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clay nanocomposites by compounding with clay gel
Composites Part B 167 (2019) 356–361
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
A novel method to prepare acrylonitrile-butadiene rubber/clay nanocomposites by compounding with clay gel
T
Shaojian Hea, Tengfei Hea, Jiaqi Wanga, Xiaohui Wub, Yang Xuec,∗, Liqun Zhangb, Jun Lina,∗∗ a
Beijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, Beijing, 102206, China State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, China c State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Acrylonitrile-butadiene rubber Clay gel Gel compounding Nanocomposite
Despite that there have been many reports on the achievement of high-performance rubber/clay nanocomposites, the preparation strategies of these nanocomposites are still quite limited. In this study, a novel method named gel compounding method was utilized to prepare acrylonitrile-butadiene rubber (NBR)/clay nanocomposites via compounding solid rubber with clay gels. As illustrated by TEM and XRD, clay was uniformly dispersed in the NBR matrix as either exfoliated platelets or very thin layered silicate stacks, indicating the separated structure was formed in the NBR/clay nanocomposite prepared by this gel compounding method. Mechanical properties were significantly improved for the NBR/clay nanocomposites, which reached the tensile strength of 12.1 MPa with 10 phr clay, more than 250% higher than that of pure NBR.
1. Introduction Clay consisting of ∼1 nm thick silicate layers with the lateral dimensions of 200–400 nm is an abundant natural resource with very low cost [1–3]. Moreover, clay does not present the environmental pollution and health risks to human. During the past years, clay has been applied to prepare rubber/clay nanocomposites which often exhibited high performance [4–7]. Therefore, considerable attention has been attracted to the development of preparation techniques and a better understanding of structure-property-processing relationship for rubber/ clay nanocomposites. To date, there are mainly three strategies employed to prepare rubber/clay nanocomposites: melt processing [8–11], solution blending [12–14] and latex compounding [15–17]. For either melting processing or solution blending methods, the clays need to be modified through the ion-exchange reaction with organic components [18,19], usually the quaternary ammonium salts. Such modification could ensure that the silicate layers can be well dispersed in the solvent for the solution blending process, or the interlayer spacing of the silicate layers can be increased to the extent that the rubber macromolecules could easily intercalate into the clay galleries during melt processing. Unfortunately, the commercial organically modified clays are usually quite expensive. Moreover, the extensive use of the solvent in solution blending may pose serious risks to both human health and the ∗
environment. In comparison, the latex compounding technique is very promising for industrialization because of its environmental friendliness and superior performance/cost ratio [17]. During this process, the pristine clay is first dispersed in water to form the clay aqueous suspension without organic modification, and then mixed with the rubber latex followed by being co-coagulated using an appropriate flocculating agent to prepare rubber/clay nanocomposites. However, the latex compounding technique requires the use of the rubbers only available in the latex form [15], which limits the wider application of this technique. Therefore, it is worth developing a general compounding method that uses the solid rubbers and the natural clay minerals without organic modification to reduce the cost of producing rubber/ clay nanocomposites. In this work, we proposed a novel method named gel compounding method to prepare acrylonitrile-butadiene rubber (NBR)/clay nanocomposites. NBR was compounded with the clay gel (containing up to 20 wt% clay) using the two-roll mill to prepare the NBR/clay “gel compound”, which was then dried under vacuum and added with the curing agents to finish vulcanization at high temperature to obtain the final product, NBR/clay nanocomposites. The mechanical properties of these nanocomposites were highly improved, and the microstructure was investigated to better understand the structure-performance relationship.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Xue), [email protected] (J. Lin).
∗∗
https://doi.org/10.1016/j.compositesb.2019.03.013 Received 19 November 2018; Received in revised form 1 February 2019; Accepted 4 March 2019 Available online 07 March 2019 1359-8368/ © 2019 Published by Elsevier Ltd.
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2. Experimental 2.1. Materials Clay (sodium-montmorillonite) with cation exchange capacity of 93 meq./100 g was supplied by Liufangzi Clay Factory, Jilin, China. NBR 1052 (35.0 wt% acrylonitrile) was supplied by Nantex Industry Co. Ltd., China. Zinc oxide, stearic acid, dibenzothiazole disulfide, N-isopropylN′-phenyl-p-phenylenediamine and sulfur were purchased from the Chemical Reagent Shop of Beijing. All materials were used as received. 2.2. Preparation Clay was swelled in deionized water at a concentration of 20 wt%, and clay gel was obtained after vigorous stirring for 12 h. Then a given amount of clay gel was mixed with NBR in a two-roll mill to obtain rubber/clay “gel compound”. To remove the water as much as possible, the gel compound was rolled into a thin sheet with the thickness less than 500 μm, which was subsequently placed at 80 °C under vacuum for 48 h to obtain NBR/clay nanocompound. After the curing agents were added in the two-roll mill, NBR/clay nanocompound was vulcanized in a standard mold using a hydraulic hot press (XLB-D 350, Huzhou Dongfang Machinery, Co., Ltd., China) at 160 °C under 15 MPa to obtain NBR/clay nanocomposite. For comparison, the NBR/clay microcomposites with various clay contents were also prepared by directly incorporating pristine dry clay into NBR following the conventional rubber processing procedure [20,21]. The recipe (parts by weight) for NBR/clay nanocomposites or micro-composites was as follows: NBR, 100.0; zinc oxide, 5.0; stearic acid, 1.0; dibenzothiazole disulfide, 1.0; N-isopropyl-N′-phenyl-p-phenylene diamine, 1.0; sulfur, 1.5; clay, variable.
