Investigation of the properties of hemp fiber reinforced polyurethane composites surface coated with polyamide-6

The distinctive properties of composites, such as impact resistance, ozone resistance, corrosion resistance, thermal conductivity, design flexibility, lightness and production diversity potential, are increasing their use in all sectors day by day. The most widely used type of composite is polymer matrix composites, which are employed in the aerospace industry, automotive sector, construction and infrastructure sector, defence industry, sports equipment, and electrical and electronic sectors. In polymer matrix composites, both natural and synthetic fibres are preferred as reinforcing phases; however, due to parameters such as global warming, lower carbon footprint, biodegradability, low cost, and good mechanical and insulation properties, research on natural fibres has increased in recent years. In this study, polyurethane (PU) matrix was used due to its resistance to chemicals and good mechanical properties, as well as its various applications and production methods. In the reinforcement phase, hemp fibre was chosen for its lightness, high strength, complete biodegradability, and low-cost production; its surface was coated with PA 6 to improve adhesion to the matrix, water absorption, and thermal stability. The coating process of hemp fibres was investigated prior to composite production. SEM, FTIR, DSC, and TGA analyses revealed that the coating process adequately covered the surface, flattened the fibre surfaces, and made them thermally more stable. Tensile tests revealed that the coating process using a 1 mm nozzle resulted in a 137% increase in surface coating, a 12% decrease in tensile strength and a 42% decrease in elongation at break compared to hemp, while the elastic modulus increased by 43%. In composite production, fibres with these values were manually placed in the mould and PU mixture was poured over them. Comparisons were made between PU, Hemp/PU and PA 6-Hemp/PU. SEM analysis revealed that the porosity of the composites increased compared to PU, and that the use of coated hemp resulted in a less porous structure. The density values were found to be 1.070 g/cm³ for PU, 0.855 g/cm³ for Hemp/PU, and 0.948 g/cm³ for PA 6-Hemp/PU. The water absorption test results were 0.4% for PU, 1.85% for hemp/PU, and 1.65% for PA 6-hemp/PU. The decrease in density and increase in water absorption due to the porous structure are consistent with the experimental data. As a result of the tensile test applied, the highest strength value was measured at 23.4 MPa in the PA 6-Hemp/PU sample, the elongation at break was 4.2% in the PU sample, and the elastic modulus was 1057.2 MPa in the PA 6-Hemp/PU sample. In the three-point bending test, the maximum stress was 47.1 MPa in the PU sample, the maximum elongation was 6.5% in the PU sample, and the elastic modulus was 1250.7 MPa in the PA 6-Hemp/PU sample. When examining the stress-strain graphs of the mechanical tests, it was observed that the PA 6-Hemp/PU composite has a rigid structure. In view of all the results obtained, it is possible to produce composites that are lighter, harder, more resistant to deformation, but with high water absorption potential. These composites can be used in furniture support units, vibration-preventing units in transportation, and internal support fillings in sports equipment. Keywords: Hemp, Lignocellulosic composite materials, Polyurethane, Composite