Flame Retardancy and Thermal Performance of Rare Earth Oxide-Doped Silicone Rubber Nanocomposites

Abstract

Room-temperature vulcanized (RTV) silicone rubber is widely used for outdoor high-voltage insulation due to its strong hydrophobicity and anti-pollution performance, but its inherent flammability limits use for fire-prone environments. This study investigates novel RTV silicone rubber composites incorporating lanthanum oxide (La₂O₃), a rare earth oxide (REO), to enhance flame retardancy via radical scavenging and char formation. Four formulations are prepared: undoped silicone rubber, silicone rubber doped with 1 wt.% and 5 wt.% nano La₂O₃, and a commercially used 45 wt.% micro aluminum trihydrate (ATH)-filled rubber. The UL-94 test shows that the 1 wt.% La₂O₃ composite achieved a V-0 rating, outperforming all other specimens, including the ATH composite, which attained V-1. Thermo-oxidative treatment and Fourier transform infrared (FTIR) analysis reveal fewer surface microcracks in the 1 wt.% La₂O₃ sample, with minimal degradation of Si–(CH₃)₂ and Si–O–Si bonds, indicating reduced bond scission and effective radical suppression. Thermogravimetric analysis (TGA) confirms enhanced thermal stability, with delayed T5%, higher thermal resistance index, and increased char yield. Inclined plane tracking (IPT) further verified improvements, as the 1 wt.% La₂O₃ composite exhibited the shortest tracking length, surpassing the ATH-based and other test specimens. Breakdown strength (BDS), volume resistivity, and Shore hardness measurements demonstrate that the 1 wt.% La₂O₃ composite offers the best dielectric performance with suitable hardness, while the ATH composite provides moderate improvement over the undoped specimen (SU). Overall, the 1 wt.% La₂O₃-filled RTV composite demonstrates superior flame resistance, dielectric reliability, tracking resistance, and mechanical stability with retained flexibility, making it a promising material for high-voltage outdoor insulation under wildfire-prone conditions.