Valorization of date palm waste and recycled HDPE into functional bio-composites via chemical fiber treatment and Nano-ZnO synergy

Abstract

The development of eco-friendly composites using recycled high-density polyethylene (rHDPE) and date palm leaf (DPL) fibers is often hindered by poor interfacial bonding. This incompatibility arises from the hydrophobic nature of the polymer matrix versus the hydrophilic nature of the natural fibers. The aim of this study is to investigate the effects of fiber treatment and nanoparticle modification on the microstructure and performance of rHDPE/DPL/ZnO composites, prioritizing efficient fiber-side treatments over matrix processing. This study was carried out by chemically treating DPL fibers with sodium hydroxide (NaOH) and stearic acid, followed by modification with zinc oxide (ZnO) nanoparticles to improve adhesion. The composites were then subjected to a comprehensive suite of characterization tests, including tensile testing for mechanical strength, thermogravimetric analysis (TGA) for thermal stability, differential scanning calorimetry (DSC) for thermal transitions, X-ray diffraction (XRD) for crystalline structure, and optical microscopy for morphological evaluation. The findings show that the treatments significantly enhanced the material properties across all metrics. Tensile testing revealed that the A5 composite achieved the highest mechanical performance, with a Young’s modulus of approximately 1.2 GPa and an elongation at break of roughly 9%. TGA results indicated improved thermal stability; specifically, sample A4 showed a degradation temperature (Td) increase of 8.7 °C (+1.83%) over A3, while sample A6 showed a gain of 5.1 °C (+1.07%) over A5. XRD analysis confirmed an efficient structural reinforcement, reaching a maximum crystallinity of 74.06%. Furthermore, melt flow index (MFI) analysis demonstrated that while fiber reinforcement naturally increases viscosity, stearic acid treatment provides a lubricating effect that maintains MFI values between 6 and 7 g/10 min, ensuring excellent processability for injection molding and 3D printing. Finally, optical microscopy and morphological studies observed superior fiber dispersion and cleaner interfaces, indicating that the fibers were evenly spread and well-separated within the rHDPE structure. It was concluded that the combination of chemical treatments and ZnO nanoparticle modification effectively bridges the compatibility gap between rHDPE and DPL fibers. These findings underscore the viability of these modified composites for high-performance structural and industrial applications.

Keywords

recycled HDPE date palm leaf fibers composite materials fiber treatment thermal stability

Introduction

In recent years, the advancement of green technologies has driven considerable interest in the development of sustainable composite materials. Among these, natural fiber-reinforced composites (NFRCs) have gained prominence due to their favorable attributes, including low density, cost-effectiveness, biodegradability, and respectable mechanical performance.^1,2 Concurrently, the agricultural sector, particularly in the Middle East and North Africa (MENA) region, faces a pressing environmental challenge in managing the substantial quantities of biomass waste generated from date palm cultivation. Each date palm tree produces approximately 3.26 to 35 kg of lignocellulosic waste annually,³ an underutilized resource that poses significant ecological and economic burdens.⁴ In response to these challenges, a paradigm shift is underway in materials science and engineering, emphasizing the valorization of agricultural residues, especially natural fibers, as viable and sustainable alternatives to synthetic reinforcements.^5,6 A key area of innovation involves the design of advanced polymer matrix composites incorporating both natural fibers and inorganic fillers to achieve enhanced mechanical strength, dimensional stability, and lightweight performance suitable for a range of structural and functional applications.⁷ Despite the inherent advantages of NFRCs, several limitations persist that hinder their deployment in high-performance and long-lifecycle applications. These include low intrinsic mechanical strength, poor moisture resistance, dimensional instability, thermal degradation, and weak fiber-matrix interfacial bonding.^8–10 To address these drawbacks, researchers have explored strategies such as hybrid reinforcement using both organic and inorganic fillers,^11,12 as well as surface modifications like alkali treatment and nanoparticle functionalization to enhance compatibility.^9,13 The incorporation of nanofillers, particularly zinc oxide nanoparticles (ZnO NPs), has emerged as a promising approach for tailoring interfacial properties and optimizing microstructural characteristics.^12,14 ZnO NPs are semiconductor materials known for their unique multifunctional properties, including high chemical stability, a broad range of radiation absorption, and a high surface-area-to-volume ratio.¹⁵ In the context of polymer composites, they serve as effective reinforcing agents that can improve thermal stability and mechanical stiffness. Furthermore, ZnO NPs provide secondary functional benefits such as photocatalytic activity, UV shielding, and antimicrobial performance,^15,16 which can protect the underlying natural fibers from environmental degradation.

However, a significant research gap remains concerning the synergistic optimization of recycled High-Density Polyethylene (rHDPE) reinforced with date palm fibers subjected to a dual-functionalization of alkaline treatment and stearic acid coating in the presence of ZnO nanoparticles. While individual treatments are well-documented, the collective impact of these specific chemical modifications combined with nano-reinforcement on the recycled matrix remains under-explored. The primary advantages of this study lie in its strong commitment to sustainability through the utilization of a 100% recycled polymer matrix and agricultural waste, which directly supports the circular economy, and its cost-effectiveness achieved by prioritizing fiber treatment over matrix modification to reduce both chemical consumption and processing time. Furthermore, the study leverages synergistic reinforcement, where dual-functionalization with alkali and stearic acid resolves the hydrophilic/hydrophobic mismatch, while the addition of ZnO NPs provides a critical “bridge” for enhanced stress transfer across the interface. Conversely, the limitations include the inherent processing complexity of the multi-step fiber modification sequence (NaOH, stearic acid, and ZnO) compared to untreated alternatives, as well as significant dispersion challenges, where achieving a uniform distribution of nanoparticles within the rHDPE/fiber interface is essential to prevent agglomeration that could otherwise lead to detrimental stress concentrations.

