Reaction to Fire of Birch Plywood Bonded with Three Different Adhesive Systems at Multiple Scales

Abstract This study investigates the reaction to fire of 9-layer birch plywood bonded with three industrially relevant adhesive systems: urea formaldehyde (UF), melamine urea formaldehyde (MUF), and phenol formaldehyde (PF). Panels were prepared under standardized conditions and evaluated at multiple scales—ranging from fuel property characterisation and thermogravimetric analysis to bench-scale cone calorimetry and ignition testing in the single flame test (SFT). A self-constructed Mini-SBI test (downscaled single burning item test) was developed to capture intermediate-scale reaction to fire with a focus on the orientation of the surface veneer layer. Results reveal that UF-bonded plywood, particularly with lengthwise-oriented surface veneers, exhibited the most critical fire response, including increased susceptibility to ignition during extended localized flame exposure and fire growth promoted by adhesive-controlled veneer separation. PF-bonded samples showed greater thermal stability and higher char formation. MUF-bonded samples exhibited comparatively favourable ignition behaviour, while smoke-production differences among the adhesive systems were not statistically significant. Differences in thermal degradation and flammability highlight the influence of adhesive chemistry and veneer orientation on the reaction to fire of the material. The Mini-SBI setup differentiated between material variants and demonstrated its potential as a material-efficient intermediate-scale method for comparative fire testing during product development. This work highlights the need for adhesive-specific consideration in fire safety assessments of engineered wood products.