Synergistic reinforcement of silanized silica-graphene oxide hybrid in natural rubber for tire-tread fabrication: A latex based facile approach

Composites Part B 161 (2019) 667–676

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Synergistic reinforcement of silanized silica-graphene oxide hybrid in natural rubber for tire-tread fabrication: A latex based facile approach

T

Lan Caoa,b,1, Tridib K. Sinhab,1, Lei Taoa, Huan Lib, Chengzhong Zonga,∗∗, Jin Kuk Kimb,∗ a b

School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong, 266042, China Elastomer Lab, Department of Materials Engineering and Convergence Technology, Gyeongsang National University, Jinju, 52828, South Korea

ARTICLE INFO

ABSTRACT

Keywords: Natural rubber latex Functionalized GO Modified silica Coupling agent Tire-tread

Graphene oxide (GO) and silica (SiO2) nanoparticles have been hybridized using KH550 modified GO (KGO) and Si69 modified SiO2 (MS) to develop natural rubber (NR)-based high-performance tire-treads. Due to donoracceptor and π-π interactions of NR with the Si69 and GO respectively, SiO2/GO hybrid (MSKGO) was homogeneously dispersed in NR-latex. During drying and curing, the composite of NR and the MSKGO (NRMSKGO) was converted to covalent bonded network structure through possible condensation and free-radical reactions. GO was converted to reduced GO (RGO). Because of the enlarged interfacial area and synergistic reinforcing effect of covalent bonded MSKRGO, lubricating effect of RGO-layers, the cured composite (NRMSKRGO-V) shows increasing storage modulus and energy dissipation capability, while decreasing loss factor and elongation at break with increasing GO content. Fabrication of tire-tread using only 10 phr of the unvulcanized green composite (NRMSKRGO-U) containing 1% of GO increases the wear resistance by 44.5% (as evaluated by running the real tires), and decreases the rolling resistance by 5.1% while increases the wet skid resistance by 14.6% (as evaluated from the dynamic mechanical analysis (DMA) data).

1. Introduction

Owing to the strong π-π interaction among the graphene layers, the graphene-rubber interfacial interaction becomes weak [18]. In contrast, chemically exfoliated GO can be easily dispersed in NR latex (NRL), due to abundant H-bonding interaction of GO-functionalities (such as, hydroxyl, carboxyl, carbonyl, epoxy, etc.) with the water of NRL [16,17]. On precipitation from the latex with simultaneous evaporation of residual water, the composite again lacks adequate interaction among NR and GO. Compatibility mismatches of both the reinforcing materials, viz., silica and GO can be resolved by developing a potential strategy, where adequate networking of NR with these materials can be achieved and high-performance NR-composite can be obtained. In some previous works, composites of rubber with silica and RGO have been accomplished using different sulfur (S)-based silane coupling agents such as alkyl silicate sulfides, sulfonated silica, etc. [5–11,15,19] Alternatively, few works have been reported to obtain silica decorated GO or RGO through H-/covalent-bridging using hydrolyzed alkyl silicates [20–22]. To enhance the compatibility of polar silica with non-polar olefinic hydrocarbon rubbers, the surface modification of silica is being accomplished using bifunctional organosilanes (i.e., (S)-based silane

NR is an indispensable material for tire-industries. However, its mechanical properties are barely good enough to meet the practical demands. In this regard, properties of NR viz., tensile strength, elasticity modulus, etc. are being tailored by reinforcing with different reinforcing agents such as carbon black (CB) and silica [1–4]. Excellent mechanical properties of CB/NR composites can usually be obtained at high CB content, leading to difficulties in the dispersion of CB during processing [4]. Also, CB is resourced from the fossil fuel, at present which is being depleted from the earth crust. It has been observed that the replacement of CB by silica improves the rolling resistance and wet grip of different composites, at equal or slightly inferior wear resistance [5–11]. However, polar silica lacks compatibility with non-polar NR [3,7], resulting agglomeration in the rubber matrix due to increase of filler-filler interaction with the increase of filler concentration. So, surface modification of silica becomes a crucial need. Alternatively, graphene can be another good reinforcing material for NR, because of its natural abundance, 2D-nanostructure, high surface area and excellent mechanical property, and its functional tailorability [12–17]. Corresponding author. Tel.: +82 10 7542 4070. Corresponding author. Tel.: +86 130 0653 9866. E-mail address: [email protected] (J.K. Kim). 1 L. C and T. K. S contributed equally to this work. ∗

∗∗

https://doi.org/10.1016/j.compositesb.2019.01.024 Received 23 April 2018; Received in revised form 2 January 2019; Accepted 2 January 2019 Available online 03 January 2019 1359-8368/ © 2019 Published by Elsevier Ltd.

