Sulfur Carriers 271
Straight sulfurized fatty acids such as sulfurized oleic acid, are used in semisynthetic metalworking
fl uids. The sulfurized fatty acid will be reacted with alkaline compounds such as amines or potassium
hydroxide to form a soap. This soap is water-dispersible and needs much less emulsifi ers than a sulfu-
rized olefi n. However, hard water stability can become a problem with this type of sulfur carrier.
9.5.2 GREASE
High demands on load-carrying capacity of machine parts require the use of EP and AW additives
to avoid material loss and the destruction of the surfaces of the friction partners. Older technology
still uses typical gear oil sulfur carriers based on short-chain olefi ns such as isobutene. These sulfur
carriers provide a high sulfur content, but their distinct, strong odor prohibits their use in open
lubricating systems.
As it is almost impossible to mask the activity of sulfurized products in greases by using yellow
metal deactivators or sulfur scavengers, inactive sulfurized products are widely used as EP addi-
tives in greases. Especially if the grease is designed for a wide application range, it is imperative
that truly inactive sulfur carriers are used, because yellow metals are widely present as friction part-
ners (e.g., brass cages in bearings). In addition, there are increasing demands on high-temperature
stability for various grease types. This also calls for inactive, oxidation-stable sulfurized products.
Typical sulfur carriers for greases are shown in Table 9.3.
Sulfurized products are also used to substitute heavy-metal-containing compounds, which are
traditionally used as EP additives in greases. Besides their excellent performance, these heavy-metal-
containing compounds show some weak points. Antimony and bismuth compounds are known to have
some weakness regarding copper corrosion, and lead compounds are toxic. In the meantime, many
of these products have been replaced by special sulfurized products either as a direct replacement or
in combination with synergistic compounds such as zinc dialkyldithiophosphates, phosphate esters,
or overbased sulfonates [41]. Sulfurized products are also used in greases for constant velocity joints
(CVJ) [42]. They are very effi cient in combination with molybdenum compounds (e.g., molybdenum
dithiocarbamate [MoDTC], molybdenum dithiophosphate [MoDTP], Mo-organic salts) as a sulfur
source to support the formation of lubricating, active molybdenum disulfi de in the friction zone.
There is an increasing demand for EP greases for environmentally sensitive applications such as
railroad wheel fl ange lubrication, railroad switches, and agricultural equipment such as tractors or
cotton picker spindles. Some sulfurized products are biodegradable and show excellent ecological
data [39]. Therefore, these products are used rather than heavy-metal-containing compounds to
enhance EP and AW properties in such applications.
9.5.3 INDUSTRIAL OILS
An increasing variety of industrial fl uids use sulfurized products as EP and AW additives.
TABLE 9.3
Typical Sulfurized Products for Greases
Type Total Sulfur Active Sulfur Features
Triglyceride 8–12 0.5–3 Mainly inactive, limited EP performance
Triglyceride 13–15 4–7 Mainly active, hard to mask Cu corrosion
long term, good EP
Olefi n 45 10–15 High EP performance, very distinct odor,
only for encapsulated systems
Triglyceride/olefi n 15 4 Mainly inactive, high EP performanceEster 9–11 1–3Mainly inactive, limited EP performance,
excellent low-temperature pumpability