Effect of Graphene Oxide Modified with Organic Amine on the Aging Resistance, Rolling Loss and Wet-Skid Resistance of Solution Polymerized Styrene-Butadiene Rubber

Graphene oxide (GO) was modified by p-phenylenediamine (PPD), aiming at improving the wet-skid resistance and reduce the rolling loss of solution polymerized styrene-butadiene rubber (SSBR). PPD with amino groups enabled GO to obtain anti-aging function. The structure of modified GO (PPD-GO) was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman spectroscopy. Mechanical tests showed that the mechanical properties of SSBR before and after aging were improved by adding PPD-GO. The results of thermogravimetric-differential scanning calorimeter synchronization analysis (TGA-DSC) indicated that SSBR/PPD-GO obtained good thermo-oxidative stability. The dynamic mechanical analysis (DMA) of SSBR composites showed that the mechanical loss factor (tanδ) peak moved to high temperature with the content of PPD-GO. The tanδ values of SSBR composites showed that it had a good effect on improving the wet-skid resistance and reducing the rolling loss of SSBR by adjusting the content of PPD-GO. In particular, with the addition of 4 phr GO, SSBR was effectively improved in mechanical properties, aging resistance, wet-skid resistance and low rolling loss.


Introduction
Solution polymerized styrene-butadiene rubber (SSBR), with good elasticity, abrasion resistance, low rolling loss and wet-skid resistance, is widely used in the field of tread adhesive [1,2]. With the continuous development of the tire industry, higher requirements are put forward for the comprehensive performance of tread adhesive. The unsaturated double bonds make it easy for the aging resistance of SSBR to be decreased in the thermal-oxidative environment [3,4]. Therefore, it is necessary to improve the aging resistance while reducing the rolling loss and improving the wet-skid resistance of SSBR.
In the past decades, nanoparticles have received increasing interest owing to its unique structural and surface properties. There are many nanoparticles have been applied to improve the performances of rubber, such as graphene [5][6][7][8], layered double hydroxide [9,10], nano silica [11], carbon nano tubes [12], montmorillonite [13,14], ect. The properties of nanocomposites are better than that of conventional composites in the same composition due to the quantum effect and surface effect caused by the small The ultrasound products were freeze-dried to obtain the GO. The pH of 1000 mL deionized water was adjusted to about 10 by dropping NH3•H2O. Then 1 g of GO and 2 g of PPD were added for reaction at a temperature of 80 °C and a time of 12 h. Finally, the reaction products were filtered, rinsed and freeze-dried to obtain the PPD-GO particles. The Schematic diagram of the synthesis of PPD-GO was showed in Figure 1.

The Preparation of SSBR Composite
SSBR, particles and additives were mixed in a two-roll mixing mill (Guangdong Zhanjiang Rubber and Plastic Machinery Factory, Zhanjiang, China). The compositions of the SSBR composites were shown in the Table 1. The vulcanizing of the mixed rubber was carried out with the flat vulcanizer (Shanghai Rubber Machinery Factory, Shanghai, China) under a pressure 10 MPa at 145 °C for 35 min.

Characterization
Fourier transform infrared spectra (FTIR) analysis was carried out by the Thermo Fisher Scientific Nicolet iS50 infrared spectrometer (Waltham, MA, USA) with the scanning speed of 4 cm −1 /min from 500 cm −1 to 4000 cm −1 . KBr pellets were used in the test.
X-ray diffraction (XRD) diffractograms were obtained using a PANalytical B.V. X Pert PRO MPD X-ray diffractometer (PANalytical, Almelo, The Netherlands), equipped with Cu Kα radiation (λ = 0.154 nm). The scanning diffraction angle was from 3 to 70 with a scanning rate of 4/min. Raman analysis was characterized by the spectrometer of ID Raman micro IM-52AV300 (Weihai Optical Instrument). The excitation source was He-Ne laser with a wavelength of 785 nm and a scanning range of 200 ~ 2000 cm −1 .
Scanning electron microscopy (SEM) analysis was performed on a JSM-6360LV instrument (Japan Electronics Co, Tokyo, Japan) to study the fracture surface morphology of SSBR composites.

