EP0603117B1 - Method for preparing coloured, printed security papers - Google Patents

Description

The invention relates to a method for producing counterfeit-proof, i.e. Color printed products, in particular securities, which cannot be reproduced true to color by means of color copying machines.

Colored printed products that fluoresce when irradiated with UV light are known, for example, from AU-A-518,156. It is also known from GB-A-1,407,065 to print sensitive documents, including securities or banknotes, with a pair of metameric printing inks at least in regions, in such a way that the metameric colors are printed side by side on the document. These colors do not differ visually in daylight, but when illuminated with UV light the two colors appear different. It is also known from EP-A-0,428,828 to print a ticket with a fluorescent and a non-fluorescent color which practically does not differ visually in daylight. When trying to copy such tickets, the fluorescent color is falsified. In the other publications mentioned, the documents can only be recognized as forgeries with the aid of technical aids (UV light) and can therefore only be assigned to recognition class 2 or 3, in which technical aids (eg UV light) are required in any case to distinguish a counterfeit or copy from the original.

Color-compatible copiers are becoming more and more widespread and have now become so powerful, particularly with regard to their color rendering, that they fully meet most of the usual requirements, especially in everyday office life. Unfortunately, this has led to such copying machines being increasingly misused to forge securities, in particular banknotes, which can be assigned to recognition class 1, in which authenticity detection should be possible visually in one to three seconds without technical aids.

The invention is now intended to show a possibility such as this misuse of color copiers, if not prevented, can be rendered useless.

According to the general idea of the invention, this object is achieved in that the colored printed products, the falsification of which is to be prevented or made useless by means of copying machines, are printed with printing inks which cannot be easily reproduced with the conventional color copying machines. The color copies made on the color copiers then differ in color, i.e. in terms of nuance, saturation and brightness clearly from the originals and are therefore easily recognizable as fakes.

More specifically, the procedure is such that at least one characteristic area of the printed product is printed with a printing ink that is formulated from a single colorant or from a mixture of two or more colorants in order to achieve a desired default color so that between the visual color impression of the with this printing ink printed characteristic area of the original printed product and the visual color impression of the corresponding area of a color copy of the original printed product by means of a color copying machine, there is a visually clearly recognizable difference.

In particular, colorants are used for the formulation of the printing ink which have significantly different optical data than the optical data (absorption / scattering as a function of the wavelength) of the colorants (toners) usually used in color copiers.

Particularly suitable colorants are those of the applicant under the names "IRGAZIN Yellow 2RLT", "IRGAZIN Yellow 3RLTN", "CROMOPHTAL Orange 2G", "CROMOPHTAL Brown 5R", "CROMOPHTAL Orange GP", "CROMOPHTAL Scarlet RN", "CROMOPHTAL Red" G "," CROMOPHTAL DPP Red BP "," CROMOPHTAL Violet B "," IRGALITH Green GLN "and" IRGALITH Green 6G ".

According to a particularly advantageous variant of the method according to the invention, the formulation of the printing ink required to achieve a desired preset color is carried out in such a way that a reference or re-creation of the desired preset color is first created by either using the preset color in a Color copying machine is usually used or similar to these colorants or a color copy of the specified color is created by means of the color copying machine. The printing ink is then formulated with the proviso that, based on two selected types of lighting, there is as large a metamerism as possible between the prescribed printing ink or the area of the original printed product printed with this printing ink on the one hand and the reference or re-adjustment of the default color on the other results in such a way that between the visual color impression of the characteristic area of the original printed product printed with this printing ink and the visual color impression of the corresponding area there is a visually clearly recognizable difference from the original printed product by means of a color copying device.

According to a further aspect of the invention, particularly clear differences between the original printed product and copies produced therefrom can be achieved if non-amorphous or anisotropic black colorants and effect colorants, in particular fluorescent colorants, are used at least in the formulation of the printing inks. The pigments sold by the applicant under the names "GRAPHITAN 7700" or "IRGAZIN Fluorescent Yellow 8501B" are preferred. The names "IRGAZIN", "CROMOPHTAL", "IRGALITH" and "GRAPHITAN" are registered trademarks of the applicant.

Further aspects and details of the invention result from the detailed explanations below.

