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Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber
Egyptian Journal of Petroleum xxx (2018) xxx–xxx
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Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber E.M. Sadek a,⇑, D.E. El-Nashar b, S.M. Ahmed a a b
Petrochemical Department, Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt Polymer & Pigment Department, National Research Centre (NRC), Dokki, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 26 October 2017 Revised 13 March 2018 Accepted 24 April 2018 Available online xxxx Keywords: Acrylonitrile butadiene rubber (NBR) Nanocomposites Cationic surfactants Rheometric characteristics Physico-mechanical properties
a b s t r a c t Solvent blending of organoclay (OC) in polystyrene in combination with melt mixing with acrylonitrile butadiene rubber (NBR) are applied to provide acrylonitrile butadiene rubber/clay nanocomposites. Clay structures based on the prepared 1,12 dibromododecylhydroxyuronium and bromo dodecyl hydroxyuronium as cationic surfactants were prepared. Colloidal suspension of each organoclay in toluene was mixed separately with polystyrene giving two types of polystyrene-organoclay composites. The prepared polystyrene-organoclay PS-OC composites were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. The results indicate the formation of polystyrene-organoclay composites with intercalated and partial exfoliated structures. The rheometric study demonstrate an increase in minimum and maximum torques with an increase in cure rate index (i.e. faster scorch and cure times) for the PS-OC (6 phr) incorporated NBR compound compared to unfilled NBR. This effect is more significant with NBR/PS-OC1 composites in comparison to NBR/ PS-OC2 composites at the same loading indicating the nanoreinforcement effect of clay on the mechanical properties of these composites. Compounds filled with PS-OC1 exhibit higher tensile strength, Young’s modulus and less elongation at break compared to those compounds filled with PS-OC2. The higher stiffness of PS-OC1/NBR composites reflects the enhancement in hardness of these compounds with respect to those containing PS-OC2. Further increasing of PS-OC loading results in a significant decrease of the properties. This is due to a possible agglomeration formation of PS-OC in rubber matrix. Furthermore, the results of reinforcement of polymers with nanosized particles indicate a promising application of the organoclay and nanocomposites that affords high performance materials. Ó 2018 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction As rubber applications demand compounds with better performance novel rubber composites with improved properties should be developed [1]. Recently, there has been much academic and industrial interest in the production of rubber nanocomposites with layered clay as a substitute for the traditional carbon black due to its high aspect ratio and high surface area. Several reports have shown that the presence of nano-dispersed clay can improve the properties of rubber like acrylonitrile butadiene rubber (NBR) [2–4], styrene butadiene rubber (SBR) [5–7], natural rubber (NR) [8–14], hydrogenated nitrile rubber (HNBR) [15], butyl rubber [16] and silicone rubber (SiR) [17].
Peer review under responsibility of Egyptian Petroleum Research Institute. ⇑ Corresponding author. E-mail address: [email protected] (E.M. Sadek).
Sadek et al. [2,3] found that maleic acid anhydride/organoclay MOC at 3 phr content had a great effect on the rheometric and swelling characteristics [2] as well as physico-mechanical properties and thermo-oxidative aging of the NBR nanocomposites [3]. Sadek et al. [5] also studied the nanoreinforcement of SBR by using organoclay modified with anionic and nonionic surfactants. The results showed simultaneous improvement in both rheometric and mechanical properties with enhancement in aging resistance of rubber nanocomposites formed with 6 phr modified organoclay with mixed surfactants. Di Credico et al. [7] prepared organoclay modified sepiolite fibers with reduced particle size and increased silanol groups on the surface layer and were used in the preparation of SBR nanocomposites with enhanced mechanical properties. Thomas et al. [8] found that the gas-barrier properties of NR was reduced on the inclusion of organically modified montmorillonite clay compared to the neat rubber properties. Thomas et al. [9] also studied the effect of clay type and content as well as vulcanizing systems on the mechanical properties of NR-clay nanocomposites.
https://doi.org/10.1016/j.ejpe.2018.04.007 1110-0621/Ó 2018 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: E.M. Sadek et al., Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.04.007
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The authors found that conventionally vulcanized nanofilled samples exhibited good improvement in mechanical properties. Guangjian and Jincheng [12] prepared and characterized a new class of effective and friendly flame-retardant organoclays based on organomontmorillonite (OMM) and its dendrimers (DOMMT). The novel product was used in the preparation of NR composites with an enhancement in mechanical, thermal properties and increasing the horizontal burning time compared with those for the pure NR. Chen et al. [13] prepared sulphur functionalized halloysite nanotubes (HNTs) as a novel functional rubber vulcanizator instead of sulphur. This novel vulcanizing agent HNTs-s-S gives rise to significant improvement in rubber-filler interfacial adhesion in comparison to NR/HNTs without supported sulphur. Incorporation of nanoclay to reinforce and compatibilize polymer blends is an innovative technology of improving the performance of rubber blends based on the concept of polymerpolymer nanocomposites. Some examples including NR/NBR blend [18], epoxy/carboxyl terminated poly (butadiene-co-acrylonitrile) liquid rubber (CTBN) [19], NR/SBR blend [20], ethylene propylene diene monomer/silicon rubber EPDM/SiR blend [21], NBR/NR blend [22], poly(lactic acid)/NR blend [23], NBR and poly(ethylene-covinyl acetate) blend [24]. The most common method used to prepare rubber/organoclay compounds is melt mixing, because it is economical, more flexible in formulation and can be used without consuming organic solvents. However, the simple melt compounding of rubber and nanofiller does not often contribute to the good filler dispersion, because the difference in their surface energy is larger. To overcome this problem different ways were used to facilitate dispersion of nanoclay and to enhance its interaction with rubber matrix. For instance, preintercalation of nanoclay by suitable surfactant [5,25] and the use of polymer component as dispersing agent during melt mixing process. Rajasekar et al. [26–28] have used epoxidized natural rubber ENR as dispersing agent for nanoclay dispersion in different rubbers like NR, (SBR and EPR) as well as NBR using solvent casting method followed by melt compounding with rubber. Zhixin et al. [29] also used ENR as dispersing agent for halloysite nanotubes in SBR following the procedure of Rajasekar et al.
In this study a simple and direct solution casting method was applied for the preparation of polystyrene-organoclay (PS-OC) composites. Then different concentrations of (PS-OC) containing two types of organoclay were compounded with NBR. The structure and morphology of the composites were examined by XRD, FTIR, and SEM. The effect of (PS-OC) content as well as organoclay modifier type on the rheological characteristics and physicomechanical properties of the prepared NBR compounds were studied in comparison with unfilled NBR compound.
NBR is a high performance rubber with a combination of excellent mechanical properties, abrasion resistance, chemical resistance especially resistance to hydrocarbons and oils. Such a high performance elastomeric material can be used in many diverse applications. It should be combined with the filler yielding utterly applicable nanocomposites. It is commonly considered as the work horse of industrial and automotive rubber products industries and is widely used in applications where oil resistance is required [30–33].
2.3. Preparation of organoclay
2. Experimental 2.1. Materials Acrylonitrile butadiene rubber (NBR) containing 32% acrylonitrile content with specific gravity 1.17 ± 0.005 was obtained from Bayer AG, Germany. Commercial grades of polystyrene (PS) Kaofu, Chemical Corp general purpose, Twain. Sodium bentonite clay powder (mesh size 300 mm), with a cationic exchange capacity (CEC) of 90 mequiv per 100 g. 1,12-dibromododecane, bromododecane and hydroxyurea were from Sigma-Aldrich Co., Germany. Compounding additives like sulphur, zinc oxide, stearic acid, tetra methyl thiuram disulfide (TMTD) and N-cyclohexyl -2benzothiazyl sulphenamide (CBS) and polymerized 2,2,4-trime thyl-1,2dihydroquinoline (TMQ) were of commercial grades and purchased from Sigma-Aldrich Co.
