Hemp fiber morphology. The SEM images revealed the surface morphology and degumming effect of treated and untreated hemp fibers. The raw hemp fibers (Raw) (Figure 3a and 3b) containing lignin, hemicellulose, oil, waxes, and pectin show a rough surface. The alkali-treated hemp fibers (A) had a noticeably clear fiber surface, due to the removal of gum from the fiber structure. The clear and smooth surface of alkali-treated fibers (Figure 3c, d) revealed good individual fibers with the whitened effect, which was due to the use of NaOH and H2O2 (for removing lignin and destruction of inherent chromophores present in the raw hemp fibers). Furthermore, the aqueous solution of hydrogen peroxide acts as a eutectic mixture therefore the depression of boiling point accelerates the degumming process of hemp fibers with a low process temperature in the alkaline-peroxide solution (Kačer et al. 2012). As the electron delocalization occurred in the conjugated double bonds of fibers allow to absorb visible light, therefore, a whitening effect can be found on the degummed fibers. As shown in Figure 3e, f, i, and j, MWE-DES treated fibers (B and D) showed an exceptionally smooth and clear surface. Even though DES-treated fiber at 1:10 ratio (Figure 3e) had a comparatively less clear surface, this might suggest the presence of low content of gummy materials. However, after increasing the microwave heating temperature and DES-fiber ratio (SEM micrographs in Figure 3i and j of DES-treatment at 1:20 DES–fiber ratio), the hemp fibers had a clean surface with a white glowing appearance which was comparable with alkali-treated fiber surfaces. MWE-DES treated fibers (D) (Figures 3i and j) had very few cracks and voids even after increasing the process temperature and time up to 120°C and 1.5 hours; this might be caused due to the uniform dispersion of microwave heating throughout the hemp fiber structure and low freezing temperature of DES (12°C). On the other hand, subsequent alkali treatment of MWE-DES-treated fibers (C and E) revealed some pits and cracks (Figure 3g, h, k, and l) in the fiber surface due to the direct attack of alkali into the cellulose of the fiber structure since DES treatment removed most of the gummy matters. In addition, lignin is strongly linked by the hydrogen bonds and formed lignin-carbohydrate complexes. However, these strong hydrogen bonds can be weakened due to the formation of competing hydrogen bonds between the hydroxyl groups of lignin-carbohydrate complexes and the chloride ions of the DES network; thus, lignin could be removed from the MWE-DES treated fiber structures. However, the H+ protons (from the hydrogen bond donor of DES) can selectively cleave the ester and/or ether linkages of lignin-carbohydrate complexes without the destruction of C-C bonds of cellulose, removing hemicellulose and lignin from the fiber structure.
Table 1
Wavenumbers of FT-IR spectra of both raw and degummed hemp samples and assignment of the functional groups
Wavenumber
(cm−1)
|
Functional Group
|
Assignment
|
3316
2916
1741
1624
1543
1376
1317
1245
1135
1014
894
|
OH stretching
C-H symmetrical stretching
C=O stretching
OH (water)
C=C aromatic ring symmetrical stretching
CH bending
CH2 Wagging
CO stretching
C-O-C asymmetric stretching
C-C, C-OH, C-H vibrations of side groups and ring
Glycosidic linkages symmetric ring stretching
|
Polysaccharides
Polysaccharides
Hemicellulose
Cellulose
Lignin
Cellulose
Cellulose
Lignin
Cellulose
Hemicellulose
Polysaccharides
|
