Лако-красочные материалы — производство

7.2.3.1 Metal Oxide-Mica Pigments

The dominant class of pearlescent pigments is based on platelets of natural mica coated with thin films of transparent metal oxides [2-8, 16]. The mica substrate acts as a template for the synthesis and as a mechanical support for the deposited thin optical layers of the pearlescent pigments. Mica minerals are sheet layer sili­cates. Pearlescent pigments are usually based on transparent muscovite but some are based on natural or synthetic phlogopite. Although muscovite occurs world­wide, few deposits are suitable for pigments. Mica is biologically inert and approved for use as a filler and colorant.

Selection and preprocessing of the mica substrate is one of the key factors which determine the quality and appearance of nacreous pigments. The aspect ratio of the final pigment depends on the particle size distribution of the mica platelets, which have a thickness of 300-600 nm and various diameter ranges (e. g., 5-25, 10-50, 30-110 pm). Since light is regularly reflected from the planes of the metal-oxide-coat­ed mica and scattered from the edges, brilliance and hiding power are inversely related to each other.

A mica pigment coated with a metal oxide (Figure 7.3) has three layers with differ­ent refractive indices and four phase boundaries P1-P4: (P1) TiO2 (P2) mica (P3) TiO2 (P4). Interference of light is generated by reflections of all six combinations of phase boundaries, some of which are equal: P1P2 = P3P4, P1P3 = P2P4, P1P4, and P2P3. The thickness of the mica platelets varies in accordance with a statistical distribution. Con­sequently, interference effects involving the phase boundaries between the mica sub­strate and the oxide coating add together to give a white background reflectance. The interference color of a large number of particles therefore depends only on the thickness of the upper and lower metal oxide coating layers.

The development of the mica-based pigments started with pearlescent colors (Figure 7.4a, TiO2-mica). This was followed by brilliant, mass-tone-colored combi­nation pigments (i. e., mica TiO2, and another metal oxide) with one color (inter­ference color same as mass tone) or two colors (interference and mass tone differ­ent) that depend on composition and viewing angle (Figure 7.4b). In the 1980s further development was made by coating mica particles with transparent layers ofiron(III) oxide (Figure 7.4c).

Titanium Dioxide-Mica

The first multilayer pigments were marketed in the 1960s as TiO2-coated musco­vite micas. Two different processes are used for coating mica in aqueous suspen­sion on a commercial basis:

(1) Homogeneous hydrolysis

100 °C

TiOSO4 + mica + H2O ^ TiO2-mica + H2SO4

(2) Titration

TiOCl2 + 2 NaOH+ mica ^ TiO2-mica + 2 NaCl + H2O

The pigments are then dried and calcined at 700-900 °C. The titration (chloride) process is preferred for interference pigments with thick TiO2 layers because it is easi — erto control. Chemical vapor deposition in a fluidized bed has also been proposed: > 100 °C

TiCl4 + 2 H2O + mica ^ TiO2-mica + 4 HCl

When TiO2 is precipitated onto muscovite under reaction conditions unfavor­able for side precipitation, e. g., pH > 1.5, only the anatase modification is formed. Even after annealing at 1000 °C, no rutilization is found in the layer, whereas the free titania turns completely into rutile at about 700 °C.

Rutile has a higher refractive index and therefore yields a stronger pearlescence than anatase. Therefore, processes have been developed to create a rutile layer on mica. A thin layer of SnO2 is precipitated as a continuous layer onto the substrate, and then the TiO2 layer is created using the usual process. SnCl2 or better SnCl4

can be used as precursors for the SnO2 precoating. SnO2 acts as a template because its lattice parameters are close to those of rutile.

The desired interference color determines the thickness of the titania layer. For a silver white pigment, 50 nm of anatase is needed and for a blue interference color about 120 nm. The sequence of interference colors obtained with increasing TiO2 layer thickness agrees with theoretical calculation in the color space (Figure 7.5). A cross section of a TiO2-mica pigment is shown in Figure 7.6.

TiO2-mica pigments are used in all color formulations of conventional pig­ments where brilliance and luster are required in addition to color, i. e., in plastics, coatings, printing, and cosmetics. Table 7.3 contains a comparative overview of TiO2-mica, basic lead carbonate, bismuth oxychloride, and natural fish silver pig­ments. Some further physical data are summarized in Table 7.4.