18 Lubricant Additives: Chemistry and Applications
abrasive boric acid. Following attempts have been made to address the issue with varying degrees
of success:
- Incorporation of HP moiety to sterically inhibit the boron–oxygen bonds from hydrolytic
attack. Commonly used HPs are 2,6-dialkyl phenols [169], 2,2′-thiobis(alkylphenols) and
thiobis(alkylnaphthols) [170]. - Incorporation of amines that have nonbonding pairs of electrons. The amines coordinate
with the electron-defi cient boron atom, thus preventing hydrolysis. U.S. Patents 4,975,211
[171] and 5,061,390 [172] disclose the stabilization of borated alkyl catechol against
hydrolysis by complexing with diethylamine. Signifi cant improvement in hydrolytic sta-
bility was reported for borate esters incorporated with a N,N′-dialkylamino-ethyl moi-
ety [156]. It was hypothesized that the formation of a stable fi ve-member ring structure
in molecules involving coordination of nitrogen with boron substantially inhibited the
hydrolytic attack from water. - Use of certain hydrocarbon diols or tertiary amine diols to react with boric acid to form
stable fi ve-member ring structures [173].
1.9 MISCELLANEOUS ORGANOMETALLIC ANTIOXIDANTS
More recently, a number of oil-soluble organometallic compounds, for example, organic acid
salts, amine salts, oxygenates, phenates and sulfonates of titanium, zirconium, and manganese
have been claimed to be effective stabilizers for lubricants [174,175]. Some of the compounds are
essentially devoid of sulfur and phosphorus, therefore, suitable for modern automotive engine
oils where lower contents of the two elements are desired. In one example [174], lubricating oils
having 25 to ~100 ppm of titanium derived from titanium (IV) isopropoxide exhibited excellent
oxidative stability in the high-temperature (280°C) Komatsu hot tube test and ASTM D 6618
test evaluate engine oils for ring sticking, ring and cylinder wear, and the accumulation of piston
deposits in a four-stroke cycle diesel engine. In another example [175], titanium (IV) isopropoxide
was used to react with neodecanoic acid, glycerol mono-oleate, or polyisobutenyl bis-succinimide
to form respective titanated compounds. These compounds, when top-treated in a SAE 5W30
engine oil to result in 50 to ~800 ppm of titanium in oil, improved the deposit control capability
of the oil as tested by using the TEOST (ASTM D 7097). Similar antioxidant effect was observed
for neodecanoates of zirconium and manganese in the same oil.
Oil-soluble or dispersible tungsten compounds, more specifi cally, amine tungstates and tungsten
dithiocarbamates, have been attempted as antioxidants for lubricants and found to be synergistic
with secondary diarylamine and alkylated phenothiazines. The mixtures, when added to an engine
crankcase lubricant to result in ~20 to 1000 ppm of tungsten, were highly effective in controlling oil
oxidation and deposit formation [176].
Sulfur-free molybdenum salts such as molybdenum carboxylates have been attempted as anti-
oxidants and found to be synergistic with ADPAs in lubricating oils [177,178]. The synergistic
mixtures improved oxidation stability of crankcase lubricants while providing additional friction
modifi cation characteristics.
1.10 MECHANISMS OF HYDROCARBON OXIDATION
AND ANTIOXIDANT ACTIONIt is now understood that oxidation of hydrocarbon-based lubricants undergoes autoxidation, a process
that leads to the formation of acids and oil thickening. To a more severe extent, oil-insoluble sludge
and varnish may be formed, causing poor lubrication, reduced fuel economy, and increased wear.