MXPA00005620A - Ink composition comprising first and second optically variable pigments - Google Patents

Abstract

This invention relates to an ink composition comprising two different types of optically variable pigments having a viewing angle dependent shift of colour.

Description

COMPOSITION OF INK THAT COMPRISES FIRST AND SECOND OPTICALLY VARIABLE PIGMENTS FIELD OF THE INVENTION This invention relates to a printing ink composition comprising first and second multilayer thin film interference pigments showing a color change that depends on the angle of observation.

BACKGROUND OF THE INVENTION Pigments consisting of thin multilayer film interference structures showing a color change dependent on the angle of observation have been described in several publications, such as, for example, L. Schmid, M. ronga, V. Radtke, O. Seeger "Luster pigments with optically variable properties", European Coatings Journal, 7-8 / 1997 and patents, for example, U.S. Patent Nos. 4,434,010, 5,059,245, 5,084,351 and 5,281,480. The general principle of these types of interference pigments is basically a sequence of alternating thin layers parallel to each other consisting of partial and / or fully reflective materials and low refractive index material. The hue, the color change and the chroma of these multilayer interference pigments, which will be abbreviated as POV (Optically Variable Pigments), depend on the material of the layers, the sequence of the layers, the number of layers and the thickness of the layer but also of the production process. A POV can be produced by two different process categories: I 1. Physical vapor deposition (PVD) technologies: i In summary, the method consists of forming a thin film, multilayer coating using advanced PVD techniques as rollers for coating, ion deposition techniques, etc. on a flexible web of a material, which is preferably soluble in a predetermined solvent. The weft is usually a polymeric material, such as polyvinyl alcohol or polyethylene terephthalate. After separating the plot of the film coating I thin multilayers, flakes are produced from there fragmenting or grinding them to the desired flake size. The separation can be carried out by peeling off the multilayer coating of the weft. For this, a layer is preferably deposited I can be peeled off in the frame before the other layers. Heat and / or solvent may be used to facilitate the separation process. Alternatively, instead of taking off, the screen can be dissolved in a solvent I suitable to carry out the separation. The optionally coated web may be cut or fragmented before the dissolution step. As the multilayer film coating separates from the web, it typically fragments into pieces j of irregular shapes and sizes. These pieces usually require further processing to achieve the desired flake size that is suitable for use as pigment flakes in coating compositions and particularly in ink compositions. The leaflets can be ground to a size that varies between 2 and 5 micras without destroying their color characteristics. Preferably, the average particle size is between 5 and 40 microns, but not greater than about 120 microns. Flakes are produced to have a relationship! dimensional of at least 2: 1. The dimensional relationship is checked by taking the ratio of the largest dimension of a leaflet surface i parallel to the planes of the layers with respect to the thickness dimension for the leaflet (perpendicular to the leaflet). The flakes can be achieved through all the most important processing categories known in the art such as grinding or ultrasonic agitation, optionally in the presence of solvents and / or other auxiliary materials. j The POV produced by this production process is characterized in that the pigment flakes consist of a stack of flat layers that run parallel to each other with the outer surfaces of the pigment flake parallel to each flat layer. Due to the process of fragmentation and grinding, the surfaces of the pigment flake perpendicular to the plane of the layers are formed irregularly with the inner layers not covered by the outer layers. POVs that have these characteristics will be referred to hereinafter POV A. 2. By Wet Chemical Reactions or Chemical Vapor Deposition (CVD) - U.S. Patent 4,328,042: The principle of chemical synthesis of POV is to coat plate-like reflective pigments that are commercially available with a number predetermined thin films semi-opaque and weakly refractive. A typical process of this kind can be described more precisely by means of a specific production process run: In a first stage the plate-type pigments are suspended in an alcohol with dispersion aids. In a continuous form, tetraethoxysilane and an aqueous solution of ammonia are added to this solution. Under these conditions, the tetraethoxysilane is hydrolyzed and the resulting hydrolysis product, the hypothetical silicic acid Si (0H) 4, condenses and forms Si02 as a uniform film on the surfaces of the plate-like pigments. The coating of Si02 can also be carried out in a fluidized bed reactor. In this case, the tetraethoxysilane vapors must react with water vapor. However, at the preferred temperatures of the gas phase deposition (100-300 ° C), the tetraethoxysilane does not react with satisfactory yields. Special precursors have to be used, which are more reactive. Suitable precursors are of the Si (OR) 2 (OOCR) 2 type. • These SE vaporize at 150 ° C and decompose easily with water at 200 ° C. Subsequently, in a process of chemical vapor deposition, the pigments coated with silicon oxide are coated with metallic or metal oxide films. The coating is