U.S. patent application number 10/992540 was filed with the patent office on 2005-04-07 for method for forming multilayer release liners and liners formed thereby.
This patent application is currently assigned to Avery Dennison Corporation. Invention is credited to Dordick, Robert, Guo, Hongjie, Huff, Stephen, Hulme, Adrian, Jansen, Alexander, Kettenis, Arnoud H., Kray, William, Meyer, Daniel, Potjer, Bert R., Sartor, Luigi, Shih, Frank Yen-Jer, Su, Wen-Chen, Tsai, Kuolih, van Zanten, Aad.
Application Number | 20050074549 10/992540 |
Document ID | / |
Family ID | 46281004 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050074549 |
Kind Code |
A1 |
Su, Wen-Chen ; et
al. |
April 7, 2005 |
Method for forming multilayer release liners and liners formed
thereby
Abstract
Disclosed herein is a multilayer silicone release surface
comprising a backing, a support layer on the backing, and a
silicone layer of the support layer. The various layers of the
multilayer release surface are deposited substantially
simultaneously, as for example by a dual die or using curtain
coating techniques.
Inventors: |
Su, Wen-Chen; (La Verne,
CA) ; Sartor, Luigi; (Pasadena, CA) ; Tsai,
Kuolih; (Arcadia, CA) ; Shih, Frank Yen-Jer;
(Arcadia, CA) ; Meyer, Daniel; (Glendora, CA)
; Huff, Stephen; (Chino Hills, CA) ; Potjer, Bert
R.; (Voorschoten, NL) ; Guo, Hongjie;
(Arcadia, CA) ; van Zanten, Aad; (Alphen aan den
Rijn, NL) ; Kettenis, Arnoud H.; (Alphen aan den
Rijn, NL) ; Kray, William; (Alta Loma, CA) ;
Hulme, Adrian; (Mentor, OH) ; Jansen, Alexander;
(Zoetermeer, NL) ; Dordick, Robert; (Sherman Oaks,
CA) |
Correspondence
Address: |
CASELLA & HESPOS
274 MADISON AVENUE
NEW YORK
NY
10016
|
Assignee: |
Avery Dennison Corporation
Pasadena
CA
|
Family ID: |
46281004 |
Appl. No.: |
10/992540 |
Filed: |
November 18, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10992540 |
Nov 18, 2004 |
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09918652 |
Mar 22, 2000 |
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10992540 |
Nov 18, 2004 |
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09389167 |
Sep 2, 1999 |
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10992540 |
Nov 18, 2004 |
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08616859 |
Mar 15, 1996 |
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5962075 |
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10992540 |
Nov 18, 2004 |
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08483509 |
Jun 7, 1995 |
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5728430 |
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Current U.S.
Class: |
427/154 ;
118/256; 427/294 |
Current CPC
Class: |
C09J 2203/334 20130101;
B05C 5/0254 20130101; B05C 9/06 20130101; D21H 27/001 20130101;
D21H 19/32 20130101; G03C 1/74 20130101; C09J 2483/005 20130101;
C09J 7/403 20180101 |
Class at
Publication: |
427/154 ;
427/294; 118/256 |
International
Class: |
B32B 033/00 |
Claims
What is claimed is:
1. A method of increasing the coating gap for a die-coated
silicone-containing layer, comprising: determining the maximum
coating gap where a substantially defect free release layer is
formed on a coated paper; increasing the coating gap; and applying
a vacuum upstream and adjacent to a coating bead formed by the
die.
2. A method for applying a silicone-containing release layer to a
backing, comprising: moving the backing along a selected path;
providing a die at a selected spacing from the backing and
expressing the silicone-containing release layer from the die, the
spacing between the die and the backing being selected to achieve
an acceptably low dispersion of the release layer into the backing
and an unstable feed of the release coating upstream of the die;
and applying a vacuum in proximity to the backing and the die at
locations upstream of the die for stabilizing the bead of the
release layer while maintaining an acceptably low dispersion of the
release layer in the backing.
3. The method of claim 2, wherein the vacuum is applied at between
1-250 cm of H.sub.2O.
4. The method of claim 3, wherein the vacuum is applied at between
25-200 cm of H.sub.2O.
5. The method of claim 3, wherein the vacuum is applied at between
50-75 cm of H.sub.2O.
6. An apparatus for applying the release layer to a backing,
comprising: means for moving the backing along a selected path; a
die having a die slot for releasing a layer of a
silicone-containing material onto the backing, the die being
adjustable for selectively increasing and decreasing a distance
between the die slot and the backing; and a vacuum apparatus for
generating a vacuum in proximity to the backing and the die and on
an upstream side of the die relative to a moving direction of the
backing.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 09/918,652, filed Mar. 22, 2002, which is a
continuation-in-part of U.S. patent application Ser. No.
09/389,167, filed Sep. 2, 1999, which is a continuation of U.S.
patent application Ser. No. 08/616,859, filed Mar. 16, 1996, now
issued as U.S. Pat. No. 5,962,075, which is a continuation-in-part
of U.S. patent application Ser. No. 08/483,509, filed Jun. 7, 1995,
now issued as U.S. Pat. No. 5,728,430, the entirety of each of
which are incorporated by reference as if fully set forth
herein.
FIELD OF THE INVENTION
[0002] The present invention generally relates to release surfaces
of the type used with pressure-sensitive adhesive (PSA)
constructions. More particularly, the present invention relates to
multilayer release liners and their methods of manufacture.
BACKGROUND OF THE INVENTION
[0003] A pressure-sensitive adhesive construction, such as a label,
generally comprises a facestock or label surface, an adhesive
composition adhered to the facestock, and a release liner. The
adhesive composition is typically coated on a silicone-containing
release surface of the liner. Alternately, the PSA can directly be
coated onto the facestock and then be laminated to the release
liner. In these combinations, the release liner protects the PSA
prior to the label being used and is removed immediately prior to
application of the label to another surface. Additionally, the
release liner serves to facilitate cost effective manufacture of
rolls or sheets of labels. The release liner also functions as a
carrier of labels for dispensing in automatic labeling operations
and for computer printing in EDP applications. The performance
attributes of a release liner are critical to both the manufacture
and end-use application of adhesive labels.
[0004] In conventional practice, the release liner is provided as a
silicone layer on a paper or film surface having high holdout,
i.e., the surface of the paper on which the silicone layer is
deposited is resistant to silicone penetration. This is preferred
because silicone tends to be an expensive component of a release
liner, and it is therefore desirable to minimize the amount of
silicone coated. High quality papers conventionally used in the
manufacture of release liners, such as a super-calendered or
densified glossy paper, achieve this goal by providing a surface
which absorbs much less silicone than regular open paper. However,
use of these high quality papers increases the cost of the end
product adhesive construction, because such papers are typically
much more expensive than regular open papers.
[0005] One currently accepted way of applying a silicone release
composition to a high holdout paper is by solvent coating. Growing
concern over the environment has imposed stringent restrictions
regarding recovery of the solvent used in applying the solvent
based silicone to the high-holdout backing paper or other
materials. An alternative to this is to use 100% solids silicone
release compositions. These are supplied with a viscosity (usually
<2000 cps) suitable for roll-coating techniques. When applied to
porous low cost papers, such as machine finished (MF) or machine
glazed (MG) papers, these materials soak into the paper (penetrate
the paper surface) to give ineffective coverage of the paper fibers
unless excessively high quantities of expensive silicone are used.
Ineffective coverage of the paper fibers provides unsuitable
release liners for PSA applications, especially where high speed
convertibility is an essential performance feature.
[0006] One proposed prior art solution to these problems is to use
low cost open papers which have been pre-coated with a support
layer comprising an inexpensive filler material, and then to coat
the silicone layer onto the support layer. The filler material of
the support layer flows into the pores and interstices of the open
paper surface which would otherwise absorb silicone if the silicone
were directly coated onto the paper. Consequently, less silicone is
needed to form an adequate release surface. An example of this
approach may be found in U.S. Pat. No. 4,859,511 to Patterson.
However, there are several drawbacks to this prior art process.
First, additional costs are incurred because the prior art methods
require two coating steps. The prior art teaches that the support
layer must first be coated and then dried, cured or hardened before
the silicone layer may be coated. Otherwise, there is a potential
for undesirable intermixing or destruction of the respective
layers. Second, because two separate coating steps are required,
more time is needed for the overall formation of the release liner.
These additional processing costs somewhat offset the savings
realized in materials by using support layers in combination with
lower cost open papers.
[0007] Thus, there is a need for improved methods of forming
multilayer release surfaces in which a support layer is used in
conjunction with a silicone layer to form a release liner.
SUMMARY OF THE INVENTION
[0008] The present invention advantageously provides an efficient
method of creating multilayer release liners, thereby overcoming
the problems resulting from the prior art processes. The present
invention achieves these benefits by providing a method of coating
both a support layer and a release layer on a substrate
substantially simultaneously. Consequently, separate coating steps
are eliminated, and a corresponding savings in both time and costs
are achieved.
[0009] Generally, these advantageous results may be achieved by at
least two different coating methods. The first method achieves
these results by modifying the die used to coat the support layer
and a release layer (e.g., silicone layer) so that the die can
dispense the fluids of both layers substantially simultaneously at
a single coating station. The die dispenses the support layer onto
the substrate, and substantially simultaneously, the release layer
on the support layer. There is no need for a separate drying,
hardening or curing step to prevent the layers from intermixing. By
controlling the coating gap between the die and substrate, the
processing conditions of the modified die may be optimized to
achieve the most stable and efficient deposition of these layers.
In some embodiments, application of controlled vacuum to the dual
die coating process may be used to improve coating efficiency,
increase coating tolerances and provide for less penetration of
coated fluids onto the substrate to be coated. The multilayer
release surfaces resulting from the simultaneous dual die coating
of support layer and silicone-containing layer are also believed to
have a unique morphology and advantageous properties.
[0010] Simultaneous coating of the support and release layers to
form a multilayer release surface may also be achieved by curtain
coating. For example, a slide coat die may be modified to have two
slots, with the upper slot metering the release layer and the lower
slot the support layer. The release layer and support layer combine
on the die face surface, and fall to the moving substrate as a
multilayer liquid sheet. The distance between the die and the
substrate may range from 5 cm to 50 cm, and more preferably, from
10 cm to 30 cm. Advantageously, curtain coating techniques do not
require as precise an optimization of the coating gap between the
die and the substrate to generate high speed coatings, and high
coating speeds are easily obtained.
[0011] With respect to simultaneous coatings using a dual die, the
present invention provides a method that is capable, at steady
state coating conditions, of precisely controlling the interface or
"separating streamline" between the support layer and
silicone-containing layer as these layers are being coated onto the
substrate. Unlike single-layer coating, the stability of the flow
(i.e., its tendency to exhibit only a steady, two-dimensional flow)
particularly at the separating streamline between the two layers,
is extremely important. Advantageously, this method can be used to
substantially simultaneously coat a support layer on a backing in
conjunction with a silicone-containing release layer on the support
layer. As used herein, substantially simultaneously refers to two
or more liquid layers being deposited at a single coating station
without an intermediate drying, curing, or hardening step for the
support layer. For die coating, preferably, the single coating
station comprises the dual die described herein, although this is
not essential to the present invention. For example, the single
coating station may comprise two separate dies located close enough
spatially to achieve the benefits of a dual die.
[0012] The present dual die method involves a number of preliminary
steps, the sequence of which is not particularly important. These
steps include an analysis of certain parameters of the liquids to
be coated, the particular and precise design of the geometries of
the die lips, and the assembly or setup of the die with respect to
the moving web. Following these steps, a number of experimental
release-surface coatings can be made in order to determine an
operating window for achieving successful multilayer dual die
coating. Even within this window, a higher quality window can be
determined for full production coating operation. These steps
assist in providing a stable, two-dimensional flow.
[0013] An unstable flow changes its profile with respect to time.
This can result in random fluctuations or regular oscillations in
the flow profile, thus causing irregularities in the
cross-sectional film configuration. In addition, slight
perturbations in the coating process under unstable conditions may
propagate, rather than dampen out quickly to a steady state
condition as with stable flow. Likewise, a three-dimensional flow
may result in undesirable mixing of the two layers, or in
cross-web, nonuniform layer thickness, as well as other defects
such as non-continuous layers or voids, etc. In stable,
two-dimensional flow each layer has greater uniformity, thus
resulting in a product of higher integrity and performance.
Furthermore, if the flow is perturbed, this type of flow will
return to its steady, two-dimensional flow characteristics rapidly,
thus minimizing any defects in the product.
[0014] The coating method of the dual die aspect of the present
invention achieves a stable, two-dimensional flow by controlling
the interface of the flow at its upstream most position, which is
referred to herein as the separating streamline or separating line.
This line is defined, in the sense of web travel, as the cross-web
line where the topmost streamline of the bottom flow layer (i.e.,
support layer) first meets the bottommost streamline of the top
flow layer (i.e., silicone-containing release layer). In the
opposite direction, the separating line can be viewed as the
location where the two flows separate from the die lips. Although
the separating line runs completely across the web, when the
die/web interface is shown from the side, it appears as a point. As
noted, this separating line will occur in the region of the mouth
of the downstream slot or feed gap where the flows of the bottom
layer and top layer are confluent. For ease of reference, this
region will be referred to herein as the "interface region." It
will be understood that if the combined flow of the two layers is
stable and two-dimensional in this interface region, and more
particularly at the separating line, it is likely to retain such
flow characteristics throughout the coating process, thus resulting
in an improved end product.
[0015] In order to achieve such advantageous flow characteristics
at the separating line, the multilayer coating method of the
present invention assists in positioning that line at the
downstream corner of a die middle lip. This corner presents a
straight, two-dimensional line across the die. Thus, if the
separating line is coincident at this corner, one will be assured
of achieving stable, two-dimensional flow. For this reason, this
corner is referred to herein as the "stability point." On the other
hand, it will be appreciated that unstable or three-dimensional
flow conditions can cause the separating line to occur at several
locations in the interface region. For example, "recirculations" in
the bottom layer flow can cause the top layer flow to be pulled
upstream such that it separates from a position underneath the
middle lip. Likewise, vortices or other stagnant flow in the top
layer can cause the top layer to separate from the middle lip at a
position within the feed gap of that flow.
[0016] Stable, two-dimensional flow characteristics in the dual die
interface region are achieved in the present invention due in part
to a method of regulating the pressure gradient such that the
separating line is positioned at the stability point. In accordance
with one method of the present invention, the pressure gradient can
be regulated by designing and assembling a die having a particular
middle lip geometry. This method of pressure regulation helps to
pin or lock the separating line at the stability point. This is
achieved, as the name implies, by regulating the pressure gradient
in the interface region. As is well understood, the pressure
gradient in this region is highly dependent on the coating gap and
its relationship to the downstream film thickness. In accordance
with complex but well understood principles of fluid mechanics, the
pressure gradient created at a particular longitudinal portion in
the bead is related to the coating gap at that point and the
downstream thickness of that flow. Here, however, much care must be
taken in the analysis. Indeed, for a single-layer coating the
analysis is more direct, since there is only one flow, and one
downstream film thickness. However, for a multilayer coating
process, there are two or more flows. Thus, in a method for
regulating the pressure gradient at a given point in the flow, the
coating gap at that point and the downstream film thickness of the
layer(s) formed by that flow must be analyzed in order to achieve
proper lip design and positioning parameters.
