TECHNICAL FIELD
[0001] 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
[0002] 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.
[0003] 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 Technology, VCH Publishers, New York 1992 and Satas, D., Web Processing
and Converting Technology and Equipment, Van Vorstrand Reinhold Publishing Co., New
York 1984.
[0004] 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.
[0005] 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.
[0006] 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. Patent No. 3,052,131 discloses coating an aqueous
dispersion using either roll charging or web precharging, U.S. Patent No. 2,952,559
discloses slide coating emulsions with web precharging, and U.S. Patent No. 3,206,323
discloses viscous fluid coating with web precharging.
[0007] U.S. Patent 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.
[0008] EP 390774 B 1 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.
[0009] U.S. Patent 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.
[0010] 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.
[0011] 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.
[0012] 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 voltage power supply. With a dielectric roll, the roll
can be provided with electrical charges by suitable means, such as a corona charging
assembly 43. Regardless of the type of backing roll 42 or its means of being charged,
its outer cylindrical surface 44 is adapted to deliver the electrical charges 39 to
the second side 28 of the web 20. As shown in FIG. 2, the electrical charges 39 from
the backing roll 42 are positive charges, and the coating fluid 32 is grounded by
grounding the coating fluid applicator 30. Accordingly, the coating fluid 32 is electrostatically
attracted to charges residing at the interface between the web 20 and the outer cylindrical
surface 44 of the roll 42. The air dam 40 reduces web boundary layer air interference
at the coating fluid web interface 41.
[0013] Known electrostatically assisted coating arrangements such as those shown in FIGS.
1 and 2 assist the coating process by delaying the onset of air entrainment and improving
the wetting characteristics at the coating wetting line. However, they apply charges
to the web at a location substantially upstream from the wetting line, and generate
fairly broad electrostatic fields. They are largely ineffective in maintaining a straight
wetting line when there are crossweb coating flow variations or cross-web electrostatic
field variations. For instance, in a curtain coater, if a localized heavy coating
fluid flow area occurs somewhere across the curtain, the wetting line in this heavier
coating region can move downweb in response depending on materials or process parameters.
This can create an even heavier coating in this area due to stress and strain on the
curtain, especially for fluids which exhibit elastic characteristics (more elastic
fluids have high extensional viscosity in relation to shear). In addition, if the
electrostatic field is not uniform (e.g., there is a corona web precharge non-uniformity),
the lower voltage area on the web will allow the wetting line in that area to move
downweb, thus increasing the coating weight in that area. These effects become increasingly
dominant as fluid elasticities increase. Thus, crossweb fluid flow variations and
crossweb electrostatic field variations cause non-uniformity in the wetting line and,
as a result, the application of a non-uniform coating on the web.
[0014] None of the known apparatus or methods for electrostatically assisted coating discloses
a technique for applying a focused electrical field to the web at the coating station
from an electrical field applicator to improve the characteristic of the applied fluid
coating and also to attain improved processing conditions. There is a need for an
electrostatically assisted coating technique that applies a more focused electrical
field to the web at the coating station.
SUMMARY OF THE INVENTION
[0015] The invention is a method of applying a fluid coating onto a substrate. The substrate
has a first surface on the first side thereof and a second surface on a second side
thereof. The method includes providing relative longitudinal movement between the
substrate and a fluid coating station, and forming a fluid wetting line by introducing,
at an angle of from 0 degrees through 180 degrees, a stream of fluid onto the first
side of the substrate along a laterally disposed fluid-web contact area at the coating
station. An electrical force is created on the fluid from an effective electrical
field originating from a location on the second side of the substrate that is substantially
at and downstream of the fluid wetting line, without requiring electrical charges
to move to the substrate while attracting the fluid to the first surface of the substrate
via electrical forces.
[0016] The creating step can include electrically energizing an electrode on the second
side of the substrate to form the effective electrical field from electrical charges.
In one embodiment, the effective electrical field is defined by a portion of the electrode
which has a radius of no more than 1.27 cm (or, in one preferred embodiment, no more
than 0.63 cm).
[0017] The substrate can be supported, adjacent the fluid coating station, on the second
side thereof, or can be supported by the electrode itself.
[0018] The stream of fluid can be formed with a coating fluid dispenser such as a curtain
coater, a bead coater, an extrusion coater, carrier fluid coating methods, a slide
coater, a knife coater, a jet coater, a notch bar, a roll coater or a fluid bearing
coater. The stream of coating fluid can be tangentially introduced onto the first
surface of the substrate.
[0019] The electrical charges of the electrode can have a first polarity and second electrical
charges (having a second, opposite polarity) can be applied to the stream of fluid
before the stream of fluid is introduced onto the substrate.
[0020] The creating step can include electrically energizing an electrode and also acoustically
exciting the electrode. In one preferred embodiment, the electrode is acoustically
excited at ultrasonic frequencies.
[0021] The inventive method is also defined as a method of applying a fluid coating onto
a substrate, where the substrate has a first side and a second side. The inventive
method includes providing relative longitudinal movement between the substrate and
a fluid coating station. A stream of fluid is introduced, at an angle of 0 degrees
through 180 degrees, onto the first side of the substrate to form a fluid wetting
line along a laterally disposed fluid-web contact area at the coating station. The
invention further includes attracting the fluid to the first side of the substrate
at a location on the substrate that is substantially at and downstream of the fluid
wetting line by electrical forces from an effective electrical field originating at
a location on the second side of the substrate.
