WO2024061899A2

WO2024061899A2 - Method for producing a security feature on a carrier substrate for value or security documents

Info

Publication number: WO2024061899A2
Application number: PCT/EP2023/075790
Authority: WO
Inventors: Phillip Jungk; Agnes LINK; Duncan Faulkner
Original assignee: Bundesdruckerei Gmbh
Priority date: 2022-09-22
Filing date: 2023-09-19
Publication date: 2024-03-28
Also published as: WO2024061898A2

Abstract

The invention relates to a method for producing a security feature on a carrier substrate for value or security documents, which security feature absorbs in the infrared range and is at least substantially transparent and/or not visible in the visible spectral range, wherein, in the method, IR-absorbing colourants are mixed with a binder and/or solvent to produce a colourant dispersion or solution and this is applied to the carrier substrate, wherein the carrier substrate is an organic substrate containing carbonate groups (-O-C(=O)- O-) and the IR absorbing colourants are dyes or pigments selected from the group of organometallic pigments, perylene derivatives, dithio dyes, dithiolene dyes, cyanines and phthalocyanines.

Description

Method for producing a security feature on a carrier substrate for valuable or security documents

The present invention relates to a production method for a security feature with IR-absorbing colorants on a carrier substrate that is at least largely transparent in the visible spectral range, as well as a security and valuable document. Such security features can be used in security documents such as ID documents, passports, visas, banknotes, certificates, vouchers, checks, airline tickets, high-quality admission tickets, labels for product security, credit or cash cards, but also other documents at risk of forgery.

To secure security or valuable documents, checking non-visually perceptible features is becoming increasingly important. On the one hand, these serve for automatic and therefore often quick and secure verification. On the other hand, the presence of security features outside of human perception is difficult for a potential counterfeiter to detect.

It has also been shown that the use of various properties to verify the authenticity of a security or valuable document makes it significantly more difficult to forge or falsify it.

It is known to use IR-absorbing materials such as soot or carbon particles in security or valuable documents as a security feature. These absorb under IR radiation and can therefore be detected by suitable sensors.

However, the IR-absorbing substances such as soot or carbon particles also absorb in the visible spectral range, i.e. they can be seen under white light and are also not transparent.

From DE 10 2016 201 709 A1 a security or value document is known with a security print, the security print being formed with at least two printing inks that produce different color impressions and with at least one printing ink that appears black, with at least one of the at least two different ones Printing inks that cause color impressions and the at least one printing ink that appears black each contains an IR absorber.

EP 3 757 179 A1 discloses an infrared-absorbing inkjet ink composition with an IR absorber from the group of methine dyes, binders, solvents, water and resins for security documents, which is characterized by abrasion resistance, radiability and light fastness.

EP 2 942 378 Al concerns IR-absorbing inkjet inks for security document personalization.

Security documents printed with inks containing dyes based on metal dithiolene complexes are known from WO 2022 013 081 A1.

EP 3 067 216 B1 relates to a radically polymerizable, oxidatively drying ink or printing ink with an IR absorber from the group of metal-dithiolene complexes (metal = nickel, palladium, platinum) and a protective agent with a functional sulfur group, the protective agent serving this purpose , to increase the resistance of the IR absorber to oxidation. The ink or printing ink can be used to print security documents.

Many security or valuable documents such as banknotes have colorless, transparent windows in visible light as additional security features. For better mechanical verification of security or valuable documents, it is desirable to be able to detect these transparent, colorless windows in a simple manner. For this purpose, it would be desirable to apply IR-absorbing materials to the colorless, transparent windows, which can be detected with an IR sensor but are not visible in the visible spectral range.

EP 2 670 801 B1 describes IR-absorbing materials that can be applied to security documents. The IR-absorbing materials described there include a carrier vehicle and at least at least one IR-absorbing dye applied to nanoparticles with an average size of not more than 100 nm. The carrier vehicle is a print medium. The dye-carrying nanoparticles are formed from inorganic material and dispersed in the carrier vehicle. The dispersion should be carried out using nano-milling, and the dispersion should make the IR-absorbing material transparent to visible radiation. However, the dispersed dye-bearing nanoparticles produced in this way have absorption bands in the spectra that extend into the visible spectral range, so that they are not transparent over the visible spectral range.

