Probing the Damping Property of Three-Dimensional Graphene Aerogels in Carboxylated Nitrile Butadiene Rubber/Polyurethane Blend.
With the rapid development of modern industrial technology, the damage of vibration and noise has made severely negative influence on the reliability and safety of equipment [1, 2]. Vibrations lead to serious wear and noise pollution, thereby shortening the service life of machine and causing harm to operators or others nearby. Damping material, a class of specially functional material, gains fresh appeal and becomes the research hotpot for moving mechanical systems with requirements of reliability, availability, safety and long-term service [3, 4]. Rubber as a damping material can transform the mechanical and acoustic energy to heat energy, so it takes effect to reduce or even eliminate the vibration and was widely used in transportation, construction, machinery, electronics, aerospace, and other fields [5-7]. However, the widespread use of rubber was restricted due to the shortcomings in glass-transition temperature ([T.sub.g]) with a narrow range or very low loss factor [8-10]. It is of great significance to modify the rubber and enhance the damping property.
Modification methods of rubber generally contain blending , copolymerization , hybridization of organic molecules , interpenetrating polymer network (IPN) , inorganic filler , and so on. Blending as a physical method could uniformly mix with two or more kinds of materials to upgrade material property, such as rubber/rubber blending and rubber/plastics blending [16-20]. Considering the difference of [T.sub.g] for rubber and plastic, it is theoretically ideal for improving the damping property via mixing rubber and plastic [21, 22].
Carboxylated butadiene nitrile rubber (XNBR) is a kind of nitrile rubber modified by the polar carboxyl, which has more polarity and can well mix with the polar or nonpolar resin [23, 24]. Polyurethane as a polar polymer is similar to nitrile rubber, with outstanding strength, wear resistance, and aging resistance [25-27]. Previous research has found the NBR and PU blending with excellent damping property. Ekvipoo et al. reported a thermoplastic natural rubber from blending of thermoplastic polyurethane (TPU) and maleated natural rubber (MNR) and found that the MNR/TPU blend manifested the higher tensile modulus, mechanical, and elastic properties due to the chemical interactions between MNR and TPU . Zhang et al. prepared new composites of NBR and PU-sericite hybrid material. The hybrid material with excellent compatibility to NBR/PU blend provided better mechanical property . Moreover, the addition of fillers and its interaction with matrix can further improve the damping property, mechanical property, aging resistance, processability, and so on.
It is also a common method for adding inorganic filler to improve damping property due to the reinforcing effect [30, 31]. Friction between internal components can improve the damping property at the movement of rubber molecules. Nano-inorganic fillers are widely used in damping materials due to their own surface and interface effects . Graphene, a two-dimensional honeycomb lattice nanomaterial has unique physicochemical properties, especially mechanical strength, thermal conductivity, and electron mobility. A great deal of research has been done on the addition of graphene to the polymer, ensuring that graphene as a reinforced phase in polymer matrix could improve the mechanical and damping properties [33, 34]. However, the tendency of graphene to agglomeration limits its application in this field. In recent years, self-assembly GA represents macroscopic three-dimensional (3D) materials with monolithic structure. The compressible and porous properties allow GA to be applied in sensing, energy storage, catalyze, water purification, and polymers [35-37]. However, the information on GA as filler in rubber is rare, and the detailed research on the effect of GA on damping properties of matrix is still missing. Hence, it is of primary interest in exploring the dynamic mechanical properties of GA in polymer.
In this article, GA was facilely synthesized by one-step hydrothermal reduction using graphene oxide (GO) as precursor and ammonia as reductant at 90[degrees]C for 12 h. Then GA as filler was added into the XNBR/PU blending system via melting/mechanical complex method to explore the influence on damping properties. The microscopic topography of GA and the XNBR/PU/GA composites was analyzed by scanning electron microscopy (SEM). Xray diffraction (XRD), Raman spectra, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to characterize the composition and structure. The damping property of the composites (including storage modulus, loss modulus and loss factor) was evaluated by the dynamic thermomechanical analysis (DMA).
