| With the increasingly tense supply-demand relationship of wood resources,the diversification of raw materials is one of the important directions for the development of wood-based panel industry in the future.Crop straw is regarded as an ideal substitute of wood resources due to its large yield,wide source and numerous species.However,the mechanical strength of crop straw is generally not high.In addition,the flakes possess many advantages including high strength-to-weight ratio,low adhesive spread,and good environmental stability.Thus,using the flakes and crop straw as raw materials to fabricate wood-straw composite particleboard with breaking structure,can not only alleviate the shortage of wood resources and improve the utilization rate of straw,but also solve the problem of low strength-to-weight ratio existed in traditional wood-based panel.It is believed to provide new ideas for the research of high-performance wood-based panel and the sustainable development.In this article,rice-straw particle and poplar flakes were used as raw materials,and MDI resin and PF resin were used as adhesives.With a design of three-layer breaking structure,the poplar&rice-straw composite particleboard was fabricated with high strength-to-weight ratio.The relationship between four factors,including the surface/core mass ratio,board density,the mesh of surface straw particle and the amount of PF resin applied in core layer,and the VDP,the surface characteristics and the physicochemical properties of board,was studied.And optimized raw material ratio,slab structure,and process control parameters.The mechanical failure mechanism of plates with great layered structure difference was investigated.The main results are as follows:?1?There was a big difference between the maximum density of surface layer and the minimum density of core layer in the particleboard,which straw particle was used as the surface layer and poplar flakes were used as the core layer.And the ratio of the maximum density of surface layer to the minimum density of core layer is over a range of 3.3-3.5,which is about twice as high as the traditional wood-based panel.The density curve changes steeply from the surface layer to the core layer,and the profile density shows a significant"U"distribution.?2?With the increase of surface straw mass ratio,and the decrease of core poplar flakes mass ratio,both the maximum density of surface layer and the minimum density of core layer decreased.Additionally,the increased thickness of surface layer leaded to the surface average roughness increasing from 2.393?m to 4.404?m,and the surface contact angle decreasing from 108.2°to 89.9°.Moreover,the mechanical properties of particleboard including the surface bond strength,core bond strength,MOR,MOE,and the board surface and board edge screw holding capability,were decreased.Through comprehensive comparison,the optimal mass ratio of surface layer to core layer is 4:6.?3?The density of particleboard has a significant effect on the properties of particleboard.With the decrease of board density,the degree of the whole board's plasticization and densification decreased.The surface bond strength,the core bond strength,MOR,MOE,and the board surface and board edge screw holding capability were all decreased.The surface pre-curing layer reached to 0.93mm when the board density was 500 kg/m3.The surface average roughness increased from 2.246?m to 5.137?m,and the surface contact angle decreased from 119.3°to 96.6°.Under the condition that the physical and mechanical properties meet the requirements,the density of poplar&rice-straw composite particleboard can decrease to 560kg/m3,and the strength-to-weight ratio can reach to45089N i m/kg.It shows that the reinforcement effect of flakes is obvious.?4?With the increase of straw particle mesh in surface layer,the maximum density of surface layer increased,and the minimum density of core layer decreased.And the ratio of the maximum density of surface layer to the minimum density of core layer is over a range of 3.5-3.9.The surface bond strength increased from 0.80MPa to 1.04MPa,and when the straw particle mesh is over a range of 60-80 mesh,the MOR and MOE reach the maximum value,which is 31.48 MPa and 4038MPa,respectively.The screw holding capability of board surface reduced from 2093N to 1696N,and the screw holding capability of board edge reduced from 1672N to 1510N.The 2h thickness swelling is between 1%to 3%,and the 24h thickness swelling is between 7%to 8%.According to the quality and manufacturing cost of the board,the straw mesh of surface layer is better over a range of40-80 mesh.?5?When the adding amount of PF adhesive in core layer increased from 4.5%to 9.0%,the bonding strength in core layer increased from 0.64MPa to 0.80MPa.The MOR,the board surface and board edge screw holding capability also increased.The 2h and 24h thickness swelling decreased from 9.3%to 4.6%and 16.0%to 11.7%,respectively.The formaldehyde emission increased from 0.43mg/100g to 0.59mg/100g,but lower than1.6mg/100g that meeting the F????of Japanese standard.?6?The failure process of poplar&rice-straw composite particleboard mainly includes three stages:elastic deformation,inelastic deformation,and ductile?delamination?failure,which is manifested as delamination fracture.This failure process enables the board to possess the function of fracture early warning.?7?When the surface layer used rice straw particle as raw material and MDI resin as adhesive with 5.0%adding amount,and the core layer used poplar flakes as raw material and PF resin as adhesive with 6.0%adding amount?instead of MDI resin with 4.0%adding amount?,the fabricated poplar&rice-straw composite particleboard with three-layer breaking structure could meet the requirement for the furniture?P2 type?or load-bearing?P4 type?Particleboard under dry state in the GB/T 4897-2015.And the cost of adhesive could be reduced by about 50%. |
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