HK1168372B - Piezochromic security element - Google Patents

Description

Pressure colour-changing security element

Technical Field

The invention belongs to the field of confidential documents. In particular, it relates to a reversible pressure sensing device which can be incorporated into or affixed to, or printed on, a security document and which exhibits a visible colour change under moderate applied pressure (e.g. generated by a human finger).

Background

As is well known in the art, piezochromic devices reversibly change color with applied pressure. EP-A0530369 (Myashita) discloses indoline spirobenzothiopyran derivatives (indolinone spirobenzothiopyran derivatives) obtained as fine orange-red crystals. Upon application of moderate pressure-rubbing the surface of the coating containing these derivatives, these crystals turn bright deep blue and remain in this state until exposure to visible light, whereupon the crystals return to the original orange color.

WO-A03/089227 (Lutz) discloses the use of a piezochromic material as a pressure indicator for roll covers used in paper machines.

WO-a 2005/092995(Leroux) relates to a reversible piezochromic system which can be applied to a form of printing ink, for example to prevent banknote counterfeiting. The system comprises a combination of an electron donor compound and an electron acceptor compound. The electron donor compound is an ionizable colour change substance, in this case a pH sensitive dye. The electron acceptor compound must exhibit a sufficiently high acidity to reveal the color of the ionised colour-changing compound when it comes into contact with the latter, but sufficiently low to allow reversibility of the colour change. The two types of compounds are incorporated into a coating composition and coated onto a substrate. Upon application of pressure or rubbing, a strong color develops and fades within 1 or 2 minutes.

The main disadvantages of the system of WO-a 2005/092995 in security printing applications are: the system is quite time consuming to switch to its original state or the accompanying color change after the application of pressure. It is highly desirable that the piezochromic system exhibit a rapid and reversible color change (two-way color change) with pressure.

Disclosure of Invention

The inventors have now surprisingly found a completely reversible, fast-reacting piezochromic device which can be used as security element on documents of value, banknotes and the like, which can be realized according to different physical principles (notably mechanical ones).

The reversibly piezochromic security element of the invention is based on an assembly comprising optically contrasting pigment particles in a film or in a coating of an elastic polymer.

Also disclosed are coating compositions for making reversible piezochromic security elements comprising an assembly of particles of an optical contrast pigment in a liquid or paste polymerizable precursor monomer or oligomer and capable of being cured to an elastic solid.

In the elastic solid thus obtained, the density and/or orientation of the pigment particles changes with compression or stretching of the elastic polymer, as a result of which a visible color change occurs, showing the optical contrast of the pigment particles. The visible color change in response to compression or extension is reversible in that the alignment of the pigment particles in the elastic polymer is transformed into an initial state with the release of external pressure. The visible colour effect can be perceived in the vicinity of the pressure application means, either from the back of the device if the back is visibly transparent, or through if the latter is visibly transparent.

In the specification, an aggregate of optically contrasting pigment particles refers to any kind of pigment particles or any mixture of pigment particles, which are visible in the elastomeric polymer. The pigments need not necessarily be of the same single type, and the collection of pigment particles may include a variety of different types of pigments, with one or more of the pigments of interest being selected from the following preferred choices.

Preferred pigment particles are non-spherical, in particular acicular, platy or flake-like particles.

The most preferred pigments for carrying out the invention are thin film interference pigmentsIn particular optically visible pigments as disclosed in US 4,705,300, US 4,705,356, US 4,721,271 and other related documents. These pigments comprise Fabry-Perot reflector/dielectric/absorber layer structures, wherein the reflector is preferably a metal or metal alloy such as aluminum, chromium, nickel. The dielectric layer is preferably magnesium fluoride (MgF)₂) Or silicon dioxide (SiO)₂) And the absorption layer is preferably chromium, nickel or carbon.

Preferred flakes for practicing the present invention have a diameter of 10-50 μm.

