Note: Descriptions are shown in the official language in which they were submitted.
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Ink-receiving layer having a recess
The invention relates to a security paper for producing
valuable documents, having an optically variable
security element and an ink-receiving layer having a
recess, a method for producing such a security paper
and an apparatus for implementing the production
method, and also a valuable document produced from the
security paper.
Valuable documents in the sense of the present
invention are, for example, banknotes, shares, loan
documents, certificates, coupons, checks, lottery
tickets, high-value entry tickets, passports, identity
papers, credit cards and other flat articles of value.
Such articles of value can, however, also be packaging
for possibly high-value products. The term security
paper in the sense of the present invention comprises
precursor stages of such valuable documents which, for
example, are not suitable to be circulated. The
security paper can be present in quasi endless form and
processed further at a later time.
Such valuable documents are often provided with
security elements as a safeguard, which permit the
authenticity of the valuable documents to be checked
and at the same time serve as a safeguard against
forbidden reproduction. In order to increase security
against forgery and to make imitation more difficult,
such valuable documents comprising security elements
are frequently covered with a further printed image.
The optically variable security element can already be
part of the security paper used for the production.
The security element used is, for example, optically
variable elements, whereby, in the sense of the present
invention, an optically variable security element is
understood as an optical element of which the visual
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impression given to an observer depends on the viewing
direction, which means the viewing angle of the
observer onto the optical element and possibly also the
direction of incidence of any illuminating light beam.
In order that the optically variable effect can be
detected, which means can be perceived visually by an
observer or detected by a machine, the optically
variable element must either be arranged directly on
the substrate surface of the security paper or it may
be covered only by layers having a sufficiently high
transparency. Therefore, it is generally a problem if,
for example in order to produce a valuable document,
the security paper is to be printed with one or more
further printed layers above such an optically variable
security element, since in this case the ability to
detect the optically variable effect can be impaired or
prevented completely. In order nevertheless to ensure
the ability to detect the optically variable effect, a
recess in the region of the optically variable security
element could be provided in the further printed layer.
However, this would firstly impair the stylistic
freedom of the printed image to be applied. Secondly,
this would lead to a clearly visible edge in the
printed image additionally to be applied, which would
interfere with the optical impression of the valuable
document and is therefore undesired.
This is additionally problematic if the exact position
of the optically variable security element on the
substrate surface can vary, as is the case for example
in security threads, which quite possibly have
intentional registration fluctuations. In such a case,
the recess would have to cover all the possible
positions of the optically variable security element in
order to ensure the ability to detect the optically
variable effect, which would lead to a very large
recess in the printed image to be applied.
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In order to increase the ink-receiving capacity for a
printing ink forming such a printed image, which is
advantageous in the case of film composite banknotes,
for example, or in order to configure the ink-receiving
capacity of the substrate surface of the security paper
in a uniform manner, which is advantageous in the case
of paper banknotes with an optically variable security
element applied or incorporated, for example, it is
further known to provide an ink-receiving or print-
receiving layer on the substrate surface of the
security paper. In order to obtain an ink-receiving
capacity to the desired extent, it is frequently
necessary to provide the ink-receiving layer with a
layer thickness which has a matting effect. This
matting effect is barely disruptive outside optically
variable security elements. Above optically variable
security elements, the optically variable effect
thereof can, however, be reduced or suppressed entirely
as a result.
It is therefore an object of the present invention to
specify a security paper on which a printed image can
be applied over the entire area in order to produce a
valuable document, wherein at the same time a
sufficient ability to detect the optically variable
effect of an optically variable security element
arranged in the security paper is ensured. It is also
an object of the present invention to specify a method
for producing such a security paper, an apparatus for
implementing the production method and also a valuable
document produced from the security paper.
This object is achieved by a security paper, a valuable
document, a production method and an apparatus for
implementing the production method having the features
of the independent claims. The dependent claims relate
to preferred refinements and developments of the
invention.
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The security paper according to the invention for
producing valuable documents comprises a substrate
having an optically variable security element, which
can be detected when at least a first side of the
substrate is observed, and an ink-receiving layer,
which is arranged on the first side of the substrate in
front of the optically variable security element in the
viewing direction, wherein the ink-receiving layer has
a first layer thickness in a first area of the
substrate surface, which is located outside the area in
which the optically variable security element is
arranged, and a recess or a reduced layer thickness as
compared with the first layer thickness in a second
area of the substrate surface, which circumscribes the
area in which the optically variable security element
is or possibly can be located.
