1. Introduction
Natural fillers can be obtained from forestry and agricultural wastes, and this includes olive pomace, which is a by-product of the olive oil production industry. Considerable amounts of these wastes are produced, and they present an environmental hazard in olive oil-producing countries. Therefore, it is extremely important to safely handle such materials [
1,
2]. It is estimated that one ton of olives is responsible for producing 0.6 tons of olive mill solid residue [
3,
4]. Olive pits (OP) are residues that form part of the solid wastes produced by the olive oil manufacturing industry during the processing and extraction of olive oil from olives [
5,
6]. Some of the negative effects that result from the spread of olive solid waste in the fields are (i) inhibition of microbial activities, (ii) reduction in seed germination, (iii) and alteration of the soil characteristics in terms of the porosity and humus concentration. Accordingly, research for identifying new possible uses for the by-products of olive processing, particularly the solid ones, is crucial for the economy and environment [
7,
8,
9]. The properties of olive stone flour, which include its abundance, biodegradability, ease of processing, low density, and low cost, make it a promising organic filler [
10,
11].
The properties of composites depend on a variety of factors such as the fiber–matrix adhesion, fiber length, fiber content (loading), fiber treatment, and fiber dispersion in the matrix [
12,
13,
14,
15]. When manufacturing natural-fiber-reinforced polymer composites, weak interfacial bonding occurs between the natural fibers and polymer matrices owing to the hydroxyl groups in natural fibers [
16,
17,
18]. Extensive studies have been performed to understand the effect of chemical treatment on natural fibers. The hydrophilic nature of the natural fibers and the hydrophobic nature of the polymeric matrices leads to phase separation, thus resulting in weak bonding at the matrix–fiber interfaces of natural fiber composites. Chemical treatment of natural fibers decreases the inherent hydrophilicity of the fibers and improves the adhesion between the matrix and fibers [
19,
20].
The inconsistent performance of natural fillers compared to synthetic fillers is one of the main limitations for the commercialization of natural fillers. This inconsistency is due to the source of the natural fillers, leading to variations in the chemical composition of a plant, filler processing, and production methods [
21,
22]. However, the inclusion of natural fillers to strengthen polymers depends mainly on the properties of the fillers. In addition, the geometry, defects, inconsistency, crystallinity, and structure of the fillers are other factors influencing the behavior of the fillers. Hence, the morphology, mechanical properties, and chemical composition of the fillers can be significantly controlled and improved through biological, physical, and chemical treatments [
23]. Among these, chemical treatments, including alkali treatment (mercerization), bleaching, acetylation, and benzoylation, are currently the most popular for natural fillers. The chemical treatment of fibers includes leaching out amorphous–nanocrystalline–biomass materials and removal of surface impurities and other substances. Therefore, the treated fillers reinforce the polymers by functioning as load-carrying elements owing to the improved filler–polymer compatibility, which provides strength and rigidity to the produced biocomposites [
24,
25]. Such treatments roughen the surface of the natural filler, and the removal of surface impurities promotes better filler–polymer interfacial compatibility and bonding, thus improving the overall performance of the produced biocomposites [
26].
Thermoplastic polymer composites have been studied and researched extensively owing to their low cost and excellent mechanical properties. Research has particularly focused on the utilization of renewable resources that are being integrated into composite manufacturing owing to socioeconomic pressures for producing biodegradable materials and lowering costs [
27,
28,
29]. The most common polyethylene types are low-density polyethylene (LDPE) and high-density polyethylene (HDPE). LDPE provides several advantages in the automotive industry such as fuel savings, chemical resistance, ease of processing, corrosion resistance, and electrical insulation, as reported in a previous study [
30]. However, the use of recycled LDPE (rLDPE) as a polymer matrix for producing reinforced composites can be a serious environmental problem owing to the non-biodegradable properties of LDPE. Among the few studies on rLDPE-based bio-composites, a noteworthy one conducted by Youssef et al. [
28] demonstrated that the tensile strength of the composites increases as the fiber percentage increases up to 10% and then slightly decreases. Nevertheless, as the fiber content increases, several issues related to the microstructure are observed, which deteriorate the mechanical properties [
31]. Owing to the hydrophilic nature of rice husk, the strength of rice husk/rLDPE composites decreases with the increasing natural filler content [
32]. However, the incorporation of up to 6% nanosilica and 4% nanoclay was determined to be optimal. Excessive amounts of nanoparticles can agglomerate, resulting in gaps and cracks in the prepared eco-composites [
33]. Meanwhile, incorporating rLDPE with up to 40 wt.% of cocoa waste degrades the strength and elongation, and the material rigidity increases [
34].
Hence, based on the existing studies [
3,
35], we concluded that LDPE and OP waste are abundant but not widely used in producing materials, particularly biocomposites. Therefore, this study evaluated the various properties of natural-filler-based polymer biocomposites fabricated from the residue of OP and rLDPE in the form of a powder. The effect of using chemically treated OP as the raw material on the performance of the produced biocomposite was investigated. The physical, chemical, thermal, and mechanical properties of the developed biocomposites were thoroughly analyzed in this investigation.