Oxidation Resistance of Base Stocks

Studies on the relationship between composition and the resistance of base stocks to oxidation started many years ago when solvent extracted stocks were the only ones available. World War II made this an area of particular importance and intensified the effort. Many of the significant early studies date from this period. 

Oxidation itself is a chemical process, accelerated by increased temperatures, in which oxygen reacts with base stock hydrocarbons to form chemically altered materials by dehydrogenation and conversion to oxygen-containing compounds. The physical and chemical properties of the oxidation products are very different from those of the base stock. 

Oxidation of hydrocarbons has been known for many years to involve the formation of key intermediate hydroperoxides and dialkylperoxides ( peroxides in general) from the reaction of oxygen and hydrocarbons via free radical intermediates. At low temperatures, the peroxides formed slowly accumulate and eventually decompose either thermally or by metal-induced reactions or by ionic routes. At high temperatures, formation and thermal decomposition of the peroxides occurs rapidly. Thermal decomposition leads to the production of additional free radicals (the propagation step of the reaction) and the formation of oxygen-containing products (e.g., acids, alcohols, ketones, polar compounds, and polymeric materials) that can ultimately bring about lubricant failure. 

This initiation step may occur in a number of ways by thermal decomposition of hydrocarbons with weak C-H bonds, by oxidation of hydrocarbons by metal ions via free radical routes, by reaction with oxygen gas, etc. 

This step first incorporates molecular oxygen into the hydrocarbon as an unstable peroxide radical. This is the start of oxidation. In the second step, the peroxy radical, which is short lived, attacks another hydrocarbon molecule. If this reaction is fast (i.e., the C-H bond being broken is weak), oxidation can occur quickly. 

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The fluid is formulated from a premium mineral od-base stock that is blended with the required additive to provide antiwear, mst and corrosion resistance, oxidation stabdity, and resistance to bacteria or fungus. The formulated base stock is then emulsified with ca 40% water by volume to the desired viscosity. Unlike od-in-water emulsions the viscosity of this type of fluid is dependent on both the water content, the viscosity of the od, and the type of emulsifier utilized. If the water content of the invert emulsion decreases as a result of evaporation, the viscosity decreases likewise, an increase in water content causes an increase in the apparent viscosity of the invert emulsion at water contents near 50% by volume the fluid may become a viscous gel. A hydrauHc system using a water-in-od emulsion should be kept above the freezing point of water if the water phase does not contain an antifreeze. Even if freezing does not occur at low temperatures, the emulsion may thicken, or break apart with subsequent dysfunction of the hydrauHc system. 

Paraffinic and naphthenic (cycloparaffinic) stocks may be used for the formulation of lubricating oils, each with favorable characteristics for particular uses. Paraffinic stocks are generally preferred for their superior lubricating power and oxidation resistance. Naphthenic stocks, on the other hand, have naturally lower pour points, i.e, they maintain flow characteristics at lower pour-points than paraffinics (Table 18.8) and are better solvents, features which are more important for applications such as heat transfer, metal working, and fire-resistant hydraulic fluids [33]. Any residual aromatics in the lubricating base stock will have been removed before formulation by solvent extraction, using N-methylpyrrolidone, furfural, or less frequently today, phenol (Eq. 18.39). 

Von Fuchs and Diamond (Shell Oil) pursued these results further by examining the effects of increasing the content of the aromatics on base stock oxidation rates.15 They undertook this study because of the increasing realization that while solvent extraction technology improved lubricant performance, overextraction of the base stock could make it less resistant to oxidation because of removal of the autoretardant components. Since extraction removed aromatics and nitrogen and sulfur compounds, a relationship between these levels and oxidation stability seemed likely. 

This concept of optimum aromaticity and the role of sulfur compounds as inhibitors were further established by a study by Bum and Greig (British Petroleum) of the oxidation of solvent extracted base stocks.18 They chose samples from a North African (Sahara) and three Middle East (Iran, Abu Dhabi, and Kuwait) crudes. The aromatic + heterocyclic (A + II) and paraffin + naphthene (P + N) components were separated by alumina chromatography from each base stock (Table 5.9 includes their composition and sulfur contents) and recombined in several ratios and the resistance of the blends to oxidation measured by the oxygen uptake method. 

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