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silica nanocomposites
Composites Part B 175 (2019) 107027
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Enhanced comprehensive performance of SSBR/BR with self-assembly reduced graphene oxide/silica nanocomposites Hong Zhu a, *, Zhongying Wang a, Xidai Huang a, Fanghui Wang a, **, Linghan Kong a, Baochun Guo b, Tao Ding c a b c
State Key Laboratory of Chemical Resource Engineering, School of Science, Beijing University of Chemical Technology, Beijing, 100029, PR China Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou, 510640, China Henan Engineering Laboratory of Flame-Retardant and Functional Materials, Henan University, Kaifeng, 475004, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Particle-reinforcement Polymer-matrix composites (PMCs) Mechanical properties Assembly
The dispersion of filler and properties of elastomer composites are always the focus in the rubber researching. In this work, RGO/SiO2 nanocomposites were obtained by self-assembling of silica nanoparticles (SiO2) and reduced graphene oxide (RGO), which were used to manufacture RGO/SiO2-SSBR/BR composites by means of compounding with solution-polymerized styrene butadiene rubber/butadiene rubber (SSBR/BR). The structure and morphology characterization showed that SiO2 nanoparticles were homogeneously distributed on RGO. With the adding of RGO, the mechanical properties were enhanced and the RGO/SiO2-SSBR/BR with 1% weight percentage of RGO possessed the optimal wet skid resistance and rolling resistance compared with commer cialized Zeosil 1165 MP highly-dispersed nano-SiO2 (Zeosil 1165 MP)-SSBR/BR (tan δ is 16.8% higher at 0 � C and 50.0% lower at 60 � C). In addition, the heat build-up exhibited considerable decrease with low RGO weight percentage, and wear resistance considerably enhanced. The reinforced integrated performances provide a po tential application of RGO/SiO2 for green tires.
1. Introduction Energy shortage and environmental conservation have drawn global attention because of the excessive usage of fossil fuel. The automobile industry, which plays a key role in fossil fuel consumption, is in a rev olution to achieve an energy conserved and eco-friendly industrial structural transformation, so that it is inevitable to manufacture ‘green tire’ with high wear-resistance, low rolling resistance and high wet skid resistance [1]. In the traditional tire manufacture, on account of the remarkable enhancement of the rubber matrix, carbon black has been diffusely employed as primary enhancement ingredient for more than one century [2]. However, it leads to the serious environmental prob lems owing to the oil dependence of carbon black. SiO2 nanoparticles, as a new generation of filler, have been proved to be a more suitable reinforcing agent for the rubber matrix due to the low-cost preparation, oil independence and multi-functional characteristics [3]. As previously reported that the well-known ‘magic triangle’ of the tire tread rubber has been improved remarkably, especially the reduction of rolling resistance
and the enhancement of wet skid resistance [4]. Thus, SiO2 nano particles become an important ingredient of the materials to fabricate “green tire”. In spite of the extensive application prospect of SiO2 nanoparticles in the tire industry, the agglomeration of SiO2 nano particles and the poor compatibility of SiO2 with rubber matrix are still the problems that prevent it to fabricate green tire. Graphene, as another oil independent filler, has aroused the research upsurge due to its exceptional modulus of elasticity, remarkable ther mal, mechanical, barrier properties, almost optical transmittance (98%) and large specific surface area [5]. According to the excellent properties of graphene, especially the outstanding mechanical properties, it is natural to incorporate graphene into rubber matrix and expect to enhance the comprehensive performance of elastomer composites. Thus, many graphene-based elastomer composites sprang out, and the prop erties of rubber composites have been significantly improved [6,7]. Nevertheless, graphene nanosheets tend to aggregate due to its high specific surface area and van der Waals interaction, and the compre hensive performance of rubber composites by incorporation of graphene
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Zhu), [email protected] (F. Wang). https://doi.org/10.1016/j.compositesb.2019.107027 Received 25 January 2019; Received in revised form 28 May 2019; Accepted 9 June 2019 1359-8368/© 2019 Published by Elsevier Ltd.
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Scheme 1. Scheme diagram of the self-assembly of RGO/SiO2 nanocomposites and RGO/SiO2-SSBR/BR composites.
itself has constrained the application of graphene in rubber industry. Therefore, elastomer composites filled with either graphene or SiO2 nanoparticles individually cannot meet the industrial demands. Mean ingfully, silica was dispersed on the surface of graphene by means of self-assembly to separate the graphene sheets, inhibiting the agglom eration of silica as well. Moreover, the existence of hybrid component and the enhancement of filler-rubber interaction could improve the reinforcing performance. Nevertheless, there are very few reports about the research of graphene-based nanocomposites as fillers used for tire tread rubber composites. In our previous works, we have discussed the properties of SSBR/BR that incorporated with SiO2-based nanocomposites, the enhanced properties were obtained compared with SiO2-SSBR/BR [8,9]. In this study, the two-dimensional graphene was procured by means of the reduction of graphene oxide (GO) and RGO/SiO2 nanocomposites was obtained by a self-assembly method, then the RGO/SiO2 was introduced into SSBR/BR to manufacture RGO/SiO2-SSBR/BR composites (Scheme 1). Interestingly, the assembly of RGO/SiO2 nanocomposites is similar to the CNTs/SiO2 nanocomposites we prepared before [9], so the prepa ration method shows the potential application in the preparation of
SiO2-based nanocomposites. In RGO/SiO2, the SiO2 nanoparticles of about 10–20 nm are uniformly dispersed on RGO, the RGO and SiO2 nanoparticles showed mutual promotion to the dispersion in rubber matrix. Additionally, the integrated performance of SSBR/BR compos ites was enhanced. As far as our information goes, there is no report on the application of RGO/SiO2 nanocomposites in tire tread elastomer composites so far. 2. Experimental 2.1. Materials Graphite flake (natural, 325 mesh) was provided by Alfa Aesar (Tianjin, China) Chemical Co., Ltd.. Sodium silicate (Na2O⋅3.2SiO2, 27 wt %) was obtained from Beijing Linhengtai Trade Co., Ltd.. Citric acid (AR, �99.5%) and 3-aminopropyltriethoxysilane (APTES) were manufactured by Shanghai Macklin Biochemical Co., Ltd.. SSBR (Buna VSL 5025-2HM, oil-extended) and BR (CB24) were manufactured by Lanxess Chemical Industry Co., Ltd. (Germany). Zeosil 1165 MP highlydispersed nano-SiO2 was purchased from Rhodia France (Qingdao, 2
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dispersed in citric acid solution (2.3 mol/L) and subjected to ultrasonic treatment for 1 h to protonate RGO-NH2 adequately. Sodium silicate was mixed with deionized water and the mixture was stirred at 80 � C after the addition of a defined amount of ethanol. The protonated RGO-NH2/citric acid slurry was slowly added to the sodium silicate mixture, then adjust the pH to 6 with 2.5 mol/L H2SO4 solution. After aging for 6 h, the mixture was washed by centrifugation and dried to obtain RGO/SiO2 nanocomposites. The pure SiO2 without self-assembling with RGO was prepared by the same procedure.
Table 1 The compound formulation (Unit: Parts per hundred rubber, phr). Ingredient
Loading/phr
SSBR BR ZnO Stearic acid RGO/SiO2 (SiO2) Si69 Accelerator D Accelerator CZ Antioxidant 4010NA Paraffin wax Sulfur
96.25 30.00 3.00 1.00 70 7 2.00 1.50 1.50 1.00 1.40
2.4. Preparation of SSBR/BR composites
China). Zinc oxide (ZnO), stearic acid, paraffin wax, sulfur, N-IsopropylN0 -phenyl-1,4-phenylenediamin (Antioxidant 4010NA), Bis(3triethoxysilylpropyl)tetrasulfide (Si69), 1,3-Diphenylguanidine (Accel erator D), N-Cyclohexyl-2-beozothiazole sulfonamide (Accelerator CZ) were commercial available and used as received.
