METHOD OF DYEING CHEMICAL TEXTILE FIBERS

Field of the Invention

The present invention is directed to the technical field of dyeing chemical textile fibers, also known as technofibers, which include synthetic textile fibers and artificial textile fibers.

In particular the invention relates to a method of dyeing textile fibers of the aforesaid type, for example, but not limited to, made of polyester, and a related product for dyeing such textile fibers.

Background Art

As it is known, among the wide range of textile fibers currently available there are chemical textile fibers, also known as technofibers.

Unlike the natural textile fibers, which are comprised exclusively of natural material of organic origin (such as cotton and silk), the chemical textile fibers are made by chemically processing raw materials of natural origin (in which case they are called artificial fibers) or are obtained by processing petroleum derivatives (in which case they are known as synthetic fibers).

An example of artificial fibers are acetate fibers, made by treating the cellulose with carbon dioxide, while examples of synthetic fibers are acrylic fibers, polyester fibers, polyamide (nylon) fibers or PET (polyethylene terephthalate) fibers.

The chemical textile fibers are generally subjected to a dyeing process which allows giving them the desired color appearance.

In general, apolar (non-ionic) dyes are used in these processes, which have poor solubility in water.

These dyes are known as "dispersed dyes" because, when mixed with water during the dyeing process, they form a dispersion in which the water is the dispersing phase and the dye is the dispersed phase.

The dispersed dyes may be classified in various ways based on their different characteristics and properties. For example, referring to the chemical structure, the dispersed dyes are distinguished into nitro dyes, amine ketone dyes, anthraquinone dyes, mono azo dyes, and azo dyes.

Or, the dispersed dyes may be divided into various groups based on the type of fiber with respect to which they are optimal in the dyeing process. Based on this principle, we distinguish dyes belonging to the Group A suitable for acetate and nylon fibers; dyes belonging to the Group B, which are excellent for the use with polyester, particularly for the dyeing variations associated with textured yams; dyes belonging to the Group C, suitable for all dyeing processes of polyester fibers; dyes belonging to the Group D, which have maximum resistance to sublimation and are used exclusively for dyeing PET fibers; and dyes which are not identified with a suffix, not suitable for polyester but suitable for acetate and nylon fibers.

Alternatively, another way to classify dispersed dyes takes into consideration the molecular weight and the number of polar groups in the dye molecule. These aspects are important because they affect the dyeing speed and the strength to the sublimation (key aspects in some dyeing processes).

In particular, we may distinguish:

• Low-energy dyes with molecular weight <300 g/mol and sublimation temperature <150°C;

• Medium-energy dyes with molecular weight between 300 and 400 g/mol and sublimation temperature between 150°C and 210°C;

• High-energy dyes with molecular weight above 400 g/mol and sublimation temperature above 210°C.

Over the decades, various dyeing processes have been developed that make use of the aforementioned dispersed dyes. In particular, the dyeing processes of the chemical textile fibers, henceforth identified as "textile fibers " are carried out by applying the dispersed dye described above to the textile fibers.

Among the dyeing processes there are those involving a so-called application method, which differs from the exhaustion method in that it involves a first step, in which the dispersed dye is mechanically applied to the textile fibers by appropriate devices, and a second step, in which the dispersed dye is thermally fixed to the textile fibers.

In general, the application methods involve the use of one or more so-called auxiliary agents. Such auxiliary agents are essential for one or more of the following aspects: ensuring adequate dispersion of the dispersed dyes in water (i.e., limiting the formation of aggregates), ensuring adequate viscosity to the prepared dispersion, and facilitating the fixation process of the dispersed dyes in the textile fibers.

Examples of auxiliary agents are sodium polyacrylate (thickener/viscosity corrector), alginates or synthetic polymer having anionic charge (viscosity correctors), citric acid (dispersing agent), propylene glycol (dispersing agent), non-ionic thickeners, tamarind-based thickeners, aloe vera extracts (thickener), diethylene glycol, polyethylene glycol and propylene glycol (humectants), surfactants and pH adjusters.

Going into how the application methods are implemented, it is worth underlining that in the known art they are declined into:

- printing methods, among which there are ink-jet printing and traditional printing (screen printing or cylinder printing), and

- impregnation methods, among which there are the Pad-Steam method, the Pad- Dry-Steam method and the Pad-Dry-Thermofix method, which may be likened to the Thermosol method.

In turn, the methods listed above may be divided into two groups.

The ink-jet printing belongs to a first group while the traditional printing and, in general, all the impregnation methods belong to a second group.

The division into these two groups is made on the basis of whether or not the textile fibers need to be treated with chemicals and auxiliary thickening agents before contacting them with the dispersed dye.

In particular, the ink-jet printing involves the textile fibers to be treated with auxiliary thickening agents before the dispersed dye is applied on the textile fibers; in contrast, the traditional printing and impregnation methods do not require the textile fiber to be treated before contacting the dispersed dye. In fact, in the methods belonging to the second group, the textile fibers are contacted with the dispersed dye already mixed with the auxiliary agents.

For example, an ink-jet printing process with dispersed dyes involves the preparation of a mixture comprising one or more auxiliary agents to treat the textile fibers before they contact the dispersed dye, that is, before the dispersed dye-based inkjet ink is applied on them. This is usually done in a vessel with two squeezing cylinders that bring the mixture on the textile fibers and subsequently, by squeezing, remove the excess mixture.

Usually sodium polyacrylate, which is dissolved in water at 160-170 g/1, is used as auxiliary agent. Sodium polyacrylate is precisely used to treat the textile fibers before they are contacted with the dispersed dyes.

After this treatment, the textile fibers are dried in machines known as "rameuse" by hot air at a temperature of 70°C. In this step the textile fibers are also stretched in preparation for the next printing step.

In fact, the dispersed dye is subsequently applied on the textile fibers by the inkjet technique.

This technique involves the application of the dispersed dye on the textile fibers by the use of a plurality of printing heads. The dispersed dye is formulated in a liquid ink comprising 2.76% by weight dispersing agents in total, 27.50% by weight wetting agents in total, 5.00% by weight anti-foaming agents in total, 0.05% by weight pH adjusters in total, dispersed dye in a range between 4% and 5% by weight and demineralized water to 100%.

For example, diethylene glycol and ethylene glycol, 5% by weight and 22.5% by weight, respectively, and triethanolamine as pH regulator are used as wetting agents.

In practice, the printing heads lay the dispersed dye through micrometer-sized cavities.

At the end of the printing step, drying takes place, during which the dispersed dye is dried so that the fabric is usable for the next steps in the process. After drying is complete, the fabric is placed on a roll or folded.

In detail, once printing has taken place, the thermal fixing of the dispersed dye is carried out by dry heating the textile fibers in hot air at 170°C for 120 seconds or by the use of superheated water steam.

Finally, a reducing washing of the dyed textile fibers is carried out, which consists of soaking them in a tank at 80°C with 2 g/1 hydrosulphite, 2 g/1 caustic soda (40°Be) and 1 g/1 detergent for 10 minutes, followed by a rinse at 60°C for 10 minutes. Washing allows excess dye not absorbed by textile fibers, residues and auxiliary agents from previous steps to be removed.

Eventually, the process involves drying and ironing the textile fibers dyed in rameuse.

In contrast, the traditional printing method involves the preparation of a waterbased printing paste comprising a dispersed dye and auxiliary agents. In the printing paste, the dye is dispersed in it.

For example, it is possible to prepare a water-based printing paste comprising the dispersed dye, formulated as 10% by weight liquid ink, 2% by weight citric acid, 0.5% by weight sodium acetate, 5% by weight tamarind-based non-ionic thickener, and water to 100%.

The liquid ink used in the preparation of the printing paste in turn comprises a dispersed dye, at least one thickening agent, one or more hydroscopic or hydrotropic substances, one or more catalysts, and one or more auxiliary agents such as wetting agents, dispersing agents, sequestering agents and reagents.

In practice, as set forth above, the auxiliary agents are mixed with the dispersed dye and, with it, applied to the textile fibers without the need for prior treatment.