The objective of this research is to exploit chemically treated palm leaf fibers in the development of novel rHDPE/DPL/ZnO composites designed for sustainable applications in automotive interiors and construction decking. The methodology employs a multi-stage modification process: alkaline treatment with NaOH is utilized to enhance mechanical interlocking by selectively removing amorphous hemicellulose and lignin, thereby increasing fiber surface roughness, followed by stearic acid functionalization, which imparts essential hydrophobicity to the fibers and significantly reduces moisture uptake when embedded within the non-polar polymer matrix. By coupling these specific surface treatments with maleic anhydride as a compatibilizing agent, this research seeks to optimize the interfacial bridge between the polar fibers and the non-polar rHDPE matrix. This approach not only maximizes the performance of the fabricated rHDPE/DPL/ZnO composites evaluated through tensile strength, Young’s modulus, and thermal analysis—but also promotes a circular economy. By providing a high-value ‘second life’ to both plastic waste and abundant plant residues, this study addresses both technical performance barriers and ecological sustainability challenges inherent to multi-component bio-composites.

Experimental section

Chemicals used

In this study, recycled high-density polyethylene (rHDPE), specifically the commercial grade Hostalen GC 7260 from LyondellBasell (density: 0.960 g/cm³; MFI: 8 g/10 min at 190 °C/2.16 kg), was selected as the polymer matrix due to its favorable processability and mechanical performance. The cellulosic reinforcements incorporated in this composite are derived from date palm leaves (DPLF) collected in Boussaâda (M’sila), Algeria, a region abundant in date palm biomass. The fibers were subjected to a systematic preparation protocol, beginning with washing and drying to eliminate impurities, followed by mechanical grinding. A final sieving step was employed to isolate fibers with a uniform particle size of 250 μm.

To enhance fiber–matrix interactions, two surface modification protocols were employed: an alkaline treatment using sodium hydroxide (NaOH, ≥98%, pellets; MW:40 g/mol; CAS No.8006-28-8, Biochem-Chemopharma), and a sequential treatment combining NaOH with stearic acid Stearic acid (CH₃(CH₂)₁₆COOH; MW:284.48 g/mol; CAS No. 57-11-4, Sigma- Aldrich). These treatments aimed to remove amorphous components (hemicellulose, lignin) and improve fiber roughness, thereby strengthening interfacial adhesion.

Composite formulations of the rHDPE/DPL/ZnO composites

Fiber surface modification was carried out through a multi-step protocol. Initially, fibers underwent thermal pre-treatment at 70 °C under agitation for 4 hours to remove impurities, waxes, and greases. Following filtration, they were dried at 60 °C for 24 hours (PNT). Subsequently, an alkaline treatment was applied by immersing the fibers in a 5% NaOH solution (10 g/100 mL) at room temperature for 24 hours, followed by neutralization with 2% acetic acid to achieve a pH of 6.5 and a further drying cycle at 60 °C for 24 hours (PTN). Finally, a portion of the alkali-treated fibers was modified via agitation in a 3% stearic acid/ethanol solution at 70–80 °C for 8 hours to ensure uniform coating, followed by oven-drying at 60 °C for 24 hours, yielding the fully stabilized treated fibers (PTNS).

To further promote compatibility between the hydrophilic fibers and the hydrophobic matrix, Maleic anhydride (C₄H₂O₃; MW: 98.056 g/mol; CAS No. 108-31-6, Biochem-Chemopharma) was added as a coupling agent. It introduces polar groups to the polymer chains that react with fiber hydroxyl groups, enhancing bonding, stress transfer, and fiber dispersion during melt processing.¹⁷

Additionally, zinc oxide nanoparticles (ZnO NPs) (density: 5.6 g/cm³; average size < 5 µm) were MW: 81.39 g/mol; CAS No. 1314-13-2, Sigma-Aldrich) were incorporated as multifunctional nanofillers. ZnO incorporation enhanced the composites’ tensile and flexural properties, thermal stability, and antimicrobial performance, while simultaneously improving moisture resistance, surface wettability, and processing rheology.¹⁸

A series of composite formulations based on recycled high-density polyethylene (rHDPE) were developed to assess the effects of fiber surface treatment and additive incorporation on the material’s performance. The formulation matrix was carefully structured to encompass varying component ratios and processing conditions, as outlined in Table 1.

Table 1.

Formulation of rHDPE/DPL/ZnO composites with varying fiber treatments and additive content.

Formulation	% of rHDPE resin	Type of fiber added in terms of chemical treatment at 5%	% of compatibilization agent (maleic anhydride)	% of ZnO NPs
Composite A1 (neat rHDPE)	100	0	0	0
Composite A2	95	Untreated palm (PNT)	0	0
Composite A3	92	Palm treated with NaOH (PTN)	3	0
Composite A4	90	Palm treated with NaOH (PTN)	3	2
Composite A5	95	Palm treated with NaOH-stearic acid (PTNS)	0	0
Composite A6	93	Palm treated with NaOH-stearic acid (PTNS)	0	2

Various formulations of rHDPE-based composites were prepared via melt mixing to assess the influence of fillers and additives on the final material performance. The compounding was performed using a Brabender® plastograph under optimized conditions (190 °C, 30 rpm, 8–10 min) to achieve homogeneous dispersion while minimizing thermal degradation of the constituents. The protocol systematically incorporated different fillers, including date palm fibers, along with functional additives such as maleic anhydride and ZnO nanoparticles. Following melt mixing, the composites were mechanically ground and subsequently shaped by compression molding, producing thin films and standardized specimens for characterization.

