Using an N-vinylpyrrolidone co-polymer in reactive dye printing as an alternative to urea

Abstract

A series of water-soluble co-polymer (NS) from N-vinylpyrrolidone (NVP) and sodium p-styrenesulfonate (SSS) with a low molecular weight are synthesized as substitutes for urea in the reactive printing of cotton fabrics. The effects of the monomer ratio of NVP to SSS on the color yield of the printed fabrics, the color fastness, and the solubility of the reactive dyes were investigated. The possibility of using a flocculation treatment for printing wastewater was also evaluated. When the dosage of NS-02 (NVP:SSS=7:3) in the printing paste is 1.0 wt%, the color yield and the color fastness of the printed fabric are equivalent to or better than those obtained with 3.0 wt% urea. Compared with the traditional reactive printing with urea, the total nitrogen content in the wastewater after NS-02 is reduced to 15% of that using urea when treated with a suitable amount of inorganic flocculants. It meets the national discharge requirements for printing and dyeing wastewater. The study shows that a low-molecular-weight co-polymer (NS-02) has a high potential to replace urea for the printing of cotton fabric with reactive dyes.

Keywords

Reactive dye printing cotton fabric urea water-soluble co-polymer total nitrogen content of wastewater

In conventional reactive dye printing, there is less water in the printing paste compared with the dyeing. Therefore, it is difficult to dissolve the dyes and they easily aggregate.¹,² Urea plays an important role in the reactive printing of cotton when it is added in the paste as a dye solubilizer and a swelling agent for the cellulose fiber during steaming.³,⁴ Therefore, a large amount of urea is added to the reactive dye printing paste to help to improve the printing quality. At the end, the total urea is discarded into the wastewater by a soaping process, which increases the nitrogen content in the wastewater to a high and unacceptable level. According to the national standard “Wastewater Pollutants Discharge Standard for Textile Printing Industry” (GB/T 4287-2012), the direct emission index of total nitrogen in industrial wastewater is 20–35 mg/L, and the indirect emission index of total nitrogen is 30–50 mg/L.⁵ The urea in the wastewater is difficult to remove, which results in eutrophication in local rivers and lakes.⁶,⁷ Now, reducing or replacing urea in reactive dye printing is becoming paramount for environmental protection, and the printing factories must reduce its dosage or eliminate urea in the printing paste formulation.

The current work on reducing the dosage of urea in reactive printing pastes is mainly focused on finding or developing chemicals to replace urea. Most of these chemical substances have a similar structure to urea and perform the same role during reactive dye printing. Therefore, early research indicated that dicyandiamide was a good substitute for urea. Nevertheless, its poor solubility, its high price, and its toxicity limit its practical usage.⁸ Sodium edetate (SE) is a chelating agent that was first used in the dyeing of cotton with reactive dyes by Ahmed to evaluate the possibility of using it to replace sodium sulfate in the absence of alkali for the dyeing of cotton with reactive dyes, and the results suggested that it was an excellent additive for the reactive dyeing of cotton.⁹ Next, Ahmed et al. used SE as a replacement for urea in reactive dye printing paste and showed that the quality was improved when using SE/free alkali compared with conventional urea/alkali printing.¹⁰ Because SE is biodegradable, its degradation rate is small even under optimal conditions, which creates a high risk for environmental pollution.¹¹ In addition, its high toxicity and the harm caused to human health further limit its practical use.¹² Ding et al. used triethylene glycol to replace urea; the quality of the printed fabrics was satisfactory when triethylene glycol replaced 30% of urea, but urea could not be completely replaced.¹³,¹⁴ PEG-400 (polyethylene glycol-400) is a polyether compound that has also been used as a substitute for urea and has a positive effect only on a few selective dyes.¹⁵ In addition to these studies, Lei et al. proposed a urea-free ecosteam process for cotton that was only applied to inkjet printing.¹⁶

