The effect of acrylic polymer dispersions on the water
vapour permeability and some other properties of finished leathers have been
studied. An acrylic based commercial binder AE 558 Nycil has been characterized
and its effect when applied in a finish formulation on some of the physical
properties of originally retanned leathers was investigated. The binder was
found to have an intrinsic viscosity of 227 dL/g, and a viscosity molecular
weight (Mv) of 4.03x105.
This was obtained by conducting a solution viscosity measurement of the solid
polymer in toluene at 25 oC.
The melting temperature of the solid binder has been found to be in the range
361.7 oC - 370 oC.
The results of these physical properties suggest that this is a very high
molecular weight polymer with high thermal stability. Formulations for leather
finishing was prepared containing the binder at varied proportions of 125 g,
150 g, 175 g, 200 g and 250 g and was applied on the leather substrates
corresponding to samples A1, A2, A3, A4, and A5 respectively. Tests on some of
the physical properties of these coated samples were conducted. The water vapour
permeability of the originally retanned (uncoated) leathers was reduced
significantly after the finish was applied. A1 has the lowest permeability at
125 g of the binder in the formulation, while A5 has the highest permeability
at 250 g of the binder in the formulation. Generally, the water vapour
permeability of the coated leathers increases as the factor varied in this
experiment was increased. A3 had the highest Shore A value at 175 g of the
binder in the formulation while A5 has the lowest Shore A value at 250 g of the
binder in the formulation. Distension and Bursting strength of the uncoated
leathers was improved after the leathers were coated. However, there was no
particular trend in effect as the quantity of the binder in the finish
formulation increased. The fastness of the coated samples generally increased
as the quantity of the binder in the finish formulations was increased with
sample A5 having the best resistance to wet rub action.
TABLE OF CONTENTS
Table of Contents
List of Abbreviations/Symbols
1.1 The Chemistry and Development of Finished Leathers
1.2 Statement of the Research Problem
1.3 Research Aim and Objectives
2.0 LITERATURE REVIEW
3.0 MATERIALS AND METHODS
3.1.1 Experimental equipments
3.1.2 Finishing consumables
3.2.1 Viscosity measurement of the resin binder
3.2.2 Melting point determination of the resin binder
3.2.3 Preparation of leather substrate
3.2.4 Preparation of the finish formulations
3.2.5 Application of the finish formulations on the prepared
3.2.6 Water vapour permeability test
3.2.7 Lastometer test
3.2.8 Shore A (o) hardness test
3.2.9 Wet rub fastness test
4.1. Examination of Resin Binder
4.1.1 Viscosity and molecular weight measurement of the resin
4.2 Physical Testing of the Finished Leather
4.2.1 Water vapour permeability of leather
4.2.2 Distension and bursting strength
4.2.3 Shore A (o) hardness of finished leather and melting
point of binder
4.2.4 Wet rub fastness
6.0 CONCLUSION AND RECOMMENDATIONS
– Fourier Transform Infrared-Spectroscopy SLTC – Society of Leather
Technologists and Chemists Wvp
-- Water Vapour Permeability
A1-A5 – Codes
representing five samples of coated leathers B1-B5 – Codes representing five
samples of uncoated leathers
Acrylics are esters of acrylic acids, which are the products
formed by the reaction of an acrylic acid and alcohol. The esters of acrylic
acid polymerise readily to form exceptionally clear plastics. These are widely
used in applications requiring clear durable surfaces, e.g. in the aircraft and
automobile industries. In more common use are surface coatings involving
acrylics. The physical properties of acrylics (such as gloss, hardness,
adhesion and flexibility) can be modified by altering the composition of the
monomer mixture used in the polymerisation process. Acrylics are used in a wide
range of industries, and the list below is simply a selection of some of the
more common examples: Adhesives, textile industry (e.g. making sponge fill used
jackets), paper coatings, paint
industry particularly in paints used for road markings.
The polymerisation process proceeds readily in the presence
of catalysts and may be carried out in any one of four different ways:
emulsion, bulk, solution or in suspension.
