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

The formation of wax crystals in mineral oils at low temperatures due to po or solubility of n-paraffinic components profoundly affects the oil rheolog y. For example, under quiescent cooling, wax crystals nucleate and grow to form macroscopic structures that cause development of a yield stress and pr oduce rheology that is stress history dependent. The crystal formation and growth process is temperature history dependent. We have explored the effec ts of temperature history on the flow properties of lubricating mineral oil s that do not contain performance chemical additives. The evolution of visc osity with decreasing temperature (T) is characterized by a strong increase in viscosity that occurs over a narrow 3-5 K range where the onset tempera ture (T,) corresponds to that at which wax crystals become microscopically visible. Increasing the cooling rate depresses T, 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 much less than T -c. Despite strong effects of stress history, we show that T much less than T-c steady state flow properties are determined by the cooling rate and it s apparent effects on the process occurring at the onset of crystal nucleat ion and growth. These results are discussed in the context of a homogeneous nucleation model for wax crystallization. (C) 1999 The Society of Rheology . [S0148-6055(99)01004-4].