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Hybrid composites consisting of palm ash (PA), halloysite nanotubes (HNTs) and Ethylene-propylene-diene monomer (EPDM) was prepared using a laboratory size two-roll mill and compression moulding. The effects of EPDM-g-MAH on the curing characteristics, tensile properties and morphology of PA/HNTs/EPDM hybrid composites were investigated. The results indicated that the tensile properties of the hybrid composites with EPDM-g-MAH were comparable to those without EPDM-g-MAH. Morphological studies of tensile fractured surface surfaces of the hybrid composites with EPDM-g-MAH using Scanning Electron Microscopy (SEM) indicates that better interaction of PA/HNTs with EPDM matrix. The presence of EPDM-g-MAH has a beneficial effect of prolonging the scorch time and reducing the cure time.
Keywords: A. Hybrid; A. Polymer-matrix composites; B. Mechanical properties
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The main purpose of adding reinforcements to rubber is to enhance certain properties while at the same time reducing the cost of the compound . Literatures reviews revealed that there were more than 100 types of reinforcements that were used in the study. However, only a few have been commercialized and used extensively . The main advantage of composite materials remains in the possibilities of combining the properties of the constituents to obtain enhanced properties such as physical, mechanical or optical . The huge demand of composite materials in various fields has brought to the attention the difficulties of developing modern composite materials known as hybrid composite materials which required the combinations of reinforcements from two or more types of fillers/fibres . The incorporation of hybrid fillers in polymer matrix has been a popular field of research in recent years [2, 5-10]. The Malaysian palm oil industry plays an important role as the biggest palm oil producer and exporter in the world. The mass production of palm oil generates solid wastes such as palm oil kernels and seeds which are later used as fuels, through a combustion process, to generate steam in palm oil mills. The ashes are highly abundant in Malaysia and the wastes are transported either to a dump site or dumped illegally. Dumping also is not an environmentally-friendly activity and creates a lot of problems.
Over the years, a large number of researches had been carried out by researchers in various fields on the utilization of palm ash [6, 11-14]. Halloysite, with a formula of (Al2Si5(OH)4.H2O 1:1) often occurs naturally as an ultramicroscopic hollow tube with a multi-layer wall. Halloysite nanotubes (HNTs) appear to have the same geometry like a multi-walled carbon nanotubes (MWCNTs) but their unique crystal structure, low density of hydroxyl functional groups and tubular shapes give an advantage to the HNTs by not forming agglomeration, thus causing them to be easily dispersed in a polymer matrix [15-19]. It has been reported  that the incorporation of palm ash (PA)/halloysite nanotubes (HNTs) hybrid fillers into EPDM increased certain properties. However, the main problem in the preparation of hybrid composite arises from the incompatibility of the constituents due to the nature of the fillers and polymer matrix which do not yield its optimum properties. The use of coupling agent or compatibilizer is one popular method that can be generally used to improve the compatibility between the fillers and the polymer matrix. The most common coupling agents or compatibilizers are silanes and maleated materials [7, 21-25]. In this study, maleic anhydride grafted onto EPDM (EPDM-g-MAH) was used as a compatibilizer for PA/HNTs/EPDM hybrid composites and its effect on the curing characteristics; tensile properties and morphology were investigated.
Ethylene-Propylene-Diene Monomer (EPDM), Keltan 778Z, with 67% ethylene content and 43% ENB, and ML (1+4) 125°C of 63 MM purchased from Bayer (M) Ltd. was used as the matrix. Palm ash (PA) was obtained from United Oil Palm Mill Penang, Malaysia and Halloysite nanotubes (HNTs) were supplied by Imerys Tableware Asia Limited, New Zealand. Both palm ash and halloysite nanotubes were dried in a vacuum oven at 80°C for 24 hours to expel moisture. The palm ash was then sieved with a BS 410, 75 μm aperture sieves to obtain an average particle size of 75 μm. The elemental composition of palm ash obtained with Rigaku RIX3000 X-Ray Fluoroscence Spectrometer (XRF Spectrometer) is shown in Table 1. Halloysite nanotubes particles are tubular in shape and have typical dimension of 150 nm-2 μm long, 20-100 nm outer diameter and 5-30 nm inner diameter . The elemental composition of halloysite nanotubes is as follows : (wt %) SiO2, 49; Al2O3, 34.8; Fe2O3, 0.35; TiO2, 0.12; Na2O, 0.25; MgO, 0.15. The other compounding ingredients used are zinc oxide, stearic acid, 2-mercapto benzothiazole (MBT), tetramethyl thiuram disulfide (TMTD), sulphur and maleic anhydride. They were supplied by Bayer (M) Ltd., and were all used as received. Toluene was supplied by Baker-Aldrich (M) Ltd.
