3.1. Basic Properties of Isolated Lignin
Black liquor used in this study was obtained by the kraft pulping process of the pulp and paper industry. The characteristics of black liquor and isolated lignin are shown in
Table 1. The yield of isolated lignin obtained was 35.88%, which is lower than the one-stage lignin precipitation by Hermiati et al. [
38], namely 45.76%. The yield of isolated lignin is affected by the lignin content in black liquor from the kraft pulping process and the precipitation process during isolation. A complete precipitation reaction will produce a higher lignin yield.
The moisture content of isolated lignin (L-isolated) was 5.07%. The moisture content of lignin was relatively low compared to the published work [
46], which was 8.05%. Ash content is the residual content of combustion of inorganic materials found in lignin. The low ash content of 0.31% indicated that the three-stage decantation process to obtain the lignin in this study produced fewer impurities compared to other studies that used a one-stage decantation process and obtained an ash content of 8.25–19.19% [
38,
47].
Table 1.
Characteristics of lignin isolated from black liquor.
Table 1.
Characteristics of lignin isolated from black liquor.
Analysis Parameters | Value | References |
---|
Moisture Content of Black Liquor (%) | 27.81 ± 1.11 | 10.00 [48] |
Solid Content of Black Liquor (%) | 76.79 ± 0.64 | 65.00–85.00 [49,50] |
pH of Black Liquor | 12.14 ± 0.02 | 12.00–13.00 [51,52] |
Yield of Lignin (%) | 35.88 ± 1.81 | 45.76 [38] |
Moisture Content of Lignin (%) | 5.07 ± 0.71 | 8.05 [46] |
Ash Content of Lignin (%) | 0.31 ± 0.19 | 8.25–19.19 [38,47] |
Acid-insoluble Lignin (AIL) (%) | 82.54 ± 0.96 | 53.08 [38] |
Acid-soluble Lignin (ASL) (%) | 12.77 ± 0.67 | 7.26 [38] |
Purity Levels of Lignin (%) | 95.32 ± 0.61 | 60.34 [38] |
The determination of lignin purity is based on AIL and ASL. As presented in
Table 1, the AIL content was 82.54% and ASL content was 12.77%. The high AIL and ASL values determined can be attributed to the three-stage decantation process during isolation. The decantation process involved washing with distilled water to remove impurities and other substances, thus increasing the total lignin generated from the isolation process. The level of purity of the isolated lignin was relatively high, namely 95.32%. This result was higher than the published works that used a similar source of black liquor and acid precipitation methods [
38], calculating around 60.34% of lignin purity. The difference in the level of lignin purity can be attributed to the differences in mineral content in lignin isolates.
The yield of dissolved fractionation is shown in
Table 2. The yield of lignin fractionated with MeOH (L-MeOH) was 71.25% and lignin fractionated with acetone (L-Ac) was 70.57%. The results are in agreement with the published work [
53], where the highest solubility of fractionated technical lignin was observed in methanol and acetone solvents of 70–80%. The solubility of this lignin in organic solvents depends on the type of lignin, aliphatic hydroxyl number, and molecular weight [
54].
Total phenolic hydroxyl groups of fractionated lignin were determined using the UV method. The UV method is a fast and easy way of measuring phenolic hydroxyl groups by giving the total number of hydroxyl (OH) groups without any structural specifications [
43]. The total phenolic OH group of L-Standard, L-Isolated, and fractionated lignin are shown in
Table 2. The highest total OH group was found in the L-Standard (8.109%), and the lowest was obtained in the L-MeOH (7.399%). The isolated lignin contains high phenolic hydroxyl groups and a condensed structure due to the kraft pulping process [
39]. The OH group value was further used in the formulation of lignin-based Bio-PU resin with an NCO/OH mole ratio of 0.3.
3.3. Properties of Bio-Polyurethane Resin
In this study, the Bio-PU resin was designed as an impregnation material to improve the characteristics of ramie fiber. Therefore, the Bio-PU resin must have a low viscosity to be impregnated into the ramie fiber [
59]. The viscosity and cohesion strength of Bio-PU resin was measured at 25 °C (
Figure 7). The lowest viscosity value of 77.02 mPa·s was determined for Bio-PU L-Isolated, followed by Bio-PU L-MeOH and Bio-PU L-Ac, which had values of 100.71 and 223.58 mPa·s, respectively (
Figure 7). The cohesion strength followed the results of viscosity, where the cohesion strength increased with greater viscosity of Bio-PU resin. Lower viscosity can increase the ability of Bio-PU resin to be impregnated and absorbed by ramie fiber. Viscosity is related to hydrogen bonds between isocyanate and polyol molecules (NCO/OH molar ratio). An increase in the NCO/OH ratio will result in more urethane bonds forming hard segments in Bio-PU and produce high viscosity [
59,
60].
