By Ingrid K. Meier and Bernhard Resch, Evonik Corporation
Introduction
Precipitated silicas have long been manufactured by feeding aqueous sodium silicate (water glass) and sulfuric acid into a batch reactor equipped with a mechanical stirrer. Mixing in such a manner generates regions within the reactor that differ in chemical composition. Consequently, the amorphous silica particles that precipitate vary in consistency and the final product has a broad particle size distribution.
Spherical precipitated silicas, on the other hand, can be produced via a controlled aggregation within a continuous loop reactor. The sulfuric acid is continuously fed into a loop reaction zone comprising a stream of rapidly moving liquid reaction media, and efficient mixing ensures that all nucleation sites are exposed to similar chemical environments resulting in very consistent particles. Additionally, controlled particle residence times enable narrow particle size distributions to be achieved.1,2
Further particle modification can be performed on the spherical precipitated silica particles after they are discharged from the loop reactor. For example, a surface area reduction results in particles with spherical morphologies, extremely low nitrogen surface areas, and low oil absorption values, enabling them to have a minimal impact on a coating’s viscosity.
Several studies have been conducted to understand the behavior of these spherical precipitated silicas in coatings applications.3,4 In interior architectural paints, they have been found to improve the burnish and wet scrub resistance of the coatings. They can also work synergistically with typical silica matting agents to improve the abrasion and scratch resistance of matte wood coatings. Increasing particle size appears to correlate with a decrease in coating transparency and an increase in matting behavior. However, for matte coatings, the question remains as to whether larger spherical silica particles are always better. Do they provide more efficient matting? Do they improve coating durability to a greater extent than the smaller particle sized grades?
Continue reading in the July-August 2026 Issue of CoatingsTech.