Synthesis, Characterisation and Analysis of Hibiscus Sabdariffa Fibre Reinforced Polymer Matrix Based Composites

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1 Synthesis, Characterisation and Analysis of Hibiscus Sabdariffa Fibre Reinforced Polymer Matrix Based Composites Synthesis, Characterisation and Analysis of Hibiscus Sabdariffa Fibre Reinforced Polymer Matrix Based Composites A.S. Singha and V.K Thakur Material Science Laboratory, National Institute of Technology Hamirpur , Himachal Pradesh (India) Received: 6 May 2008, Accepted: 5 January 2009 SUMMARY The scope of the present paper is to study the perspectives of using natural fibres as reinforcing materials in polymers. In the present paper, the synthesis and characterisation of Hibiscus Sabdariffa fibre reinforced phenol-formaldehyde resin matrix based polymer composites have been reported. The effects of fibre loading on mechanical properties such as tensile strength, compressive strength, flexural strength, and wear resistance have also been determined. The polycondensation reaction between different molar ratios of phenol and formaldehyde has been carried out for the synthesis of phenol-formaldehyde resin polymer matrix in the presence of a base catalyst. Reinforcement of the phenol-formaldehyde (PF) resin with Hibiscus Sabdariffa fibre was done with the filler in the form of 200 micron particles. The results reveal that mechanical properties such as: tensile strength, compressive strength, flexural strength and wear resistance of the PF resin increase up to 30% fibre loading and decrease beyond this fibre loading. INTRODUCTION Recently the growth in environmental awareness has led to the utilisation of natural fibres in various applications 1-2. Polymer composite materials made from natural fibres are significantly stronger and lighter than their synthetic counterparts. Various studies are ongoing to find ways to use natural fibres in place of synthetic ones in a variety of applications 3-5. These properties natural enable natural fibres to play an important role in resolving future environmental problems 6-8. Their specific advantages over the usual alternatives such as glass and carbon fibre fillers are low cost, low density, toughness, acceptable specific strength, enhanced energy recovery, recyclability and biodegradability Natural fibres consist of carbohydrates, lignin and extraneous components. The carbohydrate portion comprises cellulose and hemicelluloses etc. Cellulose is principally responsible for Telephone No: , Fax No E mail: assingha@gmail.com,vijaythakurnit@gmail.com Smithers Rapra Technology, 2009 the strength of natural fibres because of its specific properties such as high degree of polymerisation and linear orientation. All these properties have made natural fibres very attractive for various industries currently engaged in searching for products with comparable mechanical properties to synthetic fibre reinforced composites but which are lighter and do not cause health problems for workers. The Himalayan region is full of natural biomass, yet this precious wealth of nature is not exploited for better end products. Among various types of natural bio-material, Hibiscus sabdariffa fibres have high potential as a reinforcing material in polymer composites. Literature review has revealed scanty information on the applications of Hibiscus sabdariffa in the fabrication of polymer composite materials. In the present communication we have reported the synthesis of Hibiscus sabdariffa reinforced P-F matrix based polymer composites. The composites thus prepared have been subjected to testing for the evaluation of their mechanical, morphological and thermal behaviour. EXPERIMENTAL Material Phenol (Qualigens Chemicals Ltd), formaldehyde solution (Qualigens Chemicals Ltd.) and sodium hydroxide (Qualigens Chemicals Ltd.) were used as received. Matrix Polymer: Phenol-formaldehyde (P-F) resin was used as the matrix polymer. Reinforcing Filler: The ligno-cellulosic materials used as the reinforcing filler in the composite were Hibiscus sabdariffa (of particle dimensions 200 microns). Polymers & Polymer Composites, Vol. 17, No. 3,

