Fungal decay-provoked degradation of cross-linking networks in phenol-formaldehyde adhesive: From fragmentation process towards damaged mechanical robustness

The mechanical robustness of structural adhesives as represented by phenol formaldehyde (PF) adhesive is vital for the safe service of engineered wood/bamboo products facing environmental aggressors. Here, fungal decay-provoked hazard towards the mechanical robustness of PF adhesive was demonstrated, which was usually overlooked in previous works. Obtained results proved that the white rot ( Trametes versicolor, T. versicolor ) and brown rot ( Gloeophyllum trabeum, G. trabeum ) fungus can both grow on the surface of cured commercial PF adhesive. An evident oxidization can only be observed on the T. versicolor -decayed PF adhesive, as along with the appearance of 2,6-bis(1,1-dimethylethyl)-1,4-benzenediol after decay. It further demonstrated the ruptured and oxidized cross-linking structures. The evident degradation of PF adhesive can be assigned to the favored activity of laccase (40.9 U mL −1 ) and lignin peroxidase (60.5 U mL −1 ) during the decay of T. versicolor . The elastic modulus and hardness of PF adhesive reduced by over 31.6 % and 50.2 %, respectively, which is also accompanied by the elevation in the creep deformation (45.4 %), after a 40-day T. versicolor decay. This work revealed the decay-sensitivity of PF adhesive, and the cleavage of cross-linking structures is the major trigger for the degraded mechanical robustness of PF adhesive after decay. • Aggressive decay fungus grow unchecked on PF adhesive surface. • Enzyme-catalyzed fragmentation of cross-linking structures in PF adhesive. • Damaged mechanical robustness of decayed PF adhesive.