Dyeing of Linen Fabrics in Supercritical CO₂ Using a Reverse Micellar System with Ionic Liquid Domains

ABSTRACT

In order to realize the anhydrous dyeing of linen fabric with reactive dyes in supercritical CO₂, a supercritical CO₂ ionic liquid reverse micelle system was constructed, and the dyeing process of linen fabric in this system was studied. In this system, linen fabric can be dyed with reactive dyes without special pretreatment. The results show that the dyeing depth of linen fabric can be significantly improved by increasing the amount of surfactant. With the increase in temperature, the dyeing depth first increases and then decreases. The dyeing pressure displays little effect on the dyeing depth. After dyeing, there are a lot of floating colors on the surface of the fabric. Increasing the temperature and prolonging the dyeing time are conducive to dye fixation on the fabric. Adding a certain amount of dimethyl sulfoxide (DMSO) to the system can improve the dyeing performance significantly. In this paper, the best dyeing conditions of linen fabric are as follows: w, 1.2; surfactant conc. 3.5 × 10⁻² g/mL; T, 90°C; t, 180 min; p, 21MPa; DMSO concentration, 76.8 mmol/L.

摘要

为了实现活性染料在超临界CO2中对亚麻织物的无水染色，构建了超临界CO2离子液体反胶束体系，并对该体系中亚麻织物染色工艺进行了研究. 在这个系统中，亚麻织物可以用活性染料染色，而无需特殊的预处理. 结果表明，增加表面活性剂的用量可以显著提高亚麻织物的染色深度. 随着温度的升高，染色深度先增大后减小. 染色压力对染色深度影响不大. 染色后，织物表面有许多漂浮的颜色. 提高染色温度和延长染色时间有利于染料在织物上的固色. 在体系中加入一定量的二甲基亚砜（DMSO）可以显著提高染色性能.本文确定了亚麻织物的最佳染色条件为: w，1.2; 表面活性剂浓度, 3.5 × 10-2克/毫升; T、 90摄氏度; t、 180分钟; p、 21MPa; 二甲基亚砜浓度76.8 mmol/L.

KEYWORDS:

• Supercritical CO₂
• linen fabric
• reverse micelle
• dyeing
• ionic liquid
• fixation

关键词:

• 超临界CO2
• 亚麻织物
• 反胶束
• 染色
• 离子液体
• 固定

Introduction

The traditional dyeing process of natural fibers is usually carried out in water-based dyeing baths, and the subsequent washing process also consumes a lot of water. At the same time, the discharged wastewater contains a lot of colored compounds and electrolytes, leading to serious environmental problems. The critical conditions of supercritical CO₂ are relatively mild. The critical temperature of supercritical CO₂ is 31.1°C and the critical pressure is 7.38 MPa. Simultaneously, supercritical CO₂ is cheap, basically nontoxic, nonflammable, and easy to recover and recycle after need and use. As a dye bath medium, supercritical CO₂ has attracted the attention of many researchers as a viable alternative to water and organic solvents (Abou et al. 2023; Juan, Zheng, and Zheng 2018; Saus, Knittel, and Schollmeyer 1992; Wang et al. 2022; Zhao et al. 2022).

However, the dielectric constant of supercritical CO₂ is low, and it is difficult to dissolve most polar compounds (Yee, Fluton, and Smith 1992). Therefore, the industrial achievements of dyeing textile fibers in supercritical CO₂ are limited to the use of disperse dyes that are soluble in supercritical CO₂ to dye synthetic fibers. However, the technology of dyeing natural fibers under supercritical conditions is still immature. It is possible to dye natural fibers in supercritical CO₂ by introducing reactive groups into disperse dyes and modifying them appropriately (Abou et al. 2022; Cid et al. 2007; Penthala et al. 2022a, 2022b; Schmidt, Bach, and Schollmeyer 2003). The natural fabric can also be hydrophobically modified and then dyed in supercritical fluid (Gebert et al. 1994; Ozcan, Clifford, and Bartle 1997). In addition, to dye natural fibers with water-soluble dyes in supercritical CO₂, the dye must be soluble in supercritical CO₂ first.