Natural graphite powders (purity of 99.9%) were produced by Beijing De Ke Dao Jin Co., Ltd. Concentrated sulfuric acid ([H.sub.2]S[O.sub.4], 98%), potassium permanganate (KMn[O.sub.4]), sodium nitrate (NaN[O.sub.3]), hydrogen peroxide ([H.sub.2][O.sub.2], 30%), ammonia solution (N[H.sub.3]*[H.sub.2]O, 30%), stearic acid, and zinc oxide (ZnO) were purchased from Chengdu Kelong Chemical Co. Ltd. Sulfur and vulcanizing accelerator DM were purchased by Aladdin Co. Ltd. All solvents are reagent grade. XNBR (Krynac, Polysar, 27 wt% acrylonitrile) and polyurethane (PU, Elastolan C 90 A, Bayer) were used in this work. All chemicals were used without further purification.
Preparation of GO. GO dispersion was prepared from natural graphite powder by the modified Hummers' method. Briefly, 4 g of graphite powder and 2 g of sodium nitrate were added in the 500 mL three-necked flask. Then, 92 mL of concentrated sulfuric acid was added under stirring at room temperature. Under vigorous agitation, 12 g of potassium permanganate was added slowly, while adjusting the temperature to 0[degrees]C. After mixing processing for 3 h in an ice bath, the reaction was kept at 35[degrees]C for extra 1 h. About 184 mL of deionized water was added dropwise, maintaining the temperature of the mixture lower than 100[degrees]C for 3 h. Finally, a certain amount of deionized water and hydrogen peroxide (30%) were added to stop reaction. The as-prepared mixture was centrifugated and washed with 10% hydrochloric acid aqueous solution and deionized water several times to remove the impurity, forming GO dispersion.
Synthesis of Graphene Aerogel. As illustrated in Fig. 1, a certain amount of the GO aqueous and ammonia solution (25%) were mixed in a cylindrical vial. The mixture was ultrasonicated for 20 min to obtain a homogeneous solution. Then, the dark brown solution was sealed at 90[degrees]C for 12 h and subsequently graphene hydrogels were synthesized, which was naturally cooled down to room temperature. The graphene hydrogels were washed using water for several times and then graphene aerogels (GAs) were prepared by freeze-drying of graphene hydrogels for 48 h.
Synthesis of Graphene Aerogel Hybrid XNBR/PU Blends. As shown in Fig. 2, the XNBR/PU/GA composites were prepared by melting/mechanical complex method, which allows GAs with different addition amount to uniformly mix in the nitrile rubber matrix (as shown in Table 1). First, XNBR of 50 wt% was cut into fraction and dissolved in a beaker with a certain amount of acetone at room temperature, while PU and required amount of GA were dissolved in DMF using ultrasonication, respectively. After XNBR/acetone and PU/DMF were mixed uniformly by magnetically stirring for 12 h, GA/DMF suspension was gradually poured into XNBR/PU solution under stirring for 24 h. Finally, after the mixtures were dried in the oven at 60[degrees]C until the solvent was evaporated, the master batch was obtained.
After finishing the above steps, a two-roll mill (Kesheng Co. Ltd. Guangzhou, China) was adopted to prepare the XNBR/ PU/GA composites. Initially, the remaining components of XNBR and the as-prepared master batch are mixed for about 20 min at ambient temperature. After that, the calculated amounts of stearic acid, zinc oxide, DM, and sulfur were orderly added to the mixture. Before being vulcanized in a standard mold, the mixture was placed at indoor temperature for 24 h. The polyurethane, zinc oxide, stearic acid, 2,2'-dibenzothiazole disulfide, and sulfur were added with constant amount relative to XNBR (as shown in Table 1).