2.2 Preparation of EPDM-g-MAH and PA/HNTs/EPDM/EPDM-g-MAH hybrid composites
The EPDM-g-MAH was prepared, before compounding, through a process of grafting of maleic anhydride onto the ethylene propylene diene rubber chains with a Haake Rheomix Polydrive R600/610 according to the procedures reported by Nakason et al. . The compounding of palm ash, HNTs, EPDM, EPDM-g-MAH and other compounding ingredients as shown in Table 2, was done using a conventional 160 mm x 320 mm two-roll mill model XK160.
The curing characteristics of the hybrid composites such as cure time (t90), scorch time (t2) and maximum torque were determined by the Monsanto Rheometer, MDR 2000 according to ISO 3417 at 160°C. The hybrid composites were later compression moulded at 160°C based on the respective t90 values.
2.3 Characterization of EPDM, EPDM-g-MAH and PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH
FTIR spectra were recorded on a Fourier-Transform infrared (FTIR) spectrometer model Perkin Elmer System 2000 equipped with attenuated total reflectance (ATR) technique to characterize the possible interactions between HNTs, palm ash, EPDM-g-MAH and EPDM in a range between 550 and 4000 cm-1 with a 0.4 cm-1 resolution. Films of EPDM and EPDM-g-MAH were prepared into thin sheets using a two-roll mill. The EPDM-g-MAH films were vacuum dried at 75°C for 14 hours to evaporate the unreacted MAH before testing. The films were also prepared to characterize the PA/HNTs/EPDM and PA/HNTs/EPDM/EPDM-g-MAH hybrid composites.
2.4 Measurement of tensile properties
Dumb-bell shaped test pieces were cut from the moulded sheets that were previously conditioned for 24 hours at room temperature. Tensile tests were performed using an Instron Machine IX3366 at a crosshead speed of 500 mm/min in accordance to ISO 37. Data such as tensile modulus (M100 and M300), tensile strength and elongation at break (Eb) were obtained from the tests.
2.5 Measurement of rubber-filler interactions
Measurements of the rubber-filler interaction were determined through swelling of the hybrid composite in toluene according to ISO 1817. Test pieces of 30 mm x 5mm x 2 mm in dimensions were prepared from the moulded sheets. The initial weight of each of the test pieces was recorded prior to the testing. All the test pieces were then soaked in toluene and conditioned at 25°C in a dark environment for 72 hours. After the expiration of the conditioned period, the test pieces were weighed again. The test pieces were then dried in an oven at 70°C for 15 minutes and left to cool at room temperature for another 15 minutes before finally weighing them. The Lorenz-Park equation was applied in the study of rubber-filler interaction . The swelling index was calculated based on the following equation:
The subscript f and g refers to filled and gum vulcanizates, respectively; z is the ratio by weight of the filler to the rubber hydrocarbon in the vulcanizates and; a and b are constants. In this study, the weight of toluene uptake per gram of rubber hydrocarbon (Q) is calculated as follows:
The higher the Qf/Qg values obtained is reflective of the lower interaction between the fillers and the rubber .
