Influence Of Suspended Iso- And Anisometric Crystals On The Flow Behaviour Of Fluoroapatite Glass Melts During Extrusion

The rheological properties of glass melts containing fluoroapatite crystals were studied by means of extrusion. It was found that the morphology of the crystals has a strong influence on the flow behaviour of the corresponding melt during extrusion. At the same temperatures the Newtonian viscosities of the melt containing crystals with needle like morphology (glass ceramic series 2) are higher than those of the melt containing crystals with isometric morphology (glass ceramic series 1) although they have the same chemical composition. Series 2 is subject to a drastic reduction in viscosity at relatively small shear rates which is attributed to the orientation of needle like crystals. This is the so-called "suspension" effect.