38 Lubricant Additives: Chemistry and Applications
represents an API group from I to IV. The HP and the ADPA are the same as before. Clearly, with-
out the protection of antioxidant, all oils performed equally poor. A 0.5 wt% of the HP antioxidant
gave modest levels of protection that marginally increase from API group I to IV. When the base
oils were treated with the same level of the ADPA, a drastic performance boost is seen across the
board. The performance responses of the highly refi ned groups II, III, and the group IV to the added
ADPA appear to be particularly strong.
The superior antioxidant response of the groups II and III base oils over the conventional group I
base oils may be attributed to the removal of aromatic hydrocarbons and polar constituents and the
large presence of saturated hydrocarbons in the oils [243,244]. One school of thought hypothesized
that oxygen-, sulfur-, and nitrogen-containing polar species may exist in the form of micellar aggre-
gations in base oil. When an antioxidant is added, some of the natural polar molecules may interact
physically and chemically with the additives, leading to a reduction in additive effectiveness in
some circumstances. In those highly refi ned base oils where the natural compounds are essentially
low or absent, the added antioxidant is able to exert its maximum effectiveness [245].
ZDDP, another important class of antioxidant/antiwear agent, has been studied by others, and
the results indicated that its antioxidant performance is dependent on the base oil aromatics, alkyl-
substituted aromatics, average chain length of hydrocarbons, and the relative presence of normal
paraffi ns and isoparaffi ns [196]. In group I base oils, ZDDP gave good responses to highly saturated
hydrocarbons characterized with normal paraffi ns having shorter chain length. Isoparaffi ns were
found to decrease the antioxidant activity of ZDDP due to the steric hindrance of the side chains,
which restricts the additive molecules from interacting with the hydrocarbons. In oils with higher
monoaromatic hydrocarbons, ZDDP tends to perform better, which was believed to be related to
improved solvency.
1.14 FUTURE REQUIREMENTS
The need for antioxidants in future lubricants will continue and the demands for both quality and
quantity will likely to increase to meet new environment and performance requirements. Although
such trend is inevitably to take place in the entire lubricant industry, three specifi c areas may see
more rapid and dynamic advancements: (a) modern engine oils, (b) biodegradable lubricants, and
(c) engine oils that operate on biofuels.
New engine designs and engine oil formulations are being frequently rolled out. The primary
driving force is environmental in nature: the requirements for less oil consumption, better fuel econ-
omy, extended drain intervals, and lower emissions (particulates, hydrocarbons, CO, and NOx). To
meet these requirements, new automotive engines are designed lighter and smaller but are required
to operate under more severe conditions for higher output and speeds, which lead to higher engine
and oil temperatures. It has been well established that every 10ºC of temperature increase will
approximately double the rate of oil oxidation. Therefore, to maintain satisfactory service lifetime
in a more severe service environment, increasing the level of antioxidant and using those suitable for
high-temperature conditions, such as aminic antioxidants, are expected. Modern catalytic converters
are highly effective in reducing the emissions. However, they are vulnerable to the deterioration
effects of sulfur, phosphorus, and ash derived from engine oils and fuels. Accordingly, initiatives
have been made to reduce the ZDDP content in engine oils. The current ILSAC GF-4 specifi cation
has limited the sulfur and phosphorus levels to a maximum of 0.7 and 0.08 wt%, respectively, and
these numbers are likely to be even lower for future engine oils. With the ZDDP being reduced, it
is expected that the uses of ashless, primary antioxidants as well as secondary antioxidants (as a
substitute for ZDDP) will increase.
To meet the increasing performance requirements set for modern engine oils, high- quality
groups II and III base stocks are emerging to replace the conventional solvent-refi ned group I oils.
The hydrotreating and isodewaxing processes that are used to make these oils signifi cantly lower
the unsaturated hydrocarbons and polyaromatics, which contribute to poor oxidative stability.