Note: Descriptions are shown in the official language in which they were submitted.
WO 93/07969 PCI /U~i92/07610
-l- 2116962
ULTRASONICALI.Y ASSISTED COATING APPARATUS AND MET~IOD
5 TECHNICAL FIELD
The present invention relates to an
acoustically assisted coating apparatus and a method
for applying one or more layers of a coating material
- onto a moving web. More particularly, the present
invention relates to using ultrasonic energy to
improve the application of a smooth, uniform layer of
coating material onto a moving web.
BACKGROUND ART
Ultrasonically created fluid effects have
been noted in the literature since the early l900's.
Since the 1960's, the development of improved
transducers for~generating ultrasonic energy increased
activity in this~field. Ultrasonic phenomena which
20~ . relate to fluid processing or coating technologies
include cavitation, viscous heating, increased shear,
microturbulence,~;~and~acoustic streaming~ These
phenomena generate effects that include enhanced
wettability, micromixing, dispersion, emulsification,
,
2~5~ deaeration, agglomeration, sep~ration of compo~ents,
viscosity reduction, poIymer~chain disentanglement,
high polymer degradation, and increased chemical
reaction rates.~
::~
~ ~ Last et al , U.S. Patent No. 4,302,485, ~
:,
; 30 discloses using ultrasonic energy in an immersed
~ saturation system to excite a strip of fabric passing
;~ ~ through a bath of~liquid finishing agent. This causes
cavitation in the bath and increases the
microturbulence to thereby increase wicking. The
:
:
WO93/07g69 PCT/US92/07610
21i'~962 -2-
fabric is impregnated from both sides, and the liquid
is not metered onto the fabric.
In U.S Patent No. 4,307,128 to Nagano et
al., ultrasonic energy is used in a molten metal bath
to locally lift a portion of the molten metal surface
such that it contacts a moving surface of a substrate.
The coating is not metered. Absent ultrasonic energy,
this apparatus is apparently inoperative.
U.S.~Patent No. 3,676,216 to Abitboul
teaches applying ultrasonic energy to a previously
::
coated web to more uniformly and consistently
distribute the coating over the web and to smooth
irregularities in the coating. However, the
ultrasonic energy is transmitted through the air to
- ~ 15 excite the coated~web after the web is completely
coated.
Japanese Patent No. 57-187071 discloses
applying ultrasonic energy to the backside of a coated
web. However, the ultrasonic source is too far from
:` : ~ ~ :
20~ ~the~point of coating for the ultrasonic energy to
affect the liquid~at the~first contact between the
liquid~and the web~or at the la~st contact between the
: ~ . ,
liq~uid and the coating equipment.
In Canadian Patent No. 869,959, a nozzle for
2S~ applying a liquid~coating from a hopper onto a moving
web is ultrasonicàlly excited. A horn ultrasonically
vibrates the nozzle to prevent the coating from
sticking in and~clogging the nozzle. However, the
ultrasonic vibrations only affect the coating before
it is placed on~the web, and do not affect the process
during the initial contact between the coating and the
web or thereafter. Thus, the ultrasonic vibrations do
not affect the un~iformity of the thickness of the
coating as the coating is applied. The Canadian
:
WQ93/07969 PCT/US92/07610
-3
patent is representative of a body of art which 211 6 9 62
discloses applying ultrasonic energy to a nozzle
during coating to improve flow through and from the
nozzle. However, these apparatus are not practical
for use in large scale production applications where
wide coatinqs are being applied. In the formation of
web rolls such as adhesive tapes, it is common to form
the rolls in up to 150 cm (60 inch) widths. Rolls
this-size could not be formed while achieving uniform
ultrasonic excitation of sufficient intensity at the
nozzle due to tbè~difficulty in exciting the necessary
masses and lengths involved.
None of the known apparatus or systems
disclose metering the coating onto only one side of
l5~ the web and using acoustic energy to improve the
charaateristics of an~applied coating before the
coating of the~web~is complete.
