Note: Descriptions are shown in the official language in which they were submitted.
2152619
wossll2o3l ~ PCT~P93/02452
METHOD OF MINIMIZING SKIP COATING
ON A PAPER WEB
Background of the Invention
The present invention relates to a method of and apparatus
for applying liquid coating material onto a moving web of
paper, and in particular to a coating method and apparatus
of the fountain applicator type.
Coating a web of paper is generally effected by the
application of a liquid coating material onto a moving web.
The coating material may be comprised of a solid
constituent suspended in a liquid carrier. The quality of
the coating applied onto the paper web depends upon a
number of factors, and important one of which being how the
material is applied. The application of the coating
material should preferably result in a coating that is
continuous and uniform across the web.
One method previously used to coat paper webs was to feed
liquid coating material to applicator rolls that applied
the material directly onto the moving web. While th2 use of
applicator rolls yields a fairly uniform coating across the
web, as web speeds increase, there often occurs a film
split pattern in the coating applied onto the web, i.e.
cross-direction variations in the weight of the coating on
the web. This technique therefore does not lend itself to
coating webs at high speeds. Direct application by rolls
also creates forces in the roll/web nip that imbed or force
coating material into the web instead of covering the outer
surface of the web to enhance smoothness.
In an attempt to avoid these and other problems, the art
developed a coating process in which the liquid coating
material was jetted in a free standing curtain of coating
liquid directly onto the moving web with a fountain
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applicator. While fountain applicators overcome many of the
limitations of roll applicators, in their use, skip coating
often occurs. Skip coating is caused by air in the coating
liquid being contacted against the paper web and preventing
the coating liquid from uniformly contacting and being
uniformly applied onto the web surface. To decrease the
severity of the skip coating problem, fountain applicators
customarily include coating/air separation equipment to
remove air from coating liquid prior to delivery of the
coating liquid to the applicator, but the equipment is not
l00 % effective and some air remains entrained in the
coating liquid jetted against the web surface and causes
skip coating.
lS Obiects of the Invention
An object of the present invention is to provide an
improved fountain applicator for applying liquid coating
material onto a paper web, in which the resulting coating
on the web is substantially skip free.
Another object is to provide such a fountain applicator, in
which a sheet of coating liquid is flowed along a c~rved
surface substantially immediately prior to being impinged
against the web, to subject the sheet to centrifugal force
to cause air entrained in the coating liquid to move toward
one side of the sheet away from the curved surface.
A further object is to provide such a fountain applicator,
in which the sheet of coating liquid, after leaving the
curved surface, is directed toward the web in a free
standing jet curtain of coating liquid that is impinged
against the web, such that the side of the jet curtain of
coating liquid that corresponds to the opposite side of the
sheet that is relatively free of entrained air contacts the
web surface while the other side of the jet curtain of
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- 2152619
coating liquid is out of substantial contact with the web
surface.
Summary of the Invention
s
In accordance with the present invention, an apparatus for
applying a coating liquid onto a surface of a moving web
comprises an elongate concave curved surface that is
positionable proximate to, transversely of and spaced from
the web; and means for forming an elongate sheet of coating
liquid, for flowing the sheet along the curved surface, and
for then projecting the sheet in a free standing jet
curtain of coating liquid that is directed toward, against
and across the surface of the web. The coating liquid
sheet, upon being flowed along the curved surface, is
subjected to centrifugal force to cause air entrained in
the coating liquid sheet to move to one side of the sheet
that is away from the curved surface. The free standing jet
curtain of coating liquid is directed against the web such
that a side of the jet curtain of coating liquid, which
corresponds to an opposite relatively air-free side of the
coating liquid sheet, contacts the web surface, while the
other side of the jet curtain of coating liquid is o~t of
substantial contact with the web surface. Thus, only
coating liquid that is relatively free of entrained air
contacts the web surface to provide on the web surface a
coating that is substantially skip free.
The invention also contemplates a method of applying a
coating liquid onto a surface of a moving web, which
comprises the step of flowing a sheet of coating liquid
along an elongate concave curved surface that is proximate
to, extends transversely of and is spaced from the web, to
subject the coating liquid sheet to centrifugal force to
cause air entrained in the coating liquid to move to one
side of the sheet that is away from the curved surface.
