Abstract
Cellulose nanocrystals (CNC) were prepared from delignified kenaf bast fiber by using alkaline pulping, based on soda anthraquinone, hydrogen peroxide bleaching, and acid hydrolysis treatment with H2SO4. The size and morphology of the fibers were characterized by scanning electron microscopy (SEM), and the isolated fiber from unbleached and bleached pulp had a diameter between 9 to 30 µm. Fourier transform infrared (FTIR) spectroscopy exhibited that the content of lignin decreased in the pulping process, and the lignin was almost completely removed during hydrogen peroxide bleaching. Moreover, fibers were characterized for crystallinity using X-ray diffraction (XRD). The fiber crystallinity gradually increased at each stage of the process (raw kenaf bast, unbleached pulp, bleached pulp, and acid hydrolysis). The fiber was characterized by atomic force microscopy (AFM), which showed that the isolated pulp nanofibers had diameters of approximately 30 nm.
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Kenaf Bast Nanocrystalline Cellulose: Analysis of Morphological, Chemical, Crystalline, and Thermal
Rudi Dungani,a,* Lili Melani,a Widya Fatriasari,b Sasa Sofyan Munawar,c Firda Aulya Syamani,b Melbi Mahardika,b Tati Karliati,a Mustika Dewi,a Alpian,d and Wahyu Supriyati d
Cellulose nanocrystals (CNC) were prepared from delignified kenaf bast fiber by using alkaline pulping, based on soda anthraquinone, hydrogen peroxide bleaching, and acid hydrolysis treatment with H2SO4. The size and morphology of the fibers were characterized by scanning electron microscopy (SEM), and the isolated fiber from unbleached and bleached pulp had a diameter between 9 to 30 µm. Fourier transform infrared (FTIR) spectroscopy exhibited that the content of lignin decreased in the pulping process, and the lignin was almost completely removed during hydrogen peroxide bleaching. Moreover, fibers were characterized for crystallinity using X-ray diffraction (XRD). The fiber crystallinity gradually increased at each stage of the process (raw kenaf bast, unbleached pulp, bleached pulp, and acid hydrolysis). The fiber was characterized by atomic force microscopy (AFM), which showed that the isolated pulp nanofibers had diameters of approximately 30 nm.
DOI: 10.15376/biores.18.4.6913-6928
Keywords: Alkaline; Kenaf; Nanofibers; Pulping; SEM
Contact information: a: School Life Sciences and Technology, Institut Teknologi Bandung, Bandung 40132, Indonesia; b: Research Center for Biomass and Bioproducts, National Research and Innovation Agency, Bogor 16610, Indonesia; c: Research Center for Environmental and Clear Technology, National Research and Innovation Agency, Bandung 40135, Indonesia; d: Department of Forestry, Faculty of Agricultural, Palangka Raya University, Palangka Raya 74874, Center Kalimantan, Indonesia;
*Corresponding author: dunganir@gmail.com
INTRODUCTION
Cellulose and nano-size cellulose particles have generated industrial and scientific interest as novel biomaterials. Cellulose is environmentally friendly and renewable, and it is a multi-serviceable raw material that can substitute for numerous materials that are not renewable (Achukwu et al. 2022; Owen et al. 2022a,b). Potential applications range from the creation of new kinds of commercially useful materials and uses in medical technology and pharmaceuticals. Three types of nanocellulose that have been studied extensively are cellulose nanocrystal (CNC), cellulose nanofiber (CNF), and bacterial nanocellulose (BNC) (Hubbe et al. 2017). Cellulose nanofiber has a greater proportion of amorphous structures than cellulose nanocrystals, so that it is more flexible. The diameter of cellulose nanofibrils is 1 to 100 nm, while the length is in the micrometer range (Abdul Khalil et al. 2014). Cellulose nanofibrils are a promising material for the manufacture of bio-composites because they have high strength, light weight, flexibility, and biodegradability (Liang et al. 2023). In addition, nanocellulosic materials have a large surface area; they bind to other compounds such as anti-bacterial and antioxidants (Bideau et al. 2017).