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coupling agents). Basically, alkyl silicates on hydrolysis obtain hydroxy (-OH) functionality, that forms H-bonding with both the GO functionalities and hydrolyzed silica [7–10]. And the S-atom of alkyl silicate sulfide or sulfonated-silica performs donor-acceptor interaction with the NR or polyisoprene [19]. These non-covalent interactions can be transformed into covalently bonded network structure through thermally induced condensation and free-radical reactions [7,23]. Thus, huge scopes remain elusive for simultaneous grafting of both the GO and silica within the rubber matrix through proper chemical modification using suitable coupling agent. Here we report a latex based facile approach, involving modification of GO in aqueous medium with (3-Aminopropyl)triethoxysilane (KH550) resulting in KGO and hydrolyzed silica with Bis(triethoxysilylpropyl)tetrasulfide (Si69) resulting in MS to obtain the H-bonded MSKGO hybrid, which later self-assembled with NR latex (NRL) through the donor-acceptor interaction with pendant sulfides of Si69 and π-π interaction with the graphene backbone of GO, and well dispersed within the NR matrix. During drying and vulcanization at elevated temperature the NR-composite transforms to a covalent bonded network structure [7,23] and GO is partially reduced to RGO under prolonged heat treatment. In this case, ammonia of NR-latex, aminefunctionality of KH550, polysulfide of Si69 and elemental sulfur (during vulcanization) can be considered as reducing agents [24–29]. Enlarged interfacial area and synergistic reinforcing effect of covalent bonded MSKRGO, sulfide-bridging among the NR and MSKRGO hybrid, [7,23] and lubricating effect of RGO-layers [30] provide dual-dynamic behavior to the vulcanized NR-composite (i.e., NRMSKRGO-V), resulting in increased storage modulus and energy dissipation capability, while decreasing loss factor and elongation at break of the vulcanized composite with increasing the GO content. Observing these impressive features of the vulcanized NR-composite (i.e., NRMSKRGO-V), the unvulcanized green composite (i.e., NRMSKRGO-U) has been used here to develop a high-performance tire-tread.

within the dispersion. The dispersion was centrifuged (at 500 rpm) for 5-times using water/ethanol (1:3) mixture to wash out the unreacted KH550 (as supernatant), the modified GO was collected as solid mass from the bottom of the centrifuge tube and re-dispersed in 40 ml of the same solvent system i.e., water/ethanol (1:3). Here we have designated the modified GO as KGO. 2.2.3. Modification of silica (MS) On the other hand, 30 g of silica powder and 3 g of Si69 were dispersed into 80 ml of water/ethanol (1:3) mixture and sonicated for 1 h to obtain Si69 modified silica. Similar to the modified GO, the modified silica dispersion (in 40 ml of mixed solvent system i.e., water/ethanol (1:3)) was obtained after washing out the unreacted Si69. Modified silica is designated here as MS. 2.2.4. Formulation of SiO2/GO (MSKGO) hybrid The dispersion of both the modified GO and silica were mixed together, followed by simultaneous stirring and sonication for 4 h to obtain the suspension of H-bonded SiO2/GO hybrid. We have designated the hybrid as MSKGO. As 20 ml GO suspension (5 mg/ml) was used to obtain the hybrid, it is assumed here that the hybrid contains 0.1 g of GO. In different sets of experiments, the GO contents were varied as 0.3 g, 1 g, and 3 g respectively. Corresponding hybrids are symbolized as X% MSKGO as it will be mixed with 100 g of NR (where X stands for different GO contents). 2.2.5. Preparation of the NR-composites (NRMSKRGO) 500 ml diluted NRL (20% solid content) was gradually added to the MSKGO suspension (containing 0.1 g GO, 30 g silica and 80 ml mixed solvent i.e., water/ethanol (1:3)) with continuous stirring and sonication for 1 h. The milky white color of NRL was changed to grey color. 5% acetic acid was then added to the suspension to get the precipitation of the NR-composite (NRMSKGO), which was washed with plenty of deionized water till the pH value arrives to 7, followed by removal of residual solvent using a home-made squeeze water machine. Finally, the composite was cut into pieces and dried in vacuum oven at 60 °C for 24 h, resulting in the black colored NR-composite. The appearance of black color may be due to the thermal reduction (partial) of GO to RGO. Ammonia of NRL and ethanol vapor can be considered here as reducing agent [23–30]. The composite is designated as NRMSKRGO. Composite containing X% GO is defined as X% NR-composite (i.e., X% NRMSKRGO). The NR-composite and other additives were mixed on a two-roll mill for 10 min, and then vulcanized at 160 °C in a hot press for the optimum cure time determined by a disc rheometer. During vulcanization, at elevated temperature and in presence of elemental sulfur the reduction of GO and the formation of covalent bonded network structure (from Hbonded network structure) was enhanced. The formulation for the nanocomposite was followed as (per hundred rubber): 5 phr ZnO, 2 phr SA, 0.7 phr CBS, 2.25 ph S. The unvulcanized composite is designated as NRMSKRGO-U and the vulcanized product is designated as NRMSKRGO-V. Following the same formulation, vulcanization of only NR was accomplished to prepare a blank sample for comparison.

