Sulfur Carriers 255
esters were the fi rst high EP performance technology in gear oils. Many years later, Musgrave [8]
stated in an article on hypoid gear oils that it was just by chance that the synergistic effect of sulfur
with lead soaps had been discovered in the early 1930s.
An interesting historical dimension was added to the EP gear oil development during World
War II. Most of the EP gear oil development occurred in the United States due to the great importance
of automotive industry in the 1920s and 1930s. German gear oil technology was not as advanced as
that of United States. Eyewitnesses report of frequent gear box failures of German tanks and heavy
equipment during the attack against Russia. The reason was that in autumn Russian roads were
turning into mud and the heavy vehicles were operated most of the time at maximum power. The
only mildly additized gear oils were not just good enough to prevent scoring and welding.
9.2.4 SCIENTIFIC RESEARCH ON CHEMISTRY AND APPLICATION (1930–1949)
Between 1930 and 1950, the important basics of sulfur carrier technology had been developed. Patents
from this period include most of today’s raw materials and reaction pathways. Raw materials used
were animal oils [3], vegetable oils/organic acids [9], pine oils [10,11], whale oil (sperm oil), acrylates,
olefi ns [12], alcohols [13], synthetic esters [14], and salycilates [15]. Even thiocarbonates [16] and xan-
thogenates [17] were synthesized and used as organic, oil-soluble sulfur-containing EP additives.
Reaction pathways mentioned in patents mentioned earlier include
Sulfur fl ower reaction with and without H 2 S, aminic, and other suitable catalysts
Sulfur chlorination with S 2 Cl 2 [18]
Organic halides with alkalipolysulfi des
Mercaptan route [19]Important product properties that are still part of today’s development work were also mentioned
in that period:
Stability of sulfurized products, for example, diisobutene [20]
Active and inactive sulfur compounds
Corrosive and noncorrosive compounds
Light- and dark-colored derivatives
High- and low-odor productsParallel to new chemistry, the development of test machines for tribological research progressed
quickly along with publications on mechanistic studies of additives. In 1931, at an API meeting,
Mougay and Almen [21] presented the fi rst chemical interpretation for the load-carrying capacity of
sulfur-containing EP additives and their synergy with lead soaps. They attributed the performance
to the formation of a separating fi lm between the frictional partners—a theory generally accepted
today in tribological science. In 1939, this fi lm-forming theory of sulfur compounds was proven
using the four-ball tester [22]. In 1938, Schallbock et al. published [23] standard-setting results on
investigations in the fi eld of metalworking. Empirical correlations were found among cutting speed,
temperature, and tool life that are still valid. In 1946, synergistic effects of chlorinated additives
with sulfur additives were explained based on a chemical reaction theory [24] under the aspect
of newest generation hypoid gear formulations. Phosphorus additives (tricresylphophate [25], zinc
dialkyldithiophosphates [ZnDTP]), primary antioxidants (AOs) (phenyl-α-naphthylamine, butylated
hydroxytoluene [BHT]), and detergent/dispersants [26] (salycilates) also joined the world of lubri-
cant additives during this period and have been used since in combination with sulfur carriers.
Most of the development work at that time had been done in a deductive way in a trial-and-error
approach. Theoretical explanations, tribological, and chemical modeling always trailed behind (look-
ing back from today’s point of view, it is quite astonishing that not so much has changed in 70 years).
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