Abstract
Typha fibers were chemically retted in 5% sodium hydroxide solution for 1 h, 2 h, 4 h, and 8 h. Changes in chemical compositions of the untreated and treated fibers were monitored with Fourier transmission infrared spectroscopy, while changes in the crystallinity index were studied via X-ray diffraction. The FTIR spectra and scanning electron microscope images corroborated the successful removal of amorphous portions from the Typha leaf fibers during alkali treatment, which resulted in an enhanced crystallinity index for alkali-treated fibers. The alkali-treated Typha fiber for 1 h showed the highest water contact angle of 87.5°, while the untreated composite showed the lowest contact angle. Typha fiber treated for 4 h had high tensile strength, Young’s modulus, and elongation at break of 158 MPa, 1600 MPa, and 7%, respectively. The results showed that there was a general increase in the interfacial shear strength of Typha fiber with epoxy resin and polyester resin with increased time. Both the mechanical properties and crystallinity index of the Typha leaf fibers increased with increased time of retting until 4 h, after which further alkaline retting resulted in decreased values. The overall results showed that alkaline-extracted Typha leaf fibers are suitable for biodegradable film composites.
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Tailoring the Effective Properties of Typha Fiber Reinforced Polymer Composite via Alkali Treatment
Samsul Rizal,a Ikramullah,a Deepu A. Gopakumar,b Syifaul Huzni,a Sulaiman Thalib,a M. I. Syakir,b F. A. T. Owolabi,b,c N. A. Sri Aprilla,d M. T. Paridah,e* and H. P. S. Abdul Khalil b,*
Typha fibers were chemically retted in 5% sodium hydroxide solution for 1 h, 2 h, 4 h, and 8 h. Changes in chemical compositions of the untreated and treated fibers were monitored with Fourier transmission infrared spectroscopy, while changes in the crystallinity index were studied via X-ray diffraction. The FTIR spectra and scanning electron microscope images corroborated the successful removal of amorphous portions from the Typha leaf fibers during alkali treatment, which resulted in an enhanced crystallinity index for alkali-treated fibers. The alkali-treated Typha fiber for 1 h showed the highest water contact angle of 87.5°, while the untreated composite showed the lowest contact angle. Typha fiber treated for 4 h had high tensile strength, Young’s modulus, and elongation at break of 158 MPa, 1600 MPa, and 7%, respectively. The results showed that there was a general increase in the interfacial shear strength of Typha fiber with epoxy resin and polyester resin with increased time. Both the mechanical properties and crystallinity index of the Typha leaf fibers increased with increased time of retting until 4 h, after which further alkaline retting resulted in decreased values. The overall results showed that alkaline-extracted Typha leaf fibers are suitable for biodegradable film composites.
Keywords: Typha fiber characterization; Mechanical performance; Interfacial shear strength; Microbond test
Contact information: a: Department of Mechanical Engineering, Universitas Syiah Kuala, Banda Aceh 23111, Indonesia; b: School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia; c: Pulp and Paper Technology Laboratory, Federal Institute of Industrial Research Oshodi, Lagos, Nigeria; d: Department of Chemical Engineering, Universitas Syiah Kuala, Banda Aceh 23111, Indonesia; e: Institutes of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia;
* Corresponding author: parida.introp@gmail.com; akhalilhps@gmail.com
INTRODUCTION
Natural fibers have gained considerable interest for researchers, engineers, and scientists as an alternative reinforcement for fiber-reinforced polymer (FRP) composites due to their low cost, fairly good mechanical properties, non-abrasive character, eco-friendliness, and bio-degradability. They could be exploited as a replacement for various conventional fiber types, such as glass, aramid, and carbon. A wide range of natural fibers such as wood fibers, stem fibers, and leaf fibers have been used to strengthen polymer composites (Jawaid and Abdul Khalil 2011). Among the natural fibers, Typha fibers have received great attention due to their abundant nature. Typha fiber is widely available in most countries and is wildly grown on wetlands in the province of Aceh, Indonesia (Majeed et al. 2013). Although Typha is abundantly available in nature, its potential is still underutilized compared to other natural fibers. In recent decades, natural fiber-reinforced polymer composites have begun to compete with synthetic fiber-reinforced composites, such as glass fibers and carbon fibers, because they have many advantages over glass fibers and carbon fibers (Jawaid and Abdul Khalil 2011; Majeed et al. 2013). Natural fibers may present certain mechanical properties equal or superior to those of glass fibers (Taylor et al. 2017). Other advantages of using natural fibers compared to carbon fibers and glass fibers include low density, reduction in energy consumption, no irritability to the skin, renewability, and biodegradability (Fragassa et al. 2018; Ilyas et al. 2018b,c). Moreover, natural fibers have low cost with low density and high specific properties compared to synthetic fibers (Jawaid and Abdul Khalil 2011).
