Antioxidants 9
barium, molybdenum, or calcium oxides are usually chosen. For more than 60 years, zinc salts
of dialkylthiophosphoric acids (ZDDP) have been one of the most cost-effective antioxidants and
therefore have been included as a key component in many oxidation inhibitor packages for engine
oils and transmission fl uids. In addition, ZDDPs show good antiwear properties, especially in the
valve train area owing to the formation of sulfi de and phosphate fi lms through corrosive reactions
on metal surfaces. These fi lms can also provide protection against corrosive attack from the organic
acids formed during the oxidation process. The salts of C 4 /C 5 dialkyldithiophosphoric acid are
the most common, but a broad range of other alkyl and aryl derivatives have been developed to
meet special needs, for instance, protection at higher temperatures. The reaction scheme of making
ZDDP is shown in Figure 1.3.
A number of patents describe modifi cations to the fi rst step of the reactions shown in Figure 1.3; by
conducting preliminary condensation reaction of phosphorus pentasulfi de with unsaturated organic
compounds such as terpenes, polybutenes, wax olefi ns, fatty acids, fatty esters, sperm oil, and so
on to form high-molecular-weight intermediate products [68–89]. During these reactions, hydrogen
sulfi de is liberated, and the intermediates are usually acidic. The mechanism of the P 2 S 5 reaction
with olefi ns in these cases may be one of substitution (replacement of reactive hydrogen atoms) as
well as of addition. In preparing the fi nal additives, these acidic intermediates were neutralized by
the treatment with alkaline earth oxides or hydroxides to form metal salts. The calcium, barium, or
potassium salts are the most preferred products. Some additives may also display detergency char-
acteristics. The concept of conducting preliminary condensation reactions provides a facile route
to the synthesis of a wide variety of products from the reaction of phosphorus pentasulfi de and an
unsaturated organic moiety. Several of these, particularly the terpene and polybutene reaction prod-
ucts, have been used extensively in commercial applications.
To reduce the staining effect of ZDDP on metal parts (especially copper), addition of alkyl or
aryl phosphites during the synthesis has been attempted [90]. For example, triphenyl phosphite is
added to the dialkyldithiophosphoric acid and heated at 110ºC for an hour before the addition of
zinc oxide. In another patent, a novel dithiophosphate with improved oxidation stability is described
[91]. An acid is reacted with a glycol, to give a monoester having a hydroxyl group, which is then
reacted with P 2 S 5 to give the dialkyl dithiophosphoric acid. Zinc oxide is subsequently added to give
the novel dithiophosphates. To improve solubility, the salts can be made of lower dialkyl dithiophos-
phates by utilizing both primary and secondary alcohols, including butyl alcohols in the process
[92]. Mixed metal salts of dialkyl dithiophosphoric acids and carboxylic acids are claimed to have
higher thermal stability [93].
Many descriptions have recently appeared of organomolybdenum phosphorodithioate com-
plexes that impart excellent oxidation stability to lubricants. In certain circumstances, oil-soluble
molybdenum compounds are preferred additives owing to their multifunctional characteristics
such as antiwear, EP, antioxidant, antipitting, and antifriction properties. For instance, several
molybdenum dialkylphosphorodithioate complexes with varying alkyl chain length of amyl,
octyl, 2-ethylhexyl, and isodecyl were reported to exhibit appreciable antioxidation, antiwear,
FIGURE 1.3 Synthesis of ZDDP.
2 ROH + P 2 S 5 PS
SH + H 2 S
RO2 ROPS
SH + ZnO
RO2 RO RO P
ROSS2Zn + H 2 O