Influencing Mechanism Of Micro- And Nano- Structure Of Wood Bonding Interphase On Interfacial Mechanical Properties

Wood bonding interphase,as a scientific issue in the field of wood composites,plays a key role of transferring load and dispersing stress in multiphase systems.Its structural and mechanical properties have great influences on the overall strength,toughness and durability of wood composites.The technological development and performance improvement of wood composites mostly remain at the macroscopic level.But it is difficult to control the material properties from the macroscopic level,since the wood units of the composites show large variability in performance and structure.It would provide important theoretical basis for the controllable design of mechanical properties of wood composites to build relationships among glued factors,micro-nano interphase structures and mechanical properties of the interphase,and to realize the effect of micro-nano structures on mechanical properties of the interphase.In this study,wood(Cryptomeria fortunei Hooibenk)and adhesives of polyvinyl acetate(PVAc)and urea formaldehyde resin(UF)were used to prepare bonding interphases with different structures by adjusting the gluing parameters such as adhesive types,unit pressure,adhesive viscosity and grain direction laminated.The morphologies,microstructures and chemical structure of micro areas of different interphases were analyzed by combining micro and nano characterization means of optical microscopy,electron microscopy,laser confocal Raman spectroscopy and nanoIR technology.The mechanical properties distribution and bonding strength were evaluated by nanoindentation/nano-scratch techniques and technologies of pulling wood strip and tracheid from resins,respectively.The response mechanism of the interphase structure to the gluing parameters was analyzed,and the influence mechanism of the interphase structure on the mechanical properties was revealed.The ultraviolet(UV)laser ablation technology,as an innovative sample preparetion method,was utilized to prepare interphase samples,which effectively solved the problem of sample structure damage and cell deformation caused by the traditional sample preparation methods.The nanoIR technology was used to characterize the penetration of the adhesive in the wood cell wall,promoting the study of gluing mechanism to a nano-scale level.The main conclusions of the paper are as follows:(1)The pressure was a significant factor in the formation of interphase structure.As the pressure increases,the number of both pores in the PVAc interphase layer and the cracks between the adhesive and the cell wall gradually increased,while the size of them decreased.As for the UF interphase,the size of the pores and cracks obviously decreased,and they even disappeared.When the pressure increased from 0MPa to 1.2MPa,the penetration depth of PVAc and UF increased by 54.03% and 124.11%,respectively,and the infiltration uniformity increased by 20.23% and 68.02%,respectively.The radial compression of tracheid for PVAc and UF in the interphase was 31.21% and 68.03% under the pressure of 1.2MPa,and the deformation occurred mainly in the early tracheids.(2)The PVAc mainly distributed in the processing cracks of cell wall and the middle lamellar,having no significant effect on the mechanical properties of the interphase.The bonding strength between wood and PVAc was mainly ascribed to mechanical interlocking effect.The UF penetrated into the cell wall through the spaces between microfibrils to form nanoscale structure with a large area,and the elastic modulus and hardness of the penetrated cell wall increased by 9.63% ~ 26.33% and 4.03% ~ 26.14% comparing with the control cell wall.In addition to the mechanical interlocking effect,there was chemical bonding in wood/UF bonding interphase.(3)Under no pressure,the contact of PVAc with cell wall was better than that of UF,and the interfacial shear strength(2.35 ± 0.22MPa)of wood strip/ PVAc was greater than that of wood strip/UF(1.24 ± 0.04MPa).The critical destruction load of tracheid/PVAc(204.71±35.57 mN for chemical dissociation and 60.81±25.03 mN for mechanical detachment)was greater than that of tracheid/UF(46.02 ± 21.11 mN for chemical dissociation and 37.42 ± 11.08 mN for mechanical detachment).The above results of pulling tests were in accordance with the results that the macroscopic shear strength of PVAc(5.86MPa)was larger than that of UF(3.85MPa).(4)Under the pressure of 0.4MPa,0.8MPa and 1.2MPa,there was no significant difference between the macroscopic shear strength of PVAc and UF samples,but the failure morphology of them showed obvious differences.The flatness of UF was higher than that of PVAc,and it increased with the raise of pressure.The distribution uniformity of mechanical properties for the UF interphase was better than that for the PVAc interphase,which could be due to the better penetration uniformity and higher compactness degree of the UF interphase.