Fabrication Methods of Carbon-Based Rubber Nanocomposites

CHAPTER TWO

Fabrication Methods of Carbon-Based Rubber Nanocomposites Aleksandra Ivanoska-Dacikj1 and Gordana Bogoeva-Gaceva1,2 1

Research Center for Environment and Materials, Macedonian Academy of Sciences and Arts, Skopje, Macedonia Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University, Skopje, Macedonia

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Contents 2.1 Introduction 2.2 General Methods for the Fabrication of Carbon-Based Rubber Nanocomposites 2.2.1 Solution Casting and Latex Blending 2.2.2 Mechanical Mixing 2.2.3 Combination of Both Methods, Solution Casting, and Mechanical Mixing 2.3 Other Methods of Fabrication 2.3.1 In Situ Polymerization 2.3.2 Ball Milling 2.4 Conclusion References

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2.1 INTRODUCTION The modification of rubber properties by particulate fillers is an important phenomenon that has enabled development of a wide diversity of rubber products from tires to a great variety of industrial and consumer products, for example, motor mounts, fuel hoses, conveyor belts, profiles, membranes, etc. These relatively soft polymer networks, elastomer chains, were initially reinforced by carbon blacks (CBs), a colloidal form of carbon when Mote and Mathews in 1904 discovered its reinforcing effect. Today CBs still maintain the primacy of the most used filler in rubber compounds and approximately 95% of CBs produced in the world are handled in the rubber industry [1]. Besides fillers, the rubber material is always compounded with a number of other additives and it is through compounding that the specific rubber is designed to satisfy a given application in terms of properties, cost, and processability. The requirements on rubber materials are high and manifold, for example, high elastic behavior even at large deformation, tailored damping properties during cyclic deformations, high Carbon-Based Nanofillers and Their Rubber Nanocomposites. DOI: https://doi.org/10.1016/B978-0-12-817342-8.00002-0

© 2019 Elsevier Inc. All rights reserved.

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abrasion resistance, impermeability to air and water, in many cases a high resistance to swelling in solvents, high crack resistance, and long exploitation life. In order to produce high-performance elastomeric materials, nowadays attention is progressively shifting toward nanocomposites. The increasing availability of nanometer-scale fillers and their incorporation into the elastomer matrix is a growing area of rubber research. Among the nanofillers, carbon-based ones have been considered as an ideal nominee to be utilized as a tailoring agent in the polymeric materials owing to the compatibility with the organic matrices and certainly to the combination of their unique properties—electrical, mechanical, thermal, catalytic, optical. It is interesting to note that all investigations regarding elastomer-based nanocomposites started to appear later than other polymer nanocomposites. For instance, graphene (GE) was successfully isolated and identified by Geim et al. in 2004 [2] and in the years that followed many GE/polymer composites have been investigated [3], but the first works on GE/elastomer nanocomposites date from 2010 [4]. And, in spite of the tremendous research activities in the field of polymer nanocomposites during the last two decades, elastomeric nanocomposites are still in a stage of infancy as far as their application is concerned [5]. This is probably due to the specificity of rubber and the method of fabrication. Unlike thermoplastics and thermosetting polymers, which are normally supplied to the fabricator as pellets or liquid resins, rubbers are supplied to rubber processors mainly in the form of large bales or latex. That is why, although many of the production methods of polymers are also applicable to rubbers, rubber processing technology is different in certain respects, and the rubber industry is largely separated from other polymer industries [6]. Production of rubber goods consists of two basic steps. One is the production of rubber itself (natural or synthetic). The other is the processing into finished goods, which mainly consists of four steps: compounding, mixing, shaping, and vulcanizing. Consequently, a homogeneous dispersion of the carbon nanofiller particles throughout the elastomer matrix, which is the main requirement for utilizing the benefits from nanofiller incorporation, is a very difficult task to achieve. Additionally, carbon nanoparticles have a strong tendency to agglomerate. Another drawback is their inherent structure which results in a poor interfacial interaction with the polymer. If the latter problem could be overcome by different modifications of the nanofillers and/or matrix polymer (chemical or physical), the first issue strongly depends on the fabrication method used. Understanding the influence of the fabrication method on the resulting nanocomposite structure and properties is crucial for the development of high-performance materials with superior and tunable properties. In the first attempts to obtain elastomer nanocomposites, mainly two methods were used, melt mixing [7], which is closest to industrial production of rubber materials, and solution casting [8], which is most convenient for laboratory preparation, but is not applicable for industry. In the last couple of years, various strategies have

Fabrication Methods of Carbon-Based Rubber Nanocomposites

been adopted to control the dispersion of carbon nanofillers in elastomeric matrices. Most of these methods are the same as those used for the fabrication of other polymer nanocomposites, but some are exceptional and diverse. Sometimes, a combination of processing techniques is applied, since the preparation of elastomeric nanocomposites is not a single-step process. Here, we present an overview of the most commonly used fabrication methods of carbon-based rubber nanocomposites for different carbon-based nanofillers (carbon nanotubes (CNTs), GE, fullerene, nanodiamonds (NDs), nanofibers). For each method and type of nanofiller, some common preparation procedures and also some novel approaches are described in detail. At the end of the chapter, some different, not that commonly used, methods of preparation are presented and finally, a conclusion is made on the perspectives of the fabrication methods regarding their upscale on an industrial level.

