Polymer Testing 28 (2009) Contents lists available at ScienceDirect. Polymer Testing. journal homepage:

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1 Polymer Testing 28 (2009) Contents lists available at ScienceDirect Polymer Testing journal homepage: Material Properties Influence of maleic anhydride grafted ethylene propylene diene monomer (MAH-g-EPDM) on the properties of EPDM nanocomposites reinforced by halloysite nanotubes Pooria Pasbakhsh, H. Ismail *, M.N. Ahmad Fauzi, A. Abu Bakar School of Materials & Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Penang, Malaysia article info abstract Article history: Received 25 February 2009 Accepted 14 April 2009 Keywords: Halloysite nanotubes Ethylene propylene diene monomer Nanocomposites Maleic anhydride Transmission electron microscopy Ethylene propylene diene monomer grafted with maleic ahydride (MAH-g-EPDM) was prepared by peroxide-initiated melt grafting of MAH onto EPDM using a HAAKE internal mixer at 180 C and 60 rpm for 5 min. The effect of MAH-g-EPDM compatibilizer on the interactions, and tensile and morphological properties of halloysite nanotubes (HNTs) filled EPDM nanocomposites was investigated. The tensile properties of the nanocomposites were influenced by two major factors. The hydrogen bonding between MAHg-EPDM and HNTs, which was confirmed by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), as well as the formation of EPDM-rich and HNT-rich areas, are the dominant effects on the tensile strength of the nanocomposites at low and high HNT loading, respectively. It was found that the cure time (t 90 ), maximum torque (M H ) and minimum torque (M L ) of the compatibilized nanocomposites were increased after adding MAH-g-EPDM. The reinforcement mechanism of the compatibilized and uncompatibilized EPDM/HNT nanocomposites was also investigated based on morphological observations of the nanocomposites. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Polymer matrices (e.g., thermosets, thermoplastics, elastomers) reinforced by nanofillers have attracted considerable attention in recent times due to their higher mechanical, thermal and physical properties [1 6]. Rubber/ Clay nanocomposites are one of the most promising nanocomposite systems which are prepared by the incorporation of layered silicates such as organo modified montmorillonite (OMMT) into the rubbers [2,3,7 13]. However, the preparation of EPDM/Clay nanocomposites has been widely studied by researchers [3,8,9,11,13]. It has been reported in our previous works [14,15] that the incorporation of halloysite nanotubes (HNTs) into EPDM can increase the tensile, thermal, swelling and dynamic * Corresponding author. Tel.: þ x6113; fax: þ address: hanafi@eng.usm.my (H. Ismail). mechanical properties from 0 to 100 phr of HNT loading. Due to the fact that EPDM does not include any polar groups in its backbone, EPDM and HNTs are incompatible. It has also been reported by many researchers [3,16,17] that the properties (mechanical, thermal, rheological, barrier, etc.) of the rubber/clay nanocomposites are extremely affected by two important factors: the degree of dispersion of the nano-filler in the matrix and the compatibility between the nano-filler and the rubber. Melt grafting of unsaturated polar groups onto the polymer backbone by using organic peroxides to functionalize polyolefins has been studied by various researchers [17,18]. To improve the compatibility between a non-polar rubber such as EPDM and the nano-filler, melt grafting of maleic anhydride (MAH) onto the rubber backbone has also been done [16 18]. When a non-conjugated diene (a third monomer), is added to the copolymerization of ethylene and propylene, the resulting rubber becomes a terpolymer, ethylene /$ see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi: /j.polymertesting

