## 3 Results
### Rheology  
The G*/sin \(\delta\) vs. temperature of the modified asphalts is shown in Figure 4. In this context, complex modulus is related to the asphalt blend stiffness and G*/sin \(\delta\) represents the resistance of the modified asphalts to permanent deformations. The corresponding failure temperatures are indicated in the figure. The G*/sin \(\delta\) values of the modified asphalts are lower than that of the SBS-modified asphalt (control). The blend modified with 2% CFP (M2) is the closest to the reference value. The lowest values correspond to the samples incorporating the composites prepared at 80 \({}^{\circ}\)C and 50 or 70 rpm mixing speed. Therefore, the effect of using CFP as a modifier is more significant when it is added alone than when it is incorporated into the SBS/CFP composite, and its effect is similar to that of radial SBS. This means that CFP may equal the behavior of higher rubber loads; however, that study was not carried out in the present work. Keratin has a crosslinked structure similar to that of radial SBS, and then, it may be responsible for the similar rheological effects of both materials on the asphalt binder. The modifying effect becomes more evident above 85 \({}^{\circ}\)C, as previously reported [30; 31]. Since G*/sin \(\delta\) indicates the degree of asphalt resistance to deformation when exposed to stress, it can be inferred that the CFP addition may be equivalent to the use of radial SBS in a wide range of thermal conditions. These results have not been reported before, given that CFP has not been studied previously.  
The G*/sin \(\delta\) ratio is useful to determine the failure temperature of asphalts, i.e., the temperature at which the aforementioned parameter equals 1 kPa (Figure 4), also known as T\({}_{\rm{SHRP}}\). The resistance of modified asphalts to permanent deformations at high temperatures is rheologically described by T\({}_{\rm{SHRP}}\) as a softening point (from the physical perspective); at high temperatures, the higher the T\({}_{\rm{SHRP}}\), the better the mechanical resistance properties of the asphalt. The results suggest that CFP interactions are efficient enough to affect both the thermal and mechanical properties of the asphalt, as a consequence of the dispersion of the CFP domains within the asphalt matrix [29]. However, the obtained values are not as high as others reported in reference works.  
Figure 3: Derivative curve of weight loss signal obtained from TGA for SBS, CF and modified asphalts M1–M6.  
The tan \(\delta\) measurements are generally preferred over G* due to their higher sensitivity to physical changes, as well as to the chemical structure of asphalts. Figure 5 shows the tan \(\delta\) isochronal curves at the same angular frequency considered for the construction of G* curves. The elastic response of the modified asphalts with CFP is better compared with that of the SBS-modified asphalt, since the elastic modulus is higher than the viscous modulus. The elastic nature of the asphalt is reduced as the temperature increases because the phase angle reaches an asymptotic value of 90 degrees, indicating that the asphalt shifts from a predominantly elastic behavior to a more significant viscous component. CFP-modified asphalt has the lowest tan \(\delta\) value (even lower than that of the SBS-modified asphalt above 85 \({}^{\circ}\)C), suggesting that it will show a better elastic response before flowing.  
The SBS-modified asphalt (M1), the one modified with the composite prepared at 70 rpm (M6) and the CFP-modified sample (M2) do not exhibit a sharp elastic-viscous behavior shift, unlike the rest of the materials, which show this evident transition with increasing tan \(\delta\) above 80 \({}^{\circ}\)C. The latter behavior implies that M3, M4 and M5 would not withstand stress and thus, they are prone to develop failures like r  
Figure 4: Isocronal plots of G*/sin \(\delta\) versus temperature of modified asphalt M1–M6.  
Figure 5: Isochronal plots of tan \(\delta\) versus temperature at 10 rad/s for modified asphalt M1–M6.