ABSTRACT
The former, including chestnut, myrabolans (Terminalia and Phyllantus tree species), and dividi (Caesalpina coraria) extracts, 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). They have been used successfully as partial substitutes (up to 50 per cent) of phenol in the manufacture of phenol-formaldehyde resins (Kulvik, 1975, 1976). Their chemical behaviour towards formaldehyde is analogous to that of simple phenols of low reactivity, and their moderate use as phenol substitutes in the above-mentioned resins does not present difficulties. Their lack of macromolecular structure in their natural state, the low level of phenol substitution they allow, and their low nucleophilicity, limited worldwide production, and higher price somewhat decrease their chemical and economic interest. Condensed tannins, on the other hand, constituting more than 90 per cent of the total world production of commercial tannins (approximately 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), and Tsuga (hemlock bark extract) species, from which commercial tannin extracts are manufactured, and various pine bark extract species. Where bark and wood of trees were found to be particularly rich sources of condensed tannins, commercial development ensued through large-scale afforestation and/or industrial extraction, mainly for use in leather tanning. The main producers of commercial condensed tannin extracts are, in relative order of importance: Argentina, South Africa, Brazil, Paraguay, Zimbabwe, Indonesia, Kenya and Chile. The production of tannins for leather manufacture reached its peak immediately after World War II and has since progressively declined. This decline of their traditional market, coupled with the increased price and decreased availability of synthetic phenolic materials due to the advent of the energy crisis of the early 1970s, stimulated fundamental and applied research on the use of such tannins as a source of condensed phenolics.
