Читать книгу The Rheology Handbook - Thomas Mezger - Страница 171

Viscosity values always depend on the measuring temperature. In almost all cases, viscosity decreases if the sample is heated. Users mostly are interested in the softening or melting tempera­ture . Highly viscous materials usually show greater temperature dependence compared to low- viscosity ones.

When cooling, the solidification temperature is determined. The term crystallization temperature is used for crystal-forming materials and freezing point for water, and sometimes, congealing temperature for gel-forming materials.

Usually, the temperature is presented on the x-axis of the η(T)-diagram on a linear scale, and viscosity on the y-axis, either on a linear scale, as shown in Figure 3.46, or on a logarithmic scale, if the values are covering a wide range.

Due to the strong dependence of all rheological parameters on the temperature, for the test protocol it is recommended to specify the measuring temperature exactly for each measuring point. It is therefore essential to control the temperature carefully. For information on “freeze-thaw-cycle tests” in order to evaluate temperature stability of emulsions, see Chapter 8.6.2.2b: oscillatory tests.

Note: High-temperature test on a glass melt

Viscosity tests were performed on a standard glass sample DGG1 from PTB (Physikalisch-Tech­nische Bundesanstalt, Braunschweig, Germany), using a concentric cylinder measuring geometry made of aluminum oxide (bob diameter 15 mm, cup internal diameter 30 mm), as rotational tests in the shear rate range of 0.1 s-1 to 10 s-1. After exceeding the softening point at T = 930 °C, in each case occurred ideal-viscous flow behavior, showing the following values of the shear viscosity η (see Table 3.4) [3.77].

Table 3.4: Viscosity values of a glass sample
measuring temperature	1000 °C	1050 °C	1100 °C	1200 °C	1250 °C	1350 °C	1400 °C
viscosity of the glass melt	760 Pas	310 Pas	150 Pas	40 Pas	30 Pas	12 Pas	7 Pas

Values of the complex viscosity obtained with oscillatory tests on the same measuring sample resulted in similar results (see Note 3 in Chapter 8.6.2.2).

Figure 3.47: Temperature-dependent viscosity curve of a material showing gel formation,

hardening or curing