Also included is the step of directing the sheet of coating
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liquid, after it has been flowed along the curved surface,
in a free standing jet curtain of coating liquid toward,
against and across the surface of the web to contact the
web surface with a side of the jet curtain of coating
liquid that corresponds to an opposite relatively air-free
side of the coating liquid sheet, while maintaining the
other side of the jet curtain of coating liquid out of
substantial contact with the web surface, whereby only
coating liquid that is relatively free of entrained air
contacts the web surface.
Brief Description of the Drawin~s
Fig. 1 shows a prior art fountain applicator;
Fig. 2 shows a fountain applicator that embodies the
teachings of the present invention;
Fig. 3 illustrates a coating supply system of a type that
may be used to deliver coating liquid to the fountain
applicator of the invention;
Fig. 4 shows an alternate embodiment of a fountain
applicator that incorporates the teachings of the
invention; and
Figs. 5A and 5B are graphs that respectively show the
degrees gloss and the Parker Printsurf smoothness of a
coating applied onto a web with the fountain applicator of
Fig. 2, for various speeds of travel of the web past the
applicator.
Detailed DescriPtion
The Prior Art
A fountain applicator of a type contemplated by the prior
art is shown in Fig. l and indicated generally at 20. The
applicator is part of a paper coating machine, and extends
parallel to and coextensively with a movable support or
backing roll 22 which rotates in a direction shown by an
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Woss/12031 21 5 2 61 9 PCT~P93/02452
arrow 24 and supports a web of paper 26 during its travel
past the applicator. The applicator has front and rear
walls 28 and 30 that form an elongate metering slot 31
leading to an elongate outlet nozzle 32. The metering slot
communicates with a chamber 34 that receives liquid coating
material under pressure from a source of material, for flow
of the coating liquid upwardly to and through the outlet
nozzle, as indicated by the line and arrow. The outlet
nozzle extends coextensively with the backing roll 22 and
transversely of and across the paper web, and is proximate
to and faces the paper web where it is supported on the
backing roll. The upper end of the applicator rear wall 30
extends beyond the upper end of the applicator front wall
28 and defines a gap 36 with the web, and where it extends
beyond the applicator front wall, the applicator rear wall
has a flat surface 38. Coating liquid introduced into the
chamber 34 flows upwardly to and out of the outlet nozzle
in a sheet of coating liquid 40 that flows across the flat
surface 38 at the upper end of the applicator rear wall.
Upon leaving the flat surface, the sheet of coating liquid
is directed in a free standing jet curtain of coating
liquid against and transversely across the paper web, at an
acute included angle ~ within the web, as the web is moved
past the applicator.
In operation of the applicator 20, the free standing jet
curtain of coating liquid is impinged against the surface
of the backing roll supported paper web 26 to apply onto
the web surface an excess layer of coating liquid that is
doctored to a desired coat weight by a downstream doctor
42. In order for the applicator to apply an excess coating
layer that is reasonably free of voids or skips, it is
imperative that there not be an excessive amount of air
entrained in the coating. To minimize entrained air, a
conventional air removal system may be incorporated into
the coating supply system that delivers coating liquid to
the applicator, such air removal systems being well known
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2152619 6
in the art and two representative examples of such being
taught by U.S. patents Nos. 4,290,791 and 4,643,746.
However, even when an air removal system is employed, some
air remains entrained in the coating and contacts the web,
causing skip coating on the web, especia~ly at high speeds
of travel of the web past the applicator.
