Note: Descriptions are shown in the official language in which they were submitted.
~ 983~
This is a divisional application based on Canadian Serial
No. 524,559 filed December 4, 1986.
COMPOSITE FACESTOCKS AND LINERS
The present invention relates to web stoclc for
display products such as labels, signs and the like, and
more particularly to constructions and methods of making
film facestocks for such display products, liners for such
facestocks, and to the combined constructions including the
facestocks and liners.
BACKGROUND OF THE INVENTION
It has long been known to manufacture and
distribute pressure-sensitive adhesive stock for display
products such as labels and signs by providing a layer of
face material for the label or sign backed by a layer of
pressure-sensitive adhesive which in turn is covered by a
release liner. The liner protects the adhesive during
shipment and storage. With specific reference to labels,
the liner also allows for efficient handling and dispensing
of individual labels which have been die-cut from the layer
of face material while leaving the liner uncut.
Many label and sign applications require that the
face material be a polymeric film material which can provide
properties lacking in paper, such as weatherability (for
outdoor signs), strength, water resistance, abrasion
resistance, gloss and other properties. Because material
costs in the manufacture of such film facestocks are
relatively high, the desirability of reducing material costs
without sacrifice of quality has long been apparent, but
little or nothing has been accomplished toward this end.
Because the cost of paper generally compares
favourably with the cost of film materials, and because
paper liners also have other highly desirable
characteristics, the liners used with film facestocks have
*
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133983~
generally comprised paper web stock coated with a very thin
layer of silicone-based release agent. The paper web's
outer or "back" face has the roughness required to track
well on the smooth steel rolls used in high speed
manufacturing. The inner release-coated face of the paper
web is uneven enough to slightly roughen the surface of the
adhesive protected by the liner, thus preventing subsequent
air entrapment and bubble formation between label and
container in labeling applications.
However, paper readily absorbs and desorbs
moisture, leading to curling and distortion of film
facestock with which a paper liner is used. This is
particularly a problem with sheet facestock used, say, for
signs and decals. Moisture absorption and curling have been
reduced to a degree by coating the outside face of the paper
liner with a thin moisture barrier layer of film material,
but edge absorption or gradual moisture transmission through
the moisture barrier itself have largely thwarted efforts to
eliminate the problem of curling of the liner and consequent
distortion of the film facestock.
Another disadvantage of paper is its relative
mechanical weakness. This is particularly a drawback in
high speed packaging of high volume consumer products where
labeling machinery must dispense rolls of liner-carried
labels at high speed. A break in the paper liner forces
shutdown of the entire packaging line until the labeling
operation is properly reset. As line speeds have continued
to increase in recent years, the severity of this problem
has led some mass packagers to specify that labels are to be
carried on polyester film liner. The great strength of the
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1 3 3 9 ~ 3 ~
polyester film eliminates the liner breakage problem, but at
a price which reflects very much higher material costs than
those associated with paper.
The present invention
The present invention opens the way to substantial
cost savings in the manufacture of film facestocks while at
the same time maintaining the desirable characteristics of
the film facestocks which have been used prior to this
invention. In a word, costs are greatly reduced at little
or no sacrifice of quality, and even with a gain in quality
in some instances.
In another aspect, the present invention replaces
paper liner stock by liner stock of polymeric film
material. This is done in such a way as to simulate those
characteristics of paper that provide for good web tracking
and adequate prevention of the problem of air entrapment and
bubbling. At the same time, the problems of curling and
paper breaking are eliminated because the
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1~39~3a
film material used according to the invention is inherently
moisture-insensitive and is much stronger than paper. All this
is accomplished at little or no increase over the cost
associated with paper liners. In a word, quality is greatly
improved at little or no increase in costs, and even with a
reduction in costs in some instances.
Accordingly, there is provided in one aspect of the
invention a liner which comprises a multilayer web construction
for use with label, tape or sign stock. The web construction
comprises a coextrudate in the form of at least two firmly
adhered layers, at least one of which comprises a multiphase
layer which includes a continuous phase of polymeric film
material and a discontinuous phase of filler material. The
liner has at least one face that is roughened at least in part
by the mechanical effect of the filler material contained in
the coextrudate. A release face is provided on one face of the
liner.
According to a second aspect of the invention, there
is provided a method of manufacturing liner for label, tape or
sign stock which comprises the steps of:
(a) providing a plurality of at least two charges
of film-forming resin including a core or base charge and at
least one skin charge associated with the forming of at least
one skin;
(b) mixing a filler or fillers in at least the core
or base charge, or also additionally in at least one skin
yc/ sp
4a 1339~3~
charge, after preselecting the filler or fillers to include
lumpy filler components, that is filler components of
sufficient relative size, as compared to film thickness, and
sufficient circularity to affect the roughness of a face of
the resulting coextrudate; and
(c) coextruding the charges to thereby form a
filled coextrudate constituting a multilayer polymeric liner
with a filler-roughened face and comprising cojoined layers
including a core or base layer and at least one skin layer.
