Antioxidants 39
However, the naturally occurring sulfur species that can function as antioxidants have also been
largely removed. Previous discussion has clearly demonstrated that the superior oxidative stability
of these oils can only be realized when an appropriate synthetic antioxidant is used. Therefore, as
lubricant formulators increase the use of hydrotreated and synthetic base stocks, the requirements
for antioxidants in lubricants are expected to rise.
The environmental and toxicity issues of petroleum-based oils as well as their rising cost related
to a global shortage have led to renewed interest in the use of vegetable oils, such as soybean
oil, canola oil, sunfl ower oil, and coconut oil as lubricants and industrial fl uids. The industry has
already seen vegetable oils being utilized to make environmentally friendly automotive engine
oils, two-cycle engine oils, hydraulic fl uids, total loss lubricants, and marine lubricants. Vegetable
oils generally possess some excellent lubrication properties, for example, good inherent lubricity,
low volatility, high viscosity index, excellent solvency for lubricant additives, and easy miscibility
with other fl uids. However, vegetable oils are known for their poor oxidative stability as compared
to mineral oils. Research has found that typical soybean oil formed polymers at a rate an order of
magnitude faster than mineral oils [246]. To overcome this drawback, it is expected that vegetable
oil–based lubricants will need more antioxidants to meet the performance requirements set for
mineral oil–based lubricants. Owing to the unique hydrocarbon composition of vegetable oils, antioxi-
dant response in these new fl uids will be different from mineral oils. Recent work [218] has indicated
that ADPAs that have proven track of high-temperature performance in mineral base oils are not as
effective in stabilizing soybean oil at elevated temperatures (e.g., 170°C). In addition, environmentally
friendly lubricants need to use additives that satisfy biodegradation and bioaccumulation standards.
Antioxidants suitable for mineral oils may become problematic for use in vegetable oil–based lubri-
cants. As such, the development of new classes of biodegradable antioxidants may be needed.
The increasing use of biofuels such as biodiesel methyl esters derived from oil seeds, animal
fats, and reclaimed cooking oils, represents a new challenge to the stabilization of engine oils that
operate on such fuels. Recent work [247] has discovered that biodiesels of various vegetable sources
(canola, soybean, palm oils, coconut, etc.) promote the oxidation of in-service engine oils even at a
low dilution level, primarily shortening inhibited period, and leading to rapid oil viscosity increase.
Further aggravating the situation is the high and narrow boiling points of biodiesels, which make
them more persistent than mineral diesels after entering the crankcase. For an engine oil to perform
satisfactorily with these new fuels, antioxidant level has to be maintained at an effective level to
counteract the strong degradation impact from the biofuels.
Overall, future lubricants will favor antioxidants having high-performance, cost-effective, ashless,
multifunctional, and environmentally-friendly attributes. The ultimate driving force is environmental in
nature while taking into account the emerging base oil technologies and performance specifi cations.
1.15 COMMERCIAL ANTIOXIDANTS
Product Company Chemistry
Ethanox® 310 Albemarle Tetrakismethylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane
Ethanox 323 Albemarle Nonylphenol disulfur oligomer
Ethanox 376 Albemarle 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C 18 alkyl ester
Ethanox 4701 Albemarle 2,6-Di-t-butyl phenol
Ethanox 4702 Albemarle 4,4′-Methylene bis(2,6-di-t-butyl phenol)
Ethanox 4703 Albemarle 2,6-Di-t-butyl-alpha-dimethylamino-p-cresol
Ethanox 4716 Albemarle 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C 7 –C 9 alkyl ester
Ethanox 4733 Albemarle Mixture of mono-, di-, and tri-t-butyl phenols
Ethanox 4735 Albemarle Mixture of t-butyl phenols
Ethanox 4755 Albemarle Boron containing derivatives of Ethanox 4702
Ethanox 4872J Albemarle Multiring t-butyl phenol, 53% active
(Continued )