Stress Relaxation Of Wood/polypropylene Composite Modified With Coupling Agent

Within all investigations on wood flour/ polymer composites, interfacial compatibility is always the key problem that calls for solution. Adding coupling agent to wood flour/polymer composites is one of the approaches to improve the interfacial compatiblity. This study compared the results of compressive and tensile stress relaxation methods at first, and then investigated the effect of wood content, type and loading level of coupling agent, density and manfacuturing method on the stress relaxation of wood flour/polypropylene composites at various temperatures, as well as the phisical and mechanical properties of wood flour/polypropylene composites at room tempaerature. The apparent activation energy was also calculated according to the Eyring's absolute rate reaction theory and the interfacial compatibility between wood flour and polypropylene was then discussed. The results showed that:(1) The tensile stress relaxation appeared similar trends with compressive stress relaxation at room temperature. And the stress relaxed faster at higher temperatures, suggesting that not only pure polypropylene but also wood flour/ polypropylene composites are sensitive to temperature.(2) Wood content had a big influence on the stress relaxation behavior of wood flour/ polypropylene composites. The lowest stress relaxation rates, highest apparent activation energy and highest bending modulus of rupture (MOR) and modulus of elasticity (MOE) appeared at wood content of 40% of wood flour/ polypropylene composites without coupling agent, suggesting the best compatibility and strongest internal bonding between wood and polypropylene at around 40%.(3) At higher wood contents such as 50% and 60%, the mechanical properties and long-term performance can be both improved and apparent activation energy can be obviously increased by adding maleic anhydride grafted polypropylene (MAPP) or silane as coupling agent. However, the optimal loading of coupling agent was related to wood content and the type of coupling agent. The stress relaxation showed very good consistency with the mechanical properties at room temperature as shown in this study. And we found that at wood content of 50% and 60% the properties can reach high values at a MAPP loading of 2.0% or at a silane loading of 1.5% and 2.0%, respectively.(4) Density affected the interfacial bonding of wood flour/polypropylene composites. Slower rate of stress relaxation, better interfacial compatibility and physical and mechanical properties appeared at higher density of wood flour/polypropylene composites.Wood flour/polypropylene composites with 2% MAPP at same density had better properties than that with 1.5% MAPP while the trend of silane is contrary to MAPP.(5) Adding coupling agent can decrease the rate of stress relaxation, increase apparent activation energy and improve mechanical property for wood flour/polypropylene composites made by "granulated compression method". However, excessive coupling agent may weaken the improvement. Wood flour/ polypropylene composites made by "granulated compression method" had slower stress relaxation, higher apparent activation energy, bending modulus of rupture and modulus of elasticity than that made by "direct compression method" because wood flour, polypropylene and coupling agent can be mixed more uniformly in "granulated compression method" leading to improved dispersity and interfacial compatibility between wood flour and polypropylene.(6) In this study, the values of apparent activation energy of wood flour/PP composites with all the conditions ranged from 2.04 to 18.02KJ/mol, suggesting the occurrence of physical stress relaxation, which is mainly caused by the sliding between wood molecules and the break-down of the intermolecular hydrogen bonds.