|
The
Chemistry of Leather Making
Wondirad Seifu,
Program Officer,
The Chemical Society of Ethiopia (CSE)
Solutions,
A Biannual Newsletter of CSE, Volume 7, No. 2, August 1999
Introduction
The use of leather goes back to the time immemorial
where our primitive ancestors started to use crude tools for hunting of
wild animals for food purpose. After they had eaten the meat of the animal
they would clean the skin by scraping it and sling it around their shoulder
to serve as a crude coat and they cover their legs to protect their bare
feet from road and thorns.
But in a very short time the skins decayed and rotten
away, because these early men did not know how to preserve them. As
centuries passed, ways were discovered by pure chance which make the skins,
last longer. Gradually, by trial and error human being learn the effect of
wood smoke and some vegetable material on the durability and bacterial
resistance of the skins and also the effect of animal fat on the softness
property of skins.
|
Manual skins
fleshing operation
|
As the years went by, men grew more experienced in
leather making and found that they could use leather for many purposes,
besides foot wear and clothing. A great discovery was that water would keep
fresh and cool in leather bag, and this knowledge enabled tribes to wander
away from a spring or river bank, carrying their supply with them. This has
been claimed that it facilitates the spread of man over the earth.
Although, the word leather occurs in the Bible only
twice, there are many references, which clearly mean leather. Gradually,
through its long path of evolution and revolution, leather making has
emerged from traditional way of doing leather by craftsmen to the present
day leather making in the tanning industry.
The tanning industry may be regarded as a bridge between
production of hide and skins as a by-product of the meat industry and its
manufacturers into shoes and weaving apparel, for which it provides basic
raw material-leather. The technologies and skills involved in the
production meat and those required in the production of useable goods from
leather are widely different. The two groups of people having these
different skills have very little in common. These separations of skills
and degree of specialisation have existed in modern society except the
primitive one.
Leather
making material
Hide and skins are the basic raw materials of leather
making. The language employed to describe “hide” and “skins”, is simply to
differentiate between large and small skins respectively. Each consists,
approximately, 64% water, 30% protein, 2% fat, 0.5% mineral salt and 0.5%
other substances. Of these, the most important for the leather making is
the protein, consists of many types. The important one is collagen which on
tanning, gives leather. In addition, it is used for the production of
gelatine for photographic paper and film, food, as well as glue.
Collagen is one of the fibrillary components of
connective tissue in animal. Its macromolecules are arranged parallel and
close to one another. Biophysical and biochemical research studies have
shown that collagen has a very complex structure based on “coiled coil” or
triple helix form, having a high molecular weight of the order of 300,000.
The collagen molecule as a whole consists of three peptide chains
containing about 1000 amino acids residues in each.
Leather
making
Conventional leather- making process consists of
chemical and physical operations which may be divided into four stages.
1. Separation of the collagen
The skins are soaked in a
rotating drum containing water solution of wetting agents and
preservatives. Followed by treating in Lime-sulphide solution in order to
separate the unwanted parts like hair, non-structural protein, saponifiable
fats at higher PH value, 11-13. These operations also open-up
the collagenious fibres and prepared the resulted material called “pelt”
for fleshing operation in which excess fat and flesh are removed by
high-speed rotary blade. As a result, the leather making material,
collagen, is separated out.
2. Cleaning of the collagen
The lime is removed in de-liming operation, using salt
of strong acids towards optimum PH range, 7.5-8.5, where the
next process called “Bating” is triggered in order to loosen any remaining
hair roots, epidermal structure, pigments and fatty cells by enzymes. Depending
on the solvent-soluble fat content of the skins, degreasing operation may
be employed using organic solvents. This followed by the process called
“Pickling”. The purpose of this operation is to stop enzymatic bating and
adjust the acidity of the collagen for tanning.
3. Stabilising the collagen
This stage is the critical one, called, Tanning, which
transformed the collagen material irreversibly into leather. Tanning is a
process of introducing a tanning agent into the collagen. It is done by
putting additional cross-linkers into the collagen to bind the active group
of the tanning agent to functional groups of the collagen. In doing so, it
imparts adequate strength and resistance to various biological and physical
agents to the collagen. In practical terms tanning converts the putrefyable
collagen into stable product which has filled many useful needs of man
since the dawn of civilisation.