[0022] The invention is also an apparatus for applying a coating fluid onto a substrate
which has a first surface on a first side thereof and a second surface on a second
side thereof. The apparatus includes means for dispensing a stream of coating fluid
onto the first surface of the substrate to form a fluid wetting line along a laterally
disposed fluid contact area. A field applicator extending laterally across the second
side of the substrate (generally opposite the fluid wetting line) bears electrical
charges, and applies an effective electrical field to the substrate at a location
on the substrate that is substantially at and downstream of the fluid wetting line
to attract the fluid to the first surface of the substrate. The effective electrostatic
field primarily emanates from electrical charges on the electrical field applicator
rather than electrical charges transferred to the substrate.
[0023] The electrical field applicator can include a small diameter rod, a conductive strip,
or a conductive member with a small radius portion for use in defining the effective
electrical field. An air bearing can extend laterally across the substrate adjacent
the electrical field applicator for supporting and aligning the second side of the
substrate relative to the electrical field applicator.
[0024] In another embodiment, the invention is defined as a method of applying a fluid coating
onto a substrate which has a first surface on a first side thereof and a second surface
of a second side thereof. The method includes providing relative longitudinal movement
between the substrate and a fluid coating station, forming a fluid wetting line by
introducing, at an angle of 0 degrees through 180 degrees, a stream of fluid onto
the first surface of the substrate along a laterally disposed fluid-web contact area
at the coating station, exposing the coating fluid (adjacent the coating station)
to an electrical force to attract the fluid to the substrate, and exposing the coating
fluid (adjacent the coating station) to an acoustical force to attract the coating
fluid to the substrate.
[0025] In another embodiment, the invention is an apparatus for applying a coating fluid
onto a substrate having relative longitudinal movement with respect to the apparatus.
The substrate has a first surface on the first side thereof and a second surface on
the second side thereof. A coating fluid applicator dispenses a stream of coating
fluid onto the first surface of the substrate to form a fluid wetting line along a
laterally disposed fluid contact area. An electrical field applicator applies an electrostatic
field at a location on the substrate adjacent the fluid wetting line to attract the
coating fluid to the first surface of the substrate. An acoustical field applicator
applies an acoustical field at a location on the substrate adjacent the fluid wetting
line to attract the coating fluid to the first surface of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026]
FIG. 1 is a schematic view of a known electrostatic coating apparatus where charges
are applied to the moving web before it enters a coating station from an upweb corona
wire.
FIG. 2 is a schematic view of a known electrostatic coating apparatus where charges
are delivered to the moving web from a backing roll under the moving web at the coating
station.
FIG. 3 is a schematic view of one embodiment of the electrostatically assisted coating
apparatus of the present invention where the effective electrostatic field is defined
by a lateral electrode adjacent the coating fluid wetting line in combination with
an air bearing assembly.
FIG. 4 is an enlarged view of the air bearing assembly with the electrode of FIG.
3.
FIG. 5 is an enlarged schematic view of a portion of FIG. 2 illustrating the applied
electrostatic charges and lines of force.
FIG. 6 is an enlarged schematic view of a portion of FIG. 3 illustrating the electrostatic
lines of force of the effective electrical field.
FIG. 7 is a schematic view of another embodiment of the electrostatically assisted
coating apparatus of the present invention, illustrating one application of its use
for tangential curtain coating.
FIG. 8 is an enlarged schematic illustration of an air bearing and electrostatic field
generation system with multiple electrodes.
FIG. 9 is a schematic view of a tangential coating test arrangement with a prior art
sized powered roll.
FIG. 10 is a schematic view of another embodiment of the electrostatically assisted
coating apparatus of the present invention, in a generally tangential coating configuration.
FIG. 11 is an enlarged schematic illustration of the electrode assembly of FIG. 10.
FIG. 12 is a schematic view of another embodiment of the electrostatically assisted
coating apparatus of the present invention, where the effective electrostatic field
is defined by a one-inch diameter backing roll.
FIG. 13 is a schematic view of an inventive electrostatic field electrode which is
combined with an ultrasonic horn.
FIG. 14 illustrates the "dynamic contact angle" of fluid coating onto a web.
[0027] While some of the above-identified drawing figures set forth preferred embodiments
of the invention, other embodiments are also contemplated, as noted in the discussion.
In all cases, this disclosure presents the invention by way of representation and
not limitation. It should be understood that numerous other modifications and embodiments
can be devised by those skilled in the art, which fall within the scope and spirit
of the principles of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] This invention includes an apparatus and coating method which use more focused electrostatic
fields at the interface between a substrate (such as a web) to be coated and a fluid
coating material applied on the substrate. The inventors have found that more focused
electrostatic fields can improve the coating process by stabilizing, straightening,
and dictating the position of the coating wetting line, allowing wider process windows
to be achieved. For example, the invention makes possible a wider range of coating
weights, coating speeds, coating geometries, web features such as dielectric strengths,
coating fluid characteristics such as viscosity, surface tension, and elasticity,
and die-to-web gaps, as well as improving cross web coating uniformity. With curtain
coating, electrostatic coating assist allows lower curtain heights (and therefore,
greater curtain stability) and allows the coating of elastic solutions which could
not previously be coated without entrained air. Focused fields greatly enhance the
ability to run coating fluids (especially elastic fluids) since they more precisely
dictate the position, linearity, and stability of the wetting line, which results
in increased process stability. In addition, thinner coatings than were previously
possible can be produced, even at lower line speeds, which is important for processes
that are drying or curing rate limited.
