Reducing Long-Term Formaldehyde Emissions under Desiccator Conditions in Cold-Pressed Plywood Used as Cross-Laminated Timber (CLT) Core Layer by Using Urea-Based Formaldehyde Scavengers

A post-treatment strategy utilizing a 33.2% (w/w) urea-based formaldehyde scavenger solution was applied to mitigate the long-term formaldehyde emission trend under desiccator conditions associated with energy-efficient cold-pressed plywood used as the core layer in hybrid cross-laminated timber (CLT). The plywood was manufactured by cold-pressing with a phenol-resorcinol-formaldehyde (PRF) adhesive, and the scavenger solution was applied solely to the edge surfaces at three dosages (0.021, 0.063, and 0.105 g/cm2). The long-term emission trends of the treated samples were compared with those of untreated cold-pressed plywood, hot-pressed plywood, and other post-cold-pressing treatments (heat and water). The impact of the scavenger treatment on the physical and mechanical properties of the plywood (density, moisture content, and bending properties) and on the planar (rolling) shear performance of the resulting CLT panels was also evaluated. The treatment reduced the initial (24 h) formaldehyde emission by 65%–68% at dosages of 0.063 and 0.105 g/cm2, with the difference between these two dosages being statistically non-significant, indicating saturation of the scavenging effect. Over the 408 h monitoring period, the mean emission of the treated plywood was approximately 89% lower than that of the untreated control, satisfying the Korean E0 emission class. Neither the bending properties of the plywood nor the planar shear performance of the CLT panels was significantly affected by the treatment. These findings support the use of a urea-based edge treatment as a practical and structurally benign strategy for reconciling energy-efficient cold-pressing with stringent formaldehyde emission requirements in engineered wood production.