Sulfur Carriers 253
and used to summarize a group of additives that provide extreme-pressure (EP) and antiwear (AW)
properties and are used in gear oils, metalworking fl uids, greases, and engine oils. The vast majority
of them are sulfurized fats, esters, and olefi ns. To distinguish them from other sulfur-containing
products and avoid misunderstandings, a suitable defi nition of sulfur carriers is the following:
Sulfur carriers are a class of organic compounds that contain sulfur in its oxidation state 0 or –1,
where the sulfur atom is bound either to a hydrocarbon or to another sulfur atom
That does not contain other hetero atoms except oxygen
Produced by adding sulfur to all kinds of unsaturated, double-bond-containing compounds
such as olefi ns, natural esters, and acrylates or by substitution reaction with reactive organic
halides and alikeLubricant additives fi tting this defi nition are the main focus of this chapter.
Owing to the overwhelming versatility of sulfur chemistry, other sulfur-containing product
groups cannot be discussed in depth but are mentioned in the context where appropriate.
Although this group of additives has been used in the lubricant industry for more than eight
decades, sulfur carriers are not at all an endangered species. In fact, we are still seeing increas-
ing usage today. This is partly due to continuous ongoing R&D work done in this area, which
brings about innovation and product improvement. Also, many chemical aspects and applications
are waiting to be discovered. Furthermore, sulfur carriers are essential additives for the solution
of upcoming lubricant market requirements such as chlorinated paraffi n substitution; heavy metal
replacement; and health, safety, and environmental issues. Therefore, we expect to see substantial
future growth of light-colored, low-odor, and odor-free sulfur carriers.
9.2 HISTORY
As we look back on more than 100 years of sulfurized compounds, the authors had to rely on lit-
erature sources for the time before 1950s. During the literature studies, it turned out that one of
the most fruitful sources for the time period before 1950 is the review articles of Helen Sellei [1,2]
published in 1949. Much of what follows is based on their content, but we have tried to reinterpret
the information with today’s background knowledge.
9.2.1 FIRST SYNTHESIS AND APPLICATION (1890–1918)
Sulfurized fatty oils have been commercially produced for more than 100 years. Long before they
were used as additives in lubricants, they had become important additives for the rubber industry.
The addition of 4–8% sulfur to an unsaturated natural oil such as rapeseed oil at high temperatures
(120–180°C) gives a fl exible, gummy polymer called factis. Sulfur undergoes an addition reaction
to the double bonds of the natural oil and builds up a three-dimensional structure of sulfur bridges
between the triglyceride molecules. This is comparable to the vulcanization process of latex, which
results in rubber.
In the late nineteenth century, rubber was an expensive natural raw material, and with the rapid
industrialization in general and the growing automobile industry in particular, rubber tires were
needed in increasing amounts. It soon turned out that factis also provided special, positive proper-
ties to rubber goods during the vulcanization process. This was the starting point of smaller chemi-
cal factories producing additives for the rubber industry. In 1889, Carl Benz submitted the patent
for the world’s fi rst automobile in Mannheim, Germany. In the same year and city, Rhein Chemie
Rheinau GmbH was founded and started to produce sulfurized natural oils. Germany had seen a
special national aspect to the industrial history of sulfurized fats and rubber before 1914. Because
Germany had very few colonies, all rubber had to be imported. During the national tensions in
the fi rst decade of the twentieth century and subsequent trade boycotts, the search for alternatives