The Invention
In improving upon prior fountain applicators, the invention
provides an improved fountain applicator that-is uniquely
configured to apply onto a surface of a paper web a coating
layer that is essentially skip free. Such an applicator is
shown in Fig. 2 and is configured to cause air entrained in
a coating liquid sheet that is emitted from an elongate
fountain outlet nozzle, to move to a side of the sheet that
is opposite from the side that is impinged against the web,
so that only coating liquid that is relatively free of
entrained air contacts the web surface. This is
accomplished by flowing the coating liquid sheet along a
curved surface of the applicator, to subject the coating
liquid sheet to centrifugal force to cause the dense
coating liquid to move toward a side of the coatins liquid
sheet that is toward the curved surface and impinged
against the paper web, and air entrained in the coating
liquid to move toward an opposite side of the sheet that is
away from the curved surface and out of substantial contact
with the web. The radius of the curved surface is selected
for the magnitude of centrifugal force desired, the
magnitude also being a function of the flow velocity of the
coating liquid sheet across the curved surface. The flow
velocity of the coating liquid sheet is, in turn, a
function of the cross sectional area of the fountain outlet
nozzle and of the volume flow rate of coating liquid
through the nozzle, and must be such as to ensure that the
coating liquid applied onto the paper web completely and
uniformly covers the web surface.
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W095/12031 PCT~P93/02452
2152619
More particularly, the fountain applicator of Fig. 2 is
indicated generally at 50 and applies onto a surface of a
paper web 52, which is carried past the applicator on a
backing roll 54 that rotates in a direction as shown by an
arrow 56, an excess layer of coating liquid that is
doctored to a desired coat weight by a downstream doctor
means such as a blade 58. The fountain applicator is part
of a paper coating machine, and extends in the cross-
machine direction, parallel to the backing roll 54 and
transversely of, across and spaced from the backing roll
supported web. The applicator has front and rear walls 60
and 62, and attached to the upper end of the rear wall is a
plate 64. The front and rear walls and the plate form a
chamber 66 therewithin, into which liquid coating material
is delivered under pressure via a coating liquid
distribution pipe 68 that extends longitudinally through
the chamber and has a plurality of coating outlet openings
69 longitudinally spaced therealong. The front and rear
walls may be hinged at their lower ends for movement apart
to provide access to the chamber 66 for cleaning, for
example as taught by U.S. patent No. 4,534,309.
A metering slot 70 is defined between the front wall 60 and
the plate 64. The metering slot extends upwardly from the
chamber 66 and transversely of and across the backing roll
supported web 52, and from bottom to top is inclined toward
the front of the applicator to enhance a migration of air
entrained in the coating liquid toward the side of the
metering slot defined by the plate. A replaceable elongate
deflector tip 72 is at the upper end of the front wall and
an elongate out-let nozzle 74 from the metering slot is at
the top of the plate 64 between the plate and the deflector
tip. On its side toward the outlet nozzle, the deflector
tip has an elongate flat surface 76 and an elongate concave
curved surface 78 that is positioned proximate to,
transversely of and spaced from the web. The flat surface
begins within the metering slot, it may but does not
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WO95/12031 PCT~P93/024~2
215~619 8
necessarily need to extend upwardly beyond the outlet
nozzle, and leads to the curved surface. Coating liquid
exiting the elongate outlet nozzle flows in a sheet along
the flat surface of the deflector tip`to, along and then
off of the curved surface in a free standing jet curtain of
coating liquid that is directed against and across the web
surface at an appropriate included acute angle. If desired,
the downstream end of the coating liquid flow surface of
the deflector tip could terminate in an elongate flat
surface (not shown) of relatively limited length beyond the
curved surface 78, along which the coating liquid sheet
would flow after leaving the curved surface and before
being projected toward the web in a free standing jet
curtain of coating liquid. Also, adjustable deckle devices
(not shown) may be at opposite ends of the elongate outlet
nozzle to control its transverse extent and, therefore, the
transverse extent of the sheet of coating liquid, thereby
to control the width of the coating layer applied onto the
web.
