Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
1999-08-13
2001-03-27
Padgett, Marianne (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S554000, C264S435000, C264S482000, C264S492000
Reexamination Certificate
active
06207240
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present invention is described in the German priority application No. 198 36 885.2 filed Aug. 14, 1998.
BACKGROUND OF THE INVENTION
The invention relates to a process for producing symbols by laser irradiation on effect coatings or molding compounds comprising thermoplastic cholesteric liquid-crystal polymers (cLCPs) having optically variable properties.
It is known to inscribe surfaces of plastic by means of laser radiation, and in this context use is made, in addition to CO
2
lasers, preferably of Nd:YAG lasers (wavelength 1064 nm) or a frequency-doubled Nd:YAG laser (532 nm). In the case of the laser markings customary at present, the marking mechanism is normally a thermal process (N. J. Bruton, Opt. Photonics News (1997), 8(5), 24-30). The absorbed laser light induces a rapid local temperature rise in the polymer material at the irradiated sites. The polymer material pyrolyzes and a marking becomes visible. In the case of polymers which are transparent to the light wavelength used it is necessary to use light-absorbing additives, such as kaolin.
Initially, only light-colored inscriptions on a dark background or dark inscriptions on a light-colored background were possible. Through the incorporation of colored pigments which are not destroyed by the laser irradiation, it is possible to generate colored laser markings (EP 0 641 821 A1).
The laser markings known to date are therefore distinguished by the fact that they possess, on a white or colored polymer, a marking in a different color. Neither the color of the plastic nor that of the inscription, however, possesses any viewing-angle dependency. Cholesteric main-chain polymers (cLCPs) are known and can be prepared in analogy to nematic main-chain polymers by using an additional chiral comonomer (U.S. Pat. No. 4,412,059; EP-A-0 196 785; EP-A-0 608 991; EP-A-0 391 368) or by reacting nematic main-chain polymers (LCPS) with additional chiral comonomers (EP-A-0 283 273). A feature of cholesteric main-chain polymers is their helical superstructure. As a result of this, first, the material no longer has the anisotropy of the mechanical properties which is typical of nematic liquid-crystal polymers. Depending on the chiral monomer content, the material exhibits pronounced color effects which are based on the selective reflection at the helical superstructure. The precise reflection color in this case depends on the viewing angle and above all on the pitch of the helix. For any given viewing angle—for example, a sample viewed vertically from above—the reflection color which appears is a color having a wavelength which corresponds to the pitch of the helical superstructure. This means that the wavelength of light reflected is shorter the smaller the pitch of the helix. The developing pitch of the helix depends essentially on the proportion of the chiral comonomer, on the nature of its incorporation into the polymer, on the degree of polymerization, and on the structure of the chiral comonomer.
Thin films of cholesteric liquid crystals exhibit pronounced color effects when applied to an absorbent substrate, especially a black substrate, since otherwise, owing to an inadequately pronounced hiding power of the cholesteric liquid crystals, the nonselective component of the light is reflected at the substrate, which attenuates the perceived color. It is known, furthermore, that instead of a black substrate it is also possible to use substrates colored otherwise. In addition, it is possible to influence the hiding power and perceived color of the cholesteric layer by the incorporation of colorants (DE-A-196 43 277).
SUMMARY OF THE INVENTION
It is an object of the present invention to bring about color changes in viewing-angle dependent effect coatings or molding compounds by means of laser irradiation, and thus to induce markings in the form of symbols which given an appropriate choice of color may even be perceptible only at certain viewing angles.
It has surprisingly been found that this object can be achieved through the use of cholesteric liquid-crystalline polymers. An advantage over systems common at present is that the reflection properties of the effect polymer to be marked give it a viewing-angle dependent color, and it is therefore possible not only to produce colored markings on polymers but also, by virtue of the laser marking, to induce a color change in a viewing-angle dependent coating.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a process for generating a marking on an effect coating or molding compound comprising cholesteric liquid-crystalline main-chain or side-group polymers or comprising a combination of main-chain and side-group polymers, which comprises inducing a color change at the irradiated sites of the effect coating or molding compound by laser irradiation.
If a thin layer of the abovementioned cholesteric liquid-crystal polymers, which is applied to a substrate, is irradiated with a laser, the cholesteric arrangement of the liquid-crystal polymers is destroyed at the irradiated sites and, depending on the duration and intensity of irradiation, the substrate surface becomes more or less visible. If the substrate surface is colored, either colored right through or colored by means of a colored coating, the substrate color becomes visible and perceptible as a marking. Thus, for example, by laser irradiation of an effect coating which appears green when viewed perpendicularly and blue when viewed at an oblique angle, and has been applied to a black-coated metal panel, it is possible to generate a dark marking. This marking is therefore perceptible on the effect coating at any viewing angle.
However, it is also possible to adapt the background color of the substrate to the viewing-angle dependent colors of the cholesteric polymer. By this means it is possible for the marking to become visible only at a particular viewing angle. If the effect coating is such, for example, that it appears green when viewed perpendicularly and blue when viewed at an oblique angle, the chosen background color can be green or blue. If the background color is green, then the site marked with the laser is likewise green and the marking is perceptible only with difficulty when viewed perpendicularly owing to the lack of contrast. If, however, the coating is viewed at an oblique angle, the color of the effect coating appears blue, while the laser-marked site remains green and therefore becomes visible.
If it is desired that the marking should be visible when viewed perpendicularly but invisible when viewed at an oblique angle, the chosen background color must be blue in the above example.
If a molding compound comprising said cLCPs is marked with the laser, then depending on the duration and intensity of irradiation the perceived color of the nonoriented polymer is produced at the irradiated sites, i.e., a milky/cloudy color in the case of a polymer which has not been colored with the colorants below, or a gray or black marking induced by pyrolysis products.
The color of the laser marking can be influenced not only by the color of the substrate but also by the incorporation into the effect polymer of colorants which are stable with respect to the laser marking. A red pigment, for example, can be incorporated into an effect polymer which exhibits a flop from green to blue, for example. The effect coating or molding compound is irradiated with a laser until the color of the colorant becomes visible at the irradiated site.
Colorants which can be used are compounds from the series of the organic and inorganic pigments and from the series of the polymer-soluble dyes. Examples of inorganic pigments are titanium dioxide, iron oxides, metal oxide mixed-phase pigments, cadmium sulfides, ultramarine blue pigments or chromate-molybdate pigments. Organic pigments which can be employed are all pigments sufficiently well known to the person skilled in the art from the relevant literature, examples being carbon black, anthanthrone, dioxazine, phthalocyanine, quinacridone, diketo-py
Metz Hans Joachim
Schoenfeld Axel
Stohr Andreas
Clariant GmbH
Hanf Scott E.
Jackson Susan S.
Padgett Marianne
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