Here and below, printing ink is understood to mean the physical color (liquid or pasty) which is applied to the substrate, usually special banknote paper, by means of a suitable printing process (gravure printing, offset printing, etc.). Colorants are understood to mean the constituents of a printing ink which produce the color effect thereof; these are usually color pigments or soot (black colorants). Color is the visual and colorimetrically detectable color impression that a printing ink or a substrate printed with it produces. Formulation is understood to mean the composition of a printing ink according to the type and amount of colorants and the determination of the composition from a predetermined range of colorants which is necessary or suitable for the achievement (adjustment) of a desired default color (a desired color impression). The colorants usually used in a color copier are for better differentiation hereinafter referred to as toner. Color copiers usually work with three colored toners (yellow, cyan and magenta) and usually also with a black toner.

Known computer-assisted formulation systems are available for formulation, which have stored the optical data of the gamma of colorants used and which e.g. Using the spectrophotometer to simulate or specify the spectrum of the specified color using more or less sophisticated algorithms by combining the different colorants from the stored gamma with predeterminable accuracy. The result of the formulation is the composition of the printing ink according to the type and proportion of the individual colorants. A known formulation program of this type is the software issued by the applicant under the name "CGREC" and coordinated with the pigment program of the applicant, which software can optionally run on any personal computer in connection with a spectrophotometer. Depending on the accuracy required, formulation systems generally supply several formulations with possibly different colorants, all of which match the specified color within the specified tolerance. The selection of the most suitable formulation is then made on the basis of further considerations which are not relevant here. Some formulation systems also allow the determination of the metamerism between the specified color and the formulated colorant mixture, that is to say the adjustment of the specified color.

Metamerism with regard to two defined types of lighting is qualitatively known to mean the color difference between two physical colors (e.g. printing inks or printed surfaces or the like) for one of the defined types of lighting if the same two physical colors have no color difference for the other of the two defined types of lighting. The metamerism can be clearly determined quantitatively using known calculation methods (e.g. according to DIN 6172) from the spectra of the two colors for the two defined types of lighting. Spectrophotometers can be used to determine the spectra; the calculation of the metamerism is advantageously carried out according to the known formulas using the calculation programs available for this purpose. The most common types of lighting are daylight (D65) and incandescent light (A) in accordance with the standards of the CIE.

Colored securities, for example banknotes and the like, usually have some characteristic colored areas in their motif or are characteristic as a whole in narrow spaces Boundaries of constant colors printed. For the easy recognition of counterfeits produced by means of color copying machines, it is crucial that these characteristic color areas may not be reproduced true to the original with the color copying machine, ie they may be reproduced true to color. According to the general idea of the invention, these areas of the security are now printed with special printing inks which cannot be reproduced or at least only very poorly by means of the usual color copying machines, so that the copies produced in this way can therefore be colored, ie with regard to shade / hue, saturation and Clearly differentiate brightness from the original printed product and can be identified as counterfeit.

According to the invention, these special printing inks, starting from the respective default colors, are mixed (formulated) from different colorants (pigments) in such a way that the colorants involved in the respective formulation are as far as possible with regard to their spectral properties (absorption and scattering as a function of the wavelength) far from the colorants (toners) commonly used in color copiers.

Now, however, many different formulations are generally possible for the reproduction of a desired default color, and it is not immediately apparent from the start without trying out (printing, copying) how these different formulations will behave when copying using a color copying machine K. Surprisingly, it has now been found that a statement can be made in this regard if the metamerism M or the metamerism index M _i or an analog variable is included (see FIG.), And not the metamerism between the printing ink formulation P _{1 in question} , P ₂ , ...., P _n and the default color V, but the metamerism of the respective formulation P _i (i = 1,2, ..., n) compared to a reference R, which is either determined by the formulation ( Adjustment) of the default color V with the toners T _i (i = 1,2, ..., n) of the color copying machine K or with the toners at least approximately corresponding colorants or by a copy of the default color V made with the color copying machine is when the latter is a physical pattern. Surprisingly, it has been found that the color difference between the original O printed with the relevant printing ink formulation P _i and the copy C produced by means of the color copying machine K is most evident when the said metamerism M or the metamerism index or an analogous size is greatest between the relevant ink formulation P _i and the reference R. By including the metamerism or the metamerism index or an analog size Between formulation P _i and the aforementioned reference R, the person skilled in the art is given a means according to the invention to determine the cheapest (ie least copyable) formulation P _{i in} terms of copying behavior easily and quickly, without being cumbersome, time-consuming and ultimately relying on costly trial and error. This procedure is also illustrated by the representation in the drawing figure.