2.6 Scanning electron microscopy observation of the tensile fracture surfaces
The tensile fracture surfaces of the hybrid composite were investigated using ZEISS SUPRA™ 35VP Scanning Electron Microscope fitted with GEMINI field emission column. The fractured surfaces were mounted on aluminium stubs and sputter coated with a thin layer of gold-palladium to eliminate electrostatic charge build-up during examination of the sample.
Results and discussions
3.1 Maleic anhydride grafting efficiency
FTIR technique is often used to characterize the formation of bonds. The formation of bonds such as hydrogen bonds can be determined with this technique [23, 29]. Figure 1 shows a comparison of FTIR spectra between pure EPDM and EPDM-g-MAH in the 4000 to 550 cm-1 region.
According to Ismail et al. , the grafting of MAH onto EPDM is verified by the presence of bands at 1710-1719 cm-1 and 1770-1792 cm-1 which are attributed to the C=O symmetric stretching bonds. The presence of bands at 1791 cm-1 in the FTIR spectra (Figure 1b) indicated the grafting of MAH onto EPDM. However, the low intensity indicated that the grafting of MAH onto EPDM through procedures reported by Nakason et al.  was not efficient for EPDM. On the other hand, the presence of bands at 1280-1150 cm-1 which was related to the formation of ester bonds, was found in the spectra at 1262 cm-1 and 1150 cm-1. The presence of these bands indicated that the MAH incorporated into the EPDM formed ester bonds with the EPDM chains. The bands at 874 cm-1 in the EPDM-g-MAH spectra indicated the presence of MAH.
3.2 Characterization of PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH
Figure 2 shows a comparison between FTIR spectra of PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH. The FTIR spectra used were from hybrid composites at equal weight ratio (15PA/15HNTs) with and without EPDM-g-MAH.
The main reason for incorporating a compatibilizer into any composite is to induce an interaction between the filler and the matrix. The compatibilizer works in such a way that it creates a bridge for interactions between the matrix and the hybrid fillers. According to Ismail et al.  and Du et al. , the adsorption peaks of HNTs around 912 cm-1 and 1032 cm-1, are related to the Al-OH librations and Si-O stretching bands respectively. The adsorption peaks of palm ash around 3200-3750 cm-1 and 830-1100 cm-1 are associated with the -OH groups and the silanol group due to the presence of silica on the surface of the palm ash .
As seen in the spectra (Figure 2a), the adsorption spectra around 3393 cm-1 is assigned to -OH group of the palm ash and the adsorption peak at 912 cm-1 is associated with Al-OH librations of HNTs. When comparing both spectra (a) and (b) in Figure 2, it was found that while there were some peaks that have shifted, some new peaks were formed. The adsorption peaks of -OH groups in palm ash have shifted from 3393 cm-1 to 3360 cm-1 and the adsorption peaks of 1169 cm-1, 1159 cm-1, 1142 cm-1 and 1131 cm-1 that were associated with the formation of silane (Si-O-C) bonds were present in the compatibilized hybrid composites. The adsorption peaks of Al-OH librations for HNTs at 912cm-1 had shifted to 931 cm-1, whereas, the adsorption peaks of Si-O stretching of HNTs at 1016 cm-1, 1028 cm-1 and 1069 cm-1 have appeared again in the spectra.
The shift and the absence of peaks in the hybrid composites were associated with the formation of hydrogen bonding between the two fillers. The shift in adsorption peaks of -OH groups in palm ash from 3393 cm-1 to 3360 cm-1 can be associated with the presence of adsorption peaks of Si-O stretching in HNTs at 1016 cm-1, 1028 cm-1 and 1069 cm-1 that have shifted to 1014 cm-1, 1032 cm-1 and 1081 cm-1. The shift in the adsorption peaks of palm ash and the presence of HNTs Si-O stretching peaks were associated with the formation of hydrogen bonds between the functional group of the fillers and the compatibilizer.
The interaction between the fillers and the compatibilizer was associated with the formation of peaks at 1169 cm-1, 1159 cm-1, 1142 cm-1 and 1131 cm-1. The possible interactions of the fillers that may form silane (Si-O-C) bonds and hydrogen bonds are illustrated in Figure 3. The dotted lines in the illustration refer to the silane (Si-O-C) bonds between the functional groups. Apart from the proposed interaction, it is also believed that hydrogen bonds exist between Palm Ash/HNTs and HNTs/HNTs.