According to a third aspect of the invention,
there is provided a multilayer facestock for use in pressure-
sensitive label, or sign applications. The facestock
comprises a coextrudate of cojoined layers comprising a face
side and a back side with the face side having a printable
surface. The coextrudate includes stiffening layer means
which contributes the majority of the stiffness of the labels
or signs cut or formed from the facestock, and a pressure
sensitive adhesive combined on the backside of the
coextrudate. The stiffening layer means may include a core
layer comprising a polyolefin homopolymer or copolymer such as
polyethylene or polypropylene and may have a stiffness of
between 10 and 100 Gurley. Alternatively, the coextrudate may
comprise a relatively thick core layer of polymeric film
material of a stiffness suitable for label or sign
applications and a relatively thin skin layer of polymeric
material on the face side of the coextrudate having a surface
adaptable to the intended decorating process. A pressure-
sensitive adhesive layer is combined at the side of the
ycc/kb
1~39~3~
4b
coextrudate opposite the face side. In either case, the
facestock may be stretched to provide machine direction
orientation thereof. Generally speaking, each layer of the
facestock material has a particular function, which function
may be the same or different from the functions of the other
layers. This is known as function-oriented multi-layering in
labelling and sign applications.
In accordance with yet another aspect of this
invention, there is provided a method of economically
manufacturing roll or sheet facestock for labels or signs in
which a plurality of charges of film forming resin are
coextruded to form a construction in the form of a coextrudate
of cojoined layers having a face side and a back side. The
coextrudate includes a stiffening layer means which
contributes the majority of the stiffness of the construction.
At least one of the charges is selected on the basis of
density or flex modulus so as to provide the stiffening layer
means with a degree of stiffness suitable for the label or
sign application. A layer of pressure-sensitive adhesive is
combined on the back side of the coextrudate. The coextrudate
may be stretched to provide machine direction orientation
thereof. The charge associated with the face side may be
selected so as to provide a surface suitable for the intended
decorating process, such as printability, or surface
performance characteristics, such as weatherability. A
release liner may be combined with the coextrudate to form
linered label or sign facestock whereby the facestock may be
die-cut to form a label or sign releasably adhered to the liner
ycc/kb
133~8~n
4c
and surrounded by a matrix of excess facestock material.
This matrix may be stripped using the strength of the
stiffening layer means for self-supporting, non-tearing
stripping to pull the matrix away from the die-cut label or
sign.
Another aspect of the invention is found in a
method of manufacturing liner for label, tape or sign stock
which comprises the steps of providing at least one charge
of film-forming resin, and extruding the charge to thereby
form an extrudate constituting a polymeric liner, including
the step of roughening a face of the extrudate to a roughness
in its finally formed state greater than that of a gloss
finish as provided by finish rolling the extrudate with a
polished steel roll, and thereby, depending on the face
roughened, either imparts to the face a roughness which
improves tracking of the extrudate in guided contact with
steel drive or idler rolls, or provides the face of the
extrudate with a roughness adapted to be imparted to one face
of an adhesive layer by temporary contact therewith to
substantially prevent or minimize air entrapment when the one
face of the adhesive layer is adhered to a substrate.
Lastly, there is provided a polymeric liner for
label, tape or sign stock which comprises an extrudate of
film-forming resin. The extrudate has a face roughened to
a roughness in its finally formed state greater than that of
a gloss finish as provided by finish rolling the extrudate
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133983~
4d
with a polished steel roll, and thereby, depending on the
face roughened, either imparts to the face a roughness which
improves tracking of the extrudate in guided contact with
steel drive or idler rolls, or provides the face of the
extrudate with a roughness adapted to be imparted to one face
of an adhesive layer by temporary contact therewith to
substantially prevent or minimize air entrapment when the one
face of the adhesive layer is adhered to a substrate.
These and other advantages will become apparent
from the following description of the invention when taken
in conjunction with the drawings.
In the drawings, all of which are highly
diagrammatic, FIGS. 1 and 2 are fragmentary cross sections
of two different liner constructions embodying the invention;
FIGS. 3A-3D illustrate certain steps in the manufacture and
use of product embodying the invention; FIG. 4 is a
fragmentary cross section of a prior-art liner; FIGS. 5-7 are
fragmentary cross sections of three different facestock
constructions embodying the inventions; and FIG. 8
illustrates a manufacturing step related to the facestock of
the invention.
