Equilibrium water uptake of epoxy/carbon fiber composites in hygrothermal environmental conditions

Equilibrium water absorption behavior of epoxy/carbon fiber composites was investigated under hygrothermal conditions, The temperature dependence of t he equilibrium water uptake was quantitatively determined by assuming two d istinctly different physical states of bound water and free water, The boun d water was viewed as a reversible thermodynamic quantity and thus its temp erature dependence was described by a negative value of the exothermic heat of sorption in Henry's law. Ascribed to the Langmuir nonequilibrium kineti c process, the free water was assumed to be immobilized by irreversible mic rocavitational damage or crazing in hygrothermal environmental conditions. Accordingly, the amount of free water increased with hygrothermal temperatu re in an irreversible way and its temperature dependence was expressed by t he polymer-diluent affinity parameter in Langmuir equation. For various hyg rothermal conditions, the total amount of equilibrium water uptake was succ essfully predicted by the sum of the bound water at the last hygrothermal t emperature and the free water at the highest hygrothermal temperature.