Thixotropes and thickeners that control sag, leveling, and settling in coatings — fumed silica, surface-treated silica, amorphous silica, organoclays, and hectorite.
Rheology modifiers control how a coating flows: they hold viscosity high at rest so paint resists sag and pigment settling, then let it thin under the shear of brushing or spraying. Fumed silica (CAS 112945-52-5) is the workhorse thixotrope for solventborne systems, building a shear-reversible particle network at low loadings, often a few percent. Choose a rheology modifier by the problem you need to fix first. Sag, leveling, and anti-settling rarely respond to the same additive, so define the target before selecting a chemistry.
Fumed silica earns its place, but the trade-off is dispersion. It delivers strong thixotropy and anti-sag at low loading, yet it over-thickens easily and, if under-dispersed, can seed the film with visible specks. Hydrophobic modified fumed silica (CAS 68909-20-6) is the surface-treated grade for non-polar and moisture-sensitive systems. Start low, disperse under high shear, and adjust the loading upward rather than trying to rescue an over-thickened batch after the fact.
Amorphous silica (CAS 7631-86-9) does double duty as a matting agent and mild rheology modifier where a flat finish is wanted. Organoclays such as quaternary-ammonium bentonite (CAS 68953-58-2) and hectorite (CAS 12173-47-6) are clay-based thixotropes for solventborne and high-build coatings. HEUR and HASE associative thickeners cover most waterborne systems but are proprietary polymer blends without a single CAS, sourced to spec. Match the thickener chemistry to the carrier: clay and fumed silica for solventborne, associative polymers for waterborne.
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