2 A.S. Singha and V.K Thakur Synthesis of Phenol - Formaldehyde Resin The P-F resin was synthesised by the customary method, further developed in our materials science laboratory 7-8,13. Phenol and formaldehyde were taken in different molar ratios (1.0:1.0, 1.0:1.5, 1.0:2.0, 1.0:2.5, and 1.0:3.0) by weight, in the reaction kettle and were mixed with the help of a mechanical stirrer. NaOH solution was added slowly with constant stirring and heating, to the phenol-formaldehyde solution till the ph reached 8.5. Since the reaction is exothermic, proper care was taken to maintain the temperature between C, for the first hour. The temperature was then increased to C and the mixture was heated at this temperature, for 3-4 h after resinification started. The resulting resin was cooled and transferred to specially made moulds. Resin sheets of size 150 mm x 150 mm x 5.0 mm were prepared by a closed mould method as described elsewhere 3-8. The mould was closed and kept under pressure (4.0 MPa) until the resin had set into a hard mass. All the specimens were post-cured at 140 C for 7 h. Synthesis of Polymer Composites Dried Hibiscus sabdariffa fibres of dimension 200 micron were mixed thoroughly with phenol-formaldehyde resin using a mechanical stirrer with different loadings (10, 20, 30 and 40%) in terms of weight. Then above mixture was poured into specially made moulds. The surface of the moulds were coated on the inside with oleic acid to avoid adhesion of the mixture and allow easy removal of the composites. The mixture was then spread equally on the surface of the mould. Composite sheets of size 150 mm x 150 mm x 5.0 mm were prepared by compression moulding technique 7-8. Compression moulding was performed in a hot press using a mould preheated to 150 C. The material first placed in a hot open mould was left for about 7 min, and then the mould was closed. Composite sheets were prepared by hot pressing the mould at 150 C for 30 min. The pressure applied ranged from 3-4 MPa depending on the loading of reinforcing material. All the specimens were then post-cured at 150 C for 12 h. Analysis of Mechanical Properties of Polymer Resin and Polymer Composites Tensile Strength Test Tensile strength tests of resin and composite samples were conducted on a computerised Universal Testing Machine (HOUNSFIELD H25KS). Specimens of dimension 100 mm 10 mm 5 mm were used for analysis. Tensile test was conducted in accordance with ASTM D 3039 method. The test was conducted at a constant strain rate of the order of 10 mm/min. Tensile stress was applied till the failure of the sample and load extension curve was obtained. Each sample was tested seven times. Compressive Strength Test The compression strength test was conducted in accordance with the ASTM D 3410 method. Compressive stress was applied till the failure of sample. Flexural Strength Test Flexural strength of samples was also determined on a computerised Universal Testing Machine. The threepoint bend flexural test was conducted in accordance with the ASTM D 790 method. Wear Test Wear testing of the samples was conducted on a Wear & Friction Monitor (DUCOM-TR-20L). Wear resistance of composites was carried out as per the ASTM D 3702 method. Morphology of Samples Changes in the surface morphology of the fibres, polymer matrix and composites were evaluated by analysing the micrographs obtained by scanning electron microscopy (SEM). The excitation energy used was 5 kev. To achieve good electric conductivity all samples were first carbon-sputtered followed by sputtering a gold-palladium mixture before examination. Thermal Analysis of Samples Thermal analysis of materials gives us a fine account of their thermal stability. