Lubricant Additives

22 Lubricant Additives: Chemistry and Applications

are on the horizon, and their use is expected to grow in large scale in the coming future, especially
after the completion of nearly a dozen new group II/III oil refi nery plants worldwide [190].
It has been widely recognized that base oil composition, for example, linear and branched
hydrocarbons, saturates, unsaturates, monoaromatics, polyaromatics, together with traces of
nitrogen-, sulfur-, and oxygen-containing heterocycles, etc., plays an important role in the oxida-
tive stability of the oil. There have been quite extensive research activities attempting to establish
correlations between base stock composition and oxidative stability [191–195]. However, owing
to the large variations in the origin of the oil samples, the test methods, test conditions, and the
performance criteria employed, the conclusions are not always consistent and in some cases
contradictory to each other. In general, it has been agreed that saturated hydrocarbons are more
stable than the unsaturated toward oxidation. Of the different saturated hydrocarbons found in
mineral oils, paraffi ns are more stable than cycloparaffi ns. Aromatic compounds, due to their
complex and large variation in the chemical makeup, play a more profound role. Monocyclic
aromatics are relatively stable and resistant to oxidation, whereas bi and polycyclic aromatics
are unstable and susceptible to oxidation [196]. Alkylated aromatics oxidize more readily due to

FIGURE 1.7 High-temperature (>120°C) lubricant degradation leading to the formation of high-molecular-
weight hydrocarbons.

O

R C (CH 2 )n CH

O O

H 3 C C OR′

O

R C (CH 2 )nCH CHCOR′ + H 2 O

O

O

"R C (CH 2 )p COR"'

O O

R C (CH 2 )nCH CHCOR'

O

O

R C C(CH 2 )n− 1 CH CHCOR′

O

C

+

"R (CH 2 )p

Acid

or base

COR"'

−H 2 O

O

Sludge

precursors

+

TABLE 1.3

API Base Oil Categories

API Category Percent Saturates Percent Sulfur Viscosity Index

Group I ≤ 90 ≥0.03 ≥80 and ≤ 120

Group II ≥ 90 ≤0.03 ≥80 and ≤ 120

Group III ≥ 90 ≤0.03 ≥ 120

Group IV PAOs

Group V Includes all other base oils not included in the fi rst four groups