Sulfur Carriers 261
A great part of the in situ generated H 2 S does not leave the reaction mixture but is directly adsorbed
by the double bonds, thus producing saturated alkylmercaptans (Equation 9.2a). They react further,
oxidatively, with sulfur fl ower to generate alkyl- and dialkylsulfi des with the release of more H 2 S
(Equation 9.2b).
RHCCHR HS RHCCH(SH)R 2
Olefin
2
Alkylmercaptan interme→
ddiate(9.2a)2R H C 2 CH(SH) R Sx R H C CHR S (S ) S CH
Alkylmercaptan → 2x1 RRCH R HS 22
Dialkylpolysulfide(9.2b)The fi nal product consists of a full range of organic sulfur derivatives. Some of them are still
unsaturated, with isomerized double bonds and conjugated, chromophoric (color-deepening) sulfur
compounds such as thioketones and thiophenes, which cause the product to be dark black in color
and rather smelly. From an application point of view, these products exhibit EP/AW performance,
but because of their remaining double bonds, they have the following negative characteristics:
They will continue to polymerize during use and even under normal storage conditions.
They are easily oxidizable and form residues on fresh metal surfaces/discoloration.
They will cause a TAN increase within a short time in circulation systems and cause short
oil drain intervals.
They will even generate H 2 S/mercaptan during high-temperature usage in lubricant systems
(see Sections 9.4.2.1.7 and 9.4.2.1.2).So today’s main use of these black sulfurized products are total loss lubricants in which long-term
stability and bad smell are not an issue. It is the cheapest way of making sulfurized additives.
9.3.2.3 High-Pressure H 2 S Reaction
High-quality sulfur carriers, which have improved properties compared to the black materials, are
produced today using high-pressure/high-temperature equipment. The handling of toxic H 2 S under
high-pressure conditions requires sophisticated handling techniques and safety measures. Further-
more, H 2 S is an expensive gas. All these aspects contribute to signifi cantly higher production costs
compared to the simple black sulfurization.
In this process, the olefi ns, sulfur, and H 2 S are added to a high-pressure-resistant reactor and
heated to 120–170°C. The reaction is also catalyzed by amines, metal oxides, acids, etc. For low-
boiling olefi ns such as isobutene, the pressure may go up as high as 50–60 bar. For higher-boiling
olefi ns such as diisobutene, typical pressure is in the range of 2–15 bar. The presence of H 2 S as
reducing agent and strong nucleophile makes a total difference to the black sulfurization process.
The oxidative attack of sulfur on the vinylic carbon–hydrogen (C–H) bond is effectively suppressed.
The side reaction of the black sulfurization process becomes the main reaction here: the addition of
H 2 S to the double bonds to form mercaptans (Equation 9.2a) which then quickly react with sulfur
in a redox reaction to form dialkyldi-, tri-, tetra-, and polysulfi des and release 1 mol equivalent of
H 2 S (Equation 9.2b).
This procedure gives much more controlled reaction conditions and fi nally fewer side prod-
ucts. The most important effect of this reaction pathway is the fact that the double bonds are gone
after the reaction and no conjugated systems with chromophore (color-deepening) properties can
be formed. The sulfur carriers produced by this way are much more oxidatively stable, and they
are of light color. This one-step process is an advantage in terms of total production time and
turnover.