Polyvinylpyrrolidone (PVP) is a homopolymer of N-vinylpyrrolidone (NVP) with a typical molecular weight within 3000–40,000 g/mol. It interests researchers in the textile industry because of its excellent solubility, its hygroscopicity, and its adhesion properties.¹⁷,¹⁸ Many studies have shown that PVP has a good affinity for azo dyes.^19–21 In addition, PVP has been used as an additive to the inks used for textiles that improves the solubility of the dyestuff.²² Sodium p-styrenesulfonate (SSS) is another polymerizable monomer that has been widely used to improve the stability of emulsions.^23–25 Urea has been widely used in reactive dye printing because of its advantages of improving the solubilization and the hygroscopicity of the dyes. However, it is a non-ionic structure which makes it very hard to be removed in dyeing wastewater and causes environmental concerns. The hypothesis of this research is to design a new co-polymer with a similar structure to urea, which not only has advantages in reactive dye printing like urea, but also introduces an ionic structure to improve the solubility of the dyes and to reduce the total nitrogen of the wastewater after printing. We therefore selected NVP and SSS to build up the structure with these two parts to realize the designs in our hypothesis. In this work, a new route was proposed for the synthesis of co-polymer (NS) synthesized from NVP and SSS monomers by a free-radical co-polymerization with a low molecular weight and using them to replace urea in the printing paste. The solubilizing effect of the NS on the reactive dyes was studied by ultraviolet spectrophotometry and the printing performance of the NS used as substitutes for urea in the printing of cotton fabrics was investigated. The effects of the monomer ratio and the concentration of the NS on the color yield, the colorimetric parameter (L*, a*, b*, C*), the fastness, and the nitrogen content of the printing wastewater after a flocculation treatment were discussed.

Materials and methods

Materials

The pure cotton fabric (40 s×40 s weight 117 g/m²) was provided by Zhejiang Mizuda Printing & Dyeing Group Co., Ltd (Zhejiang, China). It was de-sized, scoured, bleached, and mercerized before reception. The dye used in this study was Remazol Brilliant Orange ED-2R from DyStar (China) and was used as received. Sodium alginate with 4000–5000 cps (5.0 wt% aqueous solution) was obtained from Bright Moon Seaweed Group Co., Ltd (Qingdao, China). Urea, sodium bicarbonate, sodium 3-nitrobenzenesulfonate, hydrogen peroxide, SSS, and triethylamine were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) with a laboratory reagent grade and were used as received. NVP was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) with a laboratory reagent grade and were purified by vacuum distillation before use. Polyaluminum chloride was supplied by Zhongke Tianze Water Purifying Material Co., Ltd. (Shandong, China) with an industrial grade and was used as received.

Synthesis of the co-polymer NS

The co-polymers were synthesized from NVP and SSS and were named NS. Purified water and parts of NVP, SSS, an initiator (H₂O₂), and an activator (triethylamine) were added to a four-necked jacketed glass reactor (250 mL) fitted with a reflux condenser, a nitrogen gas inlet tube, and a polytetrafluoroethylene stirrer. The appropriate weight of the mixing monomers (NVP/SSS solution) and the initiator solution were then added to a constant-press dropping funnel to reach the desired solid content of 30%.

Heat and agitation were provided by an appropriate thermostat-stirring apparatus until the temperature was ramped to around 75°C for 30 min. Then, the mixed monomers and the initiator were added dropwise into the reactor over 2 h and the temperature in the reactor was kept between around 75°C for 2 h. Then, the temperature was increased and kept between 80 and 85°C for 1 h after adding a small amount of initiator. The product was then cooled to room temperature and filtered using glass wool. Finally, we obtained a series of co-polymer NS in different ratios of monomers (NS-01, NS-02, and NS-03) aqueous solutions. The viscosity average molecular weights of NS-01, NS-02, and NS-03 were 1825, 2056, and 2213 g/mol, respectively.

Fourier transform infrared analysis

Fourier transform infrared (FT-IR) spectra were obtained as KBr pellets in 4000–500 cm^–1 region on a Nicolet Avatar 380 FT-IR (United States).

Preparation of the printing paste and printing procedure

The original paste was prepared with 8 g sodium alginate, 1 g resisting salt (sodium 3-nitrobenzenesulfonate), and 91 g deionized water. After the original paste was fully gelatinized, the printing paste was prepared with the following formulation: 4 g Orange ED-2R 3R, 0–3 g urea/NS, 2 g sodium bicarbonate, 50 g original paste, and a set amount of water to make up a total weight of 100 g.

