About the Mimosa Products
Vegetable tannins are natural products contained in the bark, wood and pods of trees. Vegetable tannins are composed of polymeric polyphenolic molecules covering a wide range of molecular mass ranging from 500 to 3000 units.
The tanning action of polyphenols is dependent on the molecular mass and the number of phenolic – OH hydroxyl groups. Mimosa tannins are on average about 1250 units & generally have a good tanning action.
Vegetable tannins are classified according to their chemical structure:
- Pyrogallol or hydrolysable tannins, such as Chestnut and Myrabolam extract.
- Catechol or condensed tannins, such as Mimosa (or Wattle) and Quebracho extract.
Mimosa extract is obtained from the bark of Black Wattle (Acacia Mearnsii) trees grown in plantations. It takes 7 to 10 years for the tree to grow large enough to be ready for harvesting and stripping of the bark, which is then chipped and extracted under controlled industrial conditions to extract the optimum amount of tannin with the lightest colour range. Mimosa or wattle bark contains about 30% tannin. The wood has other industrial uses.
Wattle bark is best processed immediately after stripping to deliver a light coloured extract; the older the bark, the darker the colour. Harvesting and stripping is seasonal, and largely depends on rainfall patterns in the country where it is grown. The chopped bark is extracted, using a counter current principle, in autoclaves under pressure at temperatures above 100 degrees celsius.
The liquid extract is then concentrated by evaporation and either poured into hessian bags, in which it slowly solidifies and becomes a solid, or the hot viscous concentrated liquid is spray dried as powder & the powder placed into bags, which are stitched closed & carefully stacked.
Mimosa extract has the following properties:
- A pH value of 4,6 – 4,8.
- A low concentration of salts and acids.
- A rapid rate of penetration through the pelt.
- A good stability to enzyme action, which arises from yeast or moulds, which occur in nature and could cause loss of tannin, particularly in pit tannages.
- A high solubility, low formation of sludge in pits.
- Mimosa extract gives good fixation of tannin to collagen.
- Mimosa extract produces a pleasant characteristic light pink/brown colour, much demanded by customers.
Mimosa tannin is stable against bacterial or fungal attack. At higher concentrations and lower pH values, they polymerise forming small amounts of a reddish brown coloured viscous sludge called “reds” or “phlobaphenes”.
THE USEFUL TREE
For every ton of bark harvested, the black wattle produces around 5 tons of heartwood which is sought after in a range of applications; from firewood and charcoal, to building timber, fence posts and even furniture making.
These give the extra weight and solidity to heavy leather such as sole leather if they are deposited inside the leather itself. Condensed tannins have the tendency to oxidise and darken on exposure to air and light.
Mimosa extracts are mostly used without any chemical treatment, but there are several products produced for special tanning purposes and these are chemically treated.
NTE Mimosa extracts are also dedusted to prevent the spread of dust in the air when handled, which would normally pollute the working atmosphere and make it unpleasant for the operators.
Applications in Tanning
Mimosa extract is a very versatile tanning compound and can be used for tannage on its own or in combination with other tanning compounds.
There are many specialty types of leather produced by combining Mimosa extract with other chemicals in pretannages or retannages.
Mimosa extract is used mainly in the tannage of heavy leathers, such as sole, saddlery, insole, case, belt, industrial leathers, or for light leather manufacture e.g. upholstery, garment, lining, shoe upper leathers and so on.
Mimosa extract is particularly suitable for modern tannages and is recommended for full vegetable tanning of hides or skins (for heavy and light leather) in drums, pits, paddles or processors, and for retanning of chrome tanned leathers.
POWDERED MIMOSA PRODUCTS
Figure 1 gives a diagrammatic breakdown of the various specialty Mimosa products. Note that as the molecular size decreases, the rate of tannin penetration increases.
EXTRACTION & PRODUCTION PROCESS
TYPICAL ANALYTICAL RESULTS (SLTC SHAKE METHOD) OF MIMOSA PRODUCTS
TANNING WITH NTE MIMOSA
Mimosa Tanning Principles
Tanning is essentially a two-stage process, although the two stages are not completely separate, in fact they proceed concurrently. The first stage consists of the diffusion of the tannins into the hide by means of the spaces or capillaries between the fibres and the fibre bundles and more minutely into the spaces between the individual fibrils comprising the fibres.
The second stage is the combination of the vegetable tannins with the collagen, the protein of which the hide fibres is mainly composed.
It is therefore necessary to consider first the factors affecting the diffusion of tannins into hide since it is obvious that tannin cannot combine with collagen fibres at a given point in the hide structure until it has diffused or penetrated to that point.
