16 Lubricant Additives: Chemistry and Applications
reacting alkyl or alkenyl succinic acid anhydride with an appropriate amine may impart such mul-
tifunctionalities. Product made by reacting a polyalkenylsuccinic acid or anhydride fi rst with an
aromatic secondary amine, then with an alkanol amine, was found to provide appreciable antioxid-
ancy, dispersancy, and anticorrosion effects to engine oils as tested in a Caterpillar engine test [141].
A more recent U.S. Patent literature [142] discloses materials made from the reaction of alkyl or
alkenyl succinic acid derivative with a diamino naphthyl compound for use as antioxidant, antiwear,
and soot dispersing agents for lubricating oils. By fusing a HP moiety to an alkenyl succinimide
domain, a novel dispersant having antioxidant property was obtained [143]. The product improved
the performance of engine oils in the sequence VG, an industry recognized sludge test to evaluate
the ability of a lubricant in preventing the formation of sludge and varnish deposits in a fi red engine.
U.S. Patent 5,075,383 [144] describes novel antioxidant–dispersant additives obtained by reacting
amino-aromatic polyamine compound, including aromatic secondary amines, with ethylene–pro-
pylene copolymer grafted with maleic anhydride. Engine oils containing the additives displayed
improved performance characteristics in laboratory oxidation and sludge dispersancy tests, as well
as in the sequence VE and the MWM-B engine tests.
1.7 COPPER ANTIOXIDANTS
The ability of copper compounds to function as oxidation inhibitors has been of interest to the
lubricant industry for years. Copper is usually considered to be an oxidation promoter, and its
presence is of a concern in lubricants such as power transmission oils, where fl uid contact with
copper-containing bearings and sintered bronze clutch plates takes place [145]. It has been sug-
gested that copper corrosion products, originating from surface attack of copper metal, are generally
catalysts that accelerate the rate of oxidation [146], whereas oil-soluble copper salts are antioxidants
[147]. To maximize the full antioxidant strength of a copper compound, the initial concentration
needs to be maintained at an optimum range, normally from 100 to 200 ppm [145,147]. Below this
range, the antioxidant effect of the copper compounds will not be fully realized, whereas above the
range, interference with antiwear additives may occur, leading to pronounced increase in wear on
high-stress contact points [148].
Examples of oil-soluble copper antioxidants developed in early years were a group of copper–
sulfur complexes, obtained by sulfurizing certain types of unsaturated hydrocarbons in the presence
of copper [149 –151]. A more recent patent describes lubricant compositions that are stabilized with
a zinc hydrocarbyl dithiophosphate (ZDDP) and 60–200 ppm of copper derived from oil-solu-
ble copper compounds such as copper dihydrocarbyldithiophosphate or copper dithiocarbamates
[148]. Oxidation data are given for fully formulated engine oils containing the ZDDP and various
supplemental antioxidants including amines, phenolics, a second ZDDP, and copper salts. Only the
blends with copper salts passed the oxidation test. With the other additives, the viscosity increase
was excessive. Organo-copper compounds including copper naphthenates, oleates, stearates, and
polyisobutylene succinic anhydrides have been reported to be synergistic with multiring aromatic
compounds in controlling high-temperature deposit formation in synthetic base stocks [147].
FIGURE 1.5 Examples of commercial sulfur-bridged phenolic antioxidants.
HOO
SO2SOH OHCH 3 CH 3SCH 3 CH 3HO OH