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

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

[0001] Method for producing a security feature on a carrier substrate for valuable or security documents

[0002] The present invention relates to a manufacturing 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 to 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-value tickets, product security labels, credit or debit cards, and other documents at risk of counterfeiting.

[0003] To protect security or valuable documents, the verification of non-visually perceptible features is becoming increasingly important. On the one hand, these features allow for automated and therefore often fast and secure verification. On the other hand, the presence of security features beyond human perception is difficult for a potential counterfeiter to detect.

[0004] 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.

[0005] It is known that IR-absorbing materials such as soot or carbon particles can be used as security features in security or valuable documents. These absorb IR radiation and can therefore be detected by suitable sensors.

[0006] However, IR-absorbing substances such as soot or carbon particles also absorb in the visible spectral range, i.e. they are visible under white light and are also not transparent.

[0007] From DE 10 2016 201 709 A1 a security or valuable document is known with a security print, wherein the security print is formed with at least two printing inks producing different color impressions and with at least one printing ink appearing black, wherein at least one of the at least two printing inks producing different color impressions and the at least one printing ink appearing black each contains an IR absorber.

[0008] 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, irradiability and lightfastness.

[0009] EP 2 942 378 A1 relates to IR-absorbing inkjet inks for security document personalisation.

[0010] Security documents printed with inks containing dyes based on metal dithiolene complexes are known from WO 2022 013 081 A1.

[0011] EP 3 067 216 B1 relates to a radically polymerizable, oxidatively drying ink or printing ink comprising an IR absorber from the group of metal dithiolene complexes (metal = nickel, palladium, platinum) and a protective agent with a sulfur functional group. The protective agent serves to increase the resistance of the IR absorber to oxidation. The ink or printing ink can be used for printing security documents.

[0012] Many security or valuable documents, such as banknotes, have colorless, transparent windows as additional security features in visible light. To improve machine verification of security or valuable documents, it is desirable to be able to easily detect these transparent, colorless windows. For this purpose, it would be desirable to apply IR-absorbing materials to the colorless, transparent windows. These materials are detectable by an IR sensor but not visible in the visible spectral range.

[0013] EP 2 670 801 B1 describes IR-absorbing materials that can be applied to security documents. The IR-absorbing materials described therein comprise a carrier vehicle and at least one IR-absorbing dye applied to nanoparticles with an average size of no more than 100 nm. The carrier vehicle is a printing medium. The dye-bearing nanoparticles are formed from inorganic material and dispersed in the carrier vehicle. Dispersal is to be carried out by nanomilling, and the dispersion is intended to make the IR-absorbing material transparent to visible radiation. However, the dispersed dye-bearing nanoparticles produced in this way exhibit absorption bands in the spectra extending into the visible spectral range, so that they are not transparent beyond the visible spectral range.

[0014] Most people can perceive wavelengths between 400 nm and approximately 780 nm with their eyes. However, the boundaries of the spectral range visible to humans are not clearly defined.

[0015] There is still a need for a security feature, particularly for security or valuable documents, that is absorbed in the IR and thus detectable by an IR detector, but is not or largely invisible in the visible spectral range.

[0016] 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.

[0017] This object is achieved by the inventive method according to claim 1 and a security document according to claim 14.

[0018] According to the invention, IR-absorbing colorants are mixed with a binder and / or solvent to produce a colorant dispersion or solution, wherein the IR-absorbing colorants are dyes and pigments and are selected from the group of organometallic pigments, perylene derivatives, dithio dyes, dithiolene dyes, cyanines, and phthalocyanines. 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.

[0019] The method according to the invention makes it possible to obtain security features which absorb in the IR spectral range, are detectable with corresponding IR detectors, but are at least largely transparent or invisible and machine-readable in the visible spectral range.

[0020] Within the scope of the present invention, it was found that at least largely transparent or non-cash security features can be obtained in the visible spectral range, which are very well detectable and machine-readable in the IR range, if the carrier substrate is an organic carrier substrate and has carbonate groups, and the colorants are selected from the group of pigments and dyes according to the invention, since small amounts of these colorants on these carrier substrates are essentially invisible in the visible spectral range but detectable in the IR range. Furthermore, this special selection of colorants and carrier substrates also enables a post-treatment step that improves the signal-to-noise ratio.

