Optical effect pigment

Abstract

Optical effect pigment comprising a plurality of layers and a magnetic element, the layers can be arranged in two stacks of asymmetric layers or in a single stack of layers and comprise at least an absorber layer and at least a dielectric layer and can further comprise a reflector layer. The magnetic element presents a magnetisation which is out-of-plane, i.e. predominantly perpendicular to the plane of the pigment, which allows a deposition on the printing substrate whereby the face of the pigment lying up or down on the substrate can be predetermined. Such effect pigment has applications in many fields and specifically in security printing, where due to controlled deposition for instance a double-stack pigment will produce a different optical effect on each of its faces.

Claims

1. An optical effect pigment comprising a plurality of layers and a magnetic element, the plurality of layers comprising at least an absorber layer and at least a dielectric layer, a plane of the effect pigment being defined by a boundary between the absorber and dielectric layers, wherein the magnetic element has an out-of-plane magnetisation with respect to the plane of the effect pigment.

2. The optical effect pigment according to claim 1, further comprising at least two reflector layers, wherein the plurality of layers are arranged in two stacks of layers, the two stacks of layers being asymmetrical and having the magnetic element located in between them.

3. The optical effect pigment according to claim 2, further comprising a luminescent material incorporated to both stacks of layers, wherein the luminescent material incorporated to one stack of layers is different form the luminescent material incorporated to the other stack of layers.

4. The optical effect pigment according to claim 3, wherein the luminescent material is incorporated to the dielectric layers.

5. The optical effect pigment according to claim 3, wherein the luminescent material forms an additional layer on each of the stacks of layers.

6. The optical effect pigment according to claim 1, further comprising at least a reflector layer, wherein the plurality of layers are arranged in a single stack of layers and the magnetic element is located next to the at least one reflector layer.

7. The optical effect pigment according to claim 1, wherein the plurality of layers are arranged in a single stack of layers comprising at least the absorber layer the dielectric layer and wherein the magnetic element functions also as a reflector layer.

8. The optical effect pigment according to claim 1, wherein the plurality of layers are arranged in two stacks of layers, one stack of layers comprising at least the absorber layer and the dielectric layer, the other stack of layers comprising at least another absorber layer and another dielectric layer and wherein the magnetic element functions also as a reflector layer for both stacks of layers.

9. The optical effect pigment according to claim 1, wherein the magnetic element consists of one magnetic layer.

10. The optical effect pigment according to claim 1, wherein the magnetic element consists of a plurality of magnetic layers with an out-of-plane magnetisation.

11. The optical effect pigment according to claim 2, wherein the magnetic element is made from an alloy of cobalt with either platinum or chromium.

12. The optical effect pigment according to claim 1, wherein the absorber layer is selected from the group of chromium, aluminum, nickel, silver, copper, palladium, platinum, titanium, vanadium, cobalt, iron, tin, tungsten, molybdenum, rhodium and niobium and their corresponding oxides, sulphides, and carbides; carbon, graphite, silicon, germanium, cerment and ferric oxide and combinations thereof.

13. The optical effect pigment according to claim 1, wherein the dielectric layer is selected from the group of zinc sulphide, zinc oxide, zirconium oxide, titanium dioxide, diamond-like carbon, indium oxide, indium-tin-oxide, tantalum pentoxide, ceric oxide, yttrium oxide, europium oxide, iron oxides, hafnium nitride, hafnium carbide, hafnium oxide, lanthanum oxide, magnesium oxide, neodymium oxide, praseodymium oxide, samarium oxide, antimony trioxide, silicon monoxide, selenium trioxide, tin oxide, tungsten trioxide, silicon dioxide, aluminum oxide and metal fluorides and combinations thereof.

14. The optical effect pigment according to claim 2, wherein the at least two reflector layers are selected from the group of aluminum, silver, copper, gold, platinum, tin, titanium, palladium, nickel, cobalt, rhodium, niobium, chromium, iridium, and combinations thereof.

15. An ink comprising a carrier and a pigment according to claim 1.

16. The optical effect pigment according to claim 6, wherein the magnetic element is made form an alloy of cobalt with either platinum or chromium.

17. The optical effect pigment according to claim 6, wherein the reflector layer is selected from the group of aluminum, silver, copper, gold, platinum, tin, titanium, palladium, nickel, cobalt, rhodium, niobium, chromium, iridium, and combinations thereof.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1: Effect pigment with in-plane magnetisation in different stages of rotation around a horizontal magnetic field.

(2) FIG. 2: Effect pigment with in-plane magnetisation in different stages of rotation around a vertical magnetic field.

(3) FIG. 3: Effect pigment with out-of-plane magnetisation in different stages of rotation around a horizontal magnetic field.