The formula of SSBR/BR composites is shown in Table 1. SSBR and BR were mixed with RGO/SiO2, Si69, ZnO, stearic acid, antioxidant 4010 NA and paraffin in an internal mixer at 60 � C, and the compounds suffered heat treatment at 150 � C for 5 min. Then the SSBR/BR com pound was vulcanized at 150 � C after mixing with accelerator and sulfur to form the RGO/SiO2-SSBR/BR composites. For comparison, the contrast SiO2 (pure SiO2 and Zeosil 1165 MP)-SSBR/BR and unfilled SSBR/BR composites were fabricated by the same procedure.
2.2. Preparation of reduced graphene oxide nanosheets (RGO)
2.5. Characterization
RGO was obtained by an improved method [20]. Typically, 3.0 g graphite flakes (G) was treated by mixed acid (VH3 PO4 : VH2 SO4 ¼ 1:9), stirred at room temperature for 12 h. Subsequently, 18.0 g KMnO4 was added in the mixture under ice bath and then heated to 50 � C for 12 h. Then, 10 mL 30% H2O2 was carefully added after the mixture was transferred into ice water. After no more gas is produced, the obtained GO was centrifuged, washed by deionized water and dried at 60 � C. 0.5 g GO was added into 500 mL deionized water and sonicated for 1 h, then the GO dispersion was heated to 90 � C after the dropwise addition of 10 mL hydrazine hydrate, and reacted for 6 h. The slurry was washed and freeze-dried to obtain the final RGO powder.
Transmission electron microscope (TEM) (HT7700, Hitachi Crop., Japan) and the scanning electron microscope (SEM) (JSM-7800 F, JEOL Ltd., Japan) were employed to analyze the morphologies of the samples. X-ray diffraction was recorded by XD-3A (Shimadzu) under Cu Kα ra diation. X-ray photoelectron spectroscopy (XPS) measurements were performed on Thermo Fisher LAB 250 ESCA System (USA) with Al Kα radiation 150 W. FT-IR spectra was obtained by a FT-IR meter (Nicolet 6700). Raman spectrometer (JY-HR 800) was employed to evaluate the defects of G, GO, RGO, RGO-NH2 and RGO/SiO2 with a laser of 514 nm. Atomic force microscopy (AFM) was measured on a Veeco Digital In strument Multi-Mode SPM under the tapping mode. The crosslink density is measured by the equilibrium swelling method in toluene and calculated through the classical Flory-Rehner equation [11]. Tensile tests were performed in the light of ASTM D638 on a CTM4104 (SANS). The dynamic rheological properties were analyzed on a RPA2000 rubber processing analyzer at 60 � C and 1 Hz.
2.3. Preparation of RGO/SiO2 nanocomposites The prepared RGO was modified by APTES to form the silanized RGO (RGO-NH2) as reported previously [10]. Then the RGO-NH2 was
Fig. 1. TEM images of (a) RGO and (b) RGO/SiO2; SEM images of: (c) RGO/SiO2, fracture surface of (d) pure SiO2-SSBR/BR composites, (e) 1.5% and (f) 2.5% RGO/ SiO2 –SSBR/BR composites. 3
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RGO/SiO2-SSBR/BR with a high weight percentage of RGO (2.5%) shows many RGO/SiO2 aggregates in the fracture surface. 3.2. Chemical structure of characterization of RGO/SiO2 nanocomposites XRD was used to confirm the crystal structure of the nanocomposites. As is observed in Fig. 2, the commercial Zeosil 1165 MP only presents one broad characterized diffraction of amorphous silica at 2θ about 22� . The GO shows the typical (002) diffraction peak of graphite at about 10.5� , indicating that the interlayer space changed to 0.83 nm after the oxidation of graphite. The (002) diffraction peak of RGO shows an obvious up-shift to about 24.5� . This diffraction peak position change confirms the decomposition of GO sheets to RGO sheets and the broad (002) peak illustrates that the RGO is highly disordered along the stacking direction [12]. After being modified by APTES, the (002) diffraction peak slightly shifted towards lower 2θ value due to the interlayer space change. The XRD pattern of 2.5% RGO/SiO2 nano composites exhibits a strong peak at about 22� , consistenting with the characterized diffraction of amorphous silica. Apparently, the graphite (100) diffraction peak at about 45� has disappeared because of the relatively low weight percentage of RGO. To further evaluate the actually occurred reactions among the re agents, the FT-IR spectra of GO, RGO, RGO-NH2 and 2.5% RGO/SiO2 were carried out and shown in Fig. 3. For GO, the characteristic peak of carbonyl C¼O appears at 1728 cm 1, and the peaks at 1405, 1225 and 1054 cm 1 separately correspond to carboxy C–O, epoxy C–O and alkoxy C–O [13]. After the reduction by hydrazine hydrate for 6 h, the peak at 1728 cm 1 disappears. However, O–H stretching and deforma tion vibrations at 3440 cm 1 and 1390 cm 1 maintain relatively high absorption, indicating that abundant number of hydroxyl functional groups exist on the surface of RGO, which can act as active sites to interact with APTES. For RGO-NH2, the doublet at 2921 cm 1 and 2851 cm 1 is consistent with the asymmetric and symmetric vibration of –CH2- of the alkyl chains in APTES [14]. In addition, the peak at 1562 cm 1 is considered as the –NH2 symmetric deformation vibration [15]. However, the N–H vibration in amino groups is not presented at about 3300 cm 1 mainly because of the overlapping of the O–H char acteristic peak. The peaks at 1194, 1119 and 962 cm 1 correspond to the stretching vibrations of Si–C, Si–O–Si and Si–OH, respectively [15]. The use of silane coupling agent induces Si–O–Si stretching vibration at the position of 1096 cm 1. Due to the condensation of the –OH group on the surface of RGO and the Si–OH that hydrolysed by APTES, there appears the Si–O–C peak at 1042 cm 1 [16]. For RGO/SiO2, The composition of SiO2 leads to the increase of Si–O–Si and Si–OH peak intensity and the emergence of the Si–OH peak at 962 cm 1 [17].
Fig. 2. XRD patterns of Zeosil 1165 MP, GO, RGO, RGO-NH2 and 2.5% RGO/SiO2.
Dynamic mechanical analysis of samples were detected by a VA3000 instrument in tensile mode under the strain amplitude of 0.1% and the frequency of 10 Hz. 3. Results and discussion 3.1. Morphology of the samples Fig. 1a shows the micromorphology of the RGO nanosheets. The transparent thin layers indicate the higher degree of exfoliation during the preparation of RGO, this result is further proved in AFM. As for RGO/ SiO2 that shown in Fig. 1b and c, the surface of RGO is anchored with a large amount of uniformly distributed SiO2 nanoparticles, it can be explained that the protonated–NH2 provides the interaction sites to attract SiO23 and the hydroxyl group on the surface of RGO could interact with SiO23 which result to a strong confining effect on the growth of silica and a decreased agglomeration of silica on the surface of graphene [8,9]. Moreover, the SEM images of pure SiO2-SSBR/BR and RGO/SiO2-SSBR/BR are shown in Fig. 1d, e and f for comparison. After the addition of RGO/SiO2 with a low weight percentage of RGO (1.5%), the sheet-like RGO dispersed uniformly in the rubber matrix with many SiO2 nanoparticles on the surface. However, in Fig. 1f, the
Fig. 3. FT-IR spectra of GO, RGO, RGO-NH2 and 2.5% RGO/SiO2, and magnified area of RGO, RGO-NH2. 4
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G to GO. Accordingly, the ID/IG value increases from 0.20 (G) to 1.51 (GO) as a result of the dramatically decrease in the size of in-plane sp2 domains during the process of oxidation and exfoliation [19]. The ID/IG values of RGO, RGO-NH2 and RGO/SiO2 are all increased compared with GO, suggesting the reconstruction of the conjugated structure and the increasing of disordered structure after reduction [20]. The ID/IG value of RGO-NH2 (1.78) is higher than that of RGO (1.67), implying the poor structure regularity of RGO-NH2 due to the incorporation of APTES, also an evidence of the functionalization of RGO by APTES. For RGO/SiO2 nanocomposites, the ID/IG value further increases to 1.88, indicating the covalent bond formation between RGO and SiO2 during the formation process of RGO/SiO2 and the generation of defects in the graphene [21]. XPS analysis was performed to further validate the covalent inter action between RGO and SiO2. The XPS survey scan of GO, RGO, RGONH2 and 2.5% RGO/SiO2 is shown in Fig. 5a, indicating the existence of C, O, Si and N in RGO/SiO2 nanocomposites. The weakened intensity of O relative to C from GO to RGO indicates the decreasing number of oxygen functional groups after the reduction of GO. Due to the linkage of APTES, RGO-NH2 shows an increasing intensity of O relative to C compared with RGO. Si 2p spectra in Fig. 5b shows the presence of Si–O–C bond (102.9 eV) [22,23]. The peaks of C–O–Si (285.2 eV) ap pears in the C 1s spectra of the APTES functionalized RGO (Fig. 5c), indicating the reaction between APTES and RGO [24]. As is shown in Fig. 5d, after the formation of RGO/SiO2 nanocomposites, the new peak appears at 289.5 eV corresponding to the Si–O–C¼O [37]. Also, the existence of C–O–Si in RGO/SiO2 indicates the covalent bond between RGO and SiO2 nanoparticles.
