Eco-friendly cationic modification of cotton fabrics for improving utilization of reactive dyes

Abstract

Cotton fabric was chemically modified by a prepared eco-friendly cationic polymer. The cationic polymer was synthesized by the reaction of dimethylamine and epichlorohydrin using the novel method of three-step polycondensation. And it was characterized by Fourier transform infrared (FTIR) spectroscopy, ¹H nuclear magnetic resonance (¹H NMR) and gel permeation chromatography (GPC). Moreover, the structures of both untreated and treated cotton fabrics were compared and investigated by FTIR, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The dyeing properties of the treated cotton fabrics are discussed. The results indicated that the total dye utilization of treated cotton by salt-free dyeing was much higher than that of untreated cotton by conventional dyeing. And the color fastness properties, the dyeing levelness and the tear strength of treated cotton were all satisfactory and could meet application demands. The environmental hazard caused by the dye wastewater could be decreased greatly in the dyeing process.

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1. Introduction

Cotton fabric is the most widely used textile in the world and is composed almost entirely of cellulose (90–96% based on weight of fibres).¹ Cotton fabric characteristically exhibits excellent physical and chemical properties: for example, stability, being comfortable to wear, high water absorbency and dyeability. In industrial processes, reactive dyes are widely used for cotton fabric dyeing due to their relative ease of application, wide color gamut and high wet fastness.² However, reactive dyeing systems require huge amounts of electrolyte (NaCl or Na₂SO₄) to overcome the repulsive charge between cotton and reactive dyes. These electrolytes are neither exhausted nor destroyed after dyeing and only 60–65% dye utilization is attainable.³ The residual dyes and electrolytes have caused severe environmental problems and disorders in living organisms.⁴

Over the years, many attempts have been made to enhance fabric-dye affinity for improving utilization of reactive dyes and eliminating or reducing the amount of electrolyte.^5–12 An alternative approach is either to dye cotton with new reactive dyes or to pre-treat cotton followed by reactive dyeing. These new reactive dyes contain two or three functional (reactive) groups per molecule. And the combination of reactive groups greatly improves their dyeing ability and possibility for applications.¹³ The total utilization of reactive dyes can be increased from an average of 60% to approximately 80%.¹⁴ Many researchers have recently focused on introducing cationic groups into cotton fabrics for enhancing interactions between cotton and reactive dyes. Such treated cotton fabrics can be dyed with reactive dyes under neutral or mildly acidic conditions without the addition of electrolyte, and dye utilization efficiency can be greatly increased. But there are some disadvantages which make it difficult to apply to industrial production; for instance, the high production cost, the health and safety of industrial production, the repeated pollution, and the degradation of wearability.^15,16

The epichlorohydrin–dimethylamine polymer is an eco-friendly cationic polymer. It was reported that this cationic polymer could be synthesized by two-step polycondensation, whose weight-average molecular weight was less than 10 000 g mol.^17–19 But it only had a single function, either as dye fixative agent or as flocculant. It is appreciated that the cationic polymer is useful for drinking-water treatment and no data are available regarding its toxicity.^17,20

In this context, epichlorohydrin–dimethylamine polymer was prepared by the novel process of three-step polycondensation. Based on two-step polycondensation, the polymer molecules were further polymerized under the catalysis of alkali and formed an ordered cationic polymer with higher molecular weights. The synthesis process conditions of the cationic polymer were investigated and optimized. The polymer was applied to modify cotton fabrics as cationizing reagent and also to allow disposal of dye wastewater as a flocculant. It did not result in further pollution. The dyeing effect of the modified cotton fabrics in salt-free dyeing was tested and compared with that of raw cotton fabrics in conventional dyeing. And the green modification method could greatly enhance the dyeability of cotton fabrics to achieve effluent reduction.

2. Experimental part

2.1 Materials and chemicals

Epichlorohydrin (AR), dimethylamine (CP), diethylenetriamine (AR) and other chemicals were purchased from Sinopharm Chemical Regent Co. Ltd.

The cotton fabric (bleached, desized and mercerized, 100 g m⁻²) was purchased from Qingdao Fanglian Group Co. Ltd.

The commercial reactive dyes CI Reactive Blue 19 and CI Reactive Black 5 were obtained from Shanghai Dyestuff Co. Ltd, China; CI Reactive Red 195 and CI Reactive Yellow 176 were purchased from Zhejiang Longsheng Group Co. Ltd, China. Their structures are shown in Fig. 1.