Liner stock constructions illustrating the
invention will be described first. A prototypical example
is illustrated in FIG. 1, which shows a multilayer web
construction generally indicated by the reference numeral 10.
The multilayer construction 10 includes a core or base layer
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4e 1 3 3 9 ~ 3 ~
12, and skin layers 14 and 16. A first face, generally
indicated by the reference numeral 18, is the outer or "back~
face of the liner stock. This face is identified with the
side of the web that will contact and be guided by the smooth
steel rolls of the manufacturing line in which the liner
stock is to be employed. A second face, generally indicated
by the reference numeral 20, is the inner face of the liner.
This face is identified with the side of the liner nearest
the adhesive to be protected by the liner.
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133983~
~ he layers 12, 14, and 16 comprise polymeric film
materials and are formed by simultaneous extrusion from any
suitable known type of coextrusion die such as, for example,
a Cloeren "vane" die heretofore employed, for example, to
form multilayer films used in food packaging applications.
The layers 12, 14, and 16 are firmly adhered to each other
in a permanently combined state to provide a unitary
coextrudate for all three layers, although any one or more
polymers or copolymers which will form firmly adherent films
when coextruded and which are otherwise suitable,
particularly in respect of heat resistance and hardness, may
be employed, such as polypropylene, acrylonitrile butadiene
styrene, nylon, polystyrene, and other appropriate
extrudable thermoplastics.
The core or base layer of the coextrudate is
loaded with filler material to provide a continuous phase of
the film-forming material itself and a discontinuous phase
of the filler material. Thus, in the example of FIG. 1 the
core or base layer 12 is loaded with mica filler 22 in
naturally occurring platelet form. Mica is presently
preferred as filler for its heat resistance and for its
flatness which enhances its contribution to film stiffness,
but other fillers having a high aspect ratio (ratio between
greatest and least dimensions) may be used, such as
wollastonite, glass fibers, talc, graphite platelets,
graphite fibers, boron fibers, sapphire fibers, steel
fibers, or polymeric or polyester fibers, e.g., DuPont's
Kevlar*. The filler comprises between about 5% and 40% by
weight of the core layer and is mixed into the charge of
film-forming resin which is fed to the extrusion orifice
associated with formation of the core layer 12.
*trade-mark
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1339~3~
The naturally occurring platelet form of the mica
22 is seen edge-on in the diagrammatic illustration of FIG.
1, the individual platelets having significant width as well
as length. These platelets orient themselves in the machine
direction when passing through the extrusion die so as to be
oriented lengthwise of the resulting extrudate as indicated
in the drawing. These platelets contribute significantly to
enhancing the heat resistance or dimensional stability and
the stiffness of the extrudate film. The mica as used in
commercial manufacture is a mined product containing
"impurities" of inorganic ore particles such as quartz or
feldspar which tend to be prune-shaped or lumpy rather than
being acircular (non-rounded) or plate-like as is the mica.
These lumpy particles 22a may include some particles whose
bulk approaches or exceeds a majority of the thickness of
the layer 12 in which they are contained. The particles
22a, and particularly the larger of such particles, act
through the skin 16 to roughen or contribute to the
roughness of the outer face thereof. They may also
similarly contribute to the roughening of the outer face of
the layer 14, i.e., the back face of the liner. Such filler
is mixed into the charge of film-forming resin which is fed
to the extrusion orifice associated with formation of the
skin layer 14.
In most applications, the extrudate is
hot-stretched, a stretch ratio of about 5:1 being employed,
or a somewhat lower or higher ratio may be employed, say,
from about 4:1 to about 9:1. Thus, using a 5:1 ratio,
o~erall extrudate thickness may be reduced from say 16 mils
at the extruding nozzle to say 3.2 mils after stretching.
Typical final thicknesses of the skin layer 14 and 16 may be
from about one-tenth of a mil to several tenths of a mil, or
e~en a mil or more, with the core 12 making up the remainder
of the thickness. Stretching causes voids 29 to form
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133~83~
adjacent to or around the lumpy particles 22a and, to some
extent, at the mica platelets 22. The particles act as
"seed" for the discontinuous voids. The voids form in a
one-to-one relationship with those of the particles that do
act as seeds, so that each void has its own associated
particle. Occasionally the size and position of either type
of particles as compared to film thickness may be such that
void formation incident to stretching also causes rupture of
a skin layer so that the fill particle is exposed at one
face of the construction, as is shown in connection with a
specific lumpy particle 22b in FIG. 1. This may occur at
either face. It is to be noted, however, that the voids 29
are isolated from each other. The roughening effect of the
filler may occur either indirectly through the bumpiness
caused by the filler distorting the extruded film in the
thickness direction or directly by breakthrough of filler
particles to the surface as in the case of particle 22b.