carried out in a fluidized bed reactor. The pigments coated with SiO2 are fluidized with inert gases, which are charged with gaseous metal carbonyls. At 200 ° C the carbonyls decompose. If iron carbonyl is used, it can be oxidized to Fe203, which forms uniform thin films on the surfaces of the pigment. As an alternative method, the iron oxide coating can be carried out with a sol-gel technique known from conventional micas. When chromium, molybdenum or tungsten carbonyls decompose under inert conditions, metal films can be obtained. Since Mo films are not stable to attack by water, they are converted to molybdenum sulphide. The POV produced by this process have precisely a coherent surface. The outer layers surround and encompass internal coatings and / or the central reflecting leaflet. Because of this, the outer layers are not flat, but they are practically parallel to each other. The external surface of the pigment is not continuously parallel to the first and second surfaces of the plate-like reflective pigment. The POV that shows these characteristics hereafter will be referred to as POV B. Without taking into account if they are of type POV A or POV B, POVs include a fully reflective layer of a material that in most cases is a metal such as aluminum, gold, copper or silver or a metal oxide or even non-metallic materials. The first reflective layer has a suitable thickness in the range between 50 and 150 nm but may be up to 300 nm. Deposited on the totally reflective material is a material with a low refractive index; This material is often called dielectric material. This layer of dielectric material must be transparent, with a refractive index no greater than 1.65. Si02 or MgF2 are the dielectric materials that are preferred. The subsequent semi-opaque layer or layers are of a metal, metal oxides or sulfides, for example, aluminum, chromium, MoS2 or Fe203. The opacity of the metal is a function of the thickness of the layer. For example, aluminum becomes opaque at approximately between 35 and 40 nanometers. Normally the thickness of the semi-opaque layer is between 5 and 10 nanometers. The thickness of the dielectric layer depends on the desired color. This is thicker if longer wavelengths are required. The OVPA can be of symmetric or asymmetric multilayer structure with respect to the fully reflective layer. Quantification of colorimetric properties is possible through the CIELAB color space diagram. In the CIELAB color space, L * indicates brightness and a * and b * are the chromaticity coordinates. In the diagram, + a * is the red address, -a * is green, + b * is yellow and -b * is blue. Croma C * = sqrt (a * 2 + b * 2) increases from the center of the outer circle. Angle of hue h arctg (a * / b *) is 0o along the axis + a * -, 90 ° along the axis + b * -, 180 ° along the axis -a * -, 270 ° along the axis -b * - and 360 ° C (equal to 0o) along the axis + a * - (see Rummp Chemie Lexikon, "Lacke und Druckfarben", Ed. U. Zorll, Georg Thieme Verlag Stuttgart, New York 1998). The flattening of the POV A flakes allows a parallel orientation both with respect to the underlying substrate and with each other when incorporated into an ink composition and printed. The surface covered with them consequently presents almost ideal reflective conditions at the characteristic wavelengths of the POV. In combination with the characteristics of the flake that are the result of the PVD production process (flat parallel layers, individual flat surfaces absolutely flat and uniform, minimum deviation of the thickness of the layer compared to the predetermined and desired value) a high degree of color saturation (chroma) and as much color change as possible with POV of that construction. Due to a large amount of color change the POV A has found wide use in applications that prevent the copying of security documents such as bank drafts, checks, credit cards, passports, identification cards, driving licenses, postage stamps, etc. Despite the favorable properties with respect to anti-counterfeit applications, coating compositions that have comprised POV A show disadvantages. Since POV A is obtained by spraying large areas of a multilayer interference film, the resulting flakes have open edges, where the inner layers are accessible to the chemical attack of the environment. This results in a somewhat lower POV A chemical stability, even when the flakes are incorporated in a cured ink layer. In particular this is a great disadvantage for the application in circulating values such as bank drafts. Chemical print resistance requirements were established by Interpol at the 5th International Conference on Currency and Counterfeiting in 1969 or the Bureau of Engraving and Printing test methods as set forth in BEP-88-214 (TH) section M5. The bright tones, intense shades and high chroma are very often incompatible with the artistic aspects of the design of circulating values; On the other hand, the properties of intense color change are the main quality (avoid copying) that justifies the use of POV. Therefore attempts have already been made to reduce the dramatic visual appearance of the coatings comprising POV-A. POV-A can be mixed with conventional black or colored pigments to achieve these and other related goals (see "Counterfeit deterrent features for the next-generation currency design", Publication NMAB-472, National Academy Press, Washington, 1993, pp. 55- 58 and references mentioned therein). However, mixing with black results in a dull and covered color. Another method, according to what is shown in EP 07, 36, 073, is to mix POV-A with suitable mica pigments, whereby