Extraction of binders from green ceramic bodies by supercritical fluid: influence of the porosity

Organic additives can be extracted from extruded or injection-moulded ceramic parts by using supercritical fluids leading to defect free green parts in a short time. A model previously defined, that predicts kinetics of extraction of solid binder molecules by solubilization and diffusion of solubilized species has been modified with a corrective term taking into account the capillary migration of a liquid organic phase according to the microstructure (pore size distribution, tortuosity) of the green sample and to the characteristics of the liquid phase (viscosity, surface tension). An analytical expression of this corrective term is proposed, for a paraffin binder and for three alkanes which are representative of its molecular distribution. The corrective term is related to the length scale over which the liquid binder flow occurs and to its linear velocity. The modified model allows the prediction of the kinetics of extraction of a given paraffin binder, by using the effective diffusion coefficient of the binder, determined with large pore size green samples and a capillary factor, calculated from binder and microstructure characteristics of the green part.