Sugarcane bagasse - A source of cellulosic fiber for diverse applications

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Abstract

Sugarcane bagasse is a fibrous material containing cellulose as its main component. It is produced in large quantities across the world. It is a kind of waste material that comes from the sugar industry. It is most commonly used in paper industries, but researchers have suggested that different mechanical and chemical treatments can help to extract cellulosic fibers, pure cellulose, cellulose nanofibers, and cellulose nanocrystals. These extracted materials have diverse applications in regenerated cellulosic fiber and composite material production. This paper will discuss the extraction procedures and typical applications in composite industries of these extracted materials. And an assessment will also be done on the production process and the properties of the end products to find out some common factors which can control the properties of these extracted material reinforced composites to some extent.

Cellulose, Linear Density, Tenacity, Tensile Strength, Alkali Treatment

1. Introduction

Sugarcane (Saccharum officinarum) is cultivated in considerable quantities in tropical countries. In 2017, about 1.84 billion tons of sugarcane were produced worldwide [1]. It is used in sugar mills and alcohol mills. But it cannot be consumed entirely by those mills as about 30% pulpy fibrous residue is produced after being utilized in those mills [2, 3,4]. These residues are called bagasse [5]. The bagasse is used in various applications, including paper industries, as feedstock, as biofuel, etc [6, 7]. Sugarcane bagasse is a lingo cellulosic material [8]. It is generally a kind of waste [2], which may have some particular uses. Since it contains quite a fair amount of cellulose, this cellulose can be extracted, and that cellulose can have different applications. The fibrous materials may also be used as fiber in the textile and civil engineering sector, too though they may need some unique treatments before being used. More specifically, this bagasse can be used to reinforce composite materials for creating a totally new type of material [7]. The main advantage of using bagasse is, it is pure waste material, and if this material can be utilized in any application even after a few simple pretreatments, the process still produces a very economical product, and also the product will surely be fully or partially biodegradable, which is quite an important factor these days. Also, the extracted fiber can show quite good mechanical properties if an appropriate technique is used [7]. The extracted cellulose can be used for producing sustainable regenerated textile fibers, too [9]. Other than that, the bagasse can also be a source for producing nanoparticles though that would not come cheaply as alternatives [10, 11]. In this paper, structural properties and applications of sugarcane bagasse in different sectors will be discussed, along with some essential pretreatments or fiber extraction procedures.

2. Physical structure and chemical composition of sugarcane bagasse fiber

Vegetable fibers that are derived from stalks or stems are called bast fibers [12]. Jute, Flax, Ramie, etc., are bast fibers, while these are also classified dicotyledons as these plants have net veined leaves [13]. Sugarcane contains parallel-veined leaves. The fiber bundles are randomly arranged throughout the stem of the fiber, but in bast fiber, the fiber bundles are arranged in a certain ring pattern, and that's why it is not classified as bast fiber [13]. The sugar cane stalk can be divided into two portions, the outside rind and an inner pith. The outside rind portion contains longer and finer bundles of fibers, while the inner portion contains the short fibers [12, 14]. Bagasse actually contains both types of fibers [2]. Cellulose covers about one-third of the plant tissues of sugarcane. Sugarcane bagasse contains about 40–50% cellulose and 25–35% hemicellulose. The rest contains lignin, wax, etc [[15], [16], [17]]. Cellulose has a crystalline structure (about 50–90% crystalline depending on the source of cellulose [18]), while hemicellulose is an amorphous structure containing xylose, glucose, etc [8, 19]. Cellulose is more like a natural linear polymer containing anhydroglucose units linked by β 1, 4 glycosidic bonds [8, 17]. It contains three hydroxyl groups of different reactivity as C-2 and C-3 got secondary –OH groups while a primary –OH can be found at C-6 position [17, 20]. These hydroxyl groups help to produce strong intermolecular and intramolecular hydrogen bonds [20]. These cellulose polymers are distributed in fibrils that are surrounded by hemicellulose and lignin. Lignin actually works as a glue between cellulose and hemicellulose and helps the material to gain rigidity [8]. It is a three-dimensional polymer containing three different phenyl-propane precursor monomer namely, p-coumaryl, coniferyl, and sinapyl alcohol, which are joined together by alkyl-aryl, aryl-aryl, and alkyl-alkyl bonds [8, 21]. The chemical composition of sugarcane bagasse is given below in Table 1 [7, 8, 22, 23, 24, 25, 26, 27, 28, 29, 30].

