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The influence of molybdenum disulfide nanoplatelets on the dispersion of nano silica in natural rubber composites
Applied Surface Science 359 (2015) 782–789
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The influence of molybdenum disulfide nanoplatelets on the dispersion of nano silica in natural rubber composites Peijin Weng, Qiuyan Wei, Zhenghai Tang, Tengfei Lin, Baochun Guo ∗ Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 29 July 2015 Received in revised form 13 October 2015 Accepted 25 October 2015 Available online 28 October 2015 Keywords: Nanocomposite Silica Molybdenum sulfide Dispersion Reinforcement Dynamic property
a b s t r a c t The dispersion of nanofiller in polymer composites is critical in governing the ultimate performances. Present study aimed to improve the dispersion of silica in elastomeric materials based on natural rubber (NR) composites using the nanoplatelets of molybdenum disulfide (MoS2 ), a graphene-like layered inorganic. NR latex was co-coagulated with MoS2 suspension to form NR/MoS2 compounds (1∼5 phr). Then silica (30 phr) was incorporated into NR/MoS2 compounds, followed by curing with sulfur, to obtained NR/MoS2 /silica composites. The dispersion state of silica in the composites was examined by TEM and the effects of MoS2 on the performance of the composites were investigated. It was found that a small amount of MoS2 nanoplatelets significantly improved the silica dispersion. Consequently, the static and dynamic mechanical properties of the crosslinked natural rubber materials were greatly enhanced. The improved dispersion of silica is associated with charge transfer interaction, giving rise to electrostatic repulsion among silica. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum disulfide (MoS2 ), a two-dimensional layered transition-metal dichalcogenide, has attracted extensive interest due to its potential in various applications ranging from electronic devices and sensors to catalysts [1–3]. Recent studies show that single-layered MoS2 is a flexible and strong sheet with a high Young’s modulus of 270–330 GPa and breaking strength of 16–30 GPa [4]. These excellent mechanical properties of MoS2 make it as a promising reinforcing filler for polymeric composites [5]. Recently, we demonstrated that, with appropriate surface functionalization, single-layered MoS2 exhibited high reinforcing efficiency for rubber, which is comparable to that possessed by graphene [6]. Natural rubber (NR), one of the most important biosynthesized polymers, is widely used for various applications [7–9]. Elastomeric material based on NR generally needs to be reinforced with fillers such as carbon black (CB), silica, and clays, because its mechanical properties especially its low modulus are inadequate for many applications [10–12]. Silica has been widely employed as reinforcement for rubbers, which can greatly improve the overall performances of the rubber composites [13–16]. Unfortunately, since silica surface bears abundant silanol groups and exhibits high polarity, it has poor compatibility with non-polar rubbers such
∗ Corresponding author. Tel.: +86 2087113374. E-mail address: [email protected] (B. Guo). http://dx.doi.org/10.1016/j.apsusc.2015.10.172 0169-4332/© 2015 Elsevier B.V. All rights reserved.
as NR [17]. As a result, silica tends to form severe aggregation in rubber matrix and has poor interfacial interaction with rubber macromolecules [18,19]. Therefore, the improvement of silica dispersion is critical in developing high-performance silica-filled elastomeric materials. Many methods such as addition of silanes and in situ generation of silica particles using sol–gel methods have been developed to improve the dispersion of silica [20–22]. In addition, incorporation of the hybridized fillers has been shown to be effective in improving filler dispersion. For example, previous study [23,24] showed that the dispersion of carbon black was improved in the presence of 2-D clay sheets in epoxidized natural rubber. In such composites, carbon black and nanoclay could form a “nanounit”, in which a carbon structural aggregate was positioned between two clay tactoids, resulting in improvements in the dynamic mechanical and tensile properties of the final composite materials [23]. Zhang added the hybrid fillers consisting of modified kaolinite and precipitated silica into styrene-butadiene rubber (SBR), the kaolinite sheets were covered by silica particles, improving the thermal stabilities of the SBR composites [24]. However, the use of clay in hybrid fillers requires surface treatment in order to achieve monolayer dispersion of clay in the rubber matrix. MoS2 nanoplatelets, as the layered inorganics, are expected to facilitate the dispersion of other fillers. With the assistance of lithium intercalation, single layered MoS2 can be stably suspended in water, making it possible to prepare hybrid fillers without modification of MoS2 . According our knowledge, reports on MoS2 have mainly focused on applications in catalysis and
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electronics [3,25]. In the present study, MoS2 was used as a novel dispersant for NR/silica composite. It was found that MoS2 in NR compounds effectively improved the dispersion of silica, resulting in greatly improved mechanical properties of composites after crosslinking by sulfur; in particular, the improvement on the dynamic properties was more prominent. This study investigated the effects of chemically exfoliated MoS2 on the structures and performances of rubber/silica composites, providing new insights into improved filler dispersion in rubber by layered materials such as transition-metal dichalcogenides. 2. Experimental 2.1. Materials Natural rubber latex with a solid content of 60 wt% was kindly supplied by the Hainan Rubber Group. Precipitated silica WL180 (specific surface area: 150 m2 /g) was provided by NanPingJialian Chemicals Ltd. (Nanping, China). MoS2 (powder, purity 99%) and n-butyllithium (1.6 M in hexane) were purchased from the SigmaAldrich Co., Ltd. Hexane (CP) was supplied by the Fuchen Chemical Reagent Factory, Tianjin, China. Sulfur (S) was industrial grade and the form of sulfur was powder. All the rubber ingredients were industrial grade and used as received. 2.2. Lithium intercalation of MoS2 Single-layered MoS2 sheets were obtained using lithium intercalation method [26]. Typically, MoS2 powder (1 g) was immersed in n-butyllithium solution (1.6 M in hexane, 10 mL) in a nitrogen atmosphere. The solution was stirred at room temperature for 72 h. The intercalated sample was washed with hexane (100 mL). The obtained semi-dry product was transferred to deionized water (300 mL) and subjected to sonication, yielding stable and singlelayered MoS2 aqueous dispersions. 2.3. Preparation of model silica/MoS2 mixtures To study the mechanism for the improved dispersion of silica, the model silica/MoS2 mixtures were prepared. MoS2 aqueous solution was diluted to 0.5 mg/mL. Silica was then slowly added to the MoS2 solution according to the preset MoS2 /silica weight ratios (1:0, 1:6, 1:10 and 1:30). The mixtures were first stirred, and then subjected to sonication for 1 h at room temperature. For the UV spectrum test, all the suspensions were further diluted into the same MoS2 concentration (0.1 mg/mL). For comparison, the silica suspensions in respective concentrations were also prepared by sonication for 1 h at room temperature. For Raman spectrum test, the above mentioned suspensions with same MoS2 concentration (0.5 mg/mL) and volume were subjected to lyophilization for 72 h. The freeze-dried solids were collected and ready for Raman experiments.