In the sense of the present invention, a detectable
optically variable security element is understood to
mean an optical element the optically variable effect
of which can be detected by a machine or perceived
visually by an observer. Examples of such optically
variable security elements are diffraction structures
which, in particular by means of the reconstruction of
optically perceivable patterns, generate a visual
impression that depends on viewing angle, such as
embossed or volume holograms and other kinegrams such
as achromatic matt structures. A further example of
such optically variable elements is optical elements
which exhibit what is known as a color tilt effect,
such as single-layer or multilayer thin layer
interference layers or liquid crystal layers, which can
in each case be present as a continuous layer or in
pigment form (what are known as effect pigments, such
as iriodine) Further examples are lens or micro--
mirror structures within the optically variable
element.
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Single-layer or multilayer substrates can be used as
security paper. In the case of single-layer
substrates, any type of paper or paper-like material is
suitable, in particular cotton-vellum paper. It is
also possible to use paper which contains a certain
proportion x of a polymer material in the range between
0 and 100% by weight. Furthermore, a plastic film, for
example a polyester film, can be used as substrate.
This film can additionally be monoaxially or biaxially
oriented. Such an orientation of the film leads,
amongst other things, to it being given polarizing
characteristics, which are used as a further security
feature in the optically variable security element.
A multilayer composite which, for example, has a layer
of paper or paper-like material can be used as a
multilayer substrate. A transparent plastic or polymer
layer is laminated onto this layer, for example from
both sides, which means that such a multilayer
composite exhibits extraordinarily high stability and
durability. In this case, security papers with
plastic-coated or polymer-coated paper layers are used
for producing film composite banknotes. Conversely,
the multilayer composite can also have a central layer
made of a plastic or polymer material, which is coated
on both sides with a layer of paper or of paper-like
material in each case. A multilayer, paper-free
composite material can also be used as a multi_layer
substrate material.
The materials used in such single-layer or multilayer
substrates can have additives which serve as a further
authenticity feature, such as luminescent substances,
which are preferably transparent in the visible
wavelength range and can be excited in the non-visible
wavelength range by a suitable auxiliary means, for
example a radiation source emitting UV or IR radiation,
in order to generate luminescence that is visible or at
least detectable by auxiliary means.
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In the case of a film composite banknote the surface of
which is formed by a plastic or polymer layer, for
example, which has only a low ink-receiving capacity,
the ink-receiving capacity can be improved by the use
of an additional print-receiving or ink-receiving
layer, or an adequate ink-receiving capacity for the
printing ink of the printed image additionally to be
applied can be created for the first time.
In the sense of the present invention, the term ink-
receiving layer is also understood to mean layers which
promote improved adhesion to the substrate surface, in
particular to the plastic or polymer layer of a film
composite banknote. The ink-receiving layer can also
be built up from a plurality of partial layers. For
example, a partial layer arranged directly on the
substrate surface primarily promotes increased adhesion
of a further partial layer to the security paper, while
the further partial layer primarily ensures the ink-
receiving capacity of the printing ink forming the
further printed image. The partial layer arranged
directly on the substrate surface and having an
adhesive capacity is also designated a primer layer.
The printing ink of the ink-receiving layer and the
partial layers possibly present is preferably composed
of silica gel, a flow additive, a crosslinker and a
binding agent.
Such ink-receiving layers are substantially colorless
and frequently exhibit a matting effect with increasing
layer thickness. Typically, an ink-receiving layer is
applied to the entire surface of the substrate and - at
least in the first substrate area in the sense of the
invention - has a constant layer thickness. Ideally,
the ink-receiving layer has no detectable structures
and imparts no information to the observer - as opposed
to other colored layers - for example in the form of
characters, graphic symbols or colors. An ink-
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receiving layer in the sense of the invention instead
forms a visually inconspicuous uniform base for the
printing ink of a further printed image, for example to
be applied in order to produce a valuable document.
The further printed image can be applied with the aid
of the offset process or another suitable printing
process. It can also be a background print:.
