Stress Relaxation of Wood Flour/Polypropylene Composites at Room Temperature
Keywords:Wood flour/polypropylene composite, wood flour content, coupling agent, stress relaxation
AbstractTo investigate the time-dependent property of wood flour/polypropylene (PP) composites and the effect of coupling agents on it, both tensile stress relaxation and compressive stress relaxation curves were determined at various wood contents (0, 20, 30, 40, 50, 60, and 70%) without a coupling agent and were also determined at 50 and 60% wood contents with different coupling agents such as maleic anhydride grafted polypropylene (MAPP) and silane. Bending modulus of rupture (MOR) and modulus of elasticity (MOE) of wood flour/PP composites at various wood contents without coupling agents and at 60% wood content with MAPP and silane as coupling agents were also tested to compare with stress relaxation results. All measurements were performed at 26 ± 1°C. Results showed that 1) the tensile stress relaxation appeared to have similar trends with compressive stress relaxation (They both declined obviously after adding coupling agents at the same wood content.); 2) wood content had a great influence on stress relaxation behavior of wood flour/PP composites (The lowest stress relaxation rates appeared at 40% wood content for both tensile and compressive stress relaxation of wood flour/PP composites without coupling agent, suggesting the best compatibility between wood and PP is at about 40% within the experimental conditions of this study.); 3) optimal loading level of a coupling agent for stress relaxation varied with type of coupling agents and wood content (Within the experimental conditions used in this study, the optimal loading level for MAPP was 2% at both wood contents, whereas for silane, it was 1.5% at 50% wood content and 2% at 60% wood content.); and 4) lower stress relaxation rates corresponded to higher bending MOR and MOE values at 60% wood content. This suggests that long-term performance of wood flour/PP composites would be consistent with bending strength at room temperature.
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