the hue of the mica pigment similar to either the normal color or the color of the mica is chosen. flush view of the POV-A in question. However, this results in the second color of the POV being strongly disturbed in these mixtures, which leads to an unsatisfactorily small color change that sometimes is not even noticeable to the naked eye. Therefore the mixture is unsuitable for anti-counterfeiting applications. In addition, POV A is expensive due to its expensive production, machinery and process. The POV B is cheaper, however the color change of the coating composition comprising POV B is weak, sometimes not even noticeable to the naked eye, for example, when the change is within a tone, for example, red . Therefore, printing inks comprising POV B are not suitable for applications that prevent the copying of security documents. It is the object of the present invention to overcome the disadvantages of the prior art. In particular, it is an object of the invention to decrease the chroma of an ink layer comprising POV A while retaining a color change that is sufficient for security document applications. It is another object to increase the chemical resistance of cured layers comprising POV A.

DETAILED DESCRIPTION OF THE INVENTION The object was solved with an ink composition comprising a binder of polymeric resin and flakes of optically variable dichroic first pigment consisting of a multilayer thin film interference structure shredded comprising a stack of flat and completely parallel layers wherein at least one of the layers is fully reflective, has first and second flat surfaces parallel to each other and deposited on at least one of the flat surfaces by at least one transparent dielectric layer, the ink composition further comprises second pigments optically variable multilayer thin film dichroic cells comprising a reflective plate-like central layer in which the central reflecting layer is completely surrounded by at least one transparent dielectric layer and / or semi-opaque layer of metal or metal oxide. The first and second optically variable dichroic pigments are selected so that they are not antagonistic to each other. In the CIELAB color measurement system "antagonistic" means that the tones (chroma + hue) of both the orthogonal view and the gradient of the first and second optically variable dichroic pigments are related by an investment center. POV mixing follows the rules of mixing additive colors, for example, mixing red POV and green gives yellow. The mixing of subtractive colors, using conventional red and green pigments would give black. For this reason, mixing POV pairs with partially "antagonistic" properties (that is, having either complementary colors or counteracting color changes) can yield very interesting results. By mixing POV A and POV B, a reduction of the POV A chroma is achieved. Along with the reduction of the POV A chroma, it has been found unexpectedly that a mixture of these two types of POV maintains a clear color change between two different tones (which are susceptible to being seen by the naked eye), for example, green to blue, magenta to green even when POV B dominates quantitatively in the mixture. This makes the POV mixture suitable for safety applications. Preferably, the chroma C * (orthogonal view) of the first optically variable dichroic pigment is equal to or greater than 50, more preferably equal to or greater than 55 and even more preferably equal to or greater than 60, while the chroma C * (view orthogonal) of the second optically variable pigment is less than 50, preferably less than 40 and even more preferably less than 30. An unexpected synergistic effect is the improvement of the chemical resistance of the cured ink layer even if the amount of POV pigments A is greater than the amount of POV B. Chemical resistance of cured ink layers comprising mixtures of POV A and POV B instead of POV A only is particularly reinforced against 2% sodium hydroxide solution, sodium sulphide and washing industrial. A preferred embodiment of the present invention is an ink composition comprising a polymeric resin binder and optically variable thin film dichroic pigment flakes consisting of a multilayer thin film interference structure comminuted, comprising a stack of flat layers and Parallels in which at least one of the layers is of a reflective material having first and second flat surfaces parallel to each other and at least one of the transparent dielectric layers is disposed on at least one of the flat surfaces, the ink composition comprises also second optically variable dichroic thin film pigments comprising a reflective plate-like central layer that is completely surrounded by at least one transparent dielectric layer and / or a semitransparent layer of metal or metal oxide whereby the two dichroic shades of the first and second dichroic pigments, thin film, optically variable are practically the same. The first multilayer thin film interference structure corresponds to POV A and the second multilayer interference structure corresponds to POV B. The perception of color is very subjective and what one observer might call "red" another could call it "orange-red" . However, as used throughout this specification, the names of the colors are defined as follows: red is any transmitted or reflected color of a wavelength between about 610 and 700 nm; orange is any transmitted and reflected color of a wavelength between approximately 590 and 610 nm; yellow is any transmitted or reflected color of a wavelength between about 570 and 590 nm; green is any transmitted and reflected color of a wavelength