Semisil R620 Vs Aerosil R972

Both SEMISIL R620 and AEROSIL R972 are surface-modified with hexamethyldisilazane (HMDS), which reacts with isolated and geminal silanol groups on the fumed silica surface to graft trimethylsilyl (–Si(CH₃)₃) groups. This reaction converts hydrophilic Si–OH sites into hydrophobic methyl-terminated surfaces. The base oxide for both grades is a standard 200 m²/g hydrophilic fumed silica — SEMISIL 200 and AEROSIL 200 respectively — which after HMDS treatment yields a post-treatment BET of approximately 110 ± 20 m²/g. The degree of silanol conversion depends on HMDS stoichiometry, reaction temperature (typically 250–350 °C in gas-phase processes), and residence time. Higher conversion yields lower residual silanol density and stronger hydrophobicity, measured as methanol wettability.

Methanol wettability quantifies the minimum methanol-water concentration needed to wet the treated silica surface — higher values indicate stronger hydrophobicity. AEROSIL R972 typically specifies a methanol wettability of ≥40%, with batch averages near 42–48%. SEMISIL R620 targets a slightly tighter specification window around 45–55%, reflecting a marginally higher degree of surface methyl coverage. Carbon content — a proxy for total organic surface coverage — runs 0.7–0.9% for R972 and 0.8–1.0% for R620. In practice, this 0.1–0.2% carbon difference translates to roughly 5–10% more trimethylsilyl groups per nm² on R620, which can improve moisture resistance in sealant and adhesive formulations exposed to high-humidity environments.

HMDS-treated grades are broadly compatible with condensation-cure (tin- and titanate-catalyzed) and addition-cure (platinum-catalyzed) silicone systems because trimethylsilyl groups do not poison platinum catalysts or consume crosslinker. Both R620 and R972 perform well in RTV-1 and RTV-2 silicone sealants at 4–8 wt% loading for thixotropy. However, in peroxide-cure systems above 160 °C, the slightly higher methyl density on R620 can generate marginally more volatile methyl radicals during decomposition — formulators using dicumyl peroxide (DCP) cure should verify final crosslink density at \>6 wt% loading. In epoxy and unsaturated polyester systems, both grades provide anti-settling at 1–3 wt% without interfering with amine or MEKP cure chemistry.

For thixotropy in solvent-based coatings and adhesives, both grades deliver comparable yield stress at equivalent loading. R620 shows a slight edge in low-polarity systems (xylene, toluene, aliphatic hydrocarbons) due to its higher methyl coverage, which improves initial dispersion and reduces structure-building time by approximately 10–15%. R972 remains the incumbent standard with broader global availability and established regulatory dossiers (REACH, TSCA, FDA indirect food contact). Price-sensitive formulators should note that R620 typically prices 15–25% below R972 at equivalent quality, making it attractive for high-volume applications in silicone sealants, gel coats, and industrial adhesives where the Evonik specification is not explicitly mandated by end customers.