STABILIZATION OF AQUEOUS ALPHA-AL2O3 SUSPENSIONS WITH BLOCK-COPOLYMERS

Water-soluble diblock copolymers (DBCPs) with a polyelectrolyte block are accessible by sequential anionic polymerization of tert-butylmetha crylate (TBMA) and ethylene oxide (EO), followed by polymer analogous conversion of the PTBMA into poly(methacrylic acid) (PMAA) blocks. The se materials are highly efficient dispersants for oxide ceramic powder s in aqueous media. A series of block copolymer samples with M-n rangi ng from 1300 to 38 900, and (EO:MAA) block length ratios from 0.5 to 1 1.7 were prepared with polydispersities close to 1.2. The influence of overall molecular weight and block length ratio, pH and ionic strengt h on the stability of aqueous alpha-Al2O3 suspensions was investigated by sedimentation and adsorption experiments, surface plasmon resonanc e (SPR) and electrokinetic measurements. The copolymers are capable of stabilizing alumina suspensions by a combination of specific adsorpti on of one block on the particle surface and a shielding effect provide d by the nonadsorbing block. In addition, the adsorption of the negati vely charged PMAA block on the appositely charged alumina surface reve rses the electrophoretic potential of the oxide particles, a process w hich is strongly pH dependent. With respect to the powder dispersing e fficiency, an optimum was found when the DBCP consisted of a short PMA A anchoring and an approximately tenfold longer PEO stabilizer block w ith an overall molecular weight of about 5000. Alumina suspensions wit h the DBCP added were highly dispersed and rather stable against salt addition in a much wider pH window than block-copolymer-free suspensio ns.