Low Temperature Rheology Of Lubricating Oils PDF Download

The association of the predominantly n-paraffinic components of mineral oils into wax nuclei that grow and interact to form macroscopic crystalline structures leads to highly non-Newtonian rheology that is strongly dependent on both temperature and stress history. Upon cooling to temperatures lower than the onset temperature for wax crystallization (Tc), the viscosityrtemperature curve shows a high activation energy region that persists over a narrow temperature range (~3-5 °C). In this temperature range, the oil transitions from a homogenous solution to a two-phase wax crystallite dispersion. By applying controlled stress rheometric techniques, we show that the high activation region is related to gelation index described in ASTM D 5133, and that the gelation index temperature corresponds to Tc. The high activation energy region is associated with the relief of supersaturation accrued within the oil upon cooling to temperatures below that of the saturation temperatures of the paraffin molecules in solution. Therefore, the gelation index characterizes the onset of nucleation rather than the formation of macroscopic wax crystal structures that would be associated with a yield stress and gelation. The activation energy and rheology at T