Apparatus for coating semiconductive material

Excessive heat-generating current levels produced in semiconductive materials by electrostatically assisted coating apparatus employed to, for example, improve the uniformity of a coating applied to such materials are avoided by passing an auxiliary current through said semiconductive materials in the same region and in a direction opposite to that of the current produced by said electrostatically assisted coating apparatus such that the difference between the said current produced by said electrostatically assisted coating apparatus and the said auxiliary current is less than or equal to a predetermined value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to means for coating semiconductive materials 
with electrostatically assisted coating apparatus, in general, and to such 
apparatus for coating a moving web of such materials, in particular. 
2. Description of the Prior Art 
In the manufacture of various coated products it is often essential that 
coating materials applied to such products be of uniform thickness. In, 
for example, the continuous manufacture of coated photographic sheet 
materials, a non-uniform thickness coating applied to a moving web of said 
material will require considerably more drying time for drying the thicker 
portions of a nonuniform coating than will be required for drying the 
thinner portions of said nonuniform coating. In addition, a temperature 
gradient that is optimum for drying said thicker coating portions is often 
excessive for optimum drying of said thinner coating portions. Drying time 
is usually the major factor limiting maximum production rates of many 
coated products. Also, many properties of photographic film, for example, 
such as sensitivity to light, color saturation, etc., can be adversely 
affected when constructed with nonuniformly coated sheet materials. 
Mechanical devices generally employed in the web coating art, such as 
doctor blades, scrapers and the like, have controlled the uniformity of 
web coating thickness to a limited degree. However, in the production of 
photographic film, for example, such contact devices have a propensity for 
inducing surface defects in the film coatings and in addition, these 
contact devices very often have a detrimental effect on the sensitometry 
of a finished photographic film product. 
One of the most effective coating thickness control apparatus in present 
day use in the coating industry utilizes electrostatics to uniformly 
deposit coating materials on products to be coated. In the production of 
photographic film, for example, a web or sheet of material to be coated is 
passed between an electrically conductive support or backing roller and a 
coating applicator from which coating material flows onto a surface of 
said web. An electrostatic field is established across the gap between the 
coating applicator and the backing roller by a high voltage power supply 
whose output terminals are normally connected between said applicator and 
said roller. The electrostatic field causes a coating, of uniform 
thickness, to be deposited on the web surface to be coated, and permits 
larger coating gaps to be employed between said coating applicator and the 
material to be coated. While the voltage magnitude established between 
said applicator and said roller is less than that required to generate 
corona, said magnitude often exceeds 3KV DC. 
The use of electrostatically assisted coating apparatus employing voltages 
in the vicinity of 3KV or more is relatively effective when coating 
dielectric materials or materials that have a relatively high electrical 
resistance. However, if such apparatus is employed to coat semiconductive 
materials, excessive heat-generating current levels could result because 
of the lower electrical resistance of such materials, and this excessive 
heat would have a detrimental effect on the quality of such materials. The 
greater the conductivity of the semiconductive materials, the greater the 
magnitude of harmful heat-producing current that would be generated for 
any given level of electrostatic assist. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of the present invention, a method and 
device are disclosed that will coat semiconductive materials with 
electrostatically assisted coating apparatus at higher electrostatic 
assist potentials without producing heat-generating current levels that 
could damage such materials. Excessive heat levels are precluded and 
higher coating gap potential can be achieved when electrostatically 
assisted coating apparatus is employed to coat semiconductive materials, 
by passing an auxiliary current through said semiconductive materials 
during the coating process in the same region and in a direction opposite 
to that of the current produced by the electrostatically assisted coating 
apparatus such that the difference between the said current produced by 
said electrostatically assisted coating apparatus and the said auxiliary 
current is less than or equal to a predetermined value.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
To facilitate understanding the inventive concept of the present invention, 
electrostatic coating-gap assist apparatus representative of the type 
generally employed in the prior art for coating dielectric or insulative 
materials will be described before a description of the present invention 
is initiated. Referring now to the drawings, in FIG. 1 numeral 10 
generally indicates web coating apparatus employing electrostatic 
coating-gap assist apparatus constructed in accordance with the teachings 
of the prior art. FIG. 1, web support or backing roller 12 is 
cylindrically shaped, is electrically conductive and is mounted for 
rotation about backing roller axis 14. Coating applicator 16 is mounted in 
a fixed position with respect to backing roller 12 and is spaced from said 
roller 12 by distance or gap 18. High voltage supply 20, having a DC 
voltage across its output terminals that is often in the neighborhood of 
several thousand volts, has said output terminals connected between 
backing roller 12 and applicator 16 through paths 22 and 24, respectively. 
