Water Absorption Behaviour and Mechanical Properties of High Density Polyethylene/ Pistachio Shell Flour Nanocomposites in

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1 Water Absorption Behaviour and Mechanical Properties of High Density Polyethylene/ Pistachio Shell Flour Nanocomposites in Water Absorption Behaviour and Mechanical Properties of High Density Polyethylene/ Pistachio Shell Flour Nanocomposites in M.A. Abedini Najafabadi, S. Nouri Khorasani, and J. Moftakharian Esfahani a Department of Chemical Engineering, Isfahan University of Technology, Isfahan, , Iran Received: 16 January 2013, Accepted: 16 May 2013 SUMMARY In this research, for the first time, the effects were investigated of nanoclay as a secondary reinforcement, titanium dioxide ( ) and hindered amine light stabilizer (HALS) as UV stabilizers on some properties of high density polyethylene (HDPE)/pistachio shell flour (PSF) composite as the new wood plastic composite (WPC). WPCs of HDPE/PSF with different levels of nanoclay (Cloisite 20A), and HALS were prepared. Some properties were investigated before exposure to weathering conditions. Design of experiments (DOE) was carried out to find the best formulation and decrease the number of tests. WPC granules were prepared by melt-mixing method using a twin screw extruder, and then moulded using an injection moulding machine to prepare samples. The results of X-ray diffractometry (XRD) indicated partial exfoliation and intercalation of samples containing 3 and 6 per hundred composite (phc) of nanoclay. The tensile strength and modulus of HDPE/PSF samples containing 3 and 6 phc nanoclay showed 20% increase compared to samples without nanoclay. The presence of and HALS in the formulation did not have a significant effect on tensile properties. The nanoclay presence in HDPE/ PSF formulation improved the water absorption resistance. Keywords: Nanocomposite, Mechanical properties, High density polyethylene, Pistachio shell flour, Wood plastic composite 1. Introduction In the past decades, there has been extensive interest in developing woodplastic composites (WPCs). This has been due to the inherent advantages of WPCs such as high stiffness, low density and low cost compared with other inorganic reinforcements like glass fibres, calcium carbonate or talc. Polyethylene (PE), polypropylene (PP), and poly(vinyl chloride) (PVC) are the most commonly used thermoplastic polymers in manufacture of WPCs 1. Common applications of WPCs are in furniture, decking, fencing for outdoor, flooring and siding for building components 2. These composite materials are very sensitive to flame, Correspondence to: Saied Nouri Khorasani, saied@cc.iut.ac.ir. Tel.: ; fax: Smithers Information Ltd., 2014 impact, creep, UV light, thermal ageing and water absorption 3. Therefore, there is a great concern about mechanical properties, water absorption and UV light behaviour of WPCs. There are many reports on physico mechanical properties and water absorption behaviour of natural fibres and wood flour in WPC production. These natural fibres and wood flour include rice husk flour, pine-cone flour, wood flour, cellulose fibre and sawdust 4-7. Yang 4 studied physicomechanical and morphological properties of WPCs with PP as the matrix and rice-husk flour as the filler at various processing temperatures and filler loadings. It was observed that tensile modulus of WPC improved when increasing filler loading. Also, tensile strengths of the composites slightly decreased as the filler loading increased. Addition of rice-husk flour deteriorated the impact properties of PP composites. Ayrilmis 5 investigated the effect of pine-cone flour and wood flour as reinforcements on physicomechanical properties of PP. The water resistance and flexural properties of the composites were adversely affected by an increase in pine cone-flour content. However, the flexural properties and water absorption resistance of the WPC samples were not significantly affected by the addition of 10 wt.% of the pine cone flour compared to the WPC samples prepared from wood flour. Viksne 6 evaluated the effect of wood derived fillers (WDF) on mechanical strength, water uptake in the static and dynamic condition of PP matrix. Composites containing wood fibres showed the highest flexural strength Polymers & Polymer Composites, Vol. 22, No. 4,