Nanofibers have limitless areas of application (Jiffrin et al. 2022) making them a very interesting area of research (Shen et al. 2020). The distinctive ordered architecture of natural cellulose having nanoscale fibers and crystals permits the preparation of the nano-components by chemical or mechanical methods, or a combination of both methods (Khan et al. 2022). The production of CNF is achieved by the reduction of the ordered structure of the bulk material through a breakdown process into individual nanofibers having very good crystallinity and low amorphous parts (Eichhorn et al. 2010).
Nanocellulose products have been prepared from various biomass sources such as wheat straw, sisal, hemp, soy hulls, palm oil, pineapple, wood, and kenaf (Faria et al. 2020; Franco-Urquiza and Rentería-Rodríguez 2021; Santos et al. 2021; Zhang et al. 2021; Pascoli et al. 2022; Zhang et al. 2022; Oliveira et al. 2023; Perera et al. 2023). Kenaf (Hibiscus cannabinus L., Malvaceae) is a common wild plant of tropical and subtropical Africa and Asia (Bourguignon et al. 2017). As the commercial use of kenaf continues to diversify from its historical role as a cordage crop (rope, twine, and sackcloth) to its various new applications including paper products (Azizi Mossello et al. 2010), building materials (Azzmi and Yatim 2010), absorbents (Tan et al. 2021), livestock feed (Kipriotis et al. 2015), and medical applications (Adnan et al. 2020), choices within the decision matrix will continue to increase and involve issues ranging from basic agricultural production methods to marketing of kenaf products (Bourguignon et al. 2017).
The kenaf stalk is 35% bark and 65% woody core by weight (Webber 1993). The kenaf plant consists of two distinct fibers. These are the outer layer (bast fiber), and a finer fiber in the core kenaf (Abdul Khalil et al. 2010; Silva et al. 2021). They have different lengths and chemical compositions. Apart from the chemical composition, the properties of kenaf (internal structure, microfibril angle, cell dimensions, and defects) are strongly influenced by many factors that may differ among different parts of the plant as well as among different plants (Silva et al. 2021). The kenaf core fibers are higher in holocellulose and lignin, 87.2% and 19.2%, respectively, while kenaf bast fiber is higher in α-cellulose. The α-cellulose content in bast fiber (55%) is higher than in the core (49%). A high α-cellulose content is believed to provide high strength fiber based end products (Al-Mamun et al. 2023).
The nanocellulose fibrils are isolated from the cellulosic fibers using various methods such as high-pressure homogenization, acid hydrolysis, etc. For example, Zaini et al. (2013) isolated kenaf-based CNC by using hydrolysis with H2SO4 or HCl. They obtained nano whiskers with average diameters and length of 3 nm and 100 to 500 nm, respectively. The homogenizers are used to delaminate the cell walls of the fibers and liberate the nanosized fibrils. Cellulose nanowhiskers, a more crystalline form of nanocellulose, are formed by the acid hydrolysis of native cellulose fibers using a concentrated inorganic salt, commonly sulfuric or hydrochloric acid. Song et al. (2018) extracted nanocellulose from kenaf using the combined microwave and chemical treatment showed an acicular morphology with nano-size fiber; it has a higher crystallinity but a bit lower thermal stability.
This work characterized kenaf bast nanofibers with X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscope (SEM), Fourier-transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM).
EXPERIMENTAL
Raw Materials
Kenaf was provided from National Fiber Council Indonesia (DSN) with KR 15 variety. Sodium hydroxide (NaOH), hydrogen peroxide (H₂O₂), anthraquinone (AQ), and magnesium sulphite (MgSO₄) were of analytical grade.
Preparation of Raw Materials
The kenaf bast was sun-dried until the moisture content was < 12%. The dried kenaf bast was cut to the length of 2 to 3 cm, and then it was put through a hammer mill (S-tech 2- MHC, Indonesia) to get fine particles. The particles dried kenaf bast fibers were sieved through 100 mesh screens to eliminate the fine and coarse fibers.
Pulping Process
Pulping of kenaf bast was conducted based on the method by Fatriasari et al. (2015). Kenaf bast of 200-g dry weight and the cooking liquor (36.472g Na₂O, and 0.188g anthraquinone) with a ratio of 1:7 (chip/liquor) was added to a rotary digester (model GEC-P40306, India) and run for 2.5 h at 170 °C. The screened pulp was centrifugally dewatered, and the pulps were put for oven drying at 60 °C for 24 h.
