Lubricant Additives

Antioxidants 27

an equal mole basis, in theory, ADPAs can quench twice as many alkyl peroxy radicals as a mono
HP under low-temperature conditions. At high temperatures (>120°C), the nitroxyl radical inter-
mediate can undergo one of two possible reaction pathways by either reacting with a secondary
or a tertiary alkyl radical to form an N-sec-alkoxy diphenylamine [179,186,203] or an N-hydroxyl
diphenylamine intermediate, respectively. These mechanisms are illustrated in Figure 1.15. In the
former case, the resulting alkoxy amine intermediate can thermally rearrange to form a ketone and
regenerate the starting ADPA. In the latter case, nitroxyl radical is regenerated upon reaction of
the hydroxyl diphenylamine intermediate with an alkyl peroxy radical. Thus, at high temperatures,
one molecule of ADPA can catalytically scavenge a large number of radicals before the nitroxyl

FIGURE 1.14 Resonance structures of nitroxyl radical.

N

O−

R

N

O−

R

R

N

O•

RR

R

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FIGURE 1.15 High-temperature (>120°C) function mechanism of ADPAs.

ROOH ROO•

N

H

R N

O•

R

N

O

R

CHRR′

N

OH

R

R

R

CR

ROO•

R

R

CR

H

R N

Radicals

O

R C R′ RR′IIC•

R R

R

C C

R

R

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  •