Fig. 1 Molecular structures of the reactive dyes used in the experiments.

2.2 Synthesis of epichlorohydrin–dimethylamine polymer

The cationic polymer was prepared by the reaction of dimethylamine and epichlorohydrin using the method of gradually increasing temperature and three-step polycondensation. In the first step, dimethylamine and a crosslinking agent were first added to a 250 mL glass reactor equipped with a mechanical stirrer at 0–10 °C. Epichlorohydrin was added dropwise into the reactor with constant stirring for 3 hours at a temperature below 10 °C. The system temperature was slowly raised in a gradient style, such as at 30 °C for 15 minutes, at 40 °C for 15 minutes. In the second step, polycondensation was conducted at specific temperature for 120 minutes with constant stirring. In the third step, 4 g sodium hydroxide solution (10 wt%) was added into the reactor to promote polymerization. When the viscous solution was climbing the stirrer shaft, an appropriate amount of distilled water was added into the glass reactor to inhibit the rapid increase of viscosity. When shaft-climbing phenomenon was not apparent, the reaction solution was kept at reaction temperature for 0.5 h and then hydrochloric acid solution was added to adjust the pH value to 5–6. The epichlorohydrin–dimethylamine polymer was purified by alcohol and dried in vacuum at 50 °C for 24 h. The yield was about 96%.

2.3 Chemical modification of cotton fabrics

10 g L⁻¹ epichlorohydrin–dimethylamine polymer solution was prepared with distilled water and the pH value of the polymer solution was adjusted to 13 with NaOH solution (1 mol L⁻¹). The pressure on the mangle was adjusted to give 80% wet pickup. Cotton fabrics were dipped and padded twice in the above solution. Subsequently, the treated cotton fabric was baked at 60 °C for 15 min. Then it was rinsed with water, followed by soaping with nonionic detergent (OP-10, 1 g L⁻¹) at the boil for 30 min to remove the physically adsorbed cationic polymer, then rinsed thoroughly with water and air-dried at room temperature.²¹

2.4 Characterization

The cationicity of the prepared polymer was measured using the method adopted in the literature.²² The viscosity of the prepared polymer system before purification was measured by using a NDJ-79 Rotary Viscometer.

The chemical structure of the prepared epichlorohydrin–dimethylamine polymer was confirmed by means of infrared (IR) spectroscopy using a Thermo Nicolet IR spectrophotometer model IR 460 and ¹H NMR spectroscopy using a Bruker AV-500 spectrometer. ¹H NMR measurements were performed in D₂O. The weight-average molecular weight (M_w) of the prepared polymer was determined by gel permeation chromatography (GPC) (Wyatt Dawn Heleos) with columns of Shodex OHpak SB-805HQ (8 × 300 mm, Showa Denko, Tokyo, Japan) connected in series. THF was used as the GPC solvent.

Fourier transform infrared (FTIR) spectroscopy experiments with the untreated cotton fabric and treated cotton fabric were carried out.

A LEO 1530 scanning electron microscope (SEM) was used to study the surface morphologies of cotton samples. All samples were coated by gold sputtering before SEM testing.

The cross-section of the dyeing fabrics was observed with an MIT 500 optical microscope.

The X-ray diffraction (XRD) patterns of the cotton fabrics were recorded stepwise with 2θ between 5° and 60° by a Rigaku D/max 2500 diffractometer. The relative crystallinity (C) of the cotton fabrics was calculated as described by Rabek with the following equation, where A_c is the crystalline area and A_a is the amorphous area:^23,24

Before FTIR, SEM and XRD testing, all of the treated cotton fabrics were washed until the cationicity of the washing solution was not detected.

2.5 Dyeing procedures

2.5.1 Dyeing procedure for the modified cotton. Exhaustive dyeing was performed in the absence of salt. The modified cotton fabric was immersed in 4% dye solution (o.w.f., weight percent of dye relative to fiber) with a liquor ratio of 1 : 30. The dyeing temperature was kept at 25 °C for 30 min and then was gradually raised to 60 °C in 20 minutes. Subsequently, 15 g L⁻¹ Na₂CO₃ was added to the dye solution with stirring and the dyeing temperature was kept at 60 °C for 60 min. After dyeing, the dyed cotton fabric was introduced in a solution containing 1 g L⁻¹ nonionic surfactant (Triton X-100) at 90 °C for 20 min at a liquor ratio of 1 : 15, and then rinsed and allowed to air dry.

