This document describes the design and synthesis of novel antioxidant additives called Schiff base bridged phenolic diphenylamines (SPDs) for lubricating oils. SPDs were designed to combine sterically hindered phenol and diphenylamine moieties into a single molecule using a Schiff base bridge. Two SPD compounds were synthesized and shown to have better thermal stability and antioxidant efficiency than commercial antioxidants. Formulations containing SPDs along with other additives also exhibited improved antioxidant performance at high temperatures, demonstrating their potential for use in the lubricant industry.

1. Schiff Base Bridged Phenolic Diphenylamines for Highly Efficient and
Superior Thermostable Lubricant Antioxidants
Shasha Yu,* Jianxiang Feng, Tao Cai, and Shenggao Liu*
Polymers and Composites Division, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, No.
1219 Zhongguan West Road, Ningbo 315201, China
*S Supporting Information
ABSTRACT: Normally, the sterically hindered phenols and diphenylamines, used as antioxidants, are added into the base oil
simultaneously in order to improve the antioxidant efficiency. In view of this, we have designed a new type of antioxidant which
combined the two traditional antioxidants into one molecule with Schiff base. These novel Schiff base bridged phenolic
diphenylamine (SPD) antioxidants have been prepared through a one-step synthesis method which is suitable for large-scale
production. The thermal stability and antioxidant ability were examined by thermal gravimetric analyzer (TGA) and pressurized
differential scanning calorimetry (PDSC), respectively. The results showed that they both had better thermal stability and
antioxidant efficiency than the commonly used commercial antioxidants. Importantly, the formulation of SPDs with other
additives also exhibited improved antioxidant performances, presenting good application potential in the lubricant industry.
■ INTRODUCTION
Lubricating oil, with numerous functions such as antiwear,
antifriction, heat dissipation, seal, and damping, has been widely
used in industry. However, autoxidation easily occurs to form
harmful species which can shorten its service life and even worse
damage the machinery the oil lubricates.1
For the prevention of
lubricant oxidation, antioxidants are the key additives incorpo-
rated in lubricant formulations to protect oils from oxidative
degradation and prolong its service life.
Phenols and diphenylamines (DPAs) are two of the most
widely used antioxidants incorporated in lubricant formulations.
Phenols, especially the sterically hindered phenols, are always
low toxicity and environmentally friendly which make them
prevalent in automotive lubricating oils,2
but the low operation
temperature limits its application in harsh conditions.3−5
Diphenylamines have better performance at elevated temper-
ature, and thus, they are normally employed for workhorse
machines. But the antioxidant performance still needs to be
improved.6−8
However, with the rapid progress of society, high-
speed, high-power, and heavy-loading with small size are
becoming the development trend of machines. This design
manner greatly increased the operation pressure and temper-
ature of a lubricant system thus accelerating the oxidative
degradation of oil.9
Hence, to design and develop novel
antioxidants with superior performance at high temperatures is
becoming an urgent project.
It is worth noting that in some formulations the sterically
hindered phenols and DPAs were added to the oil simulta-
neously. This physical mixing can remarkably enhance the
antioxidant efficiency due to their intermolecular synergistic
effect.10−13
This pleasant result encouraged us to design an
antioxidant in a new way which is to combine the sterically
hindered phenol and diphenylamine moieties into one molecule
to form a bifunctional antioxidant. First, theoretically speaking,
the increasing molecular weight could result in better thermal
stability at elevated temperatures.14
Second, except for the
intermolecular synergistic effect between phenolic moiety and
aminic moiety, the intramolecular synergistic effect may also exist
to further improve the antioxidant performance.15
Third, it can
reduce the amount of additives operated in formulations.
