**4. Conclusions**

An environmentally benign ethanol/water mixed solvent method was used to prepare a PP-SiP/CNF composite. The method applied in this work is very convenient, requires less energy, and sidesteps chemical modification. It is noted that a synergistic effect of the hybrid filler occurred in this composition, showing a significant enhancement of the dispersion of CNFs in the hydrophobic polymer. Morphology results confirmed the deposition of SiPs onto the CNF surface, resulting in good dispersion of the hybrid filler in the PP matrix and preventing the agglomeration of CNFs. The incorporation of filler into the PP matrixes significantly increased the mechanical properties of the composites. Sample PP-SCNF exhibited higher tensile strength at 36.8 MPa (increments 73.55%) compared to the neat PP. However, its elongation sharply reduced due to the stiffening effect of the filler. The thermal stability of PP composites was improved by the incorporation of fillers as a physical barrier. The hybrid filler exhibited synergistic effects, especially for tensile strength, and proved to be more effective than single-filler systems. The simplicity of this strategy can be applied to other fillers such as metal oxide and graphene oxide to prepare hybrid fillers for designing polymer composites, especially to enhance the mechanical properties and thermal stability of a product.

**Author Contributions:** Conceptualization, Y.A.; Methodology, Y.A. Validation, S.M.; Formal analysis, S.M.; Investigation, S.M., K.E. and S.T.S.; Resources, Y.A.; Data curation, Y.A. and H.A.; Writingoriginal draft preparation, S.M.; Writing-review and editing, J.L. and Y.A.; Visualization, S.M.; Supervision, Y.A.; Project administration, Y.A.; Funding acquisition, Y.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by aXis (Accelerating Social Implementation for SDGs Achievement) project funded by Japan Science and Technology Agency (JST), and Kyushu Institute of Technology (Kyutech) and Universiti Putra Malaysia (UPM) were supported by co-funding research program.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented are contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.