Table 1

Chemical composition of sugarcane bagasse.

This composition stands for untreated raw sugarcane bagasse. But different pretreatments can help to reduce the contents, such as hemicellulose, lignin, etc., so that 55–89% cellulose can be yielded from these bagasse samples [7, 27].

3. Extraction process of fiber and cellulose from bagasse

Sugarcane bagasse has been used for different purposes. Cellulosic fiber has been extracted from it. It has also been used for extracting pure cellulose, while cellulose nanoparticles have also been tried to be extracted from sugarcane bagasse. Different researchers have used various techniques, and the outcomes have also been different. Some extraction processes are described below.

3.1. Fiber extraction

Sugarcane bagasse contains two different types of fibers in the rind and the inner pith. Generally, the fibers of the rind can be useful due to their longer length and comparatively better mechanical properties. Fibers from inner pith can also be used in various applications, but those are not expected to show better mechanical properties due to having minimal fiber length [12, 14, 31]. The fiber extraction procedure varied by different researchers, but the main operations remained quite similar. A general process flow diagram is shown below in Figure 1 .

Process flow diagram of fiber extraction.

Collier et al. tried to extract fibers from sugarcane rind [12]. They separated the rind from the pith and then removed the nodal regions from the rind as those regions have different physical structures [14]. They pretreated the fibers at the beginning of the process. At first, 4 cm long and 1–2 mm wide rind pieces were immersed in hot water. It was done to remove the sugars. 0.1N and 1N concentrations of sodium hydroxide (NaOH) at high atmospheric pressure (2 atm) and normal atmospheric pressure for 1–2 h and 1–4 h, respectively, were used in that study. The used temperature was 121 °C. Continuous stirring was used for standard atmospheric pressure treatment. 100 ml alkaline solution was used for 1 gm of the rind. They found that higher concentration of NaOH, higher temperature, and higher atmospheric pressure removed more amount of lignin and resulted in fiber bundles of lower linear density [21.8 Tex (Tex is a unit of linear density which means mass per unit length [32]) at 2 atmospheric pressure by using 1N NaOH in 2 h of treatment, and 22.5 Tex at normal atmospheric pressure by using the same concentration of NaOH in 4 h treatment with continuous stirring]. They found the ultimate cell length of fiber about 2 mm, which was within the range of the ultimate cell length of jute fiber [33, 34]. But the fiber bundles of lower linear density (finer fiber bundles) showed comparatively inferior mechanical properties (tenacity of 28.7 cN/Tex and 15.6 cN/Tex for fiber bundles of 54.2 and 21.8 Tex, respectively).

Michel et al. also tried to extract fibers from sugarcane bagasse [2]. The process flow was similar to the given one in Figure 1 , but they did have some variations in the pretreatment of bagasse. They tried two concentrations of NaOH (0.1N and 1N) for treating bagasse at 130 °C for an hour in an autoclave. They pretreated two samples with distilled water and salty water, respectively, at the same temperature in an autoclave. They obtained comparatively smaller fibers than the previously mentioned process because they took whole bagasse instead of only fibers from the rind. Treatment with salty water (with 1N NaOH) resulted in fibers of the lowest average length (29.8 mm) but finer fiber bundle (32 Tex), while pretreatment with only distilled water (with 0.1N NaOH) resulted in fibers of the highest average length (45.6 mm) but higher linear density (39 Tex). NaOH concentration did not seem to affect the mean fiber length too much as both 0.1N and 1N NaOH solution produced fibers of 37.6 and 37.7 mm, respectively. But the fiber bundle fineness was affected to a greater extent as 49 Tex, and 35 Tex were resulted by those two solutions, respectively. Again as the previously mentioned research, the finer fiber bundle showed lower strength (tenacity 7.5 cN/Tex), and the coarser fiber bundle showed higher strength (tenacity 22 cN/Tex). Salty water pretreated fibers were found to be less rigid, while fibers treated with only 0.1N NaOH were impossible to be bent at all. It means these fibers are relatively stiff and are not really suitable for textile purposes in comparison to other textile fibers such as jute, flax, etc., where the low tenacity of bagasse fibers is mainly responsible [[35], [36], [37], [38], [39]].

