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decomposition in motor oils using NAA and NMR
S. Fergusona, J. Johnsona, D. Gonzalesa, C. Hobbsb, C. Allena, S. Williamsa*
aDepartment of Physics and Geosciences, Angelo State University, ASU Station #10904, San Angelo, Texas 76909, USA
bDepartment of Chemistry and Biochemistry, Angelo State University, ASU Station #10892, San Angelo, Texas 76909, USA
Zinc dialkyldithiophosphates (ZDDPs) are one of the most common anti-wear additives present in commercially-available motor oils. The ZDDP concentrations of motor oils are most commonly determined using inductively coupled plasma atomic emission spectroscopy (ICP- AES). As part of an undergraduate research project, we have determined the Zn concentrations of eight commercially-available motor oils and one oil additive using neutron activation analysis (NAA), which has potential for greater accuracy and less sensitivity to matrix effects as compared to ICP-AES. The 31P nuclear magnetic resonance (31P-NMR) spectra were also obtained for several oil additive samples which have been heated to various temperatures in order to study the thermal decomposition of ZDDPs.
1. Introduction
Zinc dialkyldithiophosphates (ZDDPs) are one of the most common additives present in commercially-available motor oils. ZDDPs act as antioxidants, corrosion inhibitors, and antiwear agents (Spikes, 2004). In recent years, the ZDDP-concentrations in many motor oils have decreased substantially. For example, in 1995, when the American Petroleum Institute (API) introduced the SH service rating for motor oils to be used in gasoline engines, the maximum concentration of ZDDPs allowed was 1200 ppm (American Petroleum Institute, 2010). However, the current SN service rating, introduced in 2010, only allows a maximum ZDDP-concentration of 800 ppm (American Petroleum Institute, 2010). The decreases in ZDDP-concentrations have been due (primarily) to concerns about the effects of ZDDPs on catalytic converters. There have been studies that suggest that the presence of P in motor oils can lead to phosphate deposits on the catalysts that can reduce the conversion efficiencies of catalytic converters (Kaleli, 2001; Huang et al., 2004a). Furthermore, there have been concerns about the effects of ZDDPs on the environment. Studies have shown that when ZDDPs pollute soil and ground water they can lead to toxicities when they decompose to S and P-containing poisonous compounds (Huang et al., 2004b).
