218 Lubricant Additives: Chemistry and Applications
sulfi de is a low-cost, commodity chemical, which can often offset the additional costs for safe
use. High- pressure sulfurized olefi ns can also be prepared with reagents that generate hydrogen
sulfi de within the reactor during the course of the reaction. Direct handling of hydrogen sulfi de is
thus avoided, but there can be processing penalties, usually in the area of aqueous waste handling.
Performance wise, high-pressure sulfurized olefi ns could replace conventional sulfurized olefi ns in
suitable applications. A decision to manufacture high-pressure sulfurized olefi ns by one process or
another will require a careful assessment of acceptable risks versus economic requirements.
Other olefi ns or mixed olefi ns are also used in the preparation of various sulfurized olefi ns. Among
these, di-tert-nonyl and di-dodecyl trisulfi des and penta-sulfi des are very popular additives. Diisobutyl-
ene (2,4,4-trimethyl-1-pentene) is also used extensively to make higher-viscosity sulfurized products.
In addition, sulfurized hydrocarbons such as sulfurized terpene, sulfurized dicyclopentadiene, or
sulfurized dipentene olefi n, and sulfurized wax are also widely used due to low raw material costs.
8.2.1.1.2 Applications and Performance Characteristics
Sulfurized olefi ns played a key role in establishing superior ashless sulfur/phosphorus (S/P) additive
systems for lubricating automotive and industrial EP gear oils in the late 1960s [19–21]. The early
EP gear oil additives were clearly dominated by chlorine, zinc, and lead, which had diffi culty in
adequately protecting heavy-duty equipment. On the contrary, the S/P gear oil additive technology,
based on ashless and chlorine-free components, possesses very good thermal-oxidative stability
and rust inhibition (CRC L-33 and ASTM D665B); therefore, this is a signifi cant performance
improvement over the metal- and chlorine-based technologies.
Sulfurized olefi ns function mainly through thermal decomposition mechanisms. Sulfur prevents
contact between interacting ferrous metal surfaces through the formation of an intermediate fi lm of
iron sulfi de. By doing this, sulfur usually decreases the wear rate but accelerates the smoothing of
the surfaces. This smoothing actually helps reduce the wear rate. Furthermore, a higher percent
of active sulfur in a molecule increases the chances of reaction with the metal surface and favors
EP (antiseizure) more than antiwear properties. Thus, SIB is mainly a strong antiscuffi ng addi-
tive, with outstanding scuffi ng protection properties (e.g., CRC L-42 performance). Table 8.1 shows
coeffi cients of friction and dimensions with respect to metal surface, oil molecules, and sulfi de
layers. It can be seen that the friction coeffi cients of the sulfi de layers are about half of those for
metal-to-metal surfaces. The sulfi de layers retard the welding of the moving metal surfaces, but do
not prevent wearing. Particles of iron sulfi de are constantly sloughed off from the metal surface.
This wear can be determined by an analysis of the lubricating oils (residual iron content), and
subsequent sludge formation can be controlled by the use of dispersants.
Besides heavy-duty gear oil applications [22], sulfurized olefi ns have a lso found usef ulness in ot her
lubricant areas, such as metal processing oils, greases, marine oils, and tractor transmission oils.
TABLE 8.1
Typical Surface Characteristics
Surface Coeffi cient of Friction
Steel:steel 0.78
FeS:FeS 0.39
Copper:copper 1.21
CuS:CuS 0.74Material Dimension (Å)
Size of oil molecules 50
Size of sulfi de layers 3000
Surfaces with superfi nish 1000