82 Lubricant Additives: Chemistry and Applications
Beeck et al. [5]), the data on acid phosphate, acid phosphite, and phosphoric acid did display such
minima.
The work using radioactive P^32 also allowed a study of the competition between TCP and dif-
ferent types of additives for the metal surface. This was determined by measuring the residual
surface radioactivity after wear tests. Table 3.5 shows the effect of various types of additives on the
adsorption of P^32 from TCP. The lower the number of counts, the greater the interaction between
the additive and TCP.
Radiochemical analysis was also the technique used to investigate the deposition of phosphorus
on steel surfaces in engine tests [80]. In this study, the effect of different types of aryl phosphates
(TPP, TCP, and TXP) on case-hardened tappets was examined. The results suggested that the effi -
cacy of these additives is correlated directly with their hydrolytic stability, that is, their ability to
produce acid phosphates as degradation products. This was confi rmed by tests on a series of other
phosphates (largely alkyldiaryl phosphates), which showed good correlation between antiscuffi ng
performance and hydrolytic stability (Table 3.6). Examination of the tappet surface revealed the
presence of aryl acid phosphates on the surface and the absence of phosphides. Adsorption studies
of the neutral aryl and acid phosphates on steel surfaces indicated that, although the fi lm of neutral
ester could be more easily removed, the adsorption of the acid phosphate was irreversible, suggest-
ing salt formation. These studies led the authors to conclude that the mechanism involved initial
adsorption of the phosphate on the metal surface followed by hydrolytic decomposition to give an
acid phosphate. This reacted with the surface to give iron organophosphates, which then decom-
posed further to give iron phosphates.
The importance of impurities in determining the level of activity of TCP was confi rmed in yet
another paper [81]. The composition of impurities in commercial grades of TCP was determined
using thin-layer chromatography and analysis by neutron activation. Acidic impurities, probably the
monocresyl and dicresyl acid phosphate (and also small amounts [2 × 10 −^4 %] of phosphoric acid),
were found at 0.1–0.2%, that is, at levels that had previously been shown to produce a signifi cant
reduction in wear when added to mineral oil. Other impurities ranged from 0.2 to 0.8%. This latter
category was assumed to contain chlorophosphates based on the amount of chloride ion produced.
TABLE 3.5
Effects of Various Additives on the Adsorption of P^32Additive Concentration (wt%) Activity (Counts/min)
0.5% TCP alone 280
+2% Barium sulfonate A 0
+2% Barium sulfonate B 80
+0.1% Rust inhibitor 16
+0.5% Diisopropyl acid phosphite 25
+0.1% Dilauryl acid phosphate 24
+5.5% Acryloid dispersant 82
+7.9% Polymeric thickener 78
+0.7% Sulfur–chlorine EP additive 120
+0.5% Thiophosphate 150
+0.5% 2,2′-Methylene-bis(2-methyl,
4-tertiarybutyl phenol)250+0.5% Sulfurized terpene 290
Source: Klaus, E.E., Bieber, H.E., ASLE Preprint 64-LC-2, 1964.
With permission.