The spontaneous gluing together of solid particles in a liquid, called flocculation, was discussed in the article Dispersants for Electrostatic Stabilisation. Colloidal stability, the stability against flocculation, can be assured by using the right type of dispersant in the right amount. Two mechanisms can be used to prevent flocculation. This article is about steric stabilisation.
Flocculation is prevented when all particles in a system are covered with polymeric tails that dissolve in the liquid surrounding the particles, the so-called continuous phase.
The concentration of soluble tails in the area between particles goes up when the particles approach each other. This is unfavourable from energy point of view. The system reacts by diluting the tails: solvent molecules (or water molecules in a water-based system) flow to the area between the particles. The result of this osmotic flow is that the particles move away from each other. Mutual repulsion of the particles is governed by the solubility of the polymeric tails in the liquid. Steric stabilisation can be used in both solvent-based and waterbased systems.
Dispersant molecules that provide steric stabilisation consist of two parts. An anchor block must assure that the dispersant adsorbs at the surface of the particles. Secondly, at least one soluble tail is linked to the anchor block. The tails must have a certain minimum length and the concentration of tails at the solid-liquid interface must be high enough to give the desired osmotic effect.
Disperbyk®-190, supplied by BYK-Chemie GmbH, is a comb-shaped dispersant that can be used in water-based systems to stabilise pigments and fillers against flocculation. The anchor block of each molecule contains a variety of anchoring groups. Adsorption at the surface of a range of solid particles can be obtained in this way. The additive is supplied as a ready-to-use solution of high-molecular weight block polymer in water.
Steric stabilisation versus electrostatic stabilisation
A low dosage of charged polymer is enough to give strong electrostatic stabilisation, provided that the dispersant molecules carry a negative charge and that the molecules adsorb at the surface of the particles. The complete surface of particles must be covered with dispersant that provides steric stabilisation. Otherwise, the concentration of soluble tails in the layer around the particles is too low to give the osmotic effect. The dosage of dispersant calculated on the amount of solid surface area of the particles, the dispersant load, is much higher for dispersants that provide steric stabilisation than for dispersants that give electrostatic stabilisation.
Electrostatic stabilisation, on the other hand, is sensitive to changes in pH and to the presence of salts. A system developer should know what possible destabilising influences the system might meet during its lifetime to make the right choices.
Technical Data Sheet of Disperbyk®-190
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