The long-term hydrophobic stability of silicone rubber (SiR) insulators is essential for their reliability in high-voltage direct current (HVDC) applications. This study investigates the effect of nano-silica fillers with varying specific surface areas (SSA = 120

200 and 300 m2/g) and different concentrations of alumina trihydrate (ATH) fillers (5–50 wt.%) on the dynamic hydrophobicity of SiR using the dynamic drop test (DDT) under HVDC stress. Key performance indicators—hydrophobicity retention

recovery and transfer—were evaluated. Experimental results indicate that nano-silica

particularly with an SSA of 200 m2/g

(SFS200) significantly improves hydrophobic performance

offering the best balance of retention time

recovery

transfer and reproducibility. In contrast

high ATH loadings hinder the migration of low molecular weight and reduce recovery. Moderate ATH content (10 wt.%) showed optimal performance among ATH-filled samples but was still inferior to silica-filled composites. Although higher SSA enhanced retention time

an excessively high SSA (300 m2/g) resulted in agglomeration and consequently reduced recovery. These results confirm the superiority of fumed silica in improving the hydrophobic stability of SiR insulators and highlight the critical role of filler characteristics—particularly SSA

porosity and dispersion—in optimising SiR performance under HVDC stress. The study also demonstrates that additional experimental precautions improved DDT reproducibility

supporting its use in future standardisation.