It is clear that if there is a need to dye the textile fibers with a polychromatic pattern, it is necessary to prepare several printing pastes, each comprising a respective dispersed dye.

In fact, once the printing paste is prepared, traditional printing is performed on the textile fibers.

At this step, the printing paste, or printing pastes, are spread inside a substrate imprinted with the pattern to be printed and, by means of adjustable pressure, the printing paste, with the dispersed dye, is forced out of the meshes provided on the substrate. Following the printing step, the textile fibers are heated inside an oven in order to remove the residual water and allow for the processing in the next steps.

Subsequently the dye is fixed by thermally fixing with dry heat at 210°C for 60 seconds or superheated steam at 130°C for 120 seconds or 180°C for 60 seconds.

It is also possible to carry out the step of thermally fixing by a so-called HT steaming step at 180°C for 15 minutes.

Finally, a washing is carried out, which consists of soaking the textile fibers in a tank at 80°C with 2 g/1 hydrosulphite, 2 g/1 caustic soda (40°Be) and 1 g/1 detergent for 20 minutes, followed by a rinse at 60°C for 10 minutes, and finally drying.

The method of impregnation dyeing may be performed by the Pad-Steam method, the Pad-Dry-Steam method and the Pad-Dry-Thermofix method.

In processes using the Pad-Dry-Thermofix method, dyeing is done by sublimation of the dispersed dyes. In practice, the dispersed dyes go directly from the solid state to the gaseous state without passing through the liquid state. Such a continuous process may dye the polyester at 190-220°C.

The process involves the application of dispersed dyes to the textile fibers by impregnation by a so-called Pad-batch system and then drying by using a hot-air dryer or by infrared radiation.

During such a process the textile fibers are soaked in a water-based impregnation bath comprising the dispersed dye, formulated as 10% by weight liquid ink, 2% by weight citric acid, 0.5% by weight sodium acetate, 2% by weight tamarindbased non-ionic thickener and water to 100%.

Drying of the impregnated material is done by heated cylinders. The dried textile fibers are then heated in the air, or by contact with a hot metal surface, to a temperature in the range of 190-220°C for 1-2 minutes. As the textile fibers approach the maximum temperature, the dispersed dyes begin to sublimate and the textile fibers absorb their vapors. At about 200°C, the sublimation of the solid dye transfers its steam into the textile fibers, and the penetration into the textile fibers by diffusion is quite rapid.

Also in this case, a reducing washing of the dyed textile fibers is carried out, which consists of soaking them in a tank at 80°C with 2 g/1 hydrosulphite, 2 g/1 caustic soda (40°Be) and 1 g/1 detergent for 10 minutes, followed by a rinse at 60°C for 10 minutes.

In the application methods, among the mechanisms that most influence the dyeing process there are: the degree of dispersion achieved by the dispersed dye during the different steps of the process, and the partitioning and diffusion mechanisms that the dispersed dye undergoes in the textile fibers.

In particular, the degree of dispersion of the dispersed dye in water (in the ink-jet printing ink, in the printing paste used in traditional printing and in the impregnation bath used in the impregnation methods) affects the amount of dispersed dye absorbed by the textile fibers: the less dispersed the dye, the less it is absorbed by the textile fibers. In methods according to the known art, the dye is poorly dispersed in water: this results in high amounts of dispersed dye not absorbed by the textile fibers and a decrease in the quality of dyeing. To remove the dispersed dye that is not absorbed by the fibers there is a need to perform an appropriate washing step carried out in a strongly basic and reducing environment, followed by neutralizing washing.

Furthermore, the partitioning and diffusion mechanisms are important because they play a key role in the amount of dispersed dye present in the textile fibers and the depth the dispersed dye reaches in them.

In fact, during the partitioning, the dispersed dye present in the aqueous phase is partitioned into the fiber to be dyed due to the higher chemi cal -physical affinity of the dispersed dye with the textile fiber compared to water.

In contrast, in the diffusion, there is the penetration of the dispersed dye into the textile fibers, so that dyeing does not occur only superficially on the textile fibers.

Finally, the thermal properties of the auxiliary agents, such as that of sodium polyacrylate, affect the dyeing process itself. In particular, the thermal diffusibility, thermal conductivity and heat capacity of the auxiliary agents affect the temperature and duration at which to carry out the step of thermally fixing. In practice, the auxiliary agents affect the rate of heat propagation in the system constituted by the aforesaid dispersion and the textile fibers to be dyed, the system's ability to transmit heat to the textile fibers, and the amount of heat that needs to be supplied to the system to allow the textile fibers to reach the temperature necessary for thermally fixing.

However, although efficient for the purpose, the dyeing processes that take advantage of the aforesaid application methods are not without drawbacks.

In fact, it is known that the compounds used in the aforesaid washing steps have the disadvantage of being polluting and increasing the costs of the dyeing process, also but not only for the need to dispose of wastewater from the dyeing process itself.

Furthermore, the use of the aforesaid auxiliary agents to ensure acceptable color yields and dye quality requires the supply of a large amount of energy in the step of thermally fixing the dispersed dye.

This results in high power expenditure and high costs associated with the dyeing process.

Furthermore, normally, in the dyeing processes by the ink-jet printing method, the dispersed dyes must be further ground with respect to the grinding provided for other traditional methods, in order to reduce the particle size to about 200-250 nanometers. Clearly, this additional processing results in increased costs associated with the dyeing process.

Summary of the invention

Object of the present invention is to provide a method of dyeing chemical textile fibers with dispersed dye that allows overcoming at least one of the drawbacks described above, being easy to implement, cheap and allowing to avoid or limit the use of toxic or polluting substances, particularly but not only in the washing step.

The present invention, in a first aspect thereof, relates to a method of dyeing chemical textile fibers with dispersed dye according to claim 1.

For the purposes of the present invention, by chemical textile fibers, henceforth identified only by the term "textile fibers", are meant the textile fibers commonly known as technofibers, i.e., both the artificial textile fibers made by chemically processing raw materials of natural origin, and the synthetic textile fibers obtained by processing petroleum derivatives.

For example, the present method may be used to dye artificial acetate fibers or synthetic fibers such as acrylic fibers, polyester fibers, polyamide (nylon) fibers, or PET (polyethylene terephthalate) fibers.

Preferably, the present method is directed to dyeing polyester synthetic fibers.

The present method may be applied to textile fibers as is or in the form of woven or nonwoven fabric. In other words, this method may also be applied to fabrics including the so-called nonwoven fabrics comprising the aforementioned textile fibers.

As mentioned above, the dyeing method described herein makes use of the so- called "dispersed dyes " a well-known class of dyes used in the field.

For the purposes of the present invention, by dispersed dye is meant the dye regardless of whether or not it is actually dispersed, whether or not it is used in an aqueous phase dispersion, or whether or not it is in a solubilized form.

Some examples of dispersed dyes that may be used in this dyeing method are: Disperse Orange 3, Disperse Violet 4, Disperse Violet 1, Disperse Yellow 54, Disperse Blue 359 and Disperse Red 11. The method according to claim 1 comprises: a) a step of printing on the textile fibers or impregnating the textile fibers with dispersed dye; and b) a step of thermally fixing the dye on the textile fibers.

In practice, the present method comprises a first step in which the dispersed dye is applied to the textile fibers in the way provided by the so-called application methods, that is, by printing or impregnation.

Preferably in step a), the dispersed dye is applied to the textile fibers according to one of the following techniques known in the known art: ink-jet printing or traditional printing, screen or rotary printing (in the case of printing), or Pad-Steam method, Pad-Dry- Steam method or Pad-Dry-Therm ofix method or Thermosol method (in the case of the impregnation methods).

In addition to the printing or impregnating step, the method provides a step of thermally fixing the dye on the textile fibers, that is, a step in which, by heat, the fixing of the dispersed dye in the textile fibers takes place.