2.2 GENERAL METHODS FOR THE FABRICATION OF CARBON-BASED RUBBER NANOCOMPOSITES A lot of preparation methods are employed for fabricating polymer nanocomposites, of which the most commonly used techniques for elastomer nanocomposites are melt mixing, solution casting, and latex blending. Sometimes, a combination of processing techniques is applied, since the preparation of elastomeric nanocomposites is not a single-step process.

2.2.1 Solution Casting and Latex Blending Solution casting is one of the most usual approaches for processing polymer nanocomposites on a laboratory scale. It implies that the polymer is dissoluble in a solvent such as a tetrahydrofuran (THF), dimethyl formamide (DMF), cyclohexane, toluene, acetone, etc. The typical process involves dispersion of nanofiller particles in a suitable solvent and mixing with the polymer solution, followed by film casting and solvent evaporation, leaving dry nanocomposite film. In the case of elastomers, which have to be cured, the vulcanization ingredients are also added to the solution and after solvent evaporation, the process of vulcanization under pressure and temperature is applied. The choice of the solvent is mainly governed by the solubility of the polymer matrix. It can be the same solvent used both for the matrix and the nanofiller or it can differ. In the second case, there should be a good miscibility between the different solvents in order to enable good mixing between the phases [9]. One of the

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advantages of solution casting is the possibility to achieve debundling and goodquality dispersion of the nanofiller particles in an appropriate solvent. The drawback is that this processing technique cannot be utilized for insoluble polymers [10]. Also, the removal of organic solvent after casting has environmental implications, which limits the successful transition of this fabrication method from the laboratory to industrial-scale production. Another commonly used method for fabrication of elastomeric nanocomposite, that is, very similar to the solution casting process but more environmentally friendly, is latex blending. This method was used as a new approach to reinforce a highly viscous polymer in 2004 by Regev et al. [11]. The only difference between these two approaches of preparation of nanocomposites is that the elastomer is in the form of water suspension or latex. For layered fillers, like GE, a cocoagulation step in the process of preparation is included in order to ensure better dispersion of the fillers and strong interaction between the matrix and the filler. 2.2.1.1 Rubber/Carbon Nanotube Nanocomposites Obtained by Solution Casting Solution casting is the most commonly used method to incorporate CNTs in the polymer matrix in order to prepare nanocomposites on a laboratory scale. One of the first attempts to obtain rubber/CNT nanocomposites, using this method, was done by Frogley et al. in 2003 [8]. Due to the lengthy geometric shape of the CNTs and strong van der Waals force attraction, estimated at 500 eV/µm of tube length [12], CNTs are not easily dispersed. That is why intensive mixing, such as high-power ultrasonication or another kind of mechanical agitation approach, is used before film casting. The major limitation of this step, when applied for a longer time, is that it can lead to shortening of tube lengths, thereby deteriorating the nanocomposite properties. Badaire et al. [13] investigated in detail the influence of sonication procedures on the length of CNTs in surfactant solutions by using depolarized dynamic light scattering. They concluded that high sonication power was particularly efficient at unbundling nanotubes, whereas a long sonication time at low power could be sufficient to cut the bundles with limited unbundling. However, the general guideline for the optimum sonication conditions (time, power) is not systematically established and depends on nanotube concentration and the original shape of the CNTs [8]. Many CNT/rubber nanocomposites with different kinds of elastomeric matrices and types of CNTs have been reported to be obtained via the solution casting method [14 22]. One of the commonly used solvents is toluene, which was used in the preparation of nanocomposites based on natural rubber (NR) [15,21], styrene-butadiene rubber (SBR) [15], and silicone rubber [8]. For the preparation of NR, SBR- and ethylene propylene diene monomer (EPDM)-based nanocomposites, Bokobza [19] used cyclohexane, both as a solvent and as a dispersing agent for multiwalled carbon nanotubes (MWCNTs). Having used the same preparation procedure for these three

