ISO 8130-13:2019 pdf free download – Coating powders一 Part 13: Particle size analysis by laser diffraction.
9 Analysis
9.1 General
Laser diffraction systems are based on first principles and the analysis is performed on idealized spherical particles to give the size. Particle shape or different orientations of non-spherical particles will lead to different scattering patterns and thus variable particle size results. ISO 13320 provides further in formation.
9.2 Reference materials
Particle size measurement greatly depends on the calibration of the instrument for which certified and suitable standards are required. Powder calibration standards can be used to check the performance of the instrument and to verify coating powders with similar PSD.
For the determination of particle size during process control of similar coating powders; it may be permissable to use a coating powder sample previously measured for the same PSD and under the same analytical conditions.
9.3 Accuracy
Accuracy of coating powders can be difficult to assess and depends on the sampling of the coating powder specimen as well as the instrument conditions. After sampling, a major influence on accuracy is the dispersing mechanism of the instrument which uses compressed air or vacuum to create a semi homogeneous dispersion of coating powder particles for measurement by the laser. During dispersion, the coating powder particles may be reduced by mechanical and/or physical stresses which occur due to the applied air pressure, it should be noted that incomplete dispersion will also influence the accuracy. In an ideal situation, a homogeneous dispersion of spherical particles of similar particle size can be expected. However, not all coating powders are manufactured in the same manner leading to inhomogeneous powder particles and PSD.
The effects of physical stress caused by air pressure to disperse the coating powder should be determined to find a suitable dispersion pressure. A measuring sequence can be performed in 0,5 bar increments over the complete pressure or vacuum range appropriate for the coating powder. The dispersion pressure/vacuum yielding the lowest change in the fine particle fraction, of less than 10 urn, shall be selected for the measurements and subsequent analysis.
9.4 Precision
9.4.1 Repeatability
With suitable dispersing conditions (see 9.2) as well as a consistent measurement procedure, the repeatability for the same coating powder should have the following relative standard deviation (RSD) with respect to the cumulative powder particle volume. The RSD below may be used or as agreed between the interested parties for a particular coating powder. ISO 13320 provides further information.
For median particle size xlo, representing 10 % particle volume, the RSD is <=3 %. For median particle size xo, representing 50 % particle volume, the RSD is <=1 %. For median particle size X9Ø, representing 90 % particle volume, the RSD is <=3 %. RSD values <=3 % signify an instrument under control, consistent measurement and a good sampling procedure.