Bark extracts and their use in different bonding systems

Spruce bark extracts have proved to be a possible resource for replacing phenolic resin in different gluing systems. The quality of our spruce (Picea abies) bark extract is not so good. Stiasny number is low, between 50 - 60 varying even in extractions of this kind. Carbohydrate content is high and it has a negative influence on gluing quality. 10 % NaOH-extraction (on dry bark weight basis) gave the highest yield and polyphenolic content of extract when the extraction time was 60 min and temperature 80 °C. Extractions made in bigger scale gave better extraction results than small scale (laboratory) extractions. In gluing experiments we prepared plywood, particleboard and with normal resorcinol-phenolformaldehyde resin glued close-contact joints. A plywood gluing experiment was made also in industrial scale. Nine-ply birch plywood consisting of 1300 x 1300 x 1.5 mm birch veneers was then prepared. The results were very promising eve though the replaced amount of phenolic resin was not so high. All plywoods fulfilled the Finnish plywood standard SFS 2415 and the Finply quality requirements.