[0021] The substrate preferably contains the following groups: R1-OC(=O)-O-R2, where R1, R2 denote an alkyl or aryl radical. The substrate is preferably polycarbonate, particularly preferably polycarbonate based on bisphenol A.

[0022] The dithio or dithiolene dyes preferably contain nickel, palladium, platinum compounds, or tetrakisammonium compounds. For example, LUNIR5 can be selected as the colorant, and LUNIR6, available from Luminochem, can be selected as the pigment, Epolight 4019 from Epolin, or Epolight 3116, which contains nickel compounds. Spectrasense 765 from BASF can also be used as the IR-absorbing pigment / colorant.

[0023] 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 the colorant dispersion or solution printed onto the carrier substrate and / or between 0.025 micrograms / m 2 and 100 micrograms / m 2 , preferably between 0.08 micrograms / m 2 and 50 micrograms / m 2of the IR-absorbing colorant is applied to the carrier substrate, whereby these details are related to the surface element to which the dispersion or solution is applied.

[0024] 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.

[0025] 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, ie in the IR range a significant increase in absorption due to heating can be observed.

[0026] In other words, by increasing the temperature, a significant increase in absorption in the IR range compared to the visible spectral range can be achieved, i.e. in the visible spectral range, the (low) absorption and thus the "appearance" of the applied or printed colorant dispersion does not change substantially, whereas a significant increase in absorption occurs in the infrared.

[0027] The increase in absorption in the IR range by increasing the temperature in turn results in an improvement in the signal-to-noise ratio in absorption measurements in the infrared spectral range and thus in a reduction in the amount of IR-absorbing colorants applied to a surface element of the carrier substrate.

[0028] 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 amount of colorant dispersion or solution or the applied amount of dispersion / solution per surface area 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-absorbing colorant per surface element has the effect of increasing transparency in the visible spectral range while providing good absorption in the infrared.

[0029] 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 irradiation, preferably with IR detectors or suitable cameras.

[0030] In the context of the present invention, the security elements which 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 in the visible spectral range between 500 nm and 780 nm is at least 60%, preferably at least 65% or 75% and particularly preferably at least 80%.

[0031] The security features produced according to the invention are machine-readable and can be recognized and evaluated using suitable verification devices.

[0032] In the NIR spectral range, silicon-based detectors can be used easily and cost-effectively. However, they only have sufficient sensitivity up to approximately 1100 nm. InGaAs detectors can be used efficiently up to approximately 1600 nm. Detection at higher wavelengths is relatively complex, and cooled detectors are often required.

[0033] For the purposes of the present invention, the visible spectral range is understood to be the range between 400 nm and 780 nm. In a preferred embodiment, colorant dispersions comprising IR colorants and binders, and optionally additional additives, are used for the process according to the invention.

[0034] Before mixing the colorant with the binder, the colorants should first be ground to reduce their grain size and disperse them. This is preferably done in a grinding mill, especially a 3-roll mill with an adjustable gap. However, grinding can also be done using a ball mill or similar device. Grinding breaks up pigment agglomerates and disperses them into the colorant.

[0035] The average grain size after grinding 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 can be printed, for example, by means of an offset process.

[0036] In gravure and screen printing processes, these pigment sizes can also be up to 30 pm and larger.

[0037] The dispersion consists predominantly of colorless binders, i.e., those with little or no absorption in the visible spectral range. Preferred binders are acrylates, acrylic acid, long-chain alkanes or alkenes, especially C14-C18, long-chain petroleum derivatives, low-molecular-weight resins containing reactive double bonds, alkyd resins, or linseed oil varnishes. UV-curing binders, especially oxocarbonyl-containing binders, preferably acrylate-based with up to 15% photoinitiators, or oxidatively curing binders, especially with long-chain alkanes with double bonds, oleic acid, glycerin, and free-radical formers, are preferred.

[0038] When increasing IR absorption by raising the temperature above 160 °C, little to no dependence on the selected binder was observed. In some cases, a slight increase in absorption was observed with binders containing oxocarbonyl, i.e., -OC-(=O) groups, such as acrylates, acrylic acids, or alkyd resins. Furthermore, the dispersion may also contain additives such as rheology modifiers, flow control agents (thinners / thickeners), surfactants, adhesion promoters, tackifier resins, UV absorbers, flame retardants, surface finishers that enhance gloss or mattness, or other solvents or additives to improve miscibility or dispersion, oxidative dryers for oxidatively drying inks, and photoinitiators for UV-curing inks.

