Advances on Wool Dyeing and Finishing
Nobody knows what the future will bring and how fast. The future of coloured wool may be coloured sheep after all – Chinese scientists in Urumqi have are using already gene editing techniques to alter the coat colours of sheep. However, the wool industry is a very conservative industry and the changes may not be so disruptive anytime soon.
Real and perceived environmental risks, pressure from retailers and legislation, are today the driver for change. The main challenges are metals in dyeing, especially related to chromium (VI), absorbable organic halogen (AOX) in shrink-resist finishing and pesticides in mothproof finishing. These environmental challenges and technical alternatives in wool dyeing & finishing are discussed in this article.
Wool Composition and Dyeing
Wool is an animal fibre made of polypeptides, a biodegradable natural protein fibre. Wool is mainly from sheep hair. Other wool fibres are for example cashmere, angora and alpaca. Further examples of protein fibres are human hair and silk. Proteins are built of amino acids connected to each other by peptide bonds.
The dyeing and finishing of wool is based on several physical and chemical effects such as electrostatic interactions, van der Waals interactions and entropy driven hydrophobic interactions.
Amino groups in wool play an essential role in these interactions. The amino groups can form metal complexes with metals in dyes, form electrostatic interactions between protonated ammonium groups and sulfo groups in anionic dyes, and react with reactive dyes to form covalent bonds.
The amino acids in wool which carry amino functions are lysine (3.1 mol%), arginine (6.8 mol%), histidine (0.9 mol%) and terminal primary amino groups2. In addition, wool has many carbonamido groups in peptide bonds which can form hydrogen bonds with dyes.
Dyes for Colouration of Wool
Wool can be dyed using acid, metal complex or reactive dyes. Some of the main differences between these dyestuff classes are shown in table 1.
Acid and Metal Complex Dyestuffs for Wool
Acid dyes are dyes with conventional chromophores, mostly azo dyes, having one or more sulfo groups to achieve water solubility. Depending on molecular size the substantivity and migration varies; technicians speak of levelling and milling types of acid dyes.
The levelling performance is important for selection of the machinery. Good levelling dyes can be applied in hank dyeing of yarn and piece dyeing on the winch. Dyes with lower migration should be applied on package machinery for yarns and soft-flow jet machines for piece dyeing.
Metal complex dyes are today in most cases based on Chromium (Cr) or Cobalt (Co), mostly Chromium. Metal complex dyes are azo dyes having two OH substituents in ortho position adjacent to azo linkages, being able to form a chelating complex with metals. These complexes are very stable and exhibit high fastnesses. 1:1 and 1:2 complexes are both used, the figures indicating the ratio of metal to dye molecules (Examples see Fig. 3).
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