Lubricant Additives

Zinc Dithiophosphates 57

a fi nal rapid neutral salt-induced decomposition of the hydroperoxide will occur in which the
dialkyldithiophosphoryl radical will not react with itself to form the disulfi de but will react with
hydroperoxide to form the dialkyldithiophosphoric acid as seen in the following reaction [13]:

S

(RO) 2 PS•+R′OOH

S

(RO) 2 PSH +R′OO•

(2.21)

The dialkyldithiophosphoric acid then rapidly reacts with alkyl hydroperoxide, producing oxidation
products that are inactive in oxidation chain reactions. The simplest reaction scheme for the reduction
of the hydroperoxide is seen in the following reaction:

2

S

2 (RO)PSH+ R′OOH ++R′OH H 2 O

S

(RO) 2 PS

















(2.22)

Oxidation products include the disulfi de mentioned earlier, the analogous mono- and trisulfi des,
and compounds of the form (RO)n(RS)3–n P=S and (RO)n(RS)3–n P=O [3]. These products show
little activity as either oxidation inhibitors or antiwear agents.
The literature also reveals an ionic process that will produce more dialkyl-dithiophosphoric
acid as seen in the following reaction:

2

S

Zn R′OO• R′OO−

(RO)PS

+ ++

S

(RO) 2 PS Zn+

S

(RO) 2 PS• (2.23)

followed by Reaction 2.21 [14].
At low concentrations of ZDDP, hydrolysis of the ZDDP to the zinc basic double salt and
dialkyldithiophosphoric acid becomes viable. At temperatures >125°C, the dialkyldithiophos-
phoryl disulfi de decomposes into the dialkyldithiophosphoryl radicals, which further react with
hydroperoxide to produce more dialkyldithiophosphoric acid [6]. Thus, many pathways are avail-
able to form the active dialkyldithiophosphoric acid.
The neutral ZDDP also reacts with alkyl peroxy radicals. This is an electron-transfer mechanism
that involves the stabilization of a peroxy intermediate. An attack by a second peroxy radical leads
to the intramolecular dimerization of the resulting dithiophosphate radical forming the inactive
dialkyldithiophosphoryl disulfi de as seen in the following reaction:

RO

RO

R

RO OR

RO OR

OO

PP

RO 2 •

S•

SS

S

Zn

OR

OR

Zn

PP

S

SS

S

RO 2 •

R

RO OR

RO OR

OO

P• P

S•

SS

S

Zn

• 
  

RO OR

OR

SS

SS

PP

RO

++2 RO 2 − Zn2+

(2.24)