Influence of Alkali Treatment on Microstructure Transformation and Mechanical Properties of Palm-Fiber Cell Wall

ABSTRACT

Utilizing agricultural waste and natural fibers minimizes environmental impact and can improve the acoustic and thermal conditions of buildings. Natural fibers can be an alternative to non-biodegradable synthetic sound-absorbing materials. This study aimed to investigate the acoustic and thermal properties of insulating materials made from wool and sugarcane bagasse. Thermal conductivity, thermal resistance, acoustic and moisture absorption, and fire properties of five insulating materials made from sheep wool, goat fiber, camel wool as well as pith and fiber bundles of sugarcane bagasse were determined. The measurement of the sound absorption coefficient was performed in an impedance tube. The thermal resistance and thermal conductivity coefficient were measured according to the ASTM D5334–08 Standard. The findings show that camel wool has the highest sound-absorbing performance, thermal insulation, and fire-resistant properties. The lowest value of the noise reduction coefficient (NRC) was 0.52 for goat fiber, and the highest was 0.74 for camel wool. The maximum sound absorption coefficient of camel wool was 0.95 at a frequency above 1000 Hz. Thermal conductivity varies between 0.038 and 0.046W/(M.K). Hence, all materials tested can be considered thermally insulating. The results showed insulating materials made from wool, especially camel wool, had better performance than fiber and pith of sugarcane bagasse.

摘要

利用农业废弃物和天然纤维可以最大限度地减少对环境的影响，并可以改善建筑物的声学和热条件. 天然纤维可以替代不可生物降解的合成吸声材料. 本研究旨在研究由羊毛和甘蔗渣制成的隔热材料的声学和热性能. 测定了由羊毛、山羊纤维、驼毛以及甘蔗渣髓和纤维束制成的五种隔热材料的导热性、耐热性、吸音性和吸湿性以及防火性能. 吸声系数的测量是在阻抗管中进行的. 根据ASTM D5334-08标准测量热阻和导热系数. 研究结果表明，驼毛具有最高的吸声性能、隔热性能和耐火性能. 山羊纤维的降噪系数（NRC）最低为0.52，驼毛的降噪系数最高为0.74. 在1000 Hz以上的频率下，驼毛的最大吸声系数为0.95. 导热系数在0.038-0.046W/（M.K）之间变化. 因此，所有测试材料都可以被视为隔热材料. 结果表明，以羊毛，尤其是驼毛为原料的绝缘材料，其性能优于甘蔗渣的纤维和髓.

KEYWORDS:

• Sound absorption
• thermal insulation
• wool
• fiber
• Pith
• bagasse
• sugarcane

关键词:

• 吸音
• 隔热
• 羊毛
• 纤维
• 髓
• 蔗渣
• 甘蔗

Introduction

Palm leaf sheath fiber (palm fiber), which comes from Arenga engleri Becc, is widely used in households, civil construction, water conservation, and environmental engineering because of its excellent performance and high utilization value (Moshiul Alam et al. 2012; Zhang et al. 2015). Generally, plant cell walls consist of complex polymers, such as cellulose, hemicellulose, and lignin, which are linked to one another by noncovalent and covalent bonds to form complex structures and chemical networks (Chabbert et al. 2018). Cellulose is arranged regularly as the skeleton structure of the cell wall, which is composed of crystalline and amorphous regions (Kathirselvam et al. 2019). Lignin is a complex present in the cell walls of vascular plants. Hemicellulose and lignin are cross-linked by ester and ether bonds and are filled in microfibrils to act as a binder (Sant’anna et al. 2013). Changes in polymer components can inevitably affect the microstructure of the cell wall, leading to changes in mechanical properties.

