## 4 Results and Discussion  
### BRV Results  
The BRV test results for all bitumen samples at 135 and 165 \({}^{\circ}\)C are shown in Figure 5. The results show the influences of Kraft lignin with different percentages and blending conditions on the rotational viscosity of the bitumen samples. More precisely, the measured viscosities allow quantifying the influence of Kraft lignin, as an additive, on the stiffness of bitumen, specifically at a high percentage of additive. Generally, adding Kraft lignin increases the viscosity of the virgin bitumen at 135 and 165 \({}^{\circ}\)C and enhances the high-temperature stability. As seen in Figure 5, the viscosity of virgin bitumen at 135 \({}^{\circ}\)C is 300 mPa-s, which is similar to what the manufacturer measured (value of 309 mPa-s: Table 2). The addition of 5, 10, and 20% of Kraft lignin using a high-shear mixer (HSM) increase the viscosity to 356, 400, and 544 mPa-s, respectively. Likewise, samples prepared using a mechanical mixer (MM) exhibit a similar trend (444, 492, and 610 mPa-s for 5, 10, and 20% of Kraft lignin, respectively). Figure 4: DSC methodology for samples. Figure 5: The viscosity of bitumen samples at 135 and 165 \({}^{\circ}\)C.  
The mechanisms behind the observed increase in viscosity may relate to the molecular weight of Kraft lignin and bitumen. According to the producer's data sheet, the molecular weight of Kraft lignin is 5185 g/mol. Bitumen has a wide range of molecular weights, depending on the crude oil source and composition, but it seems that bitumen has a lower molecular weight than Kraft lignin. This increase in molecular weight may lead to higher viscosity. In addition, the polarity of Kraft lignin may be a potential factor influencing the rheological properties of the modified bitumen. As bitumen is known to possess a relatively high polarity, the presence of Kraft lignin may further contribute to the increase in viscosity. Moreover, Kraft lignin is a powder, so it contains solid particles, which increases the viscosity. The influence of the shear rate on viscosity is significant. Samples prepared by a high-shear mixer (HSM) show a lower viscosity compared to the mechanical mixer (MM) samples. Figure 5 illustrates the impact of the shear rate on virgin bitumen. Processing virgin bitumen (VB) using a high-shear mixer (HSM) results in a 7% increase in viscosity, whereas using a mechanical mixer (MM) increases it by 23%. The difference in viscosity could be attributed to a greater loss of volatiles from the virgin bitumen (VB) processed by the MM. When blending Kraft lignin with bitumen using a MM, which has a three-bladed head, the rotational motion of the mixer creates a centrifugal force that pushes the bitumen outward, resulting in a non-flat surface with a funnel shape (Figure 6a). This shape increases the surface area of the bitumen in contact with air, leading to higher oxidation. However, HSM has a different head design that creates a flat surface on the bitumen during blending (Figure 6b). This flat surface reduces the contact area of the bitumen with air, thus reducing the possibility of oxidation. Further, the selection of an appropriate head for a high-shear mixer (HSM) may influence the properties of the final product. Among the various heads available, the widely used head for powder materials was used. An HSM could lead to reduced stiffness and oxidation during the blending process compared to a MM. In general, according to the test results, all samples that were processed in either mixer have higher viscosity. According to the LC 25-007 test method, the required viscosity for mixing and compaction is 170 \(\pm\) 20 mPa\(\cdot\)s and 280 \(\pm\) 30 mPa\(\cdot\)s, respectively, for binder used for hot mix asphalt (HMA) (limits presented in Figure 5). To achieve these target values, it is necessary to increase the mixing and compaction temperature. Due to its higher viscosity, bitumen requires higher temperatures to become sufficiently fluid for mixing and compaction, resulting in higher energy consumption.