2 P. Pasbakhsh et al. / Polymer Testing 28 (2009) propylene diene terpolymer (EPDM) which can then be vulcanized by sulphur. The high mechanical, dynamic and electrical properties as well as resistance to heat, aging and oxidation make EPDM the most suitable rubber for automotive sealing systems, electrical applications, building profiles, roof sheeting and under the hood applications [19,20]. Halloysite nanotubes (Al 2 Si 2 O 5 (OH) 4 $nh 2 O) are a kind of naturally occurring aluminosilicate with nanotubular structure and have been used as a new type of nanoreinforcement for polymers such as epoxy [5,21], polypropylene [22], polyamide [23], polyvinilalcohol [24] and styrene rubber [25,26] in recent years. The outer surface of halloysite is similar to SiO 2, while the properties of the inner side and edges of the tubes could be considered as Al 2 O 3 [27,28]. Due to the unique crystal structure of the HNTs, they do not need exfoliation, and due to the small basal spacing of the crystal planes, the intercalation of the HNTs by polymers and additives is hard to achieve [5,21]. Furthermore, the polarity on the surfaces of the tubules indicates that the HNTs would be well dispersed in the polymer matrices. The aim of present study was firstly to incorporate maleic anhydride via peroxide-initiated melt grafting onto EPDM and use it as a compatibilizer in EPDM/HNT nanocomposites, which would result to higher tensile properties. A further aim of this study was to compare the tensile properties and morphological characteristics of uncompatibilized and compatibilized EPDM/HNT nanocomposites and propose a reinforcement mechanism in order to explain the possible interactions inside the EPDM/ MAH-g-EPDM/HNT nanocomposites. 2. Experiments 2.1. Materials EPDM Keltan, 778Z with ethylene content of 67%, ENB of 4.3% and ML (1 þ 4) at 125 C of 63 MM was used as the matrix. The HNTs, (ultrafine grade) were contributed by Imerys Tableware Asia Limited, New Zealand, with brightness of 98.9% as measured by a Minolta CR300 using D65 light source [29]. The DCP peroxide, maleic anhydride (MAH) and the other compounding ingredients such as zinc oxide, stearic acid, sulphur, tetramethyl thiuram disulfide (TMTD) and 2-mercapto benzothiazole (MBT) were all supplied by Bayer (M) Ltd Melt grafting MAH-g-EPDM was prepared by peroxide-initiated melt grafting of MAH onto the EPDM using a HAAKE internal mixer at 180 C and 60 rpm for 5 min; all the conditions for preparation of the MAH-g-EPDM are based on Grigoryeva and Karger-Kocsis [18] who have reported them as optimum conditions to obtain the maximum grafting content of MAH-g-EPDM. The composition of the reaction recipe was typically as follows: 39 g of EPDM, 2.5 wt% of MAH, and 0.25 wt% of DCP. The components were mixed in a HAAKE mixer with optimum mixing volume of 44.1 cm 3. All the reactants (EPDM, MAH and DCP) were dry mixed together before their fast (<1 min) introduction into the preheated mixing chamber MAH grafting efficiency FTIR spectra were recorded on a FTIR spectrometer model Perkin Elmer System 2000 in a range between 550 and 4000 cm 1 with a 0.4 cm 1 resolution. Films of mm thickness were prepared by compressionmolding using a hot press at 150 C and 5 MPa pressure. The films were vacuum dried at 75 C for 14 h to evaporate the unreacted MAH Preparation of the EPDM/MAH-g-EPDM/HNT nanocomposites The mixing of MAH-g-EPDM with the EPDM, HNTs and other compounding ingredients such as zinc oxide, strearic acid, MBT, TMTD and sulphur, as shown in Table 1, was done using a laboratory-sized two-roll mill (160 mm 320 mm), model XK-160 at room temperature for 20 min. The vulcanization behaviour of composites such as cure time (t 90 ), scorch time (t S2 ), maximum torque (M H ), minimum torque (M L ) and cure rate index (CRI) were determined at 150 C using a Monsanto Moving Die Rheometer (MDR 2000). The compounds were subsequently compression moulded at 150 C, based on respective t 90 values X-Ray diffraction analysis (XRD) The XRD patterns of HNTs, EPDM/HNT and EPDM/MAHg-EPDM/HNT nanocomposites were recorded by using a Bruker Axs model D8 diffractometer. The basal spacing of the halloysite nanotubes before and after blending with EPDM and MAH-g-EPDM was calculated by using Bragg s law. The Cu K a (l ¼ Å) was operated at 40 kv and 40 ma in combination with a Ni filter. The samples were scanned from 2q ¼ 5to FTIR spectroscopy Fourier transform infrared spectroscopy (FTIR) using a Perkin Elmer System 2000 equipped with attenuated Table 1 Compositions of the HNT filled EPDM nanocomposites (phr). Sample code EPDM MAH-g HNT ZnO Stearic MBT TMTD Sulphur -EPDM acid EPDM/H EPDM/H EPDM/H EPDM/H EPDM/H EPDM/H0/ MAH-g-EPDM EPDM/H5/ MAH-g-EPDM EPDM/H10/ MAH-g-EPDM EPDM/H30/ MAH-g-EPDM EPDM/H100/ MAH-g-EPDM