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The compound NR/silica without addition of MoS2 was also prepared as a control. 2.5. Preparation of NR/silica/MoS2 elastomeric material by sulfur curing The basic formulation of the composite is listed as follows, NR 100, zinc oxide 5, stearic acid 2, N-isopropyl-N -phenyl-4phenylenediamine 1.5, N-cyclohexyl-2-benzothiazolesulfonamide 1.5, N-tert-butyl-2-benzothiazole sulphonamide 0.5, and sulfur 1.5. The compounds were compression molded at 143 ◦ C for the optimum cure time (tc90 ). 2.6. Characterization of filler mixtures, compounds and crosslinked materials Particle sizes of nanofillers were determined using a lightscattering instrument (Zeta Sizer Nano-ZS90) for stable mixed suspensions of silica and MoS2 , which were diluted to a concentration of 0.5 mg/mL, followed by sonication for 1 h at room temperature. UV–vis spectra of the suspensions of MoS2 and its mixtures with silica were collected on a Scinco S-3150 spectrometer. Raman spectra of MoS2 and its model mixtures with silica were taken using a LabRAM Aramis of HO RIBA Jobin Yvon, equipped with a He–Ne ion laser (532.0 nm) as the excitation source. The optimum cure time (tc90 ) of the NR compounds were determined at 143 ◦ C using a UR-2030SD vulcameter (U-CAN Dynatex Inc., Taiwan). The tensile properties of crosslinked composite materials were determined using a Gotech testing machine, according to ISO 37-2005. Hardness was tested using a hardness meter, following ISO standard 48:1994. The crosslinking densities of the vulcanizates were determined using the equilibrium swelling method and calculated using the Flory–Rehner equation [27,28]. The dynamic mechanical spectra of the uncured and cured compounds were obtained using a Netzsch 242C instrument. The specimens were examined from −100 to 80 ◦ C in tensile mode, with a strain of 0.5%. The frequency and heating rate were set as 10 Hz and 3 ◦ C/min, respectively. Strain sweeping of the composites was conducted using an RPA2000 rubber process analyzer (RPA; Alpha Technology Inc., USA). The frequency and temperature were fixed at 1 Hz and 60 ◦ C, respectively. The heat build-up and dynamic compression set of the vulcanizates were determined using a Goodrich flexometer (RH-2000N, Gotech Testing Machines Inc., Taiwan). The dynamic compression test was the modified version of ISO4666/31982. The load was 0.5 MPa as the samples were relatively soft. The dispersion of silica and MoS2 in the rubber matrix were examined using transmission electron microscopy (TEM; JEOL2100) at an accelerating voltage of 30 kV. 3. Results and discussion 3.1. Effect of MoS2 on the silica dispersion in NR
2.4. Preparation of NR/silica/MoS2 compounds To prepare NR/silica/MoS2 compounds and final crosslinked composite materials, the MoS2 /NR compounds were first prepared by latex co-coagulation technique. A pre-determined MoS2 aqueous suspension was added to the NR latex. After stirring for 1 h, the MoS2 /NR mixture suspension was coagulated by adding calcium chloride solution (1.0 wt%) as the flocculant. The coagulated compounds were washed with deionized water and vacuum-dried at 60 ◦ C. Then the obtained MoS2 /NR compounds were mixed with 30 phr of silica in a Haake internal mixer at room temperature with a rotation speed of 35 rpm for 7 min. At the end, NR/silica/MoS2 compounds were mixed with rubber additives by two-roll milling.
In polymer composites, a prerequisite for achieving high performance composites is the good dispersion of the reinforcement. Herein, the micro-dispersion state of silica in the crosslinked rubber matrix was examined by TEM. Shown Fig. 1a is the TEM image of NR/silica composite, it can be seen that the silica particles connect together forming chain-like aggregates in the absence of MoS2 , which is because poor polarity matching between NR and silica. These silica aggregates cause stress concentration, leading to early fracture during stretching. Interestingly, it is obvious that the dispersion of silica is greatly improved with the incorporation of only 1 phr of MoS2 (Fig. 1b). MoS2 is dispersed as nanoplatelets and the silica particles are dispersed homogeneously throughout the
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Fig. 1. TEM images of (a) neat NR/silica; (b) NR/silica/MoS2 (1 phr); (c) and (e) NR/silica/MoS2 (3 phr); and (d) and (f) NR/silica/MoS2 (5 phr).
rubber matrix. When MoS2 loading is increased to 3 phr, the silica dispersion is further improved (Fig. 1c). The higher resolution image (Fig. 1e) shows that the diameter of the dispersed silica particles is about 30 nm, which is consistent with the value for the primary silica particles, indicating that the silica is finely dispersed as individual particles with the inclusion of MoS2 . Besides, MoS2 sheets are homogeneously dispersed as nanoplatelets. If the content of MoS2 is further increased (5 phr), some MoS2 plates start to aggregate into thicker nanoplatelets in the composite (Fig. 1d and f). Dynamic-mechanical measurements were conducted on the uncured compounds to examine the filler networking. Fig. 2 shows the dependence of the elastic modulus (G ) of uncured NR/silica compounds with various MoS2 contents on the strain. It can be seen that the initial G values of the compounds increase with increasing MoS2 content, especially when the MoS2 content is higher than 1 phr, which is due to the formation of a more developed filler network in the presence of MoS2 . The G values for all the compounds sharply decrease with increasing strain, which is known as Payne effect and is mainly related to collapse of the filler network and release of the trapped rubber in the filler network on application of an oscillatory shear [29]. In addition, compared with the control sample, the G values for the compounds with MoS2 are more sensitive to strain, indicating that the three-dimensional filler network is more developed in compounds with MoS2 .
To shed light onto the mechanism for the improved dispersion of silica, we prepared the aqueous solutions of the model silica/MoS2 compounds with various ratios. The particle size of silica in the solution is investigated by light scattering method. As shown in Fig. 3, the silica has a particle size of 430 nm in the absence of MoS2 . This value is significantly higher than that for the individual
Fig. 2. Dependence of G of uncured NR/silica compounds with various MoS2 contents on strain.
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Fig. 3. Silica particle sizes at various MoS2 contents (the inset for suspension of silica and silica/MoS2 compound).
