Abstract
The Raffia textilis fiber has a specific strength of 660 MPacm3/g and scales and hollows on its surface. Thus, this fiber is a potential composite reinforcement. The objective of this study was to evaluate the effect of alkaline treatment at room temperature on its microstructure, structure, composition, thermal behavior, mechanical properties, and color. To this end, slack raw fibers were soaked in three NaOH solutions (2.5%, 5%, and 10% by weight) for 12 hours. SEM observations revealed that fibers got more and more clean and smooth when the solution concentration was increased. In comparison with the raw fiber, it was found that fiber treated with 5% NaOH solution exhibited enhanced tensile strength (129%) and strain to failure (175%), in addition to increased yellowness, redness, and thermal stability. Contrariwise, the Young modulus and lightness slightly decreased with the treatment. The Fourier transform infrared spectra and the XRD patterns suggested an incipient allotropic transformation of cellulose for 10% NaOH-treated fibers. These changes could be explained by the gradual dissolution of non-cellulosic components as revealed by the Fourier transform infrared attenuated total reflection spectra and thermal analysis.
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Effects of Alkali Treatment on the Microstructure, Composition, and Properties of the Raffia textilis Fiber
Raymond Gentil Elenga,a,* Philippe Djemia,b David Tingaud,b Thierry Chauveau,b Jean Goma Maniongui,a and Guy Dirras b
The Raffia textilis fiber has a specific strength of 660 MPacm3/g and scales and hollows on its surface. Thus, this fiber is a potential composite reinforcement. The objective of this study was to evaluate the effect of alkaline treatment at room temperature on its microstructure, structure, composition, thermal behavior, mechanical properties, and color. To this end, slack raw fibers were soaked in three NaOH solutions (2.5%, 5%, and 10% by weight) for 12 hours. SEM observations revealed that fibers got more and more clean and smooth when the solution concentration was increased. In comparison with the raw fiber, it was found that fiber treated with 5% NaOH solution exhibited enhanced tensile strength (129%) and strain to failure (175%), in addition to increased yellowness, redness, and thermal stability. Contrariwise, the Young modulus and lightness slightly decreased with the treatment. The Fourier transform infrared spectra and the XRD patterns suggested an incipient allotropic transformation of cellulose for 10% NaOH-treated fibers. These changes could be explained by the gradual dissolution of non-cellulosic components as revealed by the Fourier transform infrared attenuated total reflection spectra and thermal analysis.
Keywords: Natural fiber; Raffia; Alkali treatment; Thermal degradation; Mechanical properties; Microstructure; Crystal structure; Cellulose; Color
Contact information: a: Laboratoire des Matériaux et Energies, Faculté des Sciences, Université Marien Ngouabi; b: Université Paris 13, Sorbonne Paris Cité, LSPM-CNRS, 99 av. Jean-Baptiste Clément, 93430 Villetaneuse, France; *Corresponding author: rgelenga@gmail.com
INTRODUCTION
The necessity of preserving the environment and increasing the use of renewable natural resources has led to the use of natural fibers in several industrial sectors, including polymer composites, building materials, technical textiles, and geotextiles (Bledzki and Gassan 1999; John and Thomas 2008). It is expected that newly passed legislation, in the EU for instance, will increase their demand even further (Jawaid and Abdul Khalil 2011; John and Thomas 2008). The rising demand has increased the use of plant fibers that have previously been neglected such as coir, bamboo, curaua, bagasse, and pineapple (Jawaid and Abdul Khalil 2011; Satyanarayana et al. 2007). Many of these new fibers are grown in tropical areas, and their industrial use should contribute to improving the living conditions in these areas. However, the expansion of agriculture is currently considered the greatest threat to biodiversity, including primary forests (Koh et al. 2010; Tilman et al. 2001). Thus, the sustainable exploitation of endemic wildlife-friendly fiber plants is a compromise between the duty to preserve biodiversity and the necessity of fighting poverty. From this perspective, multifunctional plants, or the ones that can be harvested several times in a growing cycle, are the most interesting.
The raffia palm tree belongs to the above-mentioned multifunctional plants category. Many of its parts have well known uses such as textiles, medicine, carpet, broom, palm wine, raffia oil, and building material (Edem et al. 1984, 2009). The raffia fiber resembles a strap and is actually the leaflet epidermis of a young leaf. A recent investigation reported its microstructure and some interesting properties, such as its average specific strength of about 660 MPacm3/g (Elenga et al. 2009), which is comparable to other natural fibers and synthetic fibers, such as E-glass, that are being used as composite reinforcements (Bledzki and Gassan 1999; Monteiro et al. 2011). Its microstructure exhibits alveoli on the bottom face and usually scales on the top one. These features could facilitate mechanical anchoring with a polymer matrix. Therefore, this fiber has interesting potential as composite reinforcement.
