Note: Descriptions are shown in the official language in which they were submitted.
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MULTTLAYER COATING
US~NG AN APPLICATION ROLLl~R
TECHNI(~AL FIELD
The present invention relates to methods for coating thin coats of
polymeric materials on substrates.
BACKGROUND OF THE INVENTION
Thin coats of polymeric materials such as hot-melt adhesives and
radiation curable adhesives have been applied to substrates, such as paper, by
several different methods. Such polymeric materials have been applied in one or
more layers from dies directly onto substrates passing the dies. Gravure coating has
also been used. In gravure coating, an applicator roller receives the polymeric
material from a die and transfers the coating material to a substrate. Engraved
depressions, or cells, formed in the peripheral surface of the applicator roller. carry
the coating material which is transferred to the substrate at an application interface
between the applicator roller and the substrate. With either of these processes it
may be difficult to apply very thin layers of certain polymeric coating materials.
The die openings must be very narrow to apply thin layers, and impurities such as
2() gels and other contaminants in the coating material can too easily occlude the die
opening, causing unacceptable streaking and voids in the coating, and requiring
cleaning of the die.
European Patent Application 0 031 301 A1 describes a method for
applying materials fed from a die onto a substrate by dropping the materials from
2~ the die into a nip between the substrate and a chrome surfaced roller, and moving
the substrate through that nip at a speed faster than the material is exiting from the
die so that the coating materials exiting the die will be drawn into thin layers and
coated on the substrate in layers that are much thinner than the layers of material
f exiting the die. Thus the opening in the die can 11ave a width that is not easily
3u occluded, and a thin layer of material can still be applied to the substrate. While this
method may work for some materials, it presents problems when used to apply
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certain polymeric materials such as hot-melt adllesives and radiation curable
adhesives, because molecular orientation and compression of such materials, caused
when they are forced through the die can cause ribbing or breaking of the coating
material exiting the die when it is drawn or stretched as it is coated onto the
substrate.
SUMMARY OF THE rNVENTION
The present invention provides a method of applying a coating of hot
melt polymeric material from a die to a substrate which includes thinning the layer
0 of material after it exits the die without causing ribbing or breaking of the coated
layer.
The method according to the present invention comprises the steps
of (I ) providing a coating apparatus including an application roller having a
cylindrical peripheral surface about an axis, means mounting the application roller
1~ for rotation about the axis, die means having a first elongate die outlet opening, and
means for mounting the die means at a die position along the cylindrical periphery
of the application roller with the die opening extending generally parallel to the axis
of the application roller; (2) rotating the application roller about the axis to provide
a first velocity for the periphery of the application roller and movement of the2() periphery of the application roller past the die opening; (3) extruding the hot melt
polymeric material through the first die opening onto the peripheral surface of the
rotating application roller to provide a first coating layer along the peripheral
surface; and (4~ conveying the substrate past the peripheral surface of the
application roller at an application interface to cause transfer of the first coating
2:~ layer to the substrate at the application interface. The substrate is conveyed at a
velocity greater than the surface velocity of the application roller to shear and
thereby reduce the thickness of the first coating layer on the substrate with respect
to the thickness of the first coating layer Oll the peripheral surface of the application
roller. The velocity of the periplleral surface and spacing between the die position
.() and the application interface along that peripheral surface are selected to provide a
time inte~val of at least 0.12 second (and preferably at least 0.25 second) for
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movement of portions of the first coating layer from the die position to the
application interface, thereby affording some relaxation and n olecular reorientation
of the hot melt polymeric material after the extruding step to facilitate elongation of
the first coating layer at the application interface without causing ribbing or
breaking of the coating layer.
BRIEF DESCRIPTIQ~ QF THE DRAWTNG
The present invention will be further described with reference to the
accompanying drawing wherein like reference numerals refer to like parts in the
1() several views, and wherein:
Figure l is a schematic illustration of the coating method according
to the present invention; and
Figure 2 is a cross-sectional view of an alternative embodiment of a
die that could be used in the method according to the present invention.
~ETAILED DESGRIPTIQ~ QF THE I~VENTION
Referring now to Figure 1, there is schematically illustrated a
method according to the present invention for coating first and second layers ofmaterials l I and 13 onto a substrate 30.