[0039] In addition, fluorescent dyes can also be added to the dispersion, preferably in a concentration of 0.2 to 20%.

[0040] Preferably, the additives and, if necessary, dryers or photoinitiators are added and rubbed in during the rubbing process.

[0041] The colorant dispersion can, for example, be oxidatively drying (chemical drying processes), UV-curing, absorbing or evaporating (physical drying processes) and covers the physical and chemical drying processes.

[0042] 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.

[0043] For colorant dispersions for flexographic and screen printing, the colorant dispersion should be dispersed on the dispersant due to the lower viscosity of the binders. Furthermore, the colorant dispersion / solution can be applied using non-impact printing processes, particularly inkjet processes.

[0044] For flexographic printing, a surface coverage of 2 to 4 g / m 2 This corresponds to 0.03 to 0.06 g of colorant / m for a dispersion with a pigment content of 1.5% 2 .

[0045] For an offset printing ink, the colorant dispersion generally comprises between 50 wt.% and 95 wt.% and preferably between 60 wt.% and 95 wt.% and more preferably between 80 wt.% and 95 wt.% of binder and / or between 0.1 wt.% and 20 wt.% and preferably between 0.1 wt.% and 10 wt.% and more preferably between 1 wt.% and 5 wt.% of IR-absorbing colorants (dye and / or pigment).

[0046] With a surface coverage of 1 g / m 2 For a dispersion with a pigment content of 3%, this corresponds to 0.03 g colorant / m 2 .

[0047] For an intaglio / screen printing ink, the colorant dispersion generally comprises between 50 wt.% and 95 wt.% and preferably between 70 wt.% and 95 wt.% and more preferably between 85 wt.% and 95 wt.% of binder and / or between 0.1 wt.% and 20 wt.% and preferably between 0.1 wt.% and 5 wt.% and more preferably between 0.05 wt.% and 2 wt.% of IR-absorbing colorants (dye and / or pigment).

[0048] The respective dispersion is printed or otherwise applied to the carrier substrate in a thin layer thickness. The layer thickness to be applied naturally depends on the extinction coefficient of the IR colorant in the IR range and the colorant concentration in the dispersion. Once an increase in absorption in the IR range has been observed upon 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, with the security feature produced by the method according to the invention, good, preferably machine-readable, detection of the IR absorption is still possible with a sufficiently good signal-to-noise ratio.Alternatively, with a constant layer thickness, the concentration of the IR colorant can be reduced to such an extent that a good, preferably machine-evaluable detection of the IR absorption is still possible with a sufficient signal-to-noise ratio.

[0049] By potentially reducing the applied colorant dispersion layer or concentration, transparency in the visible spectral range is further increased. Should weak absorption of the IR colorant occur in the visible spectral range, i.e., the applied dispersion or solution still exhibits a slight residual color, the security feature can be applied to a correspondingly colored area on the carrier substrate and thus "hidden."

[0050] The IR-absorbing colorant dispersion can be heated, for example, by heating the printed or otherwise provided with the IR-absorbing colorant dispersion carrier substrate in an oven to a temperature of more than 160°C or by laminating the carrier substrate provided with the IR-absorbing colorant dispersion together 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 of 165-205°C for a period of at least one second, in particular at least 10 seconds, preferably between 0.5 and 15 minutes, provided the colorants used have appropriate temperature stability. The application of pressure is not necessary for the desired increase in IR absorption.

[0051] An IR-absorbing ink can also be used for the process according to the invention. The IR-absorbing pigment / dye 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).

[0052] For example, ketones with bound propyl, ethyl or methyl acetates can be used as ink solvents.

[0053] For inkjet-based printing processes, solutions of the colorants are preferably used.

[0054] In general, the colorant solution comprises between 50 wt.% and

[0055] 99 wt.% and preferably between 70 wt.% and 99 wt.% and particularly preferably between 80 wt.% and 99 wt.% solvent and / or between 1 wt.% and 20 wt.% and preferably between 0.1 wt.% and 10 wt.% and particularly preferably between 0.1 wt.% and 1 wt.% IR-absorbing colorants (dye and / or pigment).

[0056] With a pigment content of 0.5% in the ink, the area coverage at 1200 dpi is 0.01 micrometers pigment / m 2 .

