Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MULTILAYER STRUCTURE CONTATN1NG CAVITATED MATERIALS
FOR USE IN THE PRODUCTION OF SECURITY DOCUMENTS
Related Applications
s This application is a continuation in part of U. S. Application Serial Nos.
09/162,219, filed September 28, 1998 which is a continuation in part of U.S.
Application Serial Nos. 08/872,988, filed June 1 l, 1997; 09/064,993, filed
April
23, 1998; and 08/467,484, filed June 6, 1995, which is a divisional of U. S.
Application Serial No. 08/266,918, filed 3une 27, 1994, now U.S. Patent
5,698,333
to and 08/762,089, filed December 9, 1997, now U.S. Patent No. 5,716,695,
which
is a divisional of U.B. Application Serial No. 08/601, 886, filed February
15,1996,
now U.S. Patent No. 5,618,630, incorporated herein by reference.
Field of the Invention
15 The present invention relates to a multilayer film for use in the
production
of paper-like products such as bank notes, security documents, including
travelers
and bank checks, and to a method for their production. More particularly, the
present invention relates to a multilayer film having the characteristics of
the high
quality papers typically employed in the production of bank notes and security
2 o documents.
Background of the Invention
In the production of bank notes, security documents and the like, rag paper
has been employed for over 300 years. As is well known, rag paper has several
2 s properties which are highly desirable in such applications, including dead
foldability, tear resistance, printablilty and embossability.
These highly desired properties may be characterized as follows: deadfold
is the ability of a substrate to be creased or folded and to retain the fold
without
opening. Tear resistance is the ability of a substrate to resist both
initiated and
3 o uninitiated tears and punctures. Printability is the ability of the
substrate to adsorb
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and bond inks used during the lithographic printing process. Embossability is
the
ability of the substrate to deform under the pressures of the intaglio
printing
process to form a raised image on the resulting bank note or security
document,
with the intaglio ink remaining on the raised, deformed region resulting in a
high
degree of tactility or feel to the bank note or security document. As may be
appreciated, these properties combine to give bank notes and the like their
familiar
feel and functionality.
With the advent of color copiers and computer graphic scanners, the
counterfeiting of bank notes has markedly increased. While there are active
to programs underway by major currency paper producers to make their substrate
more secure through the use of watermarks, metallized threads and optical
variable
devices (OVD's), such as photochromics, holographics, and diffraction
gratings, at
this time, these efforts do not appear to hold much promise of thwarting
counterfeiters.
15 Security features can be incorporated into or onto the bank note, including
reverse printing of the note to protect the security devices and the print.
Australian Pat. No. 488,652, discloses an approach to the production of
security articles, particularly bank notes, and describes the serious problems
which
confront conventional bank notes with respect to forgery. The bank note
disclosed
2 o therein comprises a substrate of opaque thermoplastic sheet material
intimately
bonded to a web of woven or unwoven thermoplastic fibers, the substrate being
printed as desired and having bonded thereon one or more optically-variable
security devices. The fibrous web was employed to impart durability, crumple-
resistance and tear-strength to the note. Where a security device, such as a
Moire
2 s pattern, was employed which depended for its optically variable properties
upon
the transmission of light, it was necessary to punch out a hole in the
substrate,
insert the device and bond it in place with further layers of transparent
plastic sheet
material.
Although samples of bank notes formed according to the disclosure of
3o Australian Patent No. 488,652 were said to have performed most
satisfactorily
with respect to the durability and security of conventional notes, they were
found
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to be rather complex in construction and relatively expensive to produce.
Moreover, when transmission security devices were laminated between layers in
the substrate, an area of weakness and high stress was created which reduced
both
durability and security.
Other disclosures relating to anti-counterfeiting techniques include U. S.
Patent Nos. 4,095,217 and 4,281,208, which relate to the use of a liquid
crystal
device driven by a photovoltaic element, such as a solar cell or an amorphous
silicon material.
U.S. Patent No. 4,472,627 relates to currency or other valuable documents
1 o containing a liquid crystal/ photovoltaic device which produces a coded
display in
response to artificial or ambient light. The device can function both as an
anti-
counterfeiting deterrent and also as a means for permitting a user to easily
authenticate the validity of a document containing such a device.