2. Experimental 2.1. Materials NRL (20% solid content) was sourced from Qianjin State Rubber Farm (Zhanjiang, PR China); Solid natural rubber (NR) was obtained from Shanghai Judao Chemical Co., Ltd., China; Graphite oxide (48% solid content; 0.6 ± 0.1 O/C molar ratio) was procured from the Sixth Element (Changzhou) Materials Technology Co., Ltd.; Silica (BET: 140–165 m2/g) was obtained from Quecheng Silicon Chemical Co., Ltd.; The coupling agents, 3aminopropyltriethoxysilane (NH2–(CH2)3–Si(OC2H5)3 (KH550) and Bis[3(triethoxysilyl)propyl]tetrasulfide((C2H5O)3Si–(CH2)3–S4–(CH2)3–Si(OC2H5)3 or (Si69)) were provided by Nanjing Daoning Chemical Co., China. N-cyclohexylbenzothiazole-2-sulphenamide (CBS), Zinc oxide (ZnO), stearic acid (SA), sulfur (S) were provided by Tianjin Bodi Chemical co. LTD. HEXA-80 (a basic accelerator containing hexamethylene tetramine (80%), and elastomer binder and dispersing agents (20%)) was obtained from Lanxess chemical (Shanghai) co., LTD., China. Deionized water was used in all experiments. 2.2. Synthesis

2.2.6. Fabrication of tire-tread using the NR-composite 10 phr of the unvulcanized composite i.e., 1% NRMSKRGO-U was mixed with 90 phr of solid NR (i.e., the Vietnamese rubber (SVR 3L)) to make the tire-tread through the regular tire's production process, i.e., mixing, extruding, molding and curing. The details about the compositions are described in Table 1.

2.2.1. Preparation of GO The graphite oxide (5 g) was dispersed into 1000 ml of deionized water with simultaneous mechanical stirring followed by sonication for 2 h in room temperature (rt), resulting in a dark brown stable suspension of GO. 2.2.2. Modification of GO (KGO) 20 ml GO suspension (5 mg/ml) was first well dispersed in 60 ml ethanol under vigorous sonication, followed by addition of KH550 (0.5 g). Surface modification of GO was accomplished via continuous stirring and sonication for 2 h and realized by appearance of turbidity

2.3. Characterizations Fourier transform infrared spectroscopy (FTIR) of GO, modified forms of GO (i.e., KGO) and SiO2 (i.e., MS), SiO2/GO hybrid (MSKGO) 668

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Table 1 Compositions for tire-tread production with or without the NRMSKRGO. Specification

Compositions

1. Tire-tread made of NR only (NRO)

100 phr NR; 25 phr N234 (CB); 25 phr N326 (CB); 15 phr Silica; 1.2 phr Sulfur; 1.2 phr Accelerator CBS; 1.25 phr Accelerator HEXA-80; other additives 90 phr NR; 10 phr 1%NRMSKGO; 22.5 phr N234 (CB); 22.5 phr N326 (CB); 15 phr Silica; 1.2 phr Sulfur; 1.2 phr AcceleratorCBS; 1.25 phr Accelerator HEXA-80; other additives

2. Tire-tread made of NR and 1%NRMSKRGO composite (NRC)

CB: Carbon Black; CBS: N-cyclohexylbenzothiazole-2-sulphenamide; HEXA-80: A basic accelerator containing hexamethylene tetramine (80%), and elastomer binder and dispersing agents (20%).

and the vulcanized NR-composite (i.e., NRMSKRGO-V) were recorded between 400 and 4000 cm−1 using a VERTEX 70 infrared spectrometer and their morphologies were observed by transmission electron microscopy (TEM, JEM-2100) and scanning electron microscopy (SEM, Philips XL30S FEG). Micro Raman measurements were made by a RAM HR800 UV system, equipped with 532 nm wavelength incident laser. All the samples were dried in vacuum oven at ∼40 °C before the characterizations. The rheological properties of the composite materials (before and after vulcanization) were measured using the RPA2000 Rubber Process Analyzer (Alpha Technologies, USA) at 100 °C temperature, 1 Hz frequency, and 0.7%–100% strain sweep range. Crosslinking behavior and curing time of NR-composites were determined by using MDR 2000 rheometer (MDR, Alpha Technologies, USA) at 160 °C (following ASTMD6204). Following the results, tensile sheets were prepared in the hot press (Carver press) at the temperature of 160 °C and the time based on t90, as obtained in MDR measurements. Tensile properties of the vulcanized NR-composites were measured using the mechanical testing machine (ZWICK Z005). Dynamic mechanical analysis (DMA) of the composites was carried out by using a TA instrument DMA 2980 model in tension mode at a constant frequency of 10 Hz, strain of 15%, in the temperature range of −85 °C to 85 °C and at a heating rate of 3 °C/min.

