Leather Production - Pre-tannage

[Nasser B.]

Pre-tannage

The principal chemical component in leather is the protein collagen
and the pre-tannage operations are aimed to remove other proteins as
required and to prepare the collagen chains for subsequent cross-
linkage by the various tannages.

The first stage in pre-tannage is to restore the original moisture to
the skin, which will allow all the subsequent processing to be done
correctly. The alkaline chemical treatments, normally as lime, then
clean the hide structure by removing some types of proteins and giving
a degree of swelling. There is a loosening, or destruction, of the
epidermis including the hair. The fibre structure is opened up and
fats are partially removed as soaps. After the hair is removed, the
alkaline swelling is removed and there is a further opening-up of the
fibre structure by enzymes. The hide, or skin, is often call ‘pelt’ in
these pre-tannage stages.

PREPARE INPUT LOAD

The variety of shapes and sizes of hides and skins reflect the history
and health of the animals themselves. It is a good practice to select
similar weight, condition and sizes of the raw material so that the
chemical and physical processes can be more uniform and efficient. If
this is not done, then some pieces will have too much treatment and
others will have too little.

SOAK

The object is to restore the hide to its natural moisture content and
degree of swelling. There is also the removal of dirt, soluble
proteins and curing agents (mainly salt).
It may be either be done in pits (for pre-soaking dried material),
paddle (for careful soaking of delicate skins in long floats) or, more
usually, in a drum. The drum speed is low and intermittent. Chemicals
to aid re-hydration, such as biodegradable surfactants are often
included and slight alkalinity helps to achieve a limited swelling.
The salt concentration should not be allowed to fall below 2° Beaumé.
Bactericide is needed to avoid any putrefaction damage. Ideal
temperature is 26°C and pH 9-10.

GREEN FLESH OPTION - with a by-product of green fleshings and residual
fat

The mechanical operation of fleshing is an option, for hides, to cut
away flesh tissue so that the chemicals in the subsequent operations
can penetrate easier. All chemicals penetrate faster from the flesh
side of a hide or skin, compared with the grain side.

UNHAIR (lime) - with a waste by product of hair or wool

The object is to remove the hair and to open up the fibre structure.
This is normally done by lime based, sulphide containing, liquors, in
drums or paddles. The drums are slow moving at 2-4 rpm, with
intermittent running. The method can also be done by surface
application (painting); this is normally on the flesh side, by hand or
machine, to leave the hair or wool without serious attack. The grain
side is used to obtain a particularly smooth grain, when the hair is
completely destroyed. In the main drum or paddle operation, the hair
structured is destroyed. Fats are made into soaps and there is strong
swelling, or plumping, due to the high alkalinity (about pH 12-13).
Temperature of 26°C is ideal, but is not to be above 30°C. A thorough
washing is needed when liming is completed, with the temperature 4°C
above the liming temperature. This allows better fleshing and smoother
necks.

Liming for heavy leather to be vegetable tanned has an extra day in a
weak lime solution. This will increase the opening-up of the structure
to permit more filling by the larger vegetable tanning molecules. Some
sheepskins are not put into liquors for unhairing. They are kept in
rooms to produce a controlled bacterial attack. This ‘sweating’ is
used to improve the quality of the wool but does not improve the
leather quality.

As this is the most polluting of all the tannery operations, a lot of
efforts have been made to reduce the toxic effects. This includes hair-
saving instead of hair destruction, reduced sulphide dosages,
recycling and alternative chemicals. Enzyme unhairing is another
option being developed to avoid sulphide but some alkaline treatment
is still needed to open the fibre structure.

FLESH - with a waste by-product of fleshing

The limed pelts are in a swollen state and the cutting action of the
fleshing machine is more effective here than in the earlier green
fleshing operation. Handling is difficult because of the slippery
nature of the limed hides and skins. If limed pelt is exposed to the
atmosphere for several hours, there is the risk of damage to the
surface by the formation of lime-blast. This describes the formation
of calcium carbonate films, when the carbon dioxide in the air reacts
with the calcium hydroxide of the lime solution. In practice, it can
be minimised.


TRIM - with a waste by-product of the trimmings

The cutting action of the fleshing machine blades on hides can cause
strings of material, which need to be trimmed to give a clear shape.
Skins are cleaner.

SPLIT OPTION (pre-tanned state) with by-products of splits for further
processing or for Gelatine and glue stock

The final leather has to be as even as possible in thickness although
this is not the case in the original hide or skin. The object in
splitting is to obtain a more even thickness for processing and a more
uniform final leather. Hides are much thicker than skins and need to
be split either now, or later, in the tanned state. The grain (top) is
levelled by passing the hide across an endless band-knife to an
accuracy of a few millimetres; the bottom layer, known as the ‘flesh
split’, is of irregular shape and thickness. Splitting is a skilled
operation and needs experienced operators and a well-maintained
reliable machine. Although, splitting at this stage is more difficult,
and less accurate, than splitting in the tanned state, the advantage
is that the tanning chemicals penetrate easier and are absorbed more
efficiently.

Splits are processed separately and should be an important contributor
to profitability. Several layers can be produced from an exceptionally
thick hide, such as buffalo. However, in such cases, the middle layers
are weaker in structure than the outer layers.

TRIM - with a waste by-product of trimming

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 potential leather to sell and represents profit.
Hides can be kept as whole hides until after tannage, but there can be
an advantage if the hide is segmented for some specialised
productions; the best quality butt can be processed for one product
and the shoulder and/ or belly pieces for other products. In this way,
more value is added to the one hide.


DELIME AND BATE

The object of deliming is to remove the strong alkalinity of the pelts
by the use of weaker alkalis, and weak acids, so that swelling is
reduced. Bates are enzymes and the object of bating is to produce a
smooth clean grain and remove non-structured collagen and other
proteins. It is done at specific conditions of temperature and pH and
continues the deliming. The enzyme action improves the softness, grain
elasticity and colour levelness of the leather. The work is normally
done in a drum at a temperature of 28-30°C and pH to come below 8.5.
The drum speed is faster at 10-12 rpm. Maximum temperature is 35°C for
deliming.

OPTION OF DEGREASE (sheepskins)

The object is to remove excess grease from the skins to allow proper
processing. The percentage content of natural fat depends on the type
and origin of the rawhide or skin. Based on dry material, hides have
2-10%, goats 5-10%, hair sheep 8-15%, wool sheep 20-30% and pigs
30-40%. Processing does remove some of the lower levels and leathers
need to have some fat for softness, which is also added later. Gloving
and nappa skins can tolerate 5% but the excess in wool sheepskins (and
pigskins) needs to be degreased. Surfactants to emulsify the fat have
been used in combination with fat solvents, which produce satisfactory
leathers but are environmentally damaging. Kerosene has been used with
solvent recovery, but is also not acceptable in modern practice.
Enzymes are being increasingly used for degreasing together with
biodegradable surfactants. Temperature is 35-38°C.

Contributed By Mr. Woodley, Michael
International Consultant
May, 2007

[Tulipgardener]

Thanks Nasser for sharing this!