Esters of dicarboxylic acids as additives for lubricating oils

Abstract

Five samples of dibasic acid esters with varied chemical structures were synthesized. These included didecyl carbonate, didecyl adipate and didecyl sebacate as well as modern oligomeric esters of adipic acid and sebacic acid. These esters were tested in terms of their suitability as additives of fully synthetic engine oils. It was noted that an addition of 10% of the respective esters to oils based on polyalphaolefins led to an improvement of their properties. The pour point of the oils as well as their low temperature viscosity were reduced. The viscosity index rose and oil lubricity improved. Esters of oligomeric structures synthesized by the transesterification of dimethyl adipate or dimethyl sebacate with a mixture of neopentyl glycol and decanol showed particularly suitable properties. The tested esters were compatible with the other oil components, forming a stable solution in a wide temperature range.

Full text

Esters of dicarboxylic acids as additives for lubricating oils
S. Gryglewicz
a,
*
, M. Stankiewicz
a
, F.A. Oko
a
, I. Surawska
b
a
Wroclaw University of Technology, Department of Chemistry, ul. Gdanska 719, 50-344 Wroclaw, Poland
b
LOTOS Lab. ul. Elble˛ska 135, 80-719 Gdan
´sk
Received 21 March 2005; received in revised form 31 May 2005; accepted 7 June 2005
Available online 18 July 2005
Abstract
Five samples of dibasic acid esters with varied chemical structures were synthesized. These included didecyl carbonate, didecyl adipate
and didecyl sebacate as well as modern oligomeric esters of adipic acid and sebacic acid. These esters were tested in terms of their suitability
as additives of fully synthetic engine oils. It was noted that an addition of 10% of the respective esters to oils based on polyalphaolefins led to
an improvement of their properties. The pour point of the oils as well as their low temperature viscosity were reduced. The viscosity index
rose and oil lubricity improved. Esters of oligomeric structures synthesized by the transesterification of dimethyl adipate or dimethyl sebacate
with a mixture of neopentyl glycol and decanol showed particularly suitable properties. The tested esters were compatible with the other oil
components, forming a stable solution in a wide temperature range.
q2005 Elsevier Ltd. All rights reserved.
Keywords: Synthetic oils; Adipic esters; Didecyl carbonate
1. Introduction
Natural as well as synthetic esters belong to a class of
chemical compounds most commonly applied in technique.
Compounds containing the ester group in their structures
include: lubricating oils, solvents, components of liquid
fuels and cosmetic ingredients.
For many decades esters of oil consistency have been
widely applied in industry as a major supplement to
hydrocarbon oils produced from petroleum [1,2].Itis
estimated that approximately 10% of global lubricating oil
production (including esters) are fully synthetic products
[3]. Specific properties of esters enable them to meet the
vagaries of lubrication challenges posed by modern
machines both technically and with respect to environmen-
tal protection. In comparison to hydrocarbon oils, esters
exhibit strong dipole moments. This improves their lubricity
by adhering strongly to the metal surface at the friction
point. They also show excellent qualities as components
of modern, environmentally friendly synthetic engine
oils produced from saturated olefin oligomers C
8
–C
12
(polyalphaolefins). Addition of esters enhances oil lubricity
and enables them to form stable solutions with polar
additives. A great majority of ester oils are physiologically
harmless and easily biodegradable in the natural environ-
ment [4,5].
The only barrier to the wide application of esters is
their relatively high cost of production, 4–15 times
higher than that of conventional mineral oils. Possibilities
for applying compounds of unconventional structure,
like complex esters, natural fatty acid derivatives as
well as products of their chemical modification as
lubricants are being considered [6]. New methods for
synthesizing low waste and energy saving ester oils using
advanced catalytic systems including enzymes are being
designed [7].
Of particular interest are dicarboxylic acid esters-adipic
and sebacic. Oils produced from dicarboxylic acid esters
are characterized by oxidative and thermal stability,
excellent lubricity, good biodegradability and moderate
production costs. A perfect method of producing adipic
and sebacic acid esters is the transesterification of methyl
esters with appropriate alcohols conducted in the presence
of basic catalysts [8]. The process proceeds according
Tribology International 39 (2006) 560–564
www.elsevier.com/locate/triboint
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doi:10.1016/j.triboint.2005.06.001
*
Corresponding author. Fax: C48 71 3221580.
E-mail address: stanislaw.gryglewicz@pwr.wroc.pl (S. Gryglewicz).