The printing process was carried out on a MMDFR286 screen printing machine (Austria Klagenfurt). After printing, the fabrics were first dried at 105°C for 10 min, then fixed by steaming at 105°C for 8 min, washed twice in cold water (bath ratio 1:50) for 10 min, soaped twice with a 1 g/L lotion at 90°C for 10 min, washed with cold water, and dried at 105°C for 10 min.

Color yield and colorimetric analysis

The color yield is characterized by the K/S value and the colorimetric parameters (L*, a*, b*, and C*) of the printed fabrics were measured by a Datacolor 600 spectrophotometer (United States) with a D65 illuminant and a 10° standard observer. Each sample was measured randomly in five different locations (e.g. the ones in the four corners and one middle point) and the average value was calculated.

Color fastness

The color fastness to rubbing was carried out according to GB/T 3920-1997 using a crock meter (Zhejiang, China). The color fastness to washing was tested according to the GB/T 3921.1-1997 standard using a washing fastness tester (Zhejiang, China). The staining grade of the lining fabric was evaluated according to the gray scale of China Textile Association.

Solubilization effect of the auxiliaries to the reactive dye

Supersaturated solutions of dye containing different additives (urea/NS) were prepared separately, sonicated for 7 min, kept at room temperature for 6 h, centrifuged for 10 min (10,000 rpm), and left again at room temperature for 6 h. Then, the upper supersaturated solutions were pipetted out and diluted 10,000–15,000 times. The absorption spectra of the dye solution were acquired at room temperature on a U-3310 UV-Vis spectrophotometer (Hitachi) in the visible region.

Characterization of the total nitrogen content in the printing wastewater

Polyaluminum chloride is a non-toxic inorganic polymer aluminum salt that is widely used in wastewater treatment. The mechanism of the coagulation and flocculation processes have been studied for several decades.²⁶ It is a mature technology used for the flocculation of printing wastewater. After a certain concentration of urea or NS aqueous solution was flocculated with a 60 mg/L (in Al₂O₃) of polyaluminum chloride, the supernatant was pipetted out and the total nitrogen content in the solution was determined using a Multi N/C 3100 system (Germany).

Flocculation and determination of total nitrogen

Preparations of 300 mg/L NS and urea solutions were prepared to simulate printing wastewater and were added with 60 mg/L (calculated as Al₂O₃) polyaluminum chloride at a pH of 7.0–8.0. The mixture was rapidly stirred for 2 min at 160 rpm, and then slowly stirred for 3 min at 60 rpm. After settling in subsidence for 30 min, the supernatant was taken out to measure the total nitrogen content.

Results and discussion

In this work, three water-soluble co-polymers (NS) with low molecular weight in different weight ratios of NVP/SSS were synthesized. The synthetic products were named NS-01 (NVP:SSS=9:1), NS-02 (NVP:SSS=7:3), and NS-03 (NVP:SSS=5:5). Figure 1 shows the overall reaction equations.

Figure 1.

Synthesis of the co-polymer NS.

The FT-IR spectrum of the NS and urea

The FT-IR spectrum of NS-02 is shown in Figure 2. New peaks appeared at 3451.47 cm^–1 and 2946.22 cm^–1. They were ascribed to the stretching vibrations of O-H and C-H, respectively. There was a clear absorption at 1666.20 cm^–1 due to the C＝O in pyrrolidone. The characteristic peaks at 1424.17 cm^–1 and 1289.18 cm^–1 were assigned to the bending vibration of C-H and the stretching vibration of C-N, respectively. The absorption peaks at 1203.36 cm^–1, 1128.64 cm^–1, and 1040.41 cm^–1 were ascribed to the deformation vibration of the S=O bond in the sulfonic acid group. The absorption peak at 835.99 cm^–1 was the characteristic peak of the di-substituted benzene ring. The peak for the C=C stretching vibration disappeared, which confirmed that the NVP and SSS two monomers were fully consumed by the polymerization reaction.

Figure 2.

Fourier transform infrared spectrum of co-polymer NS-02.

Solubilization of reactive dyes by the NS and urea

The solubility depends on the temperature, the chemical structure, and the solvent. The presence of more hydrophilic and polar groups in chemical substances leads to a higher solubility in water. To meet the printing requirements, the reactive dyes must have a high solubility to prevent uneven colors and avoid tarnishing during soaping. The co-polymer NS synthesized in this study has cyclic amide and sulfonate groups that have a solubilizing effect on dyes and improve the reactive dyes to react with cotton fiber.