Fig. 1. XRD curves of clay/water mixture with various clay contents.
molecules between the silicate layers [15]. We term such water-intercalated clay as the “swollen clay” (Fig. 2b). When the clay content is lower than 50 wt%, with the further addition of water, no diffraction peaks can be seen in the XRD curves (Fig. 1, curve d-g), demonstrating that all the silicate layers are completely dispersed in water without aggregation. However, there clearly exists the viscosity difference among these clay/water mixtures as shown in Fig. 2c and d, the “clay gel” (clay content ≥ 10 wt%) and the “clay aqueous suspension” (clay content < 10 wt%). Compared to the clay aqueous suspension, the clay gel exhibits much higher viscosity mainly due to the formation of loosely percolated networks by the edge-to-face or edge-to-edge connection between the silicate layers [22–24]. For the clay without organic modification, it is very difficult for hydrophobic rubber macromolecules to intercalate into the galleries of silicate layers because of the hydrophilic nature of the clay and the strong ionic interaction between the neighboring silicate layers [1], which will easily cause the clay aggregation during the preparation of rubber/clay composites. A similar result will occur to the composites involved with the swollen clay, even though the silicate layers are intercalated with the water molecules. This is because the swollen clay still exits as the relatively large stacked silicate layers and rubber macromolecules cannot intercalate into the clay galleries. While for the clay gel or clay aqueous suspension, the complete exfoliation of silicate layers significantly increases the possibility of rubber chains to surround only one single layer or just a few layers of clays, thus reducing the chance for the appearance of thick silicate stacks during the water evaporation step for the preparation of rubber/clay nanocompounds. However, compared to the clay gel, the clay aqueous suspension contains a much larger amount of water, which makes it much more difficult to well mix the clay aqueous suspension with solid rubber. In addition, the water evaporation process would be time-consuming for the compound using the clay aqueous suspension. Therefore, after careful consideration and several attempts, in this work, we chose the clay gel with clay content of 20 wt% to prepare the rubber/clay nanocomposites by the gel compounding method.
2.3. Measurements X-ray diffraction (XRD) analysis was carried out using a diffractometer (D/Max-III C, Rigaku, Japan) with CuKα radiation operating at 40 kV and 200 mA, with a scan rate of 1.00°·min−1. Ultrathin sections of the nanocomposite samples for transmission electron microscopy (TEM) experiments were cut using a liquid nitrogen cooled microtome at about −100 °C, collected on a copper grid, and then observed on an H-800 TEM (Hitachi Co., Japan) with an acceleration voltage of 200 kV. According to ISO 37-2011, the mechanical behavior was tested using a GT-TC2000 electrical tensile tester (Gotech Testing Machines Inc.) at a speed of 500 mm min−1, and five specimens were tested to give the average. Shore A hardness was measured using LX-A rubber hardness apparatus (Liuling Instrument, Shanghai, China) according to ISO 7619-1-2004. Strain sweep of the cured nanocomposites was conducted using the RPA 2000 Rubber Process Analyzer (RPA) of Alpha Technologies Co. at 60 °C and 1 Hz. 3. Results and discussion 3.1. Rationale for the preparation of NBR/clay nanocomposites using the gel compounding method When the clay is dispersed in water, the dispersion characteristics of the clay is mainly determined by the clay/water ratio. The XRD curves of clay/water mixture with various clay contents are shown in Fig. 1, and the sketches illustrating the clay dispersion status are presented in Fig. 2. As can be seen from Fig. 1 (curve a), the pristine clay without water exhibits a diffraction peak at ∼7.2°, corresponding to a layer spacing (d001) of 1.24 nm, which is due to the aggregation of silicate layers resulting from the face-to-face layer stacks (Fig. 2a). After no more than 50 wt% water is added to the clay (Fig. 1, curve b and c), the diffraction peaks shift to the lower angle indicating the increase of layer spacing, which should be attributed to the intercalation of water
3.2. Structure of NBR/clay nanocomposite TEM is the most direct means to observe the dispersion status of silicate layers in the rubber matrix. Fig. 3 shows the TEM images of the NBR/clay nanocomposite and micro-composite with 10 phr clay, and the dark lines represent the cross-sections of the silicate layers. For the NBR/clay nanocomposite, numerous silicate layers are uniformly 357
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Fig. 2. Sketch graphs of (a) pristine clay, (b) swollen clay, (c) clay gel and (d) clay aqueous suspension.
Fig. 4. XRD curves of NBR/clay (a) gel compound, (b) nanocompound, (c) nanocomposite and (d) micro-composite with 10 phr clay.
diffraction peak in the XRD curve of NBR/clay nanocompound (Fig. 4, curve b). After the addition of the curing agents, the NBR/clay nanocompound is vulcanized in the two-roll mill to achieve the NBR/clay nanocomposite, and a diffraction peak at 5.9° appears in the XRD curve of this nanocomposite (Fig. 4, curve c), corresponding to a layer spacing of 1.34 nm. This demonstrates that a certain amount of silicate layers aggregate in the nanocomposite. However, such aggregation of silicate layers is not as severe as that in the pristine clay because the basal spacing of the pristine clay is only 1.24 nm, as shown in Fig. 1. Such phenomenon suggests the clay intercalation of certain molecules or ions must have taken place during the preparation of the NBR/clay nanocomposite. On the one hand, it has been well documented that the exchange of sodium cations with long-chain quaternary alkylammonium cations results in quite a significant enlargement of the basal spacing for the organic clays, which is usually larger than 1.7 nm [9–11]. Therefore, it is impossible that the rubber macromolecules would intercalate into the clay galleries in these NBR/clay nanocomposites because this should cause even larger basal spacing. On the other hand, slightly larger basal spacing (∼1.37 nm) than that of pristine clay is observed for the NBR/clay nanocomposites prepared by the latex compounding method, and such increase is attributed to the intercalation of ions from flocculating agents [15,26,27]. However, no flocculating agents are used in our work here. Therefore, the increase of the basal spacing (from 1.24 nm to 1.34 nm) may result from the incomplete removal of water molecules that have been strongly bound to the silicate layers during the preparation of nanocomposites via gel compounding method. Since these water molecules do not stay in the rubber matrix as plasticizers or defects, they may impose little effect on the mechanical performance of the rubber/clay nanocomposites as will be shown in the following section. As for the NBR/clay micro-
Fig. 3. TEM images of NBR/clay (a) (c) (e) nanocomposite and (b) (d) (f) microcomposite with 10 phr clay.
