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VEGETABLE TANNING

The most important organic tanning agents are the vegetable tannins present in tanning liquors.They are prepared from certain parts of  plants by aqueous extraction. Their tanning power has been appreciated for a long time and Babylonian texts have recorded their use.
Vegetable tanning materials occur in nearly all forms of plant life. They are used commercially where the amount of tan is high and large quantities can be extracted economically. Other considerations are color and particular properties of the tan extracted. 

Table: Parts  of plants used as sources of tannins.

Vegetable tannins are dervatives of phenol(with several OH groups). Phenols are more acidic than alcohols (pKa~10), but are weak acids therefore form salts only with strong bases. Solubility of phenol ~7% in cold water. But the sodium salt is soluble.
Vegetable tannins react with atmospheric oxygen, particularly at high pH values to form quinones (for OH groups that are ortho-para to one another).
Vegetable tanning liquors are very complex and continually changing physically, chemically and biologically. They are partly colloidal but easily aggregate and will then sediment. Yiests moulds and bacteria can grow in the liquors, the main consequence being the fermentation of sugars to acids.
Tannins are not the only constituents of vegetable tanning liquors. The non-tans include, apart from the sugars, acids and their salts, hemicelluloses, pectin and lignin, as well as compounds containing nitrogen and phosphorus. The acids and their salts are the most important for the tanner. Apart from the nature of the tannins themselves, the acids and salts are the principal means of controlling the astringency of liquors and whole process of vegetable tannage.
Several acids, such as gallic, oxalic, citric, tartaric, and phosphoric are present in the original tanning material and sugar fermentation can yield carbonic, acetic and lactic acid.Gallic and other phenolic acids can arise from the breakdown of tannins. Polyuronic acids, whether from hemicellulose or pectin, contribute to the acidity.

There are mainly two classifications for vegetable tannins:
1)  catechol tans (condensed tans)- they are similar to catechol, usually give red-brown color and are astringent. Condensed tannins are not decomposed by acids. They gradually polymerize becoming phlobaphenes, insoluble derivatives. On dilution and standing, they deposit a thick, reddish sludge called ”reds” (phlobaphanes). In order to prevent  “reds” the extract is solubilized by heating under pressure with sodium bisulphite (3-8 % on the extract) at 98 ⁰C.
 

Quebracho is obtained from the heart-wood of the quebracho tree which grows in south America chiefly in Argentina and Paraguay.Ordinary- or warm soluble Quebracho is the natural extract rich in condensed tannins(phlobaphenes) and is not easily soluble. Its use is therefore limited to tannage of sole-leather according to the process known as “hot-pitting”.It results in a red-brown color and excellent water-proofness.  

Ordinary extract is subjected to sulphitation process which transforms it into soluble tannins. The main properties are: rapid penetration into the pelt, a high tannin and low non-tannin content.The rather low acid and medium salt content characterize them as mild tanning agent(can also be deduced from their pH values).
Sulfiting as applied to the condensed tannins consists in treating their solutions with a mixture of sodium sulfite and hydrosulfite. A part of the bonds in tannin is then split. Initially phlobaphanes dissolve, then not only the size of the tannin molecule decreases but changes in the molecule occur.

