Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2000-03-02
2003-06-24
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C428S001200, C428S403000, C428S407000
Reexamination Certificate
active
06582781
ABSTRACT:
The invention relates to multilayer cholesteric pigments, to processes for their preparation and to their use.
When substances exhibiting shape anisotropy are heated it is possible for liquid-crystalline phases known as mesophases to occur. The individual phases differ in the spatial arrangement of the centers of mass of the molecules, on the one hand, and by the arrangement of the molecules with respect to the long axes, on the other hand (G. W. Gray, P. A. Winsor, Liquid Crystals and Plastic Crystals, Ellis Horwood Limited, Chichester 1974). The nematic liquid-crystalline phase is distinguished by parallel orientation of the long axes of the molecules (one-dimensional order state). Provided that the molecules forming the nematic phase are chiral, the result is a chiral nematic (cholesteric) phase in which the long axes of the molecules form a helical superstructure perpendicular thereto (H. Baessler, Festkörperprobleme XI, 1971). The chiral moiety may be present in the liquid-crystalline molecule itself or else may be added as a dopant to the nematic phase, inducing the chiral nematic phase. This phenomenon was first investigated on cholesterol derivatives (eg. H. Baessler, M. M. Labes,
J. Chem. Phys
. 52, (1970) 631).
The chiral nematic phase has special optical properties: a high optical rotation and a pronounced circular dichroism resulting from selective reflection of circularly polarized light within the chiral nematic layer. Depending on the pitch of the helical. superstructure it is possible to produce different colors which can appear different depending on the angle of view. The pitch of the helical superstructure depends in turn on the twisting power of the chiral component. In this case, it is possible, in particular by altering the concentration of a chiral dopant, to vary the pitch and hence the wavelength range of the selectively reflected light of a chiral nematic layer. Chiral nematic systems of this type have interesting possibilities for practical use.
Cholesteric special-effect pigments and compositions comprising such pigments are known.
EP-A-686 674 and its parent DE-A-44 16 191 describe interference pigments comprising molecules which are fixed in a cholesteric arrangement; the pigments feature a platelet-shaped structure and have, as one example demonstrates, a layer thickness of 7 &mgr;m. The pigments are prepared by applying highly viscous LC material to a substrate, the substrate being conveyed at a speed of about 2 m/min below a fixed doctor blade. By this means the liquid-crystalline molecules are oriented.
WO 97/30136 describes cholesteric polymer platelets which are obtainable from a chiral polymerizable mesogenic material. The platelets can be used as effect pigments. They are single-layered and have a preferred thickness of from 4 to 10 &mgr;m.
DE-A-196 39 179 discloses transparent compositions which comprise pigments whose coloredness is dependent on the viewing angle. The compositions are said to contain absorbtive colorants in an amount such that the angle-dependent color effect which occurs in the specular-angle configuration is intensified without any definite adverse effect on the transparency of the composition under all other angular configurations. The pigments are prepared as described in the abovementioned EP-A-686 674.
DE-A-196 39 229 discloses a composition in which at least one matrix, containing pigments whose coloredness is dependent on the viewing angle, is present in at least one further matrix in a two- or three-dimensionally structured format in the form of structural elements. These matrices 1 and 2 are non-identical, or contain non-identical pigments in identical concentrations. The pigments are prepared as described in the abovementioned EP-A-686 674.
DE-A-196 39 165 describes a method of obtaining new color effects by means of pigments whose coloredness depends on the viewing angle, in a matrix, where following their incorporation into the matrix the pigments are subjected to three- and/or two-dimensionally selective tilting in the matrix. The pigments are tilted preferably by means of differently directed movement of the pigments in the matrix or by using pigments differing in their concentration in the matrix. The pigments are prepared as described in the abovementioned EP-A-686 674.
DE-A-195 02 413 describes a pigment whose coloredness is dependent on the viewing angle and which has been obtained by three-dimensional crosslinking of oriented substances of liquid-crystalline structure with a chiral phase. In order to render such pigment colorfast with respect to elevated temperatures, it is proposed to conduct crosslinking in the presence of at least one additional, color-neutral compound which contains at least two crosslinkable double bonds. The uncrosslinked cholesteric mixture is applied by knife coating onto a film, for example. No details are given regarding the thickness of the layer applied by knife coating.
U.S. Pat. No. 5,364,557 describes cholesteric inks, for drawing and printing, which comprise platelet-shaped or flakelike cholesteric pigments. The pigments are prepared by coating a conveyor belt with a cholesteric melt and then smoothing and aligning the cholesteric film by means of a blade. The pigments can comprise two layers of different handedness or two layers of identical handedness with a further layer between them which inverts the direction of rotation of circularly polarized light. A light-absorbing layer is not described as a possible constituent of a layer pigment, especially since such a layer would work against the desired effect of 100% reflection of light.
U.S. Pat. No. 5,599,412 discloses a preparation process and apparatus for preparing cholesteric inks for writing and printing. Application and alignment of the melted polymer take place as described in the above-cited U.S. Pat. No. 5,364,557.
DE-A-44 18 076 describes effect coating materials and effect finishes which comprise interference pigments formed from esterified cellulose ethers. Using these interference pigments it is possible, it is said, to produce, in the case of the coating material, color changes which are dependent on the direction of incident light and on the direction of viewing, or to produce particularly intense hues of hitherto unknown brilliance on the article coated with such a material. The pigments are prepared by comminuting cholesteric layers which after curing are said to have a thickness of 5 to 200 &mgr;m. The layers are applied, for example by knife coating. In the examples, a layer thickness of approximately 10 &mgr;m is stated.
DE-A-196 297 61 describes cosmetic or pharmaceutical formulations which comprise pigments whose coloredness is dependent on the viewing angle. The pigments comprise at least one oriented crosslinked substance with a liquid-crystalline structure and with a chiral phase. The pigments are plateletlike in form and have a thickness of 1 to 20 &mgr;n. Preparation of the pigments is as described in the abovementioned EP-A-686 674, although the layer thickness is said to be 5 &mgr;m rather than 7 &mgr;m.
Known from WO 96/02597 is a method of coating or printing substrates with a composition that comprises a chiral or achiral, liquid-crystalline monomer and a non-liquid-crystalline, chiral compound. The liquid-crystalline monomers are preferably photochemically polymerizable bisacrylates. To achieve the uniform orientation of the cholesteric phase, which is required for the development of the desired optical properties, and to achieve it even on large surfaces of complex form, it is necessary to add a polymeric binder. The layers, about whose thickness nothing is stated, are prepared via various printing techniques or by spraying.
DE-A-196 02 795 discloses a process for preparing pigment particles. The pigment particles have a substantially uniform, defined form and size, being prepared either by polymerizing a polymerizable mixture in uniformly dimensioned recesses or by initial shaping, by means of a printing process, and subsequent polymerization. The layer thickness of the pigments is
Best Wolfgang
Bettinger Günter
Blaschka Peter
Heilmann Peter
Keller Harald
BASF - Aktiengesellschaft
Le H. Thi
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