The functional groups of Bio-PU resin are shown in
Figure 8. A typical strong peak at the wavenumber of 2263 cm
−1 indicated an isocyanate group (N=C=O) of pMDI. In the Bio-PU resin (L-isolated, L-MeOH, and L-Ac), no absorption of the N=C=O group was found, indicating that the N=C=O group reacted completely with the OH group of lignin during the preparation of PU resin. It has been reported that the isocyanate group derived from pMDI will react with the hydroxyl group of the polyol [
61]. The reaction between fractionated lignin and pMDI in the formation of Bio-PU resin produces a specific functional group, namely the urethane group (R-NH-C=O-R) [
62].
Bio-PU resin showed broad peaks at wavenumbers 3400–3200 cm
−1, indicating the formation of N-H groups in the aliphatic primary amine structure. In addition, this broad peak also overlapped with the -OH groups from the lignin sample and aqueous NaOH for lignin dissolution. The urethane bond formed from the lignin phenolic OH group and isocyanate (NCO group) from pMDI [
63]. Another characteristic band in the formation of the Bio-PU resin at 1710–1685 cm
−1 revealed the formation of C=O stretching from the urethane bond (R-NH-C=O-R), and the wavenumber of 1250–1020 cm
−1 showed C-N stretching. It is indicated that the Bio-PU resin was successfully prepared from fractionated lignin (L-MeOH and L-Ac) as an alternative polyol.
Thermal properties of the Bio-PU resin were determined based on the results of the DSC analysis shown in
Figure 9. The DSC thermograms show three-step endothermic reactions in the Bio-PU resin. The
Tp1 of Bio-PU (L-Isolated, L-MeOH, and L-Ac) ranged 56–63 °C, the
Tp2 ranged 103–109 °C, and the
Tp3 or
Tg ranged 137–140 °C. The
Tp1 corresponded to the initial moisture evaporation in Bio-PU resin and the evaporation peak at
Tp2. The
Tg value represents the polymer transition at a specific temperature [
64]. The
Tg of Bio-PU L-MeOH was 140 °C; meanwhile, the
Tg of Bio-PU L-isolated and L-Ac was 137 °C. High
Tg produces hard segments in Bio-PU due to the formation of urethane. Inter-urethane hydrogen bonds play an essential role in the stability of Bio-PU resin. The structure of the hard segment is more influential in thermal degradation than the soft segment [
65].
Thermogravimetric analysis was performed to evaluate the thermal degradation of lignin-based Bio-PU resin. TGA-DTG thermogram is shown in
Figure 10. The initial weight loss occurred at a temperature of 60–110 °C, caused by water evaporation and evaporation of chemicals in unreacted lignin during the polymerization process [
44,
61]. Lignin-based Bio-PU lost 10% of the initial weight with a derivative weight loss of 1.5%/°C at a temperature of 80 °C. There was significant weight loss at temperatures above 160 °C up to 300 °C. The degradation peak occurred at a temperature of 250 °C, which indicated the initial decomposition of the urethane bond with a derivative weight loss of 2.0%/°C. Further degradation occurred at ~350 °C, which is oxidative degradation by the urethane group with a derivative weight loss of 0.75%/°C [
66]. At a temperature of 450–600 °C, both the aromatic ring decomposition of lignin and the primary oxidative decomposition of lignin occurred [
66,
67].
The weight residue after heating at 750 °C of each Bio-PU resin (L-Isolated, L-MeOH, and L-Ac) was in the range of 52–61% (
Figure 10). Bio-PU L-Ac had the highest weight residue of 61.55 and bio-PU L-MeOH had the lowest weight residue of 52.76%. The bio-PU L-Ac showed lower thermal degradation compared to Bio-PU from L-Isolated and L-MeOH due to crosslinking the lignin macromonomers. The weight loss of Bio-PU from each fractionation of lignin was 25% at 300–400 °C during degradation of the urethane group. The weight loss of the fractionated lignin-based Bio-PU resin in this study was not more than 50%. Meanwhile, rigid or semi-rigid polyurethane resin has a percentage of weight loss at 340–525 °C of 62–75% [
68]. This study showed that lignin fractionation using Acetone (Ac) could produce lignin-based Bio-PU resin with lower thermal degradation than isolated lignin.