2 M.A. Abedini Najafabadi, S. Nouri Khorasani, and J. Moftakharian Esfahani at different temperature. Significant difference was observed between PP neat and composites containing HW filler regarding water uptake in dynamic tests. Jahadi et al. 7 studied mechanical properties of sawdust/hdpe composite samples. The results demonstrated that a higher diffusion of water in the matrix led to lower tensile and flexural strengths in distilled water until saturation was reached. There are limited reports on the PSF reinforced polymer, Saffarzadeh 8 investigated the effect of Pistachio twig flour content on water uptake of PP composites. In that research, the water absorption of PP/Pistachio twig flour composite containing different level of pistachio twig flour was investigated. The results showed that the water absorption of composites increased with increasing Pistachio twig flour content. On the other hand, incorporation of nanoclay additive into the polymer matrices has shown enhancements in different properties including decrease in flammability and water absorption, while mechanical properties were improved. This is mostly attributed to the nanometric scale and high aspect ratio characteristics of the individual silicate platelets of nanoclay. The effects of nanoclay on physicomechanical properties of WPC were investigated in many reports. Kord 9 investigated effect of nanoclay on physico-mechanical properties of wood flour/pp. It was reported that the flexural and tensile moduli of WPCs were increased by adding 3 wt.% nanoclay. Also, impact strength and water absorption decreased with increasing nanoclay loading until 6 wt.%. Zhang et al. 10 reported that using PE grafted maleic anhydride (PEg-MA) in wood/hdpe nanocomposite formulation increased the interlayer spacing of the nanoclay layers. Also, it was reported that the coefficient of thermal expansion and flexural properties of WPC showed 60% and 10% improvement after adding 3 wt.% nanoclay. Lei et al. 11 evaluated the crystallization behaviour, mechanical properties, water absorption and thermal stability of HDPE/pine cone flour/nanoclay composites. The crystallization temperature (T c ), crystallization rate, and the crystallinity level of the HDPE/pine cone flour composites, were decreased by adding 2 wt.% of nanoclay. The flexural and tensile strength of HDPE/ pine flour composites increased about 20 and 24% with addition of 1 wt.% nanoclay, respectively. Sheshmani 12 investigated physical properties of PP/recycled newspaper fibre composites. PE-g-MA and nanoclay were used as coupling agent and reinforcing agent respectively. The results indicated that water absorption resistance was improved by adding nanoclay. Esnaashari 13 investigated effect of nanoclay on mechanical and water absorption properties of sawdust/ LDPE composites. It was observed that increasing nanoclay loading can improve tensile and flexural properties and decreases water diffusion coefficient. WPC formulation should contain UV stabilizers including organic stabilizer and inorganic stabilizer, HALS, due to exposure to weathering condition which in turn results in failure. There are many different reports on mechanical behaviour of WPC containing UV stabilizer and pigments under weathering exposure But the effect of different UV stabilizers simultaneously on mechanical properties and water absorption behaviour of WPC before exposure to weathering condition has not been evaluated. As far as the authors of this paper are aware, no previous research in the field of WPCs has investigated the effect of nanoclay, and simultaneous effect of and HALS on properties of HDPE/PSF composites. Therefore in this research for the first time we are reporting results of above variants on some mechanical properties and water absorption behaviour of HDPE/PSF. 2. Experimental 2.1 Materials HDPE (HD-5218UA) was supplied by Tabriz Petrochemical Company of Iran with a density of g/cm 3 which has a melt flow index (MFI) of 18 g/10 min (ASTM D 1238) and HDPE-grafted maleic anhydride (PEg-MAH, Kimia Javid Company, Iran) were used. Pistachio shell flour was obtained from Kavir wood industrial company of Iran with a mesh particle size as natural reinforcement of HDPE matrix to produce WPC. Nanoclay (Cloisite 20A) is a natural montmorillonite modified with dimethyl, dehydrogenated tallow, quaternary ammonium used as organically modified clay which was obtained from Southern Clay, Texas USA. (R-931, Siba company, USA), HALS (TINUVIN 783 FDL, Siba company, USA) and polyethylene wax (PE-Wax 680, Kimia Javid Company, Iran) were used as pigment, hindered amine UV stabilizer and lubricant respectively. 