2.2. Preparation of quaternary ammonium surfactant salts 1, 12-dibromododecane or bromododecane (0.1 mol) was condensed separately with hydroxyurea (0.2 mol or 0.1 mol) in isopropanol (100 ml). The reaction mixtures were refluxed for 3 h and allowed to cool overnight. Then, the solid products were recrystallized from ethanol and dried under vacuum at 40 °C. The products were namely as 1, 12 di-bromododecylhydroxyuronium and, bromododecylhydroxyuronium with the structures shown below as formulas (I) and (II), respectively [34].
Modified bentonite was synthesized by reacting Na-bentonite with the prepared quaternary ammonium surfactant salt solutions of 1,12 dibromododecylhydroxyuronium or bromododecylhydroxyuronium. Atypical experimental procedure was as previously explained according to Sadek et al. method [5]. The modified clay samples were expressed as OC1 and OC2 according to the used surfactants I and II, respectively.
Please cite this article in press as: E.M. Sadek et al., Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.04.007
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2.4. Preparation of PS/clay nanocomposites A colloidal suspension of each orgnoclay in toluene (25 wt%) was mixed separately with a solution of PS in toluene (10 wt%) and was stirred for 24 h with a mechanical stirrer. The final solu-
tions were cast on Petri dishes and left for 5 days under hood in order to complete evaporation of the solvent. The obtained films were pelletized and dried at 80 °C under vacuum 200 mbar for the subsequent 24 h [27]. The products were termed as PS-OC1 and PS-OC2.
Table 1 Formulations of NBR compounds. Designation and Compound numbers NBR 1 Ingredients NBR Stearic acid Zinc oxide TMTDb Sulphur CBSc TMQd PS-OC1 PS-OC2 a b c d
100 1 3 0.5 2 1 1 – –
NBR/PS-OC1 2 100 1 3 0.5 2 1 1 4 –
NBR/PS-OC2 3
4
5
6
7
100 1 3 0.5 2 1 1 6 –
Contents (phr)a 100 1 3 0.5 2 1 1 10 –
100 1 3 0.5 2 1 1 – 4
100 1 3 0.5 2 1 1 – 6
100 1 3 0.5 2 1 1 – 10
Part per hundred parts of rubber. Tetramethylthiuramdisulphide. N-cyclohexyl -2-benzothiazyl sulphenamide. Polymerized 2,2,4-trimethyl-1,2-dihydroquinolin.
Fig. 1. XRD pattern of (a) OC1, PS-OC1 and NBR/PS-OC1 (6 phr) as well as (b) OC2, PS-OC2 and NBR/PS-OC2 (6 phr) nanocomposites.
Please cite this article in press as: E.M. Sadek et al., Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.04.007
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2.5. Compounding The NBR compounds were prepared in two roll-mixing mills (outside diameter 470 mm, working distance 300 mm, speed of slow roll 24 rpm and fraction ratio of 1: 1.4) in accordance with ASTM D 15 72, 2007.Vulcanization of the rubber compounds was carried out in an electrically heated hydraulic press at 162 ± 1 °C and a pressure of about 4 MPa for the optimum cure time (Tc90), previously determined from Monsanto Rheometer. The prepared NBR formulations are given in Table 1 with 7 samples. Samples with number 2,3,4 corresponding to filled NBR vulcanizate with PS-OC1 at loading 4,6,10 phr. Samples with number 5,6,7 corresponding to filled NBR vulcanizate with PS-OC2 at the same loading in comparison to sample 1 corresponding to neat NBR vulcanizate.
sample was mounted on a standard specimen stub. A thin coating of gold was deposited into the sample surface. The middle part of the cross section of the specimens was chosen as representative of the overall morphology. The rheometric behavior of the rubber compounds was determined using an oscillating disc Monsanto rheometer model 100 as per ASTM D-2084-07,2007. The physicomechanical properties (viz., the tensile strength, elongation at break and Young’s modulus) of the rubber compounds were determined using an electronic Zwick tensile testing machine, model 1425, in accordance with ASTM D-412-06, 2006. The hardness of test specimens was measured using durometer Shore A according to ASTM D2240, 2005.
3. Results and discussion 3.1. X-ray diffraction (XRD)
2.6. Characterization X-ray diffraction measurements were performed on a Philip’s Xray diffractometer PW1390 with Cu Ka radiation (k = 0.154 nm, 45 kV and 30 mA) at room temperature (25 °C). The diffraction angle, 2h, was scanned at a rate of 2° min 1. Transmission electron microscopy micrographs were taken using a (TEM; product name: JX 1230; manufactured by JEOL (Japan)) with micro-analyzer electron probe. Infrared spectroscopy was conducted using (FTIR; product name: spectrometer-FTIR-430; manufactured by JASCO (Japan)) equipped with KBr discs, and operating in 400–4000 cm 1 range. Phase morphologies were studied using a scanning electron microscope (SEM; product name: JSM-T25; manufactured by JEOL (Japan)). For scanning electron observation the surface of the
The XRD pattern of (a) OC1, PS-OC1 and NBR/PS-OC1 (6 phr) as well as of (b) OC2, PS-OC2 and NBR/PS-OC2 (6 phr) is shown in Fig. 1. Fig. 1(a) for OC1 indicated an intense peak around 2h = 6.0 99°, corresponding to the basal spacing of 14.47 Å (d001). The PSOC1 pattern depicted that the d001main diffraction peak had been shifted towards the lower angle 2h = 1.849°, corresponding to the basal spacing of 47.742 Å (d001). This observation suggested the formation of intercalated-exfoliated structure. This was in accordance to the work of Rajasekar et al. [28] upon using Cloisite 20 A nanoclay/epoxidized natural rubber for NBR nanocomposites. On the other hand, for ogranoclay (OC2) Fig. 1(b), a diffraction peak emerged at 2h = 6.78°, corresponding to the basal spacing of 13.02 Å. The PS-OC2 pattern indicated that the diffraction peak had been
b
a
c
dc
Fig. 2. TEM of (a) OC1, (b) OC2, (c) PS-OC1 and (d) PS-OC2 composites.
Please cite this article in press as: E.M. Sadek et al., Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.04.007
E.M. Sadek et al. / Egyptian Journal of Petroleum xxx (2018) xxx–xxx
shifted to the lower angle 2h = 6.289°, corresponding to the basal spacing of 14.04 Å. This assigned an intercalated structure. Thus, XRD pattern showed the correlation between the layer spacing of modified clay (OC1 and OC2) and the structure of the used surfactants as clay modifiers. It was found that the basal spacing of modified clay (OC1) increases (i.e. shift to lower angle) with incorporation of dimeric cationic surfactant (bola form) having (CH2)12 in comparison to its monomeric structure for (OC2). Final results showed that dimeric surfactant-organoclay, had better intercalation than monomeric one [25]. The absence of the (d001) diffraction peak in the small angle range (1–10°) for the compounds NBR/PS-OC (6 phr) Fig. 1(a,b), provided strong evidence of the insertion of the NBR into the silicate galleries, disrupts the regular stacked-layer structure of the organoclay and gives rise to an exfoliated structure. That is, the tactoids of layered silicates tend to disaggregate to individual sheets, which are uniformly dispersed into the rubber matrix.