Alkali treatment is a simple and effective method to improve the mechanical properties of natural plant fibers. It was found that compared with the untreated fiber, the alkali treatment significantly improved the tensile properties of Alfa fiber (Ajouguim et al. 2019), red banana peduncle fiber (Pitchayya Pillai, Manimaran, and Vignesh 2021), borasus fruit fiber (Boopathi, Sampath, and Mylsamy 2012), Ventilago maderaspatana fibers (Rathinavelu and Sethupathi Paramathma 2022), Banana Fiber (Twebaze et al. 2022) etc. The mechanical properties of fiber are believed to rely on the molecular microstructure of its cell wall (Lee, Marcus, and Paul Knox 2011). The distribution of cellulose, hemicellulose, and lignin, the way of bonding, and the properties among the cell-wall components importantly affect the microstructure of fiber.

In recent years, modern imaging technology has been used widely in the microstructure analysis of biomaterials. FTIR microscopy has been proven to be a powerful method for revealing the chemical structure and components distribution of plant tissues at the cellular level with high spatial resolution (Cao et al. 2014; Guo et al. 2015, 2017). Confocal laser scanning microscopy (CLSM) is used to investigate lignin distribution in cell walls through lignin production of autofluorescence under specific excitation and emission wavelengths (Marin-Bustamante et al. 2018). However, few studies have dealt with the components and microstructural changes in the cell wall of palm fibers after chemical treatment by imaging techniques. The relationship between microstructure and mechanical properties during alkali treatment is also been rarely reported.

The main objective of this work is to study the effect of alkali treatment on the palm fiber, and investigate the mechanism of microstructure change on mechanical properties. Herein, palm fibers were treated by alkali with different concentrations (2, 5, and 10 wt.%), and the tensile properties of the fibers were tested. Imaging FTIR microspectroscopy and CLSM were adopted to investigate the changes in components and microstructure in the fiber cell wall. Quantitative analyses on the component changes were performed to verify the imaging results. The mechanism of the effect of alkali treatment on fiber mechanical properties was explored.

Materials and methods

Materials

Palm fibers were obtained in Yunnan province, China (102° 41 “E, 23° 15” N). The fibers were washed in water to remove dust and impurities and then dried in an oven at 60°C for 24 h. The dry palm fibers were treated with 2, 5, and 10 wt.% NaOH solution separately for about 2 h at 20°C. The treated fibers were washed with fresh water to remove residual NaOH and then dried at room temperature for 24 h. By using a sliding microtome, 10 μm-thick polyethylene glycol (PEG) embedded-fiber transverse-section slides were prepared for imaging FTIR microscopy and confocal laser fluorescent microscopy analysis. Figure 1 shows the preparation for palm fibers analysis. For comparative analysis, untreated palm fibers (raw) were also subjected to the experiments.

Figure 1. The preparation process for palm fibers.

Tensile properties

Tensile testing of palm fibers was carried out with a Universal Testing Machine (gauge length, 20 mm) at a rate of 2 mm/min. Thirty specimens for each group were subjected to tensile tests, and the average values were noted. The average diameter of fiber was determined by measuring three points in the tensile zone (Xia et al. 2019). Strain – stress curves of each group were graphed with Origin software.

FTIR microspectroscopic imaging

The embedded-fiber transverse-section slides were placed in a warm water bath to expand PEG and then transferred onto ZnS slides. After drying at 50°C, PEG was removed by placing the slides in 100% (two times), 80%, 50%, and 25% ethanol – water solution for 10 min. Finally, the slides were washed three times with distilled water to remove ethanol and then freeze dried for FTIR microspectroscopic imaging analysis. The FTIR microspectroscopic images were recorded on a Spectrum Spotlight 400 FTIR microscope (PerkinElmer Inc., Shelton, CT, USA). The spectra were recorded with a 4 cm⁻¹ spectral resolution, between 4000 cm⁻¹ and 740 cm⁻¹.

Clsm

The transverse-section slides were rinsed with deionized water in a watch glass 10 times to remove PEG. After dehydration through a graded series of ethanol solution (50%, 70%, 90%, and 100%), the slides were mounted in glycerol, covered with a coverslip, and examined with an LSM 510 META laser confocal scanning microscope. The excitation wavelength was 488 nm and the emission wavelength at 568 nm for imaging lignin autofluorescence analysis.