Most people can perceive wavelengths between 400 nm and around 780 nm with their eyes. However, the boundary of the spectral range visible to humans cannot be clearly drawn.

There is still a need for a security feature, in particular for security or valuable documents, which is absorbed in the IR and can therefore be detected using an IR detector, but which is not or largely invisible in the visible spectral range.

The object of the present invention is therefore to provide a method for producing security features and security documents with such security features which absorb in the IR spectral range, are detectable with corresponding IR detectors, but are at least largely transparent or invisible in the visible spectral range and are preferably machine-readable.

This object is achieved by the method according to the invention according to claim 1 and a security document according to claim 14.

According to the invention, IR-absorbing colorants are mixed with a binder and/or solvent to produce a colorant dispersion or solution, the IR-absorbing colorants being dyes and pigments and from the group of organometallic pigments, perylene derivatives, dithio dyes, etc Dithiolene dyes, cyanines and phthalocyanines can be selected. The colorant dispersion or solution is then printed onto the carrier substrate. According to the invention, an organic substrate comprising carbonate groups is used as the carrier substrate.

The method according to the invention can be used to obtain security features that absorb in the IR spectral range, can be detected with appropriate IR detectors, but are at least largely transparent or not visible and machine-readable in the visible spectral range.

In the context of the present invention, it was found that at least largely transparent or non-visible security features can be obtained in the visible spectral range, which are very easily detectable and machine readable in the IR, if the carrier substrate is an organic carrier substrate and has carbonate groups and the colorants from the group according to the invention Pigments and dyes are selected because small amounts of these colorants on these carrier substrates are essentially not visible in the visible spectral range, but are detectable in the IR. In addition, this special selection of colorants and carrier substrates also enables a post-treatment step that promotes the signal-to-noise ratio.

The substrate preferably contains the following groups Ri-OC(=O)-O-R2, where RI, _R2 denote an alkyl or aryl radical. The substrate is preferably polycarbonate, particularly preferably polycarbonate based on bisphenol A.

The dithio or dithiolene dyes preferably contain nickel, palladium, platinum compounds or tetrakis ammonium compounds. For example, the colorant LUNIR5 and the pigment LUNIR6, available from Luminochem, Epolight 4019 from Epolin or the Epolight 3116 containing nickel compounds can be selected. Spectrasense 765 from BASF can also be used as an IR-absorbing pigment/colorant.

Preferably less than 15 g/m ² , more preferably less than 1 g/m ² and particularly preferably less than 0.05 g/m ² of the colorant dispersion or solution are printed and/or onto the carrier substrate between 0.025 micrograms/m ² and 100 micrograms/m ² , preferably between 0.08 micrograms/m ² and 50 micrograms/m ² of the IR-absorbing colorant is applied to the carrier substrate, this information being applied to the surface element on which the dispersion or solution is applied.

In a preferred variant, the carrier substrate with the applied colorant mixture is heated after printing to a temperature of at least 160 ° C and particularly preferably to between 165 ° C and 205 ° C.

Surprisingly, it was found that by increasing the temperature of the colorant dispersion or solution applied to the carrier substrate to at least 160 ° C, the reflectivity of the IR-absorbing colorant in the infrared is reduced compared to the reflectivity in the visible spectral range, i.e. in the IR range is one Significant increase in absorption due to heating can be observed.

In other words, by increasing the temperature, a significant increase in absorption in the IR range compared to the visible spectral range can be obtained, i.e. in the visible spectral range the (low) absorption and thus the "appearance" of the applied or printed colorant dispersion essentially does not change, whereas one significant increase in absorption in the infrared occurs.

