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elastomer nanocomposites
Composites Science and Technology 132 (2016) 68e75
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Rational design of covalent interfaces for graphene/elastomer nanocomposites Zhijun Yang a, 1, Jun Liu b, 1, Ruijuan Liao a, Ganwei Yang a, Xiaohui Wu b, Zhenghai Tang a, Baochun Guo a, *, Liqun Zhang b, **, Yong Ma c, Qiuhai Nie c, Feng Wang c a b c
Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou, 510640, PR China State Key Laboratory of Organic/Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR China Shandong Linglong Tire Co., Ltd., Zhaoyuan, 265406, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2016 Received in revised form 27 June 2016 Accepted 28 June 2016 Available online 1 July 2016
The energy loss of tires during service is closely related to the hysteresis of tire tread, which is governed by the dispersion and interface of the elastomer nanocomposites. However, traditional undeformable spherical fillers have approached a bottleneck in regulating the viscoelasticity and lowering the hysteresis loss. Herein, the designed covalent interface in the graphene/elastomer nanocomposite maximizes the reinforcement of the nanomaterial as well as minimizes the dynamic energy loss. The reinforced interfacial interaction, an ultralow percolation threshold of tensile strength and high reinforcement synergistically benefit these nano-organized rubber nanocomposites. The energy losses under dynamic loading are largely suppressed in this material, due to the reduced nano-scale frictions. The developed graphene/elastomer nanocomposites are applied to typical dynamic rubber product - auto tires, and the tests indicate that these tires possess energy efficiency close to the highest “A-grade”, which gave great economic and environmental improvements. The critical role of interface structure in the performance of the elastomer nanocomposites was revealed and new design strategy for low dynamic energy losses in rubber products was suggested. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Nano composites Interface Mechanical properties Strength
1. Introduction High performance elastomeric materials are strategically important in different areas including applications in extreme conditions. Reinforcement realized by inclusion of various nanoparticles into the matrix is needed for most synthetic elastomers because they are not strong mechanically. Tens of parts of carbon black or silica are needed for gaining acceptable mechanical strength in the composite [1,2]. However, there are two main drawbacks in such highly filled elastomer nanocomposites. First, it gives rise to a higher friction among the filler particles which results in energy losses and heat generation for dynamic elastomeric products. The second is associated with the reduction of specific strength and modulus due to the increased density, which is the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Guo), [email protected] (L. Zhang). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.compscitech.2016.06.015 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
current bottleneck in lightweight design of elastomeric composites. Design of polymeric nanocomposites with high specific performance is the cutting edge topic in polymer research [3]. Recently, graphene which possesses unparalleled physical properties such as outstanding mechanical properties, was proposed as a high-efficiency reinforcement for elastomers [4,5]. Many graphene-based polymer composites with significantly improved properties have been prepared and tested [6,7]. However, only a few studies have been devoted to graphene/elastomer nanocomposites [8e12]. Potts et al. incorporated reduced graphene oxide into natural rubber latex and demonstrated processingemorphologyeproperty relationships both theoretically and experimentally [13]. Zhan et al. prepared graphene/rubber nanocomposites by ultrasonically assisted latex mixing with in situ reduction processing, and observed essential reinforcement [14]. Although some progress has been demonstrated in graphene/ elastomer nanocomposites, the reinforcing efficiency is still not high and the overall mechanical performance is not satisfactory for many applications. To take a full advantage of graphene in the reinforcement of elastomers, the improved dispersion of graphene
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and higher interfacial interaction between graphene sheets and elastomeric matrix are important [15e17]. Although there are reports on the deliberately designed interface (e.g., by forming hydrogen bonds) in graphene oxide and polar elastomer systems [18e21], more efficient and stronger chemical bonding interfacial is necessary to achieve substantial reinforcement in non-polar elastomer systems. If one modifies the graphene with an active molecular layer that could chemically bound to the elastomer polymeric chains, then the interfacial interaction can be greatly improved by forming covalent interactions. Previously, tannin has been demonstrated as highly efficient in reducing graphene oxide and simultaneously decorating graphene [22]. The tannin was converted into ortho-quinone derivatives and adsorbed on the graphene sheets during the reduction of graphene oxide. It is essential that quinones are reactive towards thiols via Michael addition [23e25]. Considering that many polythiyls are generated during elastomer curing with sulfur-based compounds [26], the ortho-quinone derivatives are proposed to be suitable for constructing covalent interfacial crosslinking between graphene and rubber matrices. In this study, we develop a new strategy for constructing strong covalent interface in graphene/styrene butadiene rubber nanocomposites. A covalent ortho-quinone-mediated interface in the graphene/rubber system featuring excellent dispersion and controlled properties was realized in the rubber composites reinforced with quinone-modified graphene. Exceptionally efficient enhancement was realized in this system and the dynamic energy loss of the as-prepared graphene/rubber nanocomposites was essentially lower than those achieved in carbon black-filled rubber. The developed graphene/elastomer nanocomposites were applied to a typical dynamic elastomeric product - auto tires, and they exhibited an extremely low rolling resistance coefficient, which is a great advancement in the auto tire manufacturing process.
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50 ml ethanol and 20 ml tert-nonyl mercaptan was added into the ethanol solution of OTP. The mixture was reacted under magnetic stirring at 80 C under refluxing for half an hour. Deionized water was added to extract the product. The model compound OTP-tC9SH was obtained after vacuum evaporation of solvents. 2.3. Synthesis of TP-reduced graphene Graphite oxide (GO) was prepared by oxidizing natural graphite according to a modified Hummers method [27]. TP-reduced graphene was synthesized by chemical reduction of GO in which TPs were employed as reducer and stabilizer simultaneously. The process for the synthesis of TP-reduced graphene was similar to our previous work [28]. Typically, TPs (3 times relative to GO) were added into the GO aqueous solution (2 mg/ml). After sonication for 30 min, the mixture was vigorously stirred at 80 C for 8 h and the suspension was washed repeatedly with abundant deionized water to remove residual TPs. 2.4. Preparation of SBR composites Graphene/SBR nanocomposites were prepared by latex cocoagulation of SBR latex with suspension of graphene. Typically, certain amount of graphene aqueous suspension (about 2 mg/ml) was dropwise added into the SBR latex and stirred for 4 h. After the co-coagulation with calcium chloride aqueous solution (1 wt%), the rubber was washed with deionized water for several times. Then the compounds were dried in a vacuum oven at 60 C for 12 h. The rubber ingredients were mixed with the dried compounds with a two-roll mill and subjected to compression at 160 C for the optimum curing time determined by a vulcameter. A basic recipe (in weight part, SBR 100; zinc oxide 5; stearic acid 1; dibenzothiazoledisulfide 0.5; N-cyclohexylbenzothiazole-2-sulphenamide 1.5; sulfur 1.5) for the rubber composites was used.