Emulsion polymerisation occurs in a water/monomer emulsion
using a water-soluble catalyst. Emulsion polymerisation is the main process
used in the production of acrylic polymers.
Bulk polymerisation is carried out in the absence of any
solvent. The catalyst is mixed in with the monomer and the polymerisation is
then left to occur with time. This is the method commonly used to manufacture
Solution polymerisation is carried out in a solvent in which
both the monomer and subsequent polymer are soluble. Only low molecular weight
polymers can be manufactured by this process, as high molecular weight polymers
cause very high viscosities.
Suspension polymerisation is carried out in the presence of
a solvent usually water, in which the monomer is insoluble, and in which it is
suspended by agitation. To prevent the droplets of monomer from coalescing and
also to prevent the polymer from coagulating, protective colloids are added.
Suitable colloids include bentonite, starch, polyvinyl alcohol and magnesium
silicate. In contrast to emulsion polymerisation the catalyst is
monomer-soluble and is dissolved in the suspended droplets. The polymers are
manufactured from monomers that are formed from the reactions of acrylic acids
with alcohols. These are then polymerised using a radical initiator in a water
following components are needed for the emulsion polymerisation reaction:
are prepared by a reversible reaction between an acrylic acid and an alcohol:
CH2= CR-COOH +
CH2=CR-COOR' + H2O
The major monomers used are ethyl acrylate, methyl
methacrylate and butyl acrylate, as well as non-acrylic monomers such as vinyl
acetate and styrene which behave similarly.
A surfactant is a substance composed of mutually repellent
polar and non-polar ends. The surfactant surrounds each monomer droplet with a
layer of surfactant with the polar tails oriented towards the surrounding water
thus forming a micelle.
Water is used as the medium to disperse these micelles.
During the process the water acts as a solvent for the surfactants and
initiators, as well as a heat transfer medium.
The initiators (catalysts) usually used are water soluble
peroxidic salts such as ammonium or sodium peroxydisulfate. The reaction can be
initiated either by thermal or redox initiation.
thermal initiation the peroxydisulfate dissociates to give two SO4-radicals.
In redox initiation a reducing agent (usually Fe2+or
Ag+) is used to provide one electron,
causing the peroxydisulfate to dissociate into a sulfate radical and a sulfate
Fe2+ + -O3S—O—O—SO3- → Fe3+ + SO4- • + SO42-
The emulsion polymerisation process can be carried out in a
reaction kettle, which is fitted with a jacket for heating and cooling to allow
control of temperature during the reaction.
Surfactant and water are first charged into the kettle. The
monomer emulsion and initiator solution (containing redox agents to split the
persulphate into sulphate radicals) is then transferred from the monomer feed
tank into the kettle at a controlled rate. The mixture in the kettle is
constantly agitated while the monomer is being added. During this time the
monomer polymerises in accordance with the reactions given below:
.SO4 + CH2=CRCOOR‘
RC. COOR‘ |
SO CH C(COOR') +CH=RC
CH -RC(COOR')CH 2
RC. COOR' 4 2
Once the reaction has proceeded
far enough to use up all the available polymerisation sites, the contents of
the kettle are transferred to the stainless steel blend tank. The batch is then
cooled, adjusted and transferred to holding tanks for storage and subsequent
packing. The quality of the final product depends on the control exercised
during the production process. Routine quality control checks of the following
properties are carried out throughout the manufacturing process:
One of the most important tests of the finished polymer is
determining its 'glass transition temperature', which is a measure of its
toughness. This is done by heating the polymer at a constant rate and measuring
its temperature. When a graph of time against polymer temperature is plotted,
there will be points where the graph is flat, i.e. the polymer is being heated
but it is not getting hotter. At these points the plastic is undergoing some
sort of phase change between two different solid phases, and the heat energy is
being used to rearrange the structure of the material rather than to simply heat
it. Where these transitions occur and how many there are affects the toughness
of the plastic.