[0057] The printed substrate can then be heated to at least 160 °C.

[0058] Within the scope of the method according to the invention, a variety of printing processes such as relief printing (letterpress / letterset, flexographic printing), planographic printing (offset), screen printing, gravure printing (intaglio printing), non-impact printing processes (electrophotography, ionography, magnetography, inkjet processes and thermography) can be used.

[0059] The invention also relates to a security document having a security feature produced according to the present invention.

[0060] The invention is described in more detail below using exemplary embodiments.

[0061] 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 range below 500 nm is not possible.

[0062] 1. Preparation of a colorant dispersion

[0063] Example recipe for a colorant dispersion:

[0064] 10% additive

[0065] 80% binder

[0066] 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 range is approximately 855 nm.

[0067] The IR-absorbing pigment was ground to an average particle size of 5 pm in a three-roll mill and incorporated into an acrylate-based binder. A dispersing agent (TEGO Dispers from Evonik) was added as an additive. The pigment dispersion was deep black with a slightly bluish or greenish shimmer.

[0068] 2. Production of a colorant ink

[0069] 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)

[0070] 1% IR-absorbing dye No. 2 (Epolight 4019 from Epolin)

[0071] The IR-absorbing dye was dissolved in the solvent mixture while stirring.

[0072] 3. Absorption spectra of the unlaminated and laminated sample

[0073] The colorant dispersion prepared according to point 1 was applied in a layer thickness of approx. 1 g / m 2 printed on polycarbonate. This corresponds to a quantity of colorant per printed area of ​​approximately 100 micrograms / m 2 . Subsequently, an absorption spectrum in reflection was recorded in the spectral range between 380 nm and 1100 nm, see Figure 2.

[0074] 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.

[0075] After lamination of the sample at 195°C for approximately 3 minutes, the absorption spectrum was measured again (see Figure 3). It can be seen that the reflectivity in the IR spectral range is significantly reduced by lamination. Surprisingly, the reflectivity in the range between 800 nm and 900 nm decreases from approximately 82% to approximately 38%, whereas the reflectivity in the visible spectral range for the small band at 600 nm decreases from approximately 78% to only approximately 72%.

[0076] This proves that by heating the colorant dispersion on the carrier substrate to at least 195°C, a significant increase in absorption in the IR range can be achieved compared to an almost constant absorption in the visible spectral range, and thus the detectability of IR absorbers can be increased with no or low detectability in the visible spectral range.

[0077] 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.

[0078] 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 application of the colorant dispersion with 5% colorant is approximately 84%. The temperature increase to at least 195°C due to lamination results in an even greater increase in the absorption band between 800 and 900 nm compared to Sample 1, while maintaining almost the same reflection in the visible range.

[0079] 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%) containing 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 lead to 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 carrier substrate according to the invention.

[0080] 4. Test results on the influence of the substrate and the binder

[0081] Colorant dispersions containing 3-5% Epolight 3116 from Epolin as an IR absorber (with nickel compounds) and binder were prepared using two different binders: a UV binder and an oxidatively drying binder. The prepared colorant dispersions were applied to different substrates and heated for 2 minutes in a drying oven at 165 °C.

[0082] An IR reflection spectrum was recorded before and after heating in the drying oven at 165 °C and the two IR reflection spectra were compared.

[0083] 1) Colorant dispersion with UV binder

[0084] The UV binder is an acrylate-based binder with up to 15% photoinitiators.

[0085] In particular, it comprises 20 to 50 wt.% acrylate (trimethylolpropane triacrylate) and other components (0 and 1 wt.% (4'-(l-methylethylidene)bisphenol polymer with (chloromethyl)oxirane, 2-propenoate and between 0 and 1 wt.% glycerol-propoxylated esters with acrylic acid, 0 to 15% photoinitiators, < 1% modified polyester derivative (TEGO) for improved dispersion, and silicone oil (printing aid). a) Substrate: Polyethylene terephthalate (PET)

[0086] No change in the IR reflection spectra due to heating b) Substrate: Paper

[0087] Slight increase in absorption of 9% (23% before heating, 32% after heating c) Substrate paper consisting of 50% plastic fibers

[0088] Slight increase in IR absorption of 9% (23% before heating, 32% after heating) d) Substrate: Polycarbonate

[0089] Strong increase in IR absorption of 34% (28% before heating, 62% after heating) paint dispersion with oxidatively drying binder

[0090] The oxidatively drying binder comprises long-chain alkanes (C14

[0091] - C18) with double bonds, oleic acid and glycerol, radical formers, metal ions as driers. a) Substrate: polycarbonate, strong increase in IR absorption of 38% (33% before heating, 71% after heating). b) Substrate: paper, slight increase of 11% (12% before heating, 23% after heating). R-absorbing colorant (without binder), no substrate. a) Heating the IR-absorbing colorant itself did not lead to an increase in IR absorption. b) Heating the pure dye before grinding, followed by an oxidatively drying binder on paper.

