Leather Production - Tannage

[Annabella Webmaster]

Tannage

Tannage is the irreversible conversion of the natural fibre network of
pelt into the material leather. The collagen structure is permanently
cross-linked. This makes it resistant to bacteria, putrefaction and
higher temperatures. When dry, it does not feel hard and can normally
be rewetted.

1. PICKLE - (can be option to export from this stage)

The object is to prepare the pelt for tannage, or for export, by
adjusting the level of acidity in salt solution. This controlled
acidity is needed to allow penetration of tanning materials through
all the thickness uniformly and to combine correctly. If this was not
done, the tanning materials would react too strongly on the outer
layers, not penetrate deeper and the appearance of the surface would
be damaged. The leather would also not be tanned correctly.
The pickle is varied according to the type of tannage to be made, with
less acid conditions being used for vegetable tannage. However, salt
concentrations are increased and essential to avoid any acid swelling.
6% salt on the total volume of water is a safety level for the most
acid conditions. Temperature must not exceed 28°C.
Vegetable tannage should start at pH 4, chrome at pH 3 and the export
pickle needs to have a stronger pickle in terms of acid content
(absorbed by the pelt) and a preservative to prevent mould. This pH is
below 2. Storage is good if the pickled pelts are kept cool, below
32°C air temperature.

2. TANNAGE

The object is to convert the pelt into leather by creating a more
stable structure, which will not putrefy and maintain an attractive
appearance. The options are:

2.1 Chrome tannage

Chrome tannage is the most widely used and most important tannage, in
more than 80% of leathers. This is used in the production of shoe
upper, furniture upholstery, clothing, leather goods and gloving
leathers. Basic chromium sulphate is the main chemical, penetrating at
pH 3 in controlled stages and being fixed at pH 3.8-4.0. Process
starts at about 24°C to finish at 39°C for better chrome exhaustion.
The higher end temperature is achieved by increasing the drum speed
from 5 to10 rpm. The leather will have a shrinkage temperature of
95-100°C. The cross-linkage is identified as being made with the
carboxyl groups of the collagen. The process is done in a drum and
takes about 10 hours for hides; skins are tanned faster because they
are thinner. The colour is blue and it is often exported semi-finished
as ‘wet blue’ with the use of preservative.
Environmental protection is limiting the amount of trivalent chrome
salts in an effluent for discharge into a public waterway, because the
heavy metal affects the treatments in the waterworks. This has
resulted in many developments to reduce the levels of chrome in
effluent by increasing chrome uptake in the drums, and by various
recycling methods. There are also ecological concerns that the
trivalent blue chromium salts in leather could be converted into the
carcinogenic hexavalent state, with its danger to health.

2.2 Vegetable Tannage

Vegetable tannage uses material obtained from parts of certain plants.
These may be the bark, wood, fruit, root or leaf of the plant. The
chemical compositions are complex and are mixtures of natural
polyphenolic compounds. The shrinkage temperature is 70-85°C. The
characteristic colour varies from pale yellow-brown to an intensive
red-brown depending on the type of vegetable tanning material or
mixture of tanning materials used and the application conditions. The
nature of the materials limits the light fastness, and there is a
considerable change in colour with time and exposure to sunlight. They
have high molecular weights and probably form linkages due to a
combination of hydrogen bonding and their size. The resultant leather
is therefore, much fuller and heavier, than chrome-tanned leather.
Vegetable is the original tanning method and used for heavier, more
compact leathers from hides – sole, strap, belt, bag, harness,
upholstery – but it can make soft light leathers such as linings and
leather goods - particularly from skins. The process is slow, with
limited mechanical action from pits or slower running drums. The time
is about 4 days for lighter weight leathers and 12 days for sole
leathers. The temperature normally starts at about 20°C for
penetration, increasing later to 35°C. It has been largely replaced by
chrome tannage, which is quicker and more economical.

2.3 Alternative Tannages

Alternative tannages now receive more attention because of
environmental concern. There have been synthetic tannages in use for
many years, known as syntans. These cover a wide range of organic
chemicals, such as phenols, naphthalene, glutaraldehyde and various
polymers. This is a drum process and takes about 10 hours, similar to
chrome tannage, but at temperatures of 25-30°C and pH 3.0-3.8. The
shrinkage temperature of the leather produced is 70-80°C. It was
originally used to make speciality leathers, such as white, and to
replace part of the established tannages. The current interest is to
replace chrome tannage and ‘wet blue’. There are ‘wet white’ tannages
for export and for further processing in the same tannery. These avoid
the chromium but do not produce an exactly identical leather character
to full chrome leathers at present. However, the auto-upholstery
buyers are demanding ‘chrome-free’ leathers and such supplies are
expected to increase. The colour of the leathers is white, or
yellowish, and they have very good light fastness. These leathers are
limited to drying temperatures of 40-50°C, compared with 70°C for
chrome tanned wet blue.