3 550 P. Pasbakhsh et al. / Polymer Testing 28 (2009) Fig. 1. FTIR analysis of (a) EPDM and (b) MAH-g-EPDM. total reflectance (ATR) technique was employed to characterize the possible interactions between HNTs, EPDM and MAH-g-EPDM. FTIR spectra were conducted in a range between 550 and 4000 cm 1 with a 0.4 cm 1 resolution. HNTs were ground thoroughly with KBr at approximately 1 3% by weight and pressed into a pellet with a thickness of about 1 mm Tensile strength After 24 h of storage, dumbbell shaped specimens were punched from the moulded sheets by a tensile specimen cutter. Modulus, tensile strength and elongation at break (E b ) were measured following ISO 37 using a universal tensile testing machine Instron 3366 at room temperature (25 2 C) at a crosshead speed of 500 mm/min Swelling properties Swelling tests were done in toluene in accordance with ISO Cured test pieces of the compounds of dimensions mm were weighed using an electronic balance. The test pieces were then immersed in toluene for 72 h and the pieces were weighed again. Calculation of the change in mass is as follows: Swelling Percentage ¼½ðM2 M1Þ=M1Š100 (1) where M1 is the initial mass of specimen (g) and M2 is the mass of specimen (g) after immersion in toluene Scanning electron microscopy (SEM) observations The fracture surfaces of tensile samples of EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites were investigated by using a Supra-35VP scanning electron microscope (SEM). The main purpose of this evaluation was to observe the degree of dispersion of halloysite nanotubes in the EPDM and to evaluate the bonding between the HNTs and EPDM. To prevent electrostatic charging during observation, a thin layer of Pd Au was coated onto the samples Transmission electron microscopy A transmission electron microscope, Philips CM12 (100 KV acceleration voltage) was used to study the (g) (f) (e) (d) (c) (b) (a) Fig. 2. XRD pattern of: (a) HNT, (b) EPDM/H5, (c) EPDM/H5/MAH-g-EPDM, (d) EPDM/H10, (e) EPDM/H10/MAH-g-EPDM, (f) EPDM/H100, (g) EPDM/H100/MAHg-EPDM.

4 P. Pasbakhsh et al. / Polymer Testing 28 (2009) Table 2 Diffraction pattern characteristics of HNTs and nanocomposites. Sample 2q ( ) d (nm) HNT EPDM/H EPDM/H5/MAH-g EPDM/H EPDM/H10/MAH-g EPDM/H EPDM/H100/MAH-g dispersion of HNTs inside the EPDM matrix. To observe the EPDM/HNT naocomposites, ultra thin specimens were prepared using a cryogenic Ultra microtome Leica-Reichert supernova. 3. Results and discussion 3.1. FTIR analysis of MAH-g-EPDM Fig. 1 shows a comparison between the FTIR spectra of pure EPDM and MAH-g-EPDM in the cm 1 region. The absence of an absorption band at 700 cm 1 related to the carbon carbon double bond of the MAH [18] confirmed the elimination of unreacted MAH by vacuum drying of the MAH-g-EPDM. FTIR spectra of the pure EPDM and MAH-g-EPDM in Fig. 1 shows two absorption bands in the range of cm 1 and cm 1 which are attributed to the C]O symmetric stretching bonds. The presence of absorption bands at 1713 cm 1 and 1780 cm 1 indicate the grafting of MAH onto EPDM. As reported by Grigoryeva and Karger-Kocsis [18], the absorption bands in the region of cm 1 can be related to grafted anhydride. The absorption band at 1713 cm 1 is attributed to the presence of dimeric carboxylic acid in MAH-g-EPDM. On the other hand, the existence of OH groups in MAH-g-EPDM is confirmed by the absorption band at 922 cm XRD analysis Fig. 2 and Table 2 give the X-Ray diffraction pattern of the HNTs, un-compatibilized and MAH-g-EPDM compatibilized EPDM/HNT nanocomposites with 5, 10 and 100 phr of HNT loading. The HNTs has a peak at 2q ¼ 12.19, which corresponds to a basal spacing of nm. As given in Table 2, the EPDM/H5/MAH-g-EPDM nanocomposite shows two lower 2q peaks around 12.1 and 7.05 which correspond to basal spacing of 1.52 nm and 0.73 nm, respectively. On the other hand, as presented in Table 2, the basal spacing of the