In the case of a paper banknote which is composed of a
single-layer substrate made of paper or paper-like
material, the optically variable security element is at
least partly arranged on the surface of the substrate
and, for example, comprises a film element. Thus, the
substrate surface of the security paper has different
materials area by area and correspondingly different
ink-receiving capacities. As a result of the use of an
ink-receiving layer, a uniform ink-receiving capacity
of the substrate surface of a paper banknote can be
ensured and, if appropriate, the ink-receiving capacity
thereof can be increased.
The security element can be produced directly on the
substrate of a security paper or a valuable document to
be secured, or pro-prepared on a separate carrier.
Such a separate carrier preferably has plastic or
polymer material and, for example, can be or have a
film material, in particular a transfer material.
Plastics such as PET (polyethylene terephthalate), PUT
(polybutylene terephthalate), PEN (polyethylene
naphthalate), PP (polypropylene), PA (polyamide) and PE
(polyethylene) are suitable for the film material.
This film can also be monoaxially or biaxially
oriented.
The security element forms a transfer element and can
be applied in endless form to a carrier material.
Here, the security element is fixed to the substrate
with the aid of an adhesive layer, a hot-melt adhesive
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preferably being used for this purpose. Following the
transfer of the security element to the substrate, the
carrier material is possibly pulled off again, so that:
only the security element remains on the substrate.
The optically variable security element can be formed
as a patch or label and, for example, have a two-
dimensional form, with comparable length dimensions in
all directions on the substrate surface, or an
elongated form, for example the form of a strip, as in
the case of security threads. If such an optically
variable security element is formed as a transfer
element with a possibly transparent film as carrier
element, then it is also designated a film element,
film patch or film strip. Such a strip is applied to
the surface of a substrate, for example, in the form of
what is known as a lead strip. Film elements can also
be applied over a recess in the form of a hole in the
substrate and thus be used as a hole covering, which at
the same time ensures visibility of the optically
variable security element from both sides of the
substrate. On the other hand, the optically variable
security element can also be partly embedded in the
substrate material, for example in the form of a
security thread, and come to the surface only in some
areas, as is the case, for example, in what are known
as window security threads, in which the security
thread comes to the surface on only one side of the
substrate, in some areas and possibly periodically, and
what are known as pendulum security threads, in which
the security thread comes to the surface alternately on
both sides of the substrate.
In the case of multilayer substrates, such as film
composite banknotes, the optically variable security
element is arranged on the central layer made of paper
or paper-like material or embedded in the latter. The
optically variable security element is thus located
within the two plastic or polymer layers laminated on
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both sides and is protected by said layers. Here,
these two plastic or polymer layers laminated on
exhibit a sufficiently high transparency, which ensures
an adequate ability to detect the optically variable
effect of the optically variable security element.
Thus, the optically variable security element: is
arranged behind the transparent plastic or polymer
layer in the viewing direction, and the ink-receiving
layer is arranged in front of the transparent layer in
the viewing direction.
The exact position of the optically variable security
element in the substrate of the security paper is often
not defined exactly and/or exhibits fluctuations. Such
registration fluctuations are entirely intended, for
example in the case of security threads, since the
security thread gives rise to a thickening of the
substrate material. To improve the stacking capacity
of valuable documents having security threads, for
example banknotes, it is therefore advantageous if the
position of the security thread varies at right angles
to the longitudinal direction of the security thread.
The overall area in which the security thread can come
to lie is also designated a wobble area, and this area
is understood as a second substrate area in the sense
of the present invention.
As a result of the use according to the invention of a
reduced layer thickness of the ink-receiving layer in
this second substrate area, firstly the matting which
results from the use of the ink-receiving layer is
reduced and therefore the ability to detect the
optically variable effect is already improved. In
addition, the reduced layer thickness of the ink-
receiving layer in the second substrate area results in
a reduced ink-receiving capacity of the substrate
surface of the security thread, which also means that
the level of coverage of a layer of printing ink
forming a printed image to be applied is reduced and
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also, as a result, the ability to detect the optically
variable effect of the optically variable security
element lying underneath is improved. The layer
thickness within the second substrate area can diminish
to zero, with which a complete recess in the ink-
receiving layer is created in the second substrate
area. In this case, the ink-receiving capacity depends
on the surface properties of the substrate of the
security paper which, in particular in the case of film
composite banknotes, the surface of which is formed by
a plastic or polymer layer, is very low.
