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Ilyas, R. A., Sapuan, S. M., Ishak, M. R., and Zainudin, E. S.  (2017). "Effect of delignification on the physical, thermal, chemical, and structural properties of sugar palm fibre," BioRes. 12(4), 8734-8754.


Eco-friendly composites can be prepared by substituting man-made synthetic fibres with various types of cellulosic fibres. Sugar palm-derived nanocrystalline cellulose is a potential substitute. The most important factor in determining a good nanofiller reinforcement agent that can be used in composites is the character of the nanofiller itself, which is affected during a preliminary treatment. Thus, to gain better nanofiller properties, the delignification (NaClO2 and CH3COOH) and mercerization (NaOH) treatments must be optimized. The main objective of this study was to identify the effects of the delignification and mercerization treatments on sugar palm fibre (SPF). In addition, the characteristics of the SPF for the preparation of the hydrolysis treatment to produce nanocrystalline cellulose (NCC) for reinforcement in polymer composites were examined. Sugar palm cellulose (SPC) was extracted from the SPF, and its structural composition, thermal stability, functional groups, and degree of crystallinity were determined via field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD), respectively. The density, moisture content, chemical composition, and structure of the SPC were also analysed.

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Effect of Delignification on the Physical, Thermal, Chemical, and Structural Properties of Sugar Palm Fibre

R. A. Ilyas,a S. M. Sapuan,a,b,*, M. R. Ishak c, E. S. Zainudin b

Eco-friendly composites can be prepared by substituting man-made synthetic fibres with various types of cellulosic fibres. Sugar palm-derived nanocrystalline cellulose is a potential substitute. The most important factor in determining a good nanofiller reinforcement agent that can be used in composites is the character of the nanofiller itself, which is affected during a preliminary treatment. Thus, to gain better nanofiller properties, the delignification (NaClOand CH3COOH) and mercerization (NaOH) treatments must be optimized. The main objective of this study was to identify the effects of the delignification and mercerization treatments on sugar palm fibre (SPF). In addition, the characteristics of the SPF for the preparation of the hydrolysis treatment to produce nanocrystalline cellulose (NCC) for reinforcement in polymer composites were examined. Sugar palm cellulose (SPC) was extracted from the SPF, and its structural composition, thermal stability, functional groups, and degree of crystallinity were determined via field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and X-ray diffraction (XRD), respectively. The density, moisture content, chemical composition, and structure of the SPC were also analysed.

Keywords: Sugar palm fibre; Delignification; Mercerization; Sugar palm cellulose; Sugar palm acid-treated fibres

Contact information: a: Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia; b: Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia; c: Department of Aerospace Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia; *Corresponding author:


Environmental concerns over global warming have encouraged researchers to discover new eco-friendly biocomposites with the development of recent technologies (Reddy and Yang 2015; Jumaidin et al. 2017c,d). The huge advantages of biocomposites, such as low cost, safety, biodegradability, abundance, light weight, several other intrinsic properties, and preferable properties compared with synthetic fibres, have caused researchers to focus on natural biocomposites (Rajkumar et al. 2016; Jumaidin et al. 2017a).

However, several problems have been observed when using natural fibres as reinforcement in polymer composites because of their incompatibility. Thus, it is necessary to either modify the surface of the natural fibres or extract their highly crystalline and rigid nanoparticles. The properties of nanocrystalline cellulose (NCC) are dependent upon the type and intensity of the treatment, as well as the preliminary method used to extract the cellulose. One method that was used by several researchers was a delignification and mercerization treatment (Tawakkal et al. 2012; Tee et al. 2013; Sanyang et al. 2016a; R.A. Ilyas et al. 2016). Delignification (using sodium chlorite, NaClO2) and mercerization (using sodium hydroxide, NaOH) treatments can be used to remove the amorphous structure in fibres, such as lignin and hemicellulose, respectively, which also splits the fibres into smaller fibrils known as cellulose microfibrils. Delignification is the process of removing lignin from natural fibre by natural enzymatic or industrial chemical processes. This process penetrates and oxidizes all of the lignin (Keshk et al. 2006). Mercerization, also called alkali treatment, is the process of subjecting the natural fibre to the action of fairly concentrated strong base solution (aqueous NaOH or KOH solution), which depend on the type and concentration of solution, temperature and time of treatment, as well as tension of material, to produce great swelling with resultant changes in the fine structure, morphology, dimensions, and mechanical properties; ASTM:D1695-07 (Majeed et al. 2013). It has been reported that mercerization has four effects on the fiber: (1) it increases the number of possible reaction sites by exposing the surface of cellulose fibre (2) it increases surface roughness, resultant in better mechanical reinforcement; (3) it improves the mechanical behavior, i.e. strength and stiffness, and increases the percentage crystallinity index of alkali treated fibres; and (4) decreases in the spiral angle, i.e. closer to fibre axis, and increase in molecular orientation (Akil et al. 2011). The removal of the amorphous structure via a combination of both treatments greatly increases the crystallinity and purity of the cellulose obtained. There is a correlation between the degree of crystallinity and stiffness of the cellulose, where an increase in the crystallinity increases the stiffness of the fibres. A higher crystallinity in the chemically treated fibres is related to a higher tensile strength. Therefore, the mechanical properties of the nanocomposite material can be improved by using these treated fibres as nanofiller (Rong et al. 2001; Bhatnagar and Sain 2005).

