Abstract
To manage the plastic waste, recycling is recognized as the most environment-friendly and non-destructive method. The aim of this research is to investigate the recyclability of oak wood flour (WF) filled high density polyethylene (HDPE) composites. Two different composite formulations (30 and 50 wt% filler) were considered, each with 3 wt% coupling agent maleic anhydride (MA). Both composites were individually reprocessed six times by extrusion. Test samples were injection molded, to measure mechanical and thermo-mechanical properties. Fiber length measurement and gel permeation chromatography (GPC) were performed respectively to examine the change on fiber length and molecular weight of polymer. Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), and Fourier Transform Infra-Red Spectroscopy (FTIR) were also carried out to better understand the impact of recycling on composite properties. After reprocessing six times, WF-HDPE composites showed relative decrease in strength and stiffness properties and slight increase in strain properties as compared to corresponding virgin composite. The strain properties saw an increase in their value with recycling. The crystallinity of HDPE decreased but thermal stability of the composite increased with reprocessing.
1. Introduction
Wood Plastic Composites (WPCs) are typically manufactured from two fundamental constituents a natural fiber or filler, and a thermoplastic resin [1,2]. Nowadays, WPCs are widely accepted as building materials especially because of their durability, higher specific strength and stiffness, and zero or very low health concern along with other advantages[3–5]. Among allWPCs, wood fiber or filler(WF) based high density polyethylene (HDPE) composites are extensively used in household apparatus (e.g., doors, decking, windows, railing, and furniture), and automotive industry (e.g., door panels and seat covers) [6]. HDPE is preferred because it has a lower melting point around 130 °C as compared to the degradation temperature of most natural fibers that varies between 200 and 220 °C, which helps in processing of its composites [6]. HDPE also shows higher toughness, stiffness, chemical resistance, thermal stability, and electrical insulation [7]. For improved properties incorporation of WF in HDPE becomes necessary since elastic modulus and strength of WF, in general is 40 and 20 times respectivelyhigherthanthatofHDPE[8,9].WFis alsoconsiderably cheap since a substantial amount of wood waste is generated in wood industry [10].
4. Conclusions
WF-HDPE composites with 30% and 50% oak wood flour, HDPE, and 3% coupling agent MAPE were manufactured in the laboratory and reprocessed up to six times by extrusion followed by injection molding. The effect of reprocessing was found to be statistically significant for all mechanical and thermo-mechanical properties of both composites. With successive recycling, strength and stiffness properties of the composites decreased, but strain properties increased mainly due to the decrease in fiber length and molecular weight reduction of the polymer. Other possible reasons for the changes in these properties include – 1) decreased interfacial adhesion between the wood fiber and polymer, 2) less efficient stress transfer from the matrix to fiber, and 3) increased polymer chain mobility with consecutive reprocessing cycles. The crystallinity of HDPE decreased but thermal stability of the composite increased with increased number of reprocessing cycles. Although recycling produced a negative effect, from cycle 0 to cycle 6 the relative change or degradation was found mild in all strength and stiffness properties of composites.
