WO2024084340A1

WO2024084340A1 - Reactive dyeing process for a cellulosic substrate

Info

Publication number: WO2024084340A1
Application number: PCT/IB2023/060227
Authority: WO
Inventors: Alberto Tonani; Andrea Novello; Ettore MORANDINO; Alessandra MAIFRINI
Original assignee: Zanolo S.P.A.
Priority date: 2022-10-17
Filing date: 2023-10-11
Publication date: 2024-04-25

Abstract

Reactive dyeing process for a cellulosic substrate comprising the following steps: (i) preparing a dyeing bath, the dyeing bath comprising water, at least one wetting agent, at least one multifunctional reactive dye and, optionally, at least one chelating agent; (ii) dipping the cellulosic substrate into the dyeing bath; (iii) adding 50% w/w liquid potassium carbonate to the dyeing bath at a concentration comprised between 50 and 180% w/w with respect to the dry weight of the cellulosic substrate, (iv) heating the dyeing bath to a temperature comprised between 50 and 70 °C, preferably equal to 60 °C; (v) allowing the cellulosic substrate to react in the dyeing bath with the at least one multifunctional reactive dye for a period of time sufficient for the at least one multifunctional reactive dye to fix to the cellulosic substrate; (vi) eliminating the at least one multifunctional reactive dye not fixed to the cellulosic substrate by adding water only, obtaining a cellulosic substrate dyed with the at least one multifunctional reactive dye.

Description

"Reactive dyeing process for a cellulosic substrate"

FIELD OF THE INVENTION

The present description relates to a reactive dyeing process for a cellulosic substrate.

BACKGROUND OF THE INVENTION

Reactive dyes for dyeing and printing cellulosic materials are very common. Reactive dyes were first introduced in 1956 as Procion dyes for producing bright colours on cellulosic materials using continuous and discontinuous dyeing methods. These are water-soluble anionic dyes available in various physical forms, such as pourable granules, finished powders and highly concentrated aqueous solutions.

The general formula of the reactive dyes, divided by fixation temperature (cold 60°C, hot 80°C) , can be schematised as shown in Figure 1. The chemical reactions between the reactive dye and a cellulosic substrate, and which leads to the dye to being fixed (covalently bonded) to the cellulosic substrate, is also schematised in Figure 1.

Reactive dyes consist of four parts, (i) the chromophore (which confers the colour to the molecule) , (ii) the reactive group (which allows the dye to react with the cellulosic substrate, and which can also react with the water molecules present in the dyeing bath, a phenomenon called hydrolysis) , (iii) the solubilising group/s attached to the chromogen (which confers water solubility to the dye) and (iv) the substituting groups (which affect the reactivity and dyeing characteristics of the dyes) .

Depending on the type of reaction, reactive dyes are divided into two categories: dyes that react through the nucleophilic substitution reaction (such as for example those containing triazine as the reactive group) and dyes that react through the nucleophilic addition reaction (these are predominantly dyes that contain a vinyl sulfone in their structure as the reactive group) .

Most reactive dyes belong to the class of azo compounds, although anthraquinone-based reactive dyes are also available.

Heterobifunctional dyes, which contain both chlorotriazine reactive groups and vinyl sulfone reactive groups, are also widely used. These dyes, capable of forming a covalent bond with cellulose through the chlorotriazine group, have the advantage lying in the fact that the chlorotriazine group increases the substantivity of the dye and therefore improves the degree of exhaustion and fixation of the dye.

Among chlorotriazine-based dyes, reactivity depends on the number of chlorine atoms present. Dichlorotriazine-based dyes have a higher reactivity than monochlorotriazine dyes; therefore, the former require a lower temperature and milder alkali for their application with respect to monochlorotriazine dyes.

Further classes of triazine dyes containing three reactive groups for each dye molecule have been developed in recent years. These multifunctional reactive dyes have been specially designed for cellulosic fibres; typical examples of such dyes are C.I. Reactive Yellow 217, C.I. Reactive Red 286 and C.I. Reactive Blue 281.