3.4. Properties of Ramie Fiber Impregnated with Bio-PU Resin
The impregnation of ramie fiber with lignin-based Bio-PU was aimed at improving ramie fiber’s thermal stability as a functional material. The weight gain of ramie fiber after impregnation was calculated as a way to investigate the influence of the impregnation process on ramie fiber. The weight gain of ramie fiber after impregnation is shown in
Table 5. In general, the weight gain increased with longer impregnation time. Impregnation for 90 min resulted in more significant weight gain for each type of lignin-based Bio-PU used, followed by impregnation times of 60 and 30 min. Ramie impregnated with Bio-PU derived from L-isolated had the highest weight gain compared to Bio-PU L-MeOH and L-Ac. This could be due to the viscosity of Bio-PU L-isolated being lower than that of L-MeOH and L-Ac (
Figure 6). The lower viscosity of Bio-PU will make it easier for the solution to be impregnated into the fiber. Ramie fiber has a fiber diameter of 40–60 µm [
69]. This could facilitate the absorption of the Bio-PU solution and increase weight gain after Bio-PU impregnation into the ramie fiber.
The FTIR spectra of the impregnated ramie fiber investigated in this study are shown in
Figure 11. In non-impregnated ramie fiber, there were six prominent peaks. Firstly, a wavenumber of 3330 cm
−1 represented OH groups from cellulose, hemicellulose, and lignin that arrange the ramie fiber [
70]. A wavenumber of 2897 cm
−1 described C-H vibration carbohydrates [
44]. A wavenumber of 1740 cm
−1 indicated the presence of C=O vibration in the ester, carboxylate groups, and hemicellulose of ramie fiber [
70,
71,
72]. Wavenumbers of 1607 cm
−1, 1424 cm
−1, and 1024 cm
−1 showed the C-H stretching of carbohydrates, C-O stretching of cellulose, C-O vibrations, and O-H vibrations of cellulose, respectively [
44,
72].
The functional groups of ramie fiber after impregnation were also observed using FTIR. A peak similar to the original ramie at a wavenumber of 3330 cm
−1 indicated O-H vibration. The peak of ramie fiber at 2897 cm
−1 shifted to 2900 cm
−1, which showed N-H bending and -CH
2 stretching of bio-PU in impregnated ramie fiber with Bio-PU resins. The wavenumber of 1740 cm
−1 was not found in ramie fiber after impregnation. It can be assumed that there was a reaction between ramie fiber and Bio-PU resins, and the formation of a peak at 1600 cm
−1 indicates the formation of C=O urethane from Bio-PU [
44]. The wavenumber of 1515–1310 cm
−1 represented the C-N stretching group from primary and secondary amides of bio-PU resin in impregnated ramie fiber. Furthermore, the wavenumbers 1200 cm
−1 and 1050 cm
−1 showed C=O vibrations of Bio-PU and C-O-C ether linkages, respectively [
19], which demonstrates that lignin-based Bio-PU successfully impregnated the ramie fiber.
Micro Confocal Raman analysis was performed to investigate the formation of urethane linkages in ramie fiber after impregnation with lignin-based Bio-PU resin. As depicted in
Figure 12a, the original ramie fiber had a typical broad peak at 1350–1400 cm
−1, which indicated the presence of cellulosic materials. After impregnation with Bio-PU resin, the Raman spectra of ramie fiber remarkably changed due to the presence of a strong peak of urethane linkages at 1614 cm
−1. This result confirmed that Bio-PU resin impregnated and covered the ramie fiber. The images of ramie fiber before and after impregnation are presented in
Figure 12b,c. The results showed that impregnation of Bio-PU into ramie fiber altered the color of ramie fiber to a light brown. Based on the result of FTIR and Micro Confocal Raman Hyperspectral Spectroscopies, the possible reaction of ramie fiber with lignin-based Bio-PU resin is illustrated in
Figure 13.
DSC analysis detected two thermal events on ramie fiber before and after impregnation, namely an endothermic reaction and an exothermic reaction (
Figure 14). The endothermic reaction is a heat absorption reaction by the ramie fiber, while the exothermic reaction is a heat release reaction by the ramie fiber [
73]. The peak of the endothermic reaction (
Tp1) in the original ramie fiber occurred at a temperature of 46 °C, which is the process of evaporation of water. In general, impregnation resulted in a slightly higher
Tp1 value. Ramie fiber impregnated with L-Isolated and L-Ac had a lower
Tp1 value than that of ramie impregnated with L-MeOH. The difference in
Tp1 value between natural ramie and impregnated ramie is influenced by differences in fiber moisture content. The moisture content of natural ramie is 5.15%, and the moisture content of impregnated ramie is between 6 and 10%. No significant influence of increasing the impregnation time on the
Tp1 value of ramie impregnated with lignin-based Bio-PU resin was observed.
The exothermic reaction in ramie fiber indicates the formation of solid residues due to hemicellulose decomposition and lignin degradation [
74]. The peak of the exothermic reaction (
Tp2) generally occurred after the heating temperature reached 300 °C. In non-treated ramie, the
Tp2 value was around 335 °C. Impregnation resulted in a slightly higher
Tp2 value. Ramie fiber impregnated with L-Isolated and L-Ac had a lower
Tp2 value than that of ramie impregnated with L-MeOH. The increase in
Tp2 value in impregnated ramie compared to original ramie indicates that ramie has more heat-resistant properties after impregnation. Ramie fiber impregnated at 90 min for all types of Bio-PU generally had an increasing
Tp2 value compared to original ramie.
