Note: Descriptions are shown in the official language in which they were submitted.
CA 02515420 2005-08-08
Security paper and method for producing the same
100011 This invention relates to a security paper for producing value
documents,
such as bank notes, passports, identification documents or the like, having a
flat sub-
strate provided at least partly with a dirt-repellent protective layer for
extending the
life time and fitness for circulation. The invention further relates to a
value document
having such a security paper and to a method for producing such a security
paper.
[00021 Value and security prints, such as bank notes, shares, bonds,
certificates and
coupons, checks, high-quality admission tickets, but also other papers at risk
of for-
gery, such as passports, or other identification documents, are often provided
with
elaborate printed images to increase their falsification security. Printing
processes that
are technically sophisticated and not accessible to everyone, such as intaglio
printing,
are used at least for some of the picture elements here.
[00031 Additionally, the security prints are frequently equipped with so-
called se-
curity elements which are difficult to imitate and permit even a layman to
check the
authenticity of the print or the document. Such security elements can be for
example
windowed security threads which are visible in certain areas on the surface of
the pa-
per of value, applied foils which have a transparent or metallized embossed
hologram,
blind embossings, so-called "latent images" produced by printing technology or
by
printing and embossing technology which render different information from
different
viewing angles, prints containing optically variable pigments and producing
different
color effects depending on the viewing angles, or prints comprising metallic
effect ink
which have metallic luster for example in a gold, silver or bronze tone.
[0004] An important component of value documents, such as bank notes, is their
flat substrate which preferably consists predominantly of cotton paper and
whose typi-
cal haptics is also influenced by the one-sided or two-sided calendering
during steel
engraving. The haptic character of a bank note is manifested mainly by its
feel and its
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flexural stiffness; it furthermore has a characteristic sound when being
deformed and
creased.
[0005] It is known to provide papers of value with a dirt-repellent protective
layer
to extend the life time and fitness for circulation. For example, the print EP
0 256 170
B2 proposes providing printed bank notes with a protective layer which
contains cel-
lulose ester or cellulose ether for the greater part and micronized wax for a
lesser part
and which is applied to the bank notes all over. The micronized wax is
dispersed by
kneading or mixing with oil, an ink binder or a mixture thereof. The sheets
freshly
printed with the protective layer can be stacked without difficulties without
any black
ink from one sheet staining the sheet therebelow.
[0006] The print WO 00/00697 discloses a security paper for bank notes having
a
dirt-repellent coating which despite the coating remains largely unchanged in
its typi-
cal properties, such as printability, sound and color, compared to an uncoated
paper. A
coating composition containing only a binder and no fillers is applied to the
bank note
paper, which has a large surface area or high surface roughness due to its
porosity.
The composition is applied in a layer thickness such that a smooth surface and
thus
little possibility for dirt deposit results, on the one hand, and the coating
is thin
enough not to impair the other stated properties of the paper, on the other
hand.
[0007] Known protective layers have in common that the wearing protection is
not
particularly high. Conventional protective layers comprising water-based
lacquers
usually fail to completely meet a demanding requirement profile. For example,
very
good dirt repellence and adhesion quality go against resistance to the
penetration of
liquid, and vice versa. Water-based lacquers therefore currently meet the high
re-
quirements for a protective layer in security printing and in particular bank-
note print-
ing only if a second component in the form of a crosslinking agent is added.
Since
such crosslinking agents are very reactive themselves, the operating staff
must be sen-
sitized to risks and suitable protective measures taken.
CA 02515420 2005-08-08
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[0008] The invention is therefore based on the problem of specifying a
security pa-
per and method for producing it which avoids the disadvantages of the prior
art. In
particular, the security paper should have a longer life time due to good dirt
repellence
and high resistance to the penetration of liquid.
[0009] This problem is solved by the security paper having the features of the
main
claim. A value document and a method for producing a security paper are the
subject
matter of the coordinated claims. Advantageous developments of the invention
are the
subject matter of the subclaims.
[0010] According to the invention, the protective layer comprises at least two
lac-
quer layers, a first lower lacquer layer being formed by a physically drying
lacquer
layer applied to the substrate which makes contact with the substrate
therebelow and
closes its pores, and a second upper lacquer layer being present which
protects the
substrate from physical and chemical influences and ensures good protection
against
the penetration of liquids and ink penetration.
