Pre-treatment by Water Retting to Improve the Interfacial Bonding Strength of Sugar Palm Fibre Reinforced Epoxy Composite

Transcription

1 Pre-treatment by Water Retting to Improve the Interfacial Bonding Strength of Sugar Palm Fibre Z. Leman*, S.M. Sapuan, M.R. Ishak and M.M.H.M. Ahmad Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang, Malaysia Received: 24 June 2009, Accepted: 8 March 2010 SUMMARY Polymer composites using natural fi bres as the reinforcing agents have found their use in many applications. However, they do suffer from a few limitations, due to the hydrophilicity of the natural fi bres which results in low compatibility with the hydrophobic polymer matrices. This paper presents the alternative lowcost methods of sugar palm (Arenga pinnata) fi bre surface treatments to improve the fi bre-matrix interfacial adhesion. Fibre surface modifi cations were carried out by water retting process where the fi bres were immersed in sea water, pond water and sewage water for the period of 30 days. The results showed that the tensile and fl exural strengths of the treated fi bre reinforced composites increased as the fi bre treatment duration increased. This indicates that the treatments had signifi cantly improved the fi bre-matrix bonding strength. Keywords: Natural fibre, Polymer composites, Surface modification, Sugar palm INTRODUCTION There is a growing interest in the use of natural fibres obtained from the plants and trees as reinforcing materials for thermoplastics and thermosets in wide variety of applications. Natural fibres like kenaf, hemp, flax, jute, henequen, pineapple leaf, sisal, banana, etc. have been widely studied by many researchers. *zleman@eng.upm.edu.my Smithers Rapra Technology,

2 Z. Leman, S.M. Sapuan, M.R. Ishak and M.M.H.M. Ahmad As a tropical country, Malaysia has abundant resources of natural fibres and one of the highly promising ones is the sugar palm (Arenga pinnata) fibre. Traditionally, the local people use the fibres to make brooms, brushes, and ropes especially for cordage on ships. Although the fibre is locally well known to have high strength and durability, very little research [1-4] has been done to study the full potential of this fibre. It is well known that the performance of composites depends on the properties of the individual components and their interfacial compatibility. To improve the interfacial bonding, fibre surface modification must be performed by either physical or chemical methods [5-10]. This study focused on the biological methods of surface modification to improve the mechanical (tensile and flexural) properties of the biocomposite. The effects of the treatments on the fibre surface properties and on the mechanical properties of the biocomposites are presented in the following sections. MATERIALS AND METHODS Materials The sugar palm fibres were obtained from the rural area of the State of Pahang, Malaysia. The fibres are black/brown in colour with diameters of up to 0.50 mm and are naturally found wrapped around the tree trunk at the base of the palm leaf ribs as shown in Figure 1. Unlike many other natural fibres, sugar palm fibres are directly obtained from the tree which does not require a secondary process to yield the fibres. Other fibres like kenaf and (a) (b) Figure 1. a) Sugar palm trees b) The fi bres removed from the tree 36

3 jute are usually subjected to the water retting process before the fibres can actually be harvested. The matrix used was epoxy resin (Epoxy BBT-7893A) and epoxy hardener (BBT-7893B), supplied by Berjaya Bintang Timur Sdn Bhd. The mixing ratio used was 4:1. The fibres were subjected to biological treatments in: i) sea water at Pantai Bagan Lalang, Sepang, ii) fresh water pond at the Faculty of Engineering, UPM and iii) sewage water from a housing area water treatment pond at Taman Serdang Jaya, Seri Kembangan, Selangor. Methods Treatment Process The fibres were immersed for the durations of 6, 12, 18, 24 and 30 days in sea water and pond water treatments. The results from these two treatment methods would be compared to the sewage water treatment in which, the fibres were soaked continuously for 30 days without the six-day intervals. After each treatment the fibres were taken out and washed thoroughly with tap water to remove any foreign materials and then dried in a room with good ventilation for 5 days. It was important to ensure that the fibres were thoroughly dried because fibres with residual moisture content could cause micro cracking during composite fabrication. Test Specimen Preparation The treated fibres were then chopped using a rotational cutting machine to a size of 1.0 mm or smaller. The composite plates with 15% fibre content (by volume) were prepared using an open mould with the dimensions of 20 cm x 18 cm x 0.34 cm and a 10 MPa pressure was applied to compress and evenly distribute the fibre-matrix mixture in the mould. The composite plates were left to cure in room temperature for 24 hours. After 24 hours the composites were carefully removed from the mould as the composites were still soft before being left again to be fully hardened for another 24 hours. Test specimens were cut from the composite plates using the OKUMA CNC machine with a cutting speed of 5000 rpm and a low feed rate of 100 mm/ min. The specimens were then tested for tensile and flexural strength using the universal testing machine, Instron Model 5566 in accordance to the ASTM D638 and ASTM D790 respectively [11-12]. 37

