Abstract
In this study, the effects of chemical modifications of oil palm mesocarp fiber (OPMF) via bleaching, silane coupling agents, and combinations of the two on the composition and morphology of OPMF were investigated. The chemically modified OPMF was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The FTIR spectra showed that bleached OPMF became more hydrophilic, while silanized unbleached and silanized bleached OPMF became less hydrophilic. The TGA thermograms indicated that bleaching successfully removed hemicellulose from the OPMF, while TGA analysis showed that silanized unbleached and silanized bleached OPMF had higher thermal stabilities than unbleached or bleached OPMF. The SEM micrographs revealed that the modified OPMF surface was rougher and more porous than that of the unbleached OPMF, further indicating that OPMF was successfully modified.
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Compositional and Morphological Changes of Chemical Modified Oil Palm Mesocarp Fiber by Alkaline Bleaching and Silane Coupling Agents
Chern Chiet Eng,a Nor Azowa Ibrahim,a,* Norhazlin Zainuddin,a Hidayah Ariffin,b and Wan Md Zin Wan Yunus c
In this study, the effects of chemical modifications of oil palm mesocarp fiber (OPMF) via bleaching, silane coupling agents, and combinations of the two on the composition and morphology of OPMF were investigated. The chemically modified OPMF was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The FTIR spectra showed that bleached OPMF became more hydrophilic, while silanized unbleached and silanized bleached OPMF became less hydrophilic. The TGA thermograms indicated that bleaching successfully removed hemicellulose from the OPMF, while TGA analysis showed that silanized unbleached and silanized bleached OPMF had higher thermal stabilities than unbleached or bleached OPMF. The SEM micrographs revealed that the modified OPMF surface was rougher and more porous than that of the unbleached OPMF, further indicating that OPMF was successfully modified.
Keywords: Oil palm mesocarp fiber; Chemical modification; Bleaching; Silane coupling agent
Contact information: a: Department of Chemistry, Faculty of Science, University Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia; b: Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, University Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia; c: Chemistry Department, Centre for Defence Foundation Studies, National Defence University of Malaysia, Kem Sungai Besi, 57000 Kuala Lumpur, Malaysia; *Corresponding author: norazowa@upm.edu.my
INTRODUCTION
The oil palm (Elaeis guineensis) originates from South Africa, grows well in all tropical areas of the world, and has become one of the main industrial crops of Malaysia. The Malaysian palm oil industry has grown tremendously over the last 25 years, and Malaysia has become the world’s leading producer and exporter of palm oil (Mohammed et al. 2011). Generally, oil palm biomass products are produced from plantations and mills. Biomass from plantations is primarily trunk and fronds, whereas biomass from mills consists of empty fruit bunches (EFB), mesocarp fibers, palm kernel shells, and palm oil mill effluent (POME) (Abas et al. 2011). For every kilogram of palm oil produced, approximately four kilograms of dry biomass are produced, excluding palm oil mill effluent (POME). In 2010, the amount of mesocarp fiber available was 10.80 Mt/year (Ng et al. 2012). Thus, there is a huge amount of fiber that can be used instead of being discarded as waste. Traditionally, the mesocarp fiber is mixed with kernel shells and is used as a solid fuel to generate electricity for the mill.
Natural fiber as reinforcement filler in polymer composites has received increasing attention from researchers because natural fibers have many significant advantages over synthetic fibers. They are environmentally friendly, fully biodegradable, renewable, cheap, and have low density (Wambua et al. 2003). However, there are some disadvantages such as poor wettability, incompatibility with some polymeric matrices, and high moisture adsorption which restrict their usage in polymer composites (Riccieri et al. 1999). To make natural, hydrophilic fibers more compatible with hydrophobic polymer matrices, the fibers can be modified to enhance their interfacial adhesion. Chemical modification of natural fibers exposes more reactive groups on the fiber surface and thus promotes more efficient coupling with a polymer matrix (Dash et al. 2000). Chemical modifications such as alkali treatment (Alam et al. 2012), acetylation treatment (Bledzki et al. 2008), isocyanate treatment (George and Verpoest 1999), treatment with maleated coupling agents (Mohanty et al. 2004), treatment with silane coupling agents (Abdelmouleh et al. 2007), grafting (Ibrahim et al. 2005), and surface deposition of multiwall carbon nanotubes (MWCNTs) (Tzounis et al. 2014) have been conducted to improve fiber-matrix adhesion.
