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Polymer-Plastics Technology and Engineering, 48: 313 323, 2009 Copyright # Taylor & Francis Group, LLC ISSN: 0360-2559 print=1525-6111 online DOI: 10.1080/03602550802675736 The Effect of Halloysite Nanotubes as a Novel Nanofiller on Curing Behaviour, Mechanical and Microstructural Properties of Ethylene Propylene Diene Monomer (EPDM) Nanocomposites H. Ismail, Pooria Pasbakhsh, M. N. Ahmad Fauzi, and A. Abu Bakar School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, Penang, Malaysia Halloysite nanotubes (HNTs) with hollow nanotubular structures were used as a new type of filler for ethylene propylene diene monomer (EPDM) matrix. EPDM/HNT nanocomposites were prepared using a two roll mill by adding 0 to 100 parts per hundred rubber (phr) HNTs. The results show that the tensile properties were increased with the addition of HNTs. The curing time decreased from 0 to 15 phr loading but subsequently increased from 15 to 100 phr, whereas the maximum torque exhibited an increasing trend from 0 to 100 phr. The addition of HNTs reduced the tan d max and increased the storage modulus especially for the EPDM/HNT nanocomposites at high HNT loading. Keywords Electron microscopy; Halloysite nanotubes; Mechanical properties; Polymer-matrix composites (PMCs) INTRODUCTION Polymer clay nanocomposites have been widely investigated by various researchers [1 18]. The nanocomposites with nanosized clay particles obtained by dispersion, were reported to exhibit high mechanical properties, enhanced thermal stability, improved gas barrier properties, high flame retardency, decreased solvent uptake as well as increased chemical and shape memory properties [2,6 10]. Layered silicates, which have been widely used as reinforcing phase in nanocomposites, belong to the general family known as the 2:1 phyllosilicates [11]. On the other hand polyolefines such as polypropylene (PP), polyethylene, and ethylene-propylene rubber are the most widely used polymers in polymer clay nanocomposites [12]. The mechanical properties of rubber=clay nanocomposites, in particular ethylene propylene diene monomer (EPDM), have been widely investigated in recent years [13 18]. Address correspondence to H. Ismail, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia. E-mail: hanafi@eng.usm.my Ahmadi et al. [13] have reported that 5 parts per hundred rubber (phr) of organo montmorillonite (OMMT) and 20 phr of maleic anhydride grafted EPDM (MAHg-EPDM) could increase the tensile strength of nanocomposites to 160% higher than unfilled EPDM, but the elongation at break (E b ) of EPDM=OMMT hybrid decreased with the increase in clay content. Recently Lu et al. [15] prepared highly filled rubber=clay nanocomposites (including EPDM with OMMT up to 150 phr) which displayed great improvement in storage modulus and stiffness, a large reduction in gas permeability but having low tensile strength and E b. EPDM is an unsaturated polyolefin that is used in many applications, such as automotive tires, wires, cables, footwear, barriers and non-automotive mechanical goods. However, EPDM does not have any polar groups in its backbone; hence, it is incompatible with polar organophilic clay to achieve homogeneous dispersion of clay layers in the EPDM matrix. The long molecular chains of rubber also make it difficult to diffuse into the galleries of silicate. Hence, pre-treatment of clays with organic compounds, such as maleic anhydride (MA) functional groups to produce MAH-g-EPDM, have been used to improve compatibility between the clays and polymer matrices [13,16 18].Itis well known that the preparation process of nanoparticles, their organic treatment and preparation of the MAHg-EPDM are relatively complex and costly. Therefore the use of economical nanofiller, with high thermal and mechanical properties would be interesting. The halloysite nanotubes (HNTs) is a kind of aluminosilicate clay (Al 2 Si 2 O 5 (OH) 4 H 2 O with 1:1 layer) with hollow micro and nanotubular structure, and are mined from natural deposits in countries such as China, New Zealand, America, Brazil and France. HNTs are chemically similar to kaolinite and they are typically used in the manufacture of high quality ceramic white-ware [19]. Many researchers [19 26] have done much work on the 313

