Hygrothermal degradation of short-glass-fiber reinforced polycarbonate: effect of fiber content and orientation, and modeling

Abstract This study examines the degradation behavior of PC/GF composites with varying short glass fiber (SGF) contents (10, 20, and 30 wt%) and fiber orientations (0°, 45°, and 90°) when exposed to conditions of 85 °C/85% RH. Although the elastic modulus remained largely unchanged, both tensile strength and strain at break exhibited significant deterioration as a result of moisture ingress, which promoted interfacial debonding (after 1008 h, tensile strength decreased by up to 27.7% and strain at break by up to 45.8%). To quantify chemical degradation, a hydroxyl index (HI) was derived from ATR-FTIR spectroscopy. The temporal evolution of HI followed a power-law kinetic model that was independent of fiber content, indicating that matrix hydrolysis governs the degradation rate, with HI increasing by up to 3.57×, while the bulk-average molecular weight decreased by 5.8–8.7%. Furthermore, a phenomenological model was developed linking the HI to macroscopic mechanical properties. This model accurately captured time-dependent, anisotropic degradation based on the material’s chemical state, yielding cross-validation errors under 5%. Ultimately, these findings demonstrate that non-destructive spectroscopic techniques can reliably estimate structural integrity, providing a robust framework for durability assessment and lifetime prediction in harsh environments.