[0011] The invention is based on the finding that the advantageous properties
of ra-
diation-curing lacquers can also be used for security papers if the
depressions, uneven
areas and pores of the substrates are previously closed by a physically drying
lacquer
layer. Radiation-curing lacquers and in particular UV-drying lacquers
(hereinafter
"UV lacquers") have the disadvantage that residual monomers and free
photoinitiators
as a rule remain as very reactive components in the depressions and pores of
the sub-
strate after radiation curing in dependence on the substrate quality, the
radiated power,
the initiator system and the monomer system (UV: ultraviolet).
[0012] This problem occurs increasingly when the UV lacquer penetrates into a
pa-
per fiber composite of e.g. a security paper. Complete polymerization of the
UV lac-
quer is then no longer possible. It has now been found that the positive
properties of
UV lacquering can be fully exploited for security papers if a combination
coating
comprising at least two lacquer layers is used in which a lower lacquer layer
makes
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contact with the substrate and closes its pores, and the upper lacquer layer
applied is a
layer protecting the substrate from physical and chemical influences.
[00131 The flat substrate of the security paper is formed in particular by an
un-
printed or printed cotton paper. The cotton paper of security and value
documents,
such as bank notes, has high porosity and surface roughness with microscopic
projec-
tions and cavities where residual monomers and photoinitiators of the
radiation-curing
lacquer layer would be deposited without the inventive use of a lower lacquer
layer.
[00141 The lower lacquer layer is advantageously formed by a water-based
disper-
sion lacquer layer. It is expediently applied to the substrate in a layer
thickness such
that it forms a smooth and contiguous layer on the substrate. The lower
lacquer layer
is advantageously elastic, so as to avoid cracks from forming in the lacquer
layer
through mechanical motions or swelling of the fiber, e.g. through moisture
absorption.
This has the advantage of longer retaining the haptics typical of bank notes,
that is, the
flexural stiffness and sound, under stress. This has a positive effect in
particular under
extreme climatic and mechanical stress. The elastic lacquers preferably
comprise
polyurethane systems which give the lacquer its elasticity. These are in
particular wa-
ter-based dispersions of aliphatic polyester polyurethanes or styrene-acrylic
polyure-
thanes. Obviously, the required amount of coating depends on the lacquer used,
the
substrate material used and its roughness and pore size and porosity factor,
among
other things.
[00151 The upper lacquer layer is preferably a radiation-curing and/or
physically
drying lacquer layer and particularly preferably comprises silicones and/or
waxes to
improve the dirt-repellent properties. In particular the radiation-curing
lacquer layer is
a UV-crosslinking lacquer layer (hereinafter "UV lacquer"). Its extremely high
physi-
cal and chemical resistance permits a corresponding qualitative increase and
adapta-
tion of the requirement profile. The higher physical resistance causes in
particular
high abrasion resistance and an extension of the life time of the security
paper. The
CA 02515420 2005-08-08
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higher chemical resistance furthermore makes the protective layer a long-term
stable,
effective barrier against water vapor and liquids, such as ink.
[0016] Additionally, the use of a UV lacquer offers a multitude of
possibilities for
selectively influencing the haptics of a security paper, in particular bank
note. Adjust-
ing the parameters of brittleness, luster and smoothness of the uppermost
protective
layer makes it possible to directly influence the haptic properties of the
coated paper
in diverse ways, in particular its flexural stiffness, smoothness and sound.
The compo-
sition of the UV lacquer layer is advantageously selected particularly with
respect to
brittleness and surface tension so as to obtain a predetermined haptics of the
security
paper, in particular a predetermined smoothness, flexural stiffness and/or
sound.
[0017] Apart from radically crosslinking UV lacquers, it is also possible to
use
cationically crosslinking lacquer systems for the radiation-curing lacquer
layer.
[0018] The physically drying layer as the upper lacquer layer alternatively
com-
prises water-based dispersions, preferably without a polyurethane component,
e.g.
based on styrene-acrylic.
