Lubricant Additives

30 Lubricant Additives: Chemistry and Applications

Heterosynergism occurs when antioxidants act by a different mechanism and hence comple-
ment each other. This type of synergy usually takes place when a primary antioxidant and a sec-
ondary antioxidant are present in one lubricant system. The primary antioxidant scavenges radicals,
whereas the secondary antioxidant decomposes hydroperoxides by reducing them to more stable
alcohols. Through these reactions, chain propagation and branching reactions are signifi cantly
slowed or inhibited. A representative example of a heterosynergism is an aminic antioxidant in
combination with a ZDDP.
Autosynergism is a third type of synergistic response that results from two different antioxidant
functions in the same molecule. Usually, antioxidants having functional groups that provide radical
scavenging and hydroperoxide decomposing functions exhibit autosynergy. Examples of this type
of antioxidants are sulfurized phenols and phenothiazines.

1.11 OXIDATION BENCH TESTS

Oxidative degradation of lubricants can be classifi ed into two main reactions: bulk oil oxidation
and thin-fi lm oxidation. Bulk oil oxidation usually takes place in a larger oil body at a slower rate.
The exposure to air (oxygen) is regulated by the surface contact kinetics, and the gas diffusion is
limited. The oxidation leads to increases in oil acidity, oil thickening, and, to a more severe extent,
oil-insoluble polymers that may manifest as sludge when mixed with unburned/oxidized fuel com-
ponents, water, and other solids. Thin-fi lm oxidation describes a more rapid reaction in which a
small amount of oil, usually in the form of a thin-fi lm coating on metal surface, is exposed to ele-
vated temperatures and air (oxygen). Under these conditions, hydrocarbons decompose much more
quickly and the polar oil oxidation products formed at the oil–metal interface can rapidly build up
on the metal surface, leading to the formation of lacquer or deposits.
Over the years, many oxidation bench tests have been developed and proven to be valuable tools
for lubricant formulators, particularly in the screening of new antioxidants and the development of
new formulations. Most bench tests attempt to simulate the operating conditions of more expensive
engine and fi eld tests, when taking into consideration the oxidation mechanisms described earlier.
In addition, a third mechanism based on oxygen uptake in a closed system has been employed in
some bench tests, such as the RPVOT [206].

N

H

R R R N

RO• , ROO• ROH, ROOH

O•

R

OH

R

R

FIGURE 1.18 Mechanism of synergism between ADPA and hindered phenol.