Fabrication Methods of Carbon-Based Rubber Nanocomposites

different hydrocarbon rubbers, the author draws an interesting conclusion that the solution mixing is sensitive to the type of matrix involved. The poorer dispersion of the CNTs was observed in EPDM matrix with regard to the two other matrices (NR and SBR), despite the identical processing conditions. This was revealed by transmission electron microscope (TEM) micrographs and reflected in the mechanical reinforcement. The author found the reason for this in the higher stiffness of EPDM rubber with regard to the two other matrices and referring to literature data that softer matrices are easier to reinforce [23,24]. The production procedure for the three elastomers was as follows. The appropriate amount of MWCNTs was dispersed into cyclohexane (in an approximate ratio 1:10 by weight) by sonicating the suspension for 30 min. The gum containing the rubber (NR, SBR, or EPDM) and all the ingredients of the formulation were mixed separately in cyclohexane under magnetic stirring until complete dissolution and then mixed with the MWCNT dispersion. When examination by optical microscopy still revealed nanotube agglomeration on a micrometer scale, the mixture was submitted to further sonication for 30 min. The sonication process was followed by agitation under magnetic stirring until evaporation of the solvent. Total removal of any remaining solvent was achieved under vacuum, overnight at 50 C, before the crosslinking process and film formation. The unfilled and filled samples were then cured into plaques, EPDM samples at 170 C for 10 min; NR and SBR, at 140 C for 30 min. Bhattacharyya et al. [18] demonstrated an efficient and successful way to prepare reinforced and conducting NR-based nanocomposite material through the incorporation of up to 8.3 wt% MWCNTs activated by oxidation. For the preparation of these nanocomposites, 120 mg of MWCNTs and 120 mg of sodium dodecyl sulfate were dispersed in 20 mL of water by sonicating the mixture in a bath sonicator for 15 min. The dispersion was then added to the ammonia solution of rubber, containing 1.2 g of prevulcanized NR. The mixture was stirred magnetically for 24 h, sonicated for 15 min in a bath sonicator, and then poured into a Petri dish and dried at 60 C for 24 h to obtain freestanding composite films. Kim et al. [25] prepared highly stretchable and conductive single-walled carbon nanotube (SWCNT)/silicone rubber composites that can be used in fabricating compliant electrodes, by spraying a mixed solution of ionic liquid-based SWCNT gel and a silicone rubber onto an elastic substrate. The authors used a moderately low SWCNT content (less than 4 wt%) and the conductivity of the obtained nanocomposites was enhanced by nitric acid doping [26,27]. The preparation method was as follows, SWCNTs were ground with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (BMIM TFSI) until physical gels were formed. The SWCNT gels were dispersed in toluene at 1 mg/mL for 1 h in an ultrasonic bath. The dispersed solution was mixed with a silicone rubber (KE-441) and stirred for 3 h. To fabricate the electrodes, the SWCNT/BMIM TFSI/KE-441 solution was sprayed onto acrylic

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elastomeric substrates through a contact mask. Those sprayed samples were dried on a hot plate for 3 h and in a room-temperature vacuum oven for 1 day. For nitric acid vapor treatment, the composite electrodes were kept in a saturated acid vapor environment that was maintained at 70 C for 30 min, followed by being placed in a vacuum oven at 25 C for 1 day. The authors considered this spray coating technique a scalable, high-throughput process to prepare stretchable conductors that can be applied to a wide range of objects of arbitrary size and shape. A new mixing technique which can be categorized under the solvent mixing approach was adopted by Shin et al. [22] to prepare soft, flexible, highly stretchable, and conductive composites of MWCNTs and polyurethane (PU) (poly[4,40-methylene-bis (phenyl isocyanate)-alt-1,4-butanediol/poly(butylene adipate)]). In this study, aligned MWCNTs (MWCNT forest, diameter: 10 nm) were grown on iron-catalystcoated Si wafers using a conventional chemical vapor deposition (CVD) method. Initially, the MWCNT forests were infiltrated with a PU solution in N,N-DMF using a simple drop-casting procedure. Later, the solvent was evaporated and the obtained composite of MWCNT forest/PU was peeled off the underlying Si wafer. One side of the prepared film was black and conductive, this was the side facing the substrate (forest side) and the other whitish and insulating side (PU side). The material obtained was soft, flexible, and highly stretchable in the sheet plane. The authors concluded that this preparation procedure could be easily extended for the fabrication of multilayer samples by applying another PU or forest layer to the surface of the composite sheet. As a result, sandwich structures of the type forest PU forest (conductive on both sides) or PU forest PU (conductive layer embedded into insulating PU) could be obtained. The sandwich structures produced by this method may have applications in highly stretchable electrodes and smart clothing that require either both conducting or insulating sides. 2.2.1.2 Rubber/Graphene Nanocomposites Obtained by Solution Casting and Solution/Latex Blending Solution casting ensures good dispersion and exfoliation of the GE layers in the elastomeric matrix, even without a chemical modification [28]. In one of the first works on elastomer/GE nanocomposites performed in 2010 by Khan et al. [4], PU/GE nanocomposites were obtained by solution casting. DMF and THF were used as solvents for both PU matrix and GE. The samples were just sonicated and dried. For layered fillers like GE, solution/latex blending is also commonly used where a cocoagulation step in the process of preparation is included. This step ensures better dispersion of the fillers and a strong interaction between the matrix and the filler. The aggregation of the fillers can be prevented kinetically, by the faster coagulation process of the emulsion [29 36], and it leads to a specific partly exfoliated structure in which the layered filler can be dispersed either as an individual layer or as a layered filler without the intercalation of rubber molecules between its sheets [36].