The Preparation of SSBR Composite
SSBR, particles and additives were mixed in a two-roll mixing mill (Guangdong Zhanjiang Rubber and Plastic Machinery Factory, Zhanjiang, China). The compositions of the SSBR composites were shown in the

Characterization
Fourier transform infrared spectra (FTIR) analysis was carried out by the Thermo Fisher Scientific Nicolet iS50 infrared spectrometer (Waltham, MA, USA) with the scanning speed of 4 cm −1 /min from 500 cm −1 to 4000 cm −1 . KBr pellets were used in the test.
X-ray diffraction (XRD) diffractograms were obtained using a PANalytical B.V. X Pert PRO MPD X-ray diffractometer (PANalytical, Almelo, The Netherlands), equipped with Cu Kα radiation (λ = 0.154 nm). The scanning diffraction angle was from 3 • to 70 • with a scanning rate of 4 • /min. Raman analysis was characterized by the spectrometer of ID Raman micro IM-52AV300 (Weihai Optical Instrument). The excitation source was He-Ne laser with a wavelength of 785 nm and a scanning range of 200~2000 cm −1 .
Scanning electron microscopy (SEM) analysis was performed on a JSM-6360LV instrument (Japan Electronics Co, Tokyo, Japan) to study the fracture surface morphology of SSBR composites.
Tensile test was conducted on a MTS CMT 6104 universal tensile testing machine (MTS Systems (China) Co., Ltd., Shanghai, China) according to the standard of GB/ t528-2009. The samples were cut into a dumbbell shape with thickness of 2 mm, width of 6 mm and the middle standard length of 25 mm.
Thermogravimetric-differential scanning calorimeter synchronization analysis (TGA-DSC) was carried out on a NETZSCH STA449F3 Jupiter instrument (NETZSCH, Bavaria, Germany) with the heating rate of 10 • C /min from 40 • C to 800 • C. The measurements were done under oxygen atmosphere with a gas flow rate of 20 mL/min. Dynamic mechanical analysis (DMA) was carried out on a TA Q800 apparatus (TA Instruments, New Castle, DE, USA) with heating rate of 3 • C/min from −80 • C to 60 • C. The amplitude was 15 µm, and the frequency was 1 Hz. The size of the sample was 60 mm long, 12.8 mm wide and 2 mm thick.

The Characterization of Structure of PPD-GO
The PPD-GO was confirmed by FTIR spectroscopy. As shown in Figure 2 and Table 2, the curve of GO showed two peaks at 3423 cm −1 and 1404 cm −1 , which corresponding to the hydroxyl and carboxyl, respectively. The peaks at 1721 cm −1 , 1620 cm −1 and 1048 cm −1 were attributed to C=O in carboxyl group, C=C and C−O−C in epoxide group. In the curve of PPD, the appearance of two peaks at about 3417 cm −1 were assigned to the stretching vibration of -N-H of NH 2 . The peaks at 1623 cm -1 and 1520 cm −1 were attributed to the bending vibration of N-H of NH 2 . Compared with the curves of PPD and GO, there were two new peaks at 1173 cm −1 and 727 cm −1 appeared on the curve of PPD-GO, which corresponding the stretching vibration of C−N and the stretching vibration of N-H of C-NH-, respectively [34,35]. Meanwhile, the peak at 3210 cm −1 was attributed to the H-bond interaction between −NH 2 and oxygen-containing groups of GO [28]. It indicated the formation of C−NH−C bands due to the grafting of PPD to GO surface.
(China) Co., Ltd., Shanghai, China) according to the standard of GB/ t528-2009. The samples were cut into a dumbbell shape with thickness of 2 mm, width of 6 mm and the middle standard length of 25 mm.
Thermogravimetric-differential scanning calorimeter synchronization analysis (TGA-DSC) was carried out on a NETZSCH STA449F3 Jupiter instrument (NETZSCH, Bavaria, Germany) with the heating rate of 10 °C /min from 40 °C to 800 °C. The measurements were done under oxygen atmosphere with a gas flow rate of 20 mL/min. Dynamic mechanical analysis (DMA) was carried out on a TA Q800 apparatus (TA Instruments, New Castle, DE, USA) with heating rate of 3 °C/min from −80 °C to 60 °C. The amplitude was 15 μm, and the frequency was 1 Hz. The size of the sample was 60 mm long, 12.8 mm wide and 2 mm thick.

