Lubricant Additives

228 Lubricant Additives: Chemistry and Applications

the additive decomposition. With phosphorus sources, such as dialkyl phosphites, fi lms containing
FePO 4 , FePO 3 , as well as organic fragments are expected. When both sulfur and phosphorus are
present, both elements contribute to the nature of the fi lm, and which one predominates depends on
the S/P ratio, the decomposition mechanisms, and the operating conditions, for example, high speed
and shock or high torque/low speed.
Ashless phosphorothioates are widely used as replacements for metallic dithiophosphates in
many lubricant applications where metal is less desirable [43,44]. Phosphorothioates are often
present (generated in situ) in lubricant formulations when both sulfur and phosphorus additives are
used. Aryl phosphorothioates provide good thermal stability and good antiwear/EP properties as
evidenced by their strong FZG performance.

8.2.4 SULFUR–NITROGEN ADDITIVES

Sulfur and nitrogen-containing additives are used to provide protection against moderate to high
pressure, metal-to-metal contacts in boundary lubrication, and EHL. Both open chain and hetero-
cyclic compounds have attracted a considerable amount of research effort to explore their potential
as antiwear and EP additives. Among open chain additives, dithiocarbamates are the most widely
used. Other additives, such as organic sulfonic acid ammonium salts [45], and alkyl amine salts
of thiocyanic acid [46] are reported in the literature, but are of relatively low commercial value.
Nitrogen and sulfur-containing heterocyclic compounds, such as 2,5-dimercapto-1,3,4-thiadiazole
(DMTD, Structure E), 2-mercapto-1,3-benzothiazole (MBT, Structure F), and their derivatives,
have been used for many years as antioxidants, corrosion inhibitors, and metal passivators; gener-
ally at relatively low concentrations.

NN

HS S SH N

C

S

SH

STRUCTURE E STRUCTURE F

8.2.4.1 Dithiocarbamates

The dithiocarbamates, the half amides of dithiocarbonic acid, were discovered as a class of chemi-
cal compounds early in the history of organosulfur chemistry [47,48]. The strong metal-binding
properties of the dithiocarbamates were recognized early, by virtue of the insolubility of metal salts
and the capacity of molecules to form chelate complexes. Other than applications in lubricant areas,
dithiocarbamates have been used in the fi eld of rubber chemistry as vulcanization accelerators and
antiozonants.

8.2.4.1.1 Chemistry and Manufacture
Organic dithiocarbamates can be made by a one-step reaction of dialkylamine, carbon disulfi de, and
an organic substrate. The organic substrate is preferably an olefi n, diene, epoxide, or any other unsat-
urated compounds as exemplifi ed in the literature [49,50]. Organic dithiocarbamates can also be
made through a two-step reaction involving ammonium or metal dithiocarbamate salts and organic
halides [51]. In the case of their ammonium salts, N-substituted dithiocarbamic acids, RNHC(=S)SH
or R 2 NC(=S)SH, are formed by reaction of carbon disulfi de with a primary or secondary amine in
alcoholic or aqueous solution before they are further reacted with ammonia. To conserve the more
valuable amine, it is a common practice to use an alkali metal hydroxide to form the salt.

RNH 2 + CS 2 + NaOH ⇒ RNHC(=S)S–Na + H 2 O (8.28)

The dithiocarbamic acid can be precipitated from an aqueous solution of dithiocarbamate by
adding strong mineral acid. The acids are quite unstable but can be held below 5°C for a short time.