Fabrication Methods of Carbon-Based Rubber Nanocomposites

Luo et al. [37] used a novel approach, that is, electrostatic self-assembly integrated latex technology, to develop electrically conductive elastic graphene oxide (GO)/NR nanocomposites with well-organized GE architecture. These threedimensional (3D) interconnected GE networks were obtained by employing electrostatic adsorption between poly(diallyldimethylammonium chloride) (PDDA) modified GE (positively charged) and NR latex (NRL) particles (negatively charged) as the driving force. By this approach, strong self-aggregation of GE was suppressed and synchronously the interfacial interaction between GE filler and NR matrices was enhanced. GO was reduced in the presence of PDDA, which renders positive charge on the GE nanosheet surface (first assembly). Then the positively charged PDDA GE was assembled with negatively charged NRL particles (second assembly). The fabrication procedure was as follows. PDDA GE aqueous solution together with NRL and formulation ingredients were mechanically stirred at 1600 rpm for 2 h, accompanied with water bath sonication (40 W). Then the mixture was coagulated by addition of an acetic solution (2.0 wt%), filtered, washed, and frozen dried. Finally, the resulting composites were compress molded at 150 C for 16 min. The authors observed an ideal assembly, almost all the NRL particles were encapsulated with GE nanosheets, leaving a transparent aqueous sublayer, here the phase separation was retarded and the organization of GE at the surface of NRL particles was tuned by the strong electrostatic interfacial interaction between GE and NRL. Mao et al. [38] prepared GO/SBR nanocomposites with complete exfoliation of GO sheets by aqueous-phase mixing of GO colloid with SBR latex and a small loading of butadiene-styrene-vinyl-pyridine rubber (VPR) latex, followed by their cocoagulation. During cocoagulation, VPR not only played a key role in the prevention of aggregation of GO sheets but also acted as an interface-bridge between GO and SBR. The SBR-based nanocomposites were prepared by the following procedure. Graphite oxide (previously prepared by a modified Hummers method [39]) was exfoliated in water to form stable GO colloid by a mild ultrasonic treatment. The proper ratio of the GO colloid, containing 0.35 wt% solid with 1 nm thick sheets, SBR latex containing 20.0 wt% solid, and VPR latex containing 5.0 wt% solid were mixed by vigorous stirring for 30 min. The GO/VPR/SBR emulsion was then cocoagulated by a 1.0 wt% sulfuric acid solution. The coagulated composites were washed with water until the pH of the filtered water reached 6 7 and then dried in an oven at 50 C for 24 h. The dried composites were compounded with rubber ingredients on an open two-roll mill and subjected to compression in a standard mold at 150 C for an optimum cure time. The authors claimed that their preparation method, which is water-mediated and green, could be easily transferred to a large scale and is appropriate for preparing various GO-based rubber composites since GO sheets can form well-dispersed aqueous colloid in water [40,41] and most rubbers can exist in latex form.

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2.2.1.3 Rubber-Based/Carbon Nanofiber Nanocomposites Obtained by Solution Casting The overall performances of the carbon nanofiber (CNF)/polymer composites are largely governed by the dispersion of the CNFs in the polymer matrix. Therefore, the dispersion technique plays a key role in the synthesizing of CNF nanocomposites [42]. Very recently Zhu et al. [43] prepared polypropylene-based elastomers/CNF nanocomposites by solution casting. They used two elastomers with slightly different compositions, one containing 16 wt% ethylene content (VM1) and the other containing 15 wt% ethylene content (VM2). The polypropylene-based elastomer (VM) was initially added in xylene with a weight ratio of 1:10 and magnetically stirred for 24 h at approximately 138 C until VM was completely dissolved. Then the CNFs were added and the mixture was kept overnight without stirring to completely wet the nanofiber surface. Mechanical stirring (500 rpm) was then performed at room temperature for 10 min and followed by magnetic stirring at 138 C overnight. The resulting solutions were then subjected to ultrasonication for 0.5 h and transferred to a flat mold to allow solvent evaporation for 24 h. The authors concluded that by this method they obtained electrically conductive VM/CNF nanocomposite that can be utilized as strain sensor with large mechanical deformation. Recently, Chowdhury et al. [44] fabricated polydimethylsiloxane (PDMS)/CNFbased nanocomposites by a solvent-assisted ultrasonication method. They employed CNFs to enhance the thermal and electrical properties of PDMS. THF was used as a solvent. First, the CNFs were dispersed in it using a high-intensity ultrasonic probe with low amplitude (to avoid breakage of CNFs and excessive heat generation during mixing). The CNFs were sonicated for 4 h in an ice bath. In a separate container dimethylsiloxane and silica of required weight ratios were mixed using a mechanical mixture at 1500 rpm for 10 min and then they were added into the CNF THF mixture for further sonication and dispersion. After evaporation of THF (its content was found to be less than 1 wt%) methylhydrosiloxane dimethylsiloxane copolymer and the catalyst were manually mixed for 10 min. The mixture was degassed in a vacuum oven at room temperature for an additional 30 min, and then it was cast into the mold and cured inside a hot press at 80 C and under 1100 psi pressure for 3 h. Finally, the nanocomposites were postcured at 150 C for 2 h. The authors concluded that this fabrication and characterization of the PDMS/CNF nanocomposites could be useful for the future design of CNF-enhanced nanocomposites. 2.2.1.4 Rubber-Based/Carbon Nanodiamonds Obtained by Solution Casting The main limitation of the successful use of NDs is their poor dispersion in various media and their tendency to agglomerate [45]. Several dispersion routes have been investigated to provide adequate dispersion of carbon NDs in an elastomeric matrix. Shenderova et al. [46] employed an intermediate solvent (isopropanol) as a dispersion

