232 Lubricant Additives: Chemistry and Applications
EP, FZG, Timken, and CRC L-37). Since amine phosphates are very polar species, they interact
strongly with other additive components, making their performance very dependent on the formu-
lation. Hence, extra attention is needed when amine phosphates are used.
8.2.5.2 Amine Thiophosphates and Dithiophosphates
Amine thiophosphates and amine dithiophosphates can be found in engine oils and industrial oils
where zinc dithiophosphates and other nitrogen-containing additives are used, either as decomposi-
tion products or as in situ-produced products. They are critical to the lubricant performance because
of their high activity toward metal surfaces.
8.2.5.2.1 Chemistry and Manufacture
Amine thiophosphates are produced by reacting thiophosphoric acid with alkyl or aryl amines [75].
Likewise, amine dithiophosphates are synthesized from dithiophosphoric acid and amines.
(RO) 2 P(=S)SH + H 2 NR′ ⇒ (RO) 2 P(=S)S∙H 3 NR′ (8.36)
(RO) 2 P(=O)SH + H 2 NR′ ⇒ (RO) 2 P(=O)S∙H 3 NR′ + (RO) 2 P(=S)O∙H 3 NR′ (8.37)
8.2.5.2.2 Applications and Performance Characteristics
Amine thiophosphates and dithiophosphates are also multifunctional additives providing good rust
inhibition and antiwear properties. Owing to their high activity and low stability, amine thiophos-
phates and dithiophosphates are not as extensively used as either amine phosphates or metallic
dithiophosphates. A detailed study of their antiwear mechanisms suggested that a tribofragmenta-
tion process is involved [76,77]. Relatively poor corrosion control is one area of concern that needs
attention. With proper formulation adjustments, it is quite feasible to overcome certain intrinsic
weaknesses and apply both chemistries to various lubricant products.
8.2.5.3 Other Phosphorus–Nitrogen Additives
There are many other phosphorus–nitrogen-containing ashless antiwear additives reported in the
literature. Some are proprietary technologies, and their commercial status is unknown. Organophos-
phorus derivatives of benzotriazole (BZT) are a group of additives based on triazole and dialkyl
or dialkylphenyl phosphorochloridate chemistry [78]. Arylamines and dialkyl phosphites can be
coupled through a Mannich condensation reaction to form unique phosphonates that are used as
multifunctional antioxidant and antiwear additives [79]. Bisphosphoramides are also reported [80].
8.2.6 NITROGEN ADDITIVES
Nitrogen-containing additives are used to provide rust inhibition and cleanliness features in various
lubricant applications. For example, nitrogen-containing ashless dispersants are a key component
for engine oils, and alkoxylated amine compounds are used in lubricating greases to provide corro-
sion inhibition [81]. Furthermore, arylamines are widely used as antioxidants due to their ability to
terminate radical chain propagation and decompose peroxides. Very few nitrogen additives alone
are considered effective antiwear/EP additives, and their performance is either very specifi c to
industrial applications or fairly dependent on product formulations. However, when used in combi-
nation with other sulfur, phosphorus, or boron additives, nitrogen-containing additives can be very
effective supplements to enhance antiwear/EP performance.
8.2.6.1 Chemistry, Manufacture, and Performance
Several novel chemistries are available in the literature for nitrogen-only antiwear additives. Among
these, dicyano compounds were tested and they exhibited very good Four-Ball Wear activities [82].
Polyimide-amine salts of styrene–maleic anhydride copolymers are also reported as antiwear
additives; however, high additive concentrations (5–10%) are needed [83]. Alkoxylated amines
(Structure M) and mixtures of fatty acid, fatty acid amide, imide or ester derived from substituted