Mimosa bark, tannins

Acacia nemu Willd. (Syn. Albizia julibrissin) He Huan Pi (Mimosa) (bark) Tannins, saponins.49 Tonic, stimulant, anthelmintic. 

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. 

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... 

The vegetable tannins can be divided into two main classes, i.e. hydrolysable and condensed. The hydrolysable tannins are obtained commercially from, for example, sumac leaves, tara pods, myrabolam fruits, Turkish or Chinese galls or oak bark. Examples of condensed tannins are those from quebracho wood, mimosa bark and gambier leaves and twigs. 

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. 

Viviers P M, Botha J J, Ferreira D, Roux D G, Saayman H M 1983 Synthesis of condensed tannins. Part 7. Angular (4,6 4,8)-prorobinetinidin triflavonoids from blackwattle ( mimosa ) bark extract. J Chem Soc Perkin Trans I 17-22... 

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]. 

The gluing experiments were done with commercially available starch, proteins (casein, glutin) and tannins of mimosa Acacia spp.), Pinus radiatay and quebracho (Schinopsis spp.) and with tannins extracted from the bark of spruce (Picea abies) and common pine (Pinus sylvestris). The extractions were carried out with water or organic solvents under alkali and/or sulphite conditions on a laboratory scale aiming at optimization of yield and polyphenolic extract content. After extraction, the solutions were concentrated under reduced pressure and freeze dried. One pine bark extract was modified with phenol. A summary of the tannin extracts and their properties is given in Table I. 

No wax emulsion was added to the board to avoid the introduction of another factor that could have limited formaldehyde emission. To this glue mix were added 2 %, 5 % and 10 % UF resin solids by mass of commercial mimosa (wattle, Acacia mearnsii formerly mollissima) bark extract, a commercial flavonoid-type tannin extract. 

Tannins from mimosa, quebracho, and pine (Pinus radiata) are actually used on an industrial scale for wood gluing. The extraction itself is only performed industrially in the southern hemisphere. The tannins are produced by water extraction of the wood or of the bark. Suitable solvents are water, alcohols [248], or acetone. Some of the parameters which influence tannin extraction are ... 

Botha, J.J., D. Ferreira, and D.G. Roux Condensed Tannins Direct Synthesis, Structure, and Absolute Configuration of Four Biflavanoids from Black Wattle Bark ( Mimosa ) Extract. J. Chem. Soc., Chem. Commun., 700 (1978) Synthesis of Condensed Tannins. Part 4. A Direct Biomimetic Approach to [4,6]- and [4,8]-Biflavanoids. J. Chem. Soc., Perkin Trans. 1, 1235 (1981). 

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. 

One way to assure low raw material costs is to have multiple use of the material. If wood chips, sawdust, or bark is to be extracted, the residue should be useful for another purpose. Examples of this approach are 1) Isolation of arabinogalac-tan gum from western larch Larix occidentalis) heartwood chips followed by kraft pulping of the extracted chips 2) isolation of mimosa tannins from bark of trees in which the peeled wood Acacia sp.) serves as raw material for chemical cellulose production (South Africa) and 3) tall oil separation from the concentrated black liquor derived from kraft pulping of southern pines. In each of these cases the brunt of the raw material cost is carried by some product other than the extractive. Assurance of low raw material costs virtually dictates extractive isolation and processing ancillary to a forest products processing plant. 

Vegetable tans, however, are based on wood, bark, leaves and roots of mimosa, quebracho, chestnut and oak. - Tannin is the active ingredient, a polyphenol (sugar derivative of gallic acid), which binds to the collagen (- proteins) by hydrogen bonds. Due to a m.w. of 500-5000, the penetration into the fibrils is slower than with chromium tans and results in heavier and fuller leathers. 

T. are extracted from various parts (wood, bark, fruits, gall apples) of trees (oak, birch, mimosa, quebracho, chestnut) with water and spray-dried. Tannin is a white to yellowish powder. 

---