Fabrication Methods of Carbon-Based Rubber Nanocomposites

media for the ND particles prior to mixing with the polymer. After the introduction of ND particles into the solvent, it was sonicated to break up the agglomerates. Finally, the obtained suspension was combined with uncured PDMS; vacuum was used to remove the solvent. The authors found that the addition of 2 wt% of NDs increased the thermal conductivity of PDMS up to 15%. Kong et al. [47] studied ND silicone rubber (hydroxyl-terminated PDMS) composite samples with different filler loadings (0.5, 1.0, 1.5, and 2.0 vol.%) for its potential to be used as a thermal interface material. Similarly to the work of Shenderova and coworkers, discussed above, a dispersion medium, here toluene, was added to the nanoparticles in order to improve the dispersion of nanoparticles. The mixture was sonicated for 20 min using the ultrasonic tip in an ice bath to obtain more homogeneous dispersion and to break up large agglomerates of NDs. Finally, the silicone rubber was added into the obtained suspension; solvent was evaporated by mechanical stirring at 400 rpm for 70 min at 70 C, followed by degassing and then cold-pressing of the samples. The ND silicone specimen had a higher modulus, higher elongation at break (at any given loading level), and a higher tensile strength (about 2.5 times higher at 2 vol.%) than unfilled silicone. Despite the number of advantages of solution mixing, the effective removal of the solvents used during the procedure remains a significant problem for the extensive use of this particular method. Furthermore, the high cost of solvents and their disposal act negatively upon the scale-up and therefore the eventual adaptation of this method by industry. Finally, this is a very sensitive procedure due to the fact that the details of each component or the processing (i.e., quantity and quality of solvents, mixing time and speed, sonication, etc.) can affect dramatically the outcome of the process. Latex compounding is a more eco-friendly method but not all elastomers are available in latex form and not all rubber products can be fabricated from latex.

2.2.2 Mechanical Mixing Mechanical mixing is the most promising approach for the production of elastomer nanocomposites on an industrial scale. It is a well-established polymer-processing technique where the shear stress induced in the polymer during the process of mixing is employed for the breakdown of aggregated fillers to the nanoscale. There are two mechanisms proposed for the dispersion of CB agglomerates in the rubber matrix during the process of mixing [48,49]. One is the rupture model [48], according to which rupture occurs along a cross section in the agglomerate wherein the number of contact points of each primary particle with its neighbors is very low. The agglomerate cleaves into two nearly equal parts. The other is the “onion peeling” model [49], according to which the stresses, generated at the agglomerate surface, are large enough at any point on the surface to remove a primary particle or a group of

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primary particles (an aggregate) from the surface of the larger agglomerate. The removed aggregates form a cloud around the initial agglomerate, partially shielding it from further size reduction. Reduction in agglomerate size occurs, as aggregates are swept from the cloud, and fresh aggregates from the agglomerate replace them. In either dispersion model, when the shear stress is larger than some critical threshold value for breaking down agglomerates of fillers, a dispersive action will occur inside the kneaded polymer melt [50]. Advantages of mechanical mixing are its speed, simplicity, and easy integration into standard industrial facilities. 2.2.2.1 Rubber/Carbon Nanotube Nanocomposites Obtained by Mechanical Mixing Melt blending is another method broadly accepted in fabricating CNT elastomeric nanocomposites [51]. CNTs are dispersed within the elastomeric polymer matrix by shearing stresses. The generated shear forces help to break CNT aggregates or prevent their reaggregation during mixing. This approach is generally noted to be simple and common for industrial-based mass production. Nanocomposites based on different types of elastomer NR [7,52,53], SBR [54 56], butadiene rubber (BR) [55,56], hydrogenated nitrile butadiene rubber [57], EPDM [58,59] were processed by this method. Verge et al. [60] reported on the dispersion by simple melt blending of tiny amounts of CNTs in acrylonitrile-butadiene rubber (NBR). The CNT bundles were more effectively exfoliated and dispersed in the NBR with higher acrylonitrile content due to stronger interaction between the acrylonitrile and CNT. Wu et al. [61] fabricated styrene ethylene butylene styrene (SEBS)/CNT composites in the presence of bis(diphenyl phosphate) (BDP) by the melt mixing method and explored their flame-retardant and mechanical properties. They reported that the pretreatment of CNTs by BDP greatly improved the final dispersion in SEBS matrix. The improved CNT dispersion was reported to enhance the flame retardancy and thermal stability of the composite. Kim et al. [58] succeeded by utilizing this conventional rubber technology, mechanical mixing, to fabricate aligned CNT-based rubber sheets in a controllable direction. They obtained CNT aligned rubber sheet along the x-direction, by applying a calendaring and shaping process, and in the z-direction, by controlling the parameters of the extrusion process. The authors concluded that by varying the amount and alignment of nanotubes in a controllable direction, the mechanical/electrical/thermal/shielding properties of nanotube/rubber sheet could be controlled. Das et al. [55] investigated a novel mixing approach for achieving a good dispersion of modified and unmodified MWCNTs in a 50:50 blend of solution-styrenebutadiene rubber (SSBR) and BR. Their aim was to obtain a good dispersion of

