MELT RHEOLOGY OF ZINC ALKALI PHOSPHATE-GLASSES

The melt rheology of two zinc alkali phosphate glasses was studied wit h oscillatory flow experiments to accelerate efforts to melt process t hem with organic polymers. The glasses differed markedly in chemical d urability and melt viscosity. Melt viscosities were time independent a t temperatures near their glass transition temperature. The omega depe ndence of the complex viscosity eta conformed excellently to predicti ons, eta(omega) = eta(0)(1+iw tau), for Hookean dumbbells. Fit parame ters eta(0) and tau were used to estimate a melt viscosity-average mol ecular weight M using a model reported by Ferry and co-workers [J. App lied Phys. 26, 359 (1955)]. Arrhenius activation energies Delta H doub le dagger for viscous flow were consistent with those traditionally fo und for inorganic glasses. At higher temperatures, the melt viscosity of both glasses increased monotonically with time. The viscosity incre ase was exponential at long times, and the transition time to the expo nential dependence was found to be strongly dependent on the shear str ain. The steep viscosity rise is expected to cause significant process ing challenges. Sample crystallization is thought to be responsible fo r this behavior. (C) 1996 Society of Rheology.