2.5.2 Dyeing procedure for the untreated cotton. The untreated cotton fabric was immersed in 4% dye solution (o.w.f.) with a liquor ratio of 1 : 30. 25 g L⁻¹ NaCl was added to the dye bath with stirring. The dyeing temperature was kept at 25 °C for 30 min. Another amount of 25 g L⁻¹ NaCl was added to the dye bath and the dyeing temperature was slowly raised to 60 °C in 20 minutes. Then 20 g L⁻¹ Na₂CO₃ was put into the dye solution with stirring and the dyeing temperature was kept at 60 °C for 60 min. After dyeing, the dyed fabric was introduced in a solution containing 1 g L⁻¹ nonionic surfactant (Triton X-100) at 90 °C for 20 min at a liquor ratio of 1 : 30, and then rinsed and allowed to air dry.

2.6 Determination of dye exhaustion and fixation

The dye exhaustion (E) was determined according to eqn (2), where A₀ is the absorbance of the original dye solution and A₁ is the absorbance of the residual dye solution after dyeing. The dye fixation (F) was determined according to eqn (3), where A₂ is the absorbance of the soap bath after soaping, and the total utilization of the original dye (T) was determined according to eqn (4). The absorbance of the dye solution was measured at maximum absorbance of the dye using a Shimadzu UV-1700 spectrophotometer.

T = E × F (4)

2.7 Color yield analysis

The color yield strength expressed as K/S value was obtained from the Kubelka–Munk equation (eqn (5)). The reflectance ratio (R) was determined at the minimum reflectance of the dye using a CM-2600d spectrophotometer (Konica Minolta). Eight separate points on each cotton fabric sample were measured. The levelness of the dyed cotton fabric was evaluated using S_r(λ). A lower S_r(λ) means better levelness.^25,26 S_r(λ) was calculated according to eqn (6) and (7).

2.8 Color fastness and tear strength testing

The washing fastness test of the dyed cotton fabric was performed according to the standard (ISO 105-C06 (C2S)) with an SW-12 (Dongyuan Testing Machinery) washing machine. The rubbing fastness test was performed according to the standard (ISO 105-X12) using a Y571B (Changzhou Textile Instrument Co. Ltd) rubbing machine.

The tear strength test of cotton fabric samples was performed using a YG (B) 033A tearing instrument (Wenzhou, China) according to ASTM D 5734-1995.

2.9 Flocculation experiments

Flocculation experiments for the residual dye liquor (CI Reactive Black 5) were conducted with a jar test apparatus. After the addition of the epichlorohydrin–dimethylamine polymer, the dye solution was stirred rapidly for 3 min at 450 rpm, followed by stirring slowly for 15 min at 40 rpm and sedimentation for 30 min. The absorbance of supernatant samples was measured using a Shimadzu UV-1700 spectrophotometer. The color removal efficiency of the prepared cationic polymer was determined by comparing the absorbance difference between original and flocculated dyeing solution.¹⁸

3. Results and discussion

3.1 Characterization of the chemical structure and evaluation of the prepared polymer

Fig. 2 shows the FTIR spectrum of the prepared cationic polymer. The peak characteristic of –OH stretching was visible at 3419 cm⁻¹.²⁷ And the peaks at 1636 cm⁻¹, 1475 cm⁻¹ and 1096 cm⁻¹ indicated the presence of C–N vibration and –CH₂–N⁺ R₃ type nitrogen (quaternary ammonium groups).^28,29 The band of medium intensity at 634 cm⁻¹ was typical of the C–Cl stretching vibration in the CH₂–Cl group. The additional peaks at 1280 cm⁻¹ indicated the presence of the CH₂–Cl group.³⁰

Fig. 2 FTIR spectrum of the prepared polymer.

Fig. 3 shows the ¹H NMR spectrum of the prepared cationic polymer. The ¹H NMR spectrum of the epichlorohydrin–dimethylamine polymer had the characteristic proton peaks of a N⁺–CH₂– linkage at 3.42 ppm, an N⁺–CH₃ linkage at 3.64 ppm, a –CH₂– linkage at 1.19 ppm and a –CH₂Cl linkage at 3.78 ppm. These results confirmed that dimethylamine had reacted with epichlorohydrin to form the cationic polymer.

Fig. 3 ¹H NMR spectrum of the prepared polymer.