The acicular, plate-like or flake-like particles are preferably contained in the elastomeric polymer in an oriented state, which orientation can be caused by a corresponding application of shear forces, as disclosed for example in DE 19639165C 2. Alternatively, the pigment particles can also be oriented via the application of an external field, such as the magnetic field disclosed in EP 1641624 and WO2008/046702 a 1. To achieve this, the pigment particles need to be responsive to a selected external field. Fig. 1 schematically shows the orientation of the pigment particles in the coating.

Preferred pigment particles for the practice of the present invention are selected from magnetic or magnetizable pigment particles.

The pigment is present in the elastomeric polymer at a concentration of 5 to 20 wt.%, preferably 10 to 15 wt.%.

In a most preferred embodiment, the pigment particles, preferably pigment flakes, are oriented approximately perpendicular with respect to the plane of the coating. The "vertical" pointer-like particles disclosed in the specification have a needle axis within 30 ° of the vertical of the plane, and the flake axis of the flake-like particles is within 30 ° of the plane of the film or coating.

Elastomeric polymers are obtained by polymerization of suitable precursor monomers (precursor monomers) or oligomers (oligomers). The coating composition is formed in a liquid or paste form by dispersing pigment particles and suitable additives in a polymerizable precursor. The coating composition is applied in the form of a thin film on a substrate, produced using suitable coating or printing techniques, if desired, and the defined orientation of the pigment particles. The applied coating composition is then cured (hardened) to produce an elastomeric material comprising pigment particles. The resulting film is useful as a color changing security device under pressure.

In a preferred embodiment, the surface of the piezochromic security device is additionally covered by an at least partially transparent protective film to prevent accidental mechanical damage. The preferred protective film is a transparent polymer foil. However, the protective film can also be any other form of protective coating, such as UV varnish or the like.

In other embodiments of the piezochromic security device, the elastic polymer film containing pigment particles is contained between two at least partially transparent protective films.

A particularly preferred embodiment relates to an optically variable piezochromic element wherein the pigment, preferably a magnetic, optically variable pigment, consists of non-transparent, reflective flakes having a thickness of the order of 1 μm and a planar extension of the order of 10-50 μm, the spectrally selective reflectivity (color) of which depends on the viewing angle relative to the plane of the flakes. In the specification, "optically variable" means having a color depending on a viewing angle or an incident angle.

The optically variable pigment flakes are preferably magnetic or magnetizable flakes, applied by an external magnetic field to allow orientation of the flakes in the coating composition (before hardening them into an elastic solid).

When moderate pressure is applied (e.g., tensile or shear forces, capable of being applied by a human finger) to a cured elastomeric composition containing optically variable flakes, within the elastomeric composition, the flakes are compressed to change their orientation, which results in a localized and highly visible color change. When the pressure (tensile or shear) is released, the flakes return to their previous position immediately, i.e. the color change depending on the pressure is rapid and fully reversible.

The effect of mechanical compression on the assembly of oriented pigment flakes contained in the elastomeric coating is shown in fig. 2: in the compressed position of the elastomeric coating, the pigment flakes adopt a lower angle towards the plane of the coating and thus exhibit enhanced specular reflection.

The effect of mechanical stretching on the assembly of oriented pigment flakes contained in the elastomeric coating is shown in fig. 4: in stretched elastomeric coatings, the flakes take a lower angle towards the coating plane and therefore show enhanced specular reflection.

In a preferred embodiment, the coating composition comprising the optically contrasting pigment particles is used as a security element on a substrate, which may be, for example, a document of value, a banknote, a certificate document, an access or financial card, or a label for tax purposes.

Preferably, the piezochromic security element is covered by an at least partially transparent polymer foil, which is preferably applied prior to the curing operation. This serves to protect the elastomeric coating from being inadvertently or intentionally scratched off. The foil may also be an overlay foil of an access or financial card, or ticket name, which may have the additional function of protecting sensitive information on these documents from tampering. The foil may also be part of a stamping foil assembly.

It will be apparent to those skilled in the art that depending on the application, additional layers may be required between the piezochromic security element and the above-mentioned polymer foil, for example to increase adhesion, to provide anti-adhesive properties or other technical and/or aesthetic purposes.