[0029] With extrusion coating it has been found that electrostatics permits the use of lower
elasticity waterbased fluids (such as some waterbased emulsion adhesives) that cannot
be extrusion coated absent the electrostatics (in the extrusion mode), as well as
permitting the use of larger coating gaps.
[0030] In curtain coating, the stream of fluid is aligned with the gravitational vector,
while in extrusion coating it can be aligned with the gravitational vector or at other
angles. While coating with a curtain coating process, where longer streams of fluid
are used, the coating step involves the displacing of the boundary layer air with
coating fluid and the major force is momentum based. In contrast, with extrusion coating,
where the stream of fluid is typically shorter than for curtain coating, the major
forces are elasticity and surface tension related. When using electrostatics an additional
force results which can assist in displacing the boundary layer air, or can become
the dominant force itself.
[0031] Although the invention is described with respect to smooth, continuous coatings,
the invention also can be used while applying discontinuous coatings. For example,
electrostatics can be used to help coat a substrate having a macrostructure such as
voids which are filled with the coating, whether or not there is continuity between
the coating in adjacent voids. In this situation, the coating uniformity and enhanced
wettability tendencies are maintained both within discrete coating regions, and from
region to region.
[0032] The substrate can be any surface of any material that is desired to be coated, including
a web. A web can be any sheet-like material such as polyester, polypropylene, paper,
knit, woven or nonwoven materials. The improved wettability of the coating is particularly
useful in rough textured or porous webs, regardless of whether the pores are microscopic
or macroscopic. Although the illustrated examples show a web moving past a stationary
coating applicator, the web can be stationary while the coating applicator moves,
or both the web and coating applicator can move relative to a fixed point.
[0033] Generically speaking, the invention relates to a method of applying a fluid coating
onto a substrate such as a web and includes providing relative longitudinal movement
between the web and a fluid coating station. A stream of coating fluid is introduced
onto the first side of the web along a laterally disposed fluid wetting line at a
coating station. The coating fluid is introduced at any angle of from 0 degrees through
180 degrees. An electrical force is created on the fluid from an effective electrical
field substantially at and downstream of the fluid contact area (e.g., originating
from one or more electrodes that are located on the second side of the web). Negative
or positive electrical charges may be used to attract the coating fluid. The coating
fluid can include solvent-based fluids, thermoplastic fluid melts, emulsions, dispersions,
miscible and immiscible fluid mixtures, inorganic fluids, and 100% solid fluids. Solvent-based
coating fluids include solvents that are waterbased and also organic in nature. Certain
safety precautions must be taken when dealing with volatile solvents, for example
that are flammable, because static discharges can create hazards, such as fires or
explosions. Such precautions are known, and could include using an inert atmosphere
in the region where static discharges might occur.
[0034] Instead of precharging the web or using an energized roll support system, as are
known, the preferred embodiments of the invention use an electrical field source,
such as narrow conductive electrode extending linearly in the cross-web direction,
positioned where the fluid web contact line should occur. The narrow conductive electrode
could be, for example, a small diameter rod in the range of about 0.16 - 2.54 cm (0.06-1.0
in), either rotating or non-rotating, a narrow conductive strip, a member with a sharply
defined (small radius portion) leading edge (the wetting line will typically be located
near the sharply defined leading edge), or any electrode with a geometry that presents
a focused and effective electrical field to the wetting line that is substantially
at and downstream of the wetting line. Generally, the smaller the radius, the more
focused the field. However if the radius becomes too small, increased corona generation
can occur. Rod diameters less than 0.16 cm (0.06 in) can be used as long as the applied
voltage is not high enough to create significant corona discharge. If the discharge
is too high, the predominant electrical force can come from corona charges that are
deposited on the second surface of the web. The electrode can be supported by a small
support structure such as a porous air bearing material adjacent the electrode on
the upweb and downweb sides. The web can be supported by the air bearing surface,
or by the electrode itself. The electrode can be closely spaced from the web or can
be in physical contact with the web. The electrode can also have discrete, discontinuous
crossweb support structures, or can be supported only on its ends. The electrode can
also be made of a porous conductive material.
[0035] The main attractive force for this embodiment comes from the electrostatic field
originating from the electrode, not from charges transferred to the backside of the
web by contact or spurious corona discharge. Again, the field is focused to be effective
(as an attractant for the coating fluid) substantially at or downstream of the web-fluid
contact line. The electrode on the backside of the web creates a more focused electrical
field than known electrostatic coating assist systems. Because the field does not
extend as far upweb as in the prior art (precharged webs or energized coating rolls),
the fluid is drawn to a more sharply defined wetting line, retains a more linear crossweb
profile, and stabilizes the wetting line by tending to lock it into position. This
means that the normal balance of forces that dictate the contact line position are
less important, and that non-linearities in the wetting line are less pronounced.
Thus, process variations, such as coating flow rates, coating crossweb uniformity,
web speed variations, incoming web charge variations, and other process variations
have less effect on the coating process. Typically the smaller the diameter of the
electrode or the more sharply defined the leading edge of the electrode structure,
the more focused the leading edge of the electrostatic field and wetting line linearity
will become, as long as spurious corona discharges can be kept to a minimum.