dispersed almost everywhere as can be seen from Fig. 3a and c. Fig. 3e shows that these dispersed silicate layers are either exfoliated or tightly stacked (with a thickness of 10–20 nm) in the NBR matrix. Such morphology is quite similar to that of the rubber/clay nanocomposites prepared by the latex compounding method, which can be defined as the separated nanocomposite with both exfoliated and tightly stacked silicate layers dispersed in the matrix [15,25]. As for the NBR/clay micro-composite, all silicate layers are dispersed in the NBR matrix as the form of stacked silicate layers with a thickness of more than 200 nm. The TEM observation confirms that the silicate layers are better dispersed in the NBR matrix by gel compounding method than by conventional blending method. Moreover, the difference between the NBR/clay nanocomposite and micro-composite suggests that the water molecules play an important role in the clay dispersion for the rubber/ clay nanocomposites prepared by gel compounding method, which is quite effective and can be applied to the solid rubber. The structures of NBR/clay composites are further characterized by XRD. The XRD curves of NBR/clay gel compound, nanocompound, nanocomposite and micro-composite with 10 phr clay are shown in Fig. 4. When the clay gel is compounded with NBR, no diffraction peak is found in the XRD curve of NBR/clay gel compound (Fig. 4, curve a), indicating that the silicate layers are completely exfoliated in the gel and gel compound. After the treatment of the NBR/clay gel compound with heat and vacuum to remove the water to obtain the nanocompound, the silicate layers still remain exfoliated in the matrix without any clay aggregations, as evidenced by the absence of 358
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Fig. 5. Proposed mechanism for the preparation process of rubber/clay nanocomposites by the gel compounding method.
Fig. 6. Stress-strain curves of NBR/clay (a) nanocomposites and (b) micro-composites with various clay loadings.
(Fig. 5e). Even though the few silicate layers inside one clay droplet might aggregate to form relatively thin stacks, due to the inhibition of rubber chains, it is highly impossible that several clay droplets could collapse and connect with each other to develop the clay stacks with the thickness up to the micro-scale. Therefore, employing the clays in the exfoliated form as the starting materials could ensure the achievement of the rubber/clay nanocompounds. Such compounding method is quite different from the melt processing method using the organic clay, in which the rubber chains intercalate into the galleries of stacked silicate layers. After the curing agent is added, the rubber/clay nanocompounds are vulcanized at high temperature under pressure to form rubber/clay nanocomposites (Fig. 5f). During this process, the high vulcanization temperature facilitates the movement of rubber chains which might slip out of the interspace between the silicate layers, resulting in a certain degree of clay stacking, similar to the previous studies [28,29].
composite, no intercalation of silicate layers occurs because the diffraction peak in the XRD remained at 7.2° (Fig. 4, curve d), the same as that of the pristine clay.
3.3. Proposed mechanism for the gel compounding method Based on the TEM and XRD results, we propose the mechanism for the preparation process of rubber/clay nanocomposites by the gel compounding method (Fig. 5) as follows: The clays (Fig. 5a) are dispersed in water to form the clay gel (Fig. 5b), which is then compounded with solid rubber by mechanical blending in a two-roll mill (Fig. 5c). During this step, the clay gel would form discrete droplets that contained one single layer or a few exfoliated layers “encapsulated” by water molecules. These droplets, separated by the rubber chains, would become smaller under vigorous shearing and well dispersed in the rubber matrix to form the rubber/clay gel compound (Fig. 5d). When being dried under vacuum, the water molecules in these clay droplets begin to evaporate and the rubber chains surrounding the droplets could act as barriers to inhibit the re-stacking of the silicate layers
3.4. Mechanical properties of NBR/clay nanocomposite The stress-strain curves of NBR/clay nanocomposites and micro359
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Table 1 Mechanical properties of NBR/clay nanocomposites and micro-composites with various clay loadings. Samples
NBR
NBR/clay nanocomposites
NBR/clay micro-composites
Clay loading (phr)
0
2
5
7
10
2
5
7
10
Shore A hardness Tensile modulus (MPa) Stress at 100% strain (MPa) Stress at 300% strain (MPa) Tensile strength (MPa) Elongation at break (%)
42 3.71 1.03 1.44 3.40 653
45 4.49 1.16 2.03 5.90 726
49 5.15 1.51 3.17 9.0 733
52 6.36 1.72 4.00 11.2 730
55 7.34 2.02 5.08 12.1 663
43 3.66 0.98 1.34 2.71 606
43 3.79 1.02 1.42 3.17 648
44 3.83 1.01 1.41 3.00 627
43 3.78 0.97 1.37 2.81 636
composites with various clay loadings are illustrated in Fig. 6a and b, respectively, and the mechanical properties are presented in Table 1. Compared to that of pure NBR (653%), the elongation at break of the NBR/clay nanocomposites (Fig. 6a) increases at first to the maximum of 733% with the clay loading of 5 phr, and then decreases to 663% when the clay loading is further increased to 10 phr. In comparison, the tensile strength of the nanocomposites continuously increases with the increasing clay loading, and reaches the maximum of 12.1 MPa with 10 phr clay, more than 250% higher than that of pure NBR (3.40 MPa). Moreover, the Shore A hardness, tensile modulus and stress at 100% or 300% strain of the nanocomposites also increase with the increasing clay loading as shown in Table 1. Related to the above-discussed findings on the structure and morphology of these NBR/clay nanocomposites, these mechanical property