EPDM/H5 nanocomposite (0.77 nm, 2q ¼ ) was not increased as much as the compatibilized one (EPDM/H5/MAH-g). The reduction of 2q and increasing of the basal spacing of the HNTs may be attributed to the intercalation of the HNTs by the other materials such as EPDM, zinc oxide and stearic acid, and can clearly confirm the formation of nanocomposites. According to Table 2 and Fig. 2, a similar finding was also observed for the basal spacing of HNTs in EPDM/H10 and EPDM/H10/ MAH-g nanocomposites, which was increased to nm and 1.21 nm, respectively. It is very clear that the interlayer spacing of the HNTs in compatibilized EPDM/HNT nanocomposite at low HNT loading was much higher than uncompatibilized nanocomposites. It has been reported in our previous work [14] that HNTs may be intercalated by EPDM, ZnO and stearic acid in un-compatibilized samples. The greater intercalation of the HNTs inside the compatibilized nanocomposites may be attributed to the intercalation of the maleic anhydride into the HNTs interlayer space. On the contrary, the HNTs in EPDM/H100 and EPDM/H100/MAHg-EPDM nanocomposites were not intercalated. As shown in Fig. 2, the degree of intercalation has decreased with increasing the HNT loading. There was no intercalation of the HNTs at 100 phr loading even in compatibilized nanocomposites. As depicted in Fig. 2, between 15 and 30 those nanocomposites with low HNT loading formed a semi-crystalline structure. The formation of this semi-crystalline structure is due to the breaking of some HNT planes ((020), (110), (002)) (a) (b) (c) (d) cm -1 Fig. 3. FTIR analysis of (a) HNTs, (b) EPDM/H5, (c) EPDM/H5/MAH-g-EPDM, (d) EPDM/H10/MAH-g-EPDM.

5 552 P. Pasbakhsh et al. / Polymer Testing 28 (2009) ENB-EPDM MAH MAH-g-EPDM EPDM/MAH-g-EPDM/HNT CH CH 3 + CH CH C C OO O *CH CH 3 CH CH 2 CH CH2 C C *CH O C C O O O CH 3 OH O O OH O Hydrogen bonding Si Al Halloysite nanotubes Scheme 1. Possible interactions between MAH-g-EPDM and halloysite nanotubes. because of the penetration of the EPDM and other materials into the lumen structure of the HNTs. The disappearance of the semi-crystalline region in nanocomposites with 100 phr HNT loading (Fig. 2 (f) and (g)) may be related to the reduction of the breakage of these planes due to limited penetration of the materials into the HNTs FTIR analysis of EPDM/HNT nanocomposites Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) was used to estimate the possible interactions between HNTs, EPDM and other ingredients inside the compatibilized and un-compatibilized EPDM/ HNT nanocomposites. Fig. 3 (a d) show a comparison between ATR-FTIR spectra of HNTs, EPDM/H5, EPDM/ H5/MAH-g-EPDM and EPDM/H10/MAH-g-EPDM nanocomposites. According to the ATR-FTIR spectra of HNTs in Fig. 3 (a), the absorption peaks around 912 cm 1 and 1032 cm 1 are related to the Al OH librations and Si O stretching bands, respectively. As depicted in Fig. 3 (b), no shift was detected in the absorption peaks at 912 cm 1 and 1032 cm 1 which is attributed to the non-polar characteristics of EPDM. On the other hand, as shown in Fig. 3 (c), for the FTIR spectra of compatibilized EPDM/HNT nanocomposite with 5 phr of HNT loading, the absorption band of Si O stretching at 1032 cm 1 and Al OH librations at 912 cm 1 have shifted to 1027 cm 1 and 932 cm 1, respectively. The 5 cm 1 blueshift in Si O stretching and the 20 cm 1 redshift in Al OH group is related to the formation of hydrogen bonding between outer and inner surfaces of the HNTs with MAH-g-EPDM, according to Scheme 1. Du et al. [25] have also used ATR-FTIR to characterize the formation of hydrogen bonding between carboxylated butadiene-styrene and HNTs. By increasing the HNT loading from 5 to 10 phr loading (Fig. 3 (d)), the Al OH group spectra at 912 cm 1 appeared again. The simultaneous presence of these two peaks at 912 cm 1 and 932 cm 1 may be attributed to the incomplete entrapment of the materials inside the lumen of the HNTs due to the increase of the HNT loading. = Al atom = Si atom = O atom = Inner hydroxyl group = hydroxyl group inside the HNTs between the sheets Octahedral Tetrahedral sheet sheet Scheme 2. Crystalline structure of halloysite nanotubes.

