The Flexural, Impact and Thermal Properties of Untreated Short Sugar Palm Fibre Reinforced High Impact Polystyrene (HIPS) Composites

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1 The Flexural, Impact and Thermal Properties of Untreated Short Sugar Palm Fibre Reinforced High Impact Polystyrene (HIPS) Composites The Flexural, Impact and Thermal Properties of Untreated Short Sugar Palm Fibre Reinforced High Impact Polystyrene (HIPS) Composites D. Bachtiar 1, S.M. Sapuan 1,*, A. Khalina 2, E.S. Zainudin 1,3 and K.Z.M. Dahlan 4 1 Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia 2 Department of Biological and Agricultural Engineering, Universiti Putra Malaysia, Serdang, Selangor, Malaysia 3 Institute of Tropical Forestry and Forest Product/Faculty of Forestry, Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia 4 Radiation Processing Division, Malaysian Nuclear Agency, Bangi, 40300, Kajang, Selangor, Malaysia Received: 16 August 2010, Accepted: 12 September 2011 SUMMARY This study investigated the mechanical and thermal performance of short sugar palm (Arenga pinnata) fibrereinforced high impact polystyrene composites. Fibre sizes of mesh and five different fibre loadings from 10 to 50% by weight have been used. The melt mixing method and hot compression moulding were used as the fabrication techniques for the composites. The flexural behaviour, impact testing, dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and the moisture absorption of short SPF-HIPS composites reinforced with varying fibre loadings were studied. The results showed that increasing the loading of short SPF in the HIPS matrix improved the flexural moduli of the composites. Lower impact strength resulted when the fibre loading was increased, and the moisture content increased significantly. The DMA test results showed increases in the storage modulus and thermal stability for the SPF-HIPS composites. Finally, the TGA test showed degradation behaviour of the composites when they were exposed to a high temperature environment. With some advantage embedding these fibres can be an alternative choice of natural fibre for reinforcing thermoplastics. Keywords: Sugar palm fibre, HIPS, Flexural, Impact, Moisture, DMA, TGA 1. Introduction Research on natural fibre composites is important because of its role in solving the challenge of the development of environmental-friendly material products. Natural fibres show good reinforcing agents as substitute inorganic fibre materials. A number of real advantages associated with the use of natural fibres include low cost, low densities, biodegradability, high specific properties, reduction of the wear in processing equipment and nonabrasive processing 1. Several types of natural fibres have been reported and are known to be used as reinforcements *Corresponding author (sapuan@eng.upm.edu.my) Smithers Rapra Technology, 2012 in polymer composites such as hemp, flax, abaca, sisal, jute, henequen, kenaf, ramie, sugar palm, oil palm, pineapple leaf, banana pseudo-stem, sugarcane bagasse, coir, rice husk, wood and bamboo 2,3,4. Sugar palm or Arenga pinnata fibre is a possible source of natural fibre from the sugar palm plant, a member of the Palmae family. But sugar palm fibre still has had little attention from researchers. Applications of this fibre cover a wide range, such as ropes, brooms, paintbrushes, filters, doormats, chair/sofa cushions and roofs because of its strength and durability 5,6. The sugar palm plant is a fast-growing palm reaching maturity in 7 years. It has become a permanent fixture throughout the world and a most economically important plant in Asia. The geographical distribution of sugar palm covers the Indo-Malay archipelago and includes tropical Southeast Asia countries, from Myanmar to the Philippines 6. It is used for construction materials, sugar, sago, wine, alcohol, fibre, thatch and many other products. This plant exhibits a tall palm with long leaves and decorative trunk fibre. The plantations of sugar palm have not been fully developed yet for several reasons, but recent development shows there is a new way to cultivate the sugar palm plant more effectively. The first sugar palm factory in the world has been built in Tomohon, Indonesia 7,8. Moreover, this plant can Polymers & Polymer Composites, Vol. 20, No. 5,