In the context of composite processing, it is also important to know the thermal behavior of the fiber. Although plant fibers have almost the same major components (cellulose, hemicellulose, lignin, and pectin), their thermal behavior varies from one fiber to another (D’ Almeida et al. 2008; Yao et al. 2008). Indeed, this behavior depends on the proportion of these components and on the cellulose structure. To enhance the fiber-matrix adhesion and the fiber’s mechanical properties, various physical and chemical treatments are applied (Bledzki and Gassan 1999; George et al. 2001). Among them, treatment with sodium hydroxide is typical and cheap. Previous studies on this type of treatment have revealed that its effects depend mainly on the fiber type, the NaOH concentration, and the soaking temperature and duration (Bledzki and Gassan 1999; Saha et al. 2010; Van de Weyenberg et al. 2006). For instance, Saha et al. (2010) reported that the tensile strength of jute fiber increased continuously with the concentration between 0% and 4% NaOH. However, the maximum tensile strength was reached after 4 h of soaking in 0.5% NaOH and 30 min in 4% NaOH solution. On the contrary, the maximum tensile strength of coir fiber soaked in 5% NaOH solution was reached after 70 h of soaking (Bledzki et al. 1999). To our knowledge, the alkalinization of raffia fiber has not yet been studied. However, such studies have been performed on fibers from other palm trees species. AlMaadeed et al. (2013) reported that after 2 h soaking at 100 °C, the tensile strength of the leaflet fibers of female date palm decreased continuously with the increase of the NaOH concentration. Contrariwise, for leaflet fibers of male date palm, the tensile strength varied parabolically, and the maximum value was obtained for fibers treated in 2% NaOH. In comparison with raw fibers, the tensile strength increase of treated fibers was about 33%. For fibers that surround the date palm trunk, Alawar et al. (2009) reported that after 1 h soaking at 100 °C, the tensile strength was maximal for 1% treated fibers. The increase was about 400% compared to the raw fiber. On the contrary, the Young’s modulus decreased continuously with the increase of the concentration. At room temperature, for the same fibers, Alsaeed et al. (2013) showed that for 24 h soaking in solution with concentration between 3 to 9%, the tensile strength decreased. For oil palm empty fruit bunches fibers (OPEFBF), according to Norul Izani et al. (2013), treatment at room temperature in 2% NaOH solution improved the strength and the Young’s modulus by 23% and 9%, respectively.¶
In previous studies, the mechanical and physical properties of the raw Raffia textilis fiber were characterized (Elenga et al. 2009), and its drying kinetics were modeled (Elenga et al. 2011). The objective of the present study is to investigate the effect of sodium hydroxide treatment on the microstructure, structure, composition, thermal behavior, mechanical properties, and color of the Raffia textilis fiber.
EXPERIMENTAL
Fibers Preparation
The raw fibers were extracted from young leaves of wild Raffia textilis palm trees. The extraction process was performed by removing the upper epidermis of the leaflet with a knife. This procedure was carried out, at most, 48 h after harvest. After the extraction, the fibers were dried in air at room temperature (RT, about 25 °C) until their mass reached equilibrium. The mean value of the moisture content of dried fiber was 17% by weight, on a dry basis. The obtained fibers had dimensions of about 0.5 cm wide, 30 to 40 cm long, and 15 μm thick. Their color was light yellow.
For the sodium hydroxide treatments (NaOH) of fibers, three NaOH concentra-tions were prepared (2.5%, 5%, and 10% by weight) by dissolving sodium hydroxide pellets (98% purity) in distilled water. These concentrations were chosen to preserve the cellulose part of the fiber. Dried and slack raw fibers were completely soaked in the NaOH solutions for 12 h at RT. After immersion, the treated fibers were washed in running tap water, followed by distilled water until the wash water became neutral (pH = 7). They were then dried again at RT.
Scanning Electron Microscopy
To investigate the microstructure and the surface morphology of raw and treated fibers, samples having a surface area of about 0.5 cm2 were cut off and coated with 5 nm of carbon for scanning electron microscope (SEM) studies using a Zeiss Supra 40VP FEG SEM instrument. These investigations were performed on both the top and bottom faces of the fiber.
Infrared Spectra Measurements
The Fourier transform infrared attenuated total reflexion (FTIR-ATR) spectra were recorded on a Perkin Elmer GX spectrometer with an accessory diamond ATR. One hundred acquisitions were performed in the range 400 to 4000 cm-1. As suggested by Oh et al. (2005), the band at 894 cm-1 was used as the internal standard band to determine the crystallinity index (CI) and the band intensity ratios. Indeed, these authors reported that the CI calculated with this band as an internal standard has the best correlation coefficient with the CI determined by X-ray diffraction (XRD).
X-ray Scattering Analysis
X-ray diffraction (XRD) experiments were performed on an automated InelTM four-circle goniometer. The monochromatic beam of wavelength 1.5405 Å (CuKα1) was obtained after reflection on the {111} planes of germanium single crystal and the elimination of Kα2 radiation. Given its low degree of crystallinity, the fibers were folded several times before being fixed on the sample holder. The X-ray diffractograms were recorded from 5.5 to 100° in 2θ mode by a curved linear detector (Inel CPS 590) with an angular resolution of 0.015°.
In a first approximation and neglecting the lattice distortion, the apparent crystallite size was calculated through use of Scherer’s formula.
