Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2002-08-14
2004-02-24
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C428S690000, C252S301160, C252S30140R, C252S301360, C252S301350
Reexamination Certificate
active
06695905
ABSTRACT:
The present invention relates to pigments having a viewing angle dependent shift of color, method for producing said pigments, use of said pigments in security applications, coating composition and bulk materials comprising said pigments and a detecting device for excitation and reading said pigments.
Pigments having a viewing angle dependent shift of color, the so called optically variable pigments, have made their proof as an efficient, printable anti-copy device on bank notes and security documents since 1987. Today, a large part of the world-wide printed currency relies on optically variable copy protection devices, and among these latter, optically variable ink (OVI™) has acquired a preeminent place.
The viewing-angle dependent shift of color cannot be reproduced by color copying equipment. Various different types of OVP materials are commercially available today, all depend on interference thin film structures. The hue, the color travel and the chromaticity of the structures however depend upon on the material constituting the layers, the sequence and the number of layers, the layer thickness as well as on the production process.
Very brilliant colors are obtained with a first type of OVP, made by physical vapor deposition according to e.g. U.S. Pat. Nos. 4,705,300; 4,705,356; 4,721,217; 4,779,898; 4,930,866; 5,084,351 and related. This OVP is constructed as a thin-film vapor-deposited Fabry-Perot resonator stack. Simple-sandwich metal-dielectric-metal, as well as double-sandwich metal-dielectric-metal-dielectric-metal layer sequences are described. The middle metal layer can be realized as opaque totally reflecting layer to yield a maximum in reflectivity of the incident light. The top metal layer(s) must be partially transparent, such that light can be coupled in and out of the Fabry-Perot resonator.
Incident light falling upon an optically variable pigment flake of said metal-dielectric-metal type is partially reflected at the top metal layer. Another part of the light travels through the dielectric and is reflected at the bottom metal layer. Both reflected parts of the incident light finally recombine and interfere with each other. Constructive or destructive interference results, depending on the thickness of the dielectric layer and on the wavelength of the incident light. In the case of white incident light, some of the light components, having determined wavelengths, are reflected, whereas other components, having other wavelengths, are not reflected. This gives rise to a spectral selection, and hence to the appearance of color.
The path difference between the top-reflected and the bottom-reflected part of the light depends noteworthy on the angle of incidence, and so does the resulting interference color.
Another, second type of OVP, made according to EP 708,154; DE 195,25,503; U.S. Pat. Nos. 5,624,468, 5,401,306; 4,978,394; 4,344,987 and related, is based on coated aluminum flakes. Mechanically flattened aluminum particles are coated by chemical vapor deposition (CVD) or by wet chemical methods with a dielectric layer and a subsequent metal or second dielectric layer. Interference colors result by the same effect as described above.
This type of OVP is cheaper in manufacture than the first type, but it also exhibits less brilliant colors and less angle-dependent color shift than the first type.
Still another, third type of OVP is based on liquid crystal pigments. Such pigments are, for example, made according to EP 601,483; EP 686,674 and related, is based on polymerized cholesteric liquid crystal (LC) phases. Cholesteric LC phases exhibit a helical arrangement of molecules, resulting in a periodic variation of the material's index of refraction along the direction perpendicular to the surface. This, in turn, has a similar effect on light scattering/light transmission as a Fabry-Perot interference stack. Due to the helical arrangement of the cholesteric LC phases, light of one circular polarization is reflected in preference, whereas the other circular polarization component is transmitted in preference and must be absorbed by a dark background. This type of OVP shows less bright colors than metal-reflector based OVP. Its color shifting properties are excellent, however, due to the rather low index of refraction of the organic material.
A fourth type of OVP, based on coated mica flakes, is described in U.S. Pat. Nos. 3,874,890; 3,926,659; 4,086,100; 4,323,554; 4,565,581; 4,744,832; 4,867,793; 5,302,199; 5,350,448; 5,693,134 and related. A high-refractive material, e.g. TiO
2
, applied by wet chemical methods or by CVD, is used for the coating and acts as a partially reflecting surface on both sides of the mica flake. The mica has the role of the dielectric. Only faint colors and weak color shifting properties are obtained with this type of OVP, which is also known as “iridescent pigment”.
A fifth type of OVP is an all-polymer multi-layer light reflector/transmitter foil according to U.S. Pat. No. 3,711,176 (cf. W. J. Schrenk et al. “Critical Reviews of Optical Science and Technology”, CR39, 1997, p 35-49). This foil, too, is an interference device which exhibits angle-dependent spectral reflection and transmission properties, and could be used for the manufacturing of a fifth type of optically variable pigment.
Large amounts of optically variable pigment are produced for merely decorative purposes (automotive paints, lacquers and the like), and are thus available to the common public in the form of paints and sprays. The security potential of optically variable ink features on bank notes is considerably decreased if no distinction can be made between “security OVP” and “decorative OVP”. A counterfeiter could noteworthy reproduce bank notes on a color copier and add the missing optically variable features with the help of commercially available decorative paints or sprays.
It is an object of the present invention to overcome the drawbacks of the prior art.
In particular it is an object to provide any kind of optically variable pigments (OVP) which comprise—apart from the viewing angle dependent color shift—additional features resulting in an response upon external energy.
It is a further object to make “security OVP” materially different from “decorative OVP” while retaining good color shifting properties.
A further object consists in providing “security OVP” with means for an easy and reliable distinction in particular from “decorative OVP”.
It is a further object to provide OVP which can be authenticated with the help of a simple device, as well as machine-authenticated at low and at high speed.
It is a further object to provide methods for manufacturing “security OVP”, in particular by using the same equipment and process as are used for the production of decorative OVP without significantly increasing the production cost.
These objects are solved by the features of the independent claims.
Particularly the objects are solved by pigments comprising an interference structure of at least two thin film layers of different materials said pigments having a viewing angle dependent shift of color and at least one of said layers comprises at least one luminescent material.
In a first embodiment the OVP has a structure comprising at least one light-transmitting dielectric layer with a first and a second surface essentially parallel to each other and at least one semi-transparent, partially reflecting layer arranged on each of said first and second surfaces of the dielectric layer with the luminescent material being comprised in at least one of the dielectric layers.
In a second embodiment the OVP has a structure comprising an opaque totally reflecting layer having first and second surfaces essentially parallel to each other and at least one sequence arranged on at least one of said first and second surfaces of the opaque totally reflecting layer, said sequence comprises at least one dielectric layer and at least one semi-transparent partially reflecting layer with the dielectric layer of said sequence being adjacent to the totally reflecting layer and the lumines
Müller Edgar
Rozumek Olivier
Shoemaker and Mattare
SICPA Holding S.A.
LandOfFree
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