Лако-красочные материалы — производство

Hue, relative tinting strength, dispersibility, and rheological behavior are the prop­erties of iron blue pigments with the most practical significance. Other important properties are the volatiles content at 100 °C, the water-soluble fraction, and acidity (ISO 2495). Pure blue pigments are mostly used in their pure form (e. g. in print­ing inks) and do not need any additives to improve them. Finely divided iron blue pigments impart a pure black tone to printing ink.

Due to their small particle size iron blue pigments are very difficult to disperse (Table 3.6, Figures 3.20 and 3.21). A graph of cumulative particle size distribution is given in Figure 3.22 for a commercial quality iron blue and for a micronized grade with similar primary particle size. The micronized grade gives greater tinting strength in dry mixtures than the blues obtained from standard grinding. The average size of the aggregates in the micronized material is ca. 5 pm compared with ca. 35 pm for the normal quality product.

Iron blue pigments are thermally stable for short periods at temperatures up to 180 °C, and therefore can be used in stoving finishes. The powdered material presents an explosion hazard; the ignition point is 600-625 °C (ASTM D 93-52). The pigments are combustible in powder form, ignition in air being possible above 140 °C [3.172].

Iron blue pigments used alone have excellent light — and weatherfastness. When mixed with white pigments, these properties can disappear [3.184]. Recent investiga­tions have shown that a topcoat (as commonly applied in automobile manufacture) overcomes this problem [3.185]. Figure 3.23 shows changes in residual gloss and color after a short weathering period. The pigments are resistant to diluted min­eral acids and oxidizing agents, and do not bleed. They are decomposed by hot, concentrated acid and alkali. Other properties are listed in Table 3.6.