(1)
In Eq. 1, K = 0.89 is the Scherrer constant, θ is the diffraction angle, B is the full-width in radians at the half-height of the peak, and λ is the X-ray wavelength (Hindeleh and Johnson 1980).
The crystallinity index (CI) was estimated by using the Segal formula (Bansal et al. 2010),
(2)
where H22.5 and H18.5 are diffractogram intensities at 22.5 and 18.5°, attributed to (002) plane and amorphous part, respectively.
Thermal Degradation
For all types of fibers, thermal degradation was investigated by the measurement of the fiber mass loss from 40 to 600 °C at a heating rate of 10 °C /min. The flowing gas was air, which was selected in order to provide conditions conducive to degradation. These measurements were carried out in a TAG 24 Setaram apparatus. To improve the heat-mass transfer, small pieces of a fiber were cut out and plated against the crucible bottom and the sample weight was limited to 2.5 mg because its density was about 0.75 g/cm3. Thus, its thickness was less than 2 mm in the crucible.
Tensile Tests
The tensile tests were performed by the use of an Instron mono-column universal testing machine model 5544, with a load capacity of 2 kN and a sensitivity on the order of 0.5%. To avoid breaking the sample within the grips, the fiber was mounted by affixing the extremities of the fiber on a piece of balsa or cardboard, which was then pinched in the grips. The sample had a mean cross section of 0.015 mm × 5 mm and an initial gauge length of 60 mm. The strain rate was about 3 × 10-4 s-1. For each treatment, at least 16 replicas of the test were carried out.
Color Measurements
Color measures were performed using a Minolta spectrocolorimeter, 3200d model, with three illuminants (D65, A, and F2). The fiber color was expressed by the L*, a*, and b* coordinates of the Commission Internationale d’Eclairage color space. The values of L*, a*, and b* represent darkness-lightness, greenness-redness, and blueness-yellowness, respectively. L* varies from 100 for perfect white to zero for black, a* measures redness when positive, gray when zero, and greenness when negative, and b* measures yellowness when positive, gray when zero, and blueness when negative. Thus, an increase of L*, a*, and b* denotes more white, red chroma, and yellow chroma, respectively. The variation of color (ΔE) was estimated in comparison to the raw fiber by the formula,
ΔE*= [(L*-Lr*))2 + (a*-ar*)2 + (b*-br*)2]1/2 (3)
where the coordinates without a subscript are those of the treated fiber and those with the subscript (r) are those of the raw fiber.
RESULTS AND DISCUSSION
Microstructure
SEM micrographs of fibers treated at different NaOH concentrations are shown in Fig. 1. The main observation was that the fibers were more and more clean and smooth with increasing solution concentration. It was also observed that the alveoli (bottom face) were more sensitive to NaOH concentration than the top face. For instance, after 10% NaOH treatment of fibers, the alveoli almost disappeared and the fibrils were less tied together. As it will be shown by ATR-FTIR analysis described in the next section, alkaline treatments dissolve mainly hemicellulose and lignin. Therefore, it can be concluded that they are probably the major constituents of alveoli. Additionally, the top faces observed here did not have scales, contrary to those reported in a previous report (Elenga et al. 2009).
Fig. 1. Micrographs of the bottom (honeycomb, first line) and top faces of raffia fibers (second line); from left to right: untreated (a), treated with 2.5% (b), 5% (c), and 10% NaOH concentrations, respectively
FTIR Analysis
The FTIR-ATR spectra of the raw and NaOH-treated fibers showed characteristic bands that can be assigned to components of lignocellulosic fibers (LCFs) as shown in Fig. 2a and 2b. As can be seen in these diagrams, the signal intensity increased with the solution concentration. This effect is consistent with the evolution of the fiber’s surface with the treatments shown by the SEM micrographs. Cleanness and smoothness of the fiber increased with the solution concentration. Due to this effect and to the inherent variability of LCFs characteristics (compared to synthetic fibers), the comparison between the spectra will be based on the relative intensity of the peaks, taking as reference the band at 897 cm-1 (here at 894 cm-1) proposed by Oh et al. (2005).
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| Fig. 2. FTIR-ATR spectra of raw and NaOH-treated fibers; peaks of the major components of the fiber are shown: cellulose Iβ (719 cm-1), lignin (830, 1240, and 1462 cm-1), and hemicellulose (1161, 1247, and 1735 cm-1) | |
The band at 668 cm-1 is attributed to the celluloses I and II (Kondo and Sawatari 1996). Its intensity increased with the NaOH concentration without the peak shifting. Oh et al. (2005)reported this band’s behavior during the conversion of cellulose I to cellulose II by alkali treatment.
The absorbance at 719 cm-1 was attributed to the cellulose Iβ (Sugiyama et al. 1991). The absorbance ratio between this band and the standard band at 894 cm-1 has been reported as having the highest correlation with the XRD CI (Oh et al. 2005). Thus, it could be considered as a measure of CI. With a NaOH concentration of up to 5%, the CI did not vary significantly, as shown in Fig. 3. However, for the 10% NaOH-treated fibers, the observed CI decrease was about 60%.