Fig. 4. Raman spectra of G, GO, RGO, RGO-NH2 and 2.5% RGO/SiO2.
Fig. 4 shows the Raman spectra of various samples. All samples exhibit two characteristic peaks, corresponding to the D band (1350 cm 1) and G band (1600 cm 1), which relate to the defects of sp3 hybridized carbons or the disordered structure of graphite and the inplane vibration of the graphite lattice. The intensity ratio of the D band and G band (ID/IG) can be utilized to analyze the defect degree and the covalent functionalization of graphite [18]. As is shown in Fig. 4, the intensity of D band significantly increased after the transformation from
Fig. 5. XPS spectra: (a) Survey spectra of GO, RGO, RGO-NH2 and 2.5% RGO/SiO2; (b) Si 2p spectra of 2.5% RGO/SiO2; C 1s spectra of (c) RGO-NH2 and (d) 2.5% RGO/SiO2. 5
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Fig. 6. AFM images and height profile of (a) RGO and (b) RGO-NH2.
Fig. 8. The G0 versus strain curves of RGO/SiO2-SSBR/BR composites with various weight percentage of RGO.
Fig. 7. The Crosslink density of SSBR/BR composites with various fillers.
To confirm the silanization of RGO by APTES, the variation of the thickness of RGO and RGO-NH2 was measured by the atomic force mi croscopy (AFM). RGO and RGO-NH2 was subjected to ultrasonic treat ment to disperse in ethanol and dropped on silicon wafer with smooth surface. The silicon wafer was dried at ambient conditions for 24 h. In dividual RGO sheet is shown in Fig. 6a. The characteristic thickness of RGO is 1.17 nm, it proved that the RGO are dominated by few layers graphene through the mechanical exfoliation, indicating the complete exfoliation [25,26]. From Fig. 6b, the RGO-NH2 shows an increasing thickness (1.68 nm) compared with RGO because of the presence of APTES molecules bonded on the basal surface of RGO [13,14].
3.3. Crosslink density of SSBR/BR composites As shown in Fig. 7 that RGO/SiO2-SSBR/BR composites have higher crosslink density than SiO2-SSBR/BR composites. And the crosslink density of RGO/SiO2-SSBR/BR composites increases with the increase of RGO weight percentage. It reveals the reaction area increases during vulcanization due to the rising hybrid content of filler, which lead to the enhancement of filler-rubber interaction. These results are consistent with mechanical properties [27].
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Fig. 9. (a) Representative stress-strain curves of the SSBR/BR composites filled with different fillers; (b) 300% modulus, tensile strength and tear strength of RGO/ SiO2-SSBR/BR composites with different RGO weight percentage.
3.4. Dynamic rheological properties of SSBR/BR composites
(3%), the G0 of RGO/SiO2-SSBR/BR reduces first and increases later as the RGO weight percentage increases compared with pure SiO2. This interesting phenomenon is mainly put down to the evolution of the filler network in the rubber matrix. Specifically, the SiO2 nanoparticles trend toward aggregate severely and the three-dimensional network structure is formed in pure SiO2-SSBR/BR composites, resulting that some rubber chains are trapped in the filler network and incapable of acting as elastomers, which leads to a relatively higher value of G’ [30,31]. For RGO/SiO2-SSBR/BR with lower weight percentage of RGO, SiO2
Dynamic rheological properties were performed to explore the network structure of RGO/SiO2-SSBR/BR composites. In Fig. 8, as the strain increases, the value of G’ (elasticity modulus) for all the SSBR/BR composites significantly decreased, which is because of the ‘Payne ef fect’ that resulted from the collapse of the filler network and release of the trapped rubber. The declined G0 value suggests the decreasing ‘Payne effect’ and the weaken filler network [28,29]. At the small strain region
Fig. 10. Variation of (a) storage modulus (G0 ) and (c) loss factor (tan δ) with temperature of SSBR/BR composites filled with different fillers; (b) Amplification of marked region of (a); (d) Amplification of marked region near 0 � C and 60 � C of (c). 7
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Fig. 11. The heat build-up (a) and wear volume (b) of SSBR/BR composites with various fillers.
nanoparticles are anchored on RGO uniformly and promote the sepa ration of RGO from each other. Meanwhile, the two dimensional RGO sheets could prevent SiO2 from aggregating to a stronger filler network. Thus, the mutual promotion between RGO and SiO2 decreased the G0 of the RGO/SiO2-SSBR/BR composites. At the same time, when the weight percentage of RGO increases to 2% and above, more and more RGO sheets are assembled with SiO2 nanoparticles to form RGO/SiO2 nanocomposites, the contact area of graphene sheets increases and tend to aggregate, resulting to the agglomeration of RGO/SiO2 nano composites and a stronger three dimensional network is formed. Consequently, more rubber chains are restricted in the filler network and the G0 value is remarkably increased. These results are in agreement with the SEM image (Fig. 1d, e and f).
interaction. After the assembly of RGO and SiO2 nanoparticles, the value of G0 in the glass state shows the similar tendency with the result of RPA. As the temperature gradually rises, the rubber matrix converts to rubber state, the filler-rubber interaction become the main contribution to the increase of G’ [33]. With RGO weight percentage going up, the G0 of RGO/SiO2-SSBR/BR rises obviously, reflecting the enhanced filler-rubber interaction in the high RGO weight percentage. Fig. 10c and d shows the variations of loss factor (tan δ) with tem perature. The crest value of tan δ of the RGO/SiO2-SSBR/BR with lower RGO weight percentage is increased compared with contrast SiO2-SSBR/ BR. This is because of the breakdown of SiO2 agglomerations after a small amount of SiO2 is substituted by RGO and the increasing of the effective volume of the polymers. Thus, the inner friction between the rubber chains increased, leading to the generation of large inner con sumption, and the enhanced peak value of tan δ is performed [34,35]. At high RGO weight percentage, the efficient volume of rubber chains is remarkably decreased because of the formation of an increased strength of filler network, resulting to the dramatical reduction of peak value of tan δ [2]. It is well known that, in the case of tire tread rubber, the wet-skid resistance and rolling resistance properties can be evaluated by the values of tan δ at 0 � C and 60 � C, respectively [28,36]. The tan δ with a higher value at 0 � C and a lower value at 60 � C indicate the enhance ment of wet-skid resistance and the reduction of rolling resistance, which are key factors of “green tire”. As is shown in Fig. 10d, at the RGO weight percentage of 1%, the RGO/SiO2-SSBR/BR possesses the opti mum over-all properties (a rise of 16.8% at 0 � C and a decline of 50.0% at 60 � C from those of Zeosil 1165 MP). These results may be attributed to the following reasons. The enhanced interface interaction result to the movement of Tg to higher temperature, which could increase the value of tan δ at 0 � C. In addition, the improved dispersion of filler nano composites and the enhanced interface interaction could reduce the re-formation of filler aggregations under the dynamic loading, and decrease the breakage and re-formation process of the filler agglomer ations which lead to the reduced dynamic hysteresis loss at 60 � C. Fig. 11 shows the heat build-up and wear volume of SSBR/BR composites with diverse fillers. The heat build-up of RGO/SiO2-SSBR/ BR reduces dramatically at low RGO weight percentage compared with contrast SiO2-SSBR/BR (Fig. 11a). During the repeated deformation, the generated heat is mainly resulted from the internal friction of filler network [37]. As the results of RPA and DMA above, the filler network of SiO2 nanoparticles is reduced with a small amount of RGO and the movement of SiO2 that anchored on the surface of RGO are restricted compared with the isolated SiO2, so during the dynamic stress process which caused disruption and reformulation of the filler network, the friction among fillers is decreased. As the weight percentage of RGO rises to 2.5%, the formation of the stronger filler network mainly results in an increasing heat build-up.