314 H. ISMAIL ET AL. characterization of the halloysite clay. Recently Du et al. [20] reported that HNTs have typical dimensions of, 10 50 nm in outer diameter, 5 20 nm in inner diameter with 2 40 mm in length. Due to the hollow nanotubular structure of halloysite, the thermal stability of polypropylene was enhanced and the flammability was reduced. However there is no reported literature on the physical and mechanical properties as well as microstructural studies of EPDM=HNT nanocomposites. In this study, the characterization of HNTs, and the effect of HNTs on the curing, mechanical properties and microstructures of EPDM nanocomposites were done and reported. EXPERIMENTAL Materials The EPDM Keltan, 778Z with ethylene content of 67%, ENB of 4.3% and ML (1 þ 4) 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 [27]. The physical and chemical analyses of Ultrafine HNTs are shown in Table 1. 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. Preparation of EPDM/HNT Nanocomposites EPDM, HNT and other compounding ingredients such as zinc oxide, strearic acid, MBT, TMTD and sulphur, as TABLE 1 The specifications of halloysite [27] Title Oxide % 1 Silica (SiO 2 ) 49.02.0 2 Alumina (Al 2 O 3 ) 34.8 (min) 3 Fe 2 O 3 0.35 (max) 4 TiO 2 0.12 (max) 5 MgO 0.15 (max) 6 K 2 O 0.10 (max) 7 Na 2 O 0.25 (max) 8 Moisture 2 1 9 Ignition Loss 13.8 0.6 10 Fired Brightness (1280 C) 95 min 11 Particle size analysis <2 mm 12 % Grit retained on 63 mm sieve <0.1 13 Kaolinite >80% 14 Quartz <10% 15 Cristobalite <10% shown in Table 2, were mixed using a laboratory-sized two-roll mill (160 mm 320 mm), model XK-160, in accordance with ASTM method D 3184-80. The cure characteristics were studied using a Monsanto Moving Die Rheometer (MDR 2000) according to ASTM D 2240-93. Samples (4 g) of the respective compounds were tested at the vulcanization temperature (150 C). The respective cure time (t 90 ), scorch times (t S2 ), and torque values were obtained from the rheograph. The compounds were subsequently compression moulded at 150 C, based on respective t 90 values. The cure rate index (CRI) of the cured compounds with the same level of curative was calculated according to Eq. (1): CRI ¼ 100=ðt 90 t S2 Þ X-ray Diffraction (XRD) Analysis The XRD analysis of HNTs and EPDM=HNT nanocompoaites were performed with a Bruker AXS model D8 diffractometer to see the basal spacing of the halloysite nanotubes before and after blending with EPDM. The samples were scanned from 5 65 2h. The Cu Ka source (k ¼ 1.54060 Å) was operated at 40 kv and 40 ma in combination with a Ni filter. Scanning Electron Microscopy (SEM) Observations SEM samples of pure HNTs were prepared by stirring the 0.5 g of halloysite with 4 ml of methanol and then 2 drops of the suspension were placed on a copper plate. This would prevent the agglomeration of powders for better scanning. The fracture surfaces of tensile samples of EPDM=HNT nanocomposites were also investigated by SEM. The main purpose of this evaluation was to observe the dispersion of halloysite nanotubes in the EPDM and to access the bonding and adhesion between the HNTs and EPDM. To prevent the electrostatic charging during observation, a thin layer of gold was coated on the samples. Samples were observed using a Supra-35VP scanning electron microscope. Transmission Electron Microscopy (TEM) Nanotubular shape of the halloysite was evaluated by a transmission electron microscopy (Philips CM12), which is equipped with a DOCU version 3.2 image analyser. The halloysite was suspended in ethanol and subsequently a droplet was pipette out from suspension and placed on a carbon thin film coated 400 mesh copper grid for 2 minutes. The dispersed halloysite was then dried and observed by TEM. Swelling Test Swelling test was done in toluene in accordance to ASTM D 471-79. Cured test pieces of the compounds of ð1þ