A saturated solution of the Orange ED-2R reactive dye with deionized water at 25°C was prepared and added to the same weight of urea or NS. Then, the absorbance curve of the dye solution at the same dilution ratio was recorded (Figure 3). In Figure 3, the maximum absorption of the Orange ED-2R does not change significantly in the visible range (λ_max=480–500 nm), which indicates that the additives have no effect on the color of the dye. The solubility of the Orange ED-2R in water is the smallest with no additives, which means that the sulfonate groups in the Orange ED-2R molecule are not enough and the hydrophobic chromophore structure is in high proportion in the dye molecules. When a small amount of Orange ED-2R is added to the aqueous solution, the dye molecules stick together and separate from the aqueous solution by precipitation to make the saturated solution at a low concentration. When urea is added, the saturation solubility of the Orange ED-2R increases and the absorbance of the dye solution increases significantly. This is because the amide structure in urea forms hydrogen bonds with the polar groups of the Orange ED-2R molecules and with water molecules. When urea is adsorbed onto the Orange ED-2R molecule, the hydrophobic parts of the dye are transformed into hydrophilic ones, which prevent the dye molecules from coalescing, so the Orange ED-2R molecules are more easily dissolved in water and the solubility of the Orange ED-2R in water increases.

Figure 3.

Ultraviolet-visible spectra of dye solutions with different additives. (the additive concentration in the corresponding supersaturated solution is 3.0 wt%).

Figure 3 also shows that the saturation solubility of the Orange ED-2R also increases significantly with the NS added to the solution. This indicates that there is an effective interaction between NS and the Orange ED-2R molecules in the aqueous solution. The experimental results also show that the different proportion of monomers in NS also significantly affects the solubility of the dye in water. When the ratio of NVP to SSS in the NS is 9/1 or 7/3 by weight ratio, the saturation solubility of the Orange ED-2R in water is higher than that when urea is added. The proportion of the SSS monomer affects the saturation solubility of the Orange ED-2R. Compared to NS-01, the proportion of SSS monomer in NS-02 increases, and the saturation solubility of the Orange ED-2R increases. However, the proportion of SSS monomer in NS-03 is too high, which causes the total amount of the NVP monomers to decrease and thus reduces the absorption of the dyes. Therefore, with NS-03, the saturation solubility of the Orange ED-2R is smaller than that of NS-01 and NS-02. NS-02 corresponds to the highest saturation solubility of the Orange ED-2R, which indicates that NS-02 has the largest solubilizing effect on the Orange ED-2R. It is because there are two monomers with different structures in the NS and they interact with the Orange ED-2R in different ways in the aqueous solution. The NVP monomer is a five-member cyclic lactam structure having a high affinity to dyes.¹⁹,²⁰ Therefore, the NVP links in the NS molecular chains in the aqueous solution are easily adsorbed on the surface of the dye molecule through weak bonds such as hydrogen bonds and van der Waals force. The NS macromolecules with more NVP links in will make more NS macromolecules to be adsorbed onto the Orange ED-2R surface, so that the surface of the Orange ED-2R molecule or the small aggregates of Orange ED-2R molecules are protected by a layer of NS macromolecules. At the same time, the sulfonic acid group in the SSS monomer of the NS macromolecules is ionized in an aqueous solution to form a negative charge. The adsorption of NS increases the surface electronegativity of the Orange ED-2R molecule in the aqueous solution, so that the saturation solubility of the Orange ED-2R increases.

By comparing NS-01 and NS-02, the solubility of the Orange ED-2R increases as well when the SSS contents in the co-polymer increases. This is because the increase of the SSS contents in the NS molecular chain further increases the electronegativity of the surface of the dye molecule or the small aggregates of dye and, consequently, more dye molecules are solubilized in water. The solubility of Orange ED-2R added by NS-03 is much lower than that by NS-02 and even lower than that with urea. The analysis shows that increasing the number of SSS monomers in the NS increases the negative charge density in the NS and the solubility in aqueous solutions; at the same time the content of the NVP monomer in the NS molecule reduces correspondingly, which reduces the number of available adsorption sites in the NS molecular chain for the dyes. The NS-03 macromolecules cannot efficiently adsorb on the surface of the Orange ED-2R, which reduces the solubility of the Orange ED-2R in the aqueous phase, but the solubility of Orange ED-2R added by NS-03 is still higher than that of the comparable sample in water.