The known reactive dyeing processes essentially provide for three steps, to be carried out in an autoclave :

1. rise of the reactive dye on the fibre (also known as the exhaustion step) ;

2. fixation of the reactive dye;

3. removal of unreacted reactive dye. The dyeing process is always preceded by a step for preparing the cellulosic substrate for dyeing, called pre-washing. This step is intended to remove impurities in the substrate (fats, waxes, pectins, lignins, minerals and organic acids) , which adversely affect the dyeing process given that they confer to the substrate a lipophilic character which prevents the fixation of the water-solubilised dye to the cellulosic substrate. Pre-washing requires the use of wetting agents, detergents, emulsifiers, sequestrants and multiple aqueous rinses to remove impurities. The pre-washing step generally lasts between 30 and 60 minutes.

Figure 2 shows the dyeing processes according to the prior art, for cold dyeing (panel A) and for hot dyeing (panel B) respectively.

The reagents required for the dyeing operation are numerous and they are added at different process steps.

At the end of the pre-washing step (not shown in figure 2) , water, wetting agents, detergents and emulsifiers (reagents "A" in Figure 2) and an electrolyte, typically represented by sodium chloride powder or sodium sulphate powder (reagent "B" in Figure 2) are introduced into the autoclave at room temperature (approximately 25-30°C) . The fibre:water by weight ratio is generally equal to 1:10. The electrolyte, present in the dyeing bath at an amount comprised between 60 to 100 % by weight with respect to the weight of the fibre, serves to fix the dye on the cellulosic fibre, given that it reduces the electrostatic repulsion forces between the fibre and the dye, that is it reduces the zeta potential [1.2] , therefore allowing to form a covalent bond between the dye and fibre.

After introducing reagents A and B into the autoclave, the temperature is brought to 60 or 80 °C as a function of the type of dyeing, cold or hot respectively .

The reactive dye (Reagent "C" in Figure 2 ) is added after heating .

The temperature in the autoclave is kept constant for a period of approximately 55 minutes . This step corresponds to the rise of the dye .

Subsequently, al kalis , such as sodium carbonate powder and sodium triphosphate powder ( reagent "D" in Figure 2 ) , are introduced for the fixation of the dye on the fibre by creating a covalent bond between the dye and the fibre . The concentration of the alkali is minimal and can vary from 2 % to 10% by weight with respect to the weight of the fibre so as not to cause hydrolys is of the dye . This step is almost entirely controlled by the amount of electrolyte ( chloride or sul fate ) in the dyeing bath [ 3 ] .

In the case of hot dyeing, after the fixation step has been completed, the temperature is lowered to approximately 60 ° C to minimise hydrolysis of the dye .

At the end of the fixation step, which has an average duration of approximately 90 minutes , detergents and soaping agents ( reagent "E" in Figure 2 ) are introduced into the autoclave to carry out the step for removing the unreacted and hydrolysed dye , which provides for repeated rinsing of the fibre in cold water and then in hot water .

The time required for the washing step is variable depending on the amount of hydrolysed dye adsorbed on the fibrils .

The dyeing process , may basically last for more than 12 hours , given that some reagents ( for example B and D) are in the form of solids and require very long introduction and dissolution times . The longer the process takes , the higher the risk of hydrolysis of the dye and therefore the risk of non-fixation of the dye on the fibre , with resulting need for use of large amounts of water and wetting agents , detergents , emulsi fiers , and sequestrants for their elimination .

It should be observed that the times indicated in Figure 2 for the various steps of a reactive dyeing process are theoretical times , which do not take into account the introduction and dissolution times of the various reagents ; it should be observed that the long processing times increase the risk of hydrolysis of the dye with resulting deterioration in the final quality of the dyed fibre .

The ability of a dye to be directly fixed, without mordanting, on a determined fibre is referred to as substantivity and it is expressed as " substantivity factor" ( %S ) .

The exhaustion of the dyeing bath during the fixation step is indicated as "exhaustion factor" ( %E ) .

The total ef ficiency of the fixation step is referred to as " total fixation factor" ( %T ) and it can be spectroscopically measured determining the amount of dye not absorbed present in the dyeing bath at the end of the fixation step .

The degree of fixation of the adsorbed dye , indicated as %F, is related to E% and T% by a simple mathematical relation %T = %F x %E / 100 [ 3 ] .

