*3.5. Mechanical Properties of BF/CNF Preform Reinforced PBS Composites*

Polybutylene succinate (PBS) was used as a biodegradable polymer matrix to examine the effect of the physical properties of BF/CNF preform on the mechanical properties of the final biodegradable FRP. PBS is known to be biodegradable in a composting environment and using natural succinic acid as a monomer maximizes its valorization [39–41]. Since PBS has a lower melting temperature (~120 ◦C) than PLA and Polypropylene (PP), it

has the advantage of preventing the thermal decomposition of natural fiber preforms when manufacturing FRP composites [2,39,42]. Figure 8a displays a direct impregnation process for PBS-BF/CNF FRP composite preparation considering cost-efficiency [31]. As shown in Figure 8b, the PBS-BF/CNF FRP composite exhibited a color similar to the BF/CNF preform, implying that the preform did not undergo thermal decomposition and subsequent yellowing during the direct PBS impregnation process. Figure 8c shows the representative stress–strain curves of the PBS-BF/CNF FRP composite tested under uniaxial tension. PBS mainly exhibited elastic deformation behavior, low tensile strength, and Young's modulus of 13.4 MPa and 270 MPa, respectively. When the BF/CNF preform was used as a reinforcing agent [43,44], the FRP composite modulus began to increase rapidly, and this phenomenon was evident in BF/CNF15 with its high CNF content. As a result, PBS-BF/CNF15 improves the tensile strength by 240% (26.4 MPa), and Young's modulus by 700% (1.6 GPa), compared to PBS. The improved mechanical properties mean that the BF fiber's properties do not merely exhibit the BF/CNF preform's reinforcing effect, but the improved mechanical properties of nano/micro fibrous preforms directly participate in the reinforcing effect of FRP [35]. In addition, the nanobrush surface morphology formed on the CNFs coated on BF could maximize the interaction with the PBS matrix [19,45,46].

**Figure 8.** Mechanical properties of BF/CNF reinforced PBS composites. (**a**) Schematic of the PBS-BF/CNF composite fabrication process, (**b**) optical images of PBS-BF/CNF composites, and (**c**–**f**) tensile properties of PBS-BF/CNF composites.