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) studies of samples were carried out in nitrogen atmosphere on a thermal analyzer (Perkin Elmer) at a heating rate of 10 C/min. Differential thermogravimetry (DTG) is a type of thermal analysis in which the rate of material weight change upon heating is plotted versus temperature and is used to simplify reading of weight versus temperature thermo gram peaks that occur close together. DTG peaks are characterised by the peak maximum (T max ) and the peak onset temperature (T e ). The area under a DTG curve is proportional to the mass change and the height of the peak at any temperature gives the rate of the mass change at that temperature. DTG curves are frequently preferred when comparing results with DTA curves because of their visual similarity. RESULTS AND DISCUSSION Static mechanical properties of fibre reinforced composites depend on the nature of the matrix material, distribution and orientation of the reinforcing fibres and the nature of the fibre matrix interfaces. In order to achieve good fibre reinforcement, interfacial strength between the fibre and matrix is the most essential factor 4-7. The interface transfers load between the matrix and the reinforcing fibres, and the interfacial area plays a major role in determining the strength of a composite material. Good interfacial bonding is a result both of good wetting of the fibres by the polymer matrix and the formation of a chemical bond between the 190 Polymers & Polymer Composites, Vol. 17, No. 3, 2009

3 Synthesis, Characterisation and Analysis of Hibiscus Sabdariffa Fibre Reinforced Polymer Matrix Based Composites fibre surface and the matrix The adhesion is usually the strongest in polar polymers capable of forming hydrogen-bonds with hydroxyl groups available on the fibre surface Figure 1. Load elongation/deformation/deflection and wear resistance curve of fibre reinforced composites (a, b, c, d) Mechanism of Synthesis of Phenol Formaldehyde Resin P F resin is normally synthesised by the condensation of phenol with formaldehyde. The nature of the product formed as a result of the polymerisation reaction is primarily dependent on the types of catalyst and the mole ratio of the reactants 7-8, In the present work, P-F resin was synthesised by the reaction of phenol with a molar excess of formaldehyde in the ratio of 1:1.5. To initiate the reaction of formaldehyde with phenol, a basic catalyst sodium hydroxide was added. Effect of Reinforcement on the Mechanical Properties of PF Matrix Based Polymer Composites Tensile Strength It has been observed that tensile strength of composites increases on reinforcement with Hibiscus sabdariffa fibre. Composites with 30 wt.% loading bore the highest load, followed by 40%, 20%, and 10% loadings (Figure 1a). It was observed that polymer composites with 10; 20, 30 and 40 percent loading bore loads of 1325, 1579, 1827 and 1721 N respectively. Compressive Strength Like tensile strength, compressive strength of the PF matrix was found to increase with the fibre loading (Figure 1b). Composites with 10; 20, 30 and 40 percent loading bore loads of , , and N respectively. Flexural Strength Similar trends to those obtained in tensile strength and compressive strength tests were observed for flexural strength results. The flexural properties of samples as a function of force (in terms of load) and deflection are shown in Figure 1c. Composites with 10, 20, 30 and 40 percent loading bore loads of , , and N respectively. Wear Test As evident from Figure 1d, the wear rate of the PF matrix decreased appreciably after reinforcement with Hibiscus sabdariffa fibre. Maximum wear resistance behaviour was shown by composites with 30 wt.% loading, followed by 40, 20 and 10 wt.% loading. -Strain Behaviour Tensile/compressive and flexural stress-strain curves have been constructed from the load elongation/deformation/deflection measurements 5-7. The plots of stress vs. strain for Hibiscus sabdariffa fibre reinforced polymer composites under tensile/compression/flexural tests are shown in Figure 2 a-c. The figures clearly show elastic regions, in which stress is linearly proportional to strain. When the load exceeded a value corresponding to the yield strength, the specimens underwent plastic deformation. The deviations from linearity are an indication of the beginning of initial matrix cracking. The first major change in slope in the curves under tensile/compression/ flexural tests is the sign of a major crack in the matrix or the beginning of fibre failure. The parameters derived from tensile/compressive and flexural stress-strain curves are shown in Tables 1-3. The tensile strength of Hibiscus Sabdariffa/PF composites is mainly dependent on the strength and modulus of fibres, the strength and chemical stability of the matrix and the effectiveness of the bonding strength between polymer matrix and fibres in transferring stress across the interface 5-6,15. The ultimate Polymers & Polymer Composites, Vol. 17, No. 3,

