Developments in leather coloration and the twin challenges of fastness and levelness:
Leather is a by-product of the meat, wool and dairy-products industry and is hide or skin that has been preserved by tanning and retains its original fibre structure intact.
The hide or skin may have been split into layers or segmented either before or after tanning and if the leather has a surface coating this must be less than 0.15mm thick.
Leather is a renewable natural resource that is part of a large global trading chain, which encompasses trading in hides, skins, leather and leather footwear. The annual global production of leather amounts to some 20 billion square feet, worth over US$40 billion. The major end uses for leather, according to the UK Leather Federation, are footwear (55%), furniture (20%), garments and gloves (15%) and other uses (10%). However, individual countries vary: for example, in the UK more leather is used in furniture and automotive uses than in shoes.
Asia is now a major centre for leather manufacture and the production of leather goods. The Council for Leather Exports in India has recently estimated the global leather trade at a higher value of US$116 billion, with China’s share around US$25 billion (22%) and India at US$3.5 billion (3%). China has been more aggressive than India in acquiring markets and brands in the USA, and India is now scouting for such opportunities in the USA, which accounts for 9% of Indian leather exports.
In leather garments, the total trade volume of around 120 million pieces is mainly supplied by China (70 million pieces), followed by India (16 million pieces). Within the European Union many tanneries/leatherdyeing establishments have closed down in recent years as a result of competition from Asia, coupled with increasing pressures to limit environmental pollution from their waste water.
Leather manufacture involves a number of different processing steps designed to remove impurities (eg. blood, dung and fibres) in order to prevent putrescence (degradation) of the hide. Pretreatments may include curing, soaking, painting, liming, fleshing, deliming, bating, pickling and degreasing.
However the most important treatment is the next, namely tanning, which converts the protein (collagen I) of the raw hide or skin into a stable material that is resistant to putrefaction and renders it suitable for subsequent treatments such as retanning, fat liquoring, dyeing, drying and finishing.
Tanning of hides and skins has a major influence upon any dyeing process because the tanning materials stabilise the leather by forming chemical crosslinks between the collagen fibres. The crosslinks hold the collagen fibres apart, preventing them from shrivelling and sticking together during drying. Tanning thus stabilises the structure against the action of acids, alkalis, heat, water and the action of micro-organisms. The three principal forms of tanning materials are based upon:
Mineral tannages - principally chrome tanning (based upon Cr III) and also alum. Chrome tanning is the main method of tanning.
Aldehyde and oil tannages - used for very soft leathers, and dry cleanable and washable fashion leathers, and chamois leather.
Vegetable tannages – based upon extracts (eg. sumach, mimosa, etc) These produce brown coloured leathers which are often thick and firm e.g. used for sole leather, belting leather, shoe linings, bags and cases.
Leather Dyeing
Depending upon the tanning materials used, the colour and texture of the leather changes and hence dyeing takes place upon a substrate that differs in its initial colour and its chemical reactivity. Skin collagen is a fibrous protein that consists principally of the amino-acids glycine (26.8%), arginine (15.3%), proline (9.0%), alanine (8.0%) and hydroxyproline (8.0%). The skin consists of very small interwoven collagen fibres each of which is made up of even smaller fibrils.
The basic collagen structure consists of twined triple units of peptide chains of different lengths. The amino-acid residues are held together by peptide links, the triple helices being held together by hydrogen bonds. The dyeing of tanned leather is thus a more difficult proposition than the dyeing of natural protein fibres such as wool and silk, which are more uniform in their dyeability.
Leather is mainly dyed under aqueous conditions in drum dyeing machines at temperatures below 60°C, to avoid deterioration and shrinkage of the leather. Older wooden drums are now increasingly being replaced by polypropylene drums. The Hüni (Switzerland) New Jumbo polypropylene (PPH) drum features a greater loading capacity, up to double the load of a standard drum with the same dimension, but with claimed savings in water (30%) and in energy (50%). Leather coloration is also carried out via spray dyeing, or by pad application methods.
The major colours dyed on leather include black and brown as well as fashion colours. Much leather dyeing is now carried out in Asian countries such as China and India. Black is particularly favoured in India and may account for up to 70% of leather coloration in India.
Leather is based primarily upon the skins of cattle, goat, sheep and pigskins, and leather coloration falls into three main categories:
Aniline leather - the most natural looking leather with the unique surface characteristics of the hide remaining visible. Aniline leather is coloured using only dyes, although a light surface coating may be applied to enhance appearance and protect against spillages/soiling.
Semi-aniline leather - coloured using dyes but given increased durability through a light surface coating containing a small amount of pigment.
Pigmented leather - this is the most durable form of leather used in the majority of furniture upholstery and nearly all automotive upholstery. Durability is imparted by a polymer surface coating that contains pigments. This gives greater resistance to scuffing or fading.
Leather Dyestuffs
BASF is a major player in the leather industry with a wide range of dyestuffs, finishes and performance chemicals. BASF’s Lugafast Colorlock technology utilises azobased, metal-free dyestuffs, free from banned arylamines, that react with the collagen protein chains, forming strong covalent bonds. This enhances the colourfastness properties over conventional anionic dyestuffs that are fixed to the leather via ionic bonds, formed by attraction between the anionic groups on the dye molecules and the cationic sites in the leather.
