Ammonium polyacrylate adsorption on “aluminium hydroxides and oxyhydroxide” coated silicon nitride powders

Si3N4 powders coated with pseudoboehmite, Al hydroxide with a OH/Al mol ratio (R) lower than 3 and bayerite, were prepared from different coating solutions and dispersed in water with an ammonium polyacrylate (NH4PA) to produce 11.8 vol% slips. The coated powders showed isoelectric points at pH 7, 7.5 and 7.5 close to that of the precipitate coatings of pseudoboehmite, Al hydroxide (R<3) and bayerite, respectively, indicating that the surface coverages were almost complete. The bayerite coated powders had the lowest specific surface area values. The coatings on Si3N4 particles were shown to improve the adsorption of NH4PA dispersant on Si3N4. The maximum adsorption of the polyelectrolyte at pH 9.5 increased from 0.01 for the uncoated powder to 0.03, 0.055 and 0.08 mg/m2 for the coated powders with pseudoboehmite, Al hydroxide (R<3) and bayerite, respectively.