Applied Clay Science

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1 Applied Clay Science 48 (2010) Contents lists available at ScienceDirect Applied Clay Science journal homepage: EPDM/modified halloysite nanocomposites 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 3 June 2009 Received in revised form 18 January 2010 Accepted 18 January 2010 Available online 1 February 2010 Keywords: Halloysite γ-methacryloxypropyl trimethoxysilane Mechanical analysis Transmission electron microscopy Nanocomposites Halloysite nanotubes (HNTs) were modified by γ-methacryloxypropyl trimethoxysilane (MPS) to improve their dispersion in ethylene propylene diene monomer (EPDM). The EPDM/HNT nanocomposites were prepared by mixing 0, 5, 10 and 30 parts per hundred rubber (phr) of HNTs with EPDM on a two-roll mill. The tensile strength and tensile modulus at 100% elongation (M100) of the nanocomposites were higher than those of EPDM/unmodified HNTs (EPDM/HNT) while the elongation at break decreased a little after modification of the HNTs. SEM and TEM studies revealed the better dispersion of the modified HNTs in the EPDM matrix. FTIR and TGA indicated that γ-mps had partially penetrated into the HNTs and interacted with the Si O groups on the surfaces of the HNTs Elsevier B.V. All rights reserved. 1. Introduction The effect of functionalization of fillers such as clay minerals and carbon nanotubes (CNTs) with organosilane on the properties of polymer clay and polymer CNTs nanocomposites has been studied extensively in recent years. Functionalization of the clay minerals and CNTs with organosilanes increases their degree of dispersion within the polymer matrix, therefore improving the mechanical properties of nanocomposites (Bag et al., 2004; Ma et al., 2006a; Chen et al., 2007; Das et al., 2008). The dispersion of montmorillonite (MMT) in polymer/mmt nanocomposites and their mechanical properties are much affected by intercalation and exfoliation of the MMT particles by the polymer chains which are achieved by modification of the MMTs with modifiers such as alkylammonium ions (Liu, 2007; Yoon et al., 2007). Zheng et al. (2004) reported that the EPDM chains were intercalated into MMT after modification with trimethyl octadecylamine and dimethylbenzyl octadecylamine, and were exfoliated when methylbis (2-hydroxyethyl) cocoalkylamine was used. These modification processes of the MMTs are costly and complicated. As for the CNTs, there are various reports on using organosilanes to modify the CNTs surface. However, the self-aggregation of CNTs originated from the strong van der Waals force is a technical challenge (Velasco- Santos et al., 2002; Kim et al., 2007). Halloysite, a 1:1 clay mineral chemically similar to kaolinite except in its higher hydrated water content shows tubular morphology but also spherical, platy and semi rolled forms (Churchman and Theng, 1984; Churchman, 2000; Joussein et al., 2005; Brigatti et al., 2006; Ismail et al., Corresponding author. Tel.: ; fax: address: hanafi@eng.usm.my (H. Ismail). 2009). HNTs have been widely used during recent years to reinforcing the polymer matrices such as epoxy resins (Liu et al.,2007a;ye et al., 2007; Deng et al., 2008) polypropylene (Ning et al., 2007) polyamide (Marney et al., 2008), polyvinyl alcohol (Liu et al., 2007b) and styrene rubber (Du et al., 2008; Guo et al., 2008). In addition, HNTs is far less expensive than CNTs and can be mined from deposits as a raw material. However, using kaolinite as a clay mineral component instead of the widely used MMTs has been suggested by Ruiz-Hitzky and Van Meerbeek (2006) due to the difficulties in production of polymer MMT nanocomposites for industrial applications. In our previous works (Ismail et al., 2008; Ismail et al., 2009; Pasbakhsh et al., 2009a, b) we studied the effect of the HNT loading on the tensile, thermal and morphological properties of EPDM nanocomposites and found a very uniform dispersion of the HNTs within the EPDM even at high HNT loading ( phr). Du et al. (2006) studied the effect of surface modification of HNTs by γ-methacryloxypropyl trimethoxysilane on the thermal and morphology properties of HNT reinforced PP nanocomposites. Surface modification increased the temperature at 5% mass loss from 414 C to 444 C for nanocomposites filled with 10phr HNT loading. 2. Experimental 2.1. Materials EPDM Keltan, 778Z with ethylene content of 67%, is an ethylidene norbornene (ENB) type with ENB of 4.3% and Mooney viscosity (ML (1+4) at 125 C) of 63MU (Mooney Unit). 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 ( 2007). The /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.clay