Surface functional groups of hemp fibers. FT-IR spectra were used to investigate the presence of the functional groups and the degumming effectiveness of both raw and degummed hemp fibers (Figure 4). The band of FT-IR spectra at 894 cm−1 is ascribed to the β (1-4) glycosidic bonds of cellulose in the polysaccharides (Kalisz et al. 2021). The symmetric vibration in the plane of C-O-C glycosidic linkages appeared at the band of 894 cm−1 for all the hemp samples as seen in Figure 4. It signifies that the structure of cellulose has undergone little change in all the treated hemp fibers. The functional group assignment of FT-IR spectra wavenumbers of raw and degummed hemp fibers is represented in Table 1. The stretching of ester groups and carbonyl groups (C=O) of hemicellulose and lignin were recognized at the band of 1741 cm−1 (Liu et al. 2007). The disappearance of the peak at 1741 cm−1 for all the degummed hemp fibers indicted the removal of hemicellulose from the fiber structure (Stevulova et al. 2014). The characteristic peak of lignin was observed at the band of 1624 cm−1 (C=C stretching of the aromatic ring). However, the peak was weakened after alkali treatment, stretched and shifted a little bit for the DES treatment at 1:10 fiber-DES ratio, and gradually reduced and disappeared after the DES treatment at 1:20 fiber-DES ratio, which was a clear indication of the removal of gum. The bending of methylene carbons (CH2) of lignin and hydroxyl groups (OH) in-plane bending were observed at the 1424 cm−1 and 1376 cm−1 bands, respectively. Those two peaks appeared as straight and stretched in the treated fibers. The changes in the characteristic vibration peaks of raw hemp fibers at 2916 cm−1 and 1741 cm−1 indicated that chemical modification had occurred in the DES-treated fibers. The absence of the peak at 2831 cm−1 in the DES-treated samples signaled the removal of oil and waxes from the hemp fibers after degumming, except for the sample B. The vibration absorption peak of hydroxyl groups at the band of 3316 cm−1 was getting stronger and stretched for both alkali and MWE-DES treated samples due to the reaction that occurred between DES and fiber (for the DES-treated fibers), and sodium hydroxide and fiber (for the alkali treatment).
Crystallinity of hemp fibers. Figure 5a represents the XRD spectra of treated and untreated hemp fibers. The peaks at 14.6° and 16.4° were assigned to the characteristic peak of cellulose-I and appeared as a wide peak at raw hemp XRD spectrum due to the presence of lignin, pectin, and hemicellulose (Perel 1990). The peaks at 14.6°, 16.4°, 22.6°, and 34.8° corresponded to the crystalline phase of hemp fibers in planes110, 1\(\stackrel{-}{1}\)0, 200, and 400, respectively (Hosseinmardi et al. 2018). The peaks of the crystalline phases were intensified and strengthened after alkali and DES treatment with the strongest peak for the DES treatment at 1:20 fiber-DES ratio. This indicates the impact of DES treatment on the orientation of crystalline cellulose. In a similar fashion, the intensity of the amorphous region peak at 18° was decreased as was expected after the DES treatment at a higher ratio and temperature. The two peaks at 14.6° and 16.4° gradually became closer for the alkali-treated and MWE-DES-treated samples (as the solid: liquid ratio and time-temperature of the degumming process increased), signifying the increased crystalline lattice and the removal of lignin, pectin, and hemicellulose. The crystallinity % and crystallite size data of both raw and degummed and hemp fibers are shown in Figure 5b and c. DES treatment enables the cellulose chains to be tightened and strengthened found in the literature (Besbes, Vilar, and Boufi 2011). The crystallinity % of the DES-treated fibers was increased as compared to that of raw hemp fiber (Raw). It was an obvious indication of the removal of non-cellulosic components (non-crystalline elements) from the hemp fiber structure. It was reported from previous research that the removal of gummy materials facilitates the cellulose chains to be relaxed (increased stress relaxation) (Viscusi, Barra, and Gorrasi 2020). Therefore, microfibrils of the cellulosic components might get rearranged; the polymeric chain and surface morphology after alkali or DES treatment could be altered due to the reaction of Na+ and hydroxyl groups of fibers (for alkali-treated), and cationic component (choline+) of the DES network and hydroxyl groups of fibers (for MWE-DES treated fibers).