Fabrication Methods of Carbon-Based Rubber Nanocomposites

CNTs by bundling or isolation of individual tubes from highly entangled primary agglomerates by a novel process of predispersing the nanotubes in a liquid medium without damaging CNT unique properties. For this purpose, they first dispersed the MWCNTs in ethanol in the ratio 1:10 by weight in the case of unmodified MWCNTs. For the hydroxyl modified MWCNTs, the alcohol ratio was chosen to be 1:30. In each case, 2.5 phr of the nonionic surfactant was added to the dispersed CNTs. This mixture was then treated gently by an ultrasonic bath (200 W effective; 25 kHz) for approximately 2 h. The rubber blend was mixed in an open two-roll mixing mill for 5 min at 80 C. Then, the ethanolic CNT suspension was added very slowly. The mixing time for the incorporation of the nanotubes was about 15 min. Finally, the curing chemicals were incorporated into the composites at room temperature. After the mixing process, the stocks were cured under pressure at 160 C to the optimum cure in respect of the t90 vulcanization time, determined with a moving die rheometer. Ivanoska-Dacikj et al. [62 64] also confirmed that this predispersion of MWCNTs in ethanol and shear mixing is enough to achieve good dispersion of MWCNTs (both 2 and 6 phr) in NR matrix. But it also enables safe manipulation of the MWCNTs, which have potentially hazardous properties [65]. Lorenz et al. [56] investigated how different agents, in which CNTs are predispersed, influence the dispersion of CNTs. They evaluated the effect of the dispersing agent and mixing procedure on the macro- and microdispersion of the CNTs by reflected light microscopy and TEM. The authors concluded that the solvent used for predispersion of CNTs has a strong impact on the dispersion and consequently on the structure and several other properties of the nanocomposites. They found that ethanol gave the best macrodispersion, showing only some small agglomerates of CNTs. The results do not considerably depend on the ethanol content. They got similar results with 2-propanol, while acetone hardly allowed the CNTs to disperse, showing many agglomerates of up to 200 µm in size. Without dispersion agent (dry) CNTs also showed many agglomerates, but the structure appears to be different because of the better interaction with the polymer. Further improved dispersion of the predispersed CNTs in NR rubber was achieved by the presence of “exfoliated” organically modified montmorillonite (EOMt) [66]. These hybrid NR-based nanocomposites containing MWCNTs and EOMt were prepared by a conventional mechanical mixing technique. It was found that the addition of only MWCNTs led to remarkably enhanced low-strain tensile modulus, tensile strength, and storage modulus of NR. And then the addition of EOMt enabled fine-tuning of the above-mentioned properties, and moreover an improvement of elasticity and elongation at break. These studies [62 64] clearly demonstrated the benefits from the incorporation of a hybrid system of nanofillers (different by nature, form, and rigidity) in the elastomer matrix.

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Very recently, Le et al. [67], using this method, developed intrinsically self-healable material based on butyl imidazole modified bromobutyl rubber (BIIR)/NR blends filled with CNTs. The nanocomposites were prepared in an internal mixer by keeping the following mixing conditions: initial chamber temperatures of 35 and 80 C, respectively, rotor speed of 60 rpm, and fill factor of 0.78. After mixing, conventional sulfur curing was omitted. The mixtures were compression-molded at 120 C under 100 bar for 20 min to obtain a sheet with a thickness of 2 mm. According to the authors, the use of butyl imidazole as physical crosslinker for the BIIR phase provided the blend composites with the noncovalent bondings, which were responsible for their self-healing properties. All these studies have proved that mechanical mixing is a promising and industrially viable method to produce rubber nanocomposites on a larger scale. 2.2.2.2 Rubber/Graphene Nanocomposites Obtained by Mechanical Mixing The high shear forces that occur during mechanical mixing can lead to the breakage of the GE or GO sheets. In comparison with the other popular methods for the preparation of GE/elastomer nanocomposites, it is the one that leads to the poorest dispersion of the filler [68], as Kim et al. [69] have demonstrated for their thermoplastic PU filled with graphite and thermally reduced GE. Kumar et al. [70] prepared synthetic isoprene rubber (IR)/GO nanocomposites by dry melt mixing. A lab mixer was employed at 60 rpm to prepare masterbatches in the following order, first the IR was masticated, then GO was added, followed by addition of ZnO and stearic acid, and finally, the vulcanization additives and sulfur were introduced. In such a way, the obtained masterbatches were passed through a two-roll mill for homogenizing the ingredients. The authors found that the incorporation of GO in IR matrix resulted in significant improvement in the material properties. Wu et al. [71] used another approach, by grafting polystyrene (PS) or poly (styrene-co-isoprene) (PSI) on the surface of GO and incorporating it in SSBR and BR. They prepared nanocomposites of SSBR-BR with GE as well as SSBR-BR with GO-PS (or GO-PSI) on an open two-roll mill followed by vulcanization. Considering that the SSBR matrix is mainly made from polymerized styrene and butadiene, herein, PS and polyisoprene have been chosen to modify GO for the sake of better dispersion of GE and higher interfacial interaction between GE sheets and rubber matrix. The authors achieved remarkable reinforcing effects in enhancing the properties of SSBR-BR composites by incorporating a small amount of modified GO. Due to the uniform dispersion and strong interfacial interaction, the addition of GO-PS and GO-PSI significantly improved the mechanical and tribological properties.

Fabrication Methods of Carbon-Based Rubber Nanocomposites

2.2.2.3 Rubber/Fullerene Nanocomposites Obtained by Mechanical Mixing A method of using fullerenes to enhance the properties of rubber composites was proposed by Chow in 1996 [72]. Jurkowska and coworkers prepared composites based on NR by incorporating fullerene-containing CBs [73]. The fullerene used was in a concentration range 0.065 0.75 and the total amount of the filler was 5 phr. The composites were prepared in an internal mixer and the curatives were added on a two-roll mill. The obtained composites had increased elasticity, hardness, and modulus. In order to trace the changes caused only by fullerene, Al-Hartomy and coworkers [74] added neat fullerene to the rubber matrix, without CB or other fillers. They added fullerene in the powder form up to 1.5 phr to the NR on an open two-roll laboratory mill together with the vulcanization ingredients. The samples were then cured at 150 C up to the optimum cure time. The authors found a relatively even distribution of the filler particles in the elastomeric matrix which they marked by a satisfactory homogeneity. 2.2.2.4 Rubber/Carbon Nanofiber Nanocomposites Obtained by Mechanical Mixing Melt mixing is a commonly used method for fabrication of polymer/CNF nanocomposites, due to its low cost, simplicity, and availability [42]. High shear forces that occur during the process of mixing lead to a relatively good dispersion of the CNFs, but their aspect ratio, which is another key parameter governing the overall performances of the CNF/polymer composites, decreases during the mixing process. It was found that the decrease of the aspect ratio results in degradation of some properties [75,76]. Therefore, the investigation of the relatively low shear mixing approach without sacrificing the dispersion is still a challenge for the preparation of CNF/polymer composites by the melt mixing approach. Rybi´nski and Janowska [77] prepared NBR/CNF nanocomposites with the use of a laboratory rolling mill. They found that the incorporation of CNF increased the thermal stability of the rubber.

