Figure 1a and Scheme S1 show schematics of the amine-functionalization reaction of KL using DETA and formaldehyde. In the DETA/formaldehyde solution, formaldehyde reacted with the primary or secondary amine groups of DETA to form immonium ions, which were generated in the presence of a high electron density of carbon atoms. Subsequently, immonium ions reacted with phenolic moieties in lignin to introduce amine groups only at the ortho and/or para positions of the phenolic hydroxyl group. Thus, the H and/or G units with free phenolic hydroxyl groups were converted to C5 amine-substituted units [30, 31]. The presence of amine groups caused the dark brown KL powder to turn light brown following the reaction. FE-SEM images of the KL and AL particles are shown in Fig. 1b and 1c, respectively, where the morphologies appear to be similar. The corresponding EDS elemental mapping images of KL (Fig. S1) and AL (Fig. 1d and S2) clearly compare the distribution of the elements, indicating the grafting of amine groups on the AL particles with nitrogen after the amination reaction. The atomic percentage of nitrogen increased from 0.89 to 13.37% after the reaction, whereas the carbon content decreased from 79.94 to 68.97%.
Spectral analysis data (FT-IR spectra, XPS spectra, and TGA/DTA curves) of the KL and AL particles are shown in Fig. 2. The FT-IR spectra were used to investigate the changes in the chemical structure of the particles after the modification process (Fig. 2a). The band located at ~ 3404 cm− 1 for KL was assigned to the hydroxyl groups in the aliphatic and phenolic structures of lignin while the relatively broader band in the 3500–3300 cm− 1 range of AL was attributed to N–H stretching vibrations of the primary and secondary amines grafted by the amination reaction [32]. The peaks at 2928 and 2840 cm− 1 were originated from the C–H asymmetrical and symmetrical stretching vibrations of the methyl and methylene groups, respectively [33], and their peak intensities increased significantly after modification. This suggests the successful introduction of the molecular chains containing numerous methyl and methylene groups during the amination reaction. Because the reaction occurred at the aromatic moieties of lignin, the intensity of the peaks corresponding to the C–H vibrations of the aromatic skeleton (1603, 1506, and 812 cm− 1), aromatic in-plane bending (1146 and 1025 cm− 1), and C–O vibrations of guaiacyl ring (1269 cm− 1) of lignin decreased markedly in the spectrum of AL [33, 34]. Furthermore, a new peak appeared at 1640 and the intensity of the peak at 1357 cm− 1 was increased for AL, which arose from the N–H bending and C–N stretching vibrations of the –NH2 structure, respectively, implying that the amine groups were successfully grafted onto the lignin structure [35, 36].
The change in the chemical composition of lignin particle surface after the reaction was further confirmed by XPS (Fig. 2b–2d). When the spectra of KL and AL were compared, the C1s and O1s binding energies of KL and AL particles were found to be 285.1 and 533.1 eV, respectively, and a new N1s peak was observed at 399.5 eV for AL, as shown in Fig. 2b. Notably, AL had a high N/C ratio (0.18), which is about 18 times higher than that of KL (0.01). In addition, a large increase in the C/O area ratio from 3.32 (KL) to 3.67 (AL) was observed, indicating that alkyl amine groups were introduced into the carbon chains of AL, leading to a higher carbon content and lower oxygen content. For more detail, in the N1s core-level spectra of AL (Fig. 2c), primary (–NH2) and secondary amine groups (–NH) were detected on the AL surface, indicating that the terminal primary amine of DETA and lignin reacted to form a chemical structure, as illustrated in the inset of Fig. 2b. As shown in Fig. 2d, the deconvoluted C1s spectra of KL and AL show peaks at 284.5, 284.8, and 286.2 eV corresponding to the carbon atoms located in aromatic rings, sp2 and sp3 hybridized carbon atoms (C–C/C = C), and functional groups of lignin (C–N/C–O), respectively [37–39]. As amine groups were grafted on the AL surface mainly via the formation of C–N linkages, the area fraction of C–N/C–O increased from 31.37 to 48.51%. These changes in N1s and C1s spectra following modification suggest the presence of both primary and secondary amine groups, indicating successful grafting of amine groups on AL particles.