4 A.S. Singha and V.K Thakur Figure 2. strain curves of polymer composites under tensile, compressive and flexural tests (a, b, c) Table 1. Parameters from tensile stress-strain curves Fibre Loading (Wt. %) Table 2. Parameters from compressive stress-strain curve Fibre Loading (Wt. %) Table 3. Parameters from flexural stress-strain curve Fibre Loading (Wt. %) Ultimate Tensile Ultimate Compressive Ultimate Flexural Yield Strength Yield Strength Yield Strength compressive strength of the composite mainly depends on the strength of the resin, while the compressive modulus of the composite depends on its Fracture Fracture Fracture Standard Deviation Standard Deviation Standard Deviation Tensile Modulus PF % % % % Compressive Modulus PF % % % % Flexural Modulus UF Resin % % % % reinforcement. In general, composite specimens under compression testing fail by a shear or kinking mechanism 16. The flexural strength depends upon the fibre types at the compressive side and dispersion extent of fibres. However the flexural modulus depends only on the composite rigidity of the compressive side, and only rarely on the dispersion extent of the fibres. Normally the flexural modulus is very sensitive to the matrix properties and matrix /fibre interfacial bonding 17. It may be observed that the mechanical properties of Hibiscus Sabdariffa/PF composites increased up to 30 wt.% fibre loading and then decreased. This decrease can be attributed to fibre fibre contact at higher fibre loading. The mechanical strength of PF composites levels off at high fibre loading. Hence lower results are obtained for 40% loading. Due to poor bonding between fibre and matrix at low % of fibre loading, Hibiscus sabdariffa fibres were not capable of transferring load to one another, and hence stress accumulated at certain points of the composite, which led to lower mechanical properties. Further, for polymer composites with 30% fibre loading, Hibiscus sabdariffa fibres acted as load carriers and transferred stress from the matrix to the reinforcement which resulted in composites with good mechanical properties. Thermal Behaviour of PF Resin and its Composites Thermo-gravimetric analysis (TGA) data were recorded for raw fibre, polymeric resin and a biocomposite 192 Polymers & Polymer Composites, Vol. 17, No. 3, 2009

5 Synthesis, Characterisation and Analysis of Hibiscus Sabdariffa Fibre Reinforced Polymer Matrix Based Composites with 10% loading In case of raw fibre, at first depolymerisation, dehydration and glucosan formation took place between the temperature ranges of 25.0 C to C followed by the cleavage of C-H, C-C and C-O bonds. Initial decomposition (IDT) temperature was 199 C and final decomposition temperature was 500 C, with a final residue of 14.29% 1,6,8. On the other hand, in the case of the resin there was a single-stage decomposition and the initial decomposition temperature was 400 C; the final decomposition of the resin took place at 1195 C with a final residue of 51.47% 8. The degradation temperatures for natural fibre-reinforced composites fall between the degradation temperatures for the matrix and the fibres. It was observed that for the biocomposite the initial decomposition temperature was 330 C and the final decomposition took place at 980 C with a final residue of 41.89%; this indicates that the presence of cellulose fibres does affect the degradation process. Similar behaviour is expected with the other loading values. The initial decomposition (IDT) temperature and final decomposition temperature (FDT) of fibre, resin and biocomposite are presented in Table 4. These results are consistent with those results reported earlier 1,4-7. These studies are further supported by differential thermal analysis (DTA) and DTG as shown in Tables 5-6. The TG and DTA curves reveal that the Hibiscus sabdariffa fibre, PF resin and fibre reinforced composites decompose in different parts of the temperature