2.2 Sample Preparation In order to choose the best formulation and prepare a batch to test, design of the experiments by DOE was carried out. DOE is a statistical Taguchi method using Minitab 15 software. The effects of three factors (Cloisite 20A,, and HALS) at three levels on mechanical properties and water absorption behaviour were investigated. Nine compounds were prepared according to an L9 Taguchi orthogonal array which has nine combinations of levels. Table 1 shows the formulation of the compounds and variable factors. The results were statistically analyzed 410 Polymers & Polymer Composites, Vol. 22, No. 4, 2014

3 Water Absorption Behaviour and Mechanical Properties of High Density Polyethylene/ Pistachio Shell Flour Nanocomposites in with 95% confidence using Qualitek 4 software. five samples were tested for each formulation. 3. Results and discussion PSF and nanoclay were dried in a vacuum oven at 105 and 60 C for 24 h respectively. Batches containing nanoclay and MAPE were prepared by melt mixing while other batches were dry mixed. HDPE, nanoclay, MA-g-PE and PE wax were mixed physically and compounded in a twin screw extruder. Produced granules were dried at 100 C for 12 h and then extruded with pistachio shell flour to produce wood plastic nanocomposite compounds. Extruder used was SHJ-20 a counterrotating twin screw extruder having L/D, 40:1 which was bought from China. The screw speed was 60 rpm and the extrusion temperatures were controlled at 100 to 170 C in six heat zones. Samples for tensile, impact and water absorption tests were prepared using an injection moulding machine. 2.3 Characterization X-ray diffraction (XRD) patterns were obtained using a D8 ADVANCE XRD system (Bruker, Germany) equipped with CuKa radiation source at the generator voltage of 40 kv and generator current of 40 ma (l = A ). The XRD specimens were made into powder form and Bragg s law, nl = 2dsinθ, was used to calculate the crystallographic spacing (d-spacing), to characterize intercalation and exfoliation structure of nanoclay in nanocomposites. 2.5 Water Absorption Water absorption behaviour of different samples was determined using water immersion test. It was only to compare the effect of different factors in water uptake behaviour of the HDPE/ nanocomposites/hals/ samples. Samples of each formulation in dimensions cm 3 were prepared and dried in an oven at 110 C for 24 hours. Before testing, samples were weighed and the initial weight (m 0 ) was recorded. Samples were then immersed in distilled water at 80 C. After 24 h nanocomposite samples were removed from the water and water from surface was wiped off using blotting paper, then dried samples were weighed to record a final sample weight (m f ). The water absorption (W a ) of samples was calculated using following Equation: W a = m f m 0 m 0 (1) 3.1 XRD Analysis Figure 1 shows the XRD patterns of HDPE/Cloisite 20A nanocomposites for samples coded 5 and 8 in Table 1 at various nanoclay contents 3 and 6 phc, compared with neat Cloisite 20A. XRD pattern of Cloisite 20A shows a characteristic peak at 2θ=3.63 o A which indicates an interlayer spacing of A according Bragg s law. Results of the XRD tests are summarized in Table 2. As shown in Figure 1 and Table 2, in the diffraction peak, intensity reduction was observed for the HDPE nanocomposites. It confirms an increased disordering of nanoclay layers in the nanocomposite samples, as also discussed by Faruk 16. Also, samples containing nanoclay show a broad peak around the characteristic peak position of the neat nanoclay, which may be due to a partial exfoliation of nanoclay layers in these samples. A similar observation was reported by Esnaashari 13. Figure 1. XRD patterns of neat nanoclay and nanocomposite samples 2.4 Mechanical Properties Tensile tests on injection moulded samples were conducted according to ASTM D638 using a universal tensile machine Zwick, Germany. The tests were carried out at a crosshead speed of 5 mm/min. Five samples for each formulation were tested. Izod impact test was carried out based on ASTM D 256, using Zwick impact tester. Samples were prepared in mm 3 dimensions and Polymers & Polymer Composites, Vol. 22, No. 4,