HALLOYSITE NANOTUBES AS NOVEL NANOFILLER 315 TABLE 2 Composition of EPDM=HNT nanocomposites Samples C.1 C.2 C.3 C.4 C.5 C.6 C.7 C.8 EPDM (phr) 100 100 100 100 100 100 100 100 HNT (phr) 0 5 10 15 30 50 70 100 ZnO (phr) 5 5 5 5 5 5 5 5 Stearic Acid (phr) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 MBT (phr) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 TMTD (phr) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Sulphur (phr) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 dimension 30 5 2 mm were weighed using an electronic balance. The test pieces were then immersed in toluene for 72 hours and the pieces were weighed again. Calculation of the change in mass is as follows: Swelling percentage ¼½ðM 2 M 1 Þ=M 1 Š100 ð2þ where M 1 is the initial mass of specimen (g) and M 2 is the mass of specimen (g) after immersion in toluene. Tensile Strength After 24 hours of storage, dumb-bell shaped specimens were punched from the moulded sheets by a tensile specimen cutter. Modulus, tensile strength and percentage of elongation at break (E b ) were measured following ASTM D 412-51 using a universal tensile testing machine Instron 3366 at room temperature (25 2 C) at a crosshead speed of 500 mm=min. Dynamic Mechanical Analysis (DMA) Dynamic mechanical properties were determined using a dynamic mechanical analyzer (Perkin Elmer DMA7). The sample was subjected to a cyclic tensile strain with force amplitude of 0.1 N at a frequency of 10 Hz. The temperature was increased at a rate of 2 C=min in the range of 90 Cto60 C. RESULTS AND DISCUSSION Characterization of Halloysite Nanotubes Figure 1 shows the X-ray diffraction pattern of the halloysite nanotubes. The peak at 12.187 2h corresponds to (001) basal spacing of 0.725 nm, determined by using Bragg s Law. This result indicates that the halloysite was mainly in the dehydrated form and typically referred to as (7 Å)-Halloysite. The basal reflections are broad and this is attributed to the small crystal size, the inconsistent layer FIG. 1. XRD pattern of halloysite nanotubes.

316 H. ISMAIL ET AL. TABLE 3 The XRD data of halloysite 00l and hk indices d(calc.) Å 2h I Mineral (001) 7.25 12.187 15.795 (7 Å)-Halloysite (02),(11) 4.43 20.0585 27.585 (7 Å)-Halloysite 4.04 22.093 33.011 Cristobalite (002) 3.63 24.9183 19.602 (7 Å)-Halloysite 3.34 26.698 30.937 Quartz (13),(20) 2.56 35.0486 17.983 (7 Å)-Halloysite (15),(24),(31) 1.67 54.9327 14.858 (7 Å)-Halloysite (06),(33) 1.48 62.427 25.199 (7 Å)-Halloysite spacing and the curvature of the layers. The dehydrated state was also confirmed with the presence of the (002) basal reflection at 24.918 2h, which is equivalent to d ¼ 3.63 Å. The relatively sharp 06 diffraction peak, with d ¼ 1.48 Å at 62.427 2h, indicates that the halloysite is a dioctahedral mineral and this relates to the typical sheetstructure of HNTs [28]. As shown in Figure 1 and tabulated in Table 3, it is clear that most of the peaks belonged to the (7 Å)-Halloysite. It can also be seen that there are two other peaks at 22.093 2h, and 26.698 2h, which correspond to the presence of silica, in the forms of cristobalite and quartz, respectively. The presence of these two forms of silica was also reported in the company s data sheet as shown in Table 1. The SEM micrograph, as shown in Figure 2, also depicts the nanotubular structure of halloysite. Kaolinite and quartz plates are also seen (indicated by arrows) beside the halloysite nanotubes. Figure 3(a and b) shows the dimensions and shape of halloysite observed by TEM, while Figure 3(c) gives the average dimensions of halloysite nanotubes after observation using TEM and SEM (more than 60 images captured by TEM). The halloysite nanotubes have typical dimensions of 150 nm 2 mm long, 20 100 nm outer diameter and 5 30 nm inner diameter. The formation of hollow nanotubes and their lumen structure will increase the potential application of the halloysite nanotubes. As shown in Figure 3(c), the volume percentage of lumen structure of HNTs may be as high as 15%. The HNTs can be used in drug delivery as a time release capsule or in paints and sealants as well as all things that could benefit from their controlled release behaviour [21,22]. A novel application hence could be expected from these hollow nanotubes to prepare the new polymer nanocomposites with high thermal stability and flame retardant properties. Figure 4 shows the various morphological shapes of halloysite nanotubes. The morphological variability of halloysite is reported to be due to various factors including crystal structure, degree of alteration, chemical composition, and the effect of dehydration [21]. These morphologies are listed in Table 4. The most common HNT morphology is hollow tube. But short tubular [24], pseudo-spherical [25], platy [24,26] and semi-rolled HNTs have also been observed widely by TEM. FIG. 2. SEM image of halloysite nanotubes. FIG. 3. The dimensions of halloysite nanotubes (a,b) Average dimensions of a halloysite nanotube (c).