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3.2. Transmission electron microscope (TEM) TEM images of (a) OC1, (b) OC2, (c) PS-OC1 and (d) PS-OC2 have been shown in Fig. 2. TEM observations indicated that the modified clay is dispersed at a nanometric level. Since no multilayer bundles or agglomerates are present. It is clearly seen, the intercalated structure of modified clay by the used surfactants, each dark line is corresponding to several intercalated clay layers, as shown in TEM micrographs Fig. 2(a,b). In Fig. 2(c,d), the darker phase corresponds to dispersed organoclay particles, where it is apparent that they are uniformly dispersed in PS. The layers are not entirely orderly intercalative structure, but basically orderly and partially disorderly exfoliated morphology. The related structure can be referred as partially exfoliated and intercalated one (indicated by circles in Fig. 2(c)). While using OC2, a platelet structure led to an intercalated composite in Fig. 2(d).
Fig. 3. FTIR of (a) PS and PS-OC1, (b) NBR, NBR/PS-OC1 (6 phr) and NBR/PS-OC2 (6 phr) composites.
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This indicated that the incorporation of PS into OC1 galleries with surface polarity (i.e. more NH2 and OH groups) and (CH2)12 as hydrocarbon species led to the formation of intercalated- partially exfoliated structure for PS-OC1 layers in comparison to PSOC2 with intercalated state. Thus, TEM pattern was well corresponded to the result of increased d-spacing of PS-OC1 as previously observed at the XRD data in Fig. 1(a). 3.3. Fourier transform infrared (FTIR) spectroscopy The FTIR spectra of pure PS and PS-OC1 in the wave number at 4000–500 cm 1 are demonstrated in Fig. 3(a). The peaks at 3024, 2920, 1720, 1460, and 1056 cm 1 are the characteristic of PS. The characteristic peaks at 1595.8 and 1491 are attributed to stretching vibration for benzene ring of PS. In the case of PS-OC1, the vibration peaks (1600, 1475 cm 1) shifted to (1633, 1493 cm 1). This shift in peak position of bands might be attributed to the hydrophobic interaction of (C12) of the clay modifier with nonpolar PS via Van der Wall interaction. In Fig. 3(a) for PS-OC1, the appearance of new peaks at 2925 and 2854 cm 1 is attributed to the C-H asymmetric and symmetric stretching vibrations of the used surfactant. This was a clear evidence of the surfactant existence between the clay layers. Also, the presence of the common peaks of the clay besides the above mentioned peaks indicated that the intercalation of the surfactant between the clay layers occured without altering its chemical structure. Fig. 3(b) shows the FTIR of NBR/PS-OC1 (6 phr) and NBR/PS-OC2 (6 phr) nanocomposites as compared with that of pure NBR. The
slightly shift in the NBR peaks was attributed to the hydrophobic interaction of PS with NBR chains.
3.4. Scanning electron microscope (SEM) Fig. 4 displays SEM morphology images of (a) NBR, (b) NBR/PSOC1 (6 phr), (c) NBR/PS-OC2 (6 phr) and (d) NBR/PS-OC1 (10 phr). Fig. 4(a), revealed a very smooth surface of the unfilled NBR matrix with some small curatives particles. In addition, Fig. 4(b and c), depicted the uniform dispersion of PS-OC (6 phr) in NBR matrix. This reflected filler- rubber interaction at (6 phr) filler loading. On the other hand, at higher loading of PS-OC1 (10 phr), Fig. 4(d), the increased filler-filler interaction led to the formation of nanoparticles agglomeration with bad distribution in NBR. This observation was in agreement with Rajasekar et al. [28].
3.5. Rheometric properties The rheometric characteristics of the rubber compounds, expressed in terms of the minimum torque (ML), the maximum torque (MH), scorch time (Ts2), the optimum cure time (Tc90), and cure rate index (CRI) were calculated and the results are tabulated in Table 2. Minimum torque (i.e. the torque at the initial stage of vulcanization) is related to the viscosity of the compounds [35]. The presence of PS-OC generated an increase in the viscosity of the compounds, consequently an increase in ML.
a
b
c
d
Fig. 4. SEM of (a) NBR, (b) NBR/PS- OC1 (6 phr), (c) NBR/PS-OC2 (6 phr) and (d) NBR/PS-OC1 (10 phr) nanocomposites.
Please cite this article in press as: E.M. Sadek et al., Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.04.007
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E.M. Sadek et al. / Egyptian Journal of Petroleum xxx (2018) xxx–xxx Table 2 Rheometric characteristics of NBR compounds at 162 ± 1 °C. Sample code
ML (dN M)
MH (dN M)
T s2 (min)
Tc90 (min)
CRI
1 2 3 4 5 6 7
3.00 8.50 10.00 9.00 7.00 9.00 8.00
45.0 71.0 75.0 74.0 65.0 72.0 70.0
1.45 1.20 1.20 1.20 1.00 1.00 1.00
6.00 5.50 5.30 10.00 5.00 5.30 10.00
21.98 23.25 24.39 11.36 22.22 23.25 11.11
Minimum torque (ML), maximum torque (MH), scorch time (Ts2), optimum cure time (Tc90), and cure rate index (CRI).
Fig. 5. Mechanical properties of NBR nanocomposites, (a) Tensile strength, (b) Young’s modulus, (c) Elongation at break % and (d) Hardness Shore A.
Maximum torque depends on both the extent of crosslinking and the reinforcement by filler particles in the polymer matrix [35]. As expected the incorporation of PS-OC in NBR matrix resulted in a significant increase in the maximum torque than that of neat NBR compound. The increase in the maximum torque was most pronounced for rubber composites loaded with 6 phr of PS-OC1 (sample No. 3) in comparison to compound NBR with PS-OC2 (sample No. 6) at the same loading. This increase in the maximum torque is an indication for improved adhesion between NBR and PS-OC due to additional crosslinks formation through NBR and PS-OC at 6 phr loading. This may be due to the hydrophobic interaction of PS with NBR rubber chains via Van der Wall interaction. This consequently resulted to have reinforcement effect and increased the extent of crosslinking for the PS-OC1 (6 phr) in the matrix in comparison with PS-OC2 at the same loading. This led
to an increase in MH for compounds containing PS-OC1 with respect to that for compounds with PS-OC2. Further increase in PS-OC1 loading (10 phr, sample No. 4) led to filler agglomeration in the NBR matrix. Hence, the reinforcing efficiency of the nanofiller in the matrix was less pronounced for the compound NBR/PS-OC1 (10 phr) compared to that of NBR/PS-OC1 (6 phr). The SEM micrograph (Fig. 4d) confirmed the filler agglomeration in NBR/PS-OC1 (10 phr) and this was responsible for the decrease in maximum torque values for these compounds as indicated before in Table 2. The same trend was found for the compound NBR/PS-OC2 (10 phr sample No. 7) in comparison with compound NBR/PS-OC2 (6 phr sample No.6). Conversely, the scorch time Ts2 (the initial vulcanization time) and the cure time Tc90 (time for 90% cure) for the NBR/PS-OC composites were lower compared to that of unfilled NBR. The addition of PS-OC to the NBR produced a little effect on the scorch time of
Please cite this article in press as: E.M. Sadek et al., Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.04.007
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the compounds. Ts2 values were slightly lower in the filled vulcanizates than the unfilled NBR. This may be due to the adsorption of curatives is on the filler surface at the beginning of curing. The addition of PS-OC to the NBR accelerated the cure kinetics as observed by the reduction of the optimum cure time Tc90. This effect was proportional to the amount of the PS-OC added being maximum reduction at 6 phr. The cure rate index (CRI), which indicates the rate of cure of the compounds, was defined as 100/(Tc90-Ts2).A higher value of CRI means a higher rate of vulcanization. The incorporation of PS-OC accelerated the vulcanization process and the curing rate increased with PS-OC content up to 6 phr. This may be attributed to the basic nature of the filler that is potentially increased with the treatment with the used surfactants. It is known that a basic medium has an activating effect on the vulcanization reaction. Since the amine groups and Sulphur form a transition metal complex [20,36,37]. The possible formation of Zn complex in which sulphur and amine groups participate may facilitate the increase in cure rate and consequently faster scorch time and cure time. Thus, one can expect that modification of clay with bola surfactant I with more amine groups may cause an increase in scorch time and cure time and consequently faster cure rate in comparison with cationic surfactant II.