to Eq. (1).
CH3OOCðCH2ÞkCOOCH3C2ROH
4ROOCðCH2ÞkCOOR C2CH3OH[
kZ4adipic acid;kZ8sebacic acid;
ROH Zmonohydroxyl alcohol
(1)
Simple adipic and sebacic dialkyl esters can be
obtained in this manner. Their disadvantage is low
viscosity. When transesterification reactions of dicar-
boxylic acid methyl esters are carried out with glycols
like neopentyl glycol below, esters of oligomeric structure
terminated with methoxyl groups, are obtained (2).
CH3OOCðCH2ÞkCO½OCH2CðCH3Þ2CH2OOCðCH2ÞkCOn
OCH3kZ4adipic acid;
kZ8sebacic acid;noligomerization degree;ð2Þ
The viscosity of such compounds can be altered as
required by changing the degree of oligomerization.
However, this type of compound has a large number of
esters groups per molecule and consequently high
polarity, causing them to exhibit low miscibility with
synthetic hydrocarbon oils.
The main task of this work, among others, is the
synthesis of oligomeric esters without methoxyl groups in
their structures. The next aim is to evaluate the advantages
of the obtained dicarboxylic acid esters with respect to their
application as components of lubricating oils. Research
included the evaluation of physicochemical properties of
esters as well as laying out an assessment plan for samples
of full synthetic engine oils having them as components.
2. Experimental
2.1. Materials
Neopentyl glycol, dimethyl adipate, dimethyl sebacate
and dimethyl carbonate were purchased from Aldrich
Steinheim. Decanol (mixture of isomers) was obtained
from Fluka Chemie GMBH.
The additive package and viscosity index improver were
a gift from Lotos-Group Laboratory, Poland. The package
of additives consists of the calcium and magnesium
alkylosalicylates, silicone antifoam agent, and zinc dialky-
lodithiophosphates. The hydrogenated copolymer of styrene
and isoprene was used as the viscosity index improver.
2.2. Physicochemical properties
The physical properties of the esters and final oil samples
were determined according to the following standard test
methods: High shear rate viscosity at 150 8C (HTHS)-
ASTM D 4624, kinematic viscosity-ASTM D 445, viscosity
index (VI)-ASTM D 2270, low temperature cranking
viscosity at K35 8C-ASTM D 5293, pour point ASTM
D97, Noack volatility-ASTM D 5800 and flash point-ASTM
D 93. Volatility of esters was determined by thermogravi-
metric method according to IP 393. The ester was heated in
an argon atmosphere, from 40 to 550 8C at a rate of
10 8C/min. Mass loss was given as a function of
temperature. Ad pour point and VI
ad
are calculated as
weighted average of pour points and viscosity indices of oil
components.
The miscibility of ester oils (10% mass) in non-polar
polyalphaolefins of PAO4 base was also determined. The
tested temperature range was from K15 to 200 8C. The
result was taken as positive when the solution formed a clear
single phase.
The antiwear properties of the finished oils were tested in
a Four-Ball Test Rig. The weld load (ASTM D 2783) was
determined.
2.3. Ester synthesis
Didecyl carbonate DeCODe was synthesized by the
transesterification of dimethyl carbonate with decanol
according to the previously described method [9].
Didecyl adipate DeAdDe was formed in a reaction of
dimethyl adipate with decanol, catalysed by calcium
methoxide in accordance with the method given in [10].
Didecyl sebacate DeSeDe was formed in a reaction of
dimethyl sebacate with decanol using an identical method to
that for of didecyl adipate.
The oligomeric ester mixture of adipic acid,
DeAd[NPGAd]
n
De, was also synthesized by way of
transesterification using as starting material dimethyl
adipate, neopentyl glycol and decanol initially in the ratio
of 2:1:2.
Oligomeric ester of adipic acid, SeAd[NPGSe]
n
De was
synthesized by the transesterification reaction using as
starting material dimethyl sebacate, neopentyl glycol and
decanol in the ratio of 2:1:2.
The proposed abbreviations for the esters are a reflection
of their chemical structure. The respective parts are
defined as follows: De-decanol; NPG-neopentyl glycol;
CO-carbonic acid; Ad, adipic acid and Se, sebacic acid.
2.4. Oil samples preparation
The prepared oil samples had a composition by mass as
follows: polyalphaolefins PAO6-24.3%, polyalphaolefins
PAO4-30.0%, package of performance additives-13.7%,
viscosity improver-22.0% and ester-10.0%. For the purpose
of comparison, the oil composition without the addition of
esters (designed as Oil-0) was prepared and analyzed. In this
case the ester was replaced with PAO4. Thus, six samples of
synthetic oils were obtained with varied ester content
in their composition: Oil-DeCODe, Oil-DeAdDe,
S. Gryglewicz et al. / Tribology International 39 (2006) 560–564 561