HALLOYSITE NANOTUBES AS NOVEL NANOFILLER 317 FIG. 5. The effect of HNT on cure time, scorch time and curing rate of EPDM=HNT nanocomposites. FIG. 4. Various shapes of HNTs observed by TEM. TABLE 4 Morphologies which observed by TEM images as shown in Fig. 4 Curing Behaviour of EPDM/HNT Nanocomposites Figure 5 shows the effect of HNT loading on the cure time (t 90 ), scorch time (t S2 ) and the cure rate (CRI) of EPDM= HNT nanocomposites. The scorch time decreased from 0 to 100 phr of HNT loading. The scorch time is the measure of premature vulcanization and optimum cure of EPDM nanocomposites [29]. These results indicate that with increasing the amount of HNT, the amount of active sulphurating agents was increased. Thus, more crosslinks were formed and a shorter scorch time was recorded. Decreasing in scorch time would accelerate the onset of vulcanization. It was also observed that the cure time (t 90 ) decreased and the cure rate increased from 0 phr to 15 phr HNT loading. However the cure time (t 90 ) increased while the cure rate decreased at HNT loading higher than 15 phr. These observations might be due to the penetration of EPDM into the lumen structure of halloysite nanotubes that makes delay in the curing of the composites at high HNT loading. The effect of HNT loading on the maximum torque (M H ) of EPDM=HNT nanocomposites is shown in Figure 6. Maximum torque, which indicates the stock Outline Morphology Figure 1 Tubular elongated Fig. 4(a) 2 Uncompleted tube Fig. 4(a) 3 Short and thin tube Fig. 4(a) 4 Semi rolled tube Fig. 4(a) 5 Platy form Fig. 4(a) 6 Splitting in HNTs Fig. 4(a) 7 Double nanotube inside each other Fig. 4(a) 8 Solid nanotubular Fig. 4(b) 9 Curved edges Fig. 4(c) 10 Thin and stubby form Fig. 4(d) 11 Serrated edges Fig. 4(e) 12 Pseudo spherical Fig. 4(f) FIG. 6. The effect of HNT loading on the maximum torque, MH of HNT-filled EPDM nanocomposites.

318 H. ISMAIL ET AL. modulus, increases due to good interfacial and intertubular adhesion between EPDM and halloysite nanotubes. The interaction between HNTs and EPDM as well as the interfacial reactions between the aluminols and silanols on the surfaces and edges of the halloysite nanotubes would reduce the mobility of the macromolecular chains of the EPDM and consequently increase the composites stiffness and modulus. Figure 7 shows the effect of HNT loading on the viscous torque (S @MH) and tan d@mh of EPDM=HNT nanocomposites. Viscous torque can be related to the damping characteristics and loss modulus of the rubber compounds [30]. Curemeters MH (S @MH) generally correlates with durometer hardness and=or modulus [31].Tand@MH is derived by dividing S by S, where S is the elastic modulus [31]. This result shows that the addition of HNT increases the damping characteristics or viscous torque and tan delta of the EPDM=HNT nanocomposites. The formation of the filler-filler interaction between HNTs would decrease the mobility around the fillers and subsequently increased the hardness, modulus and stiffness of EPDM=HNT nanocomposites. XRD of EPDM/HNT Nanocomposites XRD patterns of the HNT, EPDM=HNT (15 phr) and EPDM=HNT (70 phr) compounds are displayed in Figure 8. Also Table 5 presents the 2h and their relative interlayer spacing at different basal reflection peaks. As illustrated in Figure 8 and Table 5 the (001) basal spacing of HNTs is increased from 0.725 nm to 0.836 nm, after mixing of EPDM and other ingredients with HNT (15 phr). It was also observed that the (002) basal spacing of HNTs is expanded from 0.363 nm to 0.384 nm. A little increase in HNT basal spacing at 15 phr HNT loading indicates limited intercalation of halloysite nanotubes by EPDM and other ingredients such as stearic acid and ZnO. This limited intercalation of HNT was due to FIG. 7. The effect of HNT loading on the viscous torque and tan delta of HNT-filled EPDM nanocomposites. FIG. 8. XRD patterns of: (a) pure HNT (b) EPDM=HNT-15 phr (c) EPDM=HNT-70 phr. the non-polar characteristics of EPDM as well as the dehydrated form and small galleries of halloysite. However, complex formation of EPDM and halloysite with partially or totally dehydrated halloysite was clearly influenced by the particle size, crystallinity, Fe content of the mineral and the presence of impurities [21]. On the other hand, EPDM=HNT nanocomposites at high HNT loading (70 phr) have two basal reflections. Both peaks are located at higher angles than the 0.725 nm peak of HNT; the first peak (12.333 2h) corresponds to basal spacing of 0.717 nm which is near to (0.725 nm) and the second peak (14.362 2h) relates to basal