The synthesis of novel Schiff base antioxidants to promote anti-thermal aging properties of natural rubber

Article abstract of DOI:10.1007/s11696-017-0142-7

There is of great interest in promotion of anti-thermal aging properties of natural rubber (NR) to improve the applicability. In this study, two novel Schiff base antioxidants (SBAOs) for NR were synthesized utilizing 4-aminodiphenylamine with 3,5-Di-tert

Full text of DOI:10.1007/s11696-017-0142-7

Chem. Pap.
DOI 10.1007/s11696-017-0142-7
ORIGINAL PAPER
The synthesis of novel Schiff base antioxidants to promote
anti-thermal aging properties of natural rubber
Hong-Yan Zhang1 Kai-Kai Liu2
•
Received: 26 October 2016 / Accepted: 30 January 2017
Ó Institute of Chemistry, Slovak Academy of Sciences 2017
Abstract There is of great interest in promotion of anti-
thermal aging properties of natural rubber (NR) to improve
the applicability. In this study, two novel Schiff base
antioxidants (SBAOs) for NR were synthesized utilizing
4-aminodiphenylamine with 3,5-Di-tert-butyl-2-hydroxy-
benzaldehyde or cinnamic aldehyde in an ethanol medium.
indicates potential applications of SBAOs as efficient
antioxidants for NR.
Keywords Natural rubber vulcanizates Á Schiff base
antioxidant Á Anti-thermal aging Á Mechanical properties Á
Thermal oxidative stability
1
IR, 13C-NMR, and H-NMR confirmed the structures of
SBAOs. Addition of SBAOs improved the rheometric
properties, mechanical properties and thermal oxidative
stability of NR vulcanizates. Introduction of SBAOs in NR
increased the apparent activation energy of thermal
oxidative degradation according to Kissinger and FWO
methods. Anti-thermal aging performance of SBAOs for
NR is related to the structures. The C=N double bonds in
SBAOs improve the electron density of Ar–OH and/or Ar–
NH–Ar structures, benefiting the release of active hydro-
gen. The active hydrogen could capture free radicals ini-
tiated during the thermal oxidative aging process. The lone
pair electrons on nitrogen atom are also beneficial to delay
or terminate free radical reaction. NR with SBAOs showed
high mechanical properties of the tensile strength, tensile
stress at 100% elongation and Shore A hardness compared
to commercial BHT and 4010 during aging 96 h. It
Introduction
Natural rubbers (NR) are easily attacked by heat, light, and
oxygen/ozone initiated free radicals (Mcdonel and Shelton
1959; Li and Koening 2003), causing deteriorated
mechanical properties during the processing or application
(Mcdonel and Shelton 1959; Li and Koening 2003). Like
other radical reaction, the thermal oxidative degradation of
NR proceeds by a free radical chain mechanism, where
alkyl radical (RÁ) and oxygen centered radicals (ROOÁ,
ROÁ) cause chain initiation and growth. It is necessary to
decrease or inhibit degradation by introducing antioxidants
during the processing. The role of antioxidants lies in their
reaction with oxygen centered radicals or alkyl radical to
break the normal radical chain propagation. The hindered
phenol and hindered amine antioxidants (HPAOs and
HAAOs) are two major categories of conventional
antioxidants (Al-Ghonamy et al. 2010; Herdan et al. 1995;
Ismail et al. 1999). The HPAOs, such as styrenated phenol,
2,20-methylenebis (6-tert-butyl-4-methylphenol) and 2,6-
Di-tert-butyl-4-methylphenol, have been commercially
applied in NR (Wadelin 1956; Howard and Ingold 1962;
Malshe et al. 2006). HPAOs break chain radical reactions
by the active hydrogen in phenol (Ar–OH) reacting with
ROOÁ to aryloxy radical (Ar–OÁ) and peroxy compounds
(ROOH), and Ar-OÁ radical continuously capturing ROOÁ
to form non-free radical products (ROO–O–Ar). In many
Electronic supplementary material The online version of this
article (doi:10.1007/s11696-017-0142-7) contains supplementary
material, which is available to authorized users.