2. Experimental 2.5. Preparation of graphene-based trial tires 2.1. Raw materials SBR latex (Intex 132) with solid content of 66 wt % and styrene content of 25 wt % was produced by Polimeri Europa, Italy. Nature graphite powder was supplied by Shanghai Colloidal Co. Ltd, Shanghai, China. Tea polyphenols (TPs) was manufactured by Xuancheng Baicao Plant Co. Ltd, Anhui, China. Polyphenol oxidase (PPO) was purchased from Worthington Biochemical Co. USA. Tertnonyl mercaptan (purity of 95%) was obtained from J&K Scientific Ltd. (Beijing, China). The chemicals used for the oxidation of graphite including concentrated sulfuric acid, sodium nitrate, potassium permanganate and hydrogen peroxide, were analytical reagent and used as received. Other rubber reagents were industrial grade and used as received. 2.2. Synthesis of oxidized tea polyphenol-tert-nonyl mercaptan adduct (OTP-t-C9SH) OTP-t-C9SH was adopted as the model compound to reveal the Michael addition mechanism between the polythiyl radical and the ortho-quinone derivatives based on ortho-quinone-thiol chemistry. Firstly, the ortho-quinone derivatives i.e. oxidized tea polyphenol (OTP) was obtained by enzymatic reaction of tea polyphenols with polyphenol oxidase (PPO). Tea polyphenol (TP, 1 g) was dissolved in 1000 ml of deionized water under magnetic stirring at room temperature. Afterwards, 10 mg PPO (about 10 ku) was added into the TP solution and the oxygen was fed into the mixture for 2 h continuously. OTP was obtained after the subsequent lyophilization of the above reaction mixture. Next, the as prepared OTP was dissolved in
To examine the potential use of graphene in industrial practice, graphene-based tread rubber compounds were prepared and molded into tires in Linglong Tire Co. Ltd (Shandong, China). The basic recipe is listed as follows (in weight part), SBR 70, BR 30, graphene 2, silica 28, bis-(3-triethoxysilylpropyl)-tetrasulfide 2.8, zinc oxide 5, stearic acid 1, N-isopropyl-N’-phenyl-4phenylenediamin 1.5, diphenyl guanidine 0.8, N-tert-butylbenzothiazole-2-sulphenamide 1.7, sulfur 1.7. For the sake of comparison we also prepared tire tread rubber composites according to the commercial green tire formula of Bridgestone [29]. 2.6. Characterizations High resolution transmission electron microscope (HRTEM) images were investigated by a JEM-2100 microscope. Fourier transform infrared spectra (FTIR) spectra were measured on a Bruker Vertex 70 FTIR spectrometer. X-ray spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra DLD spectrometer. 1 HNMR spectrum were recorded by Bruker AVANCE III600 MHz spectrometer using CDCl3 (for t-C9SH) or DMSO-d6 (for OTP and OTP-t-C9SH) as the solvents. Small angle X-ray scattering (SAXS) patterns was carried out on a Nanostar SAXS (Bruker-AXS) apparatus with Cu-Ka radiation (l ¼ 0.1542 nm) at 25 C. X-ray diffraction (XRD) analysis was performed on X’pret Pro diffractometer with Cu-Ka radiation (l ¼ 0.1542 nm). The curing characteristics of the SBR compounds were determined at 160 C by a UCAN UR-2030 vulcameter, Taiwan. Tensile tests were performed on U-CAN UT-2060 (Taiwan) according to ISO standard 37e2005 with
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a stretching rate of 500 mm/min. The abrasion test was carried out with the DIN abrader (GT-7012-D, GOTECH Testing Machines Co., Taiwan) according to ISO 4649 standards. Dynamic compression heat build-up was determined by RH-2000 compression heat build-up tester (GOTECH Testing Machines Co., Taiwan). Rubber wheel rolling resistance test were measured on a RSS- II model rolling resistance testing machine (Beijing Wanhuiyifang technology Co., Ltd). The rolling resistance of the graphene-based trial tires were determined by MTS Tire Rolling Resistance System (MTS Systems Corporation, USA).
3. Results and discussion 3.1. Construction of covalent interface and graphene dispersion in rubber nanocomposites The fabrication processes of graphene/rubber nanocomposites are schematically illustrated in Fig. 1a. First, tea polyphenol (TP)reduced graphene was synthesized by chemical reduction of graphene oxide (GO) with TPs, in which TPs served as a highly efficient reducer and stabilizer [28]. During the chemical reduction, the TPs were oxidized and converted partly to ortho-quinone derivatives, which can be adsorbed onto the surface of graphene through p-p and hydrogen-bond interactions. The aqueous suspension of orthoquinone derivatives modified graphene was mechanically mixed with SBR latex for 4 h. Subsequently, the compound was coagulated and the rubber ingredients were added by a two-roll mill mixer. The mixture was vulcanized under a hot press at 160 C to obtain the crosslinked graphene/rubber nanocomposites. During this process, the vulcanizing agents react with rubber chains and form activated crosslink precursor, i.e. polythiyl radicals, as shown in Fig. 1b [26]. The crosslink precursors react with each other forming the sulfide crosslinks. Additionally, the ortho-quinone modified graphene can also covalently crosslinked onto the rubber chains via Michael addition between the polythiyl radical and the orthoquinone derivatives. The covalent interfacial crosslinking also contributes to better graphene dispersibility as well as the overall crosslinking system of the composites. Thus, the combination of uniform dispersion and the interfacial and sulfur crosslinking resulted in the rationally designed high-performance SBR nanocomposites with high reinforcing efficiency and low dynamic energy loss.
The dispersion of nanofillers is critical to the performance of the polymeric nanocomposites. HRTEM was used to probe the morphology of the graphene/rubber nanocomposites filled with 5.6 phr of graphene. As shown in Fig. 2, the dark lines are the crosssections of the graphene sheets, and the gray areas are part of the SBR matrix. Overall, it can be seen that the graphene sheets are very uniformly dispersed throughout the matrix without obvious aggregation and with partially orientated structure which could be induced by the shearing during the two-roll milling process. The shear-induced orientation of highly anisotropic nanoparticles such as clay and graphene in polymer composites was also observed in others’ studies [30e32]. By careful inspection on the morphology, it is found the nanocomposites possess hierarchical structures. At lower magnitude (Fig. 2a), the loosely re-stacked graphene nanoplatelets with a typical thickness of about 20 nm are clearly observed. From HRTEM image (Fig. 2b), one can find that substantial amount of the layers remain individually dispersed graphene sheets. We can also observe that some of the graphene layers are loosely packed to form order structure, which will be further substantiated by SAXS and XRD results (Fig. S1). Consequently rather complicated hierarchical structures but uniform dispersion status are resulted in the nanocomposites, which should be ascribed to the combination effects of kinetic, thermodynamic factors and the formation of strong covalent interface (as discussed below) during the fabrication of the nanocomposites. As is well known, the interfacial interaction is of the utmost importance on the dispersion status of filler and ultimate performance of rubber nanocomposites for the following several primary considerations. Firstly, the strong interfacial interaction hinders the aggregation of filler which is driven thermodynamically during compounding. Secondly, the improved interfacial interaction facilitates the transfer of interfacial loading. Last but not least, strong interfacial interaction effectively suppress the nano-scale frictions between graphene sheet and rubber chain because of the relative slippage, thereby the dynamic energy loss of rubber nanocomposites can be reduced. Based on the above considerations, we have accordingly developed a novel covalent ortho-quiononemediated interface in order to achieve high reinforcing efficiency and low dynamic energy loss in graphene/rubber nanocomposite systems. Based on ortho-quinone-thiol chemistry, we proposed that the TP-reduced graphene were chemical crosslink onto the rubber
Fig. 1. (a) Fabrication of graphene/rubber nanocomposites and (b) schematic illustration of proposed formation mechanism of covalent interface between graphene and rubber matrix.