[0017] Therefore, an analysis of the pressure gradient within a
particular flow, and particularly the pressure gradient of the
combined flow at the interface region, is quite complex.
[0018] The dual die method of the present invention designs the
middle and downstream die lip geometries such that the pressure
gradients in the flow fix the separating line at the stability
point. In another aspect of the method, the middle lip may extend
slightly toward the web. Therefore, the profile formed by the
design of the middle and downstream lips of the die represent a
step away from the web in the direction of web travel. This step
configuration may be flat or parallel with respect to the web or
angled with respect thereto. It may even exhibit other designs. It
is only important that certain pressure gradients be maintained in
the interface region, and particularly along the middle coating gap
from the stability point toward the upstream corner of the middle
lip.
[0019] It has been observed that the magnitude of the step may be
in the range of 0 to about 100 microns inches when coating
multilayer adhesive compositions. However, for dual die coating of
a support layer and a silicone-containing layer, it is preferred to
minimize or eliminate the step. Consequently, for these multilayer
release systems, it is presently preferred that any step be in the
range of from 0 to about 50 microns, with the optimal step
approaching zero. Minimizing or eliminating the step in this manner
optimizes the multilayer coating process for silicone release
system.
[0020] When a stepped design is used, it should be appreciated that
the die lips stepped design affects the coating gap under both the
middle and downstream lips in the interface region. Because the
middle lip is stepped toward the web, the coating gap under this
lip will be less than that under the downstream lip. As a result,
for most multilayer coatings, if the die is correctly positioned
with respect to the web, the pressure gradient under the middle lip
will be very slightly positive to approximately zero, while the
pressure gradient under the downstream lip will be negative. When
these stepped dies are used with these pressure gradient
differentials, the gap under the middle lip can be from two to
three times the film thickness, with the corresponding pressure
gradient under the middle lip again being from slightly positive to
zero. Again, this relationship exists at least in the interface
region close to the mouth of the downstream feed gap. Due to other
lip designs (such as bevels) and adjustments in the angle of attack
of the die, the relationship between the pressure gradients under
the middle lip and under the downstream lip may vary differently.
However, in the interface region it is important that the pressure
gradient at or just upstream of that region not be excessively
positive in the direction of web travel.
[0021] If the pressure gradient is too high in this region, certain
instabilities in the flow may occur, resulting in coating defects.
For example, in the absence of proper pressure gradient regulation,
the bottom layer flow may exhibit "recirculation" under the middle
lip. This could occur, for example, if the downward step in the
middle lip is not properly adjusted, and an excessively large
coating gap occurs in this region. Desirable pressure gradients may
be achieved for dual die coating multilayer silicone release
systems when the step of the middle and downstream lips is
minimized. Furthermore, the coating gap of the middle and
downstream lips may be from 2 to 3 times the total wet film
thickness. A larger coating gap results in a highly positive
pressure gradient in the bottom layer flow, causing it to actually
flow upstream a short distance before turning around and flowing
downstream, causing "recirculation" of the flow. One of the most
serious disadvantages of such recirculations in the bottom layer
flow is its tendency to pull the top layer flow upstream under the
middle lip and away from the stability point. Thus, the separating
line moves upstream and there is no assurance that the line will be
formed in a straight and steady manner. Thus, mixing and diffusion
between the two layers at their interface may increase. In
addition, the flow may be mottled or blotchy. Other defects can be
caused by recirculations. Recirculations are of two types: open
loop and closed loop. Open-loop recirculations are less damaging
because any liquid entering them leaves after a short period of
time (low "residence time"), before continuing to flow downstream.
Closed-loop recirculations, however, result in high residence time
because the liquid is trapped in them. Moreover, all recirculations
are known to prefer three-dimensional flow characteristics.
[0022] On the other hand, the pressure gradient under the middle
lip cannot be too large (which might occur, for example, if the
coating gap in this region were too small). Such a large pressure
gradient is likely to result in upstream leakage of the fluid.
Also, as mentioned above, such high pressure gradients can result
in high shear stresses with other deleterious effects on the
performance of the coating.
[0023] It will also be observed that the step designed into the
middle lip can be achieved by positioning that lip at the proper
coating gap and moving the downstream lip further away from the
web. However, there is also a tradeoff in this parameter. If the
coating gap under the downstream lip then becomes too large,
recirculations or vortices in the top layer flow may result. One
additional type of defect that may occur is known as "chatter", or
a two-dimensional oscillation of the bead.
[0024] Thus, an important advantage of the method of the dual die
aspects of present invention is that it provides a proper pressure
gradient ahead of the interface region for the coating of
multilayer silicone systems. However, as explained, this advantage
can only be achieved when the die is correctly set with respect to
the web in order to exhibit proper coating gap characteristics.
Preferably, it has been found that the die should be set such that
the coating gap under the middle lip (especially in the interface
region) is approximately two to three times the bottom layer wet
film thickness downstream of the die (before drying). It should be
re-emphasized that this thickness, however, is the thickness of the
bottom layer only which is being coated from this particular flow
under the middle layer. Similarly, the coating gap under the
downstream lip (particularly in the interface region) should be
greater than one but not greater than three times the wet film
thickness downstream, to provide the least pressure under the lips
and therefore minimize flow of material into the paper. In this
latter case, this thickness is the combined thickness of both
layers as well as any previous layers. Thus, it will be understood
that these principles apply to multilayer coating of any number of
layers, with the terms "bottom layer" and "top layer" referring to
any two adjacent layers. It will also be recognized that these
relationships will slightly vary due to non-Newtonian
characteristics of the liquid, as well as other variables.
[0025] On the other hand, the method of the present invention
allows for optimization of the dual die multilayer coating process.
In one aspect of the method, the middle and downstream lips are
flat or parallel with respect to each other. Thus, any convergence
of the downstream lip can be achieved by adjusting the angle of
attack of the die. In another aspect of the method, however, the
optimization of the coating process is facilitated by beveling the
downstream lip so that it exhibits some convergence, even without
any angle of attack adjustment. With this improvement the
"operating window" of the die can be increased. This means that
successful coating can be achieved, even if certain coating
parameters cannot be accurately controlled. On the other hand, a
larger operating window increases the chance of a larger quality
window where the best coating occurs. Moreover, a large operating
window allows a technician of less skill or experience to
successfully perform the coating operation. In addition, a wider
variety of products comprised of a broader range of liquids can be
produced, even single-layer products.
[0026] In another aspect of the present invention, the upstream lip
is also designed so that it steps toward the web with respect to
the middle lip. This also achieves an increasing pressure gradient
in the upstream direction and assists in sealing the bead under the
die lips to avoid upstream leakage. There is always recirculation
in the bottom layer under the upstream lip. However, typically,
such recirculation is open so that it does not negatively affect
the quality of the bottom layer. This upstream lip can be "flat" or
parallel to the web, or it may be beveled or angled with respect
thereto. Preferably, the bevel represents a divergence in the sense
of the web travel. This profile presents a positive pressure
gradient in the upstream direction, which further assists in
sealing the bead.
[0027] When the upstream and downstream lips of the present method
are beveled, the middle lip is preferably maintained close to flat
(in the sense that it is approximately parallel to the web, not
taking into consideration any curvature). This can be achieved,
even during operation, since angle of attack adjustments are
minimized due to the beveling of the aforementioned lips. The
flatness of the middle lip, together with an appropriate coating
gap, provides a zero pressure gradient to the flow, which
advantageously avoids recirculations and still reduces shear rate
and shear stresses, as discussed above. A flat middle lip also has
the advantage of reducing the risk of upstream leakage. Moreover,
this middle lip is the most expensive to manufacture, and the
absence of a bevel assists in reducing costs.
[0028] It should be noted that other lip geometries are possible in
order to achieve the advantages of the present invention. Also,
other methods of pressure regulation are possible.
[0029] In another aspect of the present invention, pressure
gradient regulation can also be achieved with lip designs of a
particular length, especially that of the middle and downstream
lips. That is, it will be appreciated that the length of the die
lips will affect the coating gap if the angle of attack of the die
is adjusted. Typically, with a negative angle of attack (a
convergence of the die lips with the web in the downstream
direction), the coating gap at the upstream portion of each lip is
greater than at the downstream portion of each lip. This is
especially true, considering the curvature of the back-up roll. As
noted above, if coating gaps are too great, recirculations will
occur due to inappropriate pressure gradients, thus causing the
loss of control of separating line position and poor coating
quality.
[0030] In addition, as noted above, the flow experiences shear
stresses in the bead due primarily to the rapidly moving web. Even
if the shear rate is tolerable with respect to fluid properties,
the duration of the shear can have damaging effects on liquid
quality. The longer the lips, the greater the duration of the shear
stresses experienced by the liquid. Thus, it is important when
designing the die lip geometries, to consider the length of the die
lips for coating gap, as well as shear stress considerations.
[0031] Therefore, it is an important aspect of the present method
that the lip lengths are minimized, while providing sufficient
length to develop stable rectilinear flow. Perhaps the most
important die lip length is the downstream lip. This lip must be
long enough for the flow to develop. Such lip may be in the range
of 0.1-3.0 mm in length, with about 0.8-1.2 mm being preferable.
The middle lip also may range from 0.1-3.0 mm, but is preferably
about 0.3-0.7 mm in length. The upper lip, on the other hand, can
be longer without suffering shear stresses in the liquid because
the length of travel is reduced. Moreover, a longer upstream lip
assists in sealing the bead. Thus, a lip in the range of 1.0-3.0 mm
is advantageous, with 1.5-2.5 mm being preferable.
[0032] Thus, the present method of multilayer coating has a
downstream feed gap region characterized by a pressure gradient
which generates stable flow at the interface between a bottom layer
(including any previously coated layers) and a top layer. For the
embodiments described above, this pressure gradient is achieved by
a combination of middle lip and downstream lip geometries, which
result in an adequate pressure gradient at the interface region
which is not so positive as to cause recirculations.
[0033] In addition to the correct design of the die lip geometries
and the assembly and setup of the die with respect to the web so
that correct coating gaps are achieved, the present method also
involves a careful analysis of certain fluid parameters with
respect to the liquids to be coated on the web. In particular, the
present method involves an analysis of the relative viscosities of
the two liquids. Preferably, the viscosity of the top layer liquid
should be greater than the viscosity of the bottom layer liquid.
More specifically, a top layer viscosity which is about 30% greater
than the bottom layer viscosity is optimal; however, successful
multilayer coating can be achieved when the top layer viscosity
ranges from about 50% less to 100% (or even more) more than the
viscosity of the bottom layer. However, it will be recognized by
those of ordinary skill that these ranges may vary even outside of
these boundaries for a given set of coating parameters.
[0034] This balancing of viscosities is important in order to
assist the process in achieving steady, two-dimensional flow.
However, because the flow experiences such high shear rates, the
viscosity analysis must take into consideration the change in
viscosity due to such shear rates. Thus, for example, due to shear
thinning, the viscosity of any liquid being coated may vary by
several orders of magnitude of milliPascal-seconds (mPa-sec). At
the same time, the shear rate may vary by four or more orders of
magnitude with respect to the film coating parameters involved with
the present method. In particular, shear rates above 1,000 s.sup.-1
are likely to be experienced under such coating conditions.
Accordingly, the relative viscosities of the liquids being coated
should be compared at these higher shear rates.
[0035] In addition, the surface tensions of the respective liquids
should be analyzed, with the top liquid preferably having a lower
surface tension than the bottom liquid. This condition helps to
avoid the formation of voids in the top layer with respect to the
bottom layer which may be formed by de-wetting phenomena.
[0036] Once the lip geometries have been designed and set with
respect to the die, and the liquid parameters analyzed, another
important aspect of the present invention is the experimental
determination of the area of operating parameters in which
successful coating can be achieved. This area is often referred to
as the "coating window" and may be defined in terms of a graph of
coating gap versus angle of attack of the die. Thus, in order to
determine a coating window, samples of the two liquids are
experimentally coated at varying coating gaps and angles of attack
and the coating quality is observed. The area where adequate
coating is achieved is noted, including the area where very high
quality coating is achieved (usually a subset of the overall
coating window). It is preferable that the coating window be as
large as possible so that inaccuracies in coating gap and/or angle
of attack do not result in coating defects or product degradation.
In order to add another dimension to the coating window, the same
liquids being tested are also tested at various viscosities.
[0037] Once the coating window is determined, production coating
may occur preferably at a point in the middle of the range of the
angles of attack and close to the maximum coating gap and angle of
attack.
[0038] When a dual die is used to simultaneously coat a support
layer and release layer the resulting multilayer release surface
has several desirable features. First, because the support layer
and silicone layer are coated substantially simultaneously as
liquids, the interface between the support layer and silicone layer
is not as sharp and distinct as if the support layer had been cured
or hardened prior to the coating of the silicone layer. This is
beneficial for certain applications, because the increased
dispersion observed between the layers facilitates binding of the
silicone layer to the support layer, and therefore decreases the
propensity of the silicone layer to rub-off or otherwise separate
from the support layer. Second, because the coating parameters of
the support layer and silicone layer are so tightly controlled by
the present method, the degree of dispersion of the two layers is
minimized to substantially the extent necessary to achieve
desirable bonding between the support layer and the silicone layer,
without undue waste of dispersed silicone in the support layer.
Finally, dual die coating may be used to form a multilayer release
surface from a support layer and release layer which would not form
a stable curtain for curtain coating because the surface energies
differ by too much.
[0039] The improved dispersion characteristics of the supporting
and silicone layers comprising the multilayer release surfaces of
the present invention can be characterized in several ways. One
preferred way is by transmission electron microscopy (TEM). When
TEM is applied to multilayer release systems of the prior art and
the dual die constructs of the present invention, it is observed
that two distinct layers, comprising the support layer and the
silicone release layer, are formed from both processes. However,
the borders of the layers of the prior art coatings are much
sharper, indicating that there is minimal intermixing of the
support layer and the silicone release layer. In contrast, TEM
scans of multilayer release constructs of the present invention
show that, while having well defined borders, there are a small
amount of silicone domains in the support layer, which is
indicative of desirable bonding within the layers.
[0040] In summary, the method of the present invention enhances the
optimization of the coating process for multilayer release
surfaces. The method can be utilized with a wide variety of
coatings and substrates in order to produce multilayer release
surfaces on open paper surfaces which have release properties equal
to or better than those produced on high quality papers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a perspective view of a multilayer die which may
be utilized in the present method, the die being positioned
adjacent to a moving web traveling around a back-up roll.