The factors affecting diffusion are:
- Concentration of tan liquor
- Neutral salt content
- Particle size
- Mechanical action
Concentration of tan liquor
With increasing concentration of tan liquor, the tendency for a tan liquor to penetrate hide increases, provided that the astringency does not also increase and cause case hardening which may restrict the penetration. Thus it is not always possible to increase the rate of penetration by putting raw pelt into strong tan liquors. If, however, a piece of partially tanned hide is moved from one liquor to a more concentrated liquor the penetration of the tannin will be accelerated.
The time a hide is immersed in a tan liquor will determine the amount of tan that has diffused into the leather. Thus it is obvious that the longer a piece of hide is left in a given tan liquor the greater will be the diffusion of tannin into the hide, until the system reaches equilibrium.
Increase in temperature will increase the rate of diffusion of a tan liquor into the hide. This is partly due to the fact that the viscosity of a tan liquor decreases with rise in temperature. In addition, the mobility of the tannin particles is increased with an increase in temperature.
Acidity is one of the most important factors affecting the diffusion of tan liquors into the hide. The pH of a system affects diffusion in various ways. Firstly, increasing the acidity increases the tendency of hide fibres to plump, and with increased plumping of the hide fibres, the spaces between the fibres decrease and less tan liquor diffuses into the hide. Secondly, with a decrease in acidity, especially below pH 3.5, the tendency for tannin to combine with collagen increases appreciably. Consequently, in the low pH range, the diffusion or penetration of tan into hide is further retarded because of the combination taking place between the tannin and the collagen fibres. This reduces the spaces between the fibres in addition to restricting the swelling of the fibres.
The type of acid present in tan liquors is also important since a greater quantity of the weaker acids is necessary to achieve a given pH than when mineral acids are used. Thus there is a bigger reserve of acid in tan liquors acidified with weak acids (e.g. formic or lactic). It has been shown that the greater the quantity of acid present in a tan liquor at a particular pH, the slower the penetration of the tan into the hide.
Neutral Salt Content
The presence of neutral salts tends to suppress the plumping of collagen fibres in tan liquors and the type and amount of salt in the tan liquor will influence the extent to which the swelling is reduced. It will be apparent therefore that neutral salts will help to increase the rate of penetration because reduction in plumping will enlarge the spaces between the fibres and fibrils.
Since the rate of diffusion of the particles in a tan liquor is affected by the size of the openings between the fibres, it is also affected by the size of the tannin particles. Vegetable tannins are complex mixtures of organic compounds which are polymerised to varying degrees and therefore contain particles of varying size. Thus tannins with a small particle size will diffuse into hide more rapidly. Certain chemical treatments e.g. bisulphiting, increases the solubility of some tannins and increases the rate of penetration.
Mechanical action increases the rate of penetration because the flexing action causes physical movement of the liquid between the fibres and is effective in carrying tan to the inner layers of the hide. This is not a diffusion process but in practice the effect is the same.
RETANNAGE WITH NTE MIMOSA
The factors affecting combination are:
- Concentration of tan liquor
- Acidity or pH
- Neutral salt content
- Particle size
It is obvious from the list that the factors which affect penetration also affect the combination of the tannin with the hide.
Concentration of tan liquor
The concentration of the tan liquor is important because more tannin will be fixed by the hide in a given time from a strong liquor than from a weak liquor.
The combination between tannin and collagen is a physiochemical reaction which proceeds with time, the longer the reaction or tanning is allowed to proceed the greater the fixation of the tannin by the collagen fibres.
Since reaction rate is increased with temperature, the amount of tannin that will combine with a given weight of hide substance from a tan liquor under standard conditions is increased by a rise in temperature.
Acidity or pH
Acidity plays an important part in tanning because although tan fixation occurs over the whole range of pH 1 to 8, the greatest fixation is in the range 2 to 3. This is because reactive groups are made more active and accessible at these pH levels.
The type of acid is again important when dealing with fixation or combination. A tan liquor acidified to a given pH with a greater quantity of a weak acid will give a greater tan fixation than a similar tan liquor acidified to the same pH with a smaller quantity of strong acid.
Neutral salt content
The presence of neutral salt tends to reduce the fixation of tannin. This is probably because the swelling of the fibres is reduced and reactive sites in the collagen fibres are not so readily accessible. Thus neutral salt will tend to give rather empty leather, particularly if present in excessive quantities.
The small particle sized tannins tend to be more difficult to fix.
The condition of the hides and tan liquor must be such that the tans can enter the spaces between the fibres with reasonable ease throughout the entire thickness of the hide. Having entered the hide the tans gradually combine with the collagen.
Although this combination takes place after the tans have entered the hide, both processes of combination and penetration proceed simultaneously. Suitable choice of conditions at the various stages of tanning will give the necessary penetration and tan fixing effects.