[0019] It is likewise possible for the upper lacquer layer to contain a hybrid
lacquer
containing both physically drying components, such as a water-based dispersion
lac-
quer component, and a radiation-curing lacquer component. During drying of the
hy-
brid lacquer the water component is first removed physically, e.g. by heat,
and then
the radiation-curing lacquer component cured e.g. by means of UV radiation.
Suitable
hybrid lacquers are for example aqueous dispersions based on aliphatic
urethane acry-
lates and suitable monomers or reactive oligomers, in particular acrylates
with
photoinitiators.
[0020] In a preferred embodiment, the upper lacquer layer is applied directly
to the
lower lacquer layer. Alternatively, a further lacquer layer comprising water-
based dis-
persion lacquer can be provided between the upper and lower lacquer layers.
CA 02515420 2005-08-08
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[00211 The lacquer layers of the protective layer are expediently coordinated
with
each other in their adhesion properties so as to form a highly resistant bond.
In par-
ticular, when the upper lacquer layer is applied directly to the lower lacquer
layer, the
composition of the lower lacquer layer is selected so as to ensure optimal
adhesion of
the subsequently applied radiation-curing lacquer. In an advantageous
embodiment,
an optimization of the wettability of the lower lacquer layer is obtained by
reducing
the glass transition temperature of the lacquer system. This causes both
higher adhe-
sion and improved adhesion promotion.
[00221 According to further expedient embodiments, the upper lacquer layer or
the
lower lacquer layer is transparent and colorless. In particular if a printed
substrate is to
remain as readily visible as possible, both lacquer layers can also
advantageously be
formed to be transparent and colorless. The protective effect and the
adjustability of
the haptics of the security paper are still completely retained. However, it
is also pos-
sible to color at least one of the lacquer layers. This permits the value
document to be
advantageously provided with a slight tint without any need to stockpile
different sub-
strate materials.
[00231 According to another advantageous embodiment, at least the upper
lacquer
layer has antibacterial fungus proofing.
10024] It has further proved advantageous if the lower lacquer layer is
present on
the substrate in a coating weight of from I to 6 g/m2, preferably 2 to 4 g/m2.
This cor-
responds for the preferred range to an amount of approx. 5 to 10 g/m2 in the
undried,
wet state (e.g. aqueous dispersion lacquer with 40% solids content). In any
case the
layer thicknesses must suffice to close the irregular depressions and pores of
the flat
substrate. For the upper lacquer layer it suffices if it is present on the
substrate with a
slightly lower coating weight of from 0.5 to 3 g/m2, preferably 1 to 2 g/m2.
This corre-
sponds for the preferred range in the uncrosslinked state to an amount of
approx. 1 to
2 g/m2, since UV systems are so-called "100% systems" (100% solids content).
With
surfaces already smoothed and/or compressed by previous intaglio printing the
values
CA 02515420 2005-08-08
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tend to be in the lower range, while with raw paper or the backs of intaglio
printed
pages they tend to be in the upper range.
[00251 According to an expedient development, the substrate is printed with
char-
acters or patterns and the protective layer is applied to the printed
substrate. This also
protects the print. The protective layer can also contain gaps, for example in
the form
of characters or patterns, in which optically variable elements or other
security ele-
ments have been incorporated or will be incorporated at a later time.
[00261 According to a further preferred embodiment, the protective layer is
applied
to the substrate all over. It can likewise be expedient, as with a bank note
for example,
if the flat substrate of the security paper is provided with the dirt-
repellent protective
layer on its two main faces.
[00271 The invention also includes a value document, such as a bank note,
coupon,
certificate, passport, identification document or the like, which has a
security paper of
the described type.
[00281 For producing a security paper of the described type, a flat substrate
is sup-
plied in a step a), and a dirt-repellent protective layer applied to the
substrate in a step
b). The protective layer is applied by first applying a physically drying
lacquer layer
to the substrate as the lower layer of the protective layer to make contact
with the sub-
strate therebelow and close its pores in a step b1), and applying a lacquer
layer which
protects the substrate from physical and chemical influences as the upper
layer of the
protective layer in a step b2).
