Lubricant Additives

Ashless Phosphorus-Containing Lubricating Oil Additives 103

the effect of increasing the chain length of the phosphite while reducing the length of the amine.
Most of the paper concerns the behavior of a wide range of phosphonate esters (see Section 3.16).

3.5.6 PHOSPHONATE AND PHOSPHINATE ESTERS

A large group of phosphonate esters was prepared by Barber in 1975 [152] and evaluated using the four-
ball machine. Although short-chain esters were more effective in preventing scuffi ng, the most effective
products were those containing chlorine. However, even at high levels of chlorine, the performance was
still inferior to the amine phosphite reaction products reported earlier. In comparison with TCP, incipient
seizure loads were generally higher, but the weld loads were broadly similar. Unfortunately, there were
no direct comparative data under wear test conditions. A limited number of phosphinate esters were
evaluated and found (also by four-ball tests) to give similar performance to the phosphonate esters.
The activity of a range of phosphonates was studied by Sanin et al. [153], who concluded that
their effectiveness depended on their structure and the friction regime, but esters containing no
chlorine had “no effect at either low or high load.” A further study, by the same authors, of the
mechanism of activity of phosphonates again suggested the reaction of decomposition products
with iron and the formation of a protective layer. Under severe conditions, this layer is removed,
resulting in a sudden increase in friction followed by seizure or welding. Studies of the reaction of
a dibutyltrichlorophosphonate (Cl 3 CPO(OBu) 2 ) indicated that a reaction took place at 405–408K
to give chlorobutane and an iron-containing polymer. At 413K, this polymer decomposed to give
FeCl 3 , which gave additional protection.
Phosphonate (and pyrophosphonate) esters, as their metal or amine salts, have appeared in the
patent literature over many years as AW/EP additives. Amine salts of dinonylphosphonate are, for
example, claimed in aircraft gas turbine lubricants [154], and dimethyltetradecyl phosphonate has

P

RO

RO

O

H

P

RO

O

O

H

P

RO

RO

O

H

P

RO

HO

O

H

Fe

Fe

(a) Hydrolysis

(b) Reaction

Reaction with iron surface

P

HO

O

O

H

Fe

P

H

O

O

Fe

O

Adsorption Hydrolysis in solution

Antiwear region

Antiscuff region

Hydrolysis

FIGURE 3.21 Mechanism of load-carrying action of dialkyl phosphites. (From Forbes, E.S., Battersby, J.,
ASLE Trans., 17(4), 263–270, 1974. With permission.)