Leather Production - Retannage

[Annabella Webmaster]

The object is: to produce different types of leather from the one semi-
finished leather, which is usually "wet-blue". The retannage optimises
the serviceability of the leather, adapting it to meet fashion
requirements and the demands of customers. It also creates a dried
(crust) leather to allow satisfactory finishing of the dried leather
surface.

The neutralization, retannage, dyeing and fat-liquoring wet operations
are almost always carried out in that order in one operation, with a
total time of 3-7 hours. They are done in a drum, or sectioned dyeing
vessel, at a range of temperatures between 35-60° C. Drum speed is
about 12 rpm. Re-tannage is often understood to include all the four
stages.

The individual stages of the operation influence each other, so there
are adjustments needed to obtain optimum effects for high-quality
finished leather. The properties, which can be influenced by the re-
tannage include fullness, grain tightness, softness, fat distribution,
leather colour, levelness of the dyeing, light-fastness, grain
fineness, smoothness, buffing, dry-drumming, embossing, buffing, water
repellence and chemical and physical analytical results. It is
obviously of great importance in determining the final quality.

Special treatments, such as water repellency, are an integral part of
all leather process design but there will be a part of the retannage,
which involves special chemicals. It has to be realised that such
leathers have to be designed from the initial wet work to minimise all
hydrophilic chemicals.


1. NEUTRALISE

The object is to remove strong free acids from the leather by using
milder chemicals. This weakens the strong positive surface charge of
the chrome leather so that anionic tanning materials, dyestuffs and
fat-liquors agents can penetrate and are not restricted to the
surface. The leather is said to be ‘de-acidified’ because it does not
usually reach the neutral point of pH 7. Surface pH is below 5.0
externally and 4.5 internally. If a deep neutralisation is needed to
allow other chemicals to penetrate deeper, the external and internal
values are about 5.5. It controls the reactivity of wet blue leather
and has to produce the same level in all the pieces of leather in one
retanning load. It is, therefore, important for treating a selection
of wet blue from different suppliers.


2. RETAN

The object is to control and adjust the properties of the resultant
crust leather. It is the main use for the synthetic organic tanning
materials, but vegetable tannins, polymeric, resin and mineral tanning
agents (chrome included) are also used. The environmental concern for
trivalent chrome has affected how this material is used. The character
of leather is determined by the first tanning operation, but the
retannages are an adjustment to that. There are 3 main types of
result, depending on the crust required:

- Filling of the looser structure of wet blue by vegetable tannins,
replacement syntans and resin tanning materials with a selective
filling effect. This leather is also designed to have good tightness,
buffing, embossing and finishing properties. This is for corrected
grain leather, which is the lowest quality of the wet blue selection.

- The full grain selection has a good grain and cutting value. The
retannage is designed to have less filling to retain that natural
elegance with a good break, full colour shades from dyeing and an
attractive feel from a full handle over all the hide or skin. Softness
and an attractive look are often more important than the tightness of
the lower grades. The eye appeal is the main sales factor in top
quality leather.

- A compromise of the others, so that there can be further sorting in
the crust to optimise quality, value and profit for the tannery. It is
a valuable option to have a versatile retannage, suitable for
finishing as either full grain or corrected grain.

3. DYE

The object is to colour the leather as required by the customer, or
sales forecasts. This should be an even colour and should cover any
grain defects. If the leather will not have a covering finish, the
colour should be light fast, and wash fast. It is usually done in
drums, or sectioned dyeing vessels, with different levels of float and
temperatures. Not all leathers are drum dyed. This may be done to
leave more choice in deciding the product to make from the crust and
to save the cost. Dry crust leather can be dyed by through feed
continuous methods. At present, this does not produce the same
qualities as the batch dyeing due to the limitations of time,
temperature and dyestuff type. There are many controls to affect the
results and there is a wide choice of dyestuffs, which are high cost
materials. These are often in liquid form for easier, and healthier,
handling. Health concerns have also banned the use of dyes containing
known carcinogenic chemicals, and these are not produced by any
reputable company.

Dyeing can be with anionic and cationic types but the whole retannage
process needs to be designed according to the dyeing conditions.
Colour matching is an old established human art, being replaced by
instrumental colour measurement in larger tanneries.
Wet white leathers are often required to be dyed with dyes free of
heavy metals, to meet the ecological requirements.