3.3 Determination of curing characteristics
Figures 4-5 exhibits the result of scorch time and cure time of PA/HNTs/EPDM hybrid composites with and without the presence of EPDM-g-MAH. As observed in Figure 4, the scorch time of the PA/HNT/EPDM hybrid composites with and without EPDM-g-MAH decreases as the weight ratios headed towards a higher weight ratio of HNTs. The decrease in scorch time may be attributed to the formation of bonds between PA/HNTs, PA/EPDM-g-MAH and HNTs/EPDM-g-MAH as previously discussed (refer to Figure 3). Earlier studies by Du et al.  indicated that the presence of hydrogen bonding in the composite accelerated the curing process.
At a similar filler loading, the scorch time of PA/HNTs/EPDM hybrid composites with EPDM-g-MAH exhibited a longer scorch time than PA/HNTs/EPDM hybrid composites without the presence of EPDM-g-MAH. Based on this observation, the exhibition of different scorch time values might be due to the improvement in filler dispersion in the EPDM matrix with the presence of EPDM-g-MAH.
Unlike the scorch time, at a similar PA/HNTs weight ratio, the cure time of PA/HNTs/EPDM hybrid composites with EPDM-g-MAH were shorter than PA/HNTs/EPDM hybrid composites without EPDM-g-MAH (Figure 5). Based on this observation, it can be concluded that the result was due to the enhancement of the interaction between the matrix and fillers due to the presence of EPDM-g-MAH as was proposed in Figure 3.
Figure 6 shows the maximum torque values of PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH at different Palm Ash/Halloysite Nanotubes weight ratios. It can be seen that the maximum torque of PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH exhibited an increasing trend with an increasing weight ratio of halloysite nanotubes. At a similar palm ash/halloysite nanotubes weight ratio, the maximum torque of PA/HNTs/EPDM hybrid composites with EPDM-g-MAH exhibited a higher value than the value of PA/HNTs/EPDM hybrid composites without the presence of EPDM-g-MAH.
3.4 Tensile properties
Figure 7 exhibits the tensile strength of the PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH at different palm ash/halloysite nanotubes weight ratios. As observed, the tensile strength of the hybrid composites with EPDM-g-MAH exhibited a similar trend as the PA/HNTs/EPDM hybrid composites without EPDM-g-MAH. The tensile strength increased in tandem with the increase in the weight ratio of halloysite nanotubes. Halloysite nanotube is nanotubular filler with a high surface area for interaction with the EPDM matrix. The higher surface area resulted in a better interaction between the filler and matrix, and therefore provided a better load transfer from the matrix to the reinforcements.
At a similar PA/HNTs weight ratio, the tensile strength of the PA/HNTs/EPDM hybrid composites with EPDM-g-MAH exhibit a higher value than the value of the hybrid composites without EPDM-g-MAH. As discussed before, the improvement was due to the enhancement in interfacial interaction between the fillers and the rubber matrix and the presence of EPDM-g-MAH as a compatibilizer.
Figures 8-9 showed the tensile modulus at 100% elongation and 300% elongation for PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH respectively. The tensile modulus exhibited an increment with an increased in halloysite nanotubes ratios. These observations indicated that the incorporation of higher weight ratios of HNTs into the rubber matrix resulted in an increased stiffness of the hybrid composites. Again, the better tensile modulus of PA/HNTs/EPDM/EPDM-g-MAH hybrid composites was due to the strong interfacial interactions among HNTs/EPDM-g-MAH, HNTs/Palm ash, PA/EPDM-g-MAH, and the other ingredients as illustrated in Figure 3. The strong interaction of HNTs in EPDM rubber composites was previously reported by Ismail et al. . The strong interactions reduced the elasticity and restricted the movements of the rubber chains resulting in a more rigid and stiffer composite.