[0042] FIG. 2 is a graph of shear rate versus viscosity for three
sample liquids to be coated onto a web in accordance with the
present method.
[0043] FIG. 3 is a second graph of shear rate versus viscosity for
different sample liquids to be coated.
[0044] FIG. 4 is a close-up cross-sectional view of a coating gap
formed between a single layer die and a moving web illustrating
certain principles of fluid mechanics utilized in the present
method.
[0045] FIGS. 5a-5d are schematic illustrations of the velocity
profiles formed with in the coating gap illustrated in FIG. 4 under
certain coating conditions.
[0046] FIG. 6 is a close-up cross-sectional view of the coating gap
of the multilayer die shown in FIG. 1, further illustrating the
adjustment of the various coating parameters in accordance with the
method of the present invention.
[0047] FIG. 7 is a close-up cross-sectional view of the interface
region of the coating gap shown in FIG. 6 illustrating in more
detail the relationship between lip geometries and the coating gap
adjustment steps of the present method.
[0048] FIG. 8 is a schematic illustration of the recirculation that
may occur in the bottom layer liquid if the steps of the present
method are not followed.
[0049] FIG. 9 is a schematic illustration of a vortex that may be
formed in the bottom layer liquid if the steps of the present
method are not followed.
[0050] FIG. 10 is a close-up cross-sectional view of the multilayer
die of FIG. 7, illustrating the step of adjusting the die with a
negative angle of attack with respect to
[0051] FIG. 11 is a schematic illustration of the recirculations
that may occur under the die lips when the angle of attack
adjustment shown in FIG. 10 results in excessively large coating
gaps at the upstream portions of the lips.
[0052] FIG. 12 is a close-up cross-sectional view illustrating the
step of the present method of beveling the upstream and downstream
lips.
[0053] FIG. 13 is a schematic view of the recirculations that may
occur in the feed gaps if they are not properly sized in accordance
with the present method.
[0054] FIG. 14 is a graph of coating gap versus angle of attack
illustrating the step of experimentally determining a successful
coating window as well as the quality window for a particular set
of coating parameters.
[0055] FIG. 15 is TEM (6,700.times. magnification) of a cross
section of a comparative example generated by wet on wet two pass
coating of a silicone containing layer on a support layer.
[0056] FIGS. 16 and 17 are TEMs (3,700.times. magnification) of a
cross sections of multilayer release surfaces of the present
invention.
[0057] FIG. 18 is a TEM (2,700.times. magnification) of a cross
section of a comparative example generated by coating a silicone
layer on a dried support layer.
[0058] FIG. 19A-C are illustrative diagrams of a vacuum assisted
die coating process.
[0059] FIG. 20 is a cross sectional view of a vacuum box suitable
for assisting die coating.
[0060] FIG. 21 is a schematic cross-sectional view of a curtain
coating die which may be used to form multilayer release
surfaces.
[0061] FIG. 22 is a TEM of a cross-section of a multilayer release
surface formed by curtain coating.
DETAILED DESCRIPTION OF THE METHOD
[0062] Before describing in detail the various steps of the methods
of the present invention, it will be noted that the method is not
limited to the coating of release surfaces having only two layers,
but further comprises the coating of any number of a plurality of
layers that may be incorporated into a release liner. Thus, the
drawings and descriptions thereof should not be considered limiting
with respect to the scope of the method of the present invention;
moreover, such method should not be limited to any particular
sequence with respect to its steps, except where expressly
noted.
[0063] The present simultaneous coating method can advantageously
be used with a variety of different substrates, support layer
filler compositions and silicone materials. For example, the
substrate to which the supporting and silicone layers are applied
may be machine finished and machine glazed papers, such as RL-541
from Wasau Technical Papers, Data-60 and -70 from Crown Van-Gelder,
AT-45 and AT-70 from Ahlstrom Paper Group, Willamette paper #50
EDP, Georgia Pacific vellumina papers, and NL-60 from Gascogne
Paper Company, or any other type of relatively porous open paper
may be used. Other papers which may be used include bag paper,
calendared and non-calendared clay coated paper, latex saturated
paper, and the like. Although the benefits of the present invention
with respect to cost savings are more fully realized when
relatively inexpensive open papers are used, it should be
appreciated that the present invention can also be used with more
expensive finished paper substrates, including those known to have
a high holdout.
[0064] Furthermore, the substrate may comprise materials other than
paper, such as polymeric films comprising polyethylene,
polypropylene, polybutylene, or polyester films, such as
polyethylene terephthalate, polyvinyl chlorides, polyvinylidene
fluorides, polysulfides, polyamides and nylon polymers. Suitable
substrates may also include combinations of the foregoing including
combinations of paper and polymeric substrates. Preferably, when
materials other than paper are used, the substrate may have a
thickness of about 35-100 microns, and more preferably 35 to 65
microns, to more readily facilitate use with conventional coating,
converting and dispensing machines. As will be appreciated by one
of skill in the art, it is also preferred that the particular
substrate be chosen to provide a surface which will adhere well to
the filler material of the support layer, so that the resulting
construct will not delaminate.
[0065] As noted above, the support layer is applied to the surface
of the substrate, and flows into the small pores and openings which
make up the surface of the substrate. The support layer preferably
comprises a low cost filler material. A wide variety of filler
materials may be used in the present invention to form the support
layer. The filler material should be selected so that it adheres
well to the substrate upon which it is to be coated to quickly seal
the porosity of the paper, as will be appreciated by those of skill
in the art. Furthermore, the filler material should be capable of
being expressed from a dual-die or curtain coating die using the
principles outlined below. With these goals in mind, it has been
found that suitable filler materials for the present invention may
comprise emulsions or water dispersions of latexes, cross-linkable
latexes, water soluble polymers like polyvinyl alcohols,
carboxymethyl cellulose (CMC), starch, ethylene vinyl acetate
(EVA), and may include inorganic compounds and fillers like calcium
carbonate. Representative nonlimiting filler materials include
emulsions in water dispersions of styrene butadiene latex, styrene
butadiene rubber compounds (SBR), mixtures of water and Air
Products Airflex 465, and National Starch E-200 and water.
[0066] The silicone release surface may be derived from a variety
of sources. For example, an emulsion vinyl-addition silicone system
may be used, such as that described in Examples 1-6 below.
Alternately, an aqueous emulsion blend of a vinyl-addition silicone
system which includes at least one secondary or resin component may
be used. The secondary component may be a traditional thickening
agent to aid in the processing or simultaneous coating of the
silicon-release surface, such as glycol, ethylene oxide, starch,
urethane associated acrylates, cellulose polyethylene oxide,
polyvinyl oxides, as well as other thickening agents known to those
of skill in the art. Suitable vinyl-addition silicone systems with
secondary components are those described in U.S. Pat. No. 5,318,815
or 5,165,976, the entirety of each of which are incorporated herein
by reference. Representative silicone release compositions are
available, such as General Electric silicones GE 1111-11-259, GE
1192-05-117, GE 1111-13-286, GE 1111-15-307, Dow Corning silicones
7980, 7923, 5602, Wacker silicones 38197 VP, V-20 and Rhodia
Silicolease.
[0067] Furthermore, release surfaces suitable for use in the
present invention may also be formed of compositions devoid of
silicone. These release surfaces may be incorporated into the
present invention by following the teachings herein. Examples of
these non-silicone release surfaces include polyvinyl carbamates,
vinyl acetate homopolymers and copolymers, quillion (chromium
complexes), nitrocelluloses, caseing, formaldehyde modified
starches, cellulose acetate butyrate, polyvinyl chloride resins,
fluorocarbon polymers such as vinyl ethers, and waxes, all of which
will form suitable release surfaces for a variety of PSA
constructions.
EXAMPLES
[0068] The release properties of the liners of Examples 1-22 were
quantified using two methods, the 90.degree. peel release force and
the 180.degree. peel release profile. The 90.degree. peel release
force was measured on a TLMI Lab Master instrument in the liner off
mode, at a rate of 7.62 m/min, and results were measured in cN/25
mm. The 180.degree. release profile was generated by measuring the
peel release force on an Instrumentors ZPE-1000 High Rate Peel
Tester at rates of 10, 30, 100, 200, and 300 m/min, in the liner
off mode, and results were measured in cN/25 mm. Protocols for
performing the tests are as follows:
90% Peel Release Force
[0069] This test method allows the end user to determine the force
required to separate the release backing form the pressure
sensitive adhesive coated face material. The release force is
defined as the force required to separate a pressure sensitive
adhesive coated material from its release surface (or vice versa)
at an angle of 90.degree. and a jaw separation rate of 7.62 m per
minute.
[0070] A TLMI Lab Master instrument was used. The equipment was
fitted with a back plate to which the test strip can be attached in
order to maintain an angle of peel of 90.degree. throughout the
test. Pressure plates were loaded to give a pressure of 6.86 kPa
(70 g/m.sup.2) on the test piece. The strips to be tested were 25
mm wide and had a minimum length of 175 mm in the machine
direction.
[0071] The strips were placed between two flat plates and kept for
20 hours at 23 deg. C. .+-.2 deg. C. under a pressure of 6.86 kPs
(70 g/cm.sup.2) to ensure good contact between the release surface
and the adhesive. After storage in this manner, the strips were
removed from between the plates and keep for not less than 4 hours
at the standard test conditions of 23.+-.2.degree. C. and 50.+-.5%
RH.
[0072] Each strip was fixed to a plate by means of double sided
tape so that the laminate could be peeled apart at an angle of
90.degree..
180.degree. High Speed Release Force
[0073] This test method allows the label user to assess the
separation force of a laminate at speed comparable to those
typically used to convert and dispense the material. It therefore
provides a good assessment of the conversion characteristics of the
laminates being tested.
[0074] The release force is defined as the force required to
separate the backing from the adhesive coated material, at an angle
of 180.degree. and at jaw separation rate of between 10 m and 300 m
per minute.
[0075] To perform the test, an Instrumentors ZPE-1000 High Rate
Peel Tester was used, at rates of 10, 30, 100, 200 and 300 m/min.,
with results being measured in cN/25 mm. The strips were 25 mm wide
and had a minimum length of 30 mm in the machine direction. The
strips were free from damage and had clean cut edges.
[0076] The strips under test were placed between two flat plates
and kept for 20 hours at 23.+-.2.degree. C. under a pressure of
6.87 kPa (70 g/cm.sup.2) to ensure good contact between the release
paper and the adhesive. After storage in this manner, the strips
were taken from between the glass plates and kept for not less than
4 hours at the standard test conditions of 23.+-.2.degree. C. and
50%.+-.5% RH.
Examples 1 and 2
[0077] Examples 1 and 2 compare the relative release properties of
release surface formed from a coating having 100% silicone solids
to a release surface formed from a coating having 40% silicone
solids. In each case, the silicone containing layer was coated on a
release support layer (RSL), where the RSL comprised a filler
material coated on paper.
[0078] Example 1 consists of a General Electric silicone release
layer coated on synthetic SBR as a RSL support layer, which in turn
was coated on AT-70 paper. The top layer was prepared by mixing GE
1111-11-259 (62.7 g), GE 1192-05-117 (3.3 g), and water (34.0 g).
Because no solids other than silicone were coated to form the
release layer, the release layer of Example 1 is formed from 100%
silicone solids. The bottom RSL layer is 33% solid synthetic SBR
filler. The coating was applied to AT-70 paper using a dual die at
coating speed of 200 m/min, and cured at 160.degree. C. for 4
seconds to produce the finished liner. The coat weight is 1.5
g/m.sup.2 silicone on 1.5 g/m.sup.2 RSL. This release liner was
coated with S-490 pressure-sensitive adhesive (Avery Dennison
Corporation) to produce the PSA construction.
[0079] Example 2 consists of General Electric silicone mixed with
synthetic SBR at a ratio of 40/60 (Si/SBR) as a release layer,
coated on synthetic SBR as a RSL, which in turn was coated on AT-70
paper. The top layer was prepared by mixing GE 1111-11-259 (25.08
g), GE 1192-05-117 (1.32 g), synthetic SBR filler (60.0 g), and
water (13.6 g). Thus, only 40% of the release layer of Example 2 is
silicone. The bottom RSL layer is 33% solid synthetic SBR filler.
The coating was applied to AT-70 paper using a dual die at coating
speed of 200 m/min, and cured at 160.degree. C. for 4 seconds to
produce the finished liner. The coat weight of the top layer is 1.5
g/m.sup.2 silicone and 2.25 g/m.sup.2 RSL, and the bottom layer is
1.0 g/m.sup.2 RSL. This release liner was coated with S-490
pressure-sensitive adhesive to produce a PSA construction.
[0080] The following release force data was obtained:
1 TABLE 1 Release layer % Silicone 90.degree. Peel Release Peel
Release Profile (results in cN/25 mm) Example # solids Force (cN/25
mm) 10 m/min 30 m/min 100 m/min 200 m/min 300 m/min 1 100 35.3 25.4
22.7 22.7 22.8 22.1 2 40 46.8 42.8 39.6 32.7 26.8 23.4
Examples 3-6
[0081] Examples 3-6 consist of Wacker silicone as a release layer
coated on Air Products Airflex 465 as a RSL, which was coated on
either Data-70 or Willamette paper. The top layer was prepared by
mixing Wacker 38197 VP (66.0 g), Wacker crosslinker V20 (3.5 g),
10% solution of 3M Fluorad fluorochemical surfactant FC-129 (0.7
g), 1% aqueous Cellosize Hydroxyethyl Cellulose QP-100 MH (7.5 g),
and water (22.3 g). The bottom RSL layer was prepared by mixing Air
Products Airflex 465 (52.3 g) and water (47.7 g). The coating was
applied to Data-70 or Willamette papers using a dual die at a
coating speed of 400 m/min, and cured at 170.degree. C. for 3
seconds to produce the finished liners. The coat weight is 1.0
g/m.sup.2 silicone on 5.0 g/m.sup.2 RSL. These release liners were
coated with S-2000 emulsion pressure-sensitive adhesive or S-2045
hot melt adhesives (Avery Dennison Corporation) to produce the PSA
constructions of Examples 3-6.