The increase in absorption in the IR range by increasing the temperature in turn has the consequence that the signal-to-noise ratio in absorption measurements in the infrared spectral range is improved by the temperature increase and thus the amount of IR-absorbing colorants applied to a surface element of the carrier substrate is reduced can.

A reduction in the amount of IR-absorbing colorant per surface element of the carrier substrate compared to the prior art can be achieved either by reducing the layer thickness of the applied brought amount of the colorant dispersion or solution or the amount of dispersion/solution applied per surface of the carrier material or by reducing the concentration of the colorant in the colorant dispersion or solution. In all cases, the lower absolute amount of IR absorber colorants per surface element has the effect that, with good absorption in the infrared, the transparency in the visible spectral range is increased.

Thus, the security elements produced by the method according to the invention with the special colorants on the carrier substrate used according to the invention are essentially not visible to the human eye in the visible spectral range and can only be detected or made visible under IR radiation, preferably with IR detectors or suitable ones Cameras.

In the context of the present invention, security elements that are essentially transparent and/or invisible to the human eye in the visible spectral range are understood to mean that the reflectivity of the security feature applied to the (white) carrier substrate is at least 60% in the visible spectral range between 500 nm and 780 nm. , preferably at least 65% or 75% and particularly preferably at least 80%.

The security features produced according to the invention are machine readable and can be recognized and evaluated using suitable verification devices.

In the NIR spectral range, silicon-based detectors can be used easily and cost-effectively. However, these only have sufficient sensitivity up to around 1100 nm. InGaAs detectors can be used efficiently up to around 1600 nm. Detection at higher wavelengths is relatively complex; in particular, cooled detectors are often required.

In the context of the present invention, the visible spectral range is understood to mean the range between 400 nm and 780 nm. In a preferred embodiment, colorant dispersions which comprise IR colorants and binders and, if necessary, additional additives are used for the process according to the invention.

Before mixing the colorant with the binder, the colorants should first be rubbed to reduce the grain size and disperse, preferably in a rubbing mill, especially in a 3-roll mill using an adjustable gap. Grating can also be done using a ball mill or similar. take place. Rubbing breaks pigment agglomerates and disperses them into the colorant.

After grinding, the average grain size should preferably be less than 10 micrometers and more than 0.1 micrometers, preferably more than 0.5 micrometers and less than 8 micrometers and in particular between 0.5 and 3 micrometers, so that the colorant dispersion, for example can be printed using an offset process.

In gravure and screen printing processes, these pigment sizes can also be up to 30 pm and larger.

The dispersion consists predominantly of colorless binders, i.e. binders that have little or no absorption in the visible spectral range. Acrylates, acrylic acid, long-chain alkanes or alkenes, in particular C14 - C18, long-chain petroleum derivatives, low molecular weight resins containing reactive double bonds, alkyd resins or linseed oil varnishes are preferably used as binders. Preference is given to using UV-curing binders, in particular binders containing oxocarbonyl groups, preferably based on acrylate with up to 15% photoinitiators, or oxidatively curing binders, in particular with long-chain alkanes with double bonds, oleic acid, glycerol and radical formers.

When increasing the IR absorption by increasing the temperature to over 160 °C, little to no dependence on the selected binder was observed. In some cases, a slight increase in absorption was found due to oxocarbonyl, ie binders containing -OC-(=O) groups such as acrylates, acrylic acids or alkyd resins. Furthermore, the dispersion can also contain additives such as rheology modifiers, flow control agents (thinners/thickeners), surfactants, adhesion promoters, adhesive resins, UV absorbers, flame retardants, surface finishing agents that promote gloss or mattness or other solvents or additives to improve miscibility or dispersion , oxidative dryers for oxidatively drying paints, photoinitiators for UV-crosslinking paint.

In addition, fluorescent dyes can also be added to the dispersion, preferably in a concentration of 0.2 to 20%.

The additives and, if necessary, dryers or photoinitiators are preferably added during the rubbing process and rubbed in.