Fig. 5. Raman spectra of freeze-dried MoS2 solution at various silica contents (fixed MoS2 mass).
silica particle, as silica is arranged in chains or three-dimensional networks in aqueous solution, leading severe aggregation of silica [30]. The silica particle size sharply decreases to about 260 nm when the weight ratio of silica/MoS2 is 30/1. The silica particle size continuously decreases with increasing MoS2 content. The significantly increased suspension stability of silica by MoS2 can also be verified visually, as shown in the inset in Fig. 3. With the same silica concentration, silica particles are precipitated in a few hours and that with MoS2 is still stable in the same standing time. This is presumably caused by the improvement of electrostatic repulsive forces in the system by adding negatively charged MoS2 plates. As is well documented, silica is negatively charged in water due to the deprotonation of silanol groups [30–32]. Meanwhile, during the exfoliation of LiMoS2 in water, the initially neutral MoS2 layers are converted to a complex solution containing negatively charged (MoS2 )x− layers, hydroxide anions, and lithium cations, and thus the chemically exfoliated MoS2 layers are negatively charged [33]. But on account of both with different charge, the charge transfer will happen between silica and MoS2 , which gives rise to charge repulsion between silica. Therefore, we propose that MoS2 can improve silica particle dispersion as a result of charge transfer interaction between them [34]. Fig. 4 shows the UV–vis spectra of the aqueous solutions of the model silica/MoS2 compounds with various the weight ratios. For each sample, the background of silica (with the same concentration of silica) is subtracted. The spectrum of the aqueous solutions
of the model silica/MoS2 compounds has clear characteristic peaks at 256 and 305 nm, which belong to characteristic peaks of MoS2 . The intensities of characteristic peaks along the increase in silica are compared in Fig. 4. As shown, with the increasing of silica, the intensities of the characteristic peaks at 255 and 307 nm of MoS2 are substantially decreased. It is well-known that the charge transfer interaction of single-walled carbon nanotube (SWCNT) with electron acceptors or donors will reduce the characteristic peak of SWCNT [35,36]. Matsuda et al. [37] studied the charge interaction between SWCNT and nanosilicas by the optical absorption spectra. The presence of nanosilica decreases significantly the intensity of the characteristic peak of SWCNT, which can be ascribed to the charge transfer interaction between SWCNT and nanosilica. In the present study, the decreases in the intensities of the characteristic peaks of MoS2 are reasonably attributed to the charge transfer interaction between silica and MoS2 . Fig. 5 shows Raman spectra of freeze-dried the aqueous solutions of the model silica/MoS2 compounds with various the weight ratios (the mass of MoS2 is fixed). One can observe three peaks centered at 380, 405 and 449 cm−1 , which are attributed to E12g , A1g and second order phonons [38–40]. In order to further illustrate the proposed charge transfer interaction between silica and MoS2 , the evolution of the intensity of the E12g peak, A1g peak and 2LA(M) of MoS2 , is recorded. The presence of silica decreases significantly the intensities of the E12g peak, A1g peak and 2LA(M) of MoS2 . The variation in the peak intensities may be due to 1T–2H rearrangement of MoS2 [41]. Chemically exfoliated MoS2 is commonly described as being predominantly in its 1T-polytype. Because of the charge transfer interaction between silica and MoS2 , when the silica was added to MoS2 solution, the charge of MoS2 will decrease, leading to partial rearrangement of 1T-polytype into 2H-polytype. Due to the ratio of 1T/2H is decreased, the intensity of the peaks are significantly decreased [41]. Although the addition of silica leads to decreases of the peak intensities, however, the peak intensities increase with the increasing silica fraction. This could be interpreted by the steric hindrance for the rearrangement into 2Hpolytype. 3.2. Effect of MoS2 on static mechanical properties of NR/silica composites
Fig. 4. UV–vis spectra of MoS2 dispersed in water at various silica contents, after the subtraction of the corresponding silica background (in the same silica concentration).
The vulcanization curves of the NR/silica composites with variable amounts of MoS2 are shown in Fig. 6 and the curing characteristics of NR/silica composites with variable amounts of MoS2 are summarized in Table 1. As can be seen, the scorch and cure times are greatly decreased by incorporation of MoS2 . It is well known that silica generally delays vulcanization, and the scorch and cure
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Fig. 6. Curing profiles of NR/silica with various amounts of MoS2 . Table 1 Vulcanization properties of the NR/silica composites with variable amounts of MoS2 . MoS2 (phr) NR/silica/MoS2 composites 0 1 3 5
tc10 (min:s)
3:44 2:09 1:24 1:15
tc90 (min:s)
17:37 15:42 16:03 14:12
ML (dN × m)
MH (dN × m)
1.84 2.28 2.19 2.94
15.29 21.38 22.36 20.73
times increase with increasing silica loading [15]. The delay of vulcanization by silica is interpreted as adsorption of the activators and accelerators by abundant silanol groups on the silica surface [42]. However, the addition of negatively charged MoS2 can alleviate curing retardation and increase the curing rate. Furthermore, the maximum torque (MH ) values of composites with MoS2 are higher than that of the control sample. There are two origins of the increased curing rate and enhanced MH values of the composites with MoS2 layers. First, the incorporation of two-dimensional MoS2 may facilitate the formation of a filler network by overlapping with the neighboring silica particles. Another possible origin of the increased MH for the composites with MoS2 may be the increase in the crosslink density. Fig. 7 shows the evolution of the crosslink density with increasing MoS2 content. The interfacial interactions between the filler and rubber contribute to physical crosslinks in rubber composites. If the dispersion of filler improves, physical crosslinking is strengthened as a result of the increase in interfacial volume. According to Fig. 7, the crosslink densities of the composites with MoS2 are higher than that of the control, except for the sample with 1 phr of MoS2 . When sufficient MoS2 (higher
Fig. 8. Tensile properties and hardness of NR/silica composites with various MoS2 contents.
than 1 phr) is incorporated, silica dispersion is improved (as substantiated by TEM images); therefore the composites with MoS2 have higher crosslink densities. When excess MoS2 (for example, 5 phr) is included, MoS2 may aggregate, thus the crosslink density starts to decrease. In addition, because of the deformed crystal structure, the edge sites of MoS2 have higher affinities toward some specific molecules, such as thiols [6,34,43,44]. It is well documented that various thiols are yielded as intermediate during vulcanization reaction [6,34,43,44]. Therefore, the reaction between MoS2 edges and the thiols may additionally contribute to the increased crosslink density. The dependences of the tensile modulus (stress at 300% strain) and tensile strengths of the composites on MoS2 loading are shown in Fig. 8. It is clear that addition of MoS2 significantly increases the modulus and tensile strength, compared with those of the NR/silica composite. For example, the modulus and tensile strength of the composites containing 3 phr MoS2 increase by 73% and 61%, referring to NR/silica composite, respectively. The modulus increases consistently with increasing MoS2 content. The substantial improvements in modulus and strength are attributed to the improved dispersion of silica with the incorporation of MoS2 which is verified by the TEM observations as discussed above. As a result of the formation of a more developed filler network and the presence of high-strength MoS2 layers, the modulus of the composite increases continuously. However, when excess MoS2 (higher than 3 phr) is included, the tensile strength starts to decrease, which is due to the aggregation of MoS2 . In order to exclude enhancement causing by solely adding MoS2 , tensile properties of NR/MoS2 composites is also investigated in Table 2. It should be noted that MoS2 shows moderate reinforcing efficiency toward NR. For example, for NR/MoS2 composite containing 3 phr of MoS2 , the modulus and tensile strength increase by about 40% and 12%, respectively. These increments are less than those for the NR/silica/MoS2 composite with the same MoS2 loading. Therefore, it is evident that the significant enhancements in NR/silica/MoS2 are mainly originated from the improved dispersion of silica, in combination of the contributions of MoS2 as reinforcement. The hardness of the composites is also shown in Fig. 8. It can be seen that the hardness of Table 2 Tensile properties of NR/MoS2 composites.