In a particularly preferred embodiment of the security device, the elastomeric coating composition comprising the flakes is contained between two polymeric foils, at least one of which is at least partially transparent. This allows a determined pressure to be applied from the 1 st side of the security device, e.g. by a human finger, while the resulting color change is observed from the 2 nd side of the security device, i.e. the foil/elastic coating/foil assembly.

The foil/elastic coating/foil assembly can be used on banknotes in the form of security threads, windows or a foil-on foil stamp. For use as a security thread, the foil assembly is cut into stretched strips, which are incorporated into security paper during papermaking, as is well known in the art. In order to observe the visible effect of pressure, the security thread need not be embedded entirely in the paper, but rather some parts are exposed, as is the case with ｃA windowed thread (see EP-A-0400902). For applications as windows, the foil assembly is used as a base layer for security documents with an opaque coating in which no window appears (see WO 98/13211); or, incorporated into the paper during the papermaking process, as is well known in the art (see EP- ｃA-0860298). For application as a stamping foil, the foil assembly is manufactured on a thin layer of a peelable carrier, and is preferably provided with a layer of a thermally activatable glue, as is well known in the art (see WO 92/00855).

Also disclosed is a method for producing a reversible piezochromic security element for the forgery-prevention of valuable documents, comprising the following steps:

a) providing a substrate;

b) applying a coating composition comprising an assembly of optically contrasting pigment particles in a liquid or paste polymerizable precursor monomer or oligomer onto at least a portion of a substrate;

c) the coating composition is cured to an elastomeric polymer.

In a preferred embodiment of the method, the optically variable flake pigments are magnetic or magnetizable pigments, and step b) comprises magnetic orientation of the above flake pigments in the coating with the aid of an external magnetic field.

The magnetic orientation is preferably implemented using a cliche of magnetized permanent magnetic material, such as disclosed in WO2005/002866 and WO 2008/046702.

The method may also comprise the additional step of covering the applied coating composition with an at least partially transparent polymer foil.

The substrate used in the method may further be an at least partially transparent polymer foil.

The security element according to the invention can be used for security documents or goods, such as documents of value, banknotes, certificates, access cards, financial cards, or labels for tax purposes or other purposes.

Also disclosed are security documents or articles, such as value documents, banknotes, certificate documents, access cards, financial cards, or labels for tax or other purposes, which carry the security element of the invention.

Detailed Description

Polymer and method of making same

Preferably, the polymeric binder used to contain the pigment is a high molecular weight elastomeric polymer that allows a fully reversible, elastic change in size under the influence of external pressure or force, such that at room temperature, the original size will recover immediately or almost instantaneously after the pressure or force is removed. Polymers that can be used as elastic adhesives to implement the piezochromic security element include, but are not limited to, highly flexible polymers such as natural or synthetic rubbers including styrene-butadiene copolymers (styrene-butadiene copolymers), acrylate latex systems (acrylate latex systems), polychloroprene (neoprene), nitrile rubber (nitrile rubber), butyl rubber (butyl rubber), polysulfide rubber (polysufide rubber), cis-1, 4-polyisoprene (cis-1, 4-polyisoprene), ethylene-propylene terpolymers (EPDM rubber), silicone rubber (silicone rubber), polyurethane rubber (polyurethane rubber), porous silicones (porous silicones) and other suitable polymers disclosed in the art.

In order to obtain the maximum visible effect on compression or stretching of the pigment-containing elastomeric polymer, it is advantageous to use non-spherical pigment particles, for example in the form of needles or flakes, in particular in order to produce an orientation of the pigment particles in the elastomeric binder matrix.

The positional orientation of the pigment particles in the elastic binder must then be fixed via curing of the binder, resulting in an elastic state being assumed. Fast curing systems are advantageous and UV-or EB (electron beam) curing coating compositions are quite advantageous as they allow the pigment particles to be fixed in situ immediately after the coating process.

