Fourier Transform Infrared (FTIR) Spectroscopy
The FTIR spectra of the cotton burr (CB) and the extracted cellulose (EC) are demonstrated in Fig. 2, and both exhibit common cellulose bands in their spectra and reveal noticeable differences, specifically the existence of characteristic bands related to non-cellulosic compounds like hemicelluloses and lignin molecules in the EC spectrum [22]. CB and EC show a broad peak around 3440 cm− 1, corresponding to the stretching vibrations of O-H groups in cellulose and intermolecular and intramolecular hydrogen bonds. A band at 2910 cm− 1 is attributed to the C-H stretching vibration of cellulose [23]. The 1630 cm− 1 band indicates that water contains O-H stretching and bending vibrations [24]. The sharp transmittance band at 1060 cm− 1 identifies the C-O stretching vibration of primary alcohol [25], and the small peak at 893 cm− 1 indicates C-O-C vibrations linked to the β-1,4 glycosidic linkage [26].
At 2858 cm− 1 in the cotton burr, there is a peak that corresponds to C-H stretching vibrations in the aromatic methoxy, methyl, and methylene groups of lignin [27, 28]. The peaks at 1735 cm− 1 and 1248 cm− 1 link to the stretching vibration of the carbonyl (C = O) group of the acetyl and urate groups present in the hemicellulose [29, 30]. This band also indicates the ester group within the carboxylic functional group of phenolic acids like ferulic and p-coumaric, which are part of lignin [29, 31]. However, the EC exhibits only characteristic bands associated with pure cellulose material. The spectrum shows that non-cellulosic compounds were successfully removed through chemical treatments because there are no bands for hemicellulose or lignin [29, 32, 33]. Thus, the results demonstrate that the successful extraction of cellulose from cotton burr.
X-Ray Diffraction Analysis
The X-ray diffractograms of CB and EC are illustrated in Fig. 3 indicating the improved crystallinity of EC and the introduction of cellulose II into the EC structure. The diffractograms of CB (Fig. 3(a)) exhibit only cellulose type I, which is characterized by the peaks at 16.45° (110) and 22.30° (200) [34, 35]. However, the XRD of EC (Fig. 3(b)) reveals a well-defined coexistence of cellulose I and cellulose II polymorphs. The peaks observed at 2θ = 14.66° (\(\text{1}\stackrel{\text{-}}{\text{1}}\)0), 16.45° (110), 22.30° (200), and 34.20° (004) correspond to the cellulose I [36, 37]. The presence of cellulose type II is confirmed by the appearance of peaks at 2θ = 12.20° (110), 20.25° (210), and 22.30° (200) [18, 35, 37, 38]. Cellulose II was obtained by 15 wt.% NaOH, while several studies have reported that when lignocellulosic fibers are treated using 20 wt.% NaOH solutions or higher [39–42]. The overall mechanism of the cellulose II formation process involves the solubilization and recrystallization of cellulose I, which includes an alkaline pre-treatment followed by acid hydrolysis. During the alkaline pre-treatment, the cellulose fibers are exposed to an alkaline solution, typically sodium hydroxide (NaOH). The hydroxyl (OH) groups in the cellulose molecules are replaced by sodium ions (Na+), resulting in a new lattice structure known as Na-cellulose I. This substitution of OH groups with ONa groups expands the dimensions of the cellulose molecules. Therefore, the alkaline treatment causes the cellulose fibers to swell and allows sodium ions to penetrate into the fiber matrix. This swelling and penetration of sodium hydrates weaken the existing intermolecular chain links. As a result, the cellulose fibers realign and undergo recrystallization. The recrystallization is a consequence of the swelling and breaking of the existing intra and intermolecular bonds within the cellulose lattice structures and the removal of sodium ions from the cellulose by water rinsing. After the alkaline treatment, the cellulose is subjected to acid hydrolysis, typically using sulfuric acid (H2SO4). The acid hydrolysis also further disrupts the existing intermolecular bonds, leading to the realignment and complete recrystallization of the cellulose fibers and contributing to the formation of the cellulose II structure. After the acid hydrolysis, the cellulose fibers are rinsed with water to remove the linked sodium ions. The final water rinsing step eliminates remaining acid residues and facilitates the formation of new intermolecular bonds, resulting in purified cellulose II [18, 40].
