Abstract
According to the ASTM D97, the pour point is the temperature below which petroleum products cease to flow. To evaluate the relevance of pour point measurements for synthetic lubricating oils, we investigated the crystallization, melting temperature and low-temperature flow behavior of one mineral and five synthetic lubricating oils. The classification of three groups emerged from this process. The formation of paraffin crystals in mineral oils (I) below the crystallization temperature causes shear-thinning behavior and a yield point. The crystallization temperature determined in the thermal analysis and rheology correlates well with the pour point. Synthetic lubricating oils, which solidify glass-like (II), exhibit a steady viscosity increase with falling temperature. The temperature at which viscosity reaches 1000 Pas corresponds well to the pour point. Synthetic oils, especially esters, with complex crystallization behavior (III), exhibit supercooling depending on the shear rate and cooling conditions. For these lubricating oils, the pour point provides no information for low-temperature applicability.
Highlights
The pour point [1] is commonly used to predict lubricating oils’ applicability at low temperatures
Webber [2] stated that the formation of wax crystals at low temperatures in mineral oils causes a substantial increase in viscosity and leads to highly non-Newtonian flow behavior
At temperatures below the onset temperature for wax crystallization Tc, mineral oils undergo a transition from a homogenous Newtonian fluid to a suspension of crystals exhibiting predominantly elastic behavior with G >> G and a yield point [3]
Summary
The pour point [1] is commonly used to predict lubricating oils’ applicability at low temperatures. The pour point is defined as the lowest temperature at which a base oil still flows. It is determined by examining the meniscus of the base oil in 3 K temperature intervals while tilting the sample vessel. Webber [2] stated that the formation of wax crystals at low temperatures in mineral oils causes a substantial increase in viscosity and leads to highly non-Newtonian flow behavior. At temperatures below the onset temperature for wax crystallization Tc, mineral oils undergo a transition from a homogenous Newtonian fluid to a suspension of crystals exhibiting predominantly elastic behavior with G >> G and a yield point [3]. The substantial change in the rheological properties result in the pour point
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