Introduction of glass viscosity

Viscosity is one of the important properties of glass. It penetrates through the whole process of glass production and processing, directly affecting the process of melting, clarification, homogenization, forming, annealing and heat treatment of glass.

The viscosity of the glass increases with decreasing temperature. During the change of the glass from the molten state to the solid state, the change of viscosity is continuous, gradual, and there is no numerical mutation between them. This is different from crystals, where the viscosity changes abruptly when the crystal is heated to the melting point. The speed of viscosity change with temperature is an important indicator of glass production. It is often referred to as the material property of glass. The glass whose viscosity changes rapidly with temperature is called short glass, otherwise it is called long glass.

Now introduce the viscosity reference points commonly used in glass production:

(1) Strain point: The temperature at which the stress can be eliminated within a few hours corresponds to a temperature at a viscosity of 1013.6 Pa·s.

(2) Transition point: A temperature corresponding to a viscosity of 1012.4 Pa·s, which is usually expressed by Tg.

(3) Annealing point: The temperature at which the stress can be removed within a few minutes is roughly equivalent to the temperature at a viscosity of 1012 Pa·S.

(4) Deformation point: corresponds to a temperature range when the viscosity is 1010 to 1011 Pa·S.

(5) Softening temperature: It is related to the density and surface tension of the glass, and corresponds to the temperature at a viscosity of 106.6 Pa·s, which is usually expressed by Tf.

(6) Operating range: equivalent to the temperature range of the surface of the molded glass. The upper limit of T means the temperature to be molded, and corresponds to a temperature at a viscosity of 102 to 103 Pa·S; the lower limit of T corresponds to a temperature at which the shape of the product can be maintained during molding, and corresponds to a temperature at a viscosity of >105 Pa·S. The viscosity of the operating range is generally 103-106.6Pa·S

(7) Melting temperature: corresponds to a temperature at a viscosity of 10 Pa·s at which the glass can be melted at a generally required rate. Commonly used Tm.

(8) Viscosity of automatic feeder feeding: 102-103Pa·S.

(9) Artificial picking viscosity: 102.2 Pa·S.

Glass viscosity and composition

The effects of various common oxides on the viscosity of glass are summarized as follows:

(1) Silicon dioxide, aluminum oxide, zirconium dioxide, etc. increase the viscosity of the glass.

(2) The alkali metal oxide R2O lowers the viscosity of the glass.

(3) The effect of alkaline earth metal oxides on the viscosity of glass is complicated. On the one hand, similar to alkali metal oxides, it can depolymerize large tetrahedral groups and cause viscosity reduction; on the other hand, these cations have higher electricity prices and less ionic radius, so the bond force is larger than that of metal ions, and it is possible to capture The oxygen ions of the small tetrahedral group are around themselves, increasing the viscosity. The former effect is dominant at high temperatures, while the latter effect is mainly at low temperatures. The order of viscosity increase ability of alkaline earth metals is: Mg2+>Ca2+>Sr2+>Ba2+, in which CaO increases viscosity at low temperature; at high temperature, when the content is less than 10%-12%, the viscosity is lowered, and when the content is less than 10%-12%, the viscosity is lowered. Viscosity.

(4), PbO, CdO, Bi2O3, SnO2, etc. reduce the viscosity of the glass.

In addition, Li2O, ZnO, B2O3, etc. all have the effect of increasing the low temperature viscosity and lowering the high temperature viscosity.