Tannins from bark

Tannin from bark and wood of trees is me prime source for leamer processing. Romans discovered the potential of chesmut tree and used me rind, leaves, and flowers of chestnut in pharmacopoeia. 

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. 

Tanaka, T. et al.. Tannins and related compounds. CXVI. Six new complex tannins from the bark of Psidium guajava L., Chem. Pharm. Bull, 40, 2092, 1992. 

N.A. Pinus albicaulis Engelm. P. contorta Dougl. ex. Loud. P. mugo Turra var. pumilio P. palustris Mill. P. strobus L. Bishomophinolenic acid, resins, mallol, bomeol acetate, tannins, vitamin A, vitamin C, galactose, alpha-pinenes, beta-pinenes, anthocyanin.8102 Relieve fever, bronchial and nasal congestion, improve blood flow. Anthocyanin from bark has antioxidant activity, inhibits the enzymes that cause inflammation. 

Warington proposes to estimate the tannin, not only in barks, but in all other astringent substances volumetrically, hy determining what volume of a standard solution of gelatin is required to precipitate the tannin from the extract prepared from the Bample submitted to examination. In preparing the test solution, the above chemist recommends the long staple... 

Chemical processing of bark is limited and the principal chemical products produced commercially from barks are based on the barks phenolic content (1,12). Barks generally are richer than wood in quantity and complexity of extractive components, the most important being a) the monomeric polyphenols or flavonoid compounds, and b) the polymeric phenolics, such as tannin, phloba-phenes and phenolic acids. 

In essence, due to water leaching the tannins from the bark and other chemicals in the trees themselves, these water solutions must be subjected to zero discharge by 1977, according to the new guidelines by the Environmental Protection Agency. 

For many years, most of the leather in the United States was tanned with domestic tannins from hemlock and oak bark and from chestnut wood. Today only a small amount of tannin comes from these and other domestic sources. The most important source of vegetable tannin today is the wood of the quebracho tree, which grows mainly in Paraguay and Argentina. The tannin content of this tree and a few other sources of vegetable tannin are shown in Table 28.10. 

In addition to the water-soluble and insoluble members of the condensed tannin family, both of which are soluble in polar organic solvents, there is a third related fraction, usually called a phenolic acid. This material can only be isolated by extraction with aqueous alkaline solutions or with sodium sulfite or bisulfite solution at elevated temperatures and pressures. Since the tannins and phlobaphenes are also soluble in aqueous base, they will be coextracted from bark along with the phenolic acids when bark is extracted with base. This is important to remember in the subsequent discussion on bark-based adhesives. 

The reactivity of resorcinol with formaldehyde is essential for developing the cohesive strength of the interlayer and its bonding characteristics. Condensed tannins are known to be very reactive with formaldehyde (7-0), so these renewable phenolic polymers are good candidates as resorcinol replacements. Indeed, condensed tannins from wattle and pine bark extracts have been successfully used in cold-setting, wood-laminating adhesives, and the former are used extensively in the commercial production of laminated timbers in South Africa (Pizzi, A., National Timber Research Institute, Pretoria, South Africa, personal communication, 1982) (10-13). 

Four types of condensed tannins were studied in the adhesive dips 1) extracts from pecan nut pith obtained by digestion with aqueous sodium sulfite-sodium carbonate solutions, 2) purified tannins from southern pine bark, 3) extracts from southern pine bark obtained by digestion with aqueous sodium sulfite-sodium carbonate solutions, and 4) tannins extracted with acetone-water solutions from peanut skins. 

Tannins. Herb Hergert s introduction (Chapter 12) to the use of condensed tannins in adhesives is especially interesting because he provides some reasons why commercial success is lacking in the use of condensed tannins from conifer barks despite substantial effort worldwide to parallel the South African success in the use of wattle tannins. Much of the problem in the use of conifer bark tannins remains centered on our inadequate understanding of the fundamental chemistry of these polymers. For example, Lawrence Porter (Chapter 13) provided the first measurements of the viscosities of solutions of purified condensed tannin isolates of known molecular weight and the reactions of these polymers with formaldehyde. It is incredible that this has not been done previously considering the hundreds of papers that have been published on tannin use in wood adhesives. Further evidence for the comparatively limited knowledge... 

In our laboratory we have studied thermochemical treatment of biomass using hot compressed water and found solid-like biomass wastes were converted to liquidized materials by the release of water soluble compounds inside cells due to thermal rupture of cell around 175 C at 40 atm (4). Applying this liquidization process, we tried to extract radiata tannin from the bark. 

Waxes, fats and fatty aeids, oils and resins are all readily extraeted from bark using non-polar solvents, while tannins, gums, peetin, soluble earbohydrates and sugars ean be removed by aqueous extraction. The amount of extraetives that ean be removed diminishes signifieantly after seasoning of the bark. 

A wide range of adhesives are used. For the three panel types that are the focus of this discussion synthetic adhesives are used, although tannins extracted from bark... 

Tannins—Tannic acid——322.—Quite a number of different substances of vegetable origin, principally derived from barks, leaves, and seeds. Tliey are amorphous, soluble in water, astringent, capable of precipitating albumen, and of forming imputrescible compounds with the gelatinoids. They are, with one possible exception, glucosides. 

After the hides have been removed from the bathing wheel, they are placed in the tan pits to which at first is added solutions of tan liquor and then more concentrated solutions. These tan liquors contain all varieties of microbial life, but the tannin from the tan bark has an inhibitive action on the putrefactive organisms. 

There appears to be a close relationship between color and flavor. Deeply colored samples generally have an unpleasant taste. The color may result from l annins which have an astringent taste. At least part of the color in inferior grades is associated with tannins contained in the bark-contaminated product, but even gums free from bark are often colored. Some hold that tannin is derived from bark in contact with the exudation, others that it is formed in the gum by chemical changes. 

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 ... 

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