The thicknesses mentioned above are not intended
to be limiting, and it is contemplated that in general
overall thickness of the stretched extrudate may vary from
half a mil or less to 10 or more mils. To date,
constructions of from 1.5 to 6.5 mils have been made.
When the extrudate is to be hot-stretched, and in
accordance with accepted hot-stretching techniques, after
extrusion and initial chilling the extrudate is reheated to
say 275~F and stretched at the nip between two rolls, the
second of which turns at a defined multiple of the turning
speed of the first, to thereby define the desired stretch
ratio. The stretched extrudate is then further heated to
say 300 F to thereby "heat set" the stretched films.
The use of mica platelets or other
high-aspect-ratio fillers in the coextrudate of the present
invention is mentioned above. So far as we are aware, it is
new to use mica platelets or other high-aspect-ratio filler
sg/rn
1~983~
particles to remove unwanted limpness from an extruded
flexible thermoplastic film, even a monoextrusion. Also, it
is believed to be novel, even as to a non-extrusion, to
provide a multiphase film wherein high-aspect-ratio
particles, as distinguished from circularparticles such as
calcium carbonate, act as "seeds" for discontinuous void
formation.
Rather than as a byproduct of the mining of
acircular or high-aspect-ratio filler such as mica~ acircular
filler may be deliberately included in the extrudate in the
presence or absence of an acircular filler. For example,
relatively fine circular particles may be adequate to provide
desired roughening for very thin extrudates, so that, for
example, relatively fine calcium carbonate particles or
silica or other relatively fine circular particles may be
used in thin coextrudates or monoextrusions, with the
dimensional relationships of particles and film being chosen
so that roughening is accomplished.
It will be understood by those in the industry
that at least some materials used as fillers may also be
used in small amounts as additives, such as a coloring
agent, an antistatic, an antioxidant, a whitening or
coloring means, or for other similar purposes. However,
such other uses do not generally affect the mechanical
behavior or nature of the formed film, and do not represent
filling of the film as contemplated by the invention.
Release means is provided on or at the second face
or inner side 20 and may comprise a release coating 26 of
silicone or other material providing release characteristics
(e.g., fluorocarbon) on the skin layer 16, or a release
surface presented due to the presence of release components
in the skin 16 itself. When a silicone release coat 26 is
used, the skin layer 16, being of polypropylene or other
materïal having the good silicone holdout properties of
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~339~3~
plastics, helps assure good release action and avoidance of
blocking by substantially acting as a stop against
absorption of the release layer into the core-layer and
thereby maintaining the uniformity of the release coating 26
after it is applied.
FIG. 2 illustrates a liner stock construction 110
which is similar in many ways to the construction 10 of FIG.
1, and in which similar components are numbered as in FIG. 1
but with the addition of 100 to each reference number. In
the construction 110 of FIG. 2, however, there is no
separate skin layer on the "back" side or first face 118.
Instead, the first face 118 comprises the outer or exposed
side of the core layer 112.
In the constructions shown in FIGS. 1 and 2, the
first and second faces are roughened by the mechanical
effect of the filler material, predominantly the lumpy
particles 22a or 122a. Each face should have a roughness of
at least about 10 Sheffield units.
As to the minimum roughness of the back face (such
as face 18 or face 118) which contacts the steel guide rolls
on a manufacturing line, it has been found that the
coefficient of friction of the film, as measured against
glass, increases with decreasing roughness, so that one
would expect the smoothest film, having the highest
coefficient of friction, to track the best. Surprisingly
however, when roughness of the back face is reduced to less
than about the mentioned minimum Sheffield value, tracking
of the film deteriorates with decreasing roughness, even
though the coefficient of friction is being increased.
As to the minimum roughness of the other face
(such as face 20 or 120), it has been found that at least
the mentioned minimum Sheffield value is necessary to avoid
problems of air entrapment during label application, with
higher levels being required in some instances, as will be
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more fully discussed below.
Both the continuous phase of the core layer 12 (or
112) and the discontinuous phase of filler material 22 (or
122) are dimensionally stable under conditions of moisture
absorption or desorption such as may occur during long
periods of warehousing in humid or dry climates. The same
is true of the skin layer 14 of FIG. 1. The skins 16 and
116 are similarly stable. The dimensional stability of each
liner construction is therefore independent of humidity
conditions and the construction therefore remains flat (not
curled or distorted) under different humidity conditions as
encountered at different geographic locations or at the same
location at different times. The core layers, as well as
the skins of the constructions 10 and 110, are free of all
but isolated voids and are substantially free of connected
or continuous-phase voids so that the web also remains flat
and undistorted under varying web temperature conditions as
encountered in hot air drying of inks or coatings for
facestocks with which the liner is used.
