Lubricant Additives

28 Lubricant Additives: Chemistry and Applications

radical is destroyed. It has been reported that such regeneration process can provide ADPAs with
a stoichiometric effi ciency of more than 12 radicals per molecule [186].

1.10.7 MECHANISMS OF SECONDARY ANTIOXIDANTS

1.10.7.1 Organosulfur Compounds

Organosulfur compounds function as hydroperoxide decomposers through the formation of oxi-
dation and decomposition products. The mechanism is illustrated in Figure 1.16 for an alkyl sul-
fi de. The antioxidant action starts with the reduction of an alkyl hydroperoxides to a less reactive
alcohol, with the sulfi de being oxidized to a sulfoxide intermediate. A preferred mechanism for
the subsequent reaction of sulfoxide intermediate is the intramolecular beta-hydrogen elimination,
leading to the formation of a sulfenic acid (RSOH), which can further react with hydroperoxides to
form sulfur-oxy acids. At elevated temperatures, sulfi ninc acid (RSO 2 H) may decompose to form
sulfur dioxide (SO 2 ), which is a particularly powerful Lewis acid for hydroperoxide decomposition
through the formations of active sulfur trioxide and sulfuric acid. Previous work has shown that
one equivalent of SO 2 was able to catalytically decompose up to 20,000 equivalents of cumene
hydroperoxide [204]. Further enhancing the antioxidancy of sulfur compounds is that, under certain
conditions, the intermediate sulfur-oxy acids (RSOxH) can scavenge alkyl peroxy radicals, thus giv-
ing the sulfur compound primary antioxidant characteristics:

RSO Hx RSO ROOH

ROO

x

→• •

1.10.7.2 Organophosphorus Compounds

Phosphites are a main class of organophosphorus compounds being used as secondary anti-
oxidants. Phosphites decompose hydroperoxides and peroxy radicals following the reaction

RR

ROOH

−ROH

R^1 C

R^2

H 2 C

H

O

R RSOH + H 2 C=CR^1 R^2

ROOH −ROH

RSO 2 H

ROOH −ROH

RSO 3 H

RH + SO 2

ROOH −ROH

ROSO 3 H

H 2 O

H 2 SO 4

ROOH

ROOR

R 2 C=O

Decompose more

hydroperoxides

S S

FIGURE 1.16 Antioxidation mechanism of alkyl sulfi de.