Effects of accelerated aging treatment on the microstructure and mechanics of wood-resin interphase

Plywood panels prepared from loblolly pine with cured phenol resin (PF) and urea-formaldehyde resin (UF) were submitted to accelerated aging and the microstructures and mechanics of wood-resin interphase were studied by nanoindentation (NI) and nanoscale dynamic mechanical analysis (Nano-DMA). The mass loss (ML) of wood, PF and UF resins were 3.4, 5.0 and 4.6% after aging treatment, respectively, and a large amount of microcracks were observed on the surface of wood and resins after aging treatment, which also affected the static mechanics of the cell walls far from the interphase region and the resins in the interphase region. The elastic modulus (Er) and hardness (H) values of the cell wall decreased by 7.2 and 9.5%, respectively, against the untreated control. The storage and loss modulus of the resins decreased significantly after aging treatment. The significant inconsistency in the mechanics, shrinkage and swelling properties of wood cell wall and resin in the interphase region after aging treatment resulted in a decrease of about 47 and 51% on the average bonding strength of the plywood made of PF and UF resins, respectively.