Study on the Dyeing Properties of a Novel Reactive Dye with One Vinylsulfone Reactive Group, on Cotton Substrates, Before and After Its Ultrafiltration Treatment

Reactive dyes are the most important class of synthetic dyes applied for cellulosic substrates because of their high wet fastness, brilliance, exceptional application versatility, and wide range of hues [1‐5]. Fabricated to bond covalently with the substrate, reactive dyes comprise a chromophore and a reactive group, while they own their excellent wet fastness to the formation of covalent bonds with the macromolecules of the fibers [6]. They are applied either from a solution with high pH or from neutral solutions that are later alkalized through a separate step. Reactive dyes are categorized by their functional group. Industrially, the organic synthetic dyes prevail for textile dyeing; azo dyes among them are the major synthetic organic dyes covering up to 70% of all synthetic dyes used. Azo dyes are compounds containing one or more chromophore azo groups −N=N−. N-atoms are bonded to neighboring C-atoms by sp² hybridization which may belong to benzene, naphthalene or heterocyclic aromatic rings. The most important method of preparing aromatic azo compounds is “azo coupling” and involves the reaction of aromatic diazo compounds with amines or phenols [7, 8]. The water-soluble part of the dye is necessary when dyeing cellulose fibers as well as protein fibers and usually consists of 1–4 sulfo-groups —SO₃. The chromophore is the part of the molecule that provides color and must have the necessary affinity to the substrate to be dyed satisfactorily.

In a previous published work, three new azo reactive dyes of different metallic salts (Na, K, Li) were synthesized. The dyes were purified/concentrated by ultrafiltration technology. Subsequently, they were characterized as compounds and tested on cotton, wool and polyamide fabrics investigating their dyeing exhaustion and ink-jet printing [8, 9]. Reactive Red DMAVS is synthesized according to the reaction sequence shown in Fig. 1 and is destined to dye cotton fabrics. The mono-azo reactive dyestuff was synthesized by diazotizing the amine 4-amino-2,5-dimethoxyphenylene-β-hydroxyl-ethyl-sulfone sulphate ester and coupling it with equimolar amount of 1-amino-8-hydroxy-naphthalene-3,6-disulfonic acid [10]. The synthesis of the mono functional vinylsulfone reactive dye is novel in the sense that there are no commercially available CAS registered dyes of the above structure in any salt form [9]. The yield of RR_Na dyestuff has been calculated at 720.5 g/moles amine [8].

Hydroxyl groups found on the surface of the cotton fibers (Cell-OH) act as nucleophilic groups and upon their deprotonation, in the presence of alkali solution, they are converted into cellulose anions which are even more nucleophilic (Cell-O⁻). The reactive dye, with the effect of alkali solution and removal of the sulfonic group, is converted into the highly reactive vinylsulfone. The sulfonic group causes electron delocalization by activating the double bond toward nucleophilic attack. The cellulose anion reacts with the vinylsulfone to give the anionic intermediate which is stabilized by coordination. The addition reaction is completed by protonation and finally the permanent formation of a covalent C‒O bond between dye and fiber occurs [2]. Along with the dye-fiber bonding, the competing reaction of dye hydrolysis occurs in the bath. The hydrolyzed dye is slightly adsorbed to the fiber, but it is easily lost after reduction clearing.

It is calculated that approximately 55–60% of global dyestuff consumption concerns cotton fibers, since the essential simplicity of the dyeing process makes them attractive. However, the use of these dyes causes certain problems, including the use of high electrolyte concentrations, in order to enhance, if possible, their exhaustion in the dyebath [1‐5]. Despite electrolytes, the exhaustion of the dyebaths are still limited and the discharge of colored effluents leads to pollution problems [6]. For a compound to be used as a reaction dye, the rate of hydrolysis of the fiber-dye bonds must be negligible compared to the rate of retention of the dye on the fiber [6].

Apart from exhaustion, fixation parameter is crucial for their dyeing. Fixation rate is 50–70% for monofunctional and 70–85% for bifunctional reactive dyes even under optimized dyeing conditions [11]. Development of bifunctional dyes provided prominent improvements in the fixation efficiency. The introduction of commercial low-salt reactive dyes and dyes fixable at neutral pH led to reductions of the use of inorganic chemicals [12]. The most common dye auxiliaries applied are inorganic salts, organic compounds, various acids, water and surfactants [13]. NaCl and NaCO₃ promote the cationic charge on the cellulose and facilitate the movement of the dye molecules toward the fibers as well as their fixation. Additionally, the substantivity of a dye refers to the process during which dye molecules are attracted by physical forces at the molecular level to the textile. High fixation and substantivity lead to better dye utilization and results in less hydrolyzed dye and less effluent waste [6].

