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Rational design of multifunctional properties for styrene-butadiene rubber reinforced by modified Kevlar nanofibers
Composites Part B 166 (2019) 196–203
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Rational design of multifunctional properties for styrene-butadiene rubber reinforced by modified Kevlar nanofibers
T
Yang Chena, Qing Yina, Xumin Zhanga, Wanqi Zhanga, Hongbing Jiaa,∗, Qingmin Jib, Fufeng Yangc, Xiaoting Ruic a
Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing University of Science and Technology, Nanjing, 210094, China Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing, 210094, China c Institute of Launch Dynamics, Nanjing University of Science and Technology, Nanjing, 21094, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Modified Kevlar nanofibers (m-KNFs) Mechanical properties Thermal properties
In this work, a facile and effective approach is developed to obtain the water-dispersible Kevlar nanofibers (KNFs) by modifying KNFs with the assistance of epichlorohydrin (ECH). And the ECH-modified KNFs (m-KNFs) are firstly utilized to prepare styrene–butadiene rubber (SBR)/m-KNFs nanocomposites through latex co-coagulation method. The multifunctional properties of SBR/m-KNFs nanocomposites are thoroughly investigated. It is confirmed that m-KNFs have strong interactions with SBR via π-π stacking, which can generate huge enhancement on the performance of SBR nanocomposites. For example, the tensile strength, tear strength and the maximum decomposition temperature of SBR filled with 7 phr (parts per hundred rubber) m-KNFs are increased by 576%, 202% and 13.1 °C, respectively, compared with those of neat SBR. Meanwhile, the presence of m-KNFs has also improved the dielectric constant of SBR nanocomposites. This work provides a new insight into the fabrication of multifunctional KNFs-based rubber composites.
1. Introduction As a synthetic rubber, styrene-butadiene rubber (SBR) has been applied in many fields, such as tire tread, adhesive tape, rubber hose, medical equipment, etc [1]. Reinforcing fillers are always required for SBR composites to simultaneously enhance or impart multifunctional properties, such as mechanical strength, storage modulus, electrical performance, and heat resistance. Recently, various nanoparticles, including graphene oxide (GO) [2], carbon nanotubes (CNTs) [3], bacterial cellulose whiskers (BCWs) [4], have been successfully introduced into rubber matrix, and much improved multifunctional performance of SBR nanocomposites can be achieved. Kevlar nanofibers (KNFs), developed from Kevlar microfibers, i.e., poly (para-phenylene terephthalamide) (PPTA), have drawn tremendous interests, in virtue of its superior multiple properties, such as ultra-strong mechanical strength, good thermal stability, excellent dielectric properties and so on [5]. Through the deprotonation of amide groups of PPTA in the solvent of dimethyl sulfoxide (DMSO) saturated with potassium hydroxide (KOH), homogeneous dispersion of KNFs can be obtained, which possesses extremely high aspect ratio with diameter of 3–30 nm and length of 5–10 μm [5]. It has been pointed out that KNFs can be regarded as one of the most novel candidates for the ∗
reinforcement of polymer nanocomposites. In this regard, Guan et al. [6] fabricated poly (vinyl alcohol) (PVA)/KNFs nanocomposites through the solution blending in the presence of DMSO, and found that 5 wt% KNFs can simultaneously improve the tensile strength and toughness of PVA with 79.2% and 148.8%, respectively. Kuang et al. [7] used KNFs to reinforce polyurethanes (PU), and found that the PU/ KNFs multilayered films showed both significantly enhanced ultimate strength and thermal durability. On the other hand, the KNFs can also be in conjunction with other nanoparticles, e.g., graphene [8], CNTs [9], to obtain the synergistic effect on the improvement of polymer materials. For instance, Fan et al. [8] introduced the KNFs into graphene nanosheets through π-π stacking interactions, and this hybrid fillers can bring about the increase of 84.5% and 16.9 °C in the tensile strength and the maximum decomposition temperature of poly (methyl methacrylate) (PMMA), respectively. Meanwhile, this composite films also exhibited a certain degree of UV-shielding. However, to the best of our knowledge, there is rare report about KNFs reinforced rubber nanocomposites in the literature. When it comes to rubber nanocomposites, we regard KNFs as a promising reinforcement agent. Thus, we put efforts into exploring the incorporation of KNFs into rubber matrix for the first time, which is expected to endow rubber materials with high mechanical and other outstanding properties.
Corresponding author. E-mail address: [email protected] (H. Jia).
https://doi.org/10.1016/j.compositesb.2018.11.132 Received 2 July 2018; Received in revised form 13 November 2018; Accepted 28 November 2018 Available online 30 November 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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by adding NaCl aqueous solution (6.5 wt%), washed with deionized water several times and dried at 50 °C until a constant weight compound was formed. Subsequently, the addition of rubber ingredients into the dried compound was carried out on an LN-120 open two-roll mill (LINA machinery Industrial Co., Ltd., China) at room temperature. The curing formula was as follows: SBR 100.0, S 1.5, ZnO 2.0, SA 2.4, CZ 2.2 phr (phr, parts per hundred rubbers), m-KNFs variable. A series of SBR/m-KNFs composites containing 0, 1, 3, 5, 7, 10 phr m-KNFs were prepared by compression molding at 160 °C and 15 MPa for the optimum cure time. These rubber composites were abbreviated as mKNFs-x, where the x denoted m-KNFs content (phr) in composites.
Generally, for the preparation of rubber composites, latex co-coagulation method is a simple and environment-friendly approach to obtain good dispersion of fillers in rubber matrix [10]. Neverthless, in most cases, KNFs can only exist in DMSO due to the negatively charged nitrogen on the surface of KNFs. When it is directly mixed with rubber latex, there is structural recombination of KNFs by grabbing the dissociated hydrogen, causing the gelation of KNFs and the aggregation of KNFs in rubber matrix. Thereby, KNFs should be modified to shield its negative surface charge for better dispersion in other solvent, especially in aqueous solution. Herein, we chose a low-cost and efficient epichlorohydrin (ECH) as the modifying agent for KNFs, and the modified Kevlar nanofibers (mKNFs) were prepared by reacting KNFs with ECH in DMSO solution, followed by centrifugation and redistribution in water. Then, m-KNFs were firstly incorporated into SBR through latex co-coagulation method. The mechanical performance, dielectric properties as well as thermal stability of SBR/m-KNFs nanocomposites were investigated thoroughly.
2.4. Characterization and tests Fourier transform infrared spectra (FTIR) were collected on a FTIR8400S spectrometer (Shimadzu Corporation, Japan). Ultraviolet–visible spectroscopy (UV–vis) absorption spectra were performed on a UV–6100S spectrophotometer (Shanghai Mapada Co. Ltd., China). Thermogravimetric analysis (TGA) was analyzed using a DTG-60 differential thermogravimetric (Shimadzu Co. Ltd., Japan) from 30 °C to 800 °C under a nitrogen atmosphere and at the constant heating rate of 10 °C min−1. Atomic force microscope (AFM) images was taken by a Nanoscope III D Multimode scanning probe microscope (Bruker Corporation, Switzerland) in a tapping mode. The freeze-fractured surfaces of samples were observed using a JSM6380LV scanning electron microscope (SEM; JEOL Ltd., Japan). Raman spectra were recorded by an InVia-H31894 argon ion laser Raman spectrometer (Renishaw Corporation, UK) with an excitation wavelength of 768 nm at ambient temperature and a resolution of 1 cm−1. Due to the minimal amount of m-KNFs in SBR, it is hard to identity the characteristic bond of m-KNFs for composites. Thus, we prepared another SBR/m-KNFs composite (m-KNFs as the matrix), containing 5 wt% SBR, in order to explore the interactions between mKNFs and SBR. The vulcanization characteristics were obtained by an MDR-2000 Moving Die Rheometer (Liyuan Chemical Engineering Co. Ltd., Wuxi, China). The tensile and tear tests were measured on a universal testing machine (Shenzhen SANS Co. Ltd., China) at ambient temperature with a cross-head speed of 500 mm min−1 according to ASTM D-412 and ASTM D-624, respectively. The dynamic mechanical properties were measured with a Q800 dynamic mechanical analyzer (DMA) (TA Co. Ltd., USA) under a nitrogen atmosphere at a heating rate of 5 °C min−1 from −60 to 40 °C and a tensile mode at 1 Hz. Dielectric properties were investigated via a WK6550B precision impedance analyzer (Wayne Kerr, United Kingdom) with the measured frequency ranging from 100 Hz to 5 MHz. The samples were cut into disc shape with a thickness of 1 mm and a diameter of 12 mm, and placed between two parallel copper plated electrodes to record the permittivity.
