Antioxidants 35
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
(^500100015042008251230163520402445285032553660406544704875528064)
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.