DISCLOSURE OF INVENTION
~ The~present~invention overcomes these
problems and uses~acoustic energy to assist the
coating of a~smooth~continuous or discontinuous layer
of a metered quant~ity of liquid aoating material
having a substantially uniform crossweb thickness on
,
25~ one surface of a~;moving web. The apparatus includes a
device which applies a coating material onto at least
a~portion of the~surface of the web. The device may
be any type of coating system in which the coating can
be applied onto~one side of the web, such as, for
::
example, extrusion, curtain, slot-fed knife, hopper,
fluid bearing, notch bar, blade, and roll coaters. -~
A coating applicator meters and applies a
controlled amount of coating material onto one surface
of the web across the width of the web. An ultrasonic -~
~,.
9 6 2 -4- PCT/US92/07610
energy source excites the line of initial contact
between the coating material and the web preferably at
a uniform acoustic intensity, amplitude, and frequency
in the low end of the ultrasonic spectrum. Where a
downweb structure is used as part of the die or as a
separate structure to level or smooth the coating, the
ultrasonic energy source can excite the line of final
contact between the coating applicator device or
downweb structure and the coated web. Additionally,
lQ the ultrasonic energy can excite the area between the
region of initial contact of the coating material and
~ the web and the~region of final contact between the
;~ coating applicator device or downweb structure and the
coating material. The acoustic intensity is selected
lS in combination with the properties of the coating
material and the web to create a coated web having a
substantia}ly uniform crossweb thickness.
When the coating material is applied through
a die, the ultrasonic energy generator can apply
ultrasonic energy to the coatin~ material-web
interface through the die. Alternatively, ultrasonic
energy is applied through the back surface of the web,
through a backup~horn which replaces a conventional
support. The~ultrasonic energy can also be
transmitted through~the air or other coupling fluid.
.
BRIEF DESCRIPTION OF DRAWTNGS
Figure l schematically illustrates contact
extrusion. Figure lA shows contact extrusion without
acoustic excitation and Figures lB, lC, lD, lE, lF,
and IG show contact extrusion with various ways of
applying acoustic energy.
Figure 2 schematically illustrates curtain
coating. Figure 2A shows curtain coating without
W~ 93/07g6g PCr/USg2/07610
-5- 211 6~ 6
acoustic excitation and Figures 2B, 2C, 2~, and 2E
show curtain coating with various ways of applying
acoustic energy.
Figure 3 schematically illustrates slot-fed
knife coating. Figure 3A shows slot-fed knife coating
without acoustic excitation and Figures 3B, 3C, and 3D
show slot-fed knife coating with various ways of
applying acoustic energy.
~igure 4 schematically illustrates slide
coating. Figure 4A shows slide coating without
acoustic excitation and Figure 4B shows slide coating
with acoustic excitation.
Figure S schematically iliustrates roll
coating. Figure~5A shows roll coating without
acoustic excitation and Figure SB shows roll coating
with acoustic excitation.
Figure~6~schematically illustrates
non-contact extrusion~coating. Figure 6A shows
extrusion coating without acoustic excitation and
20~ Figures 6B and 6C~show extrusion coating with acoustic
excitation.
Figure~7A~is a graph of a cross web coating
thickness profilè~without ultrasonics and Figure 7B is
a graph of the~cross web coating thickness profile
~ coated with~ultrasonics.
Figure 8A is a graph comparing the average
percentage coating thickness range variation for test
runs with ultrasonics and for test runs without
ultrasonics. ~ Figure 8B is a graph comparing the
coating thickness standard deviation variation as a
: ~ :
percentage for test runs with ultrasonics and for test
runs without ultrasonics.
: .
wo9~/oi~9~63g~6æ -6- P~/US92/07610
DETAILED DESCRIPTION
The apparatus and coating method of the
present invention apply acoustic energy to the
interface between a web and a liquid coating material
applied on the web. Although acoustic energy can be
applied at various locations in all coating systems,
improved coating is best achieved in systems which
coat the web on one surface. With this system
acoustic energy is used to improve coating thickness
uniformity on the coated web, increase wettability
(the ability of a liquid to replace a gas in contact
with a substrate), reduce edge beads and streaks,
reduce viscous drag, increase the coating gap between
the coating equipment and the web, yield more stable
eguipment operation and self-cleaning equipment,
reduce the tendency for air entrainment, coat at
higher speeds, and reduce the minimum possible coating
thickneæs~ The increased coating uniformity reduces
distortion, peaking and gapping, high spots, and
20 ~ telescoping of wound rolls of coated webs.
