Antioxidants 19
Antioxidants are essential additives incorporated in lubricant formulations to delay the onset of
autoxidation and minimize its impact. The mechanisms of lubricant degradation and its stabiliza-
tion by antioxidants are discussed in the following sections.
1.10.1 AUTOXIDATION OF LUBRICATING OIL
The well-documented autoxidation mechanism involves a free-radical chain reaction [179–181]. It
consists of four distinct reaction steps: chain initiation, propagation, branching, and termination.
1.10.1.1 Initiation
R H R HOO
O 2
→ ii (1.1)RR R R
→Energy ii
(1.2)
The initiation step is characterized as the formation of free alkyl radicals (R•) from the breakdown
of hydrocarbon bonds by hydrogen abstraction and dissociation of carbon–carbon bonds. These
reactions take place when hydrocarbons are exposed to oxygen and energy in the form of heat, UV
light, or mechanical shear stress [182]. The ease of homolytic cleavage of an R–H bond follows this
order, as determined by the C–H bond strength and the stability of the resulting radical [183]: phenyl
< primary < secondary < tertiary < allylic < benzylic. Thus, hydrocarbons containing tertiary
hydrogen or hydrogen in an alpha position to a carbon–carbon double bond or aromatic ring are
most susceptible to oxidation. The reaction rate of chain initiation is generally slow under ambient
conditions but can be greatly accelerated with temperature and the presence of catalytic transition-
ing metal ions (copper, iron, nickel, vanadium, manganese, cobalt, etc.).
1.10.1.2 Chain Propagation
RO ROOii^2 → (1.3)
ROOiiRH→ROOH R (1.4)
The fi rst propagation step involves an alkyl radical reacting irreversibly with oxygen to form an
alkyl peroxy radical (ROO•). This reaction is extremely fast, and the specifi c rate is dependent on
the radical’s substituents [179]. Once formed, the peroxy radical can randomly abstract hydrogen
from another hydrocarbon molecule to form hydroperoxide (ROOH) and a new alkyl radical (R•).
Based on this mechanism, each time a free alkyl radial is formed, a large number of hydrocarbon
molecules may be oxidized to hydroperoxides.
1.10.1.3 Chain Branching
1.10.1.3.1 R adical Format ion
ROOH→ROiiHO (1.5)
ROiiRH→ROH R (1.6)
HOiiRH→H O^2 R (1.7)