(382g) Rheological Measurements on Extremely Concentrated Suspensions Using Squeeze Flow

Some bulk molding compounds consist of extremely concentrated suspensions whose behavior lies between those of concentrated suspensions and granular materials. These compounds have the consistency of wet sand and their rheological properties cannot be measured using rotational viscometers. Capillary viscometers and extrusion-type extensional viscometers cannot be used either because of particle degradation and because measurements must be performed at high temperatures at which the thermoset resins harden. Squeeze flow may provide a way of ascertaining the rheological properties of these compounds. In particular, we examine the use of the constant-force squeeze-flow technique described by Meeten (Rheol. Acta 41, 557-566, 2002) for the measurement of the rheological properties of the bulk molding compounds used in the manufacture of bipolar plates for fuel cells. These compounds consist of extremely concentrated suspensions of graphite powder in polymer resins containing hardeners and other additives. Preliminary data suggests that in some cases the molding process may be fast enough to be unaffected by the curing of the polymer resin and that the compounds remain fairly homogeneous during the molding process. The technique provides an effective way of distinguishing between compounds for quality-control purposes and can be used to test possible constitutive equations for these compounds. The effect of slip on the interpretation of the measurements will be discussed.