EFFECT OF ACRYLIC POLYMER DISPERSIONS ON WATER VAPOUR PERMEABILITY AND SOME OTHER PHYSICAL PROPERTIES OF FINISHED LEATHERS
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 4.03×105. 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 leather substrate
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
FT-IR – 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 in padded 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 acrylic sheets.
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 emulsion.
The following components are needed for the emulsion polymerisation reaction: Monomer
Monomers are prepared by a reversible reaction between an acrylic acid and an alcohol:
|CH2= CR-COOH +||R’ OH||CH2=CR-COOR’ + H2O|
|Acrylic acid||alcohol||alkyl acrylate|
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.
Surfactant: A surfactant is a substance composed of mutually repellent polar and nonpolar 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: 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.
Initiator: 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.
In thermal initiation the peroxydisulfate dissociates to give two SO4–radicals.
|–O3S—O—O—SO3– →||2SO4– •|
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 ion:
Fe2+ + –O3S—O—O—SO3– → Fe3+ + SO4– • + SO42-
Peroxydisulfate sulfate radical
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‘||–OSO3||CH2||RC.COOR‘|
|SO CH C(COOR’) +CH=RC||(COOR’)|
–SO 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:
- Solids content
- Gel levels
- Residual monomer
- Mechanical stability
- Freeze/thaw stability
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 process.
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 tannage:
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 bookbinding.
- Chrome tanning: 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 tannage: 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.
- Oil tannage: 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.
- Syntans: Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or Neodol 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.
- Aluminium tannage: 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. chrome tanning;
- Handle: some degree of softness in comparison to dried raw pelt.
- 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 Beck, 1971).
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 collagen.
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 septum.
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.
The work of Bjerrum, 1910, tells us how we can know that there `are hydroxyl bridges in these complex molecules.
Irving 1974 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 as follows:
- 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 structure:
- 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 more stable.
- 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 leather.
- 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 Williams, 1953).
- 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 complexation, 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 fatliquoring 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 neutralization.
- 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 leather.
- 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 (sulfated) 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 protective coating.
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 coating.
- Stability against repeated tensile and compressive strains and bending.
- 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
- High adhesion
- 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 finishing.
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 on the surface of the particles. In order to reduce this drawback 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 flexibility 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 (nitrocellulose). 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 flexibility at low temperature . They (Shaw, 1952) have been successfully used on upholstery leather, case leather and certain military leathers where low temperature flexibility is necessary.
- Emulsion-type Finishes: Emulsion-type finishes consists of emulsion of nitro celluloses 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 Problem
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
This 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.
- Retanning of chrome tanned leather.
- Application of the prepared formulation on the re-tanned leather.
- Testing of the finished leathers to determine the water vapour permeability, and other physical properties of the leathers.
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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