The following two experiments demonstrate (a) performance behaviors of aminic antioxidant ver-sus HP that is in agreement with the mechanisms discussed earlier, and (b) how proper selection and combinations of antioxidants can lead to synergy that further enhances performance.
In the fi rst experiment, two turbine oils, each formulated with a base oil selected from an API group I or group IV base stock, a standard additive package of metal deactivator and corrosion inhibi-tor, and 0.8 wt% of antioxidants of interest were tested by using the TOST D 943 lifetime method. The aminic antioxidant was an ADPA containing a mixture of butylated and octylated diphenylamines.
The HP was a C7–C9 branched alkyl ester of 3,5-di-tert-butyl-4- hydroxyhydrocinnamic acid.
As can be seen from Figure 1.19, in either oil, the HP signifi cantly outperformed the ADPA by
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providing longer protection time against oxidation. Mixtures of the ADPA and the HP at 0.4 wt%
each in the oils provided even stronger protection, leading to an extended lifetime of ~5000 h for the group I turbine oil and well over 8000 h for the group IV turbine oil. Thus, under the low-tempera-ture test conditions, the HP was superior to the ADPA. Proper mixing of the two additives produced synergy, in this case a homosynergism that led to the maximum protection.
In the second experiment, a turbine oil formulated with an API group I base stock, a metal deac-tivator, a corrosion inhibitor, and a 0.5 wt% of the same antioxidants as before was tested by using the RPVOT (ASTM D 2272). The results are graphically presented in Figure 1.20. At the higher
500 1000 1504 2008 2512 3016 3520 4024 4528 5032 5536 6040 6544 7048 7552 8064
Time (h)
TAN (mgKOH/g) HP, GI
ADPA, GI HP + ADPA, GI
HP + ADPA, GIV
ADPA, GIV HP, GIV
3.0
2.5
2.0
1.5
1.0
0.5
0.0
FIGURE 1.19 TOST results of turbine oils containing group I or IV base oil and 0.8 wt% of antioxidant.
0 100 200 300 400 500 600 700 800
HP HP + ADPA ADPA
RPVOT OIT (min)
FIGURE 1.20 RPVOT results of turbine oil containing a group I base oil and 0.5 wt% of antioxidant.
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.
0 20 40 60 80 100 120 140
Baseline HP ADPA ADPA + HP
Deposits (mg)
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ADPA and the HP is powerful in the inhibition of different stages of oil oxidation as demon-strated. It is, however, important to note that the generation and the magnitude of an antioxidant synergy are dependent on the formulation, base oil, and test method used. The ADPA/HP synergy appears robust as it was successfully reproduced in two oil formulations and tests that vastly dif-fer from each other in terms of base oil makeup, additive type and complexity, test conditions, and oxidation regimes. In fact, this type of synergy has been used in a wide range of lubricants.
In a more recent development, a methylene-bridged HP was utilized and found to be synergistic with ADPA in low-phosphorus engine oils [240]. Several instances of other types of synergy have been demonstrated and discussed in greater depth elsewhere. These include, but are not limited to, synergy between sulfur-bearing HP and ADPA antioxidants for hydro-treated base stocks [134,241], synergy between aminic antioxidants [242], and synergy between primary antioxidants and oragnophosphites [57].