Quantification of Active Antioxidants by FTIR Spectroscopy and the Correlation to Measured TBN Values 2001-01-3599

Total base number (TBN) measurement is traditionally used to assess the ability of an engine oil to neutralize the generated acids during engine operation. Although such measurements are informative, they can only determine the total quantity of the remaining antioxidants in the engine oil, and not the amount of active antioxidants that can effectively neutralize the acids. The work presented here investigates the activity of antioxidants in fresh and used engine oils by Fourier Transformed Infrared Spectroscopy (FTIR) and their correlation to the TBN values obtained by titration. The method is based on the reaction of an acid solution with the alkaline ingredient of the engine oil in a non-polar solvent that mimics the actual environment experienced by the antioxidants. FTIR Spectroscopy is used to monitor the consumption of the acid. The decrease in peak intensity of the acid is then employed to quantify the remaining active antioxidants through a calibration curve.

According to the experimental results, FTIR values correlate well with TBN values for fresh oils, however, lesser correlations were found for used engine oils. These results tend to indicate that oil contamination may have an inhibition effect on the activity of antioxidants and that titrometric TBN measurements do not adequately represent the neutralization ability of used engine oils. The FTIR method described herein could serve as a valuable tool to assess the remaining useful life of the oil and complement selected condition monitoring methods (such as viscosity measurements, elemental analysis, etc.) employed in preventive/predictive maintenance programs.