Study of Alternative Eco-Friendlier Wood-Plastic Composites of Recycled HDPE Enhanced with Hemp Fibers

Plastics are the most widely used materials that can be found in almost every human made object and often replace traditional resources such as wood, metal and glass. High-Density Polyethylene (HDPE) replaces many natural resources, due to the low cost and easy processing, but its ever-increasing wastes make it harmful to the environment. Thus, many processes are used to recycle it. On the other hand, the continuous consumption of natural resources that causes a downgrading of the sustainability of the natural environment led the scientific community to study alternative products as a replacement for natural wood. Such alternative materials can derive from agricultural crop residues. Hemp is an ideal replacement for wood due to its lightweight, hydrophobic characteristics, and excellent long-term mechanical stability, with the lowest carbon emissions footprint.<br/>The aim of this research is to use alternative products for the construction of wood-plastic composites (WPCs), as an eco-friendlier solution, that can be used in the construction of floor surfaces, objects, in the interior or exterior spaces, in the construction of frames, and door leaves, but also in the automotive industry in the interior of the car. So, the main goal is the use of recycled HDPE and the replacement of wood with hemp crop residues. In the present work, HDPE was recycled through mechanical and thermal processes (r-HDPE). More specifically, HDPE waste plastics were grinded through a mill and thereafter, extruded through a Haake-Buchler Rheomixer. The thermal and mechanical properties of r-HDPE have been tested compared to HDPE and it was found that after recycling the properties of r-HDPE are not affected so the enhancement with the hemp fibers followed. Composites containing 10, 20, 35 50 and 75% wt. of hemp fibers were prepared also by melt mixing. To improve the adhesion between the fibers and the polymer matrix Joncryl ADR-4400 (JC) and propylene grafted-maleic anhydride (PPgMA) compatibilizers were used.<br/>In this study the chemical, structural, thermal, and mechanical properties of the r-HDPE-hemp fiber composites are examined and the composites with the different compatibilizers are compared. Testing the structural properties of the composites by X-Ray Diffraction (XRD), it is found that the samples are semicrystalline, and their crystallinity decreases with the increase of the hemp fibers content, while there is a shift in the peaks due to the expected disruption of the arrangement of the polymer chains by the addition of the fibers. From the Fourier Transform Infrared Spectroscopy (FTIR) it is concluded that the addition of compatibilizers does not change the chemical structure of the materials. Thereafter, the thermal properties of composites are measured by Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). So, it is concluded, that the compatibilizers do not affect the thermal properties of the composites and the decrease on thermal stability will not affect their application in WPCs. Moreover, the decomposition steps were separated dur to the degradation of hemicellulose, cellulose, lignin and rHDPE, respectively. Finally, the mechanical properties are examined by tensile and impact strength experiments. A decrease in tensile and impact strength as the content of the fibers increases due to the higher creation of crack points in the polymeric matrix is observed. However, the enhanced elongation at break is encouraging for their application at WPCs.<br/>All in all, the results of this study are positive and in conjunction with other tests concerning the degradation of the samples, the construction of different WPC-based objects will be feasible in the near future.<br/><b>Acknowledgments: </b>This research has been co-financed by the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH-CREATE-INNOVATE (project code: Τ2ΕΔΚ – 00008).