Blue interference pigments

Mehrschichtige Interferenzpigmente mit blauer Körperfarbe, bestehend aus einem plättchenförmigen Trägermaterial und einer Beschichtung, die aus (i) einer ersten Schicht eines farblosen, transparenten Metalloxides mit hoher Brechzahl, (ii) einer zweiten Schicht eines farblosen, transparenten Metalloxides mit niedriger Brechzahl und (iii) einer dritten, äußeren Schicht aus Kobaltaluminat, kobalthaltigem Glas oder Kobaltoxid gebildet wird, wobei die erste und dritte Schicht auch untereinander austauschbar sind. Als Metalloxid mit hoher Brechzahl werden Titandioxid, Zirkondioxid oder Zinnoxid verwendet. Als Metalloxid mit niedriger Brechzahl werden Siliciumdioxid oder Aluminiumoxid eingesetzt. Multilayer interference pigments with a blue body color comprise a flaky base material (BM) coated with: (1) colorless, transparent metal oxide with high refractive index (RI), (2) colorless, transparent metal oxide with a low RI, and (3) a layer of cobalt aluminate, cobalt glass or cobalt oxide, in the layer sequence (1,2,3,) (IP1) or vice versa (3,2,1) (IP2). Independent claims are also included for the following: (a) a process for the production of layer sequence IP1 by: (i) suspending BM in water and heating to 50-100 degrees C, adding a water-soluble metal compound at a pH suitable for hydrolysis so that a metal oxide hydrate with a high RI is precipitated on the particles of BM (keeping a constant pH by simultaneous addition of acid or base), (ii) adjusting to pH 4-10 with base and precipitating metal oxide hydrate with a low RI by adding a water-soluble metal compound (keeping a constant pH as above), (iii) washing the product, drying at 160-180 degrees C and calcinating at 500-1100 degrees C, (iv) suspending the product in water and coating it at a constant pH of 3-10 with cobalt and aluminum oxide hydrate by addition of suitable water-soluble compounds (plus acid or base) and (v) washing, drying and calcinating as in (iii). (b) A process for the production of IP2 which involves: (1) coating the BM suspension at pH of 3-10 with cobalt (Co) and aluminum oxide hydrate by addition of suitable water-soluble compounds (plus acid or base); (2) adjusting to pH 4-10 with base and precipitating metal oxide hydrate with a low RI by adding a water-soluble metal compound (keeping a constant pH as above), (3) suspending BM in water and heating to 50-100 degrees C, adding a water-soluble metal compound at a pH suitable for hydrolysis so that a metal oxide hydrate with a high R