& Hong-Yan Zhang
daneycets@126.com
1
Department of Science and Technology, China University of
Political Science and Law, Beijing 102249, China
2
State Key Laboratory of Heavy Oil Processing, China
University of Petroleum, Beijing 102249, China
123

Chem. Pap.
instances, the HAAOs, including N-phenyl-N0-dimethyl-
butyl-p-phenylenediamine, poly (2-amino pyridine), N-
isopropy1-N0-pheny1-p-phenylenediamine and N,N0-sub-
stituted p-phenylenediamines, showed high antioxidant
activity in comparison with HPAOs (Cataldo 2001, 2002;
Experimental
Materials
The 4-aminodiphenylamine (98 wt %) was purchased from
Aladdin Industries, USA. The 3,5-Di-tert-butyl-2-hydroxy-
benzaldehyde (95 wt%) and cinnamic aldehyde were
obtained from Wuhan Xinhuayuan Fine Chemicals Co., Ltd.,
China. The anhydrous ethanol (Analytical grade) was pro-
vided by Guangzhou Chemical Reagent Co, Ltd., China.
Typical commercial antioxidants of 2,6-Di-tert-butyl-4-
methylphenol (BHT) and N-isopropyl-N0-phenyl-1,4-
phenylenediamine (4010NA) were purchased from Shang-
hai Feige Chemicals Co., Ltd, China and Guangzhou Insti-
tute of Rubber Industry Co., Ltd., China, respectively. The
NR (SVR, Vietnam), vulcanization accelerators CZ and DM
(80 wt%) were provided by Guangzhou Liben Rubber
Materials Co., Ltd., China. The zinc oxide (technical grade)
and sulfur (80 wt%) were purchased from West Long
Chemical Co., Ltd., China. The stearic acid was obtained
from Winner Functional Materials Company, China.
´
Sparks 1967; Cibulkova et al. 2005a, 2005b). Abdel-Bary
et al. found that the antioxidant efficiency of p-
phenylenediamine derivatives increased with the increas-
ing inductive effect of the substituents (Abdel-Bary et al.
1997). Hydrogen free radical produced by homolysis of –
NH– structure in secondary amine antioxidants reacts with
ROOÁ or ROÁ forming stable products. Tertiary amine
antioxidant has a pair of lone pairs of electrons of nitrogen
atom itself. Once free ROOÁ meets it, and the transfer of
electrons delays or terminates the reaction of free ROOÁ.
Development of more types and high-efficient antiox-
idants for NR has still been focused on (Wu et al. 2015;
ˇ
´
Pan et al. 2013; Cerna et al. 2012; Ahmed et al. 2012).
Ismail et al. reported that synthesized seven arylphos-
phites compounds are good antioxidants and antifatigue
agents and their antioxidant efficiency is higher than that
of the commercial 4-methyl-2,6 di-tert-butyl phenol (Is-
mail et al. 2001). Silica-supported 2-mercaptobenzimida-
zole was proved to be an efficient and ‘‘green’’
antioxidant for styrene-butadiene rubber (Zhong et al.
2014). Wu et al. synthesized two novel macromolecular
hindered phenol antioxidants containing thioether and
urethane groups by the combination of thiol-acrylate
Michael addition and nucleophilic addition (Wu et al.
2015). The thioether and urethane groups played an
important role in improving antioxidative efficiency, and
the urethane group connected with benzene ring had
better antioxidative ability than that connected with ali-
cyclic ring.
Synthesis
3.760 g of 4-aminodiphenylamine was entirely dissolved in
30 mL anhydrous ethanol in a 100 mL beaker under
magnetic stirring. The solution was named as Solution-A.
Meanwhile, 4.933 g of 3,5-Di-tert-butyl-2-hydroxyben-
zaldehyde or 2.5 mL cinnamic aldehyde was, respectively,
dissolved in 30 mL of anhydrous ethanol in a 250 mL
three-neck flask under a reflux condenser at a water-bath
temperature of 78 °C. The resulting solutions were named
as Solution-1 and Solution-2, respectively. The Solution-A
was then slowly dropped into Solution-1 or Solution-2 in
the three-neck flask and reacted for 6 h. The appeared
crystals were separated out when the reacted solution was
cooled to room temperature. After filtered and washed with
anhydrous ethanol, the target SBAOs were obtained and
named as SBAO-1 and SBAO-2, respectively (Fig. 1). The
yields of SBAO-1 and SBAO-2 were 70.9 and 68.3%,
respectively. The commercial antioxidants of BHT and
4010 NA were simultaneously investigated as comparative
studies. The Fourier transform infrared spectroscopy
(FTIR) and nuclear magnetic resonance (NMR) were used
to analyze the structure of SBAOs on a TENSOR 27 FTIR
spectroscope (Bruker, German) and an AVANCE III NMR
spectrometer (Bruker, German), respectively.
