INTRODUCTION

Nowadays, enormous studies were developed on potential of natural fibers as reinforcement in thermoplastics which has been shown in important industries such as automotive, construction or packaging industry. Natural fibers present some advantages such as lower density and lower price. They are less abrasive to the processing equipment, harmless, biodegradable, renewable and their mechanical properties are comparable to those of inorganic fibers (Habibi et al., 2008; Arib et al., 2006). There are many fibers that has been explored as reinforcement for polymer matrix. Oil palm biomass is considered a potential source of biofiber; based on Zain et al. (1994), almost more than 30 million metric ton of entire oil palm production in the world is considered by product or agriculture material and about 8 million metric ton is empty fruit bunch materials.

The main disadvantage of natural fibers/plastic composites is the poor compatibility between the mainly hydrophobic polymer-matrix and the hydrophilic fibers. This leads to the formation of a weak interface, which results in poor mechanical properties; where the stress transfer at the interface between two different phases is determined by the degree of adhesion. A strong adhesion at the interfaces is needed for an effective transfer of stress and load distribution throughout the interface. Thus, enhanced interfacial adhesion for composites containing natural fibers would provide higher mechanical properties that can be achieved either by fiber and matrix modification with chemical/physical treatments or by use of interfacial additives such as delignition, bleaching and chlorination (Spoljaric et al., 2009; Khalid et al., 2008).

Due to the utilization of chlorine and chlorine derivatives in conventional bleaching processes, large amounts of chlorinated organic compounds are generated, which presents serious environmental problems. Therefore, the use of chlorine-free bleaching processes has become increasingly important. A few studies have been performed on delignication of lignocelluloses materials with alkaline hydrogen peroxide (Bengtsson et al., 2007; Rozman et al., 2003).

The purpose of this study was to investigate the mechanical properties of composite based on untreated and treated fibers with addition of coupling agent. The present paper is focused on study of new fabrication method of fibers by alkaline peroxide treatment. Within the framework, the role interfacial morphology and thermal stability of fiber and composites were analyzed by means of Thermo Gravimetric Analyzer (TGA) and Scanning Electronic Microscopy (SEM) including mechanical properties of prepared composites.

MATERIALS AND METHODS

Materials: Homopolymer polypropylene (PP) grade G452 (Melt flow Index 45 g/10 min, density 0.90 g cm^-3) was supplied by MTBE (M) Sdn Bhd, Empty Fruit Bunch Fibers (EFBF) was obtained from Felda Palm Oil Berhad and other chemicals supplied from Merck.

Methods
Alkaline peroxide pre-treatment of Empty Fruit Bunch (EFB): Generally, preparation of EFB was carried out in two stages. The first stage is physical treatment where empty fruit bunch was dried for minimum three days, then were cut using hammer mill and crusher machine and was passed through 500 μm sieve to obtain uniform size of empty fruit bunch fiber. After that, untreated fibers were soaked with hot distilled water at 70°C at least for 4-6 h to remove impurities and large particles. Then, fiber was treated with alkaline peroxide by placing about 25 g fibers in 500 mL Erlenmeyer flask of distilled water containing 15% (w/v) of sodium hydroxide (NaOH) solution. These delignificated fibers were then soaked and maintained at 75°C about 3 h to active the OH group of the cellulose. The fiber treatment method is commonly known as mercerization. After 2 h treatment, EFB fibers were taken out and washed with distilled water. Then, was treated with 2% of Hydrogen peroxide (H₂O₂) solution in water bath at 45°C with a speed of 50 rpm for 8 h. The excess NaOH present in treated fiber residue was neutralized by using 10% (v/v) acetic acid (at room temperature) for 30 min. Then, the treated fiber was dried in an oven at 70°C for 48 h.

Preparation of composite: The compounding of polypropylene with untreated and treated EFB fibers, with and without additional of coupling agent (MAPP) was carried out using twin screw compounder Scientific (MODEL: LTE26-40) at 180°C at a roller speed around 50 rpm and pressure about 48 bar. Coupling agent (MAPP) was added as 10:1 fiber to MAPP ratio. Composite was molded into tensile and bending test specimen, respectively by using injection molding machine (Toshiba, MODEL: IS 80 EPN) at 190°C. After molding, test specimens were conditioned at 23±2°C, with 50 % relative humidity for at least 40 h according to ASTM D618-99.

Characterizations
Fibers and composites: The surface and thermal characteristics were determined by using SEM and TGA. Surface of fibers before and after pre-treatment as well as fracture surface of composites was observed with a scanning electron microscope (SEM) (Model OXFORD). Samples of air-dried cellulose fibers were fixed to a metal-base specimen holder using double-sided sticky carbon tape and then coated with gold using a vacuum sputter-coater to make them conductive prior to SEM observation.

TGA analysis was carried out using thermogravitmetric analyzer (TA instruments, TGA Q500). Each specimens (weighed about 10 mg) were run at scanning temperature range of 25-600°C and heating rate of 10°C min^-1. TGA was conducted with the compounds placed in platinum crucible in nitrogen atmosphere at flow rate of 40 mL min^-1 to avoid unwanted oxidation.

