Antioxidants 7
ammonium polythiomolybdate with appropriate tetralkylthiuram disulfi des were found to be supe-
rior to dinuclear molybdenum compounds in terms of providing lubricants antioxidant, antiwear,
and friction-reducing properties [31].
W hen combined wit h a n appropr iate a romatic a m ine, MoDTCs ca n ex h ibit synergistic a ntiox ida nt
effects in oxidation tests [32]. As a result, molybdenum dialkyldithiocarbamates (C7–24) and ADPAs
are claimed broadly for lubricating oils [33]. More restrictive are claims for molybdenum dialkyl-
dithiocarbamates (C8–23 and C3–18) and ADPAs in lubricating oils that contain <3 wt% of aromatic
content and <50 ppm of sulfur and nitrogen [34]. Molybdenum dialkyldithiocarbamates and HP anti-
oxidants are jointly claimed for lubricating oils that contain 45 wt% or more one or two ring naph-
thenes and <50 ppm sulfur and nitrogen [35]. MoDTC was used to top-treat engine oils formulated
with group I base stocks (>300 ppm S) and an additive package designed for group II base stocks.
The oils passed the sequence IIIF oxidation test, in which the oils would otherwise fail without the
molybdenum top-treatment [36]. Further demonstrated is a combination of ADPAs, sulfurized olefi n,
or HP and oil-soluble molybdenum compounds including MoDTC. The mixture is highly effective in
stabilizing lubricants, especially those formulated with highly saturated, low-sulfur base oils [37].
Thiadiazole derivatives, particularly the monomers and dimers, represent another class of
sulfur- and nitrogen-bearing multifunctional additives with antioxidant potency. For example, the
monomeric 2-alkylesterthio-5-mercapto-1,3,4-thiadiazole has been reported to increase oxidative
stability of engine oils under thin-fi lm oxidation conditions by using thin-fi lm oxygen uptake test
(TFOUT) [38]. Lithium 12-hydroxystearate grease containing 2,5-dithiobis(1,3,4-thiadiazole-
2-thiol), a dimer, exhibited superior oxidative stability in the American Society for Testing and
Materials (ASTM) D 942 pressure bomb oxidation test [39]. When used in conjunction with ADPA
and organomolybdenum compound, the thiadiazole derivative improved the thermal-oxidation
engine oil simulation test (TEOST) deposition (ASTM D 7097) characteristic of an engine oil from
the control oil containing sulfurized isobutylene instead [40]. In addition to providing antioxidant
benefi t, the thiadiazole derivatives have been widely used as ashless antiwear and EP additives.
Some of them can also provide corrosion inhibition and metal deactivation properties to nonferrous
metals such as copper.
Phenothiazines are also well-known sulfur- and nitrogen-bearing antioxidants and have been
u s e d t o s t a b i l i z e a v i a t i o n fl uids. Recent advances have lead to N-substituted thio alkyl phenothiazines,
having improved antioxidant activities and oil solubility [41]^ as well as N-aminopropylpheno-
thiazine that can be used for further derivatization of the N-amino group [42]. For example, alkyl
phenothiazines together with aromatic amines can be attached to olefi n copolymers to result in a
multifunctional antioxidant, antiwear agent, and Viscosity index (VI) improver for lubricants [43].
Diamine sulfi des, including diamine polysulfi des, can also provide effective oxidation control
when used in conjunction with oil-soluble copper. In demonstration, dimorpholine disulfi de and
di(dimethyl morpholine) disulfi de were compared to primary alkyl ZDDP and found to be superior
in controlling oil viscosity increase of engine crankcase lubricants at elevated temperatures [44].
1.4 PHOSPHORUS COMPOUNDS
The good performance of phosphorus as an oxidation inhibitor in oils was identifi ed early on in
lubrication science. The use of elemental phosphorus to reduce sludge formation in oils has been
described [45]. However, elemental phosphorus, like elemental sulfur, may have corrosive side effects
to many nonferrous metals and alloys, so it is rarely incorporated in oils in this form, rather oil- soluble
organic compounds of phosphorus are preferred. Naturally occurring phosphorus compounds such
as lecithin have been utilized as antioxidants and many patents have been issued on these materials
for single use or in combination with other additives [46–49]. Lecithin is a phosphatide that has been
produced commercially as a by-product from the processing of crude soybean oil.
The antioxidant property of synthetic neutral and acid phosphite esters has been known for
sometime. Alkyl and aryl phosphites such as tributyl phosphite and triphenyl phosphite are effi cient