This invention is described with respect to
applying smooth, continuous coatings. Nonetheless,
these results also can be attained while applying
smooth discontinuous coatings. For example,
25~ ultrasonic energy~ can be used with the coating of a
web having a~ macrostructure such as voids which are
; filled with a coating but there is not continuity
between the coating in adjacent voids. In this
situation, the coating uniformity and enhanced
wettability is maintained both within discrete coating
regions and from region to region, with the regions
separate from each other in both the downweb and
crossweb directions.
W~93/07969 PCT/US92/07610
_7_ 21169~2
The web can be any material such as
polyester, polypropylene, paper, or nonwoven
materials. The improved wetting of the coating is
particularly useful in rough textured or porous webs,
regardless of whether the pore size is microscopic or
macroscopic.
The web and the coating material are excited
at a preferably uniform ultrasonic intensity across
the width of the coated web. The intensity is
selected in combination with the coating material
properties to maximize crossweb coating thickness
uniformity. Although the frequency and amplitude can
be~varied w~ile~maintaining a uniform ultrasonic
intensity, ultrasonic waves having uniform amplitude
lS and frequency are preferred.
Acoustic waves are longitudinal waves caused -;
by periodic compression and rarefaction of the medium
through which they travel. Thése waves also can
generate other acoustic waves such as surface
20 ~ transverse acoustic waves. Acoustic waves contain
both kinetic energy~of motion and potential energy of
compressed matter. ~The acoustic energy density, E, is
a measure~of the energy per volu~e in an longitudinal
acoustic wave and is represented by:
25~ ~
E = ~2 pof2Xo
where pO is;the~density of the medium when no acoustic
waves travel throuqh it, f is the frequency of the
, ~ :
~ 30 acoustic wave, and xO is the peak-to-peak amplitude.
;~ Where differences in acoustic energy density occur`,
forces exist which can manipulate coating liquids.
The ultrasonic energy intensity, I, is a
function of the amplitude and frequency of the waves
wo23~o~ 6 2 PCT/U~92/07610
--8--
and the properties of the medium and is represented
by:
I = c ~2 pof2Xo
where c is the speed of the acoustic waves in the
medium.
When an acoustic wave encounters a boundary
between two media~, part of the wave is transmitted
through and the~rest is reflected from the boundary.
The proportion of transmission to reflectance depends
on how similar~the~acoustic impedances of the two
media are. The~characteristic acoustic impedance, R,
is as follows:
::
R = poc~
If the impedances~of;two media are similar, most of
the wave will be~`transmitted. If the impedances ;~
20 ~ di~ffer~widely, most~of the wave will be reflected.
However,~when;a thin layer is sandwiched between two
materials~with similar~acoustic impedances, the thin ;~
layer tr~ansmits~the~acoustic waves e~en though its
impèdance differs~from that of the other materials.
25~ The~application of ultrasonic energy
provides the~desired results when used with any type
` of coaters in which the coating is metered or measured
and applied to one~surface of the web. Extrusion
. ~
coaters, both-oontact and non-contact, are illustrated
in Figures l~and 6, respectively. Curtain coaters are
; illustrated in Figure 2. Knife coaters include
slot-fed knife,~hopper, fluid bearing, notch bar, and
blade coaters, and will be discussed with reference to
a slot-fed ~nife~coater as illustrated in Figure 3.
:
W~g3/07969 PCT/US92/07610
g 211 ~962
Slide coaters are illustrated in Figure 4. Roll
coaters include gravure and kiss coaters and are
generically represented in Figure 5. Although other
types of coaters are also enhanced by the application
of acoustic energy, the systems described below are
representative. The operation of the invention is
generalIy similar with all of these coating methods.