Meanwhile, Chiparus and Chen used a similar technique where they used NaOH solution for the delignification of sugarcane bagasse and found similar results and found fiber length varying from 50.2 mm to 70.2 mm and fiber fineness varying from 18.722 to 47.822 Tex [40]. There are not many pieces of research involving the extraction of fiber from sugarcane bagasse. Some have extracted fibers just for reinforcing composite materials. For this purpose, the fiber length does not have to be longer always as fibers of minimal length can also be used as reinforcements. Cao et al. used sugarcane bagasse fiber in their research as reinforcement [41]. They extracted fiber-containing a high percentage of cellulose by treating the bagasse with NaOH. They soaked bagasse in each 1% NaOH solution at 25 °C for 2 h, maintaining a liquor ratio of 20:1. NaOH treatment increased about 17% tensile strength and also produced finer fiber (fiber length was 0.9 cm).

3.2. Cellulose extraction

There have been some researches for extracting cellulose from sugarcane bagasse. Extracted cellulose has been used in various applications. The process of extracting cellulose from sugarcane bagasse can be summarized in the following process flow diagram in Figure 2 .

Process flow diagram of cellulose extraction from sugarcane bagasse.

The processes varied by different researchers in terms of the use of the chemicals and conditions. Sun et al. used three different processes to extract pure cellulose from sugarcane bagasse [17]. They ground and dewaxed sugarcane bagasse in toluene-ethanol solution before treating the samples using water at 55 °C and with or without ultrasonic radiation. Sequential treatment was done in one set of samples by using sodium hydroxide (0.5M) and hydrogen peroxide (0.5, 1, 1.5, 2, and 3%) and got 2 types of cellulose (44.7 and 45.9%). They also produced delignified samples by treating the samples sequentially with 1.3% sodium chlorite and 10% sodium hydroxide or potassium hydroxide. This process resulted in 44.7 and 44.2% cellulose yield. In the third process, they used a one-step treatment with an 80% acetic acid and 70% nitric acid mixture under controlled conditions in 20 min of treatment. This process resulted in a cellulose yield 43.0 (at 120 °C) and 43.6% (at 110 °C).

Meanwhile, Liu et al. extracted cellulose sugarcane bagasse by using quite similar processes as the previous research [42]. They dewaxed the samples by using chloroform-ethanol solution, and then they delignified the samples using chlorite and ultrasonic irradiation before treating them with 6% sodium hydroxide at 75 °C for 2 h. Then the samples were treated in two different systems. In one approach, they treated the samples sequentially by using 15 and 18% potassium hydroxide, 15 and 18% sodium hydroxide at 23 °C for 2 h, and 8 and 10% potassium hydroxide, 8 and 10% sodium hydroxide at 23 °C for 12 h. In a one-step system, they treated the samples with 10% potassium and sodium hydroxide at 23 °C for 16 h. These treatments resulted in 50.7, 49.5, 48.6, 47.8, 57.2, and 55.4% cellulose yield, respectively. They also found that crystallinity values (39.8, 40.3, 41.6, 42.7, 44.8, and 45.6%) were less than cellulose of flax, cotton, and kenaf fibers (70, 65, and 60%, respectively) [43].