[0036] Process stability is greatly enhanced with the focused electrode field system. Typically,
if an electrostatically assisted coating system is running at a particular speed,
coating thickness, and voltage, changing one of these variables changes the wetting
line position. For example, the wetting line will shift downweb if speed is increased,
coating thickness is increased, or applied voltage is decreased, depending on the
type of coating system and fluid being coated. This can cause coating uniformity problems
and can increase the potential for air entrainment. The inventive focused field system
greatly reduces the sensitivity of the process to those variables and maintains the
wetting line at a more stable straight line position.
[0037] Many configurations of the electrode can be used in practicing the invention. FIG.
3 shows an example where a laterally extending electrode 100 is supported along the
second side 28 of the web 20. The laterally extending electrode 100 is uniformly and
closely spaced from or may be contacting the second side 28 of the web 20, longitudinally
close to the coating station 24 that includes the lateral coating fluid web contact
line 52. The web 20 is supported at the coating station 24 such as between a pair
of support rolls 54, 56. Alternatively, the web 20 can be supported at the coating
station 24 by the electrode itself, an air bearing 102 (or any suitable gas bearing,
such as an inert gas bearing), or other supports. A stream of coating fluid 32 is
delivered from the coating fluid applicator 30 onto a first surface on the first side
26 of the web 20. As shown, the coating fluid applicator 30 can be grounded to ground
the coating fluid 32 relative to the electrode 100. The air dam 40 can be any suitable
physical barrier which limits boundary layer air interference at the coating fluid
web interface or the point of coating curtain formation.
[0038] The electrode 100 may be formed, for example, from a small diameter rod or other
small dimension conductive electrode (which does not necessarily need to be round).
Preferably, the electrode 100 is disposed within the adjacent air bearing 102, which
may or may not be in contact with the air bearing. The air bearing 102 stabilizes
the web position and minimizes the web vibrations which otherwise can have an adverse
effect on coating stability and uniformity. The air bearing 102 is typically radiused
and preferably has a porous material 104 (such as porous polyethylene) in fluid communication
with an air manifold chamber 106. Pressurized air is provided to the air manifold
chamber 106 via one or more suitable inlets 108, as indicated by arrow 110. The air
flows through the air manifold chamber 106 and into the porous membrane 104. The porous
membrane 104 has a relatively smooth and generally radiused bearing surface 112 positioned
adjacent a second surface of the web 20 on the second side 28 thereof. Air exiting
the bearing surface 112 supports the web 20 as it traverses the coating station 24
and electrode 100. While an active air bearing is described, a passive air bearing
(using only the air boundary layer on the second side of the web as the bearing media)
can work at sufficiently high web speeds. The air bearing can also be a solid structure
that acts as an air bearing as substrate speeds increase and boundary layer air on
the second side of the web creates the air bearing effect. The gap between the air
bearing surface and web is a function of parameters such as the radius of the air
bearing, the web tension and speed of the web. Other known ways of creating an air
bearing can also be used such as airfoil designs commonly used in drying.
[0039] The embodiment of the electrostatic coating assist system of FIG. 3 forms a more
focused electrostatic field at the fluid-web contact area which constrains the wetting
line to a more linear profile at a desired location. The embodiment "locks" the wetting
line into a stable line extending laterally across the web (as compared to the less
effective known electrostatic coating assist systems of FIGS. 1 and 2 which provide
a less focused electrostatic attraction between the coating fluid and web). The electrostatic
field emanating from the electrode creates the main electrostatic attractive (i.e.,
effective) force on the coating fluid. Electrostatic charges are not placed primarily
from the electrode onto the web itself. Rather, their presence on the charged device,
such as an elevated potential electrode, attracts the coating fluid. It is intended
that charges not be transferred to be the web from the electrode, although in practice,
some inevitably will transfer and assist in the coating process.
[0040] Instead of grounding the coating fluid 32, an opposite electrical charge can be applied
to the coating fluid 32 such as by a suitable electrode device. In addition, the applied
polarities of the electrical charges to the coating fluid 32 and web 20 can be reversed.
This method is particularly useful when using lower electrical conductivity fluids
such as certain 100% polymer melts or 100% solids curable systems. For example, for
a low conductivity fluid, charges can be applied to the fluid before coating, whether
through the die or by a corona discharge. This system can be used when insufficient
electrostatic aggressiveness is seen due to the use of low conductivity fluids. The
ability of the inventive system to retain the fluid wetting line in a more linear
fashion results in increased coating uniformity and stability. For a conductive fluid
where the conductive path is isolated, the die potential can be raised to create the
opposite polarity in the fluid. Alternatively, the opposite polarity can be applied
to the fluid anywhere along the conductive, isolated path (including, for example,
even downstream of the wetting line).
[0041] FIG. 5 is an expanded view of the prior art system in FIG. 2, and lines of force
66 generated by the electrostatic charges relative to the coating fluid 32. For curtain
coating applications, the desired wetting line is typically the gravity-determined
coating fluid wetting line (with no electrostatics applied) when the web is stationary
(or initial coating fluid wetting line (with no electrostatics applied) when the web
is stationary) and, as illustrated in FIGS. 2 and 5, is the top dead center of the
charged roll. However, other wetting line positions are common and depend on the type
of coating die, fluid properties, and web path. The lines of force 66 indicate that
for a charged roll (like the roll 42 in FIG. 2) the forces are not well focused and
the charges are exerting forces on the coating fluid substantially upweb of the wetting
line (e.g., on upweb area 67). For example, for charged rolls that are larger than
7.5 cm (3 in) in diameter, the charges exert forces on the coating fluid substantially
upweb from the desired wetting line. However, as the delivery of charges to the web
becomes more focused, say for a one-inch diameter roll given the same potential, the
charges do not exert functional forces on the coating fluid substantially upweb from
the desired wetting line that adversely affect the wetting line uniformity (i.e.,
the charges on the web are ineffective upweb relative to the coating fluid).