Alkali-treated fiber may form alkali-resistant linkages between lignin and hemicellulose, impeding the removal of lignin. Hydrogen peroxide can be used to break these linkages and delignify the fibers to promote interfacial adhesion with the polymer matrix (Salam et al. 2007). Hydrogen peroxide can also remove cementing materials from the fiber which block adhesion with the polymer matrix. Under alkaline conditions, hydroxyl ions (OH–) produced by the perhydroxyl ion (HO2–) hydrolyze hydrogen ions (H+), liberating more perhydroxyl ions. Perhydroxyl ions may decompose into hydroxyl ions and atomic oxygen where the atomic oxygen is responsible for bleaching (Modibbo et al. 2009). Due to the problem that colours of the natural fiber are mainly brown or dark brown, which are not attractive to consumer, bleaching of the natural fiber by hydrogen peroxide is an applicable approach to make the natural fiber/ polymer composites more attractive and marketable.
Silane coupling agents are multifunctional molecules used to modify the surface of natural fibers. Chemical bonding theory is the most accepted theory explaining the interfacial bonding mechanisms of silane coupling agents. In chemical bonding theory, bifunctional silane molecules form chemical linkages between the matrix and the fibers via a chemical bond with the fiber surface through a siloxane bridge. Their organofunctional group simultaneously bonds to the polymer matrix. The general chemical formula of silane is X3Si-R, where R represents a group that can react with the matrix and X represents a group that can hydrolyze to form a silanol group in aqueous solution and react with the hydroxyl groups of the fiber surface; the R-group can be vinyl-, aminopropyl-, methacreloxypropyne- (and others) while the X-group can be chloro-, methoxy-, ethoxy- (and others). Silane undergoes several stages of hydrolysis, condensation, and bond formation during the modification of fibers. When the silane-bonded fiber surface connects with the matrix, the R-groups on the fiber surface react with the functional groups of the polymer matrix to form stable covalent bonds with the matrix. Thus, silane coupling agents function as bridges between fibers and the matrix (Herrera Franco and Valadez-González 2005). Therefore, silane-treated fiber are able to improve fiber/matrix adhesion between silane-treated fiber with polymer matrix.
The major goal of the modification of OPMF by combination of alkali bleaching and silane coupling agent is to produce a suitable, cheap, and compatible OPMF-based reinforcement filler in expensive biodegradable polymer matrix. The modified OPMF is expected to improve the fiber/matrix adhesion between modified fiber and polymer matrix which eventually produce a composite with reasonable good strength and properties that are comparable with some conventional plastics while at the same time also lowering the cost of biodegradable polymer. The applications of the composites might be more suitable for single use materials that do not require very high strength; at the same time biodegradability is important for the selection of materials.
The objective of this study was to investigate the effects of chemical modification of OPMF and the changes in the composition of OPMF following bleaching, application of silane coupling agents, and the combination of these two methods. We hypothesized that the alkali bleaching of OPMF will make the surface of OPMF become rough and porous which would be possibly more favourable as a pretreatment before silanization. This is because bleaching exposure of more hydroxyl groups enables more extensive interaction between OPMF with silane. The modified OPMF was characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM).
EXPERIMENTAL
Materials
All reactions were carried out using reagent-grade chemicals (greater than 98% purity) without further purification. Raw oil palm mesocarp fibers were obtained from Felda Palm Ind., Sdn Bhd., Serting Hilir, Negeri Sembilan. Sodium hydroxide was supplied by Merck (Germany), hydrogen peroxide from J. T. Baker (USA), and 3-(Trimethoxysilyl) propyl methacrylate from Acros Organic (USA).