2. Experimental 2.1. Materials SBR 1712 (ML (1 + 4) at 100 °C = 49) latex (solid content: 23 wt%) was purchased from YPC-GPRO Rubber Co. Ltd., Nanjing, China. Kevlar 49 yarns were purchased from Dupont. Dimethyl sulfoxide (DMSO), potassium hydroxide (KOH), epichlorohydrin (ECH) and sodium chloride (NaCl) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Curing agents including sulfur (S), zinc oxide (ZnO), stearic acid (SA), and N-cyclohexyl-2-benzothiazole sulfenamide (CZ) with industry grade were purchased from Nanjing Jinsanli Rubber Plastic Co. Ltd., China. 2.2. Preparation of modified Kevlar nanofibers The Kevlar nanofibers were prepared based on the method in Ref. [5]. Here, Kevlar 49 yarns (Fig. 1a) were cut into pieces and soaked in ethyl alcohol, followed by ultrasonic treatment for 6 h. Then, the Kevlar fibers were cleaned with deionized water and completely dried in the vacuum oven. Next, 2.5 g dried Kevlar fibers and 4 g KOH were simultaneously added into a beaker with 500 ml DMSO. The dark red solution of KNFs (Fig. 1b) was obtained after magnetic stirring for 7 days at room temperature. Afterwards, 2 ml ECH was dripped into the KNFs/DMSO solution and reacted at 30 °C for 24 h. During this time, the solution color gradually changed from dark red to transparent orange (Fig. 1c), with a continuous decrease in the viscosity of solution. The prepared modified aramid nanofibers (m-KNFs) were centrifuged (12000 rpm) for 20 min, and the collected m-KNFs were further washed with deionized water until a neutral pH was achieved. Finally, the aqueous dispersion of m-KNFs (5 mg/mL) were obtained by dispersing m-KNFs in deionized water (Fig. 1d), followed by ultrasonic treatment. 2.3. Preparation of SBR/m-KNFs composites
3. Results and discussions A certain amount of m-KNFs suspensions (5 mg/mL) were added into SBR latex and stirred for 2 h. Then, the mixture was co-coagulated
3.1. Characterization of m-KNFs FTIR spectra are used to detect the successful modification of KNFs, which are illustrated in Fig. 2a. In the spectrum of KNFs, the characteristic absorption peaks are located at 3310, 1637 and 1537 cm−1, corresponding to NeH stretching vibration of amide group, the C]O stretching vibrations and NeH bending vibrations, respectively [5]. Besides, the peaks at 1509 and 1305 cm−1 are ascribed to the C]C stretching vibrations of aromatic rings and Ph-N vibrations, respectively [5]. For m-KNFs, the new appearances of typical CeOeC and
Fig. 1. Three-step procedure followed to prepare aqueously stable m-KNFs. 197
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Fig. 2c. TGA curves of KNFs and m-KNFs; Fig. 2a. FTIR spectra,
CeH stretching vibrations at around 950 and 2920 cm−1 suggest the introduction of epoxy groups onto the KNFs [11]. Moreover, the amido NeH bending peak at 1537 cm−1 gets significantly weaker in contrast to KNFs, which is attributed to the N-substitution of ECH [11], indicating the successful modification of KNFs. Fig. 2b displays the UV–vis absorption spectra of KNFs and m-KNFs in DMSO. For initial KNFs, there is an absorption peak at around 330 nm, which is mainly due to the π-π* transitions of aromatic CeC bonds of KNFs [12]. However, the peak of m-KNFs presents a blue shift to 300 nm, which may be ascribed to the decreased concentration of πelectrons caused by the nucleophilic substitution. Fig. 2c shows the TGA and DTG plot of KNFs and m-KNFs. The main weight loss of KNFs at 586 °C is attributed to the pyrolysis of PPTA [13]. For m-KNFs, the first weight loss appears at around 320 °C, which is mainly due to the decomposition of epoxy groups [11]. The second weight loss temperature at 564 °C is ascribed to the breakdown of the polymer backbone of PPTA. Judging by the weight loss of KNFs and mKNFs from 100 to 800 °C, ca. 14.4 wt% of epoxy groups are introduced onto the surface of KNFs. The morphology of m-KNFs is characterized by AFM, as shown in Fig. 2d. It can be observed that the m-KNFs maintain the typical onedimensional fibrous structure of KNFs [8], which possess a diameter of about 5 nm and a length of about 5 μm (according to the AFM image). The calculated aspect ratio of m-KNFs is about 1000. Given the analysis above, the synthetic mechanism of m-KNFs is presented in Fig. 3. The Kevlar fibers (Fig. 3a) possess high strength and stiffness in virtue of strong intermolecular bonding interactions (e.g., hydrogen bonding, π−π stacking and van der Waals forces) within molecular backbones [5]. The KNFs can be obtained by adding bulk Kevlar fibers and a certain amount of KOH into DMSO. Through the controlled deprotonation of KOH, the mobile hydrogens are extracted from amide groups of PPTA, which causes the reduction of hydrogen
Fig. 2d. AFM image of m-KNFs.
bonding interactions between polymer chains, and forms negatively charged nitrogen ions (Fig. 3b). Then, the KNFs are reacted with ECH by the nucleophilic substitution under a mild condition, so that the epoxy groups are grafted onto PPTA and the negative surface charge will be shielded (Fig. 3c). As a result, the water-dispersible m-KNFs can be achieved (Fig. 1d).
3.2. Dispersion of m-KNFs in SBR matrix The dispersion of fillers in rubber matrix is characterized by SEM. The SEM images of freeze-fractured surfaces for SBR composites with 0–10 wt% KNFs are shown in Fig. 4a-f. The fractured surface of virgin SBR (Fig. 4a) exhibits the flat and smooth texture. After the incorporation of KNFs, the fracture surfaces of SBR/m-KNFs composites gradually become rough with the increase of m-KNFs content. In addition, we should note that no obvious clusters and agglomerations are observed across the entire fracture surface of composites, when the loading amount of m-KNFs in the range from 1 to 7 phr, which suggests that KNFs are well dispersed, and firmly embedded in the SBR matrix [8]. However, upon the KNFs loading reaches 10 phr (Fig. 4f), the fractured surface of composite shows a wrinkled and folded texture, indicating that the m-KNFs agglomerate in the matrix, which will bring about the stress concentration and decrease of mechanical properties for SBR composites. It is well recognized that the orientation of fillers in matrix plays an important role in the multi-properties of the nanocomposites. In order to obtain a further understanding on the dispersion state of m-KNFs in rubber matrix, the theoretical model of Halpin–Tsai was used to compare the predicted and exactly distribution of the m-KNFs in the SBR matrix. The theoretical Young's modulus of the nanocomposites with randomly oriented (Er) and unidirectional (Eu) m-KNFs in SBR were calculated using equations (1) and (2), respectively [14],
Fig. 2b. UV–vis sepctra and 198
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Fig. 3. Synthetic mechanism of m-KNFs.
