Optimization Of The Hot Pressing Process For Preparing Flax Fiber/PE Thermoplastic Composite

Natural fiber thermoplastic composites have the advantages of low density,strong fatigue resistance,corrosion resistance,low cost,degradability,good ductility,etc.,and are widely used in medical equipment,new energy vehicles,rail transit locomotive insulation panels,and aerospace complex structures Pieces and other fields.Therefore,it is especially important to increase the research on the preparation of natural fiber thermoplastic composites.Natural fiber thermoplastic composite materials use natural fibers as reinforcements and thermoplastic resins as matrix materials to complete production under a suitable preparation process.In this paper,flax fiber is selected as the reinforcement material,and polyethylene resin is used as the matrix material for the hot press molding of the flax fiber thermoplastic composite material.However,the mechanical strength of most natural fiber composite materials is relatively poor,which is prone to defects such as delamination of composite materials and fiber breakage.In order to solve the above problems,this article mainly designed single bundle flax fiber modification experiment and flax fiber thermoplastic composite material hot press molding experiment to enhance the mechanical strength of the material.The specific work is as follows:?1?Single bundle flax fiber chemical modification treatment,mainly including:5%NaOH solution modification treatment,2%silane coupling agent solution modification treatment and5%NaOH solution and 2%silane coupling agent solution mixed modification treatment,in this paper,10 different modified fiber bundles are designed for single-bundle fiber tensile testing,infrared spectroscopy functional group testing,and scanning electron microscope micromorphology.the result shows:?1?The tensile experiment of single flax fiber found that the tensile strength of the alkali-treated flax fiber decreased due to the corrosion of the alkaline solution,and the strength of the flax fiber gradually decreased as the modification time increased.The flax fiber modified by the coupling agent has stronger tensile strength than the original fiber,and as the modification time increases,the tensile strength becomes larger.In all the above modification methods,the strength of flax fiber obtained by alkali modification for 2h with coupling agent modification for 2h is the largest.?2?Infrared spectroscopy analysis found that the flax fiber modified by NaOH solution produced a weaker vibration absorption peak of C=O bond at 1710cm^-11 due to the destruction of lignin.The functional group of the flax treated with the coupling agent changed,mainly reflected the N-H(1605cm^-1)bond absorption peak,Si-O-C(1290cm^-1)bond absorption peak and Si-O-Si(760cm^-1)bond absorption peak was added.?3?The observation of the microscopic morphology of single modified flax fiber revealed that the surface of the flax fiber after alkali treatment had more pits,the surface roughness was larger and there were obvious gullies.With the increase of alkali treatment modification time,the corrosion effect of alkali is more obvious and the roughness of the fiber surface is greater.In contrast,the flax fiber surface modified by the coupling agent has a clear layer of attachments,the fiber surface is relatively flat,and the roughness is low.The surface of the flax fiber after the alkali and coupling agent are mixed treatment is basically the same as the surface of the flax fiber treated by the single coupling agent due to the coupling effect of the coupling agent.?2?This article focuses on the optimization of the hot-pressing process of flax fiber/PE thermoplastic composites,and choose the scientific experimental design method-Taguchi orthogonal experiment method,use this method to design the hot-press experimental parameters,and optimize the experimental parameter design to reduce unnecessary hot-press experiments,improve production efficiency,and greatly reduce the experiment cost.In addition to the experimental parameters of hot pressing temperature,pressure,and hot pressing time,the silane coupling agent is also selected as the modification method of flax fiber.Design L₉?4×3?orthogonal experiment table of 4 factors?hot pressing temperature,hot pressing pressure,hot pressing time,coupling agent modification time?,3 levels?each factor corresponds to 3levels?,and select tensile strength is used as the response value of the Taguchi orthogonal table.According to the designed Taguchi orthogonal experiment table,the tensile experiment of flax fiber composites and Taguchi experimental analysis?mean analysis,signal-to-noise ratio analysis,analysis of variance?are used to predict the best hot pressing parameters and verify the best hot pressing parameters.the main conclusions are as follows:?1?Through analysis of signal-to-noise ratio,the best hot pressing parameters are obtained:hot pressing temperature is 120°C,hot pressing time is 0.25 h,hot pressing pressure is 2 MPa,silane coupling agent modifies flax fiber for 2 h.The analysis of variance showed that the contribution of 4 factors to the tensile strength of the material was:pressure>temperature>hot pressing time>coupling agent treatment time.The verification results of the optimal hot pressing process parameters show that,compared with the average tensile strength,the tensile strength under the optimal parameters is increased by about 10%.The Taguchi method effectively improves the tensile strength of flax fiber thermoplastic composites.?2?Combined with hot pressing parameters and microstructure pictures,it can be found that the coupling agent,temperature and pressure are the three key factors that affect the performance of the material.Coupling agent modification can enhance the degree of bonding between the fiber and the matrix,and the interface strength of the fiber is improved;High temperature can reduce the viscosity of the resin,improve the fluidity of the resin,and under higher pressures,the resin is easily immersed in the fiber bundle.In the assembly process of single-nail single-shear and double-nail single-shear connections,the tensile properties of flax fiber thermoplastic composites under the optimal hot pressing parameters are increased by about 10%.