Re~erring to Figure 1, a contact extrusion
coating system is shown. In Figure lA, no ultrasonic
excitation is provided.~ A coating system 10, includes
.,
an extrusion die 12 located adjacent a backup roller
14. A web 16 of material to be coated travels from
left to right in-the figure. Coating material 18 is
extruded onto and across the web 16 as shown. The
coating~material~;18 may be applied across the entire
:: : . .
width of~the web 16 or across any fraction of the
;width in the known~manner.
In Figures lB, lC, lD, lE, lF, and lG,
ultrasonic energy~is~applied to the system lO such
; that the~ energy acts on the web 16 and coating 18 in
the region of initial contact between the web 16 and
coating 18. The details of this ultrasonic excitation
are~described below. In the coating system lO' of
Figure lB, a resonant sonotrode or ultrasonic horn 20
25 ~ replaces the backup roller 14. The ultrasonic horn 20
; is a specially~designed horn which can vibrate at
selected frequencies or amplitudes of vibration. The
~; ultrasonic energy is applied directly to the web 16
and excites the~web 16 and coating 18 at the location
3Q of initial contac between the coating 18 and the web
-~ 16.
In Figure lC, both an ultrasonic horn 20 and
a backup roller 14 are used in the coating system 10'.
The backup roller 14 is located opposite the extrusion
.
WO g3/07~69 PC~/USg2/~7610
2.~ ~;962 -lo-
die 12, and the ultrasonic horn 20 is located downweb
from this location. The ultrasonic energy is applied
directly to the coated web 16 and the energy travels
through the web 16 and coating 18 to excite the line
of initial contact between the coating 18 and the web
16 Although the horn 20 is shown downweb of the die
12, it also could be located upweb of the die 12.
Additionally, although the ultrasonic energy is not
applied directly to the line of initial contact
between the coating 18 and the web 16, the energy is
applied with a sufficient intensity such that when it
reaches the initial contact line it has sufficient
energy.
The coating system 10' of Figure lD includes
similar components to the known system 10 of Figure
lA. The web 16~ passes around a backup roller 14 and
the coating material 18 is extruded onto and across
the desired width of the web 16. An extrusion die 22
applies the coating material 18 onto the web 16.
20 ~However, in Figure lD, the die 22 is ultrasonically
; ~ excited to excite the coating 18 within the die 22 and
the excited coating 18 is extruded onto the web 16.
The ultrasonic die 22 is a specially designed die
~` connected to an~ultrasonic energy generator, either in
. ~
25~ a~sinqle housing~as shown, or by externally securing
the two together as with a mounting bracket. The
ultrasonic energy travels through the co~ting 18 to
excite the region of initial contact between the
coating 18 and~the web 16.
Referring to Figure 2, a curtain coating
system is shown.~ In the coatin~ system 26 of Figure
2A, no ultrasonic excitation is provided. The curtain
coating die 28 is spaced above the bac~up roller 14.
The web 16 travels from left to right in the figure.
W093/07969 PCT/US92/07610
-11- 21169~
The coating material 18 is extruded from the die 28
and falls in a curtain onto the web l6 across the
desired width of the web 16.
In Figure 2B, ultrasonic energy is applied
S to the coating system 26' such that the energy acts on
the web 16 and coating 18 in the region of initial
contact between the web 16 and coating 18. The
ultrasonic horn 20 replaces the backup roller 14. The
. .
ultrasonic energy is applied directly to the web 16 ~'
and excites the web 16 and coating 18 at the location
of initial contact between the coating 18 and the web '
:~ 16. In Figurè 2C, both an ultrasonic horn 20 and a '~
backup roller 14 are used. The ultrasonic energy is
applied directly to the coated web 16 and the energy
~: 15 travels upweb through the web 16 and coating 18 to
:~ excite the line of initial contact between the coating
18~and the web I6. Moreover, when the curtain length
, is short, an ultrasonic die (not shown) can be used in
a manner similar to, the system 10' of Figure lD.