The apparatus illustrated in Figure 1 is adapted for applying layers of
two different coatin~ materials 11 and 13 using a manifold coating die 14, however,
the method according to the present invention can also be used to apply only one or
more than two layers of coating material. The coating die 14 has two separate
chambers 16 and l 8 which receive the coating materials l l and 13 from sources 10
2~ and 12, respectively, and has two elongate die outlet openings 20 and 22
communicating with the chambers l 6 and 18, respectively, which are proximate the
peripheral surface 24 of an application roller 26 and have their lengths generally
aligned with the axis of the application roller 26.
First and second coating layers of the two coating materials I I and
.() 13 respectively are applied onto the periphery of the application roller 26 through
the die openings 20 and ''2 to form a multi layer coating l 5, and are subsequently
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transferred from the application roller 26 to a substrate 30 at an application
interface 2~ while being sheared at the application interface 2~ to reduce the
thickness of the first and second layers of coating material compared to their
thicknesses at the die openings 20 and 22.
The first coating material 11 is extruded through the first die opening
20 wl1ere it joins tl1e second coating material 13 that has been extruded through the
second die opening 22 to form the multiple layer coating 15. (Additional die
openings may also be provided, as shown in the embodiment of a manifold die 14' in
Figure 2 which includes three chambers 16', 18', and 1 8a and three die openings 20',
22', and 22a). The multiple layer coating 15 is then carried on the peripheral face
24 of the application 26 roller toward the application interface 28, for transfer to the
substrate 30.
The substrate 30 is withdrawn from a supply roll 32, and the coated
substrate is collected by a talie-up roll 34. A backing roller 36 opposes the
15 application roller 26 at the application interface 28. When the multiple layer coating
15 reaches the application interface 28, the multiple layer coating 15 is transferred
to the substrate 30. The outermost first coating layer ofthe coating material 13contacts and adheres to tl1e substrate 30 because the attractive forces between that
outermost first coating layer and the substrate 30, and between the first and second
2() coating layers of the materials 13 and I I, are greater than the attractive force
between the application roller 26 and the second coating layer of the material 11.
The peripheral surface of the application roller 26 may be coated with a releasecoating, such as Teflon, to facilitate release of the multi-layer coating material 15 .
The application roller 26 can, optionally, be rotated at a surface
2~ velocity that is greater than the velocity of the coating materials 13 and I I exiting
the die openings '70 and 22. This causes drawing or stretching of the multiple layer
coatingr 15, resulting in less thickness for the first and second coating layers of the
materials I 1 and 13 on the periphery of the application roller 26 than would result if
the application roller ~6 were rotated at the same surface velocity as the velocity of
.. t) the coating materials 13 and 1 1 exiting the die openings 20 and 22.
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The multiple layer coating 15 is tangentially sheared and thereby
thinned by conveying the substrate 30 past the application roller 26 at a greater
velocity than the surface velocity of the application roller 26. For example, if the
substrate 30 is conveyed at a velocity of 1.0 m/s (197 ft/min.), and the peripheral
surface of the application roller 26 is traveling at a velocity of 0.5 m/s (98.4 ft/min.)
at the application interface 28, the draw ratio is appl~xi-llately 2:1. Thus, the
thickness of the multiple layer coating 15 will be approximately 1/2 as thick on the
substrate 30 as on the peripheral surface of the application roller 26.
The velocity of the peripheral surface of the application roller 26 and
lo the spacing along that periphery between the position ofthe coating die 14 and the
application interface 28 are selected to provide a time interval of at least 0.12
second (and preferably of at least 0.25 second) for movement of each portion of the
first and second coating layers from the coating die 14 to the application interface
28. This amount of time after the materials 11 and 13 are extruded through the
1S outlet openings 20 and 22 and are possibly drawn or stretched between the coating
die 14 and the periphery of the application roller 26 and before the multiple layer
coating 15 is sheared and thinned at the application interface 28 affords some
relaxation and molecular reorientation of the hot melt polymeric material 11 to at
least partially remove orientations caused by that material ] I being extruded
20 through the coating die 14, which relaxation and molecular reorientation facilitates
elongation of tlle first coating layer 15 at the application interface 28. Without such
relaxation and molecular reorientation, there is a much greater tendency for themolecular orientation in the materials 11 and 13 caused by the extrusion through the
die outlet openings 20 and 22 and possible drawing at the coating die 14 to cause
2:~ ribbing or breaking of the coating layer when it is sheared and thinned at the
application interface 28.
Althougll it is preferred that the coating materials 11 and 13 meet to
form tlle multiple layer coating 15 prior to contact with the application roller 26, the
coating~ materials 11 and 13 could instead be sequentially applied to the application
3() roller 26. The first coathlg material 11 could be extruded on or applied to the
periphery of the application roller 26 to form the first coating layer, and the second
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coatin~, material 13 could then be extruded on or applied over the first coating layer
to form the multiple coating layer 15.