Developing the antioxidants composing multifunctional
reactive groups of the structures is a reasonable orientation.
Actually, the Schiff base antioxidants (SBAOs), which
generally are synthesized by condensation of amines with
an active carbonyl group, have shown great potential in
promoting aging resistance of NR during early research
(George et al. 1993). However there is lack of further
investigation on SBAOs.
In the present study, two novel Schiff base antioxidants
SBAOs were synthesized utilizing an efficient hindered
amine and different hindered phenols. The hindered phenol
and hindered amine in the SBAO were bonded by carbon–
nitrogen (C=N) double bonds. The advantages of HPAOs
and HAAOs were expected to coexist in the SBAOs. The
influence of SBAOs on the rheometric characteristics,
mechanical properties and thermal oxidative stability of
NR vulcanizates was investigated. Thermal oxidative
degradation kinetics were further studied to analyze the
anti-thermal aging performance of SBAOs.
Measurement methods
The anti-thermal aging properties of SBAOs were evalu-
ated by measuring the rheometric properties, mechanical
123

Chem. Pap.
Fig. 1 The synthesis routes of SBAO-1 (a) and SBAO-2 (b)
properties and thermal oxidative stability of the SBAOs
added NR vulcanizates. The prescription of NR vulcan-
izates was given below: 100 g of NR, 1.875 g of acceler-
ator CZ, 0.625 g of accelerator DM, 1 g of stearic acid, 5 g
of zinc oxide, 1.875 g sulfur and 2.0 g of antioxidants.
After shaping on an S(X) K-160A rubber open two-roll
mill (Wuxi Wantailong, China), the rubber compounds
were vulcanized on a XL13-DXS flat vulcanizing machine
(Shanghai Yayue, China). The prepared NR vulcanizates
by adding SBAO-1, SBAO-2, BHT and 4010NA were
denoted as NRV-1, NRV-2, NRV-3 and NRV-4, respec-
tively. No antioxidant added NR vulcanizate was NRV-0.
Another prescription of NR vulcanizates containing addi-
tional 50 g hard carbon black in above mixing process was
tested to further investigate the properties of SBAOs.
Similarly, they were named as: CNRV-1, CNRV-2,
CNRV-3 and CNRV-4, with SBAO-1, SBAO-2, BHT and
4010NA antioxidants, respectively.
according to the Flynn–Wall–Ozawa (FWO) method
´
(Nun˜ez, et al. 2000).
Results and discussion
IR spectra of SBAO-1 showed the typical absorption peaks
of –OH (3406.17 cm-1), CH3 (2962.56 cm-1), symmetric
CH2 stretching vibration (2906.62 cm-1), asymmetric CH2
stretching vibration (2868.05 cm-1) and C=N (1604 cm-1
)
(Fig. 2a). 13C-NMR spectra of SBAO-1 exhibited 19 car-
bon lines (Fig. 2b). The chemical shifts (d) at 29.56 and
31.63 ppm were assigned to two kinds of primary carbon
peaks, while those at 34.30 and 35.20 ppm were ascribed to
two types of tertiary carbon peaks. The d values at
118.18–158.19 ppm, 161.15 and 77.2 ppm were attributed
to 14 types of carbons on the benzene ring, the carbon of
C=N and solvent CDCl3 carbon peaks, respectively.
Besides, the distortionless enhancement by polarization
transfer (DEPT) spectra (135°) of SBAO-1 displayed CH3
peaks (d = 29.56 and 31.63 ppm), CH peak in C=N
(d = 161.15 ppm) and 7 types of CH on the benzene ring
(d = 118.2687–129.6339 ppm), respectively (Fig. 2c).
