Лако-красочные материалы — производство

Interference and angle-dependent color effects can also be achieved by layers or particles based on liquid crystal polymers (LCP) [8, 35]. Such effects can, for exam­ple, be produced by small plate-like substances which consist of an LCP material itself, or by small platelets which are uniformly coated with a cross-linked liquid crystalline polymer in a chiral-nematic arrangement [36-38].

Liquid crystals are organic compounds in a state of matter intermediate be­tween that of an isotropic liquid and an anisotropic crystalline solid [37]. Most liq­uid crystalline molecules are markedly elongated and rod like. In nematic liquid crystalline materials, the directions of the long molecular axes (directors) are arranged parallel to each other. Adding a chiral molecule to a nematic phase causes a superstructure comparable to the steps of a spiral staircase. The structure can be understood as being composed of nematic layers having the director rotated by a certain angle with respect to an adjacent layer, eventually building up a helical array (Figure 7.17).

7.5 Pigments Based on Liquid Crystal Polymers 99 Figure 7.17 Scheme of a liquid crystal film.

The thickness of a 360° turn of the director represents the pitch length p of the helix [35]. The color design of LCP pigments in paint films or of pure LCP films has its origin in an interference phenomenon. In this case, only incident light with a wavelength equal to the LCP lattice separation interferes and is reflected.

Because of the change of refractive index from layer to layer, the helical struc­ture gives rise to interference effects (Figure 7.18). When white light is incident normally on a film or an oriented arrangement of platelet-like particles of a chole­steric material with the helical axis perpendicular to the substrate, selective reflec­tion of a finite wavelength band occurs similar to Bragg X-ray reflection. The reflected band is centered about a wavelength l0, which is related to the helical pitch length p of the phase and its average refractive index n by l0 = np [35, 36].

On the other hand, a structure which has a helical superstructure with no change in the refractive index can also reflect light just like cholesteric phases [38]. In this case, it is not so much a change in the refractive index that gives rise to the optical effect but rather the superstructure.

The reflected light is circularly polarized with the same sense of polarization as the helical sense of the liquid crystal phase. Light circularly polarized in the oppo­site manner is transmitted through the sample together with those wavelengths of light not being reflected. Light experiences a double refraction as a result of the

Figure 7.18 Chiral cholesteric (nematic) liquid crystal structure. The dotted line shows a helical path within the medium. Pitch length p = 360° rotation.

anisotropy of the system. The bandwidth Ak of the selectively reflected band is described by the relation Ak = pAn. The angular dependence for an incident and observed angle © is given by k© = k0 cos ©.

The reflected light waves from the layers increase the intensity of the total reflection. The maximum reflectivity of one polarization state requires at least 6 helices or a thickness of about 3 pm. The most efficient reflection is given by layers with a thickness of up to 10 pm.

Cholesteric materials are temperature sensitive and show a thermochromic effect. The reason for this is that the pitch length of the helix and the refractive index are temperature dependent [35, 36].

Liquid crystal polymer films are transparent to visible light, including when they are in the form of platelets and coatings on substrates dispersed in a trans­parent paint film. These films need to be deposited on a dark base coating. Light is transmitted through the liquid crystal polymer, but some wavelengths are absorbed by the dark substrate or base coating. The liquid crystal polymer will be aligned parallel to the substrate, either as a film coating or on the interference pig­ment platelets within the paint film. This will then show a particular color in the orthogonal view and another color when observed at an angle. Such angle-depen­dent color phenomena gives a very striking effect, which is of great interest for security applications. Paint coatings incorporating such pigments must be con­structed of several layers with the liquid crystal materials in one of the inner layers only.

A number of interference pigments of this type are based on polysiloxanes. They are formed first as a thin cross-linked film of liquid crystalline polymers, and these are then ground to small platelets. The interference pigments them­selves are colorless and transparent. The color effect is based on the regular struc­ture and on the uniform arrangement of the liquid crystalline molecules. This gives rise to the reflection and subsequently interference with light of a particular wavelength. The other parts of the light go through the pigment particles. Very interesting color effects are possible based on these optical principles.

Liquid crystalline siloxanes are limited with respect to the glass transition in comparison to other backbone systems, such as poly(meth)acrylates. Glass transi­tion temperatures of up to 80 °C can be achieved, but there is a limit for the varia­tions of the mesogenic groups [39]. Therefore, crosslinking is the preferred meth­od. The presence of at least some polymerizable moieties within the side chain groups is necessary. Typical examples for these groups are epoxides, cinnamates, or methacrylates.

After polymerization and crushing, the liquid crystal polymer platelets can be used as iridescent pigments [40]. Such platelets can be suspended in inorganic and organic media, especially lacquers. The spray technique is mostly used for the application in paint films. The color effects are very strong if black substrates are used to ensure the absorption of the transmitted light. Very interesting colors can be obtained when combined with other effect pigments or in mixtures with con­ventional pigments [41].

Interference pigments based on liquid crystalline materials are prepared by a doctor-blade coating of the polymers in the liquid or liquid crystalline state on an even surface. The doctor-blade process leads to a thin film wherein a homoge­neous orientation of the molecules takes place. It is only after this orientation pro­cess that the film shows an interference color. The films are then cured and crushed by special techniques to yield platelets of liquid crystalline polymers showing interference effects.

7.6.1