Лако-красочные материалы — производство

By far the most important coloured inorganic pigments are the iron oxides, which provide colours range from yellow and red to brown and black. They are used extensively in paints, plastics and in building ma­terials such as cement and concrete. Both natural and synthetic iron oxide pigments are used commercially. Oxides of iron are major constitu­ents of some of the most abundant minerals in the earth’s crust. Natural iron oxide pigments are manufactured from deposits of suitable purity by a milling process. The natural iron oxides, examples of which include yellow ochre, red hematite and burnt sienna, are cost-effective materials which meet many of the demands of the colour industry. Micaceous iron oxide is a natural pigment used in metal-protective coatings. Its flake-like particles laminate in the paint film, forming a reflective surface which reduces radiation degradation of the film, and providing a barrier to moisture as an aid to corrosion protection.

Synthetic iron oxide pigments offer the advantages over their natural counterparts of chemical purity and improved control of physical form. A number of different structural types are encountered. Red iron oxides (C. I. Pigments Red 101 and 102) consist principally of anhydrous iron(iii) oxide (Fe2O3) in its а-crystal modification. Yellow iron oxide pigments (C. I. Pigments Yellow 42 and 43), although often formulated as hydrated iron(iii) oxides, are better represented as iron(iii) oxide-hydroxides, FeO(OH). The principal constituent of black iron oxide pigments is a non-stoichiometric mixed Fe(ii)/Fe(iii) oxide. It is usually formulated as Fe3O4, however, as the two oxidation states are generally present in approximately equal proportions. Brown pigments may be derived from the mixed Fe(ii)/Fe(iii) oxide or from mixtures containing Fe2O3 and FeO(OH). Iron oxide pigments are characterised, in general, by excellent durability, high opacity, low toxicity and low cost. However, the yellow pigments show somewhat lower heat stability because of their tendency to lose water at elevated temperatures, in the process turning redder due to the formation of Fe2O3. The colour of iron oxide pigments has been attributed principally to light absorption as a result of ligand-metal charge transfer, although probably influenced also by the presence of crystal field d-d transitions. The main deficiency of iron oxide pigments is that the colours lack brightness and intensity.

The most important synthetic routes to iron oxide pigments involve either thermal decomposition or aqueous precipitation processes. A method of major importance for the manufacture of a-Fe2O3, for example, involves the thermal decomposition in air of FeSO47H2O (copperas) at temperatures between 500 °C and 750 °C. The principal method of manufacture of the yellow a-FeO(OH) involves the oxidative hydrolysis of Fe(ii) solutions, for example in the process represented by reaction (1).

4FeSO4 + 6H2O + O2 4FeO(OH) + 4H2SO4 (1)

The reaction is sustained by addition of iron metal which reacts with the sulfuric acid formed, regenerating Fe(ii) in solution. To ensure that the desired crystal form precipitates, a seed of a-FeO(OH) is added. However, with appropriate choice of conditions, for example of pH and tempera­ture and by ensuring the presence of appropriate nucleating particles, the precipitation process may be adapted to prepare either the orange-brown y-FeO(OH), the red a-Fe2O3 or the black Fe3O4.

The only other ‘simple’ oxide pigment of major significance is chro — mium(iii) oxide, Cr2O3, C. I. Pigment Green 17. This is a tinctorially weak, dull green pigment but it shows outstanding durability, including thermal stability to 1000 °C. The pigment is normally prepared by treat­ment of chromates or dichromates with reducing agents such as sulfur or carbon.

The mixed phase oxides are a group of inorganic pigments which were developed originally for use in ceramics but which have subsequently found widespread application in plastics because of their outstanding heat stability and weathering characteristics combined with moderate colour strength and brightness. Structurally, the pigments may be con­sidered to be formed from stable oxide host lattices, e. g. rutile (TiO2), spinel (MgAl2O4) and inverse spinel, into which are incorporated transi­tion metal ions, e. g. Cr3+, Mn2+, Fe3 + , Co2+, Ni2+. This provides a range of colours in which the excellent durability characteristics of the host crystal structures are retained. An important commercial example of a mixed oxide pigment based on the spinel lattice is cobalt aluminate blue (C. I. Pigment Blue 28), usually represented as CoAl2O4, although in practice it is found to contain slightly less cobalt than this formula would indicate. While the successful formation of a mixed-phase oxide requires that the ‘foreign’ cation must have a suitable ionic radius to be incorpor­ated into its lattice position, a similar valency to that of the metal ion replaced is not essential. For example, a metal ion of lower valency may be incorporated into the lattice provided that an element in a higher oxidation state is incorporated at the same time in the amount required to maintain statistical electrical neutrality. As an example, nickel anti­mony titanium yellow (C. I. Pigment Yellow 53), an important member of the series, is derived from the rutile TiO2 structure by partial replacement of Ti4+ ions with Ni2+ ions, at the same time incorporating antimony(v) atoms such that the Ni/Sb ratio is 1:2.

Mixed-phase oxide pigments are manufactured by high temperature (800-1000 °C) solid state reactions of the individual oxide components in the appropriate quantities. The preparation of nickel antimony titanium yellow, for example, involves reaction of TiO2, NiO and Sb2O3 carried out in the presence of oxygen or other suitable oxidising agent to effect the necessary oxidation of Sb(iii) to Sb(v) in the lattice.