Assessment of fire-retardant treatments and their impact on the fire performance and bonding properties of aspen and silver birch veneers

Abstract Plywood is a valuable material that offers superior properties compared to solid wood. However, like solid wood, its poor flammability limits its suitability for various applications. This study investigates the impact of distinct types of fire retardants (FRs), designated as B, P, and U, as well as various application techniques—roller coating, spraying, and impregnation—on the bond strength and fire performance of aspen and birch veneers. A lap shear test (LST) was conducted to compare the bond strength of veneers bonded with a newly developed lignin-substituted phenolic formaldehyde (LPF) resin against a conventional formaldehyde resin (PF) to evaluate the effects of FR treatments. The results indicated that FR retention was significantly higher with impregnation and, overall, with aspen veneers than birch, except for aspen veneers treated using the spraying method with P-FR. The P-FR exhibited strong and consistent performance with birch veneers, irrespective of the treatment method. Notably, P-FR roller-coated aspen, with an FR retention of 9.5% and an ignition time of 11 s, demonstrated the best overall reaction to fire performance. With a basic protection duration of 52 s and a thermal decay time of 157 s, this combination demonstrated improved thermal resistance. The LST further revealed that FR treatments significantly impacted birch veneers, which experienced a 30–48% decrease in shear strength with PF resin relative to untreated veneers. The LPF resin was incompatible with birch veneers when treated with P-FR formulated from a protic ionic liquid. For aspen, the overall decrease in shear strength was 40%, but with B- and U-FR treatments, the reductions reached 38% and 50%, respectively—much higher than the decreases observed in birch (25% and 37%) relative to control samples. These findings provide valuable insights into the effects of fire retardants on new resins, though further research is necessary for comprehensive validation.