Lugafast reactive dyestuffs exhibit greater resistance to colour bleeding and to staining of other materials, as well as greater resistance to migration into plasticised PVC (polyvinyl chloride), as assessed according to ISO15701. The wash fastness (ISO15703) and colour fastness to solvents and dry cleaning (ISO11643) are also generally higher than that obtained using anionic dyestuffs.
BASF’s innovative Lugafast Colorlock range comprises six dyestuffs, enabling a very wide range of shades to be matched. The range consists of:
Lugafast Yellow GG
Lugafast Brown BC
Lugafast Red RR
Lugafast Brown BB
Lugafast Blue NB
Lugafast Black AN
The BASF Lugafast Colorlock dyes have undergone extensive environmental/toxicological testing and fulfil the requirements of the SG Label of the PFI (Germany) and the Global Automotive Declarable substance list. The Lugafast Colorlock system thus enables leathers to be dyed to deep shades whilst also satisfying comfort requirements such as high breathability.
BASF also produce Luganil and Lurazol ‘A’ dyestuffs for drum dyeing of high-quality automotive, shoe-upper and upholstery leather. Luganil/Lurazol ‘A’ dyestuffs are environmentally friendly, being free of AOX, VOC, formaldehyde and chrome and offer outstanding migration fastness. A wider range of colours can be drum dyed with Luganil Black AS liquid, Luganil Brown AR, Luganil Brown AG, Lurazol Grey AB and Luganil Beige AF.
In automotive leathers, Luganil Black AS liquid contains no coloured components that migrate over time and is also free of organically bonded halogens, metals, solvents and salts. Thus, leather components dyed with Luganil Black AS liquid can be easily disposed of in accordance with the EU directive on scrap cars.
Clariant BU Leather Services has ensured that its dyestuffs and products for leather are fully in compliance with REACH regulations (EU Directive 1907/2006/EC). Clariant’s Derma Soft dyestuffs are designed to overcome the levelness problems experienced by leather tanners when pale shades are required. Clariant has launched two generations of Derma Soft pre-dispersed dyes, the second generation being characterised by providing double the colour intensity of the first generation. Used for light-medium-intensity shades, the second-generation Derma Soft dyes reach a colour intensity that cannot be achieved by adding twice the amount of the firstgeneration dyes. The new-generation type consist of:
Derma Soft Yellow Brown p
Derma Soft Red Brown p
Derma Soft Beige p
All the Derma Soft dyestuffs are predispersed homogeneous metal-complex dyestuffs, which provide excellent level dyeing properties without the need to add dispersing agents before or during dyeing.
Clariant provides a wide range of Melioderm and Melioderm HF dyestuffs that pass the BMW hydrolysis test and avoid colour migration into the finishing coat. Thus, the colour resistance of Clariant’s specially selected MIF dyestuffs to migration into finishing under very severe conditions is excellent, providing also resistance to heat and yellowing.
Clariant has also introduced Derma Black HC p, a new-generation homogeneous anionic dyestuff, which is the result of a combination of a metal-free acid-black dye with insoluble colorant matter. Used as a single dyestuff, it can be applied to all types of leather to provide a neutral black with good all-round colour-fastness properties, as well as outstanding light fastness. The powerful covering effect of this dyestuff can also be used when it is added in Top Dyeing.
Clariant has also introduced two novel, patented, black cationic dyestuffs for topping of black dyeings on leather, to achieve the highest black intensity. Leather Black C Liquid is engineered to be used on economical or fashionable leather articles. Where garment, aniline and/or vegetabletanned leather requires a black of outstanding light fastness, topping the black dyeing with Clariant’s Floriderm Black CDL Liquid is recommended. Both these dyestuffs are homogeneous black cationic dyestuffs that are metal-free and suitable for top dyeing and the sandwich technique.
DyStar has recently introduced three new Solvaderm dyestuffs, namely:
Solvaderm Brown MF-GO
Solvaderm Grey MC-CC; and
Solvaderm Black MC-FK.
Solvaderm Brown MF-GO is a warm, neutral mid-brown colour with no reddish cast. It provides good penetration, high build-up and is an economical dyestuff, particularly suitable for shoe-upper leather.
Solvaderm Grey MC-CC is a neutral grey with a slightly greenish cast that meets the demanding specifications of the automotive/furniture sectors. This dyestuff is suitable for a wide range of shades, from pale pastels with excellent penetration to a strong middle grey. Solvaderm Black MC-FK is a new, brilliant deep-black dyestuff for full-grain and velour leather and fur. Suitable for penetration dyeing and top dyeing, it provides a neutral black with high-colour fastness properties. A special feature is its suitability for double-face dyeings with wool resist.
Leather Finishing
Extensive ranges of finishes for leather are available from many chemical manufacturers. For example, a wide range of speciality materials based on, for example, cross-linking agents, thickeners, oil, wax, preservatives, cleaners and handle modifiers enable a wide range of properties and performance to be achieved after leather coloration.
Coating of leather is a particularly important field. Polymeric coatings based upon acrylic, polyurethane, butyrate emulsion and nitrocellulose are available, as well as coatings derived from sustainable raw materials rather than petrochemical technology. Polymer binders from various sources are also applied to leather. Leather is a versatile material that can be made water-repellent, stain-repellent and even flame retardant, an important performance criterion for leather used in aircraft.