U.S. Patent No. 4,536,016 discloses a security token, such as a bank note
15 or identity card, which comprised a sheet-like substrate made up from film
of
transparent biaxially oriented polymer coated with layers of opaque and heat
activated adhesive material. The opaque layer is applied in such a way as to
leave a
transparent area for inspection of a security device, for example, a
diffraction
grating, incorporated in the polymer film. The substrate could bear printed or
2 0 other identifying indicia and was protected with an intimately bonded
layer of
transparent polymeric material.
The substrate employed in U.S. Patent No. 4,536,016 was based on the use
of oriented polypropylene (OPP). After several commemorative bank note
printings, while meeting many of the requirements for a bank note substrate,
the
2s plastic bank notes were found to fail in three major areas. First, the OPP
substrate
did not dead fold, causing problems in that the film retains either a flat or
curved
form, jamming cash registers and automatic handling equipment. Second, the OPP
substrate has poor initiated tear resistance in the processing of currency,
which
quite frequently creates nicks on the edges of bills, resulting in
catastrophic tears.
3 o Finally, the OPP product did not exhibit the tactility of paper currency,
due to the
fact that OPP does not emboss well during the intaglio process.
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Oriented high density polyethylene films have been employed in the area of
plastic packaging. Such films, biaxially oriented to a degree of greater than
6.5
times in both the machine direction (MD) and the transverse direction (TD) are
described in British Patent 1,287,527. U. S. Patent 4,680,20? relates to
unbalanced
biaxially oriented films of linear low density polyethylene oriented up to six
times in
the machine direction, and up to three times in the transverse direction but
less than
in the machine direction.
U.S. Patent No. 5,618,630 relates to a three-ply mulitlayer film structure
for the production of banknotse.
1 o While the aforementioned films have been shown to offer certain
advantages over the prior art and generally meet the requirements for which
they
were designed, a need still exists for a film which provides the
characteristics of a
high quality, rag-type paper of the type typically employed in the production
of
bank notes and security products.
15 Therefore, it is an object of the present invention to provide a multilayer
film having the characteristics of a high quality rag paper.
It is another object of the present invention to provide such a multilayer
film which possesses the dead-fold characteristics of high quality papers
while
being easy to manufacture and reasonably durable.
2 o It is a fi~rther object of the present invention to provide a multilayer
film
which possesses a high degree of tear resistance to both initiated and
uninitiated
tears and punctures in at least one direction.
It is yet another object of the present invention to provide a multilayer film
which possesses the printability and embossability of a high quality paper.
2 s It is still a fizrther object of the present invention to provide a
multilayer
film which possesses a high degree of resistance to curling at higher
temperatures,
e.g., above 150°F.
Still another object of the invention is to provide a multilayer film which is
suitable for banknote production from which it is difficult to abrade inks
resulting
3 o in banknotes of long service life. Such films can achieve high levels of
durability
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without overlaying the inked surface with protective coatings such as lacquers
or
polyurethanes.
Other objects and the several advantages of the present invention will
become apparent to those skilled in the art upon a reading of the
specification and
s the claims appended thereto.
Summary of the Invention
The multilayer film of the present invention is an opaque three-ply
structure. In a preferred embodiment, at least one layer of oriented
polypropylene
(OPP) is surrounded by at least one layer of high density polyethylene (HDPE)
on
either side of the OPP layer.
The multilayer film of the present invention exhibits good performance in
repetitive fold tests. The multilayer film structure of the present invention
also has
high tensile strength in both the oriented and unoriented direction.
