CA1193226A - Coating uniformity improvement technique - Google Patents

Abstract

ABSTRACT
The uniformity of a coating on a coated material having an electrostatic charge thereon is substantially improved by subjecting said coated material to an electrosatic field after a coating fluid has been placed on said material and while said coating material is still in its fluid state.

Description

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B~CKGI~OllND OF TIIE IMVENTIO~I
1 Field o~ the Invention . . . ......... ~, The present invention relates to a method and apparatus ~or ilnproving -~he uni~ormity o~ a coating material a~ter i~ has ' 5 been applied to a ch~rge-retainincJ material, in general, and to such apparatus ~or improvin~ the uniformity o~ a coating m~terial that has been applied to a moving web of such material, in , paxticular.
~ 2. Description oE the Prior Art '~ 10 In the manuacture o~ various coated products, it is often essential that coating materials applied to such products be of uniform thickness and/or have a smooth or planar su~ace. In, for example, the continuous manufacture of coated photographic sheet materi~l, a nonuniform thickness coatlng applied to a moving web of such material would re~uire consider-ably more dryin~ time for drying the thicker portions of said nonuniform coating than would be re~uired for drying the thinner portions of said nonuniform coating. In addition, a temperature gradien~ that is optimum for dryiny said thicker coatiny portions is oEten excessive for optimum drying of said thinner coating portions. Drying time is usually the major ~actor limiting maximum production rates of many coated products.
Also, many properties of photographic Eilm such as sensitivity to liyht, color saturation, etc., for example, can be adversely a~ected whe~l constructed with nonuniormly coated sheet ma terials .
Mechanical devices generally employed in the web ~oatiny art, such as doctor blades, scrapexs and the like, h,-ve controlled the uni~ormity o~ web coating thickness to a !

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limited degree. Ilowever, in the produc-tion 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 o-f a finished photographic film product.
One of the most effective coating thickness control techniques in present day use in the coating industry involves the employment of an electrostatic field to assist in the uniform deposition of 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 materials can flow onto a par-ticular surface of said web. An electrostatic field is established across the gap between the coating applicator and said backing roller by a high voltage power supply whose output terminals are connected be-tween said applicator and said roller. The electrostatic field in said coating causes a coating of uniform thickness to be deposited on a particular web surface. The magnitude of the voltage established be-tween said applicator and said roller is normally less than that required to generate corona, but often exceeds 3KV DC.
An electrostatic coating-gap assist technique is known wherein an electrostatic charge is placed on material to be coated prior to and/
or when said material is remote from -the gap wherein the actual coating operation takes place. A relatively intense electrostatic field is produced between the electrostatically charged ~3~6 rnaterial to be coated and an electrically conductive refer~ncc memb~r conn~cted to a low or cJround potenkial as ~aid eleckro-; s-tatically charged ma-terial is rnoved through the coating cJaE) between a support or backing roller and in cl4se pxoximit~ to ¦ 5 said reference member ~or coatincJ purposes. The re~erence member may be formed b~ the applicator, the coating fluid or j by a completely separate membex. The electrostatic field causes a coating layer of uniform thickness to be deposited on the material ~o be coated across a wide range or coatinc3 gaps without presenting an explosion or shock hazard to per-sonnel, and without causing damage to or heing subject to interrup~ions by imperfections in the material to be coated.
! El~ctrostatic fields utili~ed in a manner such as those described above can greatly improve the thicknes and/or sur~ace uniformity of a layer of coatinc3 matexial. However, the use of an electrostatic field ~ox coating improvement purposes will o~ten cause changes in coating properties such as sur~ace tension and/or the residual eleckxostatic charge i on th~ material to be coated, that can limit the extenk to j 20 which coating uniformity can be improved with an electrostatic j Eield. Electrostatic charyes present on a coated material, or coatiny ~luid on a coating material having an electrostatic ield related change in such properties as surface tension, e~c., ~ for whatever reason or reasons, can also limit the extPnt ko ; 25 which the uniformity o~ a coatiny material can be improved.
SI~MMARY OF T~IE INVENTION
In accordance with the teachings o~ the present inv~n~ion, a method and apparatus are provided for substantiall~
improviny the coating uniformity of an electrically conduc-~ive coatincJ mat~rial that has been applie~d to a rnatexial to be --3~

