Ashless Antiwear and Extreme-Pressure Additives 217
The manufacture of conventional sulfurized olefi ns involves sulfur monochloride, and the fi nal
product contains some residual chlorine. The process also generates aqueous waste with halogen-
and sulfur-containing by-products that must be disposed of. Chlorine in lubricants and other
materials is increasingly becoming an environmental concern because chlorinated dioxins can be
formed when chlorine-containing materials are incinerated. Chlorinated waxes have been elimi-
nated from many lubricants for this reason. Residual chlorine content is also becoming a major con-
cern in many areas of the world. Germany currently has a 50 ppm maximum limit on the chlorine
content of automotive gear oils. This requirement is a problem for automotive gear oil suppliers as
well as additive suppliers if their technology is based on conventional sulfurized olefi ns, since the
residual chlorine content is a consequence of the chemistry required to manufacture conventional
sulfurized olefi ns. By fi ne-tuning the manufacturing process, the chlorine content of conventional
sulfurized olefi ns may be reduced from a typical 1500 ppm to <500 ppm. However, manufacturing
changes to reduce the residual chlorine content will probably slow the production process, require
additional capital investments, and possibly generate more aqueous waste.
In the late 1970s, the high-pressure sulfurized isobutylene (HPSIB) process was developed to
replace the conventional, low-pressure chlorine process. HPSIBs are usually mixtures of di-tert-butyl
trisulfi des, tetrasulfi des, and higher-order polysulfi des [13–16]. Some HPSIBs contain oligomeric
polysulfi des of poorly defi ned composition or other materials such as 4-methyl-1, 2-dithiole-3-thione
(Structure A, [8,17], and reaction 8.4). The higher-order polysulfi des generally favor EP activity at the
expense of oxidative stability and copper corrosivity compared to the monosulfi des and disulfi des of
conventional sulfurized olefi ns. In the absence of other reagents, the straight reaction of elemental
sulfur and isobutylene results in a dark-colored liquid that contains a signifi cant amount of dithiolethi-
ones (thiocarbonates). 4-Methyl-1, 2-dithiole-3-thione is a pseudoaromatic heterocyclic compound.
Owing to its rigid ring structure, dithiolethiones can be easily precipitated as yellowish solids that
cause severe staining problems. Therefore, dithiolethione is often not a desirable side product in SIB.
CH 3CH 3CH 2 Sulfur
High pressureDithiolethione
Polysulfides
Oligomeric polysulfides(8.4)SSS
CH 3STRUCTURE A
In the presence of various catalysts (or basic materials), such as aqueous ammonia, alkali metal
sulfi des, or metal dithiocarbamates, amounts of dithiolethiones (Structure A) and oligomeric
polysulfi des can be reduced, and low-molecular-weight polysulfi des (X = 2 to 6 in Structure B) are
the predominant products [18].
(CH 3 ) 3 C S C(CH 3 ) 3
XSTRUCTURE B
The use of hydrogen sulfi de in the high-pressure sulfurized olefi n process can ease the reac-
tion complexity and also yield high-quality, low-molecular-weight polysulfi des. The compositions
of products prepared from this process usually have good clarity, low odor, light color, and high
EP activity. Hydrogen sulfi de is a very foul smelling and toxic gas. It leads to collapse, coma, and
death as result of respiratory failure within a few seconds after one or two inhalations. Liquefi ed
hydrogen sulfi de has a high vapor pressure that requires additional, adequate protective equipment.
There are considerable risks associated with its routine use on an industrial scale, but hydrogen