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 [5.125, 5.130, 5.159]. 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 ofthe multilayers in the coatings 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 5.28, 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.

Semi-transparent absorber metal

Dielectric layer

Opaque
reflector metal

Dielectric layer

Semi-transparent absorber metal

Fig. 5.28 Scheme of design of optically variable pigments (OVP).

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 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 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 [5.160].

Uses

Special effect pigments are used as colorants or part of color formulations for all systems where traditional pigments are applied, but where additional color depth, brilliance, iridescence, color travel, and other spectacular effects are required [5.122, 5.123]. Mica-based pigments dominate; their combination of pearl and interference effects, brilliance, stability and behavior in different application systems is as yet un­surpassed. Pearl luster pigments require transparent or at least translucent binders. Formulations with absorption pigments have to take their transparency and color mixing rules into account.

Special care has to be taken to ensure parallel alignment ofthe platelet-shaped pig­ment particles during their application. In most cases the pigments can be dispersed homogeneously by simply and carefully stirring them into a suitable liquid system.

Today, special effect pigments are a standard component of solvent and water-based automotive and industrial coatings. The main reasons are both the decorative effect and the technical quality. The automotive designer has a wide range of color and composition possibilities at his disposal. Attractive and individual paint is provided not only by the various colors of special effect pigments, but also by using different particle sizes in combination with other absorption pigments [5.122, 5.137, 5.161].

Industrial paint applications for special effect pigments are e. g. furniture, bicycles, airplanes, rail wagons, cans and containers, building exteriors, interior decoration, artificial pearls and jewelry. The main reasons for their use are the achievable special effects, the attractiveness of the colors and the good application behavior [5.122].

Special applications of bright electro-conductive pigments based on metal-oxide — coated mica are conductive plastic surfaces and antistatic coatings [5.122, 5.148].

Plastics are one of the best-known applications for special effect pigments. The pigments are used with all thermoplastic methods such as injection molding and extrusion. They can also be applied in duromers. Fascinating marble effects can be obtained using plunger-type injection machines, injection molding with so-called marble units and with intermittent injection molding machines [5.122].

Attractive products can also be created using film blowing, blow molding and co­extrusion. Usually 0.5 to 2% of a pearl luster pigment is added to color plastics. A special application of some effect pigments is the laser marking of plastics. The pigment that is incorporated in the polymer is responsible for a dark or white coloring of the plastic material by interaction with a laser beam. Other mica-based effect pigments are designed with structures to achieve special solar reflective properties. Applications of mica pigments with high IR reflection are for example agricultural films (selective reflection of a part of the NIR, climate control in greenhouses) and IR-reflective plastic parts for domed and continuous roof lights [5.122, 5.148].

Printing with special effect pigments fulfills the highest esthetic requirements. Many products are printed with them especially when the articles should reflect a touch of luxury and artistic style. The pigments can be used with all important print­ing techniques, such as offset, screen, gravure and flexographic printing methods

and also in coating techniques such as overprint-varnishing (OPV) and paper coat­ing. They can be used in solvent and water-based printing inks as well as in UV-based systems. New application forms of special effect pigments for printing applications are preparations for liquid inks (e. g. pearlets) and special preparations for offset printing [5.122, 5.162, 5.163].

Cosmetic applications require specific effect pigments that are approved for use according to cosmetic regulations. The applications of these pigments include color cosmetics (lipsticks, eyeshadows, blushers, eye pencils, make-up, mascaras, nail lac­quers), personal care products (shampoos, body washes, lotions, creams, oral care products, hair gels), and special effect items (theatrical and costume make-up, tem­porary hair color sprays) [5.164].

For ceramic and glaze applications, the effect pigments based on substrates like mica are coated with an additional SnO2 layer to give stabilization against the aggres­siveness of the frits at high temperatures [5.165].

5.3.2