The effect of birch (Betula pendula Roth) face veneer thickness on the reaction to fire properties of fire-retardant treated plywood

Birch (Betula pendula Roth) is the primary wood used for plywood manufacturing in the Baltic and Nordic regions. Birch plywood is a valuable material with superior properties to solid wood, but its poor flammability limits its use in building construction and transportation. Several fire-retardant impregnated spruce and birch plywood solutions are available on the market, the majority of which are based on the concept of fire-retardant pressure impregnated veneers or soaked plywood. Hence, the demand for fire-resistant plywood is increasing worldwide. However, for the plywood industry, it is crucial that any fire protection enhancement process does not significantly disrupt the current production setup. This study investigates the effect of face veneer thickness (FVT) and fire retardant (FR) concentration on the fire performance of birch plywood. The FR treatment was applied by roller coating with the main and diluted (70 %) concentrations of a new protic ionic liquid, FR. Three FVTs were evaluated: 1.0 mm, 1.5 mm and 2.6 mm. The cone heater test demonstrated that the 70 % diluted FR did not provide adequate fire protection, while the plywood batches with 2.6 mm FVT provided a better reaction to fire. Conversely, the SBI test revealed that the 1.5 mm plywood meets the EUROCLASS B requirement for building applications, mainly because of the 2.6 mm FVT's 3 % higher total heat release compared to the EUROCLASS requirement. This outcome suggests magnified non-uniformity in large samples, particularly insufficient coverage of FR despite deep penetration with 2.6 mm FVT. Given the current plywood industry practices, roller coating with the main concentration of the protic ionic liquid FR and a 1.5 mm FVT is a feasible method to improve the fire performance of birch plywood for construction applications.