Original article date: Oct. 28, 2016
Pigments and fillers, both consisting of solid particles, are important raw materials for paints. The powdery materials are wetted, separated and stabilized during paint production1. Separated solid particles in a liquid have the tendency to glue together because of attractive Van der Waals forces between the particles. The spontaneous, and undesired, gluing together of solid particles in a liquid is called flocculation.
Colloidal stability, the stability against flocculation, can be assured by using the right type of dispersant2,3 in the right amount. A suitable dispersant assures that the solid particles repel each other. The repulsive forces, provided by the dispersant molecules adsorbed at the surface of the solid particles, must be stronger than the attractive Van der Waals forces.
Two mechanisms, steric stabilization and electrostatic stabilization, can be used to prevent flocculation. This article is about steric stabilization.
Flocculation is prevented via steric stabilization when all solid particles are covered with polymeric tails that dissolve in the liquid surrounding the particles, the so-called continuous phase. In a water-based paint, for example, the tails must be water-soluble to provide steric stabilization.
The concentration of soluble tails in the area between particles goes up when the particles approach each other. This is unfavorable from an 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. The principle of steric stabilization can be used in both solvent-based and water-based systems.
Dispersants that provide steric stabilization are polymeric molecules consisting of two functional parts. An anchor block must assure that the molecules adsorb at the surface of the particles. Secondly, at least one soluble tail is attached to the anchor block. The tails must have a certain minimum length, and the concentration of tails in the layer around the particles must be high enough to give the desired osmotic effect.
Disperbyk®-190 is a comb-shaped dispersant that can be used in water-based systems to stabilize a wide variety of, both organic and inorganic, pigments and fillers against flocculation. The anchor block of each molecule contains several anchoring groups, also referred to as ‘pigment affinic groups.’ Strong adsorption at the surface of a range of solid particles is obtained in this way. The additive is supplied as a ready-to-use solution of high molecular weight block copolymer in water.
Key aspects of steric stabilization
A relatively high dosage of polymeric dispersant is needed when the additive gives steric stabilization. This is because a substantial percentage of the solid surface of the particles must be covered with polymer. 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 stabilization than for dispersants that give electrostatic stabilization.
An advantage of steric stabilization, compared to electrostatic stabilization, is that it has a low sensitivity towards changes in pH and the presence of salts. This is important in water-based systems.
It is difficult to give particles an electrostatic charge in solvent-based systems. Because of that, steric stabilization is the main mechanism used in solvent-based systems.
- Article The Basics of Dispersion and Stabilization of Pigments and Fillers, Jochum Beetsma, 31 January 2020.
- Article Dispersants – The Understood Misunderstood Additive, Marc Hirsch, 10 September 2021.
- Article Understanding Dispersants, Marc Hirsch, 19 February 2016.
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