Lubricant Additives

84 Lubricant Additives: Chemistry and Applications

also found in all the x-ray analyses of the degradation products. Evaluation of the load-carrying
capacity of the additives was found to vary directly with corrosivity except for the alkyl trithio-
phosphite. The authors surmised from this that the load-carrying capacity of phosphorus-contain-
ing additives was not only due to the reactivity of the fi lms but also due to the properties of the fi lm
that was formed. The relationship between wear and reactivity also varied directly for several com-
pounds, but in the case of the neutral phosphite and the alkyl trithiophosphite, there was no correla-
tion. This was attributed to the different composition of the fi lm in these cases. In fact, the authors
proposed that the main reaction product of the phosphite could be iron phosphide. They suggested
that the load-carrying capacity of the fi lms formed by EP additives fell in the following order:

phosphide > phosphate > sulfi de > chloride

whereas the order of AW properties was

sulfi de > phosphate > phosphide

The fi rst of these sequences is, of course, different to the order in EP activity predicted from the
stability of the fi lms formed on the metal surface and from the general perception that phosphorus
is less active than either chlorine or sulfur. Similarly, for AW performance, phosphorus is normally
regarded as more active than sulfur.
A paper by Goldblatt and Appeldoorn [84] cast doubt on the theory that the activity of TCP was
due to the generation of acidic impurities. In this study, the activity of TCP in different atmospheres
and in different hydrocarbon base stocks was examined. The resulting data showed that TCP

TABLE 3.7

Effect of Concentration on the AW Properties of Phosphorus-Containing

Additives in a Synthetic Ester

Additive Concentration (wt%)

Average Wear Scar Diameter (mm)

1 kg 10 kg 40 kg

None — 0.39 0.71 0.91

TCP 1.0 0.38 0.71 0.97

3.0 0.40 0.64 0.97

0.5 0.23 0.25 0.78

Hydrolyzed TCP 0.1 0.57 0.74 —

1.0 0.17 0.25 0.46

Dilauryl acid phosphate 0.01 0.21 0.41 0.84

0.05 0.19 0.28 0.43

1.0 0.17 0.28 0.42

Diisopropyl acid phosphite 0.02 — 0.72 —

0.05 0.16 0.25 —

0.15 — 0.33 —

Phosphoric acid 0.001 0.41 0.69 0.90

0.01 0.16 0.37 0.50

1.0 0.38 0.60 0.78

Note: ASTM D 4172. Four-ball wear test conditions: test time, 1 h; test temperature, 167°C; test speed,

620 rpm.

Source: Bieber, H.E., Klaus, E.E., Tewkesbury, E.J., ASLE Preprint 67-LC-9, 1967. With permission.