The thermal oxidative aging of NR vulcanizates was
carried out on a UA-2071A aging oven tester (U-CAN,
Taiwan, China) at 100 °C and different lengths of time
based on GB/T3512-2001 (China national standard). The
rheometric properties were measured using a UR-2010SD-
A rheometer (U-CAN, Taiwan, China) at 150 °C according
to GB/T9869-1997 (China national standard). The
mechanical properties were analyzed on a UT-2080 elec-
tronic tensile testing machine (U-CAN, Taiwan, China) at a
stretching rate of 500 mm/min in accordance with GB/
T528-1998 (China national standard). The Shore A hard-
ness was recorded on an LX-A shore0s durometer
(Shanghai Liuling, China) according to GB/T531-1999
(China national standard). The thermal gravimetric analysis
(TGA) was used to analyze the thermal oxidative stability
on a TG209 F3 thermal gravimetric analyzer (NETZSCH,
German) in an air flow of 50 mL/min. Consequently, the
thermal oxidative degradation kinetics was calculated
1
The typical H-NMR spectra of SBAO-1 (Fig. 2d) showed
proton peaks in tert-butyl carbon (d = 1.48 and 1.38 ppm),
–NH– (d = 5.76 ppm), –N = CH– (d = 8.64 ppm), ben-
zene ring (d = 6.90, 7.09, 7.24 and 7.45 ppm), solvent
CDCl3 (d = 7.25 ppm) and the associated –OH
(d = 13.91 ppm). Integral hydrogen peak areas (S) in tert-
butyl carbon, –NH–, –N=CH– and benzene ring at
(d = 6.90, 7.09, 7.24 and 7.45 ppm) were 9, 1, 1 and 11 (1,
4, 5 and 1), respectively. Similarly, the characterization
results of SBAO-2 are also summarized in Table 1. The
SBAOs were successfully synthesized according to the
results of IR, 13C-NMR, DEPT (135°) and 1H-NMR
spectra.
123

Chem. Pap.
1.0
0.8
0.6
0.4
0.2
1.0
0.9
0.8
0.7
0.6
3033.92
2868.05
2906.62
3288.52
1625.94
3406.17
2962.56
1604.72
2000 1500
SBAO-1
SBAO-2
4000
3500
3000
2500
1000
500
4000
3500
3000
2500
2000
1500
1000
500
-
1
-1
Wavenumber (cm )
Wavenumber (cm )
a
b
c
d
1
Fig. 2 IR (a), 13C-NMR (b), DEPT (c) and H-NMR (d) spectra of SBAO-1 and SBAO-2
123

Chem. Pap.
Table 1 Characterization results of synthesized SBAO-2
Characterization
methods
Peak position Structure ascription
Peak position
Structure ascription
IR spectra
3288.52 cm-1 Secondary amines –NH–
1625.94 cm-1 C=N double bonds
3033.92 cm-1
C=C double bonds
Solvent CDCl3
13C-NMR spectra (15 158.9710 ppm C=N
carbon lines)
d = 77.2 ppm
DEPT spectra (135°) 158.9710 ppm CH peak in C=N
135.9628, 142.0811, 143.0482 Four quaternary carbon peaks
and 144.5897 ppm
Others
CH in the double bonds and
the benzene ring
–NH–, S = 1
C=C, S = 1
1H-NMR spectra (18
proton peaks)
5.81 ppm
8.83 ppm
C=N, S = 1
6.94 and
7.35 ppm
7.11, 7.20, 7.28, 7.39 and
7.53 ppm
Benzene ring proton peak; S = 6, 2,
2 and 2, respectively
7.25 ppm
Solvent CDCl3
7.25 ppm
Solvent CDCl3
The mechanical properties of NRV and CNRV samples
by adding various antioxidants under thermal oxidative
aging are illustrated in Figs. 3 and 4, respectively. Tensile
strength of VNR-0 and CNRV-0 gradually decreased at
aging from 0 to 96 h. Tensile strength of VNR samples
with antioxidants firstly increased at aging 24 h and 48 h,
and then gradually decreased at aging times (Fig. 3a). The
increased tensile strength at aging 24 and 48 h may be
probably ascribed to the promoted internal crosslinking
density of rubber induced by antioxidants during the aging
process. Internal crosslinking of molecules of NR contin-
uously changes with various aging time and the maxi-
mization happened at aging 24 h for NRV with
antioxidants according to the previous investigation (Li
et al. 2015). Differently, tensile strength of CNRV samples
gradually decreased at aging from 0 to 96 h (Fig. 4a). The
hard carbon black in CNRV blocks internal crosslinking of
rubber and weakens its influence on tensile strength. The
elongation at break declined with the increase of aging
time for VNR and CNRV (Figs. 3b, 4b). The tensile stress
at 100% elongation and shore A hardness increased as
aging time increased for VNR (Fig. 3c, d) and CNRV
(Fig. 4c, d). After aging 96, NRV-1 (CNRV-1) and NRV-2
(CNRV-2) displayed higher tensile strength, tensile stress
at 100% elongation and Shore A hardness than NRV-0
(CNRV-0), revealing anti-aging effects of SBAO-1 and
SBAO-2 presence. Besides the tensile strength, tensile
stress at 100% elongation and Shore A hardness of NRV-3
(CNRV-3) and NRV-4 (CNRV-4) were lower than those of
NRV-1(CNRV-1) and NRV-2 (CNRV-2) at aging 96 h.
