Study on the role of S-glass fiber in the high-temperature ablative resistance of modified phenolic resin-based composites

Abstract

Thermal Protection Systems (TPS) in aerospace and defense applications require materials capable of withstanding extreme thermal and ablative environments while maintaining structural integrity. In this study, a phenolic–borate modified resin (PBR)-based composite reinforced with S-glass fibers was successfully fabricated via a multi-stage hot-pressing process, with fiber contents varying from 65 to 80 wt%. The investigated properties included density and porosity (ASTM D792, ASTM D2734), tensile and flexural behavior (ASTM D638, ASTM D790), and high-temperature ablative performance according to ASTM E285 using an oxy-acetylene flame source. The results indicated that as the S-glass fiber content increased from 65 to 75 wt%, the tensile and flexural strengths significantly improved, reaching maximum values of 356.24 MPa and 784.35 MPa, respectively, with an elastic modulus of 31.49 GPa. The composite containing 70–75 wt% fibers exhibited optimal ablative resistance, achieving a maximum ablation duration of 275.6 s and a minimum linear ablation rate of 0.0118 mm/s. SEM observations after the ablative test revealed well-preserved fiber integrity and the formation of a thermally stable carbonaceous layer and surface deposits on the fibers, contributing to enhanced protection against thermal degradation. These findings demonstrate that the PBR/S-glass composite possesses excellent high-temperature ablative resistance within the 70–75 wt% fiber range, making it a promising candidate material for thermal protection system (TPS) applications under extreme heat flux conditions.