Abdel-Halim extracted cellulose from sugarcane bagasse by alkaline treatment with sodium hydroxide, followed by delignification/bleaching using sodium chlorite/hexamethylenetetramine solution [44]. He used dried and ground bagasse for the process. He treated bagasse with NaOH (0.1–2.5 N) and a non-ionic wetting agent (2 gm/l) while he used 1:20 material to liquor ratio at the boiling temperature for an hour. Later for the delignification process, he treated the alkali-treated bagasse with sodium chlorite (NaClO₂) (5 gm/l), hexamethylenetetramine (0–0.5 gm/l), and non-ionic wetting agent (2 gm/l) at boiling temperature with 1:30 material to liquor ratio.

Saelee et al. extracted cellulose from dried sugarcane bagasse using steam explosion and xylanase pretreatment and bleaching process [27]. They used the technique of Rocha et al., for which, first of all, the bagasse underwent a steam explosion at 13 bar pressure and 195 °C temperature for 15 min [45]. The exploded bagasse was then treated with 20 μ/g of xylanase by using fiber to liquor ratio of 1:10 for an hour at 50 °C under constant agitation. Then bleaching was done by using 0.7% sodium chlorite (NaClO₂). Other than these, Shaikh et al. also used a steam explosion for cellulose extraction from bagasse [46]. While S. M. Costa et al. used a solution containing 16% sodium oxide (Na₂O) and 0.15% anthraquinone, at a 12:1 liquor ratio [9]. The solution was heated at 160 °C for one and a half hours, as determined in another research [47]. They bleached the obtained fiber by dissolving it in N-methylmorpholine-N-oxide (NMMO) using the method used by Fink et al. [48] to produce pure cellulose fibers.

3.2.1. Nanocellulose extraction

Process flow diagram of nanocellulose extraction from sugarcane bagasse.

The chemical treatments varied by different researchers again in terms of the chemicals, the ratios, and other conditions. Mandal and Chakrabarty extracted nanocellulose in their study [50]. The bagasse fibers were bleached with 0.7% (w/v) NaClO₂ solution. Then the obtained hollow cellulose fibers were treated with NaOH (by boiling with 250 ml 17.5% (w/v) solution for 5 h) to remove any hemicellulose from the fibers. Then the fibers were dissolved in 50 ml dimethylsulfoxide solution at 80 °C, and the treatment lasted for 3 h. The obtained product was acid hydrolyzed using 60% sulphuric acid (H₂SO₄) at 50 °C (1:20 liquor ratio) for 5 h under vigorous agitation to produce nanocellulose. The particle sizes were found in the range of 18.17–220 nm.

Kumar et al. [51] extracted cellulose nanocrystals from bagasse by using the acid hydrolysis technique described by other researchers [52,53]. They hydrolyzed delignified and hemicellulose-free bagasse using H₂SO₄ solution [64% (w/w)] at 45 °C for 60 min under vigorous and constant mechanical stirring. They found that the process produced rod-shaped cellulose nanocrystals having the size in the range of 250–480 nm (length) and 20–60 nm (diameter), and the crystallinity percentage was about 72%. This process of cellulose nanocrystal preparation was again used by Slvutsky and Bertuzzi [11]. They alkali hydrolyzed bagasse by treating 10gm of sugarcane bagasse with 100 mL of NaOH (6%) at 60 °C for 4 h in a shaker before bleaching the bagasse in 200 mL of NaClO₂ (30%) solution and with shaking for 24 h at room temperature. Then they acid hydrolyzed the bagasse to produce cellulose nanocrystals. The average length and diameter of the obtained particles were 247.51 (±32.34) nm and 10.11 (±3.36) nm, respectively. de Oliveira et al. used a similar technique to produce cellulose nanocrystals [54]. They bleached the bagasse pulps by using the process of Sun et al. in which they treated the bagasse with an alkaline solution containing 24% hydrogen peroxide (H₂O₂) and 4% NaOH at 70–80 °C [17]. Then the treated the bleached samples at 50 °C in a preheated solution of 65 wt.% H₂SO₄ with mechanical stirring for 40 min (5.0 gm pulp was added in 200 mL of the acid solution) [55]. They also found rod-like particles, which are similar to other results mentioned above that also include some which have not been discussed here [[56], [57], [58], [59], [60]]. Particle size varied within the ranges of 69–117 nm length and 6–7 nm diameter. Ferreira et al. [61] used the method of de Oliveira [54] for bleaching the bagasse. Then they acid hydrolyzed the bagasse by using H₂SO₄ where 10 gm sample was dispersed in 250 mL of H₂SO₄ (65% v/v) at 45 °C for 45 min.