Leather is made from hides and skins of animals. Large
animals such as cattle have hides, small animals such as sheep have skins. The
skin of any animal is largely composed of protein referred to as collagen, so
it is the chemistry of this fibrous protein and the properties it confers to
the skin with which the tanner is
most concerned. Leather making is a traditional industry, which has been in existence
since time immemorial, certainly over 5000 years, because the industry was
established at the time of the Hammurabi Code (1795- 1750 BC) when Article 274
laid down the wages for tanners and curriers (Reed, 1972). Indeed, the use of
animal skins is one of man‘s older technologies, perhaps only predated by tool
making. In the modern world, the global leather industry exists because meat
eating exists. Hence, most leather made around the world comes from cattle,
sheep, pigs and goats. One of the byproducts of the meat industry is the hides
and skins: considering that the annual kill of cattle alone is of the order of
300 million, such a byproduct, amounting to 10-20 million tonnes in weight,
would pose a significant environmental impact if it were not used by tanners.
Skin is primarily composed of the protein collagen and it is
the properties and potential for chemical modification of this protein that
offer the tanner the opportunity to make a desirable product from an
unappealing starting material, allowing it to be converted into a product that
is both desirable and useful in modern life.
Collagen is a generic name for a family of at least 28
distinct collagen types, each serving different functions in animals,
importantly as connective tissues (Bailey and Paul, 1998; Comper, 1996; Kichy et
al., 1993; and Kadler, et al., 2007). The major component of skin is
type I collagen. Unless otherwise specified, the term ‗collagen‘ will always
refer to type I collagen.
Collagens are proteins, i.e. they are made up of amino
acids. They can be separated into alpha amino acids and beta amino acids. Each
one features a terminal amino group and a terminal carboxyl group, which become
involved in the peptide link, and a side chain attached to the methylene group
in the centre of the molecule.
In terms of the leather making,
some amino acids are more important than others, since they play defined roles:
the roles of importance are either in creating fibrous structure or involvement
in the processing reactions for protein modification. Amino acids create
macromolecules, proteins such as collagen by reacting via a condensation
An important part of the structure of collagen is the role
of water, which is an integral part of the structure of collagen and hence
their chemically modified derivatives (Bienkiewicz, 1990). Privalov, (1982)
believed that the hydrogen bonding by water at hydroxyproline is important in
stabilizing collagen but thought that the Ramachandran model (Ramachandran and
Ramakrishnan, 1976) could not solely explain the high denaturation energy-
rather the stabilization probably included wider layers of water. Privalov
stated that having in mind the tendency of water molecules to cooperate with
their neighbours; it does not seem improbable that the hydroxypropyl can serve
as an initiator to an extensive network of hydrogen bonds. This envelops the
collagen molecule and might be responsible for the exceptional thermodynamic
properties of collagen.
Leather is used for various purposes including clothing, bookbinding, leather wallpaper, and as
a furniture covering. It is produced in a wide variety of types and styles and
is decorated by a wide range of techniques. Several tanning processes transform
hides and skins into leather:
Vegetable-tanned leather is tanned using tannins and other
ingredients found in different vegetable matter, such as tree bark prepared in bark mills, wood, leaves, fruits and roots and other similar
sources. It is supple and brown in color, with the exact shade depending on the
mix of chemicals and the color of the skin. It is the only form of leather
suitable for use in
leather carving or stamping. Vegetable-tanned leather is not stable in water;
it tends to discolor, so if left to soak and then dry it will shrink and become
less supple, and harder. In hot water, it will shrink drastically and partly
gelatinize, becoming rigid and eventually brittle. Boiled leather is an
example of this, where the leather has been hardened by being immersed in hot
water, or in boiled wax or
similar substances. Historically, it was occasionally used as armor after hardening, and
it has also been used for book binding.
