*3.2. Optimization of the Preform Bamboo Fibrous Composite Process*

Manufacture of preform-type fiber reinforcement not only minimizes separation due to the difference in compatibility between the fiber and the matrix polymer, but also makes it possible to easily manufacture fiber-reinforced plastic materials through methods such as laminating and impregnation of polymer melt [31,32]. When preparing a fibrous composite preform using nanocellulose, the manufacturing process proceeds to wet the CNF aqueous solution to microfibers, dehydration, and thermocompression processes. Among these processes, thermocompression greatly influences the thickness and moisture content of the fiber preform. In addition, to design energy-efficient polymer processing, it is necessary to optimize the heat treatment temperature and compression time in the thermocompression process. To find the optimum preforming condition for the rigid fibrous preform along with an effective dehydration process, the physical properties of the BF/CNFs determined according to the thermocompression temperature were confirmed through the thickness and moisture content (Figure 2). As seen from the figure, the color change of BF/CNF did not occur significantly until heat treatment at 120 ◦C, whereas yellowing occurred when thermal compression was performed at high temperatures of 160 ◦C and 200 ◦C. This yellowing phenomenon of the BF/CNF preform due to the high heat treatment temperature was caused by thermal degradation of cellulose [33]. The more severe the yellowing, the more degradation proceeded [34]. Since sufficient dehydration does not occur in the low temperature condition of 20–80 ◦C, the thickness of the preform is thick and not uniform, which means that it does not provide sufficient structural stability when used as a reinforcing preform for fiber reinforced plastics. As the heat treatment temperature increases, the thickness and water content of the BF/CNF decreases, meaning that heat treatment at a sufficient temperature is required for dehydration of the absorbed water molecules in the wetting process. Through a sufficient dehydration process, the gap between BF and CNFs becomes close, which means that various types of interactions, including hydrogen bonds between nano and microfibers, can easily occur. In summarizing the above results, it was found that 120 ◦C could be an optimal temperature condition necessary to minimize the preform's moisture content while preventing thermal decomposition of both BFs and CNFs.

**Figure 2.** Effect of thermocompressing temperature on the physical properties of BF/CNF fibrous preforms. (**a**) Optical images, (**b**) thickness, and (**c**) water contents of BF/CNF fibrous preforms.

In fibrous preform manufacturing, the pressing time is a crucial factor along with the pressing temperature. The preform was manufactured by varying the thermocompression time from 0 to 15 min at 120 ◦C to establish an energy-friendly manufacturing process by minimizing the time while ensuring the BF/CNF composite's structural stability. Figure 3 depicts the thickness and moisture content results of the BF/CNF preform according to the thermocompression time. The optimum temperature is confirmed as 120 ◦C because the color change does not appear during the 15-min thermocompression process. As the thermal compression time lengthens, the moisture content and thickness of the preform fall because the water molecules that participate in the wetting process of BF/CNF evaporate. This drop in moisture content and thickness decreases continuously until a thermal compression time of 10 min when equilibrium is reached. Compared with the initial BF/CNF preforms, which only proceeded with 1-min thermal compression (1.47 mm thickness and 21% moisture content), the preforms that underwent 10 min of thermal compression reduced the moisture content to 1.8%. The thickness was reduced to 1.18 mm because the BF/CNFs could penetrate the evaporated water space. Taken together, the optimal condition for minimizing energy consumption, and ensuring sufficient dehydration and a rigid preform structure, is thermal compression bonding at 120 ◦C for 10 min.

**Figure 3.** Effect of thermocompression time on the physical properties of BF/CNF fibrous preforms. (**a**) Optical images, (**b**) thickness, and (**c**) water contents of BF/CNF fibrous preforms.