Pulp Bleaching Process
A peroxide solution, at specific concentrations of 35 v/v%, was prepared as the bleaching solution. The weight ratio of the bleaching solution to slurry after alkaline process was 30:1. The bleaching process was carried out at 80 ± 2 °C and 200 rpm, and allowed a 2-h reaction time. The slurry was collected on a filter mesh and then thoroughly washed with distilled water. Finally, the damp bleached pulp was wrapped in a muslin cloth and centrifugally dewatered. The bleached pulp dried in an oven at 60 °C for 24 h.
Acid Hydrolysis Process
The acid hydrolysis treatments were carried out in an autoclave (Raypa AES-110 model) by adding 10 g of bleached pulp into 250 cm3 Erlenmeyer flasks and adding 100 cm3 of sulfuric acid (H2SO4 60%wt) solution. The condition was conducted at temperatures 120 °C for 60 min. In this way, the solid–liquid ratio was kept at a constant value of 10. Flasks were sealed with fatty cotton wrapped in sterile gauze and covered, at the top, with aluminum foil. Each acid solution was mixed with fragmented bleached pulp by slight agitation, and the system was subjected to autoclave thermal processing. The acid was removed by centrifugation using an Eppendorf Centrifuge (model 5804/5804 R, Germany) at a rotating speed of 6500 rpm for 5 min.
Scanning Electron Microscope (SEM)
Scanning electron microscopy (SEM) was used to identify morphological changes on kenaf bast fibers surface. Longitudinal segments of raw kenaf bast, unbleached pulp, and bleached pulp were mounted in an aluminium substrate at an angle of 45° and coated with gold with thickness of approximately 90 Å. Surface morphology of the absorbents was identified by SEM (model EVO MA10, Carl-Zeiss SMT, Germany) in order to understand the morphology of the kenaf fibers at an accelerating voltage of 20 KV.
Fourier-transform Infrared (FTIR) Spectroscopy
The FT-IR microscope (model IS10 Nicolet, Thermo Scientific, USA) was used to obtain the spectra of fiber kenaf bast. OPUS software was used for collecting the spectrum in the mid infrared range of 400 to 400 cm-1 with a resolution of 4 cm-1. The spectra were used to determine changes in functional groups that may have been caused by treatments. Each sample was scanned for three trials and an average spectrum was obtained. After that, the spectra were smoothed automatically using OPUS software to enhance the FT-IR spectrum quality.
Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry analysis was operated by using DSC furnace model Perkin- Elmer DSC- 821, USA. The samples with the weight of 2.30 mg were used and analyzed dynamically at a heating rate of 5.4 °C/min using N2. The maximum temperature was set up to 600 °C under a nitrogen atmosphere.
X-ray Diffraction (XRD)
A Phillips PW 1840 X-ray Diffractometer was used for obtaining the X-ray diffractograms. Nickel filtered CuKα radiations at 35 KV (Kα = 1.54 Å) with the accelerating voltage of 40 kV, and current of 25 m Å were employed. The samples were scanned in the range from 5 to 90° of 2θ with a step six of 0.05°. Further, the crystallinity index (CI) was determined with the ratio of the area of all crystalline peaks (Acr) to the total area (Atotal) (Park et al. 2010).
CI = Acr / Atotal (1)
Atomic Force Microscopy (AFM)
The atomic force microscopy (AFM) model (XE-70 Park System, Germany) was used to analyze morphology of the obtained nanofibers by acid hydrolysis treatment. The sample was diluted to the desired concentration using distilled water then sonicated to obtain good dispersion. A drop of suspension was deposited on fresh cleaved mica and left to dry in a desiccator with silica gel for 12 h. AFM analysis were performed using a Digital III- Instrument under atmospheric conditions at 25 °C.
