Effects of silane coupling agents on the properties of bentonite/nitrile butadiene rubber nanocomposites synthesized by a novel green method
Introduction
In recent years, clay has attracted much attention in the development of polymer nanocomposites (Ray and Okamoto, 2003, Karger-Kocsis and Wu, 2004, Azeez et al., 2013), particularly as an alternative to carbon black in the researches of rubbery materials in academic field (Arroyo et al., 2003, Ma et al., 2004). For clay polymer nanocomposites (CPN), predominant performances are attainable including improved mechanical properties, thermal stabilities, chemical resistance and flame retardance (Suin et al., 2014).
As one type of extensively used clay, bentonite (Bent) is a low cost inorganic mineral abundant in nature. In the past few years, Bent has been proved a satisfactory reinforcement for rubber products owing to its laminar structure with high aspect ratio (Gu et al., 2009, Chakraborty et al., 2010). A layer of Bent consists of two silica tetrahedral sheets fused to a central sheet of alumina octahedrons (Bergaya and Lagaly, 2013). However, Bent exhibits hydrophilicity which is detrimental to its compatibility in hydrophobic polymers. The hydrophilic nature of Bent is originated from the abundant ions present in the interlayer spaces, such as Na+, Ca+, Li+ as well as the reactive silanol (Si–OH) and aluminol (Al–OH) groups at the surface (Alexandre and Dubois, 2000).
To improve the affinity between Bent and rubber matrix, various modifiers and compatibilizers, including polymers, surfactants, quaternary ammonium salts and silane coupling agents (SCA) have been employed to lower the surface energy of Bent, thereby enhancing the interactions between Bent and rubber matrix (Pavlidou and Papaspyrides, 2008, Das et al., 2011, Bergaya and Lagaly, 2013). Application of modifiers or compatibilizers also facilitates the exfoliation and homogeneous dispersion of the stacked layers of Bent (Kim et al., 2003a, Zheng et al., 2004), which are crucial to the performances of the rubber product. Belonging to the family of organosilanes, SCA normally contain three groups (CH3CH2O– or CH3O–) which undergo hydrolysis in acidic/basic environment. During the hydrolysis process, silanol groups could be generated which are highly reactive to the hydroxyl groups on the surface of Bent. Meanwhile, SCA always contain a group (e.g., vinyl, amino or mercapto group) which is able to interact with the macromolecules in rubber matrix, forming covalent bonds, ion-molecule interaction or chain entanglement (Manna et al., 1999, Jia et al., 2007, Noriman and Ismail, 2012). Additionally, present in the interlayer spaces or layer edges of Bent, SCA remarkably expand the interlayer spaces, providing a better dispersion state of Bent in the matrix (Bertuoli et al., 2014).
Hitherto, there have been several technical routes for the utilization of SCA. In the research work of Mathialagan and Ismail (2012), 3-aminopropyltriethoxysilane was added into ethylene-propylene-diene monomer during solid compounding, which could enable reinforced mechanical properties and lower swelling ratio. A solution mixing method was applied by López-Manchado et al. (2004) for the blending of natural rubber and organoclay in toluene. The storage modulus and the crosslinking density of the rubber were apparently increased. This protocol is evidently more effective as compared to solid mixing, however neither economical nor environment-friendly as large amounts of organic solvent involved. Melt blending technique is another option which has been utilized in previous works (Kim et al., 2003b, Hasegawa et al., 2005, Liu et al., 2006). Compared with those methods, latex mixing is considered a better choice in the aspects of good dispersion of Bent and low cost for manufacture (Wang et al., 2000, Varghese and Karger-Kocsis, 2003). Conventionally, people treat clay or clay minerals with SCA prior to the mixing step, mainly in alcohol–water cosolvents (Gu et al., 2009, Balakrishnan et al., 2011, Li et al., 2013b), which requires more steps in the fabrication.
In present work, a novel green method for the synthesis of Bent/SCA/NBR nanocomposites was developed. In this method, no organic solvent was involved while the synthesis could be carried out in brief steps. In the preliminary work, the hydrolysis–condensation of (3-mercaptopropyl)trimethoxysilane in the process was identified while the formation of the CPN was discussed (Ge et al., 2015a). In this research, eight SCA were utilized, categorized according to their various groups as follows: (1)alkyl or alkenyl groups: ethyltrimethoxysilane (ETMS), vinylmethoxysilane (VTMS) and octadecyltrimethoxysilane (OTMS); (2)amino or imino groups: 3-aminopropyltriethoxysilane (APTES) and [3-(2-aminoethylamino)propyl]trimethoxysilane (AEAPTMS); (3)sulfur-containing groups: mercaptopropyltrimethoxysilane (MPTMS) and bis-[3-(triethoxysilyl)propyl]tetrasulfide (TESPT); (4) methacryloyl group: [3-(methacryloyloxy)propyl]trimethoxysilane (MAPTMS). Their effects to the mechanical strength, tribological properties, thermal behaviors, swelling properties and rheological properties of the CPN were evaluated and investigated. Relevant influences of different interactions such as covalent bonds and ion-molecule interaction from various SCA were further explored.
Section snippets
Materials
Bentonite (Bent, ≥ 95%; water content, < 3%; Na+-montmorillonite content, ≥ 95%; cation exchange capacity, 0.85 meq/g) was originated in the mountains of Gunma prefecture in Japan and purchased from YAKURI Pure Chem. Acrylonitrile butadiene rubber (NBR) latex (KNB 35 L, solid content: 20.3%, w/v; acrylonitrile content: 34%) was supplied by Korea Kumho Petrochemical. (3-Mercaptopropyl)trimethoxysilane (MPTMS, 95%), [3-(2-aminoethylamino)propyl]triethoxysilane (AEAPTMS, ≥ 80%),ethyltrimethoxysilane
Characterization of the CPN
In this research, characterization of CPN and neat NBR was all performed after vulcanization.
FT-IR analysis was introduced to identify the composition of raw materials, neat NBR and the CPN. In Fig. 2a, the spectra of neat Bent and NBR rubber are represented. For Bent, a strong and wide peak appeared at 1003 cm− 1, corresponding to the Si–O–Si asymmetric vibration while the broad peak at 3625 cm− 1 was attributed to the stretching vibration of Si–OH (Xi et al., 2005). The peak at 1635 cm− 1 was
Formation of the CPN
Regarding to the employed approach different from the methods reported in previous literature, the formation of the CPN is schematically illustrated in Fig. 9: DBS-Na was added into the latex in advance, in case of any aggregation occurred during the synthesis. Most of the organosilanes applied in this work were unsolvable in water, except APTES and AEAPTMS, therefore once dispersed into NBR latex, a proportion of the SCA was dissolved in the inner organic phase. Subsequently, catalyzed by
Conclusion
In this work, Bent/SCA/NBR nanocomposites were successfully fabricated with a novel green method. The composition and the intercalated/exfoliated structure of Bent in the matrix were identified with FT-IR and XRD, respectively. The morphology was investigated by FE-SEM. The effects of different SCA to the performances of the CPN were evaluated and further discussed. It could be concluded that effective filler–rubber interactions and better dispersion state of filler are vital to the
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