13 сентября, 2015 
Pokraskin 6.5.2.1 Particle Size Distributions
The antiquated Fisher Sub-Sieve Sizer (FSSS) has been largely replaced by instruments that employ light scattering techniques, such as the Horiba LA910. These modern instruments measure particle size distributions (PSDs) over a size range of 0.02-1000 pm (microns or micrometers). Given proper dispersion and operating techniques, very reproducible PSD results can be obtained. The titanate pigments have mean particle sizes ranging from 0.5 to 2.5 pm, many of them <1.0 pm. High-performance pigments should have no measurable fraction >10 pm, and often have 0%>3 pm. These properties enable Hegman grinds of >7 to be achieved and high-gloss coatings to be produced.
It must be remembered that pigments are controlled for color properties. Although the particle size affects the color properties, so do other factors. Lot-to-lot variations in mean particle size of 10% are not uncommon.
Although a modern instrument may indicate 100%<3 pm, it is important to realize that there will still be a certain portion of particles >3 pm. In fact, most pig-
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Figure 6.4 Reflectance spectra of two MnSbTi DR pigments exhibiting different curve slopes in the red. |
ments show at least a trace of a residue on a 325 mesh (45 pm) screen, and a common 325 mesh residue specification is <0.1% [1]. These coarse particles, as few as they may be, can be a problem for some of the newer applications. Test procedures to measure 400 mesh and 600 mesh residues have been developed to more critically characterize the coarse tails of the particle size distributions. Specifications as tight as <0.005% (50 ppm) 600 mesh (20 pm) residue (V12100 MnSbTi brown, Ferro Corporation) can easily be met with modern processing techniques.
Optimum particle sizes can be calculated using Mie scattering theory. Such calculations show that the color properties of pigments could be significantly improved if a monosized pigment of the appropriate size could be made.
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