The weight-average molecular weight of the prepared cationic polymer was 47 150 g mol⁻¹. Its cationicity is 4.975 mmol g⁻¹.

3.2 FTIR study of the modified cotton fabric

The chemical modification process of cotton fabrics is summarized in Fig. 4. Under basic conditions, the chloromethyl group of the epichlorohydrin–dimethylamine polymer could change into an epoxy group to react with the primary hydroxyl groups of cotton fabrics. And then quaternary amine groups could be chemically attached in the cotton fabrics and enhance the fabric-dye affinity. The FTIR spectra of untreated cotton and treated cotton after sufficient washing are illustrated in Fig. 5. Compared with the untreated cotton spectrum, an absorption peak at about 1460 cm⁻¹ appeared in the spectrum of the treated cotton, which was assigned to the bending vibration of C–N. This observation clearly indicated the presence of –CH₂ –N⁺ R₃ type nitrogen (quaternary ammonium groups).²⁸ It fully demonstrated that the quaternary amine group was chemically adsorbed on the treated cotton fabrics. It indicated that the reaction between cotton fabrics and cationic polymer had occurred.

Fig. 4 Scheme of modification reaction and dyeing process.

Fig. 5 FTIR spectra of cotton fabric samples.

3.3 Surface morphology

SEM was employed to intuitively observe the surface morphology of untreated cotton and treated cotton, which could be used to evaluate the influence of the cationic modification process. SEM images of untreated cotton and treated cotton are given in Fig. 6. The wrinkle of the treated cotton surface was significantly reduced. The surface of the treated cotton was smoother and plumper than that of the untreated cotton. Generally, the cationic modification caused little change and there was no damage to the structure of the cotton fabrics.

Fig. 6 SEM images of cotton fabric samples: (a) untreated and (b) treated.

3.4 X-ray diffraction (XRD) analysis

Fig. 7 shows the XRD patterns of the untreated and treated cotton. Typical diffraction peaks appeared at 2θ = 14.5°, 16.2°, 22.3°, and 33.7° in both Fig. 7(a) and (b) which are typical of cellulose I crystalline form, in perfect accordance with a previous study.³¹ Moreover, the peak intensity of the patterns of cotton fabrics before and after cationic modification changed very little. The relative crystallinities of untreated cotton and treated cotton were 58.3% and 59.7%, respectively. This meant that cationic modification left almost unchanged the main crystalline form and crystal structure of the cotton fabrics.

Fig. 7 XRD patterns of cotton fabric samples: (a) untreated and (b) treated.

3.5 The optimum reaction conditions of the cationic polymer

The method of increasing temperature gradually could mean the polymerization reaction slowly forms regular oligomers in the first step of polycondensation. Then these regular oligomers formed relatively larger polymer molecules in the second step of polycondensation. In the third step, polymer molecules were further polymerized by the catalysis of alkali and formed ordered cationic polymer with higher molecular weights.

The effect of the monomer ratio, the reaction temperature and the crosslinking agent was evaluated with the total dye utilization (CI Reactive Black 5) of cotton fabrics modified by the prepared cationic polymer. Fig. 8 shows that the total dye utilization of modified cotton fabrics was greatly improved. Total dye utilization increased with an increase of polymer solution viscosity. That is to say, the greater the average molecular weight of the cationic polymer is, the higher the total dye utilization of modified cotton. The cationic polymer with high molecular weights had usually a high cationic degree and thus showed high substantivity. The optimized process conditions were as follows: reaction temperature was 65 °C, mole ratio of epichlorohydrin to dimethylamine was 1.3, and the crosslinking agent was diethylenetriamine whose dosage was 3%.

Fig. 8 The influence of the monomer ratio, the reaction temperature and the crosslinking agent.

3.6 Fastness properties and tear strength

Table 1 summarizes the tear strength and color fastness properties of untreated and treated cotton fabrics. The tear strength of treated cotton fabric was reduced very slightly. The treated cotton fabric showed equal or lower rub fastness and wash fastness ratings as compared to the untreated one. These results were relatively good and were up to the application standard for dyed fabrics. The cationic modification process had almost no negative influence on the tear strength and color fastness properties of cotton fabrics.