The reference R mentioned can be produced most simply by copying the preset color V, which is present as a physical pattern, by means of the color copying machine. The color copy produced in this way is then directly the reference R, from the spectrum of which the metamerism or the metamerism index can then be determined together with the (calculated) spectrum of the respective printing ink formulation in the manner already known per se.

Alternatively, the reference can also be determined by formulating (adjusting) the specified color using the toners used in the color copying machine using a known formulation system which contains the spectral data of the toners stored. The spectrum of the toner formulation determined in this way can either be taken directly from the formulation system or determined by measuring a color sample produced with this toner formulation.

If the data of the toners are not available, substitute colorants which are as close as possible to them, in particular those from the same range as that from which the colorants originate for the formulation of the printing inks, can be used instead of the toners. In this case, too, the greatest color deviation occurs when copying if the metamerism mentioned is the greatest between the respective ink formulation and the reference.

For standard colors that lie outside the color range that can be detected by the usual toners, the best results can be achieved if the printing inks are formulated with the purest possible colorants (pigments) that are as far as possible from the toners in terms of their optical data. By using fluorescence effect pigments such as "IRGAZIN Fluorescent Yellow 8501B", the effect achieved can be further enhanced, particularly in the yellow-red area and in the yellow-green area.

For standard colors within the range of colors that can be determined by the usual toners, it is either most effective if the blunting (black component) is formed by a complementary mixture or the black component is formulated using an anisotropic colorant such as the pigment "GRAPHITAN 7700" mentioned.

The metamerism is usually determined for the CIE lighting types D65 (daylight) and A (incandescent light). Of course, other types of lighting that may be more suitable for special cases can also be used.

The formulation and determination of the metamerism is preferably carried out using a suitably programmed computer.

The above procedure is illustrated using an example below. A green is selected as the desired default color V.

Color coordinates of the default color in the L, a, b color space:

76.4 (L coordinate), -37.5 (a coordinate), 33.1 (b coordinate)

Rezeptierung P₁:

30.0% IRGAZIN Fluoreszent Gelb 8501B (20%),
20.0% IRGALITH Grün 6G (8%),
50.0% Nitrozellulose-Klarlack

Rezeptierung P₂:

43.9% IRGAZIN Fluoreszent Gelb 8501B (20%)
11.6% IRGALITH Grün GLN (8%)
44.5% Nitrozellulose-Klarlack

Rezeptierung P₃:

13.1% CROMOPHTAL Gelb 3G (8%)
24.9% IRGALITH Grün 6G (8%)
62.0% Nitrozellulose-Klarlack

Rezeptierung P₄:

17.8% CROMOPHTAL Gelb 3G (8%)
14.0% IRGALITH Grün GLN (8%)
68.2% Nitrozellulose-Klarlack

Formulations P _i with which this given default color can be reproduced are, for example (percentages in each case stand for percent by weight):

Formulation P ₁ :

30.0% IRGAZIN fluorescent yellow 8501B (20%),
20.0% IRGALITH green 6G (8%),
50.0% nitrocellulose clear coat

Formulation P ₂ :

43.9% IRGAZIN fluorescent yellow 8501B (20%)
11.6% IRGALITH Green GLN (8%)
44.5% nitrocellulose clear coat

Formulation P ₃ :

13.1% CROMOPHTAL Yellow 3G (8%)
24.9% IRGALITH Green 6G (8%)
62.0% nitrocellulose clear coat

Formulation P ₄ :

17.8% CROMOPHTAL Yellow 3G (8%)
14.0% IRGALITH Green GLN (8%)
68.2% clear nitrocellulose varnish

The percentages in brackets behind the respective pigments mean that the pigment is not in pure form, but rather that the percentage (weight percent) of a nitrocellulose clearcoat added in brackets has been added to the pure pigment. The nitrocellulose clearcoat that is added to the pure pigments is the same nitrocellulose clearcoat that is used as a component for formulation P _i .

