Structural bonding of acetylated Scandinavian softwoods for exterior lumber laminates

Abstract

The bonding properties of a resorcinol-formaldehyde (RF), a phenol-resorcinol-formaldehyde (PRF), a crosslinking polyvinyl acetate (PVAx) and an emulsion polymerisocyanate (EPI) adhesive were determined in laminates of acetylated and unmodified Scandinavian pine (Pinus silvestris L.) and spruce (Picea abies Karst.). The effects of different degrees of acetylation in adjacent laminae were also examined. The effectiveness of adhesion was determined by measuring delamination after three cycles of vacuum-pressure soaking in water and drying, and by measuring shear strength and wood failure in dry and water-saturated conditions. Acetylation stabilized laminates so that essentially no stress fracturing developed in the wood during severe cyclic delamination tests. Bonds to acetylated and unmodified wood with RF and PRF adhesives resisted delamination well, whether adjacent laminae were of equal or unequal degrees of acetylation. Acetylation affected adhesion of both adhesives as indicated by lower percentages of wood failure after water saturation. The PVAx adhesive also resisted delamination well in acetylated wood so long as individual laminae had equal acetyl content. Acetylated laminates with PVAx effectively resisted stress fracturing near bondlines and within the wood, but the unmodified laminates fractured severely. The EPI adhesive bonded the unmodified laminates so well that no delamination and remarkably little stress fracturing developed during cyclic delamination tests. Acetylation affected adhesion of the EPI adhesive as indicated by low wood failure in wet shear tests; however, shear strength and wood failure were high in acetylated wood in the dry condition.