2 D. Bachtiar, S.M. Sapuan, A. Khalina, E.S. Zainudin and K.Z.M. Dahlan also produce the bioethanol. Sugar palm plant has a highest production capability of the alcohol compared with other sources such as sweet sorghum, sugarcane, and cassava. This plant will become a promising source of biofuel in the future 8,9. Once the plantation of sugar palms is growing, there will be high availability of sugar palm fibres as by-product. Some investigations about reinforcement of thermoset material by sugar palm fibre have been carried out. The studies were about the seeking of mechanical and morphological behaviour of epoxy composites reinforced with sugar palm fibres 10,11,12. The studies focus on the varied loading and arrangement of fibres. The results show that the 10 wt.% woven roving fibre has the highest value compared to other arrangements and content of the sugar palm fibres. The work of Siregar 13 has also shown that a new form of composite with good strength and rigidity can be formed by reinforcement of epoxy matrix with the sugar palm fibres. Several investigations on sugar palm reinforced epoxy composites continue to be done on aspects such as the effect of alkali treatment 14,15, environmental treatment 16, and ageing tests 17. analysis and thermogravimetric analysis) of the SPF-HIPS composites were studied in order to lay a foundation for applications of these materials in state-of-the-art composites. 2. MaterialS and MethodS 2.1 Materials Preparation The sugar palm fibres (Arenga pinnata) were obtained from traditional market in Indonesia (Aceh province). The fibres were crushed to be short form and sieved through 30 and 50 mesh screen. Five fibre contents of 10%, 20%, 30%, 40% and 50% by weight and the unreinforced HIPS matrix as reference were studied in this work. The high impact polystyrene used as the polymer matrix was Idemitsu PS HT 50, having a density of 1.04 g/cm 3 and supplied by Petrochemical (M) Sdn Bhd, Pasir Gudang, Johor, Malaysia. Figure 1a shows the short fibres and the HIPS matrix in pellet form. 2.2 Composites Manufacturing Compounding of short SPF and HIPS matrix was carried out using a melt mixer (Brabender Plasticorder intensive mixer model PL2000-6). The mixing temperature and screw speed were set at 165 C and 50 rpm respectively. HIPS was charged into the chamber and melted (3 minutes) before dried short SPFs were added. The mixing process of short SPF and HIPS took place for about 15 minutes. The material results are shown in Figure 1b. The melt-compounded mixture obtained from the previous process was placed in the hot compression mould heated at a temperature of 165 C, and it endured the process of preheating for 4 minutes. Heating was carried out for 4 minutes followed by cooling for further 4 minutes. The final form of the composites was plates with size 150 mm x 150 mm x 3 mm, Figure 1c. 2.3 Mechanical Testing Flexural Test The flexural (three-point bend) tests were carried out following ASTM D790 that provides test methods for evaluating flexural properties of unreinforced and reinforced plastics and electrical insulation materials. An Instron universal testing machine Figure 1. (a) Short SPF and HIPS pellet, (b) piece of SPF-HIPS composites after melt mixing processing, (c) plates of SPF-HIPS composites However, the characterization of the role of sugar palm fibres in the reinforcing of thermoplastic composites has not been studied yet. This research explores the new combination of sugar palm fibre with high impact polystyrene thermoplastic. High Impact Polystyrene (HIPS) is a type of polystyrene in which polybutadiene is added during polymerization. Polystyrene has known usage for high quality goods, e.g. toys, household appliances, cases, boxes, and calculators, computer housings, and a suggested application in this research is for roof tile. Some mechanical (flexural and impact) properties, physical properties (absorption behaviour) and thermal performance (dynamic mechanical (a) (c) (b) 494 Polymers & Polymer Composites, Vol. 20, No. 5, 2012

