Chemical composition of lignin
The chemical composition of lignin is presented in Table 2. Generally, lignin is composed of 60–65% carbon, 5–7% hydrogen, and 30–35% oxygen34. The lignin used in this study was composed of 61.2% carbon, 5.8% hydrogen, and 30.7% oxygen and contained no nitrogen. The ash content of the lignin was 2.3wt%. The inorganic compounds were investigated, and a small amount of sulfur (1.7 w%) was observed as a result of the kraft pulping process.
Functional groups and hydroxyl groups of lignin
The FT-IR spectra of lignin are shown in Fig. 4a. An O–H stretching vibration from the hydroxyl groups (aromatic and aliphatic chains) of lignin was observed at 3351 cm−1. A stretching vibration of C–H from the methyl and methylene groups was observed at 2935 cm−1. The aromatic ring vibration spectra observed at 1596, 1510, and 1425 cm−1 correspond to the phenylpropane monomers35. Also, specific bands attributed to the G unit moieties were observed at 1265, 855, and 815 cm−1, indicating the presence of softwood lignin36. The in plane deformation vibration at 1456 cm−1 indicates C-H superimposed on the lignin aromatic skeleton, and the stretching vibration at 1700 cm−1 is attributed to the C = O of the carboxyl group37.
FTIR spectra of (a) kraft lignin, (b) LPU, (c) lpu and (d) CPU.
The quantitative data for the hydroxyl groups in lignin gathered through the liquid 31P NMR analysis are presented in Table 2. The spectra of 31P NMR is presented in Fig. 5. The predominant presence of G units in the lignin indicates that it was from softwood, which aligns with the FT-IR data. The concentration of free hydroxyl groups in KL is 4.9 mmol/g, encompassing three types of functional groups: 62% of phenolic, 33% of aliphatic, and 5% of carboxylic acid. It is the hydroxyl groups in lignin that are expected to function as a bio-based polyol in the PU matrix38.
31P NMR spectra of soft wood kraft lignin provided by Domtar.
Hydroxyl numbers, molecular weight distributions, and viscosities of polyols
As presented in Table 3, the hydroxyl number of lignin was calculated to be 328 mg KOH g−1. The average molecular weight of the lignin samples was determined to be 4302 g/mol, consistent with typical observations in softwood KL39. The number-average molecular weight of lignin was 1483 g/mol, and the polydispersity index was 2.9, indicating structural heterogeneity. Compared with PEG 400, the KL had a significantly larger molecular weight and less uniformity, contributing to lower reactivity in its powdered form. Similarly, the castor oil exhibited a lower hydroxyl number than the PEG 400, which was accompanied by a viscosity that is six times higher and a molecular weight that is two times larger than that of PEG 400. These results suggest that biomass-derived polyols such as KL and castor oil exhibit lower reactivity with isocyanate than a conventional polyol.
Characterization of lignin-incorporated PU foams
Chemical reaction within the PU foam matrix
In Fig. 4b,c, The FTIR spectra of the LPU and lpu foams were analyzed to investigate the reactions that produced them. Urethane linkages formed through the polyaddition reaction between hydroxyl groups and isocyanate groups, resulting in the presence of an N–H bond (3300 cm−1), C–H stretching of the aliphatic chains (2870 cm−1), C=O stretching (1710 cm−1), C–N (1218 cm−1), and C–O bond (1067 cm−1) peaks in all spectra. Carbon–carbon stretching vibrations in the aromatic ring were observed at 1579 cm−1 and 1513 cm−1, and C–H deformation of the aromatic groups was present in the regions of 815 and 763 cm−140,41. The N–H bond at 3300 cm−1, characteristic of PU, became broader as the lignin content increased, suggesting that the isocyanate groups successfully reacted with the hydroxyl groups in both the polyol and lignin, and the O–H bond at 3300–3400 cm−1 was merged with the N–H bond42. The residual isocyanate groups (N=C=O) show absorption at 2275 cm−1. In both the LPU and lpu foams, N=C=O increased as the lignin content increased43. The gradually widening of the N=C=O bond indicates decreased reactivity for urethane formation in the foams due to the lower reactivity of lignin compared with the conventional polyol26. In other words, a broader N=C=O is exhibited with an increase in lignin content due to the lower crosslinking between lignin and isocyanate. Furthermore, compared with LPU, lpu has a higher NCO peak, which is expected to exist in the foams due to the excess amount of isocyanate left after the reaction. The anticipated structure of PU foam following the bond formation between lignin and isocyanate was illustrated in Fig. 6.
