Bark extractives from spruce as constituents of plywood bonding agents

Abstract

The principles of the use of tannins and resins extracted from wood bark have been known since the 1950's. But only in recent years research on tannins has become lively. The literature review at the beginning of this paper concentrates mainly on the latest investigations in this field. The experimental part investigates the amount and quality of spruce bark extracts and their suitability as extenders in plywood adhesives. The use of chemically treated bark (bark dispersion) in plywood gluing is studied in the last part. If spruce bark is extracted with distilled water, the extract yield is 11.2% at the most, but when 10% of NaOH is added the yield increases to 40.0%. The extract content of drum-debarked pulpwood bark was clearly lower than that of unfloated logs, debarked in the sawmill. The results obtained reinforce the earlier belief that spruce bark extract can substitute Quebracho as an extender for plywood adhesives. Bark dispersion can also be used for this purpose. Exterior-grade birch plywood can be manufactured, using glues which contain spruce bark extract and dispersion. The extract and dispersion can also substitute part of the phenolic resin in the glue mixture, up to 20% at best, without any deterioration in the quality of gluing.