254 Lubricant Additives: Chemistry and Applications
had been strongly pushed, leading to the development of synthetic rubber (Buna). Subsequently,
because it was cheap and based on locally available raw materials, factis had found increasing use
as rubber substitute and rubber diluent.
9.2.2 FIRST APPLICATION IN METALWORKING OILS (1920–1930)
In the very early days of modern lubrication, it had become known that sulfur is an important ele-
ment to improve frictional properties and prevent seizure under high loads. Free sulfur and sulfur-
containing heterocyclic molecules are known as part of natural crude oil (thiophenes, thioethers,
etc.). In early refi ning technology, they were not removed effectively, especially from the higher-
viscosity oils that were typically used for gear oils, which had up to 3–4% sulfur. This natural sulfur
contributed to mild EP performance (antiwelding). After the positive effects of sulfur for lubricant
oil formulations were recognized, the next step was to physically dissolve sulfur fl ower into the
lubricant oil at elevated temperatures. This sulfur, however, is very reactive and corrosive against
copper and its alloys. Also, sulfur fl ower has a limited solubility in mineral oil, which limits its
maximum dosage and fi nal EP performance achievable.
Sulfurized esters were fi rst used in metalworking. For heavy-duty operations with a high degree
of boundary lubrication conditions, it was realized that the addition of oil-soluble sulfur compounds
had a tremendous effect on the performance. The fi rst milestone literature that reports this effect
was published in 1918 by the E.F. Houghton Corporation [3] for cutting oils. It is claimed that a mix-
ture of lard oil, mineral oil, and wool fat treated with sulfur fl ower at elevated temperatures results
in a sulfurized product that increases the performance of cutting oils enormously. In particular, the
tool life is extended, and smoking of the coolant is reduced greatly due to friction and temperature
reduction. These observations are still valid today and may be considered the starting point of the
application of sulfur carriers as additives for lubricants.
In comparison with the solid, rubberlike material factis, which has been commercialized for
several decades, Houghton Corporation’s breakthrough was to produce a liquid fatty material that
was soluble in mineral base oil at any ratio. It overcomes the solubility limits of sulfur fl ower and
allows the adjustment of the EP performance level according to treat rate. They achieved this simply
by using nonreactive mineral oil and wool wax as chain-breaking agents and diluent to control the
polymerization reaction of lard oil to keep it liquid. From thereon, the use of sulfurized oils has
become quite common in metalworking.
9.2.3 SULFURIZED COMPOUNDS FOR GEAR OILS AND OTHER LUBRICANTS (1930–1945)
Some years later, the idea of improving load-carrying capacity under high-pressure and high-tem-
perature conditions had been picked up by automotive lubricant researchers and applied to oils for
the newly constructed hypoid gear boxes. With the advent of hypoid gears in automotive applica-
tions in the 1920s and 1930s, wear and seizure under high-load conditions became a major technical
problem that lubricant companies needed to solve. Most of the development work had been done
within these lubricant companies, and the new technology had not been published in detail. How-
ever, the number of patents on sulfur compounds for lubricants developed very rapidly throughout
the 1930s and 1940s.
1936 First patent review on EP lubricants [4]
1940 Patent review 1938–1939 [5]
1941 General publication on lubricating additives including an extensive patent
bibliography [6]
1946 Review article on sulfurization of unsaturated compounds [7]This clearly indicates that the ideas that had been invented and fi rst applied by the metalworking
people also worked for gears. Combinations of sulfurized products with lead soaps and lubricity