Lubricant Additives

Ashless Phosphorus-Containing Lubricating Oil Additives 79

Although single AW/EP additives can be used to meet application and specifi cation requirements,
combinations of additives can produce both synergistic and antagonistic effects. The use of mixtures
of phosphorus- and chlorine- or sulfur-containing compounds, to extend the temperature range over
which a lubricating fi lm is available, has already been mentioned. Another example of synergism was
reported by Beeck et al. [5], who described the effect of combinations of TCP and long-chain fatty
acids. It was suggested that the use of such mixtures in some way improved the packing of the fi lm on
the surface and therefore helped to reduce metal contact. Figure 3.9 [68] in fact shows that combina-
tions of phosphate and fatty acid can result in lower wear rates than either component. Such synergy
is useful in that it reduces additive costs and the possibility that the additives might have an adverse
effect on product stability, etc. An example of additive antagonism is given in Section 3.12.

3.5 INVESTIGATIONS INTO THE MECHANISM AND ACTIVITY

OF PHOSPHORUS-CONTAINING ADDITIVES

Many papers have been written about the way in which TCP and other phosphorus-containing
compounds work as AW/EP additives. As might be expected, researchers have had differences

Physical adsorption

Desorption

Regeneration

of film

Increase in

temperature

Chemical reaction

Mechanical wear

Destruction of film

FIGURE 3.8 Basic processes involved in the mechanism of action of lubricity additives.

− 11

− 10

− 9

− 8

− 7

10 100 1000 10000

Additive (none)

0.4% Organic phosphate

0.4% Fatty acid

0.4% Organic phosphate and

0.4% fatty acid

FIGURE 3.9 Effect of fatty acid and phosphate ester on wear rate. (Fundamentals of Wear, Lubrication,
12(6), 61–72, 1957. Permission from Chevron.)