Elongation at break of the PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH is shown in Figure 10. The elongation of break of the hybrid composites with and without EPDM-g-MAH exhibits a slight decreasing trend with the increasing weight ratio of halloysite nanotubes. As the weight ratio of halloysite nanotubes increased, the effects of halloysite nanotubes became more dominant, thus increasing the stiffness of the hybrid composites. At a similar PA/HNTs weight ratio, the elongation at break of hybrid composites with EPDM-g-MAH exhibited lower values than hybrid composites without EPDM-g-MAH due to the enhanced interfacial interaction and the stiffening effect that restricted the movement of the rubber chains leading to a decrease in the ability of PA/HNTs/EPDM hybrid composites to elongate.
3.5 Rubber-filler interaction
The rubber-filler interaction (Qf/ Qg) of PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH is shown in Figure 11. It can be seen that the PA/HNTs/EPDM hybrid composites with EPDM-g-MAH showed a decreasing trend of Qf/ Qg as halloysite nanotubes weight ratio increases. The decrease in toluene uptake is a sign of improvement in rubber-filler interaction.
However, at a similar PA/HNTs weight ratio, the Qf/ Qg of PA/HNTs/EPDM hybrid composites with EPDM-g-MAH exhibited lower values. These observations might be associated with the enhancement of interfacial interaction between the fillers and rubber matrix leading to a higher crosslink in the hybrid composites. The enhanced interaction between the fillers and the EPDM matrix gave chance to the formation of a crosslink to per unit of rubber chain a higher possibility . As observed in Figure 11, a higher crosslink per unit of rubber chain will decrease the ability of the rubber to be swelled by toluene and therefore, will reduce the uptake of toluene.
3.6 Morphological studies of tensile fracture surface
Figure 12 showed the SEM micrographs of PA/HNTs/EPDM hybrid composites with and without the presence of EPDM-g-MAH at 30PA/0HNTs weight ratio.
The micrographs reveal that with the addition of EPDM-g-MAH, the fracture surface that was observed exhibited an evidence of significant matrix tearing (shown by arrow) compared to similar composite without EPDM-g-MAH. The apparent roughness of the fracture surface indicated an enhancement of interfacial interaction between the fillers and EPDM rubber matrix. In addition to that, the size of the filler pull-out (as circled) in Figure 12b is smaller than the size shown (as circled) in Figure 12a.
Figure 13 and 14 show the micrographs of PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH at 15PA/15HNTs and 0PA/30HNTs weight ratios respectively. Again, the micrographs exhibited an increase in matrix tearing after incorporating EPDM-g-MAH. Apart from that, it was also observed that the fracture surfaces had become more rugged than the hybrid composites without EPDM-g-MAH (Figure 13b and 14b). There were less ruggedness of the matrix surfaces as shown in Figure 13a and 14a.
Figure 15 represented the tensile fracture of PA/HNTs/EPDM hybrid composites with and without EPDM-g-MAH at different weight ratio of 15PA/15HNTs and 0PA/30HNTs as observed at 10,000x magnification. Figure 15c and 15d showed that the HNTs were well embedded (indicated by white arrows) within the EPDM rubber matrix as compared to Figure 15a and 15b that showed that there were some evidence of detachment of fillers from the rubber matrix as shown by the dotted white circles. The embedment of the fillers is an apparent proof of the enhanced interfacial interaction of PA/HNTs/EPDM hybrid composites when EPDM-g-MAH is present.
The presence of EPDM-g-MAH enhanced the tensile properties and filler-rubber interactions of PA/HNTs/EPDM hybrid composites. The addition of EPDM-g-MAH resulted in a prolonged scorch time and reduced cure time of PA/HNTs/EPDM hybrid composites. SEM micrographs of the tensile-fractured surfaces of the composites revealed that when EPDM-g-MAH is present, the tensile-fractured surfaces of the composites exhibited an apparent ruggedness as a result of improved interfacial interaction between the fillers and EPDM matrix.