Mass Gain of Carbon Fiber Reinforced Polyimide (CFRP) by Dipping in Hot Distilled Water

Influence of isothermal dipping in hot distilled water for 200 ks on the mass gain of carbon fiber reinforced polyimide (CFRP) was investigated. Dipping in distilled water raised the mass gain. Based on the kinetics equation, both reaction index (n) and kinetic constant (k) were obtained. Since the n value is approximately 0.6 from 333 to 373 K, the reaction mode was independent on the absorption temperature. Based on the results and discussion, the water absorption was probably the directional mass transport through the fiber/polyimide cylindrical interface. On the other hand, Arrhenius linear relationship between logarithmic k (log k) and reciprocal clipping temperature (1/T) was obtained. The apparent activation energy estimated by the slope of water absorption in the CFRP was approximately equal to the activation energy of self-diffusion coefficient of water molecule. Mass gain of CFRP (CF/PI) dipped in distilled water for 100 ks at 373 K probably enhanced the molecules' density induced by water molecules' intrusion among polyimide polymers, resulting in enhancement of (d sigma/d epsilon)(max) and fracture stress. On the other hand, the mass gain by dipping for 200 ks at 373 K enriched the water concentration in polyimide matrix. Thus, it was assumed to enhance the intermolecular distance of polyimide polymers and then to reduce the intermolecular force among polyimide polymers, resulting in a drop of (d sigma/d epsilon)(max) of CFRP dipped for 200 ks. On the other hand, the dipping for 200 ks probably raised the intermolecular force between water molecules and polyimide polymers, resulting in remarkable enhancements of the fracture strain, fracture energy and impact value of the CFRP. [doi:10.2320/matertrans.M2011215]