Reverse micelle is a spontaneous aggregation of surfactant dispersed in a continuous nonpolar organic phase. It can provide a stable water microenvironment in nonpolar organic phase by dissolving a small amount of water in the micelle (Shen, Wang, and Gao 1994). Kazuya Sawada of the Kyoto Institute of Technology in Japan and others dissolved water-soluble dyes in the supercritical CO₂ water reverse micelle system for natural fiber dyeing, and good dyeing depth on silk and wool was achieved (Sawada and Ueda 2001; Sawada and Ueda 2004; Sawada and Ueda 2003a, 2003b, 2003c). However, due to the absence of alkali, the color fastness of dyed cotton fiber is poor. Even if thermal fixation is used, the color depth of cotton fabric is still only about 40% of that before fixing.

Room temperature ionic liquids (ILs) are organic salts, which are nonvolatile, lower melting point, wide liquid path, adjustable properties, strong solubility, nontoxicity and low toxicity. They are considered as ideal green solvents and have attracted wide attention (Seddon 1999). Han et al. created reverse micelles with ionic liquid domain in supercritical CO₂, so that ionic liquids can be dispersed in supercritical CO₂. This system can dissolve polar compounds (Liu et al. 2007). Furthermore, ILs have shown the ability to swell and dissolve natural cellulose (Binder and Raines 2009; Luo et al. 2012; Moulthrop et al. 2005; Song et al. 2013; Swatloski et al. 2002; Vo et al. 2012; Zhang et al. 2005; Zhu et al. 2006). It has been found that cellulose can be dissolved without derivatization in high concentrations using ILs as the solvent. If supercritical CO₂ ionic liquid reverse micelle system is used to dissolve water-soluble dyes, better dyeing results may be obtained for natural fibers, especially for cotton and linen fabrics based on cellulose structure.

The cellulose macromolecular structure of linen fiber has high degree of crystallinity and orientation. Moreover, the linen fiber presents a compact structure, with high content of non cellulose components such as pectin, hemicellulose, and lignin, which makes the dyeability of linen fiber poor in the process of aqueous dyeing (Shi 1999). In this study, we used N-ethyl perfluorooctyl sulfonimide (C₂H₅NHSO₂C₈F₁₇; N-EtFOSA) as a surfactant to construct a reverse micelle system in supercritical CO₂ for linen fiber dyeing. For the first time, supercritical carbon dioxide and ionic liquid, two green solvents, were used together for natural fiber dyeing. Through the study of dyeing pressure, dyeing temperature, dye dosage, time, and other factors, the basic knowledge of dyeing cellulose based natural fibers using reverse micelle system in supercritical CO₂ is accumulated.

Experimental

Materials

The reagents and chemicals include 1,1,3,3-tetramethylguanidinium (TMG), N-ethyl perfluoroo-ctylsulfonamide (C₂H₅NHSO₂C₈F₁₇; N-EtFOSA), acetate (CH₃COOH) were purchased from Tianjin Damao Chemical Reagent Factory Co., Ltd. (Tianjin, China). CO₂ (99.995%) were purchased from China Haohua Research & Design Institute of Chemical Industry Co., Ltd. (Dalian, China). Linen fabric (225.0 g·m⁻²), was supplied by Qiqihar Yajin Flax Industry & Trade Group (Qiqihar, China). C.I. reactive blue 194 were purchased from Jiangsu Zhenyang Dyestuff Technology Co., Ltd. (Jiangsu, China).

Synthesis of ionic liquid

1,1,3,3-tetramethylguanidinium ([(CH₃)₂N]₂C=NH₂⁺) acetate (TMGA) was synthesized by following the literature procedure (Liu et al. 2007). 100 mL ethanol and 2.3 g TMG (20 mmol) were loaded into a 250 mL round-bottomed flask in a water bath at 25°C with stirring. Twenty mmol CH₃COOH in 30 ml of ethanol was then added into the flask under stirring. After 2 hr of reaction, the mixture was evaporated under reduced pressure. The obtained product was dried under vacuum for 6 h. TMGA is a slightly amber solid material.

