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

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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.

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