coated.
According to a first broad aspect oE -the invention, there is provided appara-tus for improving the uniformity of a coating applied a-t least in part as a Eluid on charge-retaininy base material, said apparatus comprisiny charging means ~hen energiged for producing an electric field between said coating and said base material while said coating remains in an es-sentially fluid state.
According to a second broad aspect of the present invention, there is provided apparatus for improviny the uni-formity of a wet electrically conductive coatiny that has been applied to a charge-retaininy;material, comprising anielecbrode positioned acljacent said charge-retaining material such that as said coated charge-retaining material with the wet elec-trically conduc-tive coating thereon is moved past said electrode said coated material is subject to an electric field of predetermined intensity produced be-tween said electrode and said conductive coating when a difference of electrical potential is established between said electrode and said ~ coating.
According to a third broad aspect of the present invention, there is provided a method of coating charge-retaining material comprising the steps of:
applying a coating at a coating gap in at least a partly fluid state at a given potential on a given surface of said material;
advancing said material through said coatiny yap;
and applyiny an electric field downstream of said coa-tiny gap between said material and said fluid coat:ing so as to redistribute said coating~

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The invention will now be descrihed in greater detail with reEerence to the accompanying drawings, in which:
Fig. 1 is a schematic cliayram of web coating appara-tus employing a conventional high voltage electrostatic coa--ting-gap assist technique in accordance with the teachings of the prlor art;
Fig.2 is a schematic diagram of electrostatic coating-gap assist apparatus of -the type that places an electrostatic charge on the material to be coated before it applies coating Eluid to said material;
Fig. 3 is a schematic diagram of apparatus employing web coating uniformity improvement apparatus in accordance with the present invention; and Fig.4 is an enlarged detail of the electrostatic field producing conductive bristle brush of Fig.2 and a portion of the coated material in said Fig.3 having its coating uni-formi-ty improved by the electric field established be-tween said brush and the coating material.
The present invention is directed to means for re-ducing the detrimental effects on coating uniformity produced by electrostatic charges remaining on a coated material.
These residual-type charges can be produced in several ways.
In the coating industry, for example, electrostatic fields are employed to improve coating uniformity with a satisfactory though limited degree of success. r~hile coating uniformity is substantially -4a-3~

.I im~rovcd ~lith an elcctrostatic ~iel~, ~he residual clcc~ro.c;tatic i char~es remaininy on the coat~d material lirnit the extent of said improvemen~. Elec-trostatically assis-ted coating appara~us typi.cal o~ that in present use in the coatiny industry is schematic~lly illustrated in Fig. 1. In Fig. 1, numeral 10 , gen~rally indlcates coating apparatus employing conventional . electrostatic coating-gap assist apparatus constructed in acsor-. dance with the teachiny of ~he prior art. Web suppoxt or backiny roller 12 i5 cylindrically shaped, is electrically conductive , 10 and is mounted for rota~ion about backiny roller axis 14.
. Coa~iny applicator 16 is mounted in a fixed position with respect to backing roller 12 and is spaced ~rom said roller 12 , by a distance or yap 18. Iligh voltage power supply 20, having . a DC voltacJe across its output terminals that is often in thenei~hborhood o~ several thousand volts, has said output terminals connected between backing roller 12 and applicator 16 through pa~hs 22 and 24, respectively. Because the coating fluid supplied by applicatox 16 ls electrically conductive, it often maintains said applicator 16 at or near ground potential through a coatiny-fluid-supplying conduit lnot shown), the high voltage terminal oE power supply 20 is necessarily connected to said roller 12 and the low voltage terminal of said supply 23 is 1 connected to said grounded applicator 16.
, When power supply 20 is energized through path 25, i 25 electros~a-tic field 26 is produced in coating gap 18 between high potential backing xollex 12 and yrounded applicator 16.
~s charge~retainlng web 28 is moved in direction 30 through yap 18 by drive means (not shown), said web 28 is el~ctro-i statically charged by orienting its dipoles (such as by orienting di~oles 31) by said electros-tatic :Eield 26. Electrostatic char~es .5~