Fig. 7. Crosslink densities of NR/silica composites with various MoS2 content.
NR 1 phr MoS2 3 phr MoS2
Modulus (stress at 300% strain)
Tensile strength (MPa)
2.2 ± 0.1 2.5 ± 0.1 3.1 ± 0.1
25.3 ± 1.7 29.4 ± 0.8 28.4 ± 1.8
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Fig. 9. Mooney–Rivlin plots for NR/silica composites with various MoS2 contents. Fig. 11. Strain dependences of tan ı of NR/silica composites with various MoS2 contents.
the composites with MoS2 are much higher than that for the control, i.e. NR/silica composite. When only 1 phr of MoS2 is added, the hardness increases by 10. Such a significant improvement in the hardness is mainly ascribed to the formation of a developed filler network in the composites containing MoS2 . However, with further increases in the MoS2 content, the hardness increases only slightly. The Mooney–Rivlin equation was used to evaluate the elastomeric network. The equation can be written as * = /( − −2 ) = 2C1 + 2C2 −1 , where is the stress, and C1 and C2 are constants which is independent of [12]. It has been well recognized that NR can crystallize on elongation; this is known as strain-induced crystallization [45]. In the Mooney–Rivlin curve, an abrupt upturn at high strain can be observed as a result of strain-induced crystallization [46]. As shown in Fig. 9, the upturns for composites with MoS2 occur at smaller deformations than for the control sample. This is ascribed to better dispersion of both silica and MoS2 in the vulcanizates. As silica is dispersed more homogeneously in the rubber matrix, the orientation of rubber chains takes place under smaller strain, i.e., earlier strain-induced crystallization [47,48]. It should be noted that the promotion of crystallization tends to be less profound when excess MoS2 is used (5 phr), which correlates well with the mechanical performances. As mentioned above, MoS2 tends to form aggregations at high content. Consequently, the alignment and orientation of the rubber chains are hindered.
3.3. Effect of MoS2 on dynamic mechanical properties of composites based on silica and natural rubber Fig. 10 shows the changes in the storage modulus (E ) and loss factor (tan ı) as a function of temperature for the composites. As shown in Fig. 10a, on incorporation of MoS2 , the E of all the samples increases significantly in both the rubbery region and the glassy region, which is due to the formation of the more developed filler network. It is found that the glass-transition temperature (Tg ) of the composites with MoS2 is almost unchanged. In general, the tan ı of a tread compound at about 0 ◦ C and 10 Hz is assumed to approximate the wet-grip conditions, whereas tan ı at about 60 ◦ C indicates the rolling resistance property. A higher tan ı value at about 0 ◦ C and lower tan ı value at about 60 ◦ C indicate improved wet-grip and decreased rolling resistance properties of a tire tread, which are two key indicators in “green type”. In this study, the tan ı values at 60 ◦ C are much lower than that of the NR/silica composite. For example, the tan ı value at 60 ◦ C of the composite containing 3 phr of MoS2 is 28% lower than that of the NR/silica composite. To further substantiate the dynamic loss at larger strain, RPA strain sweeps were conducted. As shown in Fig. 11, tan ı at 5% strain decreases significantly on incorporation of MoS2 , although the MoS2 loading does not exert a significant influence on tan ı peak value in DMA experiments. When 3 phr of MoS2 is included, the tan ı value (at 5%
Fig. 10. Temperature dependences of E (a) and tan ı (b) for NR/silica composites with various MoS2 contents. The inset is a magnification of the tan ı curves in the temperature range 50–75 ◦ C.
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significant improvements in the modulus and tensile strengths, and much lower heat build-up values. The reinforcement effect is highest at MoS2 loading of 3 phr, and the modulus and tensile strength are increased by over 70% and 60%, respectively. RPA strain sweeping shows that the value of tan ı (60 ◦ C and 5% strain) is decreased by about 30%. Heat build-up decreases by 17 ◦ C with only 1 phr of MoS2 . These significant changes in the properties are mainly the result of greatly improved dispersion of silica in NR by using MoS2 nanoplatelets, caused by charge transfer interaction between silica and MoS2 , giving rise to electrostatic repulsion of silica. Acknowledgements
Fig. 12. Effects of MoS2 loading on heat build-up and compression permanent set values.
strain) decreases by 29% compared with the NR/silica composite. The observed lower hysteresis loss in dynamic mechanical analysis and RPA is mainly ascribed to improved dispersion of silica by MoS2 , which reduces friction among the silica particles. The heat build-up and dynamic compression set values of all the composites are shown in Fig. 12. It shows an initial decrease and subsequent increase in the heat build-up and permanent set with increasing MoS2 content. In unfilled rubber, heat generation mainly arises from molecular friction, while in rubber compounds with reinforcing fillers, the breakage and re-formation of interaggregate bonds is mainly responsible for the heat generation [49]. As the silica surface has abundant silanol groups, silica particles tend to aggregate and form a filler network in the rubber matrix; therefore silica dispersion plays an important role in heat build-up. As can be seen in Fig. 12, the heat build-up and permanent set of the control sample are the highest in NR/silica composites. However, with addition of only 1 phr of MoS2 , the heat build-up decreases by 17 ◦ C. It is accepted that the filler network breaks down and rebuilds continuously under repeated compression, causing energy dissipation as heat. So we attribute the significant decrease in heat build-up to better silica dispersion in the presence of MoS2 , which alleviates the breaking down and reforming of the filler network. Based on the previous discussion, the vulcanizate containing 3 phr of MoS2 is expected to have the minimum heat build-up value, whereas the heat build-up is actually lowest at MoS2 loading of 1 phr. The higher heat build-up in the composites containing more than 1 phr MoS2 is attributed to the much easier sliding of rubber chains with excess MoS2 sheets. When the content of MoS2 is sufficiently high (5 phr), the increase in heat build-up is caused by the aggregation of MoS2 . It should be noted that the hardness has increased in the MoS2 -incorporated composites, which in turn lowers the measured heat build-up as the applied strain during the test is actually decreased. Nevertheless, both the increased hardness and lowered heat build-up are originated from the addition of MoS2 . Besides, compared with NR/silica composite, the permanent set of NR/MoS2 /silica composites is substantially decreased. 4. Conclusion In this study, chemically exfoliated MoS2 nanoplatelets were used to improve the dispersion of silica in NR composite. The MoS2 /NR compounds were first prepared by latex co-coagulation technique and after that silica was added and finally NR/silica/MoS2 crosslinked composite materials are obtained using sulfur. On incorporation of a small amount of MoS2 , the composites show
