Low Temperature Rheology of Commercial Base Stocks

Two commercial base stocks are evaluated using a controlled stress rheometer, a cold cranking simulator and a scanning Brookfield viscometer. These oils form wax structures below 0°C that significantly influence viscosity. These structures break down with time under shear. This degradation process depends on the magnitude and duration of applied stress. A constant stress causes a continuous decrease in viscosity even after an hour of shear. Viscosity approaches a limiting value if the imposed shear stress increases at a constant rate. Repetitions of this test lead to a lower viscosity that is relatively insensitive to shear rate over the range of applied stress. In all cases, the wax structure does not readily reform after degradation for oils held at the test temperature. These results are used to suggest the domain of current procedures and their relation to higher temperature measurements.