Effect of Vinyl Silane on the Thermal Durability of Silica-Reinforced PFAS-Free Polyurethane Composites for Fluoroelastomer Replacement

 Fluoroelastomers (FKMs) are widely used in high-temperature applications; however, increasing PFAS regulations and the high cost of fluorinated materials have accelerated the search for PFAS-free alternatives. In this study, a peroxide-curable polyester-based millable polyurethane (M-PU) was investigated as a potential replacement for FKM in applications requiring thermal stability up to 200°C. Silica was incorporated as a reinforcing filler, and the effects of vinyltrimethoxysilane (VTMS) and bis[3-(triethoxysilyl)propyl] tetrasulfide (Si-69) were evaluated. Mechanical properties and thermal aging resistance at 200°C for 168 h were assessed.
 Silica reinforcement improved the mechanical performance of M-PU, while the effectiveness strongly depended on silane chemistry. VTMS-treated compounds exhibited significantly higher tensile strength than untreated and Si-69-treated formulations. The optimized VTMS formulation achieved an initial tensile strength of 185 kgf/cm² and retained 118 kgf/cm² after aging at 200°C for 168 h, indicating excellent high-temperature durability. In contrast, Si-69 provided limited reinforcement despite its widespread use in sulfur-cured elastomer systems.
 The superior performance of VTMS is attributed to the participation of vinyl groups in peroxide-induced radical reactions, resulting in stronger interfacial interactions between silica and the polyurethane matrix. Because Si-69 is primarily designed for sulfur-vulcanized elastomers, its effectiveness was limited in the peroxide-cured M-PU system.
 These results demonstrate that vinyl silane is an effective coupling agent for improving the thermal durability of silica-filled M-PU composites. The VTMS-silica system maintained practical mechanical strength after severe thermal aging at 200°C and shows strong potential as a PFAS-free, cost-effective alternative to conventional fluoroelastomers.