A novel method for preparing microfibrillated cellulose from bamboo fibers

Bamboo trees (Bambusa Blumeana J A & J H Schultes) were collected from The Phu An Ecological Bamboo Museum and Botanical Reserve (Phu An Bamboo Village), Binh Duong, VietNam. The chemical reagents used for treating bamboo fibers and preparing microfibrillated cellulose were analytical grade purchased from Merck without further purification.

Internodes of bamboo trees were cut into portions 10 cm in length (the node portions and thin layer of exodermis and endodermis bark of the bamboo were removed). The cylindrical portion of culm is peeled in the longitudinal direction to make strips approximately 0.2 cm thick, 12–15 cm in length and about 3 cm in width [17]. Bamboo strips were washed by distilled water at ambient temperature to remove the dust and impurities on the strip surface. After that, they were gently rolled to extract BFs.

The MFC was generated from the BFs by an acid catalyzed hydrolysis method. Firstly, the BFs were ground into approximately 500 μm in size, using a cutting mill (Fritsch GmbH, Germany), with a 500 μm stainless steel trapezium-shaped sieve. The isolation of cellulose fibers required the removal of other components such as lignin, hemicellulose and pectin from the BFs. This step was achieved by applying the alkali treatment and bleaching. The alkali treatment was designed to dissolve the pectins and the hemicelluloses. BFs were treated with NaOH in different concentrations, at 30 °C, for 72 h in order to study the influence of alkali treatment in removing of hemicelluloses and pectins [17, 18]. After that, dissolved components were removed by washing with deionised water. The bleaching was done to whiten the BFs by removing phenolic compounds or chromophoric molecules in lignin. In this step, BFs are treated with 5 wt% sodium hypochlorite solution (NaOCl), at 30 °C, for 3 h [10]. After bleaching, the BFs were washed with deionised water until reaching pH of 7 and centrifuged to obtained the residue which was the raw material for acid hydrolysis. After hydrolysis, the mixture was diluted ten-fold with distilled water and neutralized with an alkaline solution until pH of 7 was achieved. The residue centrifuged was dialysed against deionised water and sonicated for 45 min in an ultrasonic bath. This suspension was freeze-dried to obtain the dried products [10].

There have been many reports related to the preparation of MFC from bagasse [18], banana [10, 19, 20], peel of prickly pear fruits [6], mulberry [3], wheat straw [21], cotton, sisal, ramie [22], etc by using 64 wt% H₂SO₄ solution at a diversity of hydrolyzing condition such as reaction temperature, reaction time and original dimension of raw materials, etc. Therefore, in the present work, the BFs were treated with 64 wt% H₂SO₄ by varying reaction times, under different controlled reaction temperatures (table 1). To study the effect of initial dimension of BFs on the hydrolysis reaction, the ground BFs were sieved using test sieves of 20, 50, 90 and 200 μm aperture sizes (Fritsch GmbH, Germany). In the following discussion, the resulting samples have been referred to as I (BFs with smaller 20 μm in size), II (BFs from 20 to 50 μm in size), III (BFs from 50 to 90 μm in size), and IV (BFs from 90 to 200 μm in size), respectively (table 2).

2.4.1. FTIR spectroscopy.

2.4.2. Optical microscopy.

2.4.3. Morphology of MFC.

2.4.4. Chemical composition measurement.

Chemical composition of fibers was measured according to the following ASTM procedures: α-cellulose (ASTM D1103-55T), lignin (ASTM D1106-56) and holocellulose (ASTM D1104-56) [19, 21]. The standard deviations were calculated by conducting three replicate measurements for each sample. The percentages of α-cellulose (A), hemicelluloses (H) and lignin (L) were calculated by following equations:

$$\begin{equation} A = \frac{{W_1 }}{{WP}} \times 100\% , \end{equation} \tag{ 1 }$$

$$\begin{equation} H = \frac{{W_2 }}{{WP}} \times 100\% - A, \end{equation} \tag{ 2 }$$

$$\begin{equation} L = \frac{{W_3 }}{{WP}} \times 100\% , \end{equation} \tag{ 3 }$$

where W is the weight of the original oven dry fibers, W₁ is weight of the oven dry α-cellulose residue, W₂ is weight of the oven dry holocellulose residue, W₃ is weight of the oven dry lignin residue and P is the proportion of moisture-free content.

2.4.5. Thermogravimetric analysis.

2.4.6. XRD analysis.

XRD examinations were performed on a D8 Advance system (Bruker). The diffracted intensity of Cu-Kα radiation (0.154 nm, 40 kV and 30 mA) was measured in a 2θ range between 5° and 40°. The BFs powder and the hydrolyzed BF samples were subjected to crystallinity analysis. Crystallinity index (Ci) of MFC samples were calculated by the following equation:

$$\begin{equation} Ci = \frac{{I_{002} - I_{{\rm{Amorph}}} }}{{I_{002} }} \times \% , \end{equation} \tag{ 4 }$$

where I₀₀₂ is the maximum intensity of the 002 peak which represents both crystalline and amorphous material and I_Amorph is the lowest height between the 200 and 110 peaks (2θ ∼ 18°), which represents amorphous material only [9, 21].