However, to our best knowledge, only few such types of
antioxidants have been reported.15
Recently, Schiff bases have gained great attention as lubricant
additives due to their easy preparation, high performance, and
Received: January 22, 2017
Revised: February 28, 2017
Accepted: March 29, 2017
Published: March 29, 2017
Research Note
pubs.acs.org/IECR
© XXXX American Chemical Society A DOI: 10.1021/acs.iecr.7b00313
Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

2. environmental friendliness.16−25
Most of the work that has been
done is to connect phenol and Schiff base into one molecule, and
the obtained antioxidant is more effective than the corresponding
phenol.26−29
These highly active phenolic Schiff bases could
display multifunctional properties after combining with other
fragments such as thiourea,30
chitosan,31,32
and amino acid
derivatives.33−36
Inspired by these works, we considered
introducing Schiff base as a bridge to combine sterically hindered
phenol and diphenylamine into one molecule (Scheme 1).
Herein, we report the design and synthesis of two different
Schiff base bridged phenolic diphenylamine (SPD) antioxidants.
Their thermal stability and antioxidant ability were examined by
thermal gravimetric analyzer (TGA) and pressurized differential
scanning calorimetry (PDSC), respectively. The results showed
that they have better thermal stability than traditional ones.
Moreover, they exhibited not only improved antioxidant
performance themselves but also good compatibilities with
other additives at elevated temperatures.
■ EXPERIMENTAL SECTION
Materials. Reagents and solvents were purchased from
commercial sources and used without further purification unless
otherwise indicated. Three kinds of mineral base oils (150SN,
150N, and HVIP) are used in industry with different brands.
PAO is a synthetic base oil. According to the American
Petroleum Institute (API), 150SN, 150N, HVIP, and PAO
(polyalpha olefin) belong to group I, group II, group III, and
group IV, respectively. 150SN and HVIP were purchased from
China National Petroleum Corporation. 150N was purchased
from China National Offshore Oil Corporation. PAO was
purchased from INEOS (England). Hydraulic oil was purchased
from Zhejiang Luby Wanling oil company (Ningbo, China).
Commercial antioxidants BHT, DPA, Irganox 1076, and ODA
were purchased from Aladdin Reagent Company (Shanghai,
China). The preparation of borated PIBSI (5) was performed
according to a patent method.37
The boron linked to the polymer
(PIBSI) possibly in the form of “B−N” or “B−O” bonds, and the
boran content was 1.626%. The preparation of molybdated
PIBSI (6) was performed according to a patent method.38
The
molybdenum linked to the polymer (PIBSI) possibly in the form
of “Mo−N” or “Mo−O” bonds, and the molybdenum content
was 5.834%. The preparation of zinc dialkyldithiophosphate
(ZDDP) was performed according to a patent method,39
and the
phosphorus content was 7.8%.
Scheme 1. Design of SPD Antioxidants
Scheme 2. Synthesis Process of SPDs
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3. Characterization. All 1
H and 13
C NMR spectra were
recorded at room temperature in CDCl3 (containing 0.03%
TMS) or in DMSO-d6 on a Bruker AV/ANCE-400 MHz
spectrometer. 1
H NMR spectra were recorded with tetrame-
thylsilane (δ = 0.00 ppm) or the residual solvent peak of DMSO-
d6 (δ = 2. 50 ppm); 13
C NMR spectra were recorded with CDCl3
(δ = 77.00 ppm) or the residual solvent peak of DMSO-d6 (δ =
39.52 ppm) as the internal reference. High-resolution mass
spectra were obtained by using Waters Micromass GCT or
Agilent Technologies 6224 TOF LC/MS mass spectrometers.
The element contents were obtained by using PE Optima
2100DV ICP-OES instrument. Melting point was obtained by
using a Mettler Toledo DSC instrument. IR spectra were
obtained by using a Nicolet iS10 spectrometer. Single crystal X-
ray diffraction data were collected at 130 k for SPD1 on a Bruker
APEX-II diffractometer. The sulfur content in the base oil was
obtained by using a Guochuang TS-300 ultraviolet fluorescence
sulfur measuring instrument.