[00291 If a "wet-on-wet" application of the two lacquer layers is not
possible, the
lower lacquer layer is dried before the upper lacquer layer is applied. Drying
can be
effected simply during a sufficiently long waiting time, for example during
transport
of a sheet over a sufficiently long transport path. With regard to fast
lacquering it is
expedient in terms of production engineering and economy to accelerate
physical dry-
CA 02515420 2005-08-08
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ing by additional measures. For this purpose it is preferable to use dryers
which have a
hot-air blower and/or an infrared emitter.
[0030] The invention offers particularly great advantages if the flat
substrate sup-
plied is a printed or imprinted cotton paper.
[0031] According to a preferred embodiment, a printed image is printed on the
sub-
strate prior to application of the protective layer. Alternatively or
additionally, a
printed image can be printed on the lower lacquer layer after application of
the lower
lacquer layer to the substrate. The upper lacquer layer is then applied to the
lower lac-
quer layer and to the printed image which is typically not all-over. It is of
course also
conceivable for the upper lacquer layer to be also printed.
[0032] The lower, upper or both lacquer layers are advantageously applied by a
flexographic printing process. The lacquer layers are expediently thereby
applied in an
amount of coating of from 1 to 8 g/m2. In another advantageous embodiment, the
lower, upper or both lacquer layers are applied by a screen printing process.
In this
case the lacquer layers are expediently applied in an amount of coating of
from 5 to 15
g/m2. According to yet a further variant of the invention, it is provided that
the lower
and/or upper lacquer layer is applied by the offset printing process, by dry
offset or by
the indirect letterpress printing process.
[0033] According to a particularly preferred embodiment, the flat substrate
sup-
plied in step a) is a paper-of-value sheet comprising a multiplicity of single
copies for
which steps b), b1) and b2) are carried out at the same time. The lower and
upper lac-
quer layers are applied to the substrate especially advantageously in-line,
i.e. in one
run, in a sheet-fed lacquering machine.
[0034] The apparatus for carrying out the described method preferably
comprises a
first lacquering module for applying the lower, physically drying lacquer
layer to the
substrate, an intermediate dryer for drying the lower lacquer layer, a second
lacquer-
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ing module for applying the upper lacquer layer, and a final dryer for curing
and/or
drying the upper lacquer layer.
[00351 To ensure constant layer thicknesses, the first and/or second
lacquering
module is formed by a flexographic printing unit with a chambered doctor
blade,
anilox roller and plate cylinder. The anilox roller advantageously has small
cells
whose volume and/or density determines the lacquer application rate. The
chambered
doctor blade lies against the anilox roller, filling the cells and scraping
off surplus lac-
quer at the same time. The anilox roller transfers lacquer to the plate
cylinder which is
preferably formed by a rubber blanket. The rubber blanket finally transfers
lacquer to
the flat substrate, in particular a paper sheet or paper web.
[00361 Further, a lacquer conditioning device is preferably provided for
adjusting
the viscosity of the lacquer and the crosslinker concentration. For lacquers
with radia-
tion-curing components the lacquer conditioning device expediently has a
tempering
device for adjusting the viscosity and flow behavior of the lacquer. Since
there are
thus only two influencing variables for the lacquer application rate, namely
cell vol-
ume and viscosity of the lacquer, such a flexographic printing unit with a
chambered
doctor blade permits realization of a lacquering method by which a uniform,
homoge-
neous and cohesive lacquer film can be applied to the whole sheet reproducibly
and
over a very long time period.
[00371 The intermediate dryer is advantageously a controllable IR-hot air
combina-
tion dryer (IR: Infrared). It is likewise expedient if two dryer modules are
used in the
intermediate dryer so that sufficient drying is guaranteed even at high speed.
The final
dryer preferably has power-controlled UV drying modules which are coordinated
with
the wavelength required for curing the upper lacquer layer and with the layer
thick-
ness thereof.
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Example 1
[0038] By flexographic printing the lower elastic lacquer layer is applied to
a cot-
ton paper with a 20 cm3/m2 roller at an application rate of approx. 2.0 g/m2
dry in the
lacquering unit 1. The elastic lacquer is an aqueous dispersion based on
styrene-
acrylic polyurethane (designation: L51073 from the company Proll).
[0039] The upper, second lacquer layer is applied over the first lacquer layer
with a
9 cm3/m2 roller at an application rate of approx. 1.8 g/m2 in the lacquering
unit 2.