Figure 2e and 2f represent the TGA/DTG curves of KL and AL recorded under N2 atmosphere. Slight weight loss and the peaks detected below 150°C in the TGA and DTG curves, respectively, were attributed to the evaporation of residual moisture because lignin usually contains water in the cell walls. In the range of 150–300°C, the low-molecular-weight lignin fragments were decomposed and evaporated (Fig. 2e). Specifically, in the DTG curves (Fig. 2F), two weight loss peaks were observed at 241°C (weight loss rate, Rwt: −0.56%/min) and 277°C (Rwt: −1.05%/min) for KL, and a single bond weight loss peak was observed at 255°C (Rwt: −1.18%/min) for AL, which may be ascribed to the degradation and evaporation of low molecular weight lignin fragments from aliphatic side chains [35]. In the decomposition step above 300°C, the weight loss peak of KL was observed at 391°C, while two peaks were observed at 312 and 363°C for AL, corresponding to C–C bond cleavage and demethoxylation of aromatic rings [35, 40]. The maximum weight loss rates were observed at 392°C (Rwt: −3.68%/min) for KL and at 364°C (Rwt: −2.67%/min) for AL. The lower temperature observed for AL can be attributed to the lower C–N bond energy compared to that of C–C bond [31]. In the final stage above 500°C, only a small weight loss was observed for both samples.
Figure 3a and 3b show the evolution of the rheological properties of the rubber compounds during vulcanization according to the amount of 6PPD, KL, and AL added as an antioxidant. The curing characteristics are expressed in terms of the torque value at the initial (moment of lowest, ML) and late stages (moment of highest, MH), crosslink density, as measured indirectly from the difference in torque (ΔMH = MH − ML), scorch time (ts2), optimum curve time (tc90), and cure rate index (CRI), as listed in Table S1. The comparison of the rheological results of NB/KL-2 with those of NB/6P-2 revealed two contradictory trends; the ts2 and tc90 of NB/KL-2 were 4.30 and 6.98 min, respectively, which are 22 and 15% higher than those of NB/6P-2 (3.37 and 5.93 min). Furthermore, the CRI value of NB/KL-2 was 37.31 min− 1, which is 5% lower than that of NB/6P-2 (39.06 min− 1). These results indicated that lignin scavenged the activated sulfur radicals, thus interfering with the crosslinking reaction [21, 41]. The torque value of NB/KL-2 indicated that the ML, MH, and ΔMH of the compound increased, although the large number of hydroxyl groups in purified lignin particles could limit their interaction with the rubber matrix. This is possibly because the micron-sized lignin particles interfered with the movement of rubber chains, or the sulfur contained in lignin and rubber led to their cross-linking to render the composite harder [42].
A comparison between the NB/AL and NB/6P-2 in the rheological properties revealed a different trend from the NB/KL-2. The MH of NB/AL-2 was 24.40 dNm, which is 28 and 17% higher than those of NB/6P-2 (19.06 dNm) and NB/KL-2 (20.84 dNm), respectively, at the same filler content. The CRI value of NB/AL-2 was 61.35 min− 1, which is 57% higher than that of NB/6P-2. Furthermore, with increasing AL content for the NB/AL specimens, the torque in the whole-time range and CRI were increased. These results are presumably because the amine groups in the AL particles acted as nucleophilic activators; they not only accelerated the vulcanization reaction of rubber, but also increased the degree of crosslinking, as illustrated in Fig. 3c. A heterolytic ring-opening reaction occurred when the amine groups grafted onto the AL particles met a sulfur ring. This reaction was initiated with the nucleophilic attack of the primary and secondary amines on the sulfur ring to produce a polysulfide [43, 44], which is expected to increase the amount of polarizable sulfur in the matrix. The sulfur on the surface of AL particles could be separated from the particles allowing the rubber matrix to further vulcanized (i.e., sulfur donor) or directly reacted with the double bonds in the rubber chains to participate in the curing reaction.