Dyeing linen fabric in supercritical CO₂

The dyeing apparatus was self-made in the laboratory, and its schematic diagram is shown in Figure 1. The dyeing process was carried out in a high pressure chamber with a diameter of 4 cm and a volume of 100 mL. The linen fabric was fixed on a shelf, and loaded into the high pressure chamber, and then a certain amount of N-EtFOSA, TMGA and C.I. reactive blue 194 were loaded into the high pressure chamber. The liquid CO₂ was then injected into the high pressure chamber by the pressure pump, and the high pressure chamber is heated at the same time. When the system temperature and pressure had reached the desirable value, dyeing was started, and a constant stirring is performed with a magnetic stirrer. Release CO₂ after dyeing, and the cloth sample is taken out. The TMGA and N-EtFOSA on the fabric surface were removed by washed dyeing samples with ethanol at room temperature. And samples were dried at room temperature for analysis and detection.

Figure 1. Schematic diagram of the apparatus used for dyeing: (1) CO₂ cylinder, (2) high-pressure pump, (3) valve, (4) pressure gauge, (5) thermostat, (6) high pressure chamber.

Dyeing ability measurement

A Color-Eye 7000A spectrophotometer (X-rite, America, D65 light source, the observation angle 10°) was used to measured K/S values. The wavelength range is 360-750 nm.

The AATCC test method 8–2007 and the AATCC test method 61–2010 (1A) were used to assess rubbing fastness and washing fastness, respectively.

Determination of dye fixation

The dye fixation, i.e. the percentage of dye molecules that are not floating colors, was determined by comparison of the K/S values after soaping and after dyeing, which was calculated by the following equation:

(1) $\mathrm{F}=\frac{K/S_s}{K/S_d}\times 100\mathrm{\%}$(1)

where F represents the dye fixation, K/S_s and K/S_d represent the K/S values after soaping and dyeing, respectively.

Result and discussion

Effect of surfactant concentration on K/S values of dyed linen

Table 1 shows the influence of different N-EtFOSA concentrations in the dyeing system on the K/S values of linen fabrics. When the concentration of N-EtFOSA is lower than 0.02 g/mL, linen fabric with uniform color cannot be obtained after 180 min of dyeing. When the concentration of N-EtFOSA increases from 0.025 g/mL to 0.04 g/mL, the K/S_d values increases from 4.75 to 6.75, and the K/S_s values increases from 1.01 to 1.81. This shows that when the molar ratio of TMGA and N-EtFOSA is unchanged, the content of N-EtFOSA and TMGA in the system is increased, which will increase the number of micelles and the dye dissolution in the system, leading to an increase in the K/S_d value of the fabric. When the content of surfactant in the system is too low, it is difficult to form stable micelles in the system, or the number of micelles is too small, resulting in low dye dissolution (Sawada, Takagi, and Ueda 2003; Zulauf and Eicke 1979). Furthermore, the small amount of micelles leads to less TMGA in the system, and the linen fiber may not be well swelled, resulting in insufficient pores in the fiber and decreasing the dye diffusion rate. This may also be one of the reasons for uneven dyeing of the fabric.

Table 1. Influence of surfactants content on the K/S values of dyed linen fabric^a..