Mechanical properties of composite: The mechanical properties of composite were characterized using tensile, flexural and impact tests. The tensile tests were conducted according to ASTM D638-03 using SHIMADZU (MODEL AG-1) Universal Testing Machine with load cell of 5 kN, using crosshead speed of 10 mm min^-1 and with a gauge length of 115 mm. Test was performed until tensile failure occurred. The flexural test was carried out using SHIMADZU (MODEL AG-1) Universal Testing Machine in accordance with ASTM D790-97 standard. The support was 51 mm while the crosshead speed utilized was 1.26 mm min^-1. Eight specimens were tested and at least five replicate specimens were presented as an average of tested specimens for all batches. These tests were carried out at 25°C.

RESULTS AND DISCUSSION

Characterization of fibers before and after alkaline-pretreatment
Scanning Electron Microscope (SEM-EDS): Based on Fig. 1a, the rough surface of lignocelluloses EFB can be seen which may be due to high residual of lignin content.

Fig. 1:	Fiber surface before (a) and after (b) pre-treatment

While, by applying alkaline peroxide treatment, residual lignin was eliminated as per Fig. 1b.

Alkaline peroxide treatment can improve adhesion characteristics between fiber-PP matrix by removing surface impurities such as lignin content. Then, breakdown the fiber bundles led increasing of porosity, the effective surface area and surface charge exposed further hydroxyl and carboxyl groups, thus exposing micro-fibrils which tend better packing of cellulose chain (Pracella et al., 2006). Furthermore, some researchers reported on the change in the crystallinity through alkaline treatment on sugarcane, coir and flax (Bledzki and Gassan, 1999). The percentage crystallinity index of alkali peroxide treated fibers might be increased because of the removal on the cementing materials, which leads to better packing of cellulose chain (Sun et al., 2004). In addition, treatment with NaOH leads to decrease in the spiral angle, i.e., closer to fiber axis and increase in the molecular orientation. A fair amount of randomness is introduced in the orientation of the crystallites due to non-cellulosic matter. Moreover, the mechanical properties of composites shown to be increased through alkalization peroxide (Averous and Le Digabel, 2006; Le Digabel and Averous, 2006; Bengtsson et al., 2007) due to the improved fiber-matrix bonding.

Thermogravitmetric analysis (TGA): The thermogravitmetric analyses (TGA) are shown in Fig. 2. The onset of thermal degradation can be identified by dramatic decrease in sample weight and increase in the temperature different due to exothermic combustion reaction. The thermal degradation of untreated EFB fiber begins around 217.90°C, while treated EFB fiber starts at around 229.19°C. Untreated fiber degrades at lower temperature due to presence of thermally unstable fiber constituents such as hemicelluloses, holocelluloses, lignin, whereas the treated fiber is more stable due to the removal of these constituents (Beckermann and Pickering, 2008).

Fig. 2:	TGA thermograms of (a) Pure polypropylene, untreated and treated composite with MAPP samples

There are interactions between these fractions to identify the multiple exothermic degradation reaction of cellulose, hemicelluloses and lignins. The maximum decomposition rate of untreated fiber was found to be at 268°C is mainly attributed to the decomposition of hemicelluloses. This is likely caused by decomposition of cellulose leading to the formation and volatilization of levoglucosan and other volatile products (Beckermann and Pickering, 2008), attributed to quick devolatilization reaction (Spinace et al., 2009). It can be concluded that alkaline treatment increases the thermal stability of EFB than untreated EFB fiber.

Similar TGA pattern was found for composites, however, there was a shifting from low to high temperature compared to fiber alone. Specific interaction between fibers and maleic anhydride can established an association through esterification between the anhydride groups and the hydroxyl groups of the fibers after treatment (Spoljaric et al., 2009) which results in higher thermal stability of composites containing treated fiber.

Characterization of composites
Tensile properties: The most crucial factor that affects the mechanical properties of fiber-reinforced materials is the fiber volume fraction and fiber matrix interfacial adhesion. The quality of interfacial bonding is determined by several factors, such as the nature of fiber and polymer components, the fiber aspect ratio, the processing method and the treatment of the fiber (Gomes et al., 2007; Pracella et al., 2006). In the case of PP-lignocelluloses (EFBF) composite without pre-treatment, adhesion between the two materials was expected to be rather poor than the PP-lignocelluloses (EFBF) composite with application of treated EFBF. EFBF has polar nature whereas polypropylene is characterized by non-polar groups.

Fig. 3:	Tensile strength of lignocellulose based biocomposite

Due to the changes in physical structure it was expected for treated lignocelluloses (EFBF) to show some changes in the overall properties. The tensile properties of composites are presented in Fig. 3.

