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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 < Tc depend on temperature and stress history, the paraffin molecular weight distribution in the oil, and the concentration of pour-point depressant. We show that the gelation index parameter is non-unique, coupled to both the effects of stress history on viscosity and the dependence of nucleation on the n-paraffin mplecular weight distribution. It is not surprising then that no correlation between gelatipn index and MRV TP-1 yield stress (ASTM D 4684) is observed.
gelation index, nucleation, stress history, yield stress, crystallization, rheology, wax
Research engineer, The Lubrizol Corporation, Wickliffe, OH
senior engineer, The Lubrizol Corporation, Wickliffe, OH
senior research scientist, The Lubrizol Corporation, Wickliffe, OH