One accepted test of flatness uses a test sheet of
the stock being tested which is 36 inches long and 24 inches
wide. The sheet is considered flat if it exhibits a lift of
no more than 1/4 inch at any corner, edge or interior area
portion under the humidity condition or conditions
encountered. Such a stock may test "flat" at say 50%
relative humidity, a humidity level commonly used at present
for quality testing at the factory, but may fail the same
test under greater or lesser humidities, particularly where
an extreme change in humidity is encountered in the field.
In contrast, test sheets of the constructions of the present
invention, such as constructions 10 and 110, exhibit less
than 1/8 inch lift, and in fact little or no discernible
lift, under any humidity condition that can be expected to
be encountered, say from 5% to 100% relative humidity.
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11
In the manufacture of the liner of FIG. 1, charges
A, B, and C, corresponding respectively to layers 16, 12,
and 14, may be prepared for coextrusion through a
coextrusion die 30, as schematically illustrated in FIG. 3A.
By preselection, charge A contains no filler, charge B
contains filler within the ranges specified earlier, and
charge C contains no filler or a lower degree of filler.
Upon coextrusion through the die 30, the charges form a
multilayer extrudate to which the silicone release coating
26 (FIG. 1) may be applied at station R to provide the
multilayer web construction 10. Or, the charge for layer 16
may include release components to inherently generate a
release surface at the outer face of the layer. If the
release coating 26 is applied, it is dried or cured
following application by any suitable means (not shown).
Prior to application of the release coating at station R,
the formed films may be hot-stretched in a known manner to
provide machine direction orientation of the liner 10. This
is generally done for "roll liner," but generally not for
"sheet liner," which terms are defined below.
In accordance with well-known practice in the
industry, the release face of a release liner may be coated
with a layer of pressure-sensitive adhesive for subsequent
transfer of the adhesive to the facestock with which the
liner is employed. When the facestock is combined with the
liner, the adhesive is joined to the facestock. Later, the
liner is removed to expose the adhesive, which now remains
permanently joined to the facestock.
Thus, as indicated in FIG. 3A, adhesive may be
applied at station S following drying or cure of the release
coat previously applied at station R. This may be a tandem
coating operation, or the adhesive coating may be on a
separate coating line. Or, the adhesive may be applied at
some later time prior to the combining of the release liner
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983~
12
with facestock. The combining of the liner with a
facestock 32 is diagrammatically illustrated in FIG. 3B.
FIG. 3C diagrammatically illustrates the die-cutting of the
facestock 32, at a station T, into a series of
pressure-sensitive labels 34 carried by the release liner
10. As is well known, this step is usually performed by
rotary cutting dies and involves the stripping of the matrix
(not shown) of waste or trim surrounding the formed labels.
FIG. 3D illustrates the application of the labels 34 to
passing workpieces 36 by use of a peelback edge 38 to
dispense the labels 34 by progressively removing the liner
from them in a well-known manner to thereby expose the
adhesive side 39 of the labels and project the labels into
contact with passing workpieces.
FIG. 4 diagrammatically illustrates a film of
conventional or prior art facestock 32 with
pressure-sensitive adhesive 40 permanently combined
therewith, such facestock being employed in the methods or
procedures of the invention at the stage illustrated at the
right end of FIG. 3B or the left end of FIG. 3C. At this
stage, the adhesive 40 (not shown in FIGS. 3A to 3D) may be
releasably carried on the liner 10 of the invention (on or
with which it may have been previously coated or combined,
as by the previously mentioned coating step at station S).
Alternatively, the adhesive 40 may have been directly coated
on or combined with the facestock 32 prior to the combining
of the facestock with the liner 10. The liner 10 is not
shown in FIG. 4; if it were, this figure would illustrate
one aspect of the present invention, namely, the combining
of a conventional type of facestock with a coextruded liner
of the type taught herein.
Where the adhesive contacts the inner face of the
liner 10, either at station S or upon the combining of the
facestock with the liner 10 if the adhesive is originally
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13
coated on or combined with the facestock, the roughness of
face 20 of the liner 10 is imparted to the adhesive. When
the adhesive is later exposed, as at face 39 in the step
illustrated in FIG. 3D, the exposed adhesive face exhibits
the roughness imparted by face 20 of the liner. This
roughness performs an important function in eliminating or
minimizing air entrapment during label application and the
resultant forming of blisters or high spots on the applied
label.
As indicated above, a roughness of at least about
Sheffield units at the adhesive-contacting face is
required for avoiding or minimizing air entrapment during
label application. However greater roughness generally is
better from the standpoint of avoiding air entrapment, and
roughnesses of 150 or more may be necessary in some
applications, depending on such factors as bottle surface,
bottle shape, amount, type and temperature of adhesive, and
line speed.