results indicate that the individual or tightly stacked silicate layers uniformly dispersed in the NBR matrix can help withstand the tensile stress of the composites, leading to an effective strengthening of rubber. The improvement of tensile strength for our nanocomposites is comparable with that for the NBR/clay nanocomposites prepared by the latex compounding method which achieved 7.45 MPa at 10 phr clay, ∼300% higher than their pure NBR (1.67 MPa) [30]. As for the NBR/clay micro-composites (Fig. 6b and Table 1), the tensile strength, stress at 100% or 300% strain and elongation at break are all reduced as compared to pure NBR. This can also be correlated with the structure/morphology features demonstrated in the XRD and TEM characterization, where the clay is poorly dispersed in the matrix in the form of large stacked silicate layers and may act as defects in the micro-composites, leading to the decrease in the mechanical performance. To better understand the reinforcement mechanism in the composites, the Mooney-Rivlin equation is applied to describe the interfacial interaction between the rubber and fillers as follows: σ σ* = = 2C1 + 2C2 λ–1 [31], where σ* is the reduced stress, σ is λ−λ–2 the applied stress, λ is the extension ratio, and C1, C2 are the MooneyRivlin constants related to the rubber-filler network and the flexibility of the network chains, respectively. Based on the data from the stressstrain curves in Fig. 6a and b, both σ* and λ−1 for the NBR/clay nanocomposite and micro-composite with 10 phr clay are calculated and plotted against each other as shown in Fig. 7. The σ* of the NBR/clay micro-composite changed with λ−1 almost linearly, while that of NBR/ clay nanocomposite exhibits an abrupt upturn at λ−1 of 0.45. The appearance of such abrupt upturn is generally attributed to the restricted extensibility of rubber chains bridging neighboring filler particles during stretching [32]. For the NBR/clay micro-composite, silicate layers are dispersed in the form of micro-sized aggregates so that there are only very few filler particles bridging to the rubber chains. Thus, no abrupt upturn is observed and the reinforcement of the silicate layers to the micro-composite is very weak. As for the NBR/clay nanocomposite with 10 phr clay, XRD and TEM analysis demonstrate the existence of nano-scaled silicate layers with huge specific surface dispersed in the rubber matrix. As a result, the rubber chains are intensively restricted by the filler particles, leading to the abrupt upturns of σ*. Therefore, these results confirm the stronger interfacial interaction between NBR and silicate layers in the nanocomposite prepared by the gel
Fig. 7. σ*-λ−1 curves between NBR/clay nanocomposite and micro-composite with 10 phr clay.
compounding method, which is much more efficient in the improvement of mechanical performances than the conventional compounding method. Furthermore, such strong interfacial interaction between NBR and silicate layers can be also proved by the RPA strain sweep results. The effect of strain on the storage modulus (G′) and tan delta of pure NBR, NBR/clay micro-composite and nanocomposite with 10 phr clay is shown in Fig. 8a and b, respectively. For the filler filled rubber composite, the G′ is affected by the filler-filler network, filler-rubber network and vulcanization crosslinking network [21]. The strength of vulcanization crosslinking network is supposed to be almost the same since the same curing agents are added in our work, and the filler-filler network is usually destroyed at a relatively low strain. Therefore, for the both NBR/clay micro-composite and nanocomposite, the dominant factor in affecting the G′ should be the filler-rubber network. As shown in Fig. 8a, the NBR/clay nanocomposite exhibits the higher G’ than both NBR and NBR/clay micro-composite, indicating the stronger fillerrubber interaction in NBR/clay nanocomposite. Moreover, the strong filler-rubber interaction can also result in the restricted segment motion of rubber macromolecules, leading to the more energy loss during a dynamic strain cycle. As a result, the tan delta of NBR/clay nanocomposite is relatively higher than that of both NBR and NBR/clay micor-composite as shown in Fig. 8b. 4. Conclusions In this work, we have proposed a novel nanocompounding method named gel compounding method, using clay gels and solid rubber to prepare NBR/clay nanocomposites. By employing the clay gels in which clays were exfoliated in water to compound with solid rubber (NBR), the existence of water molecules helped prevent the collapse and aggregation of silicate layers during the compounding process. During the 360
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Fig. 8. (a) G′-strain and (b) tan delta-strain curves of pure NBR, NBR/clay micro-composite and nanocomposite with 10 phr clay.
following drying and vulcanization processes, the rubber chains acted as barriers to inhibit the re-stacking of the silicate layers, resulting in the formation of NBR/clay nanocomposites. TEM analysis verified that the nanocomposites exhibited the features of separated structures with both exfoliated and tightly stacked silicate layers dispersed in the rubber matrix, which was further confirmed by the XRD study. Compared to the conventional compounding method by which NBR/ clay micro-composites were prepared, the gel compounding method produced the NBR/clay nanocomposites with much better mechanical performance. The application of the Mooney-Rivlin equation and the RPA strain sweep results revealed that the gel compounding technique greatly improved the interfacial interaction between NBR and silicate layers in the nanocomposite, leading to the effective strengthening of the rubber. Therefore, the present study provides a promising approach to prepare high performance rubber/clay nanocomposites using solid rubbers.