Methods
Processing of raw OPMF
The raw OPMF fiber was soaked in distilled water for 24 h to remove impurities. It was then rinsed with hot water (60 °C) twice and finally with acetone to remove wax prior to drying at 60 °C in an air oven. The fiber was ground and sieved to a particle size of 150 m using a crusher machine.
Modification of OPMF by alkaline bleaching
The OPMF was soaked in a solution containing a 1:1 ratio of 5% sodium hydroxide and 10% hydrogen peroxide for 2 h. The pH of the solution was maintained at approximately 11. The bleached OPMF was washed with distilled water and dried at 60 °C in an air oven overnight.
Modification of OPMF by methacrylate silane
First, 5 wt.% silane (weight percentage compared to OPMF) was dissolved for hydrolysis in a mixture of a 3:2 ratio of ethanol and water. The pH of the solution was adjusted to 4 with acetic acid, and the mixture was stirred continuously for an hour. Then, unbleached or bleached OPMF was soaked in the solution for 3 h and dried at 60 °C in an air oven overnight.
Fourier transform infrared spectroscopy (FTIR)
A Perkin Elmer Spectrum 100-series spectrometer (USA) equipped with an attenuated total reflectance (ATR) unit was used to evaluate the functional groups and types of bonds present in the samples. The infrared spectra were recorded in the range of 280 to 4000 cm-1.
Thermogravimetric analysis (TGA)
A Perkin Elmer TGA7 apparatus (USA) was used for the thermogravimetic analysis of the samples. Samples of about 15 mg were heated from 35 to 1000 °C with a heating rate of 10 °C/min. Nitrogen gas was pumped through the apparatus at a flow rate of 20 mL/min to maintain an inert nitrogen atmosphere.
Scanning electron microscopy (SEM)
The surface morphology of modified OPMF was observed with a scanning electron microscope (JEOL JSM-6400, Japan) after the samples were sputter-coated with gold using a Bio-rad (USA) coating system.
RESULTS AND DISCUSSION
Fourier Transform Infrared Spectroscopy (FTIR)
The FTIR spectra of unbleached and bleached OPMF are shown in Fig. 1. The peak at 1723 cm-1 for the unbleached OPMF corresponds to carbonyl group stretching, predominantly due to the acid and ester functional groups of hemicellulose (Proniewicz et al. 2002). The carbonyl group stretching peak disappeared from the bleached OPMF spectra because of the splitting of the ester groups of hemicellulose by alkaline hydrolysis (Agarwal and McSweeny 1997). This indicates that bleaching removes all hemicellulose from the OPMF. The peaks at 1628 and 1638 cm-1 in the spectra of unbleached and bleached OPMF correspond to C=C stretching. The peak at 1238 cm-1 in the spectrum of unbleached OPMF, corresponding to C-O stretching, disappeared after bleaching, perhaps due to the removal of hemicellulose from the OPMF. The H-bonded O-H stretching, represented by the peaks at 3339 cm-1(unbleached) and 3337 cm-1 (bleached), show that OPMF is hydrophilic in nature. The increment in the absorbance of bleached OPMF is due to the removal of oxidized carbohydrates and lignin. The phenyl -OH groups of lignin and COOH/COONa groups of carbohydrates undergo further degradation under alkaline conditions (Wójciak et al. 2010). Thus, bleached OPMF became more hydrophilic than unbleached OPMF.
