Moisture Transport in PA6

Sammanfattning: Polyamide 6 (PA6) is known to absorb water from its environment due to its chemical structure. The water absorption leads to changes in the mechanical properties as well as an increase in volume (swelling) of the polyamide. In this work, the sorption and diffusion of water vapor are investigated through gravimetric experiments and numerical simulations. The mixture theory framework is adopted to derive a triphasic model consisting of dry solid, gas, and bound water. In this model, PA6 is considered as a porous media where water vapor diffusion is allowed in the free void spaces. Sorption is assumed to occur at small surfaces within the polymer membrane via adsorption and desorption. Governing equations are derived from mass conservation for the bound water and the water vapor as well as the energy conservation for the entire mixture. Moreover, the dissipation inequality has been exploited to account for the second law of thermodynamics. Mass exchange is allowed between the bound water and the water vapor to describe the adsorption and the desorption. The sorption isotherm and sorption kinetics are calibrated from the gravimetric measurements and the water vapor diffusion is fitted by simulating these tests. The derived model is used to simulate the ISO 62 and ISO 1110 used by IKEA to dry and condition PA6. Based on the simulations, it was found that an efficient model has been developed in order to determine moisture evolution when a specimen is exposed to various ambient climates. A key factor affecting the time it takes for a specimen to reach moisture equilibrium is the water vapor diffusivity.