[0082] The release properties of these liners were quantified using
two methods, the 90.degree. peel release force, and the 180.degree.
peel release profile. The 90.degree. peel release force was
measured on a TLMI Lab Master instrument, at a rate of 7.62 m/min,
and results were measured in cN/25 mm. The release profile was
generated by measuring the 180.degree. peel release force on an
Instrumentors ZPE-1000 High Rate Peel Tester at rates of 3, 30, 60,
100, and 300 m/min, and results were measured in cN/25 mm. The
following data was obtained:
2 TABLE 2 90.degree. Peel Release Release Profile (velocity in
m/min) Force (results in cN/25 mm) Paper Adhesive (cN/25 mm) 3
m/min 30 m/min 60 m/min 100 m/min 300 m/min Ex. 3 Data-70 S-2000
41.2 10.7 26.4 36.6 39.4 35.0 Ex. 4 Data-70 S-2045 40.9 13.2 24.5
30.4 44.5 43.4 Ex. 5 Willamette S-2000 35.3 9.5 24.4 30.2 36.2 36.2
Ex. 6 Willamette S-2045 40.5 11.2 31.6 39.2 46.2 55.1
Examples 7-10
[0083] Examples 7-10 consist of Wacker silicone coated on a
National Starch E-200 as RSL, which in turn was coated on Data-70
or Willamette paper #50 EDP. The top layer was prepared by mixing
Wacker 38197 VP (66.0 g), Wacker crosslinker V20 (3.5 g), 10%
solution of 3M Fluorad fluorochemical surfactant FC-129 (0.7 g), 1%
aqueous Cellosize Hydroxyethyl Cellulose QP-100 MH (7.5 g), and
water (22.3 g). The bottom RSL layer was prepared by mixing
National Starch E-200 (54.5 g) and water (45.5 g). The coatings
were applied to Data-70 or Willamette papers using a dual die at a
coating speed of 400 m/min, and cured at 170.degree. C. for 3
seconds to produce the finished liners. The coat weight is 1.0 g/m
silicone on 6.0 g/m.sup.2 RSL. This release liners were coated with
S-2000 emulsion pressure-sensitive adhesive or S-2045 hot melt
adhesive to produce the PSA constructions of Examples 7-10.
[0084] The release properties of these liners were quantified using
the same method as described above, and the following data was
obtained:
3 TABLE 3 90.degree. Peel Release Release Profile (results in cN/25
mm) Paper Adhesive Force (cN/25 mm) 3 m/min 30 m/min 60 m/min 100
m/min 300 m/min Ex. 7 Data-70 S-2000 32.3 12.7 37.1 41.9 46.5 55.2
Ex. 8 Data-70 S-2045 31.6 11.9 25.0 26.6 35.3 46.7 Ex. 9 Willamette
S-2000 34.6 8.5 21.5 28.4 36.1 38.7 Ex. 10 Willamette S-2045 31.2
11.4 27.6 36.4 42.9 51.9
Examples 11-14
[0085] Examples 11-14 consist of General Electric silicone coated
on Air Products Airflex 465 as RSL, which in turn was coated on
Data-70 or Willamette papers. The top layer was prepared by mixing
GE 1111-13-286 (34.7 g), GE 1111-15-307 (34.75 g), 10% solution of
3M Fluorad fluorochemical surfactant FC-129 (0.75 g), 1% aqueous
Cellosize Hydroxyethyl Cellulose QP-100 MH (8.9 g), and water
(20.85 g). The bottom RSL layer was prepared by mixing Air Products
Airflex 465 (52.3 g) and water (47.7 g). The coating was applied to
Data-70 or Willamette papers using a dual die at a coating speed of
400 m/min, and cured at 170.degree. C. for 3 seconds to produce the
finished liner. The coat weight is 1.0 g/m.sup.2 silicone on 5.0
g/m.sup.2 RSL. This release liner was coated with S-2000 emulsion
pressure-sensitive adhesive or S-2045 hot melt adhesive to produce
the PSA constructions of Examples 11-14.
[0086] The release properties of these liners were quantified using
the same method as described above, and the following data was
obtained:
4 TABLE 4 90.degree. Peel Release Force Release Profile (results in
cN/25 mm) Paper Adhesive (cN/25 mm) 3 m/min 30 m/min 60 m/min 100
m/min 300 m/min Ex. 11 Data-70 S-2000 24.9 22.5 24.8 28.9 33.8 22.4
Ex. 12 Data-70 S-2045 28.8 25.2 18.9 23.2 19.8 18.1 Ex. 13
Willamette S-2000 21.1 11.8 14.0 14.3 17.9 24.1 Ex. 14 Willamette
S-2045 22.5 23.7 19.1 21.3 23.6 27.7
Example 15
[0087] Example 15 consists of Dow Corning silicone coated on Air
Products Airflex 465 as RSL, which in turn was coated on Data-70
paper. The top layer was prepared by mixing Dow Corning 7980 (44.0
g), Dow Corning 7923 (11.0 g), Dow Corning 5602 (1.0 g), and water
(44.0 g). The bottom layer was prepared by mixing Air Products
Airflex 465 (52.3 g) and water (47.7 g). The coating was applied to
Data-70 paper using a dual die at a coating speed of 400 m/min, and
cured at 170.degree. C. for 3 seconds to produce the finished
liner. The coat weight is 1.0 g/m.sup.2 silicone on 5.0 g/m.sup.2
RSL. This release liner was coated with S-2000 emulsion
pressure-sensitive adhesive to produce the PSA construction.
[0088] The release properties of the liner were quantified using
the same method as described in example 1, and the following data
was obtained.
5 TABLE 5 90.degree. Peel Release Force Release Profile (results in
cN/25 mm) Paper Adhesive (cN/25 mm) 3 m/min 30 m/min 60 m/min 100
m/min 300 m/min Ex. 15 Data-70 S-2000 24.8 10.8 22.3 33.1 35.9
31.0
Examples 16-19
[0089] Four different silicone formulations were used to find the
lowest possible silicone coat weight which still provides
acceptable release properties for the PSAs tested.
[0090] Example 16 consists of 6.7% solid General Electric silicone
coated on National Starch E-200 as RSL coated on Data-70 paper. The
top layer was prepared by mixing GE 1111-11-259 (12.65 g), GE
1192-05-117 (0.67 g), 10% solution of 3M Flurad fluorochemical
surfactant FC-129 (0.13 g), 1% aqueous Cellosize Hydroxyethyl
Cellulose QP-100 MH (30.44 g), and water (56.11 g). The bottom RSL
layer was prepared by mixing National Starch E-200 (54.5 g) and
water (45.5 g). The coating was applied to Data-70 paper using a
dual die at a coating speed of 6.62 m/min, and cured at 155.degree.
C. for 60 seconds to produce the finished liner. This formulation
was used to produce dry coat weights with 0.2 g/m.sup.2 silicone on
6.0 g/m.sup.2 of E-200 support layer. The release liner was
laminated with S-490 pressure-sensitive adhesive to produce a PSA
construction. Samples were then Keil aged at 70.degree. C. for 20
hours for release testing. In Keil aging, samples are aged between
aluminum metal plates for 20 hours at a pressure of 6.9 KPa. This
is conveniently accomplished by placing a weight upon the plates
when the sample is in the oven. The samples were then equilibrated
at 23.degree. C. and 50% relative humidity for 24 hours.
[0091] Example 17 consists of 10% solid General Electric silicone
coated on National Starch E-200 RSL coated on Data-70 paper. The
top layer was prepared by mixing GE 1111-11-259 (18.96 g), GE
1192-05-117 (1.0 g), 10% solution of 3M Fluorad fluorochemical
surfactant FC-129 (0.2 g), 1% aqueous Cellosize Hydroxyethyl
Cellulose QP-100 MH (28.51 g), and water (51.33 g). The bottom RSL
layer was prepared by mixing National Starch E-200 (54.5 g) and
water (45.5 g). The coating was applied to Data-70 paper using a
dual die at a coating speed of 7.62 m/min, and cured at 155.degree.
C. for 60 seconds to produce the finished liner. This formulation
was used to produce coatings with 0.3 g/m.sup.2 silicone on 6.0
g/m.sup.2 E-200 as RSL. The release liner was laminated with S-490
pressure-sensitive adhesive to produce a PSA construction. Samples
were then Keil aged as described above at 70.degree. C. for 20
hours for release testing, and equilibrated as above.
[0092] Example 18 consists of 13.3% solid General Electric silicone
coated on National Starch E-200 as RSL coated on Data-70 paper. The
top layer was prepared by mixing GE 1111-11-259 (25.27 g), GE
1192-05-117 (1.33 g), 10% solution of 3M Fluorad fluorochemical
surfactant FC-129 (0.27 g), 1% aqueous Cellosize Hydroxyethyl
Cellulose QP-100 MH (25.65 g), and water (47.48 g). The bottom RSL
layer was prepared by mixing National Starch E-200 (54.5 g) and
water (45.5 g). The coating was applied to Data-70 paper using a
dual die at a coating speed of 7.62 m/min, and cured at 155.degree.
C. for 60 seconds to produce the finished liner. This formulation
was used to produce a coating with 0.4 g/m.sup.2 RSL. This release
liner was laminated with S-490 pressure-sensitive adhesive to
produce a PSA construction. Samples were then Keil aged as
described above at 70.degree. C. for 20 hours for release testing,
and equilibrated as above.
[0093] Example 19 consists of 16.7% solid General Electric silicone
coated on National Starch E-200 RSL coated on Data-70 paper. The
top layer was prepared by mixing GE 1111-11-259 (31.56 g), GE
1192-05-117 (1.66 g), 10% solution of 3M Fluorad fluorochemical
surfactant FC-129 (0.33 g), 1% aqueous Cellosize Hydroxyethyl
Cellulose QP-100 MH (22.55 g), and water (43.90 g). The bottom RSL
layer was prepared by mixing National Starch E-200 (54.5 g) and
water (45.5 g). The coating was applied to Data-70 paper using a
dual die at a coating speed of 7.62 m/min, and cured at 155.degree.
C. for 60 seconds to produce the finished liner. This formulation
was used to produce a coating with 0.5 g/m.sup.2 silicone on 6.0
g/m.sup.2 RSL. This release liner was laminated with S-490
pressure-sensitive adhesive to produce a PSA construction. Samples
were then Keil aged as described above at 70.degree. C. for 20
hours for release testing, and equilibrated as above.
[0094] The release properties of these liners were quantified using
the 90.degree. peel release force. The 90.degree. peel release
force was measured on a TLMI Lab Master instrument, at a rate of
7.62 m/min, and results were measured in cN/25 mm. The following
data was obtained:
6TABLE 6 Oxford Silicone Silicone Wt (g/m.sup.2) 90.degree. Peel
Release Measurement Example # (Theoretical) Force (cN/25 mm)
(g/m.sup.2) 16 0.2 29.2 0.17 17 0.3 26.7 0.31 18 0.4 17.6 0.50 19
0.5 12.6 0.55
Examples 20-22
[0095] Example 20 consists of a General Electric silicone coated on
Air Products Airflex 426 coated on Georgia Pacific vellumina paper.
The top layer was prepared by mixing GE 1111-13-286 (34.75 g), GE
1111-15-307 (34.75 g), 10% solution of 3M Fluorad fluorochemical
surfactant FC-129 (0.75 g), 1% aqueous Cellosize Hydroxyethyl
Cellulose QP-100 MH (8.9 g), and water (20.85 g). The bottom RSL
layer was prepared by mixing Air Products Airflex 426 (54 g) and
water (46 g). The coating was applied to Georgia Pacific vellumina
paper using a dual die at a coating speed of 91.4 m/min, and cured
at 165.degree. C. for 3 seconds to produce the finished liner. The
dry coat weight is 1.4 g/m.sup.2 silicone on 5.0 g/m.sup.2 RSL.
This release liner was coated with S-2000 emulsion
pressure-sensitive adhesive to produce a PSA construction.
[0096] Example 21 consists of a General Electric silicone, with 20%
CRA, coated on Air Products Airflex 426 coated on Georgia Pacific
vellumina paper. The top layer was prepared by mixing GE
1111-13-286 (29.6 g), GE 1111-15-307 (29.6 g), GE 1111-16-332 (14.8
g), 10% solution of 3M Fluorad fluorochemical surfactant FC-129
(0.63 g), 1% aqueous Cellosize Hydroxyethyl Cellulose QP-100 MH
(4.65 g), and water (20.72 g). The bottom RSL layer was prepared by
mixing Air Products Airflex 426 (54.0 g) and water (46.0 g). The
coating was applied to Georgia Pacific vellumina paper using a dual
die at a coating speed of 91.4 m/min, and cured at 165.degree. C.
for 3 seconds to produce the finished liner. The coat weight is 1.4
g/m.sup.2 silicone on 5.0 g/m RSL. This release liner was coated
with S-2000 emulsion pressure-sensitive adhesive to produce a PSA
construction.
[0097] Example 22 consists of General Electric silicone, with 30%
CRA, coated on Air Products Airflex 426 coated on Georgia Pacific
vellumina paper. The top layer was prepared by mixing GE
1111-13-286 (26.63 g), GE 1111-15-307 (26.63 g), GE 1111-16-332
(22.83 g), 10% solution of 3M Fluorad fluorochemical surfactant
FC-129 (0.69 g), 1% aqueous Cellosize Hydroxyethyl Cellulose QP-100
MH (3.81 g), and water (22.83 g). The bottom RSL layer was prepared
by mixing Air Products Airflex 426 (54.0 g) and water (46.0 g). The
coating was applied to Georgia Pacific vellumina paper using a dual
die at coating speed of 91.4 m/min, and cured at 165.degree. C. for
3 seconds to produce the finished liner. The coat weight is 1.45
g/m.sup.2 silicone on 5.0 g/m.sup.2 RSL. This release linear was
coated with S-2000 emulsion pressure-sensitive adhesive to produce
a PSA construction.
[0098] The release properties of these liners were quantified using
two methods, the 90.degree. peel release force, and the 180.degree.
peel release profile. The 90.degree. peel release force was
measured on a TLMI Lab Master instrument, at a rate of 7.62 m/min,
and results were measured in cN/25 mm. The release profile was
generated by measuring the 180.degree. peel release force on an
Instrumentors ZPE-1000 High Rate Peel Tester at rates of
3,30,60,100, and 300 m/min, and results were measured in cN/25 mm.
The following data was obtained:
7 TABLE 7 90.degree. Peel Release Force Release Profile (results in
cN/25 mm) Example # % CRA (cN/25 mm) 3 m/min 30 m/min 60 m/min 100
m/min 300 m/min 20 0 16.4 14.2 18.9 21.4 21.9 24.5 21 20 19.7 17.6
26.0 33.7 36.7 35.5 22 30 25.9 25.3 32.5 38.1 35.8 36.7
Examples 23 and 24 and Comparative Examples 25 and 26
[0099] In Examples 23 and 24, Transmission Election Microscopy
(TEM) was used to study the morphology of multilayer release liners
of the present invention in comparison to prior art release
liners.
[0100] For Examples 23 and 24, GE silicone emulsion 1111-11-259 and
silicone catalyst emulsion 1192-05-117 were combined to form a
total solids content of 35%. This silicone containing layer was
coated on an release support layer comprising ethylene vinyl
acetate support layer, sold as E-200 by National Starch. The total
solids content of the release support layer was 30%. The release
support layer was coated on Data 70 paper. Example 23 was coated at
a coating speed of 7.62 m/min and cured at 154.4.degree. C. for one
minute. The coat weigh ratio of silicone to RSL of Example 23 is
2.0 g/m.sup.2 silicone to 6.0 g/m.sup.2 release support layer.