4. FAT-LIQUOR

The object is to soften the leather, as required in the product, by
lubricating the wet fibres, with a fatty emulsion, so that they do not
stick together on drying. Without fat-liquors, the leather would dry
hard and any mechanical action would damage the fibre and limit the
quality potential. It controls the feel of the dry leather. It is
normally the last operation in retannage and can be combined into that
float. These emulsions of materials have different stabilities and
care has to be taken that there is the required degree of penetration
and fixation. Complete penetration produces leather with a cloth-like
feel, suitable for garments, compared with a surface effect, which
would feel greasy and be difficult to apply a top finish. Care and
experience are needed to select the correct balance of materials. The
method of drying also needs to be considered when deciding on the fat-
liquors. Fat-liquors, with suitable stability, are also applied during
tannage to obtain a deeper coating of the fibres. After the final wet
operations (retannage etc.), the leather is generally horsed up or
stacked on platforms overnight. The water content is about 70-75%, on
the leather weight.

5. SAMM AND SET OUT

The object is to reduce the water content and to spread the leather
out by stretching it in all directions. The helical blades spread the
hide, or skin, into a flat shape and squeeze out the surplus water.
Animal skin is three dimensional to cover the animal shape, so this
operation now starts to change that into two dimensions. The shapes
need to be positioned on the rollers to avoid any creases forming in
the surface; this can be difficult for the shanks. After setting out,
the leather should be easier to handle for subsequent drying.

6. DRY

The object is to eventually take the moisture level down to about 8-14
% for mechanical softening. Water evaporates from the surface in two
stages:

- The first stage is at constant rate with the surface completely wet
so that the water can migrate to the surface from the centre of the
hide at the same speed that it evaporates. This is unbound water and
the heat of the drying does not affect the leather temperature because
of the evaporation effect.

- The second stage, the falling rate stage, is when the surface is
only partially wet and the temperature of the leather itself will
start to rise. This is a critical stage and can damage the leather if
moisture is trapped in the centre.

Uncontrolled drying is not advisable. Shrinkage of the leather also
occurs during drying and is a factor in costing. Higher temperatures
create higher shrinkage. However, this shrinkage should not be
physically restricted because the leather would become hard and
unacceptable, with the fibres unable to reposition themselves at
different moisture levels. Slow drying, at a low temperature, produces
the softest leather with the lowest shrinkage, but this is not
normally economical.
It is not normally possible to dry the leather to its required
physical condition in a single operation. First there is an initial
drying and there are 4 main methods used:

- Suspension, or hang, drying where the leather is simply hung up in
the drying room or tunnel, with controlled conditions of temperature,
relative humidity and air circulation. The resultant dry leather feels
full and round with good softness, but has the serious disadvantage
that area and smoothness is lost. All such leathers must be toggled
after staking to recover sales value and profit.

- Paste drying has the set out leathers pasted onto cool glass, or non-
corrosive metal, plates with an aqueous adhesive solution and then
passed through a tunnel drier for 5-8 hours. These machines generally
have a total of 100 to 200 of these vertical plates and up to 50
metres long. The leather passes through a number of sections, each of
which have well controlled drying conditions enabling the drying rates
to be progressively matched to the different stages described above.
This resultant leather needs some softening, but it is flat and has a
better area yield than suspension drying. The residual paste film
means that full grain finishing is not normally possible, but it is
ideal for such buffed leathers as corrected grain or splits. There
also exists the possibility of special pastes, which can be removed
easily for full grain.

- Vacuum drying is the best method for most full grain leathers,
whilst it is also perfectly satisfactory for corrected types. The set
out leathers are laid flat, grain down, onto a stainless steel table,
where they are also stretched out further by hand slickers as their
shape dictates. The table top and flattened leathers are then
enveloped by a sealing hood. Reduced air pressure then allows drying
to take place at a lower temperature, with the hide, or skin, being
kept flat in position by applied pressure, reducing shrinkage. The
temperature is about 75°C, and even less. After a few minutes, the
leather is removed with the grain side dry and the flesh side slightly
damp. It must not be dried out completely to the lower limit, but
hung, without tension, in the ambient temperature to lose the final
moisture freely. These driers have been developed into multi-table
machines and the most sophisticated systems have a conveyor delivery
from the setting out through vacuum driers, to staking and toggling
operations. These have great potential for large productions. Process
time and labour content is significantly reduced.

- Toggle drying stretches the leather manually onto perforated metal
sheets, so that the shape is retained and flattened by the toggle
clips, which have pincer grips. These hold the edge of the leather and
are fixed into the perforations of the sheets by a small positioning
foot, or lug. The drying conditions and control are as for hang
drying. The frames are sometimes assembled as a type of bookcase or,
much better, as a form of conveyor, which has greatly improved this
whole operation and reduced the handling. Toggling is used for
upholstery, side clothing, splits and lining leathers. It has 10% more
area than hang drying but at the expense of quality, because of the
tension produced from the toggles. Stretching prevents some shrinkage.
It is better for vegetable tanned light leathers, which shrink less,
and for re-toggling damp leathers after mechanical softening and dry
milling. Here it keeps the leather soft and flatter.

These initial drying methods produce leathers, which are too hard and
have an uneven moisture content, but below the normal 14%. They could
not be finished in this state.


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