Colorant manufacture problems
Over the past two decades colorant manufacture has progressively moved out of Europe and the USA into Asia, especially China and India as well as to Korea and Taiwan. The strong legislatory and regulatory stance adopted within Europe has militated against colorant manufacture there, and as textile fibre and fabric production moved into Asia the colorant manufacturing industry also moved to be closer to the sources of textile production.
The massive investment in dye and pigment manufacturing capacity in China resulted in economies of scale, lowering production costs, and as global competition intensified the purchase price per kilo of dyes and pigments fell, especially for commodity products produced on a massive scale.
However in 2014 textile dyers and printers are entering into a new phase where dye and pigment prices are rising and set to rise further.1,2 The major reasons for this lie in the increasingly strict approach being taken in China and India to decrease the environmental pollution created by colorant manufacture, and also to the current and future shortfall in the production of the chemical intermediates necessary to synthesise organic colorants. This will lead to production caps for a number of colorants, and rising prices for intermediates.
For the dyer of protein fibres such as wool and silk there are likely to be supply shortages in the acid- and reactive-dye ranges which are widely used for dyeing many types of protein fibre materials, as well as significant price rises which will affect the textile dyers' profit margin.
As retailers continue to strive to reduce costs within their manufacturing chains many dyers and printers will find themselves trapped starkly between a rock and a hard place. Either their dyeing and printing costs will have to rise or, potentially, dyeing and printing companies may cease trading or move into more-profitable lines of business in order to survive.
Colorant manufacturers will have to invest to minimise environmental pollution in order to stay in business as compliance with environmental regulation is tightened. As a result cleaner and ‘greener’ production practices will emerge, but at a cost, as dye makers seek to maintain a sound business model that will invest in colorant research and development (R&D). If dye makers cease to invest in R&D then innovation in colorant manufacture will be stifled and technological development in the textile dyeing and printing industry will stagnate.
 |
Silk fibres are obtained from the cocoon of theB mori silkworm. Picture, Oxford Silk Group. |
Wool and Silk Production
The diameter of wool fibres determines the end use and the value, with fine wools (37%), medium wools (22%) and coarse wools (41%) being produced by over a billion sheep in around 100 countries, to give an estimated total annual production of around 2.1 million tonnes.3 The world production of clean raw wool was 1,102,064 tonnes in 2012.4 Some two-thirds of the wool is used in garments and about one third in carpets, upholstery and rugs. Within all the wool end-uses, the industrial uses of wool are considered to account for some 5% of the total.
The major wool producers are Australia, Argentina, China, India, Iran, New Zealand, Russia, South Africa, United Kingdom and Uruguay. As the production of manufactured fibres has increased, wool's share of the world fibre market is now probably around 2% or less.
Sericulture (the production of silk) is spread over some 60 countries, with the major producers being in Asia. Asia produces some 90% of mulberry silk and almost 100% of non-mulberry silk.5 Although silk probably constitutes only around 0.2% of the global textile market it is highly labour-intensive. Thus there are about one million workers in China's silk sector, while the silk industry provides employment to 7.6 million people in India and to 20,000 weaving families in Thailand.
China, followed by India, is the largest silk producer. Other silk-producing countries are Uzbekistan, Brazil, Japan, Republic of Korea, Thailand, Vietnam, DPR Korea and Iran. However the main silk-consuming countries are USA, Italy, Japan, India, France, China, United Kingdom, Switzerland, Germany, UAE, Korea and Vietnam.
New Textbooks on Coloration of Protein Fibres
In association with the Society of Dyers and Colourists (SDC), John Wiley has published an excellent, authoritative textbook on the coloration of wool and other keratin fibres, edited by Emeritus Professor David M. Lewis and Dr John A. Rippon.6 This textbook is a major, comprehensive contribution to the coloration of wool and hair (see book review p15).
Aspects of the coloration of silk fibres are included in a book published by Woodhead Publishing, authored by K. Murugesh Babu, on the processing, properties and applications of silk (p15).7 The use of new technologies in the dyeing of protein fibres constitutes a chapter by Riza Atav in the book on ‘Eco-friendly Textile Dyeing and Finishing’, edited by Melih Günay and published by InTech Europe.8
Recent Research on Coloration of Protein Fibres
In order to avoid the problems associated with heavy-metal content in metal-complex dyes and with ammonia aftertreatment of reactive-dyed wool in order to achieve high colour fastness, Portuguese research workers have proposed the application of nanopigments for dyeing wool ecologically.9 Ionic nanopigments were prepared with silica and anionic dyes by a sol-gel process and applied to wool by an exhaustion method.