Sugar palm trees have been utilized for hundreds of years for manufacturing, and a variety of products can be made from its fibre, trunk, fruit, and palm sap (Tomlinson 1962). This tree is very popular in Negeri Sembilan, Malaysia because palm sap is the material used for making traditional sugar blocks. Additionally, this sugar can be processed into crystal brown sugar, which is used as an alternative to commercialized granular sugar made from sugarcane. Its sap also can be used for making bioethanol, pharmaceuticals, beverages, medicine, biofuel, and several other products under certain processing conditions. Its fibre can be used to make traditional brooms as well. Another important part of the sugar palm is its fruit. The fruit can be used for making juices, canned food, pickles, and dessert. Also, its trunk can be processed for making sago (Ishak et al. 2012).

However, the crucial part of the sugar palm tree is its fibre, which is locally known as ijuk. The fibre is black in colour and it can be used for paint brushes, brushes, roofing, handcrafts, ropes, septic tank base filters, and fishing tools (Miller 1964). According to Ishak et al. (2012), there are several advantages of the sugar palm fibre (SPF); it has a high durability, good resistance to sea water, high tensile strength, low degradation rate, and is not affected much by moisture and heat compared with coir fibres. It has unique features and does not need to undergo secondary processing, such as mechanical decorticating and water retting, to yield fibre (Ishak et al. 2012). This fibre can be readily used because it can easily be found covering the base of palm leaf ribs. Several studies have focused on the properties of raw and treated SPF and its composition to reveal its potential performance and promote its use (Rashid et al. 2016; Sanyang et al. 2016b).

The main objective of this study is to understand the effects of delignification and mercerization on the physicochemical and thermal properties of SPF. Its characteristics are examined for the preparation of a hydrolysis treatment to produce NCC for the reinforcement of polymer composites. Surface-modified SPF was subjected to physical, chemical, and thermal stability analyses using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, chemical composition analysis, and thermogravimetric analysis (TGA). Field emission scanning electron microscopy (FESEM) was used to examine the surface of the fibre after treatment.



The SPF used in this study was collected from Jempol, Negeri Sembilan, Malaysia. The chemical reagents used were sodium chlorite (NaClO2), acetic acid (CH3COOH), and sodium hydroxide (NaOH), which were supplied by Sigma Aldrich (Selangor, Malaysia).


The cellulose fibres were extracted from the SPF using two main processes, which were delignification and mercerization (Tawakkal et al. 2012; Tee et al. 2013; Sanyang et al. 2016a). The initial process was performed in accordance with ASTM D1104-56 (1978) to prepare holocellulose with a chlorination or bleaching process, which is primarily designed to remove lignin from SPF. Twenty grams of SPF was rinsed with tap water to remove foreign particles and dust. The clean SPF was then soaked in a 1000-mL beaker containing 650 mL of hot distilled water, which was then transferred to a water bath and the temperature was set to 70 °C. Next, 4 mL of acetic acid and 8 g of sodium chlorite were added to the beaker every hour. There were three different treatment times, 5 h, 6 h, and 7 h, and the treated fibres were labelled as sugar palm bleached fibre 05 (SPBF05), sugar palm bleached fibre 06 (SPBF06), and sugar palm bleached fibre 07 (SPBF07), respectively. The cellulose obtained was referred to as holocellulose, and was filtered, washed, and rinsed with distilled water.

The holocellulose was further treated to produce α-cellulose according to ASTM D1103-60 (1977). The holocellulose was soaked in 500 mL of 5% w/v NaOH solution for 2 h at 23 °C ± 2 °C. The α-cellulose that was produced was filtered and immersed in 500 mL of distilled water containing approximately 7 mL of acetic acid to neutralize the alkaline cellulose. The mixture was stirred for approximately 30 s before it was allowed to settle for 5 min. It was then rinsed with water until the cellulose residue was no longer acidic, as indicated by a pH meter. The sugar palm cellulose (SPC) obtained from the fibre treated for 5 h, 6 h, and 7 h was denoted as SPC05, SPC06, and SPC07, respectively. Finally, the cellulose was oven-dried at 103 °C overnight.

