Sulfur Carriers 257
impact on their solvency for additives and thus triggered intensive adjustment work on the additive
producer’s side including sulfur carrier manufacturers.
The reduction in sulfur content however has contributed substantially to the market growth of
sulfurized additives. The reduction of naturally occurring sulfur in base oils through the desulfur-
ization units now needs to be balanced for specifi c applications through the addition of synthetic,
oil-soluble sulfur components to keep EP/AW properties as well as AO performance. This trend
started in the 1970s but is getting stronger today with the increase in the availability of XHVI base
oils/groups II and III as well as completely synthetic basestocks (polyalphaolefi ns [PAOs]).
In the late 1970s and early 1980s, a new class of sulfur carrier has been introduced into the
industrial lubrication market: dialkylpolysulfi des. They are based on C8, C9, or C12 olefi ns and
contain up to 40% sulfur in a very reactive form. They can be looked at as liquid, oil-soluble sul-
fur fl ower. The starting point for the development of these additives was the requirement of many
lubricant-blending companies for an alternative to sulfurization of base oils with sulfur fl ower. It is
a very time-consuming step and may generate toxic gas (hydrogen sulfi de, H 2 S) and sulfur dropout
during application. Sulfur fl ower can be dissolved in mineral oil just above its melting point of
115°C in concentrations of typically 0.4–0.6% and is used if appropriate in heavy-duty metalwork-
ing applications or running-in gear oils (see Section 9.5.1.3). The solution that has been offered from
additive manufacturers has been the new class of organic polysulfi des of light color and rather low
odor. Diisobutenepentasulfi de and tert nonyl- and dodecylpentasulfi de have been introduced as easy
to blend liquids and substitutes for sulfur fl ower. Today, these active type pentasulfi des have become
the most important and wide-spread class of sulfur carriers on the industrial oil side.
In 1985, it was found that sulfur carriers, preferably polysulfi de types, show a strong synergistic
EP/AW behavior when combined with high total base number (TBN) ASTM-D4739 sodium and
calcium sulfonates [31]. This has become known as the PEP technology (passive EP) in neat oil
metalworking. In the beginning, it was hoped that this combination would be a general and simple
solution to upcoming chlorinated paraffi n replacement issue that started in Western Europe and
Scandinavia in the mid to late 1980s. But as it turns out today, the PEP technology can only partially
match the universal properties of chlorinated paraffi n formulations especially under low-speed/
high-pressure operation conditions (for more details, see Section 9.5.4.2).
From the late 1980s to early 1990s, a totally new aspect of sulfurized esters and fats has gained
substantial ground—the toxicology and ecotoxicology of these chemicals. Workers’ safety, environ-
mental compatibility, biodegradability, and similar requirements need to be addressed in industrial
more than in automotive lubrication, because workers in machine shops often cannot avoid constant
direct contact with the lubricant. The fact that the use of natural, renewable raw materials and opti-
mized production procedures may give low-toxic and biodegradable sulfur carriers refreshed the
interest of development chemists in these special, environmentally safe but classic additives.
The twenty-fi rst century’s central question of additive and lubricant R&D departments is how
to further optimize energy effi ciency and reduce friction. Again, sulfur carriers play a role. Span-
ning from engine oils to wind turbine gear boxes, formulators take advantage of their multipurpose
character.
9.3 CHEMISTRY
9.3.1 CHEMICAL STRUCTURE OF SULFUR CARRIERS
For the majority of sulfur carriers, discrete structures are very hard to sketch for several reasons:
The raw materials are very often mixtures of isomers: in the case of olefi ns, for example,
diisobutene, there are fi ve main isomers; tetrapropylene shows some 35 components in the
gas chromatogram (GC). Natural fatty oils have a distribution with mono-, double- and
triple unsaturated acids with unsaponifi cable matter.