Effect of the NS concentration on the color yield of the printed fabrics

Different dosages of the NS and 3.0 wt% of urea by weight were added to the printing paste with 4.0 wt% by weight of Orange ED-2R reactive dye. Figure 4 shows the color yield (K/S value) of the printed fabrics. Compared with the blank sample, the addition of 3.0 wt% urea in the reactive dye printing paste significantly increases the color yield of the active dye on the fabrics; this means that the urea helps the Orange ED-2R to fully react and fix on the cellulose fiber. This improves the utilization rate of the dyes and reduces the pressure of the printing wastewater treatment. Therefore, adding urea to the reactive dye printing paste is recommended in the traditional printing process.

Figure 4.

Effect of additives and concentration on the color yield of the printed cotton fabric with Orange ED-2R.

After adding the NS to the printing paste, the K/S value of the printing fabric shows a complex relationship with the NS compounds and its dosage. When the dosages of NS in the printing paste increases as 0.5 wt%, 1.0 wt%, 2.0 wt% and 3.0 wt%, the K/S value of the printed fabrics first increases, then decreases. This indicates that adding an excess of NS (3.0 wt%) to the printing paste does not improve the K/S value. When the dosage of NS-01 in the color paste is 1.0 wt% and 2.0 wt%, the K/S value of the printing fabric is close to the printing K/S value of 3.0 wt% urea. When the dosage of NS-02 and NS-03 in the color paste is 0.5 wt%, 1.0 wt%, and 2.0 wt%, the K/S value of the printing fabric is larger than that of 3.0 wt% urea. Figure 4 shows that the highest K/S value of the printed fabric was the one with the dosage of NS-02 in 1.0 wt%. The results were not changed after repeating the experiment several times.

In the absence of urea (the blank group sample in Figure 4), the K/S value suggests the color intensity of the Orange ED-2R is smaller than those with urea or the NS (other samples in Figure 4). The reason might be that without urea or the NS, it becomes difficult for the Orange ED-2R aggregates to diffuse into the interior of the cotton fibers (Figure 5(a)) and they are stopped on the surface of the fibers and hardly react with the fibers during the steaming and fixing process. Due to the poor solubility of the dye in the printing paste, the color yield (K/S value) of the printed fabrics is quite low. When adding urea in the color paste, the solubility of the Orange ED-2R increases and more Orange ED-2R multimers are shifted into monomolecular state (Figure 5(b)). Therefore, the Orange ED-2R in a single-molecule state more easily diffuses into the interior of the cotton fiber to undergo the color fixation reactions. At the same time, urea is an effective hygroscopic agent that absorbs the moisture from the gas phase during the steaming and helps the swelling of the color paste. Meanwhile, water is also transferred to the cotton fiber to produce swelling by forming water channels in the cotton fiber which makes it easier for the dye to diffuse from the color paste to the interior of the cotton fiber. Additionally, urea as a small organic molecule can diffuse into the cotton fiber and swell the cotton fiber by itself during the steaming process (Figure 5(b)). This effect generated inside the cotton fiber provides the same benefit to obtain a high color yield (K/S value) as in the printing paste.

Figure 5.

Mechanism of the NS and urea during the printing steaming process.

When adding the co-polymer NS to the printing paste, the links of the NVP and the SSS monomers have a hygroscopic effect during the steaming and it increases the solubility of the Orange ED-2R dyes in the color paste to form single-molecular dyes, which makes it easy for the dyes to be absorbed on and diffused into the cotton fibers. At the same time, the sulfonic acid anion group in the NS chain and the dye anion also generate an electrostatic repulsion during steaming, which promotes the diffusion of the Orange ED-2R dyes to the cotton fiber and increases the color yield (K/S value) (Figure 5(c)). When the dosage of the NS in the color paste is 3.0 wt%, the color yield (K/S value) of the printed fabric increases and the proportion of the SSS monomer increases as well.