Because the coating fluid applied by applicator 16 maintains said 
applicator 16 at or near ground potential through a conduit (not shown) 
supplying coating fluid to said applicator 16, the high voltage terminal 
of power supply 20 is connected to said roller 12 and the low voltage 
terminal of said supply 20 is connected to said grounded applicator 16. 
When power supply 20 is energized through paths 25, electrostatic field 26 
is produced in coating gap 18 between high potential backing roller 12 and 
grounded applicator 16. As insulative or dielectric web material 28 is 
moved in direction 30 through gap 18 by drive means (not shown), said web 
28 is electrostatically charged by orienting its dipoles (such as dipoles 
31) by means of said electrostatic field 26. The electrostatic charge 
produced on web 28 by electrostatic field 26 causes fluid 32 flowing from 
applicator 16 into coating gap 18 to be attracted toward and uniformly 
deposited on said moving web 28. 
An extremely important factor in the web coating process is the maintenance 
of a proper amount of coating material 32 in gap 18 for proper web-coating 
purposes. This porion of coating material 32 is sometimes referred to as a 
coating fluid bead and is designated numeral 34 in prior art FIG. 1. The 
surface of web 28 that is to be coated moves faster than the rate at which 
coating fluid 32 moves onto said web 28 surface. This being so, as web 28 
and fluid 32 in the form of bead 34 are brought into contact with one 
another, the faster moving web 28 pulls and thereby stretches said fluid 
32 causing the thickness of coating fluid 32 to be reduced to a desired 
level. It is believed the electrostatic field 26 changes properties of 
coating fluid 32 such surface tension allowing fluid 32 to be stretched to 
a greater degree and over a large gap between web 28 and applicator 16 
without losing or breaking bead 34 than would be possible if electrostatic 
gap-assisting field 26 were not present. In addition to its primary 
contribution of providing the desired coating layer thickness on web 28, 
gap 18 in FIG. 1 must be large enough to accommodate such things as web 
splices or foreign matter so that said splices or matter do not come into 
contact with applicator 16 and thereby adversely affect web coating 
thickness and/or surface quality. 
Turning now to the present invention, and specifically to FIGS. 2A and 3, 
in FIG. 2A numeral 36 generally indicates web coating apparatus for 
coating wet or semiconductive materials. FIG. 2A is a schematic diagram of 
apparatus that employs electrostatic coating-gap assist apparatus 
constructed in accordance with said present invention. FIG. 3 
schematically depicts electrical circuit analog 37 of the electrostatic 
coating-gap assist apparatus that is schematically illustrated in said 
FIG. 2A. In FIG. 2A, web support or backing roller 38 is cylindrically 
shaped, is electrically conductive and is mounted for rotation about 
backing roller axis 40. Coating applicator 42 is mounted in a fixed 
position with respect to backing roller 38 and is spaced for said roller 
38 by distance or gap 44. Primary high voltage supply 46, having a DC 
voltage across its output terminals that is often in the neighborhood of 
several thousand volts, has said output terminals connected between 
backing roller 38 and applicator 42 through paths 48 and 50, respectively. 
Backing roller 38 and applicator 42 are also referred to herein as 
electrodes when, for example, they are so connected to said power supply 
46. Power supply 46 is a relatively high impedance device in that by 
itself it has no significant effect on the impedance of any electrical 
circuit connected between its said high voltage output terminals. As noted 
above, because the coating fluid supplied by applicator 42 maintains said 
applicator 42 at or near ground potential through conduit (not shown) 
supplying coating fluid to said applicator 42, the high voltage terminal 
of power supply 46 is necessarily connected to said roller 38 and the low 
voltage terminal of said supply 46 is connected to said grounded 
applicator 42. 
