Improving the adhesion of high-density softwoods with isocyanate based adhesives through surface incision

Abstract

Glue laminated timber is currently manufactured using classical adhesives such as resorcinol formaldehyde and phenol resorcinol formaldehyde. These are proven structural adhesives; however their long cure times and rising costs are creating opportunities for newer technology adhesives. One such class are the structural polyurethanes with decreased spread rates and faster curing times. Their limitation lies in their inability to adhere timbers of densities exceeding 800 kg/m3. When used on species including the southern pines (Pinus spp.) with a high frequency of latewood, they delaminate after accelerated weathering tests due to stresses imposed on the glue line during the drying process. Surface incision has been trialed in this study to increase the penetration of polyurethane adhesives and reduce glue line stresses. The study shows that incisions to a depth of 2 mm decreases delamination when compared to matched samples with no incisions. The significant increase in glue line surface area may result in stress reduction as the more compliant adhesive may distribute the stresses better across the glue line. Furthermore, microscopic analysis suggests that the incisions are reducing glue line stress through crack propagation into the timber pointing to the possibility of increased timber compliance at the glue line.