A composition of organic hetero compounds as an antioxidant and antiwear additive for mineral lubricating oils

Article abstract of DOI:10.1134/S0965544113040142

As a multifunctional additive exhibiting antioxidant and antiwear activity for mineral lubricating oil, a composition of ammonium dialkyldithiophosphate (ADTP) with tetraalkyl thiuram disulfide (TDS) has been proposed, which provides lower sulfated ash, phosphorus, and sulfur contents compared with the traditional additive zinc dialkyldithiophosphate used for the same functions. Ammonium dialkyldithiophosphates bearing various alkyl radicals have been synthesized using structurally different amines. The antioxidant efficiency of ADTP samples has been shown by monitoring the formation and degradation of hydroperoxides in a hexadecane medium as a model of mineral oil. The tribological parameters of these compounds have been determined under boundary lubrication conditions.

Full text of DOI:10.1134/S0965544113040142

ISSN 0965ꢀ5441, Petroleum Chemistry, 2013, Vol. 53, No. 4, pp. 262–266. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © A.V. Zolotov, G.N. Kuz’mina, V.A. Zolotov, R.V. Bartko, A.G. Sipatrov, O.P. Parenago, 2013, published in Neftekhimiya, 2013, Vol. 53, No. 4, pp. 297–301.
A Composition of Organic Hetero Compounds as an Antioxidant
and Antiwear Additive for Mineral Lubricating Oils
A. V. Zolotova, G. N. Kuz’minaa, V. A. Zolotovb, R. V. Bartkob, A. G. Sipatrovb, and O. P. Parenagoa
a Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia
eꢀmail: parenago@ips.ac.ru
b State Research Institute No. 25: Chemmotology, Ministry of Defense of the Russian Federation, Moscow, Russia
Received December 26, 2012
Abstract—As a multifunctional additive exhibiting antioxidant and antiwear activity for mineral lubricating
oil, a composition of ammonium dialkyldithiophosphate (ADTP) with tetraalkyl thiuram disulfide (TDS)
has been proposed, which provides lower sulfated ash, phosphorus, and sulfur contents compared with the
traditional additive zinc dialkyldithiophosphate used for the same functions. Ammonium dialkyldithiophosꢀ
phates bearing various alkyl radicals have been synthesized using structurally different amines. The antioxiꢀ
dant efficiency of ADTP samples has been shown by monitoring the formation and degradation of hydropꢀ
eroxides in a hexadecane medium as a model of mineral oil. The tribological parameters of these compounds
have been determined under boundary lubrication conditions.
Keywords: additives, antioxidants, organic hetero compounds, amines, tribological parameters
DOI: 10.1134/S0965544113040142
Modern engine oils generally include quite a comꢀ eral oil composition has an inhibitory effect during
plex set of additives (the soꢀcalled additive package) highꢀtemperature (180оС) oxidation and exhibits triꢀ
for various applications, in particular, antioxidant and bological activity in friction and wear processes [6].
antiwear additives, such as zinc salts of dialkyldithioꢀ
It was of interest to study the effect of the nature of
phosphoric acid. The phosphorus and sulfur atoms in
the alkyl groups in the ammonium salt on the funcꢀ
their structure and the sulfated ash (zinc atoms) proꢀ
tional properties of the composition and to determine
duced have a negative effect on the performance of
the optimal ratio of its components. Thus, the aim of
exhaust control systems (catalytic converters, exhaust
this work was to synthesize TDS and various ADTP
gas recirculation systems, particulate filters), with
derivatives, to study the antioxidant action and behavꢀ
which internal combustion engines that meet the
ior of the relevant compositions, and to determine
Euroꢀ4 and Euroꢀ5 standards are equipped. The negꢀ
their activity in the friction and wear processes.
ative effect of such additives on the operation of cataꢀ
lytic converters is associated with the deactivation of
the additives by vitreous matter, the product formed on
the surface as a result of decomposition of zinc dithioꢀ
EXPERIMENTAL
phosphates [1, 2]. As the sulfated ash and sulfur conꢀ
tents of oil increase, the amount of deposits in the
exhaust gas recirculation system increases, the pores
of particulate filters are intensively “clogged”, and
their performance declines. For these reasons,
researchꢀandꢀdevelopment works on nextꢀgeneration
motor oils, the soꢀcalled lowꢀ and zeroꢀSAPS (Low
Sulfated Ash, Phosphorus and Sulfur) oils, which are
to contain low or no sulfated ash, phosphorus, and sulꢀ
fur, have been launched and are in progress now
abroad [3, 4].