3.6. Physico-mechanical properties Fig. 5(a–d) illustrates the mechanical properties of the NBR compounds. NBR compounds corresponding to samples No. 2–7 filled with different loadings of PS-OC (i.e. 4,6,10 phr) illustrated meaningful improvements in the tensile strength (Fig. 5a) and Young’s modulus (Fig. 5b) compared to neat NBR compound (sample No. 1). The enhancement in tensile strength and modulus of the (PSOC) filled composites was found with an increase in the PS-OC loading up to 6 phr. These results are thought to be the result of improved dispersion of silicates in NBR matrix by the application of PS-OC, as confirmed before by SEM photographs Fig. 4(b,c). It is also noted that the improvement in reinforcement efficiency due to the incorporation of nanoclay occurred when there is a proper transfer of applied stress to the nanoclay platelets. Such a stress transfer is only possible with suitable adhesion of the polymer matrix on the nanoclay to be loading bearing [3,38,39]. Compounds filled with PS-OC1 corresponding to (samples No. 2–4) exhibited higher tensile strength and Young’s modulus values, compared to those with PS-OC2 (samples No. 5–7). This was consistent to the maximum torque values (Table 2) and XRD Fig. 1. The drop in tensile strength and Young’s modulus at higher PSOC loading (10 phr) might be due to inhomogenous dispersion in NBR matrix with aggregation formation as proved by SEM photograph for PO-OC1 (Fig. 4d). This led to increase defects number. These defects are prone to resulting in stress concentration and leading to strength decreasing of the elastomer [3,5]. The elongation at break of NBR compounds was found to decrease gradually with increasing filler content. This was consequence of the stiffening effect exerted by the organoclay added. Also, the reinforcing efficiency of the organoclay and the increase in the crosslinking degree should be accompanied with a reduction in elongation at break percentages. One can expect that compounds filled with 4,6,10 phr PS-OC1 (samples No. 2–4) exhibited less elongation at break values compared to those with PS-OC2 (samples No. 5–7) at the same loading as shown in Fig. 5c. Similar results were reported by Rajasekar et al. [28], and Cataldo [40]. This was mainly due to an increase in the degree of crosslinking and restriction in mobility of the rubber chains inside the intercalated nanolayers of clay as mentioned before in rheometric section.
The hardness of the NBR compounds was found to increase with increasing filler loading as shown in Fig. 5d. Hardness Shore A passed from a value of 40 in unfilled rubber to 90 and 50 in filled rubber with PS-OC1 (10 phr sample No. 4) and PS-OC2 (10 phr sample No. 7), respectively. This is obvious from the fact that with increase in filler loading the modulus increases and hardness, being a surface property, also reflects a similar trend. This enhancement in hardness for NBR/PS-OC1 compounds was related to a higher stiffness of the nanocomposites based on PS-OC1 in comparison with those based on PS-OC2 which may be due to the same reason explained earlier. 4. Conclusions The dual process of solution casting and melt mixing was used for synthesis of acrylonitrile butadiene rubber/clay nanocomposites. Two types of organically modified clay (OC1 and OC2) based on 1,12di-bromo dodecylhydroxyuronium and bromododecylhydroxyuronium cationic surfactants were prepared. Incorporation of organoclay in PS was done by solution casting method given two types of composites. PS-OC1 composite exhibited an intercalated/exfoliated structure while upon dispertion of OC2 in PS an intercalated structure was formed. Compounding of PS-OC (6 phr) in NBR matrix led to exfoliated composites with fine dispertion of PS-OC in NBR as indicated by XRD and SEM, respectively. Increasing PS-OC to 10 phr, heterogenous dispersion of nanoparticles in NBR was found leading to irregular agglomerated structure. The properties of NBR can be completely affected by the clay modifier type and content of the prepared PS-organoclay. From rheometric characteristics, acceleration activity with lowering the scorch and cure times and an increase in minimum and maximum torques had been observed for the compounds NBR/PS-OC1 (6 phr) compared to compounds containing PS-OC2 and pure NBR. The mechanical properties (i.e. tensile strength, Young’s modulus and hardness Shore A) were enhanced for these particular compounds. The elongation at break decreased consequently with increasing PS-OC1 in NBR because of increased reinforcement. References [1] K.G. Gatos, J. Karger-Kocsis, in: S. Thomas, R. Stephen (Eds.), Rubber Nanocomposites: Preparation, Properties and Applications, John Wiley & Sons (Asia) Pte Ltd, 2010, chapter 7. [2] E.M. Sadek, D.E. El-Nashar, High Perform. Polym. 24 (7) (2012) 654–663. [3] D.E. El-Nashar, E.M. Sadek, High Perform. Polym. 24 (7) (2012) 664–670. [4] J.P. Fontana, F.F. Camilo, M.A. Bizeto, R. Faez, Appl Clay Sci. 83–84 (2013) 203– 209. [5] E.M. Sadek, D.E. El-Nashar, S.M. Ahmed, Polym. Compos. 36 (2015) 1293–1302. [6] S. He, F. Bai, S. Liu, H. Ma, J. Hu, L. Chen, J. Lin, G. Wei, X. Du, Polym. Test 64 (2017) 92–100. [7] B. Di Credico, E. Cobania, E. Callone, L. Conzatti, D. Cristofori, M. Arienzo, S. Dirè, L. Giannini, T. Hanel, R. Scotti, P. Stagnaro, L. Tadiello, F. Morazzoni, Appl. Clay Sci. 152 (2018) 51–64. [8] A.P. Meera, S.P. Thomas, S. Thomas, Polym. Compos. 33 (2012) 524–531. [9] S.C. George, R. Rajan, A.S. Aprem, S. Thomas, S.S. Kim, Polym. Test 51 (2016) 165–173. [10] U. Sookyung, C. Nakason, N. Venneman, W. Thaijaroend, Polym. Test 54 (2016) 223–232. [11] U. Sookyung, C. Nakason, W. Thaijaroen, N. Vennemann, Polym. Test. 33 (2014) 48–56. [12] G. Zhang, J. Wang, J. Phys. Chem. Solids 115 (2018) 137–147. [13] L. Chen, Z. Jia, X. Guo, B. Zhong, Y. Chen, Y. Luo, D. Jia, Chem. Eng. J. 336 (2018) 748–756. [14] M. Nematollahi, A. Jalali-Arani, K. Golzar, Appl. Clay Sci. 97–98 (2014) 187– 199. [15] J. Zhang, L. Wang, Y. Zhao, Mater. Des. 50 (2013) 322–331. [16] H. Chen, Y. Li, S. Wang, Y. Li, Y. Zhou, J. Membr. Sci 546 (2018) 22–30. [17] R. Berahman, M. Raiati, M.M. Mazidi, S.M.R. Paran, Mater. Des. 104 (2016) 333–345. [18] H.J. Maria, N. Lyczko, A. Nzihou, K. Joseph, C. Mathew, S. Thomas, Appl. Clay Sci. 87 (2014) 120–128. [19] P. Vijayan, D. Puglia, H.J. Maria, J.M. Kenny, S. Thomas, RSC Adv. 3 (2013) 24634–24643.