Example 24 was coated at coating speed of 91.4 m/min, and cured at
an oven temperature of 171.1.degree. C. The coat weight ratio for
Example 24 is 2.0 g/m.sup.2 silicone to 5.0 g/m.sup.2 RSL.
[0101] For comparison to prior art release liners, Comparative
Examples 25 and 26 were made using the silicone mixtures, E-200 as
RSL, and the paper of Examples 23 and 24. To form Comparative
Example 25, the E-200 support layer was coated on a Data 70 paper
and then was dried at 154.4.degree. C. for one minute. The silicone
layer was then coated on the dried E-200 as RSL, and the multilayer
construct was cured at 154.4.degree. C. for one minute. The ratio
of silicone to E-200 as RSL was 6.0 g/m.sup.2 to 12.0 g/m.sup.2, or
1:2. To form Comparative Example 26 was made in the same manner as
Comparative Example 25, except that the E-200 as RSL was not dried
prior to application of the silicone layer.
[0102] TEMs were made of cross-sections of Examples 23 and 24 and
Comparative Examples 25 and 26. In each of the TEMs, darker regions
indicate the presence of silicone, and lighter regions indicate the
presence of the EVA support layer or paper.
[0103] Referring to FIG. 15, there is shown a TEM of Comparative
Example 26. As discussed above, Comparative Example 26 was made by
coating the liquid E-200 as RSL layer onto E-200 as RSL paper
surface, and then without permitting the RSL to dry or cure,
thereafter coating the silicone-containing layer onto the RSL. As
shown in FIG. 15, considerable undesirable mixing of silicone and
the EVA layer resulted from this sequential coating process. The
large amount of silicone in the RSL is wasted, as it does not
contribute to the release properties of the liner.
[0104] In contrast, the substantially simultaneous coating of two
liquid layers using the dual die method of the present invention is
observed to create distinct borders between the silicone-containing
layer and the RSL. This is observed in FIGS. 16 and 17, which
correspond to Examples 23 and 24. As shown in FIGS. 16 and 17, the
silicone-containing layer is neatly segregated from the RSL layer,
with the irregularity of interfacial layer and small domains of
silicone dispersed at various points in the RSL. As noted above,
this irregularity of interfacial layer provides for better bonding
between the two respective layers, thereby decreasing the
propensity for the silicone-containing layer to rub off or separate
from the RSL.
[0105] Referring to FIG. 18, there is shown a TEM of Comparative
Example 25. As shown in FIG. 18, a very sharp border exists between
the silicone-containing layer and the RSL layer. Little or no
intermixing is seen to occur between the two respective layers.
Therefore, it is believed that bonding between the respective
layers is minimized in comparison to the dual die coating
constructs, and therefore the two layers are more likely to
separate.
Silicone Dispersion Depth Profiles
[0106] For dual die coating, the degree of intermixing between the
support layer and the silicone-containing layer is somewhat
dependent on the coat weight ratio of silicone to support layer.
For dual die applications, it has been found that interlayer mixing
decreases as the ratio of silicone/support layer is increased. This
is best shown by reference to Table 8 below, which sets forth the
total percentage of silicone in the multilayer construct as a
function of depth below the upper surface of the
silicone-containing layer, as determined from TEM scans. Also shown
in Table 8 are Comparative Examples 27 and 28. The Comparative
Examples represent the prior art process of first applying the RSL
to a substrate, and then curing or drying the RSL before the
silicone containing layer is applied. Comparative Examples 27 and
28 were made by coating 6.0 g/m.sup.2 of GE 1111-13-286/GE
1111-15-307 onto 12.0 g/m.sup.2 of E-200 as RSL. The RSL was dried
prior to silicone application by heating to 154.4.degree. C. for 60
seconds. The paper used for the Comparative Examples was Data
70.
8TABLE 8 Wet-On-Dry Comparative Silicone/Filler Coatweight Ex. 27
Depth Ratio 0.4/6 gsm (% total Wet-On-Dry Below 0.5/6 gsm 2/6 gsm
silicone Comparative Ex. 28 Surface (% total silicone found found
at this (% total silicone (.mu.m) at this depth) depth) found at
this depth) <0.5 25.8% 28.3% 19.1% 6.0% 6.20 <1 40.3% 51.7%
39.5% 13.0% 12.7% <2 50.7% 69.5% 76.9% 26.4% 25.5% <3 60.9%
80.6% 90.9% 39.9% 38.5% <4 70.6% 88.0% 95.7% 53.5% 51.5% <5
79.9% 93.7% 98.4% 67.2% 64.8% <6 87.7% 97.6% 99.6% 81.0% 78.1%
<7 92.6% 99.6% 99.9% 95.0% 91.3% <8 97.5% 100.0% 100.0% 100%
97.3% <9 98.9% 100.0% 100.0% 100% 9.10% <10 99.4% 100.0%
100.0% 100% 100.00% <11 99.9% 100.0% 100.0% 100% 100.00% <12
100.0% 100.0% 100.0% 100% 100.00%
[0107] Furthermore, the distribution of silicone as a function of
depth from the release surface differs in the multilayer constructs
of the present invention formed by dual die application in
comparison to the comparative prior art constructs formed from the
two step coating processes. This is best shown by comparing the
difference in the increase in percentage of total silicone
contained between adjacent 1 micrometer depths. For Comparative
Example 27, 13% of all of the silicone coated on the substrate is
within 1 micrometer of the surface, and 26.4% silicone is within 2
micrometers of the surface. The percentage difference between the 1
and 2 micrometer measurements is 13.4% (26.4%-13%). By repeating
this process for successive depths of Comparative Example 1, it is
observed that an average increase of 13.7%.+-.0.3% occurs between
adjacent 1 micrometer depths until the total silicone content
exceeds 90%. Essentially, this is a linear distribution profile, as
would be expected from the prior art process because silicone forms
a very discrete layer on the dried support surface, and little
intermixing occurs between the support layer and the silicone
layer.
[0108] In contrast, constructs of the present invention formed by
dual die application show a silicone distribution profile that is
non-linear. For example, at 2:6 g/m.sup.2 silicone to support layer
ratio, the increase in total silicone content between successive 1
micrometer depth is as follows:
9TABLE 9 Total Silicone Difference in %/.mu.m Depth Below Surface
(%) (from 1 .mu.m above) 0.5 19.1 1 39.5 40.8 (20.4 .div. 0.5) 2
76.9 37.4 3 90.9 14.0 4 95.7 4.8 5 98.4 2.7 6 99.6 1.2 7 99.9 0.3 8
100% 0.1
[0109] A greater concentration of silicone is observed a points
nearer to the surface of the release layer. In contrast, a
relatively constant concentration of silicone was observed in
comparative Examples 27 and 28. Furthermore, looking to the 0.4:6.0
and 0.5:6.0 g/m.sup.2 embodiments, total silicone ranges from 50.7
to 76.9% at a depth of 2 micrometers, and 70.6 to 95.7% at a depth
of 4 micrometers, and 79.9 to 98.4% at a depth of 5
micrometers.
Die Coating
[0110] The principles of dual die coating to make multilayer
constructs are explained below.
[0111] In one aspect of the dual die method of the present
invention, a uniformly layered film in the cross-web direction is
achieved by the careful analysis of the viscosities and other
physical parameters of the liquids to be coated onto the web to
form a multilayer product. This uniformity results in a high
quality product. In addition to this analysis, the present method
involves the design of the die lips and their placement relative to
the web in accordance with important principles of fluid mechanics,
in order to regulate the pressure gradients of flow during
operation. Pressure gradient regulation may also be accomplished,
in addition to the foregoing, by application of vacuum upstream of
the coating bead. These steps of die lip design, die set-up and
application of vacuum provide the capability to control the
separating line of two or more contiguous liquid layers at the
stability point and the assurance of steady, two-dimensional flow.
In order to ensure successful operation, a coating window
(including a quality window) can be determined and an optimal
operating point determined.
[0112] Referring to FIG. 1, there is illustrated somewhat
schematically a typical die coating operation. The die 20 is shown
positioned adjacent to a moving substrate or web 22 traveling in
the direction of arrow 24. The web 22 travels around a back-up roll
26 as it passes across the distal end of the multilayer die 20. As
shown in FIG. 1, it will be understood that both the die 20 and the
web 22 have substantially equal widths, such that most of the
entire width of the substrate or web is coated in one pass by the
fluid flowing out of the die and onto the web.
[0113] The die 20 is modular in that it can be assembled from a
number of individual elements and then set in the coater machine as
an integral device. Each die element is comprised typically of a
manifold 19 and a more distal die section 21. The most distal
portion of the die section is referred to as the die lip 29,
described and illustrated in more detail in connection with FIG. 2.
Since the die 20 is modular, various combinations of die lips 29
can be assembled without necessitating modifications to the other
die sections and lips 29.
[0114] As illustrated by the horizontal arrow 28 in FIG. 1, the die
20 can be moved radially into or away from the back-up roll 26 in
order to adjust the coating gap 30, which is defined as the
distance between the die lips 29 and the web 22. In addition, the
angle of attack (.alpha.) of the die 20 can be adjusted, as shown
by the arrow in FIG. 1.
[0115] The elements of the die 20 are separated from each other
slightly by slots or feed gaps 32 which allow the coating material
to flow from a manifold 34 in the die 20, through these feed gaps
in the die 20, and onto the moving web 22. In the multilayer die 20
of FIG. 1, two feed gaps 32 are shown. However, as noted above, it
will be understood that the principles of the present invention are
equally applicable to a plurality of layers in addition to two.
[0116] Analysis of Coating Liquids
[0117] As noted above, in one important aspect of the present
invention, certain physical parameters of the liquids to be coated
in multiple layers onto the substrate or web are analyzed with
respect to the likelihood of achieving uniform film thicknesses in
the cross-web direction. Of these parameters, perhaps the most
important is the liquid's viscosity. More specifically, it will be
understood that the ratio of viscosities of the two contiguous
layers to be coated must be carefully analyzed and, if possible or
practical, adjusted to a value within the optimal range.
[0118] For example, it has been observed that if the viscosity of
the top layer liquid is in the range of 50% less than to 100% more
than the viscosity of the bottom layer liquid better coating
results are likely, although other ratios may also provide good
coating results if other parameters are optimized. Optimally, the
viscosity of the top layer should be about 30% greater than that of
the bottom layer. Viscosity ratios in this range provide a more
stable flow. More specifically, a higher top layer viscosity
reduces the risk of cross-web defects termed "inter-layer ribbing",
in which the top and bottom layers alternate with one another
across the web rather than forming two uniform films, one on top of
the other.
[0119] It will be understood that the relative viscosities of the
liquids to be coated are determined in large part by the nature of
the multilayer product to be produced. That is, adjustments to
viscosity in one liquid or the other may not be possible or
practical depending on cost, supply, delivery or other variables.
However, to some degree, the viscosities of the liquids may be
"matched" in order to achieve favorable coating conditions. For
example, if greater flow stability is desired, it may be possible
to increase the viscosity of the top liquid by adding thickeners.
Likewise, the viscosity of the bottom layer may be reduced by
adding thinners, such as water, solvent, etc. On the other hand,
such thinning agents, and especially, solvents, generate other
problems such as environmental concerns, increased drying time,
etc.
[0120] In analyzing viscosities, however, one must consider the
shear rates experienced by the particular liquid under typical
coating conditions. Such shear rates vary by several orders of
magnitude, but typically exceed 1000 s.sup.-1 at most locations
along the bead. Thus, at these shear rates, the relative viscosity
of the liquids can vary widely.
[0121] FIG. 3 illustrates a shear rate/viscosity graph in which it
is proposed that a top layer A be coated over a second liquid
formulated at two different viscosities (B and B'), where B' is
greater than B. In this graph, shear rates are displayed over a
range from 0.1 to 100,000 s.sup.-1; although, the area of analysis
is at shear rates above about 1000 s.sup.-1. It will be noted that
the ratio of viscosities between layer A and layer B changes
significantly at higher shear rates as compared to lower shear
rates. Furthermore, based on the foregoing analysis, one would
assume that the combination of liquid A over liquid B would coat
well since the viscosity of A is greater than that of B. Indeed,
successful coating was achieved experimentally, but initially only
at lower web speeds. At higher web speeds, the bead leaked
upstream, a defective condition described in more detail below. The
reason this condition occurred in the present example lies in the
fluid mechanics of the flow and relates to the difficulty of a
lower viscosity liquid (liquid B in this example) to generate
enough pressure drop below the upstream lip to seal a bead which
downstream is made up, in part, of a more viscous liquid (A). This
illustrates the interaction of several principles which need to be
considered in this liquid viscosity analysis. For example, this
upstream leakage condition can be corrected in several possible
ways. One involves the design of the lip geometries in accordance
with principles of the present method described in more detail
below. Another involves the adjustment of the relative viscosities
of the two liquids.
[0122] For example, when liquids A/B' were coated experimentally,
good coating results were obtained over a wide range of web speeds.
This is because, as FIG. 2 graphically illustrates, the viscosities
of the two liquids are balanced or better matched at high shear
rates. For example, the viscosity of liquid B' is more than twice
that of B. It must be noted, however, that the viscosity of B' did
not substantially exceed the viscosity of the top layer A.
[0123] This condition is illustrated in FIG. 3 which illustrates a
shear rate versus viscosity graph for two sample liquids C and D.
In this example, liquid C is to be coated on top of liquid D. In
this graph, only the high shear rate viscosities need be analyzed.
Thus, it will be observed from FIG. 3 that, for most of the typical
shear rate range, the viscosity of the bottom layer D exceeds that
of the top layer C. Under these inverse viscosity conditions, it
has been found that it is difficult to achieve stable coating, and,
although multilayer coating may be possible, it is difficult to
achieve high quality. Under proper viscosity conditions, the
coating window for a particular operation will be larger, thus
increasing the likelihood of stable flow.
[0124] It will be appreciated, by those of ordinary skill, that a
wide variety of viscosity relationships will be encountered in
producing a particular multilayer product. Thus, the foregoing
examples are not to be considered exhaustive of the scope of the
liquid analysis encompassed within the steps of the present
method.
[0125] Another aspect of liquid analysis involves the relative
surface tensions of the liquids to be coated. It has been found
that the risk of certain defects such as dewetting or voids, or
voids in one particular layer, can be reduced if the surface
tension of the top layer is less than that of the bottom layer.
Under these conditions, the local surface tension (including the
dynamic surface tension in the film forming region) will tend to
close such voids. Surface tension can be reduced in the top layer,
to some degree, by the use of effective surfactants or other
organic soluble liquids (alcohol, ketone, etc.).
[0126] Thus, the liquid analysis aspect of the present method is
important in achieving favorable coating conditions. The lip design
and die set-up aspects of the method will be discussed together
below; however, the following information relating to single layer
coating will explain how those aspects of the present method assist
in achieving stable flow.
