A new processing aid for dry-pressing: A copolymer acting as dispersant and binder

PVX copolymers containing both carboxylate (-COO-) and hydroxyl groups (vinyl alcohol) are synthesized with the objective to ensure the double function of dispersant and of binder for dry-pressed green parts. Carboxylate groups are responsible for strong adsorption of copolymers onto alumina surface and can promote sufficient electrostatic repulsive forces to achieve a good state of dispersion, similar to that obtained with a classical ammonium poly methacrylate (PMA-NH4+) for a proportion of carboxylic groups in the copolymer larger than 35%. On the other hand, hydroxyl groups confer higher mechanical strength than PVA to the green pressed parts (4.2 MPa for 1.5 wt.% PV35 (35% -COO-) versus 1.8 MPa for 1.5 wt.% PVA, with and without PEG as plasticizer, in the case of samples pressed at 120 MPa). The high density of the green parts obtained with soft PV35 copolymer and the good adsorption of this compound onto alumina surface lead to a high mechanical strength of the pressed compacts. After addition of 1.5 wt.% PV35, the amount adsorbed (0.7 wt.%) confers a high cohesion to the spray-dried granules and the non-adsorbed copolymer (0.8 wt.%) reinforces adhesion between granules through interdiffusion of the low T, polymeric films. Then, synthesized PVX copolymers have proved to be efficient dispersants for aqueous suspensions of alumina and also efficient binders for dry-pressing, even for concentrations as low as 1.5 wt.%. (c) 2006 Elsevier Ltd. All rights reserved.