Research On Supercritical Fluids MicrocellularInjection Molding Of Wheat Straw Powder/HDPE Composites

Eco-friendly microcellular foaming wood/plastic composites has drawnincreasing interest in recent years, which not only possess the feature of traditionalwood plasticcomposite materials but also has low density, sound insulation, hightoughness, heat insulation and otherpreponderances. Therefore, it is widely used infurniture, automobile interior, construction materials and even military applications.In order to reduce the density of wood plastic composite and improve itsmechanical properties, we chose high density polyethylene (HDPE) as the matrixresin and wheat straw powder as filling material in this thesis. Supercritical fluidmicrocellular foam injection molding and traditional injection molding wereintroduced to prepare the wood plastic composite samples for comparativeobservation. Firstly,the effect of the content and granularity of wheat straw powderand the content of silane coupling agent on the melt flow rate and the mechanicalproperty (tensile strength, tensile modulus, bending strength and impact strength).ofwood/plastic composites was studied. Secondly, the microstructure morphology of theimpact section was observed by SEM to establish the relationship between the abovefactors and the microstructure of. The mechanism of foam structure enhancing thetoughness of wood/plastic composites was also discussed. Lastly, the performance ofchemical foam injection molding products was compared with the physical andsupercritical fluid microcellular foam injection molding products. The main resultsare as follows:(1) The melt flow rate (MFR) of wheat straw powder/HDPE composites wasreduced substantially with increase of wheat straw powder content; the effect ofgranularity of wheat straw powder on melt flow rate was declined with the decreaseof the straw powder particle size.(2) The density of wheat straw powder/HDPE composites was increased with theincrease of the content of wheat straw powder straw powder; however, the effect ofwheat straw powder granularity and silane coupling agent on the density of wheat straw powder/HDPE composites could be ignored. Supercritical fluid microcellularfoaming molding processing has notable effect on reducing the weight of products.(3) Tensile strength and impact strength of wheat straw powder/HDPEcomposites declined with the increase of the content of wheat straw powder; howevertensile modulus and bending strength were downward trend after rising first. Whenthe wheat straw powder content was30-40wt%, the mechanical properties of woodplastic composite is best.(4) The mechanical properties of wheat straw powder/HDPEcompositesinjection samplesincreased at first then declined with the decrease ofwheat straw powder particle sizeand the best mechanical properties could be achievedwhen themesh of wheat straw particle was40-60.(5) The dispersion of wheat straw powder in HDPE matrix material decreasedwith the increase of the content of wheat straw powder variation and the foamstructure was smaller and more uniform with the increase of wheat straw powdermesh.(6) Maleic anhydride and silane coupling agent could effectively enhance thecompatibility ofwheat straw powder and HDPE and improve the foaming effect ofwheat straw powder, makingfoam structuresmaller and closer. However, maleicanhydride can improve straw powder/HDPE mechanical properties of wood plasticcomposites more efficiently compared with silane coupling agent.