Bonding strength of wood adhesives with fire-retardants under elevated temperatures

The utilization of wood and wood derivatives are growing in building construction, mainly due to sustainability and circularity reasons, despite the inherent flammability of wood. Woodworking often requires bonding elements with adhesives, which are prone to degradation under fire. A potential solution to improve the fire resistance of wood joints and their derivatives involves incorporating fire retardants into the adhesive compositions. This study explores the adhesion properties of bonded joints of medium-density fibreboard and pinewood panels using resins based on urea and polyurethanes derived from diphenylmethane diisocyanate (MDI). Various proportions of ammonium polyphosphate, borax, and expandable graphite were incorporated as fire retardants. Shear testing was conducted on lap joints under both room and elevated temperatures to evaluate their adhesive performance. At room temperature and 100°C, the predominant failure mode was panel tear failure, while at higher temperatures, cohesive failure became more prevalent. The results indicated a reduction in bonding strength and displacement with increasing temperature. In particular, the MDI-based resin demonstrated higher shear resistance at elevated temperatures in comparison to the urea-based resin. The addition of fire retardants, particularly ammonium polyphosphate, improved the shear strength of MDI joints at 100°C and 200°C. Furthermore, the determined shear modulus per meter evaluated the decreasing rigidity of the bonding joint with increasing temperature. In the reference case of urea without fire retardants, the rigidity of the bond decreased by approximately 100% at 200°C compared to room temperature. In contrast, the MDI cases with ammonium polyphosphate and borax experienced a more moderate decrease of around 60% at 230°C, indicating improved thermal stability.