reduction of GO [24–30]. Finally, the NRMSKRGO-U was vulcanized with at 160 °C. At elevated temperature, during drying and curing, the H-bonded self-assembly was converted to covalently bonded network structure through condensation reactions and simultaneous conversions of -COOH ….HO- to ester (-C(=O)O-), -COOH …. .H2N- to amide (-C(=O)-NH-), [23] -Si-OH ….HO-Si-/-Si-OH…HO-C- to silicate (Si-OSi-/Si-O-C-), etc., [5–11] along with sulfide-bridging among NR-chains (due to vulcanization in presence of elemental sulfur) and NR with the MSKRGO hybrid (because of peripheral sulfides of Si69) [7] through free radical reactions [7,11,13,15,23]. Also, the partial reduction of GO was enhanced in presence of sulfur (S) at high temperature during vulcanization [26–28]. 10 phr of the 1% NR-composite was mixed with 90 phr of solid NR through the regular tire's production process, i.e., mixing, extruding, molding and curing respectively, resulting in high-performance tiretread having the synergistic influence of both the silica and RGO along with the lubricating effect of graphene backbone. 3.2. FTIR and Raman studies The GO, modified forms of the GO (i.e., KGO) and silica (i.e., MS), MSKGO and the vulcanized 1% NR-composite (i.e., 1% NRMSKRGO-V) were thoroughly characterized using FTIR (Fig. 2(a)) and Raman spectroscopy (Fig. 2(b)) to support the proposed reaction mechanism. The spectrum of GO shows a strong absorption band at 1725 cm−1 due to the C=O stretching of -COOH group. It also exhibits a sharp band around 1630 cm−1 attributable to the vibrations of the residual water and skeletal vibration of un-oxidized graphitic domains. The vibrational bands due to stretching and bending of hydroxyl (-OH; 1402 cm−1 and 1230 cm−1 respectively), alkoxy and epoxy (C-O; 1040 cm−1) groups present in the GO are also revealed. Due to the presence of surface adsorbed water molecules, a broadband appeared at ∼3400 cm−1. The peaks at 2920 cm−1and 2850 cm−1 are found in the spectrum due to the stretching and bending of -CH2, present in the GO [17,23]. For clarity, enlarged view of FTIR spectrum of GO is provided in Fig. 2(a). In contrast to GO, KGO shows new peaks at 1565 cm−1 due to the -NH2 in KH550, 1000-1150 cm−1 due to the Si-O-Si or Si-O-C, 850-950 cm−1 for Si-OH, broadening of peaks at 2850-2900 cm−1 due to additional –CH2 of KH550, 3430 cm−1 due to adsorbed water, and presence of GO characteristic small peaks at ∼1725 cm−1 and ∼1402 cm−1, little shifted towards lower energy due to H-bonded -COOH and -OH respectively. The characteristic peaks of Si-O-Si and Si-O-C are appeared due to crosslinking among the H-bonded functional groups viz., SiOH…HO-Si and Si-OH ….HO-C (hydroxyl(OH-) functional group of GO) during drying for sample preparation [22,23]. For modified silica (MS), the presence of characteristic intense peaks of silica (i.e., 467, 790, 960 and ∼1100 cm−1) are observed [31]. In case of SiO2/GO hybrid (i.e., MSKGO), the characteristic peaks of silica are more intensified and broaden. Broad small peaks at around ∼1600 cm−1 and 2900 cm−1 are corresponding to the GO functionalities. In contrast to the only NR, the vulcanized NR-composite (NRMSKRGO-V) shows the presence of characteristic intense and broad peak of silicate (as marked by the red color rectangle) [20] along with the less-intense broad peak of bonded RGO at around ∼1700-1500 cm−1. The enlarged view of FTIR spectrum of NRMSKRGO-V has been provided in Fig. 2 (a) for