According to the present invention, the printing step or impregnating step a) is carried out by contacting said dispersed dye with a mixture comprising two or more compounds which constitute a eutectic solvent or which are adapted to constitute a eutectic solvent upon heating.

In other words, in the context of the present invention, the dispersed dye may be contacted with the aforesaid mixture:

- before the dispersed dye itself is applied to the textile fibers (in which case the dispersed dye is combined with the aforesaid mixture and the whole applied to the textile fibers), or

- after the textile fibers have been previously treated with the mixture itself (in this case the dispersed dye is contacted with the aforesaid mixture already on the textile fibers).

In practice, the method involves, during step a), the use of either a mixture in which two or more compounds constitute a eutectic solvent or a mixture in which two or more compounds do not, at the time of their use during step a), constitute a eutectic solvent but are adapted to constitute a eutectic solvent at a later step, such as in step b).

In practice, in the first case, said two or more compounds, during step a), are present in the mixture at a concentration such that they constitute a eutectic solvent, while in the second case, during step a), said two or more compounds are not present in the mixture at a concentration such that they constitute a eutectic solvent, for example because they are highly diluted in water. In the second case, said two or more compounds may constitute a eutectic solvent as a result of heating the mixture, that is, for example, as a result of partial or total evaporation of water in which said two or more compounds are present.

Said two or more compounds constitute or may constitute a eutectic solvent, meaning that they constitute or may constitute a solvent that has a melting or solidification temperature lower than the melting temperature of the two or more compounds considered individually, that is, not mixed together.

The mixture may comprise two or more compounds, meaning that it may comprise, in addition to a first compound and a second compound, also a third compound or several compounds that constitute or may constitute a eutectic solvent.

In an embodiment of the method, the printing step involves treating said textile fibers with said mixture and subsequently applying said dispersed dye to said textile fibers treated, for example by means of ink-jet printing.

Otherwise, in another embodiment of the method, the printing or impregnating step involves the dispersed dye being contacted with said mixture prior to contacting said textile fibers with said dye, for example, as provided by the traditional (screen or rotary) printing, or in the Pad-Steam method, the Pad-Dry-Steam method, the Pad-Dry- Thermofix method or the Thermosol method.

The present method allows overcoming one or more of the drawbacks described above.

In particular, said mixture is capable of solubilizing the dispersed dye, both when said two or more compounds constitute a eutectic solvent and when said two or more compounds do not constitute a eutectic solvent, for example because they are highly diluted in water.

Thanks to this ability of the mixture to solubilize the dispersed dye, the amount of dispersed dye that is not absorbed by the textile fibers during the dyeing process is significantly limited, making it unnecessary to perform washing in a strongly basic and reducing environment of the dyed textile fibers, as is the case according to the known art.

Therefore, the use of said mixture allows eliminating the use of hazardous and polluting substances from the washing step and eliminating the corresponding neutralization step traditionally carried out after the washing step, with obvious advantages in terms of environmental sustainability, process duration and related costs.

The above allows carrying out the washing step in the absence of hydrosulphite and caustic soda and eliminating the neutralization step.

Furthermore, said mixture, in addition to being able to solubilize the dispersed dyes, also has favorable solubility parameters toward the textile fibers, but without these parameters being so high as to cause the dissolution of the textile fibers themselves.

This allows the mixture to be used in the dyeing processes of the textile fibers by promoting the diffusion mechanisms of the dispersed dye in the textile fibers.

In fact, for example, comparing the Hansen solubility parameters, with respect to the dispersed dye, of a mixture in which said two or more components constitute a eutectic solvent, with the solubility parameters of acetone and tetrachloroethene, which are solvents used in the traditional dyeing methods with dispersed dye, it emerges that said mixture has a higher ability to dissolve the dispersed dye than acetone and tetrachl oroethene .

Or, comparing the Hansen solubility parameters of acetone, tetrachloroethene and said mixture with respect to the textile fibers, it emerges that said mixture has an intermediate ability to solubilize the textile fibers compared with acetone and tetrachl oroethene .

In practice, the mixture has solubility parameters that allow it, on the one hand, to be used to solubilize the dispersed dyes, and on the other hand, to be used in the dyeing processes of the textile fibers because it is not able to dissolve them.

Furthermore, unlike the organic solvents, the aforesaid two or more compounds are soluble in water or in an aqueous environment: this aspect allows using such compounds in the context of a dyeing method of chemical textile fibers, for which aqueous mixtures are traditionally used.

In addition to the advantages set forth above, thanks to the use of said mixture, the method described herein allows the step of thermally fixing to be carried out at a lower temperature and for a shorter time duration than those of the methods according to the known art, while still ensuring dyeing of the textile fibers of greater or at least equal quality than that obtained by the methods according to the known art.

Clearly, this allows keeping costs down and the textile fibers to be dyed in less time than the known methods.

For example, when applied in the context of the ink-jet printing, the present method allows switching from a step of thermally fixing carried out at 170°C for 120 seconds to a step of thermally fixing carried out, for example, at 155°C for 90 seconds, with a 12% reduction in the temperature value and 25% reduction in the duration of the fixing step.

Otherwise, when applied in the context of the traditional screen or rotary printing, the present method allows switching from a step of thermally fixing carried out at 210°C for 30-60 seconds to a step of thermally fixing carried out, for example, at a temperature between about 155°C and about 170°C for 90 seconds, with a reduction of 26% and 20% in the temperature value of the step of thermally fixing, respectively.

Otherwise, when applied in the context of the traditional HT steaming printing, the present method allows switching from a step of thermally fixing carried out at 180°C for 15 minutes to a step of thermally fixing carried out, for example, at 172°C for 5 minutes, with a 4% reduction in the temperature value and 66% reduction in the duration of the step of thermally fixing.

It is likely that these advantages are due to, among other things, higher values of thermal diffusibility and thermal conductivity and a lower value of heat capacity of said mixture, compared to traditional auxiliary agents.

Also contributing to these advantages is the fact that the aforesaid mixture allows achieving, overall, higher levels of partition and diffusion the dispersed dye in the textile fibers.

This is allowed by the fact that, in accordance with the present method, it is possible, at first, to contact the dispersed dye with a mixture in which said two or more compounds do not constitute a eutectic solvent, for example because they are present in a solution diluted in water and, secondly, to heat the mixture with the dispersed dye in such a way that said compounds constitute a eutectic solvent, by evaporation of water.

Clearly the dye in contact with the mixture will have different properties toward the textile fiber, depending on whether it is in a mixture that does not constitute a eutectic solvent or whether it is in a mixture that does constitute a eutectic solvent.

This way it is possible to favor at first, when said two or more compounds do not constitute a eutectic solvent, the process of partitioning the dye in the textile fibers and, secondly, when due to heating said two or more compounds constitute a eutectic solvent, the process of diffusion of the dye into the textile fibers. In this context, the mixture ensures the solubilization of the dispersed dye even when said two or more compounds do not constitute a eutectic solvent.

This is the case, for example, when the method that is object of the present invention is applied in the context of the ink-jet printing, in the context of the traditional screen or rotary printing or in the context of the impregnation printing. Finally, precisely because of the aforementioned advantages related to the ability to solubilize the dispersed dye even when said two or more compounds do not constitute a eutectic solvent, the present method allows the use, in most cases, of dispersed dyes with a particle size even larger than that used in processes according to the known art.

More specifically, it is observed that most of the dispersed dyes are soluble in a eutectic mixture as considered herein and it is possible to retain a high, if not total, solubilization even following relevant dilution of the aforesaid mixture, which, following dilution, will no longer be properly eutectic but adapted, for example by heating, to form a eutectic mixture thanks to the properties of the aforesaid two or more compounds.

In the rare cases in which the dispersed dye is not totally solubilized in the eutectic mixture, there is to be noted that the latter results in dispersion of the nonsolubilized dye, leading the latter to a particle size comparable to or smaller than that used in the processes according to the known art.

In practice, the present method allows dispersed dyes with particle size traditionally used in the traditional or impregnation printing methods to be used for inkjet methods as well.