The effect of metals like Al, Fe, Cu, Ni, Na, K, Co, complexed on dye molecules or present as auxiliaries in dyebaths, is also important for industrial water treatments [13]. On the other hand, the migration index defines the migration properties of an individual dye applied by a certain process, with the ideal migration index rising to 100. The process of dyeing is based on both routes of adsorption and diffusion of the molecules in the fiber. The movement of the dye from one part of the fiber to another (migration) or the formation of chemical bonds that hold the dye in the fiber (fixation) are relevant to migration index [14].

The application of filtration, and ultrafiltration specifically, to dyestuffs aims to yield ultra-pure dyes with higher color performance, brighter hues and higher solubility [8, 9]. Ultrafiltration (UF) is performed by membranes with a pore diameter of 2–50 nm and is motivated by the pressure difference on both sides of the membrane. The ideal membrane is required to have high porosity and a narrow pore size distribution. Therefore, UF is a physical process employing interactions, not a chemical one. Compounds with a MW < 1000 permeate the membrane leaving the large molecules concentrated on the side of feedstock of the membrane. Ultrafiltration, apart from pretreatment, is also used in the post-treatment of dyebath wastes [6]. In this way, water and auxiliaries (mainly NaCl) are saved, but the environment is also protected with the least deposition of harmful wastes in water resources. Regarding ultrafiltration membranes, materials such as polysulfone or polyamide are used.

The present piece of work describes the kinetic properties of dyeing on cotton substrate with the previously synthesized reactive dye RR_Na, before and after passing an ultrafiltration purification process. The properties of exhaustion, substantivity, fixation, migration index, colorimetric measurements as well as the effect of metals such as Fe, Cu and Co on dyeing, were studied.

Figure 4 shows the UV–Vis spectra of the reactive dye before and after ultrafiltration; as can be seen RR_Na shows an absorbance of 1.22 at λ_max = 531 nm and the UF-RR_Na of 1.45 at λ_max = 529 nm. While the λ_max setback between the two is negligible, the higher absorbance (about 20%) obtained for the UF-dye is attributed to this very purification/concentration process which resulted in a more concentrated dye, by the elimination of inorganic salts and other by-products of low MW. Analogous results were obtained for all dyestuffs treated with UF process, verifying the upgrading of the dye performance [19, 20].

Figure 5a shows that E% values calculated are decreasing with increasing of depth of dyeing for both RR_Na and UF-RR_Na dyes. This decrease in E% at higher depths of shade can be explained in terms of the fact that at lower % o.w.f. more dye can be absorbed by the reactive cotton dye-sites, leading to more dye retained on cotton. Likewise, as the concentration of the dye in the dyebath is increasing, the percentage of dye molecules absorbed by the cotton reactive dye-sites is decreasing. That decrease is analogous for both RR_Na and UF-RR_Na dyes, while the UF-RR_Na dye shows in general lower E% values in all cases. Thus, the purification process resulted in a much stronger dye [12]. Figure 5b shows the effect of dyeing time on exhaustion of dyes (kinetic of exhaustion) on the fiber, regarding the experiment of 1% o.w.f. As shown, E% values increase with dyeing time for both dyes, as anticipated. Figure 5b also shows that exhaustion of both RR_Na and UF-RR_Na dyes occurs mostly within the first 30 min of dyeing. The time at which 50% color retention occurred is 8.6 min for the UF-RR_Na dye and 9.0 min for the RR_Na dye. The visual evaluation of the cotton fabrics dyed, may be seen in Fig. 5c, where the rise of the % o.w.f. led to deeper purple hues on cotton for the original RR_Na dye.

Figure 6a shows the dye uptake for both RR_Na and UF-RR_Na dyes in terms of K/S values for the dyed samples. In all depths of shade the UF-RR_Na dye shows higher K/S values compared to the untreated RR_Na dye, i.e. stronger results, a behavior explained by the UF process resulting in a purer dye with higher tinctorial strength [12]. Figure 6b shows the effect of dyeing time in terms of the K/S values obtained for both dyestuffs. The same pattern is observed, where the K/S values of the UF-RR_Na dye are higher than those of the RR_Na and secondly, most of the dye has been adsorbed after 30 min of dyeing. Kinetic observation of the K/S changes was performed on a 1% o.w.f. dyeing experiment so that the changes are more evident. Similar trends would be anticipated in higher % o.w.f. of dyeings.