Conductive bristle brush 52 is mounted in a fixed position with respect to 
and has the free ends of its bristles pointed toward and spaced from said 
grounded backing roller 38. Brush 52 is also referred to herein as a third 
electrode. DC power supply 54 has its high voltage output terminal 
connected to one end of each of the bristles of said conductive bristle 
brush 52 through path 56 and has its low voltage output terminal connected 
to applicator 42 through paths 58 and 50. Power supply 54 is also a 
relatively high impedance device in that by itself it has no significant 
effect on the impedance of any electrical circuit connected between its 
said high voltage output terminal. 
Portion 60 of semiconductive web 62 is supported in gap 44 in a spaced 
relation from applicator 42 by web backing roller or support means 38. 
Portion 64 of said web 62 is supported by said backing roller 38 such that 
outer surface 66 of said web portion 64 is in direct physical contact with 
the free ends of the conductive bristles of brush 52. The function of 
brush 52 is to provide a moving or sliding electrical contact between 
surface 66 of web portion 64 and the high voltage output terminal of 
auxiliary power supply 54 through path 56 and said brush 52. Other moving 
contact arrangements may be substituted for that provided by brush 52. One 
such moving contact arrangement may take the form of that shown in FIG. 
2B. 
Turning momentarily to FIG. 2B, electrically conductive web support or 
backing roller 68 of cylindrical shape is mounted for rotation about 
backing roller axis 70. Conductive rubber roller 72, which would be 
considered a third electrode herein if employed in place of brush 52, is 
mounted for rotation about axis 74 and is spaced from web support roller 
68. A portion of web 76 is supported between rollers 68 and 72 such that 
one surface of web 76 is in contact with roller 68 and another or the 
outer surface 78 of web 76 is in contact with conductive rubber roller 72. 
High voltage output terminal 80 of auxiliary DC power supply 82 is 
connected to surface 78 of web 76 through conductive rubber roller 72 that 
is connected to said terminal 80 through conductive path 84. When web 76 
is moved in direction 86 between rollers 68 and 72, said roller 72 rotates 
about axis 74 to thereby provide a moving contact between surface 78 of 
web 76 and said conductive rubber roller 72. 
Another less desirable arrangement may take the form of an electrically 
conductive path between the high voltage terminal of power supply 54 and 
backing roller 38 in FIG. 2A that includes a resistor whose resistance 
value is equivalent to the electrical resistance of portion 64 of 
semiconductive web 62 that is presented to said power supply 54. This 
electrically conductive path together with said resistor would be 
considered a third electrode herein if employed in place of brush 52. An 
advantage of this arrangement is that said equivalent resistor can be 
selected such that it has a larger wattage or heat rating than porion 64 
of said web 62. 
Returning now to FIGS. 2A and 3 and the preferred embodiment of the present 
invention illustrated therein, when power supplies 46 and 54 are energized 
while portion 60 of semiconductive web 62 is between roller 38 and 
applicator 42, and while portion 64 of said web 62 is between said roller 
38 and the free ends of conductive bristle brush 52, as described in 
detail above, electrical currents I.sub.1 and I.sub.2 produced by power 
supplies 54 and 46, respectively, pass through portions 60 and/or 64 of 
said web 62. Current I.sub.2 flows from primary power supply 46 to web 
support or backing roller 38 through electrically conductive path 48, 
through portion 60 of semiconductive web 62, across gap 44 into grounded 
coating applicator 42 and then back to the low potential side of said 
primary power supply 46 through electrically conductive path 50. At the 
same time that current I.sub.2 is flowing from primary power supply 46 in 
the above described manner, current I.sub.1 is flowing from auxiliary 
power supply 54. Current I.sub.1 flows from the low voltage terminal of 
auxiliary power supply 54 to grounded applicator 42 through conductive 
paths 58 and 50, across gap 44, through portion 60 of semiconductive web 
62 in a direction opposite to current I.sub.2 that is flowing from power 
supply 46, through conductive support or backing roller 38, through 
portion 64 of semiconductive web 62 and then back to the high potential 
side of power supply 54 through the sliding contact provided by brush 52, 
and electrically conductive path 56. The magnitude of current I.sub.1 to 
be supplied to portion 60 of semiconductive web 62 by auxiliary power 
supply 54 is primarily though indirectly determined by the conductivity of 
semiconductive material 62. Ideally, the effective current passing through 
portion 60 of web 62 should be zero which means current I.sub.1 from 
auxiliary power supply 54 should be exactly equal in magnitude and 
opposite in direction to current I.sub.2 flowing from primary power supply 
46, a magnitude that is primarily determined by web 62 conductivity. 
