Antioxidants 17
More complex compounds obtained from further reactions of copper salts have also been
reported to be effective antioxidants in various lubrication applications. For example, copper car-
boxylate or copper thiocyanate was reacted with a mono-oxazoline, bis-oxazoline, or lactone oxa-
zoline dispersant to form coordination complexes, wherein the nitrogen contained in the oxazoline
moiety is the ligand that complexes with copper. The resulting products exhibit^ improved varnish
control and oxidation inhibition capabilties [152]. Reaction products of a copper salt (acetate, car-
bonate, or hydroxide) with a substituted succinic anhydride derivative containing at least one free
carboxylic acid group are effective high-temperature antioxidants and friction modifi ers. When
incorporated in an engine oil formulation, the oil passed rust, oxidation, and bearing corrosion
engine tests [153]. In another patent [154], a HP carboxylic acid was used as the coupling reagent.
The resulting copper compounds are reported to be effective in the controls of high-temperature
sludge formation and oil viscosity increase when used alone or in synergistic mixtures with a con-
ventional aminic or phenolic antioxidant.
1.8 BORON ANTIOXIDANTS
The search for more eco-friendly additives to replace ZDDP has led to renewed interest in boron
esters owing to their ability to improve antioxidation, antiwear, and antifriction properties of lubri-
cants when used alone or in combination with other additives. The complex tribological behavior of
boron compounds in formulated lubricants depends on their particular chemical structures and the
interactions between boron and other active elements such as sulfur, phosphorus, nitrogen, or their
combinations when present [155,156].
A number of boron–oxygen-bearing compounds have been reported to be effective oxidation
inhibitors in terms of prevention of oil viscosity increase and acid formation at elevated tempera-
ture (163°C) [157–161]. Representatives are boron epoxides (especially 1,2-epoxyhexadecane) [157],
borated single and mixed alkanediols [158], mixed hydroquinone-hydroxyester borates [159], phe-
nol esters of hindered phenyl borates [160], and reaction products of boric acid with the condensates
of phenols with aromatic or aliphatic aldehydes [161].
Borate esters with nitrogen are known for their antioxidant activity and improved antiwear
properties probably due to the formation of additional boron nitride fi lm on rubbing surface [162].
Borated adducts of alkyl diamines with long-chain hydrocarbylene alkoxides and low-molecular-
weight carboxylic acids have been reported to have antifriction properties and high inhibition abil-
ity especially at elevated temperatures [163]. Appreciable oxidation inhibition effect has also been
reported for borate esters of hydrocarbyl imidazolines [164], borates of mixed ethyoxyamines and
ethoxyamides [165], and borates of etherdiamines [166].
Synergistic antioxidant effect of borate esters with ADPAs or with zinc dithiophosphates has
been established. When tested at 180°C in a PAO using a pressurized differential scanning calo-
rimetry (PDSC), strong synergistic antioxidant action was observed between borate esters and a
dioctyl diphenylamine at a 1:1 (w/w) blending ratio [167]. Similar effect was observed in the mix-
tures of borate esters and a ZDDP [155]. The synergism with ZDDP is of practical importance as
it allows reduced phosphorus level in a fi nished lubricant without sacrifi ce of oxidative stability.
The catalytic effect of boron in enhancing antioxidant performance has led to the development of
phenolic-phosphorodithioate borates, obtained from coborating HP and alkyl phosphorodithioate-
derived alcohol. The borates were found to possess exceptional antioxidant and antiwear proper-
ties. Both the HP moiety and the phosphorodithioate alcohol moiety were believed to provide the
basis for the synergy each of which are subsequently enhanced by the integral boron coupling
moiety [168].
Despite many tribological and antioxidation benefi ts that borate esters can offer, large use of
the chemistry for lubricant applications has not taken place. One serious drawback with most borate
esters has been their high susceptibility to hydrolysis, a process that liberates oil-insoluble and