The thermal aging test is a method of confirming the characteristics of rubber that is aged by heat applied from the outside or formed inside through repeated deformation, and thus significantly important factors directly related to the lifespan of rubber. Figure 4a shows the effects of thermal aging on the tensile properties of the NB compounds. The 50% modulus value of NB/KL-2 was 1.14 MPa, which is slightly higher than that of NB/6P-2 (1.06 MPa), but the tensile strength and elongation were almost the same for the two samples. This is because, although KL has low compatibility, the sulfur contained in lignin participated in the crosslinking reaction with rubber to prevent the deterioration of the physical properties of cured rubber or the amount of added lignin was not too high to degrade the mechanical properties of the rubber. On the other hand, the 50% modulus of NB/AL-1, NB/AL-2, and NB/AL-4 were 1.25, 1.36, and 1.53 MPa, respectively. All NB/AL compounds exhibited higher stress than NB/6P-2 over the entire strain range. The 50% modulus values of NB/AL compounds were higher than those of NB/6P-2 and NB/KL-2, indicating that the amine-grafted lignin not only accelerated the curing reaction of rubber more significantly than 6PPD and KL, but also participated directly in curing to increase the crosslink density of the samples, thereby improving its mechanical properties. After the thermal aging, the modulus increased, and the tensile strength and elongation decreased for all specimens. These were due to the simultaneous occurrence of two aging processes in rubber: hardening by crosslinking and softening by chain scissioning, respectively. These interesting results mainly derived from the hybrid rubber matrix consisting of NR and BR, between which NR with bulky side groups cannot easily undergo radical recombination reactions owing to steric hindrance; it was therefore degraded by chain scissioning caused by disproportionation and hydrogen abstraction [45]. On the other hand, crosslinking dominated in BR with less-active double bonds because of the electron-withdrawing groups [46].
Figure 4b shows the degree of deterioration of the tensile properties resulting from thermal aging. The tensile strength and elongation of NB/AL-2 decreased by 36.2 and 46.9%, respectively, by a lower degree than those of NB/6P-2 and NB/KL-2. These results indicated that the AL particles imparted excellent anti-aging properties to rubber. The NB/AL compounds exhibited lesser change in tensile strength than in elongation, compared to the other samples. This is possibly because the free radical of the NB rubber chains reacted with oxygen molecules to form rubber–O• or rubber–OH, which then reacted with amine groups grafted onto the AL particles. Detailed and additional information on the variation in the tensile properties and hardness of the specimens after thermal aging was provided in Table S2, which reveals that unaged NB/AL compounds had higher hardness than the others, and that the hardness of NB/AL compounds increased by a lower degree than those of NB/6P-2 and NB/KL-2, after thermal aging.
Figure 4c and Table S3 show swelling behaviors of the rubber compounds in solvent, which exhibit the effect of antioxidants on the crosslinking of rubber. Generally, there are two main opposing forces in swelling of crosslinked polymers, i.e., solvent infiltration into the polymer network and resistance of the network to expansion [47]. NB/KL-2 exhibited higher Q% (208.3%) and lower v (3.22 × 10− 4 mol/cm3) than those of NB/6P-2 (197.9% and 3.795 × 10− 4 mol/cm3, respectively). These results can be attributed to the fact that the radical scavenging effect and micron size of lignin physically/chemically interfered with the crosslinking of rubber during the vulcanization reaction. On the other hand, the Q% and v of NB/AL-1 were 189.3% and 3.964 × 10− 4 mol/cm3, respectively, and as the content of AL particles increased, Q% decreased and v increased. It should be addressed that, at the same filler content, v of NB/AL-2 is 8 and 28% higher than those of NB/6P-2 and NB/KL-2, respectively. It is considered that the amount of activate sulfur increased during the vulcanization process because of the presence amine groups on AL particles. These results are also in good agreement with the substantially increased torque values of the NB/AL-2, compared to the NB/6P-2 and NB/KL-2 (Fig. 3).
Figure 4d shows the digital images of NB compounds immersed in a toluene after 48 h during the swelling test. It was clearly confirmed that only NB/6P-2 changed the color of the solvent to yellow. Furthermore, the s of NB/6P-2 was 10.46%, whereas those of all other samples were about 9.50% (Table S3). This is because 2 phr of 6PPD (1 wt.%), which was not chemically bound to the rubber, was almost dissolved in the toluene penetrating out of the rubber. On the other hand, lignin-incorporated samples (i.e., NB/KL-2 and NB/AL-2) maintained the solvent transparently without any color change. This is attributable to the polar characteristic of lignin in nature, making it difficult to be dissolved in non-polar solvents such as toluene, and even if the rubber swells providing increased interstices between the polymer chains, it may be difficult for micro-sized particles to escape to the rubber surface.
