- Fiber morphology
Figure 4 and Figure 5 depicted the sectional view and longitudinal surface view of the IOF respectively. Pectin and other non-cellulosic compounds present in the structure of the okra fiber bound the elementary fibers to impart strength to the whole bundle of fiber. The elementary fiber overlapped along the length of the okra fiber. (M. Lewin and E. M. Pearce; 1998). The bundle of elementary fibers is commonly called technical fiber or single fiber. (Mohanty, Misra, and Drzal 2005). In the longitudinal view, the overlapping of the elementary fiber and the existence of the impurities were observed. The elementary fibers overlapped in non-cellulosic compounds.
The IOF is found to be polygonal in shape that varies from irregular to circular shape. The range of diameter of the IOF varies from about 60-100 µm as shown in figure 6. The mean diameter and standard deviation of tested IOF were found to be 80 ± 20 µm.
Table 2 reported, the different physical properties of some other natural fibers in comparison to IOF widely used for various Textile applications which will prove the suitability of IOF for Textile applications (Satyanarayana, Guimarães, and Wypych 2007; Md. Tahir et al. 2011).
Table 2. Physical properties of some other natural fibers in comparison to Indian okra fiber.
Fiber
|
Length (m)
|
Diameter (µm)
|
Fineness (tex)
|
Cross-sectional shape
|
References
|
Indian Okra
|
0.5-2.0
|
60-100
|
6.0-13.0
|
Polygonal
|
This study
|
Foreign Okra
|
0.5-4.0
|
40-180
|
6.0-14.66
|
Polygonal
|
Satyanarayana et al., 2007.
|
Ramie
|
1.5
|
60-900
|
0.51-0.71
|
Hexagonal
|
Abaca
|
>2.0
|
10-280
|
4.2-44.44
|
Round/Oval
|
Banana
|
0.8-2.8
|
11-34
|
6.0-7.56
|
Cylindrical
|
Sisal
|
1.0
|
100-460
|
1.0-45.1
|
Cylindrical
|
Coir
|
|
100-450
|
|
Polygonal
|
Cotton
|
|
|
0.11-0.37
|
Round/Oval
|
Flax
|
0.2-1.4
|
40-620
|
0.19-1.98
|
Polygonal
|
Md. Tahir et al., 2011.
|
Jute
|
1.5-3.6
|
30-140
|
1.44-3.0
|
Round/Oval
|
Kenaf
|
|
|
5.56
|
Cylindrical
|
Hemp
|
1.0-3.0
|
|
0.33-2.22
|
Polygonal/Oval
|
- Chemical composition
The morphological microstructure and chemical composition of plant fibers are extremely complex because of two reasons. The first reason is the hierarchical organization used for all plant fiber and the second reason is the presence of different compounds in different concentrations in the fiber composition. Generally, all the vegetable fiber was composed of cellulose and non-cellulosic materials (hemicellulose, lignin, pectin, wax, and some water-soluble compounds). In this composition, lignin and pectin act as binders (Mohanty, Misra, and Drzal 2005). In the reviewed research on okra fiber, it is found that similar to other plant fibers like jute, and flax, hemp okra fiber is also composed of cellulose, hemicellulose, lignin, pectin, fats and waxes, and water-soluble compounds with the proportion of composition 60-70%, 15-20%, 5-10%, 3.4%, 3.9%, and 2.7% respectively. (Alam and Khan 2007; G. M. Arifuzzaman Khan et al. 2009). Whereas, okra plant species and geographic location, age, climate, soil condition, and fiber extraction method decide the chemical composition of the okra fiber. (Sain and Panthapulakkal 2006; Yilmaz, Çilgi, and Yilmaz 2015). Table 3 reported, the different chemical compositions of some bast fibers in comparison to okra fibers widely used for various textile applications (Mukhopadhyay, Fangueiro, and Shivankar 2009; Mohanty, Misra, and Drzal 2005; G. M. Arifuzzaman Khan et al. 2009). The comparison of the chemical composition of okra fiber with other bast fibers proves its suitability for various textile applications.
Table 3. Chemical compositions of some bast fibers in comparison to okra fibers widely used for various textile applications.