3. Results and discussion 3.1. Synthesis Here we have developed a latex-based facile methodology to prepare SiO2/RGO based NR-composite through five chemical steps, as partly represented in Fig. 1. During vigorous stirring and sonication of graphite oxide in water, the oxygen functional groups (e.g., carboxyl (-C(=O)OH), hydroxyl (-OH), epoxy ( ), etc.) of GO interact with water molecules through formation of H-bonding, resulting homogeneous aqueous dispersion of few layers GO [17]. In the second step, a requisite amount of bi-functional linker (i.e., KH550) was added to the GO dispersion after diluting it with the water-ethanol mixture. KH550 hydrolyzed in presence of water-ethanol mixture [23]. Consequently, the H-bonding interaction of GO functionalities favored with the -OH and amine (-NH2) functionalities of KH550, resulting in turbid dispersion of modified GO (KGO). Before we proceed for 3rd step, silica was modified separately by mixing the hydrolyzed silica with Si69, where Si69 was H-bonded with hydrolyzed silica through their -OH functionality, having few remnants -OH groups on the periphery of the modified silica (MS) [6,7]. In 3rd step, modified forms of the GO and silica were mixed together under stirring and sonication where their surface active groups interacted with each other through the H-bonding interaction, resulting in SiO2/GO hybrid (i.e., MSKGO). In 4th step, during dispersing the hybrid into the NRL, the MSKGO occupied the interstitial spaces among the macromolecular NR-chains through the donor-acceptor interaction (among the NR or polyisoprene and sulfides of Si69) and π-π interaction (among the NR and the graphene backbone of GO), resulting in a homogeneous dispersion of NRMSKGO composite. The dispersion was grayish in color. During drying of the NR-composite for 24 h in a vacuum oven at 60 °C, thermal reduction of GO was partly accomplished. The black coloration of the composite was appeared due to the 669

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Fig. 1. Schematic representation for synthesis of NRMSKRGO composite through formation of covalent bonding network of silica decorated GO–NR self-assembly.

clarity. The peaks at around ∼1554 cm−1 and ∼1650 cm−1 (due to presence of amide bond), [21,22,32] ∼1700 cm−1 (due to esterification) are responsible for such broadening [32,33]. In contrast to FTIR spectra, Raman spectra (Fig. 2(b)) shows that the continuous broadening of ID and IG peaks occurs from GO to the NRcomposite (through modified GO and SiO2/GO hybrid) with almost constant ID/IG ratio (∼1), indicating the occurrence of H-bonding interaction. But after vulcanization, the ratio becomes 1.2 along with broadening of the peaks, due to the reduction of the oxygen functional groups of GO and simultaneous formation of covalent bonding of GO functionalities through different coupling reactions such as amidation, esterification, etc. [24,27,29,34] Otherwise the RGO, peaks at ∼1440 cm−1 and ∼1665 cm−1 are due to the presence of NR in the composite [35]. Thus, the formulation of NR-composite has been accomplished here through a facile method where the composite self-assembly is in situ transformed to a covalently bonded network during heat treatment and vulcanization.

3.3. Morphological characterization Fig. 3 shows the morphological studies (i.e., SEM and TEM (inset of SEM with red border)) of GO, KGO, MS, MSKGO and NRMSKRGO-V respectively. The nanostructure morphology of GO reveals that we have obtained GO having few μm lengths [16,20]. The selected area electron diffraction (SAED) pattern (inset of TEM with blue border) shows the bright hexagonal spots having multiple ring corresponding to the fewlayer GO [22,27,34]. In contrast, modified GO (KGO) shows more stacking of layers, due to the H-bonding interaction that was favored among the GO-functionalities and hydrolyzed KH550. Corresponding SAED pattern also shows an increased number of multiple ring patterns because of the layer stacking. The MSKGO hybrid shows silica decorated GO sheets with lesser interlayer stacking. The de-stacking occurs due to simultaneous intercalation and exfoliation of the GO layer and occupancy of MS into the interstitial spaces of the GO-layers through the favorable H-bonding interaction among the surface active groups of

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Fig. 2. Comparative (a) FTIR (with enlarged view of GO and NRMSKRGO-V) and (b) Raman spectra of GO, KGO, MSKGO and NRMSKRGO.

the modified forms of GO and silica [20]. Corresponding SAED pattern shows polygonal ring pattern with less-intense spots, may be due to the H-bonded network of the MSKGO hybrid. Finally, the presence of silica nanoparticles along with the GO-flake in the NR-composite can easily be observed. The polygonal SAED pattern appears due to the presence of excessive rubber in the composite. From the above discussion, it can be said that the NR-composite

contains dual dynamic network viz., H-bonding or covalent bonding (among the fillers and fillers with the NR-matrix) and π-π bonding among the GO-layers. So, preservation of structural integrity within the composite under mechanical deformation is highly expected. The effect of GO content on the performance of the composite can be understood from its rheological and mechanical properties.