Oil-DeSeDe, Oil-DeAd[NPGAd]
n
De, Oil-DeSe[NPGSe]
n-
De and Oil-0.
3. Results and discussion
3.1. Esters
The results of basic physicochemical properties analysis
of esters are presented in Table 1. In general, all the tested
esters have a low pour point, below K45 8C. This property
is derived from the presence of branched structures in the
ester molecules. Didecyl carbonate and didecyl adipate, the
esters with the lowest molecular weights, have particularly
low pour points, K73 8C and K68 8C respectively. The
high viscosity index of dicarboxylic acids ranging from
117 to 172 calls for special attention. An exception here is
the ester of carbonic acid, which has the relatively low
viscosity index 64. It is known that long, straight chain
molecules are characterized by low dependence of viscosity
on temperature; the carbonic acid ester does not belong to
such group. The longer hydrocarbon chain in the sebacic
acid molecule when compared to adipic acid, exhibits a
higher viscosity index as the sebacic acid ester. This is
observed both in the case of simple dicarboxylic acid esters
and in oligomeric esters. The latter characteristically shows
the highest viscosity in comparison with other tested esters.
The volatility of ester samples was tested by the
thermogravimetric method; see Fig. 1. This is a very
important parameter when lubricating agents have to work
in high temperatures like engine oils, for example. Too low
oil volatility leads to fluid loss and emits harmful
substances into the natural environment. It was observed
that didecyl carbonate showed the highest volatility; it also
has the least molecular weight. A 50% weight loss was
already noted at a temperature of 290 8C under the test
conditions. A graph of weight loss against temperature in
this case showed a steep tangent with the X-axis. This
suggests that weight loss is a result of evaporation (low
boiling point) without any chemical decomposition.
Didecyl adipate and didecyl sebacate are characterized by
higher volatility (w350 8C). The least volatile are
oligomeric esters. Their 50% weight loss is reached at
about 380 8C. The initial slope of the thermogravimetric
graph in this case shows a relatively low gradient to the
X-axis. There are two reasons for this. Firstly, these esters
are basically a homogeneous mixture of oligomers of
varied molecular weight. Secondly, at higher temperatures
thermal degradation of their binding structures occur,
releasing the volatile products of decomposition.
Within the test temperature range of K15 to C200 8C all
the tested esters were fully miscible with polyalphaolefins.
3.2. Olis characterization
The basic physicochemical properties of the prepared
samples of synthetic oils are presented in Table 2.
It was noted that the presence of esters in the oil
composition significantly reduces its pour point, which is an
advantage. This was greatest in the case of didecyl
carbonate. This effect was always positive. It can therefore
be stated that esters are a kind of pour point depresant. This
result was also observed by Bartha in [11]. On the other
hand, the volatility of the oil blend is, as expected, very
much dependent on the type of ester used, specifically on its
volatility as determined by thermogravimetric method.
Oligomeric esters do not significantly increase the volatility
of these blended oils. This makes them ideal additives in this
respect. The content of the didecyl sebacate and didecyl
adipate in oils insignificantly increases the Noack volatility.
Under the test conditions, didecyl carbonate practically
undergoes a complete evaporation, thereby increasing the
Noack volatility index to almost 20%. As a result of this, it
can be affirmed that didecyl carbonate is not an appropriate
additive to engine oils.
The addition of esters in each of the tested samples
improved the lubricity of oils, an effect which is observed by
the increase in weld load which is determined here in four-
ball test machine. Oligomeric esters were the best in this
respect.
The tests showed that the addition of 10% esters to the
oils did not lead to the reduction of flash point. It was even
observed that flash point was increased within the range of
2–4 8C with the exception of samples containing didecyl
carbonate. The flash point essentially remained unchanged
overall.
Table 1
Properties of esters
Ester Pour
point, (8C)
Viscosity, cSt VI Miscibility
with PAO4
40 8C 100 8C
DeCODe K73 10.28 2.56 64 C
DeAdDe K68 13.1 3.25 117 C
DeSeDe K48 16.57 4.29 167 C
DeAd[NP-
GAd]
n
De
K46 30.11 6.01 150 C
DeSe[NP-
GSe]
n
De
K46 37.78 7.52 172 C
0
20
40
60
80
100
150 200 250 300 350 400 450 500 550
Temperature [˚C]
Loss of mass [g]
DeCoDe
DeAdDe
DeSeDe
DeAdNPGAdDe
DeSeNPGSeDe
Fig. 1. Thermogravimetric data of esters: $, DeCODe; 6, DeAdDe;
:, DeSeDe; B, DeAd[NPGAd]
n
De; C, DeSe[NPGSe]
n
De.
S. Gryglewicz et al. / Tribology International 39 (2006) 560–564562

