Triple Function Lubricant Additives Based on Organic–Inorganic Hybrid Star Polymers: Friction Reduction, Wear Protection, and Viscosity Modification

Abstract

Polymer-based lubricant additives for friction reduction, wear protection, or viscosity improvement have been widely studied. However, single additives achieving all three functions are rare. To address this need, we have explored the combination of polymer topology with organic–inorganic hybrid chemistry to simultaneously vary the temperature- and shear-dependent properties of polymer additives in solution and at solid surfaces. A topological library of lubricant additives, based on statistical copolymers of stearyl methacrylate and methyl methacrylate, ranging from linear to branched star architectures, was prepared using ruthenium-catalyzed controlled radical polymerization. Control over the polymerization yielded additives with low dispersity and comparable molecular weights, allowing evaluation of the influence of polymer architecture on friction reduction, wear protection, and bulk viscosity improvement in a commercial base oil (Yubase 4). Structure–performance relationships for these functions were assessed by a combination of a high-speed surface force apparatus (HS-SFA) experiments, wear track profilometry, quartz crystal microbalance analysis, and solution viscometry. The custom-built HS-SFA provides a unique experimental environment to measure the boundary lubrication performance under extreme shear rates (≈107 s–1) for prolonged times (24 h), mimicking the extreme conditions of automotive applications. These experiments revealed that the performance of the additives as boundary lubricants and wear protectants scales with the degree of branching. The branched architectures prohibit ordering of the additives in thin films under high-load conditions, leading to a thicker absorbed polymer brush boundary layer and therefore enhanced film fluidity and lubricity. Additionally, star polymers with increasing arm number lead to bulk viscosity modification, reflected by a significant increase in the viscosity index compared to the commercial base oil. Although outperformed by linear polymers for bulk viscosity improvement, the (hybrid) star polymers successfully combine the three distinct lubricant additive functions: friction reduction, wear protection, and bulk viscosity improvementin a single polymeric structure. It should also be noted that, judging from HS-SFA experiments, hybrid stars carrying a silicate-based core outperform their fully organic analogues as boundary lubricants. The enhanced performance is most likely driven by attractive forces between the silicate cores and the employed metallic surfaces. Combining three function in one minimizes formulation complexity and thus opens a route to fundamentally understand and formulate key design parameters for the development of novel multifunction lubricant additives.

A review of recent advances in tribology

“…When inorganic and organic materials are combined, the advantages of both, such as high pressure resistance of inorganic materials and good rheology property of organic additives, can be fully utilized [176][177][178]. Van Ravensteijn et al [179] prepared the (hybrid) star polymers carrying a silicate-based core and used it as additive in a commercial base oil (Yubase 4). The experiment results showed the single additive achieving all three functions-friction reduction, wear protection, and viscosity improvement.…”

Towards a Polymer-Brush-Based Friction Modifier for Oil

“…The influence of alternative anchoring chemistries will be reported in detail elsewhere. The PFMs were evaluated for their frictional performance in rolling-sliding macroscale contact in low-viscosity oils, on both steel and chromium, and their adsorption properties were studied with the quartz-crystal microbalance (QCM) [29][30][31][32][33][34][35][36][37], using iron oxide-coated sensors.…”

Towards a Polymer-Brush-Based Friction Modifier for Oil

“…Modern methods of surface science have recently been brought to bear on the frictional properties of viscosity-modifying polymers. In [22] the synthesis of star-shaped organic-inorganic hybrid copolymers is detailed, and they were analyzed with a high-speed surface forces apparatus in conjunction with quartz crystal microbalance with dissipation (QCM-D). A higher degree of branching was found to lead to thicker lms and improved frictional performance.…”