Ashless Antiwear and Extreme-Pressure Additives 219
8.2.1.2 Sulfurized Esters and Sulfurized Oils
The oldest widely used sulfur-based additive that is still found in commercial lubricants is sulfu-
rized lard oil (SLO), a sulfurized animal triglyceride. In 1939, H. G. Smith made one of the most
important discoveries in the history of lubricant additives. He found that sulfurized sperm whale oil
(SSWO) was more soluble in paraffi nic base oils, even at low temperatures, and had a much higher
thermal stability than SLO. Thus, over 60 years ago, he recognized that the improved stability of
sulfurized sperm oil resulted from its monoester structure, compared with the triester structure of
SLO. With long-chain monoesters, the sulfur has little potential to form bridges between the mol-
ecules, as it does when triglycerides are being sulfurized. SSWO is an excellent boundary lubricant
and is highly resistant to gumming, resin formation, or viscosity increase, when subjected to high
temperature and high pressure [23]. Unfortunately, from a lubricant cost-performance viewpoint,
this additive is no longer available due to restrictions on the use of sperm whale oil. Sulfurized
jojoba oil, an expensive alternative, is available; it is also a mixture of long-chain alcohol fatty acid
compounds. All these sulfurized fats or esters are usually manufactured to contain 10–15% sulfur
and are often good antiwear and mild EP agents.
8.2.1.2.1 Chemistry and Manufacture
Sperm oil is a waxy mixture of esters of fatty alcohols and fatty acids with a small amount of
triglycerides. After the separation of solid waxes by fi ltration or centrifuge, a liquid wax remains,
consisting mainly of an ester of oleic alcohol and oleic acid. Such a structure could not be better for
sulfurizing purposes.
Similar to sulfurized olefi ns, sulfurized esters can be made by either direct sulfurization
with elemental sulfur or sulfurization with hydrogen sulfi de under superatmospheric pressures.
Nowadays, they are mainly made from vegetable oils having one or more double bonds. Sulfurized
esters are made from unsaturated fatty acids such as oleic acid, and esterifi ed with an alcohol such
as methanol. Frequently the sulfurization of fats is made in the presence of an olefi n, preferably of
long chain, and the resulting commercial product is a mixture of the two types. Equation 8.5 shows
a typical example of sulfurization of methyl oleate with elemental sulfur. When sulfurized with
hydrogen sulfi de, products usually possess light color and lower odor.
SSSS
S
HeatCH 3 (CH 2 ) 7 -CH CH-(CH 2 ) 7 COOCH 3CH 3 (CH 2 ) 7 -CH--CH-(CH 2 ) 7 COOCH 3CH 3 (CH 2 ) 7 -CH--CH-(CH 2 ) 7 COOCH 3CH 3 (CH 2 ) 7 -CH-CH 2 -(CH 2 ) 7 COOCH 3CH 3 (CH 2 ) 7 -CH-CH 2 -(CH 2 ) 7 COOCH 3xx x+ other products x = 1, 2CH 3 (CH 2 ) 7 -CH=CH-(CH 2 ) 7 COOCH 3(8.5)8.2.1.2.2 Properties, Performance Characteristics, and Applications
The load-carrying property of sulfurized oils is directly linked to the amount of active sulfur in
the additive. Percent of active sulfur (which is believed to provide EP activity) and total sulfur can
be determined by proper analytical methods, and the difference is the percent of inactive sulfur.