Gambier  is a solid extract(cubes) obtained from the leaves and stems of Uncaria Gambier, a plant which occurs both wild and cultivated in the Malayan region. Besides the predominent catechol tannins it also contains sugar,salts, waxes, oils, and even minral substances. If  used alone, tends to give a rather flabby leather. When used in retannage, it imparts suppleness and a smooth feel. A peculiarity of gambier is to give leathers an excellent capacity of being glazed and very glossy dyeings.
 A lot is known about the tannins of spruce bark. These consist of polyphenols mainly in the form of glucosides. The principal polyphenol is called piceatannol.
Glucosidic groups are known to be associated with reduced tanning power and thus spruce bark is a relatively poor tanning material. Stilbenes and their glucosides have also been found in wandoo, an extract of eucalypt. 
The poor tanning properties of glcosidic groups is explained by the ease with which aglucon is oxidated. Piceatannol in an oxygen free medium does not cause a Ts increase. In presence of oxygen, like in other flavanols, quinone systems appear which are bound irriversibly to the pelt. Tanning consists of oxidizing piceatannol to its quinone and then addition of free collagen amino groups in the 1,4 position via covalent bonds. Glycosides of piceatannol do not have tanning capacity.
The great majority of condensed tanning materials contain compounds derived from flavan. Flavones, such as quercetin, are fairly stable, almost insoluble substances are known to be a frequent source of yellow colors in nature. The closely related but colorless and soluble catechin was found in gambier leaves. It is not a tannin but during the commercial preperation of the extract it is converted into tannins, the liquor darkening at the same time. (+)-catechin is also widely distributed in nature and constitutes a minor component of wattle bark and heartwood, chesnut bark, spruce bark, oak bark,quebracho bark, tea and cola. Compounds related to catechins occur in small amounts in some tanning materials. (+)-gallocatechin, which has an extra hydroxy group at the 5’-position, is present in the barks of chesnut, wattle, and oak, as well as in tea, and wattle bark also contains some (-)-robinetinidol, which is (+)-catechin without the 5-hydroxy group.
Most of the red and blue colors of flowers are due to other derivatives of flavan, the anthocyanins such as cyanidin and delphinidin chlorides. In some flowers and fruits colorless substances occur, which on treatment with strong acids give red colors due to anthocyanidines.The simplest represantatives of these colorless compounds are flavan-3,4-diols.
Thus wattle heartwood contains a small amount of (+)-mollisacacidin, whereas in quebracho sapwood there is almost 2 % of the (-) enantiomer. In oak bark 0.5 % leucodelphinidin is present and 8-10 % of an isomer in karada.
Leucocyanidines have been isolated  from some species of mangrove.
To the tanner it is more important to know the constitution of the tannins present in condensed liquors than to know the formula for the relatively simple compounds since these are present only in small amount,except in quickly and very carefully prepared extracts of freshly gathered material. Condensed tannins are formed by condensation of these simpler compounds and it is of importance whether they are a result of acid condensation or of oxidative condensation.

2)  pyrogallols (hydrolysable tans) - esters of glucose and gallic acid (glucoside tannins) and its derivatives which are easily hydrolizable.
Hydrolizible tannins, having a polyester structure, easily hydrolize to the respective sugar or polyhydric alcohol and polyhydric phenol with carboxyl group. Hydrolysis products my be classified into gallotannins, derivatives of gallic acid, and ellagitannins, derivatives of ellagic acid.
These are more yellow-brown than catechols. Their sugar content may lead to acid fermentation during long tannage, when a deposit of sand colored sludge known as “bloom” is also formed. The later is a result of enzyme action causing hydrolysis of the ester link, releasing insoluble acids, eg. Ellagic, chebulinic from the tannins. They are usually less astringent than catechol tans.

Myrobalans has been extensively studied. There up to 12 % chebulinic acid and 2 % chebulagic acid, two tannins which have been isolated in crystalline form. Chebulagic acid is identical with chebulinic acid, except for an additional link between the positions marked.

Divi-divi contains about 2.5% chebulagic acid and 5% corilagin.
Tannins of algarobilla contain other related structural units, namely the quinone of 4,4’,5,5’,6,6’- hexahydroxydiphenic acid and brevifolincarboxylic acid.The quinone is yellow and similar components may account for the color of some myrobalans tannins also.

Chestnut extract contains a convenient quantity of soluble organic non-tannins and of organic acids of natural origin and others which develop during manufacturing proces and is characterized by considerable astringency. Such astringency manifests itself in a relatively low speed of penetration of the tanning matters into the pelts and in the property in which these tanning substances possess of fixing themselves irreversibly and in large amount to the hide.
These properties make Chestnut extract especially suitable for the tannage of heavy hides and of sole leather in particular, as by its use it is possible to obtain a firm and compact yet flexible leather of good color, light resistant with low water absorption.
Sweetened Chestnut is a chestnut extract with varied astringency therefore different behaviour in the tanning process.

It is obtained from the leaves of Sumac(Rhus Coriaria). It is the purest among vegetables hydrolizable tannins. Using sumac as the only tannin a full and even leather of yellow-hazel color is obtained. In dyeing with both anionic and basic dyestuffs an excellent color levelness is achieved

Tara
The tannin of Caesalpinia Tinctoria(bear the fruits called pods) has pyrogallic character but small quantities of catechol derivatives also occur in it. It contains practically no coloring substances therefore permits very bright and light-resistant leathers. Tara gives leather fullness and softness and at the same time a fine, closed grain. In leathers tanned with tara the grain resistance to breaking load is higher than that achieved with any other vegetable tannin.

