Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2000-11-06
2003-03-11
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S403000, C428S407000, C428S001200, C428S001300, C428S690000
Reexamination Certificate
active
06531221
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 in the arrangement of the molecules with respect to the long axes, on the other hand. 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. 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.
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. The colors appear different depending on the angle of view and depend on the pitch of the helical superstructure, which in turn depends 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 thus 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-B-383 376 describes liquid-crystal pigments comprising platelet-shaped carrier particles some of which at least are coated with liquid-crystalline material. Coating takes place by dispersing the platelet-shaped particles in a solvent in which liquid-crystalline material is dissolved, and then precipitating at least some of the liquid-crystalline material onto the particles. In the course of this the platelet-shaped carrier particles become fully or partly enveloped by the cholesteric. Uniform cholesteric layers arranged exactly parallel to the middle layer cannot be prepared by this process. The pigments are apparently not fully hiding, since they are said to be applied preferably to black surfaces.
DE-A-196 19 973 outlines, in a non-imitable manner, an idea for two- or three-layer platelet-shaped interference pigments. The pigments are intended to have at least one layer which consists of liquid-crystalline polymers whose mesogens are at least approximately in chiral-nematic and/or smectic and/or cholesteric order. Also provided in the interference pigments is a light-absorbing layer which is absorbent for at least part of the visible spectrum of light. The pigments are to be obtainable by knife coating, rolling or spray application to a smooth substrate, curing of the thin film thus produced, application of the light-absorbing layer, curing of this light-absorbing layer, optional application and curing of a further film which coincides with the first film in its composition and layer thickness, and removal and comminution of the cured layer assembly. Specific pigments, however, are not disclosed. As far as the material composition of the pigments is concerned, all that is said is that “liquid-crystalline main-chain or side-chain polymers or mixtures thereof, liquid-crystalline oligomers or oligomer mixtures, or liquid-crystalline monomers or monomer mixtures, [come] into consideration” as liquid-crystalline polymers. There are no examples regarding the preparation of the pigments or the pigment-containing coating formulations. The disclosure content of DE-A-196 19 973 is therefore limited to purely theoretical discussions of the idea of two- or three-layer pigments. Consequently, no technical teaching is provided that is imitable by the skilled worker.
WO 94/22976 describes two-layer cholesteric pigments based on two different polyorganosiloxanes from the company Wacker. The pigments are prepared in an extremely complex manner by separate coating of two previously nylon-coated glass plates with solutions of the abovementioned liquid crystals; rubbing of each liquid-crystal layer in order to orient it; attachment of thermally deformable spacers to the glass plates; placing of the glass plates together with their cholesteric layers facing one another, and uniting of the cholesteric layers by thermal deformation of the spacers at elevated temperature in a vacuum, and also crosslinking of the united cholesteric layers. The film thus obtainable is said, like the pigments obtainable from it by milling, to have a thickness of approximately 10 &mgr;m. Despite the prior coating of the glass plates with nylon, detachment of the film from the glass plates is apparently incomplete, so that residues of the film have to be scratched off in order to obtain the pigments from the plates, which makes the preparation of the pigments even more complex. The idea of three-layer pigments is merely outlined. These pigments cannot be prepared by the preparation process described for two-layer pigments. WO 94/22976 therefore provides no technical teaching which is imitable by the skilled worker and which would in any way provide three-layer pigments. The disclosure content is limited to purely theoretical discussions of the structure of three-layer pigments.
The prior document DE-A-197 57 699 discloses plated-shaped cholesteric multilayer pigments having at least two cholesteric layers and at least one interlayer separating these cholesteric layers from one another and absorbing some or all of the light transmitted by the cholesteric layer. If this absorbing interlayer is made fully opaque, then given a sufficient level of pigmentation the perceived color of the pigment is entirely independent of the background, thereby making such pigments very highly suitable for use, for example, in automotive paints.
In order to absorb the transmitting wavelength range, other prior art cholesteric interference pigments must either contain additional pigments in the cholesteric matrix or be applied to a colored background. When foreign pigments are incorporated into the liquid-crystalline mass it is disadvantageous that a considerable portion of the reflecting wavelength range is absorbed or scattered by absorption and scattered light, so that the special advantage of the interference pigments on a cholesteric basis is largely removed. The same problem occurs if cholesteric pigments are mixed with absorbing pigments into coating formulations. Reflections which disrupt the perceived color can only be avoided if the absorbing pigment is dispersed very finely into the cholesteric matrix. From general experience this is only the case if the pigment is dispersed using additives tailored specifically to the pigment surface. These compounds, such as fatty acids, salts of fatty acids, soya lecithins or phosphates, however, interfere with the development of the helical orientation and this prevents optimum color development. If, on the other hand, absorption takes place over a colored underlayer, the background must be of uniformly high quality in order to provide the desired overall impression of the effect coating. Consequently, considerable effort has to be expended on pretreating the background. An ideal background for maximum brilliance would have to be black or have specular gloss, which in the case of car bodies, for example, would be extremely difficult to realize.
It is an object of the present invention to provide special-effect pigments which no longer have the above-described disadvantages of the prior art.
We have found that this object is achieved by a multilayer pigment which comprises at least one partly light-permeable layer, below that at least one cholesteric layer and, if desired, below that, at least one further par
Best Wolfgang
Blaschka Peter
Meyer Frank
Schmid Raimund
Schneider Norbert
BASF - Aktiengesellschaft
Kiliman Leszek
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
LandOfFree
Multilayer cholesteric pigments does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multilayer cholesteric pigments, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multilayer cholesteric pigments will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3020012