Meanwhile, the reverse or back face 18 of the
liner 10, also roughened at least to the specified minimum
degree, tracks smoothly and securely and without slippage on
the steel idler rolls and drive rolls (not shown) used to
guide or drive the liner 10 in any of the stages of FIGS. 3A
to 3D.
It will be understood that the operations shown in
FIGS. 3A to 3D will often be done at different locations by
different manufacturers, or they may be combined. For
example, the steps of FIG. 3A may be performed by a liner
and adhesives manufacturer, the steps of FIGS. 3B and 3C may
be performed by a label manufacturer on one continuous pass,
rather than being interrupted by a wind-unwind sequence as
illustrated, and the steps of FIG. 3D may be performed by a
packager of manufactured products.
sg/rn
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14
Facestock which is formed into labels is usually
wound and unwound in roll form and is therefore one form of
what is known as "roll stock" or "roll facestock," and the
accompanying liner is called "roll liner." Facestock in
roll form may also be utilized as continuous tape. The
foregoing relates to roll stock and roll liner. In many
respects, the invention also applies, however, to "sheet
liner" used with "sheet stock" which might be formed as
indicated in FIGS. 3A and 3B but would then be cut into
sheets and decorated (by screen printing, for example) for
use as decals, bumper stickers, thermal die-cut signs, and
the like. Materials and procedures used for sheet stock and
sheet liner may be the same or may differ to some degree
from those used for roll stock and roll liner, but the
principles of the construction and manufacture of the liner
can be similar whether it be liner for roll stock or sheet
stock.
The release liner 110 of FIG. 2 may be roll liner
or sheet liner. This liner 110, may be extruded in a manner
similar to that indicated in FIG. 3A, but with only two
charges corresponding to the layers 116 and 112. The charge
corresponding to layer 116 has no filler and the charge
corresponding to layer 112 includes the filler 122, 122a
mixed therein.
The thickness of liner 10 or 110 may be say 3.2
mils with the skin layers 14 and 16 or 116 each being one or
several tenths of a mil thick and the core or base 12 or 112
making up the remainder.
Available mined mica supplies with their
"impurities" may have the following typical particle size
distribution:
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~33983~
Table 1
Particle Size Distribution of Mica
(Percent By Weight of Sample On Or Through Screen)
Screen Size Mica No. 1 Mica No. 2 Mica No. 3
+200 0.4 0.0 5-0
+325 15.8 38.4 23.4
-325 83.4 61.6 71.6
Note: + = retained on screen
- = through screen
Mica No. 1 = acircular
Mica No. 2 = circular
Mica No. 3 = same as 2 with different grading
Since the mesh opening of a 200 mesh screen is 2.9
mils and that of a 325 mesh screen is 1.7 mils, it can be
seen that the larger particles in these samples have
dimensions exceeding the majority of core or base thickness
or even the core thickness itself. (Mica number 3 in the
above table is generally unsatisfactory due to 5% content of
particles retained on a screen with 2.9 mil mesh openings).
Thus it will be understood that considerable surface
roughness may be "thrust on" a manufacturer utilizing these
fillers to a point where the question is not one of
achieving desired minimums but of minimizing roughness. For
example, a back face roughness exceeding a relatively low
Sheffield value will cause a "dit" (dimple plus pit) problem
when the liner is combined with a very smooth soft facestock
such as flexible polyvinyl chloride and then wound for
storage or transport, since the facestock directly contacts
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16
the rough back face in the wound condition. Other only
somewhat less dit-prone facestocks include semirigid
polyvinyl chloride, polyethylene, and polypropylene, and
these may tolerate Sheffield roughnesses of up to about 150.
Facestocks of polystyrene, polycarbonate and thermoplastic
polyester are more capable of resisting the formation of
dits up to Sheffield roughnesses of about 300 or more.
Facestocks intended to have rough surfaces that wholly or
partially mask any dits that are formed will of course
tolerate considerable roughness insofar as the formation of
dits is concerned.
In accordance with generally established
post-extrusion finishing practice, the coextrudate may pass
from the extrusion nozzle through a nip formed by chilled
finishing rolls, often a polished steel roll and a silicone
rubber roll. The extrudate face on the steel roll side
receives a gloss finish and the opposite face receives a
matte finish. Generally the face with the matte finish is
the guide roll contacting face, such first face 18 or 118,
while the opposite face, such as second face 20 or 120, is
the face with the gloss finish. However, both sides may be
given a matte or a gloss finish if desired.