Synthesis Process and Structural Characterization of
Schiff Base Bridged Phenolic Diphenylamines. The two
different SPDs were prepared by the route schematically shown
in Scheme 2. Briefly, N1
-phenylbenzene-1,4-diamine (50 mmol),
aldehyde (50 mmol), and catalyst were stirred at 60 °C under an
inert atmosphere for 3 h. After cooling to room temperature, a
precipitate was obtained. The solid was collected by filtration and
recrystallized from the appropriate solvents to give the pure
product.
(E)-2,4-di-tert-Butyl-6-(((4-(phenylamino)phenyl)-
imino)methyl)phenol (SPD1). 3,5-di-tert-Butyl-2-hydroxy-
benzaldehyde was used as the starting aldehyde. Product: yellow
solid. Isolated yield: 74%. m.p.: 142.7−144.3 °C. 1
H NMR (400
MHz, DMSO-d6) δ 1.30 (s, 9H), 1.42 (s, 9H), 6.85−6.89 (m,
1H), 7.11−7.15 (m, 4H), 7.25−7.29(m, 2H), 7.35 (d, J = 2.4 Hz,
1H), 7.38−7.40 (m, 2H), 7.45 (d, J = 2.4 Hz, 1H), 8.39(s, 1H),
8.95(s, 1H), 14.21(s, 1H). 13
C NMR (100 MHz, DMSO-d6) δ
29.27, 31.28, 33.88, 34.58, 116.75, 117.29, 118.53, 120.17,
122.39, 126.48, 127.00, 129.23, 135.61, 139.10, 139.94, 142.86,
157.28, 160.92, 161.00. IR (film): 690, 716, 746, 773, 804, 833,
864, 874, 1169, 1195, 1242, 1273, 1323, 1360, 1434, 1473, 1490,
1517, 1600, 1615, 2157, 2960, 3406. HRMS (ESI) calcd for
C27H33N2O [M + H]+
: 401.2587, found 401.2589. The structure
of SPD1 was determined by X-ray single-crystal analysis.
(E)-2,6-di-tert-Butyl-4-(((4-(phenylamino)phenyl)-
imino)methyl)phenol (SPD2).40
3,5-di-tert-Butyl-4-hydroxy-
benzaldehyde was used as the starting aldehyde. Product: yellow
solid. Isolated yield: 54%. m.p.: 152.1−153.4 °C. 1
H NMR (400
MHz, CDCl3) δ 1.49 (s, 18H), 5.56 (s, 1H), 5.76 (s, 1H), 6.89−
6.92 (m, 1H), 7.04−7.27 (m, 8H), 7.73 (m, 2H), 8.41(s, 1H).
13
C NMR (100 MHz, CDCl3) δ 30.20, 34.37, 117.28, 118.88,
120.63, 122.04, 125.97, 128.11, 129.33, 136.27, 140.75, 143.47,
146.16, 156.76, 159.02. IR (film): 692, 741, 836, 1153, 1193,
1240, 1328, 1341, 1371, 1497, 1530, 1596, 1620, 2945, 2965,
3259, 3625. HRMS (ESI) calcd for C27H33N2O [M + H]+
:
401.2587, found 401.2589.
Synthesis Process and Structural Characterization of
Phenolic Sulfide 2. A mixture of 2,6-di-tert-butylphenol (2.06
g, 10 mmol), paraformaldehyde (0.6 g, 20 mmol), octane-1-thiol
(1.94 mL, 11 mmol), and dibutylamine (129 mg, 1 mmol) in
DMF (5 mL) was heated at 100 °C for 11 h under an argon
atmosphere. The solvent was evaporated in vacuo, and the
residue was purified by column chromatography on silica gel
(petroleum ether/ethyl acetate = 30/1) to afford the product as a
light yellow oil in 72% yield (2.63 g). 1
H NMR (400 MHz,
CDCl3) δ 0.86−0.89 (m, 3H), 1.26−1.38 (m, 10H), 1.43 (s,
18H), 1.53−1.61 (m, 2H), 2.42−2.46 (m, 2H), 3.66 (s, 2H), 5.12
(s, 1H), 7.09(s, 2H). 13
C NMR (100 MHz, CDCl3) δ 14.07,
22.63, 28.99, 29.19, 29.22, 29.46, 30.29, 31.74, 31.80, 34.28,
36.68, 125.43, 128.93, 135.84, 152.69. IR (film): 769, 885, 1120,
1163, 1229, 1316, 1361, 1433, 2854, 2924, 2955, 3645. HRMS
(EI) calcd for C23H40OS [M]+
: 364.2800, found 364.2791.
