Extrusion of TiO2 ceramic powder paste

The extrusion of ceramics often requires the wetting of the powder with a binder in order to produce a paste. To control and optimise the production it is interesting to model behaviour of the paste under external conditions that are closed to the actual extrusion conditions. The aim of this paper is to investigate the mechanical behaviour of a mixture made of a ceramic powder (TiO2) and an aqueous acidic binder. The viscous behaviour is investigated using: (i) A capillary rheometry which enables, for homogeneous materials, the determination of the shear viscosity as a function of the shear strain rate; (ii) a new rheometer called 'cross apparatus', developed at LMT Cachan. The experimental results evidenced two specificities of this material: (i) Wall conditions play a major role during the how through a capillary and it can be assumed that the paste neither sticks to the wall nor that it slips with a viscous friction law; (ii) the cross apparatus confirms these results: since the paste behaves in a very different way than does a viscous non-Newtonian fluid. There could be many reasons as to why these results are obtained, but probably the most important is some dissociation taking place in the material, indicating the inappropriateness of a homogeneous model. A biphasic interpretation of the flow has been developed in a similar way to that for injection molding, which enable an explanation of the particular results of the capillary test. The originality of his paper lies in experimental results that are both characteristic of pastes and different to those that are obtained with 'classical' non-Newtonian fluids such as homogeneous molten polymers. (C) 1997 Elsevier Science S.A.