Chrome-tanned leather, invented
in 1858, is tanned using chromium
sulfate and other salts of chromium. It is more
supple and pliable than vegetable-tanned leather and does not discolor or lose
shape as drastically in water as vegetable-tanned. It is also known as wet-blue
for its color derived from the chromium. More esoteric colors are possible
using chrome tanning.
Aldehyde-tanned leather is
tanned using glutaraldehyde
or oxazolidine compounds.
This is the leather that most tanners refer to as wet-white leather due to its
pale cream or white color. It is the main type of "chrome-free"
leather, often seen in automobiles and shoes for infants. Formaldehyde tanning
(being phased out due to its danger to workers and the sensitivity of many
people to formaldehyde) is another method of aldehyde tanning. Brain-tanned
leathers fall into this category and are exceptionally water absorbent.
Brain tanned leathers are made
by a labor-intensive process which uses emulsified oils, often those of animal
brains. They are known for their exceptional softness and their
ability to be washed. Chamois
leather also falls into the category of aldehyde tanning and, like brain
tanning, produces highly water-absorbent leather. Chamois leather is made by
using oils (traditionally cod oil) that oxidize easily to produce the aldehydes
that tan the leather to make the fabric the color it is. Rose tanned leather is
a variation of vegetable oil tanning and brain tanning, where pure rose otto replaces the
vegetable oil and emulsified oils. It has been called the most valuable leather
on earth, but this is mostly due to the high cost of rose otto and its
labor-intensive tanning process.
Synthetic-tanned leather is
tanned using aromatic polymers such
as the Novolac or Neradol types
(syntans, contraction for synthetic tannins). This leather is white in
color and was invented when vegetable tannins were in short supply during the Second World War. Melamine and other
amino-functional resins fall into this category as well, and they provide the
filling that modern leathers often require. Urea-formaldehyde resins
were also used in this tanning method until dissatisfaction about the formation
of free formaldehyde was realized.
Alum-tanned leather is transformed
using aluminium salts mixed with a variety
of binders and protein sources,
such as flour and egg yolk. Alum-tawed leather is technically not tanned, as
tannic acid is not used, and the resulting material will revert to rawhide if
soaked in water long enough to remove the alum salts.
Tanning by strict definition is the conversion of a putrescible
organic material into a stable material that resists putrefaction by spoilage
bacteria. Some of the features of tanning expected include the following:
Appearance: dried raw pelt is
translucent but dry tanned leather is opaque and/or may change in colour e.g.
Handle: some degree of softness in comparison to dried raw
Smell: some tanning agents will
introduce smell, e.g. cod oil for chamois leather, extracts of plant materials
for vegetable tanned leather or aldehyde compounds;
Rise in denaturation temperature;
Resistance to putrefaction by microorganisms;
A degree of performance to the changes.
The traditional way of thinking about tanning is based on
the idea that the tanning agent confers stability to the collagen by changing
the structure through crosslinking and thereby preventing the helices from
unraveling: here is the first indication that a new concept of tanning is
required, in which the function of the tanning agent is to prevent shrinking occurring
by altering the thermodynamics of the process. The basis of the chrome tanning
reaction is the matching of the reactivity of the chromium (III) salt with the
reactivity of the collagen. The availability of ionized carboxyls varies over
the range pH 2-6. This the reactivity ranges of collagen, since the metal salt
only reacts with ionized carboxyls: the rate of reaction between chromium (III)
and unionized carboxyls is so slow it can be neglected (Geher-Glucklich and
Chromium (III) salts are stable in the range pH 2-4, where
the basicity changes, but at higher values they will precipitate. The
development of modern chrome tanning went through three distinct phases:
Single bath process: the original
process used chrome alum, Cr (SO4)3.K2SO4.24H2O,
applied as the acidic salt, typically giving pH ≈ 2 in solution. Following
penetration at that
pH, when the collagen is unreactive, the system is basified to pH ≈ 4, with
alkalis such as sodium hydroxide or sodium carbonate to fix the chrome to the
Two bath process: in this process
which was an alternative approach to the single bath process, the technology of
making chromium (III) tanning salts was conducted in situ to achieve a more
astringent and efficient tannage. This means that the process is conducted in
two steps. The pelt is saturated by chromic acid in the first bath, and then it
is removed, usually to stand overnight. At this time, there is no reaction,
because Cr (VI) salts do not complex with protein. The pelt is then immersed in
a second bath, containing a solution of a reducing agent and enough alkali to
ensure the final pH reaches at least 4. At the same time, processes were also
devised that combined both valencies of chromium, exemplified by the Ochs‘
process (Ochs et al., 1953). However the dangers of using chromium (VI) drove
change back to the single bath process. Not least of these considerations was
the incidence of damage to workers by chromium (VI) compounds: the highly
oxidizing nature of the reagents typically caused ulceration to the nasal
Single bath process: with the
development of masking to modify the reactivity of the chromium (III) salt and
hence its reactivity in tanning, the global industry universally reverted to
versions of the single bath process.