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Fig. 2. TEM photos of graphene/rubber nanocomposites (5.6 phr graphene) with different magnifications.
chains as have been mentioned above. However, direct characterization of the interfacial crosslinking would be extremely difficult due to the very low content of group concentration (about 105 mol/cm3). Consequently, to confirm the formation mechanism of ortho-quionone-mediated interface in graphene/rubber system as described above, model compound oxidized TPs (OTP)-tertnonyl mercaptan (t-C9SH) were prepared to reveal the Michael addition mechanism between the polythiyl radical and the orthoquinone derivatives. Herein, OTP obtained by direct enzymatic reaction of TPs with polyphenol oxidase is used to mimic the oxidized tea polyphenol absorbed on the graphene while tert-nonyl mercaptan is adopted as the model compound of polythiyls, as it has been revealed that many mercaptans generated during curing are tert-mercaptans [26]. FTIR were performed on OTP, t-C9SH and their addition product (OTP-t-C9SH). As shown in Fig. 3a, the characteristic peak at 1720 cm1 is assigned to the C]O stretching vibration of the ortho-quinone structure on OTP, which confirmed the successful oxidization of TPs by enzymatic reaction (The oxidization of TPs is further substantiated by FTIR and XPS measurement, Fig. S2). However, in the FTIR spectrum of OTP-t-C9SH, the vibration peak of C]O of the ortho-quinone disappear, replaced by a new absorption peak at 1694 cm1, which is assign to the C]O absorption of ester carbonyl of TPs. In addition, the characteristic peak at 1610, 1517 and 1451 cm1 are strengthened, indicating the recovery of aromatic ring structure of OTP during the reaction. The absorption of O-H bending vibration at 1035 cm1 in OTP is enhanced simultaneously. Thus, we proposed that the orthoquinone groups have participated in the Michael reaction between the OTP and t-C9SH and are converted to phenolic hydroxyl after the reaction. Compared with the spectrum of OTP, three new absorption bands at 2964 cm1, 2930 cm1 and 2870 cm1, are found in the spectrum of OTP-t-C9SH, which are assigned to the asymmetric stretching vibration of -CH3, eCH2 and symmetric stretching of -CH3, respectively. These alkyl characteristic absorptions exactly correspond to those for t-C9SH. Besides, the weak absorption peak at 2576 cm1 which related to the S-H stretching vibration in tC9SH disappears in the spectrum of OTP-t-C9SH, providing further implication of the Michael addition between OTP and t-C9SH. Moreover, the emerging of new characteristic peak at 1094 cm1 in OTP-t-C9SH suggests the formation of Ar-S-C sulfide structure, which further confirms the formation of covalent bond between OTP and t-C9SH. The formation of OTP-t-C9SH is further substantiated by 1H NMR spectroscopy. Fig. 3b depicts the comparison of 1 H NMR spectrums of t-C9SH, OTP and OTP-t-C9SH. The assignments of the 1H NMR chemical shifts (d) of t-C9SH (SH-a at dH 1.5 s;
H-b, -c, -d, -e, -f at dH 1.3 s, 1.32 t, 1.23 m, 0.88 m, 0.83 t, respectively) were made and the 1H NMR chemical shifts of the OTP were similar to our previous report [28]. Tentative assignments of 1H NMR spectrum of OTP-t-C9SH (H-b’, d’, e’ at dH 1.1 m; H-c’, -f’ at dH 1.2 t, 0.84 t, respectively) were made by compare with the 1H NMR spectrum of t-C9SH. Specifically, the 1H NMR chemical shifts dH at 1.5 which represents the H on the mercapto group of t-C9SH does not arise at the spectrum of OTP-t-C9SH, which is in accordance with the result of FTIR spectrum. Besides, the emergence of chemical shifts around 5e9 ppm can be attributed to the recovery of aromatic ring structure after the addition reaction [28]. Clearly, both the results of FTIR and 1H NMR spectrums confirm that the novel covalent interface has been successfully constructed in the graphene/SBR nanocomposites. Thus the combination of uniform dispersion and both the covalent interfacial crosslinking and sulfur crosslinking systems are resulted in the as-prepared graphene/ rubber nanocomposites. 3.2. Mechanical properties of graphene/rubber nanocomposites With the introduction of covalent interface, the dispersion status of graphene in the rubber matrix and the interfacial interaction between graphene and SBR are greatly improved, which is of significant to the mechanical properties of composites. The effect of graphene on mechanical properties of graphene/SBR nanocomposites were fully studied and the stress-strain curves of neat SBR and SBR nanocomposites are presented in Fig. 4a. Fig. 4b shows the tensile modulus (stress at 300% strain) and ultimate strength normalized to the values for neat SBR as a function of graphene contents. Detailed evolution of mechanical data with graphene content are shown in Fig. 4c. Compared with the neat SBR, the tensile strength of graphene/SBR nanocomposites with only 1.1 phr of graphene is increase by 223%. With incorporation of 5.6 phr of graphene, the tensile strength of the nanocomposite can reach to 21.5 MPa, which is over 9-folded to that of the neat SBR. Such reinforcing efficiency is actually unprecedented comparing to other layered reinforcing fillers such as clay [33e35]. A comprehensive collection of other reported data concerning the tensile strength of layered fillers reinforced rubber nanocomposites is listed and compared with the present result (Table S1), and a comparison with pristine graphite filled SBR composites was also made (Fig. S3 and Table S2). Compared with these systems, the present work indeed exhibits much higher efficiency in reinforcement of SBR, which is further confirmed by percolation phenomena of the composites based on the relationship between filler loading and tensile
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Fig. 3. (a) FTIR spectrum and (b) 1H NMR spectrum of OTP, t-C9SH and OTP-t-C9SH.
strength. As have been reported by Zhang [36], percolation phenomenon could be explained by the rubber strengthening mechanism that the formation of straightened polymer chains induced by the neighbouring fillers during the stretching process. When the distance between the fillers decrease to a specific threshold which can ensure adequate rubber chains attached on the adjacent fillers, the formation of straightened rubber chains are promoted and strengthen the rubber. As can be observed in Fig. 4d, the tensile stress of the composites initially increases slightly and then shows an abrupt increase. The percolation point of graphene/SBR nanocomposite appears surprisingly at as low as 0.42 phr, which is far below that of CB/SBR composite (5.1 phr). The ultralow percolation threshold of graphene/SBR composites should be ascribed to excellent dispersion of ultrathin graphene layers and the strong covalent interface. The above obviously different result of graphene and CB in reinforcement of SBR as well as the following MooneyRivlin plots and abrasion resistance (Fig. S4) of the corresponding SBR nanocomposites, strongly evidence that graphene possess significant higher reinforcing efficiency and better reinforcing effect towards SBR. 3.3. Reduced dynamic energy loss for energy-saving graphenebased trial tires Dynamic energy loss (hysteresis loss) is one of the most important aspect for elastomer products used in dynamic
applications such as tire tread. Generally, the energy loss of tires during service is also known as rolling resistance. Due to the hysteresis of the repeated deformations, the mechanical energy of tires partially transformed into internal energy and dissipated as heat, which will lead to the energy loss and heat build-up of tires. The temperature increment of tire will inevitably deteriorate the performance of tire and may cause potential dangers during serving. Herein, the dynamic energy loss and temperature increment of graphene/SBR nanocomposite and the control samples were tested on a rotational power loss tester. As shown in Fig. 5a, RSS-II model rolling resistance testing machine is designed according to the basic theory of rubber viscoelastic. The deformation of the rubber wheel always lags behind the stress when the rubber wheel rotates on the roller, thus the energy loss can be characterized as the torque of the opposite direction on the roller. In Fig. 5b, the energy loss of graphene/SBR nanocomposites filled with 5.6 phr of graphene is measured to be 8.7 J/r, which is lower than that of the control samples of CB/SBR composites filled with 50 phr of CB (10.9 J/r), indicating 20% energy saving. Due to the dissipation of energy, the temperature of CB/SBR nanocomposites wheel is significant increased by 87 C, while the temperature increment for graphene/ SBR nanocomposites is only 67 C. In addition, the dynamic energy loss of both the graphene/SBR and CB/SBR nanocomposites is also investigated by the dynamic compression heat build-up tester (Fig. S5), and the results further indicates that graphene/SBR composites possess much lower dynamic energy loss (rolling
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Fig. 4. (a)Stress-strain curves of SBR/graphene nanocomposites, (b) Modulus and stress values for graphene/SBR nanocomposites, normalized to the values for neat SBR, (c) Tensile modulus, strength and tear strength of graphene/SBR nanocomposites and (d) Percolation phenomena of SBR nanocomposites, the intersections of dash lines represent the percolation points.