For the development and utilization of natural fibers as an effective reinforcement in composites, it is very relevant to know the characteristics of the natural fibers. Natural fibers are generally hydrophilic, which makes natural fibers less compatible with most of the hydrophobic polymeric matrices. This incompatibility between the hydrophobic polymer matrices and hydrophilic natural fibers can result in weak adhesion at the interface between polymer matrix and natural fiber (Majeed et al. 2013; Ilyas et al. 2017; Sanjay et al. 2018). This is one of the reasons why natural fibers have not completely replaced conventional synthetic fiber materials in high-load applications. Natural fiber-reinforced composites have been used in the automotive industry (Al-Oqla and Sapuan 2014), but their application generally has been limited to components such as door panels, chair backs, packaging, and other interior panels (Sanjay et al. 2016; Sanyang et al. 2018). The different kinds of natural fibers such as jute, hemp, kenaf, oil palm, and bamboo reinforced polymer composite have received a lot of attention in different automotive applications, structural components, packing, and construction (Shalwan and Yousif 2013; Sassoni et al. 2014). The wide advantages of natural fibers reinforced composites such as high stiffness to weight ratio, lightweight, and biodegradability give them suitability in different application in the building industries (Ramezani Kakroodi et al. 2013).
Due to the rapid growth of the natural fiber industry, surface modification of the natural fibers is relevant to study in order to enhance the interfacial compatibility between the polymer matrix and natural fiber (Taylor et al. 2017; Fragassa et al. 2018; Sanjay et al. 2018). The physicochemical properties of natural fiber-reinforced composites rely on the type of fiber, matrix, and matrix-fiber interface. The interface is an in-between zone of fiber-matrix bonds that is present to distribute the stress obtained by the matrix to the fiber (Loh et al. 2013; Ahmad et al. 2015; Cohen et al. 2016). It is of great importance to note that one type of failure mechanism of a composite is when the tensile force delivered to the fiber is higher than the bonding strength of the fiber-matrix, as this will result in debonding and pull-out fibers of the matrix (Karger-Kocsis et al. 2015; Zhao et al. 2018). Interfacial shear strength is the friction force received by the matrix that can still be channelled to the fiber through the interface bond. If the fiber and matrix interactions are poor, then the tensile strength of the composite would be relatively low.
An alkali treatment of the fiber is one of the least costly and most often-used methods for increasing the strength of the interface bonds with the polymer matrix (Sanjay et al. 2018). This method removes the surface impurities on fiber surface that might harm the fiber-matrix bond. Additionally, it also removes the amorphous portions such as pectin, wax, lignin, and hemicellulose from the natural fiber (Torstensen et al. 2018). However, prolonged alkali treatment would affect the cellulosic structure of natural fiber, resulting in the decrement of mechanical properties. The surface roughness of fiber plays a key role in better mechanical bonding with the matrix. Mechanical bonding is an important factor because it is responsible for the composite’s interface strength (Da Silva et al. 2018). Methods such as single-fiber pull-out test, fiber-bundle pull-out test, and microbond test were used to evaluate the interfacial shear strength (IFSS) of the fibers (Ferreira et al. 2018; Xiong et al. 2018). In this context, herein we studied the effect of alkali treatment on the mechanical and physical properties of Typha fiber. Additionally, we also determined the effect of alkali treatment on the interfacial strength of Typha fibers with both epoxy and polyester resin via microbond test.
EXPERIMENTAL
Materials
Typha latifolia plants were collected from swamps in the Darussalam local area, (Banda Aceh, Indonesia) and had lengths from 250 cm to 310 cm and diameters from 9 cm to 14 cm; they were manually decorticated, washed, and dried. Sodium hydroxide (NaOH; with > 98% purity) was purchased from local vendors (Penang, Malaysia), in pellets. Acetic acid (Merck Group, Pulheim Germany) was used for the fiber extraction process. A solution of 5% NaOH was used to soak the fiber for 1 h, 2 h, 4 h, and 8 h; then, the fiber was washed thoroughly with the excess alkali, neutralized by the acetic acid, and then thoroughly washed and combed. The epoxy and polyester resin and hardener used were Araldite LY-564 and HY 560 (mixing ratio 100:27 wt%) from Huntsman Advanced Materials, Basel, Switzerland.
