Load-dependent flow behaviour of silicate glass melts

Abstract

Starting from the basic experiments first of all the statements of the cylinder compression method will be regarded from which information will be obtained about flow behaviour, relaxation modulus and hot-fracture strength as important criteria for the workability of high-viscosity glass melts. On the basis of calibration measurements it is shown that viscosities can be determined from 1 · 10^14 to 2 · 10^5 Pa s due to the given limits of the apparatus. For the standard glass I of the Deutsche Glastechnische Gesellschaft and for a commercial float glass the purely rheological decrease of viscosity (non-Newtonian or structural viscosity) of earlier investigations could be confirmed, the investigated viscosity range could be essentially enlarged and a clear separation could be made from thermally dependent viscosity decrease by dissipation energy. The effect of non-Newtonian flow behaviour increases clearly with temperature at equal axial stresses and shows a non-monotonous course in the range 10^9 to 10^7 Pa s. An interpretation of these observations is given in connection with a load-dependent alteration of the structure of the glass melt and the non-monotonous course is thought to be connected with crystal nucleation processes.