Читать книгу The Rheology Handbook - Thomas Mezger - Страница 168

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].