Structural glue-laminating is a successful technique for eliminating or controlling naturally occurring strength reducing defects that regularly appear in larger size low-grade wooden members. Despite a wide range of adhesives currently available for laminating softwood, little comparison has been made between their performance for ambient bond strength and durability when bonding parallel to grain specimens of spruce comprising high proportions of juvenile wood.
The notched block-shear test is utilised in this study to examine the performance of a range of adhesives. Adhesives examined comprise a melamine urea formaldehyde, a polyurethane, an emulsion polymer isocyanate, a polyvinyl acetate and two phenol resorcinol formaldehydes. The durability test involved cycling specimens five times by a vacuum-pressure-soak-drying procedure. Parameters varied, depending on the adhesive being studied, included clamping time, clamping pressure, adhesive spread rate, single or double face application and quarter sawn or plain sawn grain arrangement.
Bond shear strength failure modes varied between large percentages of solid wood failure in the shear plane, failure along annular ring and adhesion failure. Adhesive type and the parameter combination proved significant in the bond integrity for ambient and accelerated-aging tests. In general, specimens subjected to the severe durability test failed with lower wood failure percentages and lower shear strengths than ambient tested specimens.