Low-Temperature Dielectric Spectroscopy Characterization of the Oxidative Degradation of Lubricating Oil

Abstract

In this study, the two-channel and differential dielectric spectroscopy (TD-DES) technique has been applied to study different degrees of oxidative degradation for two series of simulated oxidized lubricating oils at low temperatures ranging from 20 °C to −55 °C. The deep oxidation, general oxidation, nitration, and sulfation products of the degraded lubricating oil were precisely identified by the Fourier transform infrared spectroscopy (FT-IR) and increased with the level of oxidation, which agreed well with the TD-DES data at room temperature (20 °C). For the severely degraded lubricating oil, the TD-DES real and imaginary data and relaxation characteristic changes from 20 °C to −55 °C were dramatically reduced; the interlacing characteristics of Cole–Cole plots at low temperatures could be ascribed to the formation of high-molecular-weight products during the oxidative degradation process. It was found that the two-dimensional (2D) synchronous and asynchronous dielectric spectroscopy were able to qualitatively describe the degrees of simulated oxidative degradation and the formation of highly oxidized products, as well as to explain the polarization relaxation mechanism of degraded lubricating oil. The partial least-squares (PLS) and multilinear-PLS (N-PLS) regression results indicated that 2D synchronous and asynchronous dielectric spectroscopy could better predict the FT-IR deep oxidation, general oxidation, nitration, and sulfation peak areas than TD-DES real data at 20 °C with regard to lower root-mean-square error of cross-validation (RMSECV), better correlation coefficients (R), and smaller predicted errors.

Controllable Synthesis and Synergistic Antioxidation Mechanism of Poly(p-methoxyphenol-phenylamine) in Biodegradable Vegetable-Based Lubricating Oils

“…Usually, lubricating oil works at least in the temperature range of 20–30 °C below the OIT . As shown in Figure , the OIT of pure castor oil is less than 1 min at 170 °C in the PDSC test, which means that pure soybean oil cannot work at such a high temperature and quickly undergoes oxidation deterioration.…”