The various textile processing steps (pre-washing, dyeing, printing, finishing, washing, maceration and bleaching) generate most of the textile wastewater which, typically, contains a complex mixture of organic and inorganic chemical substances , such as hydrolysed dyes , metals , salts , surfactants or the auxiliary dyeing by-products thereof , etc .

The wastewater deriving from textile processing operations can be treated with physical , chemical or biological methods . The treatment technologies are classi fied based on their position in the puri fication plant and these are referred to as primary, secondary and tertiary treatments . Generally, a complete treatment process for textile wastewater consists of the following steps : ( i ) equalisation/homogenisation to stabilise the pH, the redox potential and the dissolved oxygen, ( ii ) primary treatment to reduce the oils , fats , foams , suspended solids and large particles content through coagulation/ flocculation or flotation, ( iii ) correction of pH, ( iv) secondary treatment to reduce COD through an aerobic or anaerobic biological process , and (v) tertiary treatment ( or disinfection) to oxidise the organic and inorganic chemical substances which were not degraded in the previous steps [ 4 , 5 ] .

The dyeing of the cotton in particular requires high amounts of electrolyte ( chlorides and/or sul fates ) to reduce the zeta potential , which results in a high concentration anions Cl~ and SO4²~ in the wastewater collected at the end of the step for removing the dye . The anions Cl~ and SO4²~ have a marked toxicity on the biological sludge of the puri fication . The bacterial colonies which form the activated sludge are actually particularly sensitive to the concentration of such anions : an excessive concentration causes an increase of the osmotic pressure with resulting plasmolysis of the bacterial cell . The concentration di f ference of the anions between the internal and the external of the cell generates the di f fusion of water from the internal toward the external causing the cell membrane to break and therefore a decrease in the concentration of activated sludge [ 6 ] with resulting decrease in ef ficiency of the COD reduction biological treatment . This decrease in ef ficiency generates the risk of exceeding the law restrictions relating to the concentrations of electrolytes in puri fied water provided for drainage in surface water [ 7 ] .

Systems for reducing the concentration of chlorides or sul fates in wastewater such as capacitive deionisation, reverse osmosis and concentration by evaporation, are known in literature [ 8 ] . Despite being ef fective , these processes however entail high operating costs .

Considering the problems associated with the known reactive dyeing processes for cellulosic substrate , there therefore arises the need to identi fy alternative processes which overcome these disadvantages .

SUMMARY OF THE INVENTION

The obj ect of the present description is to provide a reactive dyeing process for a cellulosic substrate which overcomes the known disadvantages of the processes .

A first obj ect of the present invention is the elimination of the step (pre-washing) for preparing the cellulosic substrate .

A second obj ect of the present invention is to reduce the hydrolysis phenomenon in the dye .

A third obj ect of the present invention is to reduce the presence of chlorides and sul fates in the wastewater collected at the end of the dyeing process so as not to adversely af fect the process for puri fying the dyeing wastewater .

A fourth obj ect of the present invention is to reduce the duration of the dyeing process in terms of time and energy cost .

According to the invention, the obj ects outlined above are attained thanks to the obj ect speci fically referred to in the claims below, which are deemed an integral part of the present description .

The present description relates to a reactive dyeing process for a cellulosic substrate comprising the following steps:

(i) preparing a dyeing bath, the dyeing bath comprising water, at least one wetting agent, at least one multifunctional reactive dye and, optionally, at least one chelating agent;

(ii) dipping the cellulosic substrate into the dyeing bath;

(iii) adding 50% w/w liquid potassium carbonate to the dyeing bath at a concentration comprised between 50 and 180% w/w with respect to the dry weight of the cellulosic substrate;

(iv) heating the dyeing bath to a temperature comprised between 50 and 70 °C, preferably equal to 60 °C;

(v) allowing the cellulosic substrate to react in the dyeing bath with the at least one multifunctional reactive dye for a period of time sufficient for the at least one multifunctional reactive dye to fix to the cellulosic substrate;

(vi) eliminating the at least one multifunctional reactive dye not fixed to the cellulosic substrate by adding water only; obtaining a cellulosic substrate dyed with the at least one multifunctional reactive dye.