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The influence of molybdenum disulfide nanoplatelets on the dispersion of nano silica in natural rubber composites Peijin Weng, Qiuyan Wei, Zhenghai Tang, Tengfei Lin, Baochun Guo ∗ Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 29 July 2015 Received in revised form 13 October 2015 Accepted 25 October 2015 Available online 28 October 2015 Keywords: Nanocomposite Silica Molybdenum sulfide Dispersion Reinforcement Dynamic property
a b s t r a c t The dispersion of nanofiller in polymer composites is critical in governing the ultimate performances. Present study aimed to improve the dispersion of silica in elastomeric materials based on natural rubber (NR) composites using the nanoplatelets of molybdenum disulfide (MoS2 ), a graphene-like layered inorganic. NR latex was co-coagulated with MoS2 suspension to form NR/MoS2 compounds (1∼5 phr). Then silica (30 phr) was incorporated into NR/MoS2 compounds, followed by curing with sulfur, to obtained NR/MoS2 /silica composites. The dispersion state of silica in the composites was examined by TEM and the effects of MoS2 on the performance of the composites were investigated. It was found that a small amount of MoS2 nanoplatelets significantly improved the silica dispersion. Consequently, the static and dynamic mechanical properties of the crosslinked natural rubber materials were greatly enhanced. The improved dispersion of silica is associated with charge transfer interaction, giving rise to electrostatic repulsion among silica. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum disulfide (MoS2 ), a two-dimensional layered transition-metal dichalcogenide, has attracted extensive interest due to its potential in various applications ranging from electronic devices and sensors to catalysts [1–3]. Recent studies show that single-layered MoS2 is a flexible and strong sheet with a high Young’s modulus of 270–330 GPa and breaking strength of 16–30 GPa [4]. These excellent mechanical properties of MoS2 make it as a promising reinforcing filler for polymeric composites [5]. Recently, we demonstrated that, with appropriate surface functionalization, single-layered MoS2 exhibited high reinforcing efficiency for rubber, which is comparable to that possessed by graphene [6]. Natural rubber (NR), one of the most important biosynthesized polymers, is widely used for various applications [7–9]. Elastomeric material based on NR generally needs to be reinforced with fillers such as carbon black (CB), silica, and clays, because its mechanical properties especially its low modulus are inadequate for many applications [10–12]. Silica has been widely employed as reinforcement for rubbers, which can greatly improve the overall performances of the rubber composites [13–16]. Unfortunately, since silica surface bears abundant silanol groups and exhibits high polarity, it has poor compatibility with non-polar rubbers such
∗ Corresponding author. Tel.: +86 2087113374. E-mail address: [email protected] (B. Guo). http://dx.doi.org/10.1016/j.apsusc.2015.10.172 0169-4332/© 2015 Elsevier B.V. All rights reserved.
as NR [17]. As a result, silica tends to form severe aggregation in rubber matrix and has poor interfacial interaction with rubber macromolecules [18,19]. Therefore, the improvement of silica dispersion is critical in developing high-performance silica-filled elastomeric materials. Many methods such as addition of silanes and in situ generation of silica particles using sol–gel methods have been developed to improve the dispersion of silica [20–22]. In addition, incorporation of the hybridized fillers has been shown to be effective in improving filler dispersion. For example, previous study [23,24] showed that the dispersion of carbon black was improved in the presence of 2-D clay sheets in epoxidized natural rubber. In such composites, carbon black and nanoclay could form a “nanounit”, in which a carbon structural aggregate was positioned between two clay tactoids, resulting in improvements in the dynamic mechanical and tensile properties of the final composite materials [23]. Zhang added the hybrid fillers consisting of modified kaolinite and precipitated silica into styrene-butadiene rubber (SBR), the kaolinite sheets were covered by silica particles, improving the thermal stabilities of the SBR composites [24]. However, the use of clay in hybrid fillers requires surface treatment in order to achieve monolayer dispersion of clay in the rubber matrix. MoS2 nanoplatelets, as the layered inorganics, are expected to facilitate the dispersion of other fillers. With the assistance of lithium intercalation, single layered MoS2 can be stably suspended in water, making it possible to prepare hybrid fillers without modification of MoS2 . According our knowledge, reports on MoS2 have mainly focused on applications in catalysis and
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electronics [3,25]. In the present study, MoS2 was used as a novel dispersant for NR/silica composite. It was found that MoS2 in NR compounds effectively improved the dispersion of silica, resulting in greatly improved mechanical properties of composites after crosslinking by sulfur; in particular, the improvement on the dynamic properties was more prominent. This study investigated the effects of chemically exfoliated MoS2 on the structures and performances of rubber/silica composites, providing new insights into improved filler dispersion in rubber by layered materials such as transition-metal dichalcogenides. 2. Experimental 2.1. Materials Natural rubber latex with a solid content of 60 wt% was kindly supplied by the Hainan Rubber Group. Precipitated silica WL180 (specific surface area: 150 m2 /g) was provided by NanPingJialian Chemicals Ltd. (Nanping, China). MoS2 (powder, purity 99%) and n-butyllithium (1.6 M in hexane) were purchased from the SigmaAldrich Co., Ltd. Hexane (CP) was supplied by the Fuchen Chemical Reagent Factory, Tianjin, China. Sulfur (S) was industrial grade and the form of sulfur was powder. All the rubber ingredients were industrial grade and used as received. 2.2. Lithium intercalation of MoS2 Single-layered MoS2 sheets were obtained using lithium intercalation method [26]. Typically, MoS2 powder (1 g) was immersed in n-butyllithium solution (1.6 M in hexane, 10 mL) in a nitrogen atmosphere. The solution was stirred at room temperature for 72 h. The intercalated sample was washed with hexane (100 mL). The obtained semi-dry product was transferred to deionized water (300 mL) and subjected to sonication, yielding stable and singlelayered MoS2 aqueous dispersions. 2.3. Preparation of model silica/MoS2 mixtures To study the mechanism for the improved dispersion of silica, the model silica/MoS2 mixtures were prepared. MoS2 aqueous solution was diluted to 0.5 mg/mL. Silica was then slowly added to the MoS2 solution according to the preset MoS2 /silica weight ratios (1:0, 1:6, 1:10 and 1:30). The mixtures were first stirred, and then subjected to sonication for 1 h at room temperature. For the UV spectrum test, all the suspensions were further diluted into the same MoS2 concentration (0.1 mg/mL). For comparison, the silica suspensions in respective concentrations were also prepared by sonication for 1 h at room temperature. For Raman spectrum test, the above mentioned suspensions with same MoS2 concentration (0.5 mg/mL) and volume were subjected to lyophilization for 72 h. The freeze-dried solids were collected and ready for Raman experiments.