The changes in the chemical composition of BFs due to the hydrolysis were analyzed through FTIR spectroscopy. Figure 1 displays the FTIR spectra of the raw BFs and alkali treated BFs with different concentration. The FTIR spectroscopic analyses of the untreated BFs, the alkali treated BFs, the bleached bamboo fibers and the hydrolyzed BFs (figure 2) indicate that composition changes occur in the fibers during hydrolysis. The hydrophilic nature of raw BFs and treated BFs is characterized by the broad band in the 3800–3000 cm⁻¹ region, which is related to the –OH groups present in their main constituent. The peak at 2912 cm⁻¹ is connected with the aliphatic saturated C–H stretching vibration in lignin polysaccharides including cellulose and hemicellulose. The peak at 1740 cm⁻¹ that is observed in the spectrum of the untreated BFs is involved either to the acetyl and uronic ester groups of the hemicellulose or to the carboxylic group of the ferulic and p-coumeric acids of lignin and hemicellulose [3, 10]. The absence of this peak is observed in the FTIR spectrum of the alkali treated BFs with 2 wt% alkaline solution (figure 1), indicating the removal of hemicellulose and lignin from the sample. As the concentration of alkali solutions increases, the FTIR spectra are unchanged. Therefore, BFs were treated with 2 wt% alkaline solution in the following experiments.

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In the spectrum of the raw BFs, the peak at 1512 cm⁻¹ is related to the aromatic $-\!\!\!-{\rm C}=\!\!\!={\rm C}-\!\!\!-$ stretch of the aromatic rings of lignin. The shortage of this peak in the hydrolyzed BFs is ascribed to the removal of lignin by acid hydrolysis. The lack of peaks at 1740, 1512 and 1250 cm⁻¹ (figure 2) expresses the effective elimination of lignin, pectin and hemicelluloses in the hydrolyzed BFs. The peak at 1460 cm⁻¹ is linked to the $-\!\!\!-{\rm CH}_2-\!\!\!-$ bending. The sharp peak observed at 1313 cm⁻¹ is in regard to $-\!\!\!-{\rm C}-\!\!\!-{\rm H}$ asymmetric deformations. The peaks in the region from 1200–950 cm⁻¹ are owing to $-\!\!\!-{\rm C}-\!\!\!-{\rm O}-\!\!\!-$ stretching. The $-\!\!\!-{\rm C}-\!\!\!-{\rm O}-\!\!\!-{\rm C}-\!\!\!-$ pyranose ring skeletal vibration leads to a prominent peak at 1022 cm⁻¹. The intensity of this peak rises, which means that the cellulose content increases [10, 21]. The peak at 897 cm⁻¹ is connected with glycosidic $-\!\!\!-{\rm C}_1-\!\!\!-{\rm H}$ deformation, a ring vibration and $-\!\!\!-{\rm O}-\!\!\!-{\rm H}$ bending. These characters imply the β-glycosidic linkages between the anhydroglucose units in cellulose [10, 21]. Figure 3 shows the FTIR spectra of the MFC that were prepared under different conditions. All the samples prepared under varying conditions show spectral patterns similar to that of cellulose.

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Figure 4 shows the microscopic images of original BFs and alkali treated BFs. The original BFs (figure 4(a)) were covered by massive cement materials, which were obviously diminished after the treatment of BFs with the akaline solution (figure 4(b)). It is clear that hemicellulose, lignin on the surface of BFs has been eliminated, which is confirmed in the FTIR results.

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Figure 5 shows the FESEM images of MFCs generated by the hydrolysis route, using 64 wt% H₂SO₄ solution at various reaction temperatures (35, 45, 55, 65 °C), with reaction time being 2 h. The results reveal that the hydrolysis temperature strongly affects the size decrease of BFs. As the reaction temperature increases, under otherwise identical conditions, the considerable reduction in size is achieved (figures 5(a)–(c)). At 65 °C, the hydrolyzed fibers convert to particle-shape microfibers from fiber-shape ones which is presented in figure 5(d) [10]. Figures 5(e) and (f) show the FESEM images of the two samples hydrolyzed at 45 and 50 °C (sample 45II20 and 55II20) with higher magnification, which indicates the presence of MFC of 20–40 nm in diameter.

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Figure 6 shows FESEM images of MFC prepared by the acid hydrolysis of BFs at the temperature of 45 °C for a period of 1, 3, 5 and 8 h, respectively. From the images, it is obvious that when the duration of hydrolysis rises, the dimension of the resulting cellulose microfibers is reduced. Figures 6(e) and (f) show the FESEM images of the two samples hydrolyzed in 3 and 5 h (sample 45II30 and 45II50) with higher magnification, which also indicates the presence of MFC of 20–40 nm in diameter.

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Figure 7 displays FESEM images of MFC that were prepared by the acid hydrolysis of BFs having different initial dimensions. Figures 7(a)–(d) show the SEM images of MFC prepared by the hydrolysis of BFs obtained after sieving in sieves of 20, 50, 90, 200 μm aperture size, respectively. Figure 7 definitely shows that the less the dimensions of the raw BFs, the easier it is to prepare MFC with smaller dimensions [10].