spacing of 0.616 nm. These peaks, in particular the second, may be attributed to some deintercalation of the HNT galleries [15,32]. The de-intercalation phenomenon could be related to the collapsed and delamination of the HNTs during the roll milling and curing. Mechanical Properties The mechanical properties of EPDM=HNT nanocomposites were evaluated and the results are shown in Figures 9 11. It can be seen that generally the tensile strength, tensile modulus (M100) and elongation at break (E b ) of the samples increased with increasing of the HNT loading. Figure 9 shows the tensile strength is continuously increasing with the addition of HNT from 0 phr to 100 phr. For instance, the tensile strength of EPDM=HNT with 70 phr is 53% higher than that of the EPDM=HNT with 50 phr. The enhancement in tensile strength is attributed to the good dispersion of halloysite nanotubes in EPDM matrix and penetrating of EPDM inside the lumen structure of HNTs, which makes a strong interfacial and intertubular interaction between halloysite nanotubes and EPDM. The good dispersion of HNTs inside the EPDM will be shown and supported later by SEM study. It can also be seen from Figure 10 that the E b similarly demonstrates an increasing trend with HNT loading from 0 phr to 100 phr. This result could be related to the

HALLOYSITE NANOTUBES AS NOVEL NANOFILLER 319 TABLE 5 Comparison between diffraction angles and basal spacing in HNT and EPDM=HNT nanocomposites Basal reflection peaks (001) (02),(11) (002) Sample 2h d (nm) 2h d (nm) 2h d (nm) HNT 12.187 0.725 20.058 0.44 24.92 0.363 EPDM=HNT (15 phr) 10.563 0.836 23.087 0.384 Intercalation EPDM=HNT (70 phr) 12.333 14.362 0.717 0.616 20.372 0.435 24.901 0.357 de-intercalation increasing of ductility and stiffness of nanocomposites with HNT content simultaneously. Filling the lumen structure of hollow tubes and making an intertubular interaction inside the tubes between HNT and EPDM have increased the toughness of EPDM=HNT nanocomposites, consequently leading them to higher resistance to break. The increasing trends in tensile strength and E b in these EPDM=HNT nanocomposites are unusual as compared to the commonly known silicate layered nanocomposites. The latter typically shows that tensile strength and E b reaches optimum values with clay loading of 5 10 phr, but subsequently decrease at higher filler loading. This is attributed to the aggregation of the silicate layered with high clay loading [13,14,16,18,33]. However, as mentioned earlier, because of the unique crystal structure of HNTs, their hollow nano tubular shape and low hydroxyl density on their surfaces, halloysite nanotubes were able to be well-dispersed in the EPDM matrix and this had resulted in the increasing of mechanical properties as observed in this study. Figure 11 shows that the tensile modulus (stress at 100% elongation, M100) increases with increasing of the HNT loading. For example, the M100 of the EPDM=HNT nanonanocomposites at 70 phr HNT loading is about 20% higher than 50 phr. The improved property is again related to the strong interaction between HNTs and EPDM. Swelling Property The effect of HNT loading on swelling % of the EPDM=HNT nanocomposites as a result of toluene uptake is shown in Figure 12. The EPDM=HNT nanocomposites exhibited superior chemical stability. The percentage of toluene absorption of nanocomposites at 50 phr is about 35% lower than pure EPDM i.e., without HNT. Improvement of barrier properties of EPDM=HNT nanocomposites could be explained by the fact that, the presence of halloysite nanotubes had increased the tortuous path of the solvent inside the matrix. Although the aspect ratio of halloysite nanotubes, in comparison to silicate layered, is relatively low but the orientation of HNTs, filling of the lumen structure by EPDM, good dispersion of HNTs inside the matrix and reducing the concentration gradiant of filler, had significantly reduced the solvent uptake. Dynamic Mechanical Analysis Measurements The storage modulus and tan d were measured to analyse the interaction between the filler and matrix in the EPDM=HNT nanocomposites. Figure 13 shows the effect of HNT on tan d and storage modulus of EPDM=HNT nanocomposites at different temperatures. The temperature at peak of tan d is taken as the glass transition temperature (T g ). Figure 13(a) shows that the T g of the EPDM=HNT nanocomposites had increased slightly with increasing of FIG. 9. The effect of HNT loading on tensile strength of EPDM=HNT nanocomposites. FIG. 10. The effect of HNT loading on the elongation at break of EPDM=HNT nanocomposites.