resistance) than CB/SBR nanocomposites. Previously, it has been well established that the visco-elastic nature of elastomer nanocomposites tailored for tire tread leads to the occurrence of the hysteresis loss under the cyclic loading-unloading deformation when tires are running on the roads, which, at the molecular level, results from the nano-scale frictions between polymer chains, between nanoparticles and between polymer chain and nanoparticle because of the relative slippage [37,38], as schematically displayed in Fig. S6. In the present work, benefit from the uniform dispersion of graphene and the elaborate interfacial crosslinking as have been
proven above, the frictions of the latter two can be effectively suppressed. Thus, low dynamic energy loss are achieved in graphene/rubber nanocomposites (further supported by Molecular Simulation in Supporting Information). Based on the significant improved mechanical performances and reduced dynamic energy loss, the as-prepared graphene/SBR nanocomposites exhibit great potential for application of “green” tire tread. To evaluate the potential use of graphene in industrial practice, graphene-based tread rubber compounds were molded into trial tires and subjected to professional tire test (Fig. 6). Due to
Fig. 5. (a) RSS-II model rolling resistance testing machine and its diagrammatic sketch and (b) Dynamic energy loss and temperature increment of CB/SBR (50 phr N330) and graphene/SBR (5.6 phr graphene) nanocomposites.
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Fig. 6. Fabrication and rolling resistance test of energy-saving graphene-based trial tires.
the high reinforcing efficiency of graphene, acceptable mechanical properties can be achieved at much lower filler content which endows the nanocomposites with lower density. Published work has reported that every 10% reduction in vehicle weight can cut fuel consumption by about 7% [39]. The density test shows that the density of the as-prepared graphene-based tread rubber is 1.1056 g/ cm3, an average of about 10% lower than that of commercial green tires. Therefore, the reduced tires weight could simply save about 0.4% fuel consumption (based on 1200 kg vehicle weight, 15 kg per tire and a set of five tires included a spare) by adopting the graphene-based rubber tires. Furthermore, the rotational energy loss (rolling resistance) of the trial tires were measured by professional MTS Tire Rolling Resistance System. The result shows that the rolling resistance coefficient (RRC) of the graphene-based trial tires is only 6.81 kg/t which is very close to the “A grade” energy efficiency according to the fuel efficiency classes for tires of UNECE Regulation (EC) No 1222/2009 (Table S4) [40]. Commonly, the RRC of ordinary (or “black”) tires is as high as 13 kg/t while those of the commercial green tires is about 10 kg/t. As has been demonstrated that every 5% reduction in rolling resistance, the fuel consumption of vehicle is reduced by 1% [41,42]. Obviously, a significant fuel saving of up to 9.5% can be obtained by substituting graphenebased tires for ordinary tires. Even compared with commercial green tires, an energy saving of 6.4% can be achieved when adopting graphene-based tires for passenger cars which is of great help to the economic and environmental improvements. The above striking results imply the potential application of graphene-based rubber nanocomposites in high-performance green tire.
4. Conclusions A novel covalent interface in graphene/SBR nanocomposites based on ortho-quinone-thiol chemistry was developed. The latex co-coagulation and interfacial crosslinking between ortho-quinones modified graphene and rubber chains constrained the restacking of the nanosheets very effectively. The uniform dispersion and strong interfacial interaction provided synergetic improvement in the nanocomposite properties: an unprecedented reinforcing efficiency combined with drastically decreased energy loss in these graphene/SBR nanocomposites have been realized. The present work revealed a critical role of interfacial adhesion in the performance of rubber nanocomposites and provided a new solution for design for low energy loss elastomeric composites used in dynamic applications. The trial tire approaching highest A-grade energy efficiency has been performed. It is the first successful attempt to fabricate graphene-based energy-saving tires in a scalable manner, which advances the application in industry and helps to cut down fuel consumption allowing for better environmental protection.
Acknowledgment This work was supported by National Basic Research Program of China (2015CB654703), National Natural Science Foundation of China (51521062, 51473050, 51320105012 and 51333003), Key Project of Beijing Municipal Science and Technology Commission (D14110300230000) and Research Fund for the Doctoral Program of Higher Education of China (20130172110001). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.compscitech.2016.06.015. References [1] M. Wagner, Reinforcing silicas and silicates, Rubber Chem. Technol. 49 (3) (1976) 703e774. [2] M.L. Studebaker, The chemistry of carbon black and reinforcement, Rubber Chem. Technol. 30 (5) (1957) 1400e1483. [3] S. Pavlidou, C.D. Papaspyrides, A review on polymerelayered silicate nanocomposites, Prog. Polym. Sci. 33 (12) (2008) 1119e1198. [4] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183e191. [5] J.C. Meyer, A.K. Geim, M. Katsnelson, K. Novoselov, T. Booth, S. Roth, The structure of suspended graphene sheets, Nature 446 (7131) (2007) 60e63. [6] S. Stankovich, D.A. Dikin, G.H. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, et al., Graphene-based composite materials, Nature 442 (2006) 282e286. [7] X. Huang, X. Qi, F. Boey, H. Zhang, Graphene-based composites, Chem. Soc. Rev. 41 (2) (2012) 666e686. [8] C.S. Boland, U. Khan, C. Backes, A. O’Neill, J. McCauley, S. Duane, et al., Sensitive, high-strain, high-rate bodily motion sensors based on grapheneerubber composites, ACS Nano 8 (9) (2014) 8819e8830. [9] J. George, A. Bhowmick, Ethylene vinyl acetate/expanded graphite nanocomposites by solution intercalation: preparation, characterization and properties, J. Mater. Sci. 43 (2) (2008) 702e708. € ckelhuber, et al., [10] A. Das, G.R. Kasaliwal, R. Jurk, R. Boldt, D. Fischer, K.W. Sto Rubber composites based on graphene nanoplatelets, expanded graphite, carbon nanotubes and their combination: a comparative study, Compos. Sci. Technol. 72 (16) (2012) 1961e1967. [11] S. Araby, I. Zaman, Q. Meng, N. Kawashima, A. Michelmore, H.C. Kuan, et al., Melt compounding with graphene to develop functional, high-performance elastomers, Nanotechnology 24 (16) (2013) 165601. [12] B. Ozbas, S. Toki, B.S. Hsiao, B. Chu, R.A. Register, I.A. Aksay, et al., Straininduced crystallization and mechanical properties of functionalized graphene sheet-filled natural rubber, J. Polym. Sci. Part B Polym. Phys. 50 (10) (2012) 718e723. [13] J.R. Potts, O. Shankar, L. Du, R.S. Ruoff, ProcessingeMorphologyeProperty relationships and composite theory analysis of reduced graphene oxide/natural rubber nanocomposites, Macromolecules 45 (15) (2012) 6045e6055. [14] Y. Zhan, J. Wu, H. Xia, N. Yan, G. Fei, G. Yuan, Dispersion and exfoliation of graphene in rubber by an ultrasonically-assisted latex mixing and in situ reduction process, Macromol. Mater. Eng. 296 (7) (2011) 590e602. [15] J.R. Potts, O. Shankar, S. Murali, L. Du, R.S. Ruoff, Latex and two-roll mill processing of thermally-exfoliated graphite oxide/natural rubber nanocomposites, Compos. Sci. Technol. 74 (0) (2013) 166e172. [16] N. Yousefi, M.M. Gudarzi, Q. Zheng, X. Lin, X. Shen, J. Jia, et al., Highly aligned, ultralarge-size reduced graphene oxide/polyurethane nanocomposites: mechanical properties and moisture permeability, Compos. Part A 49 (2013) 42e50. [17] Y. Mao, S. Wen, Y. Chen, F. Zhang, P. Panine, T.W. Chan, et al., High
