Лако-красочные материалы — производство

Optical coatings that shift color with viewing angle have been adapted into opti­cally variable films, pigments, and inks during recent years [8, 33]. These coatings are based frequently on metal-dielectric multilayer thin structures having large color shifts with angle, high chroma, a large color gamut, and light fastness. Dif­ferent colors are produced by precisely controlling the thickness of the multilayers in the coating’s structure. To maintain tight color tolerance, the layer thickness must be controlled to within a few atoms.

The metal layers frequently consist of chromium (semitransparent absorber metal) or aluminum (opaque reflector metal). Silicon dioxide or magnesium fluor­ide are the materials mostly used for the dielectric layers. In the case of pigment particles, there is a symmetrical arrangement of the layers, as shown in Figure 7.16, whereas optical coatings can also consist of a system of unsymmetrical layers. All these arrangements are the basis for an optical phenomenon called the Fabry — Perot effect.

Such multilayer interference configurations filter the spectrum into a sequence of high-reflectance regions surrounded by low-reflectance regions and thus lend them­selves, in principle, to high-purity color production. The detailed spectral charac­teristics can be widely controlled by controlling the specific design parameters.

It has been found that for flaky pigment particles suitable for strong color-shift­ing effects, a symmetrical arrangement of the layers is necessary. A minimum of five layers and, for some optical purposes, even more may be required. The need

Semitransparent absorber metal

Dielectric Layer

Opaque Reflector Metal

Dielectric Layer

Semitransparent absorber metal

for many layers tends to make such a design relatively impractical in any high — volume coating production. Therefore, for practical purposes, only the five-layer arrangement plays a role.

The pigment flakes can typically be manufactured sequentially in a series of specialized roll-coating machines. In the first machine a carrier film, the so-called release layer, is deposited on a moving polymer web (transfer foil). This release layer is soluble in organic solvents so that the later formed multilayer film can be removed from the web at the end of the process. After depositing this release layer, the transfer foil is placed in a vacuum deposition roll coater and the first metal layer (e. g., chromium) as the semitransparent absorber metal is deposited followed by the first dielectric layer (e. g., magnesium fluoride) and the opaque metal layer (e. g., chromium). The second dielectric layer and the second semi­transparent absorber metal layer follow by the same deposition process. After this symmetrical multilayer interference coating has been deposited, the coated foil is removed from the vacuum chamber. In a subsequent step, the formed thin multi­layer film is removed from the supporting transfer foil by dissolving the release layer. Transformation into pigment particles is done by grinding the removed part of the film into small platelets, which now show pigment dimensions (thickness 0.2-2 pm, diameter 1-100 pm).

Such types of effect pigments can be used for several applications and are on the market as so-called optically variable pigments (OVP). They have found a broad application especially in preventing counterfeiting of important documents such as banknotes, stock certificates, visas, passports, or car licenses. When incor­porated into security inks and printed on bank notes and other documents of val­ue, the pigments are effective against color copying by printers, copiers, or cam­eras and unauthorized lithographic reproduction [34].

7.5