320 H. ISMAIL ET AL. FIG. 11. The effect of HNT loading on M100 of EPDM=HNT nanocomposites. HNT loading. The T g at 100 phr HNT had increased to 44 C from 46 C (at 0 phr HNT loading). However the addition of HNT shows a reduction of tan d max (the tan d value at T g ) especially for the nanocomposites with 100 phr HNT content. The large reduction of tan d may be attributed to the creation of filler-filler interaction between HNTs at high filler loading (>30 phr). In contrast, the tan d is increased at high temperatures due to creation of interactions between EPDM and HNTs. For example, Table 6 shows that, the tan d at 45 C is increased with increasing HNT content. From Figure 13(b) it is also obvious that the storage modulus is increased with higher filler content. The increasing trend of storage modulus, decreasing the tan d at T g and increasing of the tan d at elevated temperatures with increasing the HNT content imply that there is a strong interaction between the EPDM and HNTs as well as interaction between the HNTs. As explained earlier, these results may be attributed to the interfacial, intertubular and filler-filler interactions. FIG. 13. Dynamic mechanical properties (a) tan d (b) storage modulus, as a function of temperature for EPDM=HNT nanocomposites at different filler loading. SEM Analysis The tensile fracture surfaces of EPDM=HNT nanocomposites are shown in Figure 14. It can be seen in Figure 14(a) that the fracture surface of pure EPDM is smooth, FIG. 12. The effect of HNT content on the swelling % of the EPDM=HNT nanocomposites. TABLE 6 The effect of HNT content on the tan d of EPDM=HNT naocomposites at 45 C Sample HNT loading (phr) Tan d C.1 0 0.048 C.3 10 0.0515 C.5 30 0.0586 C.7 70 0.0795 C.8 100 0.0822

HALLOYSITE NANOTUBES AS NOVEL NANOFILLER 321 reflecting a brittle fracture. The roughness of the fracture surface increased with increasing of HNT loading in the synthetic rubber. The tensile fracture surfaces of EPDM nanocomposites with 5, 10, 30, 50 and 70 phr HNT loading (Figs. 14(b) to (f)) demonstrate rougher fracture surface compared with the gum EPDM vulcanizate (Fig. 14(a)). The fracture roughness indicates that the resistance to propagation of crack is large in nanocomposites as compared to pure EPDM. It can also be observed that the torturous path of the propagating crack is increased. The tensile fracture surfaces in Figures 14(d) (f) show rough surfaces that consist of tear lines. These tear paths indicate that particles have peeled off from the matrix as the crack propagates, and this, creates voids at positions where the HNT agglomerates. Figures 15(a e) shows tensile fracture surfaces of EPDM=HNT nanocomposites with (5, 10, 30, 50 and 70 phr loading) respectively at higher magnification [10,000X]. As shown in Figure 15, the voids are formed at places where spherical particles or agglomerates are pulled out from the matrix. However, there is good adhesion between halloysite nanotubes and the rubber matrix with increasing HNT content. This could be a good evidence to support the increasing tensile strength and E b with increasing HNT content. Figures 15(d) and (e) shows the existence of a filler-filler network which would result in the enhancement of the dynamic mechanical properties and chemical stability of EPDM=HNT nanocomposites. Figures 15(b) and 16 also show a peeled out particle which would form a void beside itself (shown by black arrow) and a pulled out nanotube (shown by white arrows) with good interaction with matrix. These images confirm that if the halloysite totally consist of nanotubes, the mechanical properties could be much improved. Figure 16 shows a good adhesion (shown by arrows) between a pulled out halloysite nanotube and the matrix at a tensile fracture surface of EPDM nanocomposite with 30 phr HNT loading. Mechanism The increasing trends of tensile strength, E b, tensile modulus as well as the enhancement of dynamic mechanical properties and chemical stability of the