Temperature-dependent behavior with gel formation or curing
Figure 1: Temperature-dependent viscosity function during heating, with the minimum viscosity ηmin and subsequent increase in viscosity caused by a gelation or curing process.
Typical tests in this field are aimed at investigating the behavior during gel formation or chemical curing when a sample is heated. This measurement is carried out under constant shear conditions, either as a controlled-shear-rate (CSR) test or a controlled-shear-stress (CSS) test. In most cases, CSR tests are preferred. One disadvantage of CSS tests is that, with increasing viscosity, the resulting rotational speed will decrease continuously. This would result in the deformation velocity no longer being constant. However, the latter is a decisive parameter for the comparability of the effects that shearing has on the behavior of a sample.
In addition, a defined temperature profile is preset, for example with a time-dependent temperature gradient of 1 °C/min. For these tests, it is recommended that disposable measuring systems consisting of a disposable plate and a disposable dish for single use are selected. In most cases, after the curing process, both parts of the PP system will stick together firmly and cannot be separated or cleaned without substantial effort.
Evaluation is carried out using a temperature-dependent viscosity curve (Figure 1). The focus is predominantly on the evaluation of the temperature at minimum viscosity and the viscosity value at this point. If the minimum viscosity is too high, the sample might not level as well as desired; if it is too low, problems such as edge failure of the coating may occur.
Please note: It is, in principle, not possible to evaluate an entire hardening or curing process with a rotational test because the viscosity will eventually approach infinity. Therefore, oscillatory tests are recommended for the evaluation of gel formation and curing processes because, with such tests, materials can be examined in their solid state as well.