Time-Temperature-Pressure Superposition in Polymer Thickened Liquid Lubricants

Shear-dependent viscosities have been measured over a range of temperature and pressure for seven engine oils blended to have kinematic viscosity of 14 mm2/s at 100 °C with two base oils and four viscosity modifiers. Elevated pressure measurements were performed with a pressurized thin-film Couette viscometer and ambient pressure measurements were done with a PCS USV viscometer. These measurements were fitted to a generalized Newtonian model with the effective shear modulus specified by an empirical power-law shifting rule. The use of PAO-40 as a thickener delayed the shear-thinning to very high stress as compared with the polymers. The rate sensitivity of the oils thickened with nondispersant polymers was similar. Like the Tannas TBS viscometer, the PCS Instruments USV viscometer provides shear-dependent viscosity measurements, which can be essential for the most accurate time-temperature-pressure shifting. Viscosities measured at high viscous power in the ambient pressure viscometer, however, tend to be influenced by thermal softening and at high stress by shear cavitation.