[0092] A small amount of dye was heated to 160 °C for 5 minutes. After cooling, it was rubbed to create a color. Subsequent printing onto the substrate showed no change in the reflectance measurements compared to the unheated dye.

[0093] Evaluation:

[0094] Experiments 1 b) and c) with the binder containing acrylate show a slight increase in absorption on paper.

[0095] Experiment 1 d) and 2 a) demonstrate a strong increase in absorption by the substrate polycarbonate independent of the binder.

[0096] The above experiments demonstrate 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 likely due to the presence of the -OC(=O)-O- group.

[0097] The influence of the binder is far less than that of the substrate. A certain increase in absorption was observed in binders containing acrylates.

Claims

Claims 1. A method 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 substantially transparent and / or invisible in the visible spectral range, in which method IR-absorbing colorants are mixed with a binder and / or solvent to produce a colorant dispersion or solution and this is applied to the carrier substrate, wherein the carrier substrate is an organic substrate containing carbonate groups (-OC(=O)-O-) and the IR-absorbing colorants are dyes or pigments selected from the group of organometallic pigments, perylene derivatives, dithio dyes, dithiolene dyes, cyanines and phthalocyanines.

2. Process according to claim 1, characterized in that the substrate containing carbonate groups (-OC(=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. Process according to one of the preceding claims, characterized in that the dithio or dithiolene dyes contain nickel, palladium, platinum or tetrakisammonium compounds.

4. A 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 the dispersion or solution 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 to theThe printed substrate is applied to the carrier substrate, wherein these specifications refer in each case to the surface element to which the dispersion or solution is applied. Method according to one of the preceding claims, characterized in that the printed substrate is subsequently 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 dispersion or dissolution, 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 in particular between 0.5 and 3 micrometers.Process 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 are in particular UV-curing binders, 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. Process according to one of the preceding claims, characterized in that the binder and / or solvent is colorless and / or transparent. Process according to one of the preceding claims, characterized in that the colorant dispersion is an offset printing ink and contains between 50 wt.% and 95 wt.% and preferably between 60 wt.% and 95 wt.% and particularly preferably between 80 wt.% and 95 wt.% binder and / or between 0.1 wt.% and 20 wt.% and preferably between 0.1 wt.% and. 10 wt.% and particularly preferably between 1 wt.% and 5 wt.% IR-absorbing colorants (dye and / or pigment) or is an intaglio / gravure printing ink and comprises between 50 wt.% and 95 wt.% and preferably between 70 wt.% and 95 wt.% and particularly preferably between 85 wt.% and 95 wt.% binder and / or between 0.1 wt.% and 20 wt.% and preferably between 0.1 wt.% and 5 wt.% and particularly preferably between 0.05 wt.% and 2 wt.% IR-absorbing colorants (dye and / or pigment). Method according to one of the preceding claims, characterized in that, in the case of a low remaining color, the dispersion or solution is applied to a correspondingly colored area on the carrier substrate. Process according to one of claims 1 to 8 or 10, characterized in that the colorant solution contains between 50 wt.% and 99 wt.% and preferably between 70 wt.% and 99 and particularly preferably between 80 wt.% and 99 wt.% solvent and / or between 1 wt. % and 20 wt. % and preferably between 0.1 wt. % and 10 wt. % and particularly preferably between 0.1 wt. % and 1 wt. % IR-absorbing colorants (dye and / or pigment). Process according to one of the preceding claims, characterized in that the colorant dispersion is applied by means of offset, letterset, Toray, flexographic, screen, or intaglio printing. Process according to one of claims 1 to 11, characterized in that the colorant dispersion / solution is applied by means of non-impact printing processes, in particular inkjet processes, electrophotography, ionography, magnetography, and thermography. A 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.

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