1s In accordance with the present invention, there is provided a curl-
resistant,
cross-sectionally symmetrical, laminated multilayer film substrate for use in
the
production of banknotes, security papers and the like comprising:
(a) an imbalanced biaxially oriented first layer having inner and
outer sides, comprising at least about 50 weight percent of a high density
2 o polyethylene having a density of at least about 0.94, said first layer
being oriented
in at least a first direction to a degree which is at least three times less
than the
degree of orientation present in a second direction substantially normal to
the first
direction;
(b) a balanced biaxially oriented second layer having inner and
2s outer sides, comprising a cavitating agent and at least about 90 wt.%
polypropylene, said second layer being oriented in at least a first direction,
at an
orientation ratio of at least 4: l, and oriented in a second direction
substantially
normal to said first direction, at an orientation ratio of at least 6: l;
(c) an imbalanced biaxially oriented third layer having inner and
30 outer sides, comprising at least about 50 weight percent of a high density
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polyethylene having a density of at least about 0.94, said third layer being
oriented
in at least a first direction to a degree which is at least three times less
than the
degree of orientation present in a second direction substantially normal to
the first
direction; and
(d) a laminating adhesive resin disposed between the inner sides of
{a) and (c), said second layer being laminated to said first and third layers
so that
the first direction of orientation of said third layer is substantially
aligned with the
first direction of orientation of said first layer and wherein (a) further
comprises a
coextruded propylene copolymer skin on its inner side.
to
Detailed Description of the Invention
In forming the multilayer film of the present invention at least one layer of
the substrate comprises oriented polypropylene and a cavitating agent. The use
of
oriented polypropylene in the core results in high tensile properties and high
15 performance in repetitive fold tests. The use of a cavitating agent in the
core offers
good opacity which minimizes the need for a heavy opacifying coating.
It is preferred to use oriented polypropylene {OPP) in the core. However,
other biaxially oriented polymers of a comparable tensile strength, such as
linear
low density polyethylene {LLDPE), nylons or polyester may also be used. It is
2 o preferred that the OPP core be free of any additives, for example, anti-
static agents
and silicone, which have an impact on adhesion to other layers.
The core layer of the present invention comprises a plurality of voids
formed by cavitation about a solid cavitation agent, such as polybutylene
terephthalate and/or calcium carbonate. Polybutylene terephthalate, e.g., 0.2
to 2
2 s microns in diameter, as described in U. S. Patent Nos. 5,288,548;
5,264,277; and
4,632,869, the contents of which are incorporated herein by reference, is a
suitable
cavitation agent. The spherical particles form microvoids on orientation,
resulting
in a white opaque product.
The use of a cavitating agent in the core layer offers good opacity, which
3 o reduces the need for a heavy opacifying coating and thus reduces the
weight of the
banknote. The reduced weight improves set off when freshly printed banknotes
are
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stacked in sheets at the end of the press. The notes at the bottom of the
stack will
be exposed to less pressure from the reduced stack weight.
In forming the multilayer film substrates for use in the production of the
bank notes and other security documents of the present invention, at least two
s layers of the substrate are to contain a major proportion of a high density
polyethylene (HDPE) having a density of at least about 0.94, preferably at
least
about 0.945. These film layers may be composed exclusively of a single HDPE
resin, a mixture of HDPE resins, or of HDPE containing a minor proportion of
another polymeric material, such as low density polyethylene (LDPE), linear
low
Zo density polyethylene (LLDPE), polypropylene, ethylene vinyl alcohol (EVOH)
copolymer, ethylene propylene (EP) copolymer, ethylene propylene butene-1
(EPB) copolymer, polyetser or nylon, although a single HDPE resin or a blend
of
HDPE resins is particularly preferred in the practice of the present
invention. Films
made with either a blend of HDPE resins or with microcrystalline wax have been
15 found to reduce the splittiness of the film which manifests itself as the
tendency of
the film to break in the TD direction.
When blends of HDPE polymers are employed, such blends can comprise
two or more polymers all of which preferably have densities of 0.94 or
greater.
Blends of HDPE polymers advantageously comprise a major proportion of HDPE
2 o having a melt index of 0.6 to 1.2 and one or more polymers having a
different melt
index.
Terblends may also be desirable. Suitable terblends generally comprise 50
to 98 weight percent, preferably 84 to 96 weight percent of HDPE having a
density
of 0.96 or higher and a melt index of greater than 0.5 to about 2.0; 1 to 25
weight
2 s percent, preferably 3 to 8 weight percent of HDPE having a density of 0.94
or
greater and a melt index of 0.1 to 0.5; and 1 to 25 weight percent, preferably
3 to 8
weight percent, of HDPE having a density of 0.96 or higher and a melt index of
greater than 2 to about 8.
Preferably, the second and third HDPE polymers which are minor components are
3 o present in about equal amounts.