3.5. Effect of RGO on the mechanical properties of SSBR/BR composites Fig. 9 shows the promotion of RGO on the mechanical properties of the SSBR/BR composites. Compared with pure SiO2-SSBR/BR, the ten sile strength and 300% tensile modulus of RGO/SiO2-SSBR/BR are increased, especially with the weight percentage of RGO at 1% and 1.5% because of the improved dispersion of SiO2 nanoparticles in the presence of RGO. After the reaction in the internal mixer, the anchored and iso lated SiO2 nanoparticles, as well as the exposed RGO surface without SiO2, are modified by Si69, which are able to react with rubber chains. The improved dispersion of fillers and the increased effective volume of rubber chains lead to the rising number of binding sites between filler and rubber chains. However, at the weight percentage of RGO above 1%, the increasing amplitude of tensile strength of RGO/SiO2-SSBR/BR is stepped down. This can be explained by the balance between filler dispersion and filler-rubber interaction. For the lower weight percentage of RGO, the improved dispersion of RGO/SiO2 mainly causes the increasing of tensile strength. With the rising weight percentage of RGO, the enhanced interaction between fillers and rubber chains leads to the increasing crosslink density of the SSBR/BR composites, which domi nates the withstand of external force, so that there is still a slight in crease of tensile strength, even if the dispersion of the SiO2 decreases. 3.6. Dynamic mechanical properties of SSBR/BR composites Fig. 10a and b shows the relationship between storage modulus (G0 ) and temperature. Along with the increasing of temperature, the G0 slumps dramatically consistent with the conversion of the rubber matrix from glassy state to rubber state. The energy dissipation makes for the decrease of G0 and this phenomenon can be explained by the synergic movement among the molecular chains of rubber matrix [32]. Below the glass-transition temperature (Tg), due to the restricted segmental movement of rubber chains, the G0 is mainly attributed to the filler-filler 8
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The Akron abrasion is measured to explore the potential influence of RGO/SiO2-SSBR/BR in the tire tread rubber in more depth. The mech anism of the abrasion of rubber composites generally includes chemical decomposition of the rubber chains and local mechanical rupture [38, 39]. With the same rubber matrix, the abrasion caused by the chemical decomposition can be safely disregarded. Consequently, the abrasion of RGO/SiO2-SSBR/BR basically rest with the dispersion of fillers and the interaction between filler and rubber matrix. As Fig. 11b shows, the wear volume of RGO/SiO2-SSBR/BR exhibits a significant decline with the increase of RGO, this phenomenon demonstrates the strong inter action between RGO/SiO2 and the rubber matrix, as well as the enhancement of dynamic properties of SSBR/BR compared with contrast SiO2.
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Fluorescent whitening agent stabilized graphene and its composites with chitosan. J Mater Chem 2011;21(43): 17111–8. [26] Tung VC, Allen MJ, Yang Y, Kaner RB. High-throughput solution processing of large-scale graphene. Nat Nanotechnol 2009;4(1):25. [27] Chen L, Jia Z, Tang Y, Wu L, Luo Y, Jia D. Novel functional silica nanoparticles for rubber vulcanization and reinforcement. Compos Sci Technol 2017;144:11–7. [28] Kang H, Tang Y, Yao L, Yang F, Fang Q, Hui D. Fabrication of graphene/natural rubber nanocomposites with high dynamic properties through convenient mechanical mixing. Compos B Eng 2017;112:1–7. [29] Qiao H, Chao M, Hui D, Liu J, Zheng J, Lei W, Zhang L. Enhanced interfacial interaction and excellent performance of silica/epoxy group-functionalized styrene-butadiene rubber (SBR) nanocomposites without any coupling agent. Compos B Eng 2017;114:356–64. [30] Tang Z, Huang J, Wu X, Guo B, Zhang L, Liu F. Interface engineering toward promoting silanization by ionic liquid for high-performance rubber/silica composites. Ind Eng Chem Res 2015;54(43):10747–56. [31] Wang MJ. Effect of polymer-filler and filler-filler interactions on dynamic properties of filled vulcanizates. Rubber Chem Technol 1998;71(3):520–89. [32] Bhattacharyya S, Sinturel C, Bahloul O, Saboungi ML, Thomas S, Salvetat JP. Improving reinforcement of natural rubber by networking of activated carbon nanotubes. Carbon 2008;46(7):1037–45. [33] Li Y, Han B, Wen S, Lu Y, Yang H, Zhang L, Liu L. Effect of the temperature on surface modification of silica and properties of modified silica filled rubber composites. Composites Part A 2014;62:52–9. [34] Robertson CG, Lin CJ, Rackaitis M, Roland CM. Influence of particle size and polymer filler coupling on viscoelastic glass transition of particle-reinforced polymers. Macromolecules 2008;41(7):2727–31. [35] Wang MJ, Mahmud K, Murphy LJ, Patterson WJ. Carbon-silica dual phase filler, a new generation reinforcing agent for rubber -Part I. Characterization. Kautsch Gummi Kunstst 1998;51(5):348–58. [36] Chen Y, Yin Q, Zhang X, Zhang W, Jia H, Ji Q, Rui X. Rational design of multifunctional properties for styrene-butadiene rubber reinforced by modified Kevlar nanofibers. Compos B Eng 2019;166:196–203. [37] Medalia AI. Heat generation in elastomer compounds: causes and effects. Rubber Chem Technol 1991;64(3):481–92. [38] Gent AN, Pulford CTR. Mechanisms of rubber abrasion. J Appl Polym Sci 1983;28 (3):943–60. [39] Tabsan N, Wirasate S, Suchiva K. Abrasion behavior of layered silicate reinforced natural rubber. Wear 2010;269(5–6):394–404.
