## 4 Results and Discussion
### TGA Results  
The thermal behavior of Kraft lignin and lignin-modified bitumen samples was studied using thermogravimetric analysis (TGA). This study aimed at evaluating the thermal characterization of Kraft lignin under combustion (thermal-oxidative) conditions and assessing the impact of Kraft lignin addition on the thermal behavior of bitumen within the operational temperature of the bitumen and hot mix asphalt. The TGA analysis was also carried out to evaluate the effect of blending conditions on the thermal behavior of the modified bitumen samples. The TG/DTG thermogram of Kraft lignin is shown in Figure 13.  
Figure 12: ESEM images for: (**a**) VB, (**b**) VB-MM, (**c**) 5L-MM, (**d**) 10L-MM, (**e**) 20L-MM, (**f**) VB-HSM, (**g**) 5L-HSM, (**h**) 10L-HSM, and (**i**) 20L-HSM. The results show that Kraft lignin has an initial decomposition temperature (IDT) of 198 \({}^{\circ}\)C and a final decomposition temperature (FDT) of 571 \({}^{\circ}\)C. The D\({}_{10}\) value, indicating 10% decomposition of Kraft lignin, is 273 \({}^{\circ}\)C, while the D\({}_{50}\) value, representing 50% decomposition, is 464 \({}^{\circ}\)C. The weight loss process of Kraft lignin is divided into four (4) stages with respect to initial mass. The 1 st weight loss stage occurs at a temperature range between room temperature and 180 \({}^{\circ}\)C, caused by the dehydration drying process. This is due to the moisture that exists in the Kraft lignin. From room temperature to 100 \({}^{\circ}\)C, the weight loss is due to evaporation of free water. At 60 \({}^{\circ}\)C, the maximum weight loss rate is recorded. As the temperature rises, chemisorbed water begins to be removed [34; 65]. The weight loss for free and chemisorbed water is 1.3 and 2.6%, respectively. The 2 nd weight loss stage takes place from 180 to 350 \({}^{\circ}\)C, where 16.4% weight loss is observed due to thermal decomposition carbonization. During this stage, the polysaccharides and a small number of hydroxyl (-OH) and ether bonds (-C-O-C) break down [34]. During the 3 rd stage, which occurs within the temperature range of 350 to 450 \({}^{\circ}\)C, a weight loss of 26.3% is observed. This stage involves the late phases of thermal decomposition and carbonization processes. In the 4 th stage, which takes place between 450 and 590 \({}^{\circ}\)C, a significant weight loss of 53.8% is observed. This stage is characterized by the combustion of carbonized Kraft lignin. The release of these gases suggests that Kraft lignin is undergoing significant thermal degradation. The maximum rate of decomposition temperature (MRDT) of Kraft lignin is found at a temperature of 516 \({}^{\circ}\)C, which indicates the temperature range where the maximum component in the material is decomposing. The final residue (FR) of Kraft lignin is determined to be less than 1%. The analysis of the TGA and DTG curves provides a clear understanding of the thermal behavior of Kraft lignin. Thermal stability is an important property of bitumen, as it can affect its performance and durability. In this study, the thermal stability of the bitumen samples is evaluated. To quantify the thermal stability of the samples, the D\({}_{1}\) parameter is defined, which is the  
Figure 13: TG/DTG thermograms of Kraft lignin. temperature at which 1% of the sample decomposes. The D\({}_{1}\) values for all samples are determined from the TGA test results. The D\({}_{1}\) values provide valuable information on the thermal stability of the samples. A higher D\({}_{1}\) value indicates that the material is more thermally stable, as it can withstand higher temperatures before decomposing. Therefore, by comparing the D\({}_{1}\) values of the samples, the effectiveness of any modification processes on the thermal stability of the material can be evaluated. As shown in Figure 14, the blending processes increase the D\({}_{1}\) value of the virgin bitumen (VB) from 242 to 265 \({}^{\circ}\)C for mechanical mixing (MM) and from 242 to 259 \({}^{\circ}\)C for high-shear mixing (HSM).