Experimental study of application of molecules with a cyclic head group containing a free radical as organic friction modifiers

抄録

The optimal molecular design of organic friction modifier (OFM) additives allows for a reduction in the friction and wear in mechanical systems, and thus, an improvement in the energy efficiency and reduction in CO₂ emissions. In this study, we synthesized N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide, referred to as C₁₂TEMPO, which has a large and rigid cyclic head group containing a free radical, to explore its application as a new type of OFM. We also selected stearic acid and stearonitrile, which possess small head groups without free radicals but an identical tail structure and approximately the same total length as C₁₂TEMPO, to clarify the effect of head groups. The three types of OFMs were added separately into polyalphaolefin oil at 1 wt.%, and their lubrication performance was measured with a pin-on-disk type friction tester under varied normal loads or sliding speeds. C₁₂TEMPO exhibited the highest load-carrying capacity, up to roughly thrice that of stearic acid and stearonitrile. Additionally, C₁₂TEMPO gave rise to more stable sliding than stearic acid, as confirmed from comparison of instantaneous friction coefficients. These results suggested that C₁₂TEMPO could form an effective boundary film that was robust to heavy-loaded sliding. The reason for this could be attributed to not only the strong surface interaction owing to the free radical but also to the rigidity of the cyclic structure of the head group in C₁₂TEMPO. The results obtained when varying the sliding speed suggested that the efficiency to form the boundary film followed the order: stearic acid > C₁₂TEMPO > stearonitrile. We inferred that the relatively large size of the head group of C₁₂TEMPO rendered it difficult to access the solid surfaces, thereby reducing the film formation efficiency.