(4) FIG. 4: Effect pigment with out-of-plane magnetisation in different stages of rotation around a vertical magnetic field.

(5) FIG. 5: Effect pigments with in-plane magnetisation over substrate under curved magnetic field.

(6) FIG. 6: Effect pigments with out-of-plane magnetisation over substrate under radial magnetic field.

(7) FIG. 7: Effect pigments with out-of-plane magnetisation over a substrate under a curved magnetic field.

(8) FIG. 8: Effect pigments with out-of-plane magnetisation over a substrate under a polarized magnetic field.

(9) FIG. 9: Effect pigment according to a first embodiment of the invention.

(10) FIG. 10: Effect pigment according to a second embodiment of the invention.

(11) FIG. 11: Effect pigment according to a third embodiment of the invention.

(12) FIG. 12: Effect pigment according to a fourth embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

(13) With reference to FIG. 9, according to a first preferred embodiment, the effect pigment with magnetic element comprises a first (31) and a second (32) stacks of layers and a magnetic layer (33), said magnetic layer (33) located in between first (31) and second (32) stacks of layers and having an out-of-plane magnetisation with respect to the plane of the pigment.

(14) The first stack of layers (31) comprises at least an absorber layer (34a), at least a dielectric layer (35a) and at least a reflector layer (36a). The second stack of layers (32) comprises at least a reflector layer (36b), at least a dielectric layer (35b) and at least an absorber layer (34b). The first stack of layers (31) has a different configuration than the second stack of layers (32), in that at least one of the layers (34a, 34b, 35a, 35b, 36a, 36b) is different from its counterpart, e.g. it is made of a different material or it has a different thickness or a different refractive index; or in that first (31) and second (32) stacks of layers have a different number of layers, producing in any event a double optical effect.

(15) The absorber layers (34a, 34b) are made up of metallic absorbers, including chromium, aluminum, nickel, silver, copper, palladium, platinum, titanium, vanadium, cobalt, iron, tin, tungsten, molybdenum, rhodium and niobium, as well as their corresponding oxides, sulphides, and carbides. Other suitable absorber materials include carbon, graphite, silicon, germanium, cermet, ferric oxide or other metal oxides, metals mixed in a dielectric matrix, and other substances that are capable of acting as a nonselective or selective absorber in the visible spectrum. Various combinations, mixtures, compounds, or alloys of the above absorber materials, known to the skilled in the art, may be used to form the absorber layers. In this embodiment, the absorber layer preferably has a thickness of 2 to 40 nm, more preferably of 3 to 30 nm and yet more preferably of 3.5 to 15 nm, these ranges being adequate for all embodiments herein described.

(16) The dielectric layers (35a, 35b) are made up of high refractive index materials, including zinc sulphide, zinc oxide, zirconium oxide (ZrO.sub.2), titanium dioxide (TiO.sub.2), diamond-like carbon, indium oxide (In.sub.2O.sub.3), indium-tin-oxide (ITO), tantalum pentoxide (Ta.sub.2O.sub.5), ceric oxide (CeO.sub.2), yttrium oxide (Y.sub.2O.sub.3), europium oxide (Eu.sub.2O.sub.3), iron oxides such as (II)diiron(III) oxide (Fe.sub.3O.sub.4) and ferric oxide (Fe.sub.2O.sub.3), hafnium nitride (HfN), hafnium carbide (HfC), hafnium oxide (HfO.sub.2), lanthanum oxide (La.sub.2O.sub.3), magnesium oxide (MgO), neodymium oxide (Nd.sub.2O.sub.3), praseodymium oxide (Pr.sub.6O.sub.11), samarium oxide (Sm.sub.2O.sub.3), antimony trioxide (Sb.sub.2O.sub.3), silicon monoxide (SiO), selenium trioxide (Se.sub.2O.sub.3), tin oxide (SnO.sub.2), tungsten trioxide (WO.sub.3), and combinations of those materials. Also, said dielectric layers (35a, 35b) can be made up of low refractive index materials, including silicon dioxide (SiO.sub.2), aluminum oxide (Al.sub.2O.sub.3), metal fluorides such as magnesium fluoride (MgF.sub.2), aluminum fluoride (AlF.sub.3), cerium fluoride (CeF.sub.3), lanthanum fluoride (LaF.sub.3), sodium aluminum fluorides (e.g., Na.sub.3AlF.sub.6, Na.sub.6Al.sub.3F.sub.14), neodymium fluoride (NdF.sub.3), samarium fluoride (SmF.sub.3), barium fluoride (BaF.sub.2), calcium fluoride (CaF.sub.2), lithium fluoride (LiF), combinations thereof, or any other low index material having an index of refraction of about 1.65 or less. For example, organic monomers and polymers can be utilized as low index materials, including dienes or alkenes such as acrylates (e.g., methacrylate), perfluoroalkenes, polytetrafluoroethylene (Teflon), fluorinated ethylene propylene (FEP) or combinations thereof. The thickness of the dielectric layer determines the effect pigment colour and is of the order of 200 to 800 nm.