In the co-polymer NS, the NVP chains play roles of moisture absorption and dye adsorption. The increase of the NVP content in the co-polymer NS results in a decrease of the K/S value (Figure 4). This means that the adsorption between the dye and the NVP chain in the color paste during the steaming hinders the diffusion of the Orange ED-2R dye to the cotton fibers. When the dosage of NS-01, NS-02, and NS-03 in the color paste increases from 1.0 wt% to 3.0 wt%, the K/S value of the printed fabric does not increase further and the total content of NVP chains increases correspondingly in the color paste.

Comparing the results of NS-01 and urea in Figure 4, the K/S value of urea was much larger than the K/S value of NS-01 at the same dosage 3.0 wt%. It is because NS-01 is a co-polymer synthesized by free-radical polymerization and has a higher molecular weight than urea and makes it different for NS-01 to diffuse into the cotton fiber like urea to enhance the hygroscopicity and the swelling of the cotton fiber during steaming. Therefore, it is difficult for the Orange ED-2R dyes to diffuse inside the fiber, which results in a decrease of the K/S value of the printed fabric.

Color difference and fastness of the fabrics printed using the NS instead of urea

Printing pastes were prepared with 2.0 wt% of NS-01, 1.0 wt% of NS-02, 2.0 wt% of NS-03, and 3.0 wt% of urea added respectively and with Orange ED-2R 4.0 wt% for screen printing. The measured colorimetric data of the printed fabrics are showed in Table 1 and the fastness of the printed fabric was determined according to the GB/T 3920-1997 and GB/T 3921.1-1997 standards; the results are listed in Table 2.

Table 1.

Colorimetric results of the cotton fabric printed with different additives

Samples	L*	a*	b*	ΔL*	Δa*	Δb*	C*
3.0 wt% urea	59.76	56.36	59.57	0	0	0	81.38
2.0 wt% NS-01	59.81	56.26	59.88	0.05	–0.10	0.31	81.41
1.0 wt% NS-02	56.86	57.85	61.02	–2.9	1.49	1.45	83.11
2.0 wt% NS-03	57.93	55.99	60.60	–1.83	–0.37	1.03	83.01

Table 2.

Fastness of the fabrics printed with urea or the NS

Samples	Rubbing fastness		Washing fastness
Samples	(Dry)	(Wet)	(Stain on cotton)
3.0 wt% urea	4–5	3–4	4
2.0 wt% NS-01	4–5	3–4	5
1.0 wt% NS-02	4–5	4	5
2.0 wt% NS-03	4–5	3–4	5

The evaluation of color strength on the printed fabrics by comparing the results of the color lightness (L*) and the saturation value (C*) are listed in Table 1. The L* value of NS-02 in 1.0 wt% is significantly lower than that with urea in 3.0 wt%. This shows that NS-02 is a good replacement for urea in reactive dye printing to achieve a higher color yield. The highest color difference Δa* and Δb* values of the fabrics printed with the NS at the optimal dosage to fabrics printed with 3.0 wt% urea are all lower than 1.5, indicating that the NS have a little effect on the color shade of the printed fabrics.

Table 2 shows the effect of the NS and the urea on the color fastness of the printed fabrics. Table 2 shows the rubbing fastness and the washing fastness of the fabrics printed with the NS. The washing fastness of the fabrics printed with the NS was one grade better than with urea. This is because some hydrolyzed dyes are still adsorbed on the surface of the fibers by their affinity for the cotton fibers during the soaping process of the printing fabrics. Compared with small urea molecules, the NS macromolecules contain many NVP chain structures and are easy to adsorb on the surface of hydrolyzed dyes to form stable hydrolyzed dye/NS composite micelles in the aqueous solution. This composite micelle is surrounded by SSS links in the NS macromolecular chain to form negatively stable micelles and the adsorption of hydrolysis dyes to cotton fiber was prevented by an electrostatic repulsion with the negative cotton fiber, which prevents the hydrolyzed dye from fouling the fiber again. Therefore, the soaping of the hydrolyzed dye from the fiber surface to the NS micelle becomes a unidirectional trend, which results in a one grade increase in the washing fastness than urea used.