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A novel method to prepare acrylonitrile-butadiene rubber/clay nanocomposites by compounding with clay gel
T
Shaojian Hea, Tengfei Hea, Jiaqi Wanga, Xiaohui Wub, Yang Xuec,∗, Liqun Zhangb, Jun Lina,∗∗ a
Beijing Key Laboratory of Energy Safety and Clean Utilization, North China Electric Power University, Beijing, 102206, China State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, China c State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Acrylonitrile-butadiene rubber Clay gel Gel compounding Nanocomposite
Despite that there have been many reports on the achievement of high-performance rubber/clay nanocomposites, the preparation strategies of these nanocomposites are still quite limited. In this study, a novel method named gel compounding method was utilized to prepare acrylonitrile-butadiene rubber (NBR)/clay nanocomposites via compounding solid rubber with clay gels. As illustrated by TEM and XRD, clay was uniformly dispersed in the NBR matrix as either exfoliated platelets or very thin layered silicate stacks, indicating the separated structure was formed in the NBR/clay nanocomposite prepared by this gel compounding method. Mechanical properties were significantly improved for the NBR/clay nanocomposites, which reached the tensile strength of 12.1 MPa with 10 phr clay, more than 250% higher than that of pure NBR.
1. Introduction Clay consisting of ∼1 nm thick silicate layers with the lateral dimensions of 200–400 nm is an abundant natural resource with very low cost [1–3]. Moreover, clay does not present the environmental pollution and health risks to human. During the past years, clay has been applied to prepare rubber/clay nanocomposites which often exhibited high performance [4–7]. Therefore, considerable attention has been attracted to the development of preparation techniques and a better understanding of structure-property-processing relationship for rubber/ clay nanocomposites. To date, there are mainly three strategies employed to prepare rubber/clay nanocomposites: melt processing [8–11], solution blending [12–14] and latex compounding [15–17]. For either melting processing or solution blending methods, the clays need to be modified through the ion-exchange reaction with organic components [18,19], usually the quaternary ammonium salts. Such modification could ensure that the silicate layers can be well dispersed in the solvent for the solution blending process, or the interlayer spacing of the silicate layers can be increased to the extent that the rubber macromolecules could easily intercalate into the clay galleries during melt processing. Unfortunately, the commercial organically modified clays are usually quite expensive. Moreover, the extensive use of the solvent in solution blending may pose serious risks to both human health and the ∗
environment. In comparison, the latex compounding technique is very promising for industrialization because of its environmental friendliness and superior performance/cost ratio [17]. During this process, the pristine clay is first dispersed in water to form the clay aqueous suspension without organic modification, and then mixed with the rubber latex followed by being co-coagulated using an appropriate flocculating agent to prepare rubber/clay nanocomposites. However, the latex compounding technique requires the use of the rubbers only available in the latex form [15], which limits the wider application of this technique. Therefore, it is worth developing a general compounding method that uses the solid rubbers and the natural clay minerals without organic modification to reduce the cost of producing rubber/ clay nanocomposites. In this work, we proposed a novel method named gel compounding method to prepare acrylonitrile-butadiene rubber (NBR)/clay nanocomposites. NBR was compounded with the clay gel (containing up to 20 wt% clay) using the two-roll mill to prepare the NBR/clay “gel compound”, which was then dried under vacuum and added with the curing agents to finish vulcanization at high temperature to obtain the final product, NBR/clay nanocomposites. The mechanical properties of these nanocomposites were highly improved, and the microstructure was investigated to better understand the structure-performance relationship.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Xue), [email protected] (J. Lin).
∗∗
https://doi.org/10.1016/j.compositesb.2019.03.013 Received 19 November 2018; Received in revised form 1 February 2019; Accepted 4 March 2019 Available online 07 March 2019 1359-8368/ © 2019 Published by Elsevier Ltd.
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2. Experimental 2.1. Materials Clay (sodium-montmorillonite) with cation exchange capacity of 93 meq./100 g was supplied by Liufangzi Clay Factory, Jilin, China. NBR 1052 (35.0 wt% acrylonitrile) was supplied by Nantex Industry Co. Ltd., China. Zinc oxide, stearic acid, dibenzothiazole disulfide, N-isopropylN′-phenyl-p-phenylenediamine and sulfur were purchased from the Chemical Reagent Shop of Beijing. All materials were used as received. 2.2. Preparation Clay was swelled in deionized water at a concentration of 20 wt%, and clay gel was obtained after vigorous stirring for 12 h. Then a given amount of clay gel was mixed with NBR in a two-roll mill to obtain rubber/clay “gel compound”. To remove the water as much as possible, the gel compound was rolled into a thin sheet with the thickness less than 500 μm, which was subsequently placed at 80 °C under vacuum for 48 h to obtain NBR/clay nanocompound. After the curing agents were added in the two-roll mill, NBR/clay nanocompound was vulcanized in a standard mold using a hydraulic hot press (XLB-D 350, Huzhou Dongfang Machinery, Co., Ltd., China) at 160 °C under 15 MPa to obtain NBR/clay nanocomposite. For comparison, the NBR/clay microcomposites with various clay contents were also prepared by directly incorporating pristine dry clay into NBR following the conventional rubber processing procedure [20,21]. The recipe (parts by weight) for NBR/clay nanocomposites or micro-composites was as follows: NBR, 100.0; zinc oxide, 5.0; stearic acid, 1.0; dibenzothiazole disulfide, 1.0; N-isopropyl-N′-phenyl-p-phenylene diamine, 1.0; sulfur, 1.5; clay, variable.