between approximately 500 and 570 nm; blue is any transmitted or reflected color of a wavelength between about 460 and 500 nm; and violet or purple is any transmitted color of a wavelength between approximately 400 and 460 nm; In another definition, the expression "practically the same nuances" means that nuances designated by values of a * and b * do not differ from each other by more than 60 degrees in the CIELAB color measurement system. Preferably, good results are obtained when the hue values at the midpoint between the orthogonal and vertical views were approximately equal for both POV A and POV B. "Approximately equal" means that the difference between the midpoints is not greater than 30 °. Particularly good results are obtained when the first multilayer thin film interference structure (POV A) has a symmetrical construction with respect to the first reflective layer. In this case, at least one transparent dielectric layer with a refractive index not greater than 1.65 is deposited in both the first and the second surfaces of the reflecting layer in such a way that the resulting dielectric layers are flat and parallel to the surface of the reflecting layer. The visual effects of the first multilayer thin film interference structure, POV A, are intensified when a semiopaque layer of metal or metal oxide is deposited in at least one of the dielectric layers. The same is true for the second multilayer thin film interference structure, POV B, in this case the dielectric layer is surrounded by a semiopaque layer of metal or metal oxide. The preferred materials for the two multilayer interference structures are the chrome for the semi-opaque layer and the aluminum for the reflective layer. In an ink composition of the present invention, the mixing ratio of the first and second multilayer thin film interference structures (POV A to POV B) must be in a ratio of 1:10 to 10: 1, preferably at a ratio of 1: 1.5 to 1: 0.6. The ink composition of the present invention may further comprise additional non-interference pigments. Mixtures of POV A and POV B can be incorporated in any suitable ink vehicle as long as the vehicle of the ink is not detrimental to the visual appearance of the POV. In particular it should not cover the optical effects of the pigment and should not be aggressive for the materials of the layers. In general, the ink carriers comprise at least one polymeric binder that forms film, solvents, optionally water, extenders and auxiliary agents such as defoamers, humectants, rheology control agents, antioxidants, etc. The ink layer can be applied to the underlying substrate by any of the known printing techniques, in particular, gravure printing, flexographic printing, engraving and screen printing. An ink composition comprising first and second multilayer interference structures is preferably used to print security documents such as bank drafts, checks, credit cards, etc. The color change properties of a printed image using a composition of this type are not reproducible by photocopying and thus impart to the document a solid quality of security. In addition to the application in security documents, an ink of the kind described in the present invention can be used for any commercial application in which this special decorative effect is desired. The invention is further described by means of examples.

Example 1 Three different POV A (magenta to green) (M / V), green to blue (V / A) and blue to red (A / R)) (all from Flex Products Inc., Santa Rosa, USA) were mixed with two different POV-B ED 1820 and ED 1821 ( BASF AG) in different proportions. In addition, the POV A green to blue was mixed with POV B BASF ED 1819. In all cases, the chroma of the mixture decreased effectively and it was found that the mixtures show surprisingly large color change properties. Mixtures: visual effects (change of color) in daylight The colors and color changes indicated refer to mixtures of approximately 50:50. The corresponding diagrams in the color space CIELAB for the nine series of mixtures are given in the Figures 3a to 3 i. As shown in the table, the useful color change properties of the mixture were obtained with POV A and POV B that have comparable shades (values h) in the two relevant observation angles. In these cases (that is, mixtures of ED 1819 with V / A, from ED 1820 with M / V and ED 1821 M / V) the color change is very satisfactory with respect to the prevention of photocopying. Very good results were obtained when the hue values at the midpoint between the orthogonal and vertical views were approximately equal for both POV A and POV B. This is the case, for example, in the mixture of ED 1821 with M / V. Some mixtures of POV with partially antagonistic behavior of the constituents also showed, surprisingly, remarkable color changes, such as the mixture of ED 1821 with V / A (which changes from green to almost black) or the mixture of ED 1821 with A / R (which changes from violet to coffee). A screen printing ink was produced incorporating various percentages of POV A and / or POV B into a suitable ink matrix. The chroma and the corresponding color values (hue), according to the CIELAB color system, were measured in a cured and printed ink patch (serigraphic prints were made with a Hand Coater No. 3; measurements were made on an instrument PHYMA Penta Gonio PG-5, using angles of illumination / detection with respect to the normal of 22.5 ° / 0 ° for orthogonal view and 45.0 ° / 67.5 ° for ground view) and are given as a function of the composition of the mixture in the Tables the a lg and in the corresponding Figures a a lg and Figure 2.