Types of fibre
|
Cellulose (Wt.%)
|
Hemicellulose (Wt.%)
|
Lignin (Wt.%)
|
Pectin (Wt.%)
|
Moisture Content (Wt.%)
|
Wax (Wt.%)
|
References
|
Banana
|
28.3-35.8
|
12.94-17.2
|
14.4-15.8
|
4.7
|
8.5-9.5
|
|
Mukhopadhyay et al., 2009
|
Flax
|
64.1–71.9
|
16.7–20.6
|
2.0–2.2
|
1.8–2.3
|
8–12
|
1.7
|
Mohanty et al., 2005
|
Hemp
|
70.2–74.4
|
17.9–22.4
|
3.7–5.7
|
0.9
|
6.2–12
|
0.8
|
Jute
|
61–71.5
|
12.0–20.4
|
11.8–13
|
0.2
|
12.5–13.7
|
0.5
|
Kenaf
|
31–57
|
21.5
|
8–19
|
3–5
|
|
|
Ramie
|
68.6–76.2
|
13.1–16.7
|
0.6–0.7
|
1.9
|
7.5–17
|
0.3
|
Okra
|
60–70
|
15–20
|
5–10
|
3.7
|
8.2-8.8
|
3.9
|
G. M. Arifuzzaman Khan et al., 2009
|
- Thermogravimetric analysis (TG) and differential thermogravimetry (DTG)
The thermal stability of fiber has great importance in various applications such as fiber reinforcement, structural application, etc. to decide its suitability for the respective application (B. M. Reddy et al. 2020). In this respect, thermogravimetric analysis of IOF was performed to check its thermal stability. The TG and DTG curve obtained from the thermal degradation of IOF at high temperature was used to analyze the thermal stability.
The TG and DTG curves of the IOF are shown in figure 7. The TG and DTG curves in figure 7 depicted that, the IOF shows weight loss at three stages and the decomposition of IOF was observed to befall in two main stages. Initially, the approximately 8.5% weight loss of the okra fiber observed between 30-130 oC was created due to the evaporation of water from the IOF. Further, the degradation of the IOF befalls after 240 oC. It is observed that the degradation of the IOF befalls with a gradual decrease in thermal stability above 240 oC.
The DTG curve in figure 7 depicted that, the degradation of the IOF occurs in two stages TI (240-350 oC) and TII (350-500 oC). In the first stage of TI, thermal depolymerization of hemicellulose, pectin, and the cleavage of glycosidic linkages of cellulose is responsible for 16.8% weight loss. In the second stage TII, the degradation of the α-cellulose present in the IOF is responsible for 67.1% weight loss. (Albano et al. 1999).
The lignin structure is quite similar to a highly unsaturated polymer which has a low oxygen-to-carbon ratio and is composed of aromatic rings with various branches. (W. Liu et al. 2004). Therefore, during the whole temperature range, the decomposition of lignin on the IOF occurs slowly due to its complex structure. The results can also be confirmed by the DTG curve as shown in figure 7, where it is observed that the lignin decomposes slowly throughout the temperature range of 240-500 oC. Furthermore, the residual weight percentage was observed at about 7.5%. After the complete cellulose degradation in IOF, carbonaceous residues and possible undegraded fillers observed in the final product. (Arbelaiz et al. 2006) The TG and DTG analysis concluded that the IOF was stable until around 240 °C.
Table 4 shows the weight losses at the different stages and the peak temperatures for IOF. Decomposition temperatures of some natural fibers in comparison with IOF are depicted in Table 5 (Ouajai and Shanks 2005; Spinacé et al. 2009; Yao et al. 2008; Yang et al. 2007). This comparison proves the good thermal properties of the Indian okra fiber which is suitable for various textile applications.
Table 4 shows the weight losses at the different stages and the peak temperatures for okra fibers.
|
Weight loss (%) at temperature range of (30-130 oC)
|
First degradation Stage
|
Second degradation Stage
|
Residual char (wt%)
|
Ref.
|
T I (oC)
|
Weight loss (%)
|
Tpeak (oC)
|
T II (oC)
|
Weight loss (%)
|
Tpeak (oC)
|
Indian Okra fibre
|
8.5
|
240-350
|
16.8
|
305
|
350-500
|
67.1
|
340
|
7.5
|
This study
|
Foreign Okra
|
8.4
|
220-310
|
16.1
|
303
|
310-390
|
60.6
|
359
|
7.6
|
De Rosa et.al. 2010
|
Table 5 Decomposition temperatures of some natural fibers in comparison with okra fibers.
Natural Fiber
|
Initial decomposition temperature (oC)
|
Maximum decomposition temperature (oC)
|
References
|
Indian Okra
|
240
|
340
|
Present study
|
Foreign Okra
|
220
|
359
|
De Rosa et.al. 2010
|
Hemp
|
250
|
390
|
(Ouajai & Shanks, 2005; Spinacé et al., 2009; Yang et al., 2007; Yao et al., 2008
|
Curaua
|
230
|
225
|
Kenaf
|
219
|
284
|
Jute
|
205
|
283
|
- Differential Scanning Calorimetry (DSC)
Figure 8 shows the DSC curve for the Indian okra fiber. Differential Scanning Calorimetry (DSC) is a thermos-analytical technic. It is an effective technique used to analyze fibrous material properties such as oxidation behavior and thermal stability of the fibrous material. It is based on the conduction of heat inside or outside of the fibrous material measured as a function of time or temperature. During DSC analysis fibrous samples were exposed to a controlled temperature program. (Jonoobi et al. 2009)
Figure 8 depicted that, the endothermal and exothermal reactions befell due to volatilization (gases) of the molecules and the formation of charring (solid residue) respectively (Ball, McIntosh, and Brindley 2004). The endothermic peak occurs at around 100 oC in the temperature range of 50-150 oC for the evaporation of water. The heat absorbed in this event was 96.5 mJ/mg. The exothermic peak occurs at around 443 oC in the temperature range of 240-540 oC. The heat released in this event was 2123 mJ/mg. This peak is mainly responsible for the decomposition and degradation of the hemicellulose and lignin present in the IOF respectively. The endothermic and exothermic peak temperature shows the good thermal stability of the IOF; which is more than sufficient to use it for various textile applications.