Field emission scanning electron microscopy (FESEM)

The FESEM micrographs were taken using an FEI NOVA NanoSEM 230 machine (FEI, Brno-Černovice, Czech Republic) with an accelerating voltage of 3 kV to observe the micro- and nanostructure surfaces of the longitudinal cross-section of the dried SPF and treated fibre. All of the samples were coated with gold using an argon plasma metallizer (sputter coater K575X) (Edwards Limited, Crawley, United Kingdom) to avoid charging (Sheltami et al. 2012).

Moisture content

Five samples were prepared for the moisture content investigation. All of the samples were heated in an oven for 24 h at 105 °C. The weight of the samples before (Mi, gram) and after (Mf, gram) heating were measured to determine the moisture content (Jumaidin et al. 2017b), which was calculated using Eq. 1:

Moisture content (%) = (1)


The density was measured using gas intrusion under a helium gas flow with an AccuPyc 1340 pycnometer (Micromeritics Instrument Corporation, Norcross, GA, USA). Five measurements were taken at 27 °C, and the average value was computed (Jumaidin et al. 2017b).

Chemical composition

The chemical composition was analysed according to the TAPPI T222 (2006) (acid-insoluble lignin in wood and pulp) and TAPPI T203 (1999) (α-, β-, and γ-cellulose in pulp) standard methods, and the method described by Wise et al. (1946) (holocellulose in pulp).

Fourier transform infrared (FTIR) spectroscopy analysis

The FTIR spectroscopy was used to detect possible changes in the functional groups in the SPF at different stages of treatment. The spectra were obtained using an IR spectrometer (Nicolet 6700 AEM, Thermo Nicolet Corporation, Madison WI, USA). The FTIR spectra of the samples were collected in the range of 4000 cm-1 to 500 cm−1 with a resolution of 4 cm−1 with a total of 42 scans for each sample (Jumaidin et al. 2017a,c).

X-Ray diffraction (XRD)

A Rigaku D/max 2500 X-ray powder diffractometer (Rigaku, Tokyo, Japan) operated at a generator voltage of 40 kV, a current of 40 mA, and goniometer speed of 0.02 (2θ)s-1 and equipped with CuKα radiation (λ = 0.1541 nm) in the 2θ range of 10° to 40° was used to study the X-ray diffraction patterns of the raw, bleached, and alkali-treated SPF. An empirical method (Segal et al. 1959) was used to obtain the crystallinity index of the samples (Xc), as shown by Eq. 2,


where I002 and Iam are the peak intensities of the crystalline and amorphous regions, respectively.

Degree of polymerization (DP)

The degree of polymerization (DP) for each treated fibres suspension was measured based on the intrinsic viscosity. Measurements of viscosity for resulting treated fibres suspension was carried on according to TAPPI Standard Method T230 om-08 and ISO 5351-1 as reported by Chauve et al. (2013). The treated fibres were diluted in solutions containing distilled water and copper (II) ethylenediamine (CED) solution as dissolving agent with a ratio 0.01:1:1 (treated fibre: distilled water: CED). The solution was shaken until all the fibres were completely dissolved. The viscosity of this solution and the solvent was measured at 25 °C using Ubbelohde viscometer tube (Type 231, PTA Laboratory Equipment, Vorchdorf, Austria). The experiment was performed for all the samples and repeated three times. The molecular weight of treated fibre was calculated using the Mark-Houwink approach, which was using Eq. 3,


where the intrinsic viscosity and M is the molecular weight. The values of the constants were taken as K=0.42 and α=1 for the CED solvent (Yasim-Anuar et al. 2017).

Thermogravimetric analysis (TGA)

The thermal degradation behaviour of the composites was analysed with TGA according to the weight loss because of the increase in temperature. The TGA was performed on a Q series thermal analysis machine from TA Instruments (Mettler-Toledo AG, Schwerzenbach, Switzerland) to determine the thermal stability of the SPF at different stages of the extraction. The analysis was performed in aluminium pans under a dynamic nitrogen atmosphere from 25 °C to 600 °C at a heating rate of 10 °C/min (Jumaidin et al. 2017a,c).


Physical Properties

The bleaching and alkali treatments not only caused changes in the chemical composition of the treated fibres (SPBF and SPC), they also affected the structure of the surface of the fibres. Figure 1 shows the sugar palm tree, untreated raw SPF, SPBF, and SPC. The pigment of the SPF was altered from black to light brown after the bleaching treatment, and became white after the alkali treatment.

Table 1. Physical Properties of the Sugar Palm Fibre, Bleached Fibre, Alkali-treated Fibre, and Other Biomaterials


Results expressed as the mean ± the standard deviation

Fig. 1. Photographs of the (a) sugar palm tree; (b) raw SPF; bleached fibre: (c) SPBF05,

(d) SPBF06, and (e) SPBF07; and alkali-treated fibre: (f) SPC05, (g) SPC06, and (h) SPC07

The morphology of the SPF, SPBF05, SPBF06, SPBF07, SPC05, SPC06, and SPC07 were analysed using FESEM. As shown in Table 1, the average diameters of the SPF, SPBF05, SPBF06, SPBF07, SPC05, SPC06, and SPC07 were approximately 212.01 ± 2.17, 122.95 ± 0.05, 104.45 ± 0.02, 94.49 ± 0.03, 11.84 ± 2.48, 10.68 ± 2.27, and 8.81 ± 1.65 µm, respectively. The most apparent difference between the SPF, SPBF, and SPC was a reduction in the diameter.

