Study Of Coextruded Wood/HDPE Composites With A Reinforced Shell Layer

Co-extrusion process can result in different excellent performance for wood plastic composites?WPC?,and provide a simple and effective method for preparing functional WPC.However,compared to control single layer WPC,the mechanical properties of coextruded WPC decrease because of the presence of shell layer,in addition,the creep and thermal expansion behavior may increase.Therefore,in order to improve the mechanical property of coextruded WPC and the creep and thermal expansion behavior,we prepared the coextruded WPC with a reinforced shell layer which will be of positive significance to solve the problem of insufficient property in coextruded WPC and improve its value of application.Firstly,we analyzed the physico-chemical property of different compositions in WPC system in this study,and studied the effect on the dynamic viscoelasiticity of WPC system;secondly,by reinforcing shell layer of coextruded WPC,we prepared coextruded WPC with reinforced shell layer according to change the wood content in shell layer,and using organic?thermotropic liquid crystal?and inorganic?nano/micro-SiO₂?reinforcement on shell layer,by testing the mechanical,creep and thermal expansion properties incorporating with finite element analysis to analyze the effect of shell layer reinforcement on the mechanical property and dimensional stability of coextruded WPC;finally,we prepared sandwich structural WPC by structure design,and analyzed the effect of flexible core layer on the property of sandwich structural WPC.The main contents and results are as follows:?1?The dynamic viscoelasity of wood four and its components mixing with polymerThe wood flour of WPC system used in this study was measured the content of its components,and the contents of holocellulose,cellulose,lignin,and alcohol extractive were 77.22,44.61,14.34 and 3.20%,respectively.The relative crystallinity decreased as follows:cellulose,holocellulose,wood flour,and lignin.The dynamic viscoelasity of wood four and its components was measured by powder clamp,and the results showed that,the ? relaxation progress at about 10? was attributed to interaction between minor absorbed water and hydroxyl groups,and the a relaxation progress?approximately 200??was assigned to micro-Brownian motion of wood cell wall polymers in the amorphous region.The relative storage modulus decreased as follows:cellulose>holocellulose>lignin,and the ? relaxation peak magnitude of holocellulose was maximum than lignin and cellulose,which may illustrate that hydroxyl groups of hemicellulose provided main contribution to wood ? relaxation progress.The dynaMic viscoelasity of sucrose and monosaccharide?glucose,fructose and xylose?was measured and further prove that,the interaction between a little water and methylol groups was the main reason leading to ? relaxation.The dynamic viscoelasity of wood flour and HDPE or MAPE mixtures indicated that,a little content of HDPE or MAPE can realize the plastication effect on wood flour,and its relaxation progress may cover up the relaxation progress of wood flour.Interface loss tangent showed that MAPE and wood flour exhibited better interface interaction than HDPE.?2?Effect of wood contents on the properties of co-extruded WPCCompared to control WPC core layer,the flexural strength and modulus of coextruded WPC with HDPE shell layer decreased about 33%and 45%,respectively,but the impact strength increased about 55%,and the coextruded WPC showed comparative flexural and impact properties with increasing shell layer wood contents to 30%.Coextruded WPC with 20%wood content in shell layer showed considerable flexural property compared to control core,when increased the wood content of core layer to 70%,and impact strength improved about 100%.The finite element analysis indicated that,shell layer can disperse stress when the coextruded WPC under the flexural and impact force,which can avoid the stress concertration,therefore,improving the toughness.Both of the creep resistance of shell layer and coextruded samples increased with increasing the wood content in shell layer.In addition,increasing wood content can considerably decrease the thermal expansion of the coextruded samples in thickness and length direction,respectively.Compared to control sample,the water uptake and thickness swelling were decreased effctively because of the shell layer,and the wood content in core layer was the main reason that lead to water uptake.?3?The reinforcement of TLCP in shell layer on the properties of co-extruded WPCThermotropic liquid crystal can obviously increase the mechanical propertis of the HDPE blends,and the maximam improvement of tensile strength and modulus was 35%and 47%,rpectivly,the maximam improvement of lexural strength and modulus was 53%and 112%,rcpectively,and the maximam improvement of impact strength was about 3 times.Compared to the HDPE blends,WPC reinforced by thermotropic liquid crystal showed the maximam improvement of tensile strength and modulus was 20%and 15%,repectively,and the maxmam improvement of flexural strength was 10%,but the flexural modulus did not change,and the impact strength was increaed about 30%and 50%,at room and low temperature,repectively.The addition of thermotropic liquid crystal had little effect on melting point of HDPE blends and WPC,and incresed the crystalline temperature,but did not change the crystalline structure of HDPE matrix.Compared to the control sample,the tensile strength and modulus of shell layer was incresed about 40%and 10%,respectively,with the addition of thermotropic liquid crystal,and flexural strength and modulus of shell layer with increasing the thermotropic liquid crystal content,and the maximam improvement was 25%and 40%,respectively.In addition,using thermotropic liquid crystal can consideralbly improve the creep resistance of shell layer.Coextruded samples exhibited the maximam flexural strength and modulus and was increased about 12%and 20%,respectively,when the thermotropic liquid crystal content was 15%in shell layer.Compared to control,the addition of thermotropic liquid crystal can considerably decrease the thermal expansion of coextruded samples in length direction.?4?The reinforcement of SiO₂ in shell layer on the properties of co-extruded WPCSiO₂ in shell layer was grafted with HDPE matrix by VTS treatment,and coextruded WPC showed proper interfacial adhesion between core and shell layers.The flexural strength and modulus of shell layer were increased with increasing untreated and VTS treated nSiO₂ content,and the maximam improvement was 41%and 50%,respectively.However,using untreated and VTS treated mSiO₂ only showed limited improvement on flexural strength and modulus.Similar to flexural property,the tensile strength and modulus of shell layer were incresed with the addition of untreated and VTS treated nSiO₂ and the maximam improvement was 53%and 35%,respectively.The highest flexural strength of coextruded sample was comparative to the control,when the nSiO₂ content was 5%in shell layer,but the increse degree of flexural modulus was lower than strength.The maximam improvement of flexural strength and modulus was 17%and 16%,respectively,when adding VTS treated mSiO₂ in shell layer,bur was still lower than the control.The impact strength was incresed about 45%when filled nSiO₂ in shell layer compared to the control,and did not decrease the toughness of the coextruded samples.Long time creep degree of coextruded sample was comparative to the control,when adding special content of nSiO₂ in shell layer especially VTS treated nSiO₂.Using nSiO₂ or mSiO₂ in shell layer,can obviously decrased the thermal expansion in length direction,but only showed limitied decrease in thickness direction.?5?Sandwich structural WPC penals reinforced by the flexible LLDPE core layerThe sandwich structural WPC penals showed proper interfacial adhesion between LLDPE core layer and two surface WPC layers,and the density of single layer WPC decreased with increaesing wood content,while the density of sandwich structural WPC remained unchanged.Compared to single layer WPC,the flexural and impact strength of sandwich structural WPC were increased about 42%and 70%,respectively.The finite element analysis indicated that,flexiable core layer can disperse stress when the sandwich structural WPC under the force,that can avoid the stress concertration when broken.The sandwich structural WPC showed comparative creep and relaxation degree with single layer WPC.The presence of flexiable core layer can obviously decrease water uptake of the sandwich structural WPC,but show higher thermal expansion in thickness direction.