The present invention relates to a method employing steam to fix one or more dyes onto a substrate, in particular a textile substrate, the dyes previously having been applied to the substrate which may or may not have been pretreated, and the method being implemented by the substrate and the steam being brought into contact with one another within a chamber which is substantially delimited by walls.

Such a method is generally known. To a textile substrate which may or may not have been subjected to a pretreatment in which it is treated with auxiliary sub¬ stances and/or additives in order, as it were, to "capture" the dye to be applied subsequently, a dye is applied which becomes fixed onto the substrate by the latter being brought into contact with steam within a chamber delimited by walls. Alternatively, the auxiliary substances and/or additives and the dye may be applied to the substrate simultaneously in the form of a printing paste or dye medium. As a result of steam being supplied to the substrate thus provided with dye (with or without prior drying) the temperature is raised and water is supplied, the reaction of the dye with the substrate, for example textile, being accelerated as a result. This is usually achieved by a steamer being employed, slightly superheated steam being maintained at slight excess pressure within the chamber delimited by walls. In the case of the known method, the substrate is hung out in the chamber delimited by walls and thus comes into contact with the steam already present, while supplementary steam is supplied to the chamber. A portion of the steam present penetrates into the fibres of the substrate, condenses and enables the dye to enter into a bond with the substrate and to fix onto the substrate.

The abovementioned method has a number of draw-

backs, however. Firstly, the substrate must be hung out in its entirety to allow the steam to readily penetrate into the fibres. If the known method is employed, this involves a very rapid temperature increase, to the extent that the steam becomes markedly superheated. This is an important drawback, since a textile substrate absorbs considerably more moisture from saturated steam than from markedly superheated steam. Another problem is that excessive condensation of the steam may take place on the substrate, condensation heat being liberated as a result, which increases the temperature of the substrate. If the temperature of the substrate is increased by too much, the dye present on the substrate will decompose. Equally, owing to the temperature difference between the walls of the steamer and the chamber delimited by the walls, the temperature of the walls generally being lower, condensa¬ tion of the steam may take place on the walls with the formation of water droplets which may drip from the walls onto the substrate and consequently give rise to water spots on the substrate.

It is an object of the present invention to overcome the abovementioned drawbacks. To this end the method is characterized in that it comprises at least the steps of the pressure in the chamber delimited by walls being reduced, by the fluid present being substantially removed, and by steam being supplied, the successive steps being repeated at least once.

By virtue of, first of all, the pressure being reduced in the chamber delimited by walls, by means of fluid essentially comprising air being removed from the chamber, the air which is present is removed from the fibres of the substrate. As a result, the substrate is very- accessible to the steam to be supplied next, which can therefore penetrate the fibres more effectively and more rapidly. Optimum supply of steam to the fibres is thus provided. Because of said high accessibility of steam even into the fibres the substrate need not be hung out . The present method therefore enables a dye to be fixed onto a substrate while the substrate is folded. This means that

substrates having a much larger surface area, or alternatively larger amounts of substrate, can be treated. Repeating the successive steps of reducing the pressure and supplying steam at least once, and preferably a number of times, also prevents the temperature of the chamber delimited by walls and the temperature of the substrate from rising excessively, so that the abovementioned deleterious consequences thereof do not arise. By virtue of the intermittent pressure reduction and steam supply, an optimal supply of steam to the substrate takes place continuously via the method according to the present invention.

The French patent application FR-A-2 226 500 likewise discloses a method for fixing dyes onto a substrate, a substrate continuously being introduced into and withdrawn from an apparatus suitable for this purpose. Steam is introduced at the top of the treatment chamber of the apparatus and moves downwards, expelling the air present there, so that the chamber will be completely filled with steam. The pressure reduction, intermittent or not, and supply of steam is not described therein.

The French patent application FR-A-2 241 650 likewise relates to a method for fixing dyes and describes a number of steps, in which the substrate to be treated is moved into the treatment chamber, the chamber is evacuated, followed by the introduction of steam and the said steam being drawn from the apparatus with a smaller flow rate, so that continuous through-flow of steam takes place. Here, again, the repeated pressure reduction, followed by steam being supplied, is not described.

Expediently, the steam supplied in the present invention is saturated.

The substrate absorbs more moisture from saturated steam than from superheated steam. Moreover, the steam consumption is lower, compared with the steam consumption in a conventional steamer, i.e. the energy consumption per unit of substrate is lower, and no extraction system is required.

The overall process of reducing the pressure and

supplying saturated steam takes 0 - 90 seconds in a particular embodiment. Preferably, the abovementioned time span is 0 - 25 seconds and even more preferably 0 - 15 seconds. Thus the substrate is moved into the chamber delimited by walls, the pressure in the chamber is reduced by removing the fluid present, which substantially comprises air, and steam is then supplied. Subsequently the pressure in the chamber is reduced once more, the fluid present, which in this case contains mainly steam, being removed once more from the chamber, followed by the renewed introduction of steam. If required, these steps may be repeated a few more times.

Always removing the steam from the chamber before fresh steam is supplied prevents the temperature within the chamber from becoming too high and the steam from becoming superheated. The saturated steam supplied will therefore at all times remain in a saturated state, which is very beneficial, since a textile substrate absorbs considerably more moisture from saturated steam than from superheated steam. At the same time, owing to the repeated removal of steam from the chamber, an undesirably high temperature increase of the substrate, due to condensation heat being liberated, will essentially be prevented, decomposition of the dye present thus being avoided.