6 P. Pasbakhsh et al. / Polymer Testing 28 (2009) Fig. 4. Tensile strength of EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites. A crystalline structure of HNTs is shown in Scheme 2. Consequently, the Al OH group is located inside the tubes while the outer surface of HNTs is covered by the Si O. HNTs have a dioctahedral 1:1 layered alominosilicate, consisting of two different interlayer surfaces. As shown in Scheme 2, aluminium atoms makes an octahedral structure with oxygen atoms and OH group is situated on one side of the lamella, while silicon oxygen atoms are located on the other side of the lamella. As reported by Guimares et al. [30], there are two kinds of OH groups in the HNT structure, outer and inner OH groups which are located in the tetrahedral and octahedral sheets of HNTs, respectively Tensile properties As shown in Fig. 4, the tensile strength of EPDM/MAH-g- EPDM/HNT nanocomposites from 0 to 30 phr loading is increased by compatibilization of the nanocomposites with MAH-g-EPDM except at 100 phr HNT loading. The increment of tensile strength of compatibilized EPDM/HNT nanocomposites from 0 to 30 phr HNT loading in comparison to the un-compatibilized EPDM/HNT nanocomposites is due to the formation of hydrogen bonding and improvement in the interfacial interactions between HNTs and EPDM in the presence of compatibilizer, as confirmed by FTIR results. This enhancement in the tensile strength by mixing the MAH-g-EPDM with EPDM clay nanocomposites is in good agreement with the results which have been reported by Chow et al. [16]. The decrease of the tensile strength of EPDM/H100/MAH-g-EPDM nanocomposites in comparison to EPDM/H100 nanocomposites can be attributed to the bad dispersion of the HNTs inside the EPDM in the presence of MAH-g-EPDM at high HNT loading, which will be discussed later in the morphological observations. Fig. 5 demonstrates the elongation at break (E b ) of the EPDM/HNT nanocomposites with and without the MAH-g- EPDM. As shown in Fig. 5, by adding MAH-g-EPDM, the E b of the compatibilized nanocomposites were decreased at HNT loading higher than 10 phr. However, the E b of the uncompatibilized EPDM compound (EPDM/H0) in the absence of HNT is increased 43% by adding 20 phr of the MAH-g-EPDM. The tensile modulus at 100% elongation (M100) of the EPDM/HNT nanocomposites with and without MAH-g- EPDM is illustrated in Fig. 6. The presence of maleic anhydride groups makes a stronger interaction between halloysite nanotubes and EPDM in comparison to the nanocomposites without MAH-g-EPDM. This kind of improvement in the tensile modulus has also been reported by Mohammadpour and Katbab [3]. They have mentioned that the degree of reinforcement is much more significant in the presence of the MAH-g-EPDM as a compatibilizer in organo modified montmorillonite (O-MMT) filled EPDM nanocomposites. Fig. 5. E b of EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites.