Tannin obtained from Chinese galls contains apart from tannin, galic, m-digallic and trigallic acids. On hydrolysis gives D-glucose and gallic acid in a molecular proportin of roughly 1:10. It is thought that m-digalloyl groups are attached to each of the five free hydroxy groups of glucose, but it is possible that some of them are simple galloyl ones, whereas others are trigalloyl ones.
Tannins of Turkish galls and Sumac are similar, but with a lower propotion of gallic acid to glucose. Tannins such as these which yield only gallic acid and sugar on hydrolysis are called gallotannins.
Ellagitannins give on hydrolysis a precipitate of ellagic acid. Ellagic acid is not present as such in tannin molecules, but is derived from hexahydroxydiphenic acid.

Lignins:
Lignins constitute the material that fills out the spaces between the microfibrils of the cellulose in certain plant cells and stiffens the cell structure. They are the charasteristic constituents of wood, but are also present in bark and straw.
In young tissue of trees, coniferin ie. coniferyl-4-b -D-glucoside, is present; a b-glucosidase liberates coniferyl alcohol from it in the wood and bark and lignin is formed in situ. Deposition of lignin brings about the physiological death of the cells. Lignins are insoluble in water, organic solvents and even sulphuric acid.
They contain 59-67% carbon, some methoxy groups, can be oxidized to give up to about 25 % aromatic aldehydes and react with sodium bisulphite, sodium hydrosulphide and thioglycollic acid, HSCH₂COOH. It is known that coniferol alcohol can be polymerized to give a number of intermediate products, such as dehydrodiconiferyl alcohol,  a-guaiacyl glycerol  b-coniferyl ether and pinoresinol.

Sulphiting:
In paper production, lignin has to be removed from the cellulose fibers, and one way of achieving this is by treatment with bisulphite under pressure, which solubilizes the lignin, leaving the cellulose unaffected. In the process, the hydroxy groups of aromatic side groups can be replaced by sulphonic groups to give RSO₃H, SO₂ can add to double bonds to give RSO₃H and ethers may split to give -OH + H₃OS-. Such sulphite cellulose, lignocellulose or ligninsulphonicacid liquors have low tanning power but are very cheap. They are sold alone or as blends with syntans and tanning extracts. Their main function is to solubilize the less soluble components, to speed up tannage, and to act as a filler.Condensed tanning materials are subjected to the sulphiting process in order to solubilize the less soluble fractions, when reactions similar to those with lignin occur.
Theory of tannage with vegetable and synthtic tannins:
The first step in tanning is the binding of hydroxyls of vegetable tannins to the active collagen centers.The next step- the binding of further tanning molecules continues until the interfibriller spaces are filled. The collagen active centers which react with vegetable tannins are various functional groups of its side chains and peptide bonds as well. This stage ends when collagen has absorbed ½ of its weight of vegetable tannins. The difference between Cr and vegetable tanning becomes striking here, since 3% of Cr tanning agent is sufficient to form stable bonds between collagen and tanning agent.
Tanning is carried invariably under acid conditions, although it can be done at pH 1-9. The reason lies in the fact that liquors containing vegetable or synthetic tannins are naturally acidic (at higher pH s polyphenols may oxidize and dark colors may dye the leather).
Below pH 5, limed collagen is + charged, charges on basic side chains. Vegetable tannins normally carry negative charges due to dissociation of their carboxy and phenolic groups. Tannage therefore involves ionic interaction between collagen and tan.
Collagen-NH₃⁺  + tannate^- --> collagen-NH₃⁺tannate^-
In pelt, structure can cause difficulty with accesability. Because of  “case hardening” (rapid overtannage of outermost layers), pelts are not readily penetrated. It is important to distinguish between penetration and fixation. 
Apart from ionic interaction that brings the tans close to collagen fibres, other factors like H-bonds and van der Waals attractions must be involved since polyamides such as nylon can bind tannins via their -CO-NH- groups. The weak character of such binding forces could well account for the relatively low shrinkage temperature of vegetable-tanned leather.
Some firmer covalent bonds (crosslinking collagen chains- and so contributing to tanning) may arise from quinones and possible aldehydes formed by the oxidation of polyphenols.
Much of the tannin may be physically held within the leather structure and be independent of specific chemical bonding to the collagen, since electronmicrographs of vegetable tanned leather clearly show the presence of large aggregates of tannin, lying between hide fibers. Undoubtly this accounts for the well known “filling” action of vegetable tannage and the physical properties of roundness and fullness associated with this tannage.