However, as a practical matter the magnitude of current I.sub.1 is 
empirically determined by such things as the desired electrical potential 
level on backing roller 38 and/or portion 60 of web 62 by differential 
current I.sub.2 minus I.sub.1. Current I.sub.2 is dependent upon the 
conductivity of web 62 and the magnitude of current I.sub.1 is adjusted 
until it approximates current I.sub.2. Obviously, if current I.sub.2 were 
not opposed by current I.sub.1, excessive semiconductive-web 62-damaging 
heat would be generated because of the low electrical resistance 
(relatively high conductivity) offered to current I.sub.2 in the gap 
between applicator and backing roller by said semiconductive-web 62, for 
the desired level of voltage-dependent electrostatic assist. In many 
semiconductive material coating applications the heat generated by a 
differential current (I.sub.2 -I.sub.1) of up to 5ma is acceptable. As web 
62 moves in direction 86 through gap 44, and electrostatic field 88 in 
said gap 44, coating fluid 90 from coating applicator 42 is uniformly 
deposited on semiconductive web 62 with the aid of the assisting forces 
provided by electrostatic field 88. 
DISCUSSION 
A low electrical impedance in coating gap 44 in the semiconductive material 
coating apparatus of FIG. 2A will normally cause the potential on backing 
roller 38 in said FIG. 2A to be maintained at a level that is 
substantially below that necessary for effective coating-gap assist. By 
directing currents I.sub.1 and I.sub.2 through gap 44 in opposite 
directions with respect to one another the electrical impedance of gap 44 
is increased thereby enabling higher gap assisting electrical potentials 
to be employed in, for example, said backing roller 38. 
The magnitude of electrostatic field 88 in coating gap 44 of FIG. 2A and 
the coating assisting forces produced by said field 88 are primarily 
dependent upon the voltage across and not the current through said gap 44. 
Therefore, when auxiliary current I.sub.1 is passed through portion 60 of 
semiconductive web 62 in a direction opposite to that of primary power 
supply current I.sub.2 in order to neutralize the effects that would 
otherwise be produced in web 62 by said current I.sub.2 if it were not so 
neutralized by said current I.sub.1, a desired voltage differential in the 
vicinity of 3KV DC or more can be maintained across gap 44 in order to 
generate a coating assisting electrostatic field in said gap 44, and 
without causing excessibe current-related heat to be produced in 
semiconductive web 62. 
The term semiconductive material employed herein when describing the 
preferred embodiment of the present invention encompasses an extremely 
wide range of material resistances. Semiconductive materials are normally 
considered those that have an electrical resistance greater than that of a 
pure conductor but less than 1.times.10.sup.10 ohms. However, the actual 
ohmic value of the material to be coated is not the controlling factor. 
The primary considerations are the desired voltage level across the 
coating gap and/or the level of heat that would be produced in the 
semiconductive material for any given level of coating gap voltage. The 
lower the semiconductive material resistance the higher the magnitude of 
current-related heat that will be produced without an auxiliary current 
and the higher must be the magnitude of said auxiliary current to 
neutralize the effects of such heat. 
Power supplies 46 and 54 have bee described above in the preferred 
embodiment of the present invention as two separate power supplies. 
However, a single power supply capable of supplying the currents and 
voltages provided by power supplies 46 and 54 may also be utilized. 
When a potential difference is established between backing roller 38 and 
applicator 42 in, for example, FIG. 2A, said roller 38 and said applicator 
42 are sometimes referred to herein as electrodes. 
The term "electrostatic field" employed herein means one species of 
electric field. 
It will be apparent to those skilled in the art from the foregoing 
description of my invention that various improvements and modifications 
can be made in it without departing from its true scope. The embodiments 
described herein are merely illustrative and they should not be viewed as 
the only embodiments that might encompass my invention.