Dispersion and Stabilization of solid particles (pigments and fillers) in a liquid is a challenging process taking place during the production of paints, colorants, and inks. The process can be accelerated and improved when the basic principles, governing the process, are used in an effective way.
Many problems can be prevented, and time and money can be saved when fundamental knowledge, as well as key information about the raw materials and equipment, is understood and used.
The whole process of dispersion and stabilization consists of three steps starting from dry powdery material, consisting of agglomerates of solid particles.
The powdery solid is mixed with liquid and wetting of the particles takes place. The solid particles are separated from each other by applying mechanical force. Immediately after separation, the particles have to be stabilized to prevent flocculation, the gluing together of solid particles in a liquid.
The objectives of the process are to:
− Obtain optimum hiding power of the system
− Prevent flocculation and sedimentation
− Assure protective, mechanical and aesthetical properties of the system
The Three Steps of Dispersion Process
The three steps take place simultaneously but let’s discuss these steps separately…
In the wetting step air, being present in the agglomerates, is replaced by the liquid that is used in the dispersion process.
Wetting: Solid-air Interface Converts to Solid-liquid Interface
Wetting can proceed when the surface tension of the liquid is low enough compared to the surface energy of the solid particles. It is not easy to obtain surface energy values of pigments and fillers. In many cases, this key property is governed by the type and amount of post-treatment used in the production process of the solid particles.
Wetting agents are additives used to facilitate wetting in case the surface energy of the particles is low. The key functionality of wetting agents is to lower the surface tension of a liquid. The use of wetting agents can induce problems, like:
− Insufficient inter-coat adhesion
− Increased water sensitivity, and
− Reduced film hardness
In many cases, wetting agent is not needed in the process and often the use of wetting agent even retards wetting instead of improving it. Preferably, a system is made without using wetting agents. The right types of pigments and fillers have to be selected in order to be able to develop a system without wetting agents.
Wetting, including the release of air from the system, proceeds faster when the viscosity of the liquid is lower. The importance of wetting is often underestimated, thus giving problems in the steps that follow in the dispersion and stabilization process.
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