Mechanical and thermal properties of kenaf/polypropylene nonwoven composites




Hao, Ayou

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The objectives of this research are to characterize the mechanical and thermal performance of natural fiber nonwoven composites and to predict the composite strength and long-term creep performance. Three natural fibers: kenaf, jute, and sunn hemp as potential candidates were compared in terms of physical, thermal and mechanical properties. In order to see the effects of fiber surface chemical treatment, sunn hemp fiber was treated with sodium hydroxide (NaOH) agent. Kenaf fiber was selected for the following study due to the higher specific modulus and the moderate price of kenaf fiber. After alkaline treatment, the moisture content, glass-transition temperature, and decomposition temperature of sunn hemp fiber increased but not significantly. The mechanical performance of kenaf/polypropylene nonwoven composites (KPNCs) in production of automotive interior parts was investigated. The uniaxial tensile, three-point bending, in-plane shearing, and Izod impact tests were performed to evaluate the composite mechanical properties. The thermal properties were evaluated using TGA, DSC, and DMA. An adhesive-free sandwich structure was found to have excellent impact resistance performance. Based on the evaluation of mechanical and vii thermal properties, manufacturing conditions of 230 C and 120 s for 6 mm thick sample and 230 C and 60 s for 3 mm thick samples were selected. The open-hole and pin filled-hole effects on the tensile properties of KPNCs in production of automotive interior parts were investigated. Three specimen width-to-hole diameter (W/D) ratios of 6, 3 and 2 were evaluated. A preliminary model by extended finite element method (XFEM) was established to simulate the composite crack propagation. Good agreement was found between experimental and simulation results. Mechanical properties of the KPNCs in terms of uniaxial tensile, open-hole tensile (OHT), and pin filled-hole tensile (FHT) were measured experimentally. By calculating the stress concentration factor Kt for brittle materials, the net section stress factor Kn for ductile materials, and the strength reduction factor Kr, it was found that KPNC was relatively ductile and insensitive to the notch. The strain rate effects on the tensile properties of KPNC were studied. The strain rate effects confirmed the time-dependence of KPNCs. Afterward, the creep behavior of KPNC and PP performed by DMA was investigated extensively. The linear viscoelastic limit (LVL) was found to be 1 MPa in this study. The long-term creep behavior of KPNC compared to virgin PP plastic was predicted using the time-temperature superposition (TTS) principle. Three-day creep tests were also conducted to verify the effectiveness of TTS prediction. It was found that the master curve for PP fit better with the three-day creep data than KPNC, due to the multiphase thermo-rheological complexity of KPNC. The creep recovery, stress effects and cyclic creep performance were also evaluated. Two popular creep models: the four-element Burgers model and the Findley power law model were used to simulate the creep behavior in this study. It was found that KPNC had higher creep resistance and better creep recoverability than virgin PP plastics.



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