Лако-красочные материалы — производство

Instead of mica, SiO2 flakes can be used as a substrate for pearlescent pigments. The SiO2 flakes are produced by a web-coating process. Synthetic SiO2 flakes offer three advantages over the use of natural mica: (1) the thickness of the SiO2 sub­strate can be controlled in the preparation so that at the end a pigment with a true

Pearlescent Pigment on Silica Flakes

optical three-layer system is obtained, the interference color of those systems being stronger than that for the conventional mica pigments for which the effect of the mica is “wiped out” by a broad thickness distribution; (2) as synthetic sub­strates they do not have the small iron impurities that cause the slightly yellow mass tone of natural mica; (3) SiO2 has a lower refractive index (1.46) than mica (about 1.58) and, therefore, leads to a stronger interference effect [28-30].

Figure 7.7 shows a cross-sectional diagram of metal oxide coated on mica and metal oxide deposited on silica-flake pigments. The new properties available with silica-flake pigments can be summarized as follows.

Improved substrate characteristics

• uniform and controllable substrate thickness

• smooth and uniform substrate surface

• transparent substrate with no mass tone color

Improved reflection and refraction of light

• improved chromatic strength and purity

• new color travel behavior.

The silica flakes are manufactured by a web-coating process as shown in Figure 7.8. The web, moving at a controlled speed, is wet with an aqueous solution of a silica compound. The film on the web is dried to form a stable coating layer that can be selected between 50 nm and 1000 nm thick by adjustment of process pa­rameters. The layer is then removed from the web and processed further to gener­ate flakes that are fractionated and coated. The transparent flakes have a relatively low refractive index. However, they provide an excellent substrate for coating with the relatively high index metal oxides such as titanium dioxide and iron oxide. The metal oxides are applied in a standard precipitation-coating process producing high-performance pearlescent pigments.

a: Plastic Web b: Coating of the Web c: Drying Stage d: Layer Removal

Titania is deposited as hydrated titanium oxide on the surface of the SiO2 flakes. The thickness of the layer is monitored and precisely controlled. After drying and calcining, anatase is formed and a specific color is produced corresponding to the film thickness. The anatase modification is used in the cosmetic industry. In prin­ciple, the manufacturing process is similar to the “anatase” procedure. However, a thin intermediate SnO2 layer is applied to the surface of the SiO2 flakes before the TiO2 deposition. The SnO2 layer has a rutile structure which seeds epitaxy of the TiO2 rutile modification with its higher refractive index, brilliant color intensity, and superior light stability. Consequently, the rutile pigments are preferred when these properties are advantageous.

SiO2 can be coated not only with TiO2 but also with other metal oxides that are deposited from hydrolyzable salts. Iron(III) oxide is deposited as hydrated iron ox­ide on the transparent silica flakes. Subsequent drying and calcining produces a-Fe2O3 (hematite) layers on the flakes. The high refractive index of iron oxide generates strong interference effects and, in conjunction with the mass tone, bril­liant red pigments.

Figure 7.9 shows schematic diagrams of the three metal oxide/silica-flake pig­ments. Two extraordinary optical effects are achieved by the combination of trans­parent silica flakes selected for precise uniform thickness and controlled deposi­tions of metal oxide coating layers. The color of these pigments exhibits extreme angle dependence, and objects painted with them will change appearance with the direction of lighting and the location of the observer. Color changes from gold-silver to green, to green-blue, to dark blue are seen. The strong color travel is seen even under subdued lighting conditions. Figure 7.10 demonstrates how color travel is generated as a function of viewing angle with uniform silica substrates and metal oxide layer thickness. The angle-dependent color travel of three pig­ments through the “CIELAB” system is shown in Figure 7.11 for constant Fe2O3 deposition on SiO2 substrates ofdifferent thickness.

Improved color strength and very high luster are produced by the combination of precise thickness silica flake substrate materials, selected for their interference chromaticity, and by deposition of titania or iron oxide coating layers of the precise thickness required to generate the same interference color. These pearlescent pig­ments show stronger chromaticity L*a*b*-values than can be produced with mica — based designs.

Coating is possible with

> Titanium dioxide, TiO 2 (two modifications)

> Iron oxide, Fe2O3

> Other metal oxides (as used for mica pigments)

• Color Changes Depending on the Viewing Angle

• Strong Color Travel even under Subdued Light Conditions

Figure 7.12 Properties of new pearl lustre pigments on silica flakes.

A schematic diagram of the silica-flake pigments is shown in Figure 7.12. The SEM micrograph in Figure 7.13 shows a cross-section through a TiO2 silica-flake pigment. It shows clearly that both the SiO2 flake and the TiO2 layer thickness are precisely controlled.