Abstract: [Objective] The effects of the penetration of resin on the mechanical properties of wood cell wall in the interphase region were studied systematically, which provided the theoretical basis for the optimization of the manufacturing technology and the enhancement modification of wood-based composites.[Method] The elastic modulus, hardness, creep properties, dynamic storage modulus, and dynamic loss modulus of wood cell walls from loblolly pine (Pinus taeda)and resins (phenolic resin and urea-formaldehyde resin)in the interphase region were investigated by the means of nanoindentation and nanoscale dynamic mechanical analysis (Nano-DMA).[Result] In the static mechanical behavior, the effective penetration of resin contributed greatly to the elastic modulus (E_r)and hardness (H)of cell wall of wood tracheid in the interphase region. In the interphase of wood-PF, the elastic modulus (E_r)and hardness (H)of loblolly pine cell wall increased by 7% and 26%, respectively. The curve fitting goodness of fit (coefficient of determination squared value R²)was greater than 0.97, indicating Burgers model can be used to describe the mechanical indentation creep characteristics of wood tracheid cell walls. The instant elastic modulus of the wood cell wall in the interphase increased, while the viscoelastic modulus and the viscosity decreased after being penetrated with resin. In the initial period of load holding, the creep compliance of the wood cell wall decreased by about 60% and 58% after being penetrated by PF resin (phenolic resin)and UF resin (urea-formaldehyde resin), respectively, as compared to the control cell wall. In the dynamic mechanical behavior, with the increase of the loading frequency, the storage modulus E'_r of the interphase increased gradually, while the loss modulus E″_r and loss tangent tan δ decreased. When the loading frequency was 10 Hz, the storage modulus of the cell wall layer of wood tracheid penetrated with PF resin and UF resin increased by about 16% and 29%, respectively.[Conclusion] The penetration of resins made a great contribution to the elastic modulus, hardness, and short-term creep resistance of wood cell wall. The important dynamic mechanical parameters obtained by Nano-DMA indicated that wood cell wall had a higher storage modulus and loss modulus as compared to that of the adhesives. The storage modulus of the cell wall penetrated with adhesive had an increase in comparison to the control cell wall, while the loss modulus and loss tangent showed a decreasing tendency, which will have negative effect on stress transfer and dispersion in the interphase region of wood-based composite.

Key words: wood, interphase, cell wall layer of tracheid, mechanical behavior, nanoindentation