Table the Green to Blue with ED 1819 view orthogonal view flush C * h C * h Point 1: 100% V / A 66.5 132.4 ° 43.6 273.9 'medium hue < h: 203.2 ° Point 2: 100% ED 1819 49.1 95.8 ° 11.9 38.3 medium hue < h > 67. Io Point 3: 59% V / A 53.5 118.6 ° 22.2 284.6 ° 41% ED 1819 Point 4: 41% V / A 51.4 111.6o 13.9 298.2 ° 59% ED 1819 Table Ib Magenta to Green with ED 1820 orthogonal view flush view C * C * Point 1: 100% M / V 59.1 323.4 '38.8 114.7' medium shade < h: 39.1 'Point 2: 100% ED 1820 26.0 9.3 ° 45.2 78.8 ° medium hue < h > 44.1 'Point 3: 59% M / V 39.1 333.1' 39.7 97.2 ° 41% ED 1820 Point 4: 41% M / V 33.2 341.3 40.5 91.1 '59% ED 1820 Table Green to Blue with ED 1820 orthogonal view ground view C * h C * h Point 1: 100% V / A 66.5 132. 4 '43.6 273.9 ° medium hue < h > 203.2 ° Point 2: 100% ED 1820 26.0 9.3 ° 45.2 78.8th medium hue < h > 44. Io Point 3: 59% V / A 31.9 118.9o 11.2 296.9o 41% ED 1820 Point 4: 41% V / A 20.1 98.7 ° 8.7 45.9o 59% ED 1820 Table Id Blue to Red with ED 1820 orthogonal view ground view C * h C * h Point 1: 100% A / R 63.3 282.8o 24.7 13.3 ° medium shade: h > 328. Io Point 2: 100% ED 1820 26.0 9.3 ° 45.2 78.8 ° medium hue < h > 44.1 'Point 3: 59% A / R 38.1 299.0o 26.9 53.4 41% ED 1820 Point 4: 41% A / R 30.3 312.7 ° 31.3 65.1' 59% ED 1820 Table Magenta a Verde with ED 1821 orthogonal view ground view C * C * Point 1: 100% M / V 59.1 323.4 ° 38.8 114.7 'medium shade < h > 39.1 'Point 2: 100% ED 1821 34.4 7.2 ° 31.6 67.5' medium shade < h > 37.4 ° Point 3: 59% M / V 47.7 330.3 33.1 102.4 ° 41% ED 1821 Point 4: 41% M / V 40.6 336.8 ° 30.9 93.7 '59% ED 1821 Table lf Green to Blue with ED 1821 orthogonal view ground view C * h C * h Point 1: 100% V / A 66.5 132.4 '43.6 273.9' medium shade < h > 203.2 ° Point 2: 100% ED 1821 34.4 7.2 31.6 67.5 'medium shade < h > 37.4 'Point 3: 59% V / A 38.0 125.6o 21.1 283.3' 41% ED 1821 Point 4: 41% V / A 23.8 111.0 '10.0 308.5o 59% ED 1821 Table lg Blue to Red with ED 1821 orthogonal view vertical view C * h C * h Point 1: 100% A / R 63.3 282.8o 24.7 13.3 'medium shade < h > 328. Io Point 2: 100% ED 1821 34.4 7.2 ° 31.6 67.5 ° medium hue < h > 37.4 ° Point 3: 59% A / R 47.7 293.9o 22.5 36.4 ° 41% ED 1821 Point 4: 41% A / R 39.4 304.2 ° 23.3 47.7 '59% ED 1821 Chemical resistance to caustic soda (solution in 2% water, 30 minutes, 25 ° C), saturated sodium sulfate solution in water (30 minutes, 25 ° C) and industrial washing (30 minutes, 95 ° C) C) increased dramatically for mixtures. While an ink layer with a 100% POV A ink simply turns black and completely loses color change, an ink layer with an ink based on the blends only becomes a little darker in color but retains color and change of color.