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Rational design of covalent interfaces for graphene/elastomer nanocomposites Zhijun Yang a, 1, Jun Liu b, 1, Ruijuan Liao a, Ganwei Yang a, Xiaohui Wu b, Zhenghai Tang a, Baochun Guo a, *, Liqun Zhang b, **, Yong Ma c, Qiuhai Nie c, Feng Wang c a b c
Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou, 510640, PR China State Key Laboratory of Organic/Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR China Shandong Linglong Tire Co., Ltd., Zhaoyuan, 265406, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 June 2016 Received in revised form 27 June 2016 Accepted 28 June 2016 Available online 1 July 2016
The energy loss of tires during service is closely related to the hysteresis of tire tread, which is governed by the dispersion and interface of the elastomer nanocomposites. However, traditional undeformable spherical fillers have approached a bottleneck in regulating the viscoelasticity and lowering the hysteresis loss. Herein, the designed covalent interface in the graphene/elastomer nanocomposite maximizes the reinforcement of the nanomaterial as well as minimizes the dynamic energy loss. The reinforced interfacial interaction, an ultralow percolation threshold of tensile strength and high reinforcement synergistically benefit these nano-organized rubber nanocomposites. The energy losses under dynamic loading are largely suppressed in this material, due to the reduced nano-scale frictions. The developed graphene/elastomer nanocomposites are applied to typical dynamic rubber product - auto tires, and the tests indicate that these tires possess energy efficiency close to the highest “A-grade”, which gave great economic and environmental improvements. The critical role of interface structure in the performance of the elastomer nanocomposites was revealed and new design strategy for low dynamic energy losses in rubber products was suggested. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Nano composites Interface Mechanical properties Strength
1. Introduction High performance elastomeric materials are strategically important in different areas including applications in extreme conditions. Reinforcement realized by inclusion of various nanoparticles into the matrix is needed for most synthetic elastomers because they are not strong mechanically. Tens of parts of carbon black or silica are needed for gaining acceptable mechanical strength in the composite [1,2]. However, there are two main drawbacks in such highly filled elastomer nanocomposites. First, it gives rise to a higher friction among the filler particles which results in energy losses and heat generation for dynamic elastomeric products. The second is associated with the reduction of specific strength and modulus due to the increased density, which is the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Guo), [email protected] (L. Zhang). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.compscitech.2016.06.015 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
current bottleneck in lightweight design of elastomeric composites. Design of polymeric nanocomposites with high specific performance is the cutting edge topic in polymer research [3]. Recently, graphene which possesses unparalleled physical properties such as outstanding mechanical properties, was proposed as a high-efficiency reinforcement for elastomers [4,5]. Many graphene-based polymer composites with significantly improved properties have been prepared and tested [6,7]. However, only a few studies have been devoted to graphene/elastomer nanocomposites [8e12]. Potts et al. incorporated reduced graphene oxide into natural rubber latex and demonstrated processingemorphologyeproperty relationships both theoretically and experimentally [13]. Zhan et al. prepared graphene/rubber nanocomposites by ultrasonically assisted latex mixing with in situ reduction processing, and observed essential reinforcement [14]. Although some progress has been demonstrated in graphene/ elastomer nanocomposites, the reinforcing efficiency is still not high and the overall mechanical performance is not satisfactory for many applications. To take a full advantage of graphene in the reinforcement of elastomers, the improved dispersion of graphene
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and higher interfacial interaction between graphene sheets and elastomeric matrix are important [15e17]. Although there are reports on the deliberately designed interface (e.g., by forming hydrogen bonds) in graphene oxide and polar elastomer systems [18e21], more efficient and stronger chemical bonding interfacial is necessary to achieve substantial reinforcement in non-polar elastomer systems. If one modifies the graphene with an active molecular layer that could chemically bound to the elastomer polymeric chains, then the interfacial interaction can be greatly improved by forming covalent interactions. Previously, tannin has been demonstrated as highly efficient in reducing graphene oxide and simultaneously decorating graphene [22]. The tannin was converted into ortho-quinone derivatives and adsorbed on the graphene sheets during the reduction of graphene oxide. It is essential that quinones are reactive towards thiols via Michael addition [23e25]. Considering that many polythiyls are generated during elastomer curing with sulfur-based compounds [26], the ortho-quinone derivatives are proposed to be suitable for constructing covalent interfacial crosslinking between graphene and rubber matrices. In this study, we develop a new strategy for constructing strong covalent interface in graphene/styrene butadiene rubber nanocomposites. A covalent ortho-quinone-mediated interface in the graphene/rubber system featuring excellent dispersion and controlled properties was realized in the rubber composites reinforced with quinone-modified graphene. Exceptionally efficient enhancement was realized in this system and the dynamic energy loss of the as-prepared graphene/rubber nanocomposites was essentially lower than those achieved in carbon black-filled rubber. The developed graphene/elastomer nanocomposites were applied to a typical dynamic elastomeric product - auto tires, and they exhibited an extremely low rolling resistance coefficient, which is a great advancement in the auto tire manufacturing process.
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50 ml ethanol and 20 ml tert-nonyl mercaptan was added into the ethanol solution of OTP. The mixture was reacted under magnetic stirring at 80 C under refluxing for half an hour. Deionized water was added to extract the product. The model compound OTP-tC9SH was obtained after vacuum evaporation of solvents. 2.3. Synthesis of TP-reduced graphene Graphite oxide (GO) was prepared by oxidizing natural graphite according to a modified Hummers method [27]. TP-reduced graphene was synthesized by chemical reduction of GO in which TPs were employed as reducer and stabilizer simultaneously. The process for the synthesis of TP-reduced graphene was similar to our previous work [28]. Typically, TPs (3 times relative to GO) were added into the GO aqueous solution (2 mg/ml). After sonication for 30 min, the mixture was vigorously stirred at 80 C for 8 h and the suspension was washed repeatedly with abundant deionized water to remove residual TPs. 2.4. Preparation of SBR composites Graphene/SBR nanocomposites were prepared by latex cocoagulation of SBR latex with suspension of graphene. Typically, certain amount of graphene aqueous suspension (about 2 mg/ml) was dropwise added into the SBR latex and stirred for 4 h. After the co-coagulation with calcium chloride aqueous solution (1 wt%), the rubber was washed with deionized water for several times. Then the compounds were dried in a vacuum oven at 60 C for 12 h. The rubber ingredients were mixed with the dried compounds with a two-roll mill and subjected to compression at 160 C for the optimum curing time determined by a vulcameter. A basic recipe (in weight part, SBR 100; zinc oxide 5; stearic acid 1; dibenzothiazoledisulfide 0.5; N-cyclohexylbenzothiazole-2-sulphenamide 1.5; sulfur 1.5) for the rubber composites was used.