(a) Formation structure of lignin-incorporated PU foam; the traditional polyol, which takes part in soft segment int the PU foam matrix, was replaced by lignin known as a hard segment due to its aromatic ring and complex structure. Lignin, with its varying molecular weight distribution, produced polyurethane linkage (bond formation) with isocyanate while being distributed in the matrix, making PU foam matrix more heterogeneous and rigid to some extent, (b) Formation structure of castor oil-incorporated PU foam; the traditional polyol was substituted by castor oil, referred to as soft segment. Owing to its long chain structure and higher molecular weight distribution compared to the traditional polyol, the resulting foams are expected to exhibit less rigid structure.
Mechanical properties and morphology
The addition of lignin as an alternative to the conventional polyol reduced the compressive strength compared with both the PU0 (326 kPa) and pu0 (441 kPa) controls, as demonstrated in Table 4. This decline occurred because the conventional polyol underwent a more rapid chemical reaction with isocyanate, creating a homogeneous and greater crosslinking structure than the lignin-based polyol38. At the same time, KL disturbed chemical reaction between polyol and isocyanate, resulting in reduced the number of urethane bonds, which is weaker PU foam matrixes44. However, the enhancement in compressive strength from LPU5 (147 kPa) to LPU20 (207 kPa) can be attributed to various factors. First, lignin possesses an aromatic ring structure that contributes to a higher crosslinking density with the NCO groups of the isocyanate and increased chain stiffness45. Second, as the lignin content in the bio-based polyol increased, the reactivity with isocyanate is assumed to decrease because it is less uniform and more viscous than a conventional polyol46. Consequently, the remaining unreacted isocyanate reacted with another hydrogen bond while forming allophanate groups, leading to enhanced crosslinking density26. Third, the cell size decreased slightly as the lignin content increased due to the slow polyaddition reaction rate, leading to the release of more CO2 gas during formation of the foam structure, which affected the shape of the cells and cell walls, including the spherical struts and strut joints shown in the SEM images in Fig. 7a,b. Since lignin affects the porosity of the foam as well as distribution, the pore size of the resulting foams was also observed and presented in Table 4. At a 10 wt% lignin loading level, the pore size of both foams are the smallest and most homogeneous. This reduction in pore size has been reported in several studies of lignin-incorporated PU foam32.
Bio-based polyurethane foam structure obtained by SEM images. (a) LPU foams with the addition ~ 20 wt% kraft lignin (index 1.01), (b) lpu foams with the addition of ~ 20 wt% of kraft lignin (index of 1.3), (c) CPU foams with the addition of ~ 100 wt% of castor oil (index of 1.01).
The index ratios of LPU and lpu were 1.01 and 1.3, respectively. Thus, lpu exhibited overall higher compressive strength due to its higher isocyanate content, which is the hard segment in a PU matrix47. The compressive strengths of the lpu foams were 164 kPa (lpu5), 163 kPa (lpu10), 167 kPa (lpu15), and 147 kPa (lpu20), whereas that of the control pu0 foam was 441 kPa. Among the lpu samples, the compressive strength did not change significantly as the lignin content increased28. However, beyond a certain amount of lignin, as observed in lpu20, the foaming process was affected48, leading to dimensional destabilization of the PU foam structure, as shown in Fig. 7b. This destabilization occurred because a significant amount of unreacted isocyanate remained, and an excess of the solid powder form of lignin negatively affected the foam system. For these reasons, the cell shape became irregular and pore size became less uniform as the lignin content in the LPU and lpu foams increased from 15 wt% of lignin loading level. The self-association of the lignin polyol is attributed to the adherence of particles to the cell wall, which ultimately results in the distortion or rupture of the cell shape49. The apparent density of LPU and lpu foams containing 5 wt% to 20 wt% lignin, did not differ significantly. However, compared with the control foam (79.1 kg/m3 and 81.1 kg/m3), the apparent density of LPU and lpu foams decreased along with the compressive strength because of the lower crosslinking density, as explained above.
