Wood Adhesives Based on Natural Resources: A Critical Review: Part III. Tannin‐ and Lignin‐Based Adhesives

Resins based on so-called condensed tannins have been in industrial use for nearly 50 years as replacement for phenolic resins for wood based panels with high resistance against moisture and water as well as for boards with very low subsequent formaldehyde emission. Tannins react with crosslinkers, such as formaldehyde, and form hardened and crosslinked structures, similar to phenolic resins. The methylene bridges linking two tannin molecules are stable against hydrolysis due to the C-C bonds formed. Actually mainly mimosa tannins are in use. This type of tannin is mainly produced by extraction from the mimosa bark in South Africa; other sources had been used in former times, such as pine tannin in Chile or pecan nut tannin in USA. Small amounts of spray-dried mimosa tannin are currently in industrial use in Europe. Using naturally-based crosslinkers, wood adhesive systems fully based on natural resources could be prepared. A wider industrial usage of tannins suffers from the limited availability and high transportation costs, since South Africa is the only actual producer on industrial scale. Lignin as unified term for many different types of lignins (depending on wood species or pulping process) is the by-product of the pulping industry when producing cellulose, with huge amounts accumulated each year. Actually no use of lignins as sole adhesive for wood and fibres is reported, despite the fact that nature exactly shows this utilization of lignin in the middle lamella of each tree and despite the huge R&D effort in the last decades. Few decades ago lignins had been used industrially to some extent simply as liquid extender for aminoplastic and phenolic resins; actually lignins pretreated by methylolation have replaced up to half of the amount of phenol in modified phenol-formaldehyde (PF) resins in some few restricted applications, mainly for the production of plywood. Possibilities for further development, better performance, and, finally, industrial use of lignins as sole adhesive depend on the improvement of the reactivity of lignin by suitable pretreatments.