[0042] FIG. 6 is an expanded view of the inventive system of FIG. 3, showing where the electrical
field is effective as an attractant for the coating fluid, as it is more focused beneath
the coating fluid contact line. In this case, the lines of force 69 are more focused,
thus creating a more sharply defined and linear wetting line which stabilizes the
fluid-web contact line by tending to lock it into position across the web travel path.
[0043] In an inventive electrostatic coating assist system such as illustrated in FIG. 3,
the electrode 100 can be positioned directly under the laterally extending coating
fluid-web contact line, which is determined by the placement (such as by gravitational
fall) of the coating fluid 32 onto the web 20. Web movement, surface tension, and
boundary layer effects on the first side of the web 20, and the elasticity of the
coating fluid 32, can cause the coating fluid web contact line to shift downweb. Because
of the strong electrostatic attraction that can be achieved with this invention, the
location of the electrode 100 will determine the operational location of the wetting
line when the electrode 100 is activated. Thus, the location of the electrode 100
(upstream or downstream from the initial coating fluid-web contact line) can cause
a corresponding movement of the contact line, as it tends to align itself with the
opposed attracted electrical charges. Preferably, the electrode 100 is positioned
no more than 2.54 cm (1.0 in) upstream or downstream from the initial coating fluid-web
contact line.
[0044] As mentioned above, the electrode may take many forms, but it is essential that it
create an effective electrical field for highly focused attraction of the coating
fluid to a desired wetting line location. This may be accomplished by forming portions
of the electrode with certain specific geometries. For example, a leading edge or
an edge adjacent the web may be formed to have a specifically tuned radius for creating
the desired electrical force field lines. In this instance, that portion of the electrode
preferably has a radius of no greater than 1.27 cm (0.5 in), and more preferably a
radius of no greater than 0.63 cm (0.25 in). Other field focusing means are also possible.
For instance, an additional electrode could be located adjacent the first electrode
so as to modify the field from the first electrode. The second electrode may be positioned
at any location, including upstream from the first electrode 100 or even on the first
side 26 of the web 20, so long as its resultant electrostatic field has the desired
focusing effect on the electrostatic field generated from the first electrode 100.
The result of focusing the electrostatic field generated by the electrode 100 is a
straighter wetting line which is less sensitive to non-uniform fluid flow or charge
variations of the electrode or on the incoming web, thereby providing a more uniform
coating and greater process tolerance to production variations.
[0045] It will be understood that the location of the electrode can be upstream or downstream
of the fluid wetting line so long as the effective electrical field is substantially
at or downstream of the fluid wetting line. For example, an electrode can be configured
so that surface charge density is higher substantially at or downstream of the fluid
wetting line to focus the effective electrical field substantially at or downstream
of the fluid wetting line. Alternatively, the effective electrical field can be focused
substantially at or downstream of the fluid wetting line by masking the upstream electrical
field with a conductive or nonconductive shield or grounding plate, for example, as
described in US patent application Serial No.09/544,368, filed April 6, 2000, on Electrostatically
Assisted Coating Method And Apparatus With Focused Web Charge Field, by John W. Louks,
Nancy J. Hiebert, Luther E. Erickson and Peter T. Benson.
[0046] The use of a sharply defined electrode structure adjacent the wetting line to create
an effective electrical field relative to the coating fluid also lends itself well
to tangential fluid coating, especially with more elastic fluids. A tangential coating
apparatus using such an electrode is shown in FIG. 7 (using an air bearing/electrode
assembly such as illustrated in FIG. 4). Tangential curtain coating is generally capable
of running coating fluids with higher extensional viscosities than is possible with
horizontal curtain coating geometries. A tangential coating geometry also offers advantages
associated with the handling of the coating fluid in the coating process. For example,
if a web break occurs in the coating system illustrated in FIG. 3, the electrode can
become coated with coating fluid, which will result in downtime for coater cleanup.
In addition, if the coating die is to be purged before start-up, a catch pan geometry
must be present which can complicate the coating station structure. Another advantage
from tangential coating is that curtain edge bead control during coating is more easily
achieved due to the removal of space constraints between the bottom of the die or
coating fluid applicator 30 and the web support structure (e.g., the air bearing 102).
[0047] FIG. 8 illustrates another embodiment of the air bearing assembly shown in FIG. 7.
For a particular fluid an optimum curtain length exists for a particular web speed
range. In general, higher speeds or higher coat weights can require longer curtains
and lower speeds or lower coat weights can require shorter curtains. While in FIG.
7 only one electrode is shown, the multiple electrode assembly shown in FIG. 8 has
the advantage of allowing the operator to change the curtain height by energizing
the appropriate electrode. For example, a shorter curtain could be used for a thin
coating or lower web speeds, while a longer curtain could be used for higher line
speeds. Thus rather than moving the die down to define a shorter curtain length, the
electrode 100a closest to the die 30 can be energized, and rather than moving the
die up to define a longer curtain length, the electrode 100b farthest from the die
30 can be energized. The spacings of the electrodes can be selected depending on the
fluid characteristics and speed ranges desired.