The colorant dispersion can, for example, be oxidatively drying (chemical drying processes), UV-crosslinking, flashing or evaporative (physical drying processes) and covers the physical and chemical drying processes.

The IR-absorbing colorant dispersions can be used in offset, letterset, Toray, flexo, screen and intaglio printing, whereby a suitable binder must of course be selected for the respective printing process.

For colorant dispersions for flexographic printing and screen printing, the colorant dispersion should be dispersed on the dispersant due to the lower viscosity of the binder. Furthermore, the colorant dispersion/solution can be applied using non-impact printing processes, in particular inkjet processes.

For flexographic printing, an area coverage of 2 to 4 g/m ² is preferred. For a dispersion with a pigment content of 1.5%, this corresponds to 0.03 to 0.06 g of colorant/m ² .

For an offset printing ink, the colorant dispersion generally comprises between 50% by weight and 95% by weight and preferably between 60% by weight and 95% by weight and particularly preferably between 80% by weight and 95% by weight of binder and/or between 0.1% by weight and 20% by weight and preferably between 0.1% by weight and 10% by weight and particularly preferably between 1% by weight and 5% by weight of IR- absorbing colorants (dye and/or pigment).

With an area coverage of 1 g/m ² , this corresponds to 0.03 g of colorant/m ² for a dispersion with a pigment content of 3%.

For an intaglio/screen printing ink, the colorant dispersion generally comprises between 50% by weight and 95% by weight and preferably between 70% by weight and 95% by weight and particularly preferably between 85% by weight and 95% by weight of binder and/or between 0.1% by weight and 20% by weight and preferably between 0.1% by weight and 5% by weight and particularly preferably between 0.05% by weight and 2% by weight of IR-absorbing colorants (dye and/or Pigment).

The respective dispersion is printed or otherwise applied to the carrier substrate in a small layer thickness. The layer thickness to be applied obviously depends on the extinction coefficient of the IR colorant in the IR range and the colorant concentration in the dispersion. After an increase in absorption in the IR range has been observed through heating, it is fundamentally possible to reduce the thickness of the colorant dispersion layer to be applied (at a constant colorant concentration) to such an extent that the security feature produced using the method according to the invention still has a sufficiently good signal. to noise ratio, a good detection of the IR absorption, which can preferably be evaluated mechanically, is possible. Alternatively, with a constant layer thickness, the concentration of the IR colorant can be reduced to such an extent that good detection of the IR absorption, preferably by machine evaluation, is still possible with a sufficient signal-to-noise ratio.

As a result of the possible reduction of the applied colorant dispersion layer or concentration, the transparency in the visible spectral range is further increased. If there is still a weak absorption of the IR colorant in the visible spectral range, ie the applied dispersion or solution still has a small remaining color, the security feature can be applied to a correspondingly colored area on the carrier substrate and thus “hidden”.

The IR-absorbing colorant dispersion can be heated, for example, by heating the printed carrier substrate or otherwise provided with the IR-absorbing colorant dispersion in an oven to a temperature of more than 160 ° C or by combining the carrier substrate provided with the IR-absorbing colorant dispersion together is laminated with other polymer layers in a lamination system, preferably at a temperature of at least 160 ° C. The carrier substrate with the applied colorant dispersion should preferably be kept at a temperature of at least 160 ° C and particularly preferably from 165 - 205 ° C for at least a period of at least one second, in particular at least 10 seconds, preferably between 0.5 and 15 minutes, provided that the colorants used have appropriate temperature resistance. The application of pressure is not necessary for the desired increase in absorption in the IR.

An IR-absorbing ink can also be used for the method according to the invention. Here, the IR-absorbing pigment/colorant is dissolved or dispersed in a solvent mixture while stirring and the ink is applied to a polymer substrate using a non-impact process (inkjet process, electrophotography, ionography, magnetography and thermography).

For example, ketones with bound propyl, ethyl or methyl acetates can be used as ink solvents.

Colorant solutions are preferably used for inkjet-based printing processes.