Chromium is a 3d44s2
element, so chromium (III) compounds have the electronic configuration 3d3,
forming octahedral compounds. The hexaquo ion is acidic, ionizing as a weak
acid or may be made basic by adding alkali. The hydroxyl species is unstable
and dimerises, by creating bridging hydroxyl compounds because the oxygen of
the hydroxyl confirm a dative bond via a lone pair. This process is called
olation. It is a rapid, but not immediate reaction.
work of Bjerrum, 1910, tells us how we can know that there `are hydroxyl
bridges in these complex molecules.
reviewed the chemistry of chromium (III) complexes from the point of view of
the leather industry. Some of his more important observations can be summarized
The half-life of water exchange in
the Cr (III) ligand field determined by isotopic exchange of 180 is 54hrs at 27oC:
this is an associative interaction. This can be compared to other metals,
including Al (III), which have half-lives of the order of 10-2s:
these are dissociative interactions. This difference between associative and
dissociative complexation has implications not only for the stability of Cr
(III) complexes, but also for the role of Al(III) in tanning technology.
The diol complex constructed from
the two hydroxyl bridges was assumed to be the preferred form of the chromium
dimer. It was also argued that the trimer is the linear version of the bridged
The stability of transition metal
complexes can be discussed in terms of thermodynamic stability and kinetic
stability. Chromium (III) complexes with carboxylates are thermodynamically
less stable than some other complexes, such as amines, but they are kinetically
The mechanism of exchange between
ligands into an octahedral complex depends on the stability of the intermediate
crystal field; either five- coordinate square pyramidal (SN1
mechanism) or seven- coordinate pentagonal bipyramid (SN2
mechanism). From the
calculation of the crystal field
activation energies, both mechanisms exhibit high values, with a higher value
for the mechanism involving seven- coordination. Whichever is the actual
dominating mechanism, the high activation energies explain the complexes are kinetically stable.
The stability of complexes between Cr (III) and carboxylates
is inversely proportional to
the dissociation constant of
the carboxylic acid. This was first proposed by Shuttleworth (1954). A plot of
log KCrL against log KLH
does indicate a good correlation (Chemical Society Special Publication, 1964;
Tsuchiya, et al., 1964 & 1965).
The formation of chelate complexes
is favoured compared to complexes with monobasic carboxylates.
Olation occurs at the trans
positions because the rate of ionization of the aquo ligand is faster (Irving
and Williams, 1953).
Although the modern process is conventionally referred to as
‗chrome tanning‘, the reaction is most commonly conducted with basic chromium
(III) sulfate, as the commonest reagent used in the global leather industry.
The importance of that caveat lies in understanding the roles of every
component of the salt and in reviewing the alternative options.
The reasons for the popularity of the process are clear,
when the features of the process are compared with vegetable tanning:
The process time for the chrome
tanning reaction itself is typically less than 24 hrs: the vegetable tanning
reaction takes several weeks, even in a modern process.
Chrome tanning confers high
hydrothermal stability: a shrinkage temperature of 110oC
is easily attainable. This opens up new applications, compared with vegetable
tanned leather, where the maximum achievable shrinkage temperature is 85 oC,
depending on which vegetable tannin type is used.