Fig. 3. Morphological study of GO, KGO, MS, MSKGO and NRMSKGO respectively. 671

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Fig. 4. (a) Storage moduli (G′) (b) loss factor (tanδ) and (c) rheometric graph profiles (curing curves) of the unvulcanized green composites (NRMSKRGO-U); (d) Mechanical properties (stress-strain behaviors) of the cured composites (NRMSKRGO-V); (e) Storage moduli (G′) and (f) loss factor (tanδ) of the cured composites (NRMSKRGO-V). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.4. Rheological and mechanical properties

of strain changes little at higher strain values (due to the reinforcement of homogeneously dispersed hybrid filler) and after a certain strain value it decreases rapidly and reaches optimum (due to rupture of the network structure eventually at very high strain). In contrast to G′, tanδ value shows the reverse trend having some non-linearity. For each case, at lower strain, it decreases rapidly and then increases [3]. It is probably because, the low applied strain is used for arranging the randomly distributed macromolecular entanglement to a regular network structure or rupturing of the π-π bonding among the RGO-layers, which upon application of high strain increases due to the friction among the molecular chains of NR. In contrast, the presence of the MSKRGO hybrid network structure within the NR macromolecules, lubricating effect of RGO-layers minimize the friction, [30]resulting in the decrease of tanδ value with increasing the GO content. Thus, it is observed that the property of the NRMSKRGO-U is governed by the interfacial interaction among the fillers, fillers with the NR-matrix, and π-π interaction among RGO layers. During vulcanization at high temperature, the unvulcanized composite is transformed into a covalently bonded network structure. Thus, vulcanized NR-composite (NRMSKRGO-V) contains dual-dynamic network structure, containing both the covalent bonding and π-π bonding. Vulcanization characteristics were determined from comparative rheometric graph profiles of the unvulcanized NR-composite (NRMSKRGO-U) and the only NR at 160 °C (Fig. 4(c)). It is expressed in terms of the difference of minimum and maximum torque value (i.e., △M =

Fig. 4 shows comparative rheological properties (i.e., storage modulus (G′) and loss factor (tanδ)) of the NR-composite (before and after vulcanization, i.e., both the NRMSKRGO-U and NRMSKRGO-V), rheometric graph profiles of NR-composite (before vulcanization, i.e., NRMSKRGO-U) and stress-strain properties of the NR-composite after vulcanization (i.e., NRMSKRGO-V). Fig. 4(a)and (b) show the changes of G′ and tanδ respectively with sweep of applied strain in the range of 0.7–100% at 1 Hz frequency and 100 °C temperature for the unvulcanized composite (i.e., NRMSKRGO-U). It is observed that the G′ values of only rubber remain nearly unchanged with the increase of strain, while there is an increasing trend with the increase of GO content in the composite. The silane bridging among MSKRGO hybrid along with π-π interaction of graphene backbone of RGO with the NR and donor-acceptor interaction of the NR with the sulfur of Si69 are responsible factors for such increasing trend [3,7–11]. With increasing GO content in the composite, the filler-filler interaction among the modified GO and silica through favorable H-bond formation is enhanced because of increasing availability of GO functionalities to be interacted with the functional groups of modified silica, resulting in larger value of G'. Here, the increase of the strain leads to the deformation or rupture of the H-bonded composite network. When the strain is small, the rate of formation of the network is greater than the rate of network failure. The decrease of storage modulus with increase 672

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stress-strain behaviors of the vulcanized NR-composite (NRMSKRGO-V) and the inset of Fig. 4(d) shows those results derived from the stressstrain curves. It can be observed, that composite having higher GO content shows increased modulus and decreased elongation at break in comparison to the only NR. The strain-induced crystallization of NR occurs at high elongation, whereas the NR-composite contains crystalline domains made of silane bridged NR with the covalent bonded MSKRGO hybrid. This covalent bonded network structure increases the modulus and reduces the elasticity of the composite [4,36,37]. As a result, the composite (containing GO of 0.1%, 0.3%, 1%, 3% respectively during formulation) show tensile strength 24.93, 34.03, 29.25, 22.91 MPa respectively, which is 73%, 136%, 102%, 59% higher than that of the only NR. Also, the 200% modulus reaches 3.84, 6.76, 9.33, 11.48 MPa respectively, which is 156%, 351%, 522%, 665% higher than that of the NR. While, the elongation at break reaches 501%, 486%, 493%, 434% respectively, which is 3%, 2%, 13%, 24% lower than that of the only NR. Although the vulcanized NR-composite shows increased strength and modulus, its flexibility decreases with increasing the GO content. From the above result, it can be observed that the 1% NR-composite (i.e., composite containing 1% GO) shows a balanced stress-strain behavior in comparison to the other composites. Consequently, the unvulcanized composite (NRMSKRGO-U) containing 1% GO has been used here for the tire-tread fabrication. Prior to the tiretread fabrication, the changes of G′ and tanδ of the vulcanized NRcomposites (i.e., NRMSKRGO-V) with the sweep of applied strain in the range of 0.7%–100% at 1 Hz frequency and 100 °C temperature has been examined, and represented in Fig. 4(e)and (f). The composite after vulcanization shows much higher G′ values than that of the unvulcanized NR-composite. Also, there is a large difference in between the G′-values of the NR-composite and the only NR compared to that