3 The Flexural, Impact and Thermal Properties of Untreated Short Sugar Palm Fibre Reinforced High Impact Polystyrene (HIPS) Composites (model 4301) was used. The maximum load and the crosshead speed were specified as 1 kn and 1.3 mm/min respectively, while the support span was 48 mm. The samples were prepared and cut into 5 rectangular specimens of 127 mm (l) x 12.7 mm (w) x and 3 mm (t) Impact Test Izod impact test has been used extensively to determine the impact resistance of any materials. The Izod impact tests were carried out for notched samples of 63.5 (l) x 12.7 (w) x 3 mm (t). The Izod impact strength was reported as energy per unit notch (kj/m 2 ). The value of pendulum energy for the testing was 4 J, and the tests were conducted according to ASTM D256 at the Polymer Laboratory, Malaysian Nuclear Agency. 2.4 Dynamic Mechanical Analysis (DMA) DMA is a characterization method of material viscoelasticity under dynamic conditions. This is done by applying a dynamic force and capturing the response to that input corresponding to time and temperature. The usage of DMA test was to understand the influence of fibre loading on the composites. The samples were prepared and cut into rectangular specimens of 50 mm (l) x 12.7 mm (w) x and 3 mm (t). DMA measurements were carried out on a TA Instrument operating in a three-point bending mode at 1 Hz frequency, and the samples were tested from 25 C to 140 C with a heating rate of 10 deg/ min. This procedure refers to the standard of ASTM D5023. in relation to change in temperature. A TGA/SDTA 851 e Mettler Toledo instrument was used. The specimens were observed from 30 to 600 C at a heating rate of 20 C/min. The samples weight varied from 6 mg to 20 mg. 2.6 Moisture Absorption Behaviour The moisture absorption of neat HIPS matrix and SPF-HIPS composites (10%- 50%wg. fibres) were conducted according to ASTM D570, and the shape of test specimens were mm 3. The specimens were placed in an oven at 50 C for 24 h, and then were cooled and immediately weighed to get the initial weight W 0, with g accuracy. The specimens were submerged in a box of water maintained at 25 C during 24 h. Furthermore, the specimens were taken from the water, and weighed immediately to get the final weight, W 1. The increase of weight of the samples was determined by using the formula (W 1 W 0 )/W SEM Analysis Scanning electron microscopy (SEM) was used to examine the fracture surface of samples after impact loading. The instrument used for this work was a PHILIPS XL 30 ESEM, and a thin layer of gold was coated onto the specimens surface to eliminate charging effects. 3. Results and Discussion 3.1 Flexural Properties Figure 2 shows the average values of flexural stress of short SPF-HIPS composites at various fibre loadings. It is visible that the flexural strengths of SPF-HIPS composites increased with increasing fibre contents up to a maximum of MPa (fibre content of 40 wt.%). However, all the values of the short SPF reinforced HIPS composites were still lower than that of unreinforced HIPS. The decrease of the flexural properties of short SPF-HIPS composites was due to the limitation to matrix yielding caused by penetration of the sugar palm fibres 18. It may be also due to the lack of compatibility between hydrophilic short SPF and hydrophobic HIPS matrix. However, the strength value continued to increase when the loading fibre content was higher. A peak value of flexural strength of the composites was attained by composites with 40%wg. of SPF, while it started to decrease for 50%wg. of SPF. This indicated that the optimum loading was around 40%wg. The modulus properties of short SPF-HIPS composites are shown in Figure 3. The result shows that adding Figure 2. The flexural strength of short SPF reinforced HIPS composites 2.5 Thermogravimetric Analysis Thermogravimetric analysis (TGA) is a common method for understanding the thermal stability of composite materials. This testing undertaken on the specimens is used to calculate the degradability from the weight loss Polymers & Polymer Composites, Vol. 20, No. 5,

4 D. Bachtiar, S.M. Sapuan, A. Khalina, E.S. Zainudin and K.Z.M. Dahlan Figure 3. The flexural modulus of short SPF reinforced HIPS composites Figure 4. The values of impact strength of short SPF-HIPS composites at various fibre contents by weight short SPF and the hydrophobic HIPS matrix, resulting in poor dispersion of the fibres in the matrix and weak interfacial bonding between the fibres and HIPS matrix. This behaviour may also be due to the restriction to matrix yielding imposed by sugar palm fibres 18. The same results were also indicated by the work of Vilaseca et al. 20, in which the impact behaviour of hemp strand fibre as reinforcement of polystyrene composites decreased upon increasing the fibre loading. As noted by Sanadi et al. 21, the impact resistance of thermoplastics generally decreases in the presence of agro-fibres. The reason for this is that the addition of fibres creates regions of stress concentration that require less energy to initiate a crack. the fibre to the composites results in increasing the modulus or stiffness of the composites. The same