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Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber E.M. Sadek a,⇑, D.E. El-Nashar b, S.M. Ahmed a a b
Petrochemical Department, Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt Polymer & Pigment Department, National Research Centre (NRC), Dokki, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 26 October 2017 Revised 13 March 2018 Accepted 24 April 2018 Available online xxxx Keywords: Acrylonitrile butadiene rubber (NBR) Nanocomposites Cationic surfactants Rheometric characteristics Physico-mechanical properties
a b s t r a c t Solvent blending of organoclay (OC) in polystyrene in combination with melt mixing with acrylonitrile butadiene rubber (NBR) are applied to provide acrylonitrile butadiene rubber/clay nanocomposites. Clay structures based on the prepared 1,12 dibromododecylhydroxyuronium and bromo dodecyl hydroxyuronium as cationic surfactants were prepared. Colloidal suspension of each organoclay in toluene was mixed separately with polystyrene giving two types of polystyrene-organoclay composites. The prepared polystyrene-organoclay PS-OC composites were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectroscopy. The results indicate the formation of polystyrene-organoclay composites with intercalated and partial exfoliated structures. The rheometric study demonstrate an increase in minimum and maximum torques with an increase in cure rate index (i.e. faster scorch and cure times) for the PS-OC (6 phr) incorporated NBR compound compared to unfilled NBR. This effect is more significant with NBR/PS-OC1 composites in comparison to NBR/ PS-OC2 composites at the same loading indicating the nanoreinforcement effect of clay on the mechanical properties of these composites. Compounds filled with PS-OC1 exhibit higher tensile strength, Young’s modulus and less elongation at break compared to those compounds filled with PS-OC2. The higher stiffness of PS-OC1/NBR composites reflects the enhancement in hardness of these compounds with respect to those containing PS-OC2. Further increasing of PS-OC loading results in a significant decrease of the properties. This is due to a possible agglomeration formation of PS-OC in rubber matrix. Furthermore, the results of reinforcement of polymers with nanosized particles indicate a promising application of the organoclay and nanocomposites that affords high performance materials. Ó 2018 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction As rubber applications demand compounds with better performance novel rubber composites with improved properties should be developed [1]. Recently, there has been much academic and industrial interest in the production of rubber nanocomposites with layered clay as a substitute for the traditional carbon black due to its high aspect ratio and high surface area. Several reports have shown that the presence of nano-dispersed clay can improve the properties of rubber like acrylonitrile butadiene rubber (NBR) [2–4], styrene butadiene rubber (SBR) [5–7], natural rubber (NR) [8–14], hydrogenated nitrile rubber (HNBR) [15], butyl rubber [16] and silicone rubber (SiR) [17].
Peer review under responsibility of Egyptian Petroleum Research Institute. ⇑ Corresponding author. E-mail address: [email protected] (E.M. Sadek).
Sadek et al. [2,3] found that maleic acid anhydride/organoclay MOC at 3 phr content had a great effect on the rheometric and swelling characteristics [2] as well as physico-mechanical properties and thermo-oxidative aging of the NBR nanocomposites [3]. Sadek et al. [5] also studied the nanoreinforcement of SBR by using organoclay modified with anionic and nonionic surfactants. The results showed simultaneous improvement in both rheometric and mechanical properties with enhancement in aging resistance of rubber nanocomposites formed with 6 phr modified organoclay with mixed surfactants. Di Credico et al. [7] prepared organoclay modified sepiolite fibers with reduced particle size and increased silanol groups on the surface layer and were used in the preparation of SBR nanocomposites with enhanced mechanical properties. Thomas et al. [8] found that the gas-barrier properties of NR was reduced on the inclusion of organically modified montmorillonite clay compared to the neat rubber properties. Thomas et al. [9] also studied the effect of clay type and content as well as vulcanizing systems on the mechanical properties of NR-clay nanocomposites.
https://doi.org/10.1016/j.ejpe.2018.04.007 1110-0621/Ó 2018 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: E.M. Sadek et al., Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.04.007
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The authors found that conventionally vulcanized nanofilled samples exhibited good improvement in mechanical properties. Guangjian and Jincheng [12] prepared and characterized a new class of effective and friendly flame-retardant organoclays based on organomontmorillonite (OMM) and its dendrimers (DOMMT). The novel product was used in the preparation of NR composites with an enhancement in mechanical, thermal properties and increasing the horizontal burning time compared with those for the pure NR. Chen et al. [13] prepared sulphur functionalized halloysite nanotubes (HNTs) as a novel functional rubber vulcanizator instead of sulphur. This novel vulcanizing agent HNTs-s-S gives rise to significant improvement in rubber-filler interfacial adhesion in comparison to NR/HNTs without supported sulphur. Incorporation of nanoclay to reinforce and compatibilize polymer blends is an innovative technology of improving the performance of rubber blends based on the concept of polymerpolymer nanocomposites. Some examples including NR/NBR blend [18], epoxy/carboxyl terminated poly (butadiene-co-acrylonitrile) liquid rubber (CTBN) [19], NR/SBR blend [20], ethylene propylene diene monomer/silicon rubber EPDM/SiR blend [21], NBR/NR blend [22], poly(lactic acid)/NR blend [23], NBR and poly(ethylene-covinyl acetate) blend [24]. The most common method used to prepare rubber/organoclay compounds is melt mixing, because it is economical, more flexible in formulation and can be used without consuming organic solvents. However, the simple melt compounding of rubber and nanofiller does not often contribute to the good filler dispersion, because the difference in their surface energy is larger. To overcome this problem different ways were used to facilitate dispersion of nanoclay and to enhance its interaction with rubber matrix. For instance, preintercalation of nanoclay by suitable surfactant [5,25] and the use of polymer component as dispersing agent during melt mixing process. Rajasekar et al. [26–28] have used epoxidized natural rubber ENR as dispersing agent for nanoclay dispersion in different rubbers like NR, (SBR and EPR) as well as NBR using solvent casting method followed by melt compounding with rubber. Zhixin et al. [29] also used ENR as dispersing agent for halloysite nanotubes in SBR following the procedure of Rajasekar et al.
In this study a simple and direct solution casting method was applied for the preparation of polystyrene-organoclay (PS-OC) composites. Then different concentrations of (PS-OC) containing two types of organoclay were compounded with NBR. The structure and morphology of the composites were examined by XRD, FTIR, and SEM. The effect of (PS-OC) content as well as organoclay modifier type on the rheological characteristics and physicomechanical properties of the prepared NBR compounds were studied in comparison with unfilled NBR compound.
NBR is a high performance rubber with a combination of excellent mechanical properties, abrasion resistance, chemical resistance especially resistance to hydrocarbons and oils. Such a high performance elastomeric material can be used in many diverse applications. It should be combined with the filler yielding utterly applicable nanocomposites. It is commonly considered as the work horse of industrial and automotive rubber products industries and is widely used in applications where oil resistance is required [30–33].
2.3. Preparation of organoclay
2. Experimental 2.1. Materials Acrylonitrile butadiene rubber (NBR) containing 32% acrylonitrile content with specific gravity 1.17 ± 0.005 was obtained from Bayer AG, Germany. Commercial grades of polystyrene (PS) Kaofu, Chemical Corp general purpose, Twain. Sodium bentonite clay powder (mesh size 300 mm), with a cationic exchange capacity (CEC) of 90 mequiv per 100 g. 1,12-dibromododecane, bromododecane and hydroxyurea were from Sigma-Aldrich Co., Germany. Compounding additives like sulphur, zinc oxide, stearic acid, tetra methyl thiuram disulfide (TMTD) and N-cyclohexyl -2benzothiazyl sulphenamide (CBS) and polymerized 2,2,4-trime thyl-1,2dihydroquinoline (TMQ) were of commercial grades and purchased from Sigma-Aldrich Co.