The second lacquer is a UV lacquer (designation: UV L50733 + UV L50734, mixing
ratio 3: 2, from the company Proll).
Example 2
[0040] The two-ply coating is produced as described under Example 1, with the
ex-
ception that a 20 cm3/m2 roller is used and the lacquer applied at an
application rate of
approx. 2.0 g/m2 dry in the lacquering unit 2. The second lacquer is a hybrid
lacquer
(designation: L50807 + L50806, mixing ratio 1 : 1, from the company Proll).
Example 3
[0041] By flexographic printing the lower elastic lacquer layer is applied to
a cot-
ton paper with a 13 cm3/m2 roller at an application rate of approx. 1.3 g/m2
dry in lac-
quering unit 1. The elastic lacquer is an aqueous dispersion based on styrene-
acrylic
polyurethane (designation L51073 from the company Proll). The upper second lac-
quer layer is applied over the first lacquer layer with a 13 cm3/m2 roller at
an applica-
tion rate of approx. 1.3 g/m2 in the lacquering unit 2. The second lacquer is
a sty-
rene-acrylic-based aqueous dispersion.
[0042] Further embodiments as well as advantages of the invention will be ex-
plained hereinafter with reference to the figures. For more clarity, the
figures do with-
out a representation that is true to scale and to proportion.
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[0043] Fig. 1 shows a detail of a cross section through a bank note with a two-
layered protective layer according to one embodiment of the invention,
[0044] Fig. 2 shows the layer structure of the bank note of Fig. 1 in a
schematic
representation,
[0045] Figs. 3 and 4 show the layer structure of further designs of bank notes
ac-
cording to embodiments of the invention, and
[0046] Fig. 5 shows a schematic representation of a sheet-fed lacquering plant
for
performing the invention.
[0047] Figures 1 and 2 show in cross section the structure of a bank note 10
with a
two-layered protective layer 14 according to one embodiment of the invention.
The
protective layer 14 applied to the paper fiber composite 12 of the cotton
paper in-
cludes a lower lacquer layer 16 comprising a water-based dispersion lacquer
and an
upper lacquer layer 18 comprising a UV-curing lacquer applied thereto.
[0048] The lower lacquer layer 16 makes the necessary contact with the paper
fiber
composite 12 and at the same time closes its capillaries. The lacquer layer 16
is ap-
plied in an amount of coating so as to form a smooth and cohesive surface
which en-
sures optimal adhesion of the subsequently applied UV lacquer.
[0049] The composition of the UV lacquer 18 is selected so as to obtain the
desired
haptic and dirt-repellent properties of the bank note. In particular, the
brittleness of the
UV lacquer is adjusted so as to yield desired haptics and sound of the bank
note. The
dirt-repellent properties of the bank note are determined substantially by the
selected
surface tension of the UV lacquer. The high physical and chemical resistance
of the
UV lacquer gives the bank note 10 high abrasion resistance and great
resistance to the
penetration of water vapor and liquids. The variability existing in the
selection of the
material parameters of the UV lacquer also makes it possible to realize new
properties
with respect to haptics and sound of the bank notes that were hitherto hardly
attainable
in security printing.
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[0050] In the embodiment of Figures 1 and 2 the water-based dispersion lacquer
14
is applied to the cotton paper 12 in a coating weight of 3 g/m2, the UV
lacquer 18 in a
coating weight of 1.5 g/m2. The water-based dispersion lacquer 14 has a
styrene-
acrylic polymer and the UV lacquer 18 an acrylate system in this embodiment.
[0051] While the representations of Figures 1 and 2 show the protective layer
14 on
an unprinted paper, the substrate 12 can obviously also be already printed.
This is
shown schematically in Fig. 3. Therein a printed image 20 comprising
characters or
patterns is printed on the cotton paper 12 of a bank note 10, and the
protective layer
14 is applied to the printed image 20 and the substrate 12. Alternatively or
addition-
ally, a printed image 22 can also be disposed between the lower lacquer layer
14 and
the upper UV lacquer layer 16, as shown in the representation of Fig. 4.