range of C, C and C respectively. Comparison of the magnitude and location of peaks found in the DTA/DTG curves shows that there was a significant change in the thermal behaviour of polymer matrix when reinforced with cellulosic fibres 4-9,19. Table 4. Thermogravimetric analysis of PF, HS and P-Rnf-PF composites Sr. No. Sample Code IDT ( 0 C) Table 6. Differential thermal analysis of PF, HS and P-Rnf-PF composites Sr. No. Sample Code Exothermic peaks Temperature (mg/min) 1. HS 63 [-0.9]; 361 [-1.5] 2. P-F Resin 156 [112.8]; 215 [114.9] ;416 [126.1];490[235.5] 3. P-Rnf-PF 55[50]; 317[264]; 528[104] Figure 3-SEM images of (a) PF resin (b) Hibiscus Sabdariffa fibre (c, d, e and f) composite with 10, 20, 30 and 40% loadings a b c e % wt. loss d f FDT ( 0 C) % wt. loss Final Residue (%) 1. HS P-F Resin P-Rnf-PF Table 5. Differential thermal analysis of PF, HS and P-Rnf-PF composites Sr. No. Sample Code Exothermic/Endothermic peaks C( V) 1. HS 63 [-0.9]; 361 [-1.5] 2. P-F Resin 164 [9.0]; 421 [6.0] 3. P-Rnf-PF 63[-2]; 687[-27] Morphological Study of Biocomposites Morphological results (Figure 3) show that there was proper intimate mixing of Polymers & Polymer Composites, Vol. 17, No. 3,

6 A.S. Singha and V.K Thakur fibre with the resin in the biocomposites thus synthesised. Morphological results evidently demonstrate that when polymer resin matrix was reinforced with the different loadings of fibre, morphological changes took place depending upon the interfacial interaction between the fibre and the resin matrix. CONCLUSIONS The mechanical properties of phenolformaldehyde resin were enhanced by reinforcement with Hibiscus sabdariffa fibres. The mechanical properties of polymer composites have been investigated as a function of the chemical nature of matrix polymer and the content of the reinforcing material. A higher weight content of Hibiscus sabdariffa fibre enabled the composites to increase their strength in the most effective way, when the Hibiscus sabdariffa fibre were modified into the particle shape. These Hibiscus sabdariffa fibres have the potential to replace the more toxic synthetic fibres for the synthesis of green composites. These natural cellulosic fibres may be future constituents for the fabrication of eco-friendly materials which could be used in the automotive or other industries. ACKNOWLEGEMENT The authors wish to thank the Director, National Institute of Technology Hamirpur (H.P.) INDIA for providing the basic laboratory facilities. Authors would also like to thank Editor and unknown potential reviewers for their healthy comments in improving the quality of research paper. Financial assistance from University Grant Commission (New Delhi) provided through grant no. F.NO /2004 (SR) and Ministry of Human Resource Development (MHRD- New Delhi) is also highly acknowledged. REFERENCES Bulletin of Material Science, 31(5) (2008) Singha A.S., Shama A. and Misra B.N., J. Polym. Mater., 25(1) (2008) Singha A.S., Shama A. and Thakur V.K., Bulletin of Material Science 31 (1) (2008) E-Journal of Chemistry, 5(4) (2008) International Journal of Plastic Technology, 11 (2007) Iranian Polymer Journal, 17(7) (2008) International Journal of Polymeric Materials, 57(12) ( 2008 ) Singha A.S., and Thakur VK., BioResource, 3(4) (2008) Panthapulakkal S., Zereshkian A., and Sain M., Bioresource Technol., 97 (2006) Singha A.S., Shama A. and Thakur V.K., International Journal of Polymer Analysis and Characterization, 13(6) (2008) Bledzki A.K., Reihmane S., and Gassan J., J. Appl. Polym. Sci., 59(1) (1996) Gassan J., Bledzki A.K., Compos. Part A-Appl. S., 28(12) (1997) Singha A.S., and Thakur V,K., Int. Conf. on Polymeric Materials in Power Engineering (ICPMPE), IV B-8, pp73, Bangalore, India, 4th- 6th, October (2007). 14. Grenier-Loustalot M.F., Larroque S., Grande D., and Grenier P., Polymer, 37(8) (1996) Gao Z., Reinfshinder K.L., Journal of Composites Technology and Research, 14 (1992) Carlson, Leif A., Pipes R.B., Experimental Characterization of Advanced Composite Materials. New York: Hall, (1987) Jang B.J., Liau J.Y., Hwang L.R., Shih W.K., Journal of Reinforced Plastics and Composites, 9 (1990) Polymers & Polymer Composites, Vol. 17, No. 3, 2009