Preferably, after said step of thermally fixing, the method comprises a step c) of washing with water in the absence of hydrosulphite and caustic soda, precisely because of the use of the aforesaid mixture. Preferably, washing is carried out by heating the textile fibers in water at a temperature between about 70°C and about 80°C for a time range between 9 minutes and 16 minutes, optionally in the presence of an antireplicating agent, such as quatemized ethoxylated tallow amine.

In an embodiment of the method, the step of thermally fixing is carried out by heating said textile fibers with dry heat, that is, in the absence of steam, at a temperature between about 140°C and about 170°C, and preferably at a temperature between about 150°C and about 160°C, for example at 155°C.

Preferably, in this case, the step of thermally fixing is carried out for a period of time between about 80 seconds and 100 seconds. In another embodiment of the present method the thermal fixation is carried out by heating said textile fibers, with wet heat, i.e. in the presence of steam, at a temperature between about 170°C and about 175°C, preferably at a temperature of about 172°C, wherein said step of thermally fixing is preferably carried out for a period of time between about 4.5 minutes and about 5.5 minutes, preferably for 5 minutes.

Preferably, said two or more compounds are natural compounds and constitute or are adapted to constitute a eutectic solvent belonging to the NaDES “Natural Deep Eutectic Solvents’" category.

Preferably, said two compounds are citric acid and B-Alanine or citric acid and Betaine.

Preferably, citric acid and B-Alanine or citric acid and Betaine are present in a mixture in a 1 : 1 molar ratio.

Alternatively, citric acid and B-Alanine or citric acid and Betaine are present in molar ratios other than 1 : 1, such as for example 1 :2 or 2: 1.

Alternatively, said two or more compounds may be selected from other substances. For example, the following combinations of substances may be used: 1,2- propanediol, choline chloride, water; B-Alanine, citric acid, water; Betaine, citric acid, water; fructose, choline chloride, water; glucose, choline chloride, water; glucose, citric acid, water; glycerol, choline chloride, water; lactic acid, choline chloride; lactic acid, 1,2-propanediol; lactic acid, glucose, water; lactic acid, B-Alanine, water; malic acid, sorbitol, water; malic acid, L-serine, water; malic acid, choline chloride, water; proline, malonic acid, water; xylitol, choline chloride, water; xylitol, citric acid, water.

The combinations of substances described above are preferably used with the following respective molar ratios: 1,2-propanediol, choline chloride, water 1 : 1 : 1 or 1 : 1 :3; B-Alanine, citric acid, water 1 : 1 :3; Betaine, citric acid, water 1 : 1:5; fructose, choline chloride, water 1 :1 :3; glucose, choline chloride, water 2:5:5; glucose, citric acid, water 1 : 1 :5; glycerol, choline chloride, water 2: 1 : 1; lactic acid, choline chloride 1 : 1; lactic acid, 1,2-propanediol 1 : 1 or 2: 1; lactic acid, glucose, water 5: 1 :3; lactic acid, B- Alanine, water 1 : 1 :3; malic acid, sorbitol, water 1 :1 :3; malic acid, L-serine, water 1 : 1 :3; malic acid, choline chloride, water 1 : 1 :2; proline, malonic acid, water 1 : 1.6; xylitol, choline chloride, water 1 : 1 :2 or 1 :2:3; xylitol, citric acid, water 1 : 1 :3.

The method according to the present invention will be described more in depth below with reference to a mixture comprising citric acid and B-Alanine or citric acid and Betaine, without intending to limit what is set forth to these specific combinations of compounds.

Basically, step a) is carried out by contacting said dispersed dye with a mixture comprising citric acid and B-Alanine or citric acid and Betaine, in which citric acid and B-Alanine or citric acid and Betaine constitute a eutectic solvent or may constitute a eutectic solvent upon heating.

When citric acid and B-Alanine or citric acid and Betaine constitute a eutectic solvent, they are either the only constituents of the mixture (the mixture consists of citric acid and B-Alanine or consists of citric acid and Betaine) or they are present in said mixture, in their respective eutectic proportions, together with an amount of water up to about 50% by weight to the total weight of the mixture. In contrast, when they do not constitute a eutectic solvent and are adapted to constitute a eutectic solvent upon heating, citric acid and B-Alanine or citric acid and Betaine are present in said mixture, in their respective eutectic proportions, together with an amount of water approximately equal to or greater than 50% by weight to the total weight of the mixture.

In particular, when reference is made to the fact that they are adapted to constitute a eutectic solvent upon heating, reference is preferably made to the case in which the compounds are dissolved in water and due to the water evaporation caused by heating said mixture, a eutectic solvent is constituted.

Preferably, said two or more compounds are adapted to constitute a eutectic solvent following heating carried out during said step b) of thermally fixing, or following an appropriate heating or drying step carried out prior to step b), such as by bringing the mixture to a temperature preferably equal to or above 60°C and for a time preferably equal to or above 35 seconds, more preferably a temperature equal to or above 80°C for a time equal to or above 40 seconds. It is understood that the temperature and heating time may be selected as needed depending on the dilution of the mixture and the nature of said two or more compounds. Preferably, said mixture comprises at least one auxiliary agent selected from the group comprising sodium polyacrylate, propylene glycol, non-ionic thickeners, tamarind-based thickener, aloe vera extract.

The present invention, in a second aspect thereof, relates to a product for dyeing chemical textile fibers, comprising:

- a mixture including two or more compounds that constitute a eutectic solvent or are adapted to constitute a eutectic solvent upon heating;

- a dispersed dye and/or

- at least one auxiliary agent selected from the group comprising sodium polyacrylate, propylene glycol, non-ionic thickeners, tamarind-based thickener, aloe vera extract.

The aforesaid product may be, for example, an ink for ink-jet printing, a product for treating the textile fibers before they are printed by the ink-jet technique, a liquid ink intended for the preparation of a printing paste for traditional screen or rotary printing and/or intended for the preparation of an impregnation bath, a printing paste for traditional screen or rotary printing, a product to be used to prepare an impregnation bath.

Preferably, the aforesaid product comprises citric acid and B-Alanine or citric acid and Betaine.

Finally, the present invention relates to the use of a mixture for dyeing chemical textile fibers with dispersed dye, involving the use of a mixture comprising two or more compounds which constitute a eutectic solvent or which are adapted, upon heating, to constitute a eutectic solvent.

Brief list of the figures

Further characteristics and advantages of the invention will be better evident by the review of the following detailed description of some preferred, but not exclusive, embodiments shown for illustration purposes only and without limitation, with the aid of the accompanying drawings, in which:

- figure 1 is a graph of a thermogravimetric analysis (TGA) carried out on a mixture comprising, in accordance with preferred embodiments of the method according to the present invention, citric acid and B-Alanine or citric acid and Betaine, in which the graph correlates the total mass of each of these mixtures, expressed as a percentage of the initial mass, with the temperature to which the mixture is subjected.

- figure 2 is a graph which allows comparing the thermal properties of sodium polyacrylate with those of a mixture constituted by citric acid and B-Alanine, which constitute a eutectic solvent.

- figures 3a and 3b are graphs correlating the time with the concentration gradient of dye dispersed in the textile fibers in the context of thermal processes with solid/gas/textile fiber transfer carried out, in the case of graph 3a, in the traditional way, in the case of graph 3b, by using a eutectic mixture according to what provided in the method according to the present invention.

- figure 4 shows a graph denoting the Hansen distance of various compounds vs. polyester textile fibers. In particular, the graph compares the Hansen distance of the dispersed dye, the mixture comprising citric acid and B-Alanine that constitute a eutectic solvent, water, propylene glycol, citric acid and B-Alanine vs. the polyester textile fibers.

Detailed description of the invention

The present invention relates to a method of dyeing chemical textile fibers with dispersed dye and a product for dyeing chemical textile fibers for use, preferably but not exclusively, in the present method.

In detail the present method comprises: a) a step of printing on the textile fibers or impregnating the textile fibers with dispersed dye; and b) a step of thermally fixing the dye on the textile fibers.

