Effects Of Wood Fiber Size On The Creep Of Wood Fiber-Plastic Composites

Wood-plastic composites(WPC) are world widely concerned because of better use of waste resources and providing new building materials. Creep resistance is one of shortcomings of WPC. The thesis focuses on effect of size of wood fiber on creep properties of WPC. Four meshes fibers(respectively80～120mesh,40～80mesh,20～40mesh and10～20mesh) as fillers were mixed with high-density poplyethylene (HDPE) to prepare wood fiber/HDPE composites by extrusion. Short-term creep properties of the composites determined by dynamic thermomechanical analysis(DMA). Using self-designed creep measuring device, long-term creep of the composites and24hours creep/24hours recovery were tested. Change of size of fibers was measured in the composites and in natural state. At the same time, using paper mill sludge(PMS) as a substitude of wood fiber, PMS/wood fiber/HDPE composites were produced. Effects of addition of PMS on physical and mechanical properties were studyed. The following conclusions were obtained:1)With wood fiber size larger, density of composites had no change, flexural strength and modulus increased and impact strength decreased. Compared with composites of80～120mesh, flexural modulus and tensile modulus of10～20mesh increased by42.45and28.4%, but impact strength reduced by35.5%. The tensile strength reached a maximum at40～80mesh, it is49.65MPa. Analysis of DMA showed that, with wood fiber larger, storage modulus and loss modulus of composites increased, change of size of wood fiber had no effect on glass transition temperature. Rheological analysis showed that composites of80～120mesh had the largest storage modulus and complex viscosity, for the other composites, the larger size of wood fiber was,the smaller storage modulus and complex viscosity was.2) Wood fiber size was measured, it was showed that wood fiber size had a relatively change before and after extrusion.3)Short-term creep tests showed that the larger wood fiber size was, the better short-term creep of composite was. Ambient temperature had a significant effect on short-term creep of composites, the higher the temperature was, the worse the short-term creep was. Long-term creep test showed that the larger size of fiber was, the composites had a better creep resistance.24hours creep/24hours recovery tests showed that size of fiber had the same effect as long-term creep and had no effect on recovery rate. The bigger stress level, the bigger the creep deformation and stress level had little effect on recovery parts.4)When added amount of PMS is low, mechanical properties of PMS/wood fiber/HDPE composites had not significant changes. With proportion of PMS increasing, flexural strength and elastic modulus of composites decreased while impact strength was improved. DMA analysis showed that storage modulus decreased and loss tangent increased at low temperature, and there was a emergence of new relaxation peak because of the inorganic content in PMS. When proportion of HDPE was in a certain range, addtion of PMS had no effect on rheological properties under175℃.5)Creep test of PMS/wood fiber/HDPE composite showed that addition of PMS did not improve creep properties of composites. With proportion of PMS increasing, the creep deformation is bigger.