RESULTS AND DISCUSSION
Morphology of the Treated and Untreated Kenaf Bast
The morphology of the treated and untreated kenaf bast fiber, unbleached pulp and bleached pulp was studied using SEM. Figure 1 contains SEM micrographs of the raw kenaf bast, treated and untreated. The alkaline retting treatment liberated single fiber from the fiber bundles of the raw kenaf bast fiber. The bleached fiber morphology was altered little by the bleaching treatment.
Figure 1(a) shows that the raw kenaf bast fiber bundle composed of individual fiber linked with lignin. The diameter of the raw kenaf bast fiber was 30 µm. The surfaces of the raw kenaf fibers retrieved from water retting did not show any mechanical damages, despite the presence of lignin. These findings are consistent with previous reports of retted kenaf fibers (Nie et al. 2020; Rozyanty et al. 2021). Figure 1(b) and 1(c) show the structure of the fiber after pulping and bleaching processes. The SEM images can also reveal a clean surface and finer possess of the fiber by the end of the pulping and bleaching processes. These processes separate the fiber bundle into individual fibers and cause a significant decrease in their diameter. The unbleached pulp fiber diameter is 12 µm, and the diameter for bleached pulp is 9 µm. The surface and diameter of the natural fibers were aligned with the adjacent lignin, and their elimination through the pulping and bleaching process reduced its dimensions and improved fineness (Beltran et al. 2002).
Fig. 1. SEM images of (a) raw fiber, (b) unbleached pulp, and (c) bleached pulp
FTIR Spectroscopy
FTIR spectroscopy was used to identify the functional groups on the raw kenaf bast, unbleached pulp and bleached pulp. The main differences in FTIR spectra can be seen in Fig. 2. Some differences were apparent in the magnitudes of bands in the FTIR spectrum of kenaf bast fiber, unbleached pulp, and bleached pulp. The broad and dominant peaks around 3400 to 3300 cm-1 were associated with the stretching of O-H groups, including intramolecular and intermolecular hydrogen bonds (Wang et al. 2017; Lu et al. 2022). The peak located at 1737.18 cm-1 (Fig. 2a) in the raw kenaf bast was assigned to the C=O stretching of the acetyl group in hemicelluloses (Hossain et al. 2022). In the raw kenaf bast, the peak at 1249.4 cm-1 was associated to the C-O stretching of the aryl group in lignin (Zarina and Ahmad 2015). However, the lowest O-H absorption was seen in the treatment with active alkali and sulfidity (bleached pulp). In addition, a decrease in the intensity of O–H stretching vibration also indicated partial acetylation of cellulose (Fan et al. 2013). The change of O–H stretching vibrations was dependent on the extent to which the chemical treatment altered inter- and intra-molecular hydrogen bonding in polysaccharides (Martín-Alfonso et al. 2018). Furthermore, the disappearance of this peak (C-O stretching) occurred in the unbleached and bleached pulp due to the removal of lignin after the chemical treatments (Wang et al. 2018). The content of carboxyl groups in pulp first increases and then decreases in bleached pulp (Mingfu et al. 2020).
Fig. 2. FTIR spectra of the kenaf bast fiber. (a) the raw kenaf bast, (b) the unbleached pulp, (c) the bleached pulp
The peaks observed in the ranges of 1420 to 1430 cm-1 and 1330 to 1380 cm-1 in all spectra were attributed to the symmetric bending of CH2 and the bending vibrations of the C-H and C-O groups of the aromatic rings polysaccharides, respectively (Carrillo et al., 2018). After treatment (Fig. 2b, 2c) absorption peaks of ∼ 1030 cm-1 of the kenaf bast sample, which corresponded to the C-O-C vibration found in cellulose and hemicelluloses, as well as the C-C stretching vibration or COH bending seen in hemicellulose (Zhao et al. 2020), were significantly greater than those of the raw kenaf bast. By comparing the spectrum before and after the chemical treatment of the kenaf bast used, it was observed that the intensity of the typical peak of C-H groups in the cellulose at 2901.69 cm-1 was decreased (Tang et al. 2015). The obtained results confirmed that the effect of chemical treatment using a low concentration of NaOH on the degradation of coating substances (lignin, hemicellulose, and pectin) could influence the size of cellulosic fibers relative to the initial length (Nurazzi et al. 2021).