Thermogravimetric Analysis (TGA). The experiment was
performed on 5 ± 0.5 mg samples using a TA-Q500
thermogravimetric instrument. The experiments were carried
out under continuous nitrogen flow of 100 mL/min. The
temperature ramp was set at 10 °C/min. The mass loss was
recorded from 30 to 500 °C.
Pressure Differential Scanning Calorimetry (PDSC).
The PDSC test was conducted using a computerized NETZSCH
DSC204 HP instrument (Bavarian, Germany). Oxidation
induction time (OIT) was measured using isothermal PDSC.
Here, 3.0 ± 0.2 mg of oil sample was placed in a hermetically
sealed aluminum pan with a pinhole lid for interaction of the
sample with the reactant gas (high-purity oxygen). Oil samples
were heated from 50 °C to the test temperature at a heating rate
of 50 °C/min before being held in an isothermal mode. After 2
min of heat preservation, the oxygen (flow of 100 mL/min) was
added in until the pressure was 3.5 MPa. When an exothermic
peak of oxidation appeared, the test was finished. Oxidation
induction time (OIT) was measured from the start of the oxygen
added to the start of the exothermic peak.
EPR Measurements. In a typical experiment, deoxygenated
benzene solutions containing the SPDs (0.01 M) and di-tert-
butyl peroxide (10 vol %) were sealed under nitrogen in a
Suprasil quartz EPR tube. The sample was inserted at room
temperature into the cavity of an EPR spectrometer and
photolyzed with the unfiltered light from an 80 W high pressure
mercury lamp for 8 min. The EPR spectra were recorded at
regular intervals on a Bruker EMX-10/12 spectrometer by using
the following settings: microwave frequency 9.77 GHz, power
5.02 mW, modulation amplitude 0.7 G, center field 3480 G,
sweep time 84 s, and time constant 41 ms.
■ RESULTS AND DISCUSSION
The SPD antioxidants can be conveniently prepared by the
condensation reaction of N1
-phenylbenzene-1,4-diamine and the
corresponding aldehydes. Antioxidant SPD1 was characterized
by single crystal X-ray diffraction.41
The structure analysis of
SPD1 shows that there is an intramolecular hydrogen bond
between the phenolic hydroxyl group and the nitrogen atom of
the imine moiety.
The thermal stability of the traditional antioxidants and the
synthesized compounds were examined by thermal gravimetric
analyzer (TGA), shown in Figure 1. The 5% weight loss
temperature of BHT and DPA is 88 and 108 °C and undergoes
nearly complete decomposition at 146 and 172 °C, respectively.
The poor thermal stability limits their usage at high temperature
(>120 °C). Commonly used high-temperature antioxidants
ODA (bis(4-octylphenyl)amine) and Irganox 1076 (octadecyl 3-
(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) performed with
better thermal stability with the 5% weight loss temperature of
209 and 250 °C, respectively. However, SPD1 starts to degrade
5% weight at 252 °C, and the complete decomposition
temperature is up to 363 °C. Meanwhile, the 5% weight loss
temperature of SPD2 achieves 263 °C as well. Both the initial and
the final decomposition temperatures of SPDs are significantly
higher than BHT, DPA, and ODA, similar to Irganox 1076. The
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4. superior thermal stabilities make them potential ideal candidates
as lubricant antioxidants at elevated temperatures.
