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Graphic element Research > Growth > Research projects > Products & processes projects > 'Dry' dyeing points way to greener textiles
Graphic element 'Dry' dyeing points way to greener textiles

Europe's 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.

Various 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.

Environment-friendly, versatile

When 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.

Cost savings and added value

The 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 biocides.

In 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.

The team included research institutes, an equipment manufacturer and end users - thus providing the spread of expertise needed to reconcile the theoretical and practical considerations.

  Promising results

Based 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 150C, this was then used for further trials.

While 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.

Reactive 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 components.

Increasing 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 proved relevant.

According 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.

Environment-friendly, versatile
Cost savings and added value
Promising results

Key data

The 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.

Project: SUPERCOLOR : New clean process for textile dyeing - supercritical fluid dyeing system avoiding effluents and leading to energy and water savings (BRPR960252)

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