OXIDATION OF MINERAL BASE STOCKS OF PETRO LEUM ORIGIN - RELATIONSHIP BETWEEN CHEMICAL-COMPOSITION, THICKENING AND OXIDIZED DEGRADATION PRODUCTS

This survey is based on an understanding of problems linked to the hig h-temperature oxidation of base stocks used for the formulation of lub ricants for automotive engines. The oils investigated are of different origins: Middle East (MO) and the North Sea (MDN), which respectively have high and low aromatics and sulfur contents, hydroisomerized oil (HYDI) and polyalphaolefins (PAO6), which are exempt from these compou nds, and hydrocracked oil (HYDC), which has an intermediate compositio n (Table 1). By tests during which the amount of oxygen consumed is co ntroled continuously (IFP OXYTEST, modified TFOUT) and by differential scanning calorimetry analysis (DSC), we have investigated the behavio r of these different base stocks with regard to oxidation. Mineral bas e stocks with high contents of aromatics compounds and sulfur are part ially and naturally protected against oxidation. The same is not true for hydroisomerized base stocks, which contain no natural inhibitor an d so are very easily oxidized (Fig. 2). However, all such oils become oxidized quickly as soon as they are subjected to high temperatures in the presence of air or oxygen and an oxidation catalyzing metal (Fig. 3). In the presence of soluble iron salts used as catalysts, we have found that, to be partially ''self'' protected against oxidation, a ba se stock must contain a minimum of aromatic carbons (CA greater than o r equal to 5 %) and sulfur (S greater than or equal to 0.5 %), with th is sulfur generally being linked to aromatic compounds in the thiophen ic form (Fig. 4). We will also show that, whereas the oxidation of a b ase stock issuing from refining products causes an increase in the vis cosity of this base stock, this increase is linked directly to the amo unt of oxygen consumed and depends solely on this amount, no matter wh at the antioxidant additive may be or the amount of catalyst present. For a given amount of oxygen consumed, the variation in viscosity is i ndependent of the temperature and duration of oxidation as well as of the concentration and choice of the antioxidant (Table 3, Figs. 5 and 6). The antioxidant properties of natural sulfur-containing aromatic c ompounds have been demonstrated by incorporating them in polyalphaolef in PAO6 after they have been extracted from MO and MDN oils and by stu dying the oxidation stability of the mixtures thus obtained by thin-la yer oxidation by DSC (Table 5). Whereas PAO6 has no natural resistance to oxidation, the addition of aromatic compounds coming from MO and M DN oils give this base stock an oxidation stability directly linked wi th the amount and composition of the sulfur-containing aromatic compou nds added (Tables 6 and 7, Figs. 7 to 11). Paradoxically, such natural inhibitors present in base stocks become precursors of deposits when these oils are subjected to severe oxidation conditions. These sulfur- containing aromatic inhibitors react themselves according to a process involving the association of sulfur-containing molecules. This proces s leads to an increase in viscosity and the formation of insoluble com pounds (Table 10 and Fig. 20). These insoluble compounds are not detec ted with saturated base stocks such as PAO6 or HYDI. The antioxydant e ffect of the radical-inhibitor and hydroperoxide-decomposer types has also been investigated, for each type of additive (Table 9 and Figs. 1 2 to 17) then together with synergism phenomena among such products (F igs. 18 and 19). By using high-performance analytical techniques (GC/M S, GC, C-13 NMR, IR spectroscopy, SEM, etc.), we have step by step cha racterized the degradation products formed during oxidation, i.e. vola tile products, liquid products and insoluble products (Table 17). All compounds polar products contained in these three components of oxidiz ed oil are made up of hydroxyl, carbonyl and carboxyl species: The vol atile products that we condensed when cooled (-80 degrees C) are made up of a main aqueous phase and a supernatent organic phase (Table 12). These two phases contain nonsulfur-containing linear oxygenated const ituents, alkanes as well as cyclical oxygenated components (lactones) and aromatics. The aromatic components are found solely in volatile pr oducts issuing from MO and MDN mineral base stocks (Tables 13 to 16). These products have a lower molar mass than that of base stocks and st em from the oxidation of hydrocarbons, from the oxidation and reaction s of the recombination of oxidized products themselved and from termin ation reactions. Heavy soluble products are composed of species that m ay be divided into two categories (Tables 18 and 19): Nondialyzable pr oducts with oxygen contents of about 10 to 12 % and, for MO oil, 1.3% sulfur. They have a average molar mass of around 1,000 g/mol and stem mainly from the oxidation of aromatic compounds contained in mineral o ils. Dialyzable products with molar masses always greater than those o f the base stocks (400 to 500 g/mol) and oxygen contents of between 1. 8 and 3.9% mass. The increase in the average molecular mass of oxidize d compounds and their aromatics content is determined by steric exclus ion chromatography with both ultraviolet and differential refractometr ic detections (Figs. 23 and 24). The insoluble products, whether in su spension inside the oil or deposited on the walls of the oxidation rea ctor, have been identified by scanning electron microscopy and by C-13 NMR. They appear in the form of spherical clusters, with a size betwe en 0.1 and 8 mu m depending on the oxidation conditions. They are rich in oxygen (20%) and concentrate the aromatic compounds and sulfur com ing from the base stock. For example, the insolubles coming from MO oi l contain 17.5% C-A compared to 9% in the initial oil. Their average m olecular masses also depend on the initial C-A and sulfur contents of the base stock and are between 1600 and 2300 g/mol (Table 20 and Figs. 25 and 26). This undeniably demonstrates that the presence of these s ulfur-bearing aromatic compounds contributes in the formation of insol uble products during oxidation. Therefore, with regard to problems lin ked to oxidation and the formation of insoluble products, it seems des irable to formulate motor oil from base stocks that have undergone sev ere processing to remove sulfur-containing aromatic compounds such as the ones they naturally contain, and to reinforce their oxidation resi stance by choosing a suitable combination of additives.