Lubricant Additives

138 Lubricant Additives: Chemistry and Applications

impairing the proper functioning of the various equipment parts. The acidic materials such as sulfur
oxides, and organic acids resulting from lubricant oxidation can attack metal surfaces and cause
corrosion. Sulfur oxides and sulfuric acid result from the combustion of fuel sulfur or the oxidation
of sulfur-containing additives, such as sulfurized olefi ns or zinc dialkyldithiophosphates. The func-
tion of the detergent in this case is to neutralize these acids, thus causing cor rosion. The reser ve base
in detergents primarily performs this function. The soap portion keeps oil-insoluble polar products
suspended in oil.
Phenates, sulfurized phenates, and salicylates can also act as oxidation inhibitors because of the
presence of the phenol functionality. Phenols are well known for their oxidation-inhibiting action
[5]. Detergents are also effective corrosion inhibitors, especially basic detergents [25,26], because
they not only neutralize corrosive acidic products but also form surface fi lms that isolate metal
surfaces from corrosive agents [6]. The carbonate portion in the detergent performs acid neutraliza-
tion, and the soap portion forms the protective surface fi lm. NA-SUL 729® and NA-SUL CA-50®
are examples of commercial corrosion inhibitors belonging to this class of additives. The tests that
evaluate rust and corrosion are described elsewhere [3,6]. Detergents derived from fatty carboxylic
acids are good friction modifi ers, primarily because of the linear structure of their soaps [6].
Detergents fi nd primary use in engine oils, which are responsible for more than 75% of the total
consumption. The detergent level in marine diesel engine oils is the highest because marine engines
use high-sulfur fuel, which leads to highly acidic combustion products such as sulfuric acid. The
lubricants for these engines therefore require the base reserve of highly overbased detergents.
A variety of proprietary and industry-established tests are used to determine a detergent’s effec-
tiveness in lubricants. For gasoline engine oils to be used in North America, these include the CRC
L-38, TEOST (Thermo-Oxidation Engine Oil Simulation Test [60]), ASTM Sequence IIIE/IIIF,
and ASTM Sequence VE/VG tests. For European gasoline engines, in addition to performance
in the ASTM Sequence IIIE/IIIF and VE/VG Engine tests, performance in Peugeot TU3M High-
Temperature Test and MB M111 Black Sludge Test is also required. These tests are part of the
ACEA 2002 Standard. The standard also includes a sulfated ash limit that directly affects the
amount of detergent used in formulations since it is the primary contributor to sulfated ash.
The effi cacy of diesel engine oils for North American use is evaluated by the use of both the
single-cylinder and the multicylinder engine tests. Single-cylinder tests include CRC L-38 and

FIGURE 4.17 Synthesis of neutral and basic fatty carboxylates.

C

OH

O

C

O

O

C

O

O

2

M

2

M·X MCO 3

Oleic acid

Base Stoichiometricamount

Divalent metal oleate (soap)

Excess base CO 2

Overbased or basic carboxylate

M = Ca, Mg