In some circumstances, roughness at a face,
particularly the first face, may be provided simply by the
matte finishing or embossing effect of a post-extrusion roll
of the coextruder. In other words filler may not be
necessary to the required roughening effect at a face, and
the situation becomes one of avoiding over-roughening due to
use of filler.
It will therefore be understood that, looked at
from the manufacturing standpoint of minimizing roughening
when dit-prone facestock is to be employed or when for other
reasons avoidance of over-roughening in a given application
is a concern, the invention involves the concept of
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17 ~ ~ ~3~36
attenuating the roughening effect of the filler by
coextruding at least a second unfilled or relatively
unfilled charge along with the filled charge to form the
resulting coextrudate, with the second charge forming a skin
that tends to smooth the bumps caused by the filler.
Generally, the thicker the formed skin, the greater the
smoothing action. The smoothing may result from the greater
"evening out" effect of the thicker skin, and/or from the
accomplishment of full coverage without skin rupture over
filler particles which would be otherwise exposed, such as
particle 22b in FIG. 1.
Turning now to facestock, prototypical examples of
film facestocks illustrating the invention are seen in FIGS.
and 6. In FIG. 5, a multilayer web construction,
generally indicated by the reference numeral 50, comprises a
coextrudate including a core layer 52, a skin layer 54 on
the face side of the coextrudate, and a skin layer 56 on the
inner side of the coextrudate opposite the face side.
Combined on the inner side of the coextrudate is a
pressure-sensitive adhesive layer 58. In FIG. 6, a
multilayer web construction, generally indicated by the
numeral 50a, comprises layers 52a, 54a, 56a, and 58a
generally corresponding to the layers 52, 54, 56, and 58 in
FIG. 5. However, in FIG. 6, tie layers 53 join the core
layer 52a to the skin layers 54a and 56a.
The coextrudates of FIGS. 5 and 6 are similar to
the previously described liner stock in that they comprise
polymeric film materials, are formed by simultaneous
extrusion from a suitable known type of coextrusion die, and
are adhered to each other in a permanently combined state to
provide a unitary coextrudate. The FIG. 5 construction is
used when the materials of the core and skins are such that
these layers firmly adhere or bond to each other when
coextruded as adjacent film layers. The FIG. 6
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13 c~983~
18
construction, with the tie layers 53, is used when the core
and skin materials do not sufficiently adhere or bond to
each other when they are extruded together. Generally, the
construction of FIG. 5 is presently used for roll film
facestock and that of FIG. 6 for sheet film facestocks
because, while polyethylene is presently preferred as the
core material for both applications, roll film facestocks
and sheet film facestocks generally use different skin
materials, and the presently preferred material for the skin
of the roll film facestock (ethylene vinyl acetate) is
compatible with polyethylene in respect of inherent adhesion
or bonding, while the presently preferred material for the
skin of the sheet film facestock (polyvinyl chloride) is
not.
The materials of the layers of constructions 50
and 50a are selected according to the cost/benefit
characteristics of candidate materials considering the
functional or operational requirements of the layer in
question. An important concept of the invention is the
application of this principle to the manufacture of
facestock by forming the facestock as a coextrudate of
materials so selected.
Thus, the facestock at its outside surface may
require high weatherability and printability and good
uniformity and control of surface texture, whether gloss or
matte, whereas these qualities either are not necessary or
are required in far lesser degree in the core of the
facestocks. The latter, however, must be such as to give
the facestock opacity and the desired degree of stiffness,
as well as sufficient body and strength, and represents
generally the great bulk of the total material used in the
construction. The stiffness of this core material should be
between about 10 and 100 Gurley. The inner surface of the
film coextrudate must give good anchorage for the adhesive.
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19
The presently preferred material for the core
layers 54 or 54a in many facestock applications is
polyethylene of low, medium, or high density of between
about .915 and .965 specific gravity. This is a relatively
low cost, extrudable film-forming material whose stiffness
(ranging through decreasing degrees of flexibility to
semirigid) may be determined by the density selected, and
whose body and strength are sufficient for most uses.
Polyethylene of lower densities, down to a specific gravity
of .890, may be employed for greater flexibility.
Another preferred material for the core layers 54
or 54a is polypropylene (or a propylene copolymer) having a
flex modulus range of between about 130,000 and 250,000 psi
at 73~F, depending on the stiffness desired.
Polyethylene vinyl acetate is generally the
presently preferred material for both skin layers 54 and 56
in roll film applications, while polyvinyl chloride is
generally the presently preferred material for both skin
layers 54a and 56a in sheet film applications. A suitable
resin for tie layer 53 in this instance is "CXA", marketed
by DuPont. Another material for forming tie layers is
"Plexar*" marketed by Chemplex Co. Other specific materials
are also available for performing the tying function in
coextrusion operations. The outer surface of the skin layer
54 or 54a is corona-treated in a known manner to increase
printability of the skin.