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produced OJI web 28 by clectrostatic field 26 cause fluid 32 flowing from applicator 16 into coating ga-p 18 -to be attracted toward and uniformly deposited on moving web 28.
An extremely important factor in the web coating process is the maintenance oF an appropriate amount of coating material 32 in gap 18 for proper web-coating purposes. This portion of the coating material 32 is sometimes referred to as a coating fluid bead and is designated numeral 34 in prior art Figure 1. The surface of web 28 normally moves faster than the rate at which coating fluid 32 flows 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 intermediate level. It is believed that electrostatic field 26 changes properties of coating fluid 32 such as surface tension, thereby allowing said fluid 32 to be stretched to a greater degree and over a larger gap between web 28 and applicator 16 without losing (breaking) bead 34 than would be possible if electrostatic gap-assisting field 26 were not present. In addition to the primary COII-tribution of providing the desired layer thickness on web 28, gap 18 in Figure 1 must be large enough to accommodate such things as web splices and foreign matter so that such splices or matter do not come into contact with applicator 16 and thereby adversely affect web coating quality.
Another type of electrostatically assisted coating apparatus is schema-tically illustrated in Figure 2. In Figure 2, numeral 36 generally indicated web coating apparatus employing a precharged ,, ~ 6-web coating teclmique. In Figure 2, web support or backing roller 38 is cylindrically shaped, is electrically condllctive, is mounted for rotation about backing roller axis 40 and for safety purposes is electrical]y grounded through path 41 to prevent said roller from o-perating like a high potential producing Van de Graff generator.
Coating applicator 42 is mounted in a fixed position with respect to backing roller 38 and is spaced from said roller 38 by distance or gap 44. Grounded web support or backing roller 46 is cylindri-cally shaped, is electrically conductive and grounded, and is mounted for rotation about backing roller axis 48. Conductive bristle brush 50 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 46~ DC power supply 52 has its high voltage output terminal connected to one end of each of the bristles of said con-ductive bristle brush 50 through path 54 and has its low voltage out-put terminal connected to grounded backing roller 46 through path 56 and common ground points 58.
When power supply 52 is energized through path 60, a rela-tively intense electrostatic field is es-tablished between the free ends of the bristles of said conductive bristle brush 50 and roller 46 with a relatively low voltage. A similar but more limited disclo-sure of a conductive bristle brush electrostatic charge controlling technique is contained at page 70 in the February 1980 issue of Research Disclosure.

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As char~e-retainincJ web 62 is moved in direction G~
~hrou~h the rela-tively intense electrostatic field established between enercJi~ed conductive bris~le brush 50 and ~Jround~d backing roller 46 b~ drive m~ans (not shown), an electrostatic charcJe of a pxedetermined macJnitude is esta~lished on s~id ~rc~
62. This electrostatic charge results from the orientation of ¦ dipoles in web 62 (such as oriented dipoles 66) th~t were 50 ¦ oriented when web 62 was m~ved through the electrostatic fieldbetween the free ends of conductive bristle brush S0 and roller ¦ 10 46. Conductive bris~le brush 50 and backing roller 46 may be ¦ spaced a considerable distance from applicator ~2 and its associated backing roller 38 as sehematieally emphasized by the artificial break in web 62 and by par~ition 68 passing through said artificial break because of tlle relative stability of the char~e placed on web 62 by brush 50.
Undesirable residual electros~atic charges will normally remain on a material that has been coated by means of electrostatically assisted coating apparatus such as those described above and schematically illustrated in Figs. 1 and

