Particle size analysis by laser diffraction in organic pigment technology

Abstract

Organic pigments are used to colour organic coatings and indeed they are major constituents of printing and packaging inks. They are commonly supplied in powder form, the grain size distribution of which can be determined directly by laser diffraction using dry powder feed. This is advantageous in monitoring dry milling procedures used to reduce lump dry presscake to fine powder, and for comparison and matching of different types of milling equipment. Grain size distribution can affect dispersibility of a pigment in application media, especially premix dispersion in ink media. Also laser diffraction in a `wet' cell can be used to monitor dispersion of organic pigments in liquid media of low viscosity, such as nitrocellulose/alcohol-based ink millbases. Such uses are exemplified here and the principles and techniques of particle size analysis by laser diffraction are discussed in the context of organic pigments.

Introduction

Organic pigments are finely divided crystalline solids [1]that are most commonly supplied in powder form. They are prepared as batches of dried presscake which are tested at that stage for colouristic performance. The batches are subsequently blended by milling in required proportions such that the final blended powders give performance within narrow tolerances in the intended application. The milling conditions can be controlled to produce desired powder forms depending on bulk handling requirements, ideally without detriment to application performance. Powder grain size analysis by low angle laser light scattering (LALLS) referred to here as laser diffraction has proved to be a valuable technique in this context and is much quicker, cleaner and more informative than conventional sieve analysis. The authors are not aware of any previous publications on grain size analysis specifically of organic pigment powders by this method.

Laser diffraction is also useful for comparing pigment particle size distributions in liquid media of low viscosity, such as paints 2, 3and liquid packaging inks [4], where monitoring progress of dispersion by colouristic means can present difficulties. It enables comparison of the effectiveness of dispersion techniques. Use of laser diffraction has been discussed in a recent review of particle size analysis of dispersions in liquids and an extensive bibliography given [5].

The present purpose is to describe some relevant aspects of laser diffraction techniques and exemplify their uses in the context of organic pigment technology.