The nitrocellulose clear coat can be composed as follows:
8% nitrocellulose A 250 (18% dibutyl phthalate), Union Carbide
1% Ehtocel 7cp, DOW
6% synthetic resin AFS, Hüls
20% ethoxypropanol
20% ethyl acetate
45% ethanol

The color coordinates L, a, b of the formulations P _i in the L, a, b color space are:
Formulation P ₁ : 83.8 (L coordinate), -41.9 (a coordinate), 30.8 (b coordinate)
Formulation P ₂ : 82.6 (L coordinate), -44.5 (a coordinate), 32.7 (b coordinate)
Formulation P ₃ : 81.4 (L coordinate), -40.9 (a coordinate), 32.6 (b coordinate)
Formulation P ₄ : 80.2 (L coordinate), -40.9 (a coordinate), 32.9 (b coordinate)

To create the reference R of the above specified color green with the L, a, b color coordinates 76.4 (L coordinate), -37.5 (a coordinate), 33.1 (b coordinate), the template on which the Color is printed, copied using the color copier (eg CANON CL 200). The reference R then has the L, a, b color coordinates 73.6 (L coordinate), -31.5 (a coordinate), 25.3 (b coordinate).

M₁

= 3.7,

M₂

= 4.1

M₃

= 3.8

M₄

= 2.6

The metamerism indices M _i (here i = 1, ... 4) between the respective formulation P _i and the reference R according to DIN are then:

M ₁

= 3.7,

M ₂

= 4.1

M ₃

= 3.8

M ₄

= 2.6

The largest metamerism index occurs with formulation P ₂ . This is then selected according to the teaching according to the invention and used for the original printed product.

ΔL₁

= -10.5, Δa₁ = 10.2, Δb₁ = -10.2

ΔL₂

= -8.6, Δa₂ = 13.9, Δb₂ = -14.2

ΔL₃

= -9.2, Δa₃ = 7.4, Δb₃ = -7.6

ΔL₄

= -7.3, Δa₄ = 8.4, Δb₄ = -9.0

An original printed product of each formulation P _{i of} the specified color is now created for checking. Each of these printed products is then copied using the color copier. The color deviations achieved between the respective original printed product and its copy made with the color copier in the L, a, b color space are:

ΔL ₁

= -10.5, Δa ₁ = 10.2, Δb ₁ = -10.2

ΔL ₂

= -8.6, Δa ₂ = 13.9, Δb ₂ = -14.2

ΔL ₃

= -9.2, Δa ₃ = 7.4, Δb ₃ = -7.6

ΔL ₄

= -7.3, Δa ₄ = 8.4, Δb ₄ = -9.0

Since the largest metamerism index occurs between the original printed product and the reference R created from the default color by means of a color copier in the formulation P ₂ , the coloristic deviation between the original and its copy is also greatest when copied according to the teaching according to the invention.

Claims (3)

1. A method of producing forgery-proof coloured printed articles, that is to say coloured printed articles, especially security papers, that cannot be reproduced in their true colours using colour copiers, in which at least one characteristic area of the printed article is printed with a printing ink which, for the purpose of achieving a desired master colour (V), is formulated from a single dye or from a mixture of two or more dyes, in which method there is first created a reference (R) or a match of the desired master colour (V) either by matching the master colour (V) using the dyes (T_i) customarily used in a colour copier or dyes similar thereto or by making a colour copy of the master colour (V) using the colour copier, the formulation of the printing ink necessary for obtaining the desired master colour then being effected with the proviso that, on the basis of two selected types of illumination, there is obtained the greatest possible degree of metamerism (M) between the formulated printing ink (P_i) or the area of the original printed article (O) printed using that printing ink, on the one hand, and the reference (R) or match of the master colour (V), on the other hand, so that there is a visually clearly identifiable difference between the visual colour impression of the characteristic area of the original printed article (O) printed with that printing ink and the visual colour impression of the corresponding area of a colour copy (C) made from the original printed article using a colour copier.
2. A method according to claim 1 wherein a non-amorphous or anisotropic black dye is used or co-used for the formulation of the printing ink.
3. A method according to either claim 1 or claim 2 wherein effect dyes, especially fluorescent dyes, are used or co-used for the formulation of the printing ink.

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