Research On The Rheological Behavior Of Ultra-High Filled Wood Flour/Polyethylene Composites

Wood-plastic composite(WPC)is a recyclable environmental protection composite processed by extrusion,hot pressing,injection molding,and other processes with a thermoplastic plastic matrix,biomass fiber as reinforcing phase,and a specific proportion of additives.WPC is extensively used in landscape architecture,interior decoration,automotive,and construction fields.WPC is highly compatible with environmental protection and sustainable development and therefore has ubiquitous development prospects.To reduce production costs and improve the competitive market advantage of WPC,increasing the biomass fiber content is the simplest and most effective method.The development of ultra-high biomass fiber content(mass fraction more than 70%)filled wood plastic composite(UWPC)is conducive to reducing carbon emissions to meet market development and WPC's development needs.However,fiber content increase brings a series of problems,including the uneven mixing of raw materials,poor fluidity of the melt,increased viscosity,and difficulty in forming,directly affecting the production efficiency and quality of products.These problems are closely related to the rheological properties of UWPC melt.The rheological properties of the melt reveal the flow law and clarify the causes of these problems.Therefore,mastering the rheological properties of UWPC has critical theoretical and application value for processing and improving product quality.This paper selected high-density polyethylene(HDPE)as the typical matrix,fast-growing poplar wood powder as the typical biomass fiber reinforcement phase,and ultra-high filling wood/HDPE composite(UWFPE)was prepared by hot press granulation.The static rheological properties of the UWFPE system were systematically studied.First,the internal stress decomposition method calculated the internal stresses of UWFPE in the low-speed linear shear flow field.Second,the plug flow model calculated the rheological stress of UWFPE during continuous compression.Third,the uniaxial cyclic compression method characterized the structural rheological properties of UWFPE during continuous compression.Forth,the analogy method combined with the Boltzmann superposition principle solved the compression rheological model of UWFPE during continuous compression.The main research contents and results of this paper are as follows:(1)The UWFPE sample was prepared by granulation-hot pressing,and its mechanical and physical properties were characterized.The results showed that the interface and structure of the UWFPE system reached the most stable state and the best mechanical properties at 80%wood flour content.The MAPE coupling agent reduces the wood flour agglomeration and increases the uniformity of the UWFPE system.Torque and rotary rheometers tested the rheological properties of the UWFPE system.The results show that the UWFPE system is approximately "quasi-solid" when the wood powder content exceeded 80%.Moreover,the real and effective rheological data of the UWFPE system can not be obtained by using both rheometers,indicating that traditional rheological testing instruments cannot characterize the rheological properties of the UWFPE system.Therefore,to obtain effective data that can objectively reflect the rheological properties of the UWFPE system,it is necessary to explore new experimental methods.(2)Given the limitations of UWFPE rheological testing by torque and rotary rheometers,a shear rheological device with a sealed temperature control function was designed to test and analyze the rheological behavior of UWFPE.A low-speed linear-shear flow field and an internal stress decomposition method were proposed to calculate the internal stress of the UWFPE system.The results show that the self-made shear rheological device performs well in shear rheological tests of the UWFPE system.Based on the analysis calculation results of the internal stress decomposition method,the following conclusions can be drawn:(1)Wood flour and weak interfacial friction in the UWFPE system provide the main stress during the shearing process;(2)Relatively speaking,the HDPE stress can be neglected.In the molten state of UWFPE,HDPE mainly acts as the connecting carrier of wood flour and as the stress transfer and dispersion medium.According to the calculation and analysis of interfacial friction stress,the strengthening efficiency of MAPE on the UWFPE melt system weak interface reached 72.3%,81.1%,and 91.9%at 130?,150?,and 170?,respectively.