The respective advantages of SBAO-1 and SBAO-2
antioxidants are revealed compared to commercial BHT
and 4010NA antioxidants.
Table 2 Rheometric properties of NR vulcanizates by adding various
antioxidants
ML(dNm) MH(dNm) tC90(min) tS1(min) tS2(min)
VNR-0
VNR-1
VNR-2
VNR-3
VNR-4
0.43
0.40
0.40
0.43
0.42
4.15
3.87
4:57
5:22
4:59
4:50
4:41
5:26
5:21
4:40
5:21
5:07
2:33
3:04
2:50
2:38
2:33
1:27
1:31
1:15
1:27
1:23
3:04
3:40
3:24
3:11
3:01
1:51
1:55
1:36
1:53
1:48
4.03
4.09
4.30
CVNR-0 1.49
CVNR-1 1.52
CVNR-2 1.68
CVNR-3 1.32
CVNR-4 1.58
12.23
11.43
12.11
11.84
12.38
The rheometric properties of NR vulcanizates by adding
various antioxidants are listed in Table 2. The NRV-1 and
NRV-2 showed low minimum torque (ML) and maximum
torque (MH) compared to NRV-0 and NRV-3 and NRV-4.
The decreased ML by adding SBAOs suggests the
improved fluidity. The reduced MH is related to weakened
crosslinking density of NRV samples before aging. The
addition of SBAOs caused long vulcanization time (tC90
)
and scorching time (tS1 and tS2), thus improving the safety
during manufacturing of NRV samples. The newly syn-
thesized SBAOs exhibited better performance on promot-
ing rheometric properties of NRV samples relative to
commercial BHT and 4010 NA. As for CNRV samples,
increase of ML and decrease of tC90 reveals weakened
mobility and shortened the vulcanization time, respec-
tively. Introduction of SBAO-1 improves operational
safety relative to that of SBAO-2 in CNRV samples. For
various prescription of NR vulcanizates (NRV and CNRV),
SBAOs show their advantages on the improvement of
rheometric properties.
Further investigation on thermal oxidative stability and
dynamics was performed according to NRV samples. The
TGA curves (in air) of NR vulcanizates by adding various
123

Chem. Pap.
