Methods of this type have been the subject of considerable interest in recent years, since they are particularly suitable from an ecological/toxicological point of view and are technologically practicable at costs competitive with those of conventional methods which provide for the use of water-based dyeing media.

Supercritical carbon hydroxide has been used with success in methods for dyeing synthetic fibres, for example, polyesters and polyamides, but the results of its use in the dyeing of natural fibres, for example, wool, cotton or silk fibres, have not been so satisfactory.

Disperse dyes, which are normally used for dyeing synthetic fibres, are in fact apolar and soluble in supercritical carbon dioxide, thus permitting vivid and fast dyeing of chemical fibres; these dyes do not, however, have an affinity to natural fibres which are essentially polar.

To obviate this incompatibility, it has been proposed (see, for example, M. L. Colombo et al.,"Treatments of cellulosic material with natural products dissolved in SCVF-C02"- Proceeding of the 5th Meeting on Supercritical Fluid, March (1998) 357-360) to impregnate the natural fibres with bulking agents such as thiodiglycols and polyethylene glycols before the actual dyeing treatment, but without completely solving the problem. In fact, although this initially achieves a more satisfactory adhesion of the disperse dye to the fibres, the washings to which the fibres are subsequently subjected nevertheless bring about decolorization thereof.

The object of the present invention is therefore to provide a method of dyeing natural textile fibres with the use of a dyeing medium comprising supercritical carbon dioxide and a dye, which can ensure satisfactory and persistent adhesion of the dye to the fibres.

This object is achieved by means of a method of the type indicated at the beginning of the present description and characterized in that the dye is of the water-soluble, polar type, and in that the dyeing medium also comprises a polar co-solvent.

By virtue of the use of the polar co-solvent, polar dyes can be rendered soluble in supercritical carbon dioxide in which they would otherwise be insoluble, and can thus be brought into contact with the natural fibres for which they have a chemical affinity and on which they can effectively perform their dyeing action which is persistent over time and resistant to washing.

The co-solvent is preferably selected form the group consisting of methanol, ethanol, water and mixtures thereof.

The water-soluble polar dye is preferably of the reactive, direct, cationic, or acid type or the like.

The concentration of the co-solvent in the dyeing medium is advantageously between 2 and 20% and even more preferably between 5 and 10%. These concentrations, as well as all of those given in the remaining part of the present description, in the absence of any indication to the contrary, should be understood as molar concentrations relative to the moles of C02 in the working conditions.

The natural textile fibres which can be dyed by the method of the invention are of substantially any nature, for example, fibres and hair of animal origin, vegetable fibres, and mixtures thereof, particularly wool, cotton and silk fibres.

The general parameters of the dyeing method of the invention are preferably between 70 and 250 bars with regard to pressure, and between 50 and 120°C with regard to temperature.

Further advantages and characteristics of the present invention will become clear from the following examples provided by way of non-limiting indication.

EXAMPLE 1 Dyeing tests were performed in a 1 litre autoclave with a stirrer on samples in the form of small skeins of wool each weighing about 2 g, with a dyeing medium comprising: supercritical CO2, ethanol as a co-solvent at a molar concentration of about 10k relative to the CO2, and a quantity of about 1 g, or in any case such as to ensure saturation of the dye bath, of a reactive dye.

The dyeing was carried out for about 2 hours at a pressure of about 200 bars and a temperature of about 90°C. Upon completion of this operation, the autoclave was emptied by depressurization in a single step. The dyed samples were then subjected to rapid washing in running water.

The dyed samples had good colour intensity, good colour- fastness upon washing, and good light-fastness values.

EXAMPLE 2 The tests relating to this example were performed in conditions similar to those of Example 1, with the difference that skeins of cotton were dyed with a direct dye and at about 70°C.

The dyed samples had good colour intensity, reasonable colour-fastness upon washing, and good light-fastness values.

EXAMPLE 3 The tests relating to this example were carried out in similar conditions to those of Example 1 with the difference that a skein of silk was dyed. Moreover, whereas in some tests the same co-solvent was used as in Example 1, in others water at a molar concentration of 2% was used instead.

The dye samples had very good colour intensity, good colour- fastness upon washing, and good light-fastness values.

Naturally, the principle of the invention remaining the same, the details of construction and forms of embodiment may be varied widely with respect to those described purely by way of example, without thereby departing from its scope.