4 M.A. Abedini Najafabadi, S. Nouri Khorasani, and J. Moftakharian Esfahani Table 1. Formulation of different HDPE/PSF nanocomposite samples Sample code HDPE (wt.%) PSF (wt.%) MA-g-PE (wt.%) PE WAX (wt.%) Nanoclay (phc) (phc) HALS (phc) It is observed that, the interlayer spacing has increased from nm for the neat Cloisite 20A to 3.87 nm (2θ=2.29) and 3.1 nm (2θ=2.85) in HDPE nanocomposite containing 3 and 6 phc nanoclay respectively. Also, the relative intercalation (RI) of silicate layers of nanoclay by polymer chains was quantified using following equation: Table 2. XRD results of nanocomposite samples Sample Characteristic peak d (nm) RI (%) 2q (deg) Cloisite 20A HDPE/3 phc nanoclay HDPE/6 phc nanoclay Figure 2. Effect of different factors on tensile strength of HDPE/PSF %RI = (d d0) 100 d0 (2) In Equation (2), d 0 and d are interlayer spacing of neat nanoclay Cloisite 20A and nanocomposite HDPE/Cloisite 20A respectively. The percentages of relative intercalation (%RI) for samples containing 3 and 6 phc nanoclay were 59.26% and 27.57% respectively, which indicates a significant extent of intercalation of the Cloisite20A layers by HDPE. A combination of intercalation and partial exfoliation structure for nanocomposite samples containing 3 phc nanoclay occurred. Inappropriate dispersion of nanoclay layers in HDPE nanocomposites containing 6 phc nanoclay was observed. 3.2 Mechanical Properties Tensile Strength Using DOE, the effects of three different factors at three levels on the tensile strengths of samples are shown in Figures 2 and 3. As shown in Figure 2, the tensile strengths of nanocomposite samples containing 3 and 6 phc nanoclay improved by 27 and 23% respectively compared to sample without nanoclay. This improvement is related to the fact that the polymer chains can diffuse into nanoclay layers, so that a high area of intercalated silicate layer of nanoclay resulted. It is also due to a strong adhesion between polymer chains and nanoclay layers, which limits the mobility of the polymer chains. Therefore, stress in nanocomposite samples is transferred to stiff nanoclay layers and improves tensile strength of HDPE nanocomposites. A slight decrease in tensile strength for HDPE/PSF containing 6 phc nanoclay as compared to samples 412 Polymers & Polymer Composites, Vol. 22, No. 4, 2014

5 Water Absorption Behaviour and Mechanical Properties of High Density Polyethylene/ Pistachio Shell Flour Nanocomposites in containing 3 phc is related to unwanted agglomeration formation due to lack of enough coupling agent for formation of an intercalated nanoclay structure. This reduces the reinforcing efficiency of nanoclay. Figure 2 indicates that the tensile strength of specimens containing 2 and 4 phc of without nanoclay decreased by 14% and 27% respectively. This is due to the fact that low molecular weight, acts as a plasticizer and decreases the tensile strength. It was observed that samples containing pigment did not show brittle behaviour at the yield point, whereas the other samples illustrated brittle breakage. Although HALS has a relatively high molecular weight, the addition of HALS as UV stabilizer to HDPE/PSF formulation did not play a significant role in the tensile strength of samples. which is attributed to migration of nanoparticle into the PSF and polymer chain interface, caused a decreased performance. A positive effect which is related to the dispersion of nanoclay layers, enhances the modulus. In samples containing 3 phc nanoclay, it is clear that the positive effect was dominant, and the tensile modulus improved when adding nanoclay. This effect was less for nanocomposite samples containing 6 phc nanoclay, which was due to dominance by the