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The compound NR/silica without addition of MoS2 was also prepared as a control. 2.5. Preparation of NR/silica/MoS2 elastomeric material by sulfur curing The basic formulation of the composite is listed as follows, NR 100, zinc oxide 5, stearic acid 2, N-isopropyl-N -phenyl-4phenylenediamine 1.5, N-cyclohexyl-2-benzothiazolesulfonamide 1.5, N-tert-butyl-2-benzothiazole sulphonamide 0.5, and sulfur 1.5. The compounds were compression molded at 143 ◦ C for the optimum cure time (tc90 ). 2.6. Characterization of filler mixtures, compounds and crosslinked materials Particle sizes of nanofillers were determined using a lightscattering instrument (Zeta Sizer Nano-ZS90) for stable mixed suspensions of silica and MoS2 , which were diluted to a concentration of 0.5 mg/mL, followed by sonication for 1 h at room temperature. UV–vis spectra of the suspensions of MoS2 and its mixtures with silica were collected on a Scinco S-3150 spectrometer. Raman spectra of MoS2 and its model mixtures with silica were taken using a LabRAM Aramis of HO RIBA Jobin Yvon, equipped with a He–Ne ion laser (532.0 nm) as the excitation source. The optimum cure time (tc90 ) of the NR compounds were determined at 143 ◦ C using a UR-2030SD vulcameter (U-CAN Dynatex Inc., Taiwan). The tensile properties of crosslinked composite materials were determined using a Gotech testing machine, according to ISO 37-2005. Hardness was tested using a hardness meter, following ISO standard 48:1994. The crosslinking densities of the vulcanizates were determined using the equilibrium swelling method and calculated using the Flory–Rehner equation [27,28]. The dynamic mechanical spectra of the uncured and cured compounds were obtained using a Netzsch 242C instrument. The specimens were examined from −100 to 80 ◦ C in tensile mode, with a strain of 0.5%. The frequency and heating rate were set as 10 Hz and 3 ◦ C/min, respectively. Strain sweeping of the composites was conducted using an RPA2000 rubber process analyzer (RPA; Alpha Technology Inc., USA). The frequency and temperature were fixed at 1 Hz and 60 ◦ C, respectively. The heat build-up and dynamic compression set of the vulcanizates were determined using a Goodrich flexometer (RH-2000N, Gotech Testing Machines Inc., Taiwan). The dynamic compression test was the modified version of ISO4666/31982. The load was 0.5 MPa as the samples were relatively soft. The dispersion of silica and MoS2 in the rubber matrix were examined using transmission electron microscopy (TEM; JEOL2100) at an accelerating voltage of 30 kV. 3. Results and discussion 3.1. Effect of MoS2 on the silica dispersion in NR
2.4. Preparation of NR/silica/MoS2 compounds To prepare NR/silica/MoS2 compounds and final crosslinked composite materials, the MoS2 /NR compounds were first prepared by latex co-coagulation technique. A pre-determined MoS2 aqueous suspension was added to the NR latex. After stirring for 1 h, the MoS2 /NR mixture suspension was coagulated by adding calcium chloride solution (1.0 wt%) as the flocculant. The coagulated compounds were washed with deionized water and vacuum-dried at 60 ◦ C. Then the obtained MoS2 /NR compounds were mixed with 30 phr of silica in a Haake internal mixer at room temperature with a rotation speed of 35 rpm for 7 min. At the end, NR/silica/MoS2 compounds were mixed with rubber additives by two-roll milling.
In polymer composites, a prerequisite for achieving high performance composites is the good dispersion of the reinforcement. Herein, the micro-dispersion state of silica in the crosslinked rubber matrix was examined by TEM. Shown Fig. 1a is the TEM image of NR/silica composite, it can be seen that the silica particles connect together forming chain-like aggregates in the absence of MoS2 , which is because poor polarity matching between NR and silica. These silica aggregates cause stress concentration, leading to early fracture during stretching. Interestingly, it is obvious that the dispersion of silica is greatly improved with the incorporation of only 1 phr of MoS2 (Fig. 1b). MoS2 is dispersed as nanoplatelets and the silica particles are dispersed homogeneously throughout the
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Fig. 1. TEM images of (a) neat NR/silica; (b) NR/silica/MoS2 (1 phr); (c) and (e) NR/silica/MoS2 (3 phr); and (d) and (f) NR/silica/MoS2 (5 phr).
rubber matrix. When MoS2 loading is increased to 3 phr, the silica dispersion is further improved (Fig. 1c). The higher resolution image (Fig. 1e) shows that the diameter of the dispersed silica particles is about 30 nm, which is consistent with the value for the primary silica particles, indicating that the silica is finely dispersed as individual particles with the inclusion of MoS2 . Besides, MoS2 sheets are homogeneously dispersed as nanoplatelets. If the content of MoS2 is further increased (5 phr), some MoS2 plates start to aggregate into thicker nanoplatelets in the composite (Fig. 1d and f). Dynamic-mechanical measurements were conducted on the uncured compounds to examine the filler networking. Fig. 2 shows the dependence of the elastic modulus (G ) of uncured NR/silica compounds with various MoS2 contents on the strain. It can be seen that the initial G values of the compounds increase with increasing MoS2 content, especially when the MoS2 content is higher than 1 phr, which is due to the formation of a more developed filler network in the presence of MoS2 . The G values for all the compounds sharply decrease with increasing strain, which is known as Payne effect and is mainly related to collapse of the filler network and release of the trapped rubber in the filler network on application of an oscillatory shear [29]. In addition, compared with the control sample, the G values for the compounds with MoS2 are more sensitive to strain, indicating that the three-dimensional filler network is more developed in compounds with MoS2 .
To shed light onto the mechanism for the improved dispersion of silica, we prepared the aqueous solutions of the model silica/MoS2 compounds with various ratios. The particle size of silica in the solution is investigated by light scattering method. As shown in Fig. 3, the silica has a particle size of 430 nm in the absence of MoS2 . This value is significantly higher than that for the individual
Fig. 2. Dependence of G of uncured NR/silica compounds with various MoS2 contents on strain.
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Fig. 3. Silica particle sizes at various MoS2 contents (the inset for suspension of silica and silica/MoS2 compound).
