Bi-phasic water diffusion in sheet molding compound composite

Abstract

Sheet molding compound (SMC) composite has been studied under humid-aging conditions. Diffusion of water within the material, as measured by gravimetry, was found to be in good agreement with the “Langmuir-type” diffusion model developed by Carter and Kibler. In their theory, Carter and Kibler consider the existence of two types of water molecules in the material, «mobile» and «bound». In this study, these two types have been considered separately. Furthermore, Infrared spectroscopy (FTIR) analysis has been performed by decomposition of signals to study the fraction of free (mobile) and hydrogen-bounded water. Thermal analysis and microscopic observations were put forward to explain the two types of water molecules. In this contribution, we found a “bi-phasic” water diffusion. We suggest that the «mobile» water corresponding to the diffusion in the micro-porosities follows a Fickien kinetic, which turns to sigmoidal one at a specific time of immersion τ. Whereas the kinetic of «bound» water, referring to crosslinking and plasticization, follows a sigmoidal kinetic, which turns to Fickien behavior when the overall network is saturated.