Tensile properties of finger-jointed lumber under high-temperature and oxygen-free conditions

A model for engineered wood was developed that considers the parallel-to-grain tensile strength of finger-jointed lumber at high temperatures relevant to fire conditions. The finger-jointed lumber was composed of Douglas fir, larch, and poplar wood with phenol-resorcinol-formaldehyde (PRF) as an adhesive. The tensile properties of the finger-jointed lumber were evaluated at high temperatures under oxygen-free conditions, i.e. in a nitrogen atmosphere. A combination of chemical and thermal-physical property analysis of the PRF adhesive and microscopic observations on the glueline was used to discuss the reduction of tensile strength of the parallel-to-grain finger-jointed lumber at variable temperature. The results show that the tensile strength of the finger-jointed lumber decreased linearly with increasing temperature. The parallel-to-grain tensile strength of the PRF finger-jointed samples at 20 and 280 °C were 84 and 5% of the tensile strength of the solid wood at 20 °C, respectively. The thermal-physical properties and scanning electron microscopy analysis revealed that the pyrolysis intensity of the PRF adhesive was lower than that of the wood at 220 °C or higher.