Lubricant Additives

54 Lubricant Additives: Chemistry and Applications

and alkyl mercaptan. A white precipitate will also form, which has been determined to be a low-
sulfur- containing zinc pyrophosphate. The oil phase will contain varying amounts of S,S,S-tri -
alkyltetrathiophosphate, O,S,S-trialkyltrithiophosphate, and O,O,S-trialkyldithiophosphate depend-
ing on the alkyl chain and the extent of degradation. The decomposition products of ZDDPs made
from secondary alkyl alcohols, straight-chain primary alkyl alcohols, and branched primary alkyl
alcohols appear similar in content but differ in proportions. This implies a similar mechanism for
both primary and secondary ZDDP decomposition.
O-alkyl thiphosphate esters are powerful alkylating agents. The P–O–R group is susceptible
to nucleophilic attack, thus producing an alkylated nucleophile and thiophosphate anion. The
incoming nucleophile initiates the reaction by an attack on the alpha carbon. This shows a kinetic
dependence on alkyl structure

ROP Nu−

S

+ P O−

S

+ Nu−R

(2.9)

Steric hindrance to the approach of the nucleophile will play a large rate-controlling factor here.
The only nucleophile initially present is the dithiophosphate itself. The decomposition is initiated
by one dithiophosphate anion attacking another, possibly on the same zinc atom:

2 S P S−

OR

OR

S P S−

O−

OR

S P SR

OR

OR

+ (2.10)

The resulting di-anion then attacks the triester, producing O,S-dialkyldithiophosphate anion

SSRP

O−

OR

(2.11)

resulting in the migration of an alkyl group from oxygen to sulfur. This anion then, in a route analo-
gous to the dialkyldithiophosphate anion, reacts with itself in a nucleophilic attack to effect another
alkyl transfer from oxygen to sulfur producing O,S,S-trialkyldithiophosphate

RS P SR

OR

O

(2.12)

The net effect of the above reactions is a double alkyl migration from oxygen to sulfur

−S P S

OR

OR

RS P SR

O

O−

(2.13)

The major gases associated with ZDDP decomposition are dialkylsulfi de (RSR), alkyl mercaptan
(RSH), and olefi n. The relative amounts of each of these gases depend on whether the alkyl group in
the ZDDP is primary, branched primary, or secondary [9]. In the presence of mercaptide anion (RS–)