Characterization. The morphology of grapheme aerogels and XNBR/PU/GA composites was observed using SEM (JSM-6610, Japan). To acquire composition information of G A and XNBR/ PU/GA composites, XRD (PANalytical, the Netherlands) was performed using a Cu K[alpha] radiation from 5[degrees] to 70[degrees] at a speed of 0.02[degrees]/s. Raman spectra were obtained via Horiba LabRam HR800 Raman microscopy with 532 nm laser excitation and FTIR spectra were recorded on a Nicolet Fourier-transform infrared spectrophotometer (Nicolet 6700, America). XPS (ESCALAB, America) was applied to analyze the chemical states of GA. The damping properties were evaluated by DMA analyzer (Q800, America) in stretch mode over a temperature range from -50 to 40[degrees]C at a heating rate of 3[degrees]C*[min.sup.-1].
RESULTS AND DISCUSSION
Characterizations of GA
The XRD patterns of graphite, GO, and GA are shown in Fig. 3a. A sharp peak with interlayer distance of 0.82 nm at 2[theta] = 10.8[degrees] is the typical peak of the layered GO, indicating that the graphite had been successfully exfoliated during the hydrothermal reduction process . After the hydrothermal reduction process, this sharp peak completely disappeared and transformed into a weak and broad diffraction peak with the interlayer spacing of 0.36 nm at 2[theta] = 24.6[degrees] due to the restacking of graphene nanosheets. The XRD peak of GA indicates the poor ordering of graphene sheets along their stacking direction and demonstrates that the network of the GA is composed of few-layer stacked graphene sheets. Compared with natural graphite with the interlayer distance of 0.34 nm at 2[theta] = 26.5[degrees], it is illustrated that most oxygen-containing functional groups were introduced into the interlayer space of graphite to form GO sheets. Then they were removed during hydrothermal reduction process.
Raman spectra of graphite, GO and GA are shown in Fig. 3b, which displays two distinct peaks at 1,345 and 1,592 [cm.sup.-1] for GO and GA, while those at 1,345 and 1,576 [cm.sup.-1] for graphite, corresponding to D and G bands, respectively. The shift of G band from 1,576 [cm.sup.-1] of graphite to 1,592 [cm.sup.-1] of GO and GA is attributed to the charge transfer from graphite to GO. The G band manifests a shift to higher frequencies (the so-called blue shift) and broadens clearly due to graphite amorphization. The intensity ratio of D and G bands can be used to quantitatively estimate the degree of defects. Because the D band is regarded as the structural defects in graphene-based carbon materials, while the G band is related to the vibration of [sp.sup.2] carbon . [I.sub.D]/[I.sub.G] ratio of GO is 0.93 and that of GA increase to 1.06, indicating that the degree of disorder is improved from GO to GA. So the analysis of Raman spectra is matched well with that of XRD.
The changes in surface chemistry and atomic ratio of the samples were monitored by XPS, and the atomic ratio of GO and GA was summarized in Table 2. XPS survey scans in Fig. 4a display only C 1s peak of graphite at 283.9 eV, C 1s and O 1s peaks of GO at 286.7 and 532.3 eV in turn, and N 1s peak at 402.5 eV from GA, which is derived from ammonia solution and could be combined with graphene sheets during hydrothermal reduction. The elemental ratio of GO and GA was calculated using the relevant XPS peak area and their respective atomic sensitivity factors. GO contains a large percentage of oxygen (30.97%) and its C/O atomic ratio was ~2.23. GA contains the O, C, and N content of 15.70, 78.59, and 5.71%, respectively, corresponding to the C/O atomic ratio of ~5.01. It is turned out that GO was successfully reduced by ammonia solution. Meanwhile, some amine groups were grafted into graphene sheets. Owing to the ammonia solution and oxygenated functional groups, the reduced GO sheets can set up a 3D network structure during hydrothermal reduction. C 1s and N 1s spectra of GO and GA are shown in Fig. 4b and c. Four peaks of C 1s on GO are assigned to the C--C at 284.8 eV, C--O at 286.2 eV, C--O--C at 286.8 eV, and C=0 at 288.1 eV, respectively . For C Is spectrum of GA, the peak intensity of oxygen-contained groups evidently decreases while that of C--C and C--N bonds increases significantly, illustrating the reduction of GO during hydrothermal reduction . The N Is spectrum of GA can be fitted into three peaks, pyrrolic-N at 397.6 eV, pyridinic-N at 398.7 eV, and graphitic-N at 400 eV, respectively, indicating the introduction of N atoms into graphene structure . The [sp.sup.2]-hybridized bonding of pyridinic-N, graphitic-N, and pyrrolic-N suggests that N atoms have replaced some C atoms in the graphene hexagonal ring, thereby playing a dominant role in electrochemical performance of carbon materials.
