Glue-line wood cell walls increased viscoelastic flow by high moisture tolerance thermoset adhesives

Glue-lines in boards made of wood particles using high moisture tolerance tannin-based adhesives were observed to present the unusual behaviour of producing a melting effect of the wood cell walls directly in contact with the adhesive. The high moisture content of up to 29% used in the preparation of these exterior grade particleboard and OSB industrial panels at standard pressing times causes considerable how of the amorphous lignin and hemicelluloses in the wood directly in contact with the adhesive leading to a composite, compact interphase in which almost loose wood fibers are drowned in a hardened adhesive matrix. In this interphase the anatomical characteristics of the wood cell walls appear to be lost. This considerable flow is apparently due to the marked decrease of the glass transition temperature of hemicelluloses and lignin at the very high moisture contents used. This effect was replicated in boards prepared with a synthetic MUF adhesive using also a moisture content level much higher than what normally this resin is capable of tolerating by just lengthening considerably the pressing time used.