(17) The reflector layers (36a, 36b, 41) can be made up of a variety of reflective materials, including aluminum, silver, copper, gold, platinum, tin, titanium, palladium, nickel, cobalt, rhodium, niobium, chromium, iridium, and combinations or alloys thereof. Appropriate thickness is preferably from 10 to 2000 nm, more preferably from 20 to 1000 nm and yet more preferably from 50 to 100 nm, these ranges being adequate for first and second embodiments described.

(18) It will be clear to the person skilled in the art that variations of materials and/or variations of thickness, all within the acceptable ranges described, with respect to the material or thickness of its counterpart layer; and variations of refractive index in one of the layers, or a variation in the number of layers in one of the stacks (31, 32), will entail an asymmetrical layer structure and therefore the optical effect produced by the two stacks of layers (31, 32) will be different.

(19) With respect to the magnetic layer (33) with out-of-plane magnetisation, its composition is Cobalt-based, because due to this mineral crystal structure, it is highly suitable to make thin layers with a predominantly perpendicular easy axis. To increase the magneto crystalline anisotropy constants, cobalt is allied with Platinum or chromium. CoPt and CoCr monolayer and multilayer structures can be used, the monolayer structure being preferable. Stoichiometry of said alloys are: Co.sub.75Pt.sub.25 and Co.sub.90Cr.sub.10 because these proportions optimize the out-of-plane anisotropy of the layer. The thickness of the magnetic layer (33) is preferably from 20 to 1000 nm, more preferably from 30 to 150 nm and yet more preferably from 50 to 100 nm, these ranges being adequate for the first and second embodiments.

(20) In a variant of the first embodiment, the effect pigment comprises a luminescent material which is added to the at least two dielectric layers (35a, 35b). Suitable luminescent materials are disclosed in WO 02/040599 A (FLEX PRODUCTS INC.) 23 May 2002. Addition of the luminescent material is made by the same deposition process that will be described hereinafter, by including the luminescent material together with the dielectric material in the target employed for the deposition. In a further variant, the luminescent material can be incorporated in the form of a luminescent layer added to each of the stacks of layers (31,32). Suitable materials for these layers are the same described in above-mentioned WO 02040599 A. The luminescent material is added according to the same process applicable to the other layers which will be referred hereinafter. The luminescent response may or may not be in the visible spectrum. In the latter case, the response must be detected using an appropriate sensor. A key feature of the invention is that the luminescent material incorporated to the respective stacks of layers (31, 32) is different, thereby the pigment will show a double optical effect in the form of a different luminescent response on each of its sides.

(21) With reference to FIG. 10, according to a second preferred embodiment, the effect pigment with magnetic element comprises a stack of layers (37) and a magnetic layer (38), said magnetic layer having perpendicular magnetisation. The stack of layers (37) comprises an absorber layer (39), a dielectric layer (40) and a reflector layer (41), the magnetic layer (38) being located next to the reflector layer (41). The material composition and thickness of the layers making up the stack of layers (37) and the magnetic layer (38) in this second preferred embodiment are the same as described for the first embodiment.

(22) With reference to FIG. 11, according to a third preferred embodiment of the invention, the effect pigment with magnetic element comprises a stack of layers (42) comprising an absorber layer (43) and a dielectric layer (44), with the same features as described above. The stack of layers (42) does not comprise a reflector layer. The magnetic element is a layer (45) made of Al2O3 containing magnetic nanoparticles. Due to the aluminium-based composition, this element also works as reflector layer. To achieve a perpendicular anisotropy, said magnetic layer (45) contains embedded nickel particles. Approximate nickel particle size is 20 nm. The thickness of the magnetic layer (45) is preferably from 10 to 2000 nm, more preferably from 20 to 1000 nm and more preferably from 50 to 150 nm.

(23) With reference to FIG. 12, according to a fourth preferred embodiment of the invention, the effect pigment with magnetic element comprises a first stack of layers (46) and a second stack of layers (47), each of them comprising an absorber layer (48a, 48b) and a dielectric layer (49a, 49b), with the same features as described above and neither of them comprising a reflector layer. The magnetic element is a layer (50) made of Al2O3 containing magnetic nanoparticles. Due to the aluminium-based composition, this element also works as reflector layer. To achieve a perpendicular anisotropy, said magnetic layer (50) contains embedded nickel particles. Approximate nickel particle size is 20 nm. The thickness of the magnetic layer (50) is preferably from 10 to 2000 nm, more preferably from 20 to 1000 nm and more preferably from 50 to 150 nm.