Considering the Brabender mixer chamber volume of 55 cm³ and the HDPE density of 0.96 g/cm³, the total mass of raw material was set at 52.8 g (0.96 × 55). For instance, a formulation containing 5 wt.% untreated fibers comprised 2.64 g of fibers combined with 50.16 g of polymer. This method enabled the homogeneous integration of date palm leaf fibers, maleic anhydride, and ZnO NPs into the rHDPE matrix, thereby ensuring consistent material properties for subsequent shaping and characterization.

Characterizations

To characterize the structural, thermal, rheological, and mechanical properties of the developed composites, a comprehensive set of analytical techniques was employed using specialized. Chemical functional groups and interfacial interactions were identified via Fourier Transform Infrared Spectroscopy (FTIR) using a PerkinElmer Spectrum Two™ spectrophotometer (USA) across a range of 4000–400 cm^-1, while the crystalline phase and degree of crystallinity were analyzed through X-ray Diffraction (XRD) using a Bruker D2 Phaser (Germany) with CuKα radiation (λ = 0.15406 nm) over a 2θ range of 0°–80°. The thermal behavior was evaluated using PerkinElmer (USA) instrumentation, specifically a Pyris 1 TGA for thermogravimetric analysis from room temperature to 600 °C at 10 °C/min under nitrogen to assess decomposition, and a DSC 4000 for differential scanning calorimetry up to 250 °C at 2 °C/min to determine melting transitions. Processability was quantified by the Melt Flow Index (MFI) following ASTM D1238 at 190 °C under a 2.16 kg load using a Ceast/Instron (Italy)Modular Melt Flow Tester (Italy). Mechanical performance was measured according to ASTM D638-72 using an Instron 5969 Universal Testing Machine (USA) equipped with a 50 kN load cell at a crosshead speed of 2 mm/min, with results averaged over five replicates for statistical precision. Finally, the internal morphology and fiber dispersion were examined using an Oxion Euromex (Netherlands) optical microscope equipped with 10x and 16x eyepieces and high-quality glass optics offering up to 1000x magnification.

Results and discussions

Fourier-transform infrared spectroscopy

The FTIR spectra for the neat recycled high-density polyethylene (rHDPE) and its corresponding composite formulations (A1–A6) are presented in Figure 1, illustrating the characteristic functional groups and the interfacial chemical interactions induced by various fiber surface treatments. Virgin rHDPE (A1) exhibits distinct absorption bands at 2915 cm^-1 and 2845 cm^-1, corresponding to the asymmetric and symmetric stretching vibrations of –CH₂ groups, with additional peaks at 1470 cm^-1 (CH₂ bending) and 730 cm^-1 (CH₂ rocking) typical of polyethylene chains.¹⁹ Upon the incorporation of untreated date palm leaf fibers (A2), a notable shift in the –CH₂ stretching vibration to 2900 cm^-1 is observed, which signifies the initial physical interactions between the hydrophobic rHDPE matrix and the hydrophilic fibers; this is accompanied by new bands at 1730 cm^-1 and 1240 cm^-1 originating from the carbonyl (C=O) and C–O–C stretching of hemicellulose and lignin residues.²⁰ In the formulation containing NaOH-treated fibers and maleic anhydride (A3), the spectrum maintains these polyethylene characteristics but shows a significantly more pronounced band at 1730 cm^-1, which is attributed to the carbonyl stretching of grafted maleic anhydride (MA). This change, alongside a new band near 1640 cm^-1 linked to C=C or O–H vibrations, indicates enhanced chemical grafting and improved interfacial compatibility where the MA acts as a molecular bridge between the fiber and the matrix. The integration of ZnO nanoparticles (A4) introduces new vibrational features in the 500–600 cm^-1 range, confirming the successful incorporation of the nanofiller into the composite structure. While most samples showed stable intensity at 1730 cm^-1, sample A4 uniquely exhibits an additional shoulder near 1750 cm^-1, suggesting that the ZnO facilitated limited surface esterification, thereby further anchoring the phases together. Finally, for the stearic acid-treated composites (A5 and A6), the shift in the 720 cm^-1 band associated with CH₂ rocking and acid chain deformation confirms effective surface modification. This treatment hydrophobized the fiber surface, enhancing matrix–fiber interactions and improving the dispersion of the fibers within the rHDPE matrix while preserving the fundamental chemical integrity of the polyethylene backbone.²¹

