Bond Formation By Wood Surface Reactions–Part III: Parameters Affecting the Bond Strength of Solid Wood Panels

Parameters such as press conditions, activating reagent, wood species, and bridging material were investigated to determine their effects on the shear strength of bonded solid wood panels. Panels with good dry shear strength were produced at moderate temperatures and pressures with a long closed press time. The type of activating reagent and the species of wood bonded had an important effect on the final strength properties. Sugar maple (Acer saccharum) activated with aqueous nitric acid yielded the highest and most consistent dry shear strength values. However, prolonged nitric acid activation degraded the wood substrate. Anatomical features of the wood played an important role in the final strength properties. The softwood species exhibited extensive crushing of the earlywood which created an uneven wavy surface with very poor bonding properties. The ring-porous hard wood examined also showed similar crushing of the vessel elements which resulted in lowering of the shear strength. Bridging materials such as diacids, diamines, and aromatic compounds enhanced the strength properties of the bonded panels. An aqueous solution of tannin in combination with nitric acid activation produced a panel with good wet and dry shear strengths.

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