Antioxidants 37
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].
1.13 ANTIOXIDANT PERFORMANCE WITH BASE STOCK SELECTION
Driven by escalating environmental and performance requirements, the lubricant industry is rapidly
changing for the better with the advances of additive and base oil technologies. One notable change
from a formulation point of view is that the conventional solvent-extracted base oils (group I) are
gradually being replaced by high-quality, high-performance groups II and III base stocks made
from hydrotreated (hydrocracked), hydrotreating, and hydrocatalytic dewaxing processes. These
processes provide oils with low sulfur, high degree of saturation, and viscosity index (Table 1.3).
Lubricants formulated with these base stocks generally have improved performance characteristics
such as superior oxidative stability, lower volatility, improved low-temperature properties, longer
drain intervals, and improved fuel economy. Because of these benefi ts, the API group III base oils
are becoming a serious challenge to synthetic PAOs for top-tier oil formulations.
Many efforts have been made to understand the relationship between the base oil composition and
the response to added antioxidants. Such knowledge is extremely important for lubricant formulators
when comes to the selection of an appropriate antioxidant system for a given oil. Figure 1.22 shows
the RPVOT test results of four base oils with and without the presence of an antioxidant. Each oil
FIGURE 1.22 RPVOT results of HP and ADPA in API groups I–IV base oils.
020040060080010001200140016001800API group I API group II API group III API group IVRPVOT OIT (min)No AO
HP
ADPA