Lubricant Additives

216 Lubricant Additives: Chemistry and Applications

Sulfurization by addition of sulfur compounds [elemental sulfur, hydrogen sulfi de, and mercap-
tans] to unsaturated compounds has been known to the chemical industry for years [4–8]. The two
most common classes of additives are called sulfurized olefi ns [9,10] and sulfurized fatty acid esters
[11], because they are produced from reactions of olefi ns and naturally occurring or synthetic fatty
acid esters with sulfur compounds.
In the absence of initiators, the addition to simple olefi ns is by an electrophilic mechanism;
and Markovnikov’s rule is followed. However, this reaction is usually very slow and often cannot
be done or requires very severe conditions unless an acid catalyst is used. In the presence of free
radical initiators, H 2 S and mercaptans add to double and triple bonds by a free radical mechanism,
and the orientation is anti-Markovnikov. By any mechanism, the initial product of addition of H 2 S
to a double bond is a mercaptan, which is capable of adding to a second molecule of olefi n, so that
sulfi des are often produced (reaction 8.1):

C C + H 2 S

H

C C

SH CC

HC C S C CH (8.1)

8.2.1.1 Sulfurized Olefi ns

Sulfurized olefi ns are prepared by treating an olefi n with a sulfur source under proper reaction con-
ditions. The more the sulfur used, the higher is the sulfur content. Suitable olefi ns preferably include
terminal olefi ns and internal olefi ns, mono-olefi ns and polyolefi ns. However, to provide adequate
oil solubility, the olefi n should provide a carbon chain of at least four carbon atoms. Accordingly,
suitable alpha olefi ns are butenes, pentenes, hexenes, and preferably higher alpha olefi ns such as
octenes, nonenes, and decenes. Isobutylene is a very unique olefi n that not only exhibits very high
reactivity toward sulfur reagents (high conversion rate) but also can produce sulfurized products
having very good stability and lubricant compatibility. Therefore, sulfurized isobutylene (SIB) has
been by far the most cost-effective, widely used EP additive in lubricants.

8.2.1.1.1 Chemistry and Manufacture
Initially, sulfurized olefi ns were synthesized through a two-step chloride process, and often, the
products were referred to as “conventional sulfurized olefi ns.” Sulfur monochloride and sulfur
dichloride were used in the fi rst step to produce chlorinated adducts, and then the adducts were
treated with an alkali metal sulfi de in the presence of free sulfur in an alcohol–water solvent,
followed by further treatment with an inorganic base (reactions 8.2 and 8.3) [12]. The fi nal product
is a light yellow–colored fl uid with oligomeric monosulfi des and disulfi des as the main composi-
tions, as typifi ed by reactions 8.2 and 8.3.

CH 3

CH 3

2 CH 2 + S 2 Cl 2 Cl-C(CH^3 )^2 -CH^2 -S-S-CH^2 -C(CH^3 )^2 -Cl

Major adduct

(8.2)

NaSH

Na 2 S

Cl-C(CH 3 ) 2 -CH 2 -S-S-CH 2 -C(CH 3 ) 2 -Cl SIB + 2 NaCl

SIB =

n

S-C(CH 3 ) 2 -CH 2 -S-S-CH 2 -C(CH 3 ) 2 -S

(8.3)