In Table 1, the colorimetric data of the samples dyed with the RR_Na dye before and after UF treatment are given. Defined by CIE, color may be expressed by three values/coordinates: L^* for lightness, axis black-white, a^* for red-green axis, and b^* for blue-yellow axis, as perceived by human eye. Moreover, C^* represents chroma and h^o is the hue angle. Those values entirely retrieved by reflectance spectrophotometer are included in Table 1. Table 1 shows that K/S values for both dyes are increasing with increasing depth of shade. The K/S values of all dyed samples with UF-RR_Na dye are higher than the corresponding K/S values obtained with RR_Na dye, showing stronger dyestuff. UF process resulted in a purer dye with higher coloristic strength due to the displacement of inorganic salt and other impurities, which are present in dye because of its synthesis pathway (especially in industrial terms), into the permeate solution during UF process [10]. The purity of UF-RR_Na can be related to dyestuffs’ solubility too, as shown in previous work. Higher solubilities (RR_Na: 150 g/L while UF-RR_Na: 260 g/L at 95/25 °C) allow the use of UF-dyes in the production of highly stable, high coloristic value formulations [8]. The reflectance values, R%, are decreasing at higher application strengths indicating that more dye has been absorbed at high depths of shade. This decrease in R% is in line with the K/S increase at higher depths of shade. The other colorimetric parameters L^*, a^*, b^*, C^*, h^o are similar for both RR_Na and UF-RR_Na dyes indicating that there is little visible hue changes among these two dyes (Fig. 5c).

Figure 7 shows the effect of dyeing time on the fixation (kinetics of fixation) for a specific dyeing experiment. As can be seen F% values increased with dyeing time, which can be explained in terms of the reaction between cotton ‒OH groups and the vinylsulfone reactive group present in the dye, occurring at higher level and at prolonged dyeing times. As it can also be seen from Fig. 7, the RR_Na and UF-RR_Na reactive dyes show similar fixation pattern, with the UF-RR_Na dye showing slightly higher fixation rates. The final F% of reactive dyes synthesized, about 70–80% is considered very good, able to be compared with the fixation rates of bifunctional dyes [1, 21‐23]. During UF, no particle size alteration exists for the dyestuff itself. The structure of the dye and its MW is certain (see 2.1). The membrane pore size secures the elimination of by-products that are produced during synthesis and would affect the dyeing performance. On the other hand, the extent of dye-fiber reaction is very important in the textile industry since higher fixation rates result in less hydrolyzed reactive dye in the effluents [24]. The extent of dye-fiber reaction and the ultimate discharge of unfixed dye vary widely, based on the type of reactive group and the dyeing technology used. The use of two reactive groups on a dye molecule results in higher fixation efficiencies [22]. Α high percentage fixation leads to better dye utilization and results in less hydrolyzed dye and less effluent waste. Studies have shown that dyeing 1 kg of cotton could generate 200 kg of wastewater containing up to 50% of initial input of dyes in the dyebath and up to 100 g/L of salts [4, 5]. Removal of unfixed dye at the washing stages is often not easy due to the higher substantivity built into such dyes [2]. The low fixation of dyes causes environmental issue since dyes lost in dyeing process contaminate the water sheds and are toxic to living organisms.

Next, the influence of alkali environment is studied. Figure 8a shows the substantivity of both RR_Na and UF-RR_Na dyes on the dyeing of cotton samples. As can be seen, the S% value of the UF-RR_Na dye is relatively higher (43%) compared to the substantivity of the RR_Na dye (34%). It demonstrates that the UF process resulted in a purer reactive dye. It has to be noted that the substantivity is measured in the absence of alkali which explains the relatively low exhaustion values obtained. Reactive dyes with high substantivity have been considered not to be desirable because the unreacted dye is difficult to remove by washing off, after the dyeing process and such dyes have too low diffusion coefficients [25]. Figure 8a shows the effect of alkali, Na₂CO₃, on the E% of cotton samples dyed with RR_Na and UF-RR_Na too. As illustrated, the E% is higher for the UF reactive dye than the originally synthesized dye, because UF process eliminated dye impurities, resulting in a more “active” dye. Color fastness or wash fastness results will be helpful to support the E% conclusions too, when available. The E% calculations took in consideration the dyeing experiments including caustic soda. It is known that alkali is necessary for covalent bond fixation of a reactive dye on the cotton fiber [26]. Under alkaline conditions reactive dyes react with hydroxyl groups of cellulose, mostly by nucleophilic substitution or addition, to form the covalent bonds. Thus, there is no wonder that the alkali results are reported much higher than the F% values in lower pHs.