2.2. Preparation of quaternary ammonium surfactant salts 1, 12-dibromododecane or bromododecane (0.1 mol) was condensed separately with hydroxyurea (0.2 mol or 0.1 mol) in isopropanol (100 ml). The reaction mixtures were refluxed for 3 h and allowed to cool overnight. Then, the solid products were recrystallized from ethanol and dried under vacuum at 40 °C. The products were namely as 1, 12 di-bromododecylhydroxyuronium and, bromododecylhydroxyuronium with the structures shown below as formulas (I) and (II), respectively [34].
Modified bentonite was synthesized by reacting Na-bentonite with the prepared quaternary ammonium surfactant salt solutions of 1,12 dibromododecylhydroxyuronium or bromododecylhydroxyuronium. Atypical experimental procedure was as previously explained according to Sadek et al. method [5]. The modified clay samples were expressed as OC1 and OC2 according to the used surfactants I and II, respectively.
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2.4. Preparation of PS/clay nanocomposites A colloidal suspension of each orgnoclay in toluene (25 wt%) was mixed separately with a solution of PS in toluene (10 wt%) and was stirred for 24 h with a mechanical stirrer. The final solu-
tions were cast on Petri dishes and left for 5 days under hood in order to complete evaporation of the solvent. The obtained films were pelletized and dried at 80 °C under vacuum 200 mbar for the subsequent 24 h [27]. The products were termed as PS-OC1 and PS-OC2.
Table 1 Formulations of NBR compounds. Designation and Compound numbers NBR 1 Ingredients NBR Stearic acid Zinc oxide TMTDb Sulphur CBSc TMQd PS-OC1 PS-OC2 a b c d
100 1 3 0.5 2 1 1 – –
NBR/PS-OC1 2 100 1 3 0.5 2 1 1 4 –
NBR/PS-OC2 3
4
5
6
7
100 1 3 0.5 2 1 1 6 –
Contents (phr)a 100 1 3 0.5 2 1 1 10 –
100 1 3 0.5 2 1 1 – 4
100 1 3 0.5 2 1 1 – 6
100 1 3 0.5 2 1 1 – 10
Part per hundred parts of rubber. Tetramethylthiuramdisulphide. N-cyclohexyl -2-benzothiazyl sulphenamide. Polymerized 2,2,4-trimethyl-1,2-dihydroquinolin.
Fig. 1. XRD pattern of (a) OC1, PS-OC1 and NBR/PS-OC1 (6 phr) as well as (b) OC2, PS-OC2 and NBR/PS-OC2 (6 phr) nanocomposites.
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2.5. Compounding The NBR compounds were prepared in two roll-mixing mills (outside diameter 470 mm, working distance 300 mm, speed of slow roll 24 rpm and fraction ratio of 1: 1.4) in accordance with ASTM D 15 72, 2007.Vulcanization of the rubber compounds was carried out in an electrically heated hydraulic press at 162 ± 1 °C and a pressure of about 4 MPa for the optimum cure time (Tc90), previously determined from Monsanto Rheometer. The prepared NBR formulations are given in Table 1 with 7 samples. Samples with number 2,3,4 corresponding to filled NBR vulcanizate with PS-OC1 at loading 4,6,10 phr. Samples with number 5,6,7 corresponding to filled NBR vulcanizate with PS-OC2 at the same loading in comparison to sample 1 corresponding to neat NBR vulcanizate.
sample was mounted on a standard specimen stub. A thin coating of gold was deposited into the sample surface. The middle part of the cross section of the specimens was chosen as representative of the overall morphology. The rheometric behavior of the rubber compounds was determined using an oscillating disc Monsanto rheometer model 100 as per ASTM D-2084-07,2007. The physicomechanical properties (viz., the tensile strength, elongation at break and Young’s modulus) of the rubber compounds were determined using an electronic Zwick tensile testing machine, model 1425, in accordance with ASTM D-412-06, 2006. The hardness of test specimens was measured using durometer Shore A according to ASTM D2240, 2005.
3. Results and discussion 3.1. X-ray diffraction (XRD)
2.6. Characterization X-ray diffraction measurements were performed on a Philip’s Xray diffractometer PW1390 with Cu Ka radiation (k = 0.154 nm, 45 kV and 30 mA) at room temperature (25 °C). The diffraction angle, 2h, was scanned at a rate of 2° min 1. Transmission electron microscopy micrographs were taken using a (TEM; product name: JX 1230; manufactured by JEOL (Japan)) with micro-analyzer electron probe. Infrared spectroscopy was conducted using (FTIR; product name: spectrometer-FTIR-430; manufactured by JASCO (Japan)) equipped with KBr discs, and operating in 400–4000 cm 1 range. Phase morphologies were studied using a scanning electron microscope (SEM; product name: JSM-T25; manufactured by JEOL (Japan)). For scanning electron observation the surface of the
The XRD pattern of (a) OC1, PS-OC1 and NBR/PS-OC1 (6 phr) as well as of (b) OC2, PS-OC2 and NBR/PS-OC2 (6 phr) is shown in Fig. 1. Fig. 1(a) for OC1 indicated an intense peak around 2h = 6.0 99°, corresponding to the basal spacing of 14.47 Å (d001). The PSOC1 pattern depicted that the d001main diffraction peak had been shifted towards the lower angle 2h = 1.849°, corresponding to the basal spacing of 47.742 Å (d001). This observation suggested the formation of intercalated-exfoliated structure. This was in accordance to the work of Rajasekar et al. [28] upon using Cloisite 20 A nanoclay/epoxidized natural rubber for NBR nanocomposites. On the other hand, for ogranoclay (OC2) Fig. 1(b), a diffraction peak emerged at 2h = 6.78°, corresponding to the basal spacing of 13.02 Å. The PS-OC2 pattern indicated that the diffraction peak had been
b
a
c
dc
Fig. 2. TEM of (a) OC1, (b) OC2, (c) PS-OC1 and (d) PS-OC2 composites.
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shifted to the lower angle 2h = 6.289°, corresponding to the basal spacing of 14.04 Å. This assigned an intercalated structure. Thus, XRD pattern showed the correlation between the layer spacing of modified clay (OC1 and OC2) and the structure of the used surfactants as clay modifiers. It was found that the basal spacing of modified clay (OC1) increases (i.e. shift to lower angle) with incorporation of dimeric cationic surfactant (bola form) having (CH2)12 in comparison to its monomeric structure for (OC2). Final results showed that dimeric surfactant-organoclay, had better intercalation than monomeric one [25]. The absence of the (d001) diffraction peak in the small angle range (1–10°) for the compounds NBR/PS-OC (6 phr) Fig. 1(a,b), provided strong evidence of the insertion of the NBR into the silicate galleries, disrupts the regular stacked-layer structure of the organoclay and gives rise to an exfoliated structure. That is, the tactoids of layered silicates tend to disaggregate to individual sheets, which are uniformly dispersed into the rubber matrix.