2. Experimental 2.5. Preparation of graphene-based trial tires 2.1. Raw materials SBR latex (Intex 132) with solid content of 66 wt % and styrene content of 25 wt % was produced by Polimeri Europa, Italy. Nature graphite powder was supplied by Shanghai Colloidal Co. Ltd, Shanghai, China. Tea polyphenols (TPs) was manufactured by Xuancheng Baicao Plant Co. Ltd, Anhui, China. Polyphenol oxidase (PPO) was purchased from Worthington Biochemical Co. USA. Tertnonyl mercaptan (purity of 95%) was obtained from J&K Scientific Ltd. (Beijing, China). The chemicals used for the oxidation of graphite including concentrated sulfuric acid, sodium nitrate, potassium permanganate and hydrogen peroxide, were analytical reagent and used as received. Other rubber reagents were industrial grade and used as received. 2.2. Synthesis of oxidized tea polyphenol-tert-nonyl mercaptan adduct (OTP-t-C9SH) OTP-t-C9SH was adopted as the model compound to reveal the Michael addition mechanism between the polythiyl radical and the ortho-quinone derivatives based on ortho-quinone-thiol chemistry. Firstly, the ortho-quinone derivatives i.e. oxidized tea polyphenol (OTP) was obtained by enzymatic reaction of tea polyphenols with polyphenol oxidase (PPO). Tea polyphenol (TP, 1 g) was dissolved in 1000 ml of deionized water under magnetic stirring at room temperature. Afterwards, 10 mg PPO (about 10 ku) was added into the TP solution and the oxygen was fed into the mixture for 2 h continuously. OTP was obtained after the subsequent lyophilization of the above reaction mixture. Next, the as prepared OTP was dissolved in
To examine the potential use of graphene in industrial practice, graphene-based tread rubber compounds were prepared and molded into tires in Linglong Tire Co. Ltd (Shandong, China). The basic recipe is listed as follows (in weight part), SBR 70, BR 30, graphene 2, silica 28, bis-(3-triethoxysilylpropyl)-tetrasulfide 2.8, zinc oxide 5, stearic acid 1, N-isopropyl-N’-phenyl-4phenylenediamin 1.5, diphenyl guanidine 0.8, N-tert-butylbenzothiazole-2-sulphenamide 1.7, sulfur 1.7. For the sake of comparison we also prepared tire tread rubber composites according to the commercial green tire formula of Bridgestone [29]. 2.6. Characterizations High resolution transmission electron microscope (HRTEM) images were investigated by a JEM-2100 microscope. Fourier transform infrared spectra (FTIR) spectra were measured on a Bruker Vertex 70 FTIR spectrometer. X-ray spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra DLD spectrometer. 1 HNMR spectrum were recorded by Bruker AVANCE III600 MHz spectrometer using CDCl3 (for t-C9SH) or DMSO-d6 (for OTP and OTP-t-C9SH) as the solvents. Small angle X-ray scattering (SAXS) patterns was carried out on a Nanostar SAXS (Bruker-AXS) apparatus with Cu-Ka radiation (l ¼ 0.1542 nm) at 25 C. X-ray diffraction (XRD) analysis was performed on X’pret Pro diffractometer with Cu-Ka radiation (l ¼ 0.1542 nm). The curing characteristics of the SBR compounds were determined at 160 C by a UCAN UR-2030 vulcameter, Taiwan. Tensile tests were performed on U-CAN UT-2060 (Taiwan) according to ISO standard 37e2005 with
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a stretching rate of 500 mm/min. The abrasion test was carried out with the DIN abrader (GT-7012-D, GOTECH Testing Machines Co., Taiwan) according to ISO 4649 standards. Dynamic compression heat build-up was determined by RH-2000 compression heat build-up tester (GOTECH Testing Machines Co., Taiwan). Rubber wheel rolling resistance test were measured on a RSS- II model rolling resistance testing machine (Beijing Wanhuiyifang technology Co., Ltd). The rolling resistance of the graphene-based trial tires were determined by MTS Tire Rolling Resistance System (MTS Systems Corporation, USA).
3. Results and discussion 3.1. Construction of covalent interface and graphene dispersion in rubber nanocomposites The fabrication processes of graphene/rubber nanocomposites are schematically illustrated in Fig. 1a. First, tea polyphenol (TP)reduced graphene was synthesized by chemical reduction of graphene oxide (GO) with TPs, in which TPs served as a highly efficient reducer and stabilizer [28]. During the chemical reduction, the TPs were oxidized and converted partly to ortho-quinone derivatives, which can be adsorbed onto the surface of graphene through p-p and hydrogen-bond interactions. The aqueous suspension of orthoquinone derivatives modified graphene was mechanically mixed with SBR latex for 4 h. Subsequently, the compound was coagulated and the rubber ingredients were added by a two-roll mill mixer. The mixture was vulcanized under a hot press at 160 C to obtain the crosslinked graphene/rubber nanocomposites. During this process, the vulcanizing agents react with rubber chains and form activated crosslink precursor, i.e. polythiyl radicals, as shown in Fig. 1b [26]. The crosslink precursors react with each other forming the sulfide crosslinks. Additionally, the ortho-quinone modified graphene can also covalently crosslinked onto the rubber chains via Michael addition between the polythiyl radical and the orthoquinone derivatives. The covalent interfacial crosslinking also contributes to better graphene dispersibility as well as the overall crosslinking system of the composites. Thus, the combination of uniform dispersion and the interfacial and sulfur crosslinking resulted in the rationally designed high-performance SBR nanocomposites with high reinforcing efficiency and low dynamic energy loss.
The dispersion of nanofillers is critical to the performance of the polymeric nanocomposites. HRTEM was used to probe the morphology of the graphene/rubber nanocomposites filled with 5.6 phr of graphene. As shown in Fig. 2, the dark lines are the crosssections of the graphene sheets, and the gray areas are part of the SBR matrix. Overall, it can be seen that the graphene sheets are very uniformly dispersed throughout the matrix without obvious aggregation and with partially orientated structure which could be induced by the shearing during the two-roll milling process. The shear-induced orientation of highly anisotropic nanoparticles such as clay and graphene in polymer composites was also observed in others’ studies [30e32]. By careful inspection on the morphology, it is found the nanocomposites possess hierarchical structures. At lower magnitude (Fig. 2a), the loosely re-stacked graphene nanoplatelets with a typical thickness of about 20 nm are clearly observed. From HRTEM image (Fig. 2b), one can find that substantial amount of the layers remain individually dispersed graphene sheets. We can also observe that some of the graphene layers are loosely packed to form order structure, which will be further substantiated by SAXS and XRD results (Fig. S1). Consequently rather complicated hierarchical structures but uniform dispersion status are resulted in the nanocomposites, which should be ascribed to the combination effects of kinetic, thermodynamic factors and the formation of strong covalent interface (as discussed below) during the fabrication of the nanocomposites. As is well known, the interfacial interaction is of the utmost importance on the dispersion status of filler and ultimate performance of rubber nanocomposites for the following several primary considerations. Firstly, the strong interfacial interaction hinders the aggregation of filler which is driven thermodynamically during compounding. Secondly, the improved interfacial interaction facilitates the transfer of interfacial loading. Last but not least, strong interfacial interaction effectively suppress the nano-scale frictions between graphene sheet and rubber chain because of the relative slippage, thereby the dynamic energy loss of rubber nanocomposites can be reduced. Based on the above considerations, we have accordingly developed a novel covalent ortho-quiononemediated interface in order to achieve high reinforcing efficiency and low dynamic energy loss in graphene/rubber nanocomposite systems. Based on ortho-quinone-thiol chemistry, we proposed that the TP-reduced graphene were chemical crosslink onto the rubber
Fig. 1. (a) Fabrication of graphene/rubber nanocomposites and (b) schematic illustration of proposed formation mechanism of covalent interface between graphene and rubber matrix.
