Time-dependent behavior with gel formation or curing
Figure 1: Time-dependent viscosity function of a sample with gel formation or curing: The viscosity value increases significantly after reaching time-point tS because of the gel formation or curing reaction. A previously defined viscosity value is reached at time-point tv.
This test is performed under constant shear conditions either at a constant shear rate or at a constant shear stress. In most cases, tests with controlled shear rate are preferred. One disadvantage of controlled-shear-stress tests is that, with the increase in viscosity, the resulting rotational speed will continuously decrease. This would result in the deformation velocity no longer being constant.
However, the latter is a decisive parameter for comparing the effects of shearing on the behavior of a sample. The measuring temperature is kept constant, thus providing for isothermal conditions. 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 curing, the two parts of the PP measuring system will stick together firmly, unable to be separated or cleaned without substantial effort. The result is usually evaluated as a time-dependent viscosity function (Figure 1).
Two time-points are of relevance here: the starting time tS at the beginning of the curve slope indicating the start of gel formation or a chemical curing reaction, and the time tV when a previously defined high-viscosity value has been reached.
However, with rotational tests, it is not possible to reliably provide an answer to the questions as to when a sample becomes solid and when the sample has been cured completely. This means that the entire curing process cannot be evaluated because at one point the cured mass is no longer able to flow and becomes solid. If the maximum torque of the rheometer has been exceeded, the rheometer can no longer control the preset rotational speed or shear rate. As a consequence, the speed will continuously decrease and the viscosity will approach infinity.
This is because the viscosity value is defined by the law of viscosity, stating viscosity as the shear stress divided by the shear rate. In the case of curing, the shear rate would finally reach zero. This also means that the law of viscosity would not be applicable here.
Our recommendation: Oscillatory tests are preferred over rotational tests for evaluating gel formation and curing processes because, with oscillatory tests, materials can be examined in their solid state as well.