To synthesize ADTP samples containing various
alkyl groups, the following primary and secondary
amines were used: di(2ꢀethylhexyl)amine (C8H17)2NH
(
1
), dodecylamine C12H25NH2 (
C18H37NH2, ( , didodecylamine (C12H25)2NH (
and dioctadecylamine (C18H37)2NH ( ).
2
)
, octadecylamine
3)
4),
5
Ammonium dialkyldithiophosphates were syntheꢀ
sized in two stages according to a published procedure
[7]. In the first step, diꢀ2ꢀethylhexyldithiophosphoric
acid was prepared via the reaction of 2ꢀethylhexyl
alcohol with phosphorus pentasulfide at 80оС. Then,
alkylammonium salts of this acid were synthesized in
To tackle these problems, we proposed an ashꢀfree
composition of organic hetero compounds, including an almost quantitative yield by reaction of the acid
ammonium dialkyldithiophosphate (ADTP) and tetꢀ with the aforementioned structurally different amines
raalkyl thiuram disulfide (TDS) [5]. It was shown that at 100оС and a 1 : 1 molar ratio of the reactants in a tolꢀ
the composition introduced as an additive into minꢀ uene solution. The composition and structure of the
262

A COMPOSITION OF ORGANIC HETERO COMPOUNDS
263
Table 1. Amount of sulfur and phosphorus atoms in components of the additive
Amount, %
Ratio of components in mixture
Atomic ratio
Additive components
with oil, mass fraction
P
S
S
/
P
P/S
DFꢀ11
TDS
1
0.094
none
0.064
0.048
none
0.032
none
0.016
none
0.194
0.202
0.132
0.099
0.051
0.066
0.101
0.033
0.152
2.07
–
0.48
–
1
ADTPꢀ1
ADTPꢀ1
TDS
1
2.07
3.13
–
0.48
0.32
–
0.75
0.25
0.5
0.5
0.25
0.75
ADTPꢀ1
TDS
5.23
–
0.19
–
ADTPꢀ1
TDS
11.55
–
0.09
–
compounds obtained were confirmed by elemental pair reciprocating with an amplitude of 1 mm and a
analysis and mass spectrometry.
frequency of 50 Hz was used. The load on the ball
(diameter, 12.7 mm; material, ShKhꢀ15 steel) was
applied in the stepwise manner by its increasing from
50 to 175 N with a step size of 25 N. The duration of
each step was 20 min, and the total testing time was
Tetraalkyl thiuram disulfide was synthesized in two
steps as well [8]. In the first step comprising the reacꢀ
tion of di(2ꢀethylhexyl)amine with a mixture of carꢀ
bon disulfide and sodium hydroxide in toluene at 8–
10°C, sodium di(2ꢀethylhexyl)dithiocarbamate was
obtained, which was then oxidized with an aqueous
iodine solution at 16–18оС to give tetra(2ꢀethylhexyl)
thiuram disulfide.
2 h. The sample temperature was maintained at 80°С.
The antiwear properties of the samples were evaluated
at the end of testing on the UMTꢀ3 machine with a
MMIꢀ2 toolmaker’s microscope by measuring the ball
and disc wear according to the DIN 51834 method
[11] and the ball wear scar diameter on the ChMTꢀ1
tester according to GOST (State Standard) 9490.
The antioxidant activity of these ADTP samples
was examined according to two procedures. In one of
them, in accordance with the published method [9],
the hydroperoxide buildup rate in a hexadecane
medium as a model of mineral oil was determined in
the kinetic regime of its autoꢀoxidation with oxygen at
170оС. The hydroperoxide concentration [ROOH]
was measured iodometrically. To determine the charꢀ
acter of hydroperoxide degradation, hexadecane was
initially oxidized for a time corresponding to the maxꢀ
imum yield of hydroperoxides (0.25 mol/L) and their
decomposition was then studied in an oxygenꢀfree
atmosphere by bubbling nitrogen through the solution
of an ADTP sample in hexadecane. The concentraꢀ
RESULTS AND DISCUSSION
Table 1 presents data on the amount of phosphorus
and sulfur atoms in the synthesized additive compoꢀ
nents in both their individual state and various combiꢀ
nations in the lubricating oil in comparison with the
conventional additive zinc dialkyldithiophosphate
(DFꢀ11).