The microscopic structure of as-prepared GAs is analyzed by SEM and is shown in Fig. 5. GA shows the 3D interconnected macroporous structure, in which the bended and folded graphene sheets can be clearly seen. The pore size is in the range of 10 to several hundred micrometers and the pore walls are composed of curved graphene sheets randomly stacked together. GA is prepared from 2 mg*[mL.sup.-1]. GO aqueous dispersion to 4 mg*[mL.sup.-1]. Their SEM images in Fig. 5a-f give an obvious phenomenon that lower concentration of GO aqueous dispersion can form larger pore and looser crosslinking network. With the increase in the concentration of GO aqueous dispersion, the network structure with smaller pore is more closely and disorderly. From their high magnification images in Fig. 5b, d, and f, the external surface of GA had a relatively smooth skin. The formation of physical crosslinking framework for GA can be explained as the partial overlapping and crosslinking of flexible graphene sheets .
Microstructures of XNBR/PU/GA Composites
Figure 6 shows SEM images of XNBR/PU/GA composites with GA of 0, 1, and 3 wt%. All of these cross profile are obtained from the composites dealt with liquid nitrogen. The wrinkle degree of cross profile is continuously augmented with the increase in the addition amount of GA. XNBR/PU gives a relatively smooth surface and PU is apparently implanted to the XNBR matrix, manifesting two-phase interfaces between PU and XNBR. More wrinkles and partial aggregation are observed on the fracture surface with the presence of GA, illustrating that GA has an impact on the strength and toughness of composites at low temperature. The small amount of GA can be well dispersed in the matrix, increasing the friction between the molecular chain segments and improving the damping property. Therefore, the improvement of damping property mainly depends on the reinforcing effect of GA on the mechanical properties and interface interaction.
Figure 7 gives the FTIR spectra of the hybrid materials. FTIR spectrum of neat XNBR/PU shows several distinct absorption bands. The bands at 862 and 2,912 [cm.sup.-1] are correlated with C--H groups with out of plane deformation and stretching vibrations, while the bands at 1416, 1,757, and 2,240 [cm.sup.-1] are assigned to symmetric contraction of COO- groups and stretching vibration of C=0 groups as well as C=N groups, respectively. The weak and brief absorption at 3774 [cm.sup.-1] is attributed to the symmetrical stretching vibration of hydroxyl groups. XNBR/PU/GA composites exhibit the similar spectra to neat XNBR/PU, indicating that the introduction of GA into XNBR/PU via mechanical mixing did not cause the chemical changes.
