Lubricant Additives

Antioxidants 29

mechanisms. In these reactions, phosphite is oxidized to corresponding phosphate, with the
hydroperoxide and the peroxy radical being reduced to less-reactive alcohol and alkoxy radical,
respectively.

(RO) P^3 R OOH→(RO) P^3  O R OH (1.17)

(RO) P^3 ROO (RO) P O RO^3  
••→
(1.18)

Phosphites with certain substituted phenoxy groups may also behave as peroxy and alkoxy radical
scavengers as shown in Figure 1.17. The resulting phenoxy radicals can further eliminate peroxy
radicals upon resonance transformation to cyclohyxadienone radical as discussed earlier. Owing to
the steric hindrance provided by the alkyl groups on the ortho-positions, these phosphites tend to be
more stable against hydrolysis and are preferred for use in moist lubrication environment.

1.10.8 ANTIOXIDANT SYNERGISM*

Antioxidant synergism describes the effect or response of a combined use of two or more antioxi-
dants being greater than that of any individual antioxidant. Synergistic antioxidant systems offer
practical solutions to problems where using a single antioxidant is inadequate to provide satisfactory
results, or where the treatment level has to be limited due to economic or environmental reasons.
Three types of synergy have been proposed for lubricant antioxidants [205]: (a) homosynergism,
(b) heterosynergism, and (c) autosynergism.
Homosynergism occurs when two antioxidants acting by the same mechanism interact,
generally in a single-electron-transfer cascade. A classic example is an ADPA in combination with
a HP antioxidant. ADPA is initially more reactive than HP in scavenging alkyl peroxy radicals. As
illustrated in Figure 1.18, the amine is fi rst converted to an aminyl radical, which is relatively less
stable and will accept a hydrogen atom from the HP to regenerate the alkylated amine [179,182].
In consequence, the HP is converted to a phenoxy radical. The driving force for this regeneration
cycle to occur is the higher reactivity of the ADPA compared to the HP and the greater stability of
the phenoxy radical relative to the aminyl radical [201]. After the HP is consumed, the aromatic
amine antioxidant begins to deplete. By regenerating the more reactive amine, the overall effective-
ness of the system is enhanced, and the useful antioxidant lifetime is extended.

• With permission from Dong, J. and C.A. Migdal, Synergestic Antioxidant Systems for Lubricants. 12th Asia Fuels and
  Lubes Conference Proceedings, Hong Kong, 2006.

R O P(OR′) 2

R′′O•

R′′O P(OR′) 2 +R

R′′O P(OR′) 2

O

+R

R′′OO•

O•

O•

FIGURE 1.17 Alkoxy and peroxy scavenging mechanisms of phenyl phosphite.