From these data it follows that the minimal phosꢀ
phorus and sulfur content in the composition is proꢀ
vided at an equal mass ratio of the components (0.5 :
0.5) in the additive formula.
Figure 1 shows the kinetics of buildup and conꢀ
sumption of hydroperoxides during autoxidation of
hexadecane in the presence of ADTP, in which the
amine moiety includes alkyl groups of various nature.
Since the inhibitors are likely to be consumed at the
beginning of the buildup of hydroperoxides, the reacꢀ
tion of the test compounds was 1 ×
10–3 mol/L. Using
the other procedure according to [10], highꢀtemperaꢀ
ture (180оС) oxidation of a model 1 : 1 (by volume)
mixture of the industrial oils Iꢀ20 and Iꢀ40 was carried
out in the presence of a catalyst (reduced copper) or
proꢀoxidant (copper naphthenate) with air bubbling
(5 ×
10–3 m3/s) through the oil for 20 h with sample
analysis every 5 h.
The tribological characteristics of the composition tion can occur in the system with the participation of
and its components in friction and wear processes were reactant transformation products. As follows from the
determined with a CETR (Center for Tribology Inc., results presented above, the formation of hydroperoxꢀ
USA) UMTꢀ3 tribometer and a ChMTꢀ1 fourꢀball ides in the case of additiveꢀfree hexadecane occurs
tester with varying the additive concentration of 0.5– very quickly (they are present in a small fraction
1.0 wt % in the 1 : 1 (by volume) mixture of Iꢀ20 and already in the starting hydrocarbon) to reach the maxꢀ
Iꢀ40 oils. In the case of UMTꢀ3, the ball–disc friction imal concentration of these reactive intermediates well
PETROLEUM CHEMISTRY Vol. 53
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2013

264
[ROOH], mol/L
ZOLOTOV et al.
[ROOH], mol/L
0.30
0.25
0.20
0.15
0.10
0.05
0
0.30
0.25
0.20
0.15
0.10
0.05
6
6
4
1
2
3
1
2
4
5
3
5
10
15
20
Time, min
Beginning of sampling
0
10 20 30 40 50 60 70 80 90 100
Time, min
Fig. 2. Kinetics of degradation of hydroperoxides in the
–3
presence of ADTP with [ADTP] = 1.0
hexadecane: ( ) ADTPꢀ1, ( ) ADTPꢀ4, (
) hexadecane.
×
10 mol/L in
1
2
3) ADTPꢀ5, and
Fig. 1. Kinetics of buildup of hydroperoxide in ADTP
solutions in hexadecane with [ADTP] = 1.0
(
4
–3
×
10 mol/L:
(
1
) ADTPꢀ1, (
) ADTPꢀ5, and (
2
3) ADTPꢀ3, (4) ADTPꢀ4,
(5
hexadecane by the 30th min (time to reach
[ROOH]max in pure hexadecane) were calculated for
all ADTP samples. The maximal values of the hydropꢀ
eroxide buildup rate according to the tangents drawn
at the inflection points of the rate curves were also
determined. These results are summarized in Table 2.
above that for all ADTP samples, which have approxiꢀ
mately the same effect. A small exception is ADTP
that has dioctadecylamine making the amine compoꢀ
nent; there was a distinctly expressed induction period
(~10 min) in the formation of hydroperoxides in this
case.
These data show that the minimal concentration of
hydroperoxides is observed for solutions of ammoꢀ
nium dialkyldithiophosphate in which the alkyl radiꢀ
cal comes from diꢀ(2ꢀethylhexyl)amine or dioctadeꢀ
cylamine.
