Antioxidants 13
1.6.2 PHENOL DERIVATIVES
Phenols, especially the sterically hindered phenols are another class of antioxidants being extensively
used in industrial and automotive lubricating oils and greases. Based on the chemical structure,
phenols may be customarily categorized into simple phenols such as 2,6-di-tert-4- methylphenol
(also known as BHT) and complex phenols that are typically in polymeric forms having molecular
weights of 1000 or higher. The structures, important physical properties, and typical applications of
some commonly used HPs are given in Table 1.2.
Similar to the alkyl phenol sulfi des discussed earlier, the combinations of HPs and sulfur chem-
istry have been widely reported. For example, the reaction products of simple phenols such as the
2,6-di-tert-butylphenol listed in Table 1.2 with selected thioalkenes have shown effectiveness in the
prevention of acid buildup and oil viscosity increase, without causing lead corrosion [131]. Another
patent describes a process for preparing hydrocarbylthio-HPs by reacting substituted phenols with
hydrocarbyl disulfi des using an aluminum phenoxide catalyst [132]. Using a 4,4′-methylene bis(2,6-
di-tert-butylphenol) as reference, the thiophenols were found to be superior in bulk oil oxidation
tests and bench corrosion test on bearings. High oligomeric phenolic antioxidants in the form of
hindered and sulfur bridged have been developed [133]. These compounds have lower volatility, bet-
ter thermal stability, and improved seal compatibility and corrosion properties. In general, sulfur-
bridged HPs are more effective than the conventional phenolics under high-temperature oxidation
conditions and are considered particularly suitable for the lubricants formulated with highly refi ned
base stocks [134]. Figure 1.5 shows structures of some commercial sulfur-bridged HPs that have
found use in various lubricant formulations. Thioalkene-bridged hemi-HPs prepared from catalytic
reaction of HP with thioalkene have also been reported to be active in the stabilization of mineral
oils and synthetic oils [135].
1.6.3 AMINE AND PHENOL-BEARING COMPOUNDS
Given the high popularity and effectiveness of amine and phenol derivatives as lubricant anti-
oxidants, the combination of amine and phenolic moieties in one molecule represents a logic
approach to enhance performance. In a prior art [136], fusing amine with a long carbon chain 3,5-
di-tert-butyl-4-hydroxyphenalkyl group that separates the phenol group from the amino nitrogen
leads to novel products with lower volatility, better thermal stability, and higher solubility in oils.
Nelson and Rudnick [137] reacted an ethyoxylated alkyl phenol with an alkyl arylamine in the
presence of an aldehyde. The resulting product had improved antioxidant potency owing to a syn-
ergistic action between the phenolic moiety and the amine, and also showed enhanced solubility
in oils owing to the presence of alkylated aromatic moiety in the molecule. Phenolic imidazo-
lines have been prepared from polyaminophenols and carbonyl compounds [138]. In addition to
providing antioxidant activity, the products also have corrosion inhibition and metal deactivation
properties owing to the cyclic imidazoline moiety.
Multifunctional additives containing sulfur, nitrogen, and phenolic moieties in one molecule
have been reported. In this instance, mercaptobenzothiazoles or thiadiazoles are Mannich reacted
with HP antioxidants to yield oil-soluble compounds with antioxidant and antiwear properties [139].
More complex product having similar functionalities was obtained by reacting a sulfur-containing
HP ester with an ADPA [140].
1.6.4 MULTIFUNCTIONAL AMINE AND PHENOL DERIVATIVES
The industry-wide trend in the reduction of phosphorus and sulfur, in particular, ZDDP in fi nished
lubricants has led to increasing activities in the development of novel multifunctional additives
that have combined properties of antioxidancy, antiwear, and to some extent dispersancy, while
having low-to no-sulfur and phosphorus contents. It has been shown that products obtained from