Chemical compositions of hemp fibers. CP-MAS/13C-NMR is an effective technique to illustrate the structure and chemical composition of polymeric materials. The peaks in the 60-110 ppm (Figure 6c) range revealed the solid-state NMR spectra of cellulose and hemicellulose. The chemical shift at 105 ppm corresponds to the cellulose C1 carbons, and the up-field shoulder peak at 103 ppm is attributed to the hemicellulose or xylan carbons (Terrett et al. 2019). The C4 carbon of cellulose showed a peak in the spectra at 89 ppm, which is ascribed to the carbons of the crystalline region. The peak of 72 ppm to 75 ppm corresponds to the cellulose carbons superimposed to the nearby xylan peaks. Carbon C6 of crystalline and amorphous cellulose has a similar type of observations and interpretations at 63 ppm to 65 ppm by Bonatti et al.
(Bonatti et al. 2004). Carbonyl and methoxy groups of lignin showed peaks at 174 ppm and 56 ppm, respectively (Moussa et al. 2020). As shown in Figure 6b,d, raw hemp fiber (Raw) had a broad peak at 174 ppm, and 56 ppm that gradually became absent in the Microwave-DES treated fibers (as the temperature, time, and solid-liquid ratio increased) and alkali-treated fibers. It was a clear indication of the removal of lignin and breakage of hemicellulose to be turned into glucose molecules and contributed to increased crystallinity and chemical composition rate of cellulose in the fiber structure. This phenomenon has a closer agreement with the XRD spectra (Figure 5a) of the raw and degummed hemp fibers. In addition, the broad peak at 21 ppm and the small peak at 32 ppm in the raw hemp are assigned to the methyl groups and methylene carbons of lignin and xylan, respectively. However, after DES treatment, those peaks were nowhere to be found (Figure 6b). The disappearance of the peaks is indicative of the removal of gummy compounds in the treated fibers. Lignin, a complex biopolymer can be derived from the polymerization of phenyl propane monomers, synapsyl alcohol, coniferyl alcohol, and p-coumaryl alcohol (Hansen et al. 2016). These carbons of monolignol units exist in the aromatic lignin region as amino acid groups in the raw hemp fibers, disappearing after the alkali and DES treatment (Figure 6d). The signals in the NMR spectra of raw hemp fibers at 21 ppm and 174 ppm are the consequence of agglomeration and interaction of hemicellulose and methyl moieties associated with acetyl groups (Simmons et al. 2016). The signal at 81 ppm of C4 is also generated due to the interaction of cellulose and xylan present in the raw hemp fibers. This wide-ranging signal disappeared after DES and alkali treatment. This is caused by the removal of hemicellulose and lignin from the fiber structure, which is also supported by the previous literature (Hosseinmardi et al. 2018). It is possible to find the carbon ratios which existed in the lignin structure by an integration of the area between 110 ppm-160 ppm (Fu et al. 2015). According to the literature, the chemical shift at 64 ppm could be assigned to the C5 of xylan or hemicellulose (Simmons et al. 2016), and this signal was flattened after DES and alkali treatment, indicating the removal of hemicellulose. A similar explanation can be used for the NMR band at 84 ppm. Therefore, the integration of the area under those two peaks (64 ppm and 84 ppm) can be an estimation of hemicellulose content in the fiber structure. Figure 6e represents an estimation of the chemical composition of hemp fibers obtained from Solid-state NMR spectra using the Gaussian -Lorentzian model and peak deconvolution.
