Abstract:
The invention discloses a reversibly piezochromic security element for the forgery-protection of value documents, the security element being characterized in that it comprises a collection of optically contrasting pigment particles in a film or a coating layer of an elastic polymer. In a particular embodiment, the particles are optically variable pigment flakes, oriented in a position which is substantially different from an alignment in the plane of the film or coating layer.

Description:
FIELD OF INVENTION 
       [0001]    The present invention is in the field of security documents. In particular it is about a reversibly pressure-sensitive device which can be incorporated into, or affixed onto, or printed onto a security document, and which exhibits a visible color change under a moderate applied pressure, such as can be produced by a human finger. 
       STATE OF THE ART 
       [0002]    Piezochromic devices, which reversibly change color with applied pressure, are known in the art. EP-A 0 530 369 (Myashita) discloses an indolinospiroben-zothiopyran derivative which is obtained as microfine orange-red crystals. Upon application of moderate pressure—rubbing of the surface of a coating comprising them—, these crystals turn into a brilliant deep blue, and remain so until they are exposed to visible light, where upon they turn back to their initial orange color. 
         [0003]    WO-A 03/089227 (Lutz) discloses an application of piezochromic materials as pressure indicator in the cover layer of a roll used in a papermaking machine. 
         [0004]    WO-A 2005/092995 (Leroux) is about a reversible piezochromic system which can be applied in the form of a printing ink, e.g. to protect banknotes from forgery. The system comprises the combination of an electron donating compound and an electron accepting compound. The electron donating compound is a ionochromic substance, in this case a pH-sensitive dye. The electron accepting compound must exhibit acidity high enough to develop the color of the ionochromic compound when brought in contact with the latter, but low enough to allow for the reversibility of the color change. Both types of compounds are combined in a coating composition and applied to a substrate. Upon application of pressure or friction, a strong color develops, which fades away within a minute or two. 
         [0005]    The principal disadvantage of the system of WO-A 2005/092995 in security printing applications is the considerable time it takes the system to revert to its original state after the application of pressure and the concomitant color change. A piezochromic system exhibiting rapid reversible color change with pressure, in both directions, would be highly desirable. 
       SUMMARY OF THE INVENTION 
       [0006]    The present inventors have now surprisingly found that a fully reversible, rapidly reacting piezochromic device, useful for application as a security element on value documents, banknotes, etc., can be realized on the basis of a different physical, noteworthy a mechanical principle. 
         [0007]    The reversibly piezochromic security element of the present invention is based on a collection of optically contrasting pigment particles, comprised in a film or a coating layer of an elastic polymer. 
         [0008]    The present invention discloses as well a coating composition for the production of a reversibly piezochromic security element, comprising a collection of optically contrasting pigment particles in a liquid or pasty polymerizable precursor monomer or oligomer, able to be cured to an elastic solid. 
         [0009]    In the so obtained elastic solid, upon compression or elongation of the elastic polymer, the density and/or the orientation of the pigment particles changes; this results in a visible color change, given the optically contrasting property of the pigment particles. Said visible color change in response to compression or elongation is reversible, in that, upon release of the external pressure, the arrangement of the pigment particles in said elastic polymer reverts to its initial state. The visible color effect can be perceived either in the vicinity of the pressure exerting tool, or from the back side of the device if the back side is visibly transparent, or else through the pressure exerting tool, if this latter is visibly transparent. 
         [0010]    The collection of optically contrasting pigment particles, in the present context, means any kind of pigment particles or any mixture of pigment particles which are visible within the elastic polymer. The pigment does not necessarily need to be of a same single type; the collection of pigment particles may thus comprise various types of pigments, noteworthy one or more parts of pigments chosen from the following, preferred options. 
         [0011]    Preferred pigment particles are of non-spherical shape, in particular they are needle-shaped or plate- or flake-shaped particles. 
         [0012]    Most preferred pigments for embodying this invention are the thin-film interference pigments, in particular the optically variable pigments disclosed in U.S. Pat. No. 4,705,300; U.S. Pat. No. 4,705,356; U.S. Pat. No. 4,721,271 and in the thereto related documents. These pigments comprise a Fabry-Pérot reflector/dielectric/absorber layer structure, wherein the reflector is preferably of a metal, such as aluminium, chromium, nickel, or a metal alloy. The dielectric is preferably of magnesium fluoride (MgF 2 ) or of silicon dioxide (SiO 2 ), and the absorber is preferably of chromium, nickel, or carbon. 