[0048] In all embodiments of the present invention, an effective electrical field of positive
electrical charges may be exposed to the web at the coating station, while grounding
the coating fluid. In addition, a negative polarity may be applied to the coating
fluid. Further, it is possible to reverse the polar orientations of the electrical
field and the charges applied to the coating fluid. For instance, FIG. 8 illustrates
a laterally extending electrode 120 (such as a corona wire) which is aligned to apply
a positive charge to the coating fluid 32. The electrode 120 may be shielded by one
or more suitable laterally extending shields 122 to direct and focus its application
of positive charges 124 to the coating fluid 32. In that instance, the electrode 100
on the second side 28 of the web 20 has a negative charge relative to the web 20 traversed
thereby, in order to create the desired electrostatic attraction effect. The shields
122 can be formed from a nonconductive or insulating material, such as Delrin™ acetal
resin made by E. I du Pont de Nemours of Wilmington Delaware or from a semiconductive
or conductive material held at ground potential or an elevated potential. The shields
122 can formed in any shape to achieve the desired electrical shielding.
[0049] The utility of using focused fields at the fluid wetting line to achieve a more linear
and stable wetting line was demonstrated in a series of experiments comparing tangential
coating with a relatively large diameter charged roll (see, e.g., FIG. 9) versus an
experimental focused electrode assembly (see, e.g., FIG. 10). The coating fluid was
a 100% solids curable fluid having a viscosity of approximately 3,000 centipoise.
A curtain length of approximately 4.45 cm (1.75 inches) was used (the curtain length
being measured as the distance from the bottom of the die lip to the fluid contact
line). A curtain charging corona wire was used and was about 3.18 cm (1.25 inches)
vertically below the die lip and about 7.62 cm (3.0 inches) horizontally from the
falling curtain. The curtain flow rate was adjusted to give a 50 micron (0.002 inch)
coating thickness at a web speed of 91.4 m/min (300 ft/min). The charged roll system
(FIG. 9) was a 11.3 cm (4.55 inch) diameter roll 126 with a 0.51 cm (0.2 inch) ceramic
sleeve. The ceramic surface was charged by a corona wire system. The inventive focused
electrode assembly (as illustrated in FIG. 11) included a nonconductive bar 128 with
a 3.18 cm (1.25 inch) radius surface. A conductive foil 130 was adhered to the bar
128 with a leading edge 132 of the conductive foil 130 being about 0.25 cm (0.1 inches)
above the tangent point on the bar (the tangent point being that point where the coating
curtain, unaided by electrostatics, would engage the web passing over the bar 128).
A nonconductive tape 131 has an edge abutting the leading edge 132 of the conductive
foil 130. The focused field is created by the leading edge 132 of the foil 130. The
foil 130 was charged using a negative polarity high voltage power supply. Positive
and negative polarity Glassman series EH high voltage power supplies manufactured
by Glassman High Voltage, Inc. of Whitehouse Station, New Jersey were used for these
experiments.
[0050] Using the charged roll system illustrated in FIG. 9, the curtain charging corona
wire 120 was set at a negative 20 kilovolts and the roll 126 corona charger set at
a positive 20 kilovolts. The wetting line typically occurred about 1.27 cm (0.5 inches)
upweb of the tangent point on the roll created by a vertical line from the die lip
to the roll (upweb from point 134, FIG. 9). With a web speed of 76 m/min (250 ft/min)
the wetting line was wavy with a total upweb-to-downweb deviation of 1.27 cm (0.5
inches). The measured coating thickness variation related to this was about 17.9 microns
(0.0007 inches). Increasing the speed to 91.4 m/min (300 ft/min) resulted in entrained
air in the coating 34.
[0051] Using the focused field system, major improvements were seen in wetting line uniformity
and coating uniformity. The electrode assembly of FIGS. 10 and 11 was oriented in
a tangential fashion similar to that shown in FIG. 7, but with the incoming web at
a more acute angle. The curtain charging corona wire 120 was set at a positive 20
kilovolts and the conductive foil 130 was set at a negative 20 kilovolts. At 91.4
m/min (300 ft/min), excellent wetting line linearity was observed with a related measured
coating variation of about 3.6 microns (0.00014 inches). These experiments demonstrate
the improvements in wetting line linearity and coating thickness uniformity with more
focused electrostatic fields.
[0052] Two tests with the focused field setup of FIGS. 10 and 11 were performed to analyze
the process sensitivity to the coating fluid input flow rate and current charging
uniformity, running with a 50 micron (0.002 inch) coating thickness at a web speed
of 91.4 m/min (300 ft/min). First, a lateral segment of about 0.25 cm (0.1 in) was
blocked in the slot of the coating fluid applicator 30 to create a lateral low flow
rate area in the coating curtain 32. Second, a lateral section 0.33 cm (0.13 in) long
of the curtain charging wire (electrode 120) was covered in another area, creating
a lateral area of reduced charge on the coating curtain 32. With the focused field
system of bar 128 activated, no visual deflection of the coating fluid/web contact
line was observed by either of the contrived lateral discontinuities. Absent the focused
field, the curtain 32 in the low flow area would bow upweb and the curtain 32 in the
low charge area would bow downweb, with both conditions accentuating coating non-uniformities.
