Vegetable tanning

Vegetable tanning
With the industrial application of chrome tannage rapidly advancing since the end of the last century vegetable tannage has been ousted from its predominant position into second place. Moreover synthetic tanning agents were invented by the condensation of phenols with formaldehyde and further developed with great number of aromatics. In the past decades syntans having very particular properties have been produced. In many cases they improved the vegetable tanning agents in terms of tanning technique and were sometime even superior. This led to a great number of combinations of vegetable tanning agents with syntans, which modified the tanning methods and also the properties of the leather.
Vegetable tanning materials
Vegetable tanning agents most commonly used worldwide:
Barks: mimosa, mangrove, acacia negra, eucalyptus
Fruits: myrobracho, chestnut, oak
Leaves: sumac, gambir
Tanning properties of the most important tanning agents
Hydrolysable tanning agents (pyrogallol=acid former)

---------Color--------- fullness--------------tannage---------------other

Oak------yellow, brown, dark cut--- full--------very firm -----------acid forming

Chestnut--pale,yellow olive tinge-----full-------firm-------------bloom forming

Myrobalan----pale-colored---------medium------little strength-----strongly sludging

Sumac----almost white tanning------ medium -----soft supple------good light fastness

Valonea grey-brown pale-colored -----medium full------tough firm------low content of insoluble matter

Condensable tanning materials (catechol=phlobaphene former

------------Color---------Fullness----------Tanning------------- Other
Pine ------- pale –brown --------- medium fullness-----------firm hard------------become darker on exposure to light

Mangroves-intensive red brown-----medium------------loose, spongy-----------strongly sludging

Mimosa--pale, slightly reddish tinge----full---------quick firm supple-------becomes darker on exposure to light
Untreated

Mimosa-sulphited-bleaching dull beige—medium full---medium-full----medium firm fine grain—bleaching readily soluble

Quebracho-ordinary ---very intensive reddish tinge----full---slightly softer than mimosa-----strongly sludging
Quebracho sulphited- intensive reddish tinge---medium full------ slightly softer than mimosa—cold soluble



Light and heat fastness modified mimosa for vegetable tanning


Tanac specialise in providing the leather industry with Mimosa based products farmed from their FSC certified plantations of Acacia in Brazil. By modifying the extract they have devised new high-performance extracts for upholstery and upper leathers

Mimosa or Acacia are native Australian species introduced to southern Brazil in the 1920s and rapidly becoming a sustainable economic product. At present, there are 150,000 hectares of Acacia forests in Brazil of which 30,000 hectares belong to Tanac. There are also more than 30,000 small farms, which supply Tanac with enough bark to produce 36,000 tons of vegetable products per year. These products are used in various areas such as the production of leather and adhesives as well as in water and effluent treatment.
The forests belonging to Tanac are certified by the FSC (Forest Stewardship Council), which guarantees a sustainable raw material complying with
the most stringent environmental regulations. Conventional Mimosa extracts are versatile and provide a variety of characteristics such as naturally light colouration, low acid and salt content, low viscosity and natural pH ranging from 4.0 to 5.0. They also provide stability to hydrolysis and electrolytes, good resistance to iron salts, fast penetration and stability to damage caused by microorganisms.
In spite of their wide application and versatility, the Mimosa extracts which are available on the market do not fully satisfy all the needs of upholstery leather makers, especially those of automotive leather. Thanks to its chemical structure, Acacia tannin is a natural antioxidant (figure 1), being often used to this end. This feature alters the colouration of the leather produced when exposed to light or heat.
Production of furniture, automotive, aviation and aquatic upholstery articles as well as footwear upper and leathergoods requires a high specification. The demand for these leather types is growing, particularly if they have good physical and environmental credentials. Characteristics such as light shades and heat and lightfastness are important. The products currently used to this end are not extracts but ground products obtained from irregular sources and contain impurities (some insoluble ones) that affect the quality of the final goods.