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As is particularly preferred, the film substrate of the present invention
includes a) a first layer comprising at least about 50 weight percent of a
high
density polyethylene having a density of at least about 0.94, the first layer
oriented
in at least a first direction, e.g., machine direction (MD), to a degree which
is at
s least three times less than the degree of orientation present in a second
direction
substantially normal to the first direction, e.g., transverse direction (TD);
(b) a
second layer comprising at least about 90 wt.% polypropylene, the second layer
being oriented in at least a first direction e.g., machine direction (MD), at
an
orientaion ratio of at least 3:1, and oriented in a second direction
substantially
Zo normal to said first direction e.g., transverse direction (TD), at an
orientation ratio
of at least 6: l; and c) a third layer also comprising at least about 50
weight percent
of a high density polyethylene having a density of at least about 0.95, the
third
layer also oriented in at least a first direction, e.g., MD, to a degree which
is at
least three times less than the degree of orientation present in a second
direction,
15 e.g., TD, substantially normal to the first direction, the third layer
being laminated
to the film substrate so that the first direction of orientation of the third
layer is
substantially parallel to (or aligned with) the first direction of orientation
of the first
layer.
A method of producing HI~PE films with imbalanced biaxial orientation is
2 o disclosed in U. S. Patent No. 4,870,122, the contents of which are
incorporated by
reference in their entirety.
As may be appreciated, to achieve the object of improved tear resistance m
a multilayer film of the type describe herein, it has been discovered that
unbalanced
biaxially oriented HDPE films, laminated so that their primary directions of
2 s orientation are aligned substantially parallel to each other, can exhibit
increased
tear resistance where an orientable laminating resin, e.g., polyurethane, is
disposed
between the layers and subjected to orienting so that the primary direction of
orientation in the oriented resin is substantially normal to the primary
direction of
orientation of the first
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and third HDPE film layers. In one embodiment, such orientation in the primary
direction of orientation of the laminating adhesive resin is achieved during
the
laminating step itself, preferably in the machine direction (MD).
The degree of orientation of the HDPE film layers is an important
s aspect of this invention inasmuch as the proper degree of orientation
provides
desirable physical properties. Although higher density HDPE resin having a
density of 0.957 or greater can be made directly into thin films by cast
extrusion,
problems of curling, uniformity and flatness exist. Accordingly, thin HDPE
films
of about 0.8 to 1.5 mils having the best balance of properties are obtained
using
to unbalanced biaxially oriented films 'prepared from films having a cast
gauge of 4 to
15 mils which are reduced to the desired gauge by stretching, i.e.,
orientation.
The films are produced and oriented in a conventional manner. The
film is heated to its orientation temperature and first subjected to MD
orientation
between two sets of nip rolls, the second rotating at a greater speed than the
first in
is an amount equal to the desired draw ratio. Then the film is TD oriented by
heating
and subjecting it to transverse stretching in a tenter frame. Typically 1VID
orientation is conducted at 60 to 120°C and TD orientation at 110 to
145°C.
While it is preferred that the degree of orientation in a first film
direction be at least three times less than the degree of orientation present
in a
2 o direction substantially normal to the first direction, it is more
particularly preferred
that each HDPE film layer be oriented to an extent of about 1.1 to about 2.0
times
in the machine direction (MD) and about 6 to about 12 times in the transverse
direction (TD). It has been found that the HDPE film layers can be produced
with
excellent quality at caster speeds of up to about 110 feet per minute (fpm)
2 s corresponding to line speeds of 140 fpm at 1.25 times MD orientatuon. In
the
alternative, the degree of orientation in a first film direction can be at
least three
times greater than the degree of orientation present in a direction
substantially
normal to the first direction. Even unoriented blown HDPE may be used as the
first and third layers and still maintain some degree of deadfold in the
multilayer
3 o film substrate of the present invention.
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When employed, this degree of unbalanced orientation produces an
interesting effect in the HDPE components of the structure. The effect is a
visible
rippled and striated appearance, with the ripples being parallel to the
transverse
orientation direction. Under low magnification, in each square centimeter of
s HDPE film there will be seen from about 5 to about 30 discontinuous
undulating
ripples and striations generally parallel to the direction of orientation.