Fig. 5. Raman spectra of freeze-dried MoS2 solution at various silica contents (fixed MoS2 mass).
silica particle, as silica is arranged in chains or three-dimensional networks in aqueous solution, leading severe aggregation of silica [30]. The silica particle size sharply decreases to about 260 nm when the weight ratio of silica/MoS2 is 30/1. The silica particle size continuously decreases with increasing MoS2 content. The significantly increased suspension stability of silica by MoS2 can also be verified visually, as shown in the inset in Fig. 3. With the same silica concentration, silica particles are precipitated in a few hours and that with MoS2 is still stable in the same standing time. This is presumably caused by the improvement of electrostatic repulsive forces in the system by adding negatively charged MoS2 plates. As is well documented, silica is negatively charged in water due to the deprotonation of silanol groups [30–32]. Meanwhile, during the exfoliation of LiMoS2 in water, the initially neutral MoS2 layers are converted to a complex solution containing negatively charged (MoS2 )x− layers, hydroxide anions, and lithium cations, and thus the chemically exfoliated MoS2 layers are negatively charged [33]. But on account of both with different charge, the charge transfer will happen between silica and MoS2 , which gives rise to charge repulsion between silica. Therefore, we propose that MoS2 can improve silica particle dispersion as a result of charge transfer interaction between them [34]. Fig. 4 shows the UV–vis spectra of the aqueous solutions of the model silica/MoS2 compounds with various the weight ratios. For each sample, the background of silica (with the same concentration of silica) is subtracted. The spectrum of the aqueous solutions
of the model silica/MoS2 compounds has clear characteristic peaks at 256 and 305 nm, which belong to characteristic peaks of MoS2 . The intensities of characteristic peaks along the increase in silica are compared in Fig. 4. As shown, with the increasing of silica, the intensities of the characteristic peaks at 255 and 307 nm of MoS2 are substantially decreased. It is well-known that the charge transfer interaction of single-walled carbon nanotube (SWCNT) with electron acceptors or donors will reduce the characteristic peak of SWCNT [35,36]. Matsuda et al. [37] studied the charge interaction between SWCNT and nanosilicas by the optical absorption spectra. The presence of nanosilica decreases significantly the intensity of the characteristic peak of SWCNT, which can be ascribed to the charge transfer interaction between SWCNT and nanosilica. In the present study, the decreases in the intensities of the characteristic peaks of MoS2 are reasonably attributed to the charge transfer interaction between silica and MoS2 . Fig. 5 shows Raman spectra of freeze-dried the aqueous solutions of the model silica/MoS2 compounds with various the weight ratios (the mass of MoS2 is fixed). One can observe three peaks centered at 380, 405 and 449 cm−1 , which are attributed to E12g , A1g and second order phonons [38–40]. In order to further illustrate the proposed charge transfer interaction between silica and MoS2 , the evolution of the intensity of the E12g peak, A1g peak and 2LA(M) of MoS2 , is recorded. The presence of silica decreases significantly the intensities of the E12g peak, A1g peak and 2LA(M) of MoS2 . The variation in the peak intensities may be due to 1T–2H rearrangement of MoS2 [41]. Chemically exfoliated MoS2 is commonly described as being predominantly in its 1T-polytype. Because of the charge transfer interaction between silica and MoS2 , when the silica was added to MoS2 solution, the charge of MoS2 will decrease, leading to partial rearrangement of 1T-polytype into 2H-polytype. Due to the ratio of 1T/2H is decreased, the intensity of the peaks are significantly decreased [41]. Although the addition of silica leads to decreases of the peak intensities, however, the peak intensities increase with the increasing silica fraction. This could be interpreted by the steric hindrance for the rearrangement into 2Hpolytype. 3.2. Effect of MoS2 on static mechanical properties of NR/silica composites
Fig. 4. UV–vis spectra of MoS2 dispersed in water at various silica contents, after the subtraction of the corresponding silica background (in the same silica concentration).
The vulcanization curves of the NR/silica composites with variable amounts of MoS2 are shown in Fig. 6 and the curing characteristics of NR/silica composites with variable amounts of MoS2 are summarized in Table 1. As can be seen, the scorch and cure times are greatly decreased by incorporation of MoS2 . It is well known that silica generally delays vulcanization, and the scorch and cure
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Fig. 6. Curing profiles of NR/silica with various amounts of MoS2 . Table 1 Vulcanization properties of the NR/silica composites with variable amounts of MoS2 . MoS2 (phr) NR/silica/MoS2 composites 0 1 3 5
tc10 (min:s)
3:44 2:09 1:24 1:15
tc90 (min:s)
17:37 15:42 16:03 14:12
ML (dN × m)
MH (dN × m)
1.84 2.28 2.19 2.94
15.29 21.38 22.36 20.73
times increase with increasing silica loading [15]. The delay of vulcanization by silica is interpreted as adsorption of the activators and accelerators by abundant silanol groups on the silica surface [42]. However, the addition of negatively charged MoS2 can alleviate curing retardation and increase the curing rate. Furthermore, the maximum torque (MH ) values of composites with MoS2 are higher than that of the control sample. There are two origins of the increased curing rate and enhanced MH values of the composites with MoS2 layers. First, the incorporation of two-dimensional MoS2 may facilitate the formation of a filler network by overlapping with the neighboring silica particles. Another possible origin of the increased MH for the composites with MoS2 may be the increase in the crosslink density. Fig. 7 shows the evolution of the crosslink density with increasing MoS2 content. The interfacial interactions between the filler and rubber contribute to physical crosslinks in rubber composites. If the dispersion of filler improves, physical crosslinking is strengthened as a result of the increase in interfacial volume. According to Fig. 7, the crosslink densities of the composites with MoS2 are higher than that of the control, except for the sample with 1 phr of MoS2 . When sufficient MoS2 (higher
Fig. 8. Tensile properties and hardness of NR/silica composites with various MoS2 contents.
than 1 phr) is incorporated, silica dispersion is improved (as substantiated by TEM images); therefore the composites with MoS2 have higher crosslink densities. When excess MoS2 (for example, 5 phr) is included, MoS2 may aggregate, thus the crosslink density starts to decrease. In addition, because of the deformed crystal structure, the edge sites of MoS2 have higher affinities toward some specific molecules, such as thiols [6,34,43,44]. It is well documented that various thiols are yielded as intermediate during vulcanization reaction [6,34,43,44]. Therefore, the reaction between MoS2 edges and the thiols may additionally contribute to the increased crosslink density. The dependences of the tensile modulus (stress at 300% strain) and tensile strengths of the composites on MoS2 loading are shown in Fig. 8. It is clear that addition of MoS2 significantly increases the modulus and tensile strength, compared with those of the NR/silica composite. For example, the modulus and tensile strength of the composites containing 3 phr MoS2 increase by 73% and 61%, referring to NR/silica composite, respectively. The modulus increases consistently with increasing MoS2 content. The substantial improvements in modulus and strength are attributed to the improved dispersion of silica with the incorporation of MoS2 which is verified by the TEM observations as discussed above. As a result of the formation of a more developed filler network and the presence of high-strength MoS2 layers, the modulus of the composite increases continuously. However, when excess MoS2 (higher than 3 phr) is included, the tensile strength starts to decrease, which is due to the aggregation of MoS2 . In order to exclude enhancement causing by solely adding MoS2 , tensile properties of NR/MoS2 composites is also investigated in Table 2. It should be noted that MoS2 shows moderate reinforcing efficiency toward NR. For example, for NR/MoS2 composite containing 3 phr of MoS2 , the modulus and tensile strength increase by about 40% and 12%, respectively. These increments are less than those for the NR/silica/MoS2 composite with the same MoS2 loading. Therefore, it is evident that the significant enhancements in NR/silica/MoS2 are mainly originated from the improved dispersion of silica, in combination of the contributions of MoS2 as reinforcement. The hardness of the composites is also shown in Fig. 8. It can be seen that the hardness of Table 2 Tensile properties of NR/MoS2 composites.
