*3.3. Effect of CNF Addition on the Physical Properties of BF/CNF Fibrous Preforms*

It is essential to deposit a nanomaterial on a microfibrous structure when preparing a nano/micro hybrid fibrous preform. The nanomaterial deposition process can be primarily carried out through impregnation and spraying methods [35]. Separate equipment is required for the spraying method, and penetration into the fiber structure may be challenging. BF/CNFs hybrid preforms can be constructed through a dehydration technique after

thoroughly mixing and stirring the CNF aqueous solution and BF in the immersion method. This immersion mixing method also has the advantage of being able to easily control the added CNFs in the BF/CNF framework by varying the concentration of aqueous CNFs. However, a sufficient number of CNFs may not penetrate the BF nonwoven structure when a larger volume of aqueous CNFs is added over the microfiber's absorption properties or when high-viscosity CNFs are input according to a high concentration. Figure 4 illustrates the dyeing images and weight results of BF/CNF fibrous preforms prepared by varying the content of CNFs. Even with the same bamboo cellulose, there is a difference in dyeing kinetics depending on the fiber morphology. For micrometer-scale BFs, an adequate dyeing process does not undergo because the dye molecules cannot be adsorbed on the BF microfiber's surface and diffuse into the interior in a short dyeing time of 10 min [36]. On the other hand, for nanoscale CNFs with a high surface area of the nanofibrillar morphology, the dye molecules easily and rapidly adsorb onto the CNF surface, and the color remains satisfactory even after the washing process.

**Figure 4.** Effect of CNF addition on the visual inspection of BF/CNF fibrous preforms. (**a**,**b**) Optical images of Direct Red 80 dyed BF/CNF fibrous preforms, and (**c**) weight-gain ratio results.

This difference in dyeing properties between BFs and CNFs made it possible to confirm the distribution of CNFs present in the BF/CNF preform. As shown in Figure 4, as the content of CNFs increased to 15 wt.%, the red color of the BF/CNFs gradually became darker, meaning that more CNFs were present in the BF/CNFs. On the other hand, when 20 wt.% CNFs were included, there was no significant difference in color from BF/CNF15, implying that more than 15 wt.% of CNFs can no longer participate in BF fiber coating and fiber-to-fiber bonding; these CNFs are separated in the dehydration process. In addition, the difference in dyeability of the BF/CNFs preform according to the amount of CNFs added was consistent with the actual weight increase result.This pattern was similar to the results of manufacturing silk fiber/CNF fibrous preforms [37]. Altogether, the amount of CNFs in the BF/CNF preform can be easily controlled by varying the concentration of aqueous CNF solutions.

When CNFs are added to the BF fiber nonwoven fabric manufacturing process, the nanofibrils coat the microfibers, and the interfacial bonding between the microfibers is more strongly induced [38]. Since CNFs have a higher surface area (14.5 m<sup>2</sup> ·g −1 ) than wood pulp (1.8 m<sup>2</sup> ·g −1 ) [17], they can be expected to act as a sufficient binder even with a small amount of CNFs. Furthermore, among various nanocellulose morphologies, nanofibrillated CNFs with a high aspect ratio could be more effective in bonding BF microfibers than roadlike cellulose nanocrystals (CNCs). The absolute and apparent densities were measured to examine the effect of CNF binder addition on the BF/CNF fibrous preform physical properties (Figure 5). Even when CNFs were added, the absolute density did not change significantly because both BFs and CNFs were composed of the same cellulose. However, the apparent density is calculated by including the pore volume of materials, so the volume increases as the CNF rises. The quantity of CNFs grows because CNFs are located in the void structure of the BF nonwoven structure. This rise in the apparent density and the resulting fall in porosity have a high correlation. The BF nonwoven fabric has the highest

porosity of 43.4%, but when the CNFs were included as a binding material, the coating and interfibrillar bonding of CNFs onto the BF nonwoven structure were able to occur; therefore, BF/CNF15 has the lowest porosity of 35.4%.

**Figure 5.** Effect of CNF addition level on the physical properties of BF/CNF fibrous preforms. (**a**) Absolute density, (**b**) apparent density, and (**c**) calculated porosity.

To investigate the effect of CNF addition on the morphology of the BF/CNF fibrous preform, FE-SEM observations were conducted, and the corresponding images are presented in Figure 6. In the case of nonwoven BF, no change in fiber diameter and morphology was observed despite its thermocompression. It has a porous, nonwoven structure, and the fibers are connected smoothly. This means that the previously selected preforming conditions (120 ◦C for 10 min) do not cause physical damage to the preform. For BF/CNF5, the CNF bundle is coated on the surface of the BF fiber. More CNF-coated BFs were observed for BF/CNF10, and CNF bundles began to distribute in the interfibrillar space of the preform structure. CNF bundles cover the entire surface of the BF/CNF preform in BF/CNF15, and the pore structure is significantly lost. Based on the morphology findings, the addition of CNFs increases BF fiber coating and the interfacial bonding between microfibers.

**Figure 6.** FE-SEM images of BF/CNF fibrous preforms with various amounts of CNFs. The insets represent high-magnitude images. (**a**) BF, (**b**) BF/CNF5, (**c**) BF/CNF10, and (**d**) BF/CNF15.