2.2.3 Combination of Both Methods, Solution Casting, and Mechanical Mixing Due to the complexity of the rubber composites and the necessity of the presence of different ingredients in the rubber formulation, solution casting is rarely used as a single-step method in elastomer nanocomposite preparation. Mostly for the elastomers that have to be cured the preparation is done in two steps. First the solution casting method is used to obtain good dispersion of the GE in the polymer matrix, and then in the second step melt mixing is applied, commonly performed on a two-roll mill, for incorporation of the curivatives.

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Mensah et al. [78] prepared GO/NBR nanocomposites via solution casting method. The nonmodified GO sheets were dispersed in dimethylfuran by ultrasonication for about 2.5 h. The rubber, cut into smaller pieces, was dissolved in acetone by vigorously stirring by magnetic stirrer at 60 C until complete dissolution. Both solutions were then mixed together under vigorous magnetic stirring at 60 C for 12 h until a homogeneous phase was observed. In order to coagulate the GO/NBR nanocomposite formation from the water phase, deionized water was gradually added to the mixture. The curing agents were then added to the dry sample on a two-roll mill and the samples were cured in a hot press at 160 C. Very recently, He et al. [79] exploited a common rubber latex mixing process to obtain GO/bromobutyl rubber (BBR) nanocomposites. They applied the following procedure. GO dispersed in water, by sonication, was mixed with the BBR latex which contained the vulcanization activators, accelerators, and agents. This mixture was then sonicated for 1 h, coagulated, filtered, and dried in an oven at 65 C for 2 days. To obtain “segregated” morphology, the dried solid mixture was simply subjected to compression molding and then vulcanized at 170 C. To obtain “nano-segregated” morphology, the dried solid mixture was first subjected to a twin-roll mill step at room temperature for 5 min, thus destroying the “segregated” morphology, and then it was vulcanized. The authors concluded that these two simple processing routes, that determine the morphology of the nanocomposites through the segregation of GE platelets, confined among latex rubber spheroids, can be effectively exploited to tailor mass transport properties as well as the electric and dielectric behavior of GE-based rubber nanocomposites. Berki et al. [80] compared the properties of NR-based nanocomposites containing 0.5 phr GO obtained through traditional melt mixing and latex precompounding. They prepared the nanocomposites by the following routes. The required amount of aqueous dispersion of GO was introduced in the NRL and stirred for 5 min at room temperature before pouring in an aluminum tray. After a dwelling time of 20 min (to let the air bubbles escape) the NRL in the tray was coagulated at 80 C for 30 min, then this NR/GO nanocomposite was further dried in a vacuum at room temperature for 1 day. For the nanocomposites obtained by mechanical mixing the GO in a form 1 wt%, the aqueous suspension was introduced in the block NR during mixing whereby the water carrier was evaporated. For both nanocomposites, the latex precompounded NR/GO and the block NR were mixed with the sulfuric curatives and GO/curatives, respectively, on a laboratory two-roll mill. The samples were cured in a laboratory press at 160 C for the optimum curing time under 5 MPa pressure, producing sheets with 2 mm thickness. The authors concluded that the NR/GO nanocomposites, produced by the latex route, outperformed the melt compounded versions with respect to the hardness, tensile mechanical, and fracture mechanical performances. The property improvements were attributed to a better dispersion

Fabrication Methods of Carbon-Based Rubber Nanocomposites

(higher exfoliation and larger aspect ratio) of the GO layers after latex precompounding compared to the nanocomposites produced via direct melt mixing of block NR.

