Interlaminar shear properties of glulam made of heat-treated laminates

Heat-treated wood is environmentally friendly relative to other modified woods, and if processed into glulam, it is expected to optimize not only its durability and stability in color and dimension but also the large size application of glulam. Interlayer interface is the key link where the bonding quality of glulam is chiefly determined. In this study, Mongolian Scots pine ( Pinus sylvestris var. Mongolica ) laminates with varying degrees of heat treatment were respectively bonded by one-component polyurethane adhesive (PU) and aqueous polymer isocyanate adhesive (API). In order to explore the effect of heat treatment on glulam interface, the shear test method was taken to systematically examine the interlaminar bonding properties. The result showed that high-temperature heat treatment reduced the hydrophilic groups in wood and decreased the surface wettability, and indicated a negative correlation between heat treatment temperature and surface wettability. Planing resulted in a flatter surface and reduced the adverse effect of brittle layer on the surface, thus improving the surface wettability. Deep heat treatment (215 °C) was not ideal for the interlaminar shear properties of glulam. Planing the laminate surface before gluing significantly improved the interlaminar shear properties. A planing thickness of 0.6 mm was able to obtain a better bonding property. PU was more suitable for gluing heat-treated laminates compared with API. Under a high humidity environment, the interlaminar shear property of heat-treated glulam was relatively stable. The interlaminar shear strength of glulam with a higher heat treatment temperature was less influenced by environmental humidity. Moderate heat treatment (180 °C) increased the crystallinity and expanded the crystallization region of wood, which was conducive to improve the shear properties of glulam short beams. Deep heat treatment made the wood fibers hard and brittle, and the shear strength of short beams decreased slightly.