(24) Claimed pigments conforming to the described embodiments are manufactured by deposition of successive layers' materials onto a carrier substrate, according to the known technique of physical-vapor-deposition (PVD). The carrier is preferably a flexible web, e.g. a release-coated polyethylene there-phthalate (PET) foil. The vapor-deposition can be carried out as a roll-to-roll process in a high vacuum coater. The materials are evaporated using material-specific, appropriate evaporation sources and processes known to the skilled person, such as sputtering, reactive sputtering, magnetron sputtering, thermal evaporation, electron-beam, laser-beam assisted evaporation or ion-beam evaporation.

(25) Magnetic layers (33, 38) made of CoPt alloy as incorporated to the first and second embodiments are obtained by electron beam co-evaporation, a technique that can also be employed to produce the stack (37) or stacks (31, 32) of layers. The composition of the magnetic layer (33, 38) is controlled by changing the deposition rate of Co, while deposition rate of Pt is held at 0.05 nm/sec. The base pressure of the chamber must be approximately 5×10.sup.−9 Torr prior to evaporation and well below 5×10.sup.−7 Torr during evaporation. According to this process, layers of Co.sub.75Pt.sub.25 deposited onto Al.sub.2O.sub.3 substrates held at temperatures from 180° C. to 400° C. exhibit a strong perpendicular magnetic anisotropy of 1.5×10.sup.7 erg/cm3; as described in YAMADA, et al. Magnetic properties of electron beam evaporated CoPt alloy thin films. IEEE TRANSACTIONS ON MAGNETICS. September 1997, vol. 33, no. 5, p. 3622-3624.

(26) Magnetic layers (33, 38) made of CoCr alloy as incorporated to the first and second embodiments are obtained by a process whereby both elements are co-deposited by RF sputtering, from a cobalt target on which a number of electrolytic chromium pellets are placed at regular intervals in a grid pattern. The composition of the layer is controlled by changing the surface area of the chromium pellets. An alloy target of CoCr can also be used for RF sputtering. The RF sputtering is carried out in an Argon gas atmosphere after baking the vacuum chamber and the substrate holder at about 300° C. The background pressure was kept under 2×10.sup.−7 Torr. The thickness of the layer is controlled by the sputtering time. The deposition rate is mainly influenced by the RF power density and the Argon pressure. An acceptable deposition rate is 0.33 micron/hour, the Argon pressure is 0.01 Torr and the RF power density is 0.44 watt/cm.sup.2. This process is described at IWASAKI, et al. Co-Cr recording films with perpendicular magnetic anisotropy. IEEE TRANSACTIONS ON MAGNETICS. September 1978, vol. MAG-14, no. 5, p. 849-851.

(27) Magnetic layers (45) with reflector properties as incorporated to the third embodiment are obtained by sol-gel techniques described as follows in KRAUS, et al. Synthesis and magnetic properties of Ni-Al2o3 thin films. J. appl. phys. 1997, no. 82, p. 1189-1195.: sol-gel layers are deposited from NiAl2O4 spinel precursors derived by mixing stoichiometric quantities of solutions prepared from nickel 2-ethylhexanoate and aluminum tri-sec-butoxide in 2-methoxyethanol. The nickel solution is prepared by mixing nickel 2-ethylhexanoate with 2-methoxyethanol in a molar ratio of 1:5, refluxing at 140° C. for 12 h, centrifuging, and decanting to produce a 0.6M solution. In a separate flask, aluminum tri-sec-butoxide is dissolved in 2-methoxyethanol in a molar ratio of 1:10 and refluxed for 30 min at 140° C. The volume is reduced by distillation at a temperature of 140° C. and 200 mm Hg. Acetic acid is then added to the aluminum precursor in a molar ratio of 7:1. This solution is stirred at 120° C. until clear, and cooled to room temperature. Magnetic layers (45) are produced by spin casting a 0.4M NiAl2O4 precursor solution at 3000 rpm onto (100) Si wafers, (1102) Al2O3 electronic grade substrates, or polished fused silica plates. Layers of various thicknesses are formed by the successive application and drying of the precursor solution. As-deposited films are converted to spinel by heating at 1200° C. in air for 5 min. Once formed, the spinel is reduced to Ni+Al2O3 in hydrogen (low pO.sub.2) using a Kidd Electronics rapid thermal annealed (RTA). The RTA is purged three times with 99.99% hydrogen and the reduction is carried out at 950° C. using a heating rate of 50° C./s, for 5 min in 200 cc/min flowing H.sub.2.