Lube Products: Molecular Characterization of Base Oils

Abstract

Abstract Base oils used in the formulation of lubricant products are a complex mixture of paraffins, isoparaffins, aromatic, and naphthenic (cycloparaffinic) molecules ranging in carbon number from 20 to 40+. Most automotive crankcase oils, accessory fluids, and many industrial lubricants contain in excess of 80% base oil in their ‘finished’ formulation. The molecular composition of base oils has a major effect on lubricant properties, and its compositional analysis is usually obtained by a two‐step procedure. The first step separates the aromatic and saturated fractions by column chromatography (American Society for Testing and Materials, ASTM D2549). The aromatic fraction is then further characterized by mass spectrometry (ASTM D3239), and the saturated subclasses of paraffins and cycloparaffins are measured by a different mass spectrometry method (ASTM D2786). Recently, the lube base oils have been characterized at the molecular level by means of nuclear magnetic resonance (NMR) spectroscopy. The conventional NMR spectra have the problem of signal overlap caused by the presence of CH, CH 2 , and CH 3 groups due to the great number of isomers in the base oil composition. To overcome the problem of signal overlap, some selected multiplet subspectral carbon‐13 nuclear magnetic resonance ( 13 C NMR) analyses were developed, and, in particular, gated spin echo (GASPE) and distortionless enhancement by polarization transfer (DEPT) have been applied to obtain quantitative CH n subspectra in petroleum fractions. Usually the quantitative data obtained from GASPE, which is a spectral editing technique, has a better accuracy than those from DEPT, and it will be shown how GASPE is applied to study the molecular characteristics of the aliphatic moieties of base oils of a different nature. On the basis of the GASPE data and of the average molecular mass obtained via vapor pressure osmometry (VPO), the average number of branches (NB) and aliphatic rings can be calculated. Moreover, on the basis of GASPE, the distribution of side chain lengths and the positions of the methyl groups along the straight chain (methyls are the only groups for which the positions are directly obtained from NMR spectra) were obtained. These data are essential in visualizing average molecular structures of different base oil components. Besides the rich molecular diversity seen in base oils, they also possess high molecular weights (ranging from 300 to 600 Da) along with the presence of sulfur, oxygen, nitrogen, and other heteroatoms. Data from advanced analytical techniques along with the NMR data can be used to get more detailed information about the composition. Techniques such as high‐performance liquid chromatography (HPLC), two‐dimensional gas chromatography (GCxGC), and Fourier transform‐ion cyclotron resonance (FT‐ICR) mass spectrometry have been used to analyze base oil composition and are discussed in the present work. All these structural features are related to the finished lubricant's performance in both crankcase and industrial applications.