Degradation of dispersant during milling

Deterioration of the ammonium salt of poly(methacrylic acid) (PMMAA-NH3), used as a dispersant during wet ball-milling of an alumina, has been investigated. Rheological behaviour, sedimentation tests, adsorption isotherms, pH and isoelectric point measurements, and infra-red analysis have allowed us to conclude that degradation of the dispersant takes place in two stages: (i) decrease of the charge by dehydration and (ii) complete neutralization of the dispersant by the formation of monodentate of the dispersant by the formation of monodentate COOX groups, which can lead to desorption of the polymer from the alumina surface. This second phenomenon involves a strong increase of viscosity. A low viscosity can be recovered by a subsequent addition of PMAA-NH3 at the end of milling. (C) 1996 Elsevier Science Limited