Chemical-component quantitative analysis

Chemical analysis of the palm fibers was carried out according to the Method of Quantitative Analysis of Ramie Chemical Components (GB5889–86). The content changes of the cellulose, hemicellulose, and lignin before and after alkali treatment were accurately determined.

Results and discussion

Tensile properties

Fiber-diameter distribution

Table 1 lists the average diameters of raw, 2, 5, and 10 wt.% alkali-treated palm fibers. It is shown that alkali treatment contributed to decreased fiber diameter with increased alkali concentration. In fact, impurities (pectin, sugar, ash etc.), wax and lignin were removed from the fiber surface during alkali treatment (Teli and Jadhav 2017). The fiber structure was only slightly affected, resulting in more fibrillation and giving rise to finer fibers.

Table 1. Diameters of raw and alkali-treated palm fibers.

Samples	Maximum diameter (mm)	Minimum diameter (mm)	Average diameter (mm)
Raw	0.47	0.41	0.44 ± 0.03
2 wt.%	0.46	0.37	0.41 ± 0.03
5 wt.%	0.43	0.35	0.39 ± 0.03
10 wt.%	0.41	0.34	0.36 ± 0.04

Mean ± standard deviation based on 30 data.

Tensile properties of palm fiber

The mechanical properties of alkali-treated (2, 5, and 10 wt.% NaOH) and raw fibers are shown in Figure 2(a). Among them, the tensile modulus, breaking strength and elongation at break of palm fibers were best enhanced by 5 wt% alkali treatment. Similar results are also found in Hibiscus Vitifolius, Coir and Abaca fibers (Manivel et al. 2022; Valášek et al. 2021). Furthermore, the typical stress – strain curves of the alkali-treated fibers are shown in Figure 2(b). As can be seen, Alkali treatment removes hydrogen bonds in the cross-linked network composed of cellulose and lignin, resulting in large elongation (Cai et al. 2015; Ishikura, Abe, and Yano 2009). Because hemicellulose is removed on a large scale by alkali treatment, and new hydrogen bonds are formed between the cellulose fibril chains, the microfibrils are rearranged in a more compact way, thus increasing the tensile strength and Young’s modulus.

Figure 2. The mechanical properties (a) and tensile stress – strain curves (b) of raw and alkali treatment palm fiber at different conditions.

Microstructure imaging

Palm fiber cell wall FTIR imaging

FTIR microspectroscopic data can be displayed as chemical images of specific wavelengths. Red and pink regions corresponded to larger absorption intensity, whereas blue regions corresponded to smaller absorption intensity. The chemical images at peaks near 1240 cm⁻¹ showed the relative concentrations and distribution of cellulose. The area under the peak at 1508 cm⁻¹ indicated the concentration and distribution of lignin. The concentrations and distribution of hemicelluloses were found at bands near 1710 cm⁻¹. Figure 3 shows the relative concentration and distribution of cellulose, lignin, and hemicellulose of raw and alkali-treated palm fiber transverse sections. As shown in Figure 3(a), with increased alkali concentration, the red region decreased slightly, indicating that the cellulose content decreased. Figure 3(b, c) show a significant decrease in red whereas the blue regions increased with increased alkali concentration, indicating that the lignin and hemicellulose contents dropped evidently. The spatial transformation of lignin and hemicellulose distribution and concentration indicated that their contents decreased during alkali treatment.

Figure 3. The relative concentration and distribution of cellulose (a), lignin (b) and hemicelluloses(c) at peak 1240 cm⁻¹, 1508 cm⁻¹ and 1710 cm⁻¹.

