10 Lubricant Additives: Chemistry and Applications
and antifriction properties [94]. Novel trinuclear molybdenum dialkyldithiophosphates prepared
by reacting an ammonium polythiomolybdate and an appropriate bis(alkyldithiophosphoric) acid
possess excellent antioxidant as well as antiwear and friction-reducing properties [31]. Some
molybdenum compounds have been used commercially in engine oils and metal working fl uids
as well as in various industrial and automotive lubricating oils, greases, and specialties [95].
The combination of ZDDP with a molybdenum-containing adduct, prepared by reacting a phos-
phosulfurized polyisoalkylene or alpha olefi n with a molybdenum salt, has been described [96].
In this case, the molybdenum adduct alone gave poor performance in oxidation tests, but the
mixture with ZDDP provided good oxidation stability. Novel organomolybdenum complexes pre-
pared with vegetable oil have been identifi ed as synergist with ADPAs and ZDDPs in lubricating
oils [97].
Owing to increasing concerns on the use of metal dithiophosphates that are related to toxicity,
waste disposal, fi lter clogging, pollution, etc., there have been extensive research activities on the
use of ashless technologies for both industrial and automotive applications. A number of ashless
compounds based on derivatives of dialkylphorphorodithioic acids had been reported as multifunc-
tional additives. Upon reacting diisoamylphosphorodithioic acid with various primary and second-
ary amines, eight alkylamino phosphorodithioates with varying chain length from C 5 to C 18 were
obtained and found to possess excellent antiwear and antioxidant properties as compared to ZDDP
[98]. Alkylamino phosphorodithioates obtained from reacting heptylated or octylated or nonylated
phosphorodithioic acids with ethylene diamine, morpholine, or tert-alkyl (C 12 –C 14 ) amines have
been demonstrated to impart similar antioxidant and antiwear effi cacy and superior hydrolytic sta-
bility over ZDDP [99]. Phosphorodithioate ester derivatives containing a HP moiety are also known
to have antioxidant potency. This type of chemistry can be obtained by reacting metal salts of
phosphorodithioic acids with HP halides [100] or with HP aldehydes [101]. Substituting the phenol
aldehydes with hindered cyclic aldehydes, in which the carbon atom attached to the carbonyl carbon
contains no hydrogen atoms, may also result in products having excellent antioxidant and thermal
stability characteristics [102].
1.6 AMINE AND PHENOL DERIVATIVES
Oil-soluble organic amines and phenol derivatives such as pyrogallol, gallic acid, dibutylresorcinol,
hydroquinone, diphenylamine, phenyl-alpha-naphthylamine, and beta-naphthol are early examples
of antioxidants used in turbine oils and lubricating greases [103,104]. In engine oils, these types of
compounds showed only limited effectiveness. Other amines and phenol derivatives such as tetra-
met hyld ia m i nod iphenyl met ha ne a nd a l iza r i n were used to some deg ree, ra rely a lone, but more of ten
in combination with other types of antioxidants. For example, a mixture of a complex amine with
a phosphorus pentasulfi depolybutene reaction product has been reported [105]. Another reported
mixture is a complex phenol derivative such as alizarin in combination with an alkyl phenol sulfi de
and a detergent additive [106]. As technology advances, numerous amine and phenol antioxidants
have been invented, and many of them have become the most widely used antioxidants in the lubri-
cant industry.
1.6.1 AMINE DERIVATIVES
ADPAs are one of the most important classes of amine antioxidants being used today. Owing to their
higher reactivity over the unsubstituted diphenylamine, ADPAs have been workhorse antioxidants
for engine oils and various industrial lubricants for more than two decades. Figures 1.3 and 1.4
illustrate the typical synthesis routes of some commonly used ADPAs. The reactions start with
benzene, which is fi rst converted into nitrobenzene [107], followed by a high-temperature reduc-
tion to aniline [108]. Under very high-temperature (400–500°C) and high-pressure (50–150 psi)
conditions, aniline can undergo a catalytic vapor-phase conversion to form diphenylamine [109].