Table 2 Comparative rheometric-graph profiles of NR and NRMSKRGO. Sample

△M

t10

t90

CRI

NR 0.1%NRMSKRGO 0.3%NRMSKRGO 1%NRMSKRGO 3%NRMSKRGO

7.15 11.88 11.90 12.11 11.18

2.49 2.93 3.00 2.50 1.43

4.53 7.74 7.22 6.45 5.92

49.02 20.79 23.70 25.32 22.27

(MH - ML)), induction time (t10), optimum cure time (t90), and presented in Table 2. It is interesting to observe that △M increases as well as t10 (time to increase the torque 10%) and t90 (time to increase the torque 90%) decreases with increasing the GO content. Here, the increase of △M is attributed to the presence filler-filler and filler-NR strong interactions which restrict the polymer chain mobility, implies the increase of crosslink density with the increase of GO content and well corroborates with our assumption verified by spectroscopic characterizations. The possible interactions of RGO with accelerator (CBS) and activator (ZnO) facilitate the well dispersion of accelerator and activator within the NR matrix, resulting in enhanced and faster formation of polysufide linkage among the NR chains (in presence of elemental sulfur) with increase of GO content [15,29]. Also, with the increase of GO content, the silane modifying agents (KH550 and Si69) are increased within the composite and enhances the curing behavior by increasing the possibility of silicate (Si-O-Si/Si-O-C) bonds. The sulfide rich Si69 also enhances the polysulfide linkages within the NR. As a result, the t10 and t90 decreases with increase of GO. Following the t90 values (as mentioned in Table 2), vulcanization of the NR-composite has been accomplished here. Fig. 4(d) shows the

Fig. 5. Energy dissipation during 1st cycle (black line), 2nd cycle (red line), 3rd cycle (blue line). Cyclic stress-strain curves of vulcanized NR-composites ((a) 0.1% NRMSKRGO-V, (b) 0.3% NRMSKRGO-V, (c) 1% NRMSKRGO-V, (d) 3% NRMSKRGO-V). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 673

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Fig. 6. Schematic representation to elucidate comparative DMA-curve of NRC and NRO.

hysteresis becomes smaller when the same samples are subjected to further tensile cycles to the same maximum strain. This phenomenon is well known as Mullins effect, which is related to the structural changes during tensile cycles [4,40]. From the Fig. 5, it is observed that the second or third loading curve of the NR-composites are lower and not parallel to the first one. It is due to the Mullins effect on the NR-composite caused by not only the instantaneous residual stretch of NRmatrix but also by some other microstructure changes. These changes may include rupture of the filler network, chain breakage at the interface between the NR and the fillers, slipping of the molecules (especially RGO-layers) along the filler surface and disentanglement of the rubber molecules [4]. It is observed that the hysteresis area of the NR-composites (during first loading) are increased with increasing the GO content. It reflects that with increasing the GO content the bonded network is enhanced. The π-π bonding among the RGO layers is also increased with increasing the GO content in the composite. Consequently, 3% NRMSKGO-V shows highest hysteresis loss whereas 0.1% NR-composite shows lowest hysteresis loss. Under loading during the 1st cycle, the bonded network structure is ruptured, π-π bonding among the RGO-layers is broken and slipping among the RGO layers is started. For the 2nd and 3rd cycles, the loading curves of all the composites are changed very little and remains parallel. In comparison to the 1st cycle, here, the slippage among the RGO-layers is the contributing factor for 2nd and 3rd cycle. But, still the hysteresis loss is highest for the 3% NRMSKRGO-V and lowest for the 0.1% NRMSKRGO-V. These observations infer that both the filler-filler and filler-matrix bonded network, and lubrication effect of RGO-layers (which are increasing with the increase of GO content in the composites) are the contributing factors during cyclic loading-unloading hysteresis process. The experiment well corroborates with the stress-strain behavior of the NR-composites, where it is observed that the strength and modulus are increased due to the increased filler-filler and filler-matrix interfacial interaction with increasing the GO content in the composite. From the above observations, it can be said that the GO content has

Table 3 Comparative DMA-parameters of NRC and NRO. Temperature(°C)

tanδ of NRO

tanδ of NRC

Rate (%)

0 60

0.1415 0.1296

0. 1622 0.1230

+14.6 −5.1

● Rate (%) =

(Tan of NRC Tan of NRO) Tan of NRO

100 .