2 406 P. Pasbakhsh et al. / Applied Clay Science 48 (2010) Table 2 Curing properties of nanocomposites. Sample code T 90 (min) T s2 (min) M H (dn m) M L (dn m) CRI EPDM (control) EPDM/HNT EPDM/M-HNT EPDM/HNT EPDM/M-HNT EPDM/HNT EPDM/M-HNT Fig. 1. The chemical structure of, (a) EPDM; (b) γ-mps. cure time (t 90 ) which is the time for the completion of cure, scorch time (t S2 ) which is the time for the onset of cure (Ansarifar et al., 2004), maximum torque (M H ), minimum torque (M L ) and cure rate index (CRI) which is a parameter proportional to the average slope of the cure curve (100/t 90 t S2 ) in the curing step region (Das et al., 2008) were determined at 150 C using a Monsanto Moving Die Rheometer (MDR, 2000). The compounds were subsequently compression moulded at 150 C, based on the t 90 values (Table 2) Characterization of the nanocomposites Fourier-transform infrared spectroscopy FTIR model Perkin Elmer System 2000 equipped with attenuated total reflectance (ATR) technique was used. FTIR spectra were conducted between 600 and 4000 cm 1 with 0.4 cm 1 resolution. HNTs was ground thoroughly with KBr at approximately 1% to 3% by mass and pressed into a pellet with a thickness of about 1 mm Tensile properties After 24 h of storage, dumb-bell 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. Fig. 2. Modification of the halloysite nanotubes. compounding ingredients such as zinc oxide, stearic acid, sulphur, tetramethyl thiuram disulphide (TMTD) and 2-mercapto benzothiazole (MBT) were supplied by Bayer (M) Ltd. The γ-methacryloxypropyl trimethoxysilane (γ -MPS) was manufactured by Sigma Aldrich (Fig. 1) Preparation of the nanocomposites The HNTs were modified by γ-mps according to Du et al. (2006) (Fig. 2). Series of EPDM nanocomposites filled with unmodified and modified HNTs were prepared. EPDM, modified HNTs (M-HNT) and unmodified HNTs (HNT) and the other compounding ingredients such as zinc oxide, strearic acid, MBT, TMTD and sulphur were mixed using a laboratory-sized two-roll mill (160 mm 320 mm), model XK-160 (Table 1). The vulcanization behaviour of composites such as optimum Swelling properties Swelling test was done in toluene in accordance to ISO Cured test pieces of the compounds of dimension mm were weighed using an electronic balance. The test pieces were then immersed in toluene for 72 h and the pieces were weighted again to calculate the mass increase (in percent) by swelling. The crosslinking density of specimens was measured on the basis of the rapid solvent-swelling measurements by applying the Flory Rehner equation (Flory, 1953): h i h i M c = ρ P V s V 1 = 3 r = lnð1 V r Þ + V r + χv 2 r ð1þ V r =1= ð1+ Q m Þ ð2þ where M c is the molar mass, ρ is the density of the EPDM (0.823 gcm 3 ), V s is the molar volume of the solvent (107.0 ml/mol for toluene), V r is the volume fraction of the swollen rubber, Q m is the swelling mass of the EPDM/HNT compounds in toluene and χ is the Table 1 Composition of the nanocomposites. Sample codes Materials (phr) EPDM/unmodified-HNT EPDM/modified-HNT EPDM HNT ZnO Stearic acid MBT TMTD Sulphur EPDM/HNT10 EPDM/M-HNT EPDM/HNT5 EPDM/M-HNT EPDM/HNT10 EPDM/M-HNT EPDM/HNT30 EPDM/M-HNT

3 P. Pasbakhsh et al. / Applied Clay Science 48 (2010) Fig. 3. TEM micrographs showing the basal spacing between the interlayers inside the HNTs walls in the EPDM/HNT nanocomposite. interaction coefficient between the rubber network and solvent (0.49) (Eq. (3)). The degree of crosslinking density (V) was given by Eq. (4): χ = ðδ s δ r Þ V 0 = RT ð3þ V =1= ð2m c Þ ð4þ where δ s, δ r, R and T are the solubility's parameters of the solvent and rubber network, the universal gas constant (8.314 J/molK) and absolute temperature, respectively Scanning electron microscope The fractured surfaces of tensile samples of EPDM/M-HNT and EPDM/HNT nanocomposites were investigated by using a Supra-35VP SEM. To prevent the electrostatic charging during observation, the samples were coated by a thin layer of a Pd Au alloy Transmission electron microscope Specimens with a thickness of about 100 nm were prepared using the cryogenic Ultra microtome Leica-Reichert supernova to observe the nanocomposites by using a TEM Philips CM Thermogravimetric analysis TGA analysis was performed using a Perkin-Elmer Pyris 6 TGA analyser from 25 C to 600 C at a heating rate of 20 C/min under nitrogen atmosphere. were subjected to a cyclic tensile strain with force amplitude of 0.1 N at a frequency of 10 Hz. Storage modulus and mechanical loss factor (tan