Fig. 1. XRD curves of clay/water mixture with various clay contents.
molecules between the silicate layers [15]. We term such water-intercalated clay as the “swollen clay” (Fig. 2b). When the clay content is lower than 50 wt%, with the further addition of water, no diffraction peaks can be seen in the XRD curves (Fig. 1, curve d-g), demonstrating that all the silicate layers are completely dispersed in water without aggregation. However, there clearly exists the viscosity difference among these clay/water mixtures as shown in Fig. 2c and d, the “clay gel” (clay content ≥ 10 wt%) and the “clay aqueous suspension” (clay content < 10 wt%). Compared to the clay aqueous suspension, the clay gel exhibits much higher viscosity mainly due to the formation of loosely percolated networks by the edge-to-face or edge-to-edge connection between the silicate layers [22–24]. For the clay without organic modification, it is very difficult for hydrophobic rubber macromolecules to intercalate into the galleries of silicate layers because of the hydrophilic nature of the clay and the strong ionic interaction between the neighboring silicate layers [1], which will easily cause the clay aggregation during the preparation of rubber/clay composites. A similar result will occur to the composites involved with the swollen clay, even though the silicate layers are intercalated with the water molecules. This is because the swollen clay still exits as the relatively large stacked silicate layers and rubber macromolecules cannot intercalate into the clay galleries. While for the clay gel or clay aqueous suspension, the complete exfoliation of silicate layers significantly increases the possibility of rubber chains to surround only one single layer or just a few layers of clays, thus reducing the chance for the appearance of thick silicate stacks during the water evaporation step for the preparation of rubber/clay nanocompounds. However, compared to the clay gel, the clay aqueous suspension contains a much larger amount of water, which makes it much more difficult to well mix the clay aqueous suspension with solid rubber. In addition, the water evaporation process would be time-consuming for the compound using the clay aqueous suspension. Therefore, after careful consideration and several attempts, in this work, we chose the clay gel with clay content of 20 wt% to prepare the rubber/clay nanocomposites by the gel compounding method.
2.3. Measurements X-ray diffraction (XRD) analysis was carried out using a diffractometer (D/Max-III C, Rigaku, Japan) with CuKα radiation operating at 40 kV and 200 mA, with a scan rate of 1.00°·min−1. Ultrathin sections of the nanocomposite samples for transmission electron microscopy (TEM) experiments were cut using a liquid nitrogen cooled microtome at about −100 °C, collected on a copper grid, and then observed on an H-800 TEM (Hitachi Co., Japan) with an acceleration voltage of 200 kV. According to ISO 37-2011, the mechanical behavior was tested using a GT-TC2000 electrical tensile tester (Gotech Testing Machines Inc.) at a speed of 500 mm min−1, and five specimens were tested to give the average. Shore A hardness was measured using LX-A rubber hardness apparatus (Liuling Instrument, Shanghai, China) according to ISO 7619-1-2004. Strain sweep of the cured nanocomposites was conducted using the RPA 2000 Rubber Process Analyzer (RPA) of Alpha Technologies Co. at 60 °C and 1 Hz. 3. Results and discussion 3.1. Rationale for the preparation of NBR/clay nanocomposites using the gel compounding method When the clay is dispersed in water, the dispersion characteristics of the clay is mainly determined by the clay/water ratio. The XRD curves of clay/water mixture with various clay contents are shown in Fig. 1, and the sketches illustrating the clay dispersion status are presented in Fig. 2. As can be seen from Fig. 1 (curve a), the pristine clay without water exhibits a diffraction peak at ∼7.2°, corresponding to a layer spacing (d001) of 1.24 nm, which is due to the aggregation of silicate layers resulting from the face-to-face layer stacks (Fig. 2a). After no more than 50 wt% water is added to the clay (Fig. 1, curve b and c), the diffraction peaks shift to the lower angle indicating the increase of layer spacing, which should be attributed to the intercalation of water
3.2. Structure of NBR/clay nanocomposite TEM is the most direct means to observe the dispersion status of silicate layers in the rubber matrix. Fig. 3 shows the TEM images of the NBR/clay nanocomposite and micro-composite with 10 phr clay, and the dark lines represent the cross-sections of the silicate layers. For the NBR/clay nanocomposite, numerous silicate layers are uniformly 357
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Fig. 2. Sketch graphs of (a) pristine clay, (b) swollen clay, (c) clay gel and (d) clay aqueous suspension.
Fig. 4. XRD curves of NBR/clay (a) gel compound, (b) nanocompound, (c) nanocomposite and (d) micro-composite with 10 phr clay.
diffraction peak in the XRD curve of NBR/clay nanocompound (Fig. 4, curve b). After the addition of the curing agents, the NBR/clay nanocompound is vulcanized in the two-roll mill to achieve the NBR/clay nanocomposite, and a diffraction peak at 5.9° appears in the XRD curve of this nanocomposite (Fig. 4, curve c), corresponding to a layer spacing of 1.34 nm. This demonstrates that a certain amount of silicate layers aggregate in the nanocomposite. However, such aggregation of silicate layers is not as severe as that in the pristine clay because the basal spacing of the pristine clay is only 1.24 nm, as shown in Fig. 1. Such phenomenon suggests the clay intercalation of certain molecules or ions must have taken place during the preparation of the NBR/clay nanocomposite. On the one hand, it has been well documented that the exchange of sodium cations with long-chain quaternary alkylammonium cations results in quite a significant enlargement of the basal spacing for the organic clays, which is usually larger than 1.7 nm [9–11]. Therefore, it is impossible that the rubber macromolecules would intercalate into the clay galleries in these NBR/clay nanocomposites because this should cause even larger basal spacing. On the other hand, slightly larger basal spacing (∼1.37 nm) than that of pristine clay is observed for the NBR/clay nanocomposites prepared by the latex compounding method, and such increase is attributed to the intercalation of ions from flocculating agents [15,26,27]. However, no flocculating agents are used in our work here. Therefore, the increase of the basal spacing (from 1.24 nm to 1.34 nm) may result from the incomplete removal of water molecules that have been strongly bound to the silicate layers during the preparation of nanocomposites via gel compounding method. Since these water molecules do not stay in the rubber matrix as plasticizers or defects, they may impose little effect on the mechanical performance of the rubber/clay nanocomposites as will be shown in the following section. As for the NBR/clay micro-
Fig. 3. TEM images of NBR/clay (a) (c) (e) nanocomposite and (b) (d) (f) microcomposite with 10 phr clay.
