High-Performance Pigment Dispersant Additives Improve Pigment Wetting and Dispersion in Waterborne Coatings

By Ernest C. Galgoci, Khalil Yacoub, Mauricio F. Misdrahi, K. Michael Peck, Wim P. Stout, Steven Y. Chan

The process for dispersing pigments has been described as consisting of three stages: ( 1) wetting of the pigment by the continuous phase; ( 2) me­chanical milling to break up particle aggregates and agglomerates; and (3) long-term stabilization of the dispersed particles to prevent coagulation (flocculation). For waterborne coatings and inks, the dispersion process presents a number of challenges. For example, the high surface tension (YLV; subscripts refer to liquid-vapor) of water and the interfacial tension (YSL; subscripts refer to solid-liquid) between the continuous water phase and the pigment must be reduced to adequately wet the pigment particles as shown schematically in Figure 1.

Wetting of the pigment particles is necessary to displace adsorbed air from their surfaces and to facilitate particle separation, so that, in step 2, the pigment agglomerates can be more readily reduced to the desired pri­mary particles or much smaller agglomerations. The condition for wetting ( or spreading) of the aqueous phase onto the solid pigment particle is given by the spreading coefficient, S, which is defined as:
S = Ysv – (YsL + YLv)

Spreading will occur if S > 0. Therefore, for the pigment to be wet out, YsL and YLv must be reduced such that their sum is less than the surface energy (Ysv) of the pigment.