         [0013]    The preferred flakes for embodying the invention have a diameter between 10 and 50 micrometers. 
         [0014]    The needle-shaped or the plate- or flake-shaped particles are preferably comprised within the elastic polymer in an oriented state; such orientation can be effectuated through the application of corresponding shear forces, such as disclosed in DE 196 39 165 C2. Alternatively, the pigment particles can be oriented through the application of external fields, e.g. magnetic fields such as disclosed in EP 1 641 624 and in WO 2008/046702 A1. To this aim, the pigment particles need to be responsive to the chosen external fields.  FIG. 1  schematically shows how the pigment particles in the coating can be oriented. 
         [0015]    The preferred pigment particles for embodying the invention are selected from the magnetic or magnetizable pigment particles. 
         [0016]    The pigment is present in the elastic polymer in a concentration of between 5 and 20 wt-%, preferably of between 10 and 15 wt-%. 
         [0017]    In the most preferred embodiment, the pigment particles, preferably pigment flakes, are about vertically oriented with respect to the plane of the coating. “Vertically”, in the context of the present disclosure, means that the needle-axis of needle-shaped particles is within 30° from the normal to the plane, respectively that the flake-axis of flake-shaped particles is within 30° from the plane of the film or coating. 
         [0018]    The elastic polymer is obtained through the polymerization of an appropriate precursor monomer or oligomer. A liquid or pasty coating composition is formed by dispersing the pigment particles and adequate additives in the polymerizable precursor. The coating composition is applied to a substrate in the form of a film, using an appropriate coating or printing technique, to produce, if so desired, an as well a determined orientation of the pigment particles. The applied coating composition is subsequently cured (hardened) to yield an elastic material comprising the pigment particles. The resulting film is useful as a piezochromic security device. 
         [0019]    In a preferred embodiment, the surface of the piezochromic security device is additionally covered by an at least partially transparent protecting film, to prevent accidental mechanical damages. A preferred protecting film is a transparent polymer foil. The protecting film can, however, also be any other type of protecting coating, such as a UV-varnish or the like. 
         [0020]    In a further embodiment of the piezochromic security device, the film of elastic polymer containing the pigment particles is comprised between two at least partially transparent protecting films. 
         [0021]    A particularly preferred embodiment concerns an optically variable piezochromic element, wherein the pigment is a, preferably magnetic, optically variable pigment, consisting of non-transparent, reflective flakes, which are of the order of 1 micrometer thick and have a planar extension of the order of 10 to 50 micrometers, and whose spectrally selective reflectivity (color) depends on the viewing angle with respect to the plane of the flake. “Optically variable”, in the context of the present disclosure, means having a viewing- or incident-angle dependent color. 
         [0022]    Preferably, the optically variable pigment flakes are magnetic or magnetizable flakes, so as to allow for their orientation in the coating composition through the application of an external magnetic field, prior to hardening it to an elastic solid. 
         [0023]    Upon application of a moderate pressure, a stretching or a shearing force, such as can be exerted by a human finger, to the cured elastic composition comprising the optically variable flakes, the flakes subjected to the pressure change their orientation within the elastic composition, which results in a local, highly visible color change. Upon release of the pressure, the stretching or the shearing force, the flakes immediately return into their former positions, i.e. the pressure-dependent color change is rapid and fully reversible. 
         [0024]    The effect of mechanical compression on a collection of oriented pigment flakes comprised in an elastic coating is illustrated in  FIG. 2 : At the place of compression of the elastic coating, the pigment flakes adopt a lower angle towards the plane of the coating, thus showing an enhanced specular reflection. 
         [0025]    The effect of mechanical elongation on a collection of oriented pigment flakes comprised in an elastic coating is illustrated in  FIG. 4 : In the elongated elastic coating, the flakes adopt a lower angle towards the plane of the coating, and thus show enhanced specular reflection. 
         [0026]    In a preferred embodiment, the coating composition containing the optically contrasting pigment particles is used as a security element on a substrate such as a value document, a banknote, an identity document, an access- or a banking card, or on a label serving for tax collection purposes. 
         [0027]    Preferably, the piezochromic security element is covered by an at least partially transparent polymer foil, which is preferably applied before the curing operation. 