However, thermally cured elastomeric polymer systems, such as 2-component silicones, can also be utilized, in which case, at an early stage of the thermal curing process, the orientation of the pigment particles must be maintained via an applied force, such as a magnetic field, until the polymer is sufficiently cured to maintain the pigment particles in the proper position and orientation.

Furthermore, it is advantageous to maintain a low solvent content of the coating composition for health and environmental reasons. Therefore, solvent-free formulations are the preferred choice.

Pigment incorporation

The pigment concentration in the coating composition is selected such that the maximum visible effect is produced under the application of moderate pressure (e.g., with a fingertip). In the case of flake pigments, such as the optically variable pigment flakes disclosed in US 4,838,648, the pigment concentration is selected so that the maximum surface coverage in the printed film is obtained if the coated flake particles are horizontally aligned, i.e., their largest surface is parallel to the substrate surface of the stamp. For maximum visible effect, the pigment particles are preferably oriented approximately perpendicular with respect to the plane of the substrate.

Flake-form particle thin-film optical interference pigments which can be used to carry out the invention have been disclosed in US 4,705,300, US 4,705,356, US 4,721,271 and other relevant documents.

Magneto-optically variable pigments which allow magnetic orientation of pigment particles by means of an external magnetic field have been disclosed in WO 02/073250, US 4,838,648, EP-A-686675, WO 03/00801 and US 6,838,166, which are incorporated herein by reference.

On the other hand, the pigment concentration should not be too high in order to allow the flake pigment to rotate, for example so that a good visual contrast is created between the compressed and relaxed states of the elastic polymer containing the flake pigment. The optimum concentration of flake pigment in the elastomeric polymer depends on the specific pigment properties, such as particle size and specific gravity, and the coating parameters, such as final coating thickness, and should therefore be determined by one skilled in the art to obtain the best visible effect in each application. The optimum pigment concentration is usually 1 to 30% by weight, in most cases 5 to 15% by weight, of the ink.

The average particle size and size distribution in a particular pigment batch has an impact on the achievable results. The size distribution needs to be as uniform as possible for optimum results with a rather large particle size (flake diameter 10-50 μm). However, the larger the flake diameter, the thicker the coating film must be to allow for vertical orientation of the pigment in the coating film.

The coating composition comprising the flake pigment particles is preferably applied to the surface of a hard substrate by liquid ink coating techniques, such as screen printing or bar coating. The final thickness of the applied and hardened coating is highly dependent on the pigment used, preferably on the order of 50 μm or more, in order to allow the pigment flakes to assume an easily rotatable vertical position.

The randomly oriented positions of the pigment flakes are substantially different from the alignment in the plane of the film or coating and exhibit some color change under pressure. However, pigment particles have the strongest color change when placed in the elastomeric polymer close to perpendicular with respect to the plane of the substrate. It is therefore not recommended to use a coating thickness much smaller than the diameter of the pigment flakes in this particular application.

Materials and techniques for orientation of magnetic particles in coating compositions, and corresponding printing processes, have been disclosed in US 2,418,479, US 2,570,856, US 3,791,864, DE 2006848-A, US 3,676,273, US 5,364,689, US 6,103,361, US 2004/0051297, US 2004/0009309, EP- ｃA-710508, WO 02/090002, WO 03/000801, WO2005/002866, US 2002/0160194, WO2006/061301, WO 2006/117271, WO 2007/131833, WO 2008/009569, WO2008/046702, which are hereby incorporated by reference.

The coating composition may also comprise other types of pigments and/or dyes, notably 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, and mixtures thereof. The coating composition may further comprise a forensic taggant, such as disclosed in EP-B-0927750.

The reversibly piezochromic security element of the present invention is further illustrated by the figures and the following non-limiting examples.

Drawings

Fig. 1 schematically depicts the arrangement of optically variable magnetic pigment flakes in an elastomeric coating with the aid of an external magnetic field.

Fig. 2 schematically depicts the cause of the optical effect (due to compression of the coating containing oriented flake pigments) resulting from elastic deformation.

Figure 3 illustrates the effect of finger pressure, as viewed through a coated glass plate, on the optical properties of a coating containing oriented optically variable magnetic pigments.