Figure 9 shows the migration index for both RR_Na and UF-RR_Na dyes. Migration of the dye is defined as the ability of the absorbed dye to move from a heavily absorbed area of the substrate to a less absorbed area resulting in more level dyeings. The migration process comprises of adsorption of the dye on to the fiber surface, migration through the dye liquor, and re-adsorption onto the fiber with consequent diffusion of the dye to the interior of the fiber. Migration itself is heavily influenced by the dyeing temperature. The migration index defines the migration properties of an individual dye reflecting its potency in producing dyeings. Figure 9 shows that both reactive dyes show a high migration index with RR_Na dye showing higher MI%. The difference in the MI between the two states of the dye can be attributed to the higher mobility of the RR_Na dye compared to the UF-RR_Na one. The UF process has resulted in a purer reactive dye with higher fixation rates and greater covalent bond formation and thus less migration. The migration of a dye depends on the chemistry of the dye, molecular structure, physical chemistry, and stereochemistry are all important. External factors such as concentration of the dye added to the dyebath, time, temperature, liquor ratio, rate of liquor circulation, and fabric construction also play a key-role [18].

Finally, Τable 2 shows the colorimetric data of the dyeings obtained for both RR_Na and UF-RR_Na dyes in the presence of metal salts. All sulfate reagents applied were hydrolyzed so that the ions Fe³⁺, Cu²⁺, Co²⁺ were available in dyebaths. The presence of Fe³⁺ in the dyebath resulted in slightly lighter dyeings, with higher R% and lower K/S values for both RR_Na and UF-RR_Na dyes. However, the hues in the presence of Fe³⁺ are much “redder” as can be understood from the higher a^* values. The presence of Cu²⁺ and Co²⁺ metal ions had a detrimental effect in dyeing showing color shift in λ_max and big reduction in the dye uptake as can be seen from the lower K/S values of both dyes. Additionally, the color coordinates a^* and b^* have been greatly reduced, resulting in “greener” and “bluer” dyeings.

Additionally, Fig. 10 demonstrates the effect of metal salts present in the dyeing compared to blank dyeing for UF-RR_Na dye, which is anyway stronger. The effect of metal cations on the color strength is illustrated. The big drop in dye uptake in terms of K/S and the very big impact on the shade obtained can be seen (especially for the case of CuSO₄); the conclusion is in line with the colorimetric data obtained in Table 2. It is found in literature that i) the addition of either NaCl or Na₂SO₄ to a reactive dye dyebath utilizes the inherent propensity of the dyes to aggregate in solution via co-planar association, and that ii) such electrolyte-induced dye aggregation reduces the solubility of the dye in the dyebath [27]. The case seems similar herein too. Considering that knitted cotton fabric is commonly used as sheets or undergarments, it would be important to detect whether there is any metal ion residue on dyed fabric. Yet, the image demonstrates unsuccessful dyeings in these cases to go any further.

Crucial dyeing properties of a novel reactive dye with one vinylsulfone group, such as exhaustion, substantivity, fixation, migration index as well as the effect of metals in the dyeing dyebath were studied, before and after ultrafiltration process took place for the dyestuff. Regarding the kinetics of the dyeings, it is concluded that the change in E% values is small after 30 min, with the values close for two dyestuffs (always greater for UF-dye). The coloring strength (K/S) increases with time and the highest is observed for UF-dye. Substantivity and Fixation percentages show greater affinity of UF-dyes with cotton substrates under corresponding dyeing conditions. All the observations are attributed to UF process having resulted in a much purer dye by the elimination of inorganic salts and also dye by-products. Finally, the migration index appeared higher in the original dye compared to the UF-one. Concerning the effect of metals ions (Fe³⁺, Cu²⁺, Co²⁺) on the dyeing process, it was observed that Fe³⁺ slightly affects the K/S values, while Cu²⁺ and Co²⁺ drastically deteriorate the coloring ability and even cause differences in shade (a^*, b^*) for both dyes. UF process employs physical interactions for purification; this publication proves that the very same colorant performs better in dyeing, in all values calculated, without any chemical alterations, just after being ultrafiltrated.