The present invention is based on the finding that the
optically variable effect of an optically variable
security element can still be detected if an ink-
receiving layer with reduced layer thickness and an ink
layer with a correspondingly reduced level of coverage
are located in front of the optically variable element
in the viewing direction.
Advantageously, the further printed image to be applied
during the production of the valuable document can be
printed over the entire area, without taking any
account of the position of the optically variable
security element in the substrate of the security
paper, by which means the production process is
simplified and the stylistic freedom for the further
printed image increases.
Advantageously, the ink-receiving layer is applied to
the substrate of the security paper by gravure
printing. The printing form used for this purpose, for
example a press cylinder or printing plate, has a first
and a second printing area which, during the gravure
printing, in each case come to lie in the first and
second substrate area of the security paper. The
second printing area of the printing form has a
printing ink receiving capacity for the printing ink
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from which the ink-receiving layer is produced that is
reduced compared with the first printing area.
The printing ink receiving capacity is established by
using depressions in the printing form. These
depressions can be milled, for example by means of a
CNC machine, as is usual in steel gravure printing, or
etched, as is usual in intaglio printing.
In an advantageous embodiment, the depressions are
present in the form of cells, and the second printing
area has a cell size that is reduced as compared with
the first printing area, an increased cell spacing
and/or a reduced cell depth.
In an advantageous refinement of the security paper
according to the invention, the layer thickness of the
ink-receiving layer within the second substrate area,
starting from the first layer thickness in the first
substrate area, has a course which decreases in the
form of a staircase or continuously, preferably with a
constant gradient.
The course of the layer thickness of the ink-receiving
layer, decreasing in the form of a staircase or
continuously in the second substrate area, follows
seamlessly on from the first layer thickness in the
first substrate area. Such a course, having a three-
dimensional gradient, has the advantage that, in the
edge region between the first and the second substrate
area, no disruptive edges occur which impair the
optical impression of the valuable document to be
produced. In the case of a course of the layer
thickness that decreases in the form of a staircase
within the second substrate area, the step height of
the staircase-like course is preferably chosen to be so
small that these steps are imperceptible to an
observer.
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In order to produce such a gradually decreasing course
of the layer thickness of the ink-receiving layer
within the second substrate area, the printing form has
a corresponding gradual course in the printing ink
receiving capacity in the second printing area thereof.
This can be ensured by a corresponding configuration of
the depressions in the printing form. If the printing
form has depressions in the form of cells, then the
cell size is reduced gradually, the cell spacing is
increased gradually and/or the cell depth is reduced
gradually. The gradual course of cell size, cell
spacing and/or cell depth can be configured to be
continuous. The production of the printing form is
simplified if the course of the printing ink receiving
capacity of the printing form has a constant gradient,
in which cell size, cell spacing and/or cell depth are
preferably varied with a constant gradient.
The production of the printing form is also simplified
if, additionally or alternatively, the printing ink
receiving capacity of the printing form has a course
that is staircase-like and thus constant in some areas.
The depressions in the printing form are configured in
such a way that, in some areas, a uniform printing ink
receiving capacity is achieved, areas having a
different, gradually decreasing printing ink receiving
capacity being arranged immediately beside one another.
The printing ink receiving capacity of directly
adjacent areas preferably differs only slightly, so
that the step height of the staircase-like course is so
small that these cannot be perceived visually by an
observer in the resultant ink-receiving layer. In the
case of a staircase-like course, the gradient results
from the positions and heights of the steps.
In a particularly preferred refinement, a multilayer
ink-receiving layer is printed onto the substrate of
the security paper in that a plurality of printing
forms, preferably a plurality of press cylinders
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arranged one after another, in each case print a
partial layer of the ink-receiving layer onto the
substrate of the security paper. Here, each printing
form has the above-described staircase-like course of
the printing ink receiving capacity. The position of
the steps of the respective staircase-like courses is,
however, offset relative to one another in the various
printing forms, so that an ink-receiving layer is
produced on the substrate which has a high number of
steps each having a low step height. The number of
steps of the layer thickness of the ink-receiving layer
on the substrate of the security paper is equal to the
sum of the steps of the staircase-like course of the
printing ink receiving capacity over all the printing
forms used. This has the advantage that, by using only
a few graduations in the printing ink receiving
capacity in each of the printing forms, a high number
of steps can be produced in the layer thickness of the
ink-receiving layer on the substrate of the security
paper, which firstly simplifies the production of the
printing forms and secondly reduces or completely
suppresses the visual perceptibility of the staircase-
like course of the layer thickness of the ink-receiving
layer on the substrate. With the aid of such a
multilayer ink-receiving layer, it is additionally
possible, in particular in the first substrate area, to
achieve improved homogeneity of the ink-receiving layer
and of the printed image to be applied.
