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edited Nov. 12, 2021

Original article date: Oct. 7, 2016

Introduction

Several problems can occur with liquid systems that contain solid particles like pigments and fillers. One of the undesired phenomena is flocculation, the gluing together of solid particles in a liquid¹.

Flocculation is caused by the attractive forces that are always present between particles. The principle used to prevent flocculation is to assure that the particles repel each other. The repulsive forces must be stronger than the attractive forces. Next to steric stabilization², electrostatic stabilization can be used as a mechanism to arrange repulsion.

Electrostatic stabilization

Flocculation is prevented by means of electrostatic stabilization 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 stabilization, called anionic stabilization, is most often used in water-based systems.

Polymeric dispersants

The solid particles themselves can be charged, but most often, product developers assure that all particles have an identical electrostatic charge by using charged polymers that adsorb at the surface of the particles. A polymeric additive that provides stabilization against flocculation is called a dispersant^3,4. Dispersant molecules must have two sorts of groups to be able to provide electrostatic stabilization. First, anchoring groups must assure that the molecules adsorb at the surface of the particles. Secondly, the dispersant molecules must contain groups that have a negative charge. Solid particles that are surrounded by water will obtain a negative charge when a dispersant is used that complies with these criteria.

An example

Lopon^® 890 is an anionic dispersant used for electrostatic stabilization of solid particles in water-based systems. 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 neutralized with 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 stabilization

The formulator must be alert when using electrostatic repulsion as the only stabilization mechanism. The reason is that stabilization via charge is sensitive towards possible changes in pH: the charge of (the dispersant molecules on) the particles depends upon pH. Also, the repulsion of charged particles depends upon the presence of salts. The mutual repulsion of charged particles becomes worse when ions are present in the water surrounding the particles. This is because the charged layer around the particles, the so-called electric double layer, shrinks when ions are added to the water. The double-layer must have a certain minimum thickness to assure sufficient repulsion.

References

1. Article The Basics of Dispersion and Stabilization of Pigments and Fillers, Jochum Beetsma, 31 January 2020.
2. Article Dispersants for Steric Stabilization, Jochum Beetsma, 1 October 2021.
3. Article Dispersants – The Understood Misunderstood Additive, Marc Hirsch, 10 September 2021.
4. Article Understanding Dispersants, Marc Hirsch, 19 February 2016.

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