2.4 Pre-Tannages

There are also many pre-tannages which are used to follow the pickle,
but before the main tannage, so that there is an improved uptake or
performance from the tannage itself. Such examples are formate before
chrome tannage, phosphate before vegetable and syntans before ‘chrome-
free’ tannage of wet white.


After tannage, leathers contain a lot of surplus tan liquor.
Consequently, this excess is normally allowed to drain, with the
leathers piled on a ‘horse’ or stacked on platforms, overnight. This
allows further fixation of tanning material by the leathers whilst the
drained liquid is collected, for recycling or for treatment in the
effluent.

3. SAMM

The object is to remove the unbound water so that the hide can be
packed, split or shaved, with consistent uniform moisture content and
an exact thickness. The natural differences in the structure of the
hide mean that the tanning material absorption and the liquid
absorption also vary. Consequently, the leather is still not the same
thickness all over after tannage, even if it was already levelled by
splitting in the pelt. It is first sammed to reduce the water content
from about 70% to about 60%. The hides are then squeezed between the
moist felt rollers of a samming machine, which also flattens the
shape. A setting out action, to further spread the hide, is often
incorporated into the sequence on the machine, with extra rollers. The
moist leather is then sorted for export or for further production in
the tannery.

4. SIDING

This refers to cutting the hide into two halves and is done now, if it
was not done before tanning. It may be done manually with the use of a
cutting guide on a table, or by specialist equipment. It has to be
accurately done down the backbone to produce the flatness in that
area. Hides can also be cut into other shapes if required in the final
leather.

5. SORTING - (can be an option to export at this stage)

The object is to grade the hides, and skins, according to their
potential quality. Wet blue (or wet white) is normally exported
without splitting, so that the full hide thickness is available to the
buyer. Quality is sorted on an agreed basis. This will assess the
degree of damage in the hide, or skin, and how it affects the cutting
value. Each quality has a different value.
Exports may specify particular grades. The wet leathers for export
need to be carefully folded, and packed in plastic sheeting so that
the packing is completely waterproof. This is to prevent permanent
creasing of the leathers and any drying out in transit. The rewetting
of such dried leathers is extremely difficult, and it is advantageous
if a very small amount of a hydrophilic compound (fat-liquor type) can
be added to the tannage, provided there is no effect on final
quality.
If it is not for export, a similar sorting is done for the tannery's
own operations. This has the same assessments, and produces a range of
qualities. Sorting also decides whether it can be full grain leather,
or whether the grain needs to be buffed away and corrected in some way
to disguise faults. The sorting figures are then compared with
outstanding customer orders. Their requirements will show thickness,
quality and quantity. In this way, the actual customer orders start
from here, and should have a reliable completion date for a delivery
schedule.

6. SPLIT OPTION - (tanned state) producing a split for further
processing and tanned waste by-product

The object is to obtain a more even thickness for processing and a
more uniform final leather, if it were not done in the limed
condition. At this stage, the leather has a more stable structure. The
tanned hide is less swollen and so it is easier to handle. The actual
levelling is more accurate. The thickness is determined by the final
product to be made. It will allow for some final adjustment by
shaving. The machine and operators are critical to a successful
operation, from quality and profitability aspects. A good machine is a
valuable investment for the tannery.


7. TRIM - with by product of damp tanned leather waste

The object is to produce an economic shape for sale or processing
further. The grain layer (top split) needs to have any ragged edge cut
away to facilitate other machine work, whilst the lower flesh split
has to be trimmed to such a regular shape that can have a uniform
thickness. Trimming should be to retain, or improve, value. The
quantity of trim should be controlled to see that it is not excessive,
because it loses profit. The actual flesh split is larger than a flesh
limed split.


8. SHAVE - with by product of tanned shavings

The object is to make the final thickness adjustment and have an even
cutting through leather with consistent moisture. The moisture content
should be 30-45% Splitting leather can never be accurate enough and so
a shaving machine is needed to refine the produced thickness and leave
the flesh side smooth. The shaved thickness is determined by the
customer requirements, allowing for the loss in processing between the
semi-moist condition and the final despatch.


9. PREPARING LOADS for retannage and dyeing

The object is to plan the production of crust leather to meet the
demands of the customers’ and sales forecasts. Customer orders define
the quantity and quality of finished leather. The tannery identifies
the type of retanned leather (the crust), which corresponds to the
finished leather. If there is not sufficient quantity and quality
available in crust stock, there has to be more produced from the wet
blue. In this way, the work tickets from the wet blue are planned to
supply the crust stock as required, from orders or sales forecasts.
The loads will be in standard sizes for the retanning drums and
correspond to established processes.


Contributed By Mr. Woodley, Michael
International Consultant
May, 2007
http://www.ab-corporategifts.com