textile industry faces stiff competition from suppliers in low-wage eastern
economies. It is also disadvantaged by the need to comply with increasingly
stringent environmental legislation. In the three-year SUPERCOLOR project,
a consortium led by the Institut Textile de France studied the dyeing
of natural fibres in a supercritical carbon dioxide fluid (SCO2),
with a view to replacing the more energy-hungry and effluent-generating
conventional aqueous processes.
avenues have been explored to reduce the dependence of the textile industry
on consuming very large volumes of water. The use of organic solvents
has proved largely unacceptable for a number of reasons, while work continues
on improvements to the ecological and economic performance of established
aqueous processes. But total elimination of water usage would be the ideal
solution - and this could be achieved by switching to SCO2.
CO2 gas is subjected to high pressure at temperatures above
its so-called 'critical point', it forms a supercritical fluid with
an unusual combination of properties that make it a versatile solvent
and reaction medium. It is already used in the food industry and
other sectors, for example to extract caffeine and deasphalt petroleum
residues. Dyeing of polyester fabrics in SCO2 has also
been demonstrated on a semi-industrial scale. Dyestuffs or other
additives dissolved in the pressurised SCO2 impregnate
the fibre structures and are deposited when the reaction vessel
is depressurised. This method is particularly environment-friendly,
as the gaseous CO2 can be recycled with minimal losses,
and any unused dye is recovered as a reusable solid residue. The
process is faster than water-based dyeing, because of higher diffusion
of this medium and because no drying stage is involved. It is also
extremely flexible, as temperature and pressure variations easily
enable the properties of the fluid to be adjusted, without changing
the solvent itself. The ability to vary production conditions at
will would lend itself to the production of smaller batches, in
line with today's mass customisation and just-in-time delivery philosophies.
savings and added value
objective of SUPERCOLOR was to examine the possibilities for extending
the process to natural fibres - wool, cotton and their blends with
polyester - which together make up some 60% of the European textile
market. This would bring valuable economies through an estimated
energy saving of 15-20% and elimination of the increasingly costly
treatment of around 100 m3 of waste water for every tonne of textile.
In addition, it offers the potential to create novel products with
higher added value, by introducing ingredients such as perfumes,
fireproofing agents, waterproofing agents, stain-repellents and
this basic research project, completed in January 2000, the participants
focused on the search for dye formulations that would function satisfactorily
in SCO2 and be suitable for eventual use on an industrial
scale. They also examined a number of process variables, such as
fibre pre-and post-treatment, alteration of the dye molecules and
modification of the fluid's properties.
team included research institutes, an equipment manufacturer and
end users - thus providing the spread of expertise needed to reconcile
the theoretical and practical considerations.
on early laboratory experience, Swedish partner Chematur Ecoplanning
constructed a pilot unit with a 7-litre dyeing chamber suitable
for connection to any standard SCO2 extraction system.
Operating at pressures up to 400 bar and a maximum temperature of
150°C, this was then used for further trials.
existing wool and cotton dyes were found to be insufficiently soluble
in the supercritical fluid, overall results confirm the feasibility
of industrialising such a process for most popular textiles.
variants of disperse, solvent and natural dyes all proved capable
of producing intense, even tinting, with reactive disperse types
offering the best colour-fastness. For wool/polyester blends, the
use of both reactive disperse and conventional disperse dyestuffs
was necessary to obtain adequate penetration into the two fibre
the pressure, temperature, fluid density and treatment time improved
the depth and consistency of shades, as did modification of the
SCO2 with small quantities of methanol or water. Variations
in parameters such as the mode of agitation, speed of depressurisation
- and even the arrangement of ingredients within the chamber - also
to project co-ordinator Denis Couvret of the Institut
Textile de France, full-scale equipment could readily be developed
from currently available systems. However, further research in collaboration
with a dyestuff manufacturer would be necessary to develop an adequate
colour range and ensure the universal applicability of SCO2
processing. In the context of growing environmental regulation,
this technology could then give European industry the edge in innovative,
cost-competitive textiles for tomorrow's markets.
European textile industry is getting a boost from improvements
in manufacturing technology thanks to projects sponsored under
the Growth Programme's Innovative products,
processes and organisation key action.
: New clean process for textile dyeing - supercritical fluid dyeing
system avoiding effluents and leading to energy and water savings