Diffusion model based on Fick’s second law for the moisture absorption process in wood fiber-based composites: is it suitable or not?

Abstract

The diffusion model based on Fick’s second law was applied to the moisture absorption process in wood fiber-based composites. The data of moisture absorption process on wood fiberboard and wood fiber/polymer composites with different densities were analyzed using the diffusion model. Specimens were subjected to different relative humidities and temperatures. The diffusion coefficients (D) and surface emission coefficients (S) along the thickness direction were calculated by the unsteady-state method using a nonlinear curve fitting algorithm. The experimental results showed that the S obtained from Fick’s second law diffusion model did not provide an external resistance when the moisture is absorbed into the wood. While the diffusion model did not provide the best fit of the moisture absorption data, the calculated internal moisture transfer rate, D, still shows physical meanings. The temperature had a significant effect on the D. The higher the temperature, the greater the D obtained. The D showed a decreased trend as the RH (or EMC) increased. No specific trends were found on the D as a function of board density. Other limitations of the diffusion model for the moisture movement in wood are also discussed.