4. Conclusions RGO/SiO2 nanocomposites was designed and obtained by a selfassembly method, and the properties of RGO/SiO2-SSBR/BR were studied. It has been found that RGO/SiO2 homogeneously distributed in SSBR/BR matrix and the filler-rubber interfacial interaction has been significantly enhanced at low RGO weight percentage. At the same time, the enhanced comprehensive performance indicates that it could be feasible for RGO/SiO2 to be a reinforcing filler in the elastomer com posites. This work supplies crucial information for the design of prom ising fillers for high-performance green tires. Acknowledgments This work is supported by the National Basic Research Program of China (Grant No. 2015CB654700 and 2015CB654703), the National Natural Science Foundation of China (No. 21776014 and 21476020), and the Joint Funds of the National Natural Science Foundation of China (No. U1705253). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107027. References [1] Zhang C, Tang Z, Guo B, Zhang L. Significantly improved rubber-silica interface via subtly controlling surface chemistry of silica. Compos Sci Technol 2018;156:70–7. [2] Tang Z, Zhang C, Wei Q, Weng P, Guo B. Remarkably improving performance of carbon black-filled rubber composites by incorporating MoS2 nanoplatelets. Compos Sci Technol 2016;132:93–100. [3] Li Y, Han B, Liu L, Zhang F, Zhang L, Wen S, et al. Surface modification of silica by two-step method and properties of solution styrene butadiene rubber (SSBR) nanocomposites filled with modified silica. Compos Sci Technol 2013;88:69–75. [4] Hall DE, Moreland JC. Fundamentals of rolling resistance. Rubber Chem Technol 2001;74(3):525–39. [5] Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S. Graphene based materials: past, present and future. Prog Mater Sci 2011;56(8):1178–271. [6] Tang Z, Wu X, Guo B, Zhang L, Jia D. Preparation of butadiene–styrene–vinyl pyridine rubber–graphene oxide hybrids through co-coagulation process and in situ interface tailoring. J Mater Chem 2012;22(15):7492–501. [7] Li S, Li Z, Burnett TL, Slater TJ, Hashimoto T, Young RJ. Nanocomposites of graphene nanoplatelets in natural rubber: microstructure and mechanisms of reinforcement. J Mater Sci 2017;52(16):9558–72. [8] Kong L, Li F, Wang F, Miao Y, Huang X, Zhu H, Lu Y. In situ assembly of SiO2 nanodots/layered double hydroxide nanocomposite for the reinforcement of solution-polymerized butadiene styrene rubber/butadiene rubber. Compos Sci Technol 2018;158:9–18. [9] Kong L, Li F, Wang F, Miao Y, Huang X, Zhu H, Lu Y. High-performing multi-walled carbon nanotube/silica nanocomposites for elastomer application. Compos Sci Technol 2018;162:23–32. [10] Lin Y, Liu S, Peng J, Liu L. The filler–rubber interface and reinforcement in styrene butadiene rubber composites with graphene/silica hybrids: a quantitative correlation with the constrained region. Composites Part A 2016;86:19–30.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Enhanced comprehensive performance of SSBR/BR with self-assembly reduced graphene oxide/silica nanocomposites Hong Zhu a, *, Zhongying Wang a, Xidai Huang a, Fanghui Wang a, **, Linghan Kong a, Baochun Guo b, Tao Ding c a b c
State Key Laboratory of Chemical Resource Engineering, School of Science, Beijing University of Chemical Technology, Beijing, 100029, PR China Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou, 510640, China Henan Engineering Laboratory of Flame-Retardant and Functional Materials, Henan University, Kaifeng, 475004, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Particle-reinforcement Polymer-matrix composites (PMCs) Mechanical properties Assembly
The dispersion of filler and properties of elastomer composites are always the focus in the rubber researching. In this work, RGO/SiO2 nanocomposites were obtained by self-assembling of silica nanoparticles (SiO2) and reduced graphene oxide (RGO), which were used to manufacture RGO/SiO2-SSBR/BR composites by means of compounding with solution-polymerized styrene butadiene rubber/butadiene rubber (SSBR/BR). The structure and morphology characterization showed that SiO2 nanoparticles were homogeneously distributed on RGO. With the adding of RGO, the mechanical properties were enhanced and the RGO/SiO2-SSBR/BR with 1% weight percentage of RGO possessed the optimal wet skid resistance and rolling resistance compared with commer cialized Zeosil 1165 MP highly-dispersed nano-SiO2 (Zeosil 1165 MP)-SSBR/BR (tan δ is 16.8% higher at 0 � C and 50.0% lower at 60 � C). In addition, the heat build-up exhibited considerable decrease with low RGO weight percentage, and wear resistance considerably enhanced. The reinforced integrated performances provide a po tential application of RGO/SiO2 for green tires.
1. Introduction Energy shortage and environmental conservation have drawn global attention because of the excessive usage of fossil fuel. The automobile industry, which plays a key role in fossil fuel consumption, is in a rev olution to achieve an energy conserved and eco-friendly industrial structural transformation, so that it is inevitable to manufacture ‘green tire’ with high wear-resistance, low rolling resistance and high wet skid resistance [1]. In the traditional tire manufacture, on account of the remarkable enhancement of the rubber matrix, carbon black has been diffusely employed as primary enhancement ingredient for more than one century [2]. However, it leads to the serious environmental prob lems owing to the oil dependence of carbon black. SiO2 nanoparticles, as a new generation of filler, have been proved to be a more suitable reinforcing agent for the rubber matrix due to the low-cost preparation, oil independence and multi-functional characteristics [3]. As previously reported that the well-known ‘magic triangle’ of the tire tread rubber has been improved remarkably, especially the reduction of rolling resistance
and the enhancement of wet skid resistance [4]. Thus, SiO2 nano particles become an important ingredient of the materials to fabricate “green tire”. In spite of the extensive application prospect of SiO2 nanoparticles in the tire industry, the agglomeration of SiO2 nano particles and the poor compatibility of SiO2 with rubber matrix are still the problems that prevent it to fabricate green tire. Graphene, as another oil independent filler, has aroused the research upsurge due to its exceptional modulus of elasticity, remarkable ther mal, mechanical, barrier properties, almost optical transmittance (98%) and large specific surface area [5]. According to the excellent properties of graphene, especially the outstanding mechanical properties, it is natural to incorporate graphene into rubber matrix and expect to enhance the comprehensive performance of elastomer composites. Thus, many graphene-based elastomer composites sprang out, and the prop erties of rubber composites have been significantly improved [6,7]. Nevertheless, graphene nanosheets tend to aggregate due to its high specific surface area and van der Waals interaction, and the compre hensive performance of rubber composites by incorporation of graphene
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Zhu), [email protected] (F. Wang). https://doi.org/10.1016/j.compositesb.2019.107027 Received 25 January 2019; Received in revised form 28 May 2019; Accepted 9 June 2019 1359-8368/© 2019 Published by Elsevier Ltd.
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Scheme 1. Scheme diagram of the self-assembly of RGO/SiO2 nanocomposites and RGO/SiO2-SSBR/BR composites.
itself has constrained the application of graphene in rubber industry. Therefore, elastomer composites filled with either graphene or SiO2 nanoparticles individually cannot meet the industrial demands. Mean ingfully, silica was dispersed on the surface of graphene by means of self-assembly to separate the graphene sheets, inhibiting the agglom eration of silica as well. Moreover, the existence of hybrid component and the enhancement of filler-rubber interaction could improve the reinforcing performance. Nevertheless, there are very few reports about the research of graphene-based nanocomposites as fillers used for tire tread rubber composites. In our previous works, we have discussed the properties of SSBR/BR that incorporated with SiO2-based nanocomposites, the enhanced properties were obtained compared with SiO2-SSBR/BR [8,9]. In this study, the two-dimensional graphene was procured by means of the reduction of graphene oxide (GO) and RGO/SiO2 nanocomposites was obtained by a self-assembly method, then the RGO/SiO2 was introduced into SSBR/BR to manufacture RGO/SiO2-SSBR/BR composites (Scheme 1). Interestingly, the assembly of RGO/SiO2 nanocomposites is similar to the CNTs/SiO2 nanocomposites we prepared before [9], so the prepa ration method shows the potential application in the preparation of
SiO2-based nanocomposites. In RGO/SiO2, the SiO2 nanoparticles of about 10–20 nm are uniformly dispersed on RGO, the RGO and SiO2 nanoparticles showed mutual promotion to the dispersion in rubber matrix. Additionally, the integrated performance of SSBR/BR compos ites was enhanced. As far as our information goes, there is no report on the application of RGO/SiO2 nanocomposites in tire tread elastomer composites so far. 2. Experimental 2.1. Materials Graphite flake (natural, 325 mesh) was provided by Alfa Aesar (Tianjin, China) Chemical Co., Ltd.. Sodium silicate (Na2O⋅3.2SiO2, 27 wt %) was obtained from Beijing Linhengtai Trade Co., Ltd.. Citric acid (AR, �99.5%) and 3-aminopropyltriethoxysilane (APTES) were manufactured by Shanghai Macklin Biochemical Co., Ltd.. SSBR (Buna VSL 5025-2HM, oil-extended) and BR (CB24) were manufactured by Lanxess Chemical Industry Co., Ltd. (Germany). Zeosil 1165 MP highlydispersed nano-SiO2 was purchased from Rhodia France (Qingdao, 2
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dispersed in citric acid solution (2.3 mol/L) and subjected to ultrasonic treatment for 1 h to protonate RGO-NH2 adequately. Sodium silicate was mixed with deionized water and the mixture was stirred at 80 � C after the addition of a defined amount of ethanol. The protonated RGO-NH2/citric acid slurry was slowly added to the sodium silicate mixture, then adjust the pH to 6 with 2.5 mol/L H2SO4 solution. After aging for 6 h, the mixture was washed by centrifugation and dried to obtain RGO/SiO2 nanocomposites. The pure SiO2 without self-assembling with RGO was prepared by the same procedure.