Thermal stability of hemp fibers. Figures 7a, b, c, d, and e represent the TG and DSC curves of treated and untreated hemp fibers. The heterogeneous chemical composition of natural fibers makes thermal degradation complex. The thermal degradation of hemp fibers from TGA can be divided into three thermal stages including water evaporation at 50°C - 150°C, hemicellulose decomposition at 220°C - 300°C, and cellulose degradation at 300°C - 400°C. However, it is thought that the thermal degradation of lignin begins at roughly 280°C and ends at about 450°C. The complex aromatic nature of lignin enables lignin to keep thermal stability at a higher temperature; therefore, it can slow down the decomposition of cellulose to some extent in terms of the thermal degradation event of cellulose (Viscusi et al. 2021; Fisher et al. 2002). MWE-DES treated samples and alkali-treated sample both demonstrated a small amount of water evaporation as compared to the raw hemp sample because of the reduction of OH groups. It is evident from the TG thermograms (Figure 7b) that hemicellulose has lower thermal stability as the degradation of hemicellulose observed at 256°C for raw hemp and sample B (on the other side, the absence of the hemicellulose peak in the derivative TG curve indicates the removal of hemicellulose from the samples C, D, and E), which would facilitate the second stage thermal degradation (lignin thermal degradation) for a longer period (Viscusi, Barra, and Gorrasi 2020) which might help lignin to have higher thermal stability. At the third degradation stage, the peak at 300°C - 375°C represents the cellulose degradation, and 375°C - 400°C could be assigned to the oxidative degradation of cellulose. However, DES-treated fibers, especially the samples C, D, and E had higher thermal stability (shifting the degradation peak toward higher temperature in TG curves for all the degummed samples) as the degradation starts from 320°C to 375°C. The main weight loss of raw hemp fibers was recorded at about 225°C - 390°C while the treated hemp fibers showed their main weight loss in the 250°C - 390°C region as can be seen in Figure 7a. The onset points of DES-treated hemp fibers including alkali-treated fibers moved from 287°C to 310°C - 318°C, as similarly found by Fan et al. (Fan 2010) (Table 2). The thermal degradation of cellulosic components usually occurs at higher temperatures compared to that of non-cellulosic components. The increased onset temperatures in MWE-DES treated fibers indicated the removal of non-cellulosic components. The removal of non-cellulosic components helps cellulose to be rearranged in such a way to increase crystallinity with the densely packed rearrangement of glucose molecules, and it renders high thermal stability to cellulose since densely packed crystalline rearrangements slow down thermal decomposition.
Table 2. Summary of TG and DSC data of both raw and degummed hemp fibers (TD, Tmax, and Tg refer to thermal degradation, peak maximum, and glass transition temperatures in the TG and DSC curves)
Sample
ID
|
TGA
|
DSC
|
Weight Loss (%)
|
Onset Temp. (°C)
|
Tmax
(°C)
|
Tg
(°C)
|
Enthalpy (J/g)
|
1st TD
(50-150°C)
|
2nd TD
(220-300°C)
|
3rd TD
(300-400°C)
|
At
600°C
|
Raw
A
B
C
D
E
|
5.246
3.106
4.129
2.842
2.875
1.811
|
17.21
9.698
11.22
8.500
7.497
8.256
|
36.65
55.83
48.37
60.23
60.17
63.00
|
65.28
77.28
74.62
82.22
78.88
80.11
|
287
311
310
316
322
318
|
324
344
340
348
353
349
|
250
302
305
400
326
328
|
272.09
111.43
175.25
139.13
215.34
208.32
|
Bond water (OH groups) to cellulose and hemicellulose have higher thermal stability (Kabir et al. 2013), thus water evaporation occurred in the raw hemp in the range of 50°C - 150°C as seen in the DSC curves (Figure 7e). On the other hand, the range in MWE-DES treated fibers was 50°C - 140°C. Water evaporation for the alkali-treated fibers occurred at 50°C - 135°C (Figure 7a). It can be assumed that there was therefore a reduction of hydroxyl groups in the DES-treated and alkali-treated samples. The removal of hemicellulose and improved hydrophobicity of the treated fibers results in higher thermal stability for cellulosic compounds. According to TG data (Table 2), after 350°C in the first heating run, more than 75% of the hemp samples was decomposed. However, in the DSC Figure 7e, the glass transition of dry cellulose occurs at 200-250°C depending on its molecular weight, crystallinity, and structure (Kubát and Pattyranie 1967). The Tg of 70% crystalline cellulose containing 2% water (playing the role of a