Photodegradation risk evaluation of polyurethane gluelines in wood products by infrared spectroscopy and mechanical tests

During outdoor use, glued wood products are exposed to rather harsh conditions, which involve moisture and temperature fluctuations, attacks by microorganism, UV radiation etc. For some risks, the effect on glued wood products are already known, however less investigated are the risks concerning UV radiation. It has been established that weathering affects only the surface of the wood without influencing the mechanical properties. However, polyurethanes are known to undergo more significant degradation with structural changes, strength reduction and surface erosion. The objective of the study was to investigate whether photodegradation caused by UV irradiation can impair the bond quality of glued wood products bonded with one-component polyurethane adhesives. In the study, considerable attention was attributed to the investigation of the chemical changes by FTIR methods focusing on the differences as well as normalisation possibilities of the spectra. The results showed that UV significantly degraded the chemical structure and mechanical properties of the one-component polyurethane films as well as introduced a number of microcracks in the gluelines. Despite that, glued wood products after UV irradiation showed only minor or none at all reduction in the bond quality characterised by wood failure percentage. The only identified aspect that had some effect was the thickness of glueline, which should be kept at minimum. The chemical analysis revealed two stable bands, which could be used for normalisation of polyurethanes FTIR spectra during weathering investigations. The corresponding bands are attributed to the aromatic and isocyanurate ring structures.