UV-irradiation reduces the mechanical performance of wood-adhesive interphase

UV irradiation leads to various usage issues of engineered wood products (EWPs), including discoloration and declined mechanical performance. The radical generation and physico-chemical changes of the wood-phenol formaldehyde (PF) adhesive interphase during alternating UV irradiation were investigated in this paper, aiming to understand the oxidation and degradation of EWPs in daily applications. Electron spin resonance spectroscopy demonstrated that UV irradiation activates the phenol ring in lignin and PF adhesive, generating the reactive oxygen species (ROSs) composed of phenoxyl radicals, singlet oxygen, superoxide anion, and hydroxyl radicals. Experiments also showed that ROSs cleavage the weak covalent bonds in the cell wall and PF adhesive, triggering photon-oxidation and degradation. The cavities in the UV irradiated cell wall were observed after UV irradiation by scanning electron microscopy. Nanoindentation analysis also demonstrated the reduced mechanical performance of cell walls in the wood-adhesive interphase and bulk wood, leading to the significant reduction in the shearing and compression strength of wood-PF adhesive bonds. In contrast, the chemical structure and mechanical performance of PF adhesive remained after UV irradiation, suggesting better stability to UV exposure. It has been demonstrated that the degradation of wood has priority during the alternating UV irradiation.