Lubricant Additives

Ashless Phosphorus-Containing Lubricating Oil Additives 83

The authors commented that the TCP used for the investigation was the best grade available, but
even this material contained up to 25% polar impurities. It was thought typical of the TCP used in
the wear studies to date and reported in the literature.
Wear tests on TCP, acid phosphates, and phosphites in a super-refi ned mineral oil and a syn-
thetic ester (di-3-methylbutyl adipate) indicated that relatively small amounts (0.01%) of additive
can produce a signifi cant wear reduction in mineral oils and that the acidic materials were more
active. However, in the polar base stock, where there is competition for the surface, the amount of
TCP required to provide a similar reduction in wear is substantially greater. The effectiveness of
the alkyl acid phosphates is not signifi cantly reduced in the synthetic ester, suggesting that their
polarity (and hence adsorption) is greater than that of the neutral phosphate, the synthetic ester,
and its impurities (Table 3.7). The authors concluded that the activity of TCP was due to the acidic
impurities and that the neutral ester acted as a reservoir for the formation of these impurities during
the life of the lubricant.
Until about 1969, the theory regarding the production of a phosphate fi lm on the steel surface
seemed to be widely accepted. Reports then appeared suggested that the situation was more com-
plicated. One paper [82] examined and compared the corrosivity toward steel, the load- carrying
capacity, and the AW performance of several phosphorus compounds. Using the hot-wire technique
at 500°C [83] followed by an x-ray analysis of the surface fi lms that were produced, the reactivity (or
corrosivity) was studied. Perhaps, not surprisingly, the neutral phosphate and phosphite evaluated
showed relatively little reactivity with the steel, whereas the acid phosphate and phosphite produced
substantially more corrosion. The anomaly was the behavior of a neutral alkyl trithiophosphite,
which showed a very high reactivity but low load-carrying ability, suggesting a different mode of
breakdown. Analysis of the fi lms formed confi rmed the major presence of basic iron phosphate (or
principally iron sulfi de in the case of the thiophosphite), but small amounts of iron phosphide were

TABLE 3.6

Correlation between the Antiscuffi ng Performance and Ease of Hydrolysis

(Acid Formation) of Organic Phosphates

Additive (0.08% wt Added P)

Relative Ease

of Hydrolysisa

Time to Scuffi ng (min)b at a

Spring Load

305 lb 340 lb

Benzyldiphenyl phosphate 100 > 30 9

Allyldiphenyl phosphate 100 > 30 Not tested

Ethyldiphenyl phosphate 80 28 Not tested

Octyldiphenyl phosphate 50 15 6

Triphenyl phosphate 50 15 5

Tritolyl phosphate 30 8 Not tested

2-Ethylhexyldiphenyl phosphate 5 2–3 Not tested

None — 2–3 Not tested

Note: Camshaft, Ford Consul (cams phosphated); Tappet, Ford Consul (non-phosphated); Camshaft

speed, 1500 rpm (equivalent engine speed 3000 rpm); Base oil, SAE 10W/30 oil without EP

additive.

a Because of the wide range of hydrolytic stability of these compounds, it was not possible to compare

the stabilities of all these compounds in the same acid medium. Consequently, an arbitrary scale was

drawn up with benzyldiphenyl phosphate assigned a value of 100.

b Mean of several tests.

Source: Barcroft, F.T., Daniel, S.G., ASME J. Basic Eng., 64-Lub-22, 1964. With permission.