*2.4. Characterization of BF/CNFs*

Optical images of BF and CNF were obtained using a Galaxy Note 10 (Samsung, Seoul, Korea). The morphology of the BFs was observed through a Field emission scanning electron microscope (FE-SEM, SUPRA 55VP, Carl Zeiss, Oberkochen, Germany), and the morphology of the CNFs was detected using a Field emission transmission electron microscope (FE-TEM, JEM2100F, JEOL, Tokyo, Japan). The thickness of the BF/CNF preform was measured using a thickness measuring device (CD-20CP, Mitutoyo, Kawasaki, Japan). The preform's moisture content was measured using a moisture meter (MB120, OHAUS, New Jersey, United States). The BF/CNF preform was immersed in 60 mL of Direct Red 80 dye solution (0.0025 wt.%) and incubated for 30 min to visualize the distribution of CNFs within the BF nonwoven structure. Next, distilled water was applied to remove the unreacted dye molecules from the sample until no red color was observed on the paper towel, which was pressed onto the BF/CNF fibrous composite sample.

The BF/CNF preform's physical structure was confirmed by measuring the porosity, and the absolute density was determined by He pycnometry (AccuPyc 1330, Micromeritics, Georgia, United States). Samples were weighed before being loaded into the measuring chamber. The nonwoven preform's apparent density (ρe) was calculated from the mass and sheath volume of the nonwoven preform. The porosity of the preform was computed with the following Equation (1):

$$\text{Porosity (\%)} = \left\{ \frac{1 - \text{Envelope density}}{Absolute Density} \right\} \times 100 \tag{1}$$

The morphological properties of the BF/CNF preform were observed through FE-SEM. A pull-to-break experiment was performed using a universal testing machine (34SC-1, Instron, Massachusetts, United States) to evaluate the BF/CNF preform's mechanical properties. A preform sample with a width of 10 mm, a length of 20 mm, and a thickness of 0.8 mm was utilized. A 1 kN load cell was applied, and the experiment was carried out at a tensile speed of 0.5 mm·min−<sup>1</sup> . The experiment was repeated five times to obtain the average value and standard deviation of tensile strength, modulus of elasticity, elongation, and toughness. The PBS-BF/CNF FRP mechanical properties were measured

through a tensile test, and the experiment was performed under the same conditions as the BF/CNF preform.
