Natural Adhesives, Binders, and Matrices for Wood and Fiber Composites: Chemistry and Technology

Tannin Adhesives

The word tannin has been used loosely to define two different classes of chemical compounds of mainly phenolic nature: hydrolysable tannins and condensed tannins. The former are mixtures of simple phenols such as pyrogallol and ellagic acid and of esters of a sugar, mainly glucose, with gallic and digallic acids (Pizzi, 1983). Their lack of macromolecular structure in their natural state, the low level of phenol substitution they allow, their low nucleophilicity, limited worldwide production, and higher price somewhat decrease their chemical and economical interest.

Condensed tannins, on the other hand, constituting more than 90% of the total world production of commercial tannins (200,000 tons per year), are both chemically and economically more interesting for the preparation of adhesives and resins. Condensed tannins and their flavonoid precursors are known for their wide distribution in nature and particularly for their substantial concentration in the wood and bark of various trees. These include various Acacia (wattle or mimosa bark extract), Schinopsis (quebracho wood extract), Tsuga (hemlock bark extract), and Rhus (sumach extract) species, from which commercial tannin extracts are manufactured, and various Pinus bark extract species.