Effects of bio-particle reinforcement on polymer composites

In this study, it was aimed to utilize walnut, almond, and hazelnut shells—considered as industrial waste—as reinforcing agents in high-density polyethylene (HDPE) matrix biocomposites. For this purpose, all three types of shells were sieved to a particle size below 75 microns and incorporated into the HDPE matrix at concentrations of 0.5%, 1%, and 1.5%, resulting in a total of nine different composite samples. In the scope of mechanical testing, the highest tensile strength and modulus of elasticity were recorded in the sample containing 1.5% hazelnut shell, with values of 18.99 MPa and 270.13 MPa, respectively. In the three-point bending test, the same 1.5% hazelnut shell-reinforced sample stood out with a flexural strength of 24.05 MPa and a flexural modulus of 458.95 MPa. The mechanical properties of the samples reinforced with almond and walnut shells were found to be lower compared to those containing hazelnut shells. According to the results of the Differential Scanning Calorimetry (DSC) analysis, the highest melting enthalpy (ΔHm) was recorded in the sample with 1% almond shell addition, reaching 20.49 J/g. The highest melting temperature was observed in the 1.5% walnut shell-reinforced sample at 137.99 °C. Thermogravimetric Analysis (TGA) results clearly demonstrate the contribution of the additives to thermal stability. While the initial degradation temperature of pure HDPE was 119.82 °C, it increased to 207.63 °C in the sample with 1.5% almond shell and to 210.92 °C in the one with 1% walnut shell. The highest peak degradation temperature was observed in the 1.5% almond shell-reinforced sample, reaching 478.45 °C. In conclusion, both mechanical and thermal tests revealed that especially 1–1.5% additions of almond, walnut, and hazelnut shells significantly enhanced the performance of HDPE-based biocomposites.