The layer thickness of the ink-receiving layer within
the second substrate area has a minimum value. This
minimum value can be zero or different from zero and,
for example, can amount to 25% of the first layer
thickness in the first substrate area. This minimum
value can be assumed in a two-dimensional area within
the second substrate area. If the second substrate
area has an elongated shape, as is true for example in
the case of a security thread as an optically variable
security element, then the minimum value of the layer
thickness within the second substrate area is
=
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preferably assumed only in a linear area. If the
second substrate area has comparable length dimensions
in all directions on the substrate surface, as is true
for example in the case of a two-dimensional patch,
then the layer thickness of the ink-receiving layer has
the minimum value preferably only in a point-like area.
Advantageously, the course of the layer thickness of
the ink-receiving layer is symmetrical with respect to
the area in which the minimum value is assumed. This
has the advantage that the perceptibility of the
gradual course of the layer thickness within the second
substrate area is reduced further.
In an advantageous refinement, the security paper forms
the precursor stage of a film composite banknote. In
this case, a transparent plastic or polymer film is
laminated onto both sides of a central layer made of a
paper or a paper-like material. The optically variable
security element is arranged in the area of the central
layer of paper or paper-like material and is thus
located behind the transparent plastic or polymer layer
in the viewing direction. The ink-receiving layer is
arranged directly on this transparent layer and is
therefore in front of the transparent layer in the
viewing direction.
In a further preferred refinement of the document, the
optically variable security element can also be
perceived visually when observing a second side of the
substrate, located opposite the first side of the
substrate. This can be implemented, for example, by
means of a pendulum security thread, which can be
detected section by section from the front side and
reverse of a valuable document, or else by means of a
security element which forms a hole covering of a
recess in the substrate.
Further exemplary embodiments and advantages of the
invention will be explained below by way of example by
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using the accompanying figures. The examples
constitute preferred embodiments, which in no way
restrict the invention. The figures shown are
schematic representations which do not reflect the real
proportions but are used for improved clarity of the
various exemplary embodiments.
In detail:
Fig. 1 shows a banknote having an optically variable
security element;
Fig. 2 shows a cross section through a first exemplary
embodiment of a security paper having an ink-
receiving layer;
Fig. 3 shows a cross section through a second
exemplary embodiment of a security paper having
an ink-receiving layer; and
Fig. 4 shows a cylindrical press cylinder.
In fig. 1, a banknote 1 is illustrated as a valuable
document, having a window security thread 2 as
optically variable security element. The security
thread 2 has a strip width of 3 to 5 man and, as a
result of registration fluctuations, is located at an
arbitrary point within what is known as the wobble area
3, which has a strip width of about 1 cm.
In fig. 2, a first exemplary embodiment of a security
paper 7, which is used to produce a film composite
banknote 1 as illustrated in fig. 1, is illustrated in
cross section along the line A-A' in fig. 1. The
substrate 8 of the security paper 7 comprises a central
paper layer 4 and transparent PET films 5 laminated
onto both sides. Also illustrated is the security
thread 2, which, in the sectional drawing shown (along
the line A-A'), comes to the surface of the paper layer
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4 and can thus be detected from above when the security
paper 7 in fig. 2 is observed. The security thread 2
shows an optically variable effect. In the exemplary
embodiment shown in fig. 2, it is arranged centrally
within the wobble area 3. However, its position varies
arbitrarily within the wobble area 3 in the security
paper 7 and thus from banknote 1 to banknote 1.