3 1 + ηL ξVg 5 1 + 2ηT Vg ⎤ Er = Em ⎡ + ⎢ 8 1 − η ξVg 8 1 − ηT Vg ⎥ L ⎦ ⎣ Eu = Em
ηL =
ηT =
ξ=
(1)
1 + ηL ξVg 1 − ηL Vg
(2)
Eg / Em − 1 Eg / Em + ξ
(3)
Eg / Em − 1 Eg / Em + 2
2l 3d
(4) (5)
where Em is the Young's modulus of the matrix, Eg is the modulus of the m-KNFs, Vg is the volume fraction, and l/d refers to the average aspect ratio of m-KNFs. Here Em = 1.80 MPa, Eg = 90 GPa, density
Fig4g. Halpin–Tsai theoretical models of SBR/m-KNFs nanocomposites.
Fig. 4. SEM images of the fracture surfaces of (a) neat SBR, SBR nanocomposites with various m-KNFs content: (b) 1 phr, (c) 3 phr, (d) 5 phr, (e) 7 phr and (f) 10 phr, 199
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Fig. 5a. Raman spectra of m-KNFs and SBR/m-KNFs composites;
ρSBR = 0.96 g/cm3 and the ρm-KNFs = 1.44 g/cm3 [5]. The experimentally observed l/d is about 1000, which is obtained from AFM images. The experimental data and predictions of the Halpin–Tsai model with two distribution state of m-KNFs are exhibited in Fig. 4g. Through the comparison, we can find that the experimental values are much closer to the random model rather than unidirectional model. It suggests that the distribution of m-KNFs in SBR matrix is mostly random. 3.3. Interactions between m-KNFs and SBR matrix The Raman spectra of m-KNFs and SBR/m-KNFs composites are used to investigate the possible interactions between m-KNFs and SBR, as is shown in Fig. 5a. For m-KNFs, the peaks at 1178, 1319 and 1606 cm−1 belong to the CeC bond from benzene ring, while the peaks at 1504, 1647 and 1271 cm−1 are ascribed to CeN and CeH bond, respectively [13]. While m-KNFs are mixed with SBR, the CeC bond of benzene ring peak shifts from 1606 to 1596 cm−1. Similar results were also observed in SBR/GO nanocomposites [1,15]. It suggests that the ππ interactions may exist between m-KNFs and SBR matrix due to the stacking of large aromatic molecules [1]. Based on aforementioned viewpoints, the schematic diagram of interaction between m-KNFs and SBR is shown in Fig. 5b.
Fig. 6. (a, b) Vulcanization characteristics of SBR/m-KNFs nanocomposites.
Fig. 6. It can be seen from Fig. 6a, with the increase of m-KNFs content, the value of t90 gradually increases while the value of CRI decreases, which is due to the fact that the m-KNFs may influence the diffusion of vulcanizing agents and thus impede the vulcanization process of SBR [16]. Generally, ML and MH reflect crosslink degree of rubber and fillerrubber interactions [16]. As is shown in Fig. 6b, the values of ML, MH, and ΔS are improved gently with increasing m-KNFs loading, which are attributed to the strong interfacial interactions between m-KNFs and SBR matrix [1].
3.4. Vulcanization characteristics Vulcanization characteristics, including scorch time (t10), optimum curing time (t90), cure rate index (CRI), minimum torque (ML), maximum torque (MH) and the difference of maximum and minimum torque values (ΔS) for all SBR/m-KNFs composites are presented in
3.5. Mechanical properties The typical stress–strain curves of all samples are shown in Fig. 7a, and the mechanical properties including tensile strength, elongation at break, tensile modulus at 100% elongation (M100), and tear strength are abstracted in Table 1. The pure SBR shows a tensile strength of 2.78 MPa and a tear strength of 10.06 kN/m. After the incorporation of
Fig. 2b. Schematic diagram of interaction between m-KNFs and SBR.
Fig. 7a. Typical stress-strain curves of SBR/m-KNFs nanocomposites; 200
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Table 1 Mechanical properties of SBR/m-KNFs nanocomposites. m-KNFs Contents (phr)
Tensile strength (MPa)
Elongation at break (%)
M100 (MPa)
Tear strength (kN/m)
0 1 3 5 7 10
2.78 ± 0.28 4.20 ± 0.53 8.24 ± 0.80 12.03 ± 1.60 18.80 ± 0.83 11.65 ± 1.04
489 547 600 647 687 613
0.91 0.94 0.99 1.03 1.13 1.11
10.06 16.29 19.31 22.40 30.36 26.38
± ± ± ± ± ±
23 17 11 29 31 24
± ± ± ± ± ±
0.05 0.02 0.03 0.02 0.07 0.08
± ± ± ± ± ±
2.20 1.40 2.10 1.18 2.71 1.07
m-KNFs, the tensile strength and tear strength of SBR/m-KNFs composites have gradually increased with the increase of m-KNFs content, and achieve maximums at a loading of 7 phr, which are improved by 576% and 202%, respectively, compared with those of pure SBR. This indicates that the m-KNFs can be regarded as an ideal reinforcing agent for SBR matrix. However, a further increase of the m-KNFs content leads to the decrease of tensile strength and tear strength. The values of elongation at break and M100 for SBR composites show the similar trend to the tensile strength, with an increase by 40% and 24% at a loading of 7 phr in contrast to those of pure SBR, respectively. The significantly enhanced mechanical performance of SBR composites is mainly due to the fact that m-KNFs possess large aspect ratio and high strength, making it possible that effective stress transfer from SBR to m-KNFs via the interface upon suffering uniaxial tension [17]. In addition, the strong interfacial interactions improve the compatibility between mKNFs and SBR matrix, which is greatly beneficial to the increase of mechanical properties of SBR composites [1]. In previous studies, SBR composites can be reinforced by various strong and stiff nanoparticles. Yin et al. [17] introduced a biological nano-filler of BCWs into SBR, and found that the tensile strength only increased from 2.37 MPa to 9.93 MPa (increased by 319%) with 2.5 phr BCWs. Liu et al. [16] fabricated SBR/oleylamine-modified GO (NGO) composites, and found that SBR with 5 phr NGO achieved a tensile strength of only about 12 MPa. Besides, Peddini et al. [3] reported that 10 wt% MWCNTs could enhance the tensile strength values of SBR composites from 3.30 MPa to 11.70 MPa (increased by 255%), but the elongation at break decreased from 463% to 382%. Similar results are also found in relevant reports [18,19]. Encouragingly, in this work, we achieved a larger tensile strength value of 18.80 MPa and elongation at break value of 687% for SBR composites with 7 phr m-KNFs. For further study, the rubber network can be evaluated by the wellknown Mooney–Rivlin equation [20], in which the reduced stress (σ∗) is presented as a function of extension ratio (λ).
σ ∗ (λ ) =
σ = 2C1 + 2C2 λ−1 λ − λ−2
Fig. 7b. Representative Mooney–Rivlin plots of reduced stress as a function of reciprocal extension ratio of SBR/m-KNFs nanocomposites.