Their antioxidant activities were further investigated using
isothermal PDSC. The oxidation induction times (OITs)
containing 5 μmol/g of various antioxidants in PAO are shown
in Figure 2. The experiments were performed under 3.5 MPa of
high-purity oxygen in 180 °C which is extremely rigorous for
oxidation of base oil. The OIT of PAO without any antioxidant
was 2.8 min. In contrast, after adding SPD antioxidants to these
oils, the OIT achieved 20.4 min for SPD1 and 24.6 min for
SPD2. They all exhibited a better antioxidant activity than the
commercial antioxidants BHT (14.2 min), DPA (14.2 min),
Irganox 1076 (12.6 min), and ODA (14.9 min).
In order to investigate the intramolecular synergism of the two
SPDs, the antioxidant activity of the simple mixture (BHT
+DPA) was also tested in four different base oils, shown in Figure
3. When adding the mixture of BHT and DPA with the equimolar
amounts to 150N, HVIP, or PAO, their antioxidant activities
were all worse than that of SPD2 but better than that of SPD1.
An intramolecular synergism may probably exist in SPD2’s
solution, thus deeply enhancing the reaction rate between the
antioxidant and the radicals generated from base oil. However, an
intramolecular antagonistic effect may result in the poor
antioxidant efficiency of SPD1. This can be ascribed to the
strong intramolecular hydrogen bond which prevented the
phenol moiety to interact with the radicals.28−30,43−46
Interesting
results were obtained in 150SN. BHT+DPA showed the lowest
OIT of only 23.5 min among the five base oils. Oppositely, SPD
antioxidants obtained the best results and far exceeded that of
BHT+DPA.
The main difference of these oils is their saturated hydro-
carbons content and sulfur content.42
The saturated hydro-
carbons content in 150SN was less than others; thus, it could not
to be the reason for the higher performance. Then, the sulfur
contents of the base oils were tested. The data showed that the
sulfur content of 150SN reached up to 362.0 mg/L, largely
surpassing those of the other base oils which were all less than 7
mg/L (Table 1). We speculate that there may be a strong
interaction between the antioxidants and sulfur compounds.
Thus, four kinds of sulfides 1−4 were selected to investigate the
effect between the two different compounds, shown in Figure 4.
In order to minimize the impact of the active components in base
oil and thus to analyze the antioxidant efficiency more accurately,
PAO was adopted as the tested base oil which is pure without
other active compounds. The tested temperature was increased
to 190 °C so as to shorten the test time. The OITs of all the
sulfides were less than 3 min except phenolic sulfide 2 which was
8.3 min. After adding the sulfides to the solution containing BHT
+DPA, the OITs were all less than the simple sum of BHT+DPA
and the sulfides except benzyl disulfide which indicated that an
antagonistic effect on oxidation stability occurred. On the
contrary, the presence of sulfides in the SPD’s solution
dramatically increased the antioxidant efficiency to a much
greater extent which showed a strong synergistic effect between
the two compounds. For example, when dioctyl sulfide was
added, the oxidation induction time jumped to 45.4 min from
11.5 min for SPD1 and doubled to 35.2 min for SPD2. Best
results were obtained when combining with benzyl disulfide.
Figure 1. TG curves of BHT, DPA, Irganox 1076, ODA, SPD1, and
SPD2.
Figure 2. Oxidation induction time of PAO containing 5 μmol/g
antioxidants including the traditional ones and SPDs.
Figure 3. Comparison between new antioxidants (10 μmol/g) and
mixtures of BHT (10 μmol/g) and DPA (10 μmol/g) in different base
oils using isothermal PDSC (180 °C).