N-EtFOSA concentration (g/mL)	K/Sd	K/Ss	F(%)
0.015	-	-	-
0.02	-	-	-
0.025	4.75	1.01	21.27
0.03	6.02	1.31	21.76
0.035	6.34	1.75	27.6
0.04	6.75	1.81	26.82

a: owf%, 0.55; w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; T, 110°C; t, 180 min; p, 21MPa

Effect of molar ratio of TMGA to N-EtFOSA on K/S values of dyed linen

Figure 2 shows the dyeing results of linen fabric in reverse micelle system when the molar ratio (w) of TMGA and N-EtFOSA is changed. The results show that the K/S_d value of the fabric enhances with the increase of w value from 0.51 to 0.86, and decreases with the increase of w value from 0.86 to 1.89. K Sawada et al. found that when the weight of water is far less than the weight of the fabric in the reverse micelle system, increasing the amount of water has no effect on the dye concentration in the system and the color depth of the dyed fabric. However, the system with a large proportion of water is more conducive to the fiber swelling (Sawada, Takagi, and Ueda 2003). With the increase of w value, the content of ionic liquid increased, which promoted the swelling of linen fabric.

Figure 2. Influence of the ionic liquid-to-surfactant molar ratio (w) on the K/S values of dyed linen fabric. (owf%, 0.55; surfactant conc., 3.5 × 10⁻² g/mL; T, 110°C; t, 180 min; p, 21MPa).

The micelles formed in the dyeing process constantly carry ionic liquids and dyes to the fabric. There are also ionic liquids removed from the fabric surface under the action of surfactants. These two processes gradually reach equilibrium with the extension of dyeing time. Figure 3 gives a schematic representation of this phenomenon. During the research, we measured the weight change of dyed linen fabric (Δm). The influence of dye quality is very small, so under the same conditions, the change of Δm should be caused by the quality change of TMGA on the fabric, Δm first increases and then remains unchanged with the increase of w value. With the w value is increased from 0.51 to 0.86, the fabric absorbs more TMGA and dyes, which increases the K/S_d value. The increase of TMGA on the surface of the fabric also makes more TMGA act on the fabric fiber, which tends to swell the fiber better and accelerates the dye diffusion in the fiber, so the K/S_s value and dye fixation are improved (see Figure 4). The w value increased from 0.86 to 1.89, and the decrease of K/S value may be due to the fact that some dyes were dissolved in excessive TMGA, which reduced the dye concentration in the system.

Figure 3. Schematic representation of dyeing process of linen fabrics in supercritical CO₂ reverse micelle system.

Figure 4. Influence of the ionic liquid-to-surfactant molar ratio (w) on fixation of dyed linen fabric. (owf%, 0.55; surfactant conc., 3.5 × 10⁻² g/mL; T, 110°C; t, 180 min; p, 21MPa).

Influence of dye dosage on K/S values of dyed linen

Figure 5 shows the influence of different dye dosages on linen fabric dyeing in the reverse micelle system. With the increase in dye dosage, the K/S_d value and K/S_s value of the dyed linen fabric increase, but the fixation changes little. This shows that with the increase in dye dosage, the micelle will bring more dyes to the surface of the fabric. However, under the same condition, the equal amount of TMGA adsorbed on the fabric surface, and the swelling of the fiber is the same. The swelling of the fiber has a significant impact on the diffusion rate of dyes inside the fiber. Therefore, the K/S_d value of the dyed linen fabric increases significantly, while the dye fixation is almost unchanged.

Figure 5. Influence of dye concentration on the K/S values of dyed linen fabric. (w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; T, 110°C; t, 180 min; p, 21MPa).

