## 4 Results and Discussion
### ESEM Results  
Environmental Scanning Electron Microscopy (ESEM) was used to investigate the morphology of the Kraft lignin, virgin bitumen, and lignin-modified bitumen samples. To analyze the images, ImageJ software (version 1.52a) was used. The ESEM images provide valuable information about the microstructure, surface topography, and elemental composition of the samples.  
Figure 11a shows a representative ESEM image of the Kraft lignin. The Kraft lignin particle sizes were analyzed using ESEM, and the results show that the particle sizes ranged from 2 to 50 \(\upmu\)m. The particle size of Kraft lignin powder is a crucial factor that can influence the properties of lignin-modified bitumen. Smaller particle sizes promote uniform dispersion within the bitumen matrix, resulting in a homogeneous mixture. Additionally, smaller particles provide a larger interfacial area between lignin and bitumen, facilitating interaction and bonding. These findings are consistent with the observations from the laser granulometry test (Section 3.1.2 and Figure 2), indicating the reliability of the measurement methods employed. Moreover, the ESEM image reveals that the Kraft lignin particles exhibit angularity and rough surface characteristics. These features can be attributed to the inherent properties of Kraft lignin.  
Figure 11b shows the ESEM image of the Kraft lignin in bitumen for a 10L-MM sample. The visual observation of the sample reveals that there is a good dispersion of Kraft lignin in the bitumen, and there is no aggregation of Kraft lignin particles. The dispersion of Kraft  
Figure 11: ESEM image of: (**a**) Kraft lignin and (**b**) 10L-MM sample (magnification of \(\times\)1000).  
lignin particles in bitumen is a crucial factor that can significantly affect the properties of the lignin-modified bitumen. A well-dispersed Kraft lignin powder can lead to improved performance properties. On the other hand, poor dispersion can lead to several negative effects, including reduced effectiveness of Kraft lignin, decreased homogeneity, and reduced mechanical properties of modified bitumen [10]. The analysis of particle sizes shows that the visible particle sizes ranged 2 \(\pm\) 1 \(\upmu\)m. Interestingly, the measurement of the area shows that only 2.5% of Kraft lignin is visible in the modified bitumen 10L-MM, suggesting that the remaining Kraft lignin (5.7%) might have absorbed or dissolved in the bitumen (91.8%)--here, percentage by volume, not by weight. To determine the percentage of visible Kraft lignin particles in the 10L-MM sample, a representative area was randomly selected for analysis. Individual particle areas were measured using ImageJ software.  
Figure 12 shows nine ESEM images revealing the fibril (worm-shaped) structure of bitumen samples with three different percentages of Kraft lignin and two blending conditions. Upon examination of the images, it can be observed that the fibril structure of the bitumen samples is affected by both the Kraft lignin content and the blending conditions. Specifically, increasing the amount of Kraft lignin in the samples results in the more widespread distribution of visible Kraft lignin particles within the fibril structure, as shown in images c, d, e, g, h, and i (Figure 12). Furthermore, the blending process used during sample preparation also plays a crucial role in determining the fibril structure. Image b, which depicts virgin bitumen processed in a mechanical mixer (VB-MM), exhibits a thinner diameter and more voids or pores in comparison to image f, showing virgin bitumen processed in a high-shear mixer (VB-HSM). Based on the quantitative measurements, the average diameter of the fibril structure of the MM sample and HSM sample are 9.5 and 13.4 \(\upmu\)m, respectively. The increased oxidation during blending using MM may have led to the thinner fibril structure observed. Based on the findings presented in this study, it can be concluded that both the Kraft lignin content and the blending conditions significantly affect the fibril structure of bitumen samples.