         [0028]    This allows for protecting the elastic coating from being inadvertently or intentionally scratched away. Said foil may also be the over-laminating foil of a credit- or access card, or of a transportation title, which may have the additional function of protecting the sensitive information on these documents from being tampered. Said foil can also be part of a stamping foil assembly. 
         [0029]    As obvious to the skilled person, there may be, depending on the application, a need for additional layers between the piezochromic security element and the said polymer foil, such as for promoting adhesion, for providing release properties, or for still other technical and/or esthetical purposes. 
         [0030]    In a particularly preferred embodiment of the security device, the elastic coating composition containing the flakes is comprised between two polymer foils, at least one of which being at least partially transparent. This allows for applying the verification pressure, e.g. by a human finger, from a first side of the security device, whilst observing the resulting color change from the second side of the security device, i.e. the foil/elastic coating/foil assembly. 
         [0031]    Such foil/elastic coating/foil assemblies may be used on banknotes in the form of security threads, windows or affixed stamping foils. For application as a security thread, the foil assembly is cut into elongated stripes, which are incorporated into security paper during the papermaking, as known to the skilled in the art. In order to observe the visible effect of pressure, the security thread must not be buried entirely within the paper, but exposed in some parts, such as is the case with a window-thread (see EP-A-0 400 902). For application as a window, the foil assembly is either used as the base layer of the security document, which carries an opacifying coating where no window is to appear (see WO 98/13211), or, alternatively, incorporated into the paper during the papermaking process, as known to the skilled in the art (see EP-A-0 860 298). For application as a stamping foil, the foil assembly is produced on a releasable carrier foil, and preferably provided with a heat-activatable glue layer, as known to the skilled person (see WO 92/00855). 
         [0032]    Disclosed is as well a process for making a reversibly piezochromic security element for the forgery-protection of value documents, the process comprising the steps of
       a) providing a substrate;   b) applying a coating composition comprising a collection of optically contrasting pigment particles in a liquid or pasty polymerizable precursor monomer or oligomer to at least part of the substrate;   c) curing the coating composition to an elastic polymer.       
 
         [0036]    In a preferred embodiment of the process, the optically variable flake pigment is a magnetic or magnetizable pigment, and step b) comprises the magnetic orienting of said flake pigment in the applied coating with the help of an external magnetic field. 
         [0037]    Said magnetic orienting is preferably performed using an engraved plate of magnetized permanent magnetic material, such as disclosed in WO 2005/002866 and WO 2008/046702. 
         [0038]    The process may also include the additional step of covering the applied coating composition by an at least partially transparent polymer foil. 
         [0039]    The substrate used in the process may further be an at least partially transparent polymer foil. 
         [0040]    The security element according to the invention can be used for the counterfeit protection of a security document or item, such as a value document, a banknote, an identity document, an access-card, a banking card, or a label serving for tax collection or other purposes. 
         [0041]    Further disclosed is a security document or item, such as a value document, a banknote, an identity document, an access-card, a banking card, or a label serving for tax collection or other purposes, carrying a security element according to the present invention. 
       DETAILED DESCRIPTION 
     Polymer 
       [0042]    Preferably, the polymer binder used to comprise the pigment is a high molecular weight elastic polymer, which allows for a fully reversible, elastic change of dimensions under the influence of external pressure or force, such that the original dimensions are restored after removal of the pressure or force quickly or almost instantaneously at room temperature. 
         [0043]    The polymers which can be used as the elastic binder, to embody the piezochromic security element, include but are not limited to highly flexible polymers such as natural and synthetic rubbers including styrene-butadiene copolymer, acryl ate latex systems, polychloroprene (neoprene), nitrile rubber, butyl rubber, polysulfide rubber, cis-1,4 polyisoprene, ethylene-propylen terpolymers (EPDM rubber), silicone rubber and polyurethane rubber, porous silicones, as well as other suitable polymers disclosed in the art. 
         [0044]    In order to obtain a maximum of visible effect upon compression or elongation of the pigment-containing elastic polymer, it is of advantage to use non-spherical pigment particles, such as needles or flakes, and in particular, to produce an orientation of the pigment particles in the elastic binder matrix. 
         [0045]    The position orientations of the pigment particles in the elastic binder must subsequently be fixed through a curing of the binder, so as to adopt the elastic state. A rapid curing system is of advantage, and UV- or EB (electron beam) curing coating compositions are correspondingly preferred, because they allow an immediate in situ fixation of the pigment particles subsequent to the coating process. 