Other than these processes, Sofia et al. used a ball milling technique [62], while Feng et al. used ultrasonic treatment without acid hydrolyzation [63]. For the first process, dewaxed bagasse fibers were dilignified by using a 1.3% sodium chlorite solution, and then the obtained holocellulose fibers were treated with 10% potassium hydroxide (KOH) and 10% NaOH at 20 °C. They used two techniques; one was used by Kumar et al. [51], which involved acid hydrolysis, while the other approach used Spex 8000M shaker mill for milling. They placed 0.25 gm of bleached bagasse fibers in a 70mL container made of polypropylene along with 50 mL of deionized water and 20gm of cerium-doped zirconia balls (ball diameter was 0.5 mm), and the process took place for an hour. Results showed that the ball milling technique did not remove the amorphous portions, whereas acid hydrolysis removed amorphous portions and increased the crystalline part of cellulose nanocrystals. In the study of Feng et al., they bleached the fibers with 30% hydrogen peroxide at 80 °C for 3 h. Then the fibers were treated with deionized water for an hour before being disintegrated in a high-speed blender at 48000 rpm for 5 min. Then the samples were treated with ultrasonication for producing nanofibers. Kathiresan and Sivaraj extracted nanocellulose from sugarcane bagasse using sodium hydroxide [64]. The fibers were collected from the outer rind at first. The fibers were chopped into small pieces and then treated with sodium hydroxide (1% sodium hydroxide solution at 80 °C for two hours) to separate the fibers easily. The collected fibers were then milled (at 300 rpm for 5 h) in a high-energy ball mill to produce nanofibers.

4. Applications

Generally, fibrous materials are most commonly used in the textile sector. But for being suitable for textile applications, the fibers need certain qualities such as fineness, crimp, tensile strength, etc. [31], which sugarcane bagasse cannot offer, as shown in Table 2 .

Table 2

Comparison of different natural fibers with sugarcane bagasse [[65], [66], [67], [68], [69], [70]].

It can be seen from the above table that sugarcane bagasse fiber has meager strength and these fibers are quite coarser in comparison to other fibers such as cotton, flax which are extensively used in textile products. Only coir and sisal fiber has shown some similarities, and these fibers are not used in the textile application that much. But these fibers are not totally unusable. In fact, natural fibers that do not have better properties like cotton or flax are used in composites as reinforcement quite often [30]. Sugarcane bagasse fibers have also been used in composites by several researchers [10, [71], [72], [73], [74], [75], [76], [77]]. Cerqueira et al. prepared bagasse fiber reinforced polypropylene composite [72]. The fibers were pretreated with 10% H₂SO₄ and 1% NaOH. Addition of 20% fiber increased 15.5% tensile strength, 45.4% impact strength and 32.4% flexural rigidity. Monteiro et al. prepared bagasse fiber reinforced polyester composite and found an increase in flexural rigidity due to reinforcement [75]. Oladele also found an increase in tensile strength with bagasse fiber addition in polyester composite [78]. Moubarik et al. showed that delignified sugarcane bagasse fiber significantly improved the mechanical properties of low-density polyethylene composite [79]. Monterio et al. reported that bagasse fiber-reinforced multilayered epoxy composite could show similar performance to Kevlar multilayered sheet [80].