negative effect and produced a lower Figure 3. Portion of effectiveness of different factors on tensile strength Figure 4. Effects of different factors on tensile modulus of HDPE/PSF Contribution of different factors in formulation on tensile strength was shown in Figure 3. It is clear that the greatest effects are related to nanoclay, and HALS respectively. Nanoclay can play 65% effect in tensile strength of different samples while and HALS affect the tensile strength by only 7% and 5% respectively Tensile Modulus The effects of three factors on the tensile modulus of HDPE/PSF are illustrated in Figures 4 and 5. Results indicate that the presence of 3 phc of nanoclay improved the tensile modulus by 11% compared to formulations containing no nanoclay. This improvement is due to a large interfacial area (due to nanoclay presence) which leads to better stress transfer at the interface between the silicate layers of nanoclay and HDPE composites. Figure 5. Portion of effectiveness of different factors on tensile modulus On the other hand, as discussed by Kord 9, the reinforcing efficiency of nanoclay depends on two different phenomena. A negative effect, Polymers & Polymer Composites, Vol. 22, No. 4,

6 M.A. Abedini Najafabadi, S. Nouri Khorasani, and J. Moftakharian Esfahani intercalated structure as discussed above. Figure 4 shows that the tensile modulus of samples containing nanoclay was improved by adding only 2 phc of, which is related to the filler effect of the low concentration of as a pigment, but increasing the content up to 4 phc in the composite decreased the tensile modulus slightly. This can be explained on the basis that an intercalated structure of the nanocomposite samples resulted in limitation in polymer chain s mobility, and the low concentration acted as a filler which improved the tensile modulus. In HDPE/PSF composite containing 4 phc of, the filler acted as stress concentration points which in turn decreased the tensile modulus. As shown in Figures 4 and 5, HALS did not have a significant effect on the tensile modulus of HDPE/PSF samples, as explained above in the discussion of tensile strength behaviour. Figure 5 indicates that the nanoclay content is an important factor in tensile modulus compared to compounds containing and HALS Izod Impact Strength Izod impact strengths of compounds containing different levels of additives are illustrated in Figures 6 and 7. Izod impact strength of nanocomposite samples was decreased 10% by increasing nanoclay concentration. This indicates that addition of nanoclay decreased the ductility of Figure 6. Effect of different factors on Impact strength of HDPE/PSF the composites. This phenomenon is related to the formation of low clay agglomerates which led to a higher stress concentration in nanocomposites samples and accelerated crack initiation and crack propagation stage. The impact strengths of samples containing 6 phc of nanoclay show a greater drop in comparison to samples containing 3 phc nanoclay, which can be linked to higher agglomerated structure of silicate layers of nanoclay in samples containing 6 phc nanoclay. Addition of 2 phc in the presence of nanoclay to HDPE compounds can act as filler in HDPE/ PSF nanocomposites which improve stiffness of sample and its impact strength. This can be explained by considering that a higher stiffness of composite resulted in delayed crack propagation and improved the impact strength. Addition of 4 phc of in HDPE/ PSF nanocomposite samples led to low compatibility between the matrix and additives. The impact strength decreased in the presence of small cracks or flaws. Figure 7 shows that nanoclay had a greater contribution to the impact strength of nanocomposite samples compared to samples containing HALS and pigment, as discussed before in the Tensile Strength section. Figure 7. Relative effectiveness of different factors on impact strength 3.3 Water Absorption Behaviour One of the important properties that have been investigated for the HDPE/ PSF was water absorption behaviour because it limits their application. The fact that cellulosic fibres easily absorb water is one of the reasons for fibre surface treatments. The treated fibres may absorb less moisture, and thus favour adhesion to the polymer matrix, which results in a better performance in a humid environments. The high water absorption of the HDPE/PSF may cause difficulties during processing. 414 Polymers & Polymer Composites, Vol. 22, No. 4, 2014