Table 2 E′ and Tg values for SBR/m-KNFs nanocomposites. m-KNFs Contents (phr)
E' (−60 °C)/MPa
E' (25 °C)/MPa
Tg/°C
0 1 3 5 7 10
1441 1705 1883 1922 2051 2090
2.57 2.69 2.75 2.98 3.16 3.27
−38.90 −38.29 −37.26 −36.42 −35.90 −34.98
Fig. 8a. Temperature dependence of (a) storage modulus E′ and
temperature dependence on the storage modulus (E′) and the loss factor (tan δ) of SBR/m-KNFs nanocomposites, and the relevant parameters are summarized in Table 2. As is shown in Fig. 8a, with the increase of m-KNFs content, the storage modulus of SBR exhibits a gradual enhancement both in the glassy and rubbery regions, which suggests that m-KNFs can improve the stiffness and load bearing capacity of SBR matrix [15]. Specifically, E’ of m-KNFs-10 at −60 °C and 25 °C possess the highest values, which proves the reinforcing effect of m-KNFs on the SBR nanocomposites. Glass transition temperature (Tg) of SBR/m-KNFs nanocomposites are determined from the peaks of the tan δ - temperature curves as shown in Fig. 8b. In contrast to neat SBR, the Tg of nanocomposites shift to higher temperature with the increasing m-KNFs content. For example, the Tg increases gradually from −38.9 °C for neat SBR to −34.98 °C for m-KNFs-10. The addition of m-KNFs into SBR matrix could trap the rubber chain segments due to the strong interfacial interactions between fillers and matrix, which leads to the decrease in the mobility of rubber chains [20,21].
(6)
where the σ is nominal stress, C1 and C2 are constants independent of λ. As is illustrated in Fig. 7b, the curves of SBR/m-KNFs nanocomposites show a typical ‘‘U’’ shape. For all samples, the σ∗ decreased sharply in the low strain (λ−1 > 0.7), which is due to the collapse of the filler network and the release of the trapped rubber in the filler network upon stretching, i.e., Payne effect [17]. After the relative flat region, σ∗ of all samples present an abrupt upturn with the increase of strain in the high strain (λ−1 < 0.4), which is ascribed to the finite extensibility of rubber chains bridging neighboring fillers during stretching [17]. Moreover, the value of σ∗ increases consistently with the increase of filler content. For this rubber structure, the increase of m-KNFs content means the formation of stronger interfacial interactions between filler and rubber matrix, which can stiffen the rubber network [17]. 3.6. Dynamic mechanical properties Dynamic mechanical analysis is used to evaluate reinforcement effect of m-KNFs on the SBR nanocomposites. Fig. 8 presents the 201
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Fig. 8b. Tan δ for SBR/m-KNFs nanocomposites.
Fig. 10. (a) TGA and (b) partial DTG curves of SBR/m-KNFs nanocomposites. Table 3 Summary of thermal properties for SBR/m-KNFs nanocomposites.
Fig. 9. Dependence of dielectric constants on the frequency for SBR/m-KNFs nanocomposites at room temperature.
3.7. Dielectric properties The dielectrical properties of polymeric materials are highly related to its real parts of complex permittivity, which represents the storage capacity of electric energy [20]. Fig. 9 shows the frequency dependence of dielectric constant for SBR/m-KNFs nanocomposites at room temperature. With the increase of frequency, the dielectric constant of nanocomposites decrease slightly, which is due to the relaxation process of charge carriers upon field reversal [22]. Over the whole range of frequency, the dielectric constant of SBR samples increase gradually with the increase of m-KNFs content. This increasement in dielectric constant can be attributed to the accumulation of more charge carriers at the internal interfaces between fillers and rubber matrix with the increasing loading of m-KNFs, which leads to the interfacial polarization (Maxwell-Wagner-Sillars effect) [23]. In previous studies, Liu et al. fabricated SBR composites filled with a hybrid of the silica decorating on GO surface (SiO2@GO), and found that the SBR composites exhibited a dielectric constant value of 4.08 at 1 kHz when the filler content is 20 phr [18]. Herein, SBR filled with 10 phr m-KNFs gets a higher dielectric constant value of 6.92, indicating that the m-KNFs can be regarded as a promising nanoparticle for boosting the dielectric properties of polymer materials.
m-KNFs Contents (phr)
T5/°C
T50/°C
Tmax/°C
0 1 3 5 7 10
316.0 299.7 293.5 286.0 283.9 280.8
445.5 451.5 454.9 456.3 458.7 461.9
449.2 451.3 459.0 460.6 462.3 465.6
and temperature of the maximal rate of decomposition (Tmax) of SBR nanocomposites increase gradually with increasing loading amount of m-KNFs. For instance, T50 and Tmax of m-KNFs-7 are improved by 13.2 °C and 13.1 °C, respectively, in contrast to those of neat SBR. This excellent improvement in thermal stability might be due to a strong nano-filler network formed by interfacial interaction between m-KNFs and SBR, which can act as the barrier and delay the heat transmission [24]. Thus, the decomposition of SBR composites are retarded and the relevant thermal degradation temperature are improved. 4. Conclusion In conclusion, the water-dispersible m-KNFs with a diameter of ∼5 nm were successfully synthesized through chemical treatment of KNFs with ECH. The SBR/m-KNFs nanocomposites were fabricated by a simple latex co-coagulation method, followed by curing process. It was found that strong π-π interaction exists between m-KNFs and SBR matrix, which played an important role on the reinforcement of m-KNFs in SBR. The m-KNFs with low amount could be well dispersed in SBR matrix with random orientation. With incorporation of 7 phr m-KNFs, the tensile strength and tear strength were increased by 576% and 202%, respectively, in contrast to those of unfiiled SBR. In addition, the storage modulus, dielectric constant and thermal stability of
3.8. Thermal properties The TGA and DTG curves of SBR/m-KNFs composites are displayed in Fig. 10, and the results of TGA are summarized in Table 3. With the increase of m-KNFs content, temperature for 5% degradation (T5) of SBR/m-KNFs nanocomposites decreases gradually, which is resulted from the low decomposition temperature of m-KNFs at around 300 °C (as shown in Fig. 2c). However, temperature for 50% degradation (T50) 202
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nanocomposites were also greatly improved with the increase of mKNFs content. In this respect, this work should provide a valuable method into the rational design of multifunctional rubber materials.
