36 Lubricant Additives: Chemistry and Applications
test temperature (150°C), the OIT of the blend containing ADPA was ~600 min, while the ADPA
was depleted. The HP protected the oil for ~300 min, indicating that the HP is only half as effective
as the ADPA under the same test conditions. A mixture of ADPA and HP with 0.25 wt% of each
additive present provided a protection for over 700 min. Therefore, in contrast to the TOST results,
under high-temperature conditions, the ADPA was superior to the HP. Similar to what was observed
in the TOST, a synergistic mixture of the two additives provided the maximum protection.
The superiority of ADPA over HP and the benefi t of antioxidant synergy for maximum oxi-
dation protection have been further demonstrated in a GF-4 prototype passenger car motor oil
(PCMO). The oil contained an API group II base oil, a low level (0.05 wt%) of phosphorus derived
from ZDDP, and a number of other additives (detergents, dispersant, viscosity index improvers,
pour point depressant, etc.) that are commonly found in engine oil formulations. The ADPA, HP,
and their mixture were tested at 1.0 wt% in the oil on a TEOST MHT apparatus, using the ASTM D
7097 standard procedure. The results are presented in Figure 1.21. The baseline blend, which con-
tained all other additives except the antioxidant, produced a fairly high level (130 mg) of deposits.
With the addition of the HP, the deposit was substantially reduced to ~80 mg, with the ADPA, down
to ~55 mg. By properly mixing the two antioxidants while keeping the total level constantly at
1.0 wt%, the deposit was further reduced to ~40 mg. The TEOST results confi rm the superior per-
formance of ADPA and further demonstrate the benefi t of antioxidant synergy for high-temperature
oxidation conditions.
The antioxidant mechanisms discussed earlier well explain the experimental results and can
serve as a foundation to guide lubricant formulators in the selection of correct antioxidant(s) for
a particular end use. To obtain a successful formulation, other factors such as cost performance,
volatility, color, solubility, odor, physical form, toxicity, and compatibility with other additives
also need be taken into consideration. From a performance standpoint, HPs are excellent primary
antioxidants for their stoichiometric reactions with free radicals under lower-temperature con-
ditions. In contrast, ADPAs are excellent primary antioxidants for high-temperature conditions
owing to their catalytic radical scavenging actions. The homosynergism facilitated between the
FIGURE 1.21 TEOST results of a prototype PCMO containing a group II base stock and a total of 1.0 wt%
of antioxidant.
020406080100120140Baseline HP ADPA ADPA + HPDeposits (mg)