4 Z. Leman, S.M. Sapuan, M.R. Ishak and M.M.H.M. Ahmad Microscopy Analysis A scanning electron microscope (SEM) machine was used to study the surface morphology of the fibre under different types of surface treatments. The microscopic analysis of the surface morphology was important in characterising the structural changes that had occurred during treatment and how it affected the fracture behaviour of the composite. This provided vital information on the level of interfacial adhesion that existed between the fibre and the matrix later when used as reinforcement. For the SEM work, the samples were dried in an oven for two hours at a temperature of 120 C. This was important because high moisture content in the fibre could cause charging in which the image shown on the SEM would be too contrast because of the presence of moisture. The next step was to coat the sample with thin layer of gold using the coating machine to give a good image result. RESULTS AND DISCUSSIONS Fibre Surface Morphology The SEM analysis revealed that the surface of the untreated sugar palm fibre was covered by a layer of hemicelluloses and pectin (shown by the arrow in Figure 2) that protects the inner layer which mainly consists of lignin. This outer layer is mainly responsible for moisture absorption and it needs to be removed so that the epoxy could adhere to the fibre more firmly through the lignin layer. Figure 2. SEM photomicrograph of the untreated sugar palm fi bre 38

5 The surface of the fibre has become almost smooth after 30 days treatment in pond water as shown by the arrow in Figure 3. Its outer surface layer is almost free from hemicelluloses and pectin. The cellulose polymer has undergone degradation through oxidation, hydrolysis, and dehydration reaction. This reaction is the same type of reaction that takes place in the presence of acids and bases. The result from the treatment using sea water has the same pattern as the result using pond water. The outer fibre surface became much smoother due to the removal of the hemicelluloses and pectin. It is believed that the salinity of sea water has contributed to this finding. The 30 days treatment showed the best surface topography (Figure 4). Figure 3. SEM photomicrograph of the sugar palm fi bre after 30 days of pond water treatment Figure 4. SEM photomicrograph of the sugar palm fi bre after 30 days of sea water treatment 39

6 Z. Leman, S.M. Sapuan, M.R. Ishak and M.M.H.M. Ahmad The inner layer of the fibre which consists of lignins is responsible for the characteristic change in colour and adhesiveness in the cell wall. The lignins hold the cellulose cell together and this provides strength to the fibre. If these adhesive components decrease it will affect the fibre s strength. Table 1 below shows the chemical composition of sugar palm fibre. Table 1. Chemical composition of sugar palm fibre [4] Chemical composition % Holocellulose 57 Alpha-cellulose 50 Hemi-cellulose 7 Lignin 45 Moisture 9.5 Solubility in alcohol-acetone 2.16 The result of sea water and pond water treatment for 30 days had obviously modified the fibre surfaces. However, it is almost impossible to determine which method is better by examining the surface morphology only. For this reason, the tensile and flexural tests were carried out on the composites to substantiate the findings from the SEM analysis. Tensile and Flexural Strength Tensile tests results confirmed that the untreated fibres had poor adhesion with the matrix, thus giving the lowest value of tensile stress. On the other hand, the fibres with longest treatment duration (30 days) had the highest tensile stress value due to better interfacial adhesion. Figures 5 and 6 show the tensile stresses for different treatment periods for pond water and sea water treatment. Results for flexural tests for the composite with untreated fibre, treated by pond water and sea water also followed the same pattern. For the purpose of comparison, the fibres were also treated by soaking them in sewage water for 30 days and the test results showed that the composites were no better than the previous two treatment methods as shown in Figure 7. To further support these findings, SEM analysis was done again to inspect the fracture behaviour of the test specimens. Figure 8 shows the tensile fracture of the untreated fibre composite. As shown by the arrows there were voids or holes where these were supposed to be the holes of the fibres which 40

7 Figure 5. Tensile stress versus treatment period for pond water treatment Figure 6. Tensile stress versus treatment period for sea water treatment 41

