The Rheology Handbook. Thomas Mezger

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СКАЧАТЬ time (in min) is defined as: tr = tgel – tmin with tmin at the time point of the viscosity minimum. Measured is under isothermal conditions, e. g. using a PP-geometry (gap 0.5 mm) at a constant shear rate of 10 s-1.

      Some further, but often very simple test methods to determine whatsoever gel times or gelation times are briefly presented in Chapters 11.2.1d/e/f (formation and tackiness of resin filaments, temperature increase due to an exothermic reaction), 11.2.8e (falling ball), 11.2.11a/b (rotation until standstill) and 11.2.12a4/5/6 (reduction of an oscillation amplitude path, reaching the phase shift angle of 45°, deflection path by the formograph); see also Chapter 12 (guideline, 12.2.2, 12.2.4, 12.3.3, 12.3.5), and the Index. In Chapter 11.2.13 information can be found about the following terms: incubation time, vulcanization time, scorch time, rise time, cure time of uncured rubbers, crosslinked rubbers and elastomers.

      Note 1: Pot life , open time , and gelation time of resins

      The following terms are often used to analyze the period of time for the use of a resin in an open container at the processing temperature. The reason for the increasing gel formation may be drying, oxidation or air humidity, or a curing reaction with two-component resins. Typical evaluation by a rotational test is a viscosity measurement at a low constant shear rate, e. g. at γ ̇ = 1 s-1. Possible criteria for analysis are:

      1 The pot life is the period of time as long as a material is applicable [3.73]. In daily practice, many users are choosing the time point when the initial viscosity value has doubled.

      2 The open time is the period of time for which the material is still able to flow. This time is over if a previously defined, upper limiting value of the viscosity is reached; this time point is often also called the gelation time.

      Tip: It is more meaningful to evaluate here the viscoelastic properties using oscillatory tests (see Note 1 in Chapter 8.5.3b).

      3.1.2.1.5b) Comparison of controlled shear rate (CSR) , and controlled shear stress (CSS) tests

      For hardening or curing processes, CSR tests are involved with the disadvantage that the preset constant shear rate remains unchanged even if the viscosity values are continuously increasing and the sample is becoming more and more solid and therefore more inflexible. This can lead to irreversible partial destruction of the structure, therefore decisively disturbing a homogeneous hardening process. CSS tests offer an advantage here: The torque (or shear stress, resp.) is kept constant and the hardening sample causes the resulting rotational speed (or shear rate, resp.) to decrease. Using the CSS mode, the continuing hardening process is disturbed less and less, and therefore, it is less influenced by the resulting decreasing degree of deformation. Finally, if the resistance force of the solid sample is larger than the shear force which is applicable by the test instrument, the rotational speed will be displayed as n = 0 (or as shear rate γ ̇ = 0, respectively). This indicates that the measuring system has come to a standstill after all.

      Note 2: Advantage of oscillatory tests when determining the gel point

      Nowadays, rotational tests should no longer be used for accurate investigations of processes such as gel formation, hardening or chemical curing reactions, since here, the process kinetics and reaction development, and therefore the test results, are often strongly influenced by the test conditions. Instead, oscillatory tests in the linear viscoelastic (LVE) deformation range should be performed since in this range, the user can be sure that the sample’s structure is strained only to a very limited extent, therefore remaining undestroyed during the whole test. Using this mode of testing, as well the onset of gel formation as well as the gel time can be exactly determined in the form of the sol/gel transition point (see Chapter 8.5.3b with Figure 8.36). A further advantage of oscillatory tests is the fact that the test can be still continued, even if the hardened material is already showing gel-like character, behavior of a soft or even of a rigid solid. In this case, viscosity values would be displayed as already “infinitely high”, being therefore no longer detectable by rotational tests at all. Further information on gel formation can be found in [3.20] [3.21] [3.22].

3.5Temperature-dependent flow behavior and viscosity function

      This section informs about the temperature-dependence of viscosity. Here, the degree of the shear load is kept at a constant value for each single test interval.

3.5.1Test description

      Preset

      1 With controlled shear rate (CSR): γ ̇ = const (see Figure 3.34), and time-dependent temperature profile T(t), e. g. as an upward or downward ramp

      2 With controlled shear stress (CSS): τ = const (similar to Figure 3.34), and time-dependent temperature profile T(t), e. g. as an upward or downward ramp

      Result

      Function of τ (or γ ̇ , respectively) and η dependent on the temperature (as in Figures 3.35 to 3.37, if time t is replaced by temperature T on the x-axis; see also Figures 3.46 and 3.47).

      The shape of the curves of τ(T) and η(T) are similar because τ and η are proportional since

      τ = η ⋅ γ ̇ (here with γ ̇ = const). However, the curves of the functions of γ ̇ (T) and η(T) show an

      inverse shape since γ ̇ and η are inversely proportional (since η = τ/ γ ̇ , here with τ = const).

       Figure 3.46: Temperature-dependent viscosity curve

3.5.2Temperature-dependent flow behavior of samples showing no hardening

      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), СКАЧАТЬ