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Table 3 shows the chemical composition of raw BFs, alkali treated BFs, bleached BFs and hydrolyzed BFs. The result indicates that the original BFs possess the lowest percentage of α-cellulose, the highest percentage of hemicellulose and lignin. After treating BFs with the 2 wt% alkaline solution, the content of α-cellulose increased from 45.24 to 64.08%. On the contrary, hemicellulose content decreases from 24.16 to 15.87% and lignin decreases from 22.32 to 16.12%. This shows that when the raw fibers were treated with alkaline solution, the small part of hemicellulose and lignin in the surface of BFs was removed. In comparison with the alkali treated BFs, the bleaching raised α-cellulose content in the cellulose fibers to 82.23%. It also reduced the percentage of hemicelluloses and lignin to 7.52 and 5.97%, respectively. This indicates that the further dissolution of the hemicellulose and the depolymerization of the lignin components has occurred during this step [21]. During the bleaching treatment, the hemicellulose is partially broken down and the lignin is depolymerized, forming sugars and phenolic compounds that are soluble in water. However, the result reveals that the highest content of α-cellulose and the lowest content of hemicellulose and lignin were reached by acid hydrolysis, in which the contents of α-cellulose, hemicellulose and lignin in the final product were 95.53, 2.21 and 1.67%, respectively. The hydrolysis of glycosidic linkages in hemicellulose and the ether linkages in lignin are catalyzed by sulfuric acid. At the end of the hydrolysis, the cellulose is de-polymerized and defibrillated resulting in microfibrillated cellulose [19]. The removal of hemicellulose and lignin in chemically treated BFs is also verified in FTIR results.

Figure 8 shows the thermogravimetric analysis (TGA) and differential thermogravimetry (DTG) curves of original BFs, bleached BFs and MFC (sample 45II30). The initial weight loss initiated at 70 °C is attributed to the evaporation of free water in the samples. Due to the low decomposition temperature of hemicellulose, lignin and pectin [3, 10], the curve of original BFs indicates an earlier weight loss started at approximately 210 °C, and then reached a dominant peak at 348 °C on DTG curve related to the pyrolysis of cellulose. On the other hand, the TGA curve of the bleached BFs shows a higher decomposition temperature which starts at 230 °C before reaching dominant peak at 351 °C. The TGA curve of the MFC sample, however, shows significantly different degradation behavior from raw BFs. The lower temperature decomposition starts at around 220 °C and then reaches the dominant peak at 343 °C. In addition, a shoulder peak begins at around 220 °C followed by the main decomposition peak at 343 °C, suggesting there are two degradation stages. The lower temperature stage may correspond to the degradation of more accessible, and therefore more highly sulfated amorphous regions, whereas the higher temperature stage is related to the breakdown of unsulfated crystal interior. Actually, the introduction of sulfated groups into the crystals in the sulfuric acid hydrolysis process could reduce the thermostability of MFC. This result is similar to those reported by Li et al [3].

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On the other hand, less weight residue of bleached BFs than that of original BFs is due to the fact that the hemicellulose and lignin were mostly removed from the cellulose fibers, while the increased weight residue of cellulose whisker is because of the sulfate groups acting as the flame retardants [3].

The crystalline behaviors of raw BFs and the influences of hydrolysis condition on crystallinity of MFC are studied by wide angle XRD. Figure 9 shows the XRD diagrams of the initial BFs, hydrolyzed BFs in different conditions. From the results, it is obviously that cellulose has crystalline nature with an intensive peak at 2θ = 22.6° corresponding to 0 0 2 lattice plane and a shoulder peak in the region 2θ = 14–17° [10, 21]. Table 4 presents crystallinity index (Ci) for raw BFs, hydrolyzed BFs at 65 °C for 3 h (65II30), hydrolyzed BFs at 45 °C for 8 h (45II80), and hydrolysed BFs at 45 °C for 3 h (45II30). The results in figure 9 show that crystallinity increases from 56.72% for untreated BFs (curve a) to 70.50% for MFC in curve d (sample 45II30). The elimination of noncellulosic polysaccharides and the dissolution of amorphous regions lead to higher crystallinity of MFC. Hydrolysis takes place in the amorphous regions prior to the crystalline domains in that the former are more vulnerable to acids. This increase of crystallinity after acid treatment has been discussed by several authors [6, 19, 21]. The results also reveal that reaction conditions have substantial effects on crystallinity of final products. When the reaction temperature reaches 65 °C, under otherwise identical conditions, the crystallinity index substantially decreases to 49.59% (curve b in figure 9 and table 4), indicating that the hydrolysis of crystalline domains takes place at high temperature. Crystalline domains, which are broken down by hydrolysis, convert to amorphous regions and then dissolve out. As the reaction time endures to 8 h instead of 3 h, and other conditions are unchanged, the crystallinity index considerably reduces to 57.14% (curve c in figure 9). This reveals that after occurring in amorphous regions, the hydrolysis takes place in crystalline domains. The hydrolysis of crystalline domains by raising reaction temperature or enduring reaction time are also confirmed in FESEM results.

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