Influence of Nano-Additives on the Long Term Performance of Silicone Rubber Composites for High Voltage Insulation Room temperature vulcanized silicone rubber (RTV-SiR) has been widely used for high voltage (HV) electrical and non-electrical applications for the last three decades. Neat (pure: un-aged) silicone rubber exhibits poor mechanical, thermal, electrical, tracking/erosion, and water immersion properties which restrain its insulation applications for long-term performance. Environmental, electrical and chemical stresses significantly influence the electrical and non-electrical properties of polymer insulators. The variation in physical, mechanical, thermal and electrical properties over aging are major dilemmas of HV polymer insulators which are yet to be thoroughly investigated. During the past few decades, much effort has been made to enhance its insulation properties. To overcome these deficiencies, various types of micro and nano-sized inorganic particles (like alumina trihydrate (ATH), silica (SiO2), ZnO, titanium oxide, calcium carbonate, barium-titanate etc.), fiber, whisker and platelets are incorporated into polymer matrix. To enhance mechanical, thermal and electrical properties of the base material, filler optimized concentration, type and shape is necessary for any particular property enhancement. Moreover, to increase the long term performance of polymer composites under diverse environmental, mechanical and electrical stresses; detailed characterization and aging analysis is required. This thesis mainly focuses on the preparation, characterization and long term (9000 h) multi stress accelerated lab weathering evaluations of micro-ATH and SiO2 filled SiR composites. In addition, the reinforcement of SiR matrix is successfully accomplished with the combination of micro-ATH and micro/nano-SiO2 particles. All SiR-blends with different composition by weight percent (wt. %) of ATH and SiO2 are prepared by simple blending in a two roll mixing mill as per ASTM standard procedure. Moreover, for comparative analysis composites of micro/nano-SiO2 filled ethylenepropylene-dyne-monomer (EPDM) and epoxy resin are also formulated. In this thesis first, SiR composites with ATH/SiO2 particles are investigated to comprehend the effect of micro-ATH and micro/nano-SiO2 particles on theechanical, thermal and electrical properties. In addition, for comparative analysis the influence of hybrid-SiO2 (micro + nano) on mechanical, thermal and electrical properties of SiR and EPDM composites are also investigated. Experimental results shows that compared to neat and other silica filled composites, the micro-ATH filled SiR composites exhibit higher mechanical, thermal and electrical properties. Secondly, the impact of micro-SiO2, micro-ATH and mixture of micro and nano-sized particles (ATH+SiO2) on tracking/erosion and water absorption resistance of SiR novel composites are investigated using the Inclined Plane Test (IPT) and water immersion tests as per ASTM standards. Additionally, to validate and compare the amalgamation and dispersion effects of micro/nano-SiO2 on tracking/erosion and water immersion resistance of polymer composites, some blends of SiR and epoxyresin are also evaluated. Results shows that the erosion/tracking and water immersion resistance of prepared insulants were significantly modified by addition of ATH and SiO2 particles. In addition, compared to SiO2 filled SiR and epoxy composites, the ATH filled SiR composites shows higher mass loss. Moreover, IR-thermographs reveals that maximum and minimum tracking temperature is exhibited by micro-ATH blend (SMC2) and neat SiR, respectively. Despite many advantages of SiR composites, their long-term performances when exposed to environmental and electrical stresses, are not investigated in detail. This work also includes the 9000 h multi-stress accelerated lab aging of micro-ATH and micro/nano-silica filled SiR composites with the aim to quantify the weathering effects (simulating actual field environment of Islamabad (Pakistan)) on surface morphology, hydrophobicity, leakage current (LC), and structural variations. At the end of aging period, progressive discoloration, roughness, and chalking is viewed on HV side of all blends. Hydrophicity analysis indicates that compared to ATH filled composites (SMC1, SMC2 and SMNC), the silica filled blends (SR, SMC3) exhibit higher surface resistivity (hydrophobicity) over 9000 h weathering. Furthermore, variation in LC peaks are found in safe limit (˂ 1mA) over aging period. In addition, FTIR spectra of all SiR composites exhibit decrease in the concentration of their functional groups over weathering. However, compared to neat blends, the composites filled with ATH/SiO2 particles exhibit little variation in absorbance peaks %) over aging period. Generally, all samples successfully endured 9000 h lab weathering. This thesis also presents the influence of multiple stresses on mechanical, thermal electrical, and water immersion properties over 9000 h lab aging. Moreover, at the end of aging period these properties (mechanical, thermal, electrical and water immersion) of SiR composites are first time re-evaluated and compared with the characterization results of neat composites.