Fig. 7. Crosslink densities of NR/silica composites with various MoS2 content.
NR 1 phr MoS2 3 phr MoS2
Modulus (stress at 300% strain)
Tensile strength (MPa)
2.2 ± 0.1 2.5 ± 0.1 3.1 ± 0.1
25.3 ± 1.7 29.4 ± 0.8 28.4 ± 1.8
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Fig. 9. Mooney–Rivlin plots for NR/silica composites with various MoS2 contents. Fig. 11. Strain dependences of tan ı of NR/silica composites with various MoS2 contents.
the composites with MoS2 are much higher than that for the control, i.e. NR/silica composite. When only 1 phr of MoS2 is added, the hardness increases by 10. Such a significant improvement in the hardness is mainly ascribed to the formation of a developed filler network in the composites containing MoS2 . However, with further increases in the MoS2 content, the hardness increases only slightly. The Mooney–Rivlin equation was used to evaluate the elastomeric network. The equation can be written as * = /( − −2 ) = 2C1 + 2C2 −1 , where is the stress, and C1 and C2 are constants which is independent of [12]. It has been well recognized that NR can crystallize on elongation; this is known as strain-induced crystallization [45]. In the Mooney–Rivlin curve, an abrupt upturn at high strain can be observed as a result of strain-induced crystallization [46]. As shown in Fig. 9, the upturns for composites with MoS2 occur at smaller deformations than for the control sample. This is ascribed to better dispersion of both silica and MoS2 in the vulcanizates. As silica is dispersed more homogeneously in the rubber matrix, the orientation of rubber chains takes place under smaller strain, i.e., earlier strain-induced crystallization [47,48]. It should be noted that the promotion of crystallization tends to be less profound when excess MoS2 is used (5 phr), which correlates well with the mechanical performances. As mentioned above, MoS2 tends to form aggregations at high content. Consequently, the alignment and orientation of the rubber chains are hindered.
3.3. Effect of MoS2 on dynamic mechanical properties of composites based on silica and natural rubber Fig. 10 shows the changes in the storage modulus (E ) and loss factor (tan ı) as a function of temperature for the composites. As shown in Fig. 10a, on incorporation of MoS2 , the E of all the samples increases significantly in both the rubbery region and the glassy region, which is due to the formation of the more developed filler network. It is found that the glass-transition temperature (Tg ) of the composites with MoS2 is almost unchanged. In general, the tan ı of a tread compound at about 0 ◦ C and 10 Hz is assumed to approximate the wet-grip conditions, whereas tan ı at about 60 ◦ C indicates the rolling resistance property. A higher tan ı value at about 0 ◦ C and lower tan ı value at about 60 ◦ C indicate improved wet-grip and decreased rolling resistance properties of a tire tread, which are two key indicators in “green type”. In this study, the tan ı values at 60 ◦ C are much lower than that of the NR/silica composite. For example, the tan ı value at 60 ◦ C of the composite containing 3 phr of MoS2 is 28% lower than that of the NR/silica composite. To further substantiate the dynamic loss at larger strain, RPA strain sweeps were conducted. As shown in Fig. 11, tan ı at 5% strain decreases significantly on incorporation of MoS2 , although the MoS2 loading does not exert a significant influence on tan ı peak value in DMA experiments. When 3 phr of MoS2 is included, the tan ı value (at 5%
Fig. 10. Temperature dependences of E (a) and tan ı (b) for NR/silica composites with various MoS2 contents. The inset is a magnification of the tan ı curves in the temperature range 50–75 ◦ C.
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significant improvements in the modulus and tensile strengths, and much lower heat build-up values. The reinforcement effect is highest at MoS2 loading of 3 phr, and the modulus and tensile strength are increased by over 70% and 60%, respectively. RPA strain sweeping shows that the value of tan ı (60 ◦ C and 5% strain) is decreased by about 30%. Heat build-up decreases by 17 ◦ C with only 1 phr of MoS2 . These significant changes in the properties are mainly the result of greatly improved dispersion of silica in NR by using MoS2 nanoplatelets, caused by charge transfer interaction between silica and MoS2 , giving rise to electrostatic repulsion of silica. Acknowledgements
Fig. 12. Effects of MoS2 loading on heat build-up and compression permanent set values.
strain) decreases by 29% compared with the NR/silica composite. The observed lower hysteresis loss in dynamic mechanical analysis and RPA is mainly ascribed to improved dispersion of silica by MoS2 , which reduces friction among the silica particles. The heat build-up and dynamic compression set values of all the composites are shown in Fig. 12. It shows an initial decrease and subsequent increase in the heat build-up and permanent set with increasing MoS2 content. In unfilled rubber, heat generation mainly arises from molecular friction, while in rubber compounds with reinforcing fillers, the breakage and re-formation of interaggregate bonds is mainly responsible for the heat generation [49]. As the silica surface has abundant silanol groups, silica particles tend to aggregate and form a filler network in the rubber matrix; therefore silica dispersion plays an important role in heat build-up. As can be seen in Fig. 12, the heat build-up and permanent set of the control sample are the highest in NR/silica composites. However, with addition of only 1 phr of MoS2 , the heat build-up decreases by 17 ◦ C. It is accepted that the filler network breaks down and rebuilds continuously under repeated compression, causing energy dissipation as heat. So we attribute the significant decrease in heat build-up to better silica dispersion in the presence of MoS2 , which alleviates the breaking down and reforming of the filler network. Based on the previous discussion, the vulcanizate containing 3 phr of MoS2 is expected to have the minimum heat build-up value, whereas the heat build-up is actually lowest at MoS2 loading of 1 phr. The higher heat build-up in the composites containing more than 1 phr MoS2 is attributed to the much easier sliding of rubber chains with excess MoS2 sheets. When the content of MoS2 is sufficiently high (5 phr), the increase in heat build-up is caused by the aggregation of MoS2 . It should be noted that the hardness has increased in the MoS2 -incorporated composites, which in turn lowers the measured heat build-up as the applied strain during the test is actually decreased. Nevertheless, both the increased hardness and lowered heat build-up are originated from the addition of MoS2 . Besides, compared with NR/silica composite, the permanent set of NR/MoS2 /silica composites is substantially decreased. 4. Conclusion In this study, chemically exfoliated MoS2 nanoplatelets were used to improve the dispersion of silica in NR composite. The MoS2 /NR compounds were first prepared by latex co-coagulation technique and after that silica was added and finally NR/silica/MoS2 crosslinked composite materials are obtained using sulfur. On incorporation of a small amount of MoS2 , the composites show
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