Table 1 The compound formulation (Unit: Parts per hundred rubber, phr). Ingredient
Loading/phr
SSBR BR ZnO Stearic acid RGO/SiO2 (SiO2) Si69 Accelerator D Accelerator CZ Antioxidant 4010NA Paraffin wax Sulfur
96.25 30.00 3.00 1.00 70 7 2.00 1.50 1.50 1.00 1.40
2.4. Preparation of SSBR/BR composites
China). Zinc oxide (ZnO), stearic acid, paraffin wax, sulfur, N-IsopropylN0 -phenyl-1,4-phenylenediamin (Antioxidant 4010NA), Bis(3triethoxysilylpropyl)tetrasulfide (Si69), 1,3-Diphenylguanidine (Accel erator D), N-Cyclohexyl-2-beozothiazole sulfonamide (Accelerator CZ) were commercial available and used as received.
The formula of SSBR/BR composites is shown in Table 1. SSBR and BR were mixed with RGO/SiO2, Si69, ZnO, stearic acid, antioxidant 4010 NA and paraffin in an internal mixer at 60 � C, and the compounds suffered heat treatment at 150 � C for 5 min. Then the SSBR/BR com pound was vulcanized at 150 � C after mixing with accelerator and sulfur to form the RGO/SiO2-SSBR/BR composites. For comparison, the contrast SiO2 (pure SiO2 and Zeosil 1165 MP)-SSBR/BR and unfilled SSBR/BR composites were fabricated by the same procedure.
2.2. Preparation of reduced graphene oxide nanosheets (RGO)
2.5. Characterization
RGO was obtained by an improved method [20]. Typically, 3.0 g graphite flakes (G) was treated by mixed acid (VH3 PO4 : VH2 SO4 ¼ 1:9), stirred at room temperature for 12 h. Subsequently, 18.0 g KMnO4 was added in the mixture under ice bath and then heated to 50 � C for 12 h. Then, 10 mL 30% H2O2 was carefully added after the mixture was transferred into ice water. After no more gas is produced, the obtained GO was centrifuged, washed by deionized water and dried at 60 � C. 0.5 g GO was added into 500 mL deionized water and sonicated for 1 h, then the GO dispersion was heated to 90 � C after the dropwise addition of 10 mL hydrazine hydrate, and reacted for 6 h. The slurry was washed and freeze-dried to obtain the final RGO powder.
Transmission electron microscope (TEM) (HT7700, Hitachi Crop., Japan) and the scanning electron microscope (SEM) (JSM-7800 F, JEOL Ltd., Japan) were employed to analyze the morphologies of the samples. X-ray diffraction was recorded by XD-3A (Shimadzu) under Cu Kα ra diation. X-ray photoelectron spectroscopy (XPS) measurements were performed on Thermo Fisher LAB 250 ESCA System (USA) with Al Kα radiation 150 W. FT-IR spectra was obtained by a FT-IR meter (Nicolet 6700). Raman spectrometer (JY-HR 800) was employed to evaluate the defects of G, GO, RGO, RGO-NH2 and RGO/SiO2 with a laser of 514 nm. Atomic force microscopy (AFM) was measured on a Veeco Digital In strument Multi-Mode SPM under the tapping mode. The crosslink density is measured by the equilibrium swelling method in toluene and calculated through the classical Flory-Rehner equation [11]. Tensile tests were performed in the light of ASTM D638 on a CTM4104 (SANS). The dynamic rheological properties were analyzed on a RPA2000 rubber processing analyzer at 60 � C and 1 Hz.
2.3. Preparation of RGO/SiO2 nanocomposites The prepared RGO was modified by APTES to form the silanized RGO (RGO-NH2) as reported previously [10]. Then the RGO-NH2 was
Fig. 1. TEM images of (a) RGO and (b) RGO/SiO2; SEM images of: (c) RGO/SiO2, fracture surface of (d) pure SiO2-SSBR/BR composites, (e) 1.5% and (f) 2.5% RGO/ SiO2 –SSBR/BR composites. 3
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RGO/SiO2-SSBR/BR with a high weight percentage of RGO (2.5%) shows many RGO/SiO2 aggregates in the fracture surface. 3.2. Chemical structure of characterization of RGO/SiO2 nanocomposites XRD was used to confirm the crystal structure of the nanocomposites. As is observed in Fig. 2, the commercial Zeosil 1165 MP only presents one broad characterized diffraction of amorphous silica at 2θ about 22� . The GO shows the typical (002) diffraction peak of graphite at about 10.5� , indicating that the interlayer space changed to 0.83 nm after the oxidation of graphite. The (002) diffraction peak of RGO shows an obvious up-shift to about 24.5� . This diffraction peak position change confirms the decomposition of GO sheets to RGO sheets and the broad (002) peak illustrates that the RGO is highly disordered along the stacking direction [12]. After being modified by APTES, the (002) diffraction peak slightly shifted towards lower 2θ value due to the interlayer space change. The XRD pattern of 2.5% RGO/SiO2 nano composites exhibits a strong peak at about 22� , consistenting with the characterized diffraction of amorphous silica. Apparently, the graphite (100) diffraction peak at about 45� has disappeared because of the relatively low weight percentage of RGO. To further evaluate the actually occurred reactions among the re agents, the FT-IR spectra of GO, RGO, RGO-NH2 and 2.5% RGO/SiO2 were carried out and shown in Fig. 3. For GO, the characteristic peak of carbonyl C¼O appears at 1728 cm 1, and the peaks at 1405, 1225 and 1054 cm 1 separately correspond to carboxy C–O, epoxy C–O and alkoxy C–O [13]. After the reduction by hydrazine hydrate for 6 h, the peak at 1728 cm 1 disappears. However, O–H stretching and deforma tion vibrations at 3440 cm 1 and 1390 cm 1 maintain relatively high absorption, indicating that abundant number of hydroxyl functional groups exist on the surface of RGO, which can act as active sites to interact with APTES. For RGO-NH2, the doublet at 2921 cm 1 and 2851 cm 1 is consistent with the asymmetric and symmetric vibration of –CH2- of the alkyl chains in APTES [14]. In addition, the peak at 1562 cm 1 is considered as the –NH2 symmetric deformation vibration [15]. However, the N–H vibration in amino groups is not presented at about 3300 cm 1 mainly because of the overlapping of the O–H char acteristic peak. The peaks at 1194, 1119 and 962 cm 1 correspond to the stretching vibrations of Si–C, Si–O–Si and Si–OH, respectively [15]. The use of silane coupling agent induces Si–O–Si stretching vibration at the position of 1096 cm 1. Due to the condensation of the –OH group on the surface of RGO and the Si–OH that hydrolysed by APTES, there appears the Si–O–C peak at 1042 cm 1 [16]. For RGO/SiO2, The composition of SiO2 leads to the increase of Si–O–Si and Si–OH peak intensity and the emergence of the Si–OH peak at 962 cm 1 [17].
Fig. 2. XRD patterns of Zeosil 1165 MP, GO, RGO, RGO-NH2 and 2.5% RGO/SiO2.