Accordingly, the use of the electrostatic focused field to facilitate coating is very
effective in overcoming system irregularities in the coating fluid curtain.
[0053] Comparative quantitative analysis tests were also conducted to evaluate the utility
of precharging the incoming fluid to increase the aggressiveness of the electrostatic
system for fluids with limited electrical conductivity. In this series of tests, a
100% solids curable fluid was coated on a 0.0036 cm (0.0014 inch) polyester web. The
viscosity of the fluid was approximately 1,400 centipoise. A slide curtain die set
up was used such as illustrated in FIG. 12, with a conductive backing roll 200 of
only 2.54 cm (1.0 inch) diameter, attached to a positive polarity high voltage power
supply. The die 30 was located directly above the top dead center of the roll 200,
at a height of about 2.7 cm (1.06 inches). However, it was observed that the aggressiveness
of the coating method was limited by the low electrical conductivity of the coating
fluid 32. To address this, the surface of the coating fluid 32 was charged to an opposite
polarity of the energized backing roll 200. Two methods of doing this were investigated
and seen to be functional, one being to elevate the potential of the die 30, and the
other being the use of a corona wire 220 (and associated shield 222) to charge the
surface of the fluid. The curtain charging was accomplished with a 0.015 cm (0.006
inch) diameter tungsten corona wire located about 6.35 cm (2.5 inches) from the falling
curtain on the downweb side of the wetting line, about 1.27 cm (0.5 inches) above
the roll surface. The exact location of this corona wire 220 was not extremely critical,
and it could be located at different locations along the falling curtain, on the opposite
side of the curtain, or adjacent the slide surface of the die 30.
[0054] This series of tests was run on the inventive electrostatic coating assist system
of FIG. 12 to determine the maximum coating speed that could be attained at a given
curtain flow rate (a) without electrostatics, (b) with only the roll potential elevated,
and (c) with the roll potential elevated along with curtain precharging. The flow
rate of the coating fluid 32 was held constant and set to yield a dry coating thickness
of 14.3 microns (0.00057 inches) at 91.4 m/min (300 ft/min). With no electrostatics,
the wetting line occurred 1.27 cm (0.5 inches) downweb of the top dead center of the
roll 200 at a web speed of 3.1 m/min (10 ft/min). At higher web speeds, the wetting
line deflected further downweb, creating a bowed contact line, coating nonuniformity,
air entrainment and curtain breakage. With the backing roll 200 energized to a positive
20 kilovolts, the wetting line occurred at about 0. 64 cm (0.25 inches) downweb, at
a web speed of 24.4 m/min (80 ft/min). Further increases in speed resulted in the
wetting line moving further downweb. With the roll 200 energized to a positive 20
kilovolts and the curtain corona charging wire 220 at a negative 11 kilovolts, the
wetting line occurred at about 0.64 cm (0.25 inches) downweb at a web speed of 97.5
m/min (320 ft/min). These tests show the utility of charging lower conductivity coating
fluids as a way to improve the electrostatic charge attraction aggressiveness of the
inventive electrostatic coating assist system. Another set of experiments was conducted
on the electrostatic coating assist system of FIG. 12 (using the same coating fluid)
for the purpose of determining the minimum coating thickness that could be achieved
at a web speed of 91.4 m/min (300 ft/min). With no electrostatics (i.e., no charges
applied to roll 200 or electrode 220) the pumping system used was not capable of supplying
sufficient coating fluid 32 to get up to the minimum flow rate necessary to cause
the wetting line to occur at the top dead center position of the roll 200 (the flow
rate was not high enough to create the fluid momentum necessary to cause the wetting
line to occur near the top dead center of the roll 200 and the curtain to maintain
a vertical position). At this pump rate, which was less than the minimum coating thickness,
the wetting line occurred about one inch downweb of the top dead center position of
the roll 200, yielding a coating thickness of 85 microns (0.0034 inches). Using electrostatics,
with both the backing roll 200 and corona wire 220 energized as in the previous example
much thinner coatings were possible, with a minimum coating thickness of 6.5 microns
(0.00026 inches) being achieved with the wetting line occurring essentially at the
top dead center position of the roll 200.
[0055] Since it was observed that more focused electrostatic fields produced more linear
and stable coating fluid wetting lines, a tangential coating system utilizing a focused
field apparatus, similar to that shown in FIG. 7 was evaluated. The electrode 100
in the air bearing assembly 102 was a 0.157 cm (0.062 inch) diameter rod. For the
first experiment with this design, a 100% solids curable fluid having a viscosity
of approximately 3,700 centipoise was use as a coating fluid. A two inch curtain length
was used (the curtain length being measured as the distance from the bottom of the
die lip to the rod). The curtain charging corona wire 120 was about 0.75 inches vertically
above the rod and about 2.25 inches horizontally spaced from the rod. The rod electrode
was held at a negative 16 kilovolts and the curtain corona charging wire was held
at a positive 10 kilovolts. The two roll air bearing assembly was aligned to present
the web 20 for contact with the coating fluid 32 at approximately a 10-degree angle
from vertical. A 50 micron (0.002 in) thick coating was produced at a web speed of
250 feet per minute with a straight and stable contact line. Coating thickness variation
resulting from wetting line variations was only about 2 microns (0.00008 inches).
