Ind. Eng. C h e m . Res. 1987,26, 1888-1895
1888
Role of Additives and Transition Metals in Lubricating Oil Oxidation Terence Colclough E x x o n Chemical Ltd., E x x o n Chemical Technology Centre, Abingdon, Oxfordshire, O X 1 3 IBB, England
A bulk oil oxidation test has been used to investigate the pro- and antioxidant effects of iron and copper and also the effects of basic dispersants and detergents, zinc dithiophosphates, and other antioxidants. Lube oil stability is strongly influenced by the particular combinations and concentrations of transition metal or lube additive used. Zinc dithiophosphates, on their own, strongly inhibit iron-catalyzed oxidations, but their activity is greatly reduced in the presence of basic additives. Under certain conditions, soluble copper, even a t very low concentrations, is a far more potent antioxidant than conventional aromatic amines or phenols. Proposed actions of copper as a radical scavenger and zinc dithiophosphates as metal deactivators are discussed. Products from the oxidation of lubricating oils include carboxylic acids, ketones, and alcohols, which then condense to form polymeric materials. These oxidation products lead to the formation of varnish and sludge and to an increase in viscosity. The main antioxidants used in automotive lubricants are zinc dialkyldithiophosphates (ZDPs), but because of the trend to lower phosphorus levels to reduce poisoning of catalytic converters, coupled with the use of longer drain intervals and higher engine operating temperatures, lubricants are being required to operate under increasingly severe conditions. ZDPs alone cannot cope with these conditions especially when their concentrations are limited to 0.05-0.08% P, which are now becoming more common. There is therefore a need to understand more about the various factors which affect the oxidation of fully formulated lubricating oils under fired engine test conditions and also develop new phosphorus-free antioxidants to supplement the action of the ZDPs. The most severe test which measures oil oxidation and viscosity increase is the Sequence IIID fired engine test. Oil oxidation is a result of the high bulk oil temperature (149 "C) and the presence of promoters which include lead salts, nitrogen oxides and other radical species from the blow-by gases, and soluble iron. The extreme severity of the test is evidenced by the rapid depletion of the ZDP, which can be monitored by 31PNMR analysis. Typical used oil spectra (Figure 1)show ZDP present at the start (6 99-102), whereas after 8 h, complete degradation has occurred to give the phosphorus triester, (RO),(RS)PS (6 96), monothiophosphates (6 63,28), and phosphates (6 f5). The high rate of ZDP depletion in this test must be due to thermal decomposition at high localized temperatures (up to 250 "C in the piston-liner zone), hydrolysis, and participation in oxidative reactions. A 31PNMR study of the purely thermal decomposition of a ZDP in a fully formulated oil showed only a slight reduction ( < l o % ) in the ZDP concentration over 16 h at 165 "C. Most of the early work on the action of antioxidants and transition metals on hydrocarbon oxidation was carried out in comparatively simple systems. The oxidation of a fully formulated lubricant oil in a fired engine represents a much more complex environment, but the trend is now to simulate these conditions more closely in the laboratory. Bench oxidation tests used within the oil industry have been reviewed (Hsu, 1981; Warne and Vienna, 1984), and attempts have been made to develop tests which correlate with the Sequence IIIC and IIID tests (Cecil, 1973; Kuhn, 1973; Hsu et al., 1982; Klaus et al., 1982). The roles of copper and iron have been investigated using a thin film 0888-5885/87 / 2626- 1888$01.50/0
oxidation test (Clark et al., 1985),and it has been shown that the composition of the base stock, as well as the presence of succinimide dispersant and calcium high base number sulfonate, all affect the oxidation stability of a lube oil (Hsu and Lin, 1983). In the present work, a bulk oil oxidation test has been used to study the effects of iron, copper, and various lubricant additives and antioxidants on lube oil oxidation, and depending on the concentrations and combinations used, wide variations in oxidative stability are observed. The mechanisms of action of the catalysts and antioxidants studied are discussed.
Experimental Section The oxidation rig was based on that used in the ASTM D943 test. Normal conditions were as follows: 300 g of oil was oxidized for 64 h/165 "C,using an air flow rate of 1.7 L min-' with ferric acetylacetonate (40 ppm Fe) as catalyst. Two base stocks were used: a solvent 150 neutral mineral oil (S150 N, 0.3% S) and a white oil (
