Antioxidants 23
the presence of highly reactive benzylic hydrogen atoms. Kramer et al. [193] demonstrated that
the oxidative rate of a hydrocracked 500N base oil doubled when the aromatic content increased
from 1 to 8.5 wt%. Naturally occurring sulfur compounds are known antioxidants for the inhi-
bition of the early stage of oil oxidation. Laboratory experiments have shown that mineral oils
containing as little as 0.03% of sulfur had good resistance to oxidation at 165°C over sulfur-free
white oils and PAOs [145]. In hydrocracked oils that are essentially low in aromatics, better oxi-
dative stability was found with elevated sulfur concentration (>80 ppm) versus a level at 20 ppm
or lower [192]. It has been proposed that sulfur compounds act as antioxidants by generating
strong acids that catalyze the decomposition of peroxides through a nonradical route or by pro-
moting the acid-catalyzed rearrangement of arylalkyl hydroperoxides to form phenols that are
antioxidants [145,179]. Contrary to sulfur, nitrogen-bearing compounds, especially the hetero-
cyclic components (also called “basic nitrogen”), accelerate oil oxidation even at relatively low
concentrations [197]. In highly refi ned groups II and III base stocks that are essentially devoid
of heteroatom-containing molecules, aromatic and sulfur contents are considered as the main
factors which infl uence the base oil oxidative stability [192,193]. It has been shown that oxida-
tive stability of a given base stock can be enhanced when the combinations and concentrations
of base stock sulfur and aromatics are optimized [194].
1.10.5 OXIDATION INHIBITION
The proceeding mechanistic discussion makes clear several possible counter measures to control
lubricant oxidation. Blocking the energy source is one path. However, this is only effective for
lubricants used in low shear and temperature situations. A more practical approach for most lubri-
cant applications is the trapping of catalytic impurities and the destruction of alkyl radicals, alkyl
peroxy radicals, and hydroperoxides. This can be achieved through the use of a metal deactiva-
tor and an appropriate antioxidant with radical scavenging or peroxide decomposing functionality,
respectively.
The radical scavengers are known as primary antioxidants. They function by donating hydrogen
atoms to terminate alkoxy and alkyl peroxy radicals, thus interrupting the radical chain mechanism
of the auto-oxidation process. The basis for a compound to become a successful antioxidant is that
peroxy and alkoxyl radicals abstract hydrogen from the compound much more readily than they do
from hydrocarbons [198]. After hydrogen abstraction, the antioxidant becomes a stable radical, the
alkyl radical becomes a hydrocarbon, and the alkyl peroxy radical becomes an alkyl hydroperoxide.
HPs and aromatic amines are two main classes of primary antioxidants for lubricants.
The peroxide decomposers are also called secondary antioxidants [180]. They function by
reducing alkyl hydroperoxides in the radical chain to nonradical, less-reactive alcohols. Organo-
sulfur and organophosphorus compounds and those containing both elements, such as ZDDPs, are
well-known secondary antioxidants.
Since transitional metals are present in most lubrication system, metal deactivators are usually
added to lubricants to suppress the catalytic activities of the metals. Based on the functioning mech-
anisms, metal deactivators for petroleum products can be classifi ed into two major types: chelators
[180] and surface passivators [199]. The surface passivators act by attaching to metal surface to form
a protective layer, thereby preventing metal–hydrocarbon interaction. They can also minimize cor-
rosive attack of metal surface by physically restricting access of the corrosive species to the metal
surface. The chelators, however, function in bulk of the lubricant by trapping metal ions to form an
inactive or much less-active complex. With either mechanism, metal deactivators can effectively
slow the oxidation process catalyzed by those transitional metals, which in turn lends metal deacti-
vators an antioxidant effect. Table 1.4 lists examples of metal deactivators that are commonly found
in lubricant formulations.