results were also indicated by Zainuddin s work 19, that the flexural modulus of banana pseudo-stem fibre filled unplasticized PVC composites increased with the fibre loading Impact Properties Figure 4 shows the average values of impact strength of sugar palm fibre reinforced high impact polystyrene composites with loading from 0% until 50% by weight. It clearly shows that increase the fibre loading decreased the impact strength of the composites. It is well known that there are three contributions that influence the impact response of fibre composites, i.e. the interfacial bond strength, the matrix and fibre properties 1. Impact energy is the energy dissipation caused by debonding, fibre and or matrix fracture and fibre pull-out. Fibre fracture dissipates little energy compared to fibre pull-out. The former is common in composites with strong interfacial bonds while the occurrence of the latter is sign of weak bonding. The decrease of the impact properties of short SPF-HIPS composites was due to the poor compatibility of the hydrophilic SEM images of the fracture surfaces of impact specimens can be seen in Figure 5. Pulled out fibres and the corresponding holes are visible in the fracture surfaces of the composites. These figures show the impact fracture surfaces of pure HIPS and SPF reinforced HIPS composites with 10%, 20%, 30%, 40% and 50% weight loadings respectively. From these pictures it can be seen that an increased fibre loading give a wider contact area between the fibre and HIPS matrix. When poor compatibility emerged, regions of stress concentration were increased that caused a decrease of the impact resistance. Addition of fibres also contributed to increasing the voids between fibre and matrix area, as shown in Figure 6. The void-trapped gave the worst effect on impact resistance because this point triggers the initiation of a crack. It is suggested to do the some treatment like chemical and physical treatment of the fibre to enhance the impact performance of SPF-HIPS composites. Voiding can be minimized with the proper processing. And it will be the next study in characterization of sugar palm fibre reinforced high impact composites project. 496 Polymers & Polymer Composites, Vol. 20, No. 5, 2012

5 The Flexural, Impact and Thermal Properties of Untreated Short Sugar Palm Fibre Reinforced High Impact Polystyrene (HIPS) Composites Figure 5. SEM photograph of fracture surface after impact loading for the specimen of untreated sugar palm fibre reinforced HIPS composites Effect of the Temperature on the Dynamic Mechanical Analysis The storage modulus data evaluated by DMA for the short sugar palm fibre reinforced HIPS composites are depicted in Figure 7. It shows the variation of the storage modulus as a function of temperature. DMA was performed to show how exposing the composites to elevated temperatures would affect the stiffness of the composites. There is a little decrease of the storage modulus for 10% weight content of sugar palm fibre. However, in general the storage modulus values increased with increasing sugar palm fibre content in the composites at the initial temperature (25 C), and the storage modulus of SPF-HIPS composites is higher than that of pure HIPS matrix. This is due to the reinforcement imparted by the fibres, which allowed stress transfer from the matrix to the fibre. It was apparent that the change in the storage modulus values showed a significant fall at higher temperature. There is a difference between the pure HIPS with the composites reinforced by the sugar palm fibres. The storage modulus for pure HIPS starts to fall at 85 C while the reinforced composites start to fall at 105 C. It was also observed that the addition of sugar palm fibre on the matrix led to better thermal stability than that of the pure matrix. Figure 6. The micrograph of fracture surface shows the void trapped between fibre and matrix in composites Figure 8 shows the variation of the loss modulus of the sugar palm reinforced HIPS composites with temperature. The loss modulus in each composite reaches a maximum as the storage modulus decreases most rapidly. This behaviour is caused by the free movement of the polymeric chains at higher temperature. From the loss modulus curves, the glass transition temperature (T g ) was determined at the peak value 22. As seen in Figure 8, owing to the fibre present in the HIPS matrix, the T g of SPF-HIPS composites shifted to higher temperature, which can be associated with the decreased mobility of the matrix chains, due to the Polymers & Polymer Composites, Vol. 20, No. 5,