Modifying Mimosa
By means of chemical modification of Mimosa extracts, lightfastness may be increased so that significant leather colouration changes do not occur after upholstery articles have been produced. These alterations take place through chemical modification where specific radicals are added to the original molecule so that they provide the extract with the capacity to reduce oxidation when exposed to sunlight or other sources of light, resulting in a lower degree of colour alteration.
Organic chemical reactions are the best way to provide heat and light stability to raw materials by using efficient protective groups. These groupings prevent electronic resonance between the B aromatic link and its free hydroxyls thus stabilising the molecular structure and guaranteeing a lower degree of alteration of the extract colour and, consequently, that of the leather tanned with new modified Mimosa (such as Tanac’s Supertan LTA).
Using modified Mimosa with such characteristics makes it possible to produce tanned and retanned leather where a light colour as well as heat and lightfastness are possible.
These additional properties are added to other favourable characteristics of Mimosa extracts such as fast penetration, smooth open grain, good filling and excellent softness. Figure 2 shows a significant improvement in heat and lightfastness when using modified Mimosa compared with standard Mimosa extract. In figure 2 the samples were treated with heat, natural and UV light and the discolouration measured.
The colouration provided by modified Mimosa and Tara are quite similar.
Physical, chemical and mechanical tests carried out on automotive upholstery leather, comparing modified and standard products, showed a significant improvement in the results obtained from the article that used modified Mimosa compared to regular Mimosa extract. It may also be noted that both products reached
the parameters required in OEM upholstery leathermaking including a significant increase in the tensile strength in the sample, which used modified Mimosa as the retanning agent.

Footwear upper
Taking into consideration that the production of upper leather accounts for more than 50% of global leather consumption and that this market is evolving fast as far as quality of the final goods are concerned, Tanac have also developed modified Mimosa as a sustainable alternative for the market.
Table 1 highlights the results of the physical and mechanical tests carried out on footwear leather samples. It shows an improvement in the results obtained from leathers where modified Mimosa was applied compared to standard Mimosa extracts.
Vegetable tanning method
1. Old pit method
Tannage, lasting about 12-18 months, is performed according to the counter –current principle with thin tanning liquors in the color pit. Long duration required in this method now only used in isolated cases. The label “old pit method means” tanning with oak and pine bark which provides specially compact and firm sole leather having a low content of substances removable by washing. Prolonged tanning is a disadvantage.
2. Accelerated tannage with tan solutions of higher concentration
Final tanning is performed by further increasing the concentration using higher- percentage tanning agents and tan liquors of higher concentration, in the handler and lay-away respectively, and by additional mechanical agitation in the drum. Duration reduced 2-6 months. Compared to slow pit method bonding of tannin is slightly reduced and the content of substances removable by washing is considerably higher.
3. Quick tanning methods
They are all based on the principle of increased concentration, mechanical agitation, increased temperature and pH variation. The duration of tanning is reduced to 4-20 days, depending on the operational condition and type of leather.
4. Rapid tanning method
These are powder tanning substances without float. The best known methods are
1. RFP process of BAYER
2. Rapitan process of BASF
They reduced the duration of tanning to 2-3 days. The hides should be well prepared for these tanning methods and the vessels should be suitable in order to reduce possible error and avoid case-harding.
Vegetable tanning process
1. Complete soaking and liming
2. Complete removal of the subcutaneous tissue by through fleshing
3. Complete deliming of all sections of the skin. Dry deliming and the addition of sodium hydrogen sulfide as well as aromatic sulpho acids are good for thick pelts.
4. The formation of false backs is avoided by slashing the butts along the backbone line.
5. Pretanning with small particle synthetic pretanning agents of low astringency. Also preliminary treatment with glutaraldehyde, polyphosphates or pickles containing chrome salts or chromiferrous syntans.
6. Through washing after pretannage and good draining of the residual liquor.
7. Addition of the powder tanning agents for final tanning in 2-3 portions and addition of dispersing tanning agents in appropriate quantities. Temperature raises not exceeding 38 ⁰ C.
8. Addition of a slip additive or untreated oil to reduce friction.