Adhesive mimosa tannin adhesives

The only tannins in the world currently being commercially exploited for adhesive applications are those isolated by hot- (or cold-) water extraction of Acacia meamsii bark in the province of Natal, South Africa. Approximately 100,000 tons of mimosa tannin were being produced annually as reported in 1980, the latest year for which production figures were available (41)- Of this amount, about 10,000 tons were used in adhesive applications mainly in South Africa, Australia, and New Zealand. While this number is not large in light of the 300,000 to 400,000 tons of phenol used annually in resins, it does provide evidence that bark tannins can be economically used for adhesives. This application is facilitated by the relatively high cost of phenol and resorcinol in... 

Early attempts to use mimosa tannin in particleboard adhesives involved high-temperature alkaline treatment of the extract to reduce viscosity of the 40% solids level needed (43,44) Subsequent improvements followed the same course as with plywood, namely the use of phenol-formaldehyde or phenol-resorcinol-formaldehyde as crosslinking agents (45) and the use of catalysts or mix modifications to reduce press temperature requirements and to extend pot life. Recent work (46) has shown that exterior chipboard adhesives can also be prepared by crosslinking of mimosa tannins with 4,4-diphenylmethane diisocyanate. 

Wattle (mimosa extract) tannin adhesives have been industrially produced and used in South Africa for many years. From the first, mostly unsuccessful, attempts in 1968 has evolved the consistent manufacture, industrial application,... 

Tannin-based adhesives have been studied by many researchers for the past 30 years [3, 7-14]. Nowadays, tannins are commercially produced e.g., mimosa tannins in Brazil, South Africa, India, Zimbabwe and Tanzania. On the other hand, quabracho tannin is produced in Argentina, mangrove tannin in Indonesia, whilst both Italy and Slovenia produce chestnut tannin [15]. 

More recently, a modification of the system described by Kreibich has been used extensively in industry with good success. Part A of the adhesive is again a standard phenol-resorcinol-formaldehyde (PRF) cold-setting adhesive, with powder hardener added at its standard pH. Part B can be either the same PRF adhesive with no hardener and the pH adjusted to 12, or a 50 to 55% tannin extract solution at a pH of 12-13, provided that the tannin is of the condensed or flavonoid type, such as mimosa, quebracho, or pine bark extract, with no hardener [118,135-137], The results obtained with these two systems are good and the resin not only has all the advantages desired but also the use of vegetable tannins and the halving of the resorcinol content makes the system considerably cheaper [118,135-137]. 

Interest in use of condensed tannins as components of adhesive formulations began about three decades ago. While research studies have been carried out in widely scattered laboratories around the world, three major areas of activity can be distinguished. These are 1) development of bark extracts and commercial production facilities on the west coast of North America, 1953 to 1975 2) application of tannins in adhesive formulations in South Africa based on indigenously produced mimosa (wattle) tannin, early 1970 s to the present and 3) a resurgence of interest in pine bark as raw material for tannin-based adhesives, beginning in the middle 1970 s. Each of these activities has been characterized by parallel efforts on structural identification of the tannins and development of unique methods for incorporating the isolated tannins into adhesives. 

Condensed tannins constitute more than 90 per cent of the total world production of commercial tannins (200000 tons per year) [11]. Their high reactivity towards aldehydes and other reagents renders them both chemically and economically more interesting for the preparation of adhesives, resins and other applications apart from leather tanning. The main commercial species, such as mimosa and quebracho, also yield excellent heavy duty leather. 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), Rhus (sumach extract) species, and various Pinus bark extract species, from which commercial tannin extracts are manufactured. 

Most of the recent efforts to develop uses for the condensed tannins have centered on their application in wood adhesives. Reviews by Pizzi (182, 186) and others (15, 78, 87, 93, 208) provide references to several hundred papers and patents on this subject. Despite world-wide research efforts on other sources of tannins, particularly since the 1972-1973 petroleum shortage, the mimosa or wattle tannins extracted from the bark of black wattle Acacia mearnsii) remain the major source of condensed tannins exploited commercially for adhesive manufacture. Of the approximately 100000 tons of wattle tannin produced annually, only about 10000 tons are used in wood adhesives, predominantly in South Africa but also in Australia and New Zealand (186). The extensive use of wattle tannins by the wood products industry of South Africa is impressive indeed, as these tannins have partly replaced phenol and resorcinol usage in adhesives for bonding of particleboard, plywood, and laminated timbers (182, 186, 213). Three factors have contributed to the success in use of wattle tannin-based adhesives, namely the comparatively high costs of phenol and resorcinol in the Southern Hemisphere, their resorcinolic functionality and low molecular weight and, perhaps most importantly, the commitment by the research and industrial communities of these countries to reduce the reliance of the forest products industry on petroleum-based adhesives. 

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