Various materials may be applied using the method described above.
For example, the first material I ] (which forms the outermost layer on the substrate
5 30) may be a pressure sensitive adhesive comprising isooctyl acrylate (IOA),
octodecyl acrylate (ODA), acrylic acid (AA), and 4-acryloyl-oxy-benzophenone
(ABP). Alternatively, the first material 11 may be a pressure sensitive adhesivecomprising isooctyl acrylate (IOA), octodecyl acrylate (ODA), and 4-acryloyl-oxy-
benzophenone (ABP). Other suitable adhesives for use as the first material 11 are
disclosed in PCT Publication No. WO 93/13148 and in commonly assigned U.S.
Patent Application Serial Number 07/816,593, filed 31 December 1991 and entitled"Removable, Low Melt Viscosity Acrylic Pressure Sensitive Adhesives."
The second material 13 could form a second coating layer called a
"tie" layer that bonds an adhesive first coating layer to a paper substrate. A suitable
15 second material 13 for use in such a tie layer comprises maleated propylene/hexene
copolymer, such as that available from the Eastman Chemical Corporation of
Kingsport, Tennessee, under product number Pl 824-013 .
The method and apparatus according to the present invention
provide a solution to the die opening occlusion problems of the prior art described
2(~ above. The present invention overcomes problems of die opening occlusion by
affording the use of die openil1~s that are sufficiently large that they will not easily
be occluded. The multiple layer coating 15 can be relatively thick when it exits the
extruding apparatus 14. It can, optionally, be made thinner by causing drawing or
stretching at the interface between the coating die and the application roller 26, and
25 it will be made thinner by being sheared at tl1e interface 28 between the peripheral
surface of the application roller 26 and the substrate 30. As a result, a relatively
thin multiple layer coating 15 can be applied to the substrate 30. even though arelatively thicl; multiple layer coating 15 was initially extruded from the coating die
14. Gels and other contaminants that mi~,ht become lodged in smaller die openings
3() (such as those that might be used in direct die coatin~J apparatuses to form a
multiple layer coating 15 of the same thickness on the substrate 30) pass through
--6--
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the larger die openings of tl1e present invention. Thus, the streaks and voids in the
coating layer that result from occlusion of the die openings are restricted.
The manifold die 14 may be designed as known in the art, and
should be adapted to facilitate the even, smooth extrusion of the respective coating
materials. The geometry of the die and die openings may thus be selected for
optimum performance with a particular set of coating materials. For example, theextrusion pressure may be increased or decreased, the die opening dimensions maybe increased or decreased, the separation between the respective die openings may
be increased or decreased, or the partition separating adjacent chambers may be
raised or lowered to enable the respective layers to meet slightly inside of or outside
of the die. The extrusion process thus may be optimized for the materials to be
extruded, as is known in the art.
The present invention will be better understood with reference to the
followin;, Example, which is intended to be illustrative and nonlimiting.
1~ A source of a pressure sensitive adhesive comprising 58% by weight
isooctyl acrylate (IOA), 40% by weight octodecyl acrylate (ODA), 2% by weight
acrylic acid (AA), and 0.4% by weight 4-acryloyl-oxy-benzophenone (ABP) was
provided (the total percentage of the constituents does not equal 100% due to
roundin;,). The adhesive was prepared in accordance with the disclosure of U.S.
2() Patent Application Serial Number 07/816,593. The adhesive was heated to a
temperature of approximately 138~ C (280~ F) prior to being supplied under
pressure to the first chamber 16 of the manifold die 14. The first and second
chambers 16 and 18 of the manifold die 14 each included a single, slot-shaped die
opening, 20 and 22 respectively, each measurin, approximately 2.9 cm (I.14 in)
2~ long by 0.051 cm (0.02 ill) wide. The pressure was applied by a constant
displacement gear pUlllp, and was maintained at a level sufficient to produce a flow
rate of approximately 1.42 g/S (0.050 OZ/S) through the first die opening 20.
Similarly, a source of polyolefin, available from the Eastman
Chemical Company of Kingsport, Tennessee under product number P1824-013,
,u was provided, and was heated prior to being supplied under pressure to the second
chamber 1~ of the mal1ifold die 14. The pressure was applied by a second constant
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displacement gear pump, and was maintained at a level sufficient to produce a flow
rate of approxhllately 0.675 g/s (0.0238 oz/s) througll the second die opening 22.
