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Content archived on 2024-05-07

New clean process for textile dyeing - supercritical fluid dyeing system avoiding effluents and leading to energy and water savings


This 36-monthly period mainly consisted in looking for suitable formulations for wool and cotton dyeing in supercritical carbon dioxide medium, and adapt them to a larger scale device.
Solubility studies were performed to find suitable dyestuffs. Conventionally used hydrophilic dyestuffs for natural fibres are not suitable for dyeing from SCO2 because of their low solubility in SCO2. However, it was found out that hydrophobic, non-ionic dyestuffs of low molecular size are required to dye natural fibres from supercritical CO2. The highest solubility in this solvent was found for dyes with short alkyl groups. Higher-sized dyestuffs molecules were not or only slightly soluble. The presence of reactive groups in the colouring molecules is helpful to achieve high wet fastness of dyed samples. Besides, the solubility or dispersion of dyestuffs in supercritical CO2 can be achieved by addition of small amounts of modifiers or surfactants.

The routes to settle efficient dyeing recipes in SCO2 medium may be distributed among three alternatives that are the treatment of the textile substrate, the alteration of the dye molecule, and the modification of the supercritical fluid properties. In the first case, a modification of the fibre might improve its dyeability properties, either through a physical treatment of the textile surface, or by a chemical introduction of specific functional groups into the fibre chains. The second possibility that is worth investigating is the modification of the dyestuff with other chromophores or substituents that may lead to new fixation mechanisms and/or new application procedures for better binding properties to the fibres. Finally, the solvent power of the supercritical fluid may be enhanced through a combination with one or several modifiers, and tuned up by pressure, temperature and flowrate, so as to improve dye solubility and level up dye penetration into the fibres.
A good coloration with high levelness was achieved for fibres dyed from supercritical CO2 with selected dyestuffs. The coloration can be enhanced by applying high temperatures (T > 80°C for cotton or 100°C for wool), high CO2 densities, high pressures (p > 250 bar), increasing the time and by addition of a modifier. Applying a fabric pre-treatment or using a fluoro-surfactant may also bring enhancements for cotton dyeing.

Water and methanol are the best modifiers with the restriction that the presence of even small amounts of water at high temperatures (T > 100°C) causes a decrease of the quality of wool.
The whole colour gamut is covered by the studied dyestuffs. Several commercially available dyestuffs were found to be suitable for dyeing cotton and wool from SCO2, like disperse, solvent, disperse reactive and natural dyestuffs. Furthermore, modification of commercially available disperse or solvent dyes into reactive derivatives, and formation of a complex with mordanting agents are two approaches to improve the insufficient fastness properties of conventional disperse dyes.
From the data obtained from laboratory- or pilot-scale SCO2 dyeing experiments and their comparison with the conventional process, it has been proved that the dry route with supercritical carbon dioxide might be viable, once the right dyestuffs are available.
Scientific and technical objectives:
Possible industrial applications of supercritical fluids
technology, which industrial valorisation has already
been made in food industry, has been investigated in
different sectors considering the huge advantages by
comparison with traditional techniques using solvents. In
textile area, research has been done for dyeing of
synthetic fibers with disperse dyes, in supercritical
carbon dioxyde (SC02), leading to a process avoiding
water and effluents.
The objectives of the project are to extend the
applications of this new technology to natural fibers and
blends, using this "supersolvent" not only as a carrier
but also as a reaction medium, e.g. for dyeing process
with vat or reactive dyes for cellulosics and milling or
reactive dyestuffs for wool.
Industrial objectives:
The research involved in this project will allow to
develop SC02 technology within a short delay, giving some
economical advantages contributing to a higher
competitiveness for European textile industry against far
and medium east countries.
As soon as the flexibility of the process has been proved
by several applications in textile industry, e.g.
polyvalency for dyeing different nature of fibers with
different types of colourings, the industrialization
could occur quickly, considering that existing equipments
used in food industry will need only simple adaptations
for the transfer to textile (especially reactor
Industrialisation is already possible for polyester
dyeing with disperse dyes. However the success of such a
new technology will depend especially to it's multi use
possibilities, e.g. dyeing both natural and synthetic
fibers, involving commercial available dyes. More, the
targeted market involving natural fibers is much more
important: natural fibers and mixtures represents a
market share of more than 60 %. Wearing industry: 53%
cotton, 9% wool Worldwide market shares (1994): cotton
45,6%; wool 3,7%; man made fibres: 50,7% (including
technical textiles); PES/wool 9%. Europe: wool: 10 to 12
%; cotton > 25 %.
The use of vat dyes or reactive dyes for cotton and wool
dyeing will necessit a basic research using technical
informations available on chemical reactions in SC02. Lab
scale experiments will be performed by Research
Organisations as to define the process requirements and
thus those of industrial equipment. This work will be
performed together with an equipment manufacturer
(providing SC02 industrial equipment for food industry)
and technical informations and support from two textile
dyes manufacturers. An economical, environmental and
social assessment will be done by end users. Equipment
and process requirements definition will allow to define
the exploitation plans.

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