Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2001-02-20
2003-02-18
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S402000, C106S403000, C106S415000, C428S402000, C428S403000, C428S404000, C428S042100
Reexamination Certificate
active
06521036
ABSTRACT:
The invention relates to optically variable pigments providing a color shift between two distinct colours, to methods for producing the pigments, to an optically variable coating composition comprising such pigments and to a method for producing indica comprising such pigments according to the preamble of the independent claims.
Optically variable pigments having a color shift between two distinct colours with the color shift being dependent on the viewing angle, are well known. Throughout this specification the expression “optically variable” stands for such a type of optical characteristic. The production of these pigments, their use and their characteristic features, are described in various publications and patents, e.g. L. Schmidt, N. Mronga, W.Radtke, O. Seeger, “Lustre pigments with optically variable properties”, European Coatings Journal 7-8/1997, patents U.S. Pat. No. 4,434,010, U.S. Pat. No. 5,059,245, U.S. Pat. No. 5,084,351, U.S. Pat. No. 5,135,812, U.S. Pat. No. 5,171,363, U.S. Pat. No. 5,571,624, EP 0341002, EP 0736073, EP 668 329 and EP 0 741 170.
Optically variable pigments having a viewing angle dependent shift of color are based on a stack of superposed thin-film layers with different optical characteristics. The hue, the amount of color-shifting and the chromaticity of such thin-film structures depend among others on the material constituting the layers, the sequence and the number of layers, the layer thickness as well as on the production process.
Generally, optically variable pigments comprise an opaque, totally reflecting layer, a dielectric layer of a low refractive index material, i.e. with an index of refraction of 1.65 or less, deposited on top of the opaque, totally reflecting layer and a semi-transparent, partially reflecting layer applied on the dielectric layer.
The combination of a dielectric layer and a semi-transparent partially reflecting layer may be regarded as a sequence which can be repeatedly applied.
Throughout this specification the terms opaque, totally and partially-reflecting, semi-transparent and transparent relate to electromagnetic radiation in the visible range of the electromagnetic spectrum, i.e. from approximately 400 to 700 nm.
The semi-transparent, partially reflecting layer can be made of metals (aluminum, chromium) or of high-refractive materials, e.g. certain metal-oxides or metal-sulphides (MoS
2
, Fe
2
O
3
etc). Preferred materials for the dielectric layer are SiO
2
and MgF
2
. Usually the thickness of the semi-transparent, partially reflecting layer is between 5 and 25 nm, whereas the thickness of the dielectric layer depends on the colours desired. Typically it is in the range of between 200 and 500 nm. The opaque, totally reflecting layer is preferably of aluminum. Gold, copper, silver, and cobalt-nickel alloys have been alternatively proposed as materials for the opaque, totally reflecting layer. Generally, the degree of opaqueness is a function of the layer thickness. Aluminum becomes opaque at approximately 35-40 nm thickness. Therefore, a typical range of the thickness of the totally reflecting aluminum layer is between 50-150 nm.
Optically variable pigments can be produced by a number of different processes; two of them have gained major importance. In the first of them a continuous sheet of an optically variable thin-film structure is produced and ground to pigments, in the second, suitable precursor particles, e.g. Aluminum flakes, are coated with an optically variable thin-film structure.
In the first type of production process, the various layers are deposited successively on top of each other by physical vapor deposition processes to form a continuous sheet of a multilayer thin-film stack onto a flexible carrier. The carrier is preferably a web of a PET or similar polymer foil.
The resultant multilayer thin-film structure comprises an opaque, totally reflecting layer with first and second surfaces, a dielectric layer deposited on top of the opaque, totally reflecting layer and a semi-transparent, partially reflecting layer applied on top of the dielectric layer. The sequence dielectric layer/partially reflecting layer may be present either on one side of the opaque, totally reflecting layer or on both of its sides. The latter case can result in a symmetric multilayer stack, i.e. in a symmetric structure of the optically variable pigment, assumed the optical properties on both sides are chosen to be the same. To obtain the final pigment the resulting multilayer thin-film sheet is detached and comminuted to the desired pigment size by known methods of grinding.
Any suitable physical vapor deposition (PVD) technique used for the production of thin-films can be applied for the production of this type of optically variable pigments. Such techniques are described in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Verlag Chemie, Weinheim, Germany, Volume A VI, page 67 ff. and in Milton Ohring “The Materials Science of Thin Films”, Academic Press Inc., 1992; they take essentially place under high vacuum conditions.
Producing the superposed layers by physical vapor deposition results in smooth and substantially plane and parallel layers which render possible a high chromaticity of the pigment as a consequence of parallel reflected electromagnetic radiation.
Furthermore due to the absence of oxygen during the production process a potential formation of oxide layers on the metal surfaces is prevented. This is important since in presence of oxygen the surface of many metals are attacked and a thin oxide layer spontaneously is formed on the exposed surfaces. However, already a thin oxide layer can perturb the reflectivity of those metal layers which constitute the opaque, totally reflecting layer.
Grinding of the multilayer thin-film sheet to pigment particles of the desired size results in substantially flat pigments, i.e. flakes, with irregularly broken edges. At these edges, perpendicular to the plane of the layers which form the stack, the opaque totally reflecting central layer is not longer protected by the superposed dielectric layer. Moreover, the mechanical stress exerted onto the pigment during the grinding process can result in hairline cracks in the dielectric layers. As a consequence, corrosion of the inner layers of the multilayer stack may take place at multiple sites upon contact with suitable chemical reagents. Such reagents are omnipresent, as e.g. in printing ink formulations (resins, solvents, etc.), or simply stem from the environment acting on the printed inks (sweat, laundry, etc.).
In the context of the present invention the term corrosion stands for the reaction of a metal material with the environment which results in a measurable change of the material or which leads to a deterioration of the function of a metallic surface or layer of the complete system. This definition is according to DIN 50900 Tl.1 April 1982, Tl.2 January 1984 and Tl.3. September 1985.
With respect to the optically variable pigments corrosion of the metals constituting some of the layers results in a measurable change of the optical properties of that pigment. The change may go from a weakening of chromaticity to a complete deterioration (loss) of the color properties of that pigment. The by-product of the corrosion can contribute substantially to the observed optical changes.
Aluminum, with a reflectivity of about 99% over the whole spectral domain of interest, represents a quite ideal choice from the optical point of view.
However, corrosion of aluminum generally occurs with water, acids or bases, as well as with strong complex forming agents or simply in the presence of oxygen. This is due to the strongly electropositive nature of aluminium, which has a standard potential of −1.70 V against hydrogen. Thus, in contact with water or oxygen, aluminium spontaneously form a very thin aluminium oxide layer which however completely protects the underlying metal from further attack. Therefore, corrosion of a clean aluminium surface in air or water self-stops immediately. However any chemical reagent
Bleikolm Anton
Muller Edgar
Rozumek Olivier
Bell Mark L.
Manlove Shalie A.
Shoemaker and Mattare
SICPA Holding S.A.
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