The crystallinity of EC increased significantly compared with CB after being purified and acid hydrolyzed. According to the Segal method (Eq. 1) [16], the CrI (crystallinity index) of cellulose I and cellulose II in the EC are 79.67% and 68.71%, respectively, while the crystallinity of CB (cellulose-I) is only 43.11%. The higher CrI of EC indicates the successful removal of low crystalline hemicellulose and lignin from CB during alkalization and bleaching. Moreover, the CrI was increased by eliminating amorphous regions within cellulose after the acid treatment. Hydronium ions effectively penetrated the cellulose chains during the acid hydrolysis process, particularly targeting the susceptible amorphous regions. This facilitated the hydrolysis of glycoside bonds, eventually releasing individual crystallites [15, 29, 30, 43]. The size of the crystallites in EC and CB are determined using X-ray diffractograms and Scherrer's equation (Eq. 2). The average crystallite size for EC is 5.16 nm, while for CB, it is 1.64 nm, both measured at the peak point of cellulose I (Bragg's angle of 22.50). The increased crystallite size of EC may be caused by a decrease in the amorphous region and the rearrangement and recrystallization of the cellulose chains after acid hydrolysis. This process could result in longer crystal dimensions and a higher crystallinity index of cellulose [44]. Furthermore, Table 1 represents the cellulose extracted from different agricultural residues through acid hydrolysis, while the extracted cellulose shows better crystallinity than others. Cellulose crystallinity is considered a key factor since the higher crystallinity of cellulose can impart greater reinforcement in the composite materials.
Table 1
Crystallinity index (CrI) and onset temperature (Tonset) of the obtained cellulose in this study and compared with the reported literature
Sample
|
Extraction method
|
Cr. I. (%)
|
Tonset
|
References
|
CNC from Nypa Fruticans trunk
|
Acid hydrolysis
|
76.6
|
190
|
[30]
|
CNC from Coconut husk fiber
|
Acid hydrolysis
|
79.3
|
265
|
[30]
|
CNC from artemisia annua Stems
|
Acid hydrolysis
|
72
|
193
|
[15]
|
CNC from walnut shells
|
Acid hydrolysis
|
51.00
|
243.8
|
[8]
|
CNC from Calotropis procera
|
Acid hydrolysis
|
68.7
|
180
|
[45]
|
CNC from flax fiber
|
Acid hydrolysis
|
87
|
336
|
[46]
|
CNC from soy hulls
|
Acid hydrolysis
|
73.50
|
170
|
[47]
|
CNC from cotton gin motes
|
Acid hydrolysis
|
69.7
|
202
|
[5]
|
Extracted cellulose
|
Acid hydrolysis
|
79.67
|
258
|
This work
|
Scanning Electron Microscope (SEM) and Elemental analysis
SEM-EDS analyzes the morphology and elemental composition of the CB and EC. The CB surface was smooth compared with the EC, see Fig. 4. The chemical treatment resulted in rough surfaces on the EC due to the removal of lignin and hemicelluloses [15, 45]. The process began with an alkali treatment using a sodium hydroxide solution, which caused the disintegration of CB due to the removal of cementing materials from the surface. The particles in the CB are held together by hemicelluloses, lignin, and other water-soluble components that act as binders [29]. This alkali treatment disrupted some alkali-labile linkages, such as ether and ester linkages between lignin monomers or between lignin and polysaccharides. This partial disintegration facilitated the penetration of the bleaching solution during the subsequent bleaching treatment. The chlorine in the bleaching solution further oxidized the lignin, forming hydroxyl, carbonyl, and carboxylic groups, allowing residual lignin solubilization [6].
