60 Lubricant Additives: Chemistry and Applications
of the minimum phosphorus requirement, the treatment level for ZDDP in organic oils is limited to
∼0.5 to 1.5%, depending on the alkyl chain length used.
The new challenge to motor oil formulators is in passing the required ILSAC tests while keeping
the ZDDP level low. Yamaguchi et al. have shown that the antioxidant effect of ZDDP is signifi -
cantly enhanced in API group II base stocks with as much as 50% increase noted for a basic ZDDP.
An increase in antioxidancy was also noted when using ZDDPs in polyol ester [24]. Several studies
have also shown that ZDDP oxidation by-products are in effective antiwear agents. The use of these
base-stock effects to extend the oxidation life of the ZDDP may be a suitable method for the formu-
lator to reduce the level of ZDDP needed to accommodate the GF-3 limits.
The synergistic effect between organic molybdenum compounds and ZDDP in wear reduction
is currently being studied as a means of lowering phosphorus content in engine oils. In U.S. patent
5,736,491, molybdenum carboxylate is used with ZDDP to give a synergistic reduction in friction
coeffi cient by as much as 30%, thus allowing a reduction in the total phosphorus content and an
improvement in fuel economy [25]. The patent literature has sited other organic molybdenum com-
pounds such as molybdenum dithiocarbamates (MoDTC) and dialkyldithiophosphates (MoDTP) as
being useful, synergistic secondary antiwear agents [26].
ZDDPs are also used in hydraulic fl uids as antiwear agents and antioxidants. The treatment level
for ZDDP in hydraulic fl uids is lower than that used for engine oils, typically running between 0.2
and 0.7% by weight. They are used in combustion with detergents, dispersants, additional organic
antioxidants, viscosity index improvers, pour point depressants, corrosion inhibitors, defoarmers,
and demulsifi ers for a total treatment level of between 0.5 and 1.25% [27]. Primary ZDDPs are
preferred over secondary ZDDPs due to their better thermal and hydrolytic stability. One problem
faced by hydraulic fl uid formulators is the need for a fl uid that will service both high-pressure
rotary vane pumps and axial piston pumps, preferably out of the same sump. High-pressure vane
pumps require a hydraulic fl uid with antiwear properties and oxidative stability commonly achieved
through the use of ZDDPs. High-pressure piston pumps need only rust and oxidation protection and
do not require ZDDPs. ZDDPs can cause catastrophic failure to axial piston systems by adversely
affecting the sliding steel–copper alloy interfaces. The patent literature has several examples of
formulators trying to overcome this problem with the use of additional wear-moderating chemis-
tries such as sulfurized olefi ns, polyol esters or borates of them, fatty acid imidazolines, aliphatic
amines, and polyamines. Another problem faced by hydraulic fl uid formulators is the interaction
of ZDDPs with overbased alkaline earth detergent salts (as well as the interaction of carboxylic
acid and alkenyl succinic anhydride rust preventatives with these detergents) in the presence of
water to give fi lter-clogging by-products. Formulators have tried to overcome this problem of poor
“wet” fi lterability by using nonreactive rust inhibitors (i.e., alkenyl succinimides) and improving the
hydrolytic stability of ZDDP antiwear agent [28].
ZDDPs are used in EP applications such as gear oils, greases, and metalworking fl uids.
Secondary ZDDPs are preferred due to their thermal instability resulting in quick fi lm formation
under high loads. In automotive gea r oils, ZDDPs a re used at 1.5 – 4% in combination with EP agents
(such as sulf u r ized olefi ns), corrosion inhibitors, foam inhibitors, demulsifi ers, and detergents. Total
multifunctional additive package treatment levels for automotive gear lubricant additives are from
5 to 12% by weight. Industrial gear oil formulators have generally gone to ashless systems using
sulfur–phosphorus-based EP antiwear chemistries at total additive package treatment levels of
1.5–3%. In general, the recent focus in gear oil technology improvement has centered on increased
thermal stability and EP properties.
ZDDPs are used in greases in chemical systems that closely resemble gear oil formulations.
Many gear oil lubricant additives are used in EP greases. In general, the ZDDP treatment level
for greases is in the same range as that used for gear oils. ZDDP, usually secondary or a mixture
of secondary and primary, is used in combination with sulfurized olefi ns, corrosion inhibitors,
ashless antioxidants, and additional friction modifi ers. A recent advancement in grease technology
is the use of ZDDP/sulfurized olefi n synergy to replace antimony and lead in high-EP grease