In practice after step a), the textile fibers, on which a dispersed dye has been applied, undergo a step of thermal fixation during which the textile fibers are heated.

According to the invention, in the present method the printing step or impregnating step are carried out by contacting the dispersed dye with a mixture comprising two or more compounds which constitute a eutectic solvent or which are adapted to constitute a eutectic solvent upon heating. In particular, the aforesaid two or more compounds may be: citric acid and B- Alanine; citric acid and Betaine; 1,2-propanediol, choline chloride and water; B- Alanine, citric acid and water; Betaine, citric acid and water; fructose, choline chloride and water; glucose, choline chloride and water; glucose, citric acid and water; glycerol, choline chloride and water, lactic acid and choline chloride; lactic acid and 1,2- propanediol; lactic acid, glucose and water; lactic acid, B-Alanine and water; malic acid, sorbitol and water; malic acid, L-serine and water; malic acid, choline chloride and water; proline, malonic acid and water; xylitol, choline chloride and water; xylitol, citric acid and water.

In particular, a mixture where a first compound is citric acid and a second compound is B-Alanine or Betaine is preferred.

Figure 1 shows a graph of a thermogravimetric analysis (TGA) carried out on a mixture comprising citric acid and B-Alanine and on a mixture comprising citric acid and Betaine.

In the first case, the mixture is comprised of 210 g citric acid and 89.09 g B- Alanine, so as to create a mixture of citric acid and B-Alanine with a 1 : 1 molar ratio; in the second case, we start from a mixture comprising 210 g citric acid and 117.08 g Betaine, so as to create a mixture of citric acid and Betaine with a 1 : 1 molar ratio.

The graph, which correlates the total mass of each of these mixtures, which is expressed as a percentage of the initial mass, i.e., the mass before heating, with the temperature to which the mixture is subjected, shows that even subjecting said mixtures to high temperatures does not result in significant degradation of the compounds that comprise them.

For example, at 150°C, the total mass of the compounds that make up each mixture is about 95% of the initial mass. This aspect, which will be taken up below, is crucial, recalling what was mentioned above about the need to operate a step of thermally fixing on the textile fibers after the dispersed dye has been applied: the fact that these compounds have high thermal stability is an indication that they may be used in a dyeing method without being degraded, creating fumes, during the fixing step.

The chemical textile fibers dyed by the present method may be both artificial textile fibers and synthetic textile fibers.

In general, the present method may be used to dye artificial acetate fibers, or synthetic fibers such as acrylic fibers, polyester fibers, polyamide (nylon) fibers, or PET (polyethylene terephthalate) fibers.

Preferably, the present method is directed to dyeing polyester synthetic fibers. In the following, reference will be made to dyeing polyester textile fibers; however, what is described in relation to these fibers may also be extended to the other chemical fibers mentioned above.

The present method may be applied to textile fibers as is or in the form of woven or nonwoven fabric. In other words, this method may also be applied to fabrics including the so-called nonwoven fabrics comprising the aforementioned textile fibers.

The mixture comprising the aforesaid two or more compounds is contacted with the dye before the textile fibers are treated with the mixture itself or after the textile fibers have been treated with said mixture.

For example, when the method is applied in the context of the ink-jet printing, the aforesaid mixture is contacted with the dye after the textile fibers have been treated with said mixture.

In contrast, when the method is implemented in the context of the traditional screen or rotary printing or in the context of the impregnation, the mixture is contacted with the dye before the textile fibers are treated with the mixture itself.

In light of this distinction, the present method may be implemented in different embodiments described below.

A first embodiment of the method according to the present invention relates to a method of dyeing chemical textile fibers, preferably polyester fibers, by making use of the ink-jet printing technique to apply the dispersed color on the textile fibers. In particular, this first embodiment of the method provides for preparing a mixture comprising 210 g citric acid monohydrate and 89.09 g 13-Alanine, so as to create a mixture of citric acid and 13- Alanine with a 1 : 1 molar ratio.

In this mixture, citric acid and 13-Alanine constitute a eutectic solvent and at room temperature this mixture is a solid. To this mixture, 59.846 g water is added, accounting for 20% of the total weight of the mixture which has been formed.

The water, citric acid and B-Alanine thus mixed are heated to a temperature between about 70°C and about 75°C and are kept at a temperature within this range until a clear solution is obtained.

Once the citric acid and B-Alanine are solubilized in water, 222.3 g of the solution is taken and 667.7 g water is added.

In this solution, citric acid and B-Alanine do not constitute a eutectic solvent.

After mixing, one or more auxiliary agents may be added.

In particular, 50 g propylene glycol and 60 g sodium polyacrylate may be added. It is preferable to add the sodium polyacrylate after the propylene glycol and after properly mixing the mixture.

A tamarind-based thickener or Aloe Vera extract may be used as an alternative to sodium polyacrylate.

The mixture thus prepared is used to treat the textile fibers before the dispersed dye is applied on them by using an ink-jet printer for textile fibers. In practice, before applying the dye, the textile fibers are treated with such a mixture in which citric acid and B-Alanine do not constitute a eutectic solvent.

Such a mixture may be prepared in the implementation of the dyeing method described herein or may be provided as a dyeing product for use in the subsequent steps.

In addition to this first embodiment of the method, a dyeing product for textile fibers comprising a water solution of citric acid, B-Alanine or Betaine and one or more auxiliary agents is therefore also described.

Such a product is, in essence, a product for treating the textile fibers before they are printed by the ink-jet technique.

A first embodiment of this product comprises about 13% by weight citric acid monohydrate, about 5.6% by weight B-Alanine, 11% by weight adjuvant agents and water to 100%.

A second embodiment of this product comprises about 14% by weight citric acid monohydrate, about 7.8% by weight Betaine, 11% by weight adjuvant agents and water to 100%.

Returning to the method, the mixture as prepared above is fed in an impregnation pot, setting a cylinder squeeze ratio that allows 50% absorption.

After the textile fibers have been properly treated with the mixture, they are dried in hot air at a temperature between 60°C and 70°C.

Once water evaporated, citric acid and B-Alanine are present in the textile fibers as a film.

Next, the previously treated textile fibers are printed by using, as mentioned above, an ink-jet printer for textile fibers. For example, printing can be set by adjusting the resolution to 600dpi- 1200dpi. The ink-jet printing is carried out in a known way by using one or more ink-jet printing inks prepared in a known way by using dispersed dyes.

Alternatively, one or more ink-jet printing inks prepared as follows may be used.

A mixture comprising citric acid and B-Alanine is prepared by putting together 210 g citric acid monohydrate and 89.09 g B-Alanine, so as to create a mixture of citric acid and B-Alanine with a 1 : 1 molar ratio.

To this mixture, 59.846 g water is added, accounting for 20% of the total weight of the mixture.

The water, citric acid and B-Alanine thus mixed are heated to a temperature between about 70°C and about 75°C and are kept at a temperature within this range until a clear solution is obtained.

400 g of a mixture prepared in the way and with the molar ratios mentioned above is collected and 300 g powdered dispersed dye is added to it. Clearly the dispersed dye that is added is selected depending on the color the ink needs to have. The dispersed dye which is added may have a particle size with a particle population density between 80 nm and 1 micron.

Kneading is performed and once a good solubilization of the dispersed dye is achieved, 50 g propylene glycol is added. After the propylene glycol is added, kneading is performed until adequate dissolution of the powdered dye is achieved.

Once complete dissolution has been reached, 240 g water is added and, optionally, 10 g sodium polyacrylate is added.

This produces a liquid ink for use in the preparation of an ink-jet printing ink in which the dispersed dye has a concentration of about 30% by weight.

For the preparation of 1000 g ink-jet printing ink, an amount of liquid ink between 50 g and 150 g is prepared in the way described above and wetting agents (such as diethylene glycol, polyethylene glycol, propylene glycol), in an amount between 200 g and 300 g, 20 g tamarind -based non-ionic thickener, 10 g surfactant agents, 5 g pH adjusters, 100 g of a mixture prepared by mixing 210 g citric acid, 89.09 g 13- Alanine and 59.846 g water, and water to 1000 g, are added to it.