The fixing procedure is advantageously terminated by air being supplied to the chamber.

This causes the substrate to cool down to a temperature at which it can be removed manually from the chamber where fixing has taken place.

To prevent an excessively large temperature increase of the substrate due to liberated condensation heat, a particular embodiment provides for the temperature of the chamber in which the method is implemented to be below 105°C.

If the temperature of the chamber is set at a low value, for example an initial value of 80°C, it will certainly be possible for the substrate temperature to rise as a result of liberated condensation heat, but not to the

extent that decomposition of the dye will occur.

Preferably, the temperature of the chamber in which the method is implemented is below the temperature of the walls which delimit the chamber. If the temperature of the walls which delimit the chamber is higher than the temperature of the chamber itself, the steam cannot condense on the walls and therefore does not give rise to water spots on the substrate. Finally, the present invention relates to an apparatus for implementing the method according to the invention, the apparatus being characterized in that it is a pressure vessel whose walls can be set at a desired temperature. An additional advantage of the abovementioned pressure vessel is that its temperature can be set accurately and reproducibly, compared with the conventional steamer which can be set only to an accuracy of 1°C.

Advantageously, the pressure vessel is of double- walled construction.

The present invention is explained below in more detail with reference to the following, nonlimiting examples.

EXAMPLE 1 A textile substrate of 2 m x 2.5 m, comprising cellulose fibres, is subjected to a pretreatment in which at least bicarbonate is applied to the substrate. In general, such a pretreatment takes place to capture and to immobilize the dye to be fixed, use being made of auxiliary substances or additives such as, for example, thickeners and bicarbonate.

After application of the bicarbonate, the dye is applied to the textile substrate. In order to allow the dye to be fixed onto the substrate, the textile substrate is placed into a chamber, delimited by walls, of a pressure vessel and the chamber is sealed. Then the air is removed from the chamber and thus from the fibres, saturated steam then being introduced into the chamber. The temperature is

set at 102°C; the chamber delimited by walls then has a pressure of 1.08 atmospheres. The pressure is maintained by continuous injection of saturated steam. After a specific period, in the present case 15 s, the pressure in the chamber is reduced once more by substantially removing steam from the chamber. Immediately afterwards saturated steam is again injected into the chamber. The steps of removing steam from the chamber, immediately followed by saturated steam being supplied to the chamber are repeated after 30 s and once more after 45 s. Then the substrate and the steam are left in contact with one another for a specific time. During this time, fixing of dyes onto the substrate is completed. In the process, the steam penetrates into the fibres of the substrate where it condenses and dissolves the dye. In the present case the dye comprises a cationic polymer which can react with the hydroxyl group of the cellulose with the elimination of hydrochloric acid. This hydrochloric acid is neutralized by means of the previously applied bicarbonate. In the process, a covalent chemical bond is formed between the dye and the substrate, which is also water-fast. In the case of the pressure vessel used in the present example, the temperature of the walls can be adjusted. Thus it is possible for the walls to be set at a higher temperature than the chamber surrounded by the walls. In the present example, the walls have a temperature, for example, of 105°C, while steam having a temperature of 102°C is introduced into the chamber. This prevents the steam from condensing on the walls of the autoclave and being able to give rise to water spots on the textile substrate.

The process is terminated by air being supplied to the chamber. The dye is then found to be essentially completely fixed onto the substrate, to be water-fast and not to exhibit any water spots.

EXAMPLE 2

A textile substrate comprising polyacrylic fibres and having a size of 0.5 x 0.5 m is provided with a basic dye, such as Astrozon and Maxilon dyes {registered

trademarks) .

The dye is fixed onto the substrate by the method according to Example 1 being used.

On completion of the procedure a substrate is obtained comprising a dye which has been fixed onto the substrate completely and uniformly and which does not exhibit any unevenness.

EXAMPLE 3 A textile substrate comprising cellulose fibres and having a size of 2 x 7 m is printed, in accordance with the rotary screen printing method, with a reactive dye, such as: Cibacron, Procion, Basilen, Levafix and Drimaren dyes (registered trademarks) .

To fix the dye onto the substrate, the process described in Example 1 is employed.

This results in a substrate comprising a dye which has been fixed onto the substrate completely and uniformly. No unevennesses, such as water spots, are observed.

EXAMPLE 4 A textile substrate comprising polyester fibres and having a size of 1 x 1 m is printed with a disperse dye, such as: Terasil, Palanil, Samaron, Resiren dyes (registered trademarks) .

The dye is fixed onto the substrate with the aid of the method described in Example 1.

After fixing, the dye is found to be uniformly fixed onto the substrate.

EXAMPLE 5 A substrate comprising polyamide fibres and having a size of 4 x 5 m is printed with an acid and metal complex dye, such as: Avilon, Acidol, Telon, Alizarin, Erionyl and Irgalan dyes (registered trademarks) .

After implementation of the fixing method as described in Example 1, the dye is found to have been fixed onto the substrate in an essentially complete and uniform manner.

If substrates comprising wool and natural silk are employed, together with the same dyes, complete and uniform fixing of the dye onto the substrate is likewise achieved.

EXAMPLE 6 A substrate comprising cellulose fibres and having a size of 10 x 4 m is printed with a vat dye, such as: Indanthren, Cibanon, Caledon and Durindone dyes (registered trademarks) .

Fixing of the dye onto the substrate is carried out with the aid of the method as described in Example 1.

After completion of the procedure the dye is found to be fixed uniformly onto the substrate, without water spots.