Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-04-06
2002-04-09
Parker, Fred J. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S533000, C427S600000, C427S420000
Reexamination Certificate
active
06368675
ABSTRACT:
TECHNICAL FIELD
This invention relates to an electrostatically assisted coating method and apparatus. More specifically, the invention relates to using electrostatic fields at the point of coating fluid contact with a moving web to achieve improved coating process uniformity.
BACKGROUND OF THE INVENTION
Coating is the process of replacing the gas contacting a substrate, usually a solid surface such as a web, by one or more layers of fluid. A web is a relatively long flexible substrate or sheet of material, such as a plastic film, paper or synthetic paper, or a metal foil, or discrete parts or sheets. The web can be a continuous belt. A coating fluid is functionally useful when applied to the surface of a substrate. Examples of coating fluids are liquids for forming photographic emulsion layers, release layers, priming layers, base layers, protective layers, lubricant layers, magnetic layers, adhesive layers, decorative layers, and coloring layers.
After deposition, a coating can remain a fluid such as in the application of lubricating oil to metal in metal coil processing or the application of chemical reactants to activate or chemically transform a substrate surface. Alternatively, the coating can be dried if it contains a volatile fluid to leave behind a solid coat such as a paint, or can be cured or in some other way solidified to a functional coating such as a release coating to which a pressure-sensitive adhesive will not aggressively stick. Methods of applying coatings are discussed in Cohen, E. D. and Gutoff, E. B., Modern Coating and Drying Teclmology, VCH Publishers, New York 1992 and Satas, D., Web Processing and Converting Technology and Equipment, Van Vorstrand Reinhold Publishing Co., New York 1984.
The object in a precision coating application is typically to uniformly apply a coating fluid onto a substrate. In a web coating process, a moving web passes a coating station where a layer or layers of coating fluid is deposited onto at least one surface of the web. Uniformity of coating fluid application onto the web is affected by many factors, including web speed, web surface characteristics, coating fluid viscosity, coating fluid surface tension, and thickness of coating fluid application onto the web.
Electrostatic coating applications have been used in the printing and photographic areas, where roll and slide coating dominate and lower viscosity conductive fluids are used. Although the electrostatic forces applied to the coating area can delay the onset of entrained air and result in the ability to run at higher web speeds, the electrostatic field that attracts the coating fluid to the web is fairly broad. One known method of applying the electrostatic fields employs precharging the web (applying charges to the web before the coating station). Another known method employs an energized support roll beneath the web at the coating station. Methods of precharging the web include corona wire charging and charged brushes. Methods of energizing a support roll include conductive elevated electrical potential rolls, nonconductive roll surfaces that are precharged, and powered semiconductive rolls. While these methods do deliver electrostatic charges to the coating area, they do not present a highly focused electrostatic field at the coater. For example, for curtain coating with a precharged web, the fluid is attracted to the web and the equilibrium position of the fluid/web contact line (wetting line) is determined by a balance of forces. The electrostatic field pulls the coating fluid to the web and pulls the coating fluid upweb. The motion of the web creates a force which tends to drag the wetting line downweb. Thus, when other process conditions remain constant, higher electrostatic forces or lower line speeds result in the wetting line being drawn upweb. Additionally, if some flow variation exists in the crossweb flow of the coating fluid, the lower flow areas are generally drawn further upweb, and the higher flow areas are generally drawn further downweb. These situations can result in decreased coating thickness uniformity. Also, process stability is less than desired because the fluid contact line (wetting line) is not stable but depends on a number of factors.
There are many patents that describe electrostatically-assisted coating. Some deal with the coating specifics, others with the charging specifics. The following are some representative patents. U.S. Pat. No. 3,052,131 discloses coating an aqueous dispersion using either roll charging or web precharging, U.S. Pat. No. 2,952,559 discloses slide coating emulsions with web precharging, and U.S. Pat. No. 3,206,323 discloses viscous fluid coating with web precharging.
U.S. Pat. No. 4,837,045 teaches using a low surface energy undercoating layer for gelatins with a DC voltage on the backup roller. A coating fluid that can be used with this method include a gelatin, magnetic, lubricant, or adhesive layer of either a water soluble or organic nature. The coating method can include slide, roller bead, spray, extrusion, or curtain coating.
EP 390774 B1 relates to high speed curtain coating of fluids at speeds of at least 250 cm/sec (492 ft/min), using a pre-applied electrostatic charge, and where the ratio of the magnitude of charge (volts) to speed (cm/sec) is at least 1:1.
U.S. Pat. No. 5,609,923 discloses a method of curtain coating a moving support where the maximum practical coating speed is increased. Charge may be applied before the coating point or at the coating point by a backing roller. This patent refers to techniques for generating electrostatic voltage as being well known, suggesting that it is referring to the listed examples of a roll beneath the coating point or previous patents where corona charging occurs before coating. This patent also discloses corona charging. The disclosed technique is to transfer the charge to the web with a corona, roll, or bristle brush before the coating point to set up the electrostatic field on the web before the coating is added.
FIGS. 1 and 2
show known techniques for electrostatically assisting coating applications. In
FIG. 1
, a web
20
moves longitudinally (in the direction of arrows
22
) past a coating station
24
. The web
20
has a first major side
26
and a second major side
28
. At the coating station
24
, a coating fluid applicator
30
laterally dispenses a stream of coating fluid
32
onto the first side
26
of the web
20
. Accordingly, downstream from the coating station
24
, the web
20
bears a coating
34
of the coating fluid
32
.
In
FIG. 1
, an electrostatic coating assist for the coating process is provided by applying electrostatic charges to the first side
26
of the web
20
at a charge application station
36
spaced longitudinally upstream from the coating station
24
(the charges could alternatively be applied to the second side
28
). At the charge application station
36
, a laterally disposed corona discharge wire
38
applies positive (or negative) electrical charges
39
to the web
20
. The wire
38
can be on either the first or second side of the web
20
. The coating fluid
32
is grounded (such as by grounding the coating fluid applicator
30
), and is electrostatically attracted to the charged web
20
at the coating station
24
. A laterally disposed air dam
40
can be disposed adjacent and upstream of the coating station
24
to reduce web boundary layer air interference at the coating fluid web interface
41
. The corona wire could be aligned in free space along the web (as shown in
FIG. 1
) or alternatively, could be aligned adjacent the first side of the web while the web is in contact with a backing roll at the coating station.
FIG. 2
shows another known electrostatically assisted coating system. In this arrangement, a relatively large diameter backing roll
42
supports the second side
28
of the web
20
at the coating station
24
. The backing roll
42
can be a charged dielectric roll, a powered semiconductive roll, or a conductive roll. The conductive and semiconductive rolls can be charged by a high volta
Erickson Luther E.
Louks John W.
Wang Sharon
3M Innovative Properties Company
Parker Fred J.
Szymanski Brian E.
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