[0127] Single-Layer Fluid Mechanics
[0128] In order to assist in understanding the advantages of the
present method, it is important to understand the relationship
between the coating gap 30, the downstream wet film thickness, and
the liquid pressure gradient. This can best be illustrated and
explained with respect to a single-layer coating process.
[0129] Thus, referring to FIG. 4, there is shown a close-up
cross-sectional, schematic view taken through a pair of die lips 36
positioned adjacent to a moving web 22 to form a coating gap 30
("c.g."). It will be noted with respect to FIG. 1 that the die 20
has been rotated clockwise approximately 90 degrees in order to
facilitate this illustration. In addition, the web 22 is shown to
be flat or horizontal, whereas it actually will exhibit some
curvature as it conforms to the back-up roll (not shown). However,
the configuration shown in FIG. 4 is a good approximation of the
fluid mechanics occurring in the bead 42 of liquid formed in the
coating gap 30 between the die lips 36 and the moving web 22.
[0130] For ease of reference, "downstream" will refer to the
direction of web 22 travel, while "upstream" is in the opposite
direction or to the left. Thus, the upstream lip 36a is formed on
the distal-most tip of the upstream die section 38a, while the
downstream lip 36b is formed on the distal-most tip of the
downstream die section 38b. The two die sections 38a,b form between
them a coating slot or feed gap 40 out of which the liquid flows
onto the moving web 22. As shown in FIG. 4, the liquid first
travels upstream and then turns to flow downstream in an open
recirculation within the bead 42. The bead 42 is bounded on its
upstream edge by an upstream meniscus 44 and on its downstream edge
by a downstream meniscus 46 or film-forming region. If the fluid,
due to extreme conditions, escapes the bead 42 and travels
upstream, this is referred to as upstream leakage.
[0131] The coating gap 30 is shown as dimension A in FIG. 4. It
will be understood, particularly with reference to subsequent
drawings, that the coating gap 30 can vary along the longitudinal
length of the lips 36 in accordance with different lip geometries,
lip machining defects, angled or beveled lips, adjustments and
angle of attack of the die, etc.
[0132] The wet film thickness (h) of the flow is shown downstream
of the bead 42. It is defined as the thickness of the flow before
drying. The pressure gradient of the flow at various longitudinal
positions is related to the wet film thickness (f.t.) and to the
coating gap 30 at that location, it being understood that for a
given flow rate (Q) the film thickness and web velocity are
inversely proportional. Thus, for a Newtonian liquid flowing at
steady state, the velocity is given as follows: 1 u = u _ y a + a 2
2 ( p x ) [ ( y a ) - ( y a ) ]
[0133] where:
[0134] u=velocity of the liquid downstream;
[0135] =velocity of the web;
[0136] a=coating gap (c.g.);
[0137] h=wet film thickness (f.t.);
[0138] .mu.=viscosity of the liquid;
[0139] x=horizontal coordinate in the downstream direction;
[0140] y=vertical coordinate going from lip to web; and
[0141] dp/dx=pressure gradient in the downstream direction.
[0142] It will be noted from this equation that the velocity of the
flow (u) is made up of two components. The first component may be
characterized as a "drag driven" component, wherein the velocity of
flow varies in direct proportion to the speed of the web. The
second component may be referred to as a "pressure driven"
component, such that the velocity of flow is proportional to the
pressure gradient (dp/dx) at a given point. Using the definition of
flow rate (Q), one integrate the above equation to solve for the
pressure gradient, yielding: 2 p x = 12 a 3 ( u a _ 2 - Q )
[0143] Since, the pressure gradient may be expressed in terms of
the coating gap (a) and wet film thickness (h) as: 3 p x = 12 u _ a
3 ( a 2 - h )
[0144] Thus, where h=1/2a (or, in other words, the coating gap is
twice the wet film thickness), dp/dx=0. Accordingly, in accordance
with these well-known relationships, the velocity of the flow and
the related pressure gradient at a particular point in the bead can
be determined for a given coating gap/film thickness relationship.
The velocity can be plotted as a velocity profile, such as those
illustrated in the series of schematic illustrations comprising
FIG. 5. In all cases described below, it will be noted that where
y=0 (at the die lip), the velocity of flow (u) equals zero; but
while y=a (at the web), the velocity of flow equals that of the
web.
[0145] FIG. 5a illustrates a coating condition wherein the coating
gap 30 is exactly equal to twice the film thickness. In this
condition the pressure in the liquid is constant, giving a pressure
gradient of zero.
[0146] However, as noted above, coating gap conditions can change
due to a number of variables. Thus, FIG. 5b illustrates a condition
where the coating gap 30 is less than two times the downstream film
thickness. Under these circumstances the velocity profile is
concave in the downstream direction, thus exhibiting a negative
pressure gradient. This negative pressure gradient produces a
pressure drop along the downstream lip 36b in the downstream
direction. The pressures in the upstream regions are higher, thus
adding to the velocity characteristics of the liquid and causing it
to push forward or bulge the velocity profile, as shown in FIG.
5b.
[0147] On the other hand, FIG. 5c illustrates the situation where
the coating gap 30 is equal to three times the film thickness (h).
Under these conditions the downstream pressure gradient is greater
than zero, meaning that the flow sees an increasing pressure
downstream. This increase in pressure has a tendency to diminish
the velocity, making the velocity profile convex in the downstream
direction.
[0148] Finally, FIG. 5d illustrates the condition when the coating
gap 30 is greater than three times the film thickness (h). Again,
the pressure gradient is positive, but more so than that shown in
FIG. 5c. Thus, an even greater downstream pressure is seen,
actually causing the flow to travel upstream a short distance
before it turns and travels downstream. This condition illustrates
the principal cause for recirculation in the liquid. This
recirculation can occur under the upstream lip 36a, as shown in
FIG. 4, but may also occur under the downstream lip 36b if the
coating gap 30 is too great, as illustrated in FIG. 5d.
[0149] This recirculation, while not particularly damaging to the
quality of the film in single layer coating, can have disastrous
effects in multilayer coating. It has been found that such
conditions can be substantially avoided with correct lip design and
proper die assembly and set-up. Because of their interrelationship,
these aspects of the present method are discussed together
below.
[0150] Lip Design and Die Set-Up
[0151] The method of the present invention controls the pressure
gradients in the liquids under a wide variety of coating conditions
in order to achieve a stable flow. This is accomplished in large
part by the design of the lip geometries and the assembly, set-up,
and adjustment of the die.
[0152] Thus, referring to FIG. 6, there is shown a close-up
cross-sectional view of a multilayer die 20 which may be utilized
with the method of the present invention. The present method can be
utilized in accordance with dies and other coating techniques well
known to those of ordinary skill in the art to produce successful
multilayer products.
[0153] Although similar to FIG. 4, this die 20 is comprised of
upstream and downstream die sections 50a and 50c, as well as a
middle section 50b separating the two. Formed between these various
sections are an upstream feed gap 52 and a downstream feed gap 54.
The liquid from the upstream feed gap 52 flows onto the web 22 to
form a bottom layer 58, while the liquid from the downstream feed
gap 54 flows onto the bottom layer to form a top layer 56. It will
be noted that the angle formed between these two feed gaps 52, 54
is approximately 30 degrees, which advantageously provides a good
construction for the machining of a middle lip 60b formed on the
distal end of the middle land 50b. It will also be noted from FIG.
6 that the lips 60a and 60c of the upstream and downstream die
sections 50a,c form a stepped or staircase configuration with
respect to the middle lip 60b in order to regulate the pressure
gradient in this region. The importance of this relationship will
be described and illustrated in more detail in connection with FIG.
5.
[0154] It will be noted in FIG. 6 that this stepped lip
configuration results in various coating gaps. For ease of
reference, the subscript b will refer to the bottom layer 58 while
the subscript t will refer to the top layer 56. Thus, the coating
gap of the bottom layer (c.g..sub.b) is characterized by two
different values, one under the upstream lip 60a and one under the
middle lip 60b. The coating gap of the top layer (c.g..sub.t) is
characterized by a larger value. As noted above, these coating gaps
bear important relationships to the downstream film thickness of
the respective flows which are formed thereby. Thus, for example,
the bottom coating gap bears an important relationship in terms of
pressure gradient with the downstream film thickness of the bottom
layer 58 (f.t..sub.b), while the coating gap of the top layer 56
bears an important relationship with the total downstream film
thickness (f.t..sub.t) (it is perhaps helpful to note that the
subscript t may refer not only to the top layer, but also to the
"total" thickness of the downstream film) which includes the sum of
the bottom and top layers. This is because the coating gap
analysis, in determining pressure gradient, must be based on the
total flow at that gap, including the flow approaching the web 22
at that position as well as all previous flows and layers resulting
therefrom.
[0155] It will be further noted from FIG. 6 that the bottom coating
gap is less than the top coating gap in order to form the "step"
described above. It should be appreciated, however, in those
embodiments where it is desired to eliminate or minimize the step,
the differential observed in FIG. 6 will be less noticeable or be
nonexistent. This step in the middle lip 60b with respect to the
downstream lip 60c occurs in a very important interface area where
the two flows converge at the downstream feed gap 54. Thus, an
important aspect of the present invention is a design process which
results in particular middle lip 60b and downstream lip 60c
geometries, including the length of each lip in this region. These
are also described in more detail below in connection with FIG.
7.
[0156] Finally, it will be noted in FIG. 6 that the lips 60 are
each parallel to each other or, in other words, lie in parallel
planes. However, the principles of the present invention are not
limited to such design considerations. For example, the lips 60 can
be angled or beveled with respect to one another, as described
below and illustrated in more detail in connection with FIG. 7. In
addition, a wide variety of other lip geometries and other methods
for affecting the pressure gradient are within the principles of
the present invention.
[0157] Referring to FIG. 7, there is shown a close-up view of the
interface region, as illustrated more generally in FIG. 4. This
drawing illustrates the complete interface between the top layer
flow 56 from the bottom layer flow 58. The flow of each layer, as
well as its respective direction, is shown by a series of arrows.
Thus, the two layers are shown exhibiting steady, two-dimensional
flow with the separating streamline optimally positioned at the
stability point. This results in uniform layers in terms of cross
web and down web cross-sectional thickness. This type of stable,
two-dimensional flow results in good multilayer product
performance.
[0158] As noted above, in order to achieve such stable flow, it is
important to avoid mixing between the two layers. This can be
achieved, in one aspect of the present invention, by accurate
control of the separating line of the two fluids. As shown in FIG.
7, best coating results are achieved when this separating line
coincides with the downstream corner 62 of the middle lip 60b,
referred to as the stability point. The present invention comprises
a method for regulating pressure gradients in the flow to fix or
lock the separating line of the top flow at this stability point
62. Preferably, the pressure gradient under the middle lip 60b (and
in particular the downstream corner 62 of the middle lip 60b) is
not greater than the pressure gradient which would cause
recirculation under the middle lip. Thus, the flow of the top layer
does not have a tendency to invade the bottom layer coating gap in
the upstream direction. This pressure situation tends to fix the
separating line at the stability point 62 under the downstream
lip.
[0159] As noted above, this advantage is achieved in one aspect of
the present invention by stepping the die lips away from the web 22
in the downstream direction. This step is shown as dimension A in
FIG. 7. The magnitude of this step may fall within a wide range of
dimensions which may be optimized for a given set of coating
conditions. However, preferably, this distance A will fall in the
range of 0-100 micrometers, more preferably 0-30 micrometers,
optimally approaching zero when coating multilayer release
systems.
[0160] At the same time, however, as noted above, in order to
achieve the advantages of the present invention, these lips must be
appropriately positioned with respect to the web 22 in order to
achieve the proper coating gaps. For example, if the bottom coating
gap (c.g..sub.b) is greater than three times the film thickness
(f.t..sub.b), a large pressure gradient will be developed just
upstream of the interface area, as illustrated in FIG. 5d. Thus, a
negative velocity profile may occur, causing recirculation in the
bottom layer under the middle lip 60b. This recirculation may have
the effect of pulling the top layer upstream and away from the
stability point 62. This condition is illustrated in FIG. 8, and
has all the disadvantages described above. On the other hand, if
the bottom coating gap is a substantial amount less than two times
the film thickness (f.t..sub.b), although the desirable negative
pressure gradient will be generated, it may be too high, thus
resulting in upstream leakage, high shear rates, etc. Thus,
preferably, the bottom coating gap should be maintained at
approximately two to three times the film thickness.
[0161] In addition, the coating gap under the downstream lip 60c
(c.g..sub.t) should be in the range of one to three times the total
film thickness (f.t..sub.t). Again, if it is too great, the
pressure gradient under the downstream lip may be sufficiently
large to cause the separating line to move up into the downstream
feed gap and to separate from the middle die at a point on the
upstream wall of such feed gap, as illustrated in FIG. 9. This flow
condition causes a closed recirculation in the bottom layer flow
and results in film defects. Thus, there are a number of trade-offs
which require careful balancing of these parameters in order to
achieve accurate pressure gradient control.
[0162] Referring again to FIG. 7, it will be noted that the
upstream lip 60a is also stepped toward the web 22 with respect to
the middle lip 60b. This also has the result of decreasing the
coating gap and increasing the pressure gradient upstream. This
situation will assist in sealing the bead 42 under the die lips. In
fact, this coating gap is dictated by the following rationales. The
pressure drop developed along this region must match the pressure
drop through the liquid along the downstream portion of the flow,
plus any differential pressure imposed by the ambient air
surrounding the liquid at its downstream and at its upstream
interfaces. Thus, the coating gap under the upstream lip 60a can be
used to balance these pressure forces. It has been found that a
slight step (illustrated as dimension B in FIG. 7) on the order of
0-100 microns is suitable.
[0163] Moreover, because of the sensitivity of this process, it
will be appreciated that the total step between the upstream lip
60a and the downstream lip 60c (i.e., A+B) should also be carefully
regulated. Thus, it has been found that total steps in the range of
0-0.008 inches are advantageous. In addition, the feed gap
dimension should also be carefully maintained to be about not more
than five times the wet film thickness of the film being fed
through that gap. If this gap is excessive, recirculations can
occur in the feed gap, as illustrated in FIG. 13. Thus, these
dimensions (C and D in FIG. 7) can each vary in the range of 25-400
microns.
[0164] Another important aspect of the present invention which
assists in maintaining proper coating gaps and minimizing shear
rates is the length of the lips. As shown in FIG. 7, the length of
the downstream lip 60c (Ld) may be anywhere in the range of 0.1-3.0
millimeters, with about 0.8-1.2 millimeters being preferable.
However, the length of this lip should be minimized so as to reduce
the shearing of the multilayer film, which could lead to
three-dimensional flows and uneven film formation. The length of
the middle lip 60b (Lm) can also fall within the range of 0.1-3.0
millimeters, with about 0.3-0.7 millimeters being preferable. The
length of this lip should be minimized so as to reduce the
possibility that the upstream portion, when subject to changes in
die angle of attack, will approach a coating gap of three times the
film thickness. However, the lip must be long enough to allow the
bottom layer flow to develop into a rectilinear flow. Finally, the
upstream lip 60a length is less critical, since there is minimal
flow along that lip. However, an increased lip length in this
region will assist in sealing the flow.