Effect of temperature on K/S values of dyed linen

Figure 6 shows the influence of dyeing temperature on the K/S values of linen fabric in reverse micelle system. It can be seen from the figure that the dyeing process is significantly affected by temperature. With the temperature increasing from 75°C to 110°C, the dyeing process reaches equilibrium faster. This shows that higher temperature makes the adsorption and desorption of TMGA on linen fiber reach equilibrium more quickly. Moreover, temperature also affects the swelling of the fiber by ionic liquids. At higher temperature, ionic liquids are more likely to destroy the hydrogen bonds within and between cellulose molecules, making the fiber more swollen, which is conducive to the diffusion of dyes on the surface and inside of the fiber. Simultaneously, it can be seen from the figure that after 90 min of dyeing, the linen fabric dyed at 90°C displays a higher color depth than that at 110°C. This may be because under a certain pressure, CO₂ at a lower temperature has a higher density and a higher solubility for solutes. When the amount of raw materials added is the same, more TMGA and dyes are carried to the fiber surface at a lower temperature, resulting in a higher dye content on the fiber surface. Compared with 110°C and 75°C, 90°C may give consideration to both fiber swelling and solute carrying, so the K/S_d values obtained is higher. However, from the curve of fixation changing with dyeing time (see Figure 7), it can be seen that although the linen fabric dyed at 90°C has a higher K/S_d value, the increase in temperature plays a significant impact on the dye diffusion, the fixation at 110°C is slightly higher than 90°C. Generally, reactive dyes react with cellulose under alkaline conditions, otherwise the dyes can only be adsorbed on the surface of the fabric, and the fixation rate is poor (Gorenek 1999). However, K Sawada et al. found that reactive dyes can also be effectively fixed on cotton at higher temperatures. This shows that although there is no alkali involved in the dyeing process, the higher temperature can promote the reaction between reactive dyes and cellulose to achieve the fixation of dyes on cotton fabrics. If the accessibility of cotton fabric can be increased when the dyeing temperature is raised, the fixation rate may be further improved (Sawada and Ueda 2003a).

Figure 6. Influence of the time on the K/S values of dyed linen. (owf%, 0.55; w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; p, 21MPa).

Figure 7. Influence of the time on the Fixation of dyed linen. (owf%, 0.55; w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; p, 21MPa).

Effect of supercritical CO₂ pressure on K/S values of dyed linen

Figure 8 and 9 shows the effect of supercritical CO₂ pressure on the K/S values of dyed linen fabric. It can be seen from Figure 8 that under higher CO₂ pressure, the K/S value of dyed linen fabric is larger, but the increase is small. K Sawada et al. found that under a given temperature, the solubility of disperse dyes in supercritical CO₂ increases with increasing pressure. But the dyes in the reverse micelle system dissolved in the micellar pool rather than dissolved in CO₂. Once the reverse micelle is formed, the pool is immediately saturated with dyes and the dye concentration remains unchanged with increasing pressure (Sawada et al. 2002). However, in the dyeing process, under higher CO₂ pressure, the contact opportunity between dye molecules and fabric fibers is increased, so higher K/S value can be obtained (Sawada, Takagi, and Ueda 2003).

Figure 8. Influence of the pressure on the K/S values of dyed linen. (owf%, 0.55; w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; T, 90°C).

Figure 9. Influence of the pressure on the K/S values of dyed linen. (owf%, 0.55; w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; T, 90°C).

With the increase in pressure from 15MPa to 21MPa, the TMGA adsorbed on linen fabric increases, which may affect the swelling of flax fiber. Therefore, under the experimental conditions, the pressure presents a significant impact on the fixation of linen fabric.

Effect of dimethyl sulfoxide on linen dyeing

Figure 10 shows the effect of DMSO concentration in supercritical CO₂ reverse micelle system on dye performance of linen fabric. The dyeing ability of C.I. reactive blue 194 for flax fiber was improved due to the addition of DMSO. With the DMSO concentration enhancing from 0 to 76.8 mmol/L, the K/S_d value, K/S_s value, and fixation increased gradually. DMSO and DMF (N, N-dimethylformamide) seem to be the most effective solvents among various co solvents tested by Rinaldi for dissolving cellulose in ionic liquids. Adding DMSO to ionic liquids can reduce viscosity and significantly improve mass transfer. The mixed solution made of ionic liquid and DMSO in a certain proportion can dissolve cellulose faster than pure ionic liquid (Andanson et al. 2014; Lin, Yamaguchi, and Suzuki 2013; Payal et al. 2015; Xu et al. 2013; Zhao et al. 2013). Furthermore, DMSO and TMGA formed a mixture on linen fiber, which promoted the swelling of the fiber. With the increase in DMSO dosage from 76.8 mmol/L to 115.2 mmol/L, the K/S_d value is basically unchanged, while the K/S_s value and fixation decrease. This is due to the excessive addition of DMSO, which makes the TMGA content in the mixed solution on the surface of linen fabric decrease, reducing the swelling capacity of the mixed solution on the fiber.

