Structures in lubricating oil

The material offered in this chapter is intended to provide some basic insight into the manner in which petroleum lubricants are prepared from crude oil and how the composition of a lubricating fluid thus obtained correlates with its properties. It is only a summary of a subject which would require several volumes for adequate treatment. Particular emphasis will be put on the relations of chemical structures in lubricating oils to classificatory properties and to performance in service. 

Petroleum lubricating oils are sometimes characterized by their density. As will be seen when we come to the examination of chemical structures in lubricating oils later in this chapter, density is associated with structural type. Paraffinic oils are lower in density than naphthenic or aromatic oils. Since paraffinic oils show more desirable viscosity-temperature behavior and better thermal stability than naphthenic oils, in the past density was widely used as an easily measured indicator of oil quality. With the development of additives to improve viscosity and thermal stability behavior, classification of oils by their density ratings has lost much of its former importance. 

TYPE STRUCTURES IN LUBRICATING OILS BY CORRELATION WITH PHYSICAL PROPERTIES INDIRECT METHODS... 

Structure in Lubricating Oils by Direct Techniques 16.3.1. Extraction, Chromatographic Adsorption,... 

Mass Spectrography of Refinery-Run Fractions Nature of the Alkyl and Aromatic Structures Type Structures in Lubricating Oils by Correlation with Physical Properties Indirect Methods. ... 

Zinc dialkyldithiophosphates (ZDDPs), which act as antiwear additives in lubricating oils and were postulated to exist in various molecular forms (monomer, dimer or neutral form, and basic form), were studied by multi-edge (Zn K-, P K- and S K-) XAS for structural assessment [311]. Grazing incidence absorption spectroscopy measurements have provided evidence for breakdown of the ZDDP molecule following its adsorption on to a steel substrate surface [312]. XANES and CEMS were used to study the interaction of per-fluoropolyalkyl ether (PFPAE) additives with Fe-based alloys [313],... 

Pawlak, Z., Micellar Structure of Lubricating Oils in Selected Papers. The Third International Symposium on Tribochemistry, Cracow, Poland, September 10-12, 2001. 

Infrared spectroscopy is used for the analysis of almost all the fractions and products of crude oil. However, in the last century, a very interesting purpose of the infrared spectroscopy has been developed. It is the dynamic monitoring of the changes in the structure of lubricating oils as it undergoes degradation. Many processes such as oxidation or polycondensation in oils can be studied by infrared spectroscopy. 

Both trialkyl- and triarylstibine sulfides and selenides are known. Trimethylstibiae sulfide [15082-97-6], C3H9SSb, has been prepared from trimethylstibine oxide and hydrogen sulfide (164). It is monomeric in benzene and chloroform. Trialkylstibine sulfides and selenides have been prepared from tri-alkylstibines and sulfur or selenium, respectively (165). Unlike triphenylstibine oxide, the structure of triphenylstibine sulfide is tetrahedral, as shown by both Mu ssbauer and x-ray diffraction studies (153). A patent covering the synergistic use of triphenylstibine sulfide with aromatic amines as antioxidants in lubricating oils has been issued (166). 

A theory which would tie together the phenomenology of the viscosity behavior of liquids with their molecular structure would be desirable not only as an intellectual accomplishment but also as a useful aid in predicting the behavior of liquids as lubricants over a wide range of conditions. However, this goal is far from being attained because of basic deficiencies in present-day theories of liquids and because of the complex constitution of the hydrocarbon mixtures present in lubricating oils. 

Figure 16-3. Viscosity index and structural types in lubricating oil. (a) Commercial solvent-extracted lubricating stocks. (b) Aromatic-free fractions. 1 Non-condensed cycloalkanes. 2 Condensed cycloalkanes. 3 Isoparaffins. 4 Aromatics. From data by Andre and O Neal [11].

When petroleum or kerosene (as the raw materials for gas oil or lubricants) are purified by using oleum or sulfuric acid, a reaction with the aromatic compounds takes place. While these substances were originally seen as waste products, later their chemical structures and surface-active properties were identified, thus leading to special applications for such products. Nowadays, petroleum fractions with a high content of aromatic hydrocarbons are treated with sulfur trioxide to form alkylaryl sulfonates. These products are then transformed into the sodium, ammonium or alkaline-earth salts. They are soluble in oils and therefore are of some importance as additives in lubricants, oil fuels and corrosion-inhibiting oils. Further more, they are also used as auxiliaries in production of fabrics and as dispersants in enhanced oil recovery processes. 

Dispersions of finely divided solids in non-aqueous media have been important for paints, inks, reinforced polymers and lubricating oils, but with the development of liquid toner systems and "ultra-structure" processing of ceramics as fine powders dispersed in organic media, the understanding and optimization of such systems is more important than ever. 

Conventional sludge dispersants for lubricating oils have been composed from poly(isobutenyl) succinimide. The basic structure of such a PIB type is shown in Figure 6.7. 

A packed tower can successfully fractionate with a very small pressure drop, as compared to a tray. For a modern trayed tower, to produce one single theoretical tray worth of separation (that s like a single, 100 percent efficient tray), a pressure drop of about 6 in of liquid is needed. A bed of structured packing can do the same job, with one inch of liquid pressure drop, even when allowing for the vapor distributor. In low-pressure fractionators, especially vacuum towers used to make lubricating oils and waxes, this can be of critical importance. 

Crude oils are classified chemically according to the structures of tire larger molecules in the mixture. Classification methods use combinations of the words paraffinic, naphthenic, aromatic, and asphaltic. For instance, crude oil which contains a predominance of paraffinic molecules will yield very fine lubricating oils from the gas-oil fraction and paraffin wax from the residuum. Oh the other hand, if the larger molecules are aromatic and asphaltic, the heavier fractions of the crude oil are useful for pitch, roofing compounds, paving asphalts, and other such applications. 

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