Crystallinity
Crystallinity is an important factor to evaluate the cellulose properties of biomass after pretreatment. It was obtained from the 2θ peak in XRD diffractograms. The X-ray diffraction (XRD) spectra of the raw kenaf bast fiber, unbleached pulp, and bleached pulp are shown in Fig. 3. As shown in the figure, the XRD diffraction pattern of kenaf bast fiber samples showed two dominant peak intensities at the diffraction angles (2θ) of ~15.81o, and ~22.51o, while the low intensity peaks can be assigned to other elements present in the kenaf bast fiber. A peak at around ~ 22.51° (2θ) showed the crystalline peak of cellulose (Trilokesh and Uppuluri 2019; Yi et al. 2020). Meanwhile, the broad peak at 2θ around ~15.81° represented the amorphous material of lignocellulose, such as amorphous cellulose, hemicellulose, and lignin (Singh et al. 2017). The crystallinity index (CI) of kenaf bast fiber was calculated from XRD intensity data using peak deconvolution method were 49.28%, 59.10%, and 63.54% for the raw, unbleached pulp, and bleached pulp, respectively.
Based on the samples of unbleached pulp and bleached pulp there was an increase of crystallinity index (Fig. 3b, and c). The higher crystallinity index on bleached pulp sample caused by active alkali gave an increasing α-cellulose content (Solihat et al. 2017). The fiber crystallinities gradually increased at each stage of the process. Alkaline retting removes lignin and hemicelluloses, so that the percentage of the crystalline regions in cellulose increased. Hydrogen peroxide bleaching accelerated the cleavage of the cellulose molecular chains within the amorphous regions, resulting in the further increase of the crystallinity of the bleached first (Shi et al. 2011). During the bleaching treatment with the existence of alkali, the cellulose may be oxidized, and degradation of cellulose would have occurred (Qu et al. 2010). In addition, the remaining lignin was degraded by hydrogen peroxide and removed during bleaching. The chromophore structures of lignin were destroyed by oxidizing the carbonyl structure and quinoid structure of lignin side chain during H2O2 bleaching (Li et al. 2020).
Thermal Analysis
The DSC thermograms of kenaf bast fiber, unbleached pulp and bleached pulp are shown in Fig. 4. The samples were heated from 25 to 450 °C over 45 min to measure the melting temperature.
DCS was carried out to determine the thermal behaviour of the fibers. DSC analysis also enables the identification to be made of chemical activity occurring in the fibers as the temperature is increased (Agung et al. 2011). Figure 4a shows the DSC curve for raw kenaf bast fiber. The Tg of raw kenaf bast fiber was clearly observed at two peaks, peak 1 at 163.8 °C and 170.5 °C, peak 2 at 183.3 °C and 193.2 °C representing onset and midpoint temperature, respectively. In this DSC curve, the hemicellulose exhibited an exothermic peak at 165.3 °C (peak 3) with an enthalpy value of 16.5 J/g. Its amorphous structure broke down and formed residual char, whereas the thermal degradation of cellulose was associated with a peak at 185.9 °C (peak 4) with an associated enthalpy of 16.6 J/g.
Fig. 3. X-ray diffraction spectra. (a) the raw kenaf fiber, (b) unbleached pulp, and (c) bleached pulp
Tg is manifested by a drastic change in the base line, indicating the beginning of the decomposition of cellulose and hemicellulose and lignin, which takes longer to start the decomposition (Silva et al. 2016). Differential thermal analysis curves of raw kenaf bast fiber show that a broad endotherm observed in the temperature range of 150 to 175 °C, its indicates presence of water molecules in the fibers. This endothermic peak above 100 °C were observed, which is due to the dehydration of the water stored in the fiber (Shahinur et al. 2020).