The Characterization of Structure of PPD-GO
The PPD-GO was confirmed by FTIR spectroscopy. As shown in Figure 2 and Table 2, the curve of GO showed two peaks at 3423 cm −1 and 1404 cm −1 , which corresponding to the hydroxyl and carboxyl, respectively. The peaks at 1721 cm −1 , 1620 cm −1 and 1048 cm −1 were attributed to C=O in carboxyl group, C=C and C−O−C in epoxide group. In the curve of PPD, the appearance of two peaks at about 3417 cm −1 were assigned to the stretching vibration of -N-H of NH2. The peaks at 1623 cm -1 and 1520 cm −1 were attributed to the bending vibration of N-H of NH2. Compared with the curves of PPD and GO, there were two new peaks at 1173 cm −1 and 727 cm −1 appeared on the curve of PPD-GO, which corresponding the stretching vibration of C−N and the stretching vibration of N-H of C-NH-, respectively [34,35]. Meanwhile, the peak at 3210 cm −1 was attributed to the H-bond interaction between −NH2 and oxygen-containing groups of GO [28]. It indicated the formation of C−NH−C bands due to the grafting of PPD to GO surface.

GO PPD PPD-GO
In X-ray diffractograms (Figure 3), there was only one peak on the spectra of GO at 11.06 • , corresponding the crystal face (002). After PPD was grafted onto GO, the crystal face (002) transferred to 6.55 • and the basal spacing (002) increased from 0.85 nm to 1.35 nm, indicating that a certain amount of PPD entered the interlayer of GO. It was noting that there was a broad peak on the spectra of PPD-GO at 24.98 • , which coincided with the characteristic peak of graphene, indicating that PPD had a reduction effect on GO. In the curve of PPD-G, there were many small peaks between the 2θ of about 15 • -30 • , indicated that GO was exfoliated to some extent and PPD formed small polymeric crystallites on the GO sheets [36].
certain amount of PPD entered the interlayer of GO. It was noting that there was a broad peak on the spectra of PPD-GO at 24.98, which coincided with the characteristic peak of graphene, indicating that PPD had a reduction effect on GO. In the curve of PPD-G, there were many small peaks between the 2θ of about 15-30, indicated that GO was exfoliated to some extent and PPD formed small polymeric crystallites on the GO sheets [36]. The intensity ratio (ID/IG) of the D-peak to G-peak of the Raman spectra of graphene is commonly used to characterize the defect density of graphene [37][38][39][40]. In the Raman spectra of GO and PPD-GO (Figure 4), D peak and G peak appeared around 1310 cm −1 and 1580 cm −1 respectively. The ID/IG of PPD-GO was 1.49, which was 0.24 higher than that of GO. This is because the amino group of PPD reacted with the oxygen-containing group on GO, resulting in the decrease of the average size of the sp 2 domain [30]. The intensity ratio (I D /I G ) of the D-peak to G-peak of the Raman spectra of graphene is commonly used to characterize the defect density of graphene [37][38][39][40]. In the Raman spectra of GO and PPD-GO (Figure 4), D peak and G peak appeared around 1310 cm −1 and 1580 cm −1 respectively. The ID/IG of PPD-GO was 1.49, which was 0.24 higher than that of GO. This is because the amino group of PPD reacted with the oxygen-containing group on GO, resulting in the decrease of the average size of the sp 2 domain [30].

