256 Lubricant Additives: Chemistry and Applications
9.2.5 SUMMARY OF THE PAST 50 YEARS
With the fundamentals of sulfur carriers being explored so early in lubricant additive history, the
literature from 1950 until today concentrates around improvements in production procedures, com-
binations and synergies with other additives, improvement in product qualities, and search for spe-
cial applications. The use of sulfur carriers has been extended from metalworking and automotive
engine to industrial oils and greases. Ashless hydraulic oils may contain sulfurized EP additives for
special applications. Now this product group is used throughout the lubricant oil industry.
Tr i b ol og y has been defi ned as a particular fi eld of scientifi c research, and several basic models
of additive response have been worked out. A review article [27] in 1970 summarizes the state of
the art at that time, including many literature references.
Until the 1950s, sulfur carriers were mostly made by the lubricant manufacturers. However,
increasing environmental awareness, growing market, and the need for more specialized products
brought about change. As the sulfurization process involves deep chemical knowledge and produc-
tion know-how and includes extremely high safety risks, specialty chemical companies became
active in this area. It is expected that the few lubricant companies that still produce a small quantity
of black sulfurized fats in-house may discontinue sooner or later.
Since the 1950s, the sulfur carrier market was split into two segments: automotive and indus-
trial. In automotive gear oil applications, sulfurized isobutene (SIB) soon became the standard EP
product because it is high in sulfur content but low in corrosivity. The typical, rather strong smell of
SIB is no real problem in this fi eld of application because gear boxes are totally closed systems. In
any open lube system, this EP technology is not acceptable. The big oil companies had their petro-
chemical subsidiaries (Mobil Chemical, BP Chemical, Shell Chemical, Exxon Chemical, Chevron
Chemical, etc.) and added additive manufacturing as the market grew including SIB production
units. So SIB production has always been the target for those companies with focus on automotive
additives and lubricants. Over the decades, the SIBs have gone through changes in chlorine level
due to environmental requirements [28]. Starting at 2–3% in the early days, today high qualities no
longer contain chlorine because of a chlorine-free, high-pressure H 2 S production processes. Also,
the amount of active sulfur in SIBs has been reduced to improve the long-term abrasive wear of
gear oil formulations in bearings and to meet today’s fi l l - f o r - l i f e requirements. But in principle, in
automotive applications, the same sulfur chemistry is in use today as it was some 60 years ago.
The other big fi eld of application of sulfurized EP additives is industrial lubrication. Tradition-
ally this area is less regulated and restricted by Original Equipment Manufacturer (OEM) approv-
als, general specifi cations, and standards—it is particularly true for the metalworking market. Here
much more differentiated, problem-solving additives have been and still are in use. This environ-
ment has generated a greater variety of smaller volume sulfur carriers that address strongly differ-
entiated technical requirements of metalworking processes and grease applications. Subsequently
smaller, more specialized chemical companies entered the lube additive business. The fi rst products
were dark in color, but as early as 1962, Rhein Chemie commercialized its fi rst light-colored, low-
odor sulfurized synthetic ester based on chlorine-free production technology (see Section 9.3.2).
A big milestone in the history of sulfur carriers has been the international banning of sperm oil
(whale oil) in 1971. Up to this year, sperm oil and lard oil (pig fat) have been the dominant fatty raw
materials for sulfur carriers. The sperm-oil-based products in particular showed excellent solubility
and lubricity in addition to their sulfur-related EP properties. The extensive research activities of
this period resulted in various patents [29,30]. The new raw materials turned out to be vegetable oils
in combination with either synthetic esters or olefi ns.
Another aspect that strongly infl uenced the sulfur carrier market has been the change of refi n-
ery technology for base oils. The driving force behind was the necessity to improve environmental
as well as health and safety aspects of the major refi nery products: fuels. These requirements led
to drastic reduction of aromatic components and sulfur content in fuels and subsequently of base
oils. From a lubricant point of view, the reduction of aromaticity of the base oils had strong negative