Thermal behavior
Figure 8 depicts the TG and differential thermogravimetric (DTG) curves for both the LPU(a) and lpu foams(b). The char yield increased gradually from 10.2 wt% for PU0 to 15.8 wt% for LPU20 and from 12.9 wt% for pu0 to 17.5 wt% for lpu20. The char yield suggests that increasing the amount of lignin in both the LPU and lpu foams produced higher thermal stability. This behavior was attributed to the thermostability of lignin50. In detail, the foams exhibited stability below 150 °C, followed by three major thermal decomposition patterns. The initial event at 150–350 °C was attributed to the removal of the hard segment and the cleavage of urethane bonds25. The second stage, from 350 to 450 °C, was associated with weight loss from the soft segment in the foams. The third phase occurred at 450–600 °C and indicated the breakdown of the lignin aromatic ring network. Due to the intrinsic structure of lignin, in the third stage, the thermal degradation of lignin-containing foams is generally less than that of control foams40. All foam samples underwent degradation within the temperature range of 150–600 °C, with the peak degradation occurring in the range of 320–350 °C. In this range, the control foam had the minimum degradation rate because it had the highest crosslinking of urethane bonds, as shown by the FTIR spectra and compression strength51. At the same time, an increase in lignin content corresponded to a decrease in the trend of the maximum degradation rate because the intrinsic and aromatic structure of lignin enhanced the stiffness of the polymer chains, leading to a low degradation rate52.
TG and DTG curves of (a) LPU, (b) lpu and (c) CPU foams.
Characterization of castor oil-incorporated PU foams
Chemical reaction within the PU foam matrix
Figure 4d shows the infrared spectra results for PU0, CPU50, and CPU100. Similar to the LPU and lpu foams, a peak in the range of 3333–3294 cm−1, corresponding to the N–H bond, was observed, indicating the successful urethane reaction in the foam matrix. The FT-IR peaks at 2925 cm−1 and 2854 cm−1 confirmed the presence of the C–H stretching vibration53. As the castor oil content increased, the C-H spectrum widened. Specifically, the CPU foams containing 0 wt% to 100 wt% of castor oil underwent successful reactions that resulted in the absence of remaining isocyanate (N=C=O) and stretching vibrations of the O–H groups. In other words, all of the isocyanate reacted with all the polyols, leading to urethane linkages54. Moreover, as the amount of castor oil increased, a broader spectrum of stretching vibrations indicating C=O bonds was observed in the region of 1720 cm−1 due to the structure of the castor oil and not only from the urethane linkages55. The stretching vibration at 1216 cm−1 can be associated with C–N bonds in urethane linkages53. Following section will be explained about possible bond formation between castor oil and isocyanate as described in Fig. 6.
Mechanical properties and morphology
In the CPU foams, the compressive strength decreased gradually as the amount of castor oil increased. As shown in Table 4, CPU10 exhibited a compressive strength of 284 kPa, and CPU100 showed a reduced strength of 23 kPa. This decrease with higher castor oil content is due to lower crosslinking and a reduced number of hard segments because castor oil has a long chain structure, as demonstrated in Figs. 1 and 8. Therefore, it acts as a soft segment, leading to the formation of semi-rigid foams45. In other words, the polymer chains get less tightly packed together as the amount of castor oil increases, and cell morphology affects the mechanical properties. The increase in castor oil content caused larger cell sizes and void fractions, consistent with the SEM images in Fig. 7c. Notably, when the castor oil content reached 50 wt%, the foam exhibited an open cell structure, which also indicates that the soft segments increase along with the higher castor oil content. The transition from a closed-cell to an open-cell structure occurred at a castor oil content of 50 wt%. At this content, the pore size decreased to 669 µm compared to 1313–1353 µm observed at 10–40 wt% content, as shown in Table 4. However, the pore size increased again from 669 µm (CPU50), reaching 1416 µm (CPU100). These changes in cell structure and pore size were accompanied by a decrease in compressive strength33. Although the compressive strength of the CPU foams decreased as the castor oil content increased, the apparent density within the CPU foams was similar to that of the control56.
Thermal stability
Figure 8c shows the TG and DTG curves for the CPU foams. As the amount of castor oil in the foams increased, lower thermal stability and a reduced char yield were observed. The char yield decreased gradually from 10.6 wt% with CPU10 to 4.2 wt% for CPU100 (Table 4). More specifically, three distinct stages were observed during the thermal decomposition of the CPU foams. The beginning weight loss stage from 150 °C was caused by the depolymerization of some hard segments within the urethane bonds. At 150–350 °C, foams with higher castor oil content had a lower degradation temperature and faster degradation rate because of the lower hard segment content57. The second weight loss stage at 350–450 °C is mainly attributed to the chain scission of the polyols, which is the degradation of the castor oil fatty acid chains58. In this stage, the rate of weight loss in CPU foams increased with the addition of castor oil, which could be explained by the increased proportion of soft segments as the castor oil content increased from 10 to 100%. After the decomposition of the urethane bonds and the polyols mentioned above, the final stage is mainly assigned to further thermo-oxidation.
