Pigments and fillers are solid particles that are used in paints and inks.1 The chemical composition of the surface of such particles influences the physico-chemical behavior of the particles in the systems in which they are used. Formulators should be aware that solid particles might be post-treated by the producer. Post-treatment is an additional step, in the production process of pigments and fillers, in which each solid particle is coated with another material.
Post-treatment of solid particles.
In this article, a few examples of the influence of post-treatment of solid particles on their properties, and on the properties of the systems in which the particles are used, are discussed.
Stability against flocculation
Solid particles glue together because they attract each other. The spontaneous and undesired process of gluing together of solid particles in a liquid is called flocculation. The function of a stabilizer called dispersant is to provide repulsive forces between the particles, thus preventing flocculation.
Adsorption of dispersant molecules at the surface of a solid particle.
To do their job, dispersant molecules must be adsorbed at the surface of the solid particles. The chemical composition of the surface of a particle governs what dispersants will be able to adsorb strongly at that surface. Adsorption behavior can be influenced by choosing the right anchoring groups, being part of the dispersant molecules, or by applying the proper post-treatment of the particles. The latter is especially important with respect to hydrophobic particles, like most organic pigments and carbon blacks. The particles of untreated carbon black pigments, for example, are hydrophobic because the surface consists (mainly) of carbon. It is difficult to adsorb polymer molecules at the surface of hydrophobic particles.
Matching anchor sites and anchoring groups.
Carbon black can be post-treated via oxidation, thus transforming the surface from hydrophobic (nonpolar) to hydrophilic (polar). To be more precise, the surface of oxidized carbon black particles is acidic. It is said that oxidized carbon black particles are covered with acidic anchor sites. It is much easier to adsorb dispersant at such a surface, especially when the dispersant molecules contain basic amine groups. When doing so, the anchoring groups of the dispersant are adjusted to the anchor sites on the surface of the solid particles. In this example, matching anchor sites and anchoring groups enables strong acid-base interactions at the interface of particle and dispersant, resulting in strong adsorption.
Fastness of pigments
Fastness is the resistance of solid particles against external influences like chemicals, solvents, light, UV radiation and elevated temperatures. The fastness of pigments can be influenced by post-treatment. The fastness of organic pigments can become worse when the particles are post-treated. In the table, some key properties of two Pigment Blue 15 pigments of Heubach GmbH are compared.
Properties of Pigment Blue 15: untreated versus post-treated.
In this example, post-treating the blue pigment particles, going from PB 15:3 to PB 15:4, improves the dispersibility of the pigment but at the expensive of fastness against solvents, especially alcohols.
Mechanical strength of systems
Systems, like coatings, that contain solid particles can 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.
Loss of mechanical strength because of interfacial failure. (courtesy Potters Industries LLC)
The mechanical strength of coatings can often be improved by coupling the surface of the solid particles with the surrounding binder matrix of the coating. Coupling refers to chemical bonds that are formed during cure of the coating, between the surface of the particles and the resin system of the coating.
Coupling of the surface of solid particles and the binder matrix. (courtesy Potters Industries LLC)
Bifunctional silanes can be used as so-called coupling agents, to post-treat solid particles that have hydroxyl (-OH) groups on their surface.2 One type of chemical group of such silanes reacts with the surface of the solid particles, thus forming a covalent bond. The other type of reactive group is able to participate in the cure of a 2-component resin system during film formation. The principle of coupling solid particles and binder matrix is identical to the chemical modification of a substrate to enable adhesion via covalent bonds, so-called interfacial crosslinking.3
Functionalization of filler via chemical reaction with bifunctional silane
References
- High Performance Pigments for Industrial Coatings, Wally Kesler, 16 October 2020
- Superior Coatings Performance with Organosilane Components, Ron Lewarchik, 10 August 2018
- Co-dependent: Adhesion on Steel via Covalent Bonding, Jochum Beetsma, 2 March 2018
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