Dynamic mechanical analysis of samples were detected by a VA3000 instrument in tensile mode under the strain amplitude of 0.1% and the frequency of 10 Hz. 3. Results and discussion 3.1. Morphology of the samples Fig. 1a shows the micromorphology of the RGO nanosheets. The transparent thin layers indicate the higher degree of exfoliation during the preparation of RGO, this result is further proved in AFM. As for RGO/ SiO2 that shown in Fig. 1b and c, the surface of RGO is anchored with a large amount of uniformly distributed SiO2 nanoparticles, it can be explained that the protonated–NH2 provides the interaction sites to attract SiO23 and the hydroxyl group on the surface of RGO could interact with SiO23 which result to a strong confining effect on the growth of silica and a decreased agglomeration of silica on the surface of graphene [8,9]. Moreover, the SEM images of pure SiO2-SSBR/BR and RGO/SiO2-SSBR/BR are shown in Fig. 1d, e and f for comparison. After the addition of RGO/SiO2 with a low weight percentage of RGO (1.5%), the sheet-like RGO dispersed uniformly in the rubber matrix with many SiO2 nanoparticles on the surface. However, in Fig. 1f, the
Fig. 3. FT-IR spectra of GO, RGO, RGO-NH2 and 2.5% RGO/SiO2, and magnified area of RGO, RGO-NH2. 4
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G to GO. Accordingly, the ID/IG value increases from 0.20 (G) to 1.51 (GO) as a result of the dramatically decrease in the size of in-plane sp2 domains during the process of oxidation and exfoliation [19]. The ID/IG values of RGO, RGO-NH2 and RGO/SiO2 are all increased compared with GO, suggesting the reconstruction of the conjugated structure and the increasing of disordered structure after reduction [20]. The ID/IG value of RGO-NH2 (1.78) is higher than that of RGO (1.67), implying the poor structure regularity of RGO-NH2 due to the incorporation of APTES, also an evidence of the functionalization of RGO by APTES. For RGO/SiO2 nanocomposites, the ID/IG value further increases to 1.88, indicating the covalent bond formation between RGO and SiO2 during the formation process of RGO/SiO2 and the generation of defects in the graphene [21]. XPS analysis was performed to further validate the covalent inter action between RGO and SiO2. The XPS survey scan of GO, RGO, RGONH2 and 2.5% RGO/SiO2 is shown in Fig. 5a, indicating the existence of C, O, Si and N in RGO/SiO2 nanocomposites. The weakened intensity of O relative to C from GO to RGO indicates the decreasing number of oxygen functional groups after the reduction of GO. Due to the linkage of APTES, RGO-NH2 shows an increasing intensity of O relative to C compared with RGO. Si 2p spectra in Fig. 5b shows the presence of Si–O–C bond (102.9 eV) [22,23]. The peaks of C–O–Si (285.2 eV) ap pears in the C 1s spectra of the APTES functionalized RGO (Fig. 5c), indicating the reaction between APTES and RGO [24]. As is shown in Fig. 5d, after the formation of RGO/SiO2 nanocomposites, the new peak appears at 289.5 eV corresponding to the Si–O–C¼O [37]. Also, the existence of C–O–Si in RGO/SiO2 indicates the covalent bond between RGO and SiO2 nanoparticles.
Fig. 4. Raman spectra of G, GO, RGO, RGO-NH2 and 2.5% RGO/SiO2.
Fig. 4 shows the Raman spectra of various samples. All samples exhibit two characteristic peaks, corresponding to the D band (1350 cm 1) and G band (1600 cm 1), which relate to the defects of sp3 hybridized carbons or the disordered structure of graphite and the inplane vibration of the graphite lattice. The intensity ratio of the D band and G band (ID/IG) can be utilized to analyze the defect degree and the covalent functionalization of graphite [18]. As is shown in Fig. 4, the intensity of D band significantly increased after the transformation from
Fig. 5. XPS spectra: (a) Survey spectra of GO, RGO, RGO-NH2 and 2.5% RGO/SiO2; (b) Si 2p spectra of 2.5% RGO/SiO2; C 1s spectra of (c) RGO-NH2 and (d) 2.5% RGO/SiO2. 5
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Fig. 6. AFM images and height profile of (a) RGO and (b) RGO-NH2.
Fig. 8. The G0 versus strain curves of RGO/SiO2-SSBR/BR composites with various weight percentage of RGO.
Fig. 7. The Crosslink density of SSBR/BR composites with various fillers.
To confirm the silanization of RGO by APTES, the variation of the thickness of RGO and RGO-NH2 was measured by the atomic force mi croscopy (AFM). RGO and RGO-NH2 was subjected to ultrasonic treat ment to disperse in ethanol and dropped on silicon wafer with smooth surface. The silicon wafer was dried at ambient conditions for 24 h. In dividual RGO sheet is shown in Fig. 6a. The characteristic thickness of RGO is 1.17 nm, it proved that the RGO are dominated by few layers graphene through the mechanical exfoliation, indicating the complete exfoliation [25,26]. From Fig. 6b, the RGO-NH2 shows an increasing thickness (1.68 nm) compared with RGO because of the presence of APTES molecules bonded on the basal surface of RGO [13,14].
3.3. Crosslink density of SSBR/BR composites As shown in Fig. 7 that RGO/SiO2-SSBR/BR composites have higher crosslink density than SiO2-SSBR/BR composites. And the crosslink density of RGO/SiO2-SSBR/BR composites increases with the increase of RGO weight percentage. It reveals the reaction area increases during vulcanization due to the rising hybrid content of filler, which lead to the enhancement of filler-rubber interaction. These results are consistent with mechanical properties [27].
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Fig. 9. (a) Representative stress-strain curves of the SSBR/BR composites filled with different fillers; (b) 300% modulus, tensile strength and tear strength of RGO/ SiO2-SSBR/BR composites with different RGO weight percentage.
3.4. Dynamic rheological properties of SSBR/BR composites
(3%), the G0 of RGO/SiO2-SSBR/BR reduces first and increases later as the RGO weight percentage increases compared with pure SiO2. This interesting phenomenon is mainly put down to the evolution of the filler network in the rubber matrix. Specifically, the SiO2 nanoparticles trend toward aggregate severely and the three-dimensional network structure is formed in pure SiO2-SSBR/BR composites, resulting that some rubber chains are trapped in the filler network and incapable of acting as elastomers, which leads to a relatively higher value of G’ [30,31]. For RGO/SiO2-SSBR/BR with lower weight percentage of RGO, SiO2
Dynamic rheological properties were performed to explore the network structure of RGO/SiO2-SSBR/BR composites. In Fig. 8, as the strain increases, the value of G’ (elasticity modulus) for all the SSBR/BR composites significantly decreased, which is because of the ‘Payne ef fect’ that resulted from the collapse of the filler network and release of the trapped rubber. The declined G0 value suggests the decreasing ‘Payne effect’ and the weaken filler network [28,29]. At the small strain region
Fig. 10. Variation of (a) storage modulus (G0 ) and (c) loss factor (tan δ) with temperature of SSBR/BR composites filled with different fillers; (b) Amplification of marked region of (a); (d) Amplification of marked region near 0 � C and 60 � C of (c). 7
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Fig. 11. The heat build-up (a) and wear volume (b) of SSBR/BR composites with various fillers.
nanoparticles are anchored on RGO uniformly and promote the sepa ration of RGO from each other. Meanwhile, the two dimensional RGO sheets could prevent SiO2 from aggregating to a stronger filler network. Thus, the mutual promotion between RGO and SiO2 decreased the G0 of the RGO/SiO2-SSBR/BR composites. At the same time, when the weight percentage of RGO increases to 2% and above, more and more RGO sheets are assembled with SiO2 nanoparticles to form RGO/SiO2 nanocomposites, the contact area of graphene sheets increases and tend to aggregate, resulting to the agglomeration of RGO/SiO2 nano composites and a stronger three dimensional network is formed. Consequently, more rubber chains are restricted in the filler network and the G0 value is remarkably increased. These results are in agreement with the SEM image (Fig. 1d, e and f).