Fig. 2. FESEM micrographs of the raw SPF: (a) longitudinal section, (b) cross section, and (c) primary and secondary cell walls; bleached fibre: (d) SPBF05, (e) SPBF06, and (f) SPBF07; and alkali-treated fibre: (g) SPC05, (h) SPC06, and (i) SPC07

From Table 1, it was clearly seen that the diameter of the SPBF and SPC was almost 2 and 20 times smaller than the diameter of the SPF, respectively. Table 1 showed that there was a slight decrease in the diameter of SPBF05, SPBF06, and SPBF07. This was caused by the increased reaction time of NaClO2 on the fibre, which removed lignin from the SPF. The longer the reaction time of NaClO2, then the smaller the diameter of the fibres. It was reported in previous literature that the treatment method with NaClO2 under acidic conditions has successfully been used to remove lignin from natural fibres (Keshk et al. 2006; Liu et al. 2012).

The diameter obtained for the SPC was almost similar to the average diameter of kenaf cellulose (13 µm) (Tawakkal et al. 2012), coir cellulose (3 µm to 12 µm) (Sonia and Dasan 2014), cellulose microfibres (10.04 µm) (Sonia and Dasan 2014), soy hull cellulose (3 µm to 12 µm) (Alemdar and Sain 2008), and banana cellulose (10 µm) (Deepa et al. 2011). Alemdar and Sain (2008) reported a 6 µm to 7 µm diameter for cellulose microfibres, which is less than the diameter of the SPC found in this study. The decreasing trend observed for the diameter of SPBF05, SPBF06, and SPBF07 was ascribed to the removal of lignin and hemicellulose via the delignification and mercerization of the raw SPF, where the SPF bundle was cleaved and cellulose microfibrils were released (Talib et al. 2011). These trends were the same as the trends observed for the SPBF because SPC was obtained by continuing the process used to obtain SPBF.

The surface topography of the rod-like SPFs was rough, and hole-like spots that appeared were evenly arranged. Similar spots were also reported on the surface of coir and SPF by Ticoalu et al. (2012). According to Ticoalu et al. (2012), these observable spots on the surface of the fibres cover the pit on the cell wall, and they are known as tyloses. However, after the fibres went through the process of delignification and mercerization, the surface topography of the derived SPBF and SPC became smooth and groovy, respectively, and parallel lines appeared along the length of the SPBF and SPC (Tee et al. 2013; Sanyang et al. 2016a). For the surface topography, SPBF had a smooth surface. This was due to the removal of impurities, such as pectin, lignin, and waxy constituents, from the surface of the cell walls after the acid treatment (Tee et al. 2013). However, the cellulose microfibrils in the fibres were still arranged in a bundle form. The SPF is generally composed of coarse bundles of single fibres held together by lignin and pectin. Therefore, when sodium chlorite and acetic acid are used directly, the sodium chlorite penetrates and oxidizes all of the lignin (Keshk et al. 2006). The groovy surface of the SPC was due to the removal of lignin and hemicellulose (Liu et al. 2004; Sgriccia et al. 2008; Talib et al. 2011; Sanyang et al. 2016a). These observations were supported by several authors who studied the surface appearance of natural fibre-derived cellulose (Sgriccia et al. 2008; Tawakkal et al. 2012; Tee et al. 2013; Sanyang et al. 2016a).

The moisture content is an important feature that should be taken into account when determining if a natural material is an appropriate filler for polymer composites. It has been reported that the stability, dimensions, porosity formation, and tensile strength of a biocomposite can decline if the biocomposite has a high moisture content (Razali et al. 2015; Jumaidin et al. 2017b). Hence, a lower moisture content is required. The moisture contents of the SPF, SPBF, and SPC in this study (Table 1) were low (8.36 wt.%, 6.13 wt.% to 6.25 wt.%, and 3.83 wt.% to 4.19 wt.%, respectively) compared with other natural fibres (sisal, jute, flax, banana, nettle, hemp, and ramie fibres), which have a moisture content of approximately 8 wt.% to 22 wt.% (Li et al. 2000; Akil et al. 2011). This might have been due to the initial heating of the fibres that eliminated some of the moisture. The raw SPF had a slightly higher moisture content than the treated fibre (SPBF and SPC), which indicated that the loss of unstable compounds during the delignification and mercerization treatments also reduced the moisture content. Therefore, the low moisture content possessed by SPBF and SPC make them good prospective fillers in polymer composites.