7 554 P. Pasbakhsh et al. / Polymer Testing 28 (2009) Fig. 6. Tensile modulus at 100% of elongation (M100) of EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites Scanning electron microscopy The fractured surfaces of EPDM/HNT and EPDM/MAH-g- EPDM/HNT nanocomposites are presented in Fig. 7. Fig. 7 (a) and (d) compares the fractured surfaces of EPDM/H0 and EPDM/H0/MAH-g-EPDM nanocomposites. It can be seen from these two images that the EPDM/H0/MAHg-EPDM has a rougher surface in comparison to EPDM/H0, Fig. 7. Tensile fractured surfaces of EPDM/HNT nanocomposites: (a) EPDM/H0, (b) EPDM/H30 and (c) EPDM/H100; and EPDM/MAH-g-EPDM/HNT nanocomposites: (d) EPDM/H0/MAH-g-EPDM, (e) EPDM/H30/MAH-g-EPDM and (f) EPDM/H100/MAH-g-EPDM.

8 P. Pasbakhsh et al. / Polymer Testing 28 (2009) Fig. 8. Tensile fractured surfaces of EPDM/HNT nanocomposites: (a) EPDM/H10, (b) EPDM/H10/MAH-g-EPDM. Fig. 9. Tensile fractured surfaces of EPDM/HNT nanocomposites EPDM/ H100/MAH-g-EPDM. The presence of EPDM and HNT-rich areas. and this is in good agreement with the tensile properties results. The increasing of the tortuous path and roughness of the compatibilized MAH-g-EPDM nanocomposites in comparison to the un-compatibilized nanocomposites is clearly observed in Fig. 7 (b and e) and Fig. 7 (c and f). The presence of maleic anhydride which was grafted onto the EPDM increased the interfacial bonding between EPDM and HNTs. The increase of the interfacial bonding between HNTs and matrix increased the roughness of the fractured surfaces. The roughness of the fractured surfaces is increased by increasing of the HNT loading from 0 to 100 phr loading. Fig. 8 (a) and (b) compare the fractured surfaces of the un-compatibilized and compatibilized EPDM/HNT nanocomposites with 10 phr HNT loading. It is shown that both samples, EPDM/H10 in Fig. 8 (a) and EPDM/H10/MAHg-EPDM in Fig. 8 (b), have quite good dispersion of HNTs inside the matrix, indicating the ability of HNTs to disperse homogenously. It is also observed that the interface between HNTs and matrix in EPDM/H10/MAH-g-EPDM nanocomposite is blurred compared to EPDM/H10 due to the HNTs being wrapped in the matrix, and no debonded tubes and cavities can be seen in compatibilized EPDM/ H10/MAH-g-EPDM nanocomposite. This all indicates that, after compatibilization of EPDM/HNT nanocomposites with MAH-g-EPDM, the compatibility between HNTs and EPDM inside the nanocomposites is increased by formation of interfacial interactions (hydrogen bonding) between HNTs and EPDM at low HNT loading. It is noteworthy to mention that the fractured surfaces of EPDM/MAH-g-EPDM/HNT nanocomposites at high HNT loading have two different domains, as depicted in Fig. 9: HNT-rich area and EPDM-rich area. The EPDM-rich area Fig. 10. Tensile fractured surfaces of EPDM/HNT nanocomposites: (a) EPDM/H100, and, (b) EPDM/H100/MAH-g-EPDM.

9 556 P. Pasbakhsh et al. / Polymer Testing 28 (2009) Fig. 11. TEM images of EPDM/HNT nanocomposites: (a) EPDM/H10, (b) EPDM/H100; and EPDM/MAH-g-EPDM/HNT nanocomposites, (c) EPDM/H10/MAH-g- EPDM, (d) EPDM/H100/MAH-g-EPDM. was circled in Fig. 9 while the other regions belong to the HNT-rich area. The formation of these two phases at high HNT loading is attributed to the polarity difference between polar HNTs, MAH-g-EPDM and non-polar EPDM due to the high concentration of the HNTs and low quantity of the MAH-g-EPDM. The MAH-g-EPDM is more polar and the HNTs prefer to interact with OH groups of maleic anhydride by hydrogen bonding. Ye et al. [5] have reported that two phase structures, HNT-rich and epoxy-rich phases, were formed in epoxy/hnt nanocomposites. Wang et al. [17] have also demonstrated the preference of silicate layers to be dispersed and exfoliated by the EPDM-g-MAH rather than non-polar poly (trimethylene terephthalate) PTT matrix inside PTT/EPDM-g-MAH/organoclay ternary nanocomposites, which led to creation of two different phases. Fig. 10 shows the tensile fractured surfaces of EPDM/ HNT nanocomposites which confirm the lower tensile strength and E b of the compatibilized EPDM/HNT nanocomposites at 100 phr HNT loading (Fig. 10 (b)) in comparison to un-compatibilized EPDM/H100 nanocomposites (Fig. 10 (a)). Although the creation of fibril structures (which are shown by arrows) due to the compatibilization effect of maleic anhydride is clearly seen in EPDM/H100/MAH-g-EPDM, there are still some unfilled cavities in EPDM/H100/MAH-g-EPDM nanocomposites in Fig. 12. TEM images of EPDM/MAH-g-EPDM/HNT nanocomposites (a) EPDM/H10/MAH-g-EPDM showing a very good dispersion of HNTs inside the EPDM at low HNT loading, (b) EPDM/H100/MAH-g-EPDM showing the formation of two phases of HNT-rich and EPDM-rich regions.

