Equilibrium Water Uptake of Epoxy/Carbon Fiber Composites in Hygrothermal Environmental Conditions

Abstract

Equilibrium water absorption behavior of epoxy/carbon fiber composites was investigated under hygrothermal conditions. The temperature dependence of the equilibrium water uptake was quantitatively determined by assuming two distinctly different physical states of bound water and free water. The bound water was viewed as a reversible thermodynamic quantity and thus its temperature dependence was described by a negative value of the exothermic heat of sorption in Henry’s law. Ascribed to the Langmuir nonequilibrium kinetic process, the free water was assumed to be immobilized by irreversible microcavitational damage or crazing in hygrothermal environmental conditions. Accordingly, the amount of free water increased with hygrothermal temperature in an irreversible way and its temperature dependence was expressed by the polymer-diluent affinity parameter in Langmuir equation. For various hygrothermal conditions, the total amount of equilibrium water uptake was successfully predicted by the sum of the bound water at the last hygrothermal temperature and the free water at the highest hygrothermal temperature.