2.3 OTHER METHODS OF FABRICATION 2.3.1 In Situ Polymerization The in situ polymerization technique is the third traditional method for processing polymer nanocomposites [81]. The general principle of this fabrication method involves mixing of the nanofiller with the monomers in a solvent, followed by in situ polymerization. It has been used successfully to produce polymer/layered-silicate nanocomposites with an exfoliated structure, and due to the similar layered structure of GE, this method was also employed for the preparation of GE/elastomer nanocomposites [82,83]. Raghu et al. [84] used the in situ method to prepare waterborne polyurethane (WPU) with functionalized graphene sheets (FGSs). They first mixed FGSs with a polyol (PCL) and then reacted with other monomers for polymerization. The preparation procedure was as follows. The FGSs were dispersed in acetone and sonicated and then mixed with a PCL in the reactor. To prepare the prepolymer, dioctyl tin dilaurate and isophorone diisocyanate were charged and reacted with the PCL at 80 C under constant stirring. After 2 h, additional chain extension reactions were carried out at 80 C for 4 h with dimethylol propionic acid (DMPA), and then for 2 h with 1,4-butanediol. After cooling to 30 C, triethylamine was fed into the reactor to neutralize the DMPA unit of the prepolymer. To obtain an aqueous emulsion of the neutralized prepolymer, water was fed dropwise into the reactor. Then triethylenetetramine dissolved in water was fed to the emulsion for the final chain extension reaction for 1 h. The resulting product was a WPU with a solid content of about 30 wt%. The author compared the properties of samples produced in such a way and samples produced by melt mixing. They found that in the nanocomposites made by the in situ method the conductivity of WPU was more efficiently enhanced by the added FGSs because FGSs finely disperse as nanosize in a WPU matrix to make an effective conducting channel. The modulus improvement of WPUs by the reinforcing effect of FGSs was more evident compared to the nanocomposites made by a physical mixing method, which suggests that the interaction between FGSs and WPUs is stronger when made by an in situ method. Despite the success of the in situ method in promoting effective dispersion of nanofillers in some polymer matrices, this method exhibits some drawbacks. It involves complex procedures and processing steps and requires expensive reactants.

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It is not applicable for all types of polymers, only those insoluble in the solvent and thermally unstable, and is applicable only for a very limited number of elastomers.

2.3.2 Ball Milling Ball milling is suggested as a novel method for enhancing filler dispersion in different matrices that is environmentally and economically sustainable [85]. It is a solid-state mixing of powders, usually performed with ball mills, which enables intimate mixing of the constituents through repetitive plastic deformation. By applying discontinuously high strain rates, ball milling enables the reduction of particle size in addition to the effective mutual dispersion of the processed phases. This results in the fabrication of finely dispersed metastable solids under far-from-equilibrium conditions [86 88]. By tuning the milling intensity and processing conditions (the frequency and energy of the collisions, number of milling tools, powder charge, atmosphere, temperature, time) the properties of the final product can be tailored. This method was successfully applied in the fabrication of carbon nanofiller-based elastomer nanocomposites. Tang et al. [89] and Witt et al. [90] reported on the preparation and characterization of a conductive silicone rubber composite, filled with both CB and CNTs, prepared by the ball milling method. Because of the good dispersion and synergistic effects of CB and CNTs, the obtained nanocomposite showed improvement in mechanical properties and conductivity in both studies. Alimardani et al. [91] focused on the preparation and rheological investigations of carboxylated styrene-butadiene rubber latex (XSBR) MWCNT nanocomposites. They used the concurrent ball milling method to embed CNTs into XSBR latex. The samples were ball milled for an optimized time of 6 h and then dried at a temperature of 40 C in an oven for 48 h. To eliminate the possible effect of other ingredients on the microstructure of the XSBR nanotube nanocomposite, the samples were prepared free of any extra materials generally added to rubber latex compounds. They used a small homemade ball mill composed of a stainless steel container with a diameter and height of 8 and 12 cm, respectively, and 400 small stainless steel balls in two different sizes of 0.6 and 0.8 mm. The study of rheological properties of nanocomposites showed that the simultaneous milling of the latex and CNTs can lead to relatively better dispersion of CNTs in the latex compared to the dispersion obtained by ultrasonication, which provides a new opportunity to produce latex nanocomposites in a large scale using industrial ball mills. Potts et al. [92] reported the dispersion of reduced graphene oxide (RGO) into NR employing two methods: solution treatment and two-roll milling. They demonstrated that the processing approach had an important impact on the composite morphology and thus on the final properties. Solution treatment (implantation of peroxide, curing agent by swelling the NR in toluene) preserved the segregated filler

Fabrication Methods of Carbon-Based Rubber Nanocomposites

network morphology produced by the cocoagulation procedure, whereas the milling process destroyed this network and generated a homogeneous dispersion of RGO platelets in the NR matrix. The segregated network morphology was advantageous for conductivity properties. The reinforcement in the solution-treated nanocomposites was attributed to the formation of a sample-spanning network of strongly interacting RGO platelets located in the interstitial regions between latex particles. In the milled nanocomposites, on the other hand, the reinforcement was due to mechanical restraint, along with the promotion of strain-induced crystallization by the high aspect ratio of the RGO platelets.

2.4 CONCLUSION After the compressive overview of the fabrication methods used to obtain carbon nanofiller-based elastomer nanocomposite, it can be concluded that those fabrication methods that can be easily up-scaled to an industrial level are the most promising. In this sense, the solution mixing method, although resulting in well-structured nanocomposites, has a number of disadvantages, like high cost of the solvents and the problem with their disposal. On the other hand, latex compounding is a more ecofriendly method, but its disadvantage is that not all elastomers are available in latex form. The in situ method is applicable only for a very limited number of elastomers. It involves complex procedures and processing steps and requires expensive reactants. Ball milling is a promising method that is environmentally and economically sustainable, but it cannot be easily adapted to the existing production equipment in the rubber industry. The most favorable fabrication method, which is industrially viable and which can easily enable the production of rubber nanocomposites on a larger scale, is the standard melt mixing method, but still for a good dispersion a pre-preparation of the nanofiller in a liquid medium is needed. Finally one can conclude that in order to produce cost-effective high tech rubber products with a specific use still smart combination of the existing methods of the fabrication, their optimization or invention of new ones is still needed.

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