Mechanical and Optical Properties of Thermoplastic Composite from Recycled Polypropylene Reinforced with Wood Fillers

Abstract

Plastic waste management systems have become crucial importance due to an increase of household plastic waste. Plastics are reinforced with the natural fillers (NF) to realise Wood plastic composite (WPC), which has widespread use in construction, decking, and making automotive parts, owing to their superior strength over recycled polymers. Recent research reveals a critical issue of inadequate encapsulation of fibers by the polymer matrix, that allows water uptake through film cavities. This is caused by poor compatibility between the hydrophobic polymer and highly hydrophilic wood flour (WF) resulting in diminished mechanical properties. Additionally, uncertainties persist regarding the optical properties of WPCs, tied to fluctuations in refractive index due to variations in NF composition and packing density. This study presents the viability of utilizing recycled polypropylene (PP), coded under Resin Identification Code (RIC) 5, to manufacture composite panels, focusing on enhancing mechanical properties and optical properties for diverse applications. WF was ground to fine particles using varying sandpaper grades, to study its effects on the composite, while sorted recycled plastics and composite blend underwent injection molding to create the panels. Optical microscopy was used to determine WF particle sizes. Chemical modification was done with 5% NaOH aimed to booster interfacial adhesion between WF and the polymer matrix. The optical microscope images were analyzed using image J software for the three different WF sanded by three different sandpapers; P80, P60, and P40. The WF particle size of 0.268 µm, 0.742 µm and 1.23 µm were compounded with the plastic at a ratio of 1: 9. Fourier Transform Infra-Red (FTIR) spectra analysis, indicated a big reduction in the absorption peak of the spectrum for the smallest WF particles after the alkali treatment as compared to large particles. The stress-strain curves for the samples were obtained from the Tensile Testing Machine (TTM) and used to analyze the mechanical properties of the composites. The samples optical properties were investigated using Ultra Violet-visible (UV-Vis) spectrophotometer and Tauc‘s plots. The study found that WPCs with finer WF particles (0.268 µm) exhibited highest ultimate tensile strength, achieving up to 15.36 MPa, compared to WPCs with coarser particles (1.23 µm), which had strength as low as 7.5 MPa. These results demonstrate that incorporating finely ground WF into recycled polypropylene composite enhance their mechanical properties and reduces their band energy gap, attributed to improved interfacial bonding, effective load transfer, and altered electronic properties. The product offers a solution to the mechanical weakening of improperly disposed non-biodegradable plastics, transforming them into practically applicable products with determined electrical conductivities, offering a solution to environmental concerns while advancing practical applications.