58 Lubricant Additives: Chemistry and Applications
The zinc metal atom provides an easy route for heterolysis of the radical intermediate; thus, the
disulfi de, by itself, has little antioxidant functionality [15]. ZDDP acts as an oxidation inhibitor
not only by trapping the alkyl radicals, thus slowing the chain reaction mechanism, but also by
destroying alkyl hyperoxides and inhibiting the formulation of alkyl radicals. Empirical determi-
nation of the relative antioxidant capability of the three main classes of ZDDP shows secondary
ZDDP > primary > aryl ZDDP. The relative performance of each ZDDP type may correlate with
the stabilization of the dialkyl(aryl)dithiophosphoryl radical and its subsequent reactivity with
alkyl hydroperoxide to produce the catalyzing acid.
Commercial ZDPs are a mixture of both neutral and basic salts. It has recently been
determined that neutral and basic ZDDPs give essentially equivalent performance with respect to
antioxidant behavior. This can be explained by the equilibrium shown in Reaction 2.8. At elevated
temperatures, as would occur in an oxidation test, the basic ZDDP is converted into the neutral
ZDDP. As the temperature is lowered, the equilibrium shifts back toward the formation of the
basic ZDDP, indicating that the concentration of basic ZDDP as a function of temperature. The
solvent used and the presence of other additives also play a role in this equilibrium. Thus, the exact
composition of neutral versus basic salts at any time in an actual formulation is a complex function
of many variables.
2.6 ANTIWEAR AND EXTREME-PRESSURE FILM FORMATION
ZDDPs operate mainly as antiwear agents but exhibit mild EP characteristics. As an antiwear
agent, ZDDP operates under mixed lubrication conditions with a thin oil fi lm separating the metal
parts. Surface asperities, however, intermittently penetrate the liquid fi lm, giving rise to metal-on-
metal contact. The ZDDP reacts with these asperities to reduce the contact. Likewise, when the load
is high enough to collapse the oil fi lm, the ZDDP reacts with the entire metal surface to prevent
welding and to reduce wear. A great deal of study has been done to determine the nature of this
protective fi lm and the mechanism of deposition, where the thermal degradation products of the
ZDDP are the active antiwear agents.
The antiwear fi lm thickness and composition are directly related to temperature and the
extent of surface rubbing. Initially, ZDDP is reversibly absorbed onto the metal surface at low
temperatures. As the temperature increases, catalytic decomposition of ZDDP to dialkyldithio-
phosphoryl disulfi de occurs, with the disulfi de absorbed onto the metal surface. From here, the
thermal degradation products (as described in Section 2.3) are formed with increasing tempera-
ture and pressure until a fi lm is formed on the surface [16]. The thickness and composition of this
fi lm have been studied using many different analytical techniques, but no analysis gives a concise
description of the fi lm size and composition for the various kinds of metal-to-metal contact found
in industrial and automotive lubrication regimes. In general, the antiwear/EP ZDDP fi lm can be
said to be composed of various layers of ZDDP degradation products. Some of these degradation
products are reacted with the metal making up the lubricated surface. The composition of the lay-
ers is temperature-dependent.
The fi rst process that takes place is the reaction of sulfur (from the ZDDP thermal degrada-
tion products) with the exposed metal leading to the formation of a thin iron sulfi de layer [17].
Next, phosphate reacts to produce an amorphous layer of short-chain ortho- and metaphosphates
with minor sulfur incorporation. The phosphate chains become longer toward the surface, with
the minimum chain length approaching 20 phosphate units. Some studies have indicated that this
region is best described as a phosphate “glass” region in which zinc and iron cations act to stabilize
the glass structure. At the outermost region of the antiwear fi lm, the phosphate chains contain
more and more organic ligands, eventually giving way to a region composed of organic ZDDP
decomposition products and undegraded ZDDP itself. The thickness of the fi lm has been analyzed
to be as small as 20 nm using ultra thin fi lm interferometry and as large as 1 μm using electrical
capacitance [18–21].