Tensile shear strength of wood adhesives with fire-retardants under elevated temperatures

The utilization of wood and wood derivatives is growing in building construction due to sustainability and circularity, 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 room and elevated temperatures to evaluate their adhesive performance. MDI-based resin demonstrated higher shear resistance at elevated temperatures than the urea-based resin. The addition of fire retardants to MDI revealed a complex mechanism: they decreased strength at room temperature but improved it at 100 and 200 °C through protective actions and a later reverse behaviour at 230 °C, suggesting the onset of catalytic degradation. The transition from panel tear to cohesive failure at higher temperatures confirmed that adhesive degradation became the limiting factor. The MDI formulations proved competitive with, and in some cases superior to, established formaldehyde-based structural adhesives at elevated temperatures. The study concludes that the optimal formulation is application-dependent: for fire-resistance applications, the MDI resin without fire retardant proved to be the most robust at 230 °C, whereas for high-temperature service, MDI with APP maximizes strength at 100–200 °C.