Compared with the ink-jet printing ink made according to the known art, the ink described herein is mostly prepared by making use of environmentally friendly raw materials.

It should be underlined that regardless of how the ink is formulated, thanks to the use of the mixture described above comprising citric acid and 13-Alanine, it is possible to use dispersed dyes having a particle population density between 1 micron and 80 nm instead of using dispersed dyes processed in such a way as to have particle densities in the range of 200-250 nm, as is the case in the known art.

Following the printing step, the textile fibers either undergo a drying step followed by a step of thermally fixing or directly undergo a step of thermally fixing.

The drying step may be carried out at 80°C for a time range between 40 seconds and 50 seconds.

During the step of thermally fixing, the textile fibers on which the ink- formulated dispersed dye was printed are heated in hot air at 155°C for 90 seconds.

The graph shown in Figure 1 shows that, at this temperature, the citric acid and 13-Alanine are poorly degraded; in fact, at this temperature there is more than 95% of the mass of citric acid and 13-Alanine initially present.

After the step of thermally fixing, the textile fibers undergo a washing step in water heated to a temperature of 70°C-80°C for 5-10 minutes in the presence of an antireplicating auxiliary agent. For example, quaternized ethoxylated tallow amine at an amount of 2 g/1 may be used. Lastly, a final drying step is performed.

Thanks to the use of the mixture described above comprising citric acid and 13- Alanine, this method allows, compared to an ink-jet dyeing method according to the known art, to significantly limit the use of sodium polyacrylate and to implement the thermally fixing and washing steps more advantageously, providing dyeing results of equal or higher quality than the fibers dyed by the traditional methods.

For example, compared with a dyeing method by ink-jet printing according to the known art involving carrying out the step of thermally fixing at 170°C for 120 seconds, the present method allows the thermal fixation to be carried out at 155°C for 90 seconds, with a 12% reduction in temperature and 25% reduction in the time required for the thermal fixing to take place. Clearly, this translates into saving time or costs associated with the method.

Or, compared with a dyeing method of ink-jet printing according to the known art, the present method allows avoiding the use of hydrosulphite, caustic soda and detergent during the washing step, so that these potentially toxic substances are not used. Accordingly, the subsequent neutralization step, carried out according to the known art at 60°C for 10 minutes, can also be avoided, with obvious cost and time savings.

Finally, comparing the end result of the dyeing method described above with the result of a dyeing method of ink-jet printing according to the known art, it emerges that the textile fibers dyed by the ink-jet method according to the present invention have higher quality dyeing.

For example, by using the mixture described above, the textile fibers have higher yield, higher resolution, gloss and definition. Furthermore, unlike the textile fibers dyed following the method according to the known art, the textile fibers have four-color components that are homogeneous with each other.

Furthermore, a test carried out by dyeing textile fibers using the dyeing products employed in the known art and carrying out the step of thermally fixing at 155°C for 90 seconds, instead of 170°C for 120 seconds, showed that the result of such process had a lower dyeing yield than the result obtained by the method according to the present invention, even though the step of thermally fixing was carried out under the same conditions.

This means that the benefits associated with this method are actually given by the use of the aforesaid mixture.

Finally, by performing a comparison on rub and washing fastness carried out according to UNI EN ISO 105-X12 standards, it was found that the method described herein results in higher rub and washing fastness.

A second embodiment of the method according to the present invention relates to dyeing textile fibers, preferably but not exclusively made of polyacrylate, by employing the traditional screen or rotary printing technique.

This embodiment involves the initial preparation of a printing paste intended to be conveyed into an appropriate printing device and then applied to the textile fibers.

The printing paste is made by using a liquid ink prepared with a mixture comprising citric acid and B-Alanine (first mixture) or with a mixture comprising citric acid and Betaine (second mixture).

The liquid ink prepared with a mixture comprising citric acid and B-Alanine is prepared in the following way.

210 g citric acid and 89.09 g B-Alanine are put together, so as to create a mixture of citric acid and B-Alanine with a 1 : 1 molar ratio.

To this mixture, 59.846 g water is added, accounting for 20% of the total weight of the mixture.

The water, citric acid and B-Alanine thus mixed are heated to a temperature between about 70°C and about 75°C and are kept at a temperature within this range until a clear solution is obtained.

400 g of a mixture prepared in the way and with the molar ratios mentioned above is collected and 300 g powdered dispersed dye is added to it. Clearly the dispersed dye that is added is selected depending on the color the printing paste needs to have once prepared.

Kneading is performed and once a good solubilization of the dispersed dye is achieved, 50 g propylene glycol is added. After the propylene glycol is added, kneading is performed until the complete dissolution of the powdered dye.

Once complete dissolution has been reached, 240 g water is added and, optionally, 10 g sodium polyacrylate is added.

This way a liquid ink, in which the dispersed dye has a concentration of 30% by weight to be used in the preparation of the printing paste, is obtained. In the liquid ink thus prepared, citric acid and 13-Alanine constitute a eutectic solvent and the dispersed dye is solubilized in the mixture.

The liquid ink prepared with a mixture comprising citric acid and Betaine is prepared in a similar way as described for the liquid ink comprising citric acid and 13- Alanine.

The differences are the following. First, we start from a mixture comprising 210 g citric acid and 117.08 g Betaine, so as to create a mixture of citric acid and Betaine with a 1 : 1 molar ratio, and then we add 65.44 g water to the mixture, which is 20% of the total weight of the mixture. Furthermore, after adding the dispersed dye, instead of 10 g sodium polyacrylate, 10 g of a non-ionic, tamarind-based anti-migration agent or Aloe Vera extract may optionally be added.

The ink described above constitutes a second dye product to be used in the preparation of the printing paste.

In a first embodiment, the aforesaid ink comprises: 28% by weight citric acid, 12% by weight 13- Alanine, 30% by weight powdered dye, 6% by weight auxiliary agents (for example, 5% propylene glycol and 1% anti-migration agent relative to the total weight of the ink) and water to 100%.

In a second embodiment, the aforesaid ink comprises: 26% by weight citric acid, 14% by weight Betaine, 30% by weight powdered dye, 6% by weight auxiliary agents (for example, 5% propylene glycol and 1% anti-migration agent relative to the total weight of the ink) and water to 100%.

Clearly, depending on the color range of the dye you want to obtain, you need to prepare different inks by using different dispersed dyes.

Making use of a liquid ink prepared according to the recipe described above, a printing paste is prepared in the following manner. 100 g liquid ink prepared as described above is taken, and:

- if it has been prepared by using a mixture comprised of citric acid and B- Alanine, 100 g of a mixture prepared by mixing 210 g citric acid monohydrate and 89.09 g B-Alanine and adding 59.846 g water, equal to 20% of the total weight of the mixture, is added to the liquid ink (water, citric acid and B-Alanine thus mixed are heated to a temperature of about 70°C-75°C and kept at a temperature within this range until a clear solution is obtained).

- if it has been prepared by using a mixture comprised of citric acid and Betaine, 100 g of a mixture prepared by mixing 210 g citric acid monohydrate and 117.08 g Betaine and adding to the mixture 59.846 g water, equal to 20% of the total weight of the mixture, is added to the liquid ink (water, citric acid and Betaine thus mixed are heated to a temperature of about 70°C-75°C and kept at a temperature within this range until a clear solution is obtained).

After this step, 50 g tamarind-based non-ionic thickening agent and 750 g water are further added.

In the printing paste, citric acid and B-Alanine or citric acid and Betaine do not constitute a eutectic solvent and the dispersed dye is solubilized in the printing paste itself.

In a first embodiment, the aforesaid printing paste comprises: about 8.6% by weight citric acid, about 3.7% by weight B-Alanine, 3% by weight dispersed dye, about 5.5% by weight auxiliary agents and water to 100%.