(3)The compression method is suitable for rheological tests of high-viscosity melts,and provides the continuous deformation characteristics and the controllable deformation rate in a large strain range.The UWFPE system rheological behavior during the compression process was calculated and analyzed using the self-made compression rheological mold combined with the plug flow model.The results show that the plug flow model is suitable for calculating the rheological stress of the UWFPE system within a limited range of axial strain.However,the model is no longer applicable to the continuous compression process when the axial compression deformation reaches a certain threshold,and the edge begins to rupture.During continuous compression,the specimens with 70 wt%,75 wt%,and 80 wt%wood flour content displayed a window period after the compaction stage,which can be used to characterize the stable rheological behavior of the melt.The stable rheological window period still exists in systems with wood flour content as high as 85%;however,it becomes narrow.When the wood flour content increases to 90%,it is difficult to observe the stable rheological window period.The compressive rheological stress equation for UWFPE melt with wood flour content as a variable was determined and validated by nonlinear fitting.(4)The uniaxial cyclic compression method was used to study the structural rheological properties of the UWFPE melt during continuous compression.The strain range of viscoelastic and plastic deformations was determined by residual strain.The elastic modulus of the loading stage was obtained by using the effective data in the elastic stage.Finally,the yield stress of the UWFPE melt during the loading process was estimated and compared.The strain energy density,dissipation energy index,adhesive strength,and adhesive energy were calculated under different conditions such as content and particle size of wood flour,compression rate,temperature,and mixed matrix.The results show that wood flour content is an important factor affecting the rheological properties of the UWFPE system.The increase in wood flour content reduces the influence of other factors,except for the matrix,on the structural changes of the UWFPE system.With the compression rate increase,the yield stress and energy consumption of UWFPE melt during the deformation process increase,but the elastic modulus,residual strain,and dissipation index did not change significantly.Increasing temperature and adding polypropylene(PP)to the matrix significantly improved the fluidity of the matrix,conducive to the flow and molding of UWFPE.The decrease of wood flour particle size reduced the melt's fluidity and plasticity while increasing the deformation energy.However,when the wood flour particle size is 80?100 mesh,the adhesive energy of melts with 80%and 85%wood flour content is almost zero,indicating that mechanical bonding resistance decreases during the melt and flow processes.(5)To explore the rheological model suitable for UWFPE during continuous compression,the uniaxial compression process of UWFPE melt was analogized to the creep process with continuous stress change by the analogy method and combined with the Boltzmann superposition principle.The effects of wood flour content and particle size,compression rate,and mixed matrix on the rheological behavior of UWFPE melt during uniaxial compression were analyzed using the compression rheological model.The results show that the compression rheological model obtained by the combined analogy-Boltzmann superposition principle simulates the continuous compression of the UWFPE melt.The elastic modulus E1 in the unloading stage is noticeably larger than in the loading stage due to structural recombination and compaction stages.However,the structural change of these stages does not change the relaxation time of viscoelastic deformation.With the increase of wood flour content,the E1 increased significantly,and the system's relaxation time and viscosity also increased.Increasing the compression rate,the modulus of the two stages does not change significantly,indicating that the modulus is a parameter independent of the compression rate.Contrary,the viscosity of the UWFPE melt decrease with the increase of the compression rate,proving that UWFPE melts retain the shear thinning property.The compression rheological model parameters are all sensitive to the fluidity of the matrix.The E1,relaxation time,and viscosity of the system decrease evidently with the increase of the fluidity of the matrix.As the wood powder particle size decreases,the E1,relaxation time,and viscosity of the loading and unloading stages increase significantly.However,due to the reorganization and compaction of the system structure,the difference of these three parameters becomes smaller under differentinfluencing factors in the unloading stage.In conclusion,compared with conventional HDPE-based WPC,the rheological behavior of UWFPE can be summarized as(1)The solid-like property is strong,but the melting property of shear-thinning is retained;(2)The melt is very unstable after being stressed,but maintains a window period of stable flow;(3)The melt processing fluidity is extremely poor,but the surface easily forms a wall slip with the inner wall of the processing equipment.These characteristics give UWFPE great application and commercial potential for continuous extruder production.