Melt flow index

The Melt Flow Index (MFI) results, illustrated in Figure 2, provide critical insights into the rheological behavior and processability of rHDPE-based composites reinforced with date palm leaf fibers. As shown in the data, the neat recycled HDPE (A1) exhibits the highest MFI at 8.3 g/10 min, indicating low melt viscosity and excellent flowability; this behavior is particularly advantageous for high-shear processing techniques, such as extrusion and injection molding, as it promotes efficient mold filling while minimizing energy consumption.²² Upon the incorporation of untreated date palm fibers (A2), a significant reduction in MFI is observed, a trend primarily caused by increased melt viscosity. This decline stems from the inherent incompatibility between the hydrophilic fibers and the hydrophobic matrix, which leads to poor fiber dispersion and a tendency for fibers to agglomerate, thereby creating microstructural irregularities that disrupt homogeneous polymer chain mobility and restrict melt flow. In contrast, alkali (NaOH) and MA treatments (A3/A4) stabilize the MFI values by enhancing interfacial compatibility; the chemical etching of the fiber surface and subsequent coupling action allow for a more uniform fiber distribution, though the presence of solid fiber reinforcement naturally keeps the MFI lower than the neat polymer. Notably, the composites containing stearic acid-treated fibers (A5 and A6) exhibit the highest MFI values among the reinforced systems, which is scientifically attributed to the lubricating effect of the stearic acid molecules. By reducing internal friction between the fibers and the matrix and acting as a processing aid, these molecules facilitate a smoother and more stable melt flow.¹⁹ Despite these fluctuations, all tested composite systems (A2–A6) maintained MFI values within a narrow range of 6 to 7 g/10 min (at 190°C, 2.16 kg load). This consistency is vital for practical manufacturing, as maintaining an MFI below 10 g/10 min is considered essential to ensure consistent extrusion flow and dimensional accuracy in both injection molding and Fused Filament Fabrication (FFF)-based 3D printing applications.²³

TGA analysis

The results of the thermal stability tests for the rHDPE-based composites, expressed in Figure 3 and Table 2, demonstrate a clear correlation between fiber surface chemistry and the resulting thermal resistance of the material. Thermogravimetric Analysis (TGA) confirms that the specific reinforcement strategies employed significantly influence the onset degradation temperatures (T_d), incorporating reference values for DPL fibers from the work of Al-Oqla et al.²⁴

Table 2.

Thermal properties of rHDPE/DPL/ZnO composites.

Simples	T onset (°C)	T of maximum decomposition (°C)	T offset (°C)	Residual mass (%)
DPL	227	326	354	27,7
A1	408.62	463.8	486.92	0
A2	384.06	447.12	450.56	2.20
A3	425.03	474.6	497.26	0.90
A4	435.45	477.4	512.99	2.93
A5	442.40	483.3	505.60	1.27
A6	449.19	482.5	507.25	1.46

Initially, the neat recycled HDPE (A1) established a baseline stability with a T_d of 463.8°C, reflecting the inherent thermal robustness of the polymer matrix; however, the introduction of untreated date palm fibers (A2) caused a sharp decline in this property, with T_d dropping to 447°C. This initial reduction is scientifically attributed to the presence of non-cellulosic impurities, such as hemicellulose, volatile waxes, and surface contaminants, which possess significantly lower decomposition temperatures and serve as premature degradation sites that compromise the composite’s integrity at lower thermal loads.^25–29 In contrast, all chemically modified composites (A3–A6) exhibited a significant upward trend in stability, outperforming both the untreated fiber composite and the neat polymer matrix, with the most remarkable enhancements observed in the ZnO-nanoparticle-modified samples. Specifically, the transition from A3 to A4 yielded a T_d increase of 8.7°C (+1.83%), while the transition from A5 to A6 resulted in a gain of 5.1°C (+1.07%), reaching peak degradation temperatures of approximately 483.3°C.

This superior thermal performance is explained by the synergistic effects of constituent purification and advanced interfacial engineering. Alkali (NaOH) treatment effectively removes thermally unstable hemicellulose, while stearic acid and MA enhance the interfacial adhesion between the fibers and the rHDPE, creating a tighter, more uniform interface that acts as a barrier to heat transfer and limits localized thermal stresses.^30,31 The significant thermal boost in samples A4 and A6 is primarily due to the dual role of the ZnO nanoparticles; firstly, their surface modification with stearic acid ensures excellent dispersion, creating a physical “thermal maze” that hinders the diffusion of volatile degradation products. Secondly, ZnO acts as a high-efficiency thermal stabilizer by scavenging the free radicals generated during the initial thermo-oxidative degradation of the polyethylene chains, thereby interrupting the auto-catalytic degradation cycle and significantly raising the energy required for thermal decomposition.^32–35 Consequently, these modifications not only delay the onset of thermal events but also fundamentally stabilize the composite structure, extending the service life and durability of the material for high-performance industrial applications.

Differential Scanning Calorimetry

The thermal characteristics and phase transitions of the rHDPE-based composites were investigated via Differential Scanning Calorimetry (DSC), with the resulting thermograms and melting temperatures (T_m) illustrated in Figure 4. The analysis, conducted up to a maximum temperature of 250°C, revealed a consistent endothermic event centered at approximately 150°C, corresponding to the melting of the semi-crystalline rHDPE matrix. Data trends indicate that the incorporation of treated date palm leaf (DPL) fibers resulted in a slight but consistent increase in T_m values compared to the neat matrix and untreated fiber composites. Specifically, the formulations containing treated fibers (A5 and A6) exhibited sharper and more well-defined endothermic peaks than the untreated composite (A2), suggesting a more uniform crystalline structure and enhanced thermal stability.

