Lubricant Additives

6 Lubricant Additives: Chemistry and Applications

with metals to form oil-soluble metal phenates. These metal phenates play the dual role of detergent
and antioxidant.
Multifunctional antioxidant and extreme pressure (EP) additives with heterocyclic structures
were prepared by sulfurizing norbornene, 5-vinylnorbornene dicyclopentadiene, or methyl cyclo-
pentadiene dimer [16]. Heterocyclic compounds such as n-alkyl 2-thiazoline disulfi de in combi-
nation with zinc dialkyldithiophosphate (ZDDP) exhibited excellent antioxidant performance in
laboratory engine tests [17]. Heterocyclic sulfur- and oxygen-containing compositions derived from
mercaptobenzthiazole and beta-thiodialkanol have been found to be excellent antioxidants in auto-
matic transmission fl uids [18]. Novel antioxidant and antiwear additives based on dihydrobenzothio-
phenes have been prepared through condensation of low-cost arylthiols and carbonyl compounds in
a one-step high-yield process [19].

1.3 SULFUR–NITROGEN COMPOUNDS

The dithiocarbamates were fi rst introduced in the early 1940s as fungicides and pesticides [20]. Their
potential use as antioxidants for lubricants was not realized until the mid-1960s [21], and since then,
there have been continuous interests in this type of chemistry for lubricant applications [22]. Today,
dithiocarbamates represent a main class of sulfur–nitrogen-bearing compounds being used as antioxi-
dants, antiwear, and anticorrosion additives for lubricants.
Depending on the type of adduct to the dithiocarbamate core, ashless and metal-containing
dithiocarbamate derivatives can be formed. Typical examples of ashless materials are methylene
bis(dialkyldithiocarbamate) and dithiocarbamate esters with general structures being illustrated in
Figure 1.2. Both are synergistic with alkylated diphenylamine (ADPA) and organomolybdenum
compounds in high-temperature deposit control [23]. In particular, methylene bis(dialkyldithiocar
bamate) in combination with primary antioxidants such as arylamines or HPs and triazole deriva-
tives is known to provide synergistic action in stabilizing mineral oils and synthetic lubricating oils
[24–26]. This material has been used to improve antioxidation characteristics of internal combus-
tion engine oils containing low levels (<0.1 wt%) of phosphorus [27]. In another effort to reduce
phosphorus content in aviation gas turbine lubricants, methylene-bridged bis(dialkyl) or bis(alkylar
yldithiocarbamate) was used as high-temperature antioxidant and antiwear agent to replace tricresyl
phosphates that are of a concern to produce neurotoxic ortho-cresol isomers in trimethylolpropane
triester base oil under high-temperature service conditions [28].
It has been known that metal dithiocarbamates such as zinc, copper, lead, antimony, bismuth,
and molybdenum dithiocarbamates (MoDTCs) possess desirable lubricating characteristics includ-
ing antiwear and antioxidant properties. The associated metal ions affect the antioxidancy of the
additives. Within the group, MoDTCs are of greater interest particularly for engine crankcase lubri-
cants. Certain molybdenum additives posses good oxidation resistance and acceptable corrosion
characteristics, when prepared by reacting water, an acidic molybdenum compound, a basic nitro-
gen complex, and a sulfur source [29,30]. Oil-soluble trinuclear MoDTCs prepared by reacting

FIGURE 1.2 Ashless dithiocarbamates for lubricants.

N

C

S

S

R

R

S

C

S

N

R

R

N

C

S

S

R

R

C

C

C

R

C

O

O

C

O

O

Bis(disubstituted dithiocarbamate) Dithiocarbamate ester

R R

R