Brief description of the drawings

Now, the invention will be described in detail, solely by way of non-limiting example, with reference to the attached figures, wherein:

Figure 1: general formulae of the cold and hot reactive dyes and typical diagram of the reaction of a reactive dye with a cellulosic substrate.

Figure 2: Diagrams of the cold (A) and hot (B) reactive dyeing processes according to the prior art. Figure 3: Diagram of the dyeing process according to the present invention.

Detailed description of the invention

In the following description, numerous specific details are provided to provide an exhaustive understanding of the embodiments. The embodiments may be implemented with or without one or more of the specific details, or with other processes, components, materials etc. In other cases, well-known structures, materials, or operations are not described in detail so as to avoid confusing aspects of the embodiments.

Within the present application, reference to "one (= numeral adjective) embodiment" or "an (= indefinite articles) embodiment" indicates that a particular version, structure, or characteristic described with reference to the embodiment is included in at least one embodiment. Therefore, the forms of the expressions "in one (= numeral adjective) embodiment" or "in an ( = indefinite articles) embodiment" in various points within the present application do not all necessarily refer to the same embodiment. Furthermore, the particular versions, structures , or characteristics may be combined in any appropriate fashion in one or more embodiments .

The titles provided herein are just for the sake of convenience and they do not interpret the scope of the various embodiments.

The present description relates to dyeing process for a cellulosic substrate comprising the following steps :

(i) preparing a dyeing bath, the dyeing bath comprising water, at least one wetting agent, at least one multifunctional reactive dye and, optionally, at least one chelating agent; ( ii ) dipping the cellulosic substrate into the dyeing bath;

( iii ) adding 50% w/w liquid potassium carbonate to the dyeing bath at a concentration comprised between 50 and 180% w/w with respect to the dry weight of the cellulosic substrate ;

( iv) heating the dyeing bath to a temperature comprised between 50 and 70 ° C, preferably equal to 60 ° C ;

(v) allowing the cellulosic substrate to react in the dyeing bath with the at least one multi functional reactive dye for a period of time suf ficient for the at least one multi functional reactive dye to fix to the cellulosic substrate ;

(vi ) eliminating the at least one multi functional reactive dye not fixed to the cellulosic substrate by adding water only; obtaining a cellulosic substrate dyed with the at least one multi functional reactive dye .

The reactive dyeing process for a cellulosic substrate according to the present invention is characterised by the non-use of chlorides and/or sul fates such as electrolytes for fixing the multi functional dye to the cellulosic substrate . The dyeing bath according to the present invention is actually substantially free of chlorides and/or sul fates . In this manner, the puri fication treatment of the dyeing wastewater is ef ficient , and it should not provide for further processes for removing chlorides and/or sul fates so as to be able to introduce the puri fied dyeing water in surface water .

The reactive dyeing process of a cellulosic substrate according to the present invention is further characterised by the omission of a step for pre-washing the cellulosic substrate . As a matter of fact , such step can be omitted by using a high concentration of potassium carbonate in the dyeing bath which allows to remove impurities present on the cellulosic substrate , which would af fect the fixation of the at least one multi functional dye to the cellulosic substrate . The impurities naturally present on the cotton fibrils consist of fats , waxes , pectins , lignins , minerals and organic acids which adversely af fect the dyeing process given that they confer to the substrate a lipophilic character which prevent the contact between the aqueous bath and fibre and - as a result - prevent the fixation of the dye to the cellulosic substrate . The dissolution of these impurities carried out at high alkaline concentrations facilitates the penetration of the dye into the fibrils and - as a result - the reactivity thereof .

The step for removing from the dyeing bath of the at least one multi functional dye not fixed to the cellulosic substrate is carried out without adding further compounds , such as detergents and soaping agents , but by adding water only . The high concentration of potassium carbonate , the reduced processing times and the selection of speci fic multi functional reactive dyes actually allows to minimise the amount of hydrolysed dye and sali fy the unreacted dye with respect to the prior art with resulting simpli fication of the washing step which can therefore be carried out with water alone .

In an embodiment , the wetting agent is selected from non-ionogenic surfactants with HLB comprised between 5 and 13 .