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Acknowledgements This work was financially supported by Aeronautical Science Foundation of China (2016ZF9009) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] Chen Y, Yin Q, Zhang X, Jia H, Ji Q, Xu Z. Impact of various oxidation degrees of graphene oxide on the performance of styrene–butadiene rubber nanocomposites. Polym Eng Sci 2017https://doi.org/10.1002/pen.24729. [2] Yin B, Wang J, Jia H, He J, Zhang X, Xu Z. Enhanced mechanical properties and thermal conductivity of styrene–butadiene rubber reinforced with polyvinylpyrrolidone-modified graphene oxide. J Mater Sci 2016;51(12):5724–37. [3] Peddini SK, Bosnyak CP, Henderson NM, Ellison CJ, Paul DR. Nanocomposites from styrene–butadiene rubber (SBR) and multiwall carbon nanotubes (MWCNT) part 2: mechanical properties. Polymer 2015;56:443–51. [4] Chen Y, Li G, Yin Q, Jia H, Ji Q, Wang L, Wang D, Yin B. Stimuli‐responsive polymer nanocomposites based on styrene‐butadiene rubber and bacterial cellulose whiskers. Polym Adv Technol 2018;29(5):1507–17. [5] Yang M, Cao K, Sui L, Qi Y, Zhu J, Waas A, Arruda EM, Kieffer J, Thouless MD, Kotov NA. Dispersions of aramid nanofibers: a new nanoscale building block. ACS Nano 2011;5(9):6945–54. [6] Guan Y, Li W, Zhang Y, Shi Z, Tan J, Wang F, Wang Y. Aramid nanofibers and poly (vinyl alcohol) nanocomposites for ideal combination of strength and toughness via hydrogen bonding interactions. Compos Sci Technol 2017;144:193–201. [7] Kuang Q, Zhang D, Yu J, Chang Y, Yue M, Hou Y, Yang M. Toward record-high stiffness in polyurethane nanocomposites using aramid nanofibers. J Phys Chem C 2015;119(49):27467–77. [8] Fan J, Shi Z, Zhang L, Wang J, Yin J. Aramid nanofiber-functionalized graphene nanosheets for polymer reinforcement. Nanoscale 2012;4(22):7046–55. [9] Fan J, Wang J, Shi Z, Yu S, Yin J. Kevlar nanofiber-functionalized multiwalled carbon nanotubes for polymer reinforcement. Mater Chem Phys 2013;141(2–3):861–8. [10] Kang H, Zuo K, Wang Z, Zhang L, Liu L, Guo B. Using a green method to develop graphene oxide/elastomers nanocomposites with combination of high barrier and mechanical performance. Compos Sci Technol 2014;92:1–8.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Rational design of multifunctional properties for styrene-butadiene rubber reinforced by modified Kevlar nanofibers
T
Yang Chena, Qing Yina, Xumin Zhanga, Wanqi Zhanga, Hongbing Jiaa,∗, Qingmin Jib, Fufeng Yangc, Xiaoting Ruic a
Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, Nanjing University of Science and Technology, Nanjing, 210094, China Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing, 210094, China c Institute of Launch Dynamics, Nanjing University of Science and Technology, Nanjing, 21094, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Modified Kevlar nanofibers (m-KNFs) Mechanical properties Thermal properties
In this work, a facile and effective approach is developed to obtain the water-dispersible Kevlar nanofibers (KNFs) by modifying KNFs with the assistance of epichlorohydrin (ECH). And the ECH-modified KNFs (m-KNFs) are firstly utilized to prepare styrene–butadiene rubber (SBR)/m-KNFs nanocomposites through latex co-coagulation method. The multifunctional properties of SBR/m-KNFs nanocomposites are thoroughly investigated. It is confirmed that m-KNFs have strong interactions with SBR via π-π stacking, which can generate huge enhancement on the performance of SBR nanocomposites. For example, the tensile strength, tear strength and the maximum decomposition temperature of SBR filled with 7 phr (parts per hundred rubber) m-KNFs are increased by 576%, 202% and 13.1 °C, respectively, compared with those of neat SBR. Meanwhile, the presence of m-KNFs has also improved the dielectric constant of SBR nanocomposites. This work provides a new insight into the fabrication of multifunctional KNFs-based rubber composites.
1. Introduction As a synthetic rubber, styrene-butadiene rubber (SBR) has been applied in many fields, such as tire tread, adhesive tape, rubber hose, medical equipment, etc [1]. Reinforcing fillers are always required for SBR composites to simultaneously enhance or impart multifunctional properties, such as mechanical strength, storage modulus, electrical performance, and heat resistance. Recently, various nanoparticles, including graphene oxide (GO) [2], carbon nanotubes (CNTs) [3], bacterial cellulose whiskers (BCWs) [4], have been successfully introduced into rubber matrix, and much improved multifunctional performance of SBR nanocomposites can be achieved. Kevlar nanofibers (KNFs), developed from Kevlar microfibers, i.e., poly (para-phenylene terephthalamide) (PPTA), have drawn tremendous interests, in virtue of its superior multiple properties, such as ultra-strong mechanical strength, good thermal stability, excellent dielectric properties and so on [5]. Through the deprotonation of amide groups of PPTA in the solvent of dimethyl sulfoxide (DMSO) saturated with potassium hydroxide (KOH), homogeneous dispersion of KNFs can be obtained, which possesses extremely high aspect ratio with diameter of 3–30 nm and length of 5–10 μm [5]. It has been pointed out that KNFs can be regarded as one of the most novel candidates for the ∗
reinforcement of polymer nanocomposites. In this regard, Guan et al. [6] fabricated poly (vinyl alcohol) (PVA)/KNFs nanocomposites through the solution blending in the presence of DMSO, and found that 5 wt% KNFs can simultaneously improve the tensile strength and toughness of PVA with 79.2% and 148.8%, respectively. Kuang et al. [7] used KNFs to reinforce polyurethanes (PU), and found that the PU/ KNFs multilayered films showed both significantly enhanced ultimate strength and thermal durability. On the other hand, the KNFs can also be in conjunction with other nanoparticles, e.g., graphene [8], CNTs [9], to obtain the synergistic effect on the improvement of polymer materials. For instance, Fan et al. [8] introduced the KNFs into graphene nanosheets through π-π stacking interactions, and this hybrid fillers can bring about the increase of 84.5% and 16.9 °C in the tensile strength and the maximum decomposition temperature of poly (methyl methacrylate) (PMMA), respectively. Meanwhile, this composite films also exhibited a certain degree of UV-shielding. However, to the best of our knowledge, there is rare report about KNFs reinforced rubber nanocomposites in the literature. When it comes to rubber nanocomposites, we regard KNFs as a promising reinforcement agent. Thus, we put efforts into exploring the incorporation of KNFs into rubber matrix for the first time, which is expected to endow rubber materials with high mechanical and other outstanding properties.
Corresponding author. E-mail address: [email protected] (H. Jia).
https://doi.org/10.1016/j.compositesb.2018.11.132 Received 2 July 2018; Received in revised form 13 November 2018; Accepted 28 November 2018 Available online 30 November 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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by adding NaCl aqueous solution (6.5 wt%), washed with deionized water several times and dried at 50 °C until a constant weight compound was formed. Subsequently, the addition of rubber ingredients into the dried compound was carried out on an LN-120 open two-roll mill (LINA machinery Industrial Co., Ltd., China) at room temperature. The curing formula was as follows: SBR 100.0, S 1.5, ZnO 2.0, SA 2.4, CZ 2.2 phr (phr, parts per hundred rubbers), m-KNFs variable. A series of SBR/m-KNFs composites containing 0, 1, 3, 5, 7, 10 phr m-KNFs were prepared by compression molding at 160 °C and 15 MPa for the optimum cure time. These rubber composites were abbreviated as mKNFs-x, where the x denoted m-KNFs content (phr) in composites.
Generally, for the preparation of rubber composites, latex co-coagulation method is a simple and environment-friendly approach to obtain good dispersion of fillers in rubber matrix [10]. Neverthless, in most cases, KNFs can only exist in DMSO due to the negatively charged nitrogen on the surface of KNFs. When it is directly mixed with rubber latex, there is structural recombination of KNFs by grabbing the dissociated hydrogen, causing the gelation of KNFs and the aggregation of KNFs in rubber matrix. Thereby, KNFs should be modified to shield its negative surface charge for better dispersion in other solvent, especially in aqueous solution. Herein, we chose a low-cost and efficient epichlorohydrin (ECH) as the modifying agent for KNFs, and the modified Kevlar nanofibers (mKNFs) were prepared by reacting KNFs with ECH in DMSO solution, followed by centrifugation and redistribution in water. Then, m-KNFs were firstly incorporated into SBR through latex co-coagulation method. The mechanical performance, dielectric properties as well as thermal stability of SBR/m-KNFs nanocomposites were investigated thoroughly.