Claims (12)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following CLAIMS t 1 is claimed as property. An ink composition comprising a polymeric resin binder and flakes of a first optically variable dichroic pigment consisting of a multi-layered, shredded, thin film interference structure comprising a stack of completely flat and parallel layers, wherein at least one of the layers is fully reflective and has first and second parallel flat surfaces each other, at least one of the flat surfaces is deposited by at least one transparent dielectric layer, wherein the ink composition further comprises second, optically variable multilayer thin film dichroic pigments, comprising a reflective plate-like core layer, where the reflecting central layer is completely surrounded by at least one transparent dielectric layer and / or a semiopaque layer of metal or metal oxide, and the first and second optically variable dichroic pigments are selected so that they are not antagonistic to each other.
2. 2. An ink composition according to claim 1, characterized in that the chroma C * of the first optically variable dichroic pigment in orthogonal view is equal to or greater than 50 and the chroma C * of the second optically variable dichroic pigment in orthogonal view is smaller to 50.
3. 3. An ink composition according to claims 1 to 2 characterized in that the midpoint of the shades between the orthogonal and vertical view of the first and second optically variable pigments do not differ by more than 30 °.
4. 4. An ink composition comprising a binder of polymeric resin and flakes of a first optically variable dichroic pigment consisting of a shredded thin film interference structure, comprising a stack of completely flat and parallel layers in which at least one of the layers is totally reflective and having first and second surfaces parallel to each other, at least one of the transparent surfaces is deposited on at least one of the flat surfaces, the ink composition further comprises second multilayer thin film dichroic pigments, optically variables, comprising a reflective plate-like central layer, where the reflecting central layer is completely surrounded by at least one transparent dielectric layer and / or a semi-opaque layer of metal or metal oxide, whereby the two dichroic shades of the first dichroic pigment optically variable and the second optical dichroic pigment The variable ones are practically the same.
5. 5. An ink composition according to one of claims 1 to 4, wherein the first multilayer thin film interference structure is symmetrical and has at least one transparent dielectric layer having an index of at least two surfaces of the reflective layer. refraction no greater than 1.65 with parallel and flat surfaces with respect to the surface of the reflecting layer.
6. 6. An ink composition according to one of claims 1 to 5, wherein the first multilayer thin film interference structure further comprises, disposed in at least one of the dielectric layers, a semiopaque layer of metal or metal oxide.
7. 7. An ink composition according to any of claims 1 to 6, wherein the second multilayer thin film interference structure further comprises, disposed in at least one of the dielectric layers, a semiopaque layer of metal or metal oxide.
8. 8. An ink composition according to one of claims 1 to 7, wherein the reflective layer of the first and / or the second multilayer thin film interference structure is a metal or metal oxide.
9. 9. An ink composition according to one of claims 1 to 8, wherein at least one layer of the second multilayer thin film interference structure is produced by chemical vapor deposition.
10. 10. An ink composition according to one of claims 1 to 9, characterized in that the ratio between the first and the second optically variable dichroic pigments is in the range between 1:10 and 10: 1, preferably in a range between 1: 1.5 and 1: 0.6.
11. 11. An ink composition according to any of the preceding claims, wherein the color change of the second multilayer structure with the viewing angle is smaller and the chroma is weaker compared to the first interference structure.
12. 12. A mixture of optically variable dichroic first pigment flakes consisting of a multilayer, shredded, thin film interference structure comprising a stack of completely flat and parallel layers in which at least one of the layers is fully reflective and has first and second plane surfaces parallel to each other, at least one transparent dielectric layer is deposited on at least one of the flat surfaces, characterized in that the mixture comprises second, optically variable multilayer thin film dichroic pigments, comprising a reflective plate-like central layer When the reflecting central layer is completely surrounded by at least one transparent dielectric layer and / or semi-opaque layer of metal or metal oxide, whereby the first and second optically variable dichroic pigments are selected in such a way that the color effect does not it is destroyed by antagonistic behavior. 11. A security document having first and second surfaces, at least part of the area of one of the surfaces is coated with an ink composition according to any of the preceding claims.