Fig. 4 schematically depicts the cause of the optical effect resulting from elastic deformation (due to stretching of the coating containing oriented flake pigments).

Fig. 5 illustrates the effect of elongation on the optical properties of a coating containing oriented optically variable magnetic pigments: a) no stretching; b) under tension.

FIG. 6 schematically depicts the use of the pressure sensitive coating of the present invention as a security element on an ID card.

Example 1: optically variable magnetic pigments in 2-component silicon elastomers (silicon elastomers)

The coating composition used to make the pressure sensitive optically variable security element of the present invention is formulated by dispersing optically variable magnetic pigment particles in a thermally curable solventless 2 component Silicone elastomer Sylgard 527 Primerless Silicone Dielectric Gel (Dow Corning).

The two components of Sylgard 527 were thoroughly mixed at room temperature in a weight ratio of 0.9: 1.1. Sylgard 527 gel contains components a and B in a set in separate containers. The two components are generally mixed in a weight ratio of 1: 1. A slightly harder gel may be obtained by increasing the ratio of part B to part a in the initial mixture.

Then, at a concentration of 10 wt%, a magnetic optically variable pigment (Flex products Inc.), Santa Rosa, Calif. "Green to blue", 5-layer design Cr/MgF was applied₂/Ni/MgF₂As disclosed in US 4,838,648) was dispersed in Sylgard 527 mixture and the pigmented coating composition was attached to a clear polymer foil (100 μm PVC, from Puetz-Folien) or glass plate (microscope slide) at a thickness of about 100 μm using a coating rod (hand coater).

The film thus obtained was pre-dried on a hot plate at 80 ℃ for 5 minutes to increase the viscosity of the Sylgard 527 adhesive. The pigment particles in the coating are then oriented approximately perpendicular to the plane of the substrate, using for example the "plastoferrite" magnet as disclosed in WO2008/046702 a 1. The resulting film appeared uniformly gray and partially transparent. The film was held on the magnet until the viscosity of the Sylgard binder was high enough to maintain the position and orientation of the pigment particles contained therein, and then cured in an oven at 150 ℃ for 30 minutes. The cured film is highly flexible and exhibits mechanical elastic properties. In order to protect the film thus obtained from mechanical damage (scratches), it may be covered with an adhesive foil which is itself transparent.

Upon compression of the elastic film between the finger tip and the substrate, a clear and fully reversible color change from dark gray to light green was observed from the back of the substrate (fig. 3).

Example 2: optically variable magnetic pigments in UV curable dielectric gels

The coating composition used to make the pressure sensitive optically variable security element of the present invention is formulated by dispersing optically variable magnetic pigment particles in a UV curable 1-component solvent-free silicone dielectric gel X3-6211 encapsulating agent (dow corning).

The same magnetic optically variable pigment as in example 1 was dispersed in silica gel X3-6211 at a concentration of 7.5% by weight, and the pigmented coating composition was attached to a transparent polymer foil (100 μm PVC from Puetz-Folien) or a glass plate (microscope slide) with a thickness of about 100 μm using a coating rod (hand coater).

The pigment particles in the X3-621 binder were then oriented to form an angle of approximately 60 ° relative to the plane of the substrate, using a magnet such as that disclosed in WO2008/046702 a1, and then dried in situ using a conventional UV radiation curing unit as is well known in the art.

The cured film is highly flexible and has elastomeric properties. In order to protect the film from mechanical damage, it may be covered with an adhesive foil which is transparent per se.

Upon compression of the elastic film between the fingertip and the glass plate, a reversible and clear change from dark gray to blue-green was observed.

Example 3: optical diffraction pigments in 2-component silicon elastomers

The coating composition used to make the pressure sensitive security element of the present invention was formulated by dispersing the aluminum flake pigment in a thermally curable solvent-free 2-component silicone elastomer Sylgard 527 primerless silicone resin dielectric gel (dow corning), as shown in example 1.