Additionally, one of the most vital factors that should be taken into account before considering a new material as a filler in polymer composites is their weight, as this property may affect the attainment of the end product. The density of the material is directly associated with the weight. Table 1 displays the average density of the SPF, SPBF05, SPBF06, SPBF07, SPC05, SPC06, and SPC07, which were approximately 1.50, 1.36, 1.33, 1.30, 1.36, 1.32, and 1.28 g/cm3, respectively. These values were similar to other fibres, such as abaca, flax, cotton, sisal, and ramie, that have a density of 1.5 g/cm3(Akil et al. 2011; Faruk et al. 2012). However, the density of the SPF, SPBF, and SPC were lower compared with man-made fibres, such as E-glass (2.55 g/cm3) and carbon (1.78 g/cm3), but slightly higher than other natural fibres, such as oil palm empty fruit bunch (0.7 g/cm3), coir (1.15 g/cm3), and bagasse (1.25 g/cm3) (Akil et al. 2011; Faruk et al. 2012). Moreover, it was observed that the treated fibre (SPBF and SPC) had slightly lower densities than the raw SPF. In addition, the density value of the bleached fibres (SPBF05, SPBF06, and SPBF07) and alkali-treated fibres (SPC05, SPC06, and SPC07) showed a decreasing trend. This decreasing trend in density might have been because of the removal of the main components from the fibres, such as lignin and hemicellulose, as shown in Table 1. The removal of cellulosic components during the bleaching and alkali treatments created voids in the fibre structure, which caused fibre swelling to occur. The constituents of the fibres then became very separated. The increased volume and weight loss caused the density value to decrease (Ray and Sarkar 2001).

Chemical Compositions

The chemical composition is a crucial criterion that influences the thermal, physical, and mechanical properties of natural fibres. Plant fibres are composite constituents constructed by nature. Generally, fibres are composed of a matrix of crystalline cellulose microfibrils reinforced with hemicellulose and amorphous lignin (Cristaldi et al. 2010). However, the different amounts of these compounds depend on the growth conditions (sources, climate, soil features, and dietary and aging conditions) and fibre processing/extraction methods (Mukherjee and Radhakrishnan 1972; Cristaldi et al. 2010; Ishak et al. 2012; Razali et al. 2015). Table 2 displays the results of the chemical composition analysis of the raw and treated SPF.

Table 2. Comparison of the Chemical Compositions of the Treated and Untreated Sugar Palm Fibre


From Table 2, it was seen that SPBF07 and SPC07 had the highest cellulose contents in the SPBF and SPC groups, and were 56.67% and 82.33%, respectively. The cellulose contents for SPC06, SPC05, SPBF06, SPBF05, and SPF were 80.47%, 80.96%, 56%, 54.08%, and 43.88%, respectively. In addition, higher amounts of cellulose were observed in the treated fibre (SPBF and SPC) compared with the raw SPF. This finding may have been due to the removal of some extractive components from the raw SPF during the delignification and mercerization treatments, which separated a higher proportion of the insoluble compounds in the treated fibre (Tan and Lee 2014). Thus, SPBF07 and SPC07 are the preferred fibres to be used for the preparation of NCCs via hydrolysis.

FTIR Spectroscopy Analysis

Figure 3 shows the fingerprints of the functional groups in the raw and treated SPF, and Table 3 displays a summary of the assigned FTIR bands. The band located at 1719 cm−1 in the raw SPF spectrum is attributed to the C=O stretching of the acetyl and uronic ester groups of the hemicellulose or the ester linkage of the carboxylic groups of ferulic and p-coumaric acids of lignin and/or xylan in the hemicellulose (Sun et al. 2005; Alemdar and Sain 2008; Fabiyi and Ogunleye 2015). This band was still present in the FTIR spectra of the fibres after the bleaching treatment, but it was no longer present after the alkali treatment. The disappearance of this band could have been caused by the removal of hemicellulose and lignin from the SPF during chemical extraction (Alemdar and Sain 2008; Jonoobi et al. 2009; Sheltami et al. 2012). This was supported by the results obtained from the chemical composition analysis of the fibres (Chemical Compositions and Table 2), in which it was indicated that lignin was almost completely removed after the bleaching treatment. However, hemicellulose was still present after the bleaching and alkali treatments. For this reason, the disappearance of the C=O stretching band from the spectra could have been caused by the cleavage of the ester-linked substances of the hemicellulose via the alkali treatment. The ether bonds between lignin and hemicellulose were not affected by this treatment (Xiao et al. 2001; Sheltami et al. 2012).