2.4. Characterization and tests Fourier transform infrared spectra (FTIR) were collected on a FTIR8400S spectrometer (Shimadzu Corporation, Japan). Ultraviolet–visible spectroscopy (UV–vis) absorption spectra were performed on a UV–6100S spectrophotometer (Shanghai Mapada Co. Ltd., China). Thermogravimetric analysis (TGA) was analyzed using a DTG-60 differential thermogravimetric (Shimadzu Co. Ltd., Japan) from 30 °C to 800 °C under a nitrogen atmosphere and at the constant heating rate of 10 °C min−1. Atomic force microscope (AFM) images was taken by a Nanoscope III D Multimode scanning probe microscope (Bruker Corporation, Switzerland) in a tapping mode. The freeze-fractured surfaces of samples were observed using a JSM6380LV scanning electron microscope (SEM; JEOL Ltd., Japan). Raman spectra were recorded by an InVia-H31894 argon ion laser Raman spectrometer (Renishaw Corporation, UK) with an excitation wavelength of 768 nm at ambient temperature and a resolution of 1 cm−1. Due to the minimal amount of m-KNFs in SBR, it is hard to identity the characteristic bond of m-KNFs for composites. Thus, we prepared another SBR/m-KNFs composite (m-KNFs as the matrix), containing 5 wt% SBR, in order to explore the interactions between mKNFs and SBR. The vulcanization characteristics were obtained by an MDR-2000 Moving Die Rheometer (Liyuan Chemical Engineering Co. Ltd., Wuxi, China). The tensile and tear tests were measured on a universal testing machine (Shenzhen SANS Co. Ltd., China) at ambient temperature with a cross-head speed of 500 mm min−1 according to ASTM D-412 and ASTM D-624, respectively. The dynamic mechanical properties were measured with a Q800 dynamic mechanical analyzer (DMA) (TA Co. Ltd., USA) under a nitrogen atmosphere at a heating rate of 5 °C min−1 from −60 to 40 °C and a tensile mode at 1 Hz. Dielectric properties were investigated via a WK6550B precision impedance analyzer (Wayne Kerr, United Kingdom) with the measured frequency ranging from 100 Hz to 5 MHz. The samples were cut into disc shape with a thickness of 1 mm and a diameter of 12 mm, and placed between two parallel copper plated electrodes to record the permittivity.
2. Experimental 2.1. Materials SBR 1712 (ML (1 + 4) at 100 °C = 49) latex (solid content: 23 wt%) was purchased from YPC-GPRO Rubber Co. Ltd., Nanjing, China. Kevlar 49 yarns were purchased from Dupont. Dimethyl sulfoxide (DMSO), potassium hydroxide (KOH), epichlorohydrin (ECH) and sodium chloride (NaCl) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Curing agents including sulfur (S), zinc oxide (ZnO), stearic acid (SA), and N-cyclohexyl-2-benzothiazole sulfenamide (CZ) with industry grade were purchased from Nanjing Jinsanli Rubber Plastic Co. Ltd., China. 2.2. Preparation of modified Kevlar nanofibers The Kevlar nanofibers were prepared based on the method in Ref. [5]. Here, Kevlar 49 yarns (Fig. 1a) were cut into pieces and soaked in ethyl alcohol, followed by ultrasonic treatment for 6 h. Then, the Kevlar fibers were cleaned with deionized water and completely dried in the vacuum oven. Next, 2.5 g dried Kevlar fibers and 4 g KOH were simultaneously added into a beaker with 500 ml DMSO. The dark red solution of KNFs (Fig. 1b) was obtained after magnetic stirring for 7 days at room temperature. Afterwards, 2 ml ECH was dripped into the KNFs/DMSO solution and reacted at 30 °C for 24 h. During this time, the solution color gradually changed from dark red to transparent orange (Fig. 1c), with a continuous decrease in the viscosity of solution. The prepared modified aramid nanofibers (m-KNFs) were centrifuged (12000 rpm) for 20 min, and the collected m-KNFs were further washed with deionized water until a neutral pH was achieved. Finally, the aqueous dispersion of m-KNFs (5 mg/mL) were obtained by dispersing m-KNFs in deionized water (Fig. 1d), followed by ultrasonic treatment. 2.3. Preparation of SBR/m-KNFs composites
3. Results and discussions A certain amount of m-KNFs suspensions (5 mg/mL) were added into SBR latex and stirred for 2 h. Then, the mixture was co-coagulated
3.1. Characterization of m-KNFs FTIR spectra are used to detect the successful modification of KNFs, which are illustrated in Fig. 2a. In the spectrum of KNFs, the characteristic absorption peaks are located at 3310, 1637 and 1537 cm−1, corresponding to NeH stretching vibration of amide group, the C]O stretching vibrations and NeH bending vibrations, respectively [5]. Besides, the peaks at 1509 and 1305 cm−1 are ascribed to the C]C stretching vibrations of aromatic rings and Ph-N vibrations, respectively [5]. For m-KNFs, the new appearances of typical CeOeC and
Fig. 1. Three-step procedure followed to prepare aqueously stable m-KNFs. 197
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Fig. 2c. TGA curves of KNFs and m-KNFs; Fig. 2a. FTIR spectra,
CeH stretching vibrations at around 950 and 2920 cm−1 suggest the introduction of epoxy groups onto the KNFs [11]. Moreover, the amido NeH bending peak at 1537 cm−1 gets significantly weaker in contrast to KNFs, which is attributed to the N-substitution of ECH [11], indicating the successful modification of KNFs. Fig. 2b displays the UV–vis absorption spectra of KNFs and m-KNFs in DMSO. For initial KNFs, there is an absorption peak at around 330 nm, which is mainly due to the π-π* transitions of aromatic CeC bonds of KNFs [12]. However, the peak of m-KNFs presents a blue shift to 300 nm, which may be ascribed to the decreased concentration of πelectrons caused by the nucleophilic substitution. Fig. 2c shows the TGA and DTG plot of KNFs and m-KNFs. The main weight loss of KNFs at 586 °C is attributed to the pyrolysis of PPTA [13]. For m-KNFs, the first weight loss appears at around 320 °C, which is mainly due to the decomposition of epoxy groups [11]. The second weight loss temperature at 564 °C is ascribed to the breakdown of the polymer backbone of PPTA. Judging by the weight loss of KNFs and mKNFs from 100 to 800 °C, ca. 14.4 wt% of epoxy groups are introduced onto the surface of KNFs. The morphology of m-KNFs is characterized by AFM, as shown in Fig. 2d. It can be observed that the m-KNFs maintain the typical onedimensional fibrous structure of KNFs [8], which possess a diameter of about 5 nm and a length of about 5 μm (according to the AFM image). The calculated aspect ratio of m-KNFs is about 1000. Given the analysis above, the synthetic mechanism of m-KNFs is presented in Fig. 3. The Kevlar fibers (Fig. 3a) possess high strength and stiffness in virtue of strong intermolecular bonding interactions (e.g., hydrogen bonding, π−π stacking and van der Waals forces) within molecular backbones [5]. The KNFs can be obtained by adding bulk Kevlar fibers and a certain amount of KOH into DMSO. Through the controlled deprotonation of KOH, the mobile hydrogens are extracted from amide groups of PPTA, which causes the reduction of hydrogen
Fig. 2d. AFM image of m-KNFs.
bonding interactions between polymer chains, and forms negatively charged nitrogen ions (Fig. 3b). Then, the KNFs are reacted with ECH by the nucleophilic substitution under a mild condition, so that the epoxy groups are grafted onto PPTA and the negative surface charge will be shielded (Fig. 3c). As a result, the water-dispersible m-KNFs can be achieved (Fig. 1d).