dispersed almost everywhere as can be seen from Fig. 3a and c. Fig. 3e shows that these dispersed silicate layers are either exfoliated or tightly stacked (with a thickness of 10–20 nm) in the NBR matrix. Such morphology is quite similar to that of the rubber/clay nanocomposites prepared by the latex compounding method, which can be defined as the separated nanocomposite with both exfoliated and tightly stacked silicate layers dispersed in the matrix [15,25]. As for the NBR/clay micro-composite, all silicate layers are dispersed in the NBR matrix as the form of stacked silicate layers with a thickness of more than 200 nm. The TEM observation confirms that the silicate layers are better dispersed in the NBR matrix by gel compounding method than by conventional blending method. Moreover, the difference between the NBR/clay nanocomposite and micro-composite suggests that the water molecules play an important role in the clay dispersion for the rubber/ clay nanocomposites prepared by gel compounding method, which is quite effective and can be applied to the solid rubber. The structures of NBR/clay composites are further characterized by XRD. The XRD curves of NBR/clay gel compound, nanocompound, nanocomposite and micro-composite with 10 phr clay are shown in Fig. 4. When the clay gel is compounded with NBR, no diffraction peak is found in the XRD curve of NBR/clay gel compound (Fig. 4, curve a), indicating that the silicate layers are completely exfoliated in the gel and gel compound. After the treatment of the NBR/clay gel compound with heat and vacuum to remove the water to obtain the nanocompound, the silicate layers still remain exfoliated in the matrix without any clay aggregations, as evidenced by the absence of 358
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Fig. 5. Proposed mechanism for the preparation process of rubber/clay nanocomposites by the gel compounding method.
Fig. 6. Stress-strain curves of NBR/clay (a) nanocomposites and (b) micro-composites with various clay loadings.
(Fig. 5e). Even though the few silicate layers inside one clay droplet might aggregate to form relatively thin stacks, due to the inhibition of rubber chains, it is highly impossible that several clay droplets could collapse and connect with each other to develop the clay stacks with the thickness up to the micro-scale. Therefore, employing the clays in the exfoliated form as the starting materials could ensure the achievement of the rubber/clay nanocompounds. Such compounding method is quite different from the melt processing method using the organic clay, in which the rubber chains intercalate into the galleries of stacked silicate layers. After the curing agent is added, the rubber/clay nanocompounds are vulcanized at high temperature under pressure to form rubber/clay nanocomposites (Fig. 5f). During this process, the high vulcanization temperature facilitates the movement of rubber chains which might slip out of the interspace between the silicate layers, resulting in a certain degree of clay stacking, similar to the previous studies [28,29].
composite, no intercalation of silicate layers occurs because the diffraction peak in the XRD remained at 7.2° (Fig. 4, curve d), the same as that of the pristine clay.
3.3. Proposed mechanism for the gel compounding method Based on the TEM and XRD results, we propose the mechanism for the preparation process of rubber/clay nanocomposites by the gel compounding method (Fig. 5) as follows: The clays (Fig. 5a) are dispersed in water to form the clay gel (Fig. 5b), which is then compounded with solid rubber by mechanical blending in a two-roll mill (Fig. 5c). During this step, the clay gel would form discrete droplets that contained one single layer or a few exfoliated layers “encapsulated” by water molecules. These droplets, separated by the rubber chains, would become smaller under vigorous shearing and well dispersed in the rubber matrix to form the rubber/clay gel compound (Fig. 5d). When being dried under vacuum, the water molecules in these clay droplets begin to evaporate and the rubber chains surrounding the droplets could act as barriers to inhibit the re-stacking of the silicate layers
3.4. Mechanical properties of NBR/clay nanocomposite The stress-strain curves of NBR/clay nanocomposites and micro359
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Table 1 Mechanical properties of NBR/clay nanocomposites and micro-composites with various clay loadings. Samples
NBR
NBR/clay nanocomposites
NBR/clay micro-composites
Clay loading (phr)
0
2
5
7
10
2
5
7
10
Shore A hardness Tensile modulus (MPa) Stress at 100% strain (MPa) Stress at 300% strain (MPa) Tensile strength (MPa) Elongation at break (%)
42 3.71 1.03 1.44 3.40 653
45 4.49 1.16 2.03 5.90 726
49 5.15 1.51 3.17 9.0 733
52 6.36 1.72 4.00 11.2 730
55 7.34 2.02 5.08 12.1 663
43 3.66 0.98 1.34 2.71 606
43 3.79 1.02 1.42 3.17 648
44 3.83 1.01 1.41 3.00 627
43 3.78 0.97 1.37 2.81 636
composites with various clay loadings are illustrated in Fig. 6a and b, respectively, and the mechanical properties are presented in Table 1. Compared to that of pure NBR (653%), the elongation at break of the NBR/clay nanocomposites (Fig. 6a) increases at first to the maximum of 733% with the clay loading of 5 phr, and then decreases to 663% when the clay loading is further increased to 10 phr. In comparison, the tensile strength of the nanocomposites continuously increases with the increasing clay loading, and reaches the maximum of 12.1 MPa with 10 phr clay, more than 250% higher than that of pure NBR (3.40 MPa). Moreover, the Shore A hardness, tensile modulus and stress at 100% or 300% strain of the nanocomposites also increase with the increasing clay loading as shown in Table 1. Related to the above-discussed findings on the structure and morphology of these NBR/clay nanocomposites, these mechanical property results indicate that the