The observed increase in melting temperature and the preservation of crystallinity is primarily attributed to the “nucleation effect” provided by the treated fibers. In these composite systems, the chemically modified DPL fibers act as heterogeneous nucleation sites, which lower the energy barrier for polymer chain arrangement and promote the growth of stable crystalline lamellae during solidification.^32,35 The surface treatments, particularly with stearic acid and ZnO, improve the fiber’s surface energy and topography, thereby strengthening the interfacial interactions with the rHDPE matrix. This improved compatibility allows the fibers to facilitate more efficient crystal growth at the interface, which not only contributes to the refined endothermic peaks observed in A5 and A6 but also enhances the overall thermal resistance of the material.²⁷

This behavior aligns with the decisive role of interfacial compatibility in natural fiber-reinforced systems. As established by Tablit et al. and Jamadi et al., the use of specialized surface treatments and additives significantly enhances fiber dispersion and stress transfer, resulting in superior thermal transitions and structural stability.^36–38 Furthermore, the transition from broad peaks in A2 to well-defined peaks in the modified samples suggests that the reduction of non-cellulosic impurities and the introduction of ZnO nanoparticles minimize structural defects within the matrix.³⁹ Consequently, the strong interfacial adhesion achieved in these modified composites promotes greater durability and thermal performance, supporting their suitability for high-performance applications.⁴⁰

X-ray diffraction analysis

The crystallographic structure of the rHDPE matrix and its various composite formulations was analyzed using X-ray diffraction (XRD), with the resulting patterns illustrated in Figure 5. The diffractogram of the neat rHDPE (A1) displays a characteristic semi-crystalline orthorhombic lattice, evidenced by two prominent diffraction peaks at 2θ ≈ 25.5° and 28.4°, corresponding to the (110) and (200) crystallographic planes, respectively. Upon the incorporation of date palm leaf fibers, a slight shift in these peaks toward lower angles was observed, indicating a marginal expansion of the lattice or the influence of the filler load on the polymer’s internal structure. As summarized in Table 3, the crystallinity values (Xc) fluctuate significantly based on the fiber state. Specifically, the incorporation of untreated fibers (A2) led to a reduction in peak intensity and a broader diffraction profile, whereas composites reinforced with chemically modified fibers (A3–A6) exhibited a marked enhancement in crystallinity, reaching a maximum value of 74.06% (Table 4).

Table 3.

Melting temperatures and corresponding enthalpies of the composites.

Samples	Tm (°C)	Area (ΔHm)
A1	142,77	788,28
A2	139,40	1069,59
A3	142,13	1343,92
A4	144,17	1036,55
A5	148,05	1504,61
A6	146,69	972,99

Table 4.

Crystallinity values (Xc%) of rHDPE/DPL/ZnO composites derived from X-ray diffraction analysis.

Composite	Xc (%)
A1	62.71
A2	60.47
A3	62.73
A4	68.15
A5	74.06
A6	64.14

The observed trends in crystallinity are directly linked to the quality of the fiber-matrix interface and the molecular mobility of the polymer chains. In the untreated composite (A2), poor interfacial adhesion and fiber agglomeration disrupt the regular folding of rHDPE chains, creating amorphous domains and a less organized molecular structure that weakens the diffraction signal. Conversely, the superior crystallinity in samples A3 through A6 is a result of the effective surface modifications (alkali and stearic acid), which remove surface impurities and improve the compatibility between the hydrophobic matrix and the fibers. This improved interface allows the fibers to function as effective heterogeneous nucleation sites, facilitating the orderly arrangement of polymer chains at the fiber surface.^31,41

The inclusion of ZnO nanoparticles, particularly in the stearic acid-modified samples (A4 and A6), further accelerates this crystalline growth by acting as a secondary nucleating agent. This structural reinforcement correlates strongly with the thermal behavior observed in the DSC analysis; the improved molecular ordering associated with higher crystallinity requires greater thermal energy to disrupt, thus explaining the elevated melting transitions noted earlier. This synergy between XRD and DSC findings, as supported by the work of Hachaichi et al., confirms that optimized fiber treatment and nanoparticle integration significantly enhance the structural integrity and thermal performance of bio-composites by promoting a more robust and organized crystalline network.⁴²

Mechanical properties

The mechanical properties of the developed rHDPE-based composites, as illustrated in Figure 6, demonstrate a significant transition from the baseline behavior of the neat matrix to a highly reinforced structural material. Tensile testing, conducted across 30 specimens with five replicates per formulation (A1–A6) to ensure statistical reliability, revealed that the A5 composite (treated with NaOH and stearic acid) achieved the most superior mechanical profile, reaching a Young’s modulus of 1.2 GPa and an elongation at break of 9%. This performance represents a highly optimized balance of stiffness and ductility, whereas the untreated fiber composite (A2) exhibited suboptimal mechanical performance characterized by premature failure and poor stress absorption. This dramatic enhancement in the A5 formulation is scientifically attributed to a dual-action interfacial mechanism resulting from the synergistic NaOH and stearic acid treatments; the alkali treatment removes non-cellulosic components to increase surface roughness for superior mechanical interlocking, while the stearic acid reduces the surface energy gap between the hydrophilic fibers and the hydrophobic matrix.