The Mechanical Properties of SSBR Composites Before and After Aging.
Tensile tests were conducted to evaluate the mechanical properties of SSBR composites. It can be seen from Figure 5 (a) and Table 3 that SSBR/PPD-GO composites had better mechanical properties than SSBR without PPD. In particular, the tensile strength and the elongation at break of SSBR/PPD-GO4 were 21.5 MPa and 350%, respectively. When the amount of PPD-GO reached 5 phr, the mechanical properties of SSBR/PPD-GO decreased to the values comparable to those for SSBR. This is due to the fact that PPD-GO agglomerated in the SSBR matrix when the amount of PPD-GO increased, resulting in a decrease in mechanical properties. In order to investigate the effect of PPD-GO on the thermo-oxidative aging resistance, SSBR composites were selected to conduct the thermal-oxygen aging test at 90 °C for 96 h. After thermal oxygen aging, mechanical properties of rubber will change significantly. In Figure 5(b) and table 3, the elongation at break of SSBR/PPD-GO composites was almost equivalent to that of SSBR, which showed a slight trend of increasing first and decreasing later. The tensile strength of SSBR/PPD-GO composites was about 1.3 MPa higher. It was indicated that PPD-GO can not only significantly improve the mechanical properties of SSBR, but also effectively enhance the aging resistance of SSBR.

The Mechanical Properties of SSBR Composites Before and After Aging.
Tensile tests were conducted to evaluate the mechanical properties of SSBR composites. It can be seen from Figure 5a and Table 3 that SSBR/PPD-GO composites had better mechanical properties than SSBR without PPD. In particular, the tensile strength and the elongation at break of SSBR/PPD-GO4 were 21.5 MPa and 350%, respectively. When the amount of PPD-GO reached 5 phr, the mechanical properties of SSBR/PPD-GO decreased to the values comparable to those for SSBR. This is due to the fact that PPD-GO agglomerated in the SSBR matrix when the amount of PPD-GO increased, resulting in a decrease in mechanical properties. In order to investigate the effect of PPD-GO on the thermo-oxidative aging resistance, SSBR composites were selected to conduct the thermal-oxygen aging test at 90 • C for 96 h. After thermal oxygen aging, mechanical properties of rubber will change significantly. In Figure 5b and Table 3, the elongation at break of SSBR/PPD-GO composites was almost equivalent to that of SSBR, which showed a slight trend of increasing first and decreasing later. The tensile strength of SSBR/PPD-GO composites was about 1.3 MPa higher. It was indicated that Materials 2020, 13, 1025 6 of 10 PPD-GO can not only significantly improve the mechanical properties of SSBR, but also effectively enhance the aging resistance of SSBR.
increased, resulting in a decrease in mechanical properties. In order to investigate the effect of PPD-GO on the thermo-oxidative aging resistance, SSBR composites were selected to conduct the thermal-oxygen aging test at 90 °C for 96 h. After thermal oxygen aging, mechanical properties of rubber will change significantly. In Figure 5(b) and table 3, the elongation at break of SSBR/PPD-GO composites was almost equivalent to that of SSBR, which showed a slight trend of increasing first and decreasing later. The tensile strength of SSBR/PPD-GO composites was about 1.3 MPa higher. It was indicated that PPD-GO can not only significantly improve the mechanical properties of SSBR, but also effectively enhance the aging resistance of SSBR.