Mechanical and Damping Properties of XNBR/PU/GA Composites
GA as inorganic filler can play a role in damping performance due to its mechanical property and interface interaction between the fillers and matrix. And GA in the XNBR/PU can hinder the movement of macromolecular chain. These factors are beneficial to increase internal friction for improving the damping performance of the composites. In order to explore the effect of GA on the damping properties of XNBR/PU blends, GA with different addition amount was introduced into the blend. Figure 8 shows the storage modulus (E') and loss modulus (E") of the XNBR/PU/ GA composites. With the increase in content of GA, the peak values of E' first increase from 1,731 to 2,689 MPa at the amount of 0-1 wt% and finally reach to maximum of 3,219 MPa with GA of 3 wt%. E' of XNBR/PU/GA is higher than that of XNBR/PU, because GA with low concentration can well disperse in the matrix and enhance mechanical performance. The addition of GA with high content could not only improve the dispersion of graphene in XNBR/PU but also significantly enhance the stiffness and interfacial interaction of composites (3 wt. % GA, especially), which is beneficial for dissipation of energy. Moreover, E" of the composites gives the same trend, which continuously increase with the increase of GA, as shown in Fig. 8b and d. The E" of the XNBR/PU was 169 MPa. Surprisingly, E" of the composites with 1 and 3 wt% GA was 250 and 316 MPa, which was increase by 47.9% and 87.0%, respectively. It is indicates that GA can boost the tenacity of the polymer chain of XNBR at temperature of -29[degrees]C to -26[degrees]C and prompt the friction between the polymer chains.
Figure 9 gives the temperature effect on the tan[delta] of XNBR/ PU/GA composites with different GA content, which shows two peaks at around -25 and 0.6[degrees]C. At low temperature below 0[degrees]C, the value of tan[delta] increased with the increase in the addition amount of GA. The value of tan[delta] is significantly improved from minimum 0.42 (without GA) to maximum 0.70 (with 3 wt% GA) at 0.6[degrees]C, ensuring that the addition of GA can further enhance the damping properties. Besides, the maximum value of tan[delta] represents at [T.sub.g]. In addition, for temperature of above the 0[degrees]C, the [T.sub.g] range of XNBR/PU/GA composites increase with the addition of 3 wt% GA. For the XNBR/PU with 3 wt% of GA, tan[delta], E', and E" were always occupies the highest value, indicating the composite containing 3 wt. % GA exhibited better comprehensive damping performance. Therefore, GA greatly improves the damping performance and damping temperature range due to the increase in stiffness and interfacial interaction as well as [T.sub.g].
Thermal Aging Test of XNBR/PU/GA Composites
Thermal aging of the XNBR/PU/GA composites is investigated under 120[degrees]C for 7 days in oven. Figure 10 gives the storage modulus and tan[delta] of the composites after thermal aging. The E' of thermal aging-composites greatly improves, which increases to 2,728, 2,856m and 3,221 MPa for the addition amount of GA with 0, 1, and 3 wt%, respectively, possibly because the effects of post-vulcanization in heating process boosted the crosslinking density of vulcanized rubber. And the crosslinking structure of molecular chain is more complete. The tan[delta] curve of XNBR/PU/GA composites after thermal aging in Fig. 10c is similar to those of the untreated materials with a shoulder peak at low temperature and a sharp peak at higher temperature than 0[degrees]C. For the sharp peak part, the tan[delta] of XNBR/PU is only 0.39. However, that of XNBR/PU with 1 and 3 wt% arrive at 0.43 and 0.51, respectively, indicating that XNBR/PU/GA shows an excellent thermal aging resistance. Their tan[delta] values are lower than the composites before thermal aging. The crosslinking degree of composites was improved during the heat preservation process at 120[degrees]C, thereby the increase in the crosslinking density. So the crosslinking effect allows the short range molecular to have difficulty in being broken. And temperature of tano peaks moves forward high temperature.
03D GA was facilely prepared via one-step self-assembly hydrothermal reduction using GO as precursor and ammonia as reductant, which presents a spatial porous structure and network skeleton. G A as filler added into XNBR/PU blends can improve the damping performance depending on such network structure and mechanical properties of GA, which enhances the mechanical properties and interface interaction of composites. Hence, 3D GA from a structure-controllable preparation method with graphene-like performance characteristics takes another step forward in real-world applications for graphene-like materials.