In an embodiment , the at least one wetting agent is selected from alcohols having a linear or branched, saturated or unsaturated alkyl chain, comprising 5- 18 carbon atoms with degree of ethoxylation in the range 3- 12 . Wetting agents that can be used in the reactive dyeing process according to the present invention are for example Alcohol C12C14 3/9 OE, Oleic alcohol 4/10 OE, Alcohol C12C18 3/10 OE, Alcohol CIO 5/7 EO, 2- ethylhexyl alcohol 3/5 OE, Synthetic alcohol C12C13 2/8 OE, Synthetic alcohol C12C15 3/7 OE, Alcohol C9 011 6/8 OE, Oxo alcohol C10C13 6/9 OE . This list shall not be deemed to be limiting, given that numerous wetting agents can be used in the present process.

In an embodiment, the at least one chelating agent is selected from sodium polyacrylate, citric acid, tartaric acid, ascorbic acid, gluconic acid and polyphosphates .

The chelating agent has the function of chelating calcium and magnesium ions contained in water so as to prevent these ions from coordinating with the at least one multifunctional dye. The multifunctional dyes actually contain the anionic groups, for example SO3A which - binding to the mono- and bi-valent cations present in the water generate partially insoluble salts, which would reduce the efficiency of the dyeing process.

The preparation of the dyeing bath, which is carried out in the autoclave intended for dyeing, provides for allowing some minutes to elapse between the addition of one reagent and the other. Specifically, the at least one wetting agent is introduced into the autoclave in the presence of water and after 5-15 minutes, preferably 10 minutes, the at least one chelating agent is added and 5 to 10 minutes, preferably 5 minutes, are allowed to elapse. Then there is added at least one multifunctional dye (pre-solubilised in water) .

Subsequently, the cellulosic substrate and water are introduced, the water being added in a ratio by weight with respect to the dry weight of the cellulosic substrate comprised from 5:1 to 20:1, preferably equal to 10:1, and 5 to 15 minutes, preferably 10 minutes, are allowed to elapse under stirring.

Then, potassium carbonate is added and 5 to 15 minutes, preferably 5 minutes, are allowed to elapse under stirring.

Subsequently, the temperature in the autoclave is brought to 50-70 °C, preferably to 60 °C, with a temperature gradient comprised between 2 and 5 °C/min and kept at 60°C for a period comprised between 10 and 60 minutes depending on the desired tone intensity.

In an embodiment, the at least one dye is contained in the dyeing bath in an amount comprised in the range 0.1-8% by dry weight with respect to the dry weight of the cellulosic substrate.

In an embodiment, the step for preparing the dyeing bath (i) is conducted at room temperature.

In an embodiment, the step for preparing the dyeing bath (i) has a duration comprised between 10 and 30 minutes .

In an embodiment, the step for preparing the dyeing bath (i) provides for:

- adding - in the presence of water - the at least one wetting agent,

- allowing 5-15 minutes to elapse,

- adding the at least one chelating agent,

- allowing 5-10 minutes to elapse,

- adding the at least one multifunctional dye.

In an embodiment, the step (ii) for dipping the cellulosic substrate in the dyeing bath is conducted by adding water to the dyeing bath in a ratio by weight with respect to the dry weight of the cellulosic substrate comprised between 5:1 and 20:1, preferably 10:1, and keeping it stirring.

In an embodiment, the step (iii) for adding 50% w/w liquid potassium carbonate to the dyeing bath is conducted under stirring and it has a duration comprised between 5 and 15 minutes.

In an embodiment, the step for fixing the dye to the cellulosic substrate (v) has a duration comprised between 10 and 60 minutes.

In an embodiment, the step for removing the dye not fixed to the cellulosic substrate (vi) provides for: a first continuous rinse with water at room temperature, a second rinse with recirculating water at 50-70 °C, preferably 60 °C, and a third continuous rinse with water at room temperature.

In an embodiment, the step for removing the dye not fixed to the cellulosic substrate (vi) has a duration of about 20-30 minutes.

In an embodiment, the at least one multifunctional dye is selected from multifunctional dyes comprising chlorotriazine reactive groups. Preferably, the at least one multifunctional dye is selected from multifunctional dyes comprising at least two, more preferably at least three chlorotriazine reactive groups per molecule.

