Investigating the effects of base oil type on microstructure and tribological properties of polyurea grease

The soaring prices and physiological toxicity of lithium-based grease make polyurea grease a promising alternative due to its balanced cost and performance. Employing preformed thickeners to avoid toxic raw materials has become a preferred method for eco-friendly preparation of polyurea grease. However, the underlying mechanism of base oil modulates the microstructure of polyurea greases prepared with preformed thickeners, subsequently influencing their macroscopic performance remains unclear. Herein, three polyurea greases were synthesized by regulating the base oil types ((poly(alpha-olefin) oil (PAO40), alkyl naphthalene oil (AN30), and polyether oil (OSP320)) with the same preformed thickener. The real-time polarizing microscope observation of the grease structure evolution process revealed that the preformed thickeners gradually expanded in the base oil and formed a fiber structure. An increased polarity disparity between thickener and base oil leads to a reduced swelling rate and increased fiber length. As a result, compared with high polarity OSP320 and low polarity AN30, non-polar PAO40 induces a more interconnected 3D network composed of long fiber in obtained grease (PG). Low field nuclear magnetic resonance characterization and molecular simulations reveal this network exhibits significant binding ability towards base oil, reducing the movement ability of the base oil, endowing PG with the highest structural strength, achieving effective lubrication and low noise performance. These in-depth understandings will help promote the research and development of polyurea greases.