Study On The Preparation Of Microcellular Polypropylene Based Wood-polymer Composites By Extrusion Foaming Method

As one of the green and environment-friendly materials,wood-polymer composites(WPC)can be widely applied in construction,logistics,automobile and other fields,which can protect the forest resources and alleviate the current shortage of social resources.In addition,in order to reduce the density and improve the toughness of WPC,the microcellular structure is introduced into the samples by extrusion foaming technology,which can restrain the expansion of micro-cracks and increase the impact strength of WPC products.In this paper,the experimental platform used for the preparation of microcellular WPC was built based on the traditional single screw extruder with the optimization design of screw structure and the introduction of dynamic mixer and melt pump.Besides,the formulation and molding process parameters were optimized to improve the bubble structure of samples,and the change rules of cell morphology were also explored.The main research work is listed as follows:1.The preparation requirements of microcellular WPC were first analyzed and compared,then the separation screw and barrier screw head were introduced to improve the mixing and plasticizing ability of the extruder.Besides,the cavity transfer dynamic mixer(CTM)and melt gear pump were also mounted for good dispersion mixing effect,stable extrusion and high melt pressure in the experimental platform.As a result,the experimental platform,which was used for the preparation of microcellular WPC,was built for the following research.2.The crystallization characteristics and rheological behaviors of PP/POE/WF composites,PP/HDPE/WF composites and PP/microcrystalline wax/WF composites were analyzed and compared,respectively.In addition,the effect of polymer blends on the bubble structure,mechanical strength and heat resistance of the samples were also investigated with the purpose of exploring the interaction among material characteristics,microstructure and product performances.3.The effects of wood flour,compatibilizer,nucleating agent and foaming agent on the foaming behaviours and other properties of samples were investigated,and the formula was optimized to improve the bubble structure.Besides,the gas solubility and diffusion rate in the extrusion process were adjusted by the die temperature and screw speed,which could also affect the starting point and density of bubble nucleation.As a result,the bubble structure and mechanical properties of the samples were further optimized and improved.4.The effect of molecular chain structure on the crystallization characteristics and rheological behaviors of PP and PP/POE/WF composites were investigated.Besides,the subsequent bubble nucleation,growth and setting process of composites with different PP were also analyzed and compared.At last,the effect of PP molecular chain structure on the bubble structure and mechanical properties of the samples was explored.5.The axial and radial bubble structure of the samples was observed and analyzed,which contributed to the establishment of the physical model describing the process of bubble nucleation,growth and confinement under different process conditions.And the preparation of PP based microcellular WPC with high impact strength and high heat resistance was successfully realized.