STRUCTURAL BONDING OF ACETYLATED SCANDINAVIAN SOFTWOODS FOR EXTERIOR LUMBER LAMINATES

The bonding properties of a resorcinol-formaldehyde (RF), a phenol-res orcinol-formaldehyde (PRF), a crosslinking polyvinyl acetate (PVAX) an d an emulsion polymer-isocyanate (EPI) adhesive were determined in lam inates of acetylated and unmodified Scandinavian pine (Pinus silvestri s L.) and spruce (Picea abies Karst.). The effects of different degree s of acetylation in adjacent laminae were also examined. The effective ness of adhesion was determined by measuring delamination after three cycles of vacuum-pressure soaking in water and drying, and by measurin g shear strength and wood failure in dry and water-saturated condition s. Acetylation stabilized laminates so that essentially no stress frac turing developed in the wood during severe cyclic delamination tests. Bonds to acetylated and unmodified wood with RF and PRF adhesives resi sted delamination well, whether adjacent laminae were of equal or uneq ual degrees of acetylation. Acetylation affected adhesion of both adhe sives as indicated by lower percentages of wood failure after water sa turation. The PVAX adhesive also resisted delamination well in acetyla ted wood so long as individual laminae had equal acetyl content. Acety lated 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 tha t no delamination and remarkably little stress fracturing developed du ring cyclic delamination tests. Acetylation affected adhesion of the E PI adhesive as indicated by low wood failure in wet shear tests, howev er, shear strength and wood failure were high in acetylated wood in th e dry condition.