Time-temperature resistance of transverse stressed lap joints of glued spruce and thermal analysis of adhesives

Lap joints were prepared by gluing spruce plates with three types of adhesives (phenol-resorcinol-formaldehyde, PRF; one-component polyurethane, PUR; and epoxy, EP). The joints were exposed to 140 °C and 170 °C for 24 h (20, 40, 60, 80, 180 and 1140 min) and then loaded in four-point bending to verify the behavior of glued lap joints in building structures exposed to fire in its initial stage and stressed in transverse direction. The lap joint strength was compared to that of spruce specimens exposed to the same conditions and of the same stressed area. The failure mode of lap joints was evaluated visually; the failure mode was caused by exceeding the transverse load capacity of bulk wood. The strength of spruce fell by 40% after exposure to both temperatures for 20 min and next decrease began after 80-min exposure at 170 °C as a result of the beginning of hemicellulose decomposition. Only PUR improved the spruce strength at 20 °C (by 30%) and at both temperatures except that after exposure to 170 °C for long time (3 and 24 h). The PUR-spruce lap joints revealed very good fire resistance for the initial fire exposure (80 min at 140 °C and 40 min at 170 °C). The strength of both PRF and EP lap joints was the same as that of spruce at 20 °C but PRF improved the spruce strength at 140 °C after 20-min exposure (by 45%) and at 170 °C, where PRF lap joints bore the load irrespective of wood degradation. The EP lap joints revealed the worse thermal resistance due to the rubber state of incompletely post-cured and degraded EP. To evaluate an adhesive structure and its prospective change after thermal exposure and to evaluate the influence of an adhesive thickness, the adhesives were cured in a form of bulk and thin films and tested by Thermogravimetry (TGA), Differential Scanning Calorimetry, Fourier Transformed Infrared Spectroscopy (FTIR) and evolved gas analysis (EVA) using TGA-FTIR. Curing and post-curing process was shown to be thickness dependent only in case of EP. Low-molecular components detected during adhesives heating were products of PUR and PRF post-curing and of unreacted hardnener decomposition in case of EP. All adhesives were degraded after thermal exposure to both elevated temperatures.