Examples of some multifunctional dyes to be used in the dyeing process according to the present invention are: C.I. Reactive Yellow 217, C.I. Reactive Red 286, C.I. Reactive Blue 281. C.I. Reactive Black 5.

The aforementioned list of multifunctional dyes which can be used in the reactive dyeing process described herein shall not be deemed exhaustive in any manner whatsoever.

Figure 3 schematically shows the dyeing process for a cellulosic substrate according to the present invention .

The reactive dyeing process for a cellulosic substrate according to the present invention provides for the use of multifunctional dyes and 50% w/w liquid potassium carbonate as the fixation agent, excluding the use of chlorides and sulfates.

The process provides for introducing the following reagents into the autoclave, in a single step at room temperature, without carrying out the step for preparing the substrate (pre-washing) :

A: a wetting agent, preferably a non-ionogenic surfactant with HLB comprised between 5 and 13, and optionally a chelating agent;

B: 50% w/w liquid potassium carbonate, in proportions variable from 50 to 180% w/w with respect to the weight of the substrate as a function of the tone intensity; and

C: at least one multifunctional dye.

The process does not provide for carrying out the step for pre-washing the cellulosic substrate, given that the high alkaline concentrations of potassium carbonate used in the bath where the dyeing is carried out allow the salification and dissolution of the various impurities (e.g. waxes, fats, etc.) present on the substrate .

Therefore, the process provides for a heating step, which is carried out by increasing the temperature up to about 60°C with a temperature gradient comprised between 2 and 5°C per minute.

The step for fixing the dye, which already starts at 25°C, and continues slowly during the heating step, is carried out without using further products (such as chlorides and/or sulfates) and it has a duration comprised between 10 and 60 minutes as a function of the tone intensity. The concentration of potassium carbonate is actually capable, starting from room temperature, of obtaining a salt concentration sufficient to reduce the zeta potential required for the interaction of the dye with the cellulosic substrate. Furthermore, potassium carbonate is capable of limiting hydrolysis in the multifunctional dye reducing - as a result - the amount of dye not fixed to the substrate to be removed in the last step of the dyeing process.

The removal of the dye is carried out easily and at an unexpected rate (approximately 20-30 minutes) without adding further reagents (e.g. Detergents and soaping agents) but water only (at low amounts) . Without being restricted to any theory on this subject-matter, the inventors strongly believe that the high concentration of potassium carbonate allows the latter to salify the unreacted dye, therefore facilitating the removal thereof .

Besides the aforementioned advantages, the dyeing process described herein allows to overcome the problem relating to handling chlorides and sulfates in wastewater. In the step for homogenising the wastewater treatment process (i.e. treatment which brings the pH from about 12 to an approximately neutral value) , potassium carbonate is balanced between bicarbonate and carbonate, as shown in diagram 1 reported below.

Diagram 1

2 K⁺

K₂CO₃ Bicarbonate and carbonate ions , although in high concentrations , do not cause toxicity in biological sludge [ 9 ] . Therefore , potassium carbonate does not alter the ef fectiveness of the biological treatment and it does not lead to a risk of exceeding the salt concentration values provided for by the law for introducing puri fied dyeing wastewater into surface water .

Furthermore , the present process has halved theoretical and practical times with respect to the prior art , given that it provides for using liquid reagents ( therefore eliminating the times required for the movement and dissolution of solid reagents ) and the reactions between the dye and substrate tend to already start at room temperature therefore reducing the duration of the step for fixing the dye to the cellulosic substrate .

Example 1

The dyeing of the cotton with reactive dyes is carried out in an autoclave with capacity of 250 kg of fibre in a fibre : water by weight ratio of about 1 : 10 , that is using 2 , 500 kg of water for dyeing 250 kg of fibre . The dyeing process does not provide for the step for pre-washing the fibre , but exclusively :

- the preparation of the dyeing bath,

- the dyeing of the cellulosic substrate , and

- the removal of the unreacted dye from the dyeing bath .