" 20 Additionally, a downweb structure such as a
rigid leveling bar 30 shown in Figure 2D, or a
flexible leveling pad 32 shown in Figure 2E may be
used to smooth or level the coating material 18 after
it is applied to~improve the thickness uniformity.
25: When a downstream element such as the leveling bar 30
or leveling pad:32 is used as part of the coat'ing
system 26', application of ultrasonic energy can be
beneficially applied in the region of final contact
between the-coated web 16 and the downweb leveling
: 30 structure. Thus, the ultrasonic energy need not reach
the region of initial contact between the web 16 and
the coating 18~as~long as it reaches the region of
final contact between the coated web 16 and the
leveling bar 30 or leveling pad 32. The web beneath
9 62 -12- PCT/USg2/07610
the leveling bar 30 and the leveling pad 32 can be
supported (as shown) or unsupported. These devices
can be directly ultrasonically excited. An
ultrasonically excited unsupported structure could
also be used to meter the fluid.
Referring to Figure 3, a slot-fed knife die
coating system 36 IS shown. In Figure 3A, no
ultrasonic excitation is provided. The coating system
36 includes a slo~ ~éd knife die 38 located adjacent
to the backup r~ller 14. The web 16 of material to be
coated travels from left to right in the figure, and
the coating material 18 is deposited onto the web 16
across the desired web width as shown.
In Figures 3B, 3C, and 3D, ultrasonic energy
15~ is applied to the system 36' such that the energy acts
on the web 16 and coating 18 in the region of initial
contact between~the web 16 and coating 18. In Figure
3B, the ultrasonic horn 20 replaces the backup roller
14. The ultrasonic energy is applied directly to the
web 16 and excites the web 16 and coating 18 at the
location of~initial contact between the coating 18 and
the web 16, as well~as the coating 18 between the die
38 and the horn~2~0. In Figure 3C, both an ultrasonic
horn 20 and a~backup roller 14 are used. The
ultrasonic energy~is applied directly to the coated
web 16 and the energy travels through the web 16 and
coating 18 to excite the line of initial contact
between the coating 18 and the web 16. In Figure 3D,
the knife die is ultrasonicalIy excited and is shown
as knife die 40. The coating 18 i5 excited while
still within the knife die 40 and the energy travels
through the coating 18 to the region of initial
contact between the coating 18 and the web 16.
W093/07~9 PCT/USg2/07610
-13- 2116962
Additionally, the ultrasonic energy can ~-
excite the area between the region of initial contact
of the coating material and the web and the region of
final contact between the coating applicator device or
downweb structure and the coating material. This
applies to all discussed coating methods when downweb
structures are used.
Referring to Figure 4, a slide coating
.
system 44 is shown. In Figure 4A, no ultrasonic
excitation is provided. The coating system 44 is a
, slide die 46, and~is located adjacent the backup
; roller 14. The'web~16 of material to be coated ~'
travels from~left to~right in the figure and the
' coating material 18 is coated onto the web 16 as
shown. ~The coating is applied across the desired
width of the web 16.
In Figure 4B, ultrasonic energy is applied
to the~system 44'~suc~ that the energy acts on the web
16,and~coating~18 in the region of initial contact
2~0~ between the web~16~and coating 18. The ultrasonic
horn 20~replaces~the~backup roller 14 and the
ultrasonic energy~is applied directly to the web 16
and excites the web 16 and coating 18 at the location
of initial contact~between the coating 18 and the web
ZS ~16. Moreover,~ an~ultrasonic slide die (not shown) in
which the coating, 18 is excited while still within the
slide~die and~the~energy travels through the coating
~; 18 to the region of initial contact between the
coating 18 and the~web~l6 can be used.
Referring to Figure 5, a roll coating system
: : : ~
,~ ~ 50 is shown. In~Figurë 5A, no ultrasonic excitation
is provided. The~coating system 50 includes a pan 52 '
containing liqu~id coating material 18 and a roll 54
mounted for rotation within the pan 52. The backup
:
W093/07969 PCT/USg2/07610
~1 lG~2 -14-
roller 14 is located adjacent the roll 54. The web 16
of material to be coated travels from left to right in
the figure. The coating material 18 is applied to the
web 16 across the desired width and a smoother or
doctor blade 56 may be used to wipe off excess coating
18 and level or smooth the coating 18 on the web 16.