The average diameters of cotton burrs are 40 ± 5µm and reduced to 5 ± 2µm after cellulose extraction. These significant changes could result from acid hydrolysis, which successfully removes the amorphous phase by releasing hydronium ions, leading to the breakdown of glycosidic bonds within amorphous regions along the cellulose [48, 49].
Table 2
Elemental composition of CB and EC
Sample
|
C (%)
|
O (%)
|
Cu (%)
|
Ca (%)
|
Pt (%)
|
O/C ratio %mass
|
Cotton burr
|
48.85
|
39.10
|
1.92
|
0.21
|
9.93
|
0.80
|
Extracted cellulose
|
67.01
|
32.99
|
-
|
-
|
-
|
0.49
|
The elemental composition of CB and EC is shown in Table 2 and Fig. 4c and d. In both samples, the peaks for oxygen (O) and carbon (C) were prominent, which are typical components of cellulose. The disappearance of Cu, Ca, and Pt in EC indicates that these were eliminated with the removal of non-cellulosic elements during the extraction process. Table 2 shows the decrease of the O/C ratio in the EC sample, which indicates the reactive bonds on the particle surface. This relative decrease in oxygen content can be suggested as owing to the removal of oxygen-containing non-cellulosic components (lignin, hemicellulose) during bleaching and acid hydrolysis [50, 51].
Thermal Property Analysis
The thermal stability of cellulose is essential for some industrial and commercial applications such as the composites and products that require to endure at high temperatures. The TGA thermograms and DTG curves (a derivative of TGA, showing the rate of weight loss) of CB and EC are displayed in Fig. 5. From the TGA, it is found that at 120°C, the CB shows a weight loss of 7.10%, while the EC exhibits a weight loss of 3.41%. The reason for this weight loss was the evaporation of moisture that loosely bound the surfaces of the materials, which is related to their hydrophilic nature. The moisture content of EC is lower than that of CB because of the dehydration of the cotton burr during acid hydrolysis, which is introduced by sulfate groups on the outer surface of EC [30, 52]. The introduction of sulfate groups can disrupt these hydrogen bonding interactions, making it more difficult for water molecules to bind to the cellulose structure and reducing moisture content [49, 53]. The hydrophilicity of hemicellulose, lignin, and other non-cellulosic components contributes to higher moisture retention in CB [54, 55]. The thermal parameters, such as the onset temperature (Tonset), the maximum degradation temperature (Tmax), and char residue, are listed in Table 3.
Table 3
Mass loss and degradation temperature of cotton burr and extracted cellulose obtained from TGA
Sample
|
Tonset (°C)
|
DTG Tmax (°C)
|
Residue (%) at 475°C
|
Cotton burr
|
202
|
301
|
37.84
|
Extracted cellulose
|
258
|
346
|
16.58
|
The thermal stability of EC (extracted cellulose) is greater than that of the CB (cotton burrs) sample, as evident from their respective characteristics. The onset temperature (Tonset) of CB and EC are found at 202°C and 258°C, respectively, which is 56°C (Tonset) higher than that of CB see Fig. 5a and Table 3. Previous literature has reported that cellulose materials with a high crystallinity index exhibit greater thermal stability see in Table 1. This is because their thermal decomposition initiates from the disordered domains and progresses toward the more ordered regions of the material [36, 56] since the crystalline regions resist degradation at lower temperatures. Our findings align with these observations and can be explained by the fact that cellulose materials with higher crystalline order of EC tend to have increased thermal stability than lower crystalline CB. The residual char is observed in the CB sample (37.84% at 475°C) as opposed to the EC sample (16.58% at 475°C). This difference is due to the absence of charred residue resulting from the breakdown of lignin and hemicelluloses in the EC sample since these components were effectively removed during the bleaching treatment [22, 32, 57]. The DTG analysis of the CB sample revealed a multistep degradation process occurring between temperatures 170°C and 301°C (Fig. 5b). A shoulder appeared at 250°C in the DTG curve (Fig. 5b) is associated with the decomposition of hemicellulose [58]. In contrast, the thermal degradation behavior of EC underwent a one-stage thermal degradation process characterized by a single DTG peak at 346°C. The EC sample started to degrade at 230°C, and its degradation rate peaked at 346°C. The breakdown of pure cellulose molecules is responsible for this degradation, which involves processes such as dehydration, decarboxylation, depolymerization, and decomposition of glycosyl units [59]. There was no shoulder around 250°C in EC due to the successful removal of non-cellulosic portions during the treatment process of CB [18, 60].
