With the constantly improving quality of color photocopies and printings, and in an attempt to protect security documents such as banknotes, value documents or cards, transportation tickets or cards, tax banderols, and product labels that have no reproduceable effects against counterfeiting, falsifying or illegal reproduction, it has been the conventional practice to incorporate various security elements in these documents. Typical examples of security elements include security threads, windows, fibers, planchettes, foils, decals, holograms, watermarks, security inks comprising optically variable pigments, magnetic or magnetizable thin-film interference pigments, interference-coated particles, thermochromic pigments, photochromic pigments, luminescent, infrared-absorbing, ultraviolet-absorbing or magnetic compounds.
Colorshifting elements (also referred in the art as optically variable elements or goniochromatic elements) exhibit a viewing-angle or incidence-angle dependent color, and are used to protect banknotes and other security documents against counterfeiting and/or illegal reproduction by commonly available color scanning, printing and copying office equipment.
Materials having a liquid crystal structure with a chiral phase, also known as cholesteric liquid crystal materials, are known and used as optically variable elements.
Cholesteric liquid crystal polymers show a molecular order in the form of a helical super-structure perpendicular to the longitudinal axes of its molecules. The helical superstructure provides for a periodic refractive index modulation throughout the liquid crystal material, which in turn results in a selective transmission/reflection of determined wavelengths of light (interference filter effect). Cholesteric liquid crystal polymers can be obtained by subjecting one or more crosslinkable substances (nematic compounds) with a chiral phase to alignment and orientation. The particular situation of the helical molecular arrangement leads to cholesteric liquid crystal materials exhibiting the property of reflecting a circularly polarized light component within a determined wavelength range, wherein said circularly polarized light may be left-handed or right-handed, depending on the sense of rotation of the molecular helices. The range of wavelengths reflected by a cholesteric liquid crystal polymer is determined by the geometry of its periodic refractive index modulation, i.e. the pitch of the molecular helices, as known to the skilled man. The pitch (i.e. the distance over which a full rotation of 360° of the helical arrangement is completed) can be tuned in particular by varying selectable factors including the temperature and solvents concentration, by changing the nature of the chiral component(s) and the ratio of nematic and chiral compounds. The pitch of the material can finally be frozen by a cross-linking (polymerization) reaction, such that the color of the resulting cholesteric liquid crystal polymer is no longer depending on external factors such as the temperature.
The range of colorshifts of cholesteric liquid crystal polymers available for security documents might be limited, and with the aim of increasing the color gamut of cholesteric liquid crystal polymers, it can be of advantage to modify the reflection characteristics and thereby increasing the range of available colors.
As mentioned above, the reflected light from a cholesteric liquid crystal polymer depends on the pitch of its helical structure and is thereby dependent on the nature and the concentration of the chiral component(s). In particular, adding an appropriate quantity of the chiral component(s) to the precursor mixture results in a colorshift modification to shorter wavelengths. However, it is neither straightforward nor cost-effective to customize the cholesteric liquid crystal material during manufacture for each specific application and for each desired colorshift effect. Moreover, the fine-tuning of colorshift properties of cholesteric liquid crystal polymers is a delicate step often resulting in an important waste of time and/or material.
US 2007/0224341 discloses a method in which a chiral liquid-crystal layer is brought into contact with an extraction medium (extractant) by coating or printing methods in such a way that diffusion of substances out of the liquid-crystal layer into the extractant occurs. The disclosed method comprises the steps of i) applying a first layer of a polymerizable or curable chiral liquid crystal material to a support, ii) partially or completely polymerize or cure the first layer of the polymerizable chiral liquid-crystal material, iii) apply at least one further layer of one or more extraction media to the partially or fully polymerized or cured first liquid-crystal layer, and, iv) where appropriate, completely polymerize or cure the first liquid-crystal layer and/or one or more of the further layers.
WO 2007/138255 A1 discloses a method of forming a customizable security device comprising the steps of forming on a substrate a liquid crystal layer, an at least partially absorbing layer overlapping with at least a part of one side of the liquid crystal layer, and at least one customizing region overlapping at least a part of an opposite side of the liquid crystal layer in selected regions. The customizing region modifies the colorshifting properties of the liquid crystal layer, and this modification is apparent to the observer as a change in the angle of view at which the different colors are observed.
U.S. Pat. No. 7,033,653 discloses a birefringent marking comprising a liquid crystal material having discrete regions with different thickness, said marking being prepared by applying droplets of a solution of a liquid crystal material to a substrate and allowing the solvent to evaporate. However, the disclosed method may suffer from a poor reproducibility since the thickness of the liquid crystal layer can be easily altered.
WO 2012/076533 A1 discloses a method of changing a position of a selective reflection band exhibited by a chiral liquid crystal precursor composition in a cured state. The disclosed method comprises a step of incorporating in the precursor composition at least one salt that is capable of changing a position of a selective reflection band exhibited by the precursor composition in a cured state.
WO 2012/076534 A1 discloses a method changing a position of a selective reflection band exhibited by a cured chiral liquid crystal precursor composition comprising (i) one or more nematic compounds, (ii) one or more chiral dopant compounds which are capable of giving rise to a cholesteric state of the cured composition, and (iii) at least one salt that changes a position of a selective reflection band exhibited by the cured composition compared to a position of a selective reflection band exhibited by a cured composition that does not contain the at least one salt. The disclosed method comprises a step of contacting the composition with a modifying resin which is made from one or more polymerizable monomers, at least one of the monomers comprising a heteroatom selected from O, N, and S, and is capable of changing a position of a selective reflection band exhibited by the cured chiral liquid crystal precursor composition comprising the at least one salt.
US 2013/0029169 discloses a method of producing multicolored coatings on substrates using two different liquid crystalline coating compositions, said multicolored coating exhibiting at least three different colors. The disclosed method consists of coating a first coating composition containing a first polymerizable nematic liquid crystal material onto a substrate and subsequently coating a second coating composition containing a second polymerizable nematic liquid crystal material onto the substrate while the first coating composition is still in an unpolymerized state, wherein the first and the second coating composition overlap in at least one defined area, followed by polymerization of the resulting coating.
Therefore, there remains a need for a method for producing multi-colored optically variable security feature based on cholesteric liquid crystal polymers in an easy, economic, improved, predictable, reproduceable and controlled manner.