Лако-красочные материалы — производство

The patent literature shows that DR pigments date back to at least 1934. At that time their primary application was the coloration of ceramic ware. In the USA, however, it was the development of the vinyl siding market that created a large market for these pigments. Like the white TiO2 rutile pigments, they have a com­bination of strong UV absorbance and high near-infrared (NIR) reflectance. These properties give them the excellent weathering characteristics needed for this out­door application. The current commercial pigments have been significantly improved and optimized to meet the needs of this and similar markets. In fact, the MnSbTi DR browns of today are twice as strong as they were 20 years ago.

In 1962, Franz Hund was issued a patent on the preparation of DR pigments [9]. His patent demonstrated the ability of rutile to form solid solutions with many com­pounds. Three fundamental rules were given for the formation of a DR pigment:

1. Substitutional atoms must have ionic sizes similar to Ti4+

(0.68 A) or O2- (1.38 A),

2. charge balance (electroneutrality) should be maintained, and

3. the cation:anion ratio should remain at 1:2, as in TiO2.

More than 200 formulation examples are given in Hund’s patent. An acceptable range of cation ionic radii from 0.46 to 0.91 A is cited. However, use of a different set of ionic radii, specifically an average of Pauling’s and Belov’s [28], generates a more realistic range of ionic radii of 0.62-0.80 A for the same list of cations. The patent also has many examples substituting F — for O2-.

The rules of electroneutrality and a 1:2 cation:anion ratio require the use of at least two dopants for the commercial pigments. For the NiSbTi DRs, partial sub­stitution of Ni2+ for Ti4+ is accompanied by co-substitution of an ion with a valence greater than 4, such as Sb +, Nb +, or W6+, so that the average valence is 4. For the NiSbTi pigments, an Sb +/Ni2+ mole ratio of 2 is required to maintain electroneu­trality, while a W6+/Ni2+ ratio of 1 is needed for the NiWTi pigments. These substi­tutions are simply solid solutions of either NiSb2O6 or NiWO4 in TiO2.

The CrSbTi buff DR pigment was previously studied by ESCA, EPR, and diffuse reflectance spectroscopy to determine the oxidation states of the Cr in the pigment [29]. It was concluded that both Cr3+ and Cr4+ can be contributing species in the pigment, with Cr4+ producing redder shades. Contamination by CrO3-x species, where x= 0.4-0.8, was said to contribute a black color.

Finally, environmental and health concerns have prompted a number of agen­cies to develop lists and limits for regulated elements. Often referred to as “Heavy Metals”, probably because Pb and Hg attracted most of the earlier attention, “Regulated Metals” is a more appropriate title. Originally, the regulations for food — contact applications precluded the presence of Sb in the pigments. This led to the development of “Heavy Metal Free” pigment product lines that included NiNbTi and CrNbTi DR pigments [10]. However, both NiSbTi and CrSbTi DR pigments have now been approved for most food contact applications at limited levels.

6.2.2