Low temperature rheology of lubricating mineral oils: Effects of cooling rate and wax crystallization on flow properties of base oils

Abstract

The formation of wax crystals in mineral oils at low temperatures due to poor solubility of n-paraffinic components profoundly affects the oil rheology. For example, under quiescent cooling, wax crystals nucleate and grow to form macroscopic structures that cause development of a yield stress and produce rheology that is stress history dependent. The crystal formation and growth process is temperature history dependent. We have explored the effects of temperature history on the flow properties of lubricating mineral oils that do not contain performance chemical additives. The evolution of viscosity with decreasing temperature (T) is characterized by a strong increase in viscosity that occurs over a narrow 3–5 K range where the onset temperature (Tc) corresponds to that at which wax crystals become microscopically visible. Increasing the cooling rate depresses Tc and causes the activation energy in the transition region to increase. These changes correlate to a decrease in the average crystal size and a strong increase in the apparent steady state viscosity of the wax crystal dispersions at T≪Tc. Despite strong effects of stress history, we show that T≪Tc steady state flow properties are determined by the cooling rate and its apparent effects on the process occurring at the onset of crystal nucleation and growth. These results are discussed in the context of a homogeneous nucleation model for wax crystallization.The formation of wax crystals in mineral oils at low temperatures due to poor solubility of n-paraffinic components profoundly affects the oil rheology. For example, under quiescent cooling, wax crystals nucleate and grow to form macroscopic structures that cause development of a yield stress and produce rheology that is stress history dependent. The crystal formation and growth process is temperature history dependent. We have explored the effects of temperature history on the flow properties of lubricating mineral oils that do not contain performance chemical additives. The evolution of viscosity with decreasing temperature (T) is characterized by a strong increase in viscosity that occurs over a narrow 3–5 K range where the onset temperature (Tc) corresponds to that at which wax crystals become microscopically visible. Increasing the cooling rate depresses Tc and causes the activation energy in the transition region to increase. These changes correlate to a decrease in the average crystal size and a st...