7 Water Absorption Behaviour and Mechanical Properties of High Density Polyethylene/ Pistachio Shell Flour Nanocomposites in This can be due to incomplete curing of the thermosetting matrices the presence of voids or cracks or even poor matrix fibre adhesion as also discussed by Sheshmani 12. Figure 8. Effects of different factors on water absorption behaviour of HDPE/PSF Figure 8 shows the water absorption as a function of different contents of nanoclay, and HALS. The results indicate HDPE/PSF samples without nanoclay had higher water absorption compared to nanocomposite samples. This is due to hydrogen bonding of the water molecules to the free hydroxyl groups present in the cellulosic cell wall materials and the diffusion of water molecules into the filler-matrix interface. Also, as discussed by Gassemi et al. 17, nanoclay can play two different roles in barrier properties and water absorption resistance of WPC samples. Firstly, the hydrophilic nature of the clay surface tends to immobilize some of the moisture, and secondly, surfactant-covered clay platelets form a tortuous path for water transport. Of course these two mechanisms could be more efficient when the silicate platelets of the nanoclay are exfoliated. The water absorption resistance of HDPE/PSF samples containing 3 phc nanoclay is higher than that for samples containing 6 phc nanoclay. This is justified by XRD analysis of nanocomposites. As discussed regarding XRD analysis, samples containing 6 phc nanoclay showed intercalated structures which led to agglomerate formation in polymer matrix and resulted in low water absorption resistance compared to samples containing 3 phc of nanoclay. The effect of nanoclay on water absorption behaviour of HDPE/PSF samples was higher than for samples containing and HALS. HALS and had a considerable effect on UV absorption behaviour of WPC samples and did not show remarkable effects on the water absorption of nanocomposite samples. It is notable that water absorption behaviour of nanocomposite samples with different contents of and HALS did not follow a meaningful pattern. 4. CONCLUSIONS The results of this study indicate that HDPE/PSF nanocomposite could be used as new WPCs with superior mechanical properties and water absorption resistance behaviour. This research documented morphological structure, some mechanical properties and water absorption behaviour of HDPE/PSF nanocomposite containing different level of nanoclay, and HALS. The experimental results point to the following conclusions: 1. X-ray diffraction analysis demonstrated exfoliated and intercalated silicate layers of the nanoclay particles in HDPE/PSF samples containing 3 phc and 6 phc nanoclay respectively. 2. The tensile strengths of nanocomposite samples containing 3 and 6 phc nanoclay improved by 27 and 23% compared to samples without nanoclay respectively. The tensile moduli of HDPE/PSF nanocomposite samples were higher than those of samples of HDPE/PSF. 3. The Izod impact strength of nanocomposite samples was decreased by 10% when increasing the nanoclay loading from 3 to 6 phc. This was an indication that addition of nanoclay to HDPE/PSF formulation decreased the ductility of the samples. 4. The results show that nanoclay presence in HDPE /PSF composite improved water absorption resistance by 13% in comparison to HDPE/PSF. 5. and HALS did not have significant effects on the mechanical properties and water absorption behaviour. References 1. Marathe D.S. and Joshi P.S., Journal of Applied Polymer Science, 114(1) (2009) Abu Bakar M.B., Mohd Ishak Z.A., Mat Taib R., Rozman H.D. and Mohamad Jani S., Journal of Applied Polymer Science, 116(5) (2010) Ndiaye D., Fanton E., Therias S.M., Vidal L., Tidjani A., and Gardette J.L., Composite Science and Technology, 68(13) (2008) Yang H.S., Kim H.J., Son J., Park H.J., Lee B.J., and Hwang T.S., Composite Structures, 63(3-4) (2004) Polymers & Polymer Composites, Vol. 22, No. 4,

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