## 4 Results and Discussion
### FTIR Results  
The FTIR spectroscopy analysis was performed to investigate the influence of Kraft lignin addition on the chemical structure of bitumen under different blending conditions. Figure 9 shows the FTIR spectral results of VB and HSM samples. The FTIR spectrum presented in this study was obtained as an average of three (3) replicates. To ensure consistency and comparability, all spectra were normalized to the band with the highest intensity in the bitumen spectrum using a min-max normalization approach [60].  
Figure 8: Norm of complex shear modulus \(\,\mid\)G*\(\,\mid\) and phase angle (8) of specimens using DSR test.  
As shown in Figure 9, significant changes observed are between 2000 to 400 cm\({}^{-1}\), which are typically referred to as the \"fingerprint region\" [61]. This region contains a unique pattern of absorption peaks that can identify specific functional groups present in the modified bitumen sample.  
From a chemical reaction point of view, the FTIR spectra of Kraft lignin and modified bitumen with 5, 10, and 20% of Kraft lignin for HSM samples in the fingerprint region are shown in Figure 10. A shift to higher wavenumbers (2000 to 400 cm\({}^{-1}\)) in an FTIR spectrum can indicate a change in chemical structure and an increase in bond strength, but does not necessarily mean a chemical reaction has occurred [62,63]. The shift in peak position (\(y\)-axis) can also be due to changes in intermolecular interactions.  
According to the test results, new peaks are generated at 2 wavenumbers of 1260 and 1510 cm\({}^{-1}\), which intensified with increasing Kraft lignin percentage. The wavenumber of 1260 cm\({}^{-1}\) is associated with the stretching vibration of C-O bonds in aliphatic and aromatic  
Figure 10: FTIR spectra of Kraft lignin, VB, 5L-HSM, 10L-HSM, and 20L-HSM samples (wavenumber of 400 to 2000 cm\({}^{-1}\)).  
Figure 9: FTIR spectra of VB, 5L-HSM, 10L-HSM, and 20L-HSM binders (wavenumber of 400 to 4000 cm\({}^{-1}\)) (average of three replicates per sample type).  
eters, while the wavenumber of 1510 cm\({}^{-1}\) is commonly attributed to the stretching vibration of aromatic ring structures [64]. Meanwhile, Kraft lignin has peaks in the same wavenumbers, indicating the presence of Kraft lignin in the bitumen, not a new chemical reaction. On the other hand, the peak values of the curves for other wavenumbers are slightly different, but the peak positions of the curves are basically the same, indicating that the Kraft lignin does not produce new functional groups, but intensifies the existing peaks. It can be speculated that the physical interaction between the two components leads to the observed changes in the FTIR spectra, and no chemical reaction occurs. It is important to note that the FTIR results for the MM samples are consistent with those of the HSM samples, indicating that the blending conditions do not significantly affect the chemical composition of the modified bitumen. Based on these findings, it can be concluded that the addition of Kraft lignin to modified bitumen can alter the chemical structure of the bitumen without causing any chemical reaction between the two components. The results suggest an increase in the strength of certain chemical bonds due to the presence of Kraft lignin. These findings have important consequences for the use of Kraft lignin as a sustainable modifier for bitumen.