Fig. 3. FTIR spectra of the (a) raw SPF; bleached fibre: (b) SPBF05, (c) SPBF06, and (d) SPBF07; and alkali-treated fibre: (e) SPC05, (f) SPC06, and (g) SPC07

The peaks in the 1520 cm−1 to 1510 cm−1 region were determined to be aromatic skeletal vibrations of the lignin and lignocellulosic functional groups. The bands observed at 1227 cm−1, 1507 cm−1, and 1593 cm−1 corresponded to lignin (Faix et al. 1992; Arya et al. 2012). These peaks disappeared after the bleaching treatment, which suggested that this treatment effectively removed lignin from the fibres. This conclusion was supported by the chemical composition analysis of the fibres, shown in Table 2. Additionally, the peaks at 1600 cm−1 to 1475 cm-1 were determined to be the structural polymer stretching of the aromatic groups present in the form of lignin (Himmelsbach et al. 2002; Sahari et al. 2012).

Based on Fig. 4 and Table 3, the absorbance peaks in the region from 1650 cm−1 to 1630 cm−1 and at approximately 2900 cm−1 reflected the stretching of O–H and C–H groups, respectively. The peaks in the 3700 cm−1 to 3100 cm−1 region were assigned to adsorbed water. The peaks at 897, 1030, 1160, 1316, 1370, and 1424 cm−1 were attributed to the C–H rocking vibrations, C–O stretching, C-O-C asymmetric valence vibration, C-H2 rocking vibration, C-H2 deformation vibration, and cellulose in the carbohydrates, respectively (Alemdar and Sain 2008; Sheltami et al. 2012). These different bands were seen in all of the spectra, regardless of the treatment of the fibres. The intense peak at 1656 cm-1signified C=C stretching of unsaturated acids or sterols corresponding to tannin (Sun and Tomkinson 2002).

Table 3. Summary of the IR Bands Observed for the Bleached and Alkali-treated Sugar Palm Fibre


The intense peaks at 3500 cm−1 to 3200 cm-1 indicated the presence of O-H groups in the untreated and treated fibres because of the presence of hydroxyl groups in the cellulose, hemicellulose, and lignin. The peaks at 1800 cm−1 to 1600 cm-1 signified carbonyl groups (C=O) in the lignin and hemicellulose (Kazayawoko et al. 1997). The high peaks at 1300 cm−1 to 1000 cm-1 existed in all of the fibre types, and signified C-H stretching and C-O groups (Faix et al. 1992).

XRD Measurements

The crystallinity of individual fibres can influence the thermal and mechanical properties of a composite. Therefore, an XRD study was conducted to investigate the crystallinity of the raw and treated SPF. The XRD graph (Fig. 4) showed that all of the diffractograms displayed sharp peaks at 2θ values of approximately 16° and 22.8°, which were assumed to signify the typical cellulose I form. In contrast, the amorphous region was characterized by low peaks at a 2θ value of approximately 18°(Segal et al. 1959). This showed that the crystalline cellulose structure was not altered by the chemical treatment.

Fig. 4. X-ray diffraction patterns of the (a) raw SPF; bleached fibre: (b) SPBF05, (c) SPBF06, and (d) SPBF07; and alkali-treated fibre: (e) SPC05, (f) SPC06, and (g) SPC07

Table 4. Properties of Cellulose from various Sources


The crystallinity index of each untreated and treated sample was also calculated and listed in Table 4. A noticeable increase in the crystallinity from 55.8% for the raw SPF to 76.0% for the chemically treated cellulose fibres was observed. The crystallinity indices of the raw SPF, SPBF05, SPBF06, SPBF07, SPC05, SPC06, and SPC07 were 55.8%, 64.9%, 64.8%, 65.9%, 73.4%, 74.2%, and 76.0%, respectively. The increase in the crystallinity of the fibres was due to the removal of amorphous substances, such as lignin and hemicellulose, during the delignification and mercerization treatments. This increase in crystallinity after the chemical treatments was supported by several other studies (Alemdar and Sain 2008; Chen et al. 2011). Compared with other treatments, SPBF07 and SPC07 had higher crystallinity indices with 65.9% and 76.0%, respectively. This was because of the prolonged delignification treatment time, which removed most of the lignin content from the fibres. The crystallinity index for SPC07 after the alkali treatment was higher than the values of mengkuang leaf (69.5%) and pineapple leaf (54%) fibres reported by Sheltami et al. (2012) and Cherian et al. (2010), respectively. Additionally, the diffraction peak at 22.6° became sharper after the chemical treatment. This observation was related to the better crystalline domains.

Degree of Polymerization

Degree of polymerization (DP) is a crucial parameter for evaluating the length and branching of cellulose chains. Besides that, it has been stated by Audrey et al. (2007), that DP and molecular weight may affect the properties of cellulose such as spinnability, solubility, and the mechanical properties of cellulose based materials. The degree of polymerization and viscosity-average molecular weight of the treated fibres were determined using an intrinsic viscosity measurement. Table 4 gives the DP value for the various treated fibres prepared in this study. The degree of polymerization of the SPBF05, SPBF06, SPBF07, SPC05, SPC06 and SPC07 were 3417.9, 3164.5, 2963.3, 1165.7, 1047.9 and 946.4, respectively, and the molecular weight were 554,200 g/mol, 513,100 g/mol, 480,500 g/mol, 189,000 g/mol, 169,900 g/mol, and 153,500 g/mol, respectively.