Denitrification of printing wastewater with the NS by flocculation

The wastewater from printing and dyeing factories contains a large amount of toxic and harmful substances and cannot be directly discharged into the river.³,⁴,⁶ Generally, the wastewater needs to undergo multiple biochemical treatments before discharge is possible. The flocculation–sedimentation method is an important water treatment method. The flocculation treatment is carefully controlled; a large number of toxic and harmful substances are adsorbed to the opposite ions of the flocculants and the flocculent precipitate is separated and removed from the wastewater solution by mechanical separation method.

In this work, the determination results of total nitrogen after flocculation of simulated printing wastewater are shown in Table 3. Before the flocculation treatment, the total nitrogen content in the simulated printing wastewater with the NS was only 21–35% of that of urea. This is because the nitrogen content in the co-polymer NS is lower than in the urea molecule. In addition, the nitrogen content in the NS macromolecular chain decreases gradually as the amount of SSS monomer increases, so that the total nitrogen content in the simulated soap wastewater is lower than that in the urea wastewater solution with the same dosage. The total nitrogen content of the simulated printing wastewater containing the NS is reduced by 33% to 54% after flocculation with polyaluminum chloride, whereas that for urea showed almost no reduction.

Table 3.

Denitrification of the printing wastewater with urea or the NS by flocculation

Additive	Urea	NS-01	NS-02	NS-03
TN^a (mg/L)	150.75	54.20	43.10	32.35
TN^b (mg/L)	150.75 --^c	35.86 ↓^d	23.83 ↓^d	14.71 ↓^d
TN^e (%)	0	33	44	54
TN^f (%)	100	35	28	21
TN^g (%)	100	23	15	9

TN: total nitrogen.

The required TN of the dyeing wastewater by national standard (GB/T 4287-2012) is no more than 35 mg/L.

^aBefore treatment.

^bAfter flocculation treatment.

^cNo precipitation occurs (--).

^dPrecipitation occurs (↓).

^eTN content percentage reduction.

^fTN content percentage reduction before treatment compared with the TN with the urea (as 100%).

^gTN content percentage reduction after treatment compared with the TN with the urea (as 100%).

When comparing the variation of total nitrogen for NS-01, NS-02, and NS-03, increasing the SSS monomer in the NS increases the removal rate of the total nitrogen content. This is because polyaluminum chloride used for the flocculation is a positively charged colloid in an aqueous solution that easily adsorbs the negatively charged NS via interionic attraction to form flocculation. When the volume of flocculant particles gradually increases and the Brownian motion becomes insufficient to support the weight of the particles to be dispersed in water, the flocculant particles are precipitated and separated from the water.

In the NS-03 simulated wastewater, the total nitrogen content is only 9% of that of the corresponding urea wastewater solution after the flocculation treatment. On the contrary, the urea molecules are electrically neutral and flocculation precipitation cannot occur using positively charged polyaluminum chloride colloids. Therefore, the total nitrogen content in the wastewater solution containing urea does not change.

Conclusion

The addition of the NS in the reactive dye printing paste of cotton fabric improves the solubility of the dye without changing its absorption spectrum. The K/S values of the fabrics printed with NS-02 and NS-03 at the optimal dosage were higher than that printed with 3.0 wt% urea. The NS had no significant effect on the shade and the color fastness of the printed fabrics. The fabrics printed with the NS had a better tarnishing fastness than those printed with urea, which indicates the NS are tarnishing-proof in the soaping process. The total nitrogen content of the printing wastewater with the NS was significantly lower than that with urea. Additionally, it can be reduced by 33% to 54% after flocculation with polyaluminum chloride, which satisfies the indirect emission index for total nitrogen in printing wastewater. Therefore, the NS are good substitutes for urea in the reactive dye paste. In particular NS-02 at a concentration of 1.0 wt% had better color fastness and higher K/S value than that with 3.0 wt% urea, and the printing wastewater with NS-02 can meet the national wastewater discharge standards after the flocculation treatment. Therefore, NS-02 has a potential feasibility to replace urea in the green printing of cotton fabrics with reactive dyes.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Min Jie

He Jin-xin

References

Coates

EJ.

Aggregation of dyes in aqueous solutions. Color Technol 2010; 85: 355–368.

Vickerstaff

Lemin

DR.

Aggregation of dyes in aqueous solution. Nature 1946; 157: 373.