EPDM=HNT nanocomposites from 0 to 100 phr HNT loading is a rare phenomenon that was observed in this study. To the knowledge of authors, there has been no report of this kind of improvement. The reinforcement mechanism of rubber matrix with halloysite nanotubes is affected by various factors and is schematically proposed as in Figure 17. For filler-rubber interaction: there are two kinds of interaction between EPDM and halloysite nanotubes. The first type is the interfacial interaction between EPDM and halloysite nanotubes which is attributed to the physical interaction such as the van der Waals forces. The in-rubber FIG. 14. Tensile fracture surfaces of EPDM=HNT nanocomposites with (a) 0 phr, (b) 5 phr, (c) 10 phr, (d) 30 phr, (e) 50 phr, (f) 70 phr, [Magnification: 100X]. FIG. 15. Tensile fracture surfaces of EPDM=HNT nanocomposites with (a) 5 phr, (b) 10 phr, (c) 30 phr, (d) 50 phr, (e) 70 phr, [Magnification 10,000X].

322 H. ISMAIL ET AL. FIG. 18. Proposed model for the filler filler contacts at high specific surface area [34]. FIG. 16. White arrows: Good adhesion between pulled out halloysite nanotubes and matrix; Black arrow: A peeled out particle which would form a void beside itself, at a tensile fracture surface of EPDM=HNT nanocomposite with 30 phr HNT loading. structure model which was proposed by Frohlich et al. [34] and investigation of dielectric properties by Lanzl [35], indicated that the rubber had formed three zones around the filler as shown in Figure 18. At high filler loading, the rubber chains which normally penetrate between the fillers and then filling the void space within each aggregate, will act partly as reinforcement rather than as a matrix. The confined thin rubber shell with low mobility around the fillers improves the interaction between the nanotubes. The second type of filler-rubber interaction, which specifically belongs to nanotubes, is the inter-tubular interaction between the HNTs and penetrated rubber inside them. As mentioned before, HNTs have typical dimensions of 150 nm 2 mm long, 20 100 nm outer diameter and 5 30 nm inner diameter. Thus the volume percentage of the lumen structure of HNTs may be as high as 15%. The diffusion of rubber inside the lumen structure of the HNTs, the good dispersion of HNTs inside the EPDM and the partial intercalation of HNTs at 15 phr HNT loading which was confirmed by the XRD results, play the main role in improving the mechanical and curing properties of EPDM=HNT nanocomposites at low HNT loading. However, because of the nonpolar characteristics of the EPDM, the intertubular interaction and intercalation are relatively low. For filler-filler interaction: with increasing HNT content, the distance between halloysite nanotubes becomes smaller. The presence of penetrated rubber inside the HNTs with the creation of a strong filler-filler network in the highly filled EPDM=HNT nanocomposites, improved the mechanical properties and chemical stability of these nanocomposites. As reported before by Lu et al. [15] in the highly filled rubber clay nanocomposites (RCNs), strong clay layered network would enhance the stiffness and modulus of the RCNs but decrease the tensile properties. FIG. 17. The proposed mechanism of filler-rubber and filler-filler interaction inside the EPDM=HNT nanocomposites. CONCLUSIONS The most common HNTs morphology is hollow nanotubes. However, short tubular, pseudo spherical, platy and semi-rolled HNTs can also be seen in TEM observations. The holloysite nanotubes have typical dimensions of 150 nm 2 mm long, 20 100 nm outer diameter and 5 30 nm inner diameter. The volume percentage of the lumen structure of the HNT is as high as 15%. The scorch time and cure time (t 90 ) decreased with increasing HNT loading (up to 15 phr). However, increasing the cure time (t 90 ) at HNT loading higher than 15 phr, was due to the penetration of EPDM in to the lumen structure of halloysite nanotubes, which

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