From the same table , it can be seen that the DP obtained for the SPC07 was almost similar to the DP of oil palm cellulose (967) (Yasim et al. 2017) and bamboo cellulose (891) (Wang et al. 2010). According to Yasmin et al. (2017) the molecular weight and degree of polymerization of bio-cellulose based material were ranged from approximately 90,000 to 300,000 g/mol and 400 to 3000, respectively. The sugar palm cellulose (SPC05, SPC06 and SPC07) were in the range of the cellulose DP, thus in good agreement with those reported in the literature.

Besides, it has been reported by Kumar et al. (2009), that delignification by the acid-chlorite process has a significant effect on the cellulose chain, in which it causes a huge reduction in the average degree of polymerization of treated fibres. Extensive delignification of SPBF07 process reduced the DP to 2963, and further mercerization process caused the DP to decrease to 946.4. The significant reduction in DP is consistent with the reduced treated fibre lengths observed and previously mentioned. In this study, the DP was reduced by more than 68% after 7 h of delignification and mercerization process. The decreasing trend observed for the DP of SPC05, SPC06, and SPC07 was attributed by the removal of lignin and hemicellulose via the delignification and mercerization of the raw SPF, where the SPF bundle and SPBF fibres was cleaved and cellulose microfibrils were released (Talib et al. 2011). Moreover, the DP reduction of cellulose was also due to the acid catalysed cleavage during acid-chlorite delignification (Hubbell and Ragauskas 2010).

Thermogravimetric Analysis (TGA)

Generally, the thermoplastic handling temperature increases above 200 °C, so it is important to know the thermal properties of the fibres before they are used in biocomposites to determine their compatibility with the thermoplastic itself. The thermal degradation characteristics of the treated and untreated SPF are shown with the TG and DTG curves in Fig. 5. It was observed that the thermal degradation of the treated and untreated fibres had four stages.

Fig. 5. TG and DTG curves of the (a) raw SPF, bleached fibre, (b) SPBF05, (c) SPBF06, and (d) SPBF07, and alkali-treated fibre: (e) SPC05, (f) SPC06, and (g) SPC07

The first stage was evaporation of the moisture content (45 °C to 123 °C) within the fibre, followed by degradation of the lignocellulosic components of the hemicellulose (220 °C to 315 °C), cellulose (300 °C to 370 °C), lignin (160 °C to 900 °C), and finally the ash (1723 °C). When the fibres were heated, the weight of the fibres gradually decreased because of the loss of water and volatile extractives (Ishak et al. 2012). Also, the volatile extractives were less likely to move to the fibre surface. This lack of movement of volatile extractives was because of the movement of water from inside the fibre to the fibre surface, as the water on the fibre surface was vaporized during the heating process. The low-temperature heating process resulted in a low mass loss with only losses of water in the cell lumen and cell wall, and volatile extractives in the fibres. It was observed that the evaporation of moisture from the SPF was completed at 106.78 °C, which was higher compared with the treated fibre (approximately 101 °C), as shown in Table 5. This was because of the high moisture content of the SPF (8.36%), which resulted in a higher weight loss compared with the treated fibres (3.83% to 6.87%), as shown in Table 1. It was observed in Table 5 that the weight loss for the treated fibres ranged from approximately 8.58 wt.% to 10.36 wt.% at approximately 101 °C, while for the SPF, the weight loss was 10.38 wt.% at 106.78 °C. This remarkable weight loss was because of the degradation of the basic lignocellulosic components, starting with hemicellulose, cellulose, and lastly lignin, which generally degrades at 100 °C to 140 °C and higher (Yang et al. 2007). It was observed that the first and secondary thermal degradation onset temperatures of the raw SPF occurred at 210.58 °C and 308.05 °C, respectively. Meanwhile, the DTG curve showed that the onset temperature of the treated fibres shifted to a lower temperature (approximately 195 °C). A slight decrease in the second thermal degradation onset temperature (approximately 284 °C) was also observed. The higher thermal stability of the SPF might have been because of the higher lignin content, which gives rigidity to the plant materials.