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Fig. 2. TEM photos of graphene/rubber nanocomposites (5.6 phr graphene) with different magnifications.
chains as have been mentioned above. However, direct characterization of the interfacial crosslinking would be extremely difficult due to the very low content of group concentration (about 105 mol/cm3). Consequently, to confirm the formation mechanism of ortho-quionone-mediated interface in graphene/rubber system as described above, model compound oxidized TPs (OTP)-tertnonyl mercaptan (t-C9SH) were prepared to reveal the Michael addition mechanism between the polythiyl radical and the orthoquinone derivatives. Herein, OTP obtained by direct enzymatic reaction of TPs with polyphenol oxidase is used to mimic the oxidized tea polyphenol absorbed on the graphene while tert-nonyl mercaptan is adopted as the model compound of polythiyls, as it has been revealed that many mercaptans generated during curing are tert-mercaptans [26]. FTIR were performed on OTP, t-C9SH and their addition product (OTP-t-C9SH). As shown in Fig. 3a, the characteristic peak at 1720 cm1 is assigned to the C]O stretching vibration of the ortho-quinone structure on OTP, which confirmed the successful oxidization of TPs by enzymatic reaction (The oxidization of TPs is further substantiated by FTIR and XPS measurement, Fig. S2). However, in the FTIR spectrum of OTP-t-C9SH, the vibration peak of C]O of the ortho-quinone disappear, replaced by a new absorption peak at 1694 cm1, which is assign to the C]O absorption of ester carbonyl of TPs. In addition, the characteristic peak at 1610, 1517 and 1451 cm1 are strengthened, indicating the recovery of aromatic ring structure of OTP during the reaction. The absorption of O-H bending vibration at 1035 cm1 in OTP is enhanced simultaneously. Thus, we proposed that the orthoquinone groups have participated in the Michael reaction between the OTP and t-C9SH and are converted to phenolic hydroxyl after the reaction. Compared with the spectrum of OTP, three new absorption bands at 2964 cm1, 2930 cm1 and 2870 cm1, are found in the spectrum of OTP-t-C9SH, which are assigned to the asymmetric stretching vibration of -CH3, eCH2 and symmetric stretching of -CH3, respectively. These alkyl characteristic absorptions exactly correspond to those for t-C9SH. Besides, the weak absorption peak at 2576 cm1 which related to the S-H stretching vibration in tC9SH disappears in the spectrum of OTP-t-C9SH, providing further implication of the Michael addition between OTP and t-C9SH. Moreover, the emerging of new characteristic peak at 1094 cm1 in OTP-t-C9SH suggests the formation of Ar-S-C sulfide structure, which further confirms the formation of covalent bond between OTP and t-C9SH. The formation of OTP-t-C9SH is further substantiated by 1H NMR spectroscopy. Fig. 3b depicts the comparison of 1 H NMR spectrums of t-C9SH, OTP and OTP-t-C9SH. The assignments of the 1H NMR chemical shifts (d) of t-C9SH (SH-a at dH 1.5 s;
H-b, -c, -d, -e, -f at dH 1.3 s, 1.32 t, 1.23 m, 0.88 m, 0.83 t, respectively) were made and the 1H NMR chemical shifts of the OTP were similar to our previous report [28]. Tentative assignments of 1H NMR spectrum of OTP-t-C9SH (H-b’, d’, e’ at dH 1.1 m; H-c’, -f’ at dH 1.2 t, 0.84 t, respectively) were made by compare with the 1H NMR spectrum of t-C9SH. Specifically, the 1H NMR chemical shifts dH at 1.5 which represents the H on the mercapto group of t-C9SH does not arise at the spectrum of OTP-t-C9SH, which is in accordance with the result of FTIR spectrum. Besides, the emergence of chemical shifts around 5e9 ppm can be attributed to the recovery of aromatic ring structure after the addition reaction [28]. Clearly, both the results of FTIR and 1H NMR spectrums confirm that the novel covalent interface has been successfully constructed in the graphene/SBR nanocomposites. Thus the combination of uniform dispersion and both the covalent interfacial crosslinking and sulfur crosslinking systems are resulted in the as-prepared graphene/ rubber nanocomposites. 3.2. Mechanical properties of graphene/rubber nanocomposites With the introduction of covalent interface, the dispersion status of graphene in the rubber matrix and the interfacial interaction between graphene and SBR are greatly improved, which is of significant to the mechanical properties of composites. The effect of graphene on mechanical properties of graphene/SBR nanocomposites were fully studied and the stress-strain curves of neat SBR and SBR nanocomposites are presented in Fig. 4a. Fig. 4b shows the tensile modulus (stress at 300% strain) and ultimate strength normalized to the values for neat SBR as a function of graphene contents. Detailed evolution of mechanical data with graphene content are shown in Fig. 4c. Compared with the neat SBR, the tensile strength of graphene/SBR nanocomposites with only 1.1 phr of graphene is increase by 223%. With incorporation of 5.6 phr of graphene, the tensile strength of the nanocomposite can reach to 21.5 MPa, which is over 9-folded to that of the neat SBR. Such reinforcing efficiency is actually unprecedented comparing to other layered reinforcing fillers such as clay [33e35]. A comprehensive collection of other reported data concerning the tensile strength of layered fillers reinforced rubber nanocomposites is listed and compared with the present result (Table S1), and a comparison with pristine graphite filled SBR composites was also made (Fig. S3 and Table S2). Compared with these systems, the present work indeed exhibits much higher efficiency in reinforcement of SBR, which is further confirmed by percolation phenomena of the composites based on the relationship between filler loading and tensile
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Fig. 3. (a) FTIR spectrum and (b) 1H NMR spectrum of OTP, t-C9SH and OTP-t-C9SH.
strength. As have been reported by Zhang [36], percolation phenomenon could be explained by the rubber strengthening mechanism that the formation of straightened polymer chains induced by the neighbouring fillers during the stretching process. When the distance between the fillers decrease to a specific threshold which can ensure adequate rubber chains attached on the adjacent fillers, the formation of straightened rubber chains are promoted and strengthen the rubber. As can be observed in Fig. 4d, the tensile stress of the composites initially increases slightly and then shows an abrupt increase. The percolation point of graphene/SBR nanocomposite appears surprisingly at as low as 0.42 phr, which is far below that of CB/SBR composite (5.1 phr). The ultralow percolation threshold of graphene/SBR composites should be ascribed to excellent dispersion of ultrathin graphene layers and the strong covalent interface. The above obviously different result of graphene and CB in reinforcement of SBR as well as the following MooneyRivlin plots and abrasion resistance (Fig. S4) of the corresponding SBR nanocomposites, strongly evidence that graphene possess significant higher reinforcing efficiency and better reinforcing effect towards SBR. 3.3. Reduced dynamic energy loss for energy-saving graphenebased trial tires Dynamic energy loss (hysteresis loss) is one of the most important aspect for elastomer products used in dynamic
applications such as tire tread. Generally, the energy loss of tires during service is also known as rolling resistance. Due to the hysteresis of the repeated deformations, the mechanical energy of tires partially transformed into internal energy and dissipated as heat, which will lead to the energy loss and heat build-up of tires. The temperature increment of tire will inevitably deteriorate the performance of tire and may cause potential dangers during serving. Herein, the dynamic energy loss and temperature increment of graphene/SBR nanocomposite and the control samples were tested on a rotational power loss tester. As shown in Fig. 5a, RSS-II model rolling resistance testing machine is designed according to the basic theory of rubber viscoelastic. The deformation of the rubber wheel always lags behind the stress when the rubber wheel rotates on the roller, thus the energy loss can be characterized as the torque of the opposite direction on the roller. In Fig. 5b, the energy loss of graphene/SBR nanocomposites filled with 5.6 phr of graphene is measured to be 8.7 J/r, which is lower than that of the control samples of CB/SBR composites filled with 50 phr of CB (10.9 J/r), indicating 20% energy saving. Due to the dissipation of energy, the temperature of CB/SBR nanocomposites wheel is significant increased by 87 C, while the temperature increment for graphene/ SBR nanocomposites is only 67 C. In addition, the dynamic energy loss of both the graphene/SBR and CB/SBR nanocomposites is also investigated by the dynamic compression heat build-up tester (Fig. S5), and the results further indicates that graphene/SBR composites possess much lower dynamic energy loss (rolling
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Fig. 4. (a)Stress-strain curves of SBR/graphene nanocomposites, (b) Modulus and stress values for graphene/SBR nanocomposites, normalized to the values for neat SBR, (c) Tensile modulus, strength and tear strength of graphene/SBR nanocomposites and (d) Percolation phenomena of SBR nanocomposites, the intersections of dash lines represent the percolation points.