3D structure GA was successfully prepared via one-step self-assembly hydrothermal reduction. As-prepared GA as filler was added into XNBR/PU blends to verify the damping performance and the prospect for real application of such structural materials in polymer. The following conclusions can be drawn:
(a) The macroporous network structure of GA can be effectively controlled via regulating the content of precursor and reductant, reaction condition.
(b) The mechanical performance of XNBR/PU is significantly improved by the addition of GA. The storage modulus and loss modulus increase up to 3,219 MPa and 316 MPa, and tan[delta] displays two peaks (values are 0.22 and 0.61, respectively) at around -25[degrees]C and 0.6[degrees]C. GA enhances the thermal aging resistance and comprehensive performance of the composites, especially the addition amount of 3 wt% GA.
(c) The improvement of damping performance for the composites with addition of GA mainly depends on the nature and reinforcing effect of filler on the mechanical properties and interface interaction.
The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 51705435), Key Project of Sichuan Department of Science and Technology (No. 2018JZ0048) and Key Projects of Guangxi Province Science and Technology plan (NO. AA18242002).
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Han Yan, (1) Hao Li, (1) Wen Li, (1) Xiaoqiang Fan, (1) Lin Zhang, (1) Minhao Zhu (1,2)
(1) Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China
(2) Tribology Research Institute, School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China
Correspondence to: X.Q. Fan; e-mail: firstname.lastname@example.org Contract grant sponsor: Guangxi Province Science and Technology; contract grant number: AA18242002. contract grant sponsor: Department of Science and Technology; contract grant number: 2018JZ0048. contract grant sponsor: National Natural Science Foundation of China; contract grant number: 51705435.
Published online in Wiley Online Library (wileyonlinelibrary.com).
Caption: FIG. 1. Schematic illustration of the synthesis process of GA. [Color figure can be viewed at wileyonlinelibrary.com!
Caption: FIG. 2. Flowchart for the fabrication process of XNBR/PU/GA composites. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 3. (a) XRD patterns and (b) Raman spectra of graphite, GO and GA. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 4. (a) Full-range XPS spectra of graphite, GO and GA, (b) high-resolution C 1s spectra of GO and GA, (c) high-resolution N 1s spectrum of GA. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 5. Typical scanning electron microscope images of GAs prepared by GO concentration of (a,b) 2 mg*[mL.sup.- 1] (c,d) 3 mg*[mL.sup.-1], (e,f) 4 mg*[mL.sup.-1]. [Color figure can be viewed at wileyonlinelibrary.com!
Caption: FIG. 6. Morphology of XNBR/PU/GA composites without GA (a, b), with the addition amount of GA 1 wt% (c, d), 3 wt% (e,f). [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 7. FTIR spectra of XNBR/PU/GA composites with the addition amount of GA at 0, 1, and 3 wt%. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 8. (a) Storage modulus and (b) loss modulus (inset image are the high-definition picture of the peak for the loss modulus) curve of XNBR/PU/GA composites, (c) the peak value of storage modulus and (d) that of loss modulus for XNBR/PU/GA composites. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 9. (a) The loss factor (tan[delta]) curve (inset image are the high-definition picture of the peak of the tan[delta] at different temperature) and (b) the peak value of the tan[delta] of XNBR/PU/GA composites. [Color figure can be viewed at wileyonlinelibrary.com]
Caption: FIG. 10. Storage modulus and tan[delta] of XNBR/PU/GA composites after thermal aging. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Formulation of XNBR/PU/GA composites (wt%). Ingredients XNBR PU ZnO Stearic Sulfur DM GA acid Amounts 100 20 5 1 1.5 0.5 0, 1, 3 TABLE 2. The atomic ratio of the GO and GA measured by XPS analysis. Element content (at%) Ratio of elements Samples C 0 N C/O GO 69.03 30.97 0 2.23 GA 78.59 15.70 5.71 5.01