δ) were determined in the temperature range from 90 C to 60 C at a heating rate of 2 C/min. 3. Results and discussion 3.1. Halloysite nanotubes (HNTs) The basal spacing of the HNTs walls is around nm (Churchman, 2000; Joussein et al., 2007; Ismail et al., 2009) and the thickness of the HNTs wall is around nm (Fig. 3). Fig. 3 shows the dimensions of the lumen of the unmodified HNTs in EPDM/HNT5 nanocomposite and their walls which consisted of tens of layers. HNTs have two different interlayer surfaces; the Al OH group is located inside the tubes while the outer surface of HNTs is covered by the siloxane group (Joussein et al., 2005; Liu et al., 2007a) (Fig. 4). The lumen space inside the halloysite tubes can be intercalated and interacts with γ-mps, vulcanization ingredients and the other materials such as EPDM Fourier-transform infrared spectroscopy Table 3 gives the wavenumbers and assignments of major IR vibration bands of FTIR spectra of Fig. 5 (Joussein et al., 2005; Yuan et al., 2008; Pasbakhsh et al., 2009a). Modification of the HNTs by Dynamic mechanical analysis Dynamic mechanical properties (DMA) were measured by using a dynamic mechanical analyzer (Perkin Elmer DMA7). The samples Table 3 FTIR bands and assignments of HNT and EPDM/HNT nanocomposites. Assignments HNT M-HNT EPDM/ HNT5 Position (cm 1 ) EPDM/ M-HNT5 Fig. 4. Schematic structure of a halloysite nanotube. Perpendicular Si O stretching Symmetric stretching of Si O O H deformation of inner hydroxyl groups In-plane Si O stretching In-plane Si O stretching (CH 2 ) 3 scissoring band 1470 O H deformation of water C=C bond 1637 C=O stretching vibrations (ester) 1720 C H symmetric stretching bands C H asymmetric stretching bands C H stretching vibration 2958 O H stretching of inner hydroxyl groups O H stretching of inner-surface hydroxyl groups

4 408 P. Pasbakhsh et al. / Applied Clay Science 48 (2010) Fig. 5. FTIR spectra of (a) HNTs, (b) M-HNTs, (c) EPDM/HNT5, (d) EPDM/M-HNT5. γ-mps created some new FTIR bands such as 1470, 1720 and 2958 cm 1 which were assigned to the (CH 2 ) 3 scissoring, C O and C H stretching vibration. The C C adsorption band at 1637 cm 1 overlapped with the OH deformation band of water. The absence of a band at 1700 cm 1 in Fig. 5b and the presence of OH vibration bands at 912, 3620 and 3695 cm 1 confirmed that hydrogen bonds were not formed between halloysite and γ-mps because the OH groups of HNTs were not easily accessible to the coupling agent. However the increasing of the intensity of the band at 912 cm 1 from HNTs to M-HNTs and on the other hand the broadening and intensifying of the band at 1088 cm 1 may be indicative of the possibility of RSi O Si and RSi O Al bonds between RSi OCH 3 and RSi OH groups of γ-mps with Si O groups at the surface and Al OH groups at the edges of the HNTs. These observations indicate the partial grafting of the γ-mps on the surface and edge of the HNTs (Fig. 6). According to ATR-FTIR spectra of HNTs in Fig. 7a, the absorption bands around 912 cm 1 and 1032 cm 1 were related to the Al OH vibrations and Si O stretching bands(du et al., 2008; Pasbakhsh et al., 2009a). The band at 912 cm 1 was shifted to 931 cm 1 and 940 cm 1 for EPDM/HNT5 and EPDM/M-HNT5 nanocomposites (Fig. 7c, d). The shift can be attributed to the interaction of the OH groups inside the lumen of HNTs indicating that γ-mps was penetrated to some extent into the tubes. The 1032 cm 1 belongs to the Si O groups at the surface of the tubes. However, the band at 1047 cm 1 (Fig. 7d) was attributed to the formation of Si O Si groups which indicated the interaction between γ-mps and the surface of the HNTs. The significant reduction in intensity of the bands at 3620 and 3694 cm 1 was related to the appearance of new bands at 2918 and 2850 cm 1 (C H asymmetric and symmetric stretching bands). This reduction indicated that the hydroxyl groups inside the lumen reacted with EPDM. Thus, due to the orientation of OH groups at the edges and inside the lumen of the HNTs, RSi OH and RSi OCH 3 groups of γ-mps interacted with the Al OH groups of HNTs located inside and at the edges of the nanotubes. In addition, some silane groups were linked to the surface of the tubes forming Si O Si bonds as confirmed by the presence of 1047 cm 1 absorption band Curing properties The curing time (t 90 ), scorch time (t s2 ), M H and M L of the nanocomposites increased after the modification of the HNTs. Increased curing times due to the modification of montmorillonite by organosilanes was reported (Choi et al., 2003; Ma et al., 2006b; Das et al., 2008; Du et al., 