Лако-красочные материалы — производство

Molybdate red and molybdate orange [12656-85-8], C. I. Pigment Red 104:77605, are mixed-phase pigments with the general formula Pb(Cr, Mo, S)O4 [3.115]. Most com­mercial products show a MoO3 content of 4-6%, a refractive index of 2.3-2.65 and

densities of about 5.4-6.3 g cm-3). Their hue depends on the proportion of molybdate, crystal form, and particle size.

Pure tetragonal lead molybdate, which is colorless, forms orange to red tetragonal mixed-phase pigments with lead sulfochromate. The composition of molybdate red and molybdate orange pigments can be varied to give the required coloristic proper­ties; commercial products usually contain ca. 10% lead molybdate. Lead molybdate pigments have a thermodynamically unstable tetragonal crystal modification that can be transformed into the undesirable stable yellow modification merely by dispersing [3.129]. This is especially true of the bluish varieties of molybdate red which have larger particles whose color can be changed to yellow by shear forces. The tetrago­nal modification of the lead molybdate pigments must therefore be stabilized after precipitation [3.130, 3.131].

The fastness properties of the molybdate orange and molybdate red pigments are comparable with those of the chrome yellows. As with the chrome yellows, the pigment particles can be coated with metal oxides, metal phosphates, silicates, etc., to give stabilized pigments with high color brilliance and good fastness properties, as well as highly stabilized grades with very good resistance to light, weathering, sulfur dioxide, and temperature, andwitha very low content of acid-soluble lead (DIN 55770, 1986 or DIN/ISO 6713, 1985).

The colors of lead molybdate pigments vary from red with a yellow hue to red with a blue hue. Since chrome orange is no longer available (see Section 3.4.3), molybdate orange has become much more important.

3.4.2.1