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3.2. Transmission electron microscope (TEM) TEM images of (a) OC1, (b) OC2, (c) PS-OC1 and (d) PS-OC2 have been shown in Fig. 2. TEM observations indicated that the modified clay is dispersed at a nanometric level. Since no multilayer bundles or agglomerates are present. It is clearly seen, the intercalated structure of modified clay by the used surfactants, each dark line is corresponding to several intercalated clay layers, as shown in TEM micrographs Fig. 2(a,b). In Fig. 2(c,d), the darker phase corresponds to dispersed organoclay particles, where it is apparent that they are uniformly dispersed in PS. The layers are not entirely orderly intercalative structure, but basically orderly and partially disorderly exfoliated morphology. The related structure can be referred as partially exfoliated and intercalated one (indicated by circles in Fig. 2(c)). While using OC2, a platelet structure led to an intercalated composite in Fig. 2(d).
Fig. 3. FTIR of (a) PS and PS-OC1, (b) NBR, NBR/PS-OC1 (6 phr) and NBR/PS-OC2 (6 phr) composites.
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This indicated that the incorporation of PS into OC1 galleries with surface polarity (i.e. more NH2 and OH groups) and (CH2)12 as hydrocarbon species led to the formation of intercalated- partially exfoliated structure for PS-OC1 layers in comparison to PSOC2 with intercalated state. Thus, TEM pattern was well corresponded to the result of increased d-spacing of PS-OC1 as previously observed at the XRD data in Fig. 1(a). 3.3. Fourier transform infrared (FTIR) spectroscopy The FTIR spectra of pure PS and PS-OC1 in the wave number at 4000–500 cm 1 are demonstrated in Fig. 3(a). The peaks at 3024, 2920, 1720, 1460, and 1056 cm 1 are the characteristic of PS. The characteristic peaks at 1595.8 and 1491 are attributed to stretching vibration for benzene ring of PS. In the case of PS-OC1, the vibration peaks (1600, 1475 cm 1) shifted to (1633, 1493 cm 1). This shift in peak position of bands might be attributed to the hydrophobic interaction of (C12) of the clay modifier with nonpolar PS via Van der Wall interaction. In Fig. 3(a) for PS-OC1, the appearance of new peaks at 2925 and 2854 cm 1 is attributed to the C-H asymmetric and symmetric stretching vibrations of the used surfactant. This was a clear evidence of the surfactant existence between the clay layers. Also, the presence of the common peaks of the clay besides the above mentioned peaks indicated that the intercalation of the surfactant between the clay layers occured without altering its chemical structure. Fig. 3(b) shows the FTIR of NBR/PS-OC1 (6 phr) and NBR/PS-OC2 (6 phr) nanocomposites as compared with that of pure NBR. The
slightly shift in the NBR peaks was attributed to the hydrophobic interaction of PS with NBR chains.
3.4. Scanning electron microscope (SEM) Fig. 4 displays SEM morphology images of (a) NBR, (b) NBR/PSOC1 (6 phr), (c) NBR/PS-OC2 (6 phr) and (d) NBR/PS-OC1 (10 phr). Fig. 4(a), revealed a very smooth surface of the unfilled NBR matrix with some small curatives particles. In addition, Fig. 4(b and c), depicted the uniform dispersion of PS-OC (6 phr) in NBR matrix. This reflected filler- rubber interaction at (6 phr) filler loading. On the other hand, at higher loading of PS-OC1 (10 phr), Fig. 4(d), the increased filler-filler interaction led to the formation of nanoparticles agglomeration with bad distribution in NBR. This observation was in agreement with Rajasekar et al. [28].
3.5. Rheometric properties The rheometric characteristics of the rubber compounds, expressed in terms of the minimum torque (ML), the maximum torque (MH), scorch time (Ts2), the optimum cure time (Tc90), and cure rate index (CRI) were calculated and the results are tabulated in Table 2. Minimum torque (i.e. the torque at the initial stage of vulcanization) is related to the viscosity of the compounds [35]. The presence of PS-OC generated an increase in the viscosity of the compounds, consequently an increase in ML.
a
b
c
d
Fig. 4. SEM of (a) NBR, (b) NBR/PS- OC1 (6 phr), (c) NBR/PS-OC2 (6 phr) and (d) NBR/PS-OC1 (10 phr) nanocomposites.
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E.M. Sadek et al. / Egyptian Journal of Petroleum xxx (2018) xxx–xxx Table 2 Rheometric characteristics of NBR compounds at 162 ± 1 °C. Sample code
ML (dN M)
MH (dN M)
T s2 (min)
Tc90 (min)
CRI
1 2 3 4 5 6 7
3.00 8.50 10.00 9.00 7.00 9.00 8.00
45.0 71.0 75.0 74.0 65.0 72.0 70.0
1.45 1.20 1.20 1.20 1.00 1.00 1.00
6.00 5.50 5.30 10.00 5.00 5.30 10.00
21.98 23.25 24.39 11.36 22.22 23.25 11.11
Minimum torque (ML), maximum torque (MH), scorch time (Ts2), optimum cure time (Tc90), and cure rate index (CRI).
Fig. 5. Mechanical properties of NBR nanocomposites, (a) Tensile strength, (b) Young’s modulus, (c) Elongation at break % and (d) Hardness Shore A.
Maximum torque depends on both the extent of crosslinking and the reinforcement by filler particles in the polymer matrix [35]. As expected the incorporation of PS-OC in NBR matrix resulted in a significant increase in the maximum torque than that of neat NBR compound. The increase in the maximum torque was most pronounced for rubber composites loaded with 6 phr of PS-OC1 (sample No. 3) in comparison to compound NBR with PS-OC2 (sample No. 6) at the same loading. This increase in the maximum torque is an indication for improved adhesion between NBR and PS-OC due to additional crosslinks formation through NBR and PS-OC at 6 phr loading. This may be due to the hydrophobic interaction of PS with NBR rubber chains via Van der Wall interaction. This consequently resulted to have reinforcement effect and increased the extent of crosslinking for the PS-OC1 (6 phr) in the matrix in comparison with PS-OC2 at the same loading. This led
to an increase in MH for compounds containing PS-OC1 with respect to that for compounds with PS-OC2. Further increase in PS-OC1 loading (10 phr, sample No. 4) led to filler agglomeration in the NBR matrix. Hence, the reinforcing efficiency of the nanofiller in the matrix was less pronounced for the compound NBR/PS-OC1 (10 phr) compared to that of NBR/PS-OC1 (6 phr). The SEM micrograph (Fig. 4d) confirmed the filler agglomeration in NBR/PS-OC1 (10 phr) and this was responsible for the decrease in maximum torque values for these compounds as indicated before in Table 2. The same trend was found for the compound NBR/PS-OC2 (10 phr sample No. 7) in comparison with compound NBR/PS-OC2 (6 phr sample No.6). Conversely, the scorch time Ts2 (the initial vulcanization time) and the cure time Tc90 (time for 90% cure) for the NBR/PS-OC composites were lower compared to that of unfilled NBR. The addition of PS-OC to the NBR produced a little effect on the scorch time of
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the compounds. Ts2 values were slightly lower in the filled vulcanizates than the unfilled NBR. This may be due to the adsorption of curatives is on the filler surface at the beginning of curing. The addition of PS-OC to the NBR accelerated the cure kinetics as observed by the reduction of the optimum cure time Tc90. This effect was proportional to the amount of the PS-OC added being maximum reduction at 6 phr. The cure rate index (CRI), which indicates the rate of cure of the compounds, was defined as 100/(Tc90-Ts2).A higher value of CRI means a higher rate of vulcanization. The incorporation of PS-OC accelerated the vulcanization process and the curing rate increased with PS-OC content up to 6 phr. This may be attributed to the basic nature of the filler that is potentially increased with the treatment with the used surfactants. It is known that a basic medium has an activating effect on the vulcanization reaction. Since the amine groups and Sulphur form a transition metal complex [20,36,37]. The possible formation of Zn complex in which sulphur and amine groups participate may facilitate the increase in cure rate and consequently faster scorch time and cure time. Thus, one can expect that modification of clay with bola surfactant I with more amine groups may cause an increase in scorch time and cure time and consequently faster cure rate in comparison with cationic surfactant II.