Viscosity properties of composed oils were also
determined and the results are given in Table 3.
The addition of esters leads to an insignificant reduction
in the viscosity of the final products. It was observed,
however, that the presence of esters improved the viscosity
index of the oil samples. This was also noted in the case of
oil compositions containing didecyl carbonate, which itself
has a low viscosity index. Increase in viscosity index was
seen to be higher than what it should have been from the
summation of all the components. We therefore note a
synergistic effect of ester addition to the other components
of oils.
The results of low-temperature viscosity analysis are also
of interest. These tests are a simulation of the cold start of
engines. At a temperature of K35 8C under which the tests
were conducted, all the esters had a decreasing effect on the
viscosity of oils tested.
Another important function of added esters was
ensuring good ability for form a hydrodynamic film at
high temperatures and high shear velocity. These are
conditions under which engines work when they are
heated up, in motion and under increased load.
Appropriate properties of oils under these conditions
are determined by the value of high shear rate viscosity
(HTHS). The maximum HTHS value for oils in the SAE
40 range is 3.7 mPa s at a temperature of 150 8C and a
shear rate of 10*6/s. All the tested oil compositions had
a HTHS value less than 3.7 thereby meeting the above
demand.
4. Conclusions
The most suitable esters as components of engine oils
were oligomeric esters of adipic acid and sebacic acid. The
presence of these esters improved the low temperature
properties of oils; both the pour point and low-temperature
cranking viscosity (K35 8C) were reduced. Moreover, the
addition of esters increases the viscosity index of oils and
improves their lubricity. In terms of rheological properties,
oils containing oligomeric esters meet performance
demands for the SAE 5W/40 range of oils.
The content of simple dibasic acid esters-didecyl
carbonate, didecyl adipate and didecyl sebacate, positively
influence oil properties just like the tested oligomeric esters,
except that they show relatively higher volatility. In this
respect, didecyl carbonate particularly exhibits a negative
influence. This is a compound with low molecular weight
and low boiling temperature.
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Table 2
The influence of esters content on some physicochemical and performance properties of oils
Sample Pour point, 8C Ad pour point
a
8C Flash point, 8C Noack volatility, % Weld load, kG
Oil-DeCODe K52 K43.3 236 22 84.5
Oil-DeAdDe K43 K42.8 233 12.8 93.1
Oil-DeSeDe K45 K40.8 232 12.1 105
Oil-DeAd[NPGAd]
n
De K42 K40.6 234 11.1 114.5
Oil-DeSe[NPGSe]
n
De K45 K40.6 234 10.8 109.6
Oil-0 K40 K40 236 10.4 80
a
Ad pour point, calculated value.
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at 150 8C, mPas
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ada
Cranking viscosity
at K35 8C, mPas
40 8C 100 8C
Oil-DeCODe 3.37 68.15 12.42 183.2 165.1 7600
Oil-DeAdDe 3.4 71.50 12.85 182.4 170.4 7800
Oil-DeSeDe 3.45 72.76 13.13 184.3 175.4 7800
Oil-DeAd[NPGAd]
n
De 3.62 78.66 13.51 176.1 173.7 8800
Oil-DeSe[NPGSe]
n
De 3.66 78.76 13.73 179.6 175.9 8300
Oil-0
b
3.4 76.19 13.19 176.3 – 8900
a
VI
ad
, calculated value.
b
Prepared without addition of ester.
S. Gryglewicz et al. / Tribology International 39 (2006) 560–564 563

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