Lubricant Additives

26 Lubricant Additives: Chemistry and Applications

1.10.6.2 Aromatic Amines

A particularly effective class of aromatic amines useful as primary antioxidants is the ADPAs.
The reaction of the antioxidants begins with hydrogen atom abstraction by alkyl peroxy radi-
cal and alkoxy radical, the mechanism of which is illustrated in Figure 1.12. Owing to the rapid
reaction of alkoxy radicals with oxygen, the resulting alkyl peroxy radical is present at higher
concentration, and its reaction with ADPA predominates. The aminyl radical formed can undergo
a number of possible reaction pathways depending on temperature, degree of oxidation (relative
concentration of peroxy radicals versus alkyl radicals), and the chemical nature of the ADPA
[201]. Figure 1.13 shows the low-temperature (<120°C) oxidation inhibition mechanism [179,202],
starting with aminyl radical attacking a second alkyl peroxy radical to form a nitroxyl radical
and alkoxy radical. The nitroxyl radical is stabilized through three possible resonance structures
[182,202], as illustrated in Figure 1.14. Next, a third alkyl peroxy radical reacts with the nitroxyl
radical to form a nitroxyl–peroxide complex, which can further eliminate an ether molecule, form-
ing a nitroxyl cyclohexadienone. Following reactions involve a fourth alkyl peroxy radical being
added to the nitroxyl cyclohexadienone to form nitroxyl cyclohexadienone peroxide, followed by
a dissociation reaction to form 1,4-benzoquinone and an alkylated nitrosobenzene. Therefore, on

O

CH 3 OOR

Heat

O

O

+ RO• + •CH 3

FIGURE 1.11 Decomposition of cyclohexadienone alkyl peroxide at higher temperatures.

FIGURE 1.12 ADPA as a hydrogen donor.

R N

H

RO , ROO R N R + ROH, ROOH

R

• • •

FIGURE 1.13 Low-temperature function mechanism of ADPA.

ROO

R N

O•

R N R + RO•

ROO•

R N

O−

OOR

R

+

−ROR

R N

O−

O

+

R N

O•

O R O

ROO•

−RO• O+ O

ROO•

R

N