These mechanical results are fundamentally linked to the structure–property relationships validated by the previous characterizations: the chemical modification of the fibers in A5 serves as a catalyst for structural reorganization where the treated fibers act as high-efficiency heterogeneous nucleation sites. This is corroborated by the XRD data (68.15% crystallinity) and the sharper DSC melting peaks, indicating a robust crystalline network that effectively “anchors” the polymer chains and restricts their mobility. This molecular anchoring is rheologically confirmed by the Melt Flow Index (MFI) data, where the reduced flow in A5 indicates that the polymer chains are tightly bound to the DPL/ZnO reinforcements. This restricted mobility prevents localized stress concentrations and fiber pull-out, instead enabling the composite to distribute external loads across the entire reinforced network, thereby delivering the highest observed durability and structural integrity for high-performance industrial applications.^23,31,43

Optical microscopy

The morphology of the fracture surfaces for the rHDPE/DPL composites, as captured by optical microscopy micrographs in Figure 7, provides visual evidence of the impact of fiber treatments on dispersion and interfacial adhesion. In the composite reinforced with raw fibers, A2(a), the micrographs reveal a heterogeneous structure characterized by fiber aggregates and the presence of significant surface impurities. These impurities, combined with the inherent hydrogen bonding between natural fibers, promote agglomeration within the hydrophobic rHDPE matrix. In stark contrast, the micrographs for the treated composites, specifically A3(b) and A5(d), demonstrate a transition to a significantly cleaner and more uniform fiber distribution. Furthermore, the composites modified with ZnO nanoparticles, A4(c) and A6(e), exhibit the highest level of homogeneity, with a dense and well-integrated morphology.

The observed morphological differences are scientifically explained by the removal of non-cellulosic constituents and the optimization of surface energy. The presence of waxes, hemicellulose, and pectin on the untreated fibers in sample A2 creates a physical barrier that prevents the polymer matrix from wetting the fiber surface, leading to the observed fiber-to-fiber “clumping” and weak interfacial bonding. However, the alkali and stearic acid treatments effectively strip away these contaminants, exposing the cellulose fibrils and increasing the surface roughness.⁴⁴ This surface activation facilitates better mechanical interlocking and intimate contact between the fiber and the matrix, as seen in the “cleaner” interfaces of A3 and A5.

The superior homogeneity noted in A4 and A6 is attributed to the synergistic role of the treated ZnO nanoparticles; by acting as a secondary filler that bridges the gaps at the interface, these nanoparticles prevent the formation of voids and stabilize the dispersion. This morphological refinement is a visual confirmation of the structure-property relationship discussed in earlier sections: the absence of large fiber aggregates ensures that external loads are distributed evenly rather than concentrating at defect sites. Consequently, this improved interfacial integrity directly supports the high tensile strength and thermal stability recorded for these modified systems, proving that the chemical and nanoparticle treatments have successfully transformed the fiber-matrix interface from a point of weakness into a point of reinforcement.

Conclusion

This study demonstrates that the incorporation of surface-modified date palm leaf fibers (DPLFs) and ZnO nanoparticles significantly improves the structural, thermal, and mechanical performance of recycled high-density polyethylene (rHDPE) composites. The systematic application of fiber treatments, particularly with sodium hydroxide (NaOH) and stearic acid, effectively enhanced interfacial adhesion and promoted a more uniform fiber dispersion within the polymer matrix.

These modifications, combined with the nucleating and stabilizing effects of ZnO nanoparticles, resulted in composites exhibiting improved thermal stability, enhanced crystallinity, and a balanced combination of mechanical strength and stiffness.⁴⁵

The synergy between optimized natural fiber treatments and nanoparticle reinforcement provides an effective and sustainable strategy for upgrading recycled plastics. The developed composites therefore show strong potential for use in high-performance structural and industrial applications.

Footnotes

Acknowledgements

The authors express their sincere appreciation to the Institute of Materials Sciences and Techniques at Ferhat ABBAS University Setif 1, the Department of Process Engineering in the Faculty of Technology at Ferhat ABBAS University Setif 1, the Department of Mechanical Engineering at the University of Biskra, Algeria, the Department of Fuels and Renewable Energies, Faculty of Science and Technology, Ahmed Draia University, Adrar, Algeria and the University Brunei Darussalam for their significant support and collaboration on this research project.

ORCID iD

Fatma Zohra Benabid

Author contributions

Hadda Benaceur: Methodology, investigation, data curation, writing – original draft.

Haroun Houicha: Analysis, review & editing.

Oum Keltoum Mallem: Supervision, writing, review & editing.

Fatma Zohra Benabid: Supervision, conceptualization, funding acquisition, writing, review & editing.

Mohammad Mansoob Khan: Conceptualization, and writing, review & editing.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. The authors declare that none of their personal traits or recognized conflicting financial interests are thought to have impacted the work presented in this study.

Data Availability Statement

Data will be accessible upon request.*

References

Dejene

Geletaw

. Development of fully green composites utilizing thermoplastic starch and cellulosic fibers from agro-waste: a critical review. Polym-Plast Technol Mater 2024; 63(5): 540–569. https://doi.org/10.1080/25740881.2023.2296649

Mishra

Mandal

Rout

, et al. A state-of-the-art review on potential applications of natural fiber-reinforced polymer composite filled with inorganic nanoparticle. Composites Part C Open Access 2022; 9: 100298. https://doi.org/10.1016/j.jcomc.2022.100298

Awad

Zhou

Katsou

, et al. A critical review on date palm tree (Phoenix dactylifera L.) fibres and their uses in bio-composites. Waste Biomass Valor 2021; 12(6): 2853–2887. https://doi.org/10.1007/s12649-020-01105-2

Elseify

Midani

Shihata

, et al. Review on cellulosic fibers extracted from date palms (Phoenix Dactylifera L.) and their applications. Cellulose 2019; 26(4): 2209–2232. https://doi.org/10.1007/s10570-019-02259-6