By applying 100% Polypropylene as benchmark, the tensile strength was found to be 38 MPa. The weak bonding between the hydrophilic lignocelluloses filler and the hydrophobic matrix polymer obstructs the stress propagation and causes the tensile strength to decrease as the filler loading increases as explained in many researches (Spoljaric et al., 2009; Habibi et al., 2008; Khalid et al., 2008). Therefore by additional of coupling agent, it enhanced interfacial adhesion between the PP matrix and EFBF filler which treated composite increased significantly rather PP-untreated EFBF composite as per expectation which also can be seen in Fig. 3. These suggests likely due to be attributed to the covalent bonds created between the fiber and the coupling agent and which were ensured by the maleate groups. Moreover, MAPP underwent esterification reaction or hydrogen bonding, at the interface, between the hydroxyl groups of the fibers on one side and the carboxylic groups of the MAPP diffused matrix polymer on the other side (Habibi et al., 2008; Arib et al., 2006; Khalid et al., 2008). Based on Fig. 3 it can be seen that the tensile strength of the MAPP-treated EFBF composite were significantly greater than those of the MAPP-untreated EFBF composite with increasing of fibers till 50 wt% loading. These possible mechanisms have been illustrated in Fig. 4.

It can be seen on Fig. 4 of SEM study that the untreated EFB composite fracture as Fig. 4a had occurred predominantly by transversal fracture in the plane matrix. It is observed that fibers oriented in random arrangement. There are gaps between fibers and matrix which some deep holes left, with more fibers breakage and less pull out tends weak adhesion and inefficient stress transfer from matrix.

Fig. 4:	Biocomposite before (a) and after (b) after pre-treatment with addition of MAPP

Fracture surface of composite with treated fibre (Fig. 4b) showed modification in topographic aspects of the mark developed at matrix surface and clearly improved strong bonding. In addition more of fibers breakage can be seen which indicated improvement adhesion between fiber and matrix. It can be seen in treated composite with MAPP is covered by the layer of matrix material that had been pulled out together with the fiber. This is also a clear indication that treated composite has an effective interaction with PP matrix and have better capability to support stress transfer than the untreated composites from the polymer matrix.

Flexural modulus properties: Modulus (stiffness) is another basic property of composites; the primary intention of filler incorporation is usually to increase the stiffness of the resultant material. The elastic modulus of bulk natural fibers such as wood and hemp are about 1.0±0.5 GPa (Khalid et al., 2008; Luz et al., 2008). Composites with additional MAPP as coupling agent, provided better flexural modulus rather than without coupling agent.

It can be seen as per Fig. 5 that increasing filler content increased the stiffness of composites significantly till loading 50% of fibers. PP-treated EFBF with MAPP composite showed significant increase compared to untreated EFBF till fiber loading is 50wt% which increased from 2.7 to 3.5GPa. There are many factors affecting the modulus of the composites. Those factors are filler content, modulus and aspect ratio. As per expectation, the elastic modulus of fibers, for instance are expected to increase with increasing degree of molecular orientation. Well-oriented cellulosic fibers such as flex have much higher Young’s modulus than fibers with medium orientation, such as cotton (Colom et al., 2003).

Not only that, it is may be because alkaline reacts with cementing materials of the fiber, splitting the fibers into sfiner filaments. In addition, combination of alkaline peroxide enhanced the porosity and pore size on the fiber where improved physical interlocking leading better interfacial bonding between fiber and matrix which illustrated as per Fig. 1a and b.

Fig. 5:	Flexural modulus of lignocellulose based biocomposite

By addition of coupling agent as compatibilizer as per Fig. 4b indicated better interfacial bonding between fiber and polypropylene as matrix which enhanced mechanical properties.

The addition of MAPP with EFBF composite has enhanced the efficiency of stress transfer from matrix to the EFB. This is may come about as the result of better bridging between these two component at interfacial region. It is known that compatibility, in general, between polymer matrix and reinforcing agent is one of the main criteria to produce a good mechanical characteristic of a composite (Bengtsson et al., 2007; Georgopoulos et al., 2005).This bridging may be formed either by Van der Waals Covalent bonding between MAPP which attached to EFBF and PP could be through the opening C = C group in MAPP through radical reaction of dicumyl peroxide (Spoljaric et al., 2009). This can be illustrated in Fig. 4b. Hence, the increased adhesion brought by the C=C group’s reaction with the PP matrix, together with other means of enhancing compatibility such as Van der Waals interaction, should be reflected in the mechanical properties. Moreover, due to better compatibility and wetting of MAPP to the PP matrix polymer improved interfacial bonding between the filler and the matrix polymer resulted in good stress propagation and improved the tensile strength.

CONCLUSION

This study showed that addition of coupling agent with pre-treatment of EFB fiber provided better interfacial interaction with polypropylene matrix in term of mechanical and thermal properties. The improvement of ensile strength was significantly increased by filler loading till 50wt% after addition of coupling agent. Flexural modulus was also indicated that treated EFB composite with coupling agent provides better interfacial bonding chain rather than untreated EFB composite till 50wt% fiber loading. SEM and TGA showed evidence of better adhesion and compatibility between the EFB fibers and the PP after modification matrix with MAPP as coupling agent as well as better thermal stability.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the staffs of Felda Palm Industries Berhad (Felda Lepar Hilir 3), Fiber and Biocomposite Development Centre (FIDEC) and Polypropylene Laboratory of MTBE (M) Sdn Bhd for their technical support to carry out this research.