In Figure 5B, ultrasonic energy is applied
to the system 5~' such that the energy acts on the web
16 and coating 18 in the region of initial contact
between the web 16 and coating 18. This is
accomplished by replacing the backup roller 14 with an
: ultrasonic horn 20. The ultrasonic energy is applied
directly to the~web 16 and excites the web 16 and
coating 18 at the location of initial contact between
~ 15 ~ the coating lg and the web 16. Alternatively, when a
:: ~dootor blade 56~is used as part of the coating
:: applicator device lO to level or smooth the coating 18
on the web 16, application of ultrasonic energy can be
beneficially applied:in the region of final contact
20:~ between the coated web 16 and the downweb doctor
blade. Thus, when;the doctor blade is used, the
ultrasonic~energy need not reach the region of initial
contact:between~the:web 16 and the coating 18 as long
as it reaches the region of final contact between the
25~ ~coated web 16 and~;the doctor blade. Ultrasonic energy
also performs:well with other coating systems
including those~with a plurality of rolls.
: Figures 6A, 6B, and 6C correspond to Figures
: : lA, lB, and lC, respectively, and illustrate
non-contact extrusion coating systems 60, 60'.
~: : In one~arrangement for all of the coating
configurations,~the ultrasonic source is located at
the line of initial contact between the coating
material and the web. Preferably, the ultrasonic
W~93/07~6g PCT/~S~2/07610
-15- ~1 1 G9 ~2
energy is applied at the backside of the web through
an ultrasonic horn used in place of a backup roll or
other support. However, the ultrasonic source can be
located remotely from the initial contact line to
S apply energy to the coated or uncoated web as long as
sufficient ultrasonic energy reaches the line of
initial contact. The maximum distance is about 15 cm
although the best results have been found to occur
within 8 cm. Alternatively, as discussed with respect
to Figure 2, the ultrasonic energy can be applied
within 15 cm of the location of any downweb leveling
or-smoothing structure. Also, the ultrasonic energy
can excite the area between the region of initial
contact of the coating material and the web and the
lS~ region of final contact between the coating applicator
device or downweb structure and the coating material.
The ultrasonic energy can be applied at any one or a
combination of these areas.
Regardless of the location of the u}trasonic
energy source, the ultrasonic energy adds energy to
the coating liquid. As the acoustic energy intensity
increases, the coating quality and processability,
including the thickness uniformity, improves until an
optimum acoustic intensity level is reached. Acoustic
energy preferably is applied near this optimum level
which is at intensity levels between 0.1 W/cm2 and 40
W/cm2, depending on the kind of coater and the type of
material being coated. However, the application of
ultrasonic energy can create web vibrations such as
surface acoustic waves which apply energy to the
coating. Depending on the magnitude of the vibration,
this can improve or degrade the coating quality. Care
must be taken to avoid adverse affects such as lower
W093/07969 PCT/USg2/07610
2~ GQ62 -16-
frequency standing waves which yield coating
nonuniformity.
The application of ultrasonic energy through
a backup horn that generally replaces a backup roller
S is the preferred arrangement in all coating
configurations. The ultrasonic energy can be applied
to the web by direct contact or through any medium
which transmits a~sufficient amount of energy such as
a coupli:ng fluid. The working surface of the horn
itself, and also the web in contact with the horn, is
at or near a pressure node in the acoustic standing
wave. As the ultrasonic energy is transmitted and
, :
reflected by the~web and the coating material, the
combined waves~pull coating material toward the horn
pressure node and toward the web. This improves
drawdown in extrusion coating and provides a more
stable liquid contact line in both extrusion and
curtain coating.~The coating material is urged to the
web and reduces~the~tendency for air entrainment
20 ~ between the~coating and the web. Other desirable
effects that improve wettability include phenomena
such~as ultrasonic viscosity reduction and contact
line and~bulk fluid~dynamics with the associated fluid
momentum contributions. Furthermore, because the horn
2~5 is a rigidly mounted, nonrotating, low friction
surface, backup roll runout and the associated downweb
variations are~eliminated. If desired, a carrier web
could be used;to shield the moving coated web from the
-~ stationary ultrasonic horn.
