Ashless Phosphorus-Containing Lubricating Oil Additives 89
and the pure isomers increased with steel and other metals with increasing oxidation state of the
metal/oxide. In comparison with little or no degradation in the absence of metal/metal oxides,
limited degradation took place in the presence of metal, but almost complete breakdown of the
phosphate occurred (at the same temperature—in the range 440–475°C) in the presence of Fe 2 O 3
and Fe 3 O 4. The isomeric forms of TCP also displayed different levels of reactivity with tris-ortho-
cresyl phosphate (TOCP), more active than the meta and para isomers. The authors indicated that
these relativities are consistent with the oxide’s free energy of formation; those oxides with the high-
est free energy of formation show the lowest level of activity, and vice versa. Different types of steel
surface also displayed different levels of reactivity, with 316C stainless steel being the least active.
Surface analysis of the steel specimens used indicated that, depending on whether the metal
surface was oxygen-rich or poor, different mechanisms of degradation predominate. When excess
oxygen was present, the fi lm produced was a polyphosphate with good lubricating properties, whereas
a surface with only a thin oxide coating produced iron phosphate, which has poor lubrication proper-
ties. No phosphide was found in the surface coating, but an iron/amorphous carbon layer, possibly
rich in fused aromatics, arising from the degradation of the aromatic part of the phosphate was found
when using the TBPP, but not when TCP was examined. Since these aromatics have a planar struc-
ture, they may assist with lubrication by allowing the surface to move more easily over one another.
However, it is likely that the end result is a composite of the behavior of the polyphosphate and the
carbonaceous fi lm, if formed. Indeed the author suggests that the polyphosphate may be acting as a
“binder” for the carbon, and it is the latter that is providing the lubrication. The proposed mechanism
for the formation of the polyphosphate fi lm was thought to involve the cleavage of the C–O bond
on one of the pendant groups as the phosphate attaches itself to the surface (presumably through
the –P=O function), eliminating a cresyl radical. This is followed by the elimination of another
cresyl radical as the second C–O bond breaks, and an Fe–O bond is formed. In this way a “lattice of
cross-linked PO 3 is formed with the Fe surface.” Wear of the fi lm is not a problem as it appears to be
self-healing due to diffusion of Fe ions through the polyphosphate layer to the surface where reaction
with phosphate continues. There was no suggestion that hydrolysis of the phosphate is involved.
3.5.2.3 Recent Commercial Developments
Although the majority of phosphates used as AW/EP additives are relatively low-viscosity prod-
ucts, interest has been expressed in materials of high molecular weight for aerospace applications,
where low volatility is important; for example, high-temperature lubricants for aero-derivative gas
turbines and greases for space vehicles. Three products have become commercially available and
have been evaluated: an ISO 100 tertiarybutylphenyl phosphate with low TPP content, resorcinol
tetraphenyl bisphosphate (Figure 3.12), and isopropylidene di-p-phenylenetetraphenyl bisphosphate
(Figure 3.12). The hydrolytic stability of the resorcinol diphenyl phosphate is relatively poor, but
this would not be of major concern for aerospace applications, for example, in greases. However,
this material has been claimed as an AW additive for fuels and lubricants [101], whereas the TBPP
has been incorporated into an aerospace grease formulation [102].
As part of an assessment of the high-molecular-weight additives for use in high-temperature
aviation gas turbine oils, they were compared under coking, four-ball wear, and oxidation test
conditions. The results are given in Table 3.9. Although the AW performance of the butylphenyl
phosphate is not as good as that of TCP, the reduced impact on deposit formation and magnesium
corrosion performance has made it the most promising candidate.
Although much of the recent focus of activity has been on aryl phosphates, there have also been
developments with alkyl phosphates. TBP, for example, is now used as an EP additive for EP steam
and gas turbine oils used when the turbine is driving a reduction gear (Ertelt, R. Private Communica-
tion, September 2001). About 1.5% of the additive is used to increase the FZG gear test performance
(DIN 51354) from a load stage failure of about 6–8 to 10–11. Again, the neutral nature of the mol-
ecule is of advantage in minimizing interaction with other components of the formulation.