The electrostatic coating assist thus minimized process variations and enhanced coating
uniformity.
[0056] U.S. Patent Nos. 5,262,193 and 5,376,402 disclose that acoustically exciting the
line of initial contact between the coating fluid and the web during coating increases
uniformity and wettability of the coating fluid. The inventors here have found that
applying both the acoustic and electrostatic fields simultaneously have an additive
effect on the desirable forces on the wetting line. For example, FIG. 13 illustrates
a test conducted using a 0.076 cm (0.03 in) inner diameter hollow needle 225 as the
coating die and a combined ultrasonic and electrostatic electrode 228 beneath the
second side 28 of the web 20. The combined electrode consisted of an ultrasonic horn
230, having on its horn face 232 layers of nonconductive polyester tape 234 and a
layer of conductive aluminum tape 236. As shown, the needle 225 was oriented perpendicular
to the horn face 232 on the first side 26 of the web 20, and the horn 230 was on the
second side 28 of the web 20, similar to the orientation shown in FIG 3, with the
web 20 passing over aluminum tape 236 on the horn surface 232. The needle 225 is aligned
to dispense a stream of coating fluid 238 onto the first surface of the web 20 opposite
the electrode 228. In fluid coating, the "dynamic contact angle" or "DCA" is a measure
of the resistance of the coating system to failure due to air entrainment. Generally,
the dynamic contact angle (see, FIG. 14) increases with increasing web speed until
the onset of air entrainment occurs, generally near 180 degrees.
[0057] The application of ultrasonic or electrostatic forces reduces the dynamic contact
angle. The ultrasonic aluminum horn was 1.91 cm (0.75 inches) wide with a 1.27 cm
(0.5 inch) radius. The applied frequency was 20,000 kilohertz and the amplitude was
20 microns (0.0008 in) peak to peak. The electrostatic electrode was constructed by
attaching two layers of adhesive tape (polyester 234) plus an outer layer of aluminum
tape 236 which was coupled to a positive high voltage power supply. The coating fluid
238 was a glycerine and water solution having a viscosity of 100 centipoise. It was
seen that at a web speed of 3 m/min (10 ft/min), the "dynamic contact angle" without
electrostatics or ultrasonics was 135 degrees, while with ultrasonics alone it was
reduced to 105 degrees, with electrostatics field applied alone it was reduced to
90 degrees, and with electrostatic and ultrasonic forces applied simultaneously it
was reduced to 70 degrees, showing the additive effects of the two coating assist
forces. As the web speed was increased to 30 m/min (100 ft/min) without ultrasonics
or electrostatics, the "dynamic contact angle" increased to about 160 degrees, where
air entrainment occurred. With electrostatics alone at a web speed of 30 m/min (100
ft/min) the dynamic contact angle was only 110 degrees. With ultrasonics alone, the
dynamic contact angle was also only 110 degrees. With both ultrasonics and electrostatics
applied, the dynamic contact angle was reduced to 100 degrees, further showing the
additive effects of the two coating assist forces. To illustrate the effect of the
external forces which reduce the dynamic contact angle on coating speed, at a web
speed of 3 m/min (10 ft/min), the "dynamic contact angle" without electrostatics or
ultrasonics was 135 degrees, while with electrostatics alone, the "dynamic contract
angle" did not increase to 135 degrees until a web speed of 76 m/min (250 ft/min)
was reached. The benefits of acoustically exciting can be attained at other frequencies
as well, including both sonic and ultrasonic frequencies.
[0058] The benefits of combining acoustics and electrostatics in a coating environment are
not limited to the specific application detailed above. The beneficial additive effects
of exposing the coating fluid to electrical forces and acoustical forces adjacent
the coating station will be found in many coating applications. For example, even
if the electrostatic system and ultrasonic system are being used where the forces
are not substantially at and down-web of the fluid line, increases in desirable effects
such as reduced air entrainment and higher coating speeds can be seen. If, however,
the electrostatic or ultrasonics (or both) are configured to apply the forces substantially
at and downstream of the fluid contact area, further improvements can be realized.
The application of both an electrostatic field and an acoustical field adjacent the
fluid wetting line to attract the coating fluid to the substrate being coated results
in significant advantages, and is not limited in structure or methodology to the specific
electrostatic and acoustical embodiments and force applicators disclosed herein.
[0059] Also incorporated herein by reference is US patent application Serial No. 09/544,368,
filed April 6, 2000, on Electrostatically Assisted Coating Method And Apparatus With
Focused Web Charge Field, by John W. Louks, Nancy J. Hiebert, Luther E. Erickson and
Peter T. Benson.
[0060] Various changes and modifications can be made in the invention without departing
from the scope or spirit of the invention. For example, any method may be used to
create the focused electrode field. The electrostatic focused field can also be made
to be laterally discontinuous, to coat only particular downweb stripes of the coating
fluid onto the web, or can be energized to begin coating in an area and de-energized
to stop coating in an area, so as to create an island of coating fluid on the web
or patterns of coating fluid thereon of a desired nature. The electrostatic field
can also be made to be non linear, for example by a laterally non linear electrode
so as to create a non linear contact line and non uniform coating. Thus if the electrode
has a downweb curvature in a particular laterally disposed area, the coating in that
area can be thicker in that area as compared to adjacent areas.
[0061] All cited materials are incorporated into this disclosure by reference.