226 Lubricant Additives: Chemistry and Applications
describes the S-terpene ester, whereas U.S. Patent 2,976,308 describes an anti-Markovnikov addition
of phosphorodithioic acid ester to various olefi ns, both aromatic and aliphatic. Amine dithiophosphates
and other novel dithiophosphate esters are reported in the literature [34–38]. Coupling with vinyl
pyrrolidinone, acrolein or alkylene oxides (to make hydroxyl derivatives) are also known [39–41].
8.2.3.1.1 Chemistry and Manufacture
Similar to metallic dithiophosphates, ashless dithiophosphates are also based on phosphorus
pentasulfi de (P 2 S 5 ) chemistry. They can be prepared from the same precursor of ZnDTP, dithio-
phosphoric acid (reaction 8.22) through the reaction of alcohol (or alkylphenol) and P 2 S5.
4ROH + P 2 S 5 ⇒ 2(RO) 2 P(=S)SH + H 2 S (8.22)
The dithiophosphoric acids are further reacted with an organic substrate to generate ashless
derivatives. Typical organic substrates are compounds such as olefi ns, dienes, unsaturated esters
(acrylates, methacrylates, vinyl esters, etc.), unsaturated acids, and ethers. The effi ciency and stability
of the ashless dithiophosphates very much depends on components used in their manufacture and
the reaction conditions.
The most common ashless dithiophosphate used in the marketplace is a dithiophosphate ester
made from ethyl acrylate and o,o-diisopropyl dithiophosphoric acid as described in the following:
[C 3 H 7 – O–] 2 – P(=S)S–CH 2 – CH 2 – C(=O)O–C 2 H 5
Treatment of terpenes, polyisobutylene (PIB), or polypropylene (PP) with phosphorus pentasulfi de
and hydrolysis give thiophosphonic acids [R–P(=S)(OH) 2 where R = PIB, terpenes, or PP).
They can be further reacted with propylene oxide or amines to reduce acidity. However, this type of
additive belongs to the same class of chemicals called ashless dispersants. Hence, they can be dual
functional dispersants with improved antiwear/EP properties.
8.2.3.1.2 Applications and Performance Characteristics
Unlike ZnDTP, ashless dithiophosphates are usually not as versatile, and therefore cannot be con-
sidered as multifunctional additives. Although ashless dithiophosphates have fairly good antiwear
and EP properties, their anticorrosion properties are not as good as ZnDTP. This is closely related
to the stability and decomposition mechanisms of ashless dithiophosphates. Relatively weak cor-
rosion protection also limits their application at high concentrations in engine oils as well as some
industrial oils.
Ashless dithiophosphates can be useful in metalworking fl uids, automotive transmission fl uids
(ATF), gear oils, greases, and non-zinc hydraulic fl uids [42,43].
8.2.3.2 Ashless Phosphorothioates and Thiophosphates
Numerous esters of the phosphorothioic acids are known. In salts and esters of these oxygen/sulfur
(O–S) acids, there may be a preferred location of the multiple bonds, but in general, this is not well
known. Thus in principle, there are two series of possible acids, each of which might give rise to
salts and esters as described in the following:
Phosphorothionic acid Phosphorothiolic acid Phosphorothioic acid
(Thionophosphoric) (Thiolophosphoric) (Thiophosphoric)HO OHHO SHO SHHO O
P P H 3 PO 3 S