Thermal degradation of ramie fiber before and after impregnation with lignin-based Bio-PU resin was monitored using TGA-DTG analysis. As displayed in
Figure 15, the initial stage of the degradation process occurred at a temperature of 25–100 °C, which is attributed to the release of water contained in ramie fiber and the degradation of low-molecular-weight components. At this stage, the ramie fiber lost 5% weight with a derivative weight loss of around 1.0–2.0%/°C. Significant weight loss was observed after heating at 250–450 °C, which caused the ramie fiber to experience an extreme weight loss of around 75% at a derivative weight loss of 13%/°C. The results showed that the impregnation of ramie fiber with Bio-PU remarkably enhanced the thermal stability by reducing the weight loss value to 50% with a lower derivative weight loss of 6.0–7.0%/°C. At a temperature of 200–400 °C, a complex reaction occurs from the degradation of the constituent components of lignocellulose fibers, such as cellulose, hemicellulose, and lignin [
75]. Hemicellulose begins to degrade at a temperature of 200–290 °C, at a temperature of 240–350 °C, cellulose degradation occurs, and lignin degradation begins at a temperature of 280–500 °C. Several related studies also stated that the degradation of lignin, cellulose, and hemicellulose in ramie fiber occurred at a temperature of 300–400 °C, whereas at a temperature of 320–400 °C, the cellulose glycosidic bond was broken, and the decomposition of the lignin structure by a high molecular weight followed at a temperature of 360–750 °C [
44,
72,
76].
The impregnated ramie fiber lost 75% of its weight at a temperature of 550–660 °C, while the non-impregnated ramie fiber experienced a similar weight loss at 469 °C. This can be attributed to the occurrence of lignin degradation at that temperature. Maximum decomposition occurred at a temperature of 300–400 °C, and has also been noted that lignin degradation in ramie fiber is the limit for determining thermal stability [
76]. This study showed that lignin-based Bio-PU can increase the thermal stability of ramie fiber so that it is better than natural ramie. The impregnation time affects the thermal degradation of the ramie fiber remarkably. The longer the impregnation time, the lower the thermal degradation of the impregnated fiber. The weight residue of the original ramie fiber was 14.3%. Meanwhile, impregnated ramie has a higher residue weight, at 18.4–21.7%. An impregnation time of 90 min produced lower thermal degradation of ramie fiber with greater weight residue than those at 30 and 60 min. The lowest thermal degradation was obtained in both ramie fiber impregnated with L-MeOH and L-Ac for 90 min with an average weight residue of around 21.7%.
Mechanical properties are essential characteristics for ramie fiber as a textile and composites material. A graphical representation of the stress-–strain curves of ramie fiber impregnated with different lignin-based Bio-PU resins for different impregnation times is presented in
Figure 16. The results showed that increasing the impregnation time generally enhanced the maximum stress. Ramie impregnated with Bio-PU L-Ac showed the highest maximum stress after 90 min of impregnation, followed by Bio-PU L-isolated and L-MeOH. The lowest stress was obtained in the original ramie fiber. This indicated that the impregnation of Bio-PU resin into ramie fiber could enhance the mechanical properties of the fiber. The fiber deformation can be divided into the following three stages: (i) A linear part related to the deformation of the fiber cell wall; (ii) a non-linear part interpreted as a thicker cell wall deformation (S2) and known as elasto-visco-plastic; and (iii) a final linear section, which is the elastic response aligned with the tensile strain [
33].
The values obtained for the modulus of elasticity (MOE) and tensile strength of non-impregnated and impregnated ramie fibers are presented in
Table 6. In general, the mechanical properties of impregnated ramie fibers increased compared to the original ramie. The tensile strength of the original ramie was 397.72 MPa with an MOE value of 10.45 GPa. The tensile strength of impregnated ramie was in the range of 397.72–648.48 MPa with MOE values ranging from 13.06 to 31.10 GPa. Increasing the impregnation time generally increased the elastic modulus and tensile strength of ramie fiber. Ramie fiber impregnated with Bio-PU L-isolated for 90 min resulted in the highest MOE value of 31.10 GPa. Meanwhile, ramie impregnated with Bio-PU L-Ac for 90 min had the highest tensile strength of around 577.61 MPa. Several factors besides chemical modification affected the mechanical properties of ramie fiber, namely parameters related to relative environmental humidity, fiber length, fiber microstructure, moisture content and drying, and fiber diameter [
77,
78].