Faiad

Alsmari

Ahmed

, et al. Date palm tree waste recycling: treatment and processing for potential engineering applications. Sustainability 2022; 14(3): 1134. https://doi.org/10.3390/su14031134

Jayaprakash

Osama

Rajagopal

, et al. Agriculture waste biomass repurposed into natural fibers: a circular bioeconomy perspective. Bioengineering 2022; 9(7): 296. https://doi.org/10.3390/bioengineering9070296

Ramesh

Rajeshkumar

Srinivasan

, et al. Influence of filler material on properties of fiber-reinforced polymer composites: a review. e-Polymers 2022; 22(1): 898–916. https://doi.org/10.1515/epoly-2022-0080

Amjad

Anjang Ab Rahman

Awais

, et al. A review investigating the influence of nanofiller addition on the mechanical, thermal and water absorption properties of cellulosic fiber reinforced polymer composite. J Ind Text 2022; 51(1): 65S–100S. https://doi.org/10.1177/15280837211057580

Dejene

. Exploring the potential of ZnO nanoparticle-treated fibers in advancing natural fiber reinforced composites: a review. J Nat Fibers 2024; 21(1): 2311304. https://doi.org/10.1080/15440478.2024.2311304

10.

Akter

Haris Uddin

Rahman Anik

. Plant fiber-reinforced polymer composites: a review on modification, fabrication, properties, and applications. Polym Bull 2024; 81(1): 1–85. https://doi.org/10.1007/s00289-023-04733-5

11.

Bag

Kumar

Bansiwal

, et al. Polymer matrix composites (PMCs) for defense applications. In: Namburi

Wanhill

RJH

Setua

(eds) Novel Defense Functional and Engineering Materials (NDFEM). Springer, 2024, Vol. 2, pp. 1–37. https://doi.org/10.1007/978-981-99-9795-4_1

12.

Rashid

Haque

Islam

, et al. Nanotechnology-enhanced fiber-reinforced polymer composites: Recent advancements on processing techniques and applications. Heliyon 2024; 10(2): e24692. https://doi.org/10.1016/j.heliyon.2024.e24692

13.

Mohammed

Rahman

Mohammed

, et al. Surface treatment to improve water repellence and compatibility of natural fiber with polymer matrix: Recent advancement. Polym Test 2022; 115: 107707. https://doi.org/10.1016/j.polymertesting.2022.107707

14.

Siddiqui

Rabbi

Rahman

. An experimental review on enhancing the low and high-velocity impact resistance of NFCs through nanoparticle reinforcement. J Reinf Plast Comp. Epub ahead of print 2024; 44: 1270–1298. https://doi.org/10.1177/07316844241240569

15.

Lorero

Campo

Del Rosario

, et al. New manufacturing process of composites reinforced with ZnO nanoparticles recycled from alkaline batteries. Polymers 2020; 12(7): 1619. https://doi.org/10.3390/polym12071619

16.

Dejene

. Advancing natural fiber-reinforced composites through incorporating ZnO nanofillers in the polymeric matrix: a review. J Nat Fibers 2024; 21(1): 2356015. https://doi.org/10.1080/15440478.2024.2356015

17.

Akonda

El-Dessouky

. Effect of maleic-anhydride grafting on the properties of flax reinforced polypropylene textile composites. J Text Sci Technol 2019; 5(4): 69–85. https://doi.org/10.4236/jtst.2019.54007

18.

Şomoghi

Semenescu

Pasăre

, et al. The impact of ZnO nanofillers on the mechanical and anti-corrosion performances of epoxy composites. Polymers 2024; 16(14): 2054. https://doi.org/10.3390/polym16142054

19.

Ichim

Muresan

Codau

. Natural-fiber-reinforced polymer composites for furniture applications. Polymers 2024; 16(22): 3113. https://doi.org/10.3390/polym16223113

20.

Hayeemasae

Viet

Masa

, et al. The use of kenaf fibre as a natural anti-degradant in recycled high-density polyethylene and natural rubber-based thermoplastic elastomers. Polymers 2023; 15(5): 1237. https://doi.org/10.3390/polym15051237

21.

Alsayed

Badawi

, et al. Polymeric composite materials for radiation shielding: a review. Environ Chem Lett 2021; 19: 2057–2090. https://doi.org/10.1007/s10311-021-01189-9

22.

Chafidz

Rizal

Kaavessina

, et al. Processing and properties of high-density polyethylene/date palm fiber composites prepared by a laboratory mixing extruder. J Mech Eng Sci 2018; 12(3): 3771–3785. https://doi.org/10.15282/jmes.12.3.2018.2.0333

23.

Wang

Capoen

D’hooge

, et al.

Can the melt flow index be used to predict the success of fused deposition modelling of commercial poly (lactic acid) filaments into 3D printed materials?

Plast Rubber Compos 2018; 47(1): 9–16. https://doi.org/10.1080/14658011.2017.1397308

24.

Al-Oqla

Hayajneh

Al-Shrida

MAM

. Mechanical performance, thermal stability and morphological analysis of date palm fiber reinforced polypropylene composites toward functional bio-products. Cellulose 2022; 29(6): 3293–3309. https://doi.org/10.1007/s10570-022-04498-6

25.

Jain

Mallya

Nayak

, et al. Influence of alkali and silane treatment on the physico-mechanical properties of Grewia serrulata fibres. J Korean Wood Sci Technol 2022; 50(5): 325–337. https://doi.org/10.5658/WOOD.2022.50.5.325

26.

