A SIMPLE-MODEL FOR MOISTURE SORPTION IN EPOXIES WITH SIGMOIDAL AND 2-STAGE SORPTION EFFECTS

In the present paper the moisture absorption in an epoxy is considered for two cases, very often present simultaneously: (i) the sorption is sigmoidal with a considerable deviation from Fickian behaviour for small values of time and (ii) two-stage sorption takes place for large values of time. A simple mathematical model for the sorption process is proposed, which seems to cover satisfactorily the experimental data. It is based on the idea that besides the diffusion, fast and slow changes in the properties of material are taking place due to the interaction between the epoxy and the fluid. So the model utilises three constants: (i) the average diffusion coefficient, (ii) one characteristic time for the fast processes and (iii) one for the slow processes. The values of the parameters are determined as a function of the relative humidity with a general curve fitting type procedure. The model also offers a generalisation of the standard Fickian determination of the average diffusivity from the sorption curve for the case of small time sigmoidal behaviour.