Particle Size and Zeta Potential Analysis
The ability of cellulose to disperse evenly and maintain colloidal stability is an essential characteristic for its use in suspended forms [6]. Zeta potential (ζ) reflects the electrostatic repulsion between particles attributed to their charged state, preventing their agglomeration and ensuring the dispersion stability of cellulose in aqueous solutions. Dynamic light scattering (DLS) was utilized to determine the particle size distribution of the cellulose in suspension. After centrifugation, the cellulose dispersed in water was analyzed using a DLS particle size analyzer. The hydrodynamic particle size and zeta potential value of CB and EC were measured and the obtained results are illustrated in Figs. 6 and 7. The minimum particle size is found to be 171.7 nm which accounts for 2%, then a gradual and steady increase in the size of the particles which reaches a peak at 192.4 nm accounting for about 56%, and 39.28% of particles have the size of 213.6 nm (Fig. 6). The size of CB reduced from an average of 402 nm due to the breaking of the amorphous cleavage of cellulose.
The average zeta potential values for CB and EC are − 63mV and − 76mV (Fig. 7), respectively. This demonstrates that the acid hydrolysis process causes the accumulation of negative charges on the EC surface, resulting in a rise in the negative zeta potential [43]. Additionally, it is anticipated that the enhancement of the surface area of the extracted particles might lead to a higher negative charge. Due to their average zeta potential value of -76mV, the cellulose particles or surfaces demonstrate strong electrostatic repulsion, resulting in enhanced dispersion stability. Higher zeta potential values generally indicate more substantial repulsive forces, enhancing stability by preventing particle aggregation or flocculation.
Moisture Content
Moisture content can affect the barrier properties of packaging materials and polymeric composite materials, such as interfacial bonding and mechanical strength. The lower moisture content in cellulose improves the interfacial bonding in the polymeric composites, as well as strength and durability. Due to high moisture, composites prepared with non-cellulosic compounds become swollen when wet and thus lose dimensional stability. This is a major challenge of untreated lignocellulosic polymeric composites. However, if the moisture content is reduced, the swelling property of composites can be controlled. The moisture content of CB and EC are 12.65% and 5.06%, respectively. The significant reduction in moisture content in extracted cellulose is due to the removal of non-cellulosic components and an increase in crystallinity. The raw biomass contains cellulose and other non-cellulosic compounds that can retain more water. The improved crystallinity of particles leads to less moisture content since crystalline regions can hold less water. The moisture content of cellulose aligns with the findings obtained in the XRD. The moisture content is proportional to crystallinity. Moreover, this extracted cellulose has the potential to be used in polymeric composites that require negligible swelling when exposed to moisture.
Cellulose Content of Cotton Flower Burr
Table 4 shows the cellulose content of CB, cotton gin trash, and some other agricultural residue. Cotton gin trash consists of cotton burrs, leaves, and sticks. Among all these parts, CB solely possesses comparatively higher cellulose than other parts.
Table 4
Cellulose content of cotton flower burr and different agricultural waste
Sample
|
Cellulose (%)
|
References
|
Cotton gin waste
|
31
|
[14]
|
Cotton gin trash
|
25
|
[61]
|
Cotton stalk bark
|
30
|
[62]
|
Banana rachis
|
33
|
[63]
|
Barley st
|
30
|
[63]
|
Sunflower seed husks
|
31.67
|
[43]
|
Grape stalk
|
30
|
[64]
|
Corn husk
|
24.2
|
[64]
|
Corn cob
|
26.2
|
[64]
|
Cotton burr (CB)
|
35.21
|
This work
|