[0165] As mentioned, it is well known to place a slight negative
angle of attack of the die 20 with respect to the web 22 in order
to produce a converging downstream lip 60c. Thus, FIG. 6
illustrates the multilayer die 20 of the present invention turned
clockwise at a negative angle of attack (.alpha.) with respect to
the web 22. Thus, angles of attack in the range of zero to negative
5 degrees have been found to be appropriate for this purpose. It
will also be appreciated that this angle of attack changes the
coating gap at the upstream edge of all of the lips, thus affecting
the performance of the pressure gradient regulator of the present
invention. Thus, even if the coating gap at the downstream edges
remains the same at its appropriate dimension, depending upon the
length of the lips and taking into consideration the curvature of
the roll 26, the coating gap at the upstream edges of the lips may
exceed the desired value and bring the operation outside the
coating window. Thus, the longer the lips and the greater the
negative angle of attack, the more likely it is for coating
conditions to fall outside the operating window. This situation is
illustrated in FIG. 11, which illustrates recirculations under both
the middle and downstream lips.
[0166] Accordingly, in another aspect of the present invention the
upstream and the downstream lips of the die 20 may be beveled in
order to minimize these effects. Thus, for example., if the
downstream lip 60c is beveled by an angle .gamma., as shown in FIG.
7., then the need to rotate the die 20 to a negative angle of
attack is possibly eliminated. This allows greater control in the
coating gap (c.g..sub.t) along this downstream die lip. Likewise,
with a convergent beveled downstream lip 60c, the middle lip 60b
can be maintained preferably flat, as illustrated. Again, the
coating gap under this important middle lip 60b (c.g..sub.b) can be
carefully controlled in the absence of angle of attack adjustment.
That is, it is much less likely for the coating gap (c.g..sub.b) to
exceed three times the film thickness (f.t..sub.b), especially at
the upstream edges of the middle lip 60b. However, it should still
be noted that the step between the middle and downstream lips, as
discussed above in connection with FIG. 7, still exists.
[0167] Likewise, certain advantages can be achieved by beveling the
upstream lip 60a in a diverging manner by an angle .beta., as shown
in FIG. 7. This divergent angle can be used to seal the bead 42 and
adjust pressure drop across the bead. Thus, it has been found that
downstream lip 60c bevels in the range of 0-5 degrees are
appropriate, while upstream lip 60a bevels in the range of 0-2
degrees are preferable. As noted, these bevels improve the
optimization of the coating process, increase the size of the
operating window, and reduce the precision which would otherwise be
required in coating.
[0168] Design Process
[0169] In designing the lip geometries for a given set of coating
and liquid parameters, any particular sequence of analysis or
calculation is possible. One approach is to begin with the
downstream lip and move upstream, calculating each coating gap and
lip length in the process.
[0170] To begin, the wet film thicknesses for the various layers
must be determined. Typically, the dry film thickness for each
layer is obtained from product specifications in terms of coat
weight (such as grams per square meter), and the solid fraction
(the percentage of solids in the liquid), the density and viscosity
of the liquid formulation to be coated are known. Thus, to arrive
at wet film thickness, the coat weight is divided by the product of
the solid fraction and the density. This number can then be used,
in accordance with the ranges and dimensions set forth above, to
compute all coating and feed gaps in the die. The lip lengths and
angles of bevel (or angle or attack) may also be computed in
accordance with the present method to optimize the coating
operation.
[0171] Beginning at the downstream edge of the downstream lip, the
coating gap may be set at one time the total wet film thickness. At
this value, the sufficiently negative pressure gradient in the
sense of the web travel should be achieved such that smooth film
surface characteristics are achieved. As discussed above, the
length of this lip is then designed. Whether the lip is to be
beveled or a whether a negative angle of attack is applied to the
die, this lip should be convergent in the direction of web travel.
With the angle and length of the downstream lip known, the coating
gap at the upstream portion of that lip can be calculated so as to
ensure that it falls within acceptable ranges.
[0172] In designing the downstream lip, some consideration should
be given to the issue of angle of attack versus beveling. As noted
above, beveling is usually advantageous since it virtually
eliminates the negative trade-offs associated with angles of
attack. However, beveled lips are more difficult to machine than
flat lips; thus, there is some sacrifice in accuracy. There are
also increased cost considerations.
[0173] Turning to the middle lip, the coating gap at the downstream
region is critical, as explained above. It should be maintained at
around two to three times the bottom-layer film thickness, and
should not be so excessively positive as to cause recirculation
under that lip. The length of this lip should be minimized to
reduce the likelihood of developing an excessively positive coating
gap whenever an angle of attack is applied to the die, but not to
the extent that a rectilinear flow cannot develop.
[0174] The design of the upstream lip is dictated by pressure drop
considerations along the bead. Any design adequate to seal the bead
is sufficient. A divergent bevel in the web direction is preferred
since the pressure drop varies quadratically with distance along
the bead. This means that the position of the upstream meniscus of
the bead can be controlled more easily with respect to
perturbations.
[0175] Once the length and angles of the lips have been determined
and desirable coating gaps calculated, the die can be assembled
from its various sections. This is accomplished in accordance with
well known techniques, using shim stock, etc. At the same time,
however, it is important that the steps of the lips relative to one
another be correctly positioned. The feed gaps must also be formed
by the correct positioning of the die lands. In order to avoid
recirculation, the feed gaps should not be excessively wide.
Lastly, the die can be set to an initial angle of attack, as
determined by the foregoing computations or the development of a
coating window, discussed below.
[0176] Coating Window
[0177] If considered necessary or desirable, ranges of various
operating parameters for the die as thus designed and set-up can be
determined. This is typically accomplished by experimentally
coating the web using various samples of the liquids to be used in
production, and by stepping through various angles of attack and
coating gaps. Liquids of different viscosities may also be coated.
The resulting information can be illustrated with a "coating
window" indicating the parameter field within which good coating
results are obtained.
[0178] FIG. 14 illustrates a typical coating window for a
multilayer construction to be coated at a given web speed. As
shown, various points for coating gap and angle of attack are
plotted to give the boundaries of the coating window. Outside of
this window, the defects noted on the graph occurred. Thus,
clearly, it is desirable to maintain the operation within the
coating window.
[0179] It will be noted that more negative angles of attack usually
result in lower downstream coating gaps due to the rotation of the
die with respect to the web. For the graph of FIG. 14, a larger
downstream coating gap is represented by an angle of attack which
is less negative (less convergent in the direction of web travel).
Thus, in accordance with another aspect of the present method, it
is desirable to attempt to maintain the coating operation at those
regions within the coating window where greater downstream lip
coating gaps occur and where the angle of attack is just sufficient
to avoid the ribbing defect. Operation in these regions will reduce
elevated shear stresses that result in poor coating quality.
However, at the same time, the coating gap must be sufficient to
avoid recirculation below the middle lip.
[0180] These regions comprise a subset of the coating window which
is referred to as the "quality window," and represents the area
where coating quality is best. In addition, higher coating gaps
(but not those that may result in excessively positive pressure
gradients) are, in another way, desirable because they reduce the
pressure drop along the bead and make it easier to seal at the
upstream meniscus, and produce less penetration of the coating into
the web.
[0181] The trade-off here is a larger risk with respect to
perturbations. That is, in the quality window, especially at a
lower angle of attack, operation occurs near a defect boundary
("ribbing" in the example of FIG. 14). A perturbation may cause
coating conditions, at least for some duration, to fall outside the
coating window, thus resulting in a defective product. Thus, it is
optimal to pick a point of operation which is in the quality window
but far enough away from the defect boundary such that common
perturbations will not cause operations to fall outside the coating
window.
[0182] It will be appreciated by those of ordinary skill that
coating windows comprising graphs of other parameters are possible.
For example, it is common to graph web speed versus layer thickness
ratio. Any combination of two or three relevant coating parameters
may be graphed in order to determine a coating window and an inner
quality window.
[0183] Vacuum--Assisted Coating
[0184] It has been discovered that application of a uniform vacuum
adjacent to and upstream over of the width of the coating bead
facilitates formation of a stable steady state coating conditions.
Furthermore, vacuum application may be used to enlarge the coating
window, thereby increasing the ease and efficiency of the coating
operation. Advantageously, increasing the coating gap results in
less coated material penetrating the surface of the substrate, such
as silicone into a paper surface.
[0185] Conceptually, vacuum-assisted coating may be described with
reference to FIGS. 19A-C. Referring to FIG. 19A, there is shown a
schematic illustrative diagram of a single layer die 100 coating a
layer 110 onto a paper substrate 120. The coating gap 130 has been
selected in view of the principles discussed above, to provide for
a proper pressure gradient to promote stable coating. Plotted below
die 100 is the pressure gradient underneath the die and immediately
downstream of the die 100. It should be noted that the pressure
peaks just under the slot of die 100, then rapidly diminishes to
atmospheric pressures at points downstream of die 100. Shown
schematically in FIG. 19A, the pressure underneath die 100 is so
great that it forces a portion 112 of layer 110 into the paper 120.
This is undesirable, as material that is forced into the paper does
not contribute to the desired properties of the layer. For example,
where layer 110 is silicone, portion 112 does not contribute to
release, and therefore is wasted.
[0186] To reduce the pressure under die 100, the coating gap
between the die and the paper may be increased. This is depicted in
FIG. 19B, where it is observed that the coating gap between the
lips of die 100 and the surface of paper 120 has been increased
from gap 135 to gap 136. Plotted below the schematic diagram of the
die is the pressure gradient underneath the die and immediately
downstream of the die. As expected, the resulting pressure gradient
from the increased coating gap reflects a lower pressure underneath
die 100. Consequently, the portion 112 of coated layer 110 forced
into the surface of the paper is much less than that observed in
FIG. 19A. However, larger gap 136 renders the coating bead upstream
of the slot of die 100 unstable. This may lead to coating defects,
detrimentally affecting coating efficiency.
[0187] The benefits of vacuum assisted coating are shown in FIG.
19C, where the stability of the coating bead has been reestablished
by application of a vacuum immediately upstream of die 100. This is
done using vacuum box 150, having opening 155. Vacuum box 150
preferably has a width equal to or greater than the cross-web width
of die 100. Opening 155 extends at least along the width of die 100
as well. Opening 155 is positioned upstream and adjacent to the
coating bead, increasing the pressure gradient underneath the die
until coating stability is established. However, the larger gap
reduces the amounts of coated material forced into paper surface
120. Thus, using vacuum box 150, larger coating gap 136 may be used
to coat layer 110, with little waste of coated material in portion
112 of the paper 120. Vacuum assisted coating is most applicable to
low viscosity liquid coatings, which tend to be more responsive to
vacuum assist.
[0188] Referring to FIG. 20, there is shown a cross-section of one
embodiment of a vacuum box for use in the vacuum-assisted coating
embodiments of the present invention. Vacuum box 200 comprises a
main body 210, side plates 220, blade 230, vacuum tube 240 and
mounting bracket 250. Main body 210 preferably has a combined width
equal to or greater than the width of the slot of the coating die.
Side plates 220 are attached to main body 210 to form a
substantially fluid tight seal capable of preserving the vacuum
within the housing of vacuum box 200. Main body 210 and side plates
220 define a chamber 225 therein. Chamber 225 is in fluid
communication with opening 260, such that an application of vacuum
in the chamber will result vacuum box 260 introducing a vacuum
force to the exterior environment via opening 260.
[0189] A blade 230 may also be inserted into vacuum box 200, as
shown in FIG. 20. Preferably, blade 230 has a beveled bottom edge
235. In one embodiment, a vacuum is introduced into chamber 225 by
a vacuum tube 240, which extends along the width of vacuum box 200
within chamber 225. Vacuum tube 240 is in fluid communication with
a vacuum source (not shown). The vacuum source may be those
conventionally known, such as vacuum pump and venturi and the like.
Preferably, the vacuum source is at least capable of creating a
vacuum of 1 to 250, more preferably from 25 to 200, and most
preferably at least 50 to 75 cm of H.sub.2O. In this embodiment,
holes are drilled along vacuum tube 240 on its side opposite
opening 260, such that application of a vacuum in tube 240 creates
a vacuum in chamber 225 and thus through opening 260. Opening 260
may then be placed upstream and adjacent to the coating bead to
affect the pressure gradients produced by the coated fluids.
[0190] It should be appreciated by those of skill in the art that
many other embodiments can be used to form a vacuum box suitable to
assist coating.
Production Example
[0191] A vacuum box, 24 inches across, made in accordance with the
description above, was modified to allow measurements of vacuum at
the coating bead in seven locations across the box. Uniformity of
vacuum across the vacuum box was verified by offline measurements.
Vacuum variability across the box was measured at less than 25 mm
of H.sub.2O column (2.5.times.10.sup.-4 bar) standard deviation for
all variable conditions.
[0192] A coating window was generated for AT-70 paper without
vacuum assist to establish the maximum coating gap before the
defect of chatter occurs. The single layer material coated
consisted of a mixture of silicone and SBR, at 35% solids, for a
40:60 Si to SBR ratio. The die angle of attach was set at
-2.0.degree., and the coating speed was set at 30 m/minute. The
coating was applied at a target weight of 1.5 gsm dried silicone. A
coating gap of 74 microns without vacuum was established. The bead
was stable at this coating gap, and chatter and shirlastains were
not observed. Coating gaps greater than 74 microns were observed to
result in coating defects.
[0193] To establish the effects of vacuum on chatter, the gap was
increased while increasing the level of vacuum. A coating window
was established by adjusting the vacuum blade and level of vacuum
such that the coating defect of chatter was completely eliminated.
Good coating quality was established and the shirlastains were
excellent (absence of pinholes), indicating improved coating. An
increase in the coating gap of 70 .mu.m to 100 .mu.m was achieved.
The die angle of attack (AOA) was -2.0.degree., and the level of
vacuum was 25.6 cm of H.sub.2O (0.0251 bar). A later study at
-4.0.degree. AOA indicates an increase in the range of coating gap
is possible. Moreover, samples with good coating quality and
excellent shirlastains were generated up to 90 m/minute with vacuum
assist.
Curtain Coating
[0194] The present inventors have also found that curtain coating
techniques may be used to form multilayer release surfaces, wherein
a supporting layer is coated substantially simultaneously with a
release layer. As generally known to those of skill in the art, in
curtain coating, a liquid sheet is expressed from an apparatus such
as a die, and falls freely over a distance until it impinges upon a
moving substrate to be coated. The liquid sheet impacts the
surface, and if coating conditions are properly controlled, forms a
layer thereon. Curtain coating can be used to coat multiple layers
by forming a multilayered liquid sheet to be expressed from the
curtain coating apparatus. Those of skill in the art are directed
to Kistler, et al. "Liquid Film Coating," published by Chapman
& Hall, London (1997), the entirety of which is incorporated
herein by reference, for its teachings on multilayer curtain
coating techniques. See also Kistler, S. F., "The Fluid Mechanics
of Certain Coating and Related Viscous Free Surface Flows with
Contact Lines," Doctoral Thesis, University of Minnesota, November
1983.