Applied to the surface of the PET film 5 shown at the
top in fig. 2 is an ink-receiving layer 6. Outside the
wobble area 3, said ink-receiving layer 6 has a first
uniform layer thickness. Within the wobble area 3, the
layer thickness of the ink-receiving layer 6 decreases
continuously down to a minimum value, starting arom the
edge region of the wobble area 3. Such an ink-
receiving layer can be created by common coating and
application methods, for example with the aid of a
press cylinder in gravure printing, the printing ink
receiving capacity of which, in an area which comes to
lie on the wobble area 3 of the security paper 7 during
the gravure printing, shows a correspondingly
continuous course of the printing ink receiving
capacity.
The second exemplary embodiment, shown in fig. 3,
differs from the first exemplary embodiment, shown in
fig. 2, in a course of the layer thickness of the ink-
receiving layer 6 within the wobble area 3 that is like
a staircase instead of continuous. An ink-receiving
layer 6 configured in this way can in principle
likewise be produced during gravure printing with the
aid of a press cylinder which shows a correspondingly
staircase-like course of the printing ink receiving
capacity.
However, in the exemplary embodiment illustrated in
fig. 3, the print-receiving layer 6, at least outside
the wobble area 3, comprises three partial layers 6a,
6b, 6c, which each have a layer thickness of about one
third of the total layer thickness of the ink-receiving
=
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layer 6. Within the wobble area, the partial layers 6b
and 6c have recesses, while the partial layer 6a has no
recess and is applied to the entire area of the
security paper V. Thus, in the second exemplary
embodiment illustrated, within the wobble area a
minimum value of the layer thickness of the ink-
receiving layer 6 is assumed which corresponds to the
layer thickness of the partial layer 6a, that is to say
of about one third of the layer thickness outside the
wobble area 3. The three partial layers 6a, 6b and 6c
are applied by three different press cylinders DW1,
DW2, DW3 to the banknote 1, the press cylinder which
prints the partial layer 6a showing no recesses in the
printing ink receiving capacity, the press cylinder
which produces the partial layer 6b having a printing
ink receiving capacity of zero in a printing area which
is narrower than the wobble area 3, and the press
cylinder which produces the partial layer 6c havinq a
printing ink receiving capacity of zero in a printing
area the width of which corresponds to the width of the
wobble area 3. The three press cylinders are arranged
suitably in relation to one another, so that the
staircase-like course of the layer thickness of the
print-receiving layer 6, illustrated in fig. 2, is
produced within the wobble area 3. The upper partial
layers 6b and 6c are primarily used to improve the
reception of the printing ink forming the printed image
still to be applied. They are also designated print-
receiving layers. The lower partial layer 6a, arranged
directly on the surface of the substrate 8, is
primarily used to improve the mechanical adhesion of
the partial layers 6b and 6c lying above to the
substrate surface of the security paper. To this end,
the partial layer 6a exhibits a suitable adhesive
capacity. It is also designated a primer layer.
In an exemplary embodiment that is not illustrated,
each of the three press cylinders has a staircase-like
graduation of the printing ink receiving capacity. The
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printing ink receiving capacity in two of the three
press cylinders DW2, DW3 has five graduations with 0,
25, 50, 75 or 100% of the maximum printing ink
receiving capacity, which means that on each of these
press cylinders four steps are produced during the
transition from 100% to 0% printing ink receiving
capacity. In the third press cylinder. DWI, the
printing ink receiving capacity has only four
graduations with 25, 50, 75 or 100% of the maximum
printing ink receiving capacity, which means that in
this press cylinder three steps are produced during the
transition from 100% to 25% printing ink receiving
capacity. The steps of the staircase-like courses of
the three press cylinders are arranged offset suitably
relative to one another, so that the resulting overall
ink-receiving layer has a total of eleven steps that
are uniform with respect to height and position. Here,
the layer thickness of the ink-receiving layer
decreases gradually from a unit-free value of 300
outside the wobble area 3 to a minimum value of 25.
The fine staircase-like graduation resulting from this
in the layer thickness can no longer be perceived by an
observer, by which means edges interfering optically
with the visual impression of the resulting banknote 1
are avoided but, at the same time, the ability to
detect the optically variable effect of the security
thread 2 is ensured. The course of the printing ink
receiving capacity of the three press cylinders DW1,
DW2 and DW3 and also the resultant total layer
thickness of the ink-receiving layer. 6 are reproduced
numerically in the following table.