a
b
750
700
650
600
550
500
450
400
350
NRV-0
NRV-1
NRV-2
NRV-3
NRV-4
26
24
22
20
18
16
14
12
NRV-0
NRV-1
NRV-2
NRV-3
NRV-4
0
24
48
72
96
0
24
48
72
96
Thermal oxidative aging time (h)
Thermal oxidative aging time (h)
d
c
NRV-0
46
44
42
40
38
36
34
NRV-0
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
NRV-1
NRV-2
NRV-3
NRV-4
NRV-1
NRV-2
NRV-3
NRV-4
0
24
48
72
96
0
24
48
72
96
Thermal oxidative aging time (h)
Thermal oxidative aging time (h)
Fig. 3 Tensile strength (a), elongation at break (b), tensile stress at 100% elongation (c) and Shore A hardness (d) of NRV samples by adding
various antioxidants during thermal oxidative aging
antioxidants are illustrated in Fig. 5 and temperatures at
various weight losses are listed in Table 3. The TGA
curves of NR vulcanizates showed two stages. The stage of
250–410 °C probably represented the degradation of
organic components in NR, and the stage of 410–540 °C
presumably signified the pyrolysis of NR vulcanizates. The
addition of four antioxidants significantly promoted the
initial degradation temperatures (T0), maximum degrada-
tion temperature (Tmax), and degradation temperatures
(T0.1, T0.15, T0.2, and T0.3) at various weight losses of 10,
15, 20 and 30% for NR vulcanizates, respectively. It
reveals that the function of antioxidants is in delaying
degradation of NR vulcanizates in thermal oxidative pro-
cess. Besides, degradation temperatures gradually
increased from T0 to Tmax for NRV-0. However, for NRV-
1, NRV-2, NRV-3, and NRV-4 vulcanizates, degradation
temperatures displayed two stages: a little increase or
decrease from T0 to T0.1, and subsequent quick increase. T0
(Tmax) increased by 8.7 °C (6 °C), 18.6 °C (12.8 °C),
4.8 °C (2.3 °C) and 12.4 °C (26 °C) for NRV-1, NRV-2,
NRV-3, and NRV-4, respectively, relative to that for NRV-
T0.2, and T0.3 increased by 4.9–6.8 °C, 6.9–10.8 °C,
0.1–3.5 °C, and 4.7–8.1 °C for NRV-1, NRV-2, NRV-3,
and NRV-4, respectively, compared to that for NRV-0. In
contrast to the degradation temperatures of NRV-0 at
various weight losses, the largest increase of degradation
temperature of NRV-1, NRV-2, and NRV-3 occurred at T0,
whereas that of NRV-4 happened at Tmax (Table S1). For
NRV-1, NRV-2, and NRV-3 with SBAO-1, SBAO-2, and
BHT antioxidants, its degradation is difficult at initial
stage, and comparatively easy at subsequent stage. For
NRV-4 with 4010NA antioxidant, its degradation is as
follows: difficult at initial stage, comparatively easy at
intermediate stage, and harder at later stage. Probably, the
divergence is due to different mechanism of action of
various antioxidants.
NRV-1 and NRV-2 showed higher degradation tem-
peratures at various weight losses than NRV-3 vulcanizate.
It reveals that SBAO-1 and SBAO-2 significantly delay the
degradation of NR vulcanizate in thermal oxidative process
compared to BHT. Besides, SBAOs and 4010NA antioxi-
dants show their respective advantages. Anti-thermal-ox-
idative aging of SBAO-1 is likely ascribed to the structures
0 (Table S1). The degradation temperature of T0.1, T0.15
,
123

Chem. Pap.
a
b
30
28
26
24
22
20
18
16
14
12
10
550
500
450
400
350
300
250
200
CNRV-0
CNRV-1
CNRV-2
CNRV-3
CNRV-4
CNRV-0
CNRV-1
CNRV-2
CNRV-3
CNRV-4
0
24
48
72
96
0
24
48
72
96
Thermal oxidative aging time (h)
Thermal oxidative aging time (h)
d
c
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
76
72
68
64
60
56
CNRV-0
CNRV-1
CNRV-2
CNRV-3
CNRV-4
CNRV-0
CNRV-1
CNRV-2
CNRV-3
CNRV-4
0
24
48
72
96
0
24
48
72
96
Thermal oxidative aging time (h)
Thermal oxidative aging time (h)
Fig. 4 Tensile strength (a), elongation at break (b), tensile stress at 100% elongation (c) and Shore A hardness (d) of CNRV samples by adding
various antioxidants during thermal oxidative aging
Ar–OH structure. As an electron donating group, the C=N
NRV-0
double bonds improved the electron density of Ar–NH–Ar
NRV-1
structures, promoting the release of active hydrogen to
NRV-2
NRV-3
NRV-4
capture more free radicals. Besides, the lone pair electrons
on nitrogen atom are also beneficial to delay or terminate
free radical reaction.
The thermal oxidative degradation kinetics were studied
to analyze the anti-thermal oxidative aging of SBAOs. The
addition of four antioxidants in NRV samples all increased
the apparent activation energy according to the Kissinger
and FWO methods (Fig. 6). Actually, the apparent acti-
vation energy by Kissinger is calculated by differential
calculation at the maximum degradation temperature.
