Ind. Eng. Chem. Res. 2002, 41, 4889-4898
4889
Pressure Viscosity Coefficient of Lubricant Base Oils As Estimated by Nuclear Magnetic Resonance Spectroscopy Brajendra K. Sharma and Arthur J. Stipanovic* Department of Chemistry, State University of New YorksCollege of Environmental Science and Forestry, One Forestry Drive, 123 E.C. Jahn Laboratory, Syracuse, New York 13210
The pressure viscosity coefficient (PVC) is an important parameter for base oils in defining their lubricating capacity because it reflects the extent of “thickening” that occurs under high hydrodynamic loads. In this study, spin-lattice relaxation times (T1) derived from NMR experiments are used to probe the dynamic motional environment for a series of compositionally different base oils to better understand the relationship between chemical structure and bulk physical properties such as viscosity and PVC. Using a multivariable statistical analysis, it was demonstrated that the T1 times for certain 13C NMR resonances combined with the viscosity index can accurately predict the PVC of a base oil (R2 ) 0.99). Further, 13C NMR-derived “average structural parameters” including certain paraffin, isoparaffin, and naphthenic structures, also predicted PVC very accurately (R2 ) 0.99). Collectively, these results illustrate that the “energyconserving” and “traction” properties of lubricants can be predicted from the dynamics and structure of base oil molecules at ambient temperatures and pressures. Introduction Most organic fluids, including lubricant base oils, exhibit a reversible viscosity increase with the application of high pressure due to molecular mobility restrictions imposed by the forces being exerted.1 This phenomenon is especially important for lubricants because, in most mechanical applications, films of fluid are compressed between sliding or rolling surfaces under very high loads. The degree to which a fluid thickens under pressure is approximated by eq 1,1,2 where η )
log(η/η0) ) PR
(1)
viscosity at an elevated pressure (P), η0 ) viscosity at atmospheric pressure, and R ) pressure viscosity coefficient (PVC). Although this equation is widely employed, it strictly applies for most lubricants only at modest pressures (90
>10
