Characterization of hazelnut, pistachio and apricot kernel shell powders and ashes, and determination of their composite properties

Traditional materials such as iron, steel, and aluminum are inadequate with technological developments, and composite materials are used instead of these materials. Because composite materials offer advantages such as low weight/high strength ratio. In composites, fiber reinforcements such as glass, carbon, and aramid, and ceramic particles such as silicon carbide and boron carbide are generally used as reinforcing elements. However, due to the difficulties in the recycling such reinforcing elements, and their negative impacts on the environment, and their high production costs, scientists have been directed to the composite materials produced with natural reinforcing elements. In this study, composite materials were produced by using hazelnut, pistachio, and apricot kernel shells as reinforcement elements. In this respect, powder particles of 0-300 µm, 300-600 µm and 600-850 µm grain sizes were obtained by grinding the reinforcing elements. Some of the powder particles were turned into ash at 600 °C and also 900 °C. Cellulose, ash, humidity and metal contents with the chemical analysis of these powder particles and ashes, and structural properties with X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectrometer (FT-IR) analyses were characterized. In addition, composite materials were produced by adding 0%, 10%, 20% and 30% by weight of these powder particles and ashes to the polyester matrix material. The effect of chemical and structural properties of powder particles and ashes on the physical, thermal and mechanical properties of these composites such as density, thermal conductivity, tensile strength, flexural strength and compressive strength were analyzed. The XRD analysis revealed that the cellulose structure observed in hazelnut, pistachio and apricot kernel shell powders was changed by being broken down with the ashing process. The peaks observed in surface functional structures of hazelnut, pistachio and apricot kernel shell powders with FT-IR were mainly caused by cellulose and hemicellulose structures. The amounts of cellulose (28.40%), hemicellulose (24.12%) and lignin (54.63%) in the apricot kernel shell powder were observed to be higher than those of hazelnut and pistachio shell powders. The humidity content of hazelnut shell powder (10.04%) was obtained as the highest. This was followed by pistachio shell powder (8.19%) and apricot kernel shell powder (6.48%), consecutively. The highest amount of ash was observed in apricot kernel shell powder with 8.57%, while the lowest amount of ash was observed in hazelnut shell powder with 1.53%. Increase in ashing temperature slightly decreased the amount of ash obtained, though. Nitrogen, carbon, hydrogen, and oxygen elements were seen in the elemental analyzes of hazelnut, pistachio and apricot kernel shell powders. In the heavy metal analyses, heavy metals such as Sn, Ca, K, Na, Mg, Fe, Ni, Mn, Cu, Zn, and Si were found. It was observed that the amount of the heavy metal found in the apricot kernel shell was higher than that of hazelnut and pistachio shells. The density of the pure polyester material (1.145 g/cm3) was lower than the density of the composites produced with hazelnut, pistachio and apricot kernel shell powders, and ashes. As the reinforcement ratio in the composite material increased, the density of the composite increased. The highest density in the composite materials was obtained from 30% apricot kernel shell ash reinforced composite as 1.406 gr/cm3, while the lowest density was obtained from 10% hazelnut shell powder reinforced composite as 1.205 gr/cm3. In composite materials, the highest density was obtained from 30% apricot kernel ash reinforced composite as 1.406 gr/cm3, while the lowest density was obtained from 10% hazelnut shell powder reinforced composite as 1.205 g/cm3. The coefficient of thermal conductivity of the pure polyester material (0.151 W/mK) increased with the reinforcement element added to the pure polyester material. The highest coefficient of thermal conductivity was obtained from 30% hazelnut shell ash reinforced composite as 0.207 W/mK, while the lowest coefficient of thermal conductivity consisted of 10% apricot kernel shell powder reinforced composite, as the same of pure polyester material. Hazelnut, pistachio and apricot kernel shell powders added to polyester material adversely affected the tensile strength of the matrix material, which had at 31.54 MPa. However, powder particles added to the matrix material at low rates had a positive effect on flexure and compressive strength. In general, values obtained from 300-600 µm grain size gave better results than grain sizes of 0-300 µm and 600-850 µm. All the mechanical properties of the ash reinforced composites were considerably worse than those of pure polyester.