In general, the colorant solution comprises between 50% by weight and

99% by weight and preferably between 70% by weight and 99% by weight and particularly preferably between 80% by weight and 99% by weight of solvent and/or between 1% by weight and 20% by weight and preferably between 0.1% by weight and 10% by weight and particularly preferably between 0.1% by weight and 1% by weight of IR-absorbing colorants (dye and/or pigment).

If the pigment content of the ink is 0.5%, this results in an area coverage of 0.01 micrometers of pigment/m ² at 1200 dpi.

The printed substrate can then be heated to at least 160 °C.

As part of the method according to the invention, a variety of printing processes such as letterpress printing (letterpress/letterset, flexographic printing), planographic printing (offset), screen printing (screen printing), gravure printing (gravure printing), non-impact printing processes (electrophotography, ionography, magnetography, inkjet printing) can be used. procedures and thermography) can be used.

The invention also relates to a security document with a security feature produced according to the present invention.

The invention is described in more detail below using exemplary embodiments.

The spectra shown below were recorded with an IR spectrometer and not with a UV/VIS spectrometer, so that an evaluation of the spectra in the UV and short-wave visible ranges below 500 nm is not possible.

1. Preparation of a colorant dispersion

Example recipe for a colorant dispersion:

10% additive

80% binder

10% IR-absorbing colorant No. 1 (LUNIR 6, Luminochem) The absorption spectrum of the IR-absorbing colorant No. 1 in chloroform is shown in Figure 1. The absorption maximum in the IR is around 855 nm.

The IR-absorbing pigment was ground in a three-roll mill to an average grain size of 5 pm and incorporated into an acrylate-based binder. A dispersing aid (TEGO Dispers from Evonik) was added as an additive. The pigment dispersion was deep black with a slightly bluish or greenish shimmer.

2. Preparation of a colorant ink

99% solvent mixture (mixture of: mesitylene, l-methoxy-2-propanol acetate, 1,2,4-trimethylbenzene, ethyl 3-ethoxypropionate, cumene, naphtha (as described in DE 10 2008 012423 Al)

1% IR-absorbing dye No. 2 (Epolight 4019 from Epolin)

The IR-absorbing dye was dissolved in the solvent mixture with stirring.

3. Absorption spectra of the unlaminated and laminated sample

The colorant dispersion prepared according to point 1 was printed onto polycarbonate in a layer thickness of approximately 1 g/m ² . This corresponds to an amount of colorant per printed area of approximately 100 micrograms/m ² . An absorption spectrum was then recorded in reflection in the spectral range between 380 nm and 1100 nm, see Figure 2.

Above 430 nm, more than 75% of the light is reflected - even in the IR range - so absorption is low - even in the IR range.

After lamination of the sample at 195° for a period of about 3 minutes, the absorption spectrum was measured again (see Figure 3). It can be seen that the reflectivity was increased by lamination in the IR spectral range is significantly reduced. Surprisingly, the reflectivity in the range between 800 nm and 900 nm drops from approx. 82% to approx. 38%, whereas the reflectivity in the visible spectral range in the small band at 600 nm only drops from approx. 78% to approx. 72%.

This proves that the fact that the colorant dispersion on the carrier substrate was heated to at least 195 ° C achieved a significant increase in absorption in the IR range compared to an almost constant absorption in the visible spectral range and thus the detectability of IR absorbers no or low detectability in the visible spectral range can be increased.

This effect is also demonstrated by the absorption spectra of an IR-absorbing colorant No. 2, a dark blue powder. The absorption spectrum of colorant No. 2 in chloroform is shown in Figure 4. Figure 5 shows the reflection spectrum of the dispersion printed on polycarbonate with colorant 2 (sample 2) before and Figure 6 shows sample 2 after lamination.

Sample 2 differs from sample 1 in that a different IR absorber is used and the IR absorber concentration has been reduced to 5%. In sample 2, the reflectivity after applying the colorant dispersion with 5% colorant is about 84%. The increase in temperature to at least 195°C as a result of lamination results in an even greater increase in the absorption band between 800 and 900 nm compared to sample 1, with almost the same reflection in the visible range.