interaction. After the assembly of RGO and SiO2 nanoparticles, the value of G0 in the glass state shows the similar tendency with the result of RPA. As the temperature gradually rises, the rubber matrix converts to rubber state, the filler-rubber interaction become the main contribution to the increase of G’ [33]. With RGO weight percentage going up, the G0 of RGO/SiO2-SSBR/BR rises obviously, reflecting the enhanced filler-rubber interaction in the high RGO weight percentage. Fig. 10c and d shows the variations of loss factor (tan δ) with tem perature. The crest value of tan δ of the RGO/SiO2-SSBR/BR with lower RGO weight percentage is increased compared with contrast SiO2-SSBR/ BR. This is because of the breakdown of SiO2 agglomerations after a small amount of SiO2 is substituted by RGO and the increasing of the effective volume of the polymers. Thus, the inner friction between the rubber chains increased, leading to the generation of large inner con sumption, and the enhanced peak value of tan δ is performed [34,35]. At high RGO weight percentage, the efficient volume of rubber chains is remarkably decreased because of the formation of an increased strength of filler network, resulting to the dramatical reduction of peak value of tan δ [2]. It is well known that, in the case of tire tread rubber, the wet-skid resistance and rolling resistance properties can be evaluated by the values of tan δ at 0 � C and 60 � C, respectively [28,36]. The tan δ with a higher value at 0 � C and a lower value at 60 � C indicate the enhance ment of wet-skid resistance and the reduction of rolling resistance, which are key factors of “green tire”. As is shown in Fig. 10d, at the RGO weight percentage of 1%, the RGO/SiO2-SSBR/BR possesses the opti mum over-all properties (a rise of 16.8% at 0 � C and a decline of 50.0% at 60 � C from those of Zeosil 1165 MP). These results may be attributed to the following reasons. The enhanced interface interaction result to the movement of Tg to higher temperature, which could increase the value of tan δ at 0 � C. In addition, the improved dispersion of filler nano composites and the enhanced interface interaction could reduce the re-formation of filler aggregations under the dynamic loading, and decrease the breakage and re-formation process of the filler agglomer ations which lead to the reduced dynamic hysteresis loss at 60 � C. Fig. 11 shows the heat build-up and wear volume of SSBR/BR composites with diverse fillers. The heat build-up of RGO/SiO2-SSBR/ BR reduces dramatically at low RGO weight percentage compared with contrast SiO2-SSBR/BR (Fig. 11a). During the repeated deformation, the generated heat is mainly resulted from the internal friction of filler network [37]. As the results of RPA and DMA above, the filler network of SiO2 nanoparticles is reduced with a small amount of RGO and the movement of SiO2 that anchored on the surface of RGO are restricted compared with the isolated SiO2, so during the dynamic stress process which caused disruption and reformulation of the filler network, the friction among fillers is decreased. As the weight percentage of RGO rises to 2.5%, the formation of the stronger filler network mainly results in an increasing heat build-up.
3.5. Effect of RGO on the mechanical properties of SSBR/BR composites Fig. 9 shows the promotion of RGO on the mechanical properties of the SSBR/BR composites. Compared with pure SiO2-SSBR/BR, the ten sile strength and 300% tensile modulus of RGO/SiO2-SSBR/BR are increased, especially with the weight percentage of RGO at 1% and 1.5% because of the improved dispersion of SiO2 nanoparticles in the presence of RGO. After the reaction in the internal mixer, the anchored and iso lated SiO2 nanoparticles, as well as the exposed RGO surface without SiO2, are modified by Si69, which are able to react with rubber chains. The improved dispersion of fillers and the increased effective volume of rubber chains lead to the rising number of binding sites between filler and rubber chains. However, at the weight percentage of RGO above 1%, the increasing amplitude of tensile strength of RGO/SiO2-SSBR/BR is stepped down. This can be explained by the balance between filler dispersion and filler-rubber interaction. For the lower weight percentage of RGO, the improved dispersion of RGO/SiO2 mainly causes the increasing of tensile strength. With the rising weight percentage of RGO, the enhanced interaction between fillers and rubber chains leads to the increasing crosslink density of the SSBR/BR composites, which domi nates the withstand of external force, so that there is still a slight in crease of tensile strength, even if the dispersion of the SiO2 decreases. 3.6. Dynamic mechanical properties of SSBR/BR composites Fig. 10a and b shows the relationship between storage modulus (G0 ) and temperature. Along with the increasing of temperature, the G0 slumps dramatically consistent with the conversion of the rubber matrix from glassy state to rubber state. The energy dissipation makes for the decrease of G0 and this phenomenon can be explained by the synergic movement among the molecular chains of rubber matrix [32]. Below the glass-transition temperature (Tg), due to the restricted segmental movement of rubber chains, the G0 is mainly attributed to the filler-filler 8
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The Akron abrasion is measured to explore the potential influence of RGO/SiO2-SSBR/BR in the tire tread rubber in more depth. The mech anism of the abrasion of rubber composites generally includes chemical decomposition of the rubber chains and local mechanical rupture [38, 39]. With the same rubber matrix, the abrasion caused by the chemical decomposition can be safely disregarded. Consequently, the abrasion of RGO/SiO2-SSBR/BR basically rest with the dispersion of fillers and the interaction between filler and rubber matrix. As Fig. 11b shows, the wear volume of RGO/SiO2-SSBR/BR exhibits a significant decline with the increase of RGO, this phenomenon demonstrates the strong inter action between RGO/SiO2 and the rubber matrix, as well as the enhancement of dynamic properties of SSBR/BR compared with contrast SiO2.
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4. Conclusions RGO/SiO2 nanocomposites was designed and obtained by a selfassembly method, and the properties of RGO/SiO2-SSBR/BR were studied. It has been found that RGO/SiO2 homogeneously distributed in SSBR/BR matrix and the filler-rubber interfacial interaction has been significantly enhanced at low RGO weight percentage. At the same time, the enhanced comprehensive performance indicates that it could be feasible for RGO/SiO2 to be a reinforcing filler in the elastomer com posites. This work supplies crucial information for the design of prom ising fillers for high-performance green tires. Acknowledgments This work is supported by the National Basic Research Program of China (Grant No. 2015CB654700 and 2015CB654703), the National Natural Science Foundation of China (No. 21776014 and 21476020), and the Joint Funds of the National Natural Science Foundation of China (No. U1705253). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107027. References [1] Zhang C, Tang Z, Guo B, Zhang L. Significantly improved rubber-silica interface via subtly controlling surface chemistry of silica. Compos Sci Technol 2018;156:70–7. [2] Tang Z, Zhang C, Wei Q, Weng P, Guo B. Remarkably improving performance of carbon black-filled rubber composites by incorporating MoS2 nanoplatelets. Compos Sci Technol 2016;132:93–100. [3] Li Y, Han B, Liu L, Zhang F, Zhang L, Wen S, et al. Surface modification of silica by two-step method and properties of solution styrene butadiene rubber (SSBR) nanocomposites filled with modified silica. Compos Sci Technol 2013;88:69–75. [4] Hall DE, Moreland JC. Fundamentals of rolling resistance. Rubber Chem Technol 2001;74(3):525–39. [5] Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S. Graphene based materials: past, present and future. Prog Mater Sci 2011;56(8):1178–271. [6] Tang Z, Wu X, Guo B, Zhang L, Jia D. Preparation of butadiene–styrene–vinyl pyridine rubber–graphene oxide hybrids through co-coagulation process and in situ interface tailoring. J Mater Chem 2012;22(15):7492–501. [7] Li S, Li Z, Burnett TL, Slater TJ, Hashimoto T, Young RJ. Nanocomposites of graphene nanoplatelets in natural rubber: microstructure and mechanisms of reinforcement. J Mater Sci 2017;52(16):9558–72. [8] Kong L, Li F, Wang F, Miao Y, Huang X, Zhu H, Lu Y. In situ assembly of SiO2 nanodots/layered double hydroxide nanocomposite for the reinforcement of solution-polymerized butadiene styrene rubber/butadiene rubber. Compos Sci Technol 2018;158:9–18. [9] Kong L, Li F, Wang F, Miao Y, Huang X, Zhu H, Lu Y. High-performing multi-walled carbon nanotube/silica nanocomposites for elastomer application. Compos Sci Technol 2018;162:23–32. [10] Lin Y, Liu S, Peng J, Liu L. The filler–rubber interface and reinforcement in styrene butadiene rubber composites with graphene/silica hybrids: a quantitative correlation with the constrained region. Composites Part A 2016;86:19–30.
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