Estimation of diffusion and solubility coefficients for water and CO₂ in reaction injection molded parts

Abstract The evolution of gases from Reaction Injection Molded (RIM) parts during painting causes “pinhole” surface defects, which result in scrap. The transport of gases plays a major role in this outgassing. The diffusion rates of CO₂ and water through reaction injection molded parts is measured in this work. Despite the presence of glassy hard domains and dispersed microbubbles of size <25 μm (nucleated parts), the diffusion showed Fickian characteristics, and diffusion coefficients were independent of the concentration of water and CO₂ in nucleated and non‐nucleated RIM parts. The presence of microbubbles enhances diffusion, which could be predicted from the diffusion rate through the non‐nucleated material by using a simplistic one‐dimensional model. The diffusion coefficients of water and CO₂ follow an Arrhenius relationship. The solubility coefficients follow a van't Hoff relationship over a wide range of 20 to 150°C. Apart from predicting diffusion and solubility coefficients, for various RIM materials and processing conditions, the estimated parameters will be used to interpret the outgassing phenomenon in a subsequent paper.