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

The dependence of both the shear and the extensional viscosity of the glass melt, namely, NaPoLi melt (Na2O . Li2O . 2P(2)O(5)), on the shear and exte nsional rate, respectively, was studied by means of a capillary method at a low shear viscosity near the working point at eta(s) = 10(3) Pa s. To accu rately calculate shear viscosities, the Bagley end correction was made by d etermining the pressure drop at the entrance of the capillary. The extensio nal viscosity was estimated using the Cogswell model. It was found that a d ilatant flow behaviour occurred at low sheer rates, whereas a shear-thinnin g flow behaviour did at high shear rates. At the same time, an extension-th inning flow took place, which onsets at a very low extensional rate and the n 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. All the observed phenomena imply that the NaPoLi glass melt during extrusion is an anisotropic and viscoelastic fluid.