It should be noted that the hydroperoxide buildup
and consumption rate curves are selfꢀoscillating in
character in all cases. This pattern was reported by
many investigators previously [12, 13], and it was
explained in terms of the mechanism by which the
highꢀtemperature oxidation of hydrocarbons is
accompanied by profound changes in the structure of
the liquid phase, which changes are due to the formaꢀ
tion and degradation of the micellar associates [14].
It is known that the effectiveness of oxidation
inhibitors is determined not only by their ability to
inhibit the formation of hydroperoxides in the system,
but also the possibility to actively influence the rate of
ROOH decomposition into molecular (nonradical)
products. In accordance with this, we conducted
experiments on the decomposition of hydroperoxides
formed in hexadecane in the presence of various
ammonium dialkyldithiophosphates (Fig. 2). On the
basis of these data, the maximal rates of decomposiꢀ
tion of hydroperoxides were calculated from the initial
portions of the rate curves and the concentration of
hydroperoxides remaining in the system by the 20th
min were determined (Table 3).
On the basis of the kinetic data (Fig. 1), the maxiꢀ
mal concentration of hydroperoxides produced in
Table 2. Buildup of hydroperoxides in the presence of difꢀ
ferent ammonium dialkyldithiophosphates
5
[ROOH]
,
V
×
max
10 ,
max
System
mol/L
mol/(L s)
As can be seen from the data in Table 3, the highest
hydroperoxide degradation rate was in the case of
ADTP containing the 2ꢀethylhexyl radical. The miniꢀ
mal concentration of undecomposed ROOH was
approximately the same for all test ammonium dialkyꢀ
ldithiophosphates, although it was much lower than
that in pure hexadecane.
Hexadecane (HD)
GD + ADTPꢀ1
GD + ADTPꢀ2
GD + ADTPꢀ3
GD + ADTPꢀ4
GD + ADTPꢀ5
0.252
0.060
0.063
0.076
0.072
0.060
26.2
5.5
5.0
7.9
4.8
5.4
During the catalytic highꢀtemperature (180оС
)
oxidation of the model mixture of industrial oils and
their solutions with additives, their viscosity increment
PETROLEUM CHEMISTRY Vol. 53
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2013

A COMPOSITION OF ORGANIC HETERO COMPOUNDS
265
Table 3. Degradation of hydroperoxides in the presence of
different ammonium dialkyldithiophosphates at 170°C
mixed in equal proportions by mass, a fact that may
suggest the synergistic effect.
4
Being introduced as additives into mineral oil, the
synthesized compounds were also examined in friction
and wear processes using the ChMTꢀ1 tester. Table 5
presents the tribological characteristics obtained for
different ammonium dialkyldithiophosphates in a
white oil (WO) solution (0.5 wt %); the volumetric
wear of the disc was determined on the UMTꢀ3 triꢀ
bometer.
V
×
10 ,
[ROOH]
,
min
max
System
mol/(L s)
mol/L
Hexadecane (HD)
GD + ADTPꢀ1
GD + ADTPꢀ4
GD + ADTPꢀ5
1.67
3.12
2.94
2.86
0.0897
0.0397
0.0268
0.0301
As can be seen from these data, all the ADTP salts
exhibit antiwear activity, lowering the wear scar diamꢀ
eter as compared with the additiveꢀfree oil. By this
parameter, the most efficient additive among the samꢀ
ples is the salt containing the di(2ꢀethylhexyl)amine
moiety. Note a high antiwear activity of the salts in
comparison with the pure oil as determined by the volꢀ
umetric disc wear; in this case, the best result was
obtained for the salt containing the dioctadecylamine
as one of the characteristic indicators of oxidizability
of lubricants was determined (Table 4).
The data in Table 4 show that the additive compoꢀ
nents and their composition exhibit inhibitory properꢀ
ties, which are particularly distinct in the presence of
reduced copper wire, an oxidation catalyst. The best
result was achieved in the case of the components moiety.