Table 5. Onset Temperature (TOnset), Degradation Temperature of the Maximum Weight Loss Rate (TMax), Weight Loss (WL), and Char Yield for the Sugar Palm Fibre


Data was obtained from the TG and DTG curves

The second stage was the degradation of hemicellulose. This typically occurs when the temperature is increased to 220 °C, and is completed at 315 °C (Yang et al. 2007). However, the results obtained from this study showed that the degradation of hemicellulose occurred from 195.66 °C to 197.54 °C for SPBF05, SPBF06, and SPBF07, and for SPF, hemicellulose began degradation at 210 °C and was completely degraded by approximately 300 °C, as shown in Fig. 5. This stage only occurred for the SPBF and SPF, and not for the SPC because hemicellulose was removed during the alkali treatment. The onset degradation temperature was shifted from 210 °C (SPF) to 195.66 °C (SPBF) after the bleaching treatment. This may have been because of the lower hemicellulose content (7.24%) in the SPF compared with the SPBF (19.8% to 23.05%), as seen in Table 2. Several reasons were determined by Yang et al. (2007) for the sequence of lignocellulosic degradation, which starts with hemicellulose. Hemicellulose is comprised of various saccharides, such as mannose, galactose, xylose, and glucose. It appears in an amorphous state and is easy to remove at low temperatures. Also, its lower thermal stability than for cellulose causes hemicellulose to degrade to a greater extent compared with cellulose and lignin (Shafizadeh and Chin 1977).

The third stage was the degradation of cellulose, which started shortly after the hemicellulose had completely degraded. The degradation of cellulose requires higher temperatures, but once the required temperature is attained, degradation happens at a very high rate. It was seen from Table 5 that the cellulose had the highest rate of degradation of the entire degradation process. It was determined by several authors, such as Yang et al. (2007) and Ishak et al. (2012b), that the highest degradation rate occurred during the degradation of cellulose. In this study, it was observed from the DTG curves of the fibres (Fig. 5) that cellulose degraded from 210 °C to 390 °C, and the highest rate of degradation occurred at an average temperature of 330 °C. This was supported by Kim et al. (2001) and Ishak et al. (2012), who reported that the critical temperature for the degradation of cellulose is 320 °C and 330 °C, respectively. A similar temperature was found in this study because the cellulose content in the raw SPF (43.88%) was lower compared to the treated fibres (54.08% to 82.33%) (Table 2) and the weight loss of the cellulose in the raw SPF was found to be lower (43.76 wt.% at 345.45 °C) than for the treated fibre (73.71 wt.% at 345.09 °C). It was also seen in Fig. 5 that there was a noticeable difference in the TG and DTG curves of the treated and untreated fibres. The SPC samples had two more peaks compared with the SPF and SPBF. This was because of the removal of hemicellulose from the fibres during the alkali treatment. It was also seen that among the SPBF, the maximum cellulose degradation (324.44%) occurred for SPBF07. The thermal degradation onset temperature for SPF was similar to that of other natural fibres, such as roselle (220 °C) and kenaf (244 °C).

The fourth phase was the degradation of lignin. Lignin was difficult to degrade compared with hemicellulose and cellulose. The degradation of lignin started as early as 160 °C and continued until 900 °C. The difficulty in degrading lignin may have been because of the toughness of lignin, which mainly functions as support for plants and bonds individual cells together in the middle lamella region. However, lignin degradation occurred at a low weight loss rate (< 0.14 wt.%/°C) from the ambient temperature to 900 °C (Shafizadeh and Chin 1977; Yang et al. 2007; Ishak et al. 2012). It was observed in Fig. 5 that there were slight differences in the TG and DTG curves of the treated and untreated fibres. The second SPF curve shifted to the right compared with the SPBF curve. This might have been due to the higher lignin content (33.24%) in the SPF fibres compared with the SPBF (0.27% to 2.78%), which prevented the hemicellulose from being easily degraded.

When the lignin had completely degraded, only the leftover material, called char or ash, remained. This char consists of inorganic material, such as silica, which can be degraded at 1723 °C (Ishak et al. 2012). Table 5 clearly indicated there was a higher char content for the raw SPF than for the other fibres. Additionally, char also blocked the reactive chemical groups and suppressed the percent weight loss of the fibre. This was the reason why the raw SPF had a higher thermal stability than the treated fibres. These results were consistent with the results obtained from the chemical composition, XRD, and FTIR analyses.


  1. The treated SPF, particularly SPBF07 and SPC07, possessed certain advantages, such as a higher crystallinity, higher purity of cellulose, and better thermal stability, compared with the other fibres.
  2. The raw SPF had a slightly higher density and moisture content than the treated fibres. The XRD and FTIR analyses indicated that there were changes to the structure of the fibres during treatment.


The authors would like to thank the Universiti Putra Malaysia for the financial support through the Graduate Research Fellowship (GRF) scholarship. The authors are grateful to Dr. Muhammed Lamin Sanyang for guidance throughout the experiment. The authors also thank the Forest Research Institute Malaysia (FRIM) and Dr. Rushdan Ibrahim for their advice and fruitful discussions.


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Article submitted: July 9, 2017; Peer review completed: August 26, 2017; Revised version received: September 19, 2017; Accepted: September 27, 2017; Published: October 4, 2017.

DOI: 10.15376/biores.12.4.8734-8754