6 D. Bachtiar, S.M. Sapuan, A. Khalina, E.S. Zainudin and K.Z.M. Dahlan Figure 7. Storage modulus curves from dynamical mechanical analysis of short sugar palm fibre reinforced HIPS composites Figure 8. Loss modulus curves from dynamical mechanical analysis in short sugar palm fibre reinforced HIPS composites addition of fibres 23. The positive shift in the T g value shows the effectiveness of the fibre as a reinforcing agent. Figure 9 shows that the height of the tan d peak decreased with the presence of the fibres. Damping or tan d is the ratio between the loss modulus and the storage modulus. Damping in the transition region measures the imperfections in the elasticity and how much of the energy used to deform a material during DMA testing is transformed directly into heat. In Figure 9, we can observe that the SPF-HIPS composites have smaller energy dissipation coefficients than the pure HIPS matrix. The damping in the composites has decreased considerably because the polymeric chains attach in the fibres diminish mobility, decreasing the friction among them. Hence, the molecular mobility of the composites decreased and the mechanical loss required to overcome inter-friction between molecular chains was reduced after adding fibres. Incorporation of the reinforcing fibres restricted the mobility of the polymer molecules, raised the storage modulus values and reduces the viscoelastic lag between the stress and the strain, hence the tan d values decreased 22. The lowered values of tan d in composites also because there was less matrix to dissipate the vibrational energy. Figure 9. Tan d curves from dynamical mechanical analysis of short sugar palm fibre reinforced HIPS composites Thermogravimetric Analysis Thermogravimetric (TGA) curves as a function of temperature of pure HIPS, sugar palm fibre and the composites with variation fibre loading from 10% to 50% weight are given in Figure 10. Thermal decomposition of the samples takes place in the temperature range 30 to 600 C. In the case of sugar palm fibre the first weight loss between 60 and 100 C corresponds to the content of water in the sample. This moisture loss is common in natural fibres. The second weight loss at about 280 C is due to thermal depolymerization of lignin and hemicelluloses and the cleavage of 498 Polymers & Polymer Composites, Vol. 20, No. 5, 2012

7 The Flexural, Impact and Thermal Properties of Untreated Short Sugar Palm Fibre Reinforced High Impact Polystyrene (HIPS) Composites the glucosidic linkages of cellulose 24. The third weight loss at about 337 C may be due to the further breakage of the decomposition product of stage II (cellulose) leading to the formation of tar through levoglucosan. The degradation temperature of cellulose can be confirmed by looking at the thermal degradation curve of the neat cellulose which shows a single step mass loss between 350 to 400 C 25. For pure HIPS, decomposition starts at 390 C and ends at a temperature of 470 C, which indicates a better thermal stability compared to the sugar palm fibre. In the case of the SPF- HIPS composite, Figure 10 indicates that the major degradation begins at a higher temperature (329 C) and the decomposition is almost complete at a temperature of 447 C. This shows that the thermal stability of the composite is higher. Figure 11 also shows that pure HIPS and SPF-HIPS composites do not show much difference as the curves are similar to one another. For both pure HIPS and SPF-HIPS composites, the weight percentage drops significantly starting from 380 C, which is mainly due to degradation of the fibre material. second peak indicates the degradation of hemicellulose content, at about 280 C. The third peak indicates the degradation of cellulose and lignin 25 at temperature 338 C for sugar palm fibre and 350 C for the SPF-HIPS composites. The fourth peak indicates the degradation of the pure HIPS matrix, at C. From this, the thermal stability of the SPF-HIPS Figure 10. TGA curve for sugar palm fibre, pure HIPS, and SPF-HIPS composites Figure 11. Derivative thermogravimetric {DTG) curve for sugar palm fibre, pure HIPS, and SPF-HIPS composites Table 1 shows the results of TGA/ DTG data for sugar palm fibre, pure HIPS and SPF-HIPS composites with loading fibre varied from 10% to 50%. This table summarizes the result from DTA by showing the peak of the derivative thermogravimetric (DTG) curve. Generally, there are four peaks appearing from the curves. The first peak indicates the evaporation of the water content at 100 C. The Table 1. TGA/DTG results for sugar palm fibre, pure HIPS and SPF-HIPS composites SPF HIPS 10%SPF 20%SPF 30%SPF 40%SPF 50%SPF Residue, % Peak 1, C Peak 2, C Peak 3, C Peak 4, C Polymers & Polymer Composites, Vol. 20, No. 5,

