Langmuir-Type Model for Anomalous Moisture Diffusion In Composite Resins

Considerable evidence has recently been presented for anomalous mois ture diffusion in epoxy matrix composites. In an earlier paper we noted that there is no a priori reason why moisture should obey simple diffusion theory rather than Kirkwood's (linear) generalization of the Boltzmann transport equation. With the intent of keeping the analytical aspects of the moisture diffusion problem as simple as possible, we here present a slightly generalized but linear model which involves sources and sinks of diffusing water molecules. With respect to diffusive characteristics, our model is related to the simplest form of neutron transport theory. With respect to bound and unbound particles it is similar to the Langmuir theory of ad sorption isotherms, although we here assume bulk absorption in the resin with no implication that surfaces are involved. An approximation to our exact solution of the coupled linear differential equations is used to fit mildly anomalous moisture uptake curves for 5208 resin exposed to several rclative humidities for two years. The fact that the same parameters give equally good fits to the data at all humidities suggests that the absorption anomaly does not result from nonlinear (e.g. concentration-dependent) effects.