Chrome tanning alters the structure
of the collagen in only a small way: the usual chrome content of fully tanned
leather may contain up to 30 % tannin and hence the handle and physical
properties are inevitably modified, restricting applications of the leather.
Vegetable tanning creates
hydrophilic leather, because of the chemical nature of the plant polyphenols
that constitute the tanning materials, but chrome tanning makes collagen more
hydrophobic, so the tannage allows water resistance to be built into the
Chromium (III) can act as a mordant
(fixing agent for dyes) and its pale colour allows bright deep and pastel
shades (even though the base colour of the leather is pale blue). Tanning with
plant polyphenols has the effect of making the dyeing effect dull, which ever
vegetable tannin or dye types are used- the leather becomes dull (Irving and
Vegetable tanned leather may exhibit
poor light fastness, depending on the type of vegetable tannin, but chrome
tanned leather will retain its colour better.
Chrome tanning in summary, is faster, better in all sorts of
ways and offers more versatility to the tanner, with regard to the leather that
can be made from wet blue, the name given to leather after the chrome tanning
reaction is complete.
In essence, the chrome tanning reaction is the creation of
covalent complexes between collagen carboxyl groups, specifically the ionized
carboxylate groups and the chromium (III) molecular ions. In this way, the
reaction is no different to making any other carboxylate complex, such as
acetate or oxalate, although tanners tend to think of the reaction as fixation
of chrome onto collagen. The one difference between simple complexatiion, such
as acetate and chromium (III), and the typical tanning reaction is the ability
of the reactants to come together. For a simple reaction, the reactants are in
solution and can come together without hindrances, limited only by diffusion.
In the case of the tanning reaction, the substrate has finite thickness, so the
additional parameter of penetration through the cross-section comes into play.
The term ‗post tanning‘ refers
to the wet processing steps that follow the primary tanning reaction. This
might refer to following tannage with chromium (III), as is usually the case in
industry, but equally it applies to vegetable tanning or indeed any other
tannage used to confer the primary stabilization to pelt. The combination of
post tanning processes may not always be the same for all tannages: the choice
of post tanning processes depends on the primary tannage and the type of
leather the tanner is attempting to make. In all cases, post tanning can be
separated into generic processes:
Neutralisation: The process
of deacidification of the excess of free or easily liberated strong acid in
leather, prior to dyeing, retanning and fatliqouring is called neutralization.
Especially from chrome and other mineral tanned leathers this process is of
great importance if consistently satisfactory results are to be achieved in
leather manufacture. Collagen bound acids from the surface of the skins and the
major part of the acid associated with unfixed or the loosely fixed chrome may
be removed by relatively prolonged but simple washing before neutralization. Their
removal by washing before neutralization prevents interferences in dyeing,
fatliquoring or retanning and may be regarded as a part of the process of
neutralization, effecting economy of alkaline salts and time required for
Retanning: This may be a single
chemical process or may be a combination of reactions applied together or more
usually consecutively. The purpose is to modify the properties and performance
of the leather. These changes include the handle, the chemical and hydrothermal
stability or the appearance of the leather. The effects are dependent on both
the primary tanning chemistry and the retanning reactions. Retanning can
involve many different types of chemical
reactions. These include mineral tanning with metal salts (including chromium
(III) applied to chrome tanned leather), aldehydic reagents, hydrogen bondable
polymers, electrostatic reactions with polymers or resins or any other type of
synthetic tanning agent (syntan).
Dyeing: This is the colouring step.
Almost any colour can be struck on any type of leather despite the background
colour, although the final effect is influenced by the previous processes.
