Lubricant Additives

Ashless Antiwear and Extreme-Pressure Additives 227

Phosphorothiolothionic acid Phosphorodithiolic acid Phosphorodithioic acid

(Thiolothionophosphoric) (Dithiolophosphoric) (Dithiophosphoric)

HO SH

HO S

HS SH

HO O

P P H^3 PO^2 S^2

The “thionic” acids contain the group P=S, whereas the “thiolic” acids contain the group P–SH.
The term “thioic” is often used when the molecular form is unknown or when specifi cation is not
desired. One form of these acids is usually more stable than the other, and it may not be possible to
prepare both esters as, for example, the isomers of phosphorothioic acid.

RO OH

RO S

RO SH

RO O

P P

In the case of some esters, the thiolo form is the most stable, but the phenyl ester exists 80% in
thiono, (PhO) 2 P(=S)OH, and 20% in thiolo, (PhO) 2 P(=O)SH forms. The equilibrium of these com-
pounds is liable to be dependent on the nature of the R groups, the solvent used, and even the concen-
tration. Intermolecular hydrogen bonding may be expected to play a part in such equilibrium [33].

8.2.3.2.1 Chemistry and Manufacture
The creation of a compound with a phosphorus–sulfur linkage can often be carried out simply by
heating the appropriate phosphorus compound with sulfur [44]. Likewise, the replacement of oxygen
by sulfur in compounds containing P–O linkages can also be achieved simply by heating them
with P 2 S 5. Inorganic phosphorothioates (thiophosphates) are usually prepared from sulfur-containing
phosphorus compounds. They are produced during the hydrolytic breakdown of phosphorus sulfi des
and are often themselves unstable in water. They hydrolyze to the corresponding oxy compounds
with the evolution of H 2 S. Phosphorus–sulfur compounds are often thermally less stable than their
oxy analogues. A few examples are listed as follows:

P 4 S 10 + 12NaOH ⇒ 2Na 3 PO 2 S 2 + 2Na 3 PS 3 O + 6H 2 O (8.23)

(BuO) 2 P(=O)SH + RI ⇒ (BuO) 2 P(=O)SR + HI (8.24)

(PhO) 3 P + S ⇒ (PhO) 3 P=S (8.25)

(PhO) 3 P + PSCl 3 ⇒ (PhO) 3 P=S + PCl 3 (8.26)

Hydrolysis of phosphorothioate esters results in a progressive loss of sulfur as hydrogen sulfi de
(H 2 S) and its replacement by oxygen.

(RO) 3 P=S + H 2 O ⇒ (RO) 3 P=O + H 2 S (8.27)

8.2.3.2.2 Applications and Performance Characteristics
It has been known for many years that sulfur compounds form a fi lm of iron sulfi de, and phosphorus
compounds form iron phosphate, on the mating metal surfaces. Generally, the fi lms formed from
sulfur sources such as SIB are expected to contain FeS, FeSO 4 , as well as organic fragments from