Laser diffraction measurements of pigment size distributions of coatings and mill bases: implications of dilution medium and dispersion process parameters

Coating properties, such as gloss, rheology, and exterior durability, are strongly affected by the particle size distribution (PSD) of pigment agglomerates. Consequently, moving from the industrial practice of a maximum agglomerate size evaluation after dispersion (i.e. grindometer readings) to an accurate measurement of the entire PSD, holds a promising potential for quality control, product optimization, and research.

The aim of the present work was to develop an analysis procedure, based on laser diffraction, for PSD measurements of coatings. In the experiments, acrylic-based TiO₂ or Cu₂O pre-dispersions (i.e., mill bases), with variations in the composition and dispersion parameters, were investigated.

Results show that the laser diffraction measurements are influenced by the so-called obscuration value of the diluted sample, the equipment-input refractive index values, and the shape assumption) for the pigment agglomerates. Furthermore, the unavoidable sample dilution, prior to a measurement, strongly affects the stability of primary particles and agglomerates. When using a mixture of the pertinent binder and solvent, as opposed to the pure solvent only, and taking rheology issues into consideration, solvent shock was avoided and reliable PSDs could be obtained.

The new analytical procedure was used on selected dispersions with variations in coating formulations and equipment operational parameters and allowed for precise detections of the developments in PSDs and volume-moment mean diameters.

In summary, the principle of laser diffraction, with proper control of the measurement conditions, was demonstrated to be a reliable technique for PSD evaluation of coatings and pre-dispersions.