Colouring almost invariably means dyeing. Applying dye in solution or pigment,
to confer dense, opaque colour, can be performed in the drum or colouring
agents may be sprayed or spread by hand (padding) onto the surface of the
Fatliquoring: This step is primarily
applied to prevent fibre sticking when the leather is dried after completion of
the wet processes. A secondary effect is to control the degree of softness
conferred to the leather. One of the consequences of lubrication is an effect
on the strength of the leather. Fatliquoring is usually conducted with
self-emulsifying partially, sulfated or sulfonated (sulfited) oils, which might
be animal, vegetable, mineral or synthetic. This step might also include
processing to confer to the leather a required degree of water resistance.
Finished leather reflects all
the operations and processes which have taken place from the stage of flaying
the carcass to the final finishing operations before sorting. It is difficult
and in some cases impossible to correct faults which have occurred in process
prior to leather ‗finishing‘. The term ‗leather finishing‘ relates to those
operations which give the leather its final appearance and make it useful,
attractive and appealing to its users. Finishes on leather also serve as a
Formerly leather finishing was
done by coating varying mixtures of natural dye-woods, mucilages, oxblood, milk
and white eggs on the leather surface. This gave an even and pleasing appearance
to the finished leather. This method of finishing continued for a long time
until the period 1916-1918 when American Leather manufacturers introduced first
pigment finished leathers in the market. The introduction of pigment finishing
created a revolution in the technology of leather finishing and has thus made
it possible to produce leather of uniform appearance even from raw materials
with defective grain.
While in use, leathers are
subjected to various mechanical stress and strain. In order that the finishing
coats on leather can also stand the severe mechanical handling, they should
satisfy the following physico-chemical requirements (Eliseyeva, 1959):
An elongation adequate to maintain
the coating when the leather is stretched to the maximum permissible degree.
A modulus of elasticity in line with the hardness of the
Stability against repeated tensile and compressive strains
Elasticity, ensuring the coating
will return to its original condition when the deforming forces no longer act.
Durability against weathering and ageing
Durability against rubbing on dry and wet leather
Permeability to water vapour and air, ensuring the hygienic
properties of leather.
Finishing is one of the main
processes for the preparation of leather. The application results of the
aqueous polymer dispersions are affected not only by the properties of the
polymer dispersions, but also by the coating conditions. Tanners usually care
very much about the properties of the coating on the surface of leather, such
as softness, touch, toughness, covering grain damage and mending
properties, adhesive force to leather, wet fastness, solvent resistance, flex
resistance(Zhu, 1998; and Price, 2001). This study mainly focuses on the study
of acrylic based finishing formulations suitable for application in leather
1.1 The Chemistry and Development of Finished Leathers
Leather is a collagen- mineral or vegetable tannin substrate
possessing either positive or negative ionic charges respectively. Control of
these surface charges is necessary for retanning, fatliquoring and dyeing
purposes but necessarily the case in their surface coatings. Leather finishes
are used as surface coats to enhance their aesthetics, comfort, softness,
stiffness, flexibility, etc. leather ―dressing‖ as finishing used to be called
was an art, but presently it is a technology that uses guided procedures for
leather surface coatings. It is the duty of ―coating specialist‖ to apply a
well-defined blend of materials that makes up a coating which best accomplishes
the desired effect on the surface in question. Blends or formulations of
surface coatings of synthetic polymers, polyacrylates, polybutadiene and
polyurethanes all in dispersion forms are used as film forming materials. The
superiority of polyurethane dispersions over polyacrylate dispersions is
principally shown, in many cases, in their considerably improved physical
fastness properties. For example, they can be used virtually alone for
finishing high quality leather splits as well as in the manufacture of
extremely soft leathers (Walther, 1988). The dispersed solutions of
polyacrylates, polybutadienes and polyurethanes are used for providing a base
for the top coat, top coatings of leathers and binding colouring materials to
the leather surface. Acrylic resins are dispersions of polyacrylates such as
ethyl acrylate, methyl methacrylate, and methyl acrylate. These dispersed
particles form surface films that tends to allow water gain access to the
interstices, because the hydrophilic group (COOH) accumulates
the surface of the particles. In order to reduce this draw back to the bearest
minimum, crosslinkers or waxes are usually added to the formulation.