Lian-Ju

LI.

Reduction of urea addition in rayon printing with reactive dyes. Dyeing & Finishing 2004.

Prášil

Urea reduction in reactive dye printing. Ecotextile 1999; 98: 137–140.

Zheng

Wang

, et al. Treatment of real high‐concentration dyeing wastewater using a coagulation‐hydrolysis acidification‐multilevel contact oxidation system. Environ Prog Sustain Energy 2015; 34: 339–345.

Finlay

Patoine

Donald

, et al. Experimental evidence that pollution with urea can degrade water quality in phosphorus-rich lakes of the Northern Great Plains. Limnol Oceanogr 2010; 55: 1213–1230.

Sostar-Turk

Simonic

Petrinic

Wastewater treatment after reactive printing. Dyes Pigm 2005; 64: 147–152.

Zhou

HX.

Methods to reduce the dosage of urea in the textile dyeing and printing. Textile Auxiliaries 1995; 12: 1–5 (in Chinese).

Ahmed

NSE.

The use of sodium edate in the dyeing of cotton with reactive dyes. Dyes Pigm 2005; 65: 221–225.

10.

Ahmed

NSE

Youssef

El-Shishtawy

, et al. Urea/alkali‐free printing of cotton with reactive dyes. Color Technol 2006; 122: 324–328.

11.

Hong

PKA

Banerji

et al. Extraction, recovery, and biostability of EDTA for remediation of heavy metal-contaminated soil. J Soil Contam 1999; 8: 81–103.

12.

Meltzer

Kitchell

Palmon

Jr.

The long term use, side effects, and toxicity of disodium ethylenediamine tetraacetic acid (EDTA). Am J Med Sci 1961; 242: 11–17.

13.

Ding

Wang

Fang

, et al. Study on printing process of silk with reactive dyes and few urea. Silk 2010; 12: 7–10.

14.

Ding

Wang

Lin

, et al. Application of TEG replacing urea in silk printing with reactive dyes. Adv Mat Res 2011; 175–176: 691–695.

15.

Zhang

Gao

Song

, et al. Cleaner production applied to urea-free printing of cotton fabrics using polyethylene glycol polymers as alternative additives. J Clean Prod 2016; 124: 126–131.

16.

Wang

Yan

Cleaner production of inkjet printed cotton fabrics using a urea-free ecosteam process. J Clean Prod 2017; 143: 1215–1220.

17.

Mccorquodale

Williams

Busch

Compact detergent composition containing polyvinylpyrrolidone. European Patent, EP0508034A1, 1992.

18.

Lorenz

DH.

Adhesive compositions including polyvinylpyrrolidone acrylic acid polymers, and polyamines. United States Patent, US5645855A, 1997.

19.

Frank

Barkin

Eirich

FR.

The interaction of polyvinylpyrrolidone with some azo dyes. J Phys Chem 1957; 61: 1375–1380.

20.

Hamada

Fujita

Mitsuishi

Aggregation behaviour of an azo dye containing a trifluoromethyl group on poly(vinylpyrrolidone). J Chem Soc Faraday Trans 1990; 86: 4031–4035.

21.

Sheth

GN.

Studies in interaction between poly(vinyl pyrrolidone) and azo dyes. J Appl Polym Sci 1985; 30: 4659–4668.

22.

Park

Hirata

Hamada

Dye aggregation and interaction of dyes with a water‐soluble polymer in ink‐jet ink for textiles. Color Technol 2012; 128: 184–191.

23.

Yang

She

Humidity sensors using in situ synthesized sodium polystyrenesulfonate/ZnO nanocomposites. Talanta 2004; 62: 707–712.

24.

Han

Sun

Synthesis of sodium-carboxymethylcellulose/poly methyl allyl polyoxyethylene ether/poly sodium p-styrenesulfonate and its application in cement-based materials. Adv Cem Res 2019; 31: 290–296.

25.

Wang

Tang

CY.

Water-soluble graphene grafted by poly(sodium 4-styrenesulfonate) for enhancement of electric capacitance. Nanotechnology 2012; 23: 475704.

26.

Hongxiao

Zhaokun

Features and mechanism for coagulation－flocculation processes of polyaluminum chloride. J Environ Sci 1995; 15: 204–211.