         [0046]    However, thermally curing elastic polymer systems, such as 2-component silicones, can also be employed; in this case, the orientation of the pigment particles must be maintained during the initial stages of the thermal curing process, through external forces, such as a magnetic field, until the polymer is sufficiently solidified to maintain the pigment particles in place and orientation. 
         [0047]    Furthermore, for health and environmental reasons, it is of advantage to keep the solvent content of the coating composition low. Therefore, solvent-less formulations are a preferred option. 
       Pigment Incorporation 
       [0048]    The pigment concentration in the coating composition should be chosen such that a maximum of visible effect is produced upon application of a moderate pressure, such as possible with a fingertip. In case of a flake pigment, e.g. the optically variable pigment flakes disclosed in U.S. Pat. No. 4,838,648, the pigment concentration should be chosen such that a maximum surface coverage would be obtained in the printed film if the flake particles were to align horizontally after printing, i.e. with their large surface parallel to the imprinted substrate surface. For obtaining a maximum visible effect, the pigment particles are preferably oriented close to vertically with respect to the substrate plane. 
         [0049]    Flake-shaped thin-film optical interference pigments which can be used to embody the present invention are described in U.S. Pat. No. 4,705,300; U.S. Pat. No. 4,705,356; U.S. Pat. No. 4,721,271 and thereto related disclosures. 
         [0050]    Magnetic optically variable pigments, allowing for a magnetic orientation of the pigment particles by the means of an external magnetic field, have been disclosed in WO 02/073250; U.S. Pat. No. 4,838,648; EP-A-686675; WO 03/00801 and U.S. Pat. No. 6,838,166; these documents are incorporated herein by reference. 
         [0051]    On the other hand, the pigment concentration should not be excessively high, in order to allow the flake pigment to rotate, such as to yield a good visible contrast between the compressed and the released state of the flake-pigment containing elastic polymer. The optimum concentration of the flake pigment in the elastic polymer depends on the particular pigment properties such as the particle size and the specific weight, as well as of coating parameters such as the final coating thickness, and should therefore be determined ad casum by the skilled person so to obtain the best visual effect in each application. The optimal pigment concentration is generally somewhere between 1 and 30 weight percent of the ink, in most cases between 5 and 15 wt %. 
         [0052]    The mean particle size and the size distribution in a particular pigment lot have an influence on the achievable result. A rather large particle size (flake diameter in the range of 10 to 50 μm) and a size distribution as homogenous as possible are required for obtaining an optimum effect. However, the larger the flake diameter, the thicker the coating must be to allow for a vertical orientation of the pigment in the coating film. 
         [0053]    The coating composition comprising the flake pigment particles is preferably applied onto a rigid substrate surface via a liquid-ink printing technique, such as screen-printing or bar-coating. The final thickness of the applied and hardened coating layer is highly depending on the used pigment and is preferably of the order of 50 μm or higher, so as to allow for the easy rotation of the pigment flakes to adopt a vertical position. 
         [0054]    Any orientation of the pigment flakes in a position which is substantially different from an alignment in the plane of the film or coating layer will exhibit a certain color change upon the application of pressure. However, the color change is strongest with the pigment particles disposed in the elastic polymer in a position close to vertical with respect to the substrate plane. It is further not advisable to use, for this particular application, a coating thickness which is much less than the diameter of the pigment flakes. 
         [0055]    Materials and technology for the orientation of magnetic particles in coating compositions, as well as corresponding printing processes, have been disclosed in U.S. Pat. No. 2,418,479; U.S. Pat. No. 2,570,856; U.S. Pat. No. 3,791,864; DE 2006848-A; U.S. Pat. No. 3,676,273; U.S. Pat. No. 5,364,689; U.S. Pat. No. 6,103,361; US 2004/0051297; US 2004/0009309; EP-A-710508, WO 02/090002; WO 03/000801; WO 2005/002866, US 2002/0160194; WO 2006/061301; WO 2006/117271; WO 2007/131833; WO 2008/009569; WO 2008/046702; these documents are incorporated herein by reference. 