Figure 10. Influence of DMSO concentration on the K/S values of dyed linen fabric. (w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; T, 110°C; t, 180 min; p, 21MPa).

As revealed in Figures 11 and 13, the addition of DMSO increased the K/S values of linen fabric, accelerated the dye diffusion to the interior of the fiber, and greatly increased the fixation of linen fabric. Figure 12 shows photos of original linen fabric, linen fabric dyed with and without DMSO.

Figure 11. Influence of adding DMSO on the dying process of dyed linen. (owf%, 0.55; w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; T, 90°C).

Figure 12. Photos of linen fabric (a), linen fabric dyed without DMSO (b, 20 min, c, 40 min, d, 60 min, e, 90 min, f, 120 min, g, 180 min), and linen fabric dyed with DMSO (b, 20 min, c, 40 min, d, 60 min, e, 90 min, f, 120 min, g, 180 min). (owf%, 0.55; w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; T, 90°C).

Figure 13. Influence of adding DMSO on the fixation of dyed linen. (owf%, 0.55; w, 1.2; surfactant conc., 3.5 × 10⁻² g/mL; T, 90°C).

Washing and rubbing fastness

As the linen fabric dyed with DMSO has the best dyeing effect, the washing fastness and rubbing fastness of the dyed linen fabric are evaluated, and the results are shown in Table 2. The staining fastness of linen fabric dyed with DMSO can reach grade 4; But the fading fastness result is not satisfactory, about 3–4 grades.

Table 2. The washing and rubbing fastness of linen dyed with DMSO.

DMSO concentration/mmol/L	Washing fastness		Rubbing fastness
	Fading	Staining	Dry	Wet
0	2	1–2	2	1–2
19.2	2–3	3	3	2
38.4	3	3	3	2–3
57.6	3	3–4	3–4	3
76.8	3–4	4	4	3
96.0	3	3–4	3	3
115.2	2–3	3	3	2

Conclusion

Supercritical CO₂/ionic liquid reverse micelle system was used to dye linen fabrics. The effects of N-EtFOSA concentration, w value, dye dosage, dyeing temperature, dyeing pressure, and DMSO addition on the K/S values and dye fixation of dyed flax fabric were investigated. The results showed that the unmodified linen fabrics were dyed with reactive dyes. However, due to the high viscosity of ionic liquid and slow diffusion speed, a large number of colors are adsorbed on the fabric surface as floating colors. The w value and the dyeing temperature will affect the swelling degree of the fabric and the dye diffusion on the fabric. Due to the addition of DMSO to the system, the viscosity of ionic liquid was reduced, the dye diffusion speed on the fabric was increased, and the dye fixation degree was improved.

Highlights

• We studied the dyeing depth and fixation of unmodified flax fabric in supercritical CO₂/ionic liquid reverse micelle system.

• The effects of surfactant concentration, dye dosage, dyeing temperature, and pressure on the dyeing depth and fixation of flax fabric were investigated.

• Due to the addition of DMSO to the system, the viscosity of ionic liquid was reduced, the dye diffusion speed on the fabric was increased, and the dye fixation degree was improved.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was financial supported by the National Natural Science Foundation of China (21908015), Basic Scientific Research Project of Higher Education Department of Liaoning Province (J202152), Liaoning Province Applied Basic Research Project (20230101), The Seventh Batch “Hundred People Plan” of Talent Introduction in Fujian Province and Fujian Provincial Regional Development Project (2022H4015), Dalian High-level Talent Innovation Support Project (2020RQ019), Dalian Key Research and Development Program (2022YF12G×057), Dalian Science and Technology Innovation Fund Project (2022JJ12SN055) as well as Dalian Science and Technology Innovation Fund Project (2022J12G×045).