The FTIR spectra of raw and alkali-treated palm fibers were analyzed to investigate the effect of alkali treatment on the chemical characteristics of fiber cell wall. Figure 4 shows the FTIR spectra of the cell wall in the fingerprint region (1800 cm⁻¹ to 800 cm⁻¹). Specific spectral signals assigned to absorption bands of cellulose, hemicelluloses, and lignin were examined. Typical bands assigned to cellulose were located at 1424 and 1368 cm⁻¹ for CH₂ scissor motion and C-H bending vibrations, respectively, at 1336 cm⁻¹ for OH in-plane bending of amorphous cellulose, and at 1316 cm⁻¹ for CH₂ wagging vibrations in crystalline cellulose, which can be used to assess structural changes in cellulose. We determined the Crystallinity Index (I₁₃₁₆/I₁₃₃₆) from the absorption ratios, which provides additional information on the difference in the degradation process of amorphous and crystalline cellulose, wherein an increase in the ratio indicates increased crystallinity (Colom et al. 2003). Table 2 shows the Crystallinity Index of raw and alkali-treated palm fibers, which are 1.06, 1.08, 1.1, and 1.09, respectively. Based on the results, we concluded that alkali treatment increased the crystalized-cellulose content, which helped improve the tensile properties.

Figure 4. FTIR spectra of the cell wall of fibers in the fingerprint region.

Table 2. The crystallinity index of raw and alkali-treated palm fibers.

Sample	Raw	2 wt.%	5 wt.%	10 wt.%
I1316	0.9376	1.1172	0.9134	0.8342
I1336	0.8835	1.0321	0.8304	0.7642
I1316/I1336	1.06	1.08	1.10	1.09

For hemicelluloses, the characteristic peaks at 1740 and 1710 cm⁻¹ were assigned to the C=O stretching vibration in the O=C-O group of the glucuronic acid unit in xylan (Akerholm and Salmen 2003; Stevanic and Salme 2009). As for lignin, the characteristic peaks at 1600 cm⁻¹ can be ascribed to the aromatic skeletal vibrations together with C=O stretching, and those at 1508 cm⁻¹ can be ascribed to the aromatic skeletal vibration and guaiacyl ring vibration. The xylan band at 1456 cm⁻¹ can be ascribed to CH₂ symmetric bending on the xylose ring, whereas that at 1264 cm⁻¹ can be ascribed to C=O stretching (Song et al. 2013).

The relative intensities of the absorption peaks at 1740, 1710, 1600, and 1508 cm⁻¹ in palm as a function of treatments are shown in Fig .5. The density at 1740 cm⁻¹ decreased by 24% for treatment at 2 wt.% NaOH, 63.5% for treatment at 5 wt.% NaOH, and 65% for treatment at 10 wt.% NaOH. The density at 1710 cm⁻¹ decreased by 3.2% for treatment at 2 wt.% NaOH, 33.3% for treatment at 5 wt.% NaOH, and 38.7% for treatment at 10 wt.% NaOH. This significant loss in absorption of the carbonyl group, which comprised the backbone of xylan, at higher alkali concentrations most likely resulted from the degradation and loss of hemicelluloses from the cell wall. A similar observation was reported by Reddy (Reddy et al. 2013). The lignin-band intensity at 1600 cm⁻¹ showed a slight increase by 4.6% for treatment at 2 wt.% NaOH, a decrease by 14.7% for treatment at 5 wt.% NaOH, and a decrease by 18.9% for treatment at 10 wt.% NaOH. For the aromatic skeletal vibration band at 1508 cm⁻¹, relative intensities increased by 2.7% for treatment at 2 wt.% NaOH, 13.4% for treatment at 5 wt.% NaOH, and 14% for treatment at 10 wt.% NaOH. A loss of the C=O group linked to the aromatic skeleton may have probably occurred. This finding indicated the occurrence of cross-linking among the aromatic units in the lignin probably caused by the alkali treatment (Yin, Berglund, and Salmén 2011).

We observed that the bands at 1600 and 1508 cm⁻¹, which were assigned to lignin, increased slightly upon 2 wt.% NaOH treatment (Figure 5). This result indicated that the significant degradation of hemicelluloses subsequently caused a slightly increase in the lignin component of cell wall. A similar observation has been made by Huang (Huang et al. 2013). The removal of hemicelluloses and lignin in cell walls may have likely enhanced the exposure of cellulose microfibrils, consequently increasing the tensile properties of the fibers.

Figure 5. The relative intensities of the absorption peaks at 1740, 1710, 1600, and 1508 cm⁻¹ in palm as a function of treatments.