plasticizer) drops to 160°C (Szcześniak, Rachocki, and Tritt-Goc 2008). The Tg of the raw hemp sample is clearly observed right around 250°C, but this transition is absent in all other alkali and MWE-DES treated samples, obviously due to the plasticizing effect of some remnants after heating to 150°C (Table 2) since below Tg, all materials become rigid, one may conclude that the rigidity of alkali and MWE-DES treated samples is practically absent and these fibers might exhibit a certain degree of elasticity. An exothermic peak was observed at around 370°C for the raw hemp fiber in the DSC curve (Figure 7e). The exothermic peak for the MWE-DES treated fibers was shifted towards a higher temperature and sample D demonstrated an exothermic peak around 450°C, which confirmed the higher thermal stability of the cellulose in the treated fibers. Hemicellulose decomposition occurred between 50°C - 320°C with an exothermic peak at 287°C for raw hemp fibers. The disappearance of this exothermic peak in the treated fibers validated the removal of hemicellulose. MWE-DES treated and alkali-treated fibers showed a small endothermic peak at around 376°C - 390°C (Figure 7e), thus, it can be concluded that the removal of hemicellulose resulted in higher thermal stability of the treated fibers as the broad exothermic peak of cellulose in the raw hemp fibers at about 372°C moved toward 435°C for the alkali-treated fibers, while MWE-DES treated fibers had a peak between 450°C - 473°C. Lignin in natural fibers normally has a decomposition range between 250°C -450°C (Gargol et al. 2021), which is followed by the raw hemp fibers in this study. The aromatic structure of the lignin (aromatic hydrocarbons, hydroxy phenolics, guaiacyl, and syringyl compounds) enables lignin to have a broader range of degradation temperatures, which is supported by the MWE-DES treated fibers and alkalized fibers. However, DES treated fibers and alkalized fibers showed degradation of residual lignin up to 520°C (Figure 7e), indicating higher thermal stability of the treated fibers. In summary, it can be concluded that purification and less variation occurred with MWE-DES treated hemp fibers, which resulted in a higher thermal stability of the treated fibers.
UV shielding performance of hemp fibers. It is evident that all the hemp samples had excellent UV protection properties (all the hemp fiber samples have the transmittance % at a range of 0% - 0.06%) (Figure 8a). Among all the hemp samples, sample C had the lowest UPF value, even though other MWE-DES treated samples also had excellent UPF values (Figure 7b). It is obvious that there is a tendency to have a lower value of UPF after treating with NaOH since aggressive alkali treatment removes most of the lignin content from the fiber surface. The aromatic structure of lignin aids to block the UV radiation in the raw hemp fibers. The phenolic hydroxyl, carboxyl, and carbonyl groups in the lignin structure are responsible for generating heat when the fiber is subjected to the photon energy of UV light. However, these phenolic groups, especially hydroxyl groups in the lignin could absorb the generated heat and also are able to quench the active radicals through the mechanics of electron transfer (Dean et al. 2013). The presence of lignin in the fiber structure facilitates improved UV blocking performance. For instance, MWE-DES treated fibers at 1:10 fiber-DES ratio (sample B) had a UPF value of about 118 and after NaOH treatment (sample C), the UPF value was reduced to 102.43. A similar trend was observed for samples D and E (Figure 8b).
MWE-DES treatment at 1:20 fiber-DES ratio led to increased UPF values (UPF=183.67) compared with the 1:10 ratio data (UPF=127.47). There are few published studies about the contribution of DES solvent in the degummed lignocellulosic materials in terms of UV protection. However, DES made of glycerol and choline chloride with alpha hydroxylate anions exhibited crosslinking and plasticizing ability in the polysaccharide matrix (Zdanowicz, Jędrzejewski, and Pilawka 2019). The improved UV blocking performance of MWE-DES treated samples could thus be explained by the involvement of DES solvent in the fiber structure. This phenomenon probably happens due to the interference of the DES solvent as a crosslinking and plasticizing agent, forming the DES-fiber complex when it comes to contact with the fiber structure. In addition, the presence of low lignin content in the degummed fibers accelerated the UV shielding performance in the MWE-DES treated fibers.