**1. Introduction**

A polymer matrix composite is a material consisting of a polymer matrix with a reinforcing dispersed phase. Incorporating inorganic fillers into a polymer matrix can give the composite unique properties such as rigidity, high thermal stability, good mechanical property, flexibility, and ductility [1]. Fillers with particle sizes in the 1–100 nm range are defined as nanofillers. Generally, nanofillers are categorized into three types based on their geometries: one-dimensional (1D, rod-like), two-dimensional (2D, platelet-like), and three-dimensional (3D, spherical) materials [2]. Important factors used to determine the reinforcing effect of these fillers are the polymer matrix's properties, the nature and type of filler, concentration of polymer and filler, and particle size as well as particle distribution [1]. Moreover, the most common nanofillers include metallic nanoparticles, polyhedral oligomeric silsesquioxane, carbon nanomaterials, graphite nanoplates, silica nanoparticles, and nanocellulose [3–5]. These nanofillers are inserted into polymer matrixes improve the mechanical properties, gas and solvent barrier properties, thermal degradability, and chemical resistance of the polymer [6,7]. In contrast, traditional micro-fillers can lead to polymer embrittlement, loss of transparency, and loss of lightness.

**Citation:** Mustapha, S.; Lease, J.; Eksiler, K.; Sim, S.T.; Ariffin, H.; Andou, Y. Facile Preparation of Cellulose Fiber Reinforced Polypropylene Using Hybrid Filler Method. *Polymers* **2022**, *14*, 1630. https://doi.org/10.3390/polym14081630

Academic Editors: Wei Wu, Hao-Yang Mi, Chongxing Huang, Hui Zhao, Tao Liu and Carlo Santulli

Received: 1 March 2022 Accepted: 15 April 2022 Published: 18 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Cellulose fiber has drawn considerable interest as natural filler in polymer composites due to its superior mechanical properties, eco-friendliness, processability, biodegradability, biocompatibility, low toxicity, cost savings, and improved fuel efficiency [8,9]. However, it is difficult to disperse it properly in hydrophobic polymer because of their different surface properties. Fillers with various properties are used as hybrid fillers along with reinforcing fibers to further enhance physical and mechanical properties.

Researchers have utilized more than one filler material to investigate the synergistic effect of fillers on the final properties of polymer composites [10]. Anwer et al. [11] prepared nanocomposites using epoxy resin with carbon nanofibers (CaNFs), graphene nanoplatelets (GNPs), and a hybrid combination of CaNFs/GNPs as fillers. These composites were processed with and without the use of surfactants. It was proposed that GNPs prevented agglomeration of CaNFs during the process, leading to larger particle aspect ratios in the nanocomposite. In addition, Kwak et al. [12] successfully prevented the agglomeration of CaNFs using fish-derived gelatin during the dehydration process. According to Thomas et al. [13], a hybrid filler of carbon nanotubes (CNTs)/clay enhanced dispersion through the synergism and prevented agglomeration in the composite blend. A hybrid filler can improve crosslink density, tensile strength, and tear resistance due to the large contact area between clay and CNTs.

Typically, silica/cellulose-reinforced polymer composites have been prepared using the sol-gel method. Li et al. [14] synthesized cellulose nanocrystal/silica hybrids using TEOS as the silica precursor. This hybrid material was melt-blended with ultra-high molecular weight polyethylene (UHMWPE) polymer in a twin-screw extruder. The nanocomposite showed improvement in its flexural modulus and tensile and flexural strength. Although this sol-gel method is widely used to produce hybrid fillers, the precursors, such as TEOS, are relatively expensive and contain high amounts of embedded energy [3,15].

Herein, an ethanol/water mixed-solvent method was proposed as an accessible, fast, and low-cost protocol to prepare silica particles (SiPs) with cellulose nanofibers (CNFs) without chemical modification. Ethanol is miscible with water at any ratio, and the addition of ethanol to water can easily change its physicochemical properties. The in situ nucleation and growth of silica onto cellulose occurs rapidly in an aqueous solution. However, the addition of ethanol to water can restrain the diffusion of ions and the nucleation and growth of the silica. It is well known that rapid nucleation and slow growth favor the formation of particles with narrow size distribution. Thus, we chose ethanol/water mixed solvents as the solvent system.

Commercialized SiPs and CNFs were dispersed in the mixed solvent, and SiPs were deposited onto the CNF surface. During the solvent evaporation process, it was expected that the SiPs could prevent the agglomeration of CNFs. The interaction between the silica surface group and cellulose chain prevented immediate aggregation. To the best of our knowledge, there are no studies on incorporating cellulose fibers into hydrophobic polymers without surfactants or chemical modifications [3].

In this study, polypropylene (PP) was used as a polymer matrix. It is a widely utilized polymer with various advantages including low cost, superior transparency, good moisture barrier properties, and high recyclability compared to other polymers [16]. The composites, PP with fillers (SiPs, CNFs or SiP/CNF) were prepared by melt-blending in a twin-screw extruder. The effects of fillers and hybrid fillers in PP matrixes were investigated by the analysis of the matrixes' morphological, chemical, thermal, and mechanical properties. The performance of pulverized SiPs and pulverized CNFs as filler in PP composites was also identified.
