Oxydation des huiles de bases minérales d'origine pétrolière. Relation entre leur composition chimique, l'épaississement et la composition de leur produits de dégradation
Oxidation of Mineral Base Stocks of Petroleum Origin. Relationship Between Chemical Composition, Thickening and Oxidized Degradation Products
Elf Antar France
2 Institut Français du Pétrole
3 Renault SA
Cette étude est basée sur la compréhension des problèmes liés à l'oxydation à haute température des huiles de base utilisées dans la formulation des lubrifiants pour moteurs automobiles. Les huiles étudiées sont d'origines différentes : huile Moyen-Orient et huile mer du Nord, respectivement à fortes et faibles teneurs en composés aromatiques et en soufre, huile hydroisomérisée et polyalphaoléfine, exemptes de ces composés, et huile hydrocraquée de composition intermédiaire. L'influence de la teneur et de la composition en produits aromatiques et en soufre des huiles sur la dégradation thermo-oxydante est montrée. Ces composés protègent naturellement les huiles de base contre l'oxydation, mais deviennent des précurseurs de dépôts dans des conditions d'oxydation sévères, en présence d'un catalyseur, tel le fer, qui est toujours présent dans un moteur. L'action des additifs antioxydants, inhibiteurs radicalaires et décomposeurs d'hydroperoxydes, est étudiée à l'aide d'essais d'oxydation en couche mince et en volume. Les produits d'oxydation sont identifiés, ce qui a permis d'améliorer la connaissance des processus de dégradation des composés en cours d'oxydation, et de montrer l'influence de ces processus sur la formation d'espèces oxydées volatiles, l'augmentation de la viscosité de l'huile et la formation d'espèces oxydées de haute masse molaire qui, en se condensant, participent à la formation de composés insolubles et de dépôts.
This survey is based on an understanding of problems linked to the high-temperature oxidation of base stocks used for the formulation of lubricants 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 (PA06), which are exempt from these compounds, and hydrocracked oil (HYDC), which has an intermediate composition (Table 1). By tests during which the amount of oxygen consumed is controled continuously (IFP OXYTEST, modified TFOUT) and by differential scanning calorimetry analysis (DSC), we have investigated the behavior of these different base stocks with regard to oxidation. Mineral base stocks with high contents of aromatics compounds and sulfur are partially and naturally protected against oxidation. The same is not true for hydroisomerized base stocks, which contain no natural inhibitor and 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 havefound that, to be partially selfprotected against oxidation, a base stock must contain a minimum of aromatic carbons (CA > ou = 5 %) and sulfur (S > ou = 0. 5 %), with this sulfur generally being linked to aromatic compounds in the thiophenic form (Fig. 4). We will also show that, whereas the oxidation of a base stock issuing from refining products causes an increase in the viscosity of this base stock, this increase is linked directly to the amount of oxygen consumed and depends solely on this amount, no matter what the antioxidant additive may be or the amount of catalyst present. For a given amount of oxygen consumed, the variation in viscosity is independent 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 compounds have been demonstrated by incorporating them in polyalphaolefin PA06 after they have been extracted from MO and MDN oils and by studying the oxidation stability of the mixtures thus obtained by thin-layer oxidation by DSC (Table 5). Whereas PA06 has no natural resistance to oxidation, the addition of aromatic compounds coming from MO and MDN oils give this base stock an oxidation stability directly linked with the amount and composition of the sulfur-containing aromatic compounds 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 process leads to an increase in viscosity and the formation of insoluble compounds (Table 10 and Fig. 20). These insoluble compounds are not detected with saturated base stocks such as PA06 or HYDI. The antioxydant effect of the radical-inhibitor and hydroperoxide-decomposer types has also been investigated, for each type of additive (Table 9 and Figs. 12 to 17) then together with synergism phenomena among such products (Figs. 18 and 19). By using high-performance analytical techniques (GC/MS, GC, 13C NMR, IR spectroscopy, SEM, etc. ), we have step by step, characterized the degradation products formed during oxidation, i. e. volatile products, liquid products and insoluble products (Table 17). All compounds polar products contained in these three components of oxidized oil are made up of hydroxyl, carbonyl and carboxyl species :- The volatile products that we condensed when cooled (- 80°C) are made up of a main aqueous phase and a supernatent organic phase (Table 12). These two phases contain nonsulfur-containing linear oxygenated constituents, alkanes as well as cyclical oxygenated components (lactones) and aromatics. The aromatic components are found solely in volatile products 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 stem from the oxidation of hydrocarbons, from the oxidation and reactions of the recombination of oxidized products themselved and from termination reactions. Heavy soluble products are composed of species that may be divided into two categories (Tables 18 and 19) :- Nondialyzable products 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 oils. - Dialyzable products with molar masses always greater than those of 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 oxidized compounds and their aromatics content is determined by steric exclusion chromatography with both ultraviolet and differential refractometric detections (Figs. 23 and 24). The insoluble products, whether in suspension inside the oil or deposited on the walls of the oxidation reactor, have been identified by scanning electron microscopy and by 13 C NMR. - They appear in the form of spherical clusters, with a size between 0. 1 and 8 µm depending on the oxidation conditions. They are rich in oxygen (20%) and concentrate the aromatic compounds and sulfur comingfromthe basestock. For example,the insolubles coming from MO oil contain 17. 5% CA compared to 9% in the initial oil. Their average molecular masses also depend on the initial CA 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 sulfur-bearing aromatic compounds contributes in the formation of insoluble products during oxidation. Therefore, with regard to problems linked to oxidation and the formation of insoluble products, it seems desirable to formulate motor oil from base stocks that have undergone severe processing to remove sulfur-oontaining aromatic compounds such as the ones they naturally contain, and to reinforce their oxidation resistance by choosing a suitable combination of additives.
© IFP, 1995