Table 4. Results of highꢀtemperature catalytic oxidation of a 1 : 1 (vol.) mixture of mineral oils (MO) Iꢀ20 and Iꢀ40 conꢀ
taining various components
Viscosity
ν
during oxidation
in the presence of
+100°C
Sample composition
Oxidation time, h
without catalyst
copper naphthenate
(0.1 wt %)
copper
MO
0
20
0
12.29
15.20
12.20
15.37
12.33
15.30
12.33
14.78
12.29
13.16
12.20
13.20
12.33
12.86
12.33
13.09
12.29
13.74
12.20
13.15
12.33
13.15
12.33
12.51
MO + 0.5% TDS
20
0
MO + 0.5% ADTPꢀ1
20
0
MO + 0.5% TDS + 0.5%ADTPꢀ1
20
Table 5. Tribological characteristics of ammonium dialkyldithiophosphates
Critical nonseiꢀ
Volumetric disc
Wear scar diameꢀ
ter (WSD), mm
Weld load,
w, N
Load wear index
(LWI)
Formulation of system
zure load,
wear, V ×
103,
P
P
cr, N
mm3
White oil (WO)
0.77
0.37
0.65
0.56
0.66
0.58
80
135
141
150
150
150
150
36.90
52.60
46.39
46.43
38.02
45.00
1.7
0.5
0.6
0.3
0.7
0.2
WO + ADTPꢀ1 (0.5%)
WO + ADTPꢀ2 (0.5%)
WO + ADTPꢀ3 (0.5%)
WO + ADTPꢀ4 (0.5%)
WO + ADTPꢀ5 (0.5%)
100
106
100
84
89
PETROLEUM CHEMISTRY Vol. 53
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266
ZOLOTOV et al.
Table 6. Tribological properties of the composition of different formulas
Coefficient
+of friction,
Wear scar diameꢀ Critical nonseiꢀ
ter (WSD), mm zure load, Pcr, N
Load wear index
(LWI)
Formulation
Weld load, Pw, N
f
(60 min)
Mixture of Iꢀ20 + Iꢀ40 (base oil)
Base oil + TDS (1 wt %)
0.92
0.71
0.46
0.50
100
100
106
106
100
100
112
112
41.00
42.22
43.39
42.76
0.133
0.142
0.131
0.145
Base oil +ADTPꢀ1 (1 wt %)
Base oil + ADTP (0.25 wt %) +
TDS (0.75 wt %)
Base oil + ADTP (0.75 wt %) +
TDS (0.25 wt %)
0.43
0.43
0.39
106
106
106
112
112
112
43.12
43.44
44.02
0.146
0.135
0.128
Base oil + ADTP (0.5 wt %) +
TDS (0.5 wt %)
Base oil + DFꢀ11 (1 wt %)
5. V.A. Zolotov, O.P. Parenago, R.V. Bartko, et al., RU
Patent No. 2442820 (2010); Byull. Izobret., No. 5
(2012).
Chosen for further testing of the compositions with
various formulas was the salt ADTPꢀ1 as having the
best antiwear performance in terms of linear wear of
the metal surface (Table 6).
6. A. V. Zolotov, O. P. Parenago, R. V. Bartko, et al., Trenie
Smazka Mashin. Mekhanizm., No. 3, 16 (2011).
The results showed that the antifriction properties
of the composition and its components are comparaꢀ
ble with those of the initial mineral oil. The extremeꢀ
pressure activity also did not reveal the benefits of the
composition. The antiwear effect is displayed to the
greatest extent in the case of fifty–fifty ratio between
the components of the additive, and is comparable
with that of zinc dithiophosphate; however, unlike the
latter, the composition is metalꢀfree, i.e. is ashless, and
the ratio of sulfur to phosphorus atoms is nearly three
times below that of DFꢀ11.
7. S. B. Borshchevskii, E. V. Shebanova, A. A. Markova,
and I. Yu. Rebrov, Khim. Tekhnol. Topl. Masel, No. 10
(1984).
8. I. V. Shkhiyants, E. I. Markova, M. V. Voevoda, et al.,
Neftekhimiya 11, 919 (1971).
9. R. G. Shelkova, G. N. Kuz’mina, P. I. Sanin, and
O. P. Parenago, Kinet. Catal., 31, 1094 (1991).
10. A. B. Vipper, O. P. Parenago, V. A. Zolotov, et al.,
Neftepererab. Neftekhim., No. 2, 30 (1998).
11. DIN 51834: Prufung von Schmierstoffen Tribologische
⎯
Prufungen im translatorischen OszillationsꢀPrufgeratꢀ
Teil 3. Bestimmung des tribologischen Verhaltens von
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