Tanning agents do not form a distinct class of compounds
chemically. On the contrary, substances of very dissimilar composition and
nature possess tanning properties. These include basic chromium salts,
vegetable tannins, aldehydes, certain condensed phenols containing sulfonic
acids and unsaturated oils. Among them, the chromium compound, notably the
basic chromium salts, hold an unrivalled position in the tanning industry.
Chrome tanning is the most important method used to
obtain light leather with high thermal and bacterial resistance. A usual
procedure is to introduce Cr +3 salt into the collagen, adjusted
to a PH of about 3 in pickling process, and then make the
collagen-chromium complex cross-linking reaction to occur with gradual
increase of the acidity to PH 4 .
Two-bath chrome tanning process was initially used based
on saturation of the collagen with Cr+6 compound, followed by
its reduction to Cr +3 . This method is no longer used since
chrome at +6 oxidation state is biologically active and can cause cancer.
Hence the method is now replaced by one-bath method with the application of
ready-made chrome salt at +3 oxidation state. Chromium cation, (Cr+3 )
in aqueous solution occurs as hydrated hexaquochromium ion, Cr (OH2)6
+3 . The most important property of this ion
from the point of view of tanning chemistry is that the water held by its
legends can be exchanged for other ions of acid residues or hydroxyls of
collagen to form cross-linking.
4. Modification of the collagen
To obtain a utilisable product, further operations are
necessary. This includes: fat-liquoring, dyeing and finishing. During
fat-liquoring, oils are introduced into the leather in the form of
oil-in-water emulsion. This makes some important changes in the properties
of the leather. It plays a fundamental role in governing the softness,
stretch and the ability to take up or resist water. Sulphonated oils are
very commonly used because they give good and fine oil dispersion. These
oils include castor oil, code oil and neatsfoot oil.
Leather dyeing is a transition process between tanning
and finishing in which different types of colours are given to the leather.
Dyestuffs used in leather dyeing are mainly aromatic organic compounds. Two
groups of dyestuffs of very specific way of action have gained importance
for leather dyeing are metal complex and reactive dyestuffs.
The saying goes that “if leather is free from surface
defects, it does not need to go through finishing process”. Unfortunately,
the reality on the ground is not as such and finishing operation is
inevitable. A wide variety of coloured pigments are applied on the leather
during the finishing process. In addition to concealing the leather’s
defects, they impart such properties as water and scuff resistance. As the
pigments, by itself, are not capable of adhering to the leather or forming
a continuous film, other materials must be incorporated. These materials
are known as binders. Developments of polymer chemistry and technology have
provided a wide range of such materials
.
Tanning
industry and the Environment
As far as the tanning industry is concerned, there were
a big shift of tanneries between 1950’s- 1960’s from developed nations to
developing nations for reasons of stringent environmental regulation. At
this time the shift of tanneries were not complete reallocations. They
shifted only the most pollutant part of the leather making process called
“wet process”, mainly the sulphide-liming process which yields lower value
intermediate products.
The waste generated from the tanning industry is mainly
classified into three groups; namely, solid, emission and liquid wastes.
The solid waste composed of trimming, fleshing and shaving. The emission
mainly composed of volatile organic compounds and fine dust particles from
the leather. The liquid waste contains a wide range of processing chemicals
that are not exhausted or utilised in the leather making process due to the
limitation of the conventional leather making process. These chemicals
include sulphide, sodium chloride, acids, ammonium salts, chromium, etc. In
Ethiopia, it was reported that several hundred tonnes of tannery chemicals
are directly disposed to the environment.
On the other hand, the aggregate analysis of the liquid
waste as measured by biological oxygen demand (BOD) and chemical oxygen
demand (COD) have shown an average value of 3500 parts per million (ppm)
and 7000 ppm, respectively. It is very far from the accepted environmental
standard. Therefore, the problem calls for the use of cleaner technology
which focus on reduction of pollutants at the sources and/or clean- up
technology which focus on the end of the pipe treatment usually urges
installation of waste treatment plant.
References
1.
Beeby K.J. The Wonderful Story
of Leather, The Leather Institute: London
2.
Thorstensen T C. Practical
Leather Technology, Robert E. Krieger Pub.
3. Physical
Chemistry of Leather Making
llllll
|