2. Even i~ such electrostatic charge producing coating apparatus are not employed, coating uniformity can be adversely ~ affected by electrostakic charges present on coated material

3 produGed by other means such as by handling or by a coating machine, as said material is routed through same for coating purpos s~ In Fig. 3 a coated web is illustrated that is assumed to have th.is undesirable elec~rostatic charge khereon. The ; primary significance of this chaxge is the detrimental effectstha-t it has on such -thinys as coating thickne.ss and/or sur~ace uniformity and not ~he actual mechanism that produced such a charcJe z~

Turning to FicJ. 3, numeral 70 generally indicated ~Jeb coating apparatus employincJ coating uniformity improvement means constructed in accordance with the presen1- invention. In Fig. ~, web support or backiny roller 72 is c~lindrically shaped, is electrically conductive and is mounted ~or rotation about back-incJ roller axis 74. Backing roller 72 may or rnay no~ be yrounded depending upon whether or not an electrostatically assisted coatiny technique is employed and if employed, the particular type of electrostatic assist ~echnique selected. Coating appli-CatQr 76 is electrically grounded through either the coating ~luid conduit ~not shown~ or through path 77, is mounted in a fixed position with respect to backing roller 72 and is spaced ~rom said roller 72 by distance or gap 78. An intermediate portion o~ elongated sheet or web of charge retaining material ~0 is supported by backing roller 72 in said gap 78 in a spaced relation from said applicator 76. Conduc~ive bristle brush 82 is mounted in a ~ixed positi~n with respect to, and has the ~ree ends of its bristles spaced from surface 84 of said web 800 ~C power supply 86 has its high voltage output terminal connected to one end o~ each o~ the bristles o~ said conductlve brush 82 through path 88 and has its low voltage output terminal electrically grounded through path 90.
, As charge retaining material or web 80 is moved in direction 92 through coating gap 78, coating fluid 94 is deposited on said web 80 by coatiny ~luid applicatox 76. The coating process may or ma~ not be assisted by an electrostatic Eield.
llowever, under normal conditions a substantially larger residual , electrosta-tic charge and substantially greater change in coating fluid properties will be present in a coated material and its ¦ 30 coatin~, respectivel~, when an electrostati~ field is employed in a web coatiny process than when such a field is not so employed.

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When power supply 86 is energized through path 96, a rela-tively in-tense electrostatic f;eld is established betwcen the Eree ends of the bristles of said conductive bristle brush 82 and elec-trically conduct-ive coating fluid 94 grounded through applicator 76 and i-ts associated fluid-transporting conduit (not shown), or through path 77, when a portion of coating material 94 is eventually moved into the vicinity of brush 82 by moving web 80 to which it has been applied. The method of application and the e:Efects of the electro-static field established between brush 82 and coating fluid 94 on said fluid 94 are schematically illustrated in Figure 4.
Turning now to Figure 4, which is an enlarged detail of energized conductive bristle brush 82 and a portion of coated web 80 immediately adjacent said brush 82, said coated web 80 is moved in direction 92 through the electrostatic field established between said brush 82 and coating fluid 94 on said web 80. As shown in said Figure