Table 1 The tear strength and color fastness properties of dyed cotton fabrics

Fabric	Wash fastness			Rub fastness		Tear strength (N)
	Shade change	Staining		Dry	Wet	Warp	Weft
		Cotton	Wool
Untreated	4–5	4–5	4–5	4–5	3–4	22.9	17.4
Treated	4	4–5	4–5	4	3–4	22.7	17.3

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3.7 Suitability of modified cotton fabrics with other reactive dyes

In this section, other reactive dyes were used to assess the suitability of cationic modification. The cationic cotton was dyed without the addition of salt and untreated cotton was dyed with the conventional method (adding 50 g L⁻¹ NaCl). Table 2 shows that the color yield and the total dye utilization of the cationic cotton with four reactive dyes were all much higher than those of untreated cotton. In comparison with untreated cotton, the cationic cotton displayed excellent color strength using salt-free dyeing. Though S_r(λ) of the cationic cotton fabric was slightly higher than that of untreated cotton, the levelness of the cationic cotton was relatively good. The cross-section of the dyed cotton sample was examined under an optical microscope, as shown in Fig. 9. It was found that the inside of the cationic cotton was largely colored and darker than the untreated cotton, which demonstrated that the reactive dye had penetrated into the center of the fabrics under salt-free dyeing conditions.

Table 2 Dyeing effect of untreated and treated cotton fabrics

Reactive dye name	Untreated cotton			Treated cotton
	T (%)	K/S	Sr(λ)	T (%)	K/S	Sr(λ)
CI Reactive Black 5	52.9	21.68	0.065	92.7	25.32	0.076
CI Reactive Blue 19	56.8	16.08	0.061	88.6	22.84	0.068
CI Reactive Yellow 176	64.2	16.56	0.045	93.3	19.41	0.046
CI Reactive Red 195	67.4	19.33	0.058	85.5	20.21	0.053

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Fig. 9 Microscope photos of cotton fabrics dyed with CI Reactive Black 5: (a) untreated cotton after conventional dyeing and (b) treated cotton after salt-free dyeing (×400).

The dyeing mechanism was postulated and is shown in Fig. 4. It was thought that most of the reactive dyes were easily absorbed and diffused into the modified cotton by opposite charges attracting in the exhaust dyeing process. The above-mentioned absorption could greatly increase the concentration of reactive dyes inside the treated cotton, which enhances dye-fiber fixation reaction in the dye-fixation process. To some degree, the positive charge of the modified cotton could temporarily restrict the movement of anions, especially the hydroxyl anion, which accordingly decreases the hydrolysis of reactive dyes. So the total utilization of reactive dyes was improved greatly. On the other hand, the sulfatoethyl sulfonyl dye could be easily changed into a vinyl sulfone reactive dye by the elimination reaction probably due to the existence of positive charges in the modified cotton. And the generated small molecule with vinyl sulfone groups could move and attack cellulosate anions to form a conventional dye-fibre bond. Therefore the dyeing levelness of the treated cotton could be secured.

3.8 Decolorization performance of the prepared cationic polymer

A flocculation experiment for the removal of residual dyes (CI Reactive Black 5) was performed. The composition of residual dyes was that the reactive dye concentration was 0.1 g L⁻¹ and the nonionic surfactant (Triton X-100) concentration was 0.3 g L⁻¹. The variations of color removal efficiency with flocculant dosage are presented in Fig. 10. Almost 94% efficiency was achieved at 60 mg L⁻¹ dosage. The result indicated that the prepared cationic polymer was highly effective for treating CI Reactive Black 5 wastewater.

Fig. 10 Effect of cationic polymer dosage on color removal in the residual dye liquor (CI Reactive Black 5).

In the washing liquid of the modifying process, the cationic polymer concentration was about 101–126 mg L⁻¹ (per gram of cotton fabrics) from measurement of cationicity. So the washing liquid could be collected to flocculate dye wastewater. And the modifying solution could be recycled and the residue also could be used to dispose of dye wastewater. So the cationic polymer has good flocculating property and possesses significant potential to modify cotton fabrics in a green manner.

4. Conclusions

A novel epichlorohydrin–dimethylamine polymer with controlled molecular weight and controlled cationicity was successfully synthesized. FTIR analysis and ¹H NMR analysis confirmed the expected structure.

This study confirmed that after the chemical modification of the cotton fabrics, the total utilization of reactive dyes was improved greatly in the absence of salt. Besides, the color fastness properties and levelness of modified cotton fabrics were both satisfactory.

The cationic polymer had good flocculating property and did not result in repeated pollution. The investigation provided a feasible and eco-friendly way to realize salt-free dyeing with reactive dyes.

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