244 Lubricant Additives: Chemistry and Applications
The impetus for signifi cantly improved lubricant additives is found on a number of fronts.
Governmental and regulatory requirements continue to challenge the industry for improved products
with lower toxicity. New engine developments, such as increased use of diamond-like carbon (DLC)–
coated engine parts and ceramic components for wear resistance and higher contact temperatures, are
on the horizon and present opportunities for antiwear additives that can function at very high operating
temperatures. Space technology and other advanced transportation needs present new challenges to the
industry. And, of course, there will always be a need for low product costs and ease of production.
Four particular developments may have a major impact on the lubricant industry in the near
term: (1) a move toward low-sulfur hydroprocessed (groups II and III), sulfur-free gas-to-liquid
(GTL) and synthetic (groups IV and V) base stocks; (2) the imminent trend toward lower ash, sulfur,
and phosphorus in engine oils; (3) a desire to reduce or eliminate chlorine in lubricants, particularly
in metalworking fl uids; and (4) a move to eliminate heavy metals and achieve low ash or even ash-
free in both engine and industrial oils.
To meet the growing needs for better thermal/oxidative stability and better viscometrics, synthetic
base stocks such as polyalpha olefi ns, together with hydrotreated petroleum base stocks and GTLs are
continuing to expand in all lubricant sectors. These types of materials have no aromatic hydrocarbons
or greatly reduced amounts of aromatic hydrocarbons, which are potentially problematic for additive
solvency; as a result of removing these solubilizing aromatics, the additives tend to precipitate out
of the oil. This is particularly true for surface-active, polar components such as antiwear additives.
Therefore, greater compatibility with nonconventional base stocks (groups II–IV) will be an essen-
tial requirement for all ashless antiwear/EP additives. Meanwhile, there are noticeable synergies
identifi ed among certain ashless antiwear/EP additives and nonconventional base stocks in a number
of lubricant applications. Therefore, the choice of proper ashless additives will be vitally important.
Because of the large number of automobiles equipped with catalytic converters that are sensitive
to phosphorus derived from zinc dithiophosphates in the crankcase oil (possible reduction of cata-
lytic effi ciency), strong needs exist for engine oils with lower phosphorus content. Initially, ILSAC
GF-4 aimed to reduce phosphorus levels to as low as 0.05% (about one-half of the former GF-3 level),
but settled on a maximum phosphorus level of 0.08% instead (a 20% reduction). In addition to phos-
phorus limits, GF-4 oils also offered improved oxidation stability (including nitration control), high-
temperature wear discrimination, high-temperature deposits control and used oil pumpability [103].
As vehicle emission regulations become more challenging, increasing restrictions are likely to
be placed on other lubricant elements besides phosphorus that can impact emission control systems.
Sulfur and metals are also under scrutiny as sulfur is suspected as a poison of DeNOx catalysts,
and ash (from metals) may plug after-treatment particulate traps. Modern engine oils rely heavily
on ZDDP to provide antiwear, antioxidation, and anticorrosion protection. Since ZDDP is rich in
phosphorus, sulfur, and zinc, it becomes an obvious target for emission control. In fact, at former use
level, ZDDP was almost solely accountable for more than two-thirds of sulfur and all the phosphorus
and zinc present in engine oils, excluding sulfur from base oils. Oils with ZDDP at former levels
could make it diffi cult for OEMs to optimize (for cost and life) their exhaust after-treatment systems.
Therefore, the future trend will likely be toward further reduced ZDDP in engine oils providing that
the performance integrity can be maintained through the use of alternate additives [104].
To satisfy performance requirements in terms of oxidation control and deposit levels, more
antioxidants could be added to the engine oil formulation. These ashless antioxidants (hindered
phenols and arylamines) may effectively compensate for the loss of oxidation protection due to the
reduction in ZDDP concentrations. However, since ZDDP is such a cost-effective additive and is
the sole antiwear component used in many engine oils, a reduction in ZDDP treat levels may not
provide the needed wear protection. Recently, engine builders are requiring even greater antiwear
protection, and more demanding test protocols are being put in place to ensure that lubricants can
meet these more stringent specifi cations. Therefore, there is a strong need for advanced ashless
antiwear systems to replace or supplement ZDDP to satisfy emission regulations while ensuring
high levels of wear protection [105,106].