ESTIMATION OF DIFFUSION AND SOLUBILITY COEFFICIENTS FOR WATER AND CO2IN REACTION INJECTION-MOLDED PARTS

The evolution of gases from Reaction Injection Molded (RIM) parts duri ng painting causes ''pinhole'' surface defects, which result in scrap. The transport of gases plays a major role in this outgassing. The dif fusion 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 mu m (nucleated parts], the dif fusion showed Fickian characteristics, and diffusion coefficients were independent of the concentration of water and CO2 in nucleated and no n-nucleated RIM parts. The presence of microbubbles enhances diffusion , which could be predicted from the diffusion rate through the non-nuc leated material by using a simplistic one-dimensional model. The diffu sion coefficients of water and CO2 follow an Arrhenius relationship. T he solubility coefficients follow a van't Hoff relationship over a wid e range of 20 to 150 degrees C. Apart from predicting diffusion and so lubility coefficients, for various RIM materials and processing condit ions, the estimated parameters will be used to interpret the outgassin g phenomenon in a subsequent paper.