Previous work had shown that dyeing with nanopigments of silica could achieve light colours on wool, but the use of ionic nanopigments enabled darker colours to be achieved, with higher diffusion inside the wool fibres and similar exhaustion levels to that obtained with reactive dyes.
A linear relationship was established between absorption and concentration for the nanopigments, enabling the exhaustion to be calculated. The colour fastness achieved was similar to that obtained for medium-dark colours using reactive dyes without any need for an alkaline aftertreatment.9
The effects of UV radiation on the dyeing of wool with acid dyes and with 1:1 metal-complex dyes has been studied by Italian research workers.10,11 The work on UV pretreatment followed by acid dyeing at 70°, 80° and 90°C demonstrated that UV radiation pretreatment on the substrate, for five minutes at low power (50W/cm2) or for 30 seconds using a high-power lamp (900W/cm2), imparted faster dyeing kinetics and a higher final-bath exhaustion for wool dyed at 80°C compared with untreated wool dyed at 90°C.10
UV-radiated samples gave the best colour-fastness results in wet/dry rubbing tests and domestic laundering. It was concluded that UV radiation was a valid ecofriendly pretreatment that could enable wool to be dyed at a temperature 20°C lower than in conventional dyeing.
The research work on 1:1 metal-complex dyes used a medium-pressure UV lamp that generated oxidised species (eg. cystine oxides and cysteic acid), mainly in the cuticular region of the wool fibre (ie. epi-, exo- and endocuticle) as well as in the wool cortex. UV-treated wool showed a decrease of water-contact angle, then an increase in fibre hydrophilicity coupled with better dye-absorption kinetics. Using selected 1:1 metal-complex dyes, the maximum colour difference between UV-radiated and non-irradiated areas was evaluated.
The main interest on wool fabrics was focused upon obtaining one colour, double face, same shade but different depth (eg. higher on the UV-irradiated side) or two shades, double face using different colours. The colour fastness values obtained were similar to those obtained via conventional premetallised 1:1 dyeings.
An interesting approach to the dyeing of wool-polyester blend fabrics has been taken by Iranian research workers who have studied the application of novel mono azo-napthalimide dyes containing both polar and nonpolar groups in a one-bath, single-stage dyeing method.12 Dyes containing the butyric acid group and with N-ethyl-n-(2-hydroxyethyl)-aniline, N,N-diethylaniline and N,N-diethyl-m-toluidine substituents showed reasonably good buildup (K/S>8) and gave red to bluish-red dyed fabrics. Dyed at pH 4 at temperatures up to 100°C, the colour fastness to wet/dry rubbing and perspiration fastness (alkaline/acid) were medium to excellent, but the colour fastness to light was only poor-moderate (3-4).
 |
| Wool ComfortMeter |
Research on Finishing of Protein Fibres
Silk fabric has been made more flame retardant by surface grafting of a flame retardant (dimethyl-2-(methacryloyloxyethyl) phosphate, initiated by the use of UV radiation at 254nm, and a photocatalyst, namely methyl 2-benzoylbenzoate.13 Various physical methods of analysis demonstrated the presence of surface grafting, which lowered the initial thermal decomposition temperature by 41°C as well as decreasing carbon-monoxide emissions and promoting solid and rigid char formation. In addition, the heat release rate was also decreased by this flame-retardant treatment.
A special issue of the Journal of the Textile Institute has been devoted to covering research on the comfort and photostability of wool.4 Of particular interest is the comprehensive review on the tactile properties of fabrics, with a special focus on next-to-skin, knitted wool fabrics.15
The avoidance of prickle in wool fabrics is an important topic and work using the Wool ComfortMeter has examined this phenomenon.14 The Wool ComfortMeter scans the surface of a wool fabric, registering the signals triggered by fabrics protruding from the fabric surface that exceed a threshold in bending stiffness.
Such fibres are capable of triggering nerves in the skin and there is thus a good correlation between the Wool ComfortMeter results and fibre-evoked prickle sensations. Clearly, such an evaluation method could have many applications for the evaluation of surface finishes on wool fabrics.
Future Developments in Dyeing and Finishing Protein Fibres
Because of the relatively small size of the silk and wool markets compared with cotton and manufactured fibres, there have been few commercial developments in the dyeing and finishing of these fibres over the last year. Indeed it is likely that coloration research in the future will concentrate upon polyester and cotton fibres, so that coloration and finishing developments in wool and silk will result as a spin-off from research on other fibres, eg. acid dyes for polyamides, reactive dyes for cellulosics, and from general research on textile finishes.