Ozone resistance is one of the critical properties in rubber compounds that resists the formation of microcracks through chemical attack to rubber, thereby suppressing deterioration of rubber performance and extending lifespan. The initiation of cracks under stretching is due to the reaction occurring between double bonds of rubber chain and ozone, i.e., ozonolysis. The rapid reaction of ozone with olefinic double bonds forms an ozonide ring, which is subsequently broken causing macroscopic surface cracks. Furthermore, ozone cracking occurs not only in surface of rubber compounds but also in interfaces between filler and matrix because ozone can penetrate faster within the rubber structure at a higher temperature.
In the case of NB/KL-2, due to the low compatibility of KL, air gaps were formed between the particles and the matrix, increasing the exposed area vulnerable to ozone. As a result, cracks were simultaneously generated both internally and externally, and the growing cracks eventually led to specimen cutting, as shown in Fig. S3. Figure 5a and 5b show the digital and optical microscope images of NB/6P-2 and NB/AL-2, respectively, after ozone aging under 20% stretching. In general, 6PPD with a relatively small molecular weight (298.38 g/mol) blooms on the surface and exerts a sufficient barrier effect against ozone attack [48, 49]. However, after ozone aging, numerous small cracks with sizes of 100–200 µm and large cracks over 1 mm were formed on the surface of NB/6P-2 due to the severe aging conditions. On the other hand, it is noteworthy that the surface of NB/AL-2 showed intermediate cracks with sizes ranging from 0.1 to 1 mm. As the AL was crosslinked with rubber and had restricted particle movement, it is considered that cracks may occur in other parts except for the part where the AL particles were dispersed near the surface. In addition, since NB/AL-2 had a small amount of unreacted double bonds of rubber chain to be attacked by ozone, micro-cracks were hardly generated on the surface.
Figure 5c and 5d show the 3D images and surface profiles of rubber surfaces using optical profilometry, which is a non-contact metrology technique for surface analysis and topographical characterization. In case of NB/6P-2, large crack with a depth of 162.05 µm and a width of 56.90 µm and small cracks of < 10 µm were detected on the surface. The surface roughness parameter values of NB/6P-2 were Sa = 4.737 µm, Sq = 13.199 µm, and Sz = 213.217 µm, respectively. In contrast, NB/AL-2 had a smaller crack (depth = 67.76 µm and width = 22.26 µm) and lower surface roughness parameter values (Sa = 2.077 µm, Sq = 4.047 µm, and Sz = 115.138 µm) than those of NB/6P-2 (Table S4). The exceptional ozone resistance of NB/AL-2 can be attributable to the reduction of the uncured area of matrix and radical scavenging performance of AL particles. These results suggest that the amination modification of lignin significantly improves the reactivity with radicals generated from ozone, imparting rubber with excellent ozone-aging resistance.
Figure 6 shows the results of predicting the fatigue life of rubber by the fracture of unaged or thermally aged specimens under repeated deformation conditions. The mechanical theory of the fatigue failure mechanism is that, in the process of fatigue, the molecular chains of rubber are broken by mechanical force, and the free radicals produced react with oxygen to induce oxidative aging, causing the fracture of molecular chains and formation of microcracks, which gradually expand with time [50, 51]. Considering this mechanism, the number of cycles was taken as a measure of the service life of the tested rubber compounds. The number of cycles to failure (Nf) of NB/6P-2 was 90,001, because 6PPD imparted excellent fatigue resistance properties by quickly reacting with the radicals of the rubber chains that were physically generated and removing them. In the case of NB/KL-2, the weak bonds between the rubber and lignin were first broken during the fatigue process, resulting in the formation of large cracks in the rubber matrix. Consequently, NB/KL-2 performed poorly (16,234 cycles) under fatigue conditions. On the other hand, the Nf of NB/AL-2 was 89,934 cycles, which is similar to that of NB/6P-2. Remarkably, after thermal aging, the NB/AL-2 (84,931 cycles) exhibited much better fatigue resistance compared to that of the NB/6P-2 (79,052 cycles). These improved fatigue resistances of NB/AL-2 were attributed to the interconnected composite structures derived from the presence of the AL particles, which were physically entangled and chemically crosslinked into the matrix and thereby effectively endured to external stress. Combined with the excellent fatigue resistance evidenced by the flex fatigue test, it can be concluded that the AL is one of the best candidate antioxidants that can be incorporated to rubber.