CLSM imaging of palm lignin autofluorescence

To further study lignin’s microstructure transformation and distribution characteristics in palm fiber cell wall, CLSM was used to investigate the lignin distribution and relative lignin concentration. Lignin concentration is linearly proportional to image brightness, which can be evaluated by image brightness (Ding et al. 2016). Fluorescence in cell walls originated from lignin autofluorescence, which comprised monolignols. Figure 6 shows the autofluorescence images of raw and alkali-treated samples at different concentrations. Figures 6(a, e) demonstrate the heterogeneous distribution of lignin in raw fibers, with higher lignin autofluorescence intensity occurring in the cell corner middle lamella and compound middle lamella regions. For the sample treated by 2 wt.% NaOH, fluorescence intensity in cell wall increased probably due to the aggregation-induced emission effect of lignin. Furthermore, a continuous increase in NaOH concentration to 10 wt.% caused an obvious decrease in fluorescence intensity, which may be attributed to a significant degradation of hemicelluloses linked to lignin by ester and ether bonds. Overall, the transformation of fluorescence reflected changes in lignin component and organization in addition to changes in interactions with other polymers, particularly with hemicelluloses. This conclusion was consistent with the FTIR microspectroscopic analysis.

Figure 6. Fluorescence images of transverse section of raw, alkali-treated at 2 wt.%, alkali-treated at 5 wt.%, and alkali-treated at 10 wt.% palm cells.

Chemical components of palm fiber

The chemical components of raw and alkali-treated fibers are presented in Table 3. Raw fiber consisted of cellulose (28.04%), hemicelluloses (23.5%), lignin (38.18%), and impurities (10.28%). Hemicellulose was significantly affected by the concentration of the alkaline solution, which decreased from 23.5% to 10.82%. Lignin content decreased from 38.18% to 28.66%, and the impurity content decreased from 10.28% to 5.36%. Correspondingly, cellulose content increased from 28.04% to 55.16%. These results were consistent with those of FTIR microspectroscopy and CLSM analyses.

Table 3. Chemical composition of raw and alkali-treated palm fibers.

Samples	Hemicellulose (%)	Lignin (%)	Cellulose (%)	Impurities (%)
Raw	23.5	38.18	28.04	1.28
2 wt.%	17.25	36.18	38.43	8.14
5 wt.%	13.2	34.1	45.57	7.13
10 wt.%	10.82	28.66	55.16	5.36

Conclusion

The microstructure transformation of alkali-treated palm fiber and the influence of this transformation on fiber mechanical properties were studied in this paper. The results indicated that the alkali treatment concentration is an important factor influencing the chemical composition, microstructure transformation and tensile properties of palm fibers. The conclusions are as follows:

1. The alkali treatment lead to the microstructure transformation of fiber’s cell wall. FTIR microspectroscopy and CLSM analyses provided new information on cell-wall microstructure transformation resulting from the removal of hemicelluloses and lignin.

2. The removal of hemicellulose and lignin helps to rearrange the fibrils in a more compact manner, and increases the crystallinity index of palm fiber cellulose molecules.

3. The crystallinity index analysis indicates that the crystallinity of 5% NaOH treated fiber was the highest and the result was consistent with CLSM analyses.

4. The alkali treatment lead to the increased tensile properties. The tensile modulus, breaking strength and elongation at break of 5% NaOH treated fiber was found to be maximum and was found to reach 3718.07 MPa, 283.76 MPa and 44.32% when compared to values 3032.19 MPa, 216.64 MPa and 39.10% of untreated fiber.

These findings may benefit the high-value utilization of palm in the field of materials.

Highlights

• Imaging technologies reveal fiber’s structure characteristics at the cellular level.

• Tensile properties of palm fiber are improved significantly during alkali treatment.

• The alkali treatment lead to the microstructure transformation of fiber’s cell wall.

• The removal of hemicellulose and lignin contributes to the improved crystallinity.

Disclosure statement

No potential conflict of interest was reported by the author(s).