This effect
gives the film a slight translucent appearance, which tends to slightly blur
distant
objects viewed through the film. This effect indicates that the layers have
been
oriented in an imbalanced manner. The high density polyethylenes contemplated
1o for use in the practice of the present invention include those disclosed in
U.S.
Patent No. 4,870,122.
In the oriented polypropylene layer the orientation is adjusted to give
essentially balanced tensile properties in both directions.
To achieve the desired surface characteristics required of the paper-like
1 s products of the present invention, one or more skin layers can be applied,
in any
known manner, to the multilayer HI~PE substrate material, for example by
coating
or coextrusion before orientation or by coating the HDPE after one or both of
the
orientation operations. The skin layer can be any of the conventional
materials
used for this purpose in conjunction with polyolefin films, particularly
polyethylene
2 o films. For example, to achieve a press-ready surface, a polymeric resin
could be
blended with fillers, fibers, pigments or the like, as necessary.
Additionally, voided
films, such as those disclosed in U.S. Patent Nos. 4,377,616, 4,632,869,
4,758,462
and others, may be laminated to the multilayer HDPE substrate to impart the
opacifying properties of those structures to the films of the present
invention.
2 s The HDPE-containing layers (a) and (c) as disclosed above can further
comprise copolymer polypropylene skins, e.g., ethylene-propylene-butene-1
terpolymer, provided on at least one side thereof, preferably on both the
inner and
outer sides thereof. In one embodiment, the skins themselves can also comprise
a
component which is similar to components in the adhesive resin used to
laminate
3 0 layers (a), (b) and (c). For example, low density polyethylene (LDPE) can
make
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up from 10 to 20 wt% of the skins, with the balance being a copolymer
polypropylene.
A suitable adhesion-promoting primer which provides greater adhesion
between the laminated surfaces, e.g., polymers selected from the group
consisting
of polyethyleneimide, epoxy, polyurethane, and acrylic, can be provided
between
the copolymer polypropylene skins (on the inner sides of (a) and (c)) and the
laminating adhesive resin. Primer compositions are disclosed in U.S. Pat. Nos.
4,447,494 and 4,681,803, which are incorporated herein by reference.
It is also envisioned that the coated substrate can be embossed, dyed,
1 o printed, texturized or otherwise treated before or after lamination; this
being done
on the internal or external surfaces of the laminated layers, so as to
provide, for
example, visual andlor tactile identification of the nature of a banknote, its
significance or value. Printing processes include, but are not limited to,
lithography,
UV-screen, flexography, gravure and intaglio printing processes. Any flexo or
1 s gravure printable ink, either colors or machine readable ink, such as
lithographic,
IR, LTV, magnetic and intaglio inks, can be used. Printing on the core is also
a
security feature as well.
The opaque cavitated three-ply structure of the present invention can be
surface-printed without losing opacity. This is a surprising result for
intaglio
2 o processes which employ relatively high temperatures (greater than
70°C) and high
pressures {greater than 1000 psi). Unlaminated cavitated films would generally
have their cavitation crushed and lose opacity under similar conditions. The
cavitation acts as a cushion that diminishes the abrasion force seen by the
ink,
making the ink more abrasion resistant.
2 s The laminating techniques which can be employed to effect the present
invention are known in the art and include: adhesive-bonding or cementing,
e.g.,
with laminating adhesive resins, preferably with a transparent agent; solvent-
bonding, where a mist of solvent is sprayed over the surfaces to be bonded
together; thermal lamination by heat-bonding where thermoplastic sheets are
3o subject to a hot rolling or pressing operation; cast-lamination where one
layer is
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cast onto the second and the second forms a substrate; or, extrusion or draw-
lamination as in calendering operations known in the art.
The use of solventless or 100% solids adhesive resins, such as a two-part
polyurethane resin, WD4110, available from H.B. Fuller Co., is particularly
preferred. 100% solids laminating adhesives are an effective alternative to
solvent-
based adhesives. 100% solids laminating adhesives impart excellent clarity,
enhancement of printing, high bond strength and heat sealing resistance to the
multilayer film laminated structure of the present invention.