Prepara ti on of the dyeing ba th

The dyeing bath consists of an aqueous solution containing the auxiliaries and the dyes required for the dyeing step . The auxiliaries comprise a wetting agent , that is a non-ionogenic surfactant with HLB comprised between 5 and 13 required to facilitate the wettability and therefore the penetration of the liquid into the textile fibre, and a chelating agent, such as sodium polyacrylate, capable of sequestrating the calcium and magnesium ions present in the water, therefore reducing the hardness of the water.

The multifunctional reactive dyes used in this dyeing process belong to the class of Avitera SE (Huntsman Corporation) .

2.5 kg of wetting agent and, after 10 minutes, 7.5 kg of sodium polyacrylate (MW 4500) , are introduced into the autoclave. The dyes, which were previously solubilised in water, are introduced after another 5 minutes. The amount of solid dye varies, and it is comprised in the range from 0.1-8% by dry weight with respect to the dry weight of the cellulosic substrate depending on the tone and intensity of the colour to be obtained. In the specific case of a dyeing of a black cotton ribbon there were used 8 kg di C.I. Reactive blue 281, 0.5 kg di C.I. Yellow reactive 217 and 0.3 kg of C.I. red reactive 286.

The solution containing the auxiliaries, dyes and water at room temperature, is introduced into the autoclave, which already contains the cotton. Once through with filling the autoclave, a mechanical pump is used to circulate the mixture for 10 minutes so as to obtain the correct mixing of the products and the saturation of the fibre.

Subsequently, 350 kg of a 50% w/w of potassium carbonate solution are added still at room temperature; the mixture thus obtained is circulated in the autoclave for 5 minutes, therefore obtaining a dissolution of the waxes and of the fats present on the raw cotton fibre, therefore eliminating the pre-washing step. Despite the concentration of potassium carbonate being very high, being a weak base the pH remains at approximately 12 throughout the entire process.

The preparation of the dye bath has a duration of about 20-30 minutes.

Dyeing the cellulosic substrate

The reactions required for the dye to be fixed to the textile fibre through covalent bonds are carried out in this step.

The dye is fixed by heating the bath from 25°C up to 60°C following a + 2.5 °C/min gradient; after reaching the temperature of 60°C, it is kept constant for 60 minutes .

The partial fixation of the dye at room temperature and the high content of potassium carbonate in a weak alkaline environment allows the dye to already react at temperatures below 30°C without causing hydrolysis in the dye; this allows to half the fixation times with respect to the prior art.

The dyeing step ends by draining the bath, "exhausted", by now with 90% yield.

Removal of the unreacted dye from the dyeing bath

The dyed fibre, still in the autoclave, saturated with high concentrations of potassium carbonate, is cleaned of the unreacted dyes and residual auxiliaries through rinse cycles. Given that the pH conditions are at around 12, the (presumed) salification of the unreacted dyes with potassium carbonate allows a quick cleaning of the fibre.

A first continuous rinse is carried out with water at room temperature for 10 minutes; then the autoclave is drained, filled with water at 60 °C recirculated for 10 minutes. The final continuous washing is carried out with water at room temperature . In order to proceed to the subsequent textile treatments , the fibre is usually dried using centri fugal water extractors and ovens .

The step for removing the unreacted dye from the dyeing bath has a duration of about 20-30 minutes .

The quality control , usually carried out at the end of the processing, provides for a colorimetric analysis through spectrophotometry for the colour point and resistance checks , that is the resistance of the dyeing to various mechanical and chemical stresses provided for by the technical speci fications and according to regulated methods .

The resistance indices of the most signi ficant dyes for each of the categories provided for are "discharge on cotton" and "degradation" . The method to be used for carrying out tests to establish such indices are described in the of ficial ISO standards which provide speci fic recommendations on how to prepare the sample ( test tubes ) and the reagents (where applicable ) , the equipment and relative speci fications , the operating conditions and the outcome evaluation methods .

The resistance of the colour to domestic and industrial washing, the resistance of the colour to dry washing, the resistance of the colour to sweat or rubbing are evaluated in terms of "colour degradation" of the sample and "discharge" of the colour of the sample from the fabrics subj ect matter of the test . The "degradation" evaluation is based on the degree of contrast between the test tube after the test and that of the original fabric . The evaluation of the "discharge" is carried out through the visual evaluation of the colour change of the fabric subj ect matter of the test . Two scales are used to assign the resistance indices : gray scale to establish colour degradation; gray scale to establish colour discharge . Both scales consist of a base scale from 1 (worst ) to 5 (best ) .

