Лако-красочные материалы — производство

The shape of the pigment particle is extremely important for ensuring good magnetic properties. Isometric iron oxide pigments produced by direct precipitation are seldom used. Since 1947, needle-shaped y-Fe2O3 pigments have been prepared with a length to width ratio of ca. 5 : 1 to 20 : 1 and a crystal length of 0.1-1 pm [5.1].

Anisometric forms of Fe3O4 with the spinel structure or y-Fe2O3 with a tetrag­onal superlattice structure do not crystallize directly. They are obtained from iron compounds that form needle-shaped crystals (usually a — and y-FeOOH, see Section

3.1.1 Iron Oxide Pigments) [5.2-5.4]. An alternative method for the production of the a-FeOOH precursor starts from an aqueous suspension of Fe(OH)2, oxidizing that suspension to needle-shaped particles of a-FeOOH with an oxidizing gas while maintaining the pH greater than 11 at a temperature between 30 and 45 °C under intensive stirring [5.5, 5.6]. The oxide hydroxides are converted to Fe3O4 by dehydra­tion and reduction. Reducing agents may be gases (hydrogen, carbon monoxide) or organic compounds (e. g., fatty acids). The particle geometry is retained during this process.

Industrial Inorganic Pigments. Edited by G. Buxbaum and G. Pfaff Copyright © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30363-4

Since the pigments are subjected to considerable thermal stress during this con­version, the FeOOH particles are stabilized with a protective coating against sintering (usually silicates [5.7], phosphates [5.8], chromates [5.9], or organic compounds such as fatty acids [5.10]).

Finely divided stoichiometric Fe3O4 pigments are not stable to atmospheric oxi­dation. They are therefore stabilized by partial oxidation or by complete oxidation to Y-Fe2O3 below 300 °C.

In an alternative process, the starting material consists of needle-shaped particles of a-Fe2O3 instead of FeOOH pigments [5.11, 5.12]. The synthesis is carried out in a hydrothermal reactor, starting from a suspension of Fe(OH)3 and crystal growth is controlled by means of organic modifiers.

5.1.1.2