56 Lubricant Additives: Chemistry and Applications
Aryl ZDDPs, due to the stability of the aromatic ring, are not susceptible to nucleophilic
attack. Thus, the initial thermal decomposition reaction described in Reaction 2.9 cannot occur.
Also the formation of olefi n from an acid-catalyzed elimination reaction cannot occur. Aryl ZDDPs
are, therefore, very thermally stable.
A rating of various ZDDPs in terms of thermal stability would, therefore, be aryl > branched
primary alkyl > primary alkyl > secondary > tertiary. The varying amounts of decomposition
products that depend on the heat history and the alkyl or aryl chain involved will directly control the
amount of EP and wear protection the ZDDP will provide in a given circumstance [10].
Hydrolysis of ZDDP begins with cleavage of the carbon–oxygen bond of the thiophosphate
ester, with the hydroxide anion displacing the thiophosphate-anion-leaving group. The stability
of the intermediate alkyl cation determines the ease with which this cleavage takes place. The
secondary alkyl cation is more stable and more easily formed than the primary alkyl cation;
therefore, hydrolysis of secondary ZDDP occurs more easily than hydrolysis of primary ZDDP.
For the case of an aryl ZDDP, the carbon–oxygen bond cannot be broken, and the site of hydrolytic
attack is the phosphorus–oxygen bond with the displacement of phenoxide anion with hydroxide
anion. The order of hydrolytic stability is, therefore, primary > secondary > aryl.
2.5 OXIDATION INHIBITION
Base oils used in lubricants degrade by an autocatalytic reaction known as auto-oxidation. The
initial stages of oxidation are characterized by a slow, metal-catalyzed reaction with oxygen to form
an alkyl-free radical and a hydroperoxy-free radical as seen in the following reaction:
RH O R HOO
M
2
+
→ ** (2.18)This reaction is propagated by the reaction of the alkyl-free radical with oxygen to form an alkylp-
eroxy radical. This radical further reacts with the base oil hydrocarbon to form alkyl hydroperoxide
and another alkyl radical as seen in the following reaction:
R O ROO ROOH R
**RH
2 →→
* (2.19)This initial sequence is followed by chain branching and termination reactions forming high-
molecular-weight oxidation products [11].
The antioxidant functionality of ZDDP is ascribed to its affi nity for peroxy radicals and
hydroperoxides in a complex pattern of interaction.
The initial oxidation step of ZDDP by hydroperoxide is the rapid reaction involving the oxidative
formation of the basic ZDDP salt as seen in the following reaction:
2
2 6S
Zn ′OOHR Zn 4 O R′OH
(RO) 2 PS4++ +
S(RO) 2 PSS
(RO) 2 PS (2.20)In this reaction, 1 mol of alkyl hydroperoxide converts 4 mol of neutral ZDDP to 1 mol of basic ZDDP
and 2 mol of the dialkyldithiophosphoryl radical (which subsequently reacts to produce the disul-
fi de) [12]. The rate of hydroperoxide decomposition slows during an induction period during which
the basic zinc thermally breaks down into the neutral ZDDP and zinc oxide [6]. This is followed
by the neutral ZDDP further reacting with hydroperoxide to produce more dialkyldithiophosphoryl
disulfi de and more basic ZDDP. When the concentration of the basic ZDDP becomes low enough,