Layer buildup on two-layered porous substrate by dip-coating: Modeling and effect of additives on growth rate

A model was developed to describe layer buildup on a two-layered substrate during dip-coating. In this model, the growth rate can be used to estimate the permeability of the cast layer. Layers of submicron alumina were dip-coated onto the asymmetric porous substrate to evaluate the model. The layer buildup and properties were studied with different slip additives. Slips were deflocculated with nitric acid or sodium poly(methacrylic acid) (Na-PMAA) to achieve electrostatic or electrosteric stabilization. Properties of the slips with electrostatic stabilization were varied with poly(vinyl alcohol) (PVA) concentration. Properties of the slips with electrosteric stabilization were varied with Na-PMAA concentration. PVA addition decreased the growth rate of the layer by reducing capillary pressure and increasing the viscosity of the filtrate. Electrostatic stabilization of the slip with PVA addition also led to almost complete deflocculation and a rather high density of layers. Na-PMAA addition led to fast growth of the layer. Na-PMAA addition increased the capillary pressure and had a minor effect on the viscosity of the liquid phase. Na-PMAA addition led to an incomplete dispersion of particles and therefore a high porosity of layers. Permeability values of coated layers obtained from the model were compared with permeability values predicted by the Kozeny-Carman equation. In the case of complete deflocculation of the slip, the permeability values estimated with the model correlated reasonably well with the predicted values. Incomplete deflocculation led to higher permeability values than predicted by the Kozeny-Carman equation. (C) 2000 Academic Press.