Category Tech Guide

Fumed silica is a synthetic amorphous silicon dioxide produced via flame hydrolysis of silicon tetrachloride (SiCl₄). Primary particles of approximately 7nm fuse into aggregates during the high-temperature process, resulting in an ultra-high surface area material (100–400 m²/g) used as a rheology modifier and reinforcing filler.

Hydrophilic fumed silica has untreated surface silanol (Si-OH) groups that attract water, making it ideal for aqueous systems and polar resins. Hydrophobic grades are surface-treated with silanes or siloxanes to repel moisture, suited for non-polar systems, moisture-sensitive formulations, and anti-settling applications.

Silica matting agents create a micro-rough surface topology on the dried coating film. The irregular surface scatters incident light rather than reflecting it specularly, reducing measured gloss. Particle size (typically 3–12 μm) and loading level (2–8% by weight) control the final gloss value.

Fumed silica is widely used in industrial coatings, adhesives, sealants, elastomers (silicone rubber), polyester and epoxy resins, printing inks, and pharmaceutical formulations. It provides thixotropy, anti-sagging, and anti-settling properties across solvent-borne and solvent-free systems.

Fumed silica should be dispersed under high shear (e.g., dissolver disc at 10–20 m/s tip speed) to break apart agglomerates and develop the thixotropic network. Pre-wetting with a small portion of the resin carrier before high-shear mixing improves dispersion efficiency. Over-shearing can degrade the rheological network and should be avoided.