In a second embodiment, the aforesaid printing paste comprises: about 8% by weight citric acid, about 4.4% by weight Betaine, 3% by weight dispersed dye, about 5.5% by weight auxiliary agents and water to 100%.

The printing paste thus prepared is used in the rotary or screen printing process. The printing paste is therefore an additional product used for dyeing textile fibers.

The screen or rotary printing is carried out in a known way.

Clearly printing pastes of different colors will have to be prepared and employed depending on the colors used.

After printing, a drying step can be performed or the textile fibers are directly subjected to a step of thermally fixing carried out at 155°C for 90 seconds, in case a mixture comprising citric acid and 13-Alanine is used, or carried out at 170°C for 90 seconds if a mixture comprising citric acid and Betaine is used.

The graph shown in Figure 1 shows that, at this temperature, citric acid and 13- Alanine or citric acid and Betaine are poorly degraded; in fact, at 155°C and 170°C there is more than 95% of the total mass of citric acid and 13-Alanine and of citric acid and Betaine initially present, respectively.

After the step of thermally fixing, the textile fibers undergo a washing step in water heated to a temperature of 70°C-80°C for 15 minutes in the presence of an antireplicating auxiliary agent. For example, quaternized ethoxylated tallow amine at an amount of 2 g/1 may be used.

Thanks to the use of the mixture described above comprising citric acid and 13- Alanine or citric acid and Betaine, this method allows, compared to a dyeing method by rotary or screen printing according to the known art, to significantly limit the use of sodium polyacrylate in the preparation of the printing paste and to advantageously implement the steps of thermally fixing and washing, thus obtaining dyed products of equal or better quality than the ones obtained in a known way.

For example, compared with a dyeing method according to the know art involving carrying out the step of thermally fixing with dry heat at 210°C for 30-60 seconds, the present method allows carrying out the thermal fixation at 155°C for 90 seconds, with a 26% reduction of the temperature needed for the thermal fixation to occur, or at 170°C for 90 seconds, with a 20% reduction of the temperature needed for the thermal fixation to occur. Clearly, this translates into saving costs associated to the dyeing method.

Or, compared with a dyeing method according to the known art, the present method allows avoiding the use of hydrosulphite, caustic soda and detergent during the washing step, so that these potentially toxic substances are not used, and the related neutralization step traditionally carried out at 60°C for 10 minutes is not performed, with obvious savings in costs and time.

Finally, comparing the end result of the dyeing method described above with the result of an analogous dyeing method according to the known art, it emerges that the textile fibers dyed by the method according to the known art have lower quality dyeing.

For example, by using the mixture described above, the textile fibers have higher yield, higher resolution, gloss and definition. Furthermore, unlike the textile fibers dyed with the method according to the known art, the textile fibers have four-color components that are homogeneous with each other.

A variation of the embodiment of the method described above involves carrying out the step of thermally fixing by HT steaming.

In this variation, the printing paste is made from the liquid ink prepared and formulated as described above. The printing paste is also prepared in a similar way as presented above with the difference that 500 g aloe vera extract instead of 50 g tamarind-based thickener and 300 g water instead of 750 g water are added.

In particular, the aforesaid printing paste may comprise: about 8% by weight citric acid, about 4.4% by weight Betaine, 3% by weight dispersed dye, about 50.5% by weight auxiliary agents and water to 100%.

The printing step is carried out in a known way, according to the rotary or screen printing technique.

The step of thermally fixing is carried out by HT steaming at 172°C for 5 minutes.

Thanks to the use of the mixture described above comprising citric acid and 13- Alanine or citric acid and Betaine, this method allows achieving significant advantages over a similar dyeing method according to the known art from the steps of thermally fixing and washing.

For example, compared with an analogous dyeing method according to the know art involving carrying out the step of thermally fixing by HT steaming at 180°C for 15 minutes, the present method allows carrying out the thermal fixation by HT steaming at 172°C for 5 minutes, with a 4% reduction in temperature and 66% reduction in the time required for the thermal fixation to take place. Clearly, this translates into saving costs associated to the dyeing method.

Or, compared with an analogous dyeing method according to the known art, the present method allows avoiding the use of hydrosulphite, caustic soda and detergent during the washing step, so that these potentially toxic substances are not used, and the related neutralization step traditionally carried out at 60°C for 15 minutes and draining step at 40°C are not performed, with obvious savings in costs and time.

A third embodiment of the method according to the present invention relates to dyeing textile fibers, preferably but not exclusively made of polyacrylate, by employing the impregnation dyeing technique.

Such embodiment involves the preparation of an impregnation bath that must be compatible with acidic pH and must have a non-ionic character.

The impregnation bath is prepared by mixing 100 g liquid ink prepared as described above with reference to the printing paste, 20 g tamarind-based non-ionic thickener, 780 g water and 100 g of a mixture prepared by mixing 210 g citric acid monohydrate, 89.09 g 13-Alanine and 59.846 g water, or prepared by mixing 210 g citric acid monohydrate, 117.08 g Betaine and adding 59.846 g water.

In a first embodiment, the aforesaid impregnation bath comprises: about 8.6% by weight citric acid, about 3.7% by weight 13- Alanine, 3% by weight dispersed dye, about 2.5% by weight auxiliary agents and water to 100%.

In a second embodiment, the aforesaid impregnation bath comprises: about 8% by weight citric acid, about 4.4% by weight Betaine, about 3% by weight dispersed dye, about 2.5% by weight auxiliary agents and water to 100%.

Once the textile fibers are impregnated with this mixture, they undergo a step of thermally fixing.

The advantages described above, with reference to the embodiments of the method, are achieved thanks to the use of the aforesaid mixture with which the dispersed dye is contacted.

In particular, during the ink-jet printing, the dispersed dye formulated as liquid ink for ink-jet printing is contacted with the mixture comprising citric acid and 13- Alanine during the printing step. In this context, the mixture comprising citric acid and 13- Alanine constitutes a eutectic solvent and when the ink-jet ink is applied to the textile fibers treated with this mixture, the dispersed dye contained in the ink is solubilized in the eutectic mixture present on the textile fibers.

During the traditional rotary or screen printing, the dispersed dye is contacted with the mixture comprising citric acid and B-Alanine or comprising citric acid and Betaine prior to the printing step on the textile fibers, and particularly during the preparation of the liquid ink and during the preparation of the printing paste intended to be applied on the textile fibers.

In the printing paste, citric acid and B-Alanine or citric acid and Betaine do not constitute a eutectic solvent, nevertheless the dispersed dye is solubilized in the printing paste itself.

What has been described for the traditional printing can be extended to impregnation and, in particular, to the impregnation bath made as described above.

In practice, regardless of the application technique used, the eutectic mixture comprising citric acid and B-Alanine or citric acid and Betaine is particularly effective in solubilizing the dispersed dye. It is noted that the dispersed dye is solubilized in the mixture even after dilution of the mixture itself.

In essence, unlike the traditionally used auxiliary agents such as citric acid or propylene glycol, which are capable of dispersing the dispersed dye in water, such a mixture is instead capable of solubilizing the dispersed dye in water.

Thanks to this ability to solubilize the dispersed dye, the amount of dispersed dye that is not absorbed by the textile fibers during the dyeing process is significantly limited, thus making it unnecessary to perform a washing in a strongly basic and reducing environment of the dyed textile fibers, as is the case of the methods according to the known art.

Therefore, the use of said mixture allows eliminating the use of hazardous and polluting substances from the washing step and eliminating the corresponding neutralization step traditionally carried out after the washing step, with evident advantages in terms of environmental sustainability, process duration and related costs.

Furthermore, when the mixture constitutes a eutectic solvent, it enables the diffusion mechanism of the dispersed dye in the textile fibers.

Considering a mixture prepared by dissolving 210 g citric acid monohydrate and 89.09 g 13-Alanine in 59.846 g water or dissolving 210 g citric acid monohydrate and 117.08 g 13- Alanine in 65.44 g water, the following tests were performed.

