Pigment particles aggregation

Pigments are morphologically described by a standardized terminology [2] they can occur as single or primary particles, aggregates, or agglomerates. 

Already dispersed pigment particles may for various reasons reassemble and form loosely combined units with various shapes. The most important among these are flocculates (Fig. 4), assemblies of wetted crystallites and/or aggregates or smaller agglomerates. They usually form in a low viscosity medium which fills the interior cavities of the pigment flocculates. Flocculates are therefore mechanically more labile than agglomerates and can usually be broken up by weak shear such as stirring. 

For inks which contain pigments, the most common problem is aggregation of the pigment particles due to the inherent instability of most dispersion systems. Since most modern inkjet inks for graphic applications contain dispersed pigments, the stabilization mechanisms of dispersions will be briefly discussed below. 

The ink supply system and most print heads have filters which are aimed at preventing arrival of large particles to the nozzles. Aggregation of pigment particles usually causes an increase in viscosity, which can interfere with the ink flow through the ink supply system. The aggregates can block the filters and thus may decrease the flow rate over time, eventually causing starvation of ink in the print head. 

A higher soHds loading may be achieved with thinner adsorbed layers, but this may also result in interparticle attraction resulting in particle aggregation. Clearly, a compromise is needed such that an appropriate steric stabihser is chosen for the particle size of the pigment... 

An analysis of the kinetic curves of the pigments samples sedimentation has been done in accordance with the method [5]. Calculated differential curves of particles aggregates distribution per sizes in water and butyl acetate are represented on Fig. 4. 

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