individual or tightly stacked silicate layers uniformly dispersed in the NBR matrix can help withstand the tensile stress of the composites, leading to an effective strengthening of rubber. The improvement of tensile strength for our nanocomposites is comparable with that for the NBR/clay nanocomposites prepared by the latex compounding method which achieved 7.45 MPa at 10 phr clay, ∼300% higher than their pure NBR (1.67 MPa) [30]. As for the NBR/clay micro-composites (Fig. 6b and Table 1), the tensile strength, stress at 100% or 300% strain and elongation at break are all reduced as compared to pure NBR. This can also be correlated with the structure/morphology features demonstrated in the XRD and TEM characterization, where the clay is poorly dispersed in the matrix in the form of large stacked silicate layers and may act as defects in the micro-composites, leading to the decrease in the mechanical performance. To better understand the reinforcement mechanism in the composites, the Mooney-Rivlin equation is applied to describe the interfacial interaction between the rubber and fillers as follows: σ σ* = = 2C1 + 2C2 λ–1 [31], where σ* is the reduced stress, σ is λ−λ–2 the applied stress, λ is the extension ratio, and C1, C2 are the MooneyRivlin constants related to the rubber-filler network and the flexibility of the network chains, respectively. Based on the data from the stressstrain curves in Fig. 6a and b, both σ* and λ−1 for the NBR/clay nanocomposite and micro-composite with 10 phr clay are calculated and plotted against each other as shown in Fig. 7. The σ* of the NBR/clay micro-composite changed with λ−1 almost linearly, while that of NBR/ clay nanocomposite exhibits an abrupt upturn at λ−1 of 0.45. The appearance of such abrupt upturn is generally attributed to the restricted extensibility of rubber chains bridging neighboring filler particles during stretching [32]. For the NBR/clay micro-composite, silicate layers are dispersed in the form of micro-sized aggregates so that there are only very few filler particles bridging to the rubber chains. Thus, no abrupt upturn is observed and the reinforcement of the silicate layers to the micro-composite is very weak. As for the NBR/clay nanocomposite with 10 phr clay, XRD and TEM analysis demonstrate the existence of nano-scaled silicate layers with huge specific surface dispersed in the rubber matrix. As a result, the rubber chains are intensively restricted by the filler particles, leading to the abrupt upturns of σ*. Therefore, these results confirm the stronger interfacial interaction between NBR and silicate layers in the nanocomposite prepared by the gel
Fig. 7. σ*-λ−1 curves between NBR/clay nanocomposite and micro-composite with 10 phr clay.
compounding method, which is much more efficient in the improvement of mechanical performances than the conventional compounding method. Furthermore, such strong interfacial interaction between NBR and silicate layers can be also proved by the RPA strain sweep results. The effect of strain on the storage modulus (G′) and tan delta of pure NBR, NBR/clay micro-composite and nanocomposite with 10 phr clay is shown in Fig. 8a and b, respectively. For the filler filled rubber composite, the G′ is affected by the filler-filler network, filler-rubber network and vulcanization crosslinking network [21]. The strength of vulcanization crosslinking network is supposed to be almost the same since the same curing agents are added in our work, and the filler-filler network is usually destroyed at a relatively low strain. Therefore, for the both NBR/clay micro-composite and nanocomposite, the dominant factor in affecting the G′ should be the filler-rubber network. As shown in Fig. 8a, the NBR/clay nanocomposite exhibits the higher G’ than both NBR and NBR/clay micro-composite, indicating the stronger fillerrubber interaction in NBR/clay nanocomposite. Moreover, the strong filler-rubber interaction can also result in the restricted segment motion of rubber macromolecules, leading to the more energy loss during a dynamic strain cycle. As a result, the tan delta of NBR/clay nanocomposite is relatively higher than that of both NBR and NBR/clay micor-composite as shown in Fig. 8b. 4. Conclusions In this work, we have proposed a novel nanocompounding method named gel compounding method, using clay gels and solid rubber to prepare NBR/clay nanocomposites. By employing the clay gels in which clays were exfoliated in water to compound with solid rubber (NBR), the existence of water molecules helped prevent the collapse and aggregation of silicate layers during the compounding process. During the 360
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Fig. 8. (a) G′-strain and (b) tan delta-strain curves of pure NBR, NBR/clay micro-composite and nanocomposite with 10 phr clay.
following drying and vulcanization processes, the rubber chains acted as barriers to inhibit the re-stacking of the silicate layers, resulting in the formation of NBR/clay nanocomposites. TEM analysis verified that the nanocomposites exhibited the features of separated structures with both exfoliated and tightly stacked silicate layers dispersed in the rubber matrix, which was further confirmed by the XRD study. Compared to the conventional compounding method by which NBR/ clay micro-composites were prepared, the gel compounding method produced the NBR/clay nanocomposites with much better mechanical performance. The application of the Mooney-Rivlin equation and the RPA strain sweep results revealed that the gel compounding technique greatly improved the interfacial interaction between NBR and silicate layers in the nanocomposite, leading to the effective strengthening of the rubber. Therefore, the present study provides a promising approach to prepare high performance rubber/clay nanocomposites using solid rubbers.
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