Figure 7 is an absorption spectrum of a third colorant. The pure substance is a dark green powder with an absorption maximum at 975 nm and Figure 8 shows the reflection spectrum of the polycarbonate coated with the third colorant dispersion (5%) with colorant No. 3. This shows good absorption in the spectral range of 700 - 1000 nm with high reflection in the visible range. With this colorant, lamination does not cause an increase in absorption in the spectral range of 700 - 1000 nm. The absorption remains constant, which means that heating the substrate or carrier material does not lead to an increase in absorption in the IR for every colorant and that the desired security feature can be achieved simply by selecting the colorants and the carrier substrate according to the invention.

4. Experimental results on the influence of the substrate and the binder

Colorant dispersions were produced with 3 - 5% Epolight 3116 from Epolin as an IR absorber (with nickel compounds) and binder, using two different binders, a UV binder and an oxidatively drying binder. The colorant dispersions produced were applied to different substrates and heated in a drying oven at 165 °C for 2 minutes.

An IR reflection spectrum was recorded before and after heating in the drying cabinet at 165 °C and the two IR reflection spectra were compared.

1) Colorant dispersion with UV binder

The UV binder is an acrylate-based binder with up to 15% photoinitiators.

In particular, it comprises 20 to 50% by weight of acrylate (trimethylolpropane triacrylate) and other components (0 and 1% by weight of (4'-(l-methylethylidene bisphenol polymer with (chloromethyl)oxirane, 2-propenoate and between 0 and 1% by weight of glycerol-propoxylated esters with acrylic acid, 0 to 15% photoinitiators, < 1% modified polyester derivative (TEGO) for better dispersion and silicone oil (printing aid). a) Substrate: polyethylene terephthalate (PET)

No change in IR reflection spectra due to heating b) Substrate: paper

Slight increase in absorption of 9% (23% before heating, 32% after heating c) Substrate paper consisting of 50% plastic fibers

Slight increase in IR absorption of 9% (23% before heating, 32% after heating) d) Substrate: Polycarbonate

Strong increase in IR absorption of 34% (28% before heating, 62% after heating) arbmitteldispersion with oxidative drying binder

The oxidative drying binder includes long-chain alkanes (C14

- C18) with double bonds, oleic acid and glycerol, free radical generator, metal ions as dryers. a) Substrate: Polycarbonate strong increase in IR absorption of 38% (33% before heating, 71% after heating) b) Substrate: Paper small increase of 11% (12% before heating, 23% after heating) R-absorbing colorant (without binder), no substrate a) Heating the IR-absorbing colorant as such did not lead to any increase in absorption in the IR. b) Heating the pure dye before rubbing the dye, then using an oxidatively drying binder on paper

A small amount of dye was heated to 160 °C for 5 minutes. After cooling, a color was rubbed with it. The Subsequent printing on the substrate showed no change in the reflectance measurements compared to unheated dye.

Evaluation:

Experiments 1 b) and c) with the binder containing acrylate demonstrate a slight increase in absorption on paper.

Experiments 1 d) and 2 a) demonstrate a strong increase in absorption by the polycarbonate substrate regardless of the binder.

The above experiments prove that the increase in absorption after heating in a drying oven is highly dependent on the substrate. The largest increase in absorption was observed on polycarbonate and is probably due to the presence of the -O-C(=O)-O- group.

The influence of the binder is far less than that of the substrate. Some increase in absorption was observed in binders containing acrylates.

Claims

Expectations

1. Process for producing a security feature on a carrier substrate for valuable or security documents, which security feature absorbs in the infrared range and is at least essentially transparent and / or not visible in the visible spectral range, in which process IR-absorbing colorants are used to produce a colorant dispersion or solution is mixed with a binder and/or solvent and this is applied to the carrier substrate, the carrier substrate being an organic substrate containing carbonate groups (-O-C(=O)-O-) and the IR-absorbing colorants being dyes or pigments, which are selected from the group of organometallic pigments, perylene derivatives, dithio dyes, dithiolene dyes, cyanines and phthalocyanines.