10 P. Pasbakhsh et al. / Polymer Testing 28 (2009) a b c d HNTs HNT rich EPDM rich = Sulphur crosslinking = Al-hydrogen bonding = EPDM chains = Si-hydrogen bonding Scheme 3. Reinforcement mechanism of un-compatibilized EPDM/HNT nanocomposites: (a) low HNT loading, (b) High HNT loading; and compatibilized EPDM/ HNT nanocomposites: (c) low HNT loading, (d) high HNT loading. comparison to the EPDM/H100 nanocomposites, which may be related to an insufficient amount of EPDM-g-MAH (20 phr) for compatibilizaton of EPDM/HNT nanocomposites at 100 phr loading of HNTs. MAH-g-EPDM is not sufficient for compatibility of HNTs at high loading. This mismatch created two phases inside the nanocomposites, particularly at high HNT loading (100 phr) Transmission electron microscopy Fig. 11 demonstrates the comparison between the dispersion of the HNTs inside the compatibilized and uncompatibilized nanocomposites with 10 and 100 phr HNT loading. As shown in Fig. 11 (a) and (c), the dispersion of the HNTs inside the EPDM/H10/MAH-g-EPDM is increased in comparison to EPDM/H10 nanocomposites. On the other hand, it is very clear from Fig. 11 (b) and (d) that the HNTs are more dispersed in EPDM/H100 in comparison to EPDM/ H100/MAH-g-EPDM. A better depiction of good and bad dispersion of HNTs at 10 and 100 phr HNT loading in compatibilized EPDM/HNT nanocomposites is shown in Fig. 12 (a) and (b), respectively. As described before, the HNTs prefer to interact with the polar MAH-g-EPDM instead of non-polar EPDM due to the formation of hydrogen bonding between Si O and Al OH groups of HNTs and OH groups of MAH-g-EPDM. By increasing the HNT loading from 0 to 100 phr, the fraction of MAHg-EPDM to HNT loading is decreased, and the amount of 3.7. Reinforcement mechanism The proposed reinforcement mechanism of compatibilized and un-compatibilized EPDM/HNT nanocomposites at low and high HNT loading is illustrated in Scheme 3. This mechanism is concluded from the FTIR, XRD, SEM and TEM results which have been discussed earlier. For uncompatibilized EPDM/HNT nanocomposites, as illustrated in Scheme 3 (a) and (b) and reported in our previous work [14], because of the straight and tubular morphology and unique crystal structure of the HNTs, they can be homogenously and easily dispersed inside the EPDM matrix. According to Scheme 3(b) and depicted in Fig. 11(b), in those regions with high concentration of HNTs, they tend to form zig-zag structures due to the edge-to-edge and faceto-edge interactions between them. Regarding Scheme 3 (c) and (d) and earlier discussion, there are two effects which compete with each other: The hydrogen bonding between MAH-g-EPDM and HNTs, and the formation of EPDM and HNT-rich areas which is

11 558 P. Pasbakhsh et al. / Polymer Testing 28 (2009) influenced by the ratio of MAH-g-EPDM to HNT loading. As shown in Scheme 3 (c), at low HNT loading the effect of hydrogen bonding plays the leading role which results in increased tensile strength, but at high HNT loading the formation of EPDM/HNT-rich areas has the dominant effect resulting in reduction of the tensile strength and very large decrease in E b Curing properties Table 3 gives the comparison between the curing properties of EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites. From Table 3, it can be concluded that the curing time (t 90 ), scorch time (t s2 ), M H and M L of the samples are increased due to adding 20 phr of MAH-g- EPDM. The increase of the curing time of the compatibilized nanocomposites in comparison to un-compatibilized nanocomposites at similar HNT loading may be attributed to the cure retardency effect of maleic anhydride [31]. The reactions between some maleic anhydride groups of the