The preferred identity of the outer and inner skin
layer material at present is partly a choice of convenience
in reduction to practice, and it is contemplated that these
materials often will not be identical in actual manufacture.
For example, polyethylene vinyl acetate might be the
material of choice for the outer skin, but polyethylene
*trade-mark
sg/rn
1339~3~
acrylic acid might be used on the inner skin for better
anchorage to, say, an acrylic adhesive of choice.
Other materials for the skin layers include
meltable film-forming substances used alone or in
combination, such as polyethylene methyl polyacrylic acid,
polyethylene ethyl acrylate, polyethylene methyl acrylate,
acrylonitrile butadiene styrene polymer, nylon,
polybutylene, polystyrene, polyurethane, polysulfone,
polyvinylidene chloride, polypropylene, polycarbonate,
polymethyl pentene, styrene maleic anhydride polymer,
styrene acrylonitrile polymer, ionomers based on sodium or
zinc salts of ethylene/methacrylic acid, polymethyl
methacrylates, cellulosics, fluoroplastics,
polyacrylonitriles, and thermoplastic polyesters.
While the foregoing examples of facestocks have
employed skin layers on each side of the core, there are
instances where a skin layer is employed only on the outer
side of the construction, such as the construction 60 shown
in FIG. 7, which employs the single skin layer 66 on the
outer side of a core layer 62. In this instance, the
pressure-sensitive adhesive layer 68 is directly adjacent
the core layer. For example, such a construction could be
used for the manufacture of high durability labels.
Material presently preferred for the core layer in such
instance is polyvinyl chloride or acrylonitrile butadiene
styrene, and for the skin layer, polyvinylidene fluoride.
It will be understood from the foregoing that
multilayer film facestocks have been provided having a
relatively thick core layer of polymeric film material which
contributes the majority of the stock's dimensional
stability and stiffness, having a cojoined, relatively thin,
ink-printable skin layer at least at the face side of the
construction, and having a pressure-sensitive adhesive layer
combined at the sides of the construction opposite the face
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21
side. From a method standpoint, this is accomplished by
coextruding a plurality of at least two charges of
film-forming resin to form a coextrudate having a relatively
thick core layer and at least one relatively thin skin layer
after preselecting the charge for the core layer, as by
selection of density or flex modulus, to provide the degree
of stiffness suitable for the label or sign application, and
after preselecting the charge for the skin layer to provide
a skin adapted to the intended decorating process, and
combining the coextrudate with a pressure-sensitive adhesive
layer.
Thus, in the manufacture of the facestock 50 seen
in FIG. 5, charges D, E, and F, corresponding respectively
to layers 52, 54, and 56, may be prepared for coextrusion
through a coextrusion die 70, as schematically illustrated
in FIG. 8. Charge E for the core layer is preselected to
provide the suitable degree of stiffness, charge D is
preselected to allow for good printability (usually
following corona treatment of the formed film) and for
weatherability if indicated, and charge F is preselected for
good adhesive anchorage. As previously indicated, often
charges D and F for the skin layers may be the same, and in
some applications, the skin layer on the inner or adhesive
side, corresponding to charge F, is eliminated. The
coextrudate 54, 52, 56 forming the facestock may be
hot-stretched.
The coextrudate may be directly coated with the
adhesive 58, or the adhesive 58 may be transferred from a
liner with which the facestock is combined. In particular,
the coextrudates of cojoined facestock layers 54, 52, 56 may
be substituted for the facestock 32 of FIGS. 3B to 3D, and
the adhesive 58 may be the adhesive applied at the coating
station S in FIG. 3A. The result is an all plastic
facestock/liner combination in which both the facestock and
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liner are multilayered.
Instead of being coated or combined on the formed
coextrudate as just described, the adhesive 58 may be
coextruded along with the film-forming layers, 54, 52, 56.
The invention also contemplates simultaneously extruding
both liner and facestock as by simultaneously extruding all
the charges A through F, together with a charge of adhesive
58, which would for example be extruded through an
additional orifice adjacent to the orifice for charge F.
This would require provision of release means for the liner
prior to contact of the liner by the adhesive.
The facestock construction 50a is manufactured in
a manner similar to the manufacture of facestock 50. The
additional tie layers 53 are coextruded along with the
layers 52a, 54a, and 56a.
It should be evident that this disclosure is by
way of example and that various changes may be made by
adding, modifying or eliminating details without departing
from the fair scope of the teaching contained in this
disclosure. The invention is therefore not limited to
particular details of this disclosure except to the extent
that the following claims are necessarily so limited.
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