There are three
different types of leather finishes which are commonly used by leather
finishers. They are:
Water- type Finishes: these may be
based on pigments, protein binders, such as casein, shellac, gelatin, egg and
blood albumen, waxes and mucilaginous substances like decoction of linseed. The
finishes are mainly used for glazed-finish leathers which are required to be
glazed by glazing machine. The binders in the finish are intended to hold the
pigments or dyes in suspension and be bound firmly on the leather surface.
Softness, glazing properties and handle are contributed by water-soluble
plasticisers, waxes and mucilaginous matters. Water-type finishes based on
pigments, dyes and resin dispersions including urethane are increasingly used
to achieve special effects on the finished leather. The use of such finishes
has produced many improvements over the conventional protein based finishes
such as better adhesion and flexibilty of the finish, improved filling and
sealing properties and greater uniformity of the finish.
Solvent-type Finishes: In contrast
to water-type finishes solvent based finishes contain as a binder either
polyurethane or colloid cotton (nitro-cellulose). These finishes are dissolved
in organic solvents such as butyl acetate, cyclohexanone, etc. these finishes
are widely used for finishing upholstery leather, bag and case leather etc.
solvent finishes based on vinyl resin instead of nitrocellulose have shown
improved resistance to flexing and better flexibilty at low temperature . They
(Shaw, 1952) have been successfully used on upholstery leather, case leather
and certain military leathers where low temperature flexibilty is necessary.
Emulsion-type finishes consists of emulsion of nitrocelluloses or resins. Such
emulsions are being widely used to confer ‗combining properties of water and
lacquer finish‘. Lacquer/emulsion topcoats for upper, garment and glove leather
are gaining wide acceptance (Shaw, 1952).
In leather finishing the three types of finishes mentioned
above can either be used alone or in combination with one another. The choice
depends on the specific effects desired on the finished leather.
The suitability of leather for shoe manufacture is based
upon the twin abilities of being able to exclude water, but allow air and water
vapour to pass through the cross-section of the upper. This is the basis of
foot comfort when shod. The properties are so important that attempts have been
made to mimic them in synthetic materials, by the so-called poromerics of which
to date, none has been successful. One typical example is the failure of Corfam
in the 1960s (Kanigel, 2007). The properties of leather depend on the origin of
the raw materials, how the pelt is prepared for chemical modification, how that
modification is conferred chemically, how the leather is lubricated and finally
how the surfaces are prepared. Leather can be made as stiff and as tough as
wood, as soft and flexible as cloth and anything in between. It is the
traditional art and craft of the leather technologist to control the parameters
and variables of processing to make leathers with defined desired or required
properties. It is from the creativity of the leather scientists that the range
of leathers that can be made is continually widening.
1.2 Statement of the Research
The interest on this study stemmed from the increasing
incidence of worn shoe complaints involving lack of finish fastness, and other
performance defects thus leading to investigation of the influence of finish
components, and impregnating resins. It would be impossible to investigate a
comprehensive range of resin dispersions, thus simple formulations of acrylic
based binder have been selected.
1.3 Research Aim and Objectives
research is aimed at preparing a formulation of acrylic polymer dispersions
suitable for application in leather finishing.
The aim of this research was
achieved through the following objectives:-
Preparation of acrylic polymer formulations.
Retannage of chrome tanned leather.
Application of the prepared formulation on the retanned
Testing of the finished leathers to
determine the water vapour permeability, and other physical properties of the
Among various leather finishing materials, acrylic resin
finishing agents are popular in leather industry due to its good film-forming
performance, good adherence, simple production process and low production cost,
large market share in both product kinds and yields, and has a bright market
prospect and application prospect. The suitability of leather for shoe
manufacture is based upon its twin ability of being able to exclude water and
allow air and water vapour to pass through the cross-section of the upper. This
is the basis of foot comfort when shod (Kanigel, 2007). Therefore the testing
for the water vapour permeability of the finished leathers is necessary.
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