- X-Ray Diffraction Analysis (XRD)
X-ray diffraction analysis (XRD) is a significant and non-destructive technique used in material science to analyze the crystallographic structure of the material. It can be used to analyze the structure of the atoms within the material for larger crystals in macromolecules and inorganic compounds. Also, it is used to analyze the material composition, crystallinity, and phase purity for the too-small crystal size.
Figure 9 shows the X-ray spectrum of IOF. In the X-ray spectrum of IOF 3 undistinguishable peaks were observed at 2θ = 15.70, 22.20, and 34.50. These observed peaks depicted the presence of a semicrystalline region in IOF.
The peak observed at 2θ = 15.70, and 2θ = 22.20 shows the identity of cellulose I and IV in IOF (Sreenivasan et al. 2011). The peaks at 15.70, 22.20, and 34.50 cellulosic crystallographic planes (2 0 0), (1 1 0), and (0 4 0) respectively observed in IOF (Yilmaz et al. 2016; G. M. Arifuzzaman Khan et al. 2009). The peaks observed at 15.70 and 22.20 depicted the non-cellulosic material or maybe the presence of impurities in the IOF (N. Reddy and Yang 2005).
The XRD analysis shows that the Indian okra fiber is composed of 25.8% amorphous region and 74.2% crystalline region. A greater crystalline region reduces the chemical reactivity, elongation, and water absorption capacity of the Indian okra fiber (Elenga et al. 2009). Whereas, the strength and refractive index of the Indian okra fiber is more with improved orientation; which was obligatory to use for various textile applications.
- Fourier transform infrared spectrometry
In this study, Fourier transform infrared spectrometry (FTIR) used to detect the functional group in the material working in attenuated total reflectance mode (ATR) was used to analyze the chemical structure of the IOF. IR spectrum of absorption, emission, and photoconductivity obtained in a range 4000 and 400 cm-1 analyzed for IOF. Figure 10 shows the main absorbance peaks that occur due to the presence of various chemical groups in the lignocellulosic IOF compound.
The wide absorption band which is centered at 3400 cm-1, is in the region of 3600-3100 cm-1 depicted the presence of stretching vibration of the O-H bond and hydrogen bond of the hydroxyl group. (Spinacé et al. 2009). The cellulose and hemicellulose components present in the IOF are depicted by the characteristics band at 2922 cm-1, 2852 cm-1, and 2372 cm-1 which shows the C-H stretch vibration of CH and CH2 (Alvarez and Vázquez 2006). The presence of the carbonyl C-O bond of the carboxylic acid in lignin or the ester group in hemicellulose is represented by the absorbance at 1739 cm-1. (G. M. Arifuzzaman Khan et al. 2009; W. Liu et al. 2004). The band at 1654 cm-1 depicted the presence of a certain amount of moisture in the IOF (Paiva et al. 2007). The C-C bond present in the aromatic compound of the lignin is depicted by the band at 1560 cm-1 (W. Liu et al. 2004). The absorbance peak at 1458 cm-1 depicted the existence of CH2 bond in the cellulose (Sgriccia, Hawley, and Misra 2008). The C-O bond exists in the acetyl group of the lignin compound is depicted by the peaks at 1410 cm-1 and of the hemicellulose depicted by the peak at 1251 cm-1 (W. Liu et al. 2004; Mwaikambo and Ansell 2002). The C-H bending vibration was observed at the peak of 1381 cm-1 whereas, C-O stretching present in the aromatic ring of polysaccharides was depicted by a peak at 1319 cm-1. (Jonoobi et al. 2009). The unbalanced distortion of C-O-C stretching vibration is represented by the absorbance of the peak at 1157 cm-1 (Cyras et al. 2004). The strong absorption peak at 1111 cm-1, 1047 cm-1, and 1022 cm-1 shows the CO and O-H bond which is due to the presence of polysaccharides in cellulose (D. Liu et al. 2009). The b-glycosidic bonding among the monosaccharides and C-OH bending is depicted by the peaks at 898 cm-1 and 669 cm-1 consecutively. (Mwaikambo and Ansell 2002).