Comparative mechanisms of hydrothermal versus hydrolytic degradation of polypropylene and nylon 6 in aqueous and saline-alkaline environments

Polypropylene (PP) and nylon 6 (PA6) are widely employed in engineering applications, yet their durability under combined hygrothermal and saline-alkali conditions remains insufficiently understood. In this work, the long-term aging behaviors of PP and PA6 were systematically studied through immersion in water and saline-alkaline solution at 20, 40, and 60 °C for up to 120 days. Water absorption, tensile performance, dynamic mechanical analysis, and micro-structural characterizations (FTIR, DSC, SEM) were employed to elucidate the degradation mechanisms. Results demonstrated that both polymers exhibited non-Fickian diffusion. PP showed limited water uptake (≤1.6 %) and a diffusion coefficient of ∼10⁻¹¹ cm² s⁻¹, with aging dominated by reversible physical plasticization. In contrast, PA6 absorbed 6–8 times more water than PP due to hydrogen-bonding with amide groups, and its hydrolysis (k = −1.56 < 0) led to irreversible chemical degradation. After 120 days in 60 °C saline-alkaline solution, PP retained ∼73.5 % of its tensile strength with only superficial pitting, while PA6 suffered a 39.3 % strength loss accompanied by corrosion holes (up to 50 μm) and a 20 °C drop in glass-transition temperature. The crystallinity of PA6 decreased from 45.7 % to 32.1 %, confirming chain scission and reduced cross-linking density. These findings highlighted that PP degraded primarily through hydrothermal plasticization, whereas PA6 underwent hydrolytic chain cleavage, which was markedly accelerated by elevated temperature and alkaline media. This study had revealed the degradation differences between non-polar and polar polymers under harsh conditions, providing a theoretical basis for material selection and life prediction in hygrothermal and saline-alkali engineering, and clarifying the fundamental differences in the mechanism of “physical plasticization - chemical hydrolysis".