Ashless Antiwear and Extreme-Pressure Additives 225
8.2.2.4 Acid Phosphates
Acid phosphates are also potent additives, useful in similar areas of antiwear and EP to the dialkyl
phosphites. Orthophosphoric (monophosphoric) acid (H 3 PO 4 ), the simplest oxyacid of phosphorus,
can be made by reacting phosphorus pentoxide with water. It is widely used in fertilizer manufac-
ture. Orthophosphoric acid has only one strongly ionizing hydrogen atom and dissociates according
to the following reaction
H 3 PO 4 ⇔ H+ + H 2 PO 4 – ⇔ H+ + HPO 4 2 – ⇔ H+ + PO 4 3 – (8.18)
Since the fi rst dissociation constant, K 1 (7.1 × 10 −^3 ), is much larger than the second (K 2 = 6.3 ×
10 −^8 ), very little of the H 2 PO 4 produced in the fi rst equilibrium goes on to dissociate according to
the second equilibrium. Even less dissociates according to the third equilibrium since the third
constant K 3 is very small (K 3 = 4.4 × 10 −^13 ). The acid gives rise to three series of salts containing
these ions, for example, NaH 2 PO 4 , Na 2 HPO 4 , and Na 3 PO 4.
8.2.2.4.1 Chemistry and Manufacture
Alkyl (aryl) acid phosphates are made from alcohol (phenol) and phosphorus pentoxide. Generally,
a mixture of monoalkyl (aryl) and dialkyl (aryl) phosphates is produced.
3ROH + P 2 O 5 ⇒(RO) 2 P(=O)OH + (RO)P(=O)(OH) 2 (8.19)
Pure monoalkyl or dialkyl (aryl) phosphates can be synthesized through different reaction routes
as follows:
ROH + POCl 3 ⇒ ROP(=O)Cl 2 ⇒ (Hydrolysis) ⇒ (RO)P(=O)(OH) 2 (8.20)
(RO) 2 P(=O)H + Cl 2 ⇒ (RO) 2 P(=O)Cl ⇒ (Hydrolysis) ⇒ (RO) 2 P(=O)(OH) (8.21)
8.2.2.4.2 Properties, Performance Characteristics, and Applications
Phosphoric acids tend to hydrolyze further when exposed to humidity. The extent of hydrolysis
depends on the moisture content of the ambient atmosphere and the duration of exposure. Wherever
possible, phosphoric acids should be handled in a dry nitrogen atmosphere to prevent hydrolysis.
Therefore, for applications where incidental moisture contact is inevitable, acid phosphates are not
recommended.
Acid phosphates are used as rust inhibitors and antiwear additives. However, they are not as
widely used as their amine-neutralized derivatives, for example, amine phosphates.
8.2.3 SULFUR–PHOSPHORUS ADDITIVES
Sulfur–phosphorus additives are used to provide protection against moderate to high pressure,
metal-to-metal contacts in boundary lubrication, and EHL. Metallic sulfur–phosphorus additives,
such as zinc dithiophosphates (ZnDTPs), are the most important antiwear/EP components used in
engine oils. Ashless sulfur–phosphorus additives are used less extensively, and the most commonly
available S/P additives in the marketplace are based on chemistries of dithiophosphates, thiophos-
phates, and phosphorothioates. Other important applications of S/P compounds are in matches,
insecticides, fl otation agents, and vulcanization accelerators.
8.2.3.1 Ashless Dithiophosphates
Numerous patents were issued on the use of phosphorodithioic acid esters in lubricating oils in the early
days. U.S. Patent 2,528,732 describes alkyl esters of phosphorodithioic acid. U.S. Patent 2,665,295