Several problems can occur with liquid systems that contain solid particles like pigments and fillers. The undesired phenomenon of gluing together of solid particles in a liquid is called flocculation.
Flocculation is caused by the attractive forces that are always present between particles. The key to preventing flocculation is assuring that the particles repel each other. The repulsive forces must be stronger than the attractive forces. Two mechanisms can be used to arrange repulsion. This article is about electrostatic stabilisation. Check back in the Prospector Knowledge Center for a future article dedicated to steric stabilisation.
Flocculation is prevented when all particles in a system carry an electrostatic charge of the same sign. The amount of charge on the particles must be sufficient to overrule the attractive forces. In most paints and inks the particles have a negative charge. This type of stabilisation, called anionic stabilisation, is most often used in water-based systems.
The particles themselves can be charged, but most often the system developers prefer that all particles have an identical charge, using charged polymers that adsorb at the surface of the particles. A polymeric additive that provides stabilisation against flocculation is called dispersant. A dispersant molecule must have two sorts of groups to be able to do its job. First, anchoring groups ensure that the dispersant adsorbs at the surface of the particles. Secondly, groups that carry a negative charge must be present. Solid particles, that are surrounded by water, will obtain a negative charge when a suitable dispersant is used that complies with these two criteria.
Electrostatic stabilisation dispersant example
Lopon® 890, supplied by BK Giulini GmbH, is an anionic dispersant used for electrostatic stabilisation of solid particles in water-based systems1. The additive consists of linear polyacrylate molecules dissolved in water. The molecules carry a negative charge because part of the acid groups (-COOH) of the polymer are neutralised by means of sodium hydroxide (NaOH).
The acid groups and the carboxylate groups (-COO-) act as anchoring groups and the carboxylate groups provide the negative charge.
Critical aspects of electrostatic stabilisation
The formulator must be alert when using electrostatic repulsion as the only stabilisation mechanism. Stabilisation via charge is sensitive towards possible changes in pH: the charge of the particles depends on pH. Also, the repulsion of charged particles depends upon the presence of salts. Repulsion becomes worse when ions are present in the water surrounding the particles. This is because the charged layer around the particles, the so-called double layer, shrinks when ions are added to the water. The double layer must have a certain minimum thickness to ensure sufficient repulsion.
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