## 2 Background
### Lignin-Modified Bitumen  
The characteristics of lignin make it a highly promising material with potential applications in various industries. Its renewability, affordability, non-toxic nature, wide availability, and similarity in chemical structure to bitumen are key properties that make lignin a suitable modifier or alternative extender for use in the asphalt industry [14; 15]. In recent years, there has been a growing interest in lignin-modified bitumen. One area of investigation has been the rheological properties of lignin-modified bitumen. Numerous studies have been conducted to evaluate the effects of several types and concentrations of lignin additives on the viscosity, stiffness, and high- and low-temperature performance of bitumen. It has been observed that the addition of lignin can improve certain properties of bitumen, such as the high-temperature performance and resistance to fatigue. However, the use of lignin can also have drawbacks, such as increased brittleness and reduced low-temperature performance [16; 17; 18; 19]. [PERSON] et al. (2023) conducted research to evaluate the impact of Kraft lignin as an additive at various concentrations ranging up to 30% on the rheological properties of the bitumen [20]. Their test results for Brookfield Rotational Viscosity (BRV), Dynamic Shear Rheometer (DSR), Ring-and-Ball, and Multiple Stress Creep Recovery (MSCR) showed that the addition of Kraft lignin increased the viscosity, stiffness, softening point, and high-temperature performance of the bitumen, respectively. However, this also led to a decrease in low-temperature performance, as shown by their Bending Beam Rheometer (BBR) test results [20]. Additionally, [PERSON] et al. (2023) investigated the effect of lignin and formic acid lignin additives at concentrations of 5 and 10% [21]. Their study revealed that the inclusion of these additives could signifiificantly enhance the properties of the asphalt at high temperatures, while also improving its resistance to fatigue and cracking at low temperatures [21]. Chemical reactions between lignin with bitumen and the microstructure of lignin-modified bitumen are also the subjects of some studies. [PERSON] et al. (2019) investigated the use of lignin as an antioxidant in bitumen and evaluated its chemical and rheological properties [22]. The results of the Fourier-Transform Infrared Spectroscopy (FTIR) test on bitumen samples with 10% Organosolv lignin showed that the various functional groups of lignin did not produce new chemical functional group peaks. Moreover, the microstructures of the bitumen with and without lignin were found to be almost identical, according to the Environmental Scanning Electron Microscope (ESEM) test [22]. In another study conducted by [PERSON] et al. (2022), a bio-oil was used as an additive in lignin-modified bitumen [23]. FTIR spectra analysis indicated that there were no newly generated chemical functional groups in the blend of lignin, virgin bitumen, and oil. It was concluded that the blending process did not create a new chemical composition. As a result, there was either no chemical reaction or the reaction was negligible [23]. Scanning Electron Microscope (SEM) and Environmental Scanning Electron Microscope (ESEM) are important techniques for investigating lignin-modified bitumen, as they provide high-resolution images of the surface [24]. Many research studies have been conducted in this area [25; 26; 27]. For instance, [PERSON] et al. (2022) used SEM to investigate the shape and particle size distribution of lignin powder [28]. Finally, one of the noteworthy research areas of lignin-modified bitumen is the investigation of thermal properties, which are essential for its performance under high- and low-temperature conditions [29]. The thermal stability and decomposition behavior of lignin-modified bitumen can be assessed using Thermogravimetric Analysis (TGA). Numerous research studies have been conducted to investigate the thermal properties of modified bitumen [30; 31; 32]. For example, [PERSON] et al.