Spectra flair pigment silver 1500-20(FLEX product, JDSU, ca) was dispersed in Sylgard 527 mixture at a concentration of 8 wt% and the pigmented coating composition was attached to a glass plate (microscope slide) at a thickness of about 100 μm using a coating rod (hand coater).

The resulting film was cured in an oven at 150 ℃ for 30 minutes and then covered with an adhesive foil which was itself transparent. When the elastic film is compressed between the fingertips and the substrate, bright rainbow colors ranging from silver to multicolor are observed from the back surface of the substrate.

Example 4: stretching effect of elastomeric coatings containing oriented optically variable pigment flakes

The coating composition used to make the shear sensitive security element of the present invention was formulated by incorporating optically variable magnetic pigment particles into a UV curable 1-component solvent free silicone dielectric gel X3-6211 encapsulating agent (dow corning), as described in example 2.

A dispersion tape (a band soft dispersion) was attached to a transparent polymer foil (100 μm PVC from Puetz-Folien) with a thickness of about 100 μm using a coating rod (hand coater). After the pigment particles are approximately perpendicularly oriented with respect to the plane of the substrate, the film is partially dried by UV curing and then a second polymeric foil is placed on the surface of the film to form a sandwich-like arrangement. The elastomeric film is then further cured with UV. Fig. 5a illustrates an unstretched, oriented coating between two flexible substrates, which has a dull gray appearance. Fig. 5b shows the mechanical stretching effect on the coating of fig. 5 a: a clear and fully reversible color change from dark gray to light green is observed.

Example 5: examples of applications of optically variable magnetic pigments in UV curable dielectric gels

As shown in FIG. 6, the pressure-sensitive coating composition disclosed in example 2 can be used, for example, as a security element on an ID card. The manufacture of plastic cards typically involves 4 steps: i) plastic compounding/molding of the core sheet; ii) printing; iii) laminating; and iv) cutting/embossing. To obtain the double-sided pressure-sensitive properties, after molding step i), three circles were cut (as shown) into a core plastic sheet and filled with the pressure-sensitive coating composition prepared in example 2 above. After UV curing of the pressure sensitive coating, the core plastic sheet was laminated on both sides with a layer of transparent foil each. The card can also be manufactured in the usual way (printing, cutting, etc.).

When touched from the back side, the pressure sensitive element of this plastic card clearly showed a transition from dark to green when viewed from the front side. Alternatively, by means of the laminated cladding, the circle in the middle of the front face can be touched by the mechanical transmission of pressure resulting in a color change from dark to green in the peripheral 2 circles (when viewed from the front side).

The disclosed embodiments illustrate how a piezochromic security element is produced by orientation and fixing of flake-like pigment particles in a highly flexible and elastic-rich elastomeric polymer layer, which is preferably produced by application of a solvent-free, UV-curable precursor material. Depending on the thickness of the elastic polymer layer, optimum optical effects can be obtained with pigment concentrations of 5 to 15% by weight. The improved effect can be obtained with thicker films, however, the achievable film thickness is limited by the process factors of the printing process and by drying.

Other embodiments of the disclosed invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein.

Claims (30)

1. A security element for security against reversible piezochromic security elements for value documents, characterized in that it comprises an assembly of optically contrasting pigment particles in a film or coating of an elastic polymer,

wherein at least a portion of the pigment particles (i) are selected from acicular and platy or flake-like particles and (ii) are oriented in a position substantially different from the arrangement in the plane of the film or coating.

2. Security element according to claim i, wherein at least a part of the pigment particles are selected from thin-film interference pigment particles.

3. Security element according to claim 1 or 2, wherein at least a part of the pigment particles are selected from optically variable pigment particles.

4. Security element according to claim 1 or 2, wherein at least a part of the pigment particles comprise a fabry-perot reflector/dielectric layer/absorbing layer configuration.

5. Security element according to claim 1 or 2, wherein at least a part of the pigment particles are flakes having a diameter of 10-50 μm.

6. Security element according to claim 1 or 2, wherein at least a part of the pigment particles are selected from magnetic or magnetizable pigment particles.

