Long‐Term Behavior of Carbon Fiber‐Reinforced Thermoplastic Composites for Type V Hydrogen Storage Tanks: Effects of Hygrothermal Aging on Physicochemical and Mechanical Properties

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ABSTRACT This study investigates the hygrothermal aging behavior of carbon fiber‐reinforced polyphthalamide (CF/PPA) composites used in Type V hydrogen tanks, which are exposed to humid environments during service. Water absorption was evaluated at 25°C, 50°C, 80°C, and 90°C over aging periods up to 240 h. It followed a Dual‐Fick model, with diffusion coefficients ranging from 1 × 10 −12 to 5 × 10 −12 m 2 /s (D 1 ) and 2 × 10 −13 to 5 × 10 −13 m 2 /s (D 2 ), and saturation levels up to 3.9 wt%. This dual‐phase diffusion reflects water ingress both into the polymer bulk and along fiber/matrix interfaces via capillary transport, leading to a localized plasticized interphase near the fibers. Physicochemical and mechanical consequences of water absorption were studied. A notable decrease in the glass transition temperature ( T g ) was observed, confirming the plasticization of the PPA matrix. Tensile tests were conducted to ascertain the effect of water content on ultimate stress. The results revealed a decrease in ultimate stress with increasing water content at different temperatures. Microscopic analysis confirmed interfacial decohesion between fiber and matrix after 72 h of aging at 20°C and 90°C, with more severe decohesion observed at the higher temperature. These findings emphasize the key role of the fiber/matrix interface in regulating moisture‐induced degradation and highlight the importance of considering dual‐phase diffusion mechanisms.

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