Methods
Fabrication of composite
The untreated and alkali treated Typha fibers were dried in the sun for 4 h. Then, the fiber was cut to 200 mm length and compacted by placing fibers between two metal plates. Then the fiber was pressed. A rectangular shape of metal plate with 170 mm x 150 mm x 3 mm was employed as a composite mold. Typha fibers of different dimensions were spread on polyester and epoxy resin by a hand layup method. After this, the molds were pressed with 200 kg/cm2 in the compression molding machine.
Fourier transform infrared spectroscopy (FTIR)
The chemical functional groups of untreated and alkali-treated Typha fibers were studied using a Nicolet iS10 Fourier Transform Infrared Spectrometer device equipped with an attenuated total reflectance (ATR) microscope recorded within a wavelength range of 500 cm-1 to 4000 cm-1 (ThermoFisher, Waltham, MA, USA). The Nicolet FTIR spectrometer was used to record the absorption spectra of Typha fibers after 1 mg of the fiber powders were pelletized with 100 mg KBr.
X-ray diffraction (XRD)
The X-ray diffractometer was used to observe the crystalline index of the fiber. The wide-angle XRD spectra of the fibers were recorded on a Bruker D8 Advance (Billerica, USA) diffractometer. All samples were scanned in the range of 2θ from 0 ° to 30 °.
Scanning electron microscopy (SEM)
Detailed morphological images of the untreated and alkali-treated Typha fibers’ surfaces and microdroplets were observed using a Carl Zeiss Leo Supra 50 VP scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) that operated with an accelerating voltage of 15 kV. The sample surface was coated with gold using a Polaron SC515 sputter coater (Ladd Research Industries Inc., Williston, VT, USA) prior to observation.
Tensile testing
The mechanical properties of the fibers were evaluated for their tensile strength, ductility, and modulus of Typha fiber elasticity. These tests were performed using a tensile testing machine (Inspect micro S500; Hegewald & Peschke, Nossen, Germany) equipped with a load cell of 10 kN, with a crosshead speed of 2 mm/min with according to ASTM D3379-75 standard. The diameters of Typha fibers were measured using Olympus Cell B software (Olympus Corporation, Tokyo, Japan). Ten measurements were taken for each fiber in order obtain the average value of tensile strength.
Contact angle studies
The water contact angle of the fiber was determined using a KSV CAM 101 (Biolin Scientific, Gothenburg, Sweden) optical contact angle meter to examine the surface wettability of the fiber. Hypodermic syringes were employed to drop water onto the fiber surface. The contact angle was measured on the side of the water droplet. The image was recorded for 40 s with a speed of one frame every 10 s. Three measurements were obtained for each sample and the average value was calculated.
Microbond test
The microbond test developed by Miller et al. (1987) was conducted to determine the IFSS value. Epoxy and polyester resins were dripped onto the surface of Typha fiber, which is called a microdroplet, as shown in Fig. 1, to determine the interfacial shear strength (IFSS) of Typha fibers in both matrices.
Fig. 1. Microdroplet of polyester resin on the surface of Typha fiber
The fibers were withdrawn from the matrix using the tensile test machine. The embedded length of the microdroplet resin was in the range of 1.58 mm to 2.19 mm and the fiber diameters were 0.17 mm to 0.46 mm. The microbond test schematic is presented in Fig. 2. The interfacial shear strength (τ) of the Typha fiber was calculated using Eq. 1,
(1)
where Fmax is the maximum load (N), Df is the fiber diameter (mm), and Le is the fiber embedded length in the matrix (mm).
