*3.3. Optical Properties of the PP Composite with Fillers*

The effect of different types of silica and CNFs on optical properties was elucidated. The PP composite was melt-blended through a twin-screw extruder. Figure 4 shows the PP composite after melt-blending and hot-pressing for analyses and the study of mechanical properties. Pristine PP (Figure 4a) acted as a reference. Figure 4b,c demonstrates the PP composite with SiP SS and SiP SP, while Figure 4e,f shows the PP-SS/CNF and PP-SP/CNF composites. Figure 4d presents the PP composite with only CNFs as the filler.

Based on observations, excellent dispersibility was noticed for the PP composite with SiPs and also the PP composite with SiPs/CNF. However, significant agglomeration or poor dispersion of CNFs can be observed in PP/CNF composites. This scenario was posited to be due to the different surface properties and the hydrophilic nature of CNFs. Inherently strong intra- and inter-molecular hydrogen bonds of CNFs led to a strong affinity to itself. Achieving good dispersion of fillers within polymer matrixes is the most critical factor in determining their resultant mechanical performance [22].

**Figure 4.** Digital images showing the transparency of (**a**) PP, (**b**) PP-8.75 SS, (**c**) PP-8.75 SP, (**d**) PP-1.25 CNF, (**e**) PP-SS/CNF, and (**f**) PP-SP/CNF.

UV-vis spectroscopy analysis was carried out to further support the optical transparency results of the PP nanocomposites. Figure 5 shows the UV-vis transmittance spectra of the PP composite with the hybrid fillers, and the percentages of visible light transmittance values of the PP composite are summarized in Table 2.

**Figure 5.** UV–vis transmittance spectra for PP and PP composite.

**Table 2.** Optical properties of PP and PP composite.


The UV-visible spectra of PP nanocomposites were analyzed in the wavelengths between 200 and 800 nm. Based on data given in Table 2, the transmittance of plain PP was about 65.3 %T at 660 nm. After adding the hybrid fillers, the transmittance value was reduced. The transmittance efficiency decreased due to blocking by the hybrid filler. The dispersity of the reinforcement will affect the transparency of the nanocomposite. The transmittance level of the PP SP/CNF (44.6 %T) was higher than the PP SS/CNF (19.9 %T), even provided the same hybrid filler loading level. This scenario was due to the higher hydrophobicity of SP making it easier to incorporate with the hydrophobic PP polymer matrix.
