The effect of processing parameters on the properties of gamma-alumina washcoats deposited on ceramic honeycombs

The coating of cordierite honeycomb specimens with gamma-alumina slurries f or the preparation of washcoats for automotive applications was investigate d. The dependence of slurry viscosity on factors such as the solids content , the pH and the particle size distribution of the powder used, was determi ned. Slurry viscosity was correlated to the loading percentage achieved, as well as to the quality of the washcoat in terms of homogeneity and reprodu cibility. It was found than an adequate solids content in the slurry is nec essary for the achievement of satisfactory loading per impregnation. When, though, the particle size of the powder employed is of colloidal dimensions , high solids content leads to extremely high viscosity values. Adjustment of the slurry viscosity is therefore necessary and this was achieved with t he use of either HCl or ammonium poly-methacrylate, an organic polyelectrol yte. Optimum loading conditions were achieved when the slurry viscosity lie d between 50-150 mPa.s. For a specific solids content, the organic polyelec trolyte led to lower viscosity slurries and resulted in better reproducibil ity of the loading percentage. With the use of ammonium poly-methacrylate, slurries of fine particles, up to 40 wt% solids content could be handled, r esulting in reproducible loading percentages of the order of 15 wt%. (C) 20 00 Kluwer Academic Publishers.