In evaluating the Hansen solubility parameters of acetone, tetrachloroethene and said mixture with respect to the dispersed dye, it emerges that said mixture has a high ability to dissolve the dispersed dye compared with acetone and tetrachloroethene. In fact, the eutectic solvent has a distance according to Hansen parameters, relative to the dispersed dye, of 3.67 while acetone and tetrachloroethene of 9.77 and 9.15, respectively.

In contrast, by comparing the Hansen solubility parameters of acetone, tetrachloroethene and said mixture with respect to the textile fibers made of polyester, it emerges that said mixture has a distance according to Hansen parameters of 5.60 relative to the textile fibers made of polyester. Such distance indicates an affinity of the mixture to the polyester that is intermediate between acetone, which has a distance of 5.24, and tetrachloroethene, which has a distance of 8.08.

The distances were calculated by using the following formula:

In practice, the mixture has solubility parameters that allow it, on the one hand, to be used to solubilize the dispersed dyes, and on the other hand, to be used in the dyeing processes of the textile fibers because it is not able to dissolve them.

The advantages described above with reference to the step of thermally fixing are also likely achieved by, among other things, the fact that the eutectic mixture has higher thermal conductivity and thermal diffusibility values than the traditional auxiliary agents, and a lower heat capacity than the traditional auxiliary agents.

For example, the heat capacity of the mixture comprised exclusively of citric acid and 13-Alanine, calculated on a total mass of 299.23 g comprised of 70.23% citric acid and 29.77% 13-Alanine, is 1144.58 J/K.

Or, the heat capacity of the mixture described above added with propylene glycol turns out to be 1203.04 J/K. This mixture has a total mass of 314.23 g and is comprised of citric acid (210.14 g) at 66.87%, B-Alanine (89.09 g) at 28.35% and propylene glycol (15 g) at 4.77%.

Otherwise the heat capacity of the above described mixture added with propylene glycol and sodium polyacrylate turns out to be 1253.33 J/K. This mixture has a total mass of 319.63 g and is comprised of citric acid (210.14 g) at 65.74%, B-Alanine (89.09 g) at 27.87%, propylene glycol (15 g) at 4.69% and sodium polyacrylate (5.4 g) at 1.6%.

Such values are significantly lower than the heat capacity of sodium polyacrylate, which is found to be 5852 J/K.

The heat capacity values stated above were calculated in a known way taking into account that heat capacity is given by the product of "specific heat per unit mass" and the mass of the substance sample.

The fact that the mixture used, even with auxiliary agents added, has a lower heat capacity than that of polyacrylate indicates that the use of the aforesaid mixture in a dyeing method allows a predetermined temperature to be reached by supplying less energy than would be required using sodium polyacrylate.

Regarding the thermal conductivity and diffusivity parameters, the mixture comprised of citric acid and B-Alanine has higher values than those of sodium polyacrylate, which is usually used in the dyeing methods according to the known art.

The comparison between this mixture and sodium polyacrylate was carried out by a simulation performed with the Comsol Multiphysic simulator. Such simulation involves the simulated use of thermal detection probes present at the heat source (air), on the matrix (mixture or polyacrylate) and on the polyester textile fiber.

The graph in Figure 2 shows the temperature trend values as a function of time, employing a mixture of citric acid/B-Alanine and a mixture constituted by sodium polyacrylate.

Logarithmic scales or normalized curves were used for comparison.

From this test it can be seen that the eutectic mixture shows a velocity factor by which the mixture increases the polyester temperature of 1.8.

Furthermore, in the context of the thermal processes with solid/gas/textile fiber transfer, the use of the present mixture allows a greater concentration gradient of dye to be achieved between the vapor phase and the surface of the textile fibers than a method according to the known art.

This is because dye steam reaching the surface of the textile fiber, and the molecular adsorption is regulated by the formula

Ap ₀ = RT ln Cf/ Cg

In this formula Ap ₀ is the standard affinity (free energy variation) of the dye with the fiber at the temperature T, R is the constant of the ideal gas law, T is the temperature, Cf is the concentration of the dye on the textile fibers, Cg is the concentration of the dye steam, and the ratio Cf / Cg = K is the partition coefficient.

Since the Cf/Cg ratio coincides with the partition coefficient, using the eutectic mixture means that a greater concentration gradient of dye develops between the vapor phase and the surface of the textile fibers than in a system in the absence of the eutectic mixture. In the methods according to the known art, such values would be reached at higher temperatures. Furthermore, since the diffusion of the dye molecules from the surface inside the textile fibers (Df) begins as soon as the concentration gradient of the dye is established, employing the aforesaid mixture achieves this condition in less time than a known method under the same thermal conditions. And since the Df value is also increased in a eutectic system, the rate at which the dye molecules diffuse through the polymer chains also increases, further establishing a gain not only in time but also in temperature, i.e., we will have a higher concentration of dispersed dye in the textile fibers than in a known method under the same thermal conditions.

This appears by comparing the graph in Figure 3a, which shows the amount of dispersed dye in the textile fibers by using a known dyeing method, with the graph in Figure 3b, which shows the amount of dispersed dye in the textile fibers by employing the aforesaid mixture in accordance with the present invention.

Furthermore, the calculation of the degree of penetration into the section of the polyester textile fibers shows that the mixture constituted by citric acid and B-Alanine penetrates in the textile fiber 2.08 times more than sodium polyacrylate.

The degree of penetration can be calculated by the formula I) k\al in which D is the depth of penetration, K is the constant, "a" is the thermal diffusivity and " " the time.

The thermal diffusivity of the aforesaid mixture is 0.297 mm ² /s while that of sodium polyacrylate is 0.068 mm ² /s.

Therefore, for the same time and constant K value, the penetration depth of the eutectic mixture is 2.08 greater than that of sodium polyacrylate.

Furthermore, the eutectic mixture has a higher thermal conductivity value than that of sodium polyacrylate, that is, 0.484 W/m*K compared to 0.398 W/m*K (1.2 times higher).

Furthermore, it was found that such a mixture increases temperature 3.35 times faster than sodium polyacrylate for the same surface area and energy supplied.

Finally, when the method involves the use of a mixture comprising the aforesaid compounds that do not constitute a eutectic solvent it allows, at first, to promote the partition of the dispersed dye in the fibers, and secondly, after heating, to promote the diffusion of the dispersed dye in the textile fibers.

This can be appreciated with the help of the graph shown in Figure 4, from which it emerges that when the mixture comprises water, dispersed dye, citric acid and 13-Alanine, which do not constitute a eutectic solvent, the affinity of the dispersed dye for the polyester textile fibers is greater than the affinity of water, citric acid and 13- Alanine: this favors the mechanism of partitioning the dispersed dye in the textile fibers.

On the other hand, when, upon heating the mixture, there is the formation of a mixture in which citric acid and 13-Alanine constitute a eutectic solvent, the affinity of the dispersed dye with respect to the textile fibers is similar to that of the mixture itself: this promotes the diffusion mechanism of the dispersed dye in the textile fibers. This graph also shows that the affinity of the mixture for the dispersed dye, in which citric acid and 13-Alanine constitute a eutectic solvent, is greater than the affinity for the dispersed dye of water, citric acid and 13-Alanine that do not constitute a eutectic solvent.

Such affinity values were calculated with the following formula: and by using as values those in the following table: This change in the affinity of the mixture to the textile fibers occurs when the step of thermally fixing is carried out after, in the context of traditional printing, the printing paste is applied on the textile fibers, or after the impregnation of the textile fibers is carried out.

Contacting the textile fibers with a dispersed dye, initially, in a mixture in which the aforesaid compounds do not constitute a eutectic mixture and, subsequently, in a mixture in which the aforesaid compounds do constitute a eutectic mixture, allows achieving a better dyeing result because textile fibers are obtained with a greater amount of dye dispersed inside them (thanks to the greater partition) and with dispersed dye penetrated more deeply (thanks to the greater diffusion). In the embodiments depicted and described, a technician in the art may make a number of changes and modifications to the present invention, with the purpose of satisfying contingent and specific needs, all of which are however included within the scope of protection of the invention as defined by the following claims.