## 4 Results and Discussion
### TGA Results
The increase in the D\({}_{1}\) value of the VB-MM and VB-HSM suggests that the blending process may have improved the thermal stability of the material. The higher D\({}_{1}\) value indicates that the processed bitumen sample can withstand higher temperatures before decomposing, compared to the VB sample. VB-MM has a higher D\({}_{1}\) value in comparison to VB-HSM. The increase in the D\({}_{1}\) value could be attributed to the removal of volatile components and increased oxidation levels in the MM sample during the blending process, which can increase the molecular weight and stiffness and enhance resistance to thermal changes. Further investigation is required to fully understand the underlying mechanisms contributing to this observation. The D\({}_{1}\) values for MM are 257 \({}^{\circ}\)C, 251 \({}^{\circ}\)C, and 243 \({}^{\circ}\)C for 5, 10, and 20% of Kraft lignin, respectively. One of the reasons for the decrement in the D\({}_{1}\) value can be attributed to the lower thermal stability of Kraft lignin compared to bitumen. Moreover, the use of HSM results in a further decrease in thermal stability. The D\({}_{1}\) values for HSM samples are 229, 220, and 186 \({}^{\circ}\)C for 5, 10, and 20% of Kraft lignin, respectively. Possible reasons for the lower thermal stability in lignin-modified bitumen samples include alterations to the chemical composition and microstructure of the bitumen, as well as the introduction of impurities or degradation during the Kraft lignin modification process. FTIR test results (Section 4.3) show no significant changes in functional groups, suggesting that chemical modifications to the bitumen are not the primary reason for the lower D\({}_{1}\) value. It is likely that multiple factors are contributing to the lower thermal stability of lignin-modified bitumen samples. Further investigation is required to determine the exact mechanisms involved. The effect of different percentages of Kraft lignin on the operational temperature of the hot mix asphalt (<200 \({}^{\circ}\)C) was investigated by thermogravimetric analysis (TGA). The TG and DTG thermograms of the Kraft lignin, VB, and HSM samples are presented in Figure 15.  
Figure 14: D\({}_{1}\) value of bitumen samples. The VB sample has more mass loss compared to the VB-HSM sample in the temperature range of 100-200 \({}^{\circ}\)C. This indicates that the blending process caused the loss of volatile components from the bitumen. The lignin-modified bitumen samples showed more mass loss than the VB and VB-HSM samples, and the mass loss increased with increasing Kraft lignin percentage. This result suggests that the presence of Kraft lignin in the bitumen affects its thermal behavior, which can be attributed to the thermal decomposition of Kraft lignin. Interestingly, a thermal event was observed at 131 \({}^{\circ}\)C in the 20L-HSM sample, which is not present in other samples. The reason for this thermal event is unclear, and further investigation is needed to understand the underlying mechanism. One possible explanation is that it could be related to the interaction between Kraft lignin and bitumen with a higher amount of Kraft lignin, leading to a new phase formation. Overall, the results prove that the addition of Kraft lignin to bitumen affects its thermal behavior, and the Kraft lignin percentage in the modified bitumen plays a significant role in determining the mass loss curve.