3.2. Dispersion of m-KNFs in SBR matrix The dispersion of fillers in rubber matrix is characterized by SEM. The SEM images of freeze-fractured surfaces for SBR composites with 0–10 wt% KNFs are shown in Fig. 4a-f. The fractured surface of virgin SBR (Fig. 4a) exhibits the flat and smooth texture. After the incorporation of KNFs, the fracture surfaces of SBR/m-KNFs composites gradually become rough with the increase of m-KNFs content. In addition, we should note that no obvious clusters and agglomerations are observed across the entire fracture surface of composites, when the loading amount of m-KNFs in the range from 1 to 7 phr, which suggests that KNFs are well dispersed, and firmly embedded in the SBR matrix [8]. However, upon the KNFs loading reaches 10 phr (Fig. 4f), the fractured surface of composite shows a wrinkled and folded texture, indicating that the m-KNFs agglomerate in the matrix, which will bring about the stress concentration and decrease of mechanical properties for SBR composites. It is well recognized that the orientation of fillers in matrix plays an important role in the multi-properties of the nanocomposites. In order to obtain a further understanding on the dispersion state of m-KNFs in rubber matrix, the theoretical model of Halpin–Tsai was used to compare the predicted and exactly distribution of the m-KNFs in the SBR matrix. The theoretical Young's modulus of the nanocomposites with randomly oriented (Er) and unidirectional (Eu) m-KNFs in SBR were calculated using equations (1) and (2), respectively [14],
Fig. 2b. UV–vis sepctra and 198
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Fig. 3. Synthetic mechanism of m-KNFs.
3 1 + ηL ξVg 5 1 + 2ηT Vg ⎤ Er = Em ⎡ + ⎢ 8 1 − η ξVg 8 1 − ηT Vg ⎥ L ⎦ ⎣ Eu = Em
ηL =
ηT =
ξ=
(1)
1 + ηL ξVg 1 − ηL Vg
(2)
Eg / Em − 1 Eg / Em + ξ
(3)
Eg / Em − 1 Eg / Em + 2
2l 3d
(4) (5)
where Em is the Young's modulus of the matrix, Eg is the modulus of the m-KNFs, Vg is the volume fraction, and l/d refers to the average aspect ratio of m-KNFs. Here Em = 1.80 MPa, Eg = 90 GPa, density
Fig4g. Halpin–Tsai theoretical models of SBR/m-KNFs nanocomposites.
Fig. 4. SEM images of the fracture surfaces of (a) neat SBR, SBR nanocomposites with various m-KNFs content: (b) 1 phr, (c) 3 phr, (d) 5 phr, (e) 7 phr and (f) 10 phr, 199
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Fig. 5a. Raman spectra of m-KNFs and SBR/m-KNFs composites;
ρSBR = 0.96 g/cm3 and the ρm-KNFs = 1.44 g/cm3 [5]. The experimentally observed l/d is about 1000, which is obtained from AFM images. The experimental data and predictions of the Halpin–Tsai model with two distribution state of m-KNFs are exhibited in Fig. 4g. Through the comparison, we can find that the experimental values are much closer to the random model rather than unidirectional model. It suggests that the distribution of m-KNFs in SBR matrix is mostly random. 3.3. Interactions between m-KNFs and SBR matrix The Raman spectra of m-KNFs and SBR/m-KNFs composites are used to investigate the possible interactions between m-KNFs and SBR, as is shown in Fig. 5a. For m-KNFs, the peaks at 1178, 1319 and 1606 cm−1 belong to the CeC bond from benzene ring, while the peaks at 1504, 1647 and 1271 cm−1 are ascribed to CeN and CeH bond, respectively [13]. While m-KNFs are mixed with SBR, the CeC bond of benzene ring peak shifts from 1606 to 1596 cm−1. Similar results were also observed in SBR/GO nanocomposites [1,15]. It suggests that the ππ interactions may exist between m-KNFs and SBR matrix due to the stacking of large aromatic molecules [1]. Based on aforementioned viewpoints, the schematic diagram of interaction between m-KNFs and SBR is shown in Fig. 5b.
Fig. 6. (a, b) Vulcanization characteristics of SBR/m-KNFs nanocomposites.
Fig. 6. It can be seen from Fig. 6a, with the increase of m-KNFs content, the value of t90 gradually increases while the value of CRI decreases, which is due to the fact that the m-KNFs may influence the diffusion of vulcanizing agents and thus impede the vulcanization process of SBR [16]. Generally, ML and MH reflect crosslink degree of rubber and fillerrubber interactions [16]. As is shown in Fig. 6b, the values of ML, MH, and ΔS are improved gently with increasing m-KNFs loading, which are attributed to the strong interfacial interactions between m-KNFs and SBR matrix [1].
3.4. Vulcanization characteristics Vulcanization characteristics, including scorch time (t10), optimum curing time (t90), cure rate index (CRI), minimum torque (ML), maximum torque (MH) and the difference of maximum and minimum torque values (ΔS) for all SBR/m-KNFs composites are presented in
3.5. Mechanical properties The typical stress–strain curves of all samples are shown in Fig. 7a, and the mechanical properties including tensile strength, elongation at break, tensile modulus at 100% elongation (M100), and tear strength are abstracted in Table 1. The pure SBR shows a tensile strength of 2.78 MPa and a tear strength of 10.06 kN/m. After the incorporation of
Fig. 2b. Schematic diagram of interaction between m-KNFs and SBR.
Fig. 7a. Typical stress-strain curves of SBR/m-KNFs nanocomposites; 200
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Table 1 Mechanical properties of SBR/m-KNFs nanocomposites. m-KNFs Contents (phr)
Tensile strength (MPa)
Elongation at break (%)
M100 (MPa)
Tear strength (kN/m)
0 1 3 5 7 10
2.78 ± 0.28 4.20 ± 0.53 8.24 ± 0.80 12.03 ± 1.60 18.80 ± 0.83 11.65 ± 1.04
489 547 600 647 687 613
0.91 0.94 0.99 1.03 1.13 1.11
10.06 16.29 19.31 22.40 30.36 26.38
± ± ± ± ± ±
23 17 11 29 31 24
± ± ± ± ± ±
0.05 0.02 0.03 0.02 0.07 0.08
± ± ± ± ± ±
2.20 1.40 2.10 1.18 2.71 1.07
m-KNFs, the tensile strength and tear strength of SBR/m-KNFs composites have gradually increased with the increase of m-KNFs content, and achieve maximums at a loading of 7 phr, which are improved by 576% and 202%, respectively, compared with those of pure SBR. This indicates that the m-KNFs can be regarded as an ideal reinforcing agent for SBR matrix. However, a further increase of the m-KNFs content leads to the decrease of tensile strength and tear strength. The values of elongation at break and M100 for SBR composites show the similar trend to the tensile strength, with an increase by 40% and 24% at a loading of 7 phr in contrast to those of pure SBR, respectively. The significantly enhanced mechanical performance of SBR composites is mainly due to the fact that m-KNFs possess large aspect ratio and high strength, making it possible that effective stress transfer from SBR to m-KNFs via the interface upon suffering uniaxial tension [17]. In addition, the strong interfacial interactions improve the compatibility between mKNFs and SBR matrix, which is greatly beneficial to the increase of mechanical properties of SBR composites [1]. In previous studies, SBR composites can be reinforced by various strong and stiff nanoparticles. Yin et al. [17] introduced a biological nano-filler of BCWs into SBR, and found that the tensile strength only increased from 2.37 MPa to 9.93 MPa (increased by 319%) with 2.5 phr BCWs. Liu et al. [16] fabricated SBR/oleylamine-modified GO (NGO) composites, and found that SBR with 5 phr NGO achieved a tensile strength of only about 12 MPa. Besides, Peddini et al. [3] reported that 10 wt% MWCNTs could enhance the tensile strength values of SBR composites from 3.30 MPa to 11.70 MPa (increased by 255%), but the elongation at break decreased from 463% to 382%. Similar results are also found in relevant reports [18,19]. Encouragingly, in this work, we achieved a larger tensile strength value of 18.80 MPa and elongation at break value of 687% for SBR composites with 7 phr m-KNFs. For further study, the rubber network can be evaluated by the wellknown Mooney–Rivlin equation [20], in which the reduced stress (σ∗) is presented as a function of extension ratio (λ).
