Abstract
The interaction between dissolved Ba+2 and dissociated ammonium salt of poly(acrylic acid) (PAA-NH4) in aqueous suspension has been studied. It is found that the dissolved Ba+2 causes flocculation of dissociated PAA-NH4, thus degrading its dispersing effectiveness in aqueous BaTiO3 suspensions. The concentration of PAA-NH4 required to stabilize aqueous BaTiO3 suspensions increases with increasing Ba+2 concentration at a given pH. A stability map, which is determined by rheology study, is constructed to describe the amount of PAA-NH4 required to have well-dispersed aqueous BaTiO3 suspensions as a function of Ba+2 concentrations at different pH values. The interactions between added boric oxide (B2O3) and organic additives, and their effects on the colloidal stability of aqueous barium titanate (BaTiO3) suspensions have been investigated. The initially Newtonian or pseudoplastic aqueous BaTiO3 suspension with poly(vinyl alcohol) (PVA) becomes dilatant and its viscosity also increases dramatically, when a critical amount of B2O3, 0.4 wt%, is added. This has been attributed to a chemical reaction between dissociated borate ion (B(OH)4-) and PVA, which forms a gel-type structure in the aqueous BaTiO3 suspension. The dilatancy and viscosity of aqueous BaTiO3 suspension with PVA and B2O3 increase significantly when a dispersant of ammonium salt of poly(acrylic acid) (PAA-NH4) is present. The ionization of B2O3 is enhanced by the basicity of dissociated PAA-, promoting the formation of PVA-B(OH)4- gel-type structure in the aqueous BaTiO3 suspensions. With added plasticizer of 1,2- (e.g., ethylene glycol (EG)) or 1,3-diol (e.g., 1,3-propanediol (PPDL)) molecular structure, both the dilatancy and viscosity of aqueous BaTiO3 suspensions with PVA, B2O3 and PAA-NH4 reduce dramatically. Stable compounds of diol-B(OH)4- are formed before the dissociated B(OH)4- reacts with PVA, reducing the degree of gelation of aqueous BaTiO3 suspensions. A stability map is constructed to describe the amount of EG required to obtain colloidally stable aqueous BaTiO3 powder suspensions with PVA, PAA-NH4 and B2O3.