Лако-красочные материалы — производство

Numerous reactions occur during the synthesis of DR pigments. Some of the important reactions in the formation of an NiSbTi pigment are shown in Scheme 6-1. Note that A represents the anatase TiO2 form and R represents the rutile TiO2 form.

Sb2O3 + O2 —У Sb2O4 Sb2O4 + O2 —— Sb2O3 Sb2O3 melts Sb2Ox sublimes Sb2O4 + A-TiO2 — A-(Sb, Ti)O2 A-(Sb, Ti)O2 — R-(Sb, Ti)O2 NiCO3 — NiO + CO2 NiO + Sb2O5 — NiSb2O6 NiO + TiO2 — NiTiO3

(Sb, Ti)O2 + NiO + O2 — (Ni, Sb, Ti)O2 pigment Particle faceting

Particle growth or agglomerate fusion

Scheme 6-1: Reactions that occur during the calcination process to form NiSbTi pigments.

Sb2O3 is the usual Sb source for the production of MSbTi pigments. Based upon the concept of electroneutrality presented in Hund’s patent, oxidation of Sb2O3 to Sb2O5 is required for pigment formation. Equilibria between Sb2O3, Sb2O4, and Sb2O5 are dependent on the temperature and the partial pressure of oxygen. The equilibria need to be controlled to enable pigment formation and pre­vent any melt formation. Oxidation of Sb2O3 is usually accomplished with an ade­quate airflow (O2 source), but is often assisted with added oxidizers. However, depending on the firing conditions, alkali and alkaline earth based oxidizers can also lead to the formation of metal antimonates, resulting in weaker pigments.

Formation of other secondary phases can also occur and need to be minimized. The phases NiTiO3 and MnSb2O6 have been observed in NiSbTi and MnSbTi DR pigments, and can be somewhat difficult to eliminate once formed. Once again, the presence of secondary phases generally correlates to weaker pigment strength. In addition, the secondary phases can potentially be deleterious to the weathering properties of the pigment.

Anatase TiO2 is generally the Ti source used for the production of DR pigments. The lower density of anatase enables a more facile formation of an Sb2O4:TiO2 sol­id solution that precedes pigment formation. In some cases, rutile or titanium hydrates of various types can be used as the Ti source.

A new study has examined the reactions involved in the formation of the CrSbTi buff DR pigments [31]. Based on DTA/TGA results and XRD analysis of samples pulled at different temperatures, a proposed reaction sequence is given in Scheme 6-2. It was also observed that any free Sb2Ox reduces and volatilizes at higher tem­peratures.

1. Sb2O3 + O2 —— Sb2O4

2. Sb2O4 + TiO2(A) — (Sb, Ti)O2(A):expanded lattice

3,4. Sb2O4 + O2 — Sb2O5, and

(Sb, Ti)O2(A):expanded lattice — (Sb, Ti)O2(R):expanded lattice

5. (Sb, Ti)O2(R):expanded lattice + Cr2O3 — (Cr, Sb, Ti)O2:normal lattice

Scheme 6-2: Reaction pathway for the formation of CrSbTi-DR pigments

Anatase is capable of forming solid solutions containing up to 14 mol% SbO2. The expanded anatase lattice that is formed slowly converts to an expanded rutile lattice in about the same time that the Sb2O4 undergoes further oxidation to Sb2O5. This oxidation can be observed as an exothermic peak and an increase in mass at ~920 °C, as seen in the DTA and TGA curves in Figure 6.1. The chromium appears to diffuse into the lattice after the oxidation has begun, in a partially con­certed manner. As the chromium diffuses into the lattice, a lattice contraction is observed, and there is color development of the pigment.

In addition to the patent literature, there are journal articles that discuss the forma­tion of DR pigments. Hund discusses mixed-phase DR pigments [32] based on his patent work. Krause discusses more specifically the NiSbTi DR pigments [33, 34].

Tavala discusses the CrWTi DR pigments [35] and NiMTi DR pigments [36]. Hugue — nin discusses new TiO2 precursors for making DR pigments [37]. Solc discusses syn­thesis ofVSbTi gray DR pigments [38]. Tena discusses MNbTi DRpigments [39].

6.3.2