         [0056]    The coating composition can further comprise other types of pigments and/or dyes; thus it may noteworthy comprise non-magnetic optically variable pigments, additive-color-mixing pigments, iridescent pigments, liquid crystal polymer pigments, metallic pigments, magnetic pigments, UV-, visible- or IR-absorbing pigments, UV-, visible- or IR-luminescent pigments, UV-, visible- or IR-absorbing or luminescent dyes, as well as mixtures thereof. The coating composition may further comprise forensic taggants, e.g. as disclosed in EP-B-0 927 750. 
     
    
     
         [0057]    The reversible piezochromic security element of the present invention is now further illustrated by the figures and by the following, non limiting examples. 
           [0058]      FIG. 1  schematically depicts the alignment of optically variable magnetic pigment flakes in an elastic coating with the help of an external magnetic field. 
           [0059]      FIG. 2  schematically depicts the origin of the optical effect resulting from an elastic deformation due to compression of a coating comprising oriented flake pigments. 
           [0060]      FIG. 3  illustrates the effect of finger pressure on the optical properties of a coating comprising oriented optically variable magnetic pigments, as seen through a glass plate carrying the coating. 
           [0061]      FIG. 4  schematically depicts the origin of the optical effect resulting from an elastic deformation due to elongation of a coating comprising oriented flake pigments. 
           [0062]      FIG. 5  illustrates the effect of elongation on the optical properties of a coating comprising oriented optically variable magnetic pigment: a) without stretch; b) under stretch. 
           [0063]      FIG. 6  schematically depicts an application of the pressure sensitive coating of the present invention as security element on an ID-card. 
       
    
    
     EXAMPLE 1   
     Optically Variable Magnetic Pigment in a 2-Component Silicon Elastomer 
       [0064]    A coating composition for producing a pressure-sensitive optically variable security element according to the present invention was formulated by dispersing optically variable magnetic pigment particles in the heat curable solvent-less 2-component silicon elastomer Sylgard 527 Primerless Silicone Dielectric Gel (Dow Coming). 
         [0065]    The two components of Sylgard 527 were thoroughly mixed at room temperature in a 0.9:1.1 by weight ratio. The Sylgard 527 gel comes as a kit, comprising components A and B in separate containers. The two components are typically mixed in a ratio of 1:1 by weight. A somewhat firmer gel can be obtained by increasing the ratio of part B to Part A in the initial mixture. 
         [0066]    Subsequently, magnetic optically variable pigment (Flex Products Inc., Santa Rosa, Calif., “green-to blue”, 5-layer design Cr/MgF 2 /Ni/MgF 2 /Cr, as disclosed in U.S. Pat. No. 4,838,648) was dispersed in the Sylgard 527 mixture at a concentration of 10 wt-%, and the pigment-containing coating composition was deposited at about 100 μm thickness with the help of a coating bar (hand-coater) onto a transparent polymer foil (100 μm PVC from Puetz-Folien) or onto a glass plate (microscopy slide). 
         [0067]    The so obtained films were pre-dried on a hot plate for 5 min at 80° C., in order to increase the viscosity of the Sylgard 527 binder. The pigment particles in the coating were then oriented to a close to vertical position with respect to the substrate plane, using a “plastoferrite” magnet such as described in WO 2008/046702 A1. The resulting film appeared homogenously grey and partly transparent. The film was kept on the magnet until the viscosity of the Sylgard binder was high enough to retain the positions and orientations of the pigment particles comprised in it, and was then cured in an oven for 30 minutes at 150° C. The cured film was highly flexible and showed a mechanically resilient behavior. In order to protect the so obtained film against mechanical damage (scratching), it was covered with a transparent self adhesive foil. 
         [0068]    Upon compressing the elastic film between a fingertip and the substrate, a clear and fully reversible color change from dark grey to bright green was observed from the back side of the substrate ( FIG. 3 ). 
       EXAMPLE 2 
     Optically Variable Magnetic Pigment in a UV-Curable Dielectric Gel 
       [0069]    A coating composition for producing a pressure-sensitive optically variable security element according to the present invention was formulated by dispersing optically variable magnetic pigment particles in the UV-curable 1-component solventless silicon dielectric gel X3-6211 Encapsulant (Dow Corning). 
         [0070]    The same magnetic optically variable pigment as in example 1 was dispersed in the Silicon gel X3-6211 at a concentration of 7.5 wt-%, and the pigment-containing coating composition was deposited at about 100 μm thickness with a coating bar (hand-coater) onto a transparent polymer foil (100 μm PVC from Puetz-Folien) or onto a glass plate (microscopy slide). 