The Thermal Properties of SSBR Composites
The synchronous thermal analysis of SSBR composites was tested under oxygen atmosphere. The content of PPD-GO was 4 phr. In Figure 6a, the TGA curve of SSBR composites was mainly divided into two stages. The first stage was from 200 • C to 440 • C, which mainly attributed to the primary degradation of rubber chains. The second stage was from 440 • C to 600 • C, which attributed to the further degradation of rubber and thermal oxidation degradation of carbon black. In the first stage, the temperature corresponding to the maximum degradation rate of SSBR/PPD-GO was 10 • C higher than that of SSBR. This is because some of the free radicals generated in the thermal oxygen degradation process were captured by the amino group on PPD-GO [6]. Figure 6b showed the DSC curve of SSBR/PPD-GO. The exothermic peak of SSBR in the whole degradation process was more obvious than that of SSBR/PPD-GO, indicating that the PPD-GO effectively slowed down the thermal oxygen degradation of SSBR.

The Thermal Properties of SSBR Composites
The synchronous thermal analysis of SSBR composites was tested under oxygen atmosphere. The content of PPD-GO was 4 phr. In Figure 6a, the TGA curve of SSBR composites was mainly divided into two stages. The first stage was from 200 °C to 440 °C, which mainly attributed to the primary degradation of rubber chains. The second stage was from 440 °C to 600 °C，which attributed to the further degradation of rubber and thermal oxidation degradation of carbon black. In the first stage, the temperature corresponding to the maximum degradation rate of SSBR/PPD-GO was 10 °C higher than that of SSBR. This is because some of the free radicals generated in the thermal oxygen degradation process were captured by the amino group on PPD-GO [6]. Figure 6b showed the DSC curve of SSBR/PPD-GO. The exothermic peak of SSBR in the whole degradation process was more obvious than that of SSBR/PPD-GO, indicating that the PPD-GO effectively slowed down the thermal oxygen degradation of SSBR.

The Micromorphology of SSBR Composites
Variations in the surface morphology of SSBR composites were illustrated by SEM. In Figure 7a, there were obvious pores between GO and the SSBR matrix. In Figure 7b, the PPD-GO was embedded in the fracture surface and maintained a good interaction with the SSBR matrix. It can be

The Micromorphology of SSBR Composites
Variations in the surface morphology of SSBR composites were illustrated by SEM. In Figure 7a, there were obvious pores between GO and the SSBR matrix. In Figure 7b, the PPD-GO was embedded in the fracture surface and maintained a good interaction with the SSBR matrix. It can be attributed to the grafting of PPD, which weakened the agglomeration of GO.

The Dynamic Mechanical Properties of SSBR Composites
It can be seen from Figure 8 that the storage modulus (Eʹ) of SSBR/PPD-GO composite was higher than that of the SSBR and the peak in mechanical loss factor (tanδ) of SSBR/PPD-GO composites moved to high temperature, indicating that the crosslinking density of SSBR/PPD-GO composite was more than that of SSBR. This was due to the strong interaction between PPD-GO and SSBR matrix, which formed many physical crosslink points [41][42][43]. The tanδ value at 0 °C and 60 °C was the indicator for evaluation of high performance rubber tire materials [44]. It can be considered that the larger the tanδ value at 0 °C, the better wet-skid resistance, and the smaller the tanδ value at 60 °C, the less the rolling loss. As shown in Table 4, the tanδ value of SSBR/PPD-GO composites at 0 °C was increased. The tanδ value of SSBR/PPD-GO1 was 0.153, an increase of 5.5% over that of SSBR. At 60 °C, the tanδ value of SSBR/PPD-GO composites was decreased with the content of PPD-GO. The tanδ value of SSBR/PPD-GO5 was the smallest, which was 8.9% lower than that of SSBR. This indicated that it had a good effect on improving the wet-skid resistance with the low content of PPD-GO and a positive contribution on reducing the rolling loss with the high content of PPD-GO. Both of wet-skid resistance and low rolling loss of SSBR/PPD-GO4 had been obviously improved, which were 2.1% and 6.5% higher than that of SSBR without PPD-GO.

