Inorganic Nano-Particle Network Functionalized Wood Flour-HDPE Composites

Incorporation of inorganic nano-particles in wood plastic composites(WPC)is an efficient method to enhance the mechanical properties and dimensional stability and endow specific functions to the WPC.The nano-particles are usually distributed in WPC uniformly using the traditional one-step melt-blending process,however,the nano-particles is easy to agglomerate due to the electrostatic force and a small amount of chemical bond effect.Nano-particles often distribute in WPC in the form of aggregates especially at higher content.In order to fabricate WPC with proper functions,higher content of nano-particles is required by the traditional one-step melt blending(10-40 wt%).However,filling high content of nano-particles leads to higher cost and lower strength of WPC.To address the above problem,firstly,the SiO2 particles were used to distribute uniformly in WPC,and the influence of SiO2 on WPC was investigated under the premise of uniform dispersion.Then,using three high-density inorganic nano-particles(nano-SiO2,nano-scale conductive carbon black and nano-ZnO)to form "network structure" by cladding WPC pellets(millimeter-scale)as thin layer,to enable a small amount of inorganic nano-particles to be distributed at the junction between WPC pellets with a higher density which can result in higher effectiveness.The prepared network-structural WPC based on three inorganic nano-particles can realize the reinforcement of the mechanical properties,the dimensional stability of WPC,while giving WPC flame retardancy,conductive and electromagnetic shielding properties at lower loading.The main contents and results are as follows:(1)Effect of homogeneous distribution of SiO2 on the properties of WPC:Six methods were used to distribute nano-SiO2 uniformly in the WPC,the effect of these methods on the mechanical properties of WPC was evaluated.Selecting the optimal scheme,then the systematic experience was carried out with the dosage of micro-and nano-SiO2 as variable,the effect of SiO2 dispersion methods(dry dispersed and wet dispersed)and interfacial modification on the WPC properties.The results demonstrate that the fracture occurs at the interface between the SiO2 and HDPE when the WPC with SiO2 was prepared via dry dispersed method,indicating a weak bonding;compared to dry dispersed method,SiO2 were distributed in WPC more homogeneous us wet dispersed method,and the fracture section is occurred at the HDPE regions,suggesting a firm interfacial combination between SiO2 and HDPE.The peak heat release of WPC was decreased by 14.7%and 17.1%when adding 9%micro-SiO2 using dry dispersion and wet dispersion,respectively.The total smoke release was all decreased about 19.3%for two methods.The peak heat release rate of WPC was decreased by 26%and 30%by dry dispersion and wet dispersion with 9%nano-SiO2,respectively,but the smoke release was increased.The above results show that the flame retardant effect of SiO2 on WPC was very limited even though SiO2 were uniformly dispersed and had considerable compatibility with HDPE.The mechanical results showed SiO2(0.5%-9wt%)could improve mechanical properties of WPC at a moderate level(15%-30%)on the premise of uniform dispersion.The creep results showed that the creep strain of nano-SiO2 filled WPC could be reduced obviously by adding nano-SiO2 via wet dispersed method.(2)The effect of network distribution of nano-SiO2 on the properties of WPC:Three kinds of WPC panels were prepared by moulding method:1)the network-structural WPC panels(WPCs)based on the VTS modified nano-SiO2(VTS-SiO2,0.55 wt%content),2)the flame retardant modified WPC(WPCF),3)the flame retardant modified network-structural WPC(WPCSF,0.75 wt%content).The fabrication process of network-structural WPC was introduced as follows:the wood flour(unimpregnated or impregnated with flame retardant),HDPE and additives were compounding together in a blender,then the mixture was melted and blended in a twin-screw extruder and then crushed,screened,to prepare WPC particles with 4-6 mm,after that,VTS-SiO2 was coated on the surface of WPC particles by solution stirring and rotating evaporation,finally,the WPC particles adsorbed a layer of VTS-SiO2 were hot-pressed into panels.The cone calorimeter results demonstrated that the heat releases of the three kinds of WPC panels were all decreased compared with the unfilled WPC,and the decrements of WPCs and WPCSF(VTS-SiO2 content is 0.55%and 0.75%respectively)were comparable with the 9%nano-SiO2 filled WPC prepared by the homogeneous method;in addition,the smoke releases of WPCs and WPCSF exhibited slight increase and decrease,respectively,compared to unmodified WPC.The flexural strength of WPCs increased slightly,the flexural strength of WPCF and WPCSF were slight decrease;the flexural modulus were all increased for the three WPC panels compared with the unmodified WPC.The creep and stress relaxation results showed that the network-structural WPC was favorable to resist the deformation caused by external force.(3)Preparation of conductive and electromagnetic shielding(EMS)WPC with nework-structural nano-conductive carbon black(CCB):The nework-structural WPC based on the unmodified and VTS-modified CCB were prepared by simple mechanical mixing and hot-pressing.The properties of the nework-structural WPC were compared with the CCB uniformly distributed WPC prepared by the traditional melt blending.The optical microscope proved that a continuous CCB network was formed in the WPC.The conductivity and the EMS of the nework-structural WPC were 1.5×10-2 S cm-m1(conductive level)and 20.2 dB at 3%CCB content respectively,however,the conductivity and the EMS of the uniformly distributed WPC were 8.3×10-14S cm-1(dielectric level)and 5.0 dB.This method of in-situ formation of continuous CCB conductive pathway in the WPC can effectively reduce the amount of conductive particles and significantly increase the conductivity and EMS of WPC.The results of conical calorimeter showed that the heat and smoke release of the nework-structural WPC filled with 3%CCB was obviously lower than that of the uniformly distributed WPC.The SEM proved that the cracks did not grow along the CCB network during the WPC failure,indicating the CCB network had high strength.There was no significant difference of the tensile strength between CCB nework-distribution and uniform-distribution WPC,but both of them were higher than the unfilled WPC;the tensile modulus of nework-distribution WPC was higher than the uniform-distribution WPC due to the formation of rigid CCB network in situ;but the impact strength of nework-distribution WPC was lower than the uniform-distribution WPC;the tensile and impact properties of nework-structural WPC based on VTS modified CCB was higher than the unmodified CCB.The CCB nework-distributed WPC was favorable to resist the creep deformation and reduce the thermal expansion compared to the uniform-distributed WPC.(4)The effect of network distribution of nano-ZnO on the properties of WPC:SEM showed that the destruction of the network-structural WPC was along the nano-ZnO network,indicating that the weak strength of the ZnO network.There was no obvious difference in tensile strength and modulus between network-distributed WPC and uniformly-distributed WPC.The impact strength of network-distributed WPC was obviously higher than that of uniformly-distributed WPC,and the impact strength of network-distributed WPC is 30.2%higher than that of unfilled WPC.The ability of network-distributed WPC to resist the external force deformation was greater than that of uniformly-distributed WPC.Both the network-distributed WPC and uniformly-distributed WPC had no obvious effect on resistance of escherichia coli and etaphylococcus aureus.The heat release and smoke release of network-distributed WPC was lower than that of ZnO uniformly-distributed WPC due to its low strength.The method of constructing nano-particles network structure in WPC can greatly reduce the amount of nano-particles and give WPC excellent functionality at the same time.The method for forming the nano-particle rigid network in-situ can balance the contradiction between the mechanics and the functionality of WPC,and the method of the embedded nano-particle network structure is more favorable for reducing the dimensional stability of the wood-plastic nanocomposites relative to the traditional uniform distribution method.