The resistance index values of the dyes measured for the dyed cotton sample according to the present example are reported in table 1 .

Table 1

The results obtained by applying the dyeing process according to the present invention can be perfectly compared with the dyeing method according to the prior art . The final dyed fibre actually has a degree of commercial acceptability, as clearly observable by analysing the values reported in table 2 .

Furthermore , the present invention allows to obtain a drastic reduction of energy costs , reduce processing times (hal f of the time ) and a resulting increase in productivity .

The wastewater content , in the absence of Cl~ and SO₄ ²~, comply with the environmental law requirements and they do not af fect the standard primary, secondary and tertiary processes of the wastewater treatment process . The bicarbonates (HCG>3²~ ) and the carbonate ( CG>3²~ ) present in the wastewater do not j eopardise the toxicity of the wastewater in any manner whatsoever . REFERENCES

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9. Henri C. Silberman "Effects of Carbonate Detergent on Biological Wastewater Treatment" Journal (Water Pollution Control Federation) , Vol. 47, No. 3, Part I (Mar. , 1975) , pp . 524-534.

Claims

1. A reactive dyeing process for a cellulosic substrate comprising the following steps:

(ii) dipping the cellulosic substrate into the dyeing bath;

(iii) adding 50% w/w liquid potassium carbonate to the dyeing bath at a concentration comprised between 50 and 180% w/w with respect to the dry weight of the cellulosic substrate,

(vi) eliminating the at least one multifunctional reactive dye not fixed to the cellulosic substrate by adding water only, obtaining a cellulosic substrate dyed with the at least one multifunctional reactive dye.

2. The reactive dyeing process according to claim 1, wherein step (ii) is conducted by adding water to the dyeing bath in a ratio by weight with respect to the dry weight of the cellulosic substrate comprised between 5:1 and 20:1 and keeping it stirring.

3. The reactive dyeing process according to claim 1 or claim 2, wherein the wetting agent is selected from non-ionogenic surfactants with HLB comprised between 5 and 13.

4. The reactive dyeing process according to any one of the preceding claims, wherein the at least one wetting agent is selected from alcohols having a linear or branched, saturated or unsaturated alkyl chain comprising 5-18 carbon atoms with degree of ethoxylation comprised in the range 3-12.

5. The reactive dyeing process according to any one of the preceding claims, wherein the at least one chelating agent is selected from chelating compounds of calcium and magnesium ions.

6. The reactive dyeing process according to any one of the preceding claims, wherein step (i) provides for:

- adding - in the presence of water - the at least one wetting agent,

- allowing 5-15 minutes to elapse,

- optionally, adding the at least one chelating agent, and allowing 5-10 minutes to elapse,

- adding the at least one multifunctional reactive dye .

7. The reactive dyeing process according to any one of the preceding claims, wherein step (iii) is conducted under stirring and it has a duration comprised between 5 and 15 minutes.

8. The reactive dyeing process according to any one of the preceding claims, wherein the at least one multifunctional reactive dye is contained in the dyeing bath in an amount comprised in the range 0.1-8% by dry weight with respect to the dry weight of the cellulosic substrate .

9. The reactive dyeing process according to any one of the preceding claims, wherein step (i) is conducted at room temperature.

10. The reactive dyeing process according to any one of the preceding claims, wherein step (v) has a duration comprised between 10 and 60 minutes.

11. The reactive dyeing process according to any one of the preceding claims, wherein step (vi) provides for: a first continuous rinse with water at room temperature, a second rinse with recirculating water at 50-70 °C, and a third continuous rinse with water at room temperature.

12. The reactive dyeing process according to any one of the preceding claims, wherein step (vi) has a duration of about 20-30 minutes.

13. The reactive dyeing process according to any one of the preceding claims, wherein the at least one multifunctional reactive dye is selected from multifunctional reactive dyes comprising at least two chlorotriazine reactive groups.