Compositions – Liquid crystal compositions
Patent
1997-01-21
1998-09-15
Wu, Shean C.
Compositions
Liquid crystal compositions
2522994, 2522995, 106493, 106499, 106505, 106506, 428 1, C09K 1952, C08K 500
Patent
active
058074970
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to interference pigments based on liquid-crystalline polymers. This invention also relates to an effect coating material and to commodity articles, especially motor vehicle bodies, which are painted therewith. Moreover, the invention comprises a process for the preparation of the interference pigments. Liquid-crystalline polymers are already known, for example, from DE 40 08 076 A, EP 66 137 A and U.S. Pat. No. 5,188,760.
The customary automotive solid-colour paints generally involve, incorporated into a clear vehicle comprising synthetic resin, colour pigments which determine the desired coloration of the paint. The colour effect of these pigments is based on a spectrally selective absorption effect, so that a spectrally broad fraction of the incident--white--light is absorbed by the pigments and only a spectrally narrow fraction is reflected.
In the effort for brighter perceived colours, especially for vehicle bodies, the so-called effect finishes have been developed. In the case of one group thereof, the metallic paints, small metal flakes, inter alia, as pigments are incorporated by mixing into the paint vehicle. In the case of another group, coated mica particles are used as pigments. With these effect finishes it is possible to achieve a better brightness of colour than with the solid-colour paints; moreover, depending on the direction of incidence of light and/or direction of viewing, there is a slightly altered perceived colour, which is desirable. EP 383 376 A1 describes such an effect finish, in which small mica flakes are coated uniformly on all sides with a crosslinked, liquid-crystalline polymer (LCP) in chiral nematic arrangement. The coloured appearance of such pigments comes about by means of an interference phenomenon. The only light waves of the incident light which are reflected are those whose wavelength interferes with the equidistant interplanar spacings of the liquid-crystalline polymers, whereas the light fractions of other wavelengths pass through the transparent body of the paint and are absorbed by the--preferably--dark substrate. Plateletlike interference pigments of this kind, aligned parallel to the painted surface, have a defined first colour--base colour--when viewed orthogonally and a second, shorter-wave colour when viewed from an inclined direction. This viewing angle-dependent coloured appearance of the painted surface makes the coating system highly effective, and, for specific applications in which such colour effects are perceived positively by the customer, highly desirable. A disadvantage is the complex preparation of the interference pigments, making the effect coating material and, correspondingly, the effect finish on the commodity article very costly.
The preparation of interference pigments based on polysiloxanes has become known, confidentially, to the Applicant, these pigments each consisting wholly of small fragments of a thin crosslinked film of liquid-crystalline polymer. These interference pigments are colourless and transparently see-through. The colour effect which can be achieved with them is based on the regular structure and on the uniform arrangement of the molecules in the form of a liquid crystal and on the interference, which can be traced back thereto, of a certain spectral light fraction for which the pigment has a reflective action. The other light fractions pass through the pigment. By this means it is possible to achieve stunning colour effects of different kinds, depending on the configuration of the coating system and/or on the interference pigments and mixing thereof. Aside from the novel aesthetic colour effect of the coating system and of the coating material used therefor, this coating system also offers a series of technical advantages. Owing to their chemical composition, the interference pigments have a specific weight which is approximately equal to that of the vehicle of the pigments and/or the paint base. Therefore, there is no mass-related separation of pigments and paint base d
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Gailberger Michael
Mueller-Rees Christoph
Stohr Andreas
Strohriegl Peter
Daimler-Benz AG
Wacker-Chemie GmbH
Wu Shean C.
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