7. Security element according to claim 1 or 2, wherein the pigment particles are present in the film or coating at a concentration of 5-25% by weight.

8. Security element according to claim 7, wherein the pigment particles are present in the film or coating in a concentration of 10-15% by weight.

9. Security element according to claim 7, wherein at least a part of the pigment particles are approximately perpendicularly oriented with respect to the plane of the substrate, such that the needle axis of the needle-shaped particles is within 30 ° of the perpendicular to the plane and the plate axis of the flake-shaped particles is within 30 ° of the plane of the film or coating.

10. Security element according to claim 1 or 2, wherein the elastic polymer is selected from highly flexible polymers selected from natural rubber, synthetic rubbers including styrene-butadiene copolymers, acrylate latex systems, polychloroprene, nitrile rubber, butyl rubber, polysulfide rubber, cis-l, 4-polyisoprene, ethylene-propylene terpolymers, silicone rubber, polyurethane rubber and porous silicone.

11. Security element according to claim 1 or 2, wherein the elastic polymer is selected from UV-cured and e-beam cured polymers.

12. Security element according to claim 1 or 2, wherein the elastic polymer is a two-component silicone elastomer or a one-component silicone dielectric gel.

13. Security element according to claim 1 or 2, wherein the film of the elastic polymer containing pigment particles is covered by an at least partially transparent protective film.

14. Security element according to claim 1 or 2, wherein a film of an elastic polymer containing pigment particles is included between two at least partially transparent protective films.

15. A coating composition for the manufacture of a reversibly piezochromic security element for the forgery-prevention of documents of value, comprising an assembly of optically contrasting pigment particles in a liquid or paste-like polymerizable precursor monomer or oligomer, at least a part of said pigment particles being selected from the group consisting of needle-like and plate-like or flake-like particles, the coating composition being capable of being cured to an elastic polymer.

16. The coating composition of claim 15, wherein at least a portion of the pigment particles are selected from thin film interference pigment particles.

17. Coating composition according to claim 15 or 16, wherein at least a part of the pigment particles are selected from optically variable pigment particles.

18. The coating composition of claim 15 or 16, wherein at least a portion of the pigment particles comprise a fabry-perot reflector/dielectric layer/absorber layer construction.

19. The coating composition of claim 15 or 16, wherein at least a portion of the pigment particles are platelets having a diameter of 10-50 μ ι η.

20. Coating composition according to claim 15 or 16, wherein at least a part of the pigment particles are selected from magnetic or magnetizable pigment particles.

21. A coating composition according to claim 15 or 16, wherein the pigment particles are present in a concentration of 5-20 wt%.

22. The coating composition of claim 21, wherein the pigment particles are present at a concentration of 10-15 wt%.

23. A method for manufacturing a security element according to any one of claims 1 to 14, the method comprising the steps of:

a) providing a substrate;

b) applying a coating composition comprising optically contrasting pigment particles in a liquid or paste polymerizable precursor monomer or oligomer onto at least a portion of a substrate, at least a portion of the pigment particles being selected from needle-like and plate-like or flake-like particles;

c) orienting the pigment particles; and

d) curing the coating composition into an elastomeric polymer.

24. The method of claim 23, wherein the optically variable flakes are inherently magnetic or magnetizable pigments, and step b) comprises magnetically orienting the flake pigments in the applied coating by using an external magnetic field.

25. The method of claim 24, wherein the magnetic orientation is performed using a clich é of magnetized permanent magnetic material.

26. The method of any one of claims 23-25, wherein the coating composition is covered by an at least partially transparent polymeric foil.

27. The method of any one of claims 23-25, wherein the substrate is an at least partially transparent polymer foil.

28. Use of a security element as claimed in any one of claims l to 14 for the protection of security documents or goods against forgery.

29. The use of claim 28, wherein the security document or article is selected from the group consisting of a value document, a banknote, a certificate document, an access card, a financial card, and a label for tax purposes.

30. A security document carrying a security element as claimed in any one of claims 1 to 14.