According to the Kissinger method, the apparent activation
energy of NRV samples increased as follows: NRV-0
0
200
400
600
800
Temperature (䉝)
Fig. 5 TGA curves of NR vulcanizates by adding various
antioxidants
(97.11 kJ/mol) \ NRV-3
(108.93 kJ/mol) \ NRV-4
(107.84 kJ/mol) \ NRV-1
(154.69 kJ/mol) \ NRV-2
of both Ar–OH and Ar–NH–Ar. Both structures provide
more active hydrogen under the steric effect of benzene
rings. Anti-thermal-oxidative aging of SBAO-2 probably
benefits from the structures of Ar–NH–Ar, C=N double
bonds, and the interaction between them despite the lack of
(156.99 kJ/mol) (Fig. 6a). The difference of apparent
activation energy among NRV-1, NRV-2, NRV-3, and
NRV-4 is probably related to different mechanisms of
action of antioxidants. By FWO method, apparent activa-
tion energy decreased and subsequently increased for NRV
123

Chem. Pap.
Table 3 Temperatures at
various weight losses of NR
vulcanizates during TGA testing
NR vulcanizates
T0 (°C)
Temperature at various weight loss (wt %)
Tmax (°C)
10 (T0.1
)
15 (T0.15
)
20 (T0.2
)
30 (T0.3)
NRV-0
NRV-1
NRV-2
NRV-3
NRV-4
310.4
319.1
329.0
315.2
322.8
314.5
319.4
321.4
314.6
319.2
322.8
329.3
333.0
323.5
328.7
330.5
337.3
341.3
331.5
337.1
343.9
349.4
352.8
347.4
352.0
347.6
353.6
360.4
349.9
373.6
150
140
130
120
110
100
90
160
a
b
140
120
100
80
60
NRV-0
NRV-1
NRV-2
NRV-3
NRV-4
40
20
0
0.05
0.10
0.15
0.20
0.25
0.30
NRV-0
NRV-1
NRV-3
NRV samples
NRV-4
NRV-2
Weight loss(wt%)
Fig. 6 Apparent activation energy of NRV samples: Kissinger method (a) and FWO method (b) at various weight mass losses during TGA
testing
samples. The former probably corresponds to the auto-ac-
celeration of thermal oxidation process, and the latter is
likely due to decomposition of thermal oxidation products
combining with intensified thermal oxidation process.
NRV-1 and NRV-2 showed higher apparent activation
energy than NRV-3 and NRV-4 at various weight loss,
suggesting the better thermal oxidative stability of SBAOs
added NRV samples. It also indicates the potential appli-
cations of SBAOs as efficient antioxidants for NR.
density of Ar–OH and/or Ar–NH–Ar structures, promot-
ing the release of active hydrogen. The active hydrogen
could capture free radicals initiated during the thermal
oxidative aging process. The lone pair electrons on
nitrogen atom are also beneficial to delay or terminate the
free radical reaction. NR with SBAOs showed high
mechanical properties of the tensile strength, tensile stress
at 100% elongation and Shore A hardness compared to
commercial BHT and 4010 during aging 96 h. This study
indicated potential applications of SBAOs as efficient
antioxidants for NR.
Conclusions
Acknowledgements This project was supported in part by Academic
Innovation Team foundation of China University of Political Science
and Law (No. 1000-10814340).
Two novel Schiff base antioxidants (SBAOs) for NR were
synthesized utilizing 4-aminodiphenylamine with 3,5-Di-
tert-butyl-2-hydroxybenzaldehyde or cinnamic aldehyde
in an ethanol medium. The structures of SBAOs were
characterized by IR, 13C-NMR, and 1H-NMR Rheometric
properties, mechanical properties and thermal oxidative
stability of NR vulcanizates were improved due to the
addition of SBAOs antioxidants. Introduction of SBAOs
in NR increased the apparent activation energy of thermal
oxidative degradation according to the Kissinger and
FWO methods. The anti-thermal aging performance of
SBAOs for NR vulcanizates lies in the structures. The
C=N double bonds in SBAOs improve the electron
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