The Dynamic Mechanical Properties of SSBR Composites
It can be seen from Figure 8 that the storage modulus (E') of SSBR/PPD-GO composite was higher than that of the SSBR and the peak in mechanical loss factor (tanδ) of SSBR/PPD-GO composites moved to high temperature, indicating that the crosslinking density of SSBR/PPD-GO composite was more than that of SSBR. This was due to the strong interaction between PPD-GO and SSBR matrix, which formed many physical crosslink points [41][42][43].

The Dynamic Mechanical Properties of SSBR Composites
It can be seen from Figure 8 that the storage modulus (Eʹ) of SSBR/PPD-GO composite was higher than that of the SSBR and the peak in mechanical loss factor (tanδ) of SSBR/PPD-GO composites moved to high temperature, indicating that the crosslinking density of SSBR/PPD-GO composite was more than that of SSBR. This was due to the strong interaction between PPD-GO and SSBR matrix, which formed many physical crosslink points [41][42][43]. The tanδ value at 0 °C and 60 °C was the indicator for evaluation of high performance rubber tire materials [44]. It can be considered that the larger the tanδ value at 0 °C, the better wet-skid resistance, and the smaller the tanδ value at 60 °C, the less the rolling loss. As shown in Table 4, the tanδ value of SSBR/PPD-GO composites at 0 °C was increased. The tanδ value of SSBR/PPD-GO1 was 0.153, an increase of 5.5% over that of SSBR. At 60 °C, the tanδ value of SSBR/PPD-GO composites was decreased with the content of PPD-GO. The tanδ value of SSBR/PPD-GO5 was the smallest, which was 8.9% lower than that of SSBR. This indicated that it had a good effect on improving the wet-skid resistance with the low content of PPD-GO and a positive contribution on reducing the rolling loss with the high content of PPD-GO. Both of wet-skid resistance and low rolling loss of SSBR/PPD-GO4 had been obviously improved, which were 2.1% and 6.5% higher than that of SSBR without PPD-GO. The tanδ value at 0 • C and 60 • C was the indicator for evaluation of high performance rubber tire materials [44]. It can be considered that the larger the tanδ value at 0 • C, the better wet-skid resistance, and the smaller the tanδ value at 60 • C, the less the rolling loss. As shown in Table 4, the tanδ value of SSBR/PPD-GO composites at 0 • C was increased. The tanδ value of SSBR/PPD-GO1 was 0.153, an increase of 5.5% over that of SSBR. At 60 • C, the tanδ value of SSBR/PPD-GO composites was decreased with the content of PPD-GO. The tanδ value of SSBR/PPD-GO5 was the smallest, which was 8.9% lower than that of SSBR. This indicated that it had a good effect on improving the wet-skid resistance with the low content of PPD-GO and a positive contribution on reducing the rolling loss with the high content of PPD-GO. Both of wet-skid resistance and low rolling loss of SSBR/PPD-GO4 had been obviously improved, which were 2.1% and 6.5% higher than that of SSBR without PPD-GO.

Conclusions
In this study, GO grafted by PPD was added into SSBR to give SSBR/PPD-GO composites. The interaction between GO and the SSBR matrix was enhanced by introducing PPD. PPD-GO can be considered the good particle to increase the mechanical properties and aging resistance of SSBR. When the content of PPD-GO was 4 phr, the tensile strength and elongation at break were 21.5 MPa and 350% respectively, which were 29.5% and 16.7% higher than those of SSBR. After aging, the mechanical properties of SSBR/PPD-GO composites were also better than that of SSBR, in which the tensile strength was about 1.3 MPa higher. PPD-GO effectively improved the thermal oxidative aging resistance of SSBR. The better wet-skid resistance and lower rolling loss of SSBR was increased by adjusting the content of PPD-GO. In particular, both of wet-skid resistance and low rolling loss of SSBR/PPD-GO4 had been improved, which were 2.1% and 6.5% higher than that of SSBR respectively. This research can be thought of as an effective method to obtain the SSBR composite with good integrated properties in aging resistance, wet-skid resistance and low rolling loss.