When discrete security devices are incorporated within the substrate, e.g.,
optically-variable devices (OVD's), they can be enclosed in pouches axed to
the
substrate. On the other hand, the optically-variable devices themselves may be
incorporated in one (or both) layers of the laminated substrate or between the
layers, it not being necessary to incorporate a physically discrete device
within a
clearly defined pouch formed between the layers.
1 s Any suitable security device can be employed in the present invention such
as one selected from the group consisting of optically variable devices
(OVDs),
magnetic devices, electronic devices, and rare earth element-containing
devices,
with OVDs particularly preferred.
As employed in the present specification, the term "optically-variable" is
2 o used to denote any device which can readily be made to change appearance
in a
reversible, predictable and reproducible manner. The appearance of such
devices
may be altered, for example, by the application of body-heat or manual
pressure,
the variation of the angle of viewing and, the lighting conditions under which
viewing takes place. The type of devices envisioned by the present invention
are:
2 s diffraction gratings, liquid crystals, moue patterns and similar patterns
produced by
cross-gratings with or without superimposed, refractive, lenticular and
transparent
grids, such as Fresnel lenses, spaced partially-reflective, and partially
transparent,
coatings yielding variable interference patterns or the like, bi-refringent or
polarizing layers, zone-plates and the like.
s o Generally, optically-active devices of this nature are readily recognized
by
unskilled persons and are yet extremely difficult to reproduce by photographic
and
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printing techniques. Moreover the production of any one such device in a
reproducible fashion and the incorporation of such a device in a plastic
laminate as
described by the present invention is likely to be beyond the resources of the
great
majority of would-be forgers. Where a flexible paper-like product such as a
bank-
note is sought, it is of course preferable that the optically-variable devices
should,
themselves, be sheet-like, flexible and thin; it is also preferable for such
devices to
be compatible with the plastic material employed for the laminae to facilitate
bonding and mitigate against reactive changes occurnng with time.
According to the present invention, one preferred form of optically variable
1 o device may be a reflecting diffraction grating consisting of a metallized
thermoplastic film embossed with a diffraction pattern. To prevent access to
the
embossed pattern for the purpose of illicit replication, it is preferable
according to
the present invention to employ a layer of thermoplastic material on each side
of
the metallized film which has similar solubility characteristics to that of
the metal
15 layer so that separation by preferential etching will be rendered extremely
difficult.
Another preferred device is a moire pattern formed by photographically
reproducing fine line or dot patterns on each side of a thin film. The
spacings of
the dots and lines can be readily made too fine to be reproduced by printing
techniques and yet the moire pattern can be displayed upon a much larger
scale.
2 o Unique diffraction and moire patterns will often be preferred for use in
bank notes
and techniques are available for producing those by computer and photo-
reduction
methods.
Security features can be incorporated into or onto the bank note, including
reverse printing of the note to protect the security devices and the print.
2 s It has been found that films similar to those of the present invention but
which are "cross-oriented," can be susceptible to curling at temperatures
above
150°F. Such cross-oriented films are similar to those of the present
invention
except that the second layer is laminated to the film substrate so that the
first
(primary) direction of orientation of the second layer is substantially normal
to the
3 o first (primary) direction of orientation of the first layer. It is
believed that such
curling can result from differences in shrinkage at high temperatures in the
machine
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direction and transverse direction of each layer. Additional curling
susceptibility
can result where coatings or skins of the layers have different coefficients
of
contraction from the HDPE component of the layers. Such imbalanced contraction
and its attendant curling can be avoided by counterbalancing the overall
shrinkage
s properties of one layer by laminating thereto a second layer identical to
the first
layer which is provided as a mirror image of the first layer. In other words,
one
half of the layered film structure is a mirror image of the other, with the
plane of
symmetry being along the horizontal midline of the layered film structure
cross-
section. This provides a cross-sectionally symmetrical layered film structure.
1 o Examples of such films include those of ABA, ABBA, ABCCBA, ABCDCBA, etc.
construction where each letter represents a film layer, skin, coating, or
adhesive
layer. Such a cross-sectionally symmetrical layered film structure is
necessarily a
"parallel-oriented" structure, i.e., the primary direction of orientation of
the first
layer is parallel to the primary direction of orientation of the second layer
in order
15 to meet the mirror image requirement. Such a construction provides a
symmetrical
structure wherein the opposing shrinkage forces counteract each other to a
significant degree. However, such a two layer parallel construction can be
susceptible to poor tear properties in one direction, e.g, TD where two TD
oriented films are employed.