2008). The increased curing and scorch time of EPDM/M-HNT may be related to the absorption of some curatives such as zinc oxide to the surface of M-HNTs. The enhancement of the M H and M L of the EPDM/M-HNT nanocomposites in comparison to EPDM/HNT nanocomposites was due to improved interfacial interactions. Due to the modification of the HNTs by γ-mps the curing rate of the nanocomposites was reduced Scanning electron microscope Fig. 6. Possible interaction mechanism between HNTs and γ-mps. The SEM micrographs revealed that the HNTs were dispersed more homogenously after modification (Fig. 8). Most of the cavities and free tubes disappeared as well as the interface between HNTs and matrix

5 P. Pasbakhsh et al. / Applied Clay Science 48 (2010) Fig. 7. FTIR spectra from 870 cm 1 to 1150 cm 1 of (a) HNTs, (b) M-HNTs, (c) EPDM/HNT5, (d) EPDM/M-HNT5. was blurred due to the better interactions between HNTs and the matrix. More embedded and well dispersed tubes within the EPDM matrix were the reason of higher tensile properties Swelling measurements Swelling is a measure of the degree of crosslinking and decreased after modification of the halloysite (Fig. 9). Due to the increased interactions with EPDM increasing the interaction between polymer and filler would increase the crosslinking density and reduce the solvent uptake (Yan et al., 2005; Valadares et al., 2006; Bhattacharya and Bhowmick, 2008). The increased crosslinking density of the nanocomposites by modification of the HNTs was in agreement with the tensile modulus and maximum torque. As shown in Fig. 10, after modification of the HNTs, the crosslinking densities of nanocomposites increased by around 12%, 15% and 35% for 5, 10 and 30 phr of HNT loading Transmission electron microscope TEM micrographs also revealed the better dispersion of modified HNTs in EPDM matrix. The unmodified HNTs had a tendency to form edge-to-edge and face-to-edge contacts (Fig. 11). Modification of the HNTs reduced the aggregation of the HNTs in the EPDM matrix Tensile properties The tensile strength and tensile modulus (M100) of EPDM/M-HNT nanocomposites increased after the modification of HNTs, while the elongation at break (E b ) slightly reduced (Fig. 12). The better dispersion of the HNTs within the matrix as well as the inter-tubular and interfacial bonding between the M-HNTs and the EPDM matrix play the leading role in increasing of the tensile strength compared to EPDM/HNT nanocomposites. The improvement of interactions between HNTs and EPDM matrix increased the stiffness and tensile strength of the nanocomposites which consequently reduced their ductility. Hedicke-Höchstötter et al. (2009) reported the increasing of strength and stiffness as well as enhancing the E b at low HNT loading in polyamide6/hnt nanocomposites Dynamic mechanical analysis The EPDM/M-HNT nanocomposites exhibited higher values of the storage modulus (Fig. 13a). The increasing of the storage modulus of the EPDM/M-HNT nanocomposites indicated the improvement in stiffness due to the better interactions and compatibility between EPDM and HNTs. The elasticity of a material is measured by tan δ, which is the ratio of energy dissipated to energy stored per cycle. Thus, elasticity is inversely proportionate to tan δ. The tan δ of the EPDM/M-HNT5 and EPDM/M-HNT30 was lower than with unmodified HNTs (Fig. 13b). The reinforcing of the EPDM nanocomposites by modified nanotubes made the nanocomposites more elastic as shown by lower tan δ. The better dispersion of the HNTs as well as enhanced interactions between HNTs and EPDM improved the elasticity of EPDM/M-HNT nanocomposites. Zheng et al. (2004) also reported that the reduction of tanδ max of the EPDM/OMMT nanocomposites by addition of the OMMT can be related to the strong interfacial interaction between the

6 410 P. Pasbakhsh et al. / Applied Clay Science 48 (2010) Fig. 8. SEM micrographs of fractured surfaces of EPDM/HNT and EPDM/M-HNT nanocomposites: (a) EPDM/H5, (b) EPDM/H10 and (c) EPDM/H30; (d) EPDM/M-HNT5, (e) EPDM/M- HNT10 and (f) EPDM/M-HNT30. EPDM matrix and the OMMT. Generally, the stronger interactions between the rubber and filler resulted in the higher storage modulus and lower tan δ, and the glass transition temperature (T g ) is shifted to a higher temperature (López-Manchado et al., 2004). T g was determined by the maximum peak value of the plots of tan δ vs. Fig. 9. Swelling of the EPDM/M-HNT and EPDM/HNT nanocomposites at different HNT loading. Fig. 10. Crosslinking densities of the EPDM/M-HNT and EPDM/HNT nanocomposites at 5, 10 and 30phr of HNT loading.