DW3 100 100 100 75 75 75 50 50 50 25 25 25 0 0
DW2 100 100 100 100 75 75 75 50 50 50 25 25 25 0
DWI 100 100 100 100 100 75 75 75 50 50 50 25 25 25
Total 300 300 300 275 250 225 200 175 150 125 100 75 50 25
In fig. 4, a press cylinder for gravure printing is
illustrated perspectively. It contains depressions in
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the form of cells 9 for receiving the printing ink
forming the ink-receiving layer 6. In a printing area
10, the size of the cells 9 decreases continuously down
to a minimum value, and the printing ink receiving
capacity of the press cylinder exhibits a corresponding
three-dimensionally gradual course. During gravure
printing, the printing area 10 comes to lie on the
wobble area 3 of the security thread 2 of the security
paper 7. The length of the printing area 10 in the
1.0 direction of the axis of the press cylinder is less
than or equal to the width of the wobble area 3 on the
substrate 8 of the security paper 7. By using the
press cylinder illustrated, it is thus possible to
produce an ink-receiving layer 6 which has a reduced
layer thickness within the wobble area 3. In
particular, as a result, to a good approximation
continuous courses of the layer thickness of the ink-
receiving layer 6 can be implemented within the wobble
area 3. A plurality of press cylinders are preferably
combined in order to implement a plurality of partial
layers of the ink-receiving layer 6.
In an exemplary embodiment that is not illustrated,
instead of a press cylinder, use is made of a printing
plate having a correspondingly gradual course of the
printing ink receiving capacity. While in the case of
the press cylinder the gradual course of the printing
ink receiving capacity is present only in the axial
direction, the printing plate exhibits such a gradual
course in various directions on the surface of the
printing plate, it being possible for the gradients to
be different in different directions. Thus, the area
in which there is a gradual course of the printing ink
receiving capacity on the printing plate can be matched
to the shape of the optically variable security element
to be overprinted. This permits, for example, the
overprinting of an optically variable security element
in the form of a two-dimensional patch or a label.
z
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In a further exemplary embodiment, not illustrated, the
ink-receiving layer comprises a total of 4 partial
layers, three print-receiving layers, which are
primarily used to improve the reception (absorption) of
the printing ink forming the printed image to be
applied, and a primer layer, which is primarily used to
improve the mechanical adhesion of the print-receiving
layers to the substrate surface of the security paper,
the substrate surface consisting of a PET film and the
primer layer being arranged directly on the latter.
The three print-receiving layers have, besides one or
various silica gels as flow additive, a longer-chain
alcohol, for example Dowanoltype or tertiary
butoxypropanol, pentaerythrityl glycidyl ether as cross
linker, and an aqueous polyurethane or polyacrylate or
urethane acrylate dispersion as binder. If
appropriate, further additives such as waxes, surface
additives, anti-foam additives can be used. The print-
receiving layers have 0 - 5% crosslinker, 0 - 5% flow
additive, 5 - 50%, preferably 10 - 25%, silica gel and
40 - 80%, preferably 65 - 75%, binder.
The composition of the three print-receiving layers can
vary. For instance, the uppermost layer on the
security paper has a lower quantity of silica gel than
the layers lying underneath. This has the advantage
that the uppermost print-receiving layer is stable with
respect to abrasion and at the same time is still
sufficiently porous to let the printing ink of the
printed image to be applied through to the print-
receiving layers lying underneath. The print-receiving
layers lying underneath have higher proportions of
silica gel and therefore act like a sponge, a possibly
lower stability with respect to abrasion being of no
consequence.
The primer layer likewise has, besides one or various
silica gels as flow additive, a longer-chain alcohol,
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for example Dowanoltype or tertiary butoxypropanol,
polyfunctional aziridine as crosslinker, and an aqueous
acrylate, styrene acrylate or PU dispersion, for
example Alberdingk U 520, as binder. The primer layer
has 0 - 5% crosslinker, 0 - 1.0% flow additive, 0 - 20%,
preferably 0 - 10%, silica gel, and 80 - 100%,
preferably 85 - 90%, binder.
By means of a suitable, for example chemical, pre-
treatment of the PET film of the security paper for a
film composite banknote, adequate adhesion of the
print-receiving layers can also be achieved directly on
the PET film, which means that the primer layer can be
dispensed with.