The FTIR spectra of silanized unbleached and silanized bleached OPMF are illustrated in Fig. 2. The peaks at 3335 and 3338 cm-1 represent H-bonded O-H stretching for silanized unbleached and silanized bleached OPMF, respectively. The decrement in the intensities of both peaks compared to those of unbleached and bleached OPMF indicates a reduction in the hydrophilicity OPMF following silase treatment. This was due to the reaction of fiber with the alkoxy functional groups of silane (Singh et al. 1998). The hydrolysis of silane forms silanol, which reacts with the -OH groups of fibers to form stable, covalent bonds to the cell wall. These bonds are chemisorbed onto the fiber surface, thus reducing their hydrophilicity (Huda et al. 2008). The peaks representing carbonyl stretching (at 1721 cm-1) and C-O stretching (at 1252 cm-1), present in the spectrum of silanized unbleached OPMF, disappeared from the silanized bleached OPMF spectrum due to the removal of hemicellulose via bleaching. The peaks at 1160 and 1157 cm-1 were attributed to the asymmetric stretching of -Si-O-Si or -Si-O-C- bonds present in silanized unbleached and silanized bleached OPMF, respectively. The -Si-O-Si bonds indicate the existence of polysiloxanes deposited on the fiber, while -Si-O-C- bonds indicate the condensation reaction between silane and fibers (Pothan et al. 2006).
Fig. 1. FTIR spectra of unbleached and bleached OPMF
Fig. 2. FTIR spectra of silanized unbleached and silanized bleached OPMF
Thermogravimetric Analysis (TGA)
The TG thermograms for unbleached and bleached OPMF are shown in Fig. 3. The thermal degradation of both types of fiber was a gradual process with two main steps in their respective TG curves, similarly to those of other lignocellulosic compounds (Ruseckaite and Jiménez 2003). Thermal degradation of hemicellulose began at around 180 °C and reached a maximum rate at around 280 °C (Lv et al. 2010). Cellulose decomposed throughout the temperature range of 240 to 390 °C, while lignin decomposed slowly at low temperatures and at a maximum degradation rate at approximately 900 °C (Vamvuka et al. 2003). Initial weight losses of about 3.58% and 2.69% at around 140 °C for unbleached and bleached OPMF, respectively, corresponded to the volatilization of absorbed moisture present in the fiber (Ciannamea et al. 2010).
Fig. 3. TG thermogram of unbleached and bleached OPMF
The DTG thermogram in Fig. 4 shows that the thermal decomposition of unmodified OPMF was a 3-stage process.
Fig. 4. DTG thermogram of unbleached and bleached OPMF
The first stage took place within the temperature range of 140 to 300 °C; the second, from 300 to 410 °C; and the third, in which the maximum degradation rate was achieved, at around 820 °C. Thermal decomposition of bleached OPMF was a 2-stage process with stages at temperatures ranging from approximately 130 to 410 °C and 410 to 900 °C. According to Stefani et al. (2005), depolymerisation of hemicellulose and amorphous cellulose occurs fastest at a temperature of about 300 °C. Random cleavage of the glycosidic linkage remaining in cellulose occurs until around 400 °C. This suggests that bleaching the OPMF successfully removed part of the hemicellulose and amorphous cellulose, as the degradation peak disappeared from the DTG thermogram. The degradation of unbleached and bleached OPMF at temperatures above 400 °C was mainly via the degradation of lignin over a broad temperature range. The char yields for unmodified and bleached fiber were 4 and 9%, respectively, as the inorganic nutrients in plants become char (Van De Velde and Kiekens 2002). Table 1 lists the thermal degradation temperatures and weight losses of unbleached and bleached OPMF.
