Tanning materials can be classified into three main groups according to their chemical nature:
1 Aromatic tanning material (eg synthetic and vegetable tanning agents)
2 Mineral tanning material (eg chrome tanning agent)
3 Aliphatic tanning material (eg aldehyde tanning agent)
The vegetable tannins are widely distributed in the plant kingdom and occur in different concentrations in all parts of the plant materials, whether bark, fruit, wood, or roots. But those parts of plants which contain a large amount of tannin are technically feasible.
Some of the popular veg tans can be seen in Table 1.
Chemistry of vegetable tannins
The chemistry of vegetable tannins is little known. Therefore, a firm definition of what constitutes vegetable tannin is not easy to find. Probably the most acceptable definition is still that of Bate-Smith and Swain, formulated in 1962. They adopted the earlier ideas of White and described vegetable tannins as water soluble polyphenolic compounds having molecular weights between 500 to 3,000 and, besides giving the usual phenolic reactions, they have special properties such as the ability to precipitate alkaloids and proteins (Haslam, 1988).
The non-tans are other water-soluble phenolic/non-phenolic compounds, which may disperse the large aggregated molecules of ‘true tans’. The non-tans consist of sugary matter, salts, flavones, gallic acid and other acids.
Vegetable tannins are generally classified into two groups:
(1) Pyrogallol tannins or hydrolysable tans and (2) Catechol tannins or condensable tans. Their basic structures are shown in Figure 1.
Hydrolysable type
The hydrolysable tans have been further subdivided into two groups:
(A) Gallotannins
On hydrolysis these yield gallic acid and glucose, eg galls (pathological growths, called galls, present on the leaves of certain species)
a) Turkish galls are found on oak species
b) Chinese galls are found on sumac species
(B) Ellagitannins
On hydrolysis these give ellagic acid and glucose, eg myrobalans and chestnut wood
This type of tannin has the characteristic property of undergoing hydrolysis to form ‘sludge/bloom’.
Hydrolysable type
The condensable tans are not prone to hydrolysis but they are liable to oxidation and polymerisation to form insoluble products known as ‘Tannin reds/phlobaphenes’
The commercially most important tanning materials like avaram, babul, hemlock, mangrove, mimosa, quebracho etc, belong to this group.
The major differences between the two types of tannins are displayed in Table 2.
Mechanism of tanning
In mechanism of vegetable tanning, acidic groups of vegetable tannins may combine with the basic groups of the hide collagen by polyfunctional cross-linking via so-called hydrogen bridges. The mechanism of vegetable tannins is indicated in Figure 2.
The factors responsible for influencing the tanning mechanism during leather processing are as follows:
Condition of pelt
The condition of a pelt is said to be the most ideal if the collagen fibres are clearly separated. This inter-fibre separation is dependent on liming, deliming and any pretanning processes.
Particle size
Tanning solutions are colloidal in nature and they contain tannin particles of different molecular weights.
The low molecular weight compounds are too small for effective cross-linking and high molecular weight compounds are either insoluble or too large for cross-linking with polypeptide chains.
So tannin solutions in the range of 500-3,000 molecular weights can suitably associate with protein molecules to form stable cross-linked structures.
pH
Tanning takes place over a wide range of pH, but fixation varies considerably at different pH levels. If the pH is lowered sufficiently, fixation is so rapid on the outer layers of the pelt that further penetration virtually ceases. This state is called ‘case hardening’. It should be noted that at the iso-electric point of collagen, tan fixation is at its lowest. For this reason, tanning is recommended at about pH 5 where tannin fixation is at its minimum and penetration at its maximum. After full penetration is achieved, the pH of the whole system is lowered, usually with an organic acid which has the effect of ‘fixing’ the tan in the interior of the leather.
Low pH – High Fixation
Acid and salt content
The major non-tannin constitutes of a tanning liquor are natural acids and salts. The amount of acid and salt varies in different tanning liquors and these substances greatly influence the character of leather. If the salt content is too high the resultant leather will be soft while high acid contents gives firm leather.
Generally speaking the catechol tanning materials contain low acid content whereas the pyrogallol tanning materials have comparatively high acid content.
The most astringent tanning materials, eg quebracho and mimosa contain low acid and salt contents.
Relation of tannin to acid and salt contents
Low acid content – High diffusion or penetration
Low salt content – High fixation or combination
Temperature
The rate of diffusion is closely bound up with temperature. If the temperature is allowed to fall to freezing point, tannage virtually ceases. Increase in temperature up to a certain limit will accelerate diffusion of the tan liquor into the pelt.
Good diffusion is a necessary condition for achieving correct tanning fixation.
Tannin concentration
The amount of tannin penetration and combination with the hide substance is governed largely by the actual concentration of tan liquor in contact with the fibre. In the early stages of the tanning process, tannin concentration in the outside liquor is stronger than that of the internal liquor between the fibres, the faster diffusion will occur.
If the tan concentration in the outside phase is doubled, fixation is increased sufficiently. Thus, diffusion and fixation depend to a large extent on each other. A 20-35 % tannin concentration of 80-140 barkometer is found beneficial for rapid tanning.
Importance of vegetable tannins
Most types of leather, from compact, firm sole leather to soft lining leather, can be manufactured by vegetable tannage. Of all the vegetable tannins produced in the world 90% are consumed by the leather industry. Of this, 65-70% is used by the heavy leather industry and the rest by the light leather industry. Vegetable tannage of light and heavy leather require 18-20% and 25-30% tannins respectively.
In addition, the recent increase in the awareness of the use of ecofriendly materials in processing industries has seen a growing demand for vegetable tanning materials as they are one of the few environmentally friendly products causing minimum pollution/effluent problems. The effluents thus generated in the process are biodegradable.
