Lubricant Additives

230 Lubricant Additives: Chemistry and Applications

Oxidative coupling

DMTD + 2RSH + 2H 2 O 2 ⇒ RS–S–(TD)–S–SR + 4H 2 O (8.30)

Mercapto alkylation and Mannich alkylation

DMTD + 2CH 2 =O + 2RSH ⇒ RS–CH 2 – S–(TD)–S–CH 2 – SR + 2H 2 O (8.31)

MBT + CH 2 =O + RNH 2 ⇒ (BT)–S–CH 2 – NHR + H 2 O (8.32)

Amine salt formation

DMTD + 2RNH 2 ⇒ RNH 3 – S–(TD)–S–NH 3 R (8.33)

8.2.4.2.2 Applications and Performance Characteristics
MBT is a light yellow powder with limited solubility in hydrocarbons. It is more soluble in aromatic
solvent (~1.5% in toluene), polar solvents, and highly aromatic oils. MBT is used as a copper
corrosion inhibitor in fuels as well as a corrosion inhibitor/deactivator in numerous industrial lubri-
cants such as heavy-duty cutting and metalworking fl uids, hydraulic oils, and lubricating greases.
DMTD is also a light yellow powder with very limited solubility in hydrocarbons. It is considered a
versatile chemical intermediate suitable for making various oil-soluble derivatives.
Both MBT and DMTD derivatives are widely used as copper passivators and nonferrous metal
corrosion inhibitors. Some proprietary load-carrying additives are substituted MBT and DMTD
compounds that are used in various applications either as a component or as a part of additive
packages with a specifi c purpose [65,66]. In the absence of any phosphorus moiety in MBT and
DMTD, their oil-soluble derivatives are suitable for replacing zinc dithiophosphates in some lubri-
cant applications. For example, a commercial, high-density, powder-like MBT and DMTD deriva-
tives is used as a dual functional antioxidant/EP agent in greases.

8.2.4.3 Other Sulfur–Nitrogen Additives

In addition to DMTD, MBT, and dithiocarbamate additives, there are other sulfur–nitrogen- containing
additives available in the marketplace or reported in the literature. Among these, phenothiazine
derivatives (Structure H, PTZ), substituted thiourea additives (Structure I, TU), thionoimidazolidine
derivatives (Structure J, TIDZ), thiadiazolidine and oxadiazole (ODZ) derivatives (Structures K
and L), thiuram monosulfi des, thiuram disulfi des, and benzoxazoles a re of pa r ticula r interest because
they are all sulfur- and nitrogen-rich molecules [67–73]. Thiuram disulfi des, chemically similar to
dithiocarbamates, can be used in the rubber industry as vulcanizers. 2-Alkyldithio-benzoxazoles
also offer good frictional properties in addition to strong antiwear/EP properties [74].

S

H

N

R R′

STRUCTURE H, PTZ

R 3

X N N

R 2

S

R 1 R 4

STRUCTURE I, TU