Table 1. Sulfur Content Comparison among Different Base
Oils
base oil 150SN 150N HVIP PAO
sulfur content(mg/L) 362.0 6.6 3.9 <1
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5. OIT achieved a 5.8-fold increase for SPD1 and a 2.6-fold increase
for SPD2, far beyond that of BHT+DPA. Compared to the
mono-sulfide, it seemed that the more sulfur content there is, the
stronger the synergistic effect is.
A possible mechanism of the synergism was shown in Scheme
3. The autoxidation of base oil is well known as a free-radical
chain reaction involving initiation, propagation, branching, and
termination.1,13
Alkyl radical (R•), alkoxy radical (RO•),
hydroxyradical (HO•) and alkyl peroxy radicals (ROO•) were
the most pernicious radicals to accelerate the degradation of base
oil. In this regard, SPDs acted as the radical scavengers donated
hydrogen atoms to terminate ROO• to afford hydroperoxides
(ROOH) so as to inhibit its trend of trapping hydrogen atoms
from hydrocarbon. It can further capture RO• and HO• which is
easily generated by the cleavage of ROOH at elevated
temperatures. When sulfide was added to the oil simultaneously,
Figure 4. Antioxidant response in combination with different sulfides (a: 1; b: 2; c: 3; d: 4) in PAO base oil using isothermal PDSC (190 °C).
Scheme 3. Possible Mechanism of Synergism Between SPD
and Sulfide
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6. the second step (the cleavage of ROOH) would be interrupted
owing to the prominent capability of decomposing hydro-
peroxides by the sulfide. ROOH was reduced to more stable
alcohol (ROH) instead of producing RO• and HO•. On the
other hand, the preserved SPD could continue scavenging
ROO• additionally, thus improving its antioxidant capacity
deeply. In this system, the role of SPD was to trap ROO•, and the
obtained ROOH was then rapidly decomposed by sulfide. Their
synergism significantly slowed the oxidation process of the oils.
This unique excellent performance of the combination suggested
a favorable employment in lubricant oils as formulas.
In order to confirm the radical intermediate of SPD, we treated
them with photolytically generated tert-butoxyl radicals from di-
tert-butyl peroxide in deoxygenated benzene solutions inside the
cavity of an EPR spectrometer.47−49
The results are shown in
Figure 5. In the case of SPD1, only a single paramagnetic species
was obtained with spectral parameters consistent with the
nitrogen radical derived from abstraction of hydrogen atom from
the NH group. This highly persistent radical is characterized by a
Figure 5. Room temperature EPR spectrum observed under continuous irradiation of a benzene solution containing di-tert-butyl peroxide (10% v/v)
and SPDs (0.01 M).
Figure 6. (a) Antioxidant response in combination with borated PIBSI (5) in PAO using isothermal PDSC (200 °C). (b) Antioxidant response in
combination with molybdated PIBSI (6) in PAO using PDSC (210 °C). (c) Antioxidant response in combination with ZDDP in PAO using isothermal
PDSC (200 °C). (d) Antioxidant activity of SPDs in hydraulic oil (HO) using isothermal PDSC (190 °C).
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7. g-factor of 2.0061 and a particular coupling of nitrogen (aN = 10.0
G). The reason why no phenoxyl radical was detected is mainly
due to the poor reactivity of the OH group arising from the
intramolecular hydrogen bonding with the imine nitrogen
atom.44
Thus, the mechanism of antioxidation is possibly similar
to that of DPA, shown in the Supporting Information.
Interestingly, photolysis of SPD2 in the presence of the peroxide
gave rise to a different species, characterized by a particular g-
factor of 2.0049 which is similar to that of reported phenoxyl
radicals.47−49
But this phenoxyl radical was probably not formed
directly, and the reaction underwent a synergisitic mecha-
nism.1,15
Due to the higher reactivity of diphenyl amine
compared to phenol and the greater stability of phenolic radical
relative to nitrogen radical, the tert-butyl radical first abstracted a
hydrogen atom from the diphenyl aminic moiety to afford a
nitrogen radical, followed by a rapid hydrogen shifting from a
phenolic moiety to nitrogen radical. Thus, the more reactive
amine was regenerated continuously and the useful antioxidant
lifetime can be largely extended. In the SPD2’s system, an
intramolecular synergism may also take place to further
accelerate the speed of a hydrogen shifting step compared to
the system of BHT+DPA (only undergoes intermolecular
synergism),1
thus resulting in higher antioxidant activity.
