
reduces turbulence around the application roller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Shown in FIGS. 1 and 2 is a two roll coating apparatus which includes an 
open mouth hopper or pan 10 having an elongated trough with a flat base 
11, end walls 13 and side walls 12 adjoined by a bottom wall 14 which 
along with the end walls forms a reservoir or a supply of liquid coating 
solution S. An application roller 15 is rotatably mounted in the hopper by 
a shaft 16 suitably journaled in the end walls of the hopper. The bottom 
portion of the application roller is immersed in the coating solution S 
and the top portion extends through the open mouth of the hopper. When the 
roll is rotated in the direction of the arrow by any suitable means 
including gear 17 at the end of shaft 16, a layer of coating solution S is 
transferred from the hopper to a bead 18 that is contacted by and coats 
the web 19 moved and supported by roller 20 which moves in a direction 
opposite to that of the application roller. In some situations, it is 
preferable to rotate the application roller in the direction opposite 
shown by the arrow. Coating solution is supplied to the hopper 10 through 
conduit 22. Suitable control means control the level of coating solution 
in the hopper, such as through conduit 22' and a control valve (not 
shown). As is well known in the art, instead of the application roller 
transferring the coating solution to a bead 18 across which the web is 
moved thereby coating the web, the application roller may transfer the 
coating solution directly to the web surface or to another roller which in 
turn transfers the coating solution to the web directly or indirectly. 
The present invention is a device which is inserted into the two roll 
coating apparatus discussed above. The device shown in FIG. 3 includes two 
elongated bars 31, 32 with an arcuate surface indicated generally as 33 
running the entire length of the bar. The bars are attached with cross 
pieces 35 on each end to fill in the area between the end of the 
application roller and the ends of the open mouth hopper. The cross pieces 
also act as brackets for mounting the device on the two roll coating 
apparatus. The arcuate surfaces 33 of the bars 31, 32 are spaced between 
0.04 inches and 0.06 inches away from the application roller surface. The 
arcuate surfaces of the bars are immersed in the coating solution, thus 
eliminating the air-liquid interface. The tight gap created around the 
application roller reduces the Reynolds number at the interface from a 
turbulent glow situation of 3,000 to 5,000 to one of laminar flow at 300 
to 500. This ten fold reduction in Reynolds number eliminates turbulent 
eddies and currents and particles and debris can be flushed out of the 
open mouth hopper. An additional benefit of the device is that evaporation 
is prevented and there is no place for environmentally deposited dirt to 
collect. 
FIG. 4 shows a cross sectional view of the stagnation prevention device of 
the present invention inserted in the two roll coating apparatus. As 
shown, the stagnation prevention device is not submerged in the coating 
liquid. The coating liquid level is shown generally by S'. Rather, the 
arcuate surfaces 33 which closely match the curvature of the application 
roller 15 are partially immersed while the top side of the application 
device is not in the coating solution. This eliminates the air-liquid 
interface that occurs in the prior art device. The stagnation prevention 
device mounts to the hopper by the cross piece 35. The web and support 
roller are not shown in FIG. 4. 
The present invention has been tested in production coatings and has been 
found to eliminate production waste problems, mainly gel nitrate specs 
which form in the coating solution. Quantitative data is not easily 
collected, although the use of this device has eliminated many waste 
problems due to coating defects. 
While there has been shown and described what are at present considered to 
be the preferred embodiments of the present invention, it will be apparent 
to those skilled in the art that various modifications and alternations 
may be made herein. All such modifications and alterations are considered 
to be within the scope of the following claims.