Sulphited tannins for exterior wood adhesives

Abstract

Condensed tannins such as wattle (Mimosa) extract can be partially modified, through hydrolysis of a benzyl ether link and consequent opening of the flavonoid heterocyclic ring, through sulphitation. The introduction of a sulphonic group in the flavonoid structure and the heterocyclic ring opening produces tannin solutions of lower viscosity. While the sulphonic group introduced is deleterious to the strength and water resistance of simple tannin-formaldehyde adhesives, a moderate degree of sulphitation is inconsequential on the strength and water resistance of tannin-formaldehyde adhesives fortified with small amounts of simple phenolic resins used to enhance cross-linking in such a network. Their low viscosity renders these sulphited tannin adhesives particularly useful in applications, like particleboard manufacture, where this is needed.