[0052] Fig. 5 shows a sheet-fed lacquering machine 30 for applying an
inventive
combination coating comprising two lacquer layers. The sheet-fed lacquering
machine
30 comprises a lacquer conditioning device (not shown), two lacquering units
32 and
36, an intermediate dryer 34 and a final dryer 38.
[0053] The lacquer conditioning device serves to adjust the viscosity of the
lacquer
and the crosslinker concentration. A tempering device adjusts the viscosity
and flow
behavior of the UV lacquer.
[0054] The first and second lacquering units 32, 36 are each formed by a
modern
flexographic printing unit with a chambered doctor blade, anilox roller and
plate cyl-
inder. The anilox roller has tiny cells whose volume determines the lacquer
applica-
tion rate. The chambered doctor blade lies against the anilox roller, filling
the cells
and scraping off surplus lacquer at the same time. The anilox roller transfers
lacquer
to the plate cylinder which is formed by a rubber blanket in this embodiment.
The
rubber blanket finally transfers the lacquer to the paper-of-value sheets to
be coated.
[0055] The protective layer can also be applied to a continuous web. This is
pre-
ferred in particular for unprinted paper webs.
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[0056] The IR-hot air combination dryer 34 used as the intermediate dryer has
two
dryer modules to guarantee sufficient drying even at high lacquering speed. In
the final
dryer 38 the UV lacquer layer is cured by irradiation with intensive UV light
and the
protective layer additionally dried with infrared radiation and hot air. The
power and
wavelength of the drying modules of the final dryer 38 is coordinated with the
required
wavelength of the UV lacquer and with the layer thickness on the paper-of-
value sheet.
[0057] An aspect of the present disclosure provides a security paper for
producing
value documents comprising a flat substrate provided at least partly with a
dirt-repellent
protective layer for extending the life time and fitness for circulation. The
protective
layer comprises at least two lacquer layers. A first lower one of the lacquer
layers is
formed by a physically drying liquid lacquer layer applied to the substrate
which makes
contact with the substrate therebelow and closes its pores. A second upper one
of the
lacquer layers protects the substrate from physical and chemical influences.
The first
lower lacquer layer is based on a water-based dispersion of aliphatic
polyester polyure-
thanes or styrene-acrylic polyurethanes. The second upper layer is formed as
any one of
the following layers: a radiation-curing UV-crossliked lacquer layer, a
physically drying
water-based dispersion lacquer layer based on styrene-acrylic without a
polyurethane
component, and a hybrid lacquer layer containing both physically drying
components
and a radiation-curing lacquer component, and based on aqueous dispersions on
the ba-
sis of aliphatic urethane acrylates and acrylates with photoinitiators.
[0058] Another aspect of the present disclosure provides a method for
producing a
security paper for a value document. The method includes supplying a flat
substrate
and applying a dirt-repellent protective layer to the substrate. The
protective layer is
applied by applying a physically drying liquid lacquer layer to the substrate
as a first
lower layer of the protective layer to make contact with the substrate
therebelow and
close its pores, and applying a second upper layer of the protective layer to
protect the
CA 02515420 2011-02-16
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substrate from physical and chemical influences. The first lower layer of the
protective
layer is based on a water-based dispersion of aliphatic polyester
polyurethanes or sty-
rene-acrylic polyurethanes. The second upper layer is formed as any one of the
follow-
ing layers: a radiation-curing UV-crossliked lacquer layer, a physically
drying water-
based dispersion lacquer layer based on styrene-acrylic without a polyurethane
compo-
nent, and a hybrid lacquer layer containing both physically drying components
and a
radiation-curing lacquer component, and based on aqueous dispersions on the
basis of
aliphatic urethane acrylates and acrylates with photoinitiators.
[0059] In one embodiment, the UV-crosslinked lacquer layer is based on an
acrylate
system, the water-based dispersion lacquer layer on a styrene-acrylic system,
and the
hybrid lacquer layer on a system comprising aliphatic urethane acrylates and
acrylates
with photoinitiators.
[0060] In one embodiment, the lower lacquer layer is applied to the substrate
in an
amount from 2.5 to 15 g/m2, preferably 5 to 10 g/m2 (wet weight).
[0061] In one embodiment, the lacquer layers applied by a flexographic
printing
process are applied in an amount of coating of altogether 3 to 12 g/m2.
