*3.1. Silica/CNF Filler Mechanism in PP Polymer*

Fine particles aggregated due to van der Waals forces or chemical bonds [17], and CNFs agglomerated due to strong fiber–fiber hydrogen bonding coupled with their polar nature, especially in a non-polar polymeric environment [18]. We hypothesized that the silica particles would adhere to the CNFs' surface, which weakens the hydrogen bonds and prevents agglomeration. Figure 1 shows the interaction of the fine particles and CNFs if mixing in solvents. When the ratio of the fine particles is low, CNFs will agglomerate, and if too high, the fine particles will agglomerate. To further elucidate the respective mechanisms, SiPs were utilized and mixed with CNFs in ethanol/water mixed solvent. In this research, we only focus on the specific ratio of CNFs and SiPs first in order to investigate the basic interactions of the hybrid filler.

**Figure 1.** Interaction of fine particles and CNFs in ethanol/water mixed solvent.

Figure 2a,c shows the hybrid filler preparation in the ethanol/water mixed solvent and the possible hybrid filler mechanism in the hydrophobic polymer. Hydrophilic CNFs caused irreversible agglomeration during drying (Figure 2b), and due to the formation of additional hydrogen bonds between fibers, hydrophilic CNFs induced aggregation in the non-polar matrix [19]. Therefore, SiP powder was used to prevent the CNFs' aggregation from occurring when incorporated into the hydrophobic polymer. From the observation of the hot-press film, the hybrid filler prevented the aggregation of CNFs in PP composites, and a good dispersion rate and transparency were achieved (Figure 2c).

**Figure 2.** Images of SiP/CNF filler; (**a**) hybrid filler preparation, (**b**) agglomerated CNFs without SiPs and (**c**) hybrid filler mechanism in PP polymer.

Based on Figure 2a, the SiPs were mostly localized at each of the CNF fibers. Interaction between SiPs and CNFs was posited to be due to the presence of hydrogen bonds, due to the large surface areas and high amount of hydroxyl groups present in SiPs [20,21]. To further study the effect of hybrid fillers in hydrophobic polymer, PP polymer as a polymer matrix was melt-blended with fillers using a twin-screw extruder. Then, a hot-press sheet of the composite was prepared for further analysis. The chemical structures, thermal stabilities, and degree of substitution of MCC with different reaction temperatures and times were analyzed and investigated.
