Non-Newtonian flow behaviour of a phosphate glass melt at a viscosity close to the working point

Abstract

The dependence of both the shear and the extensional viscosity of the glass melt, namely, NaPoLi melt (Na2O ∙ Li2O ∙ 2P2O5), on the shear and extensional rate, respectively, was studied by means of a capillary method at a low shear viscosity near the working point at ηs = 10^3 Pa s. To accurately calculate shear viscosities, the Bagley end correction was made by determining the pressure drop at the entrance of the capillary. The extensional viscosity was estimated using the Cogswell model. It was found that a dilatant flow behaviour occurred at low shear rates, whereas a shear-thinning flow behaviour did at high shear rates. At the same time, an extension-thinning flow took place, which onsets at a very low extensional rate and then gradually tends to a constant value. With increasing the extensional rate, the Trouton ratio strongly deviates from the value of pure inelastic and isotropic fluids, i.e. 3. Αll the observed phenomena imply that the NaPoLi glass melt during extrusion is an anisotropic and viscoelastic fluid.