[0195] For multilayer release surfaces, curtain coating has certain
advantages over the dual die coaters discussed above. First, in
curtain coating, the distance traveled by the liquid sheet from the
die to the substrate may be hundreds of times greater than the gap
between the die and the substrate used for dual die coating. Thus,
curtain coating does not require as precise a control over the
coating gap as is needed for dual die coating. Consequently, less
experienced operators can/may successfully curtain coat multilayer
release surfaces, and the coating efficiency may be higher. Indeed,
it is quite common for gaps in certain coating to vary from 5 cm to
50 cm. Second, by its very nature, the fluid dynamics of curtain
coating require one to exceed the low flow limit, i.e., a minimum
volume (m. min) is needed to maintain curtain integrity. It can be
readily understood that coating thickness and coating speed are
coupled. A decrease in coating thickness from 30 .mu.m to 25 .mu.m
requires a 30/25 increase (compensational in line speed to maintain
curtain integrity. This explains why curtain coating is considered
high speed coating processes. Thus, curtain coating techniques may
be used to forms multilayer release surfaces more quickly than dual
die techniques.
[0196] The support, release layers and substrates which may be
curtain coated using the present invention are those described
above. It has been observed, however, that stable multilayer liquid
sheets are easier to achieve when the dynamic surface tension of
the liquid layers making up the multilayer sheet are approximately
the same. Where dynamic surface tensions are too different, the
curtain tends to break apart prior to contacting the moving web
underneath. To minimize or perhaps overcome problems associated
with surface tension differences, surfactants may be used to
optimize curtain coating processes of the present invention. In
some instances, addition of surfactants may not be sufficient to
establish a stable curtain. For these types of materials, the dual
die coating techniques discussed previously can generally be used
to form multilayer release surfaces, as the close proximity of the
dual die to the substrate makes surface tension effects much less
important.
[0197] Referring to FIG. 21, there is shown a schematic cross
section of slide coater 300 which may be used to curtain coat
multilayer release surfaces. Slide coater 300 comprises an upper
solid portion 302, a middle solid portion 304, and a lower solid
portion 306. The solid portions define the outer die surface. Solid
portions 302, 304 and 306 may be made out of any material known to
those of skill in the art to be useful for forming precision dies,
such as stainless steel, 316 stainless steel. 15-5 HP steel, and
other non-corrodible metals used to make dies. The dimensions of
the solid portions may vary, depending particular coating needs.
Solid portions 302, 304 and 306 should, however, be at least as
wide as necessary to accommodate slots of the desired width of the
liquid sheet to be coated on the substrate. Moreover, where it is
desirable to coat the entirety of the substrate between its lateral
edges, the die used should be wide enough to accommodate slots
wider than the substrate. One particular set of dimensions found
useful for successfully coating multilayer release constructs
appears in FIG. 21. Solid portions 302, 304 and 306 may be joined
together by conventional methods known to those of skill in the
art, such as body bolts.
[0198] A horizontal or inclined first slot 330 is formed between
upper portion 302 and middle portion 304. An incline prevents air
pockets from being trapped inside the die with the risk of air
bubbles being generated along with the metered fluid, which could
lead to curtain break-up and/or ellipsoid shaped coating defects.
Slot 330 is in fluid communication with manifold 310, such that
fluids passing through supply pipe 325 and into manifold 310 will
enter slot 330 and be expressed/metered as a first layer from die
300 along face surface slide portion 315. A horizontal (or
inclined) second slot 340 is formed between middle portion 304 and
lower portion 306. Slot 340 is in fluid communication with manifold
320, such that fluids passing through supply pipe 326 into manifold
320 will enter slot 340 and be expressed/metered as a second layer
from die 300 along face surface slide portion 315. Manifolds 310
and 320 extend across slide coater 300 to feed slots 330 and 340.
Preferably, manifolds 310 and 320 slope downward from the center of
slide coater 300 to its lateral edges (i.e., shallow inverted v
shape), to facilitate fluid flow along the length of the
manifold.
[0199] As known to those of skill in the art, fluids flowing
through slots such as slots 330 and 340 experience parabolic fluid
flow. When fluids are expressed/metered from slots 330 and 340 and
slide across the face of the die, the fluid flow converts from
parabolic flow to semi-parabolic flow. Once the fluid flow has
converted to semi-parabolic, it is capable of forming a multilayer
liquid sheet described herein.
[0200] The first and second layers meet at slot 340, to form the
multilayer liquid sheet to be coated. The fluid metered from slot
330 flows over the fluid metered from slot 340. In one preferred
embodiment, slot 330 and slot 340 are substantially parallel,
although parallel alignment of such slots is not essential to
successful curtain coating. Shims 350 or other means may be
inserted/used between the solid portions to adjust the dimensions
of slots 330 and 340. The slots used in curtain coating dies may
vary in opening as necessary to establish a successful multilayer
curtain. Suitable openings for slots of slide coater 250 are from
200 to 1000 microns, with 300-600 microns being preferred. The
distance along face 315 between slot 330 and 340, known as the
"slide distance," should be sufficient to permit fluid flow to
convert from parabolic flow to semi-parabolic flow. For slide
coater 300, this slide length is about 60 mm. Depending upon the
materials being coated, slide distances of from 10 to 100 mm are
suitable between slots. Unduly long distances between slots should
be avoided, so as not to create opportunities for fluid
instabilities. As a general rule, the slide distance should be
about 5-20 times the fluid thickness of the layer on the slide.
[0201] Although die 300 is depicted as having only two slots, it
should be appreciated by those of skill in the art that the
teachings set forth herein may be used to create dies having three
or more slots. For example, a manifold may be formed in upper
portion 302, and a cap portion applied thereover to form a third
slot (not shown) for expression of an additional fluid layer.
[0202] As shown in FIG. 21, the angle of face surface 315 with
respect to the bottom surface of bottom portion 306 is 45.degree..
This angle may be varied to optimize the curtain coating process to
the particular multilayer release materials being coated.
Generally, the more viscous the material being coated, the greater
the tilt angle desired for the face of the die. Tilt angles may
vary from 10 to 60 degrees on average, with a range of 20-45
degrees being preferred for silicone release systems comprising a
silicone containing release layer being coated within an SBR
support layer. It should be appreciated by those of skill in the
art that angles outside of these ranges may still work, albeit
sometimes not as effectively.
[0203] In order to optimize curtain coating conditions, it is
preferred to adjust the design of lip 365 of slide coater 300. If
unmodified, fluid flowing to lip 365 would tend to migrate under
bottom portion 306, disrupting stable curtain flow. To correct for
this, a slide block 370, is mounted to the bottom surface of bottom
portion 306. Slide block 370 bends downward at a sharper angle than
face surface 315, to form a more stable curtain. The underside 375
of slide block 370 is formed to have a sharp inclination, to
prevent the liquid sheet material leaving the edge of slide block
370 from flowing under block 370. The angle formed between the
front and back face of block 370 may vary, with 0 to 35 degrees
being preferred.
[0204] In use, die 300 produces a liquid sheet having two layers as
it falls from slide block 370. Conventionally, the substrate is a
moving web of paper traveling first underneath the manifold and
then away from block 375. Thus, the first and uppermost layer is
formed by liquid metered from slot 330. For multilayer release
surfaces then, slot 330 will meter the silicone containing
compositions described previously. A second layer is formed by
metering a liquid through slot 340. The second layer is covered by
the first layer as the first flows over slot 340. For multilayer
release surfaces, the second layer is formed by the supporting
layer materials described previously.
[0205] As shown in FIG. 21, upper, middle and lower solid portions
302, 304 and 306 are joined to form a face surface which is flat.
In an alternate embodiment, middle portion 304 may be adjusted to
extend outward from face 315 by a distance equal to the thickness
of the layer formed from slot 340. By making this adjustment, the
layer formed from slot 330 may flow directly onto and over the
layer formed from slot 340, minimizing the opportunity for
undesirable interfacial effects between the layers where they
meet.
[0206] Curtain coating efficiency is greatly improved if edge
guides are used in conjunction with the coating die. Edge guides
are rods or struts which extend down from the lateral edges of the
slots of the die, to almost touch the surface of the moving web
underneath. The edge guides provide a surface for the lateral edges
of the multilayer liquid sheet to flow upon. In the absence of such
a surface, the curtain tends to collapse inward. Thus, edge guides
promote stability in curtain coating. For purposes of the present
invention, many different types of edge guides as know to those of
skill in the are suitable. One example is an edge guide sold by
Bachofen+Meier AG, Bulach, Germany as Seitenblech zur Duse. Other
suitable edge guides include those described in the many issued
U.S. patents, such as U.S. Pat. No. 5,976,251, incorporated herein
by reference.
[0207] A metering pump (or pumps) may be used to pump the fluids
making up the two layers into the manifolds of slide coater 300. A
preferred metering pump is pulse free and precise, such as those
sold by Zenith as series C9000, Waukesha Universal 15, or other
suitable metering pumps may be used. The metering pump is adjusted
to provide the desired flow rate for the coating application, such
that a steady stream is supplied to the manifolds and slots 330 and
340. Depending on the specific process conditions, fluid rheology
and the die design, the fluid pressure in the manifolds may vary
from 0.15 to 10 psi, depending upon the viscosity of the liquid
being coated. As should be appreciated, higher viscosity fluids,
such as liquids with a high solids content, require higher
pressures to establish steady state flow through.
[0208] The flow rate of the material is dictated by the web speed
and designed coating thickness. Generally, there is a minimum flow
rate of from 6 or less L/(m.min) to establish a stable curtain. The
maximum flow rate, which is dictated by line speed and coating
thickness requirements, may exceed 30 L/m.min. Within these
extremes, the speed of the web may be adjusted to achieve the
desired coating thickness, as is known to those of skill in the
art.
[0209] For curtain coating, the web moving underneath the die
typically travels from 1.0 or less up to 20 m/s or more. This
generates a great deal of air pressure near the surface of the web
moving toward the falling liquid curtain. To prevent this moving
air from disrupting the curtain, an air shield should be used. The
present inventors have found a soft rubber material, attached to a
plexi glass, having a width greater than the substrate and a height
of 120 mm, to be sufficient, but other more optimum devices may be
used. The air shield is positioned from 1.0 cm (or less where
practical) to 10 cm behind the curtain. A plastic material of
suitable thickness and stiffness may be used to keep the laminar
air layer, dragged in by the moving substrate, away from the liquid
curtain. The soft rubber material makes very light/sufficient
contact with the web. Alternate designs may include those disclosed
in U.S. Pat. No. 5,224,996, the entirety of which is incorporated
herein by reference.
[0210] Another important variable to control is the air content of
the fluids being coated. Air bubbles in the curtain will
transiently disrupt it, greatly diminishing coating efficiency.
Consequently, the fluids to be curtain coated should be deaerated
prior to being pumped into the curtain coating die. Any of the
known methods of deaeration are acceptable. For example, one
suitable apparatus is a Versator sold by Cornell Machine,
Springfield, N.J. Alternatively, other deaeration methods may be
used, such as those sold by Fryma A G, Rheinfelder, Switzerland, or
thin film evaporators.
[0211] It should be readily appreciated that dies of different
designs may also be used to curtain coat the multilayer release
constructs of the present invention. For example, Liquid Film
Coating, Kistler et al, describes several different designs of
curtain coating dies which may be adapted to coat multilayer
release surfaces. These alternate designs include inverted slot-fed
type curtain dies (Kistler et al, FIG. 11c.4), reverse slide type
curtain dies (Kistler et al, FIG. 11c.5), slide-fed type curtain
dies (Kistler FIG. 11c.3) and merging slide-fed curtain coating
dies (Kistler et al, FIG. 11c.6). Among these various types of
coaters, the present inventors have found that it is easier to
optimize the coating process if slide type coaters are used.
Production Example
[0212] A multilayer release surface was created using curtain
coating techniques as follows. A release layer was formed from a
silicone/SBR mixture, with a ratio of 30:70 silicone to SBR,
undiluted to 50% solids. A release support layer was formed from
SBR, undiluted to 50% solids. The release support layer was coated
to a weight of 6 g/m.sup.2, and the release layer was coated to a
weight of 4 g/m.sup.2. The flow rate was 7 liters/meter-minute.
Each layer was coated substantially simultaneously onto Data-70
paper using a curtain coating die similar to that described above.
The die was mounted on a pilot coater. The coating web speed was
350 m/min. A tungsten wire was placed underneath the web at the
position where the curtain impacts the substrate. A voltage
differential of 25 kV was applied during the coating operations.
This (additional) body force, which pins the dynamic contact line
between the fluid and the moving substrate contributes in achieving
the desired coat quality over a much wider operating window. The
voltage differential causes the falling multilayer liquid web to
impact the with slightly more force than that imparted by gravity.
A TEM of a cross section of the resulting multilayer release
surface is shown in FIG. 22. As shown in FIG. 22, the boundary
between the release layer and the release support layer is much
sharper than that generated by dual die coating, with less
intermixing of adjacent layers. Thus, curtain coating techniques
may be used when it is desirable to achieve a more well defined
boundary between the support layer and the release layer.
[0213] Trouble Shooting
[0214] During production, as just noted, perturbations or other
irregularities may occur that introduce defects into the quality of
the film. Thus, it is advantageous, in accordance with the method
of the present invention, to be able to correct such defects as
soon as possible, in order to minimize their degree and duration.
If possible, such "trouble shooting" should occur during coating so
that operations do not have to cease.
[0215] One of the more common defective conditions, as described
above, is upstream leakage. If this occurs during operation, the
coating gap may be increased to reduce the pressure drop along the
bead. Alternatively, the elimination of upstream leakage may be
accomplished by a change of die angle of attack which produces a
higher downstream coating gap and a lower upstream coating gap
(i.e., a less negative angle of attack). Other means, such as
liquid viscosity adjustment, can be used to control upstream
leakage.
[0216] Another defect is "de-wetting." If, in the film forming
region, a perturbation affects the surface of the film, one or more
layers may retract from the underlying layers or substrate leaving
a void. This condition can be corrected by lowering the surface
tension of the upper layers by, for example, increasing the
surfactant in those layers. Also, the coating speed can be reduced
in order to maintain the dynamic surface tension of the liquid of
the film forming region at or below the stable level.
[0217] In conclusion, the method of the present invention
represents a marked advancement in the multilayer coating art. It
should be understood that the scope of the present invention is not
to be limited by the illustrations or foregoing description
thereof, but rather by the appended claims, and certain variations
and modifications of this invention will suggest themselves to one
of ordinary skill in the art.
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