3.6. Physico-mechanical properties Fig. 5(a–d) illustrates the mechanical properties of the NBR compounds. NBR compounds corresponding to samples No. 2–7 filled with different loadings of PS-OC (i.e. 4,6,10 phr) illustrated meaningful improvements in the tensile strength (Fig. 5a) and Young’s modulus (Fig. 5b) compared to neat NBR compound (sample No. 1). The enhancement in tensile strength and modulus of the (PSOC) filled composites was found with an increase in the PS-OC loading up to 6 phr. These results are thought to be the result of improved dispersion of silicates in NBR matrix by the application of PS-OC, as confirmed before by SEM photographs Fig. 4(b,c). It is also noted that the improvement in reinforcement efficiency due to the incorporation of nanoclay occurred when there is a proper transfer of applied stress to the nanoclay platelets. Such a stress transfer is only possible with suitable adhesion of the polymer matrix on the nanoclay to be loading bearing [3,38,39]. Compounds filled with PS-OC1 corresponding to (samples No. 2–4) exhibited higher tensile strength and Young’s modulus values, compared to those with PS-OC2 (samples No. 5–7). This was consistent to the maximum torque values (Table 2) and XRD Fig. 1. The drop in tensile strength and Young’s modulus at higher PSOC loading (10 phr) might be due to inhomogenous dispersion in NBR matrix with aggregation formation as proved by SEM photograph for PO-OC1 (Fig. 4d). This led to increase defects number. These defects are prone to resulting in stress concentration and leading to strength decreasing of the elastomer [3,5]. The elongation at break of NBR compounds was found to decrease gradually with increasing filler content. This was consequence of the stiffening effect exerted by the organoclay added. Also, the reinforcing efficiency of the organoclay and the increase in the crosslinking degree should be accompanied with a reduction in elongation at break percentages. One can expect that compounds filled with 4,6,10 phr PS-OC1 (samples No. 2–4) exhibited less elongation at break values compared to those with PS-OC2 (samples No. 5–7) at the same loading as shown in Fig. 5c. Similar results were reported by Rajasekar et al. [28], and Cataldo [40]. This was mainly due to an increase in the degree of crosslinking and restriction in mobility of the rubber chains inside the intercalated nanolayers of clay as mentioned before in rheometric section.
The hardness of the NBR compounds was found to increase with increasing filler loading as shown in Fig. 5d. Hardness Shore A passed from a value of 40 in unfilled rubber to 90 and 50 in filled rubber with PS-OC1 (10 phr sample No. 4) and PS-OC2 (10 phr sample No. 7), respectively. This is obvious from the fact that with increase in filler loading the modulus increases and hardness, being a surface property, also reflects a similar trend. This enhancement in hardness for NBR/PS-OC1 compounds was related to a higher stiffness of the nanocomposites based on PS-OC1 in comparison with those based on PS-OC2 which may be due to the same reason explained earlier. 4. Conclusions The dual process of solution casting and melt mixing was used for synthesis of acrylonitrile butadiene rubber/clay nanocomposites. Two types of organically modified clay (OC1 and OC2) based on 1,12di-bromo dodecylhydroxyuronium and bromododecylhydroxyuronium cationic surfactants were prepared. Incorporation of organoclay in PS was done by solution casting method given two types of composites. PS-OC1 composite exhibited an intercalated/exfoliated structure while upon dispertion of OC2 in PS an intercalated structure was formed. Compounding of PS-OC (6 phr) in NBR matrix led to exfoliated composites with fine dispertion of PS-OC in NBR as indicated by XRD and SEM, respectively. Increasing PS-OC to 10 phr, heterogenous dispersion of nanoparticles in NBR was found leading to irregular agglomerated structure. The properties of NBR can be completely affected by the clay modifier type and content of the prepared PS-organoclay. From rheometric characteristics, acceleration activity with lowering the scorch and cure times and an increase in minimum and maximum torques had been observed for the compounds NBR/PS-OC1 (6 phr) compared to compounds containing PS-OC2 and pure NBR. The mechanical properties (i.e. tensile strength, Young’s modulus and hardness Shore A) were enhanced for these particular compounds. The elongation at break decreased consequently with increasing PS-OC1 in NBR because of increased reinforcement. References [1] K.G. Gatos, J. Karger-Kocsis, in: S. Thomas, R. Stephen (Eds.), Rubber Nanocomposites: Preparation, Properties and Applications, John Wiley & Sons (Asia) Pte Ltd, 2010, chapter 7. [2] E.M. Sadek, D.E. El-Nashar, High Perform. Polym. 24 (7) (2012) 654–663. [3] D.E. El-Nashar, E.M. Sadek, High Perform. Polym. 24 (7) (2012) 664–670. [4] J.P. Fontana, F.F. Camilo, M.A. Bizeto, R. Faez, Appl Clay Sci. 83–84 (2013) 203– 209. [5] E.M. Sadek, D.E. El-Nashar, S.M. Ahmed, Polym. Compos. 36 (2015) 1293–1302. [6] S. He, F. Bai, S. Liu, H. Ma, J. Hu, L. Chen, J. Lin, G. Wei, X. Du, Polym. Test 64 (2017) 92–100. [7] B. Di Credico, E. Cobania, E. Callone, L. Conzatti, D. Cristofori, M. Arienzo, S. Dirè, L. Giannini, T. Hanel, R. Scotti, P. Stagnaro, L. Tadiello, F. Morazzoni, Appl. Clay Sci. 152 (2018) 51–64. [8] A.P. Meera, S.P. Thomas, S. Thomas, Polym. Compos. 33 (2012) 524–531. [9] S.C. George, R. Rajan, A.S. Aprem, S. Thomas, S.S. Kim, Polym. Test 51 (2016) 165–173. [10] U. Sookyung, C. Nakason, N. Venneman, W. Thaijaroend, Polym. Test 54 (2016) 223–232. [11] U. Sookyung, C. Nakason, W. Thaijaroen, N. Vennemann, Polym. Test. 33 (2014) 48–56. [12] G. Zhang, J. Wang, J. Phys. Chem. Solids 115 (2018) 137–147. [13] L. Chen, Z. Jia, X. Guo, B. Zhong, Y. Chen, Y. Luo, D. Jia, Chem. Eng. J. 336 (2018) 748–756. [14] M. Nematollahi, A. Jalali-Arani, K. Golzar, Appl. Clay Sci. 97–98 (2014) 187– 199. [15] J. Zhang, L. Wang, Y. Zhao, Mater. Des. 50 (2013) 322–331. [16] H. Chen, Y. Li, S. Wang, Y. Li, Y. Zhou, J. Membr. Sci 546 (2018) 22–30. [17] R. Berahman, M. Raiati, M.M. Mazidi, S.M.R. Paran, Mater. Des. 104 (2016) 333–345. [18] H.J. Maria, N. Lyczko, A. Nzihou, K. Joseph, C. Mathew, S. Thomas, Appl. Clay Sci. 87 (2014) 120–128. [19] P. Vijayan, D. Puglia, H.J. Maria, J.M. Kenny, S. Thomas, RSC Adv. 3 (2013) 24634–24643.
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Please cite this article in press as: E.M. Sadek et al., Influence of modifying agents of organoclay on the properties of nanocomposites based on acrylonitrile butadiene rubber, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.04.007