resistance) than CB/SBR nanocomposites. Previously, it has been well established that the visco-elastic nature of elastomer nanocomposites tailored for tire tread leads to the occurrence of the hysteresis loss under the cyclic loading-unloading deformation when tires are running on the roads, which, at the molecular level, results from the nano-scale frictions between polymer chains, between nanoparticles and between polymer chain and nanoparticle because of the relative slippage [37,38], as schematically displayed in Fig. S6. In the present work, benefit from the uniform dispersion of graphene and the elaborate interfacial crosslinking as have been
proven above, the frictions of the latter two can be effectively suppressed. Thus, low dynamic energy loss are achieved in graphene/rubber nanocomposites (further supported by Molecular Simulation in Supporting Information). Based on the significant improved mechanical performances and reduced dynamic energy loss, the as-prepared graphene/SBR nanocomposites exhibit great potential for application of “green” tire tread. To evaluate the potential use of graphene in industrial practice, graphene-based tread rubber compounds were molded into trial tires and subjected to professional tire test (Fig. 6). Due to
Fig. 5. (a) RSS-II model rolling resistance testing machine and its diagrammatic sketch and (b) Dynamic energy loss and temperature increment of CB/SBR (50 phr N330) and graphene/SBR (5.6 phr graphene) nanocomposites.
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Fig. 6. Fabrication and rolling resistance test of energy-saving graphene-based trial tires.
the high reinforcing efficiency of graphene, acceptable mechanical properties can be achieved at much lower filler content which endows the nanocomposites with lower density. Published work has reported that every 10% reduction in vehicle weight can cut fuel consumption by about 7% [39]. The density test shows that the density of the as-prepared graphene-based tread rubber is 1.1056 g/ cm3, an average of about 10% lower than that of commercial green tires. Therefore, the reduced tires weight could simply save about 0.4% fuel consumption (based on 1200 kg vehicle weight, 15 kg per tire and a set of five tires included a spare) by adopting the graphene-based rubber tires. Furthermore, the rotational energy loss (rolling resistance) of the trial tires were measured by professional MTS Tire Rolling Resistance System. The result shows that the rolling resistance coefficient (RRC) of the graphene-based trial tires is only 6.81 kg/t which is very close to the “A grade” energy efficiency according to the fuel efficiency classes for tires of UNECE Regulation (EC) No 1222/2009 (Table S4) [40]. Commonly, the RRC of ordinary (or “black”) tires is as high as 13 kg/t while those of the commercial green tires is about 10 kg/t. As has been demonstrated that every 5% reduction in rolling resistance, the fuel consumption of vehicle is reduced by 1% [41,42]. Obviously, a significant fuel saving of up to 9.5% can be obtained by substituting graphenebased tires for ordinary tires. Even compared with commercial green tires, an energy saving of 6.4% can be achieved when adopting graphene-based tires for passenger cars which is of great help to the economic and environmental improvements. The above striking results imply the potential application of graphene-based rubber nanocomposites in high-performance green tire.
4. Conclusions A novel covalent interface in graphene/SBR nanocomposites based on ortho-quinone-thiol chemistry was developed. The latex co-coagulation and interfacial crosslinking between ortho-quinones modified graphene and rubber chains constrained the restacking of the nanosheets very effectively. The uniform dispersion and strong interfacial interaction provided synergetic improvement in the nanocomposite properties: an unprecedented reinforcing efficiency combined with drastically decreased energy loss in these graphene/SBR nanocomposites have been realized. The present work revealed a critical role of interfacial adhesion in the performance of rubber nanocomposites and provided a new solution for design for low energy loss elastomeric composites used in dynamic applications. The trial tire approaching highest A-grade energy efficiency has been performed. It is the first successful attempt to fabricate graphene-based energy-saving tires in a scalable manner, which advances the application in industry and helps to cut down fuel consumption allowing for better environmental protection.
Acknowledgment This work was supported by National Basic Research Program of China (2015CB654703), National Natural Science Foundation of China (51521062, 51473050, 51320105012 and 51333003), Key Project of Beijing Municipal Science and Technology Commission (D14110300230000) and Research Fund for the Doctoral Program of Higher Education of China (20130172110001). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.compscitech.2016.06.015. References [1] M. Wagner, Reinforcing silicas and silicates, Rubber Chem. Technol. 49 (3) (1976) 703e774. [2] M.L. Studebaker, The chemistry of carbon black and reinforcement, Rubber Chem. Technol. 30 (5) (1957) 1400e1483. [3] S. Pavlidou, C.D. Papaspyrides, A review on polymerelayered silicate nanocomposites, Prog. Polym. Sci. 33 (12) (2008) 1119e1198. [4] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183e191. [5] J.C. Meyer, A.K. Geim, M. Katsnelson, K. Novoselov, T. Booth, S. Roth, The structure of suspended graphene sheets, Nature 446 (7131) (2007) 60e63. [6] S. Stankovich, D.A. Dikin, G.H. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, et al., Graphene-based composite materials, Nature 442 (2006) 282e286. [7] X. Huang, X. Qi, F. Boey, H. Zhang, Graphene-based composites, Chem. Soc. Rev. 41 (2) (2012) 666e686. [8] C.S. Boland, U. Khan, C. Backes, A. O’Neill, J. McCauley, S. Duane, et al., Sensitive, high-strain, high-rate bodily motion sensors based on grapheneerubber composites, ACS Nano 8 (9) (2014) 8819e8830. [9] J. George, A. Bhowmick, Ethylene vinyl acetate/expanded graphite nanocomposites by solution intercalation: preparation, characterization and properties, J. Mater. Sci. 43 (2) (2008) 702e708. € ckelhuber, et al., [10] A. Das, G.R. Kasaliwal, R. Jurk, R. Boldt, D. Fischer, K.W. Sto Rubber composites based on graphene nanoplatelets, expanded graphite, carbon nanotubes and their combination: a comparative study, Compos. Sci. Technol. 72 (16) (2012) 1961e1967. [11] S. Araby, I. Zaman, Q. Meng, N. Kawashima, A. Michelmore, H.C. Kuan, et al., Melt compounding with graphene to develop functional, high-performance elastomers, Nanotechnology 24 (16) (2013) 165601. [12] B. Ozbas, S. Toki, B.S. Hsiao, B. Chu, R.A. Register, I.A. Aksay, et al., Straininduced crystallization and mechanical properties of functionalized graphene sheet-filled natural rubber, J. Polym. Sci. Part B Polym. Phys. 50 (10) (2012) 718e723. [13] J.R. Potts, O. Shankar, L. Du, R.S. Ruoff, ProcessingeMorphologyeProperty relationships and composite theory analysis of reduced graphene oxide/natural rubber nanocomposites, Macromolecules 45 (15) (2012) 6045e6055. [14] Y. Zhan, J. Wu, H. Xia, N. Yan, G. Fei, G. Yuan, Dispersion and exfoliation of graphene in rubber by an ultrasonically-assisted latex mixing and in situ reduction process, Macromol. Mater. Eng. 296 (7) (2011) 590e602. [15] J.R. Potts, O. Shankar, S. Murali, L. Du, R.S. Ruoff, Latex and two-roll mill processing of thermally-exfoliated graphite oxide/natural rubber nanocomposites, Compos. Sci. Technol. 74 (0) (2013) 166e172. [16] N. Yousefi, M.M. Gudarzi, Q. Zheng, X. Lin, X. Shen, J. Jia, et al., Highly aligned, ultralarge-size reduced graphene oxide/polyurethane nanocomposites: mechanical properties and moisture permeability, Compos. Part A 49 (2013) 42e50. [17] Y. Mao, S. Wen, Y. Chen, F. Zhang, P. Panine, T.W. Chan, et al., High
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