2 o It has now been found that using a low density polyethylene (LDPE) and/or
a linear low density polyethylene (LLDPE) laminating resin oriented in the
primary
direction of orientation which is substantially perpendicular to the primary
direction
of orientation of the layers which it ties, evinces reduced susceptibility to
tearing.
The laminating resin can itself be subjected to orienting to some extent
during
2 s lamination, preferably after the resin has at least partially or
completely solidified,
and preferably in the machine direction. Such orienting can be from 1.5 to 10
times, preferably from 4 to 6 times, corresponding to a total drawdown of the
laminating adhesive resin of greater than 10%, preferably greater than 20%,
say,
75%.
3 o It has also been found that by using a 100% solids resin as the laminating
adhesive resin employed, orientation effects resulting from the lamination
CA 02369748 2001-11-30
WO 00/74936 PCT/US00/15321
procedure are improved, resulting in a multilayer film exhibiting curl
resistance and
improved chemical resistance.
One or both of the HDPE containing layers may be weakened in terms of
tear resistance in one direction to a point where they are significantly
weaker than
the laminating adhesive resin and the OPP containing core layer. When
attempting
to delaminate the structure for counterfeiting purposes, only small strips of
the
outer printed HDPE containing layers would be removed. The entire HDPE
containing layers would not be removable.
The HDPE containing layer can be tear weakened using process conditions
1 o during manufacture. Microperforating with laser technology and/or nicking
will
also cause the HDPE containing layers to weaken. The microperforations are
either diagonal or unidirectional, e.g. in the range of from about 50 to about
300
dots per inch (dpi).
The HDPE layers can also be tear weakened through the addition of
15 incompatible additives that would cause the layer to fibrilate or fracture
during
orientation. Suitable incompatible polymer additives which erect
crystallization
include polyester (PET), polybutylene terephthalate (PBT), polystyrene or a
mixture thereof. Generally, from about 1 to about 10 wt.%, preferably from
about
4 to about 8 wt.% of incompatible additive is added to the I~PE layer(s).
2 o Fibrillation results in regions of oriented HDPE surrounding long, planar
regions of
the incompatible polymer resulting in low tear regions.
The OPP core layer is not weakened and essentially provides the tensile
properties and tear resistance of the structure as a whole.
The invention is fi~rther illustrated by the following non-limiting examples.
Example 1
This example demonstrates the preparation of a multilayer film substrate
produced in accordance with the present invention which is suitable for
preparing
banknotes having good dead fold characteristics.
s o A multilayer oriented film substrate having a 1.15 mil final thickness is
prepared by coextruding HDPE with copolymer polypropylene skins on both sides
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16
to form a first layer (a). The HDPE employed is Oxychem M-621 l, available
from
Occidental Chemical Corp., Dallas, Tex., having a density of 0.96 and a melt
index
of 1Ø The copolymer polypropylene skins comprise 90 wt.% Chisso 7510, an
ethylene-propylene-butene-1 terpolymer, available from Chisso Corp. of Japan,
and
s 10 wt.% Nobil LKA-753, a low density polyethylene available from Mobil
Chemical Co., Norwalk, Conn. HDPE comprises about 90 wt.% of the resulting
film layer (a) while the skins comprise 10 wt.% (5 wt.% on each side). The
film (a)
is then oriented 1.4 times in the MD at about 1 I S°C and 6 to 12
times, e.g. 10
times in the TD at about 115-140°C in a tenter frame.
~ o Layer (a) is 100% solids adhesively laminated to an OPP layer containing
polybutylene teraphthalate ( 1.20 mil OPPalyteTM420 HTW) using a two-part
polyurethane resin, WD4110, available from H.B. Fuller Co.
The two-ply layer is again 100% solids adhesively laminated with Fuller
WD4110 to another HDPE layer described above.
is Although the present invention has been described with preferred
embodiments, it is to be understood that modifications and variations may be
utilized without departing from the spirit and scope of this invention, as
those
skilled in the art will readily understand. Such modifications and variations
are
considered to be within the purview and scope of the appended claims.
