Лако-красочные материалы — производство

It has been largely assumed that the spectral absorptions of the DR pigments are due to d-d transitions of the dopant chromophores, specifically the Ni2+, Cr3+ and Mn2+ ions. However, it has also been proposed that the transition metal dopants have energy levels intermediate in the TiO2 band gap, and that the optical response is due to charge transfer transitions. These transitions can either be from the occupied TiO2 band to unoccupied M orbitals or from occupied M orbit­als to the unoccupied TiO2 band.

The charge transfer argument appears to be more logical. An article on calcu­lated energy levels of Ru-doped TiO2 supports this approach [46]. A CrNiSbTi-DR pigment shows a shift of the absorption band intermediate between that of a NiSbTi-DR and a CrSbTi-DR, and does not appear to be a composite of two sepa­rate absorption bands. However, more definitive evidence is needed.

There are two components to the reflectance spectra as defined by Kubelka — Munk theory, namely absorption and scatter. Rutile pigment particles exhibit max­imum scatter at approximately 0.2 pm. The scatter decreases slowly with increas­ing particle size, but drops off rapidly with finer particle size, the particles becom­ing transparent at sizes <50 nm. Therefore, the reflectance spectra will be a func­tion of the particle size distribution of the pigment.

Color values are a derived set of numbers calculated from the reflectance spec­tra. A commonly used system is CIE L*a*b* with values calculated for Illuminant D65 and a 10° observer. The Lvalue, with a range from 0(black) to 100(white), cor­relates to lightness, +a*=redness, — a*=greenness, +b*=yellowness, and — b*=blue — ness. Texts can be consulted for greater detail [47].

The effect of particle size on the reflectance curve is shown in Figure 6.2. It can be seen that the finer-sized pigment exhibits more scatter in the masstone, caus­ing an increase in reflectance across the spectrum. This has a significant effect on the color values, increasing the lightness and decreasing the yellowness, as shown in Table 6.5. Therefore, coarser pigments can give brighter but more transparent mass tones.

The effect of particle size on the tinting strength of the pigment can also be seen in Figure 6.2. For tints, the scatter of the two pigments is nearly equalized because it is dominated by the scatter of the TiO2. On the other hand, the finer — sized pigment presents a larger absorption cross-section (per mass). This results in significantly greater absorption where it absorbs in the blue. Therefore, the tint­ing strength generally increases as particle size is reduced. However, this argu­ment assumes perfect dispersions. Finer-sized particles form stronger agglomer­ates, making them more difficult to disperse and keep dispersed. This effect can reduce the expected increase in tinting strength of the finer-sized pigment.

For bright masstone applications, it is important to optimize the cleanness or color purity of a pigment. The cleanness can be roughly interpreted from the reflectance spectra by the difference between the maximum and minimum reflec­tances. A more accurate approach is to develop modified color values. Correlations can be developed from grind studies to account for changes in scatter due to dif­ferences in particle size distributions. For the NiSbTi yellows, a formula to calcu­late b’ values (e. g. b’=b*-3DL*) can be developed for a particular polymer system to account for the increased scatter and whiteness from finer-sized pigments.

The cleanness can also affect the tinting strength. Use of cap “Y” (a measure of lightness) to calculate tinting strength favors dirtier pigments. Therefore, while clean­er pigments will enable generation of a broader palette, dirtier pigments can still be used in many tint applications. However, secondary phases responsible for the dirtier pigment could deleteriously affect a crucial property such as weathering.

Figure 6.3 shows the reflectance spectra for an NiSbTi, an NiCrSbTi, and a CrSbTi DR pigment. The shade becomes redder as the absorption band moves towards lower energy (longer wavelength) in going from Ni to Cr. This behavior is similar to the CdSSe pigments that can be fine-tuned within a wide range of band gaps. However, unlike the cadmium pigments, the sharp rise in reflection is broadened and reduced going from yellow to buff.

Figure 6.4 exhibits the reflectance spectra of two different MnSbTi brown pig­ments. Note that the tilt of the reflectance line can be controlled to some degree. At the present time, four manufacturers have pigments of similar strength that exhibit slight differences in the tilt of the reflectance curve.

6.5.1.1 UV and NIR Spectral Characterization

Figure 6.5 shows UV-Vis-NIR reflectance spectra of some DR pigments. The DR pigments exhibit good UV absorbance and high IR reflectance — properties that enhance their performance in outdoor weathering systems.

6.5.2