The chemical surface composition of solid particles that are used in coatings and inks, pigments and fillers, governs the physico-chemical behaviour of the particles in the systems in which they are used. Formulators should be aware that solid particles can be post-treated by the manufacturer. This implies that each solid particle is coated with a specific other material.
In this article a few examples of the influence of post-treatment on properties of systems are discussed.
Colloidal Stability
Particles in a liquid attract each other. In order to prevent the spontaneous gluing together of solid particles, called flocculation, the particles must be covered with a layer of stabiliser called dispersant. The functionality of a dispersant is that it arranges a repulsive force between the solid particles, thus preventing flocculation.
To do their job, the dispersant molecules must adsorb strongly onto the surface of the solid particles. The chemical composition of the surface of the particles governs what (anchoring groups of) dispersants will be able to adsorb strongly onto that surface. Adsorption behaviour can be influenced by choosing the right anchoring groups or by selecting the proper post-treatment of the particles. The latter is especially important with respect to hydrophobic particles like most organic pigments.
Fastness
Fastness is the resistance of solid particles against external influences like chemicals, solvents, light or elevated temperatures. The fastness of pigments can be influenced by post-treatment. Often, the fastness of organic pigments goes down when particles are post-treated. In the table below, some key properties of two commercial organic Pigment Blue 15 pigments of Heubach GmbH are compared.
In this example, post-treating the primary particles improves the dispersibility of the pigment but at the expensive of fastness against solvents, especially alcohols.
Strength of systems
Systems that contain solid particles often fail at the interface of the surface of the solid particles and the binder matrix. The problem, anchoring failure, can be identified by studying the system under a microscope.
The mechanical strength of systems can often be improved by arranging coupling of the surface of the solid particles and the surrounding binder matrix. Coupling refers to chemical bonds that are formed, between the surface of the particles and the matrix, during cure. Most often silane functional coupling agents are used as post-treatment for the solid particles.
Sovitec supplies glass beads with a variety of post-treatments under the trade name Omicron®. Because of the post-treatment, these spherical glass particles are able to crosslink together with the binder system during cure. For example, for epoxy-amine binder systems glass beads with either amine-rich surface or epoxy-rich surface can be used.
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very good information, will be helpful to know that raw material can be used for coating pigments and make post-treatment.