before vulcanization. Along with, the sulfide-linkages among the NRchains, this enhancement of G′-value occurs due to the enhanced crosslink density (through possible covalent bonding) among the participating fillers (i.e., GO and silica) as well as fillers with the NR matrix upon application of temperature during vulcanization [3,7]. In contrast to the G′, tanδ shows a reverse trend, which is also different from that of the NRMSKRGO-U. Before vulcanization, only NR exhibits naturally the highest tanδ level, due to the viscous nature of an unfilled raw polymer, whereas after vulcanization it shows lowest tanδ-level due to formation of spring like 3D-network structure having bonded sulfides within the NR-chains, which minimizes the energy consumption during strain. For the vulcanized composite, the presence of bonded MSKRGO makes it stiffer and hinders the chain movement during application of strain, resulting in friction induced increased tanδ-value. But with increasing the GO content in the composite, the lubricating effect among the RGOlayers lowers the friction, resulting in decreased tanδ-value. Thus, it is observed that both the mechanical and rheological properties of the vulcanized NR-composite (NRMSKRGO-V) are governed by both the covalent bonding and the π-π bonding of RGO. Further, we have examined this dual-dynamic network of NRMSKRGO-V by cyclic loading and unloading experiment (as shown in Fig. 5). When the NR-composites are subjected to cyclic loading and unloading process, different types of hysteresis are observed due to their different viscoelastic energy dissipation capabilities. The Table 4 Comparative experimental results after running of NRO- and NRC-tires. Specification NRO NRC

Tire position Left front file Right front file

Wear resistance (%) =

Distance travelled(D) (Km)

Treadpattern depth before running (Tb) (mm)

Tread pattern depth after running (Ta) (mm)

Accumulation wear average

52593 52593

12 12.1

7.8 9.2

12522 18098

(Accumulation wear average of NRC Accumulation wear average of NRO) Accumulation wear average of NRO

100 . 674

D Tb

Ta

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a significant effect on the mechanical and rheological property of the composite. It is therefore necessary to optimize the GO content in the composite during the real applications.

Notes The authors declare no competing financial interest. Acknowledgment

3.5. Fabrication and application of tire-tread

This work was supported both by the Natural Science Foundation of Shandong Province (project No.: ZR2016XJ002), People’s Republic of China and R&D Center for Valuable Recycling (Global-Top R&BD Program) of the Ministry of Environment, Republic of Korea (Project No.: 2016002240002).

Here, we have fabricated the NR-composite based tire-tread (i.e., NRC) by adding only 10 phr of the 1% NRMSKGO-U and compared with that without the NRMSKGO-U composite. Here we have designated the other NR-based tire-tread (having no NRMSKGO-U composite) as NRO. Vulcanized NR-composite, having NR macromolecular chains containing interstitially occupied hybrid network can be speculated as a spring-like structure when investigated under TEM (as depicted in Fig. 6) [38,39]. This spring structure is composed of sulfide bridged NR connected with the hybrid, where RGO-layers are silane-bridged with the modified silica, which is again interconnected through silicate bonding. This kind of spring-like structures embedded within the tire matrix can provoke extra wear-resistance, resulting in enhanced longevity of the NRC. Also, under different deformations (e.g., applying break to the running tire), different types of sliding viz., filler (SiO2/ RGO)-matrix (NR) and filler-filler are additionally expected for NRC along with the normal polymer-polymer sliding (which is the only case of NRO) [40]. These extra sliding behaviors are expected to reduce the rolling resistance of NRC-tire. As, NRC is composed of many more polar functionalities, interconnected with each other, restrict the molecular movement at lower temperature, resulting an increase of wet-skidding resistance also. As a consequence, NRC-tire containing only 10 phr of 1% NRMSKRGO-U, shows 44.5% increase of wear resistance, 5.1% decrease of rolling resistance and 14.6% increase of wet-skidding resistance, when compared with NRO. Table 3 shows the dynamic mechanical analysis (DMA) parameters of the NRC and NRO, as derived from the Fig. 6. Here, tanδ values at 0 °C and 60 °C has been considered to evaluate the changes in wet-skidding and rolling resistance respectively [41]. After running the pneumatic tires for 52593 Km, experimental results are tabulated in Table 4, which shows a significant difference in the depth of the pattern, resulting in the increase of wear resistance by 44.5% for NRC tire in comparison to that of NRO. Thus, wet-skidding resistance and rolling resistance are calculated from the DMA analysis, whereas the wear resistance is calculated from depth of the pattern after running the pneumatic tire.

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4. Conclusion We have successfully incorporated modified silica (MS) grafted RGO (i.e., MSKRGO hybrid composite) into the natural rubber matrix using a latex based facile and green approach, which has been used to develop the tire-tread of superior properties (e.g., high-grip performance, superior rolling and wear resistance). In this novel approach, the Hbonded NRMSKGO converts to the mechanically strong covalent bonded NRMSKRGO-V by the temperature during drying and curing, as proven by spectroscopic (i.e., FTIR and Raman), morphological (i.e., SEM and TEM) and mechanical analyses (i.e., stress-strain behavior, rheological properties, etc.). This work will not only lead to the futuristic scalable production of both the silica and GO-based high-end environment-friendly tire-tread but also provides a new insight into the use of coupling agents (i.e., Si69 and KH550), for modifications of silica and graphene oxide for different engineering applications. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 675

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