Table 1. Thermal Degradation of Unbleached and Bleached OPMF
The TG and DTG thermograms for silanized unbleached and silanized bleached OPMF are shown in Figs. 5 and 6, respectively. The thermal analysis shows that silanized unbleached and silanized bleached OPMF were more thermally stable than unbleached and bleached OPMF, respectively. Both categories of silane-treated fibers had greater thermal stability, which is in agreement with the findings of Arbelaiz et al. (2006). The initial weight losses of about 4.96 and 7.58% occurred at around 120 °C for silanized unbleached and silanized bleached OPMF, respectively. These small weight losses were attributed to absorbed moisture present in the fiber. The DTG thermogram shows that the thermal decomposition of silanized unbleached OPMF was mainly a 3-stage process taking place in temperature ranges of 120 to 340 °C, 340 to 490 °C, and with maximum degradation temperature at around 900 °C. This temperature was higher than that for unbleached OPMF. The thermal decomposition of silanized bleached OPMF was a 2-stage process taking place throughout temperature ranges of approximately 120 to 500 °C and 500 to 950 °C, also higher than those of bleached OPMF. The thermal degradation stages of both types of silane-treated OPMF are shown in Tables 4 and 5. The char yields from silanized unbleached and silanized bleached OPMF were around 4.1 and 9.6%, respectively. Due to the complex structure of lignin, it is difficult for all of the reactive sites of lignin react to with silane, which could be one of the reasons why the thermal degradation of lignin at temperatures above 500 °C was a 2-step process for both types of silane-treated OPMF. The first step of the degradation, the degradation of unreacted lignin, took place from around 500 to 800 °C. The second step of the degradation took place from 800 to 900 °C and might be due to the degradation of OPMF strongly bonded to silane via siloxane bridges, imparting the lignin with greater thermal stability. Table 2 lists the thermal degradation temperatures and weight losses of silanized unbleached and silanized bleached OPMF.
Fig. 5. TG thermogram of silanized unbleached and silanized bleached OPMF
Fig. 6. DTG thermogram of silanized unbleached and silanized bleached OPMF
Table 2. Thermal Degradation of Silanized Unbleached and Silanized Bleached OPMF
Scanning Electron Microscopy (SEM)
Figure 7 shows SEM micrographs of (a) unbleached OPMF and (b) bleached OPMF. Figure 7a shows that the surface of unbleached OPMF was smooth and sleek with no signs of external fibrillation or fibrils. Figure 7b shows that the surface of bleached OPMF was rough and heterogeneous with many pores. This indicates that bleached OPMF was in the process of peeling and underwent morphological changes (Roncero et al. 2005). The SEM results agree with the TGA results; lignocellulosic components were removed after bleaching, leading to greater exposure of hydroxyl groups and favoring chemical interactions with the polymer matrix.
Fig. 7. SEM micrographs of (a) unbleached (b) bleached OPMF
The SEM micrographs of silanized unbleached and silanized bleached OPMF are shown in Fig. 8. Figure 8a shows that the surface of silanized unbleached OPMF was rough and had many pores. Similarly, the surface of silanized bleached OPMF, as shown in Fig. 8b, was also rough and heterogeneous, with many pores. However, the pores present on the surface of silanized bleached OPMF were much deeper and larger than those of the silanized unbleached OPMF. This is because bleaching exposed more hydroxyl groups and enabled more interaction with silane which thus support the hypothesis that bleaching as pretreatment before silanization was more favourable as silane coupling are more easily to react with OH group that expose on the fiber surface.
Fig. 8. SEM micrographs of (a) silanized unbleached (b) silanized bleached OPMF
CONCLUSIONS
- The FTIR spectra and TGA thermograms showed that alkaline peroxide bleaching of OPMF successfully removed hemicellulose from its fibers.
- The SEM micrographs showed that the surface of bleached OPMF was rougher with more pores than that of the unbleached OPMF, which was smooth.
- Unbleached and bleached OPMF were treated with silane coupling agents. The FTIR spectra indicated that the silanized unbleached and silanized bleached OPMF were less hydrophilic than unbleached and bleached OPMF.
- The TGA thermograms showed that both types of silane-treated OPMF had greater thermal stability than the unbleached and bleached OPMF.
- The SEM micrographs revealed that the pores on the surface of silanized bleached OPMF surface were much deeper and larger than those on the surface of silanized unbleached OPMF. This was due to bleaching’s exposure of more hydroxyl groups, enabling more extensive interaction with silane.
ACKNOWLEDGMENTS
The authors would like to thank the Research University Grant Scheme (RUGS) of UPM for financial support. The technical staff of the Department of Chemistry, Faculty of Science, Universiti Putra Malaysia are thanked for their assistance.
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Article submitted: April 3, 2014; Peer review completed: July 8, 2014; Revised version received and accepted: July 9, 2014; Published: July 17, 2014.