3~ I~f the~region of initial contact between the coating material and web is confined by another
structure, as with the slot-fed knife system of
Figures 3B and 3D, additional effects may occur.
Because the coating material forms a thin layer
:,
W093/0796g -17- 7f f 6 9 6 2
between two acoustically-matched materials, the
transmission of acoustic energy is greatly enhanced.
The acoustic energy density in the coating material
between the die and the web is much greater than that
outside this region. Moreover, if a low coating
weight or void streak occurs in the coating area, the
acoustic energy density in this area is lower and an
increased fluid crossflow occurs which fills in the
streak. The increased energy density of the fluid in
the coating area increases the crossweb flow, reduces
streaks, reduces the tendency for air entrainment, and
results in better~crossweb uniformity and a flow
configuration~which is more resistant to external
disturbances. Additionally, the system can operate
15~ with larger gaps between the die and the web. This
permits operating with larger process tolerances as
the die position is not as critical-as when ultrasonic
;energy is not used. The use of larger coating gaps
reduces~web tear-out problems. Also, machining
20 ~ variations on the;die faces become a smaller
percentage of the~`total coating qap and their adverse
; effect on coating uniformity is reduced.
:
The preferred frequency of vibration for the
acousti~ enerqy ~is~at the low end of the ultrasonic
~25 ~ spectrum at 20,000 Hz. However, because the benefits
of ultrasonically-assisted coating are not highly
dependent on frequency, a broad range of high and low
frequencies is functional. Although lower frequencies
are audible and~present noise control problems, they
can be used when higher amplitudes are required as
with more viscous liquids or for scale-up of larger
systems. Higher frequency ultrasonic systems present
scale-up problems because they are smaller due to the
shorter wavelengths that accompany higher frequencies.
WOg3/07g~9 PCT/US92/07610
21 i ~ ~ -18-
However, high frequency systems may be preferred for
lower viscosity (less than 500 cps) liquids as they
generate fewer low frequency resonances.
Peak-to-peak amplitudes of ultrasonic
vibration between 0.002 mm and 0.20 mm have been
tested in ultrasonically-assisted coating. The higher
amplitudes are more useful for highly viscous liquids
or thin layers whereas lower viscosity liquids or
thick layers require lower amplitudes. For example,
in a slot-fed knife system with a S,000 cps
solvent-based rubber coating, a peak-to-peak amplitude
of 0.03 mm at 20,000 Hz is sufficient to observe the
desired improvements in coating quality. If the
amplitude is too large, coating uniformity can be
disrupted by~localized nonuniformities such as
rippling ef~fects.
The~angle~of input of the ultrasonic waves
preferably is perpendicular to the direction of web
travel, as shown ~in Figures ~B and lC. However, while
20~ this orientation~is preferred, the angle of input can
range from perpendicular to parallel to the plane of
;the web l6. Figures lE and lF show systems similar to
Figures lB and lC~in which the ultrasonic energy is
transmitted through an ultrasonic horn 20 and an
25~ ultrasonic die~2~2,~respectively. In these
embodiments, the horn 20 and the die 22 transmit the
;~ ultrasonic enerqy at an angle between 0 and 90. ln
` Figure lG, the~ultrasonic horn 20 transmits the -~
uItrasonic energy in a direction parallel to the plane
~ .
of the web 16 such that the amplitude of vibration of
the ultrasonic energy lies in the direction of web 16
tra~el. ~
If~ultrasonic~energy is applied through the `-
coating die (as in Figures lD and 3D) it also effects
.
