Лако-красочные материалы — производство

The phenomena of light interference can also be produced using diffraction tech­nology. Diffractive pigments are manufactured by vacuum deposition on a spe­cially patterned surface [49, 50].

Diffraction can be observed when light waves encounter obstacles of dimen­sions similar to their wavelength. A diffraction grating is regarded as an optical component made by a periodic assembly of reflecting or transmitting obstacles (grooves) separated by a distance comparable to the wavelength of light applied [49]. Diffraction gratings can be used to disperse beams of white light into the component wavelengths, producing a spectrum. Two types of diffraction gratings are, the ruled and the holographic type, distinguished. Various shapes, such as triangular symmetrical, triangular blazed, square-wave with different top plateau sizes, or sinusoidal gratings, can be produced to achieve ruled gratings. Further­more, a variety of groove frequencies, blazed angles, and depth profiles can be used. Depth, shape, and orientation of the grooves determine the relative intensity of various orders for a given freqency [49].

Diffracitve pigments are produced on thin flakes with a microstructure resolu­tion within the wavelength of visible light. Interesting optical effects within the visible spectrum are achieved when diffraction gratings have regularly spaced grooves at uniform depths on a reflective surface. Directional illumination on the diffractive surfaces leads to a better performance in regard of the color effects seen by the human eye.

The manufacture of diffractive pigments requires a structured polymer film as a template for vacuum deposition of layer sequences like MgF2/Al/MgF2 or SiO2/ Al/SiO2. These layer packages are separated from the polymer film after the deposition process by dissolving a supporting release layer [49, 50]. The resulting particles are fractionated into special size distributions. The typical thickness of the pigment particles is below 1 pm. The periodicity of the structure is in the order of 1 pm and its depth is several hundreds of nanometers. MgF2 or SiO2 act as a mechanical support for the thin structured aluminum film. The deposition technique and the one-way use of the expensive structured film are responsible for the extremely high production costs. Figure 7.19 shows a scanning electron micrograph of a diffractive pigment with its typical surface embossment.

By controlling the particle diemnsions and the surface microstructure, diffrac­tive pigments generate the appearance of multiple, bright rainbow-producing

prisms moving over a liquid silver color. The exciting color effects achievable with the pigments in binder systems open up application possibilities especially for high-end products like banknotes.

7.6.2