Effect of PAO Polymerization Degree on the Structure and Elevated-Temperature Tribological Performance of Preformed Polyurea Greases

Conventional poly-α-olefin (PAO) base oils are widely used in grease formulations, but how their degree of polymerization affects oil-phase confinement and release within polyurea thickener networks at elevated temperatures remains unclear. Here, PAO20, PAO40, and PAO100 were used to prepare PGA, PGB, and PGC, respectively, each containing 10 wt % of the same preformed polyurea thickener. Microscopy and X-ray scattering characterized thickener swelling, network aggregation, and local packing. Rheological measurements assessed network strength, LF-NMR at 125 °C assessed oil-phase mobility, and pressure oil-separation tests quantified oil release after high-temperature treatment. PGA exhibited the greatest thickener swelling and network dispersion, together with the highest oil-phase mobility and oil separation (13.29 wt %). PGC showed more pronounced fiber aggregation, the highest rheological strength, the most restricted oil-phase mobility, and the lowest oil separation (0.60 wt %). PGB exhibited intermediate oil-phase mobility, oil separation (8.43 wt %), and network strength. At 125 °C, 35 N, and 5 Hz, PGB showed the shortest running-in period (876 s), the lowest whole-test average coefficient of friction (0.130), and the narrowest wear scar (1470 μm), together with reduced metal transfer and a lower metal-oxide contribution on the worn surface. Tribological performance therefore did not vary monotonically with PAO polymerization degree: neither the highest oil-phase mobility nor the strongest oil confinement corresponded to the lowest friction and wear. These findings provide a physicochemical basis for selecting conventional PAO base oils for polyurea greases under the investigated elevated-temperature conditions.