Fig. 2. Schematic of microbond test
RESULTS AND DISCUSSION
FTIR Analysis
The results of the FTIR spectroscopic analysis of the raw and chemically retted Typha leaf fibers are presented in Fig. 3. Despite the similarities in the spectra plot, as evidenced by the two main absorbance regions, i.e., the broad region corresponding to the range 3200 cm-1 to 3400 cm-1 and the functional group region corresponding to 500 cm-1 to 1800 cm-1, the various changes observed in the spectra absorption confirmed that there were some changes in the chemical compositions of the treated Typha fibers due to the chemical retting process. The FTIR spectra analysis revealed that after the retting process all of the spectra showed a broad band in the region of 3400 cm-1 to 3300 cm-1, which indicated the characteristic O-H stretching vibration of the OH group in cellulose fiber molecules (Ilyas et al. 2018a), while the spectra band shown in each spectra at around 2900 cm-1 was characteristic of C-H stretching vibration (Khalil et al. 2001).
Fig. 3. FTIR spectra of untreated and alkali-treated Typha
Additionally, the vibration peaks detected between 1360 cm-1 and 1365 cm-1 in both the unretted and chemically retted Typha fiber samples were attributable to the bending vibration of the C-O and C–H bonds in the polysaccharide aromatic rings (Nacos et al. 2006). The absorbance peak observed around 1057 cm-1 was due to the C-O-C pyranose ring skeletal vibration. The spectra results at this peak further showed a gradual increase in the intensity of this band as the treatment time increased, which showed an increase in the crystallinity of the samples (Sun et al. 2018). The absorbance band at 1640 cm-1 common to all of the spectra was attributable to the OH bending of adsorbed water (Łojewska et al. 2005). Despite all of these similarities, specific unique absorbance peaks were identified in the spectra. For instance, the spectral peak at 1735 cm-1 for the raw Typha was attributed to the C=O stretching vibration of the acetyl and uronic ester groups from pectin, hemicelluloses, or other ester linkages of the carboxylic group of ferulic and p-coumarin acids of lignin or hemicelluloses (Owolabi et al. 2017a). The absence of this spectral peak indicated the effective removal of the amorphous portions via the alkaline treatment. Lastly, the absorbance peak at 1250 cm-1 in the raw fiber spectrum was attributable to the C-O out-of-plane stretching vibration of the aryl group in the lignin (Le Troedec et al. 2008). These two peaks completely disappeared in the spectra of the chemically/alkali-treated fibers.
XRD Crystallography of Typha Fibers
X-ray diffraction was used to analyze the effect of the cellulose fiber crystallinity on the physical and mechanical properties of the treated fiber. Figure 4 shows an XRD pattern of the raw and the chemically retted Typha fibers at different times.
Fig. 4. XRD of treated and untreated Typha fiber
All of the diffraction patterns showed peaks around 2θ = 16º, 22.5º, and 24.8º, which indicated the typical cellulose I structure. The only difference in the diffraction pattern was the change in the peak’s intensity, which indicated some changes in the fiber crystallinity. The results showed that the peaks at 16º and 22.5º were most defined for the fiber treated at 4 h, and this was attributable to the fact that during chemical retting/alkali treatment the cementing materials, such as tannins, hemicelluloses, and lignin, were dissolved, and the remaining pure crystalline cellulose were isolated. The crystallinity index for both the untreated and treated Typha fibers were calculated according to Eq. 2,
(2)
The obtained crystallinity index values of the untreated Typha fiber and treated Typha fibers at 1 h, 2 h, 4 h, and 8 h were 29.6%, 47.5%, 50.3%, 55.8%, and 35.8%, respectively. This trend was expected to correspond to the increasing trend of the mechanical properties of the fibers (Owolabi et al. 2017b). The results revealed that the sample with the lowest crystallinity index was the raw Typha fiber, and the fiber retted for 4 h had the highest crystallinity index. The increase in crystallinity index was attributed to the removal of amorphous constituents, including hemicellulose, lignin, and pectin, from the Typha fiber during the alkali treatment. Therefore, because the raw fiber contained the highest amorphous content, it was expected to have the lowest crystallinity index while the crystallinity index increased with retting time. The results showed that the crystallinity index dropped for the fiber retted for 8 h, which was attributable to fiber damage and the degradation of the cellulosic structure during the prolonged treatment of alkali. It has been reported that the crystallinity index equally depends on the pre-treatment method and the level of the fiber refining (Owolabi et al. 2017b). From the SEM image in Fig. 5e, the fiber subjected to a longer retting time (8 h) suffered damage through cell wall rupture, hence accounting for the drop in the crystallinity index. This phenomenon also explained why the mechanical strength of the retted fiber at 8 h dropped compared with the 4-h retted Typha fiber.