Fig. 4. DSC thermograms. (a) the raw kenaf fiber, (b) unbleached pulp, and (c) bleached pulp
The heating thermograms of kenaf bast fiber after treatment (unbleached pulp and bleached pulp) represent the one peak in the endothermic due to crystallization state. The first peaks of treated kenaf bast fiber were at 162.0 °C (unbleached pulp) and 160.6 °C (bleached pulp), respectively. The first endothermic peak, which can be attributed to the loss of water was observed in the range of 150 to 175 °C for kenaf bast fiber, whereas for the unbleached and bleached pulp sample the first endothermic peak was observed within the range of 150 to 160 °C. In cellulose fibres, lignin degrades at a temperature around 200 °C, while the other polysaccharides such as cellulose degrade at higher temperatures (Bharath et al. 2020). As can be seen from DSC thermograms, it is evident that the peak between 250 to 350 °C belongs to cellulose melting or decomposition. This peak does not show any endothermic or exothermic reactions and are representative of the highest stable state of the kenaf bast fibers (Lemita et al. 2022). The peaks between 100 to 200 °C are simply dehydration. Although it should be a single peak, the multiplicity may be because of some constituents like some waxy materials. Therefore these exothermic peaks, which were higher than 200 °C, indicate the decomposition temperatures of the cellulose in the fibers.
Atomic Force Microscopy (AFM) Analysis
Figure 5 shows an AFM image of cellulose whiskers, which is another term meaning CNC. The surface morphology of CNC appeared as rod-shaped fragments, which were overlapping each other in the image to form larger aggregates, showing similar diameters of the particles (Fig. 5, left). The surface morphology displayed the broadening effect and possibly agglomeration of the cellulose whiskers. The cellulose whiskers are known to have high colloidal stability, which is due to the electrostatic repulsion between the sulfate group-grafted surfaces resulting from the sulfuric acid treatment (Gousse et al. 2002). The subsequent peroxide bleaching process yielded crude cellulose which contained amorphous and crystalline domain. Furthermore, during acid hydrolysis the H+ ion would penetrate the cellulose chains and interact with the cellulose.
Fig. 5. AFM image of cellulose whiskers
Most of the CNC crystals finally obtained had the length of 300 to 450 nm and diameter of 30 nm (Fig. 5, right). The AFM images of the nanocellulose obtained from kenaf bast fiber clearly showed the presence of needle-shaped CNC samples finally after acid hydrolysis at optimal condition (Masoudzadeh et al. 2019). This result is favored by another study (Lu and Hsieh 2012), where the diameter was around 50 nm. Hence, acid hydrolysis treatment with concentration and reaction time decreased the crystals’ diameter.
CONCLUSIONS
- After bleaching, the diameter of kenaf bast fiber decreased, which was attributed to the fact that such processes separate the fiber bundle to individual fibers.
- The Fourier transform infrared (FTIR) results showed the disappearance of spectral peaks in the unbleached and bleached pulps due to removal of lignin in the course of the chemical treatment.
- X-Ray diffraction analysis showed that the raw kenaf bast fiber, unbleached and bleached pulp crystallinities gradually increased at each stage of process. Alkaline retting removed lignin and hemicelluloses so that the percentage of the crystalline region in cellulose increased.
- In differential scanning calorimetry (DSC) thermograms, it was evident that the peak between 250 and 350 °C belonged to cellulose melting or decomposition, while the peak between 100 and 200 °C was simply dehydration. Although it should be a single peak, it appeared as a multiple peak, which may be because of some waxy constituents. The exothermic peaks, which were higher than 200 °C, indicate the decomposition temperature of the cellulose in the fiber. After getting the micrometer result, acid hydrolysis was conducted to produce nanocellulose.
- Atomic force microscopy of the kenaf bast-based nano-whiskers showed that the diameter of their crystals was approximately 30 nm. The CNCs were similar to those of other natural fiber-based CNCs, but their nanoscale structures diverged.
ACKNOWLEDGMENTS
The authors thank the Ministry of Education, Culture, Research and Technology (KEMENDIKBUDRISTEK), Republic of Indonesia, for providing Research Grant ” Basic Research Scheme” No. 110/E5/PG.02.00.PL/2023. The authors also thank the Integrated Laboratory of Bioproduct (iLaB) of Research Center for Biomass and Bioproducts, National Research and Innovation Agency (BRIN), Indonesia, for providing the facilities for testing.
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Article submitted: July 12, 2023; Peer review completed: July 29, 2023; Revised version received and accepted: August 3, 2023; Published: August 8, 2023.
DOI: 10.15376/biores.18.4.6913-6928