8 Z. Leman, S.M. Sapuan, M.R. Ishak and M.M.H.M. Ahmad Figure 7. The effect of different treatment methods (30 days) on the average tensile and fl exural strength Figure 8. SEM photomicrograph of the tensile fracture of the untreated fi bre composite have been pulled out during fracture. In other words, the fibres are said to have been slipped from their positions due to weak interfacial bonding with the matrix. This also explains why composites with untreated fibres had the lowest tensile and flexural strengths. On the other hand, the treated fibres had very strong interfacial bonding with the matrix due to the absence of hemicellulose and pectin. This can be seen in Figure 9 where even after the composite was fractured, the fibres were not 42

9 Figure 9. SEM photomicrograph of the tensile fracture of the sea water treated fi bre composite (30 days). a) More fi bres at the fractured surface b) The close-up of the fi bre showing fi bre was damaged during fracture, i.e. no slippage dislocated from their positions. Also, the tensile test caused the fibre to be severely damaged but there was no slippage observed due to the absence of voids or holes. This proved that the treated fibres possessed high interfacial adhesion strength with the matrix. CONCLUSIONS Sugar palm fibres treated with sea water, pond water and sewage water showed improved surface morphology, and this could be seen under the scanning electron microscope. This study showed that the treatment using sea water for 30 days removed most of the hemicelluloses and pectin the fibre surface which resulted in improved adhesion quality between the fibre and matrix in the composite material. The tensile tests also proved that the fibres treated with sea water for 30 days had the highest tensile stress value at kpa and flexural stress at kpa. This is definitely quite a significant improvement compared to the untreated fibre. The results from this study show that sugar palm fibres can be surface modified using the biological methods at virtually no cost. The fibres may well prove to be the natural and low cost substitute for the synthetic fibres in many industrial applications in the near future. ACKNOWLEDGEMENTS The authors would like to thank the staff at the Department of Mechanical and Manufacturing Engineering and Department of Biological and Agricultural 43

10 Z. Leman, S.M. Sapuan, M.R. Ishak and M.M.H.M. Ahmad Engineering, Universiti Putra Malaysia for the help with the mechanical tests. Special thanks also go the staff at the Institute of Bioscience, Universiti Putra Malaysia for the help rendered during the SEM work. REFERENCES 1. H.Y. Sastra, J.P. Siregar, S.M. Sapuan, Z. Leman and M.M. Hamdan, Flexural Properties of Arenga pinnata Fibre s, American Journal of Applied Sciences, Special Issue, (2005) Z. Leman, S.M. Sapuan, M. Azwan, M.M. H.M. Ahmad, and M.A. Maleque, The Effect of Environmental Treatments on Fibre Surface Properties and Tensile Strength of Sugar Palm Fibre-s, Polymer- Plastics Technology and Engineering, 47, (2008) Z. Leman, S.M. Sapuan, A.M. Saifol, M.A. Maleque, M.M.H.M. Ahmad, Moisture Absorption Behavior Of Sugar Palm Fibre s, Materials and Design, 29, (2008) D. Bachtiar, S.M. Sapuan and M.M.H.M. Ahmad, Chemical Composition of Ijuk (Arenga Pinnata) Fibre as Reinforcement for Polymer Matrix Composites, Journal of Applied Technology, 4, (2006) 1, 1-7. M.S. Sreekala, M.G. Kumaran, S. Joseph, M. Jacob, and S. Thomas, Oil Palm Fibre Reinforced Phenol Formaldehyde Composites: Infl uence of Fibre Surface Modifi cations on the Mechanical Performance, Applied Composite Materials, v7, (2000) J.O. Karlsson, J.F. Blachot, A. Peguy, and P. Gatenholm, Improvement of Adhesion Between Polyethylene and Regenerated Cellulose Fibres by Surface Fibrillation, Polymer Composites, 17(2), (1996) E.T.N. Bisanda, The Effect of Alkali Treatment on the Adhesion Characteristics of Sisal Fibres, Applied Composite Materials, 7, (2000) W. Liu, A.K. Mohanty, L.T. Drzal, P. Askel, and M. Misra, Effects of Alkali Treatment on the Structure, Morphology and Thermal Properties of Native Grass Fibres as Reinforcements for Polymer Matrix Composites, Journal of Materials Science, 39, (2004) A. Gomes, K. Goda, and J. Ohgi, Effects of Alkali Treatment to Reinforcement on Tensile Properties of Curaua Fibre Green Composites, JSME International Journal, Series A, 47(4), (2004) L.Y. Mwaikambo and M.P. Ansell, Chemical Modifi cation of Hemp, Sisal, Jute, and Kapok Fibres by Alkalization, Journal of Applied Polymer Science, 84(12), (2002) ASTM D Standard test method for tensile properties of plastics. American Society for Testing Materials. 44

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