Before considering the manner of operation of the fountain
applicator 50, a typical coating supply system for the
applicator will first be considered in general terms. As
seen in Fig. 3, a coating supply system may include a
covered surge tank 82 for holding a main supply of liquid
coating material that is stirred by a motor driven impeller
unit 84. Coating liquid flows from the tank through a valve
86 to a pump 88 that delivers the coating liquid under
pressure through a valve 90 and a mesh filter 92 to an air
removal device 94. The air removal device may be of a
conventional type, and operates to remove entrained air
from coating liquid supplied from the surge tank and to
deliver the removed air, carried in a small portion of the
coating liquid, through a valve 96 for return to the surge
tank. The remaining coating liquid exiting the air removal
device is flowed through a valve 98 into one end of the
distribution pipe 68 of the fountain applicator 50. At an
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opposite end of the distribution pipe there is an outlet
100 from the top of the distribution pipe (Fig. 2), that
leads back to the surge tank through a valve 102. The
outlet allows recirculation of a small portion of the
coating liquid supplied to the distribution pipe, in order
to remove accumulated air from the top of the distribution
pipe and enhance a uniform pressure of coating liquid
throughout the chamber 66 for uniform application of
coating onto the moving web. Valves 104 and 106 selectively
direct coating liquid returned from the fountain applicator
to the surge tank, to a sewer and/or to reclamation
apparatus. A valve 108 is connected between the upstream
side of the valve 90 and the surge tank, and a valve 110 at
an outlet from the surge tank leads to the sewer or the
reclamation apparatus. When the fountain applicator is
operating, the valves 86, 90, 96, 98, 102 and 104 are open
and the valves 106, 108 and 110 are closed. When the
fountain applicator is not operating, the various valves
are selectively opened or closed to accomplish a desired
result (e.g., to accomodate cleaning of the system with
wash water), as is readily understood by those skilled in
the art.
In operation of the fountain applicator 50 and with
reference to Fig. 2, coating liquid delivered to the
applicator by the coating supply system is introduced into
one end of the distribution pipe 68 and flows through the
pipe openings 69 into the chamber 66. The air removal
device 94 removes from the coating liquid much of the
entrained air, but it is not 100 % effective, so some air
remains entrained in the coating liquid delivered to the
applicator. Some of the remaining air that accumulates at
- the top of the distribution pipe passes through the outlet
100 and is removed, but some still remains entrained in the
coating, and with prior fountain applicators this limited
amount of remaining entrained air causes skip coating on a
paper web. However, in use of the applicator of the
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2152619
invention, entrained air remaining in the coating liquid
flowed from the chamber 66 and out of the outlet nozzle 74
is prevented from contacting the surface of the web, and
therefore from causing skip coating. -
~:
More specifically, coating liquid delivered into the
chamber 66 flows upwardly through the metering slot 70 and
exits the elongate outlet nozzle 74 in an elongate sheet
112 of coating liquid that extends transversely of the
paper web 52. The sheet of coating liquid flows along the
deflector tip flat surface 76 to the concave curved surface
78, where the sheet is forcefully flowed against the curved
surface as its direction of flow changes to conform to the
curved surface. Causing the coating liquid sheet to follow
the curved surface subjects it to centrifugal force that
causes the dense coating liquid to move toward a side 116
of the sheet that is toward the curved surface and the much
less dense air entrained in the coating liquid to move
toward an opposite side 118 of the sheet that is away from
the curved surface. After flowing along the curved surface,
the sheet of coating liquid is projected from the deflector
tip in a free standing elongate jet curtain of coating
liquid that is directed toward, transversely across and
against the paper web surface, such that an included acute
angle ~ is defined between the plane of the jet curtain of
coating liquid and a tangent to the web at the point of
contact of the curtain with the web. In consequence, only
the side of the jet curtain of coating liquid that
corresponds to the side 116 of the coating liquid sheet
that is relatively free of entrained air, is impinged
against the surface of the web, while the side of the jet
curtain of coating liquid that corresponds to the opposite
side 118 of the sheet, to which most of the entrained air
has moved, is out of substantial contact with the web, so
that a skip-free coating is applied onto the web. The layer
of coating liquid applied onto the web by the applicator is
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in excess and is doctored to a desired coat weight by the
downstream doctor means 58.
