Preparation And Properties Of Co-extruded Laminated Veneer Lumber-wood Flour Polyvinyl Chloride Composites With Core-shell Structure

Poplar laminated veneer lumber(LVL)has the advantages of excellent mechanical properties and high strong-heavy ratio etc,but LVL exhibits poor dimensional stability,antibacterial and corrosion resistance because of the presence of many hydrophilic functional groups.Wood plastic composites(WPC)have the advantages of waterproof,anticorrosive and moth-proof etc.However,WPC matrix molecular chain is easy to relax under long-term load,resulting in decreasing mechanical properties and increasing the creep deformation.In this paper,poplar LVL-wood flour-polyvinyl chloride composites(WPLC)with a core-shell structure was prepared via co-extrusion technique,which combining with the advantages of poplar LVL and PVC matrix WPC(WPVC).Because of the lower mechanical properties of pure PVC compared with LVL,the WPLC with different wood flour contents were prepared to investigate the reinforcement effects of different wood contents in shell layer on the WPLC.The results showed that the interface adhesion between core and shell layers was easy to peel,which has an important influence on the mechanical properties and life cycles of WPLC due to the different humidity and temperature responses of poplar LVL and WPVC.Firstly,because of the interface problem,the interface adhesion between poplar LVL and WPVC was improved by using green chemical interface treatment method with non-adhesive bonding.The results showed that this method can improve the interface adhesion but cannot completely solve the phenomenon of stripping.Secondly,in consideration of mortise-tenon structure,different cross section LVL and stronger physical structures of interface were prepared for fabricating WPLC,and the effect of different LVL structures on the whole properties of WPLCs was investigated.Finally,the necessity and feasibility of flame retardant treatment of shell WPVC and core LVL on WPLC were analyzed by modification with flame retardants of ZnO and self-made FRW on WPVC and LVL,respectively,and the double-layer structural WPVC/poplar veneer composite was prepared for flame retarding measurement.The main research results are presented as follows:(1)The influence of wood flour content in shell layer on the properties of WPLCWPVC shell-layers showed a linear increase in flexural and tensile properties,while fracture strain and impact strength decreased gradually with increasing wood flour content.The results showed the reinforcement effect of wood flour on WPVC,but caused brittle fracture of the material.The addition of wood flour in shell-layer also contributed to the improvement of WPLC flexural strength.WPLC with 40%wood flour content in shell layer has the highest flexural strength(87.8MPa),but the flexural modulus of WPLC in each group was lower than that of LVL.Compared to LVL,WPVC shell layer can improve the impact toughness of WPLC,and the low speed drop-weight impact tests showed that WPVC shell layer has effective protection on LVL core layer when WPLC subjected to local damage.The creep resistance of WPVC shell layer and co-extruded WPLC increased with increasing wood flour content,and the creep strain of WPLC60 was the lowest,which was only 85.8%and 28.9%of LVL and WPVC60.respectively,End-capped WPLC showed excellent dimensional stability after hot water soaking-drying cycle testing by comparing with LVL.However,normal temperature immersion tests of unend-capped WPLC showed that,the shell had a significant inhibitory effect on water absorption and thickness swelling of LVL.but the shell layers were still destroyed after cycle testing.Moreover.the macroscopic peeling phenomenon at interface still occurred on unend-capped WPLC when affected by humidity and temperature at long time placement.(2)The effect of chemical interface treatment on the properties of WPLCCompared with aluminate coupling agent.MAPP and MAPE,WPVCW treated by amino-propyl-triethoxy silane(KH550)coupling agent had the best interface adhesion performance,which showed the highest interface adhesion strength(1.22MPa).the highest interface shear strength(3.69MPa).the strong interface heat and humidity resistance and the highest flexural strength(129.32MPa)and flexural modulus(10.9GPa).Silane coupling agent(KH550)treatment led to improvement of interface adhesion between shell WPVC and core LVL,but the flexural strength and flexural modulus of WPLC was slightly increased.However,the stripping phenomenon at the interface between WPVC and LVL cannot be inhibited under the influence of temperature and humidity.The rapid rupture of the shell-layer was still observed when the unend-capped specimen immersed in water.(3)The influence of different section shapes of LVL core on the properties of WPLCCompared with the original LVL,flexural strength and modulus of LVL that was worked by slotting machining were greatly reduced due to the reduction of the volume of wood,and trapezoidal groove LVL(TLVL)showed the biggest flexural strength decrease(19%)and the highest creep strain(0.28%).However,the co-extruded WPLC with trapezoidal groove LVL(WPTC)showed highest flexural and the lowest creep strain(0.21%),which can be explained that trapezoidal groove machining can significantly improve flexural strength and creep resistance of the WPLC.After boiled-freeze-drying cycle testing,the flexural performance of LVL was approximate zero,and flexural properties of WPLC decreased by 25-35%but still exhibited certain mechanical property.Which explains that the presence of the shell layer have considerable positive effect on the dimensional stability and inhibiting the decrease of mechanical properties.The flexural strength of WPTC was the highest after aging testing compared to other structures,which was attributed to the mosaic structures of trapezoidal groove in the interface can more efficiently improve the maintenance mechanical performance of WPLC.The normal temperature immersion tests of unend-capped specimens showed that the groove treatment exhibited the better dimensional stability than that of WPLC specimens prepared by the original LVL.(4)The influence of flame retardant treatment on the properties of WPVC/poplar veneer compositesThe flame retardant WPVC/poplar veneer double-layer composites(WPVCW)were prepared by hot pressing to analyze the protection effect of PVC shell layer on core LVL in WPLC,due to the size of WPLC cannot satisfy the requirement of Cone test standard.By increasing the content of flame retardants,flame resistance and smoke suppression performance of WPVCW were gradually improved.WPVCW modified by adding 15wt%ZnO and 10wt%FRW had the obvious flame retardant effect,which the peak heat release decreased by 28.5%,and total smoke release reduced by 66%.When WPVCWs were carbonized,the surface of WPVC and poplar veneer presented relative complete morphology.TG tests found that ZnO and FRW accelerated the thermal decomposition of the material at the first stage,but the thermal degradation rate decreased at the second stage.The pyrolysis rate decreased and residues were also increased at high temperature,moreover,compared with the untreated samples,the residual amount of WPVCW treated with 10wt%ZnO and 10wt%FRW increased by about 91.0%at 800?,showing better thermal stability properties after ZnO and FRW flame retardant treatment.The addition of FRW and ZnO reduced the mechanical properties of WPVCW slightly,but the interface adhesion performance was not affected obviously.In summary,WPLC combined the advantages of LVL and WPVC together.By reinforcing shell-layer and the treatment of the interface adhesion between LVL and WPVC can improve the mechanical property and durability of WPLC.In addition,the flame retardants treatment of WPVC and LVL can give WPLC excellent flame retardant properties and promote wide application fowls in the interior area.