Study of degradation in lubricant oils using NMR relaxometry

M. BALLARI; F. BONETTO; E. ANOARDO

Santos, Sao Paulo, Brasil

Congreso; XXVIII Encontro Nacional de Fisica da Materia Condensda; 2005

Sociedade Brasileira de Fisica

Engineering performance of lubricant oils is closely related to its molecular structure and functionality of additives. Despite being widely studied, the connection between degradation processes and molecular dynamic or structure are still not clearly understood in these compounds.As a mineral engine oil is used, it undergoes degradation processes mainly due to oxidation, nitration, sulfating, heat and contamination by internal (soot, wear metals) and external (fuel, water) agents. In lubricant analysis, Fourier-transform infrared (FTIR) spectroscopy is commonly used to detect the presence of various components related with some of the processes above mentioned. Other standard analysis in lubricants quality control involve determination of wear metals, cinematic viscosity, high-pressure differential scanning calorimetry (PDSC) and bulk oil oxidation stability test.Nuclear magnetic resonance (NMR) techniques were scarcely employed in the past in the analysis of lubricants degradation. For instance, a combination of  C13 NMR with H NMR spectroscopy can be used to determine CH_3, CH_2, CH and quaternary carbon contents, related with oil aging. On the other side, NMR relaxometry could be highly sensitive to changes in the molecular dynamics due to the presence of solid particles, confining structures or changes in the molecular structure. One these grounds, the use of NMR relaxometry was considered as a tool to inquire about possible links between degradation processes in lubricants oils and changes in the molecular dynamics. The proton field cycling technique was employed to measure spin-lattice relaxation time (T_1) dispersions of new and aged samples of monograde and multigrade oils. Results were interpreted in terms of standard self-diffusion and molecular rotations relaxation theories. The analysis revealed a noticeable sensitivity of the involved correlation times with the oil aging, specially manifested in the low frequency range.