:
W~93/07969 PCT/US92/07610
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the flow of coating material in the die. It h1sl ~ a~2
found that in some instances when the pumping force is
held constant, the flow rate through the die is
doubled when ultrasonic energy is applied parallel to
the liquid motion and the flow rate is improved by a
factor of five when it is applied in the perpendicular
direction. In addition, ultrasonic excitation of the
die increases the temperature of the coating material
which improves the natural flow of coating from the
die. Also, debris stuck in die crevices can be coaxed
out of the die by ultrasonic excitation, thus
; eliminating the presence of streaks in the coated web
due to trapped debris. The die is preferably excited
as a standing wave. Alternatively, the ultrasonic
vibrations can be appIied as a traveling wave
propagating through the die, either with or without
the use of a coupling material.
Many series of experiments with various
fluids have been run. In one experiment, a 30 cm ~12
~`20 in) wide knife die with an ultrasonic backup horn was -
used. A rubber-based adhesive was coated at a web
speed of 7.62 mlmin (25 ft/min) at 0.0635 mm (0.0025
; in) thick. The~ultrasonic amplitude was about 0.0305
~mm (0.0012 in) peak-to-peak. One area in the die was
.
25 intentionally plugged for about 1 mm ~0.04 in) to ~-
~ simulate a clogged die and demonstrate the ability of
; the ultrasonics to compensate with sufficient
crossflow in the coating nip to mask streaks. Cross
web coating thickness profiles were taken and are
30 illustrated in Figure 7. The coating width on the web -
is shown along the x-axis and the coating thickness is
shown along the y-axis. Figure 7A shows coating
without ultrasonics. A streak at area A was caused by
the plug in the~die orifice and a dip at area B was a
W0~3~07969~ PCT/US92/07610
naturally occurring thin coating area in the web.
When the ultrasonics was turned on, the area A filled
in to within 92% of the overall coating thickness and
the area B dip was essentially eliminated, as shown in
S Figure 7B.
Pilot plant data also was obtained. A run
of 24,689 m (81,000 ft) of 61 cm ~24 in) wide
rubber-based adhesive tape was made at 15.24 to 30.48
m/min (50 to 100 ft/min) using a slot fed knife die
with an ultrasonic backup horn. The ultrasonic
amplitude was varied between 0.015 and 0~025 mm
(0.0006 and 0.001 ~inj peak-to-peak. The coating was
0.030 mm (0.0012~in)~ thick and crossweb profiles were
measured. Ten consecutive scans of 230 data points
each were taken noting the range of the coating
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thickness and~the standard deviation of the last scan,
and the average range and standard deviation of all
ten scans. (The~range is the minimum to maximum
crossweb coating~thickness.) The~ten scan ~roups were
20 ~ ~performed 17 times with ultrasonics and 9 times
without ultrasonics.
An indicatîon of transient coating thickness
variation can be~determined by considering how much
the range~of~a single scan varies from the average
;25 ~ range of severa~ scans before it. The coating range
variations that~occur with time therefore can be
indicated by subtracting the average range of the ten
scans from the tenth scan of a group of ten scans,
taking the absolute value, and dividing by the average
range. This is~performed for all of the groups of ten
scans, then averaged. Figure 8A compares the average
range variation as a percentage for the scan groups
with ultrasonics with the scan groups without
ultrasonics. Ultrasonics reduces the percent
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W~93/07969 PCT/US92/07610
-21- 211 69~2
variation from 47% to 15%, a three-fold reduction.
Figure 8B compares the standard deviation variations
of the runs with and without ultrasonics. The
standard deviation variation percentages were reduced
from 25~ to 10% when ultrasonics was used. These
figures show the improved consistency of the overall
crossweb caliper profile as a function of run time.
- Once a desired coating profile has been established,
the profile varies less with time when ultrasonics is
present than without ultrasonics.
Various~changes and modifications may be ~-
effected therein~by one skilled in the art without
departing from the scope or spirit of the invention.
~ For example, instead of using a sonotrode as the
15 ultrasonic energy source, eccentric cams or white ~-
noise genera~tors~can be used to improve coatings.
Additionally, acoustic energy can be applied to both
sides~of the web. ~
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