During the process of producing the refined oil, a variety of the
additives with different functions are introduced into the base oil
to give it the desired physical and chemical properties.
Consequently, it is indispensable to investigate the compatibility
of the new compounds with other additives. Three kinds of
additives were studied, and the results are shown in Figure 6.
Borated PIBSI (5) and molybdated PIBSI (6) are used in a wide
range of high quality lubricants recently owing to their excellent
dispersivity and antiwear properties.37,38
But their antioxidant
performances are extremely poor. The DSC curves increased
rapidly at elevate temperatures. Dramatically, the addition of
SPDs along with 5 or 6 can sharply enhance the antioxidant
efficacy. The combination with 6 resulted in superior synergism
with a 20-fold increase for SPD1 and a 4-fold increase for SPD2.
These results also apparently showed that SPD1 performed
better compatibility than SPD2, although it behaved with poorer
antioxidation activity individually. Zinc dialkyldithiophosphate
(ZDDP) is another essential multifunctional additive with
outstanding antioxidant and antiwear properties which has
been used for more than 60 years.50,51
In 200 °C, the OIT of
ZDDP (0.4 wt %) without another additive in PAO can achieve
5.5 min as well. Not surprisingly, the performance was
remarkably improved after adding in SPDs. It is similar to the
previous tendency that the combination of SPD1 and ZDDP
exhibited higher antioxidant efficiency with an OIT of 42.9 min
than that of SPD2/ZDDP (24.2 min). Inspired by these positive
results of the combinations, we finally investigated their
antioxidant activities in the refined oil. Hydraulic oil (HO) was
introduced which contained 1400 ppm of S, 202 ppm of P, and
220 ppm of Zn. In 190 °C, the OIT of the HO is 3.6 min. The
addition of 0.4 wt % SPD can dramatically improve the
antioxidant performance with an OIT of 22.2 min for SPD1
and 18.3 min for SPD2. All these results indicated that SPDs have
good compatibilities with other multifunctional additives
through a strong intermolecular synergistic effect. The activities
of the SPDs as the radical scavengers along with the peroxide
decomposing property of the multifunctional additives provided
improved protection against the oxidation of the base oil.
■ CONCLUSIONS
We have synthesized two types of Schiff base bridged phenolic
diphenylamine (SPD) antioxidants with a simple synthetic
method which is conducive to the amplified production in
industry. They both showed better thermal stability and
antioxidant performance than the traditional ones. In addition,
the superior antioxidant activity in 150SN led us to find that there
is a unique intermolecular synergism between SPDs and sulfides.
Furthermore, the improved antioxidant performances of the
formulations with other additives suggest its potential application
in the lubricant industry as a high-temperature antioxidant.
■ ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.iecr.7b00313.
Mechanism of antioxidation by SPD1, 1
H NMR spectra,
13
CNMR spectra, and MS spectra of SPDs and sulfide 3.
(PDF)
■ AUTHOR INFORMATION
Corresponding Authors
*E-mail: yushasha@nimte.ac.cn (S. Yu).
*E-mail: sliu@nimte.ac.cn (S. Liu).
ORCID
Shasha Yu: 0000-0003-0142-3825
Jianxiang Feng: 0000-0002-9015-3830
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by “Thousand Talents Program”,
National Natural Science Foundation of China (Grant No.
21606247), China Postdoctoral Science Foundation (Grant No.
2016M590555), Zhejiang Postdoctoral Sustentation Fund,
China (Grant No. BSH1502163), and the Natural Science
Foundation of Ningbo (Grant No. 2016A610260).
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