7 P. Pasbakhsh et al. / Applied Clay Science 48 (2010) Fig. 11. TEM micrographs of nanocomposites (a) EPDM/HNT10, (b) EPDM/M-HNT10. Fig. 13. Dynamic mechanical analysis of the nanocomposites; (a) temperature dependence of storage modulus of nanocomposites (b) temperature dependence of tan δ. temperature (Paroli and Penn, 1994). The T g of the EPDM/M-HNT5 and EPDM/M-HNT30 nanocomposites decreased to 40 C and 47 C (Fig. 13b). Simultaneous decreasing of T g and tan δ were reported in PP/OMMT (Liu and Wu, 2001) and nylon-6/ommt nanocomposites (Shelley et al., 2001). Shelley et al. (2001) reported that the decrease in T g of nylon6/ommt nanocomposites can be attributed to an increase in the amorphous volume fraction due to the addition of the clay mineral. T g is a property of an amorphous polymer and is defined as the temperature at which the polymer changes from rubbery to a glassy state without any change in the phase state (Paroli and Penn, 1994). The decrease of T g with increased clay mineral dispersion in epoxy nanocomposites have been reported by Koerner et al. (2006) and Ye et al. (2007): Two competitive effects, i.e. rigid phase reinforcement, and devastation and plasticization of the network topology were attributed to the changes of T g in epoxy reinforced nanocomposites. Fig. 12. Tensile properties of EPDM/HNT and EPDM/M-HNT nanocomposites; (a) tensile strength, (b) tensile modulus M100 and (c) elongation at break (E b ). Fig. 14. Thermal stability of the EPDM/HNT nanocomposites.

8 412 P. Pasbakhsh et al. / Applied Clay Science 48 (2010) Table 4 The thermal stability of the EPDM/HNT nanocomposites. Composite As mentioned above, reinforcement and destroying the network are competing effects. At temperatures NT g reinforcement rather than intertubular interactions is the dominant effect while at temperatures bt g, inter-tubular interactions and good dispersion of the HNTs destroy the EPDM network. The decreased T g means that the activation energy for the motions of the polymer chains was decreased Thermal gravimetric analysis Thermal stability was followed by a similar decomposition trend while the temperature at maximum mass loss rate was reduced from 492 C to 486 C for EPDM/HNT5 and EPDM/M-HNT5, respectively (Fig. 14 and Table 4). The modification of the HNTs did not affect the char residue of the nanocomposites but decreased the thermal resistance. The reduced thermal stability of the nanocomposites can be attributed to the penetration of γ-mps into the lumen of the HNTs above 400 C. This penetration at high temperatures would reduce the interfacial bonding between EPDM and HNTs. 4. Conclusions The modification of the halloysite nanotubes by γ-methacryloxypropyl trimethoxysilane increased the interactions with EPDM and the degree of dispersion of the HNTs within the EPDM. This increased the tensile strength and M100 of the nanocomposites. The thermal resistance of the nanocomposites decreased after the modification of the HNTs. γ-mps interacted with Al OH groups at the edges and inside the HNTs and Si O groups at the surface of the HNTs. Acknowledgment The authors wish to acknowledge the financial support provided by USM short term grant Ac No.: and also thank for the support from USM fellowship scheme. 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