Elucidating oxidative degradation of phenol-formaldehyde adhesive induced by mold colonization

Mold colonization compromises the mechanical robustness of cured phenol-formaldehyde (PF) adhesive, arising as a significant durability obstacle to the long-term services of engineered bamboo products (EBPs). Herein, the colonization behaviors and oxidative degradation of PF adhesive induced by common mold species, namely Aspergillus niger (A. niger), Trichoderma virens (T. virens), and Penicillium citrinum (P. citrinum), were investigated, aiming to reveal the plausible degradation pathways. The results demonstrated the successful colonization of A. niger, T. virens, and P. citrinum on PF adhesive surfaces, with the diffuse distribution of oxidases in which laccase occupies the most. The enzymatic assays revealed the superior laccase activity of P. citrinum (26.5 ± 1.81 U·mL^-1) as compared with A. niger and T. virens, resulting in the progressive oxidation with aliphatic carbon reduction (66.1 %) of PF adhesive. Strong binding interactions between PF adhesive and laccase were unmasked, showing a binding energy of -7.51 kcal·mol^-1. Meanwhile, the transition metal ion in laccase is closer to the phenolic hydroxyl group (15.2 Å), indicating the capability for electron transferring. The plausible degradation pathway of mold-colonized PF adhesive can be three-staged, i.e., phenolic oxidation, quinone formation, and subsequent ring-opening rearrangement. Such speculation was further supported by the experimental identification of phenolic fragments and benzene-functionalized cyclopentene derivatives. Nanoindentation quantified severe mechanical deterioration with a 65.3 % reduction in elastic modulus of PF adhesive after 20-day P. citrinum colonization, while interfacial delamination in bamboo-PF bonds was also observed. These findings elucidate the enzymatic pathways driving PF adhesive degradation from molecular function to macroscopic failure.