Fumed Silica Hydrophobic RX-200 (Surface-Modified, PDMS-Treated)

Hydrophilic fumed silica (e.g., Aerosil 200 type) has free silanol groups on the surface that are strongly hydrogen-bonding. It works well in polar systems (waterborne coatings, adhesives) and medium-polarity solvents (esters, ketones), forming a thixotropic silanol-H-bond network. In non-polar solvents (mineral spirits, aliphatic hydrocarbons), hydrophilic silica struggles to form a stable gel because solvent penetration into the silanol network is insufficient. Hydrophobic fumed silica (RX-200 type, PDMS-treated) has silanol groups replaced with polydimethylsiloxane chains. These hydrophobic surface groups allow excellent network formation in non-polar and low-polarity solvents and provide outstanding thixotropy in alkyd, epoxy, and polyurethane solvent-borne coatings. For waterborne systems, hydrophilic silica is almost always the correct choice; for solvent-borne systems, especially in aliphatic or low-polarity media, hydrophobic grades like RX-200 give superior anti-settling and thixotropy.