Analyses Of The Relaxation Processes Of Molecular Movement On Wood Flour/Polypropylene Composite Interface

To further ascertain the interface characteristics, mechanism and the optimal loading of coupling agent of the wood plastic composite, which was one of the different wood flour content, type and content of coupling agents using direct hot-pressing process and hot pressing process after granulating respectively were prepared in this study. In the research with the dynamic thermal mechanical analysis (DMA), the dielectric approach and the stress relaxation methods, the interface characteristics were investigated from relaxation process of molecular movement.The main conclusions are summarized as follows:(1) The storage modulus (E’) of Simon poplar wood flour/polypropylene composite increased with the rising of temperature and significantly increased after adding coupling agent, because coupling agent could make interface bonding strength increase by reacting with wood flour and producing entanglement with polypropylene molecular chain. The relaxation peak was observed around0℃and was higher after adding coupling agent, because coupling agent had maken more difficulty of molecular chain segments motion in amorphous region by a new combination between wood flour and polypropylene, therefore wood flour/polypropylene composite interface was more closely.(2) There was relaxation process in Simon poplar wood flour/polypropylene composite without and with coupling agent which is based on reorientation of the methynol groups (CH2OH) in amorphous region of wood cell wall was related to the different wood flour content, type and content of coupling agents. The relaxation strength (εs-ε∞) was gradually increased with the wood flour content increasing and was first decreased then increased with loading levels of coupling agent, suggesting that the number of the motive methynol groups was deduced as a result of coupling agent reacting with methynol groups. But the relaxation strength was slowly increased above the optimal loading of coupling agent (2%), which may be caused by coupling agent that did not participate in the reaction.(3) The apparent activation energy (△E) was calculated according to the Eyring’s absolute rate reaction theory in relaxation process of Simon poplar wood flour/polypropylene composite and quantitatively characterized the combination on wood flour-polypropylene interface. The apparent activation energy of the samples without coupling agent reached a maximum when wood flour content is40%, suggesting the best compatibility and strongest internal bonding between wood flour and polypropylene. The apparent activation energy of the samples with maleic anhydride grafted polypropylene (MAPP) or silane as coupling agent was almost the same change trend and first increased then decreased with coupling agent increasing, and began to increase above that value:33.52k.l/mol and28.12kJ/mol respectively, more than twice as likely to samples without coupling agent, when the optimal loading of coupling agent (2%); that was due to the strong hindrance to the reorientation of methylol groups and, therefore, suggested the best compatibility and strongest internal bonding between wood flour and polypropylene; but the apparent activation energy of samples with two coupling agents fell to22.0kJ/mol and16.0kJ/mol respectively above the optimal loading of coupling agent, especially that of sample with silane dropped to that of sample without coupling agent and, therefore, suggested the energy barrier to the reorientation of the methynol groups was lower, too much coupling agent was present in wood flour and polypropylene, which resulted in uneven interface structure and weaker internal binding force.(4) The same relaxation process of wood flour/polypropylene composites with MAPP by using hot-pressing process after granulating and using direct hot-pressing process was the same changed. It was compared with two processes that the relaxation strength of sample and the apparent activation energy of sample at MAPP loading of2%both were higher than those of the latter, because wood flour, polypropylene and coupling agent in granulating could be mixed more uniformly leading to improve dispersity and interfacial compatibility between wood flour and polypropylene.(5) The stress relaxation behavior of wood flour/polypropylene composites appeared similar trends with that of dielectric relaxation. It was the best compatibility and the strongest internal bonding between wood and polypropylene that wood flour content is40%, the mechanical properties could be obviously increased by adding MAPP or silane as coupling agent, but the optimal loading of coupling agent was related to wood content and the type of coupling agent. The trendency of both stress relaxation and stress relaxation rate were consistent with loading of coupling agent, the stress relaxation and stress relaxation rate of wood flour/polypropylene composites by using hot-pressing process after granulating was slower to by using direct hot-pressing process, therefore performance of materials is superior to by using direct hot-pressing process.