The minimum centrifugal force to which the sheet of coating
liquid 112 is to be subjected is that which just results in
application of a substantially skip-free coating onto the
paper web 52. As is known, the centrifugal force exerted on
the sheet of coating liquid is equal to the product of the
mass of the coating liquid and its flow velocity squared,
divided by the radius of the deflector tip curved surface
78. The mass of the coating liquid may be considered as a
constant, which in practical terms means that the
centrifugal force may be varied by changing either the flow
velocity of the coating liquid sheet or the radius of the
curved surface. The flow velocity of the coating liquid
sheet is a function of the cross sectional area of the
elongate outlet nozzle 74 and of the volume flow rate of
coating liquid through it, and is chosen so that the
applied coating completely and uniformly covers the web
surface. Since there are limits on the minimum volume flow
rate of coating liquid required to obtain a uniform coating
on the paper web, and since there are practical limits on
the maximum volume flow rate of coating liquid that can be
forced throught the metering slot 70 and outlet nozzle 74,
to subject the coating liquid stream to a desired
centrifugal force, it usually is most convenient to control
the radius of the deflector tip curved surface 78.
Nevertheless, while the magnitude of centrifugal force
exerted on the coating liquid sheet may be increased by
decreasing the radius of the deflector tip curved surface,
there also are practical limits on how small the radius may
be. It presently is contemplated that the curved surface
have a radius on the of about .125" to .500", which is
believed to be sufficient to properly densify the coating
liquid on the side 116 of the coating liquid sheet that is
impinged against the web or, put another way, to cause a
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215Z619 12
sufficient amount of the entrained air to move to the side
118 of the sheet that is out of substantial contact with
the web, so that skip coating does not result. It also is
contemplated that the curved surface have an arcuate extent
in the range of about 45 to 90, with about 70 likely
being optimum.
The angle of attack of the free standing jet curtain of
coating liquid against the paper web, i.e. the included
angle between the plane of the curtain of coating liquid
and a tangent to the web surface at the point of contact of
the curtain with the web, should be chosen to obtain
optimum coating results. For the applicator 50, good
coating results have been experimentally obtained with an
included angle of 50O when using an outlet nozzle 74 having
a width of .048", with the linear distance between the
upper end of the deflector tip curved surface 78 and the
point of impact of the coating liquid curtain against the
web being on the order of .312", and with the deflector tip
flat surface 76 having a length of about .125" in the
direction of flow of the coating liquid sheet. However,
these particular parameters may have other values, since
the optimum value of each parameter is influenced by and
generally dependent upon the values of the other
parameters, and it is contemplated that the outlet nozzle
have a width in the range of about .025" to .050" and also
that the flat surface 76 on the deflector tip could
eliminated, in which case the curved surface 78 would begin
immediately at the outlet nozzle 74.
While in the fountain applicator 50 shown in Fig. 2, the
coating liquid flow surfaces 76 and 78 of the deflector tip
72 are exposed to the outside of the applicator and located
downstream from the metering slot 70 and the elongate
outlet nozzle 74, the fluid flow surfaces could be part of
and located within the fluid flow path defined by the
metering slot 70. In this case, as shown in Fig. 4, the
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upper end of the plate 64 is extended along, spaced from
and curved to conform to the fluid flow surfaces 76 and 78,
so that the metering slot then extends along and includes
the fluid flow surfaces. With this arrangement the coating
liquid sheet is subjected to centrifugal force while within
the upper end of the metering slot, an elongate outlet
nozzle 74' is at the uppermost end of the deflector tip,
and the free standing jet curtain of coating liquid is
emitted directly from the elongate outlet nozzle.
To collect run-off coating liquid that is not carried away
on the paper web 52, a run-off deflector 120 is on the
outer surface o-f a chilled water jacket 122 carried on the
plate 64. The run-off deflector leads to a return pan, from
which coating liquids is returned to the surge tank 82, and
the chilled water jacket facilitates cleaning of the run-
off deflector.
Figs. 5A and 5B show coating results obtained
experimentally when coatings were applied onto the same
grade of paper with a fountain applicator constructed
according to Fig. 2 and operated according to the teachings
of the invention. Fig. 5A shows 75 gloss obtained a~
various web speeds and Fig. 5B shows Parker Printsurf
smoothness measurements obtained at various web speeds.
While one embodiment of the invention has been described in
detail, various modifications and other embodiments thereof
may be devised by one skilled in the art without departing
from the spirit and scope of the invention, as defined in
the appended claims.
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