ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 2, pp. 333!336. + Pleiades Publishing, Ltd., 2008.
Original Russian Text + Z.V. Mamarasulova, V.V. Gromova, 2008, published in Zhurnal Prikladnoi Khimii, 2008, Vol. 81, No. 2, pp. 345!347.
Effect of p-Hydroxydiphenylamine and Boron-Containing
Pentaerythritol Esters on Properties of an Ester Oil
Z. V. Mamarasulova and V. V. Gromova
St. Petersburg State Institute of Technology (Technical University), St. Petersburg, Russia
Received June 19, 2007
Abstract-The influence of the amount of incorporated boron on the physicochemical and service proper-
ties of a pentaerythritol ester used as a base of aviation oil was examined. The effect of an antioxidant
additive, p-hydroxydiphenylamine, on the temperature dependence of the oil viscosity was evaluated.
Esters of neopentanepolyols, primarily pentaeryth-
ritol, with synthetic fatty acids C
are widely used
as synthetic lubricating oils for aviation gas turbine
engines, helicopter reduction gears, and other devices
with working temperatures of oil equal to up to 200oC.
The most important requirements to oils of this kind
are high resistance to thermal oxidation, good lubricat-
ing, anticorrosive, and protective properties, slightly
sloping temperature dependence of the viscosity, and
some other standardized parameters .
Previously published data indicate that the resis-
tance of pentaerythritol esters to thermal oxidation is
mainly determined by the presence of readily oxidiza-
ble impurities in the raw material and by the degree
of pentaerythritol esterification [2, 3]. The presence
of incomplete esters considerably impairs the oil sta-
bility. At the same time, it is known that incomplete
pentaerythritol esters considerably surpass their com-
plete analogs in the lubricating properties .
One of possible ways to enhance simultaneously
the resistance of pentaerythritol esters to thermal oxi-
dation, their lubricating properties, and other charac-
teristics is afteresterification of incomplete esters
with boric acid. Published data show that boric acid
esters exhibit high antifriction, antiwear, and anticor-
rosive properties, as well as high resistance to thermal
oxidation . At the same time, boration of the in-
complete esters may alter the physicochemical proper-
ties of the oil owing to an increase in the molecular
weight of the product or appearance of nonesterified
OH groups of boric acid. A certain influence on
the physicochemical properties of oil should also be
exerted by functional groups of the commonly used
antioxidant, p-hydroxydiphenylamine (PHDA).
Therefore, our goal was to determine the optimal
concentration of boron in pentaerythritol esters and to
evaluate the effect of PHDA on the physicochemical
characteristics of oil.
Boron was introduced into the ester molecule by
esterification of free hydroxy groups of incomplete
pentaerythritol esters with boric acid. The reaction
was performed without a catalyst on a laboratory in-
stallation with vacuum separation of water released in
the reaction. Reaction conditions: 140oC, 1 h, residual
pressure 40 kPa.
The reaction products were characterized by stand-
ard methods accepted for testing Efir-2 basic oil with
pentaerythritol esters as major components  and
B-3V commercial aviation oil . The viscosity
of the oils was determined in accordance with GOST
(State Standard) 3332000 with a VPZh-4 Pinkevich
viscometer with an accuracy of 0.2%. The tribological
properties of the samples were estimated with a four-
ball friction machine according to GOST 9490375.
The error in determining the wear scar diameter was
+ 0.1 mm. The resistance to thermal oxidation was
tested according to technical specification for B-3V
synthetic oil at 200oC for 10 h, at an air flow rate of
50 ml min
. The oil stability was evaluated by
variation of the kinematic viscosity n at 100 and
3 40oC, increase in the acid number (AN), and forma-
tion of a precipitate insoluble in isooctane.
As starting materials we used pentaerythritol esters
(PEEs) of individual monocarboxylic acids and their
fractions: caprylic acid ester (PEE C
), ester of C