The Rheology of ASTM ARO Series Mechanical Shear Stability Reference Oils

Viscosities at high and low shear rates were measured both before and after mechanical shear for the 12 ARO multigrade engine oils of the ASTM D 2, R & D VII-B Mechanical Shear Stability Program. The oils were divided into two groups by ?Nsp, the difference between low shear rate specific viscosity at 98.9 and 37.8°C (210 and 100°F). The positive ΔN^sp group suffered the greater average low shear rate permanent viscosity loss. Permanent viscosity loss at high shear rateswhere most machine operation occurs-was significantly less than at low shear rates for both groups. In several cases, the low shear rate permanent viscosity loss was nearly equal to the new oil temporary viscosity loss. Several of these oils became nearly Newtonian after mechanical shear. Long-term storage produced measurable viscosity recovery in some sheared oils. These observations are consistent with the hypothesis that polymer entanglements exist in conventional multigrade oils. The disruption of entanglements by mechanical shear leads to the observed rapid initial viscosity degradation of multigrade oils. The mechanism of mechanical shear is enough different from the mechanism of sonic shear (ASTM D 2603) to account for the different results of the two techniques.