         [0071]    The pigment particles in the X3-6211 binder were then orientated so as to form an angle close to 60° with respect to the substrate plane, using a magnet such as described in WO 2008/046702 A1, and dried in-situ using a conventional UV-radiation curing unit as known in the art. 
         [0072]    The cured film was highly flexible and had a resilient behavior. In order to protect the film against mechanical damage, it was covered with a transparent self adhesive foil. 
         [0073]    Upon compressing the elastic film between a fingertip and the glass plate, a reversible clear change from dark grey to bluish green was observed. 
       EXAMPLE 3 
     Light Diffractive Pigment in a 2-Component Silicon Elastomer 
       [0074]    A coating composition for producing a pressure-sensitive security element according to the present invention was formulated by dispersing an aluminium flake pigment in the heat curable solvent-less 2-component silicon elastomer Sylgard 527 Primerless Silicone Dielectric Gel (Dow Corning) as described in example 1. 
         [0075]    The SpectraFlair pigment Silver 1500-20 (FLEX Products, JDSU, California) was dispersed in the Sylgard 527 mix at a concentration of 8 wt-%, and the pigment-containing coating composition was deposited at about 100 μm thickness with the help of a coating bar (hand-coater) onto a glass plate (microscopy slide). 
         [0076]    The obtained films were cured in an oven for 30 minutes at 150° C. and was then covered with a transparent self adhesive foil. When compressing the elastic film between a fingertip and the substrate, a change from silver to multiple, bright rainbow colors was observed from the back side of the substrate. 
       EXAMPLE 4 
     Effects of Stretching an Elastic Coating Comprising Oriented Optically  Variable Pigment Flakes 
       [0077]    A coating composition for producing a shear force-sensitive security element according to the present invention was formulated by incorporating optically variable magnetic pigment particles in the UV-curable 1-component solvent-less silicon dielectric gel X3-6211 Encapsulant (Dow Corning) as described in example 2. 
         [0078]    A band of the dispersion was deposited at about 100 μm thickness with the help of a coating bar (hand-coater) onto a transparent polymer foil (100 μm PVC from Puetz-Folien). After orientation of the pigment particles close to vertical with respect to the substrate plane, the film was partly dried by UV curing and a second polymer foil was put on the film surface to form a sandwich-like arrangement. The elastic film was then further cured with UV.  FIG. 5   a  illustrates the unstretched, oriented coating between 2 flexible substrates, which has a dark grey appearance.  FIG. 5   b  shows the effect of mechanical stretching on the coating of  FIG. 5   a:  a clear and fully reversible color change from dark grey to bright green is observed. 
       EXAMPLE 5 
     Application Example of an Optically Variable Magnetic Pigment in a UV-Curable Dielectric Gel 
       [0079]    The pressure sensitive coating composition descried in example 2 can for example be used as security element on an ID card, as illustrated in  FIG. 6 . The manufacturing of the plastic card typically includes the 4 steps of i) plastic compounding/molding of the of the core sheet, ii) printing, iii) lamination and iv) cutting/embossing. In order to obtain a two-side pressure sensitive feature, three circles were cut, as indicated, into a core plastic sheet following the molding step i), and filled with a pressure sensitive coating composition prepared as given in example 2. After UV curing of the pressure sensitive coating, the plastic core sheet was over-laminated on both sides with each a transparent foil. The card can otherwise be processed as usual (printing, cutting etc.). 
         [0080]    The pressure sensitive element of this plastic card shows a clear shift from dark to green when touched from the back while observing from the front side. Alternatively, the middle circle on the front side can be touched to induce, through mechanical transmission of pressure by the laminated cover layer, a color shift from dark to green in the outer  2  circles when observed from the front side. 
         [0081]    The given examples illustrate how a piezochromic security element can be produced through the orientation and fixation of flake-like pigment particles within a highly flexible and resilient elastic polymer layer, which is preferably produced through the application of a solventless and UV-curable precursor material. Depending on the thickness of the elastic polymer layer, optimized optical effects are obtained with pigment concentrations between 5 and 15 wt-%. Improved effects are obtained with relatively thick films; the achievable thickness is, however, limited by process factors of the printing process and by the drying limitations. 
         [0082]    Based on the information given in the description and in the examples, the skilled in the art will be able to derive further embodiments of the disclosed invention.