The second die opening ~2 measured approximately 3.175 cm (1.25 in) long by
0.0254 cm (0.01 in) wide. A partition separated the first and second chambers 16and 18 of the die 14 to prevent mixing of the respective coating materials 11 and
13.
The first die opening 20 was upstream of the second die opening 22
to enable the respective coating materials 11 and 13 to be extruded through the
respective die openings 20 and 22, and to meet to form the multiple layer coating
10 15. The application roller 26 was supported for rotation about its axis adjacent the
die openings 20 and ''2, such that the multiple layer coating 15 would collect on the
peripheral surface of the application roller 26. The application roller 26 had aradius of approximately 12.7 cm (5.0 in), and a peripheral surface width of 3.175
cm (1.25 in). Tlle peripheral surface ofthe application roller 26 included a
1~ multiplicity of projecting structures randomly distributed across the peripheral face
ofthe application roller 26, wllich structures measured approximately 0.76 mm
(0.003 in) high, and were generally hemispherical. The projecting structures
provided a discontinuous surface texture for the surface of the first layer of pressure
sensitive adllesive material 11 that contacted it. The peripheral surface of the2() application roller 26 was coated with a fluorocarbon or Teflon release coating to
facilitate transfer of the multiple layer coating 15 to the substrate at the application
interface. Further background information regarding the construction and operation
ofthe application roller 26 apparatus may be found in copending U.S. Patent
application serial number 08/056,362, filed 30 April ] 993 and entitled "Method and
2~ Apparatus for Applying a Coating Material to a Substrate," commonly assigned to
the assignee of the present application.
The application roller 26 was rotated about its central axis at a
rotational velocity sufficient to produce a velocity of 0.5 m/s (19.68 in/s or 100
ft/min.) at its peripheral surface. The first and second gear pumps were activated to
extrude the first and second coating materials 11 and 13 through the first and
second die openings 20 and 22, respectively. The e~truded layers of coating
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materials 11 and 13 bonded together to form the multiple layer coating 15, but little
or no mixing occurred between the respective layers. The first layer of pressuresensitive adhesive material 11 was approximately 0.003 cm (0.0012 in) thick, andthe second layer oftie material 13 was approximately 0.001 cm (0.0004 in) thick.s A substrate 30 or supply web of 20 pound xerographic bond paper
was provided, and the paper from the supply roller 36 was conveyed tangentially
past the periphery of the application roller 26 at the application interface 28 and
collected on a take-up roller 34. The paper measured 30.5 cm (12.0 in) wide and
0.0122 cm (0.0048 in) thick. The substrate 30 or paper web was conveyed at a
velocity of I .5 Ill/S (300 ft/min.) at the application interface 28, and thus the web
velocity was greater than the surface velocity of the periphery of the application
roller 26 by approximately a factor of 3.
The multiple layer coating 15 was transferred to the substrate 30 or
paper web at the application interface 28. The second or tie layer of material 13
s adhered to the paper web, and the first layer of adhesive coating material 11
released from the peripheral surface of the application roller 26. The resultingcoated layer thicknesses on the paper web 30 were 0.0] 02 mm (0.0004 in) for thefirst layer of adhesive material 11, and 0.0033 mm (0.00013 in) for the second layer
of tie material 13. The first layer of adhesive material 11 was exposed along the
2() paper substrate 30, and the adllesive coated paper web was adapted for adhesive
engagement witll a surface.
The distance along the peripheral surface ofthe application roller 26
between the die openings 20 and 27 and the application interface 28 was about 0.30
meters (11.76 inches) so that parts ofthe multiple layer coating IS took about 0.6
2s second to move from the die openings 20 and 22 to the application interface. No
tendency was observed for the first layer of adhesive material 11 or the second layer
of tie material 13 to either breal; or form longitudinal ribs.
It should be understood that although the present invention has been
described with reference to the transfer of a multiple layer coating directly to a
3() substrate, it is within the scope of the present invention to transfer the multiple layer
coating to one or more intermediate rollers or other apparatuses, and then to a
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substrate. That is, tlle transfer of the multiple layer coating from the application
roller to the substrate is preferably a direct transfer, but may instead be an indirect
transfer if desired.
The present invention has now been described with reference to
several embodiments thereof. However, persons of skill in the art will recognizethat variations may be made in the embodiments described without departing from
the scope of the invention. Thus, the scope of the present invention should not be
limited by the embodiments shown and described herein, but rather by the structures
described by the claims, and the equivalents of those structures.
I() ,
I ~ )