8 D. Bachtiar, S.M. Sapuan, A. Khalina, E.S. Zainudin and K.Z.M. Dahlan composites is enhanced by sugar palm fibre reinforcements which appear to act as shields for good heat insulation. The weight percentage of char (residue) at 500 C for SPF-HIPS composites increases with the increase of sugar palm fibre content. The high content of residue formation can be claimed as a higher heat resistance due to SPF reinforcements Moisture Absorption Behaviour Water absorption is one of the important characteristics of natural fibre composites exposed to environmental conditions that determine their end user applications. Figure 12 depicts the percentages of water uptake for the SPF-HIPS composites. Water absorption (%) of the composites increased with an increase in fibre loading. The hydroxyl group in sugar palm fibre is responsible for high water absorption. When the fibre loading was increased, the number of hydroxyl groups in the composites increased, which in turn increased the water absorption. The water absorption of composites is due to the hydrogen bonding of the water molecules to the free OH groups present in the cellulosic cell wall materials, and the diffusion of water molecules into the filler/ matrix interface. Additionally, a large number of porous tubular structures present in the fibres accelerates the penetration of water by capillary action. Therefore, with an increase in the fibre content, there are more water residence sites (OH groups), which result in more absorbed water 26. The level of moisture in the composites also reduces their strength. Some chemical treatment could meet the challenge to solve this moisture problem, and it is suggested to carry out the work in future investigations. 4. Conclusions Generally, the SPF-HIPS composites show good performance in mechanical and thermal characterizations. Effects of the fibre loading on the mechanical, physical, and thermal performance are summarized below: The flexural strength and flexural modulus increase. However, all of the values of flexural strength for SPF-HIPS composites were lower than the pure HIPS matrix. But for the modulus, all the values were higher than for the pure HIPS matrix. The addition of short SPF decreased the impact strength of these composites. This may be due to the Figure 12. Water absorption behaviour of SPF-HIPS composites poor compatibility of hydrophilic short SPF and hydrophobic HIPS matrix, resulting in poor dispersion of fibres in the matrix and weak interfacial bonding between the fibres and HIPS matrix. The voids also contribute to decreasing of the impact resistance. The effects of temperature on storage modulus, loss modulus and mechanical damping (tan d) were studied. The storage modulus was found to increase with increased fibre content, except for 10% fibre content. The added fibres also lead to better thermal stability for the composites. The loss modulus curves which show the value of T g at the peak, exhibited the effectiveness of the fibre as a reinforcing agent. From the tan d curve, the SPF-HIPS composites have smaller energy dissipation coefficients than the pure HIPS matrix. TGA testing results for SPF-HIPS composites show higher thermal stability compared with the pure HIPS matrix and sugar palm fibre. The moisture content of the SPF-HIPS composites increases, and this behaviour contributes to weakening the strength. Acknowledgement The authors wish to thank Universiti Putra Malaysia for financial assistance through a Graduate Research Fellowship for the principal author. References 1. Wambua P., Ivens J. and Verpoest I., Natural fibers: can they replace glass in the fibre reinforced plastics?, Composites Science and Technology, 63 (2003), Rowell RM. Natural fibres: type and properties. In: Pickering KL, editor. Properties and performance of natural-fibre reinforced polymer composites, Cambridge: Woodhead Publishing Limited; 2008, p Cheung H.Y., Ho M.P., Lau K.T., Cardona F. and Hui D., Natural 500 Polymers & Polymer Composites, Vol. 20, No. 5, 2012

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10 D. Bachtiar, S.M. Sapuan, A. Khalina, E.S. Zainudin and K.Z.M. Dahlan 502 Polymers & Polymer Composites, Vol. 20, No. 5, 2012