σ ∗ (λ ) =
σ = 2C1 + 2C2 λ−1 λ − λ−2
Fig. 7b. Representative Mooney–Rivlin plots of reduced stress as a function of reciprocal extension ratio of SBR/m-KNFs nanocomposites.
Table 2 E′ and Tg values for SBR/m-KNFs nanocomposites. m-KNFs Contents (phr)
E' (−60 °C)/MPa
E' (25 °C)/MPa
Tg/°C
0 1 3 5 7 10
1441 1705 1883 1922 2051 2090
2.57 2.69 2.75 2.98 3.16 3.27
−38.90 −38.29 −37.26 −36.42 −35.90 −34.98
Fig. 8a. Temperature dependence of (a) storage modulus E′ and
temperature dependence on the storage modulus (E′) and the loss factor (tan δ) of SBR/m-KNFs nanocomposites, and the relevant parameters are summarized in Table 2. As is shown in Fig. 8a, with the increase of m-KNFs content, the storage modulus of SBR exhibits a gradual enhancement both in the glassy and rubbery regions, which suggests that m-KNFs can improve the stiffness and load bearing capacity of SBR matrix [15]. Specifically, E’ of m-KNFs-10 at −60 °C and 25 °C possess the highest values, which proves the reinforcing effect of m-KNFs on the SBR nanocomposites. Glass transition temperature (Tg) of SBR/m-KNFs nanocomposites are determined from the peaks of the tan δ - temperature curves as shown in Fig. 8b. In contrast to neat SBR, the Tg of nanocomposites shift to higher temperature with the increasing m-KNFs content. For example, the Tg increases gradually from −38.9 °C for neat SBR to −34.98 °C for m-KNFs-10. The addition of m-KNFs into SBR matrix could trap the rubber chain segments due to the strong interfacial interactions between fillers and matrix, which leads to the decrease in the mobility of rubber chains [20,21].
(6)
where the σ is nominal stress, C1 and C2 are constants independent of λ. As is illustrated in Fig. 7b, the curves of SBR/m-KNFs nanocomposites show a typical ‘‘U’’ shape. For all samples, the σ∗ decreased sharply in the low strain (λ−1 > 0.7), which is due to the collapse of the filler network and the release of the trapped rubber in the filler network upon stretching, i.e., Payne effect [17]. After the relative flat region, σ∗ of all samples present an abrupt upturn with the increase of strain in the high strain (λ−1 < 0.4), which is ascribed to the finite extensibility of rubber chains bridging neighboring fillers during stretching [17]. Moreover, the value of σ∗ increases consistently with the increase of filler content. For this rubber structure, the increase of m-KNFs content means the formation of stronger interfacial interactions between filler and rubber matrix, which can stiffen the rubber network [17]. 3.6. Dynamic mechanical properties Dynamic mechanical analysis is used to evaluate reinforcement effect of m-KNFs on the SBR nanocomposites. Fig. 8 presents the 201
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Fig. 8b. Tan δ for SBR/m-KNFs nanocomposites.
Fig. 10. (a) TGA and (b) partial DTG curves of SBR/m-KNFs nanocomposites. Table 3 Summary of thermal properties for SBR/m-KNFs nanocomposites.
Fig. 9. Dependence of dielectric constants on the frequency for SBR/m-KNFs nanocomposites at room temperature.
3.7. Dielectric properties The dielectrical properties of polymeric materials are highly related to its real parts of complex permittivity, which represents the storage capacity of electric energy [20]. Fig. 9 shows the frequency dependence of dielectric constant for SBR/m-KNFs nanocomposites at room temperature. With the increase of frequency, the dielectric constant of nanocomposites decrease slightly, which is due to the relaxation process of charge carriers upon field reversal [22]. Over the whole range of frequency, the dielectric constant of SBR samples increase gradually with the increase of m-KNFs content. This increasement in dielectric constant can be attributed to the accumulation of more charge carriers at the internal interfaces between fillers and rubber matrix with the increasing loading of m-KNFs, which leads to the interfacial polarization (Maxwell-Wagner-Sillars effect) [23]. In previous studies, Liu et al. fabricated SBR composites filled with a hybrid of the silica decorating on GO surface (SiO2@GO), and found that the SBR composites exhibited a dielectric constant value of 4.08 at 1 kHz when the filler content is 20 phr [18]. Herein, SBR filled with 10 phr m-KNFs gets a higher dielectric constant value of 6.92, indicating that the m-KNFs can be regarded as a promising nanoparticle for boosting the dielectric properties of polymer materials.
m-KNFs Contents (phr)
T5/°C
T50/°C
Tmax/°C
0 1 3 5 7 10
316.0 299.7 293.5 286.0 283.9 280.8
445.5 451.5 454.9 456.3 458.7 461.9
449.2 451.3 459.0 460.6 462.3 465.6
and temperature of the maximal rate of decomposition (Tmax) of SBR nanocomposites increase gradually with increasing loading amount of m-KNFs. For instance, T50 and Tmax of m-KNFs-7 are improved by 13.2 °C and 13.1 °C, respectively, in contrast to those of neat SBR. This excellent improvement in thermal stability might be due to a strong nano-filler network formed by interfacial interaction between m-KNFs and SBR, which can act as the barrier and delay the heat transmission [24]. Thus, the decomposition of SBR composites are retarded and the relevant thermal degradation temperature are improved. 4. Conclusion In conclusion, the water-dispersible m-KNFs with a diameter of ∼5 nm were successfully synthesized through chemical treatment of KNFs with ECH. The SBR/m-KNFs nanocomposites were fabricated by a simple latex co-coagulation method, followed by curing process. It was found that strong π-π interaction exists between m-KNFs and SBR matrix, which played an important role on the reinforcement of m-KNFs in SBR. The m-KNFs with low amount could be well dispersed in SBR matrix with random orientation. With incorporation of 7 phr m-KNFs, the tensile strength and tear strength were increased by 576% and 202%, respectively, in contrast to those of unfiiled SBR. In addition, the storage modulus, dielectric constant and thermal stability of
3.8. Thermal properties The TGA and DTG curves of SBR/m-KNFs composites are displayed in Fig. 10, and the results of TGA are summarized in Table 3. With the increase of m-KNFs content, temperature for 5% degradation (T5) of SBR/m-KNFs nanocomposites decreases gradually, which is resulted from the low decomposition temperature of m-KNFs at around 300 °C (as shown in Fig. 2c). However, temperature for 50% degradation (T50) 202
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nanocomposites were also greatly improved with the increase of mKNFs content. In this respect, this work should provide a valuable method into the rational design of multifunctional rubber materials.
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