Volume 48, No 1, 2026, Pages 199-209
Download full text in PDFEnhanced Low Temperature Flow Behavior of Polyurea EV Greases Using Synthetic Base Oils
Authors:
Raj Shah 
,
George S. Dodos ,
Michael Lotwin 
DOI: 10.24874/ti.2068.11.25.02
Received: 15 November 2025
Revised: 8 January 2026
Accepted: 6 February 2026
Published: 15 March 2026
Abstract:
Electric and hybrid vehicle (EV/HEV) motor bearings demand greases that remain sufficiently mobile at sub-zero temperatures. In this study, a series of polyurea-thickened grease formulations was prepared with three different low-viscosity synthetic base oils: polyalphaolefin (PAO), polyalkylene glycol (PAG), and polyol ester (POE), all with International ISO VG 32. Flow pressure was measured from 0 °C to –40 °C using a tester per DIN 51805-2, where the measured pressure indicates grease yield strength and relates to start-up torque and churning resistance in sealed EV bearings. A modified protocol introduced 4- and 8-hour relaxation periods at –20 °C and –30 °C to evaluate time-dependent stiffening. Under standard conditions, all greases showed acceptable flowability (well below 1400 mbar at –30 °C), with PAO exhibiting the lowest pressures. At –40 °C, PAO and PAG remained extrudable, while POE exceeded the 3225 mbar instrument limit due to crystallization. Extended cold soak caused only modest pressure increases for PAO and PAG but a rapid rise for POE (>3000 mbar after 4 h at –30 °C). These results demonstrate that base oil composition profoundly influences low-temperature grease mobility. In particular, the ester base oil’s tendency to crystallize under prolonged cold exposure led to markedly worse flow behavior, highlighting the need for appropriate base oil selection and modified test protocols for EV applications. This test provides a screening measure of cold-flow mobility, not a direct in-bearing torque evaluation.
Keywords:
Polyurea grease, Electric vehicle lubrication, Low-temperature performance, Cold-flow behavior, Base oil viscosity, Flow pressure (DIN 51805-2), Crystallization of esters, Sustainable lubricants