But polyethylene, polypropylene, epoxy, etc., are synthetic polymers. Composites containing types of the matrix are only partially biodegradable. For producing fully biodegradable material, natural polymers have also been used, among which starch is one of the most common ones. Starch has been used for modifying textile materials, too [81]. There have been some good examples of sugarcane bagasse reinforced starch composites. Gilfillan et al. showed that the addition of sugarcane bagasse fiber increases the crystallinity of the composite [10]. Vallejos et al. demonstrated in their study that the accumulation of sugarcane bagasse fiber increases tensile strength and decreases water absorbency of the starch composite [71]. Jeefferie et al. produced a disposable food container from sugarcane bagasse reinforced starch composite [74]. Guimarães et al. [82] showed that sugarcane bagasse reinforced starch composite has better elongation property than banana fiber reinforced starch composite of the same component ratio. On the other hand, Arrakhiz et al. showed that bagasse fiber (25 and 30% fiber loading) reinforced polypropylene composite has better tensile properties than coir and alfa fiber-reinforced composites even though both coir and alfa fibers are stronger than sugarcane bagasse fiber [73].

Nanocellulose has also been used by several researchers in their studies in recent days to produce eco-friendly composite materials. Gadheri et al. extracted nanocellulose from sugarcane bagasse and used it to reinforce cellulose film for making food packaging material [83]. The nanocellulose was obtained by treating bagasse using sodium hydroxide and anthraquinone at first, then bleaching using sodium chlorite and potassium hydroxide. These processes removed all the contents except alpha-cellulose, and that was then ground to the size of the nanofiber. For making the cellulose matrix, they used DMAc (dimethylacetamide) and LiCl (lithium chloride) solution to dissolve the cellulose contents from bagasse, and then those were used to make the composite material. Considerably good tensile and vapor transmission properties were reported from the test results for those materials. Slavutsky and Bertuzzi prepared nanocellulose reinforced starch composite [11] using sodium chlorite like the previous study. The process was a bit different, but the principle was similar. They reported that the water affinity was significantly decreased with nanocellulose reinforcement. The similarity of the chemical structure of cellulose and starch was also responsible for this improvement.

Achaby et al. produced nanocellulose reinforced polyvinyl alcohol/carboxymethyl cellulose (PVA/CMC) blend composite for food packaging application where the nanocellulose was extracted from sugarcane bagasse [84]. They used the acid hydrolysis process to extract the nanocellulose. The bagasse fibers were treated with alkali and then bleached with chlorite before being hydroseed using sulfuric acid under mechanical stirring to produce nanocellulose. The composites were made using casting techniques. The resulted materials showed that the addition of 5% CNC (weight percentage) within a PVA/CMC improved the tensile strength and modulus by 83% and 141 %, respectively, while the water vapor permeability was reduced significantly by 87%. Gan and Chow also used similar techniques (acid hydrolysis for nanocellulose preparation and casting technique for composite making) to produce nanocellulose reinforced PLA (polylactic acid) composite using sugarcane bagasse [85]. They reported that this combination was helped by the presence of hydrogen bonding among cellulose and PLA in the composite and the end products showed better thermal stability than pure PLA.

Cellulose obtained from sugarcane bagasse can also be of fair use other than in paper industries. Pure cellulose is used to make regenerated cellulosic fibers which can be used as a substitute for cotton fibers in the textile sector [86]. The extracted cellulose from it can be used to make hydroxyethyl cellulose, regenerated fiber like cellulose acetate, etc [9, 44, 46, 87]. Cellulose acetate produced by Shaikh et al. [46] showed a tensile strength of 40–61 Mpa, which matches the tensile strength of commercial cellulose acetate fibers (31–55 Mpa [88]). Other than these, cellulose nanocrystals and nanofibers have been used in composite materials with some good effects [11, 89].

5. Assessment of the properties of some products

Sugarcane bagasse fibers have been found as part of many pieces of research. So, the variation between works is quite common. Various types of works have been mentioned in the previous sections. The variations resulted from various reasons, including the used chemicals, the used procedure, the used condition, etc. If some of the common variations can be compared with similar works to find out some controlling points of the procedures. The comparison starts with the fiber extraction process below.

5.1. Fiber extraction

Collier et al. [12] and Michel et al. [2] both used a similar procedure for fiber extraction but the resulted materials were slightly different in terms of yield percentage, mechanical properties, etc. The properties are shown in the Table 3 below.

Table 3

Comparative analysis of the fiber extraction processes.