Neto

JSS

de Queiroz

HFM

Aguiar

RAA

, et al. A review on the thermal characterisation of natural and hybrid fiber composites. Polymers 2021; 13(24): 4425. https://doi.org/10.3390/polym13244425

27.

Nurazzi

Asyraf

MRM

Rayung

, et al. Thermogravimetric analysis properties of cellulosic natural fiber polymer composites: a review on influence of chemical treatments. Polymers 2021; 13(16): 2710. https://doi.org/10.3390/polym13162710

28.

Rafiqah

Diyana

AFN

Abdan

, et al. Effect of alkaline treatment on mechanical and thermal properties of miswak (Salvadora persica) fiber-reinforced polylactic acid. Polymers 2023; 15(9): 2228. https://doi.org/10.3390/polym15092228

29.

Thomason

Rudeiros-Fernández

. Thermal degradation behaviour of natural fibres at thermoplastic composite processing temperatures. Polym Degrad Stabil 2021; 188: 109594. https://doi.org/10.1016/j.polymdegradstab.2021.109594

30.

Rajeshkumar

Hariharan

Devnani

, et al. Cellulose fiber from date palm petioles as potential reinforcement for polymer composites: physicochemical and structural properties. Polym Compos 2021; 42(8): 3943–3953. https://doi.org/10.1002/pc.26106

31.

Khan

Dhakal

Saifullah

, et al. Improved mechanical and thermal properties of date palm microfiber-reinforced PCL biocomposites for rigid packaging. Molecules 2025; 30(4): 857. https://doi.org/10.3390/molecules30040857

32.

Vignesh

. Green synthesis and characterization of zinc oxide nanoparticles using neem (Azardirachta indica L.) leaf extract. Asian J Plant Pathol 2025; 19(1): 27–35. https://doi.org/10.1016/j.mtcomm.2023.105942

33.

Arumugam

Ponnusamy

, et al. Enhanced fast green dye degradation and biological activity of green synthesized Se@ZnO photocatalysts using Ficus religiosa plant extract. Biomass Convers Biorefin. Epub ahead of print 2025. https://doi.org/10.1007/s13399-025-06710-6

34.

Mallem

Zerguine

Benabid

, et al. Enhancing mechanical and thermal properties of rigid poly (vinyl chloride) composites via stearic acid‐treated zinc oxide filler. J Vinyl Addit Technol 2025; 31(2): 401–412. https://doi.org/10.1002/vnl.22178

35.

Adlie

Ali

Huzni

, et al. Impact of zinc oxide addition on oil palm empty fruit bunches foamed polymer composites for automotive interior parts. Polymers 2023; 15(2): 422. https://doi.org/10.3390/polym15020422

36.

Varma

Chandran

. Surface treatment of natural fibers for enhancing interfacial adhesion and mechanical properties in biocomposites—a comprehensive review. Composite Interfaces. Epub ahead of print 2025. https://doi.org/10.1080/09276440.2025.2498795

37.

Tablit

Krache

Amroune

, et al. Effect of chemical treatments of arundo donax L. fiber on mechanical and thermal properties of the PLA/PP blend composite filament for FDM 3D printing. J Mech Behav Biomed Mater 2024; 152: 106438. https://doi.org/10.1016/j.jmbbm.2024.106438

38.

Jamadi

Razali

Petrů

, et al. Effect of chemically treated kenaf fibre on mechanical and thermal properties of PLA composites prepared through fused deposition modeling (FDM). Polymers 2021; 13(19): 3299. https://doi.org/10.3390/polym13193299

39.

Jandas

Mohanty

Nayak

, et al. Effect of surface treatments of banana fiber on mechanical, thermal, and biodegradability properties of PLA/banana fiber biocomposites. Polym Compos 2011; 32(11): 1689–1700. https://doi.org/10.1002/pc.21165

40.

Fletes

RCV

Rodrigue

. Effect of wood fiber surface treatment on the properties of recycled HDPE/maple fiber composites. J Compos Sci 2021; 5(7): 177. https://doi.org/10.3390/jcs5070177

41.

Zhao

Copenhaver

Wang

, et al. Recycling of natural fiber composites: challenges and opportunities. Resour Conserv Recy 2022; 177: 105962. https://doi.org/10.1016/j.resconrec.2021.105962

42.

Gozdecki

Kociszewski

Moraczewski

, et al. Green composite based on a polymer mixture containing biopolymer and waste coffee husks. Polymers 2025; 17(13): 1748. https://doi.org/10.3390/polym17131748

43.

Hachaichi

Nekkaa

Amroune

, et al. Effect of alkali surface treatment and compatibilizer agent on tensile and morphological properties of date palm fibers‐based high-density polyethylene biocomposites. Polym Compos 2022; 43(10): 7211–7221. https://doi.org/10.1002/pc.26784

44.

Negi

Vedrtnam

Kalauni

. A comprehensive review on natural fiber-reinforced polystyrene composites: surface treatments, mechanical performance, and sustainable applications. J Thermoplast Compos Mater. Epub ahead of print 2025. https://doi.org/10.1177/08927057251352369

45.

Adeniyi

Abdulkareem

Ighalo

, et al. Morphological and thermal properties of polystyrene composite reinforced with biochar from elephant grass (Pennisetum purpureum). J Thermoplast Compos Mater 2022; 35(10): 1532–1547. https://doi.org/10.1177/0892705720939