4, surface 96 of coating fluid 94 is relatively uneven or nonuniform after it has been applied to web 80 but before coating fluid 94 with its said nonuniform surface 96 is subjected to the electrostatic field of brush 82. The magnitude and polarity of this electrosta.tic field is normally established empirically and is primarily determined by the type of material to be coated and the type coating material to be appli.ed.
When web 80 together with coa~ing 94 moves in direction 92 through the electrostatic field between brush 82 and coating fluid 94 while said coating fluid 94 is still in its ~3~6 fluid sta-te, rela~ively nonuni~orm surface 96 of saLd co~tirlcJ
~luid 94 is t~ansformed into relatively uni~orrn surface 9~ b~
the elec-tros-tatic field of said brush 82. The electrostatic field o~ brush 82 chanyes the electrostakic charge level or charcJe-retaining web 80 and it i5 believed, changes ~he sur~ac~
tension of coating fluid 9~ while said coatiny fluid is still in its 1uid state thereby increasing coatiny fluid fluidit~ and decreasing surface roughness or nonuniformity by reason of the increased coating fluid flow resulting from the said brush 8~ electrostatic field-produced chan~e i~ coating fluid 94 fluidity.
DISCUSSION
¦ The electrostatic fielcl associated with brush 82 in the coating uniformity improvement apparatus of the present 1 15 invention must be positioned such that it in~eracts with the ¦ charge retaining material having the residual electrostatic ¦ char~es that adversely affect coating fluid surface uniformity.
With respect to sheet of charge-retaining material 80 schematically illustrated in Fig, 3, the free ends of conductive bristle brush 82 are optimally located adjacent surface 84 of ~ said sheet 80 which is the side that is directly opposite the j side on which coating material 94 is locatedO In ~his position the electrostatic field established between brush 82 and I coating fluid 94 can most effectively change the electrostatic char~e level on web 80 and it is believed, change such propexties , as -the surface tension of coatiny fluid 9~.
The web coating uniformit~ improvement apparatus of the present invention employs the electrically conductive coa~ing material itself as a ground or electrically conductive reference member in conjunction with a conductive bristle brush to establish ~3~

~he clesired charge-controlling electrostatic Eield. This use o~ coating fluid 94 is necessary because the coatiny ~luid is necessarily in its fluid st~te when it is subjected to -the electrostatic Eield of bxush 82 for coating irnprovement S purposes and i~ an alterna~ reference or ground member were employed it would adversely e~fect coating fluid thickness and surface cluali-ty if it were placed in contact with the coating fluid while said ~luid was still .in its said fluid state~
When a potertial difference is established between brush 82 and coating fluid 94 in, for example, Fig. 3, said brush 82 is sometimes referred to herein as an electrode. Also th~
term "electrostatic field" employed herein means one species of electric ~ield.
It will be apparent to those skilled in the art from the foregoing description of my invention that various improve-ments and modifications can be made in it without departing from its true scope. The em~odiments described herein are merely illustrative and should not be viewed as the only embodiments that might encompass my invention.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for improving the uniformity of a wet elec-trically conductive coating that has been applied to a charge-retaining material, comprising an electrode positioned adjacent said charge-retaining material such that as said coated charge-retaining material with the wet electrically conductive coating thereon is moved past said electrode said coated material is subject to an electric field of predetermined intensity produc-ed between said electrode and said conductive coating when a difference of electrical potential is established between said electrode and said coating.

2. The apparatus of claim 1, wherein the said potential of said electrode is more positive than the potential of said conductive coating.

3. The apparatus of claim 1, wherein the said potential of said electrode is more negative than the potential of said conductive coating.

4. The apparatus of claim 1, 2 or 3 wherein said elec-trode is a conductive bristle brush.

5. The apparatus of claim 1, wherein said electric field is an electrostatic field.

6. Apparatus for improving the uniformity of a coating applied at least in part as a fluid on charge-retaining base material, said apparatus comprising charging means when energi-zed for producing an electric field between said coating and said base material. while said coating remains in an essentially fluid state.

7. The improvement of claim 6, wherein said fluid is ap-plied on a given surface of said material at a given potential, and said charging means includes an electrode positioned in ad-joining relation to an opposite surface.

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8. The improvement of Claim 6, wherein said fluid is applied to a moving web of said material on a given surface thereof by an electrostatically assisted coating gap, and said charging means includes an electrode positioned just downstream of said coating gap in adjoining relation to a surface of said material opposite said given surface.

9. A method of coating charge-retaining material comprising the steps of:
applying a coating at a coating gap in at least a partly fluid state at a given potential on a given surface of said material;
advancing said material through said coating gap;
and applying an electric field downstream of said coating gap between said material and said fluid coating so as to re-distribute said coating.

10. The method of claim 9, wherein said field applying step includes energizing an electrode mounted in adjoining relation to an opposite surface of said web at a potential different from said given potential,