MAH-g-EPDM with accelerator species causes delay in the optimum cure time (t 90 ). On the other hand, these reactions increased interfacial bonding between HNTs and EPDM in the presence of MAH-g-EPDM. The enhancement of the M H and M L of the EPDM/MAH-g-EPDM/HNT nanocomposites in comparison to EPDM/HNT nanocomposites is due to the improvement in the interfacial interactions. M L is an indication of the processability of the compounds. As given in Table 3, M L is increased by adding MAH-g-EPDM which shows the lower processability of the compatibilized nanocomposites due to higher shearing needed for mixing. Table 3 also shows the effect of HNT loading on the curing properties of EPDM/HNT nanocomposites. The cure time is decreased from 0 to 30 phr of HNT loading while it increased again from 30 to 100 phr loading. The effect of HNT loading and the entrapment of accelerators and EPDM inside the lumen of the HNTs are two factors which would compete with each other. At low HNT loading, the presence of the HNTs accelerates the vulcanization of the nanocomposites but at high HNT loading, because the lumen structure inside the tubes is a good place to attract the accelerators, by adding more HNTs more accelerators Table 3 Curing properties of composites. Sample code T 90 (min) T s2 (min) M H (dn m) M L (dn m) CRI ¼ 100/ (t 90 t S2 ) EPDM/H EPDM/H0/ MAH-g EPDM/H EPDM/H5/ MAH-g EPDM/H EPDM/H10/ MAH-g EPDM/H EPDM/H30/ MAH-g EPDM/H EPDM/H100/ MAH-g would become entrapped inside the lumen of the HNTs which would slow the vulcanization process Swelling properties The swelling percentage is the measurement of the degree of crosslinking, reduction in swelling indicating increase of crosslink density. Fig. 13 gives a comparison between the swelling percentages of compatibilized and un-comaptibilized EPDM/HNT nanocomposites at different HNT loadings. It is observed from the figure that the swelling percentage of the nanocomposites is decreased due to both compatibilization and HNT loading effects. Compatibilization of the nanocomposites by adding MAHg-EPDM formed hydrogen bonding between HNTs and EPDM which increased the ability of EPDM chains to extend due to toluene diffusion. It has been reported by various researchers [32,33] that increasing the interaction between polymer and filler would lead to an increase in crosslink density and reduction in solvent uptake. The increase of the crosslinking density of the nanocomposites by adding MAH-g-EPDM is in good agreement with the tensile modulus (M100) and maximum torque results discussed earlier. Fig. 13. The comparison between swelling percentage of the compatibilized and uncompatibilized EPDM/HNT nanocomposites.

12 P. Pasbakhsh et al. / Polymer Testing 28 (2009) Conclusions MAH-g-EPDM has been successfully prepared by melt compounding of the EPDM, maleic anhydride and DCP. XRD patterns indicated that the degree of intercalation of the HNTs inside the compatibilized EPDM/HNT nanocomposites is much higher than the un-compatibilized nanocomposites. The enhancement of the tensile properties is due to the creation of an interphase between EPDM and HNT by MAH-g-EPDM which helps to make stronger interfacial interactions. On the other hand, the presence of this compatibilizer reduced the curing time (t 90 ) but increased the maximum and minimum torques as well as swelling resistance of the compatibilized EPDM/HNT nanocomposites in comparison to un-compatibilized EPDM/HNT nanocomposites. Morphological observations revealed the formation of two different phases of EPDM-rich and HNTrich areas which can be the main reason for reduction of the tensile strength and E b at high HNT loading. 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