Effect of silica surface state on the hygrothermal aging behavior of silicone rubber composites for composite insulators

Silicone rubber for composite insulators is susceptible to hygrothermal deterioration, with filler–matrix interphase stability governing long-term reliability. Here, silica-filled silicone-rubber systems with tailored specific surface areas and hexamethyldisilazane (HMDS) surface modification (unmodified: 200/260/380 m 2 /g; HMDS-modified: 200 m 2 /g) were aged at 95 °C/95% RH and evaluated through mechanical, electrical, and structural characterization. Aging caused concurrent deterioration in mechanical and insulating performance, accompanied by surface oxidation, pore-type defects, and matrix-component depletion. Meanwhile, the interfacial-relaxation feature in ε″ was markedly weakened, mainly due to interfacial debonding and defect accumulation disrupting interphase continuity. The specific-surface-area effect was non-monotonic: after 1500 h aging, 260 m 2 /g retained 98.7% of tear strength and 97.3% of the Weibull shape parameter ( ), while reducing matrix-component depletion ( ) by 65.1% compared with 200 m 2 /g, indicating more stable aging durability. In contrast, 380 m 2 /g developed weak zones induced by agglomeration and increased moisture-active sites, weakening its long-term reinforcement advantage. HMDS modification improved hygrothermal-aging resistance by reducing interfacial polarity and moisture affinity; it retained 92.4% of the and reduced by 74.7% compared with 200 m 2 /g. Although it showed relatively lower initial tensile strength, its initial elongation at break reached 531% and remained the highest during aging, demonstrating a typical “toughening–strength reduction” characteristic.