2. The method according to claim 1, characterized in that the substrate containing carbonate groups (-O-C(=O)-O-) is a substrate containing Ri-O-C(=O)-O-R2 with RI, R2 = alkyl, aryl and preferably polycarbonate, particularly preferably polycarbonate based on bisphenol A.

3. Method according to one of the preceding claims, characterized in that the dithio or dithiolene dyes contain nickel, palladium, platinum or tetrakis ammonium compounds.

4. The method according to any one of the preceding claims, characterized in that less than 15 g/m ² and preferably less than 1 g/m ² and particularly preferably less than 0.05 g/m ² of the dispersion or solution are applied to the carrier substrate , and/or between 0.025 micrograms/m ² and 100 micrograms/m ² and preferably between 0.08 micrograms/m ² and 50 micrograms/m ² of the IR-absorbing colorant on the Carrier substrate are applied, whereby this information refers to the surface element on which the dispersion of the solution is applied. Method according to one of the preceding claims, characterized in that the printed substrate is then heated to a temperature of at least 160 °C and particularly preferably to between 165 °C and 205 °C. Method according to one of the preceding claims, characterized in that the IR-absorbing colorants are ground during dispersing or dissolving, preferably to a grain size of less than 10 micrometers and more than 0.1 micrometers and particularly preferably to a grain size between 0.5 Micrometers and 8 micrometers and especially between 0.5 and 3 micrometers. Method according to one of the preceding claims, characterized in that the binders are selected from the group of acrylates, acrylic acids, long-chain alkanes, long-chain petroleum derivatives, low molecular weight resins containing reactive double bonds, alkyd resins or linseed oil varnishes and in particular UV-curing binders, preferably on Acrylate base with up to 15% photoinitiators, or oxidatively hardening binders, especially with long-chain alkanes with double bonds, oleic acid, glycerol and radical formers. Method according to one of the preceding claims, characterized in that the binder and/or solvent is colorless and/or transparent. Method according to one of the preceding claims, characterized in that the colorant dispersion is an offset printing ink and between 50% by weight and 95% by weight and preferably between 60% by weight and 95% by weight and particularly preferably between 80% by weight and 95% by weight .% binder and/or between 0.1% by weight and 20% by weight and preferably between 0.1% by weight and 10% by weight and particularly preferably between 1% by weight and 5% by weight of IR-absorbing colorants (dye and/or pigment) or is an intaglio/gravure printing ink and between 50% by weight and 95% by weight and preferably between 70% by weight and 95% by weight and particularly preferably between 85% by weight and 95% by weight of binder and/or between 0.1% by weight and 20% by weight and preferably between 0.1% by weight and 5% by weight .% and particularly preferably between 0.05% by weight and 2% by weight of IR-absorbing colorants (dye and / or pigment). Method according to one of the preceding claims, characterized in that the dispersion or solution is applied to a correspondingly colored area on the carrier substrate in the event of little remaining color. Method according to one of claims 1 to 8 or 10, characterized in that the colorant solution contains between 50% by weight and 99% by weight and preferably between 70% by weight and 99% and particularly preferably between 80% by weight and 99% by weight of solvent and/or between 1% by weight and 20% by weight and preferably between 0.1% by weight and 10% by weight and particularly preferably between 0.1% by weight and 1% by weight of IR-absorbing colorants (dye and/or or pigment). Method according to one of the preceding claims, characterized in that the colorant dispersion is applied by means of offset, letterset, Toray, flexo, screen or intaglio printing. Method according to one of claims 1 to 11, characterized in that the colorant dispersion/solution is applied using non-impact printing processes, in particular inkjet processes, electrophotography, ionography, magnetography and thermography. Security document containing a security feature produced according to one of claims 1 to 13.

15. Security document according to claim 14, characterized in that the security feature is machine readable.