Comprehensive Study on Cellulose Swelling for Completely

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Comprehensive Study on Cellulose Swelling for Completely Recyclable Nonaqueous Reactive Dyeing Luyi Chen,†,‡ Bijia Wang,†,‡ Jiangang Chen,†,‡ Xinhui Ruan,†,‡ and Yiqi Yang*,†,§,∥ †

Key Laboratory of Science and Technology of Eco-Textiles, Ministry of Education, Donghua University, 2999 North Renmin Road, 201620 Shanghai, China ‡ College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Road, 201620 Shanghai, China § Department of Textiles, Merchandising and Fashion Design, University of Nebraska-Lincoln, HECO Building, Lincoln, Nebraska 68583-0802, United States ∥ Department of Biological Systems Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, HECO Building, Lincoln, Nebraska 68583-0802, United States S Supporting Information *

ABSTRACT: The swelling of cotton by non-nucleophilic organic solvents was investigated to achieve completely recyclable reactive dyeing. The degree of swelling was determined and correlated to the Hansen Solubility Parameter distance (Ra) of cellulose to the solvents and the dielectric constant of the solvents (ε). The effect of swelling temperature was also investigated. Preswelling of cotton fabrics by 150 °C N,N-dimethylacetamide (DMAc) for 1 h was found to be sufficient to accelerate dye sorption. Dyeing was carried out using C.I. Reactive Red 24 in a 40/60 mixture of DMAc and dimethylcarbonate (DMC), a cosolvent selected to facilitate dye exhaustion. The efficiency of unfixed dye removal was found to predominantly correlate to swelling (R2 = 0.9236). Excellent colorfastness was achieved with 4 rinses by 95 °C DMAc. A 10-cycle repeated dyeing sequence was demonstrated to give 43% and 90% reduction in dye consumption and disposal. The overall reduction in material disposal was estimated to be over 99.99%. The favorable results indicated that discharge-free reactive dyeing could be made possible.

1. INTRODUCTION Large amounts of water are involved in conventional textile dyeing and finishing both in terms of fresh water intake and wastewater disposal.1 To cut or eliminate process and wastewater, extensive research effort had been directed to utilization of nonaqueous media in textile processing.2−8 To go waterless in reactive dyeing is sensible given the sheer volume of cotton dyed reactively each year. More importantly, waterless reactive dyeing potentially offers the alternative to recycle and reuse the spent processing liquors. Traditional reactive dyeing effluents, high in salt and colorant contents, are not only among the hardest to treat textile wastewater9−12 but also unrecyclable because hydrolysis renders the unfixed dyes unreactive. Getting rid of water could greatly enhance the atom economy of the nucleophilic dye fixation process. However, attempts to reactively dye cotton in nonaqueous media had not been very successful. ScCO2,13 the most studied medium, and other solvents reported for nonaqueous dyeing, such as silicone oil14 and chlorohydrocarbons,15 are generally much less polar than water and inefficient in swelling cellulose. Swelling plays a major part in chemical modification of cellulose.16 In reactive dyeing, swelling opens up sites on fibers to allow effective sorption, fixation, and desorption of dye molecules. Insufficient swelling of cotton by ScCO2 was considered to be responsible for the observed poor color buildup.17,18 To improve color shade and fastness, research had been focused on preswelling of the cellulose using a polar protic cosolvent such as methanol19,20 ethanol, and even water itself.21−24 The merits © 2015 American Chemical Society

of nonaqueous dyeing were compromised because these nucleophilic cosolvents inevitably competed for the dye against cellulose. To realize fully recyclable reactive dyeing, it is crucial to retain dye reactivity by strictly excluding nucleophilic solvents throughout the dyeing process. Rinsing is also crucial in reactive dyeing because the superior fastness of reactively dyed cotton relies on thorough removal of unfixed dyes. As mentioned above, water and other nucleophilic solvents should also be strictly excluded in rinsing to ensure recyclability. In nonaqueous rinsing, insufficient swelling could harm the dye-removal efficiency because dye desorption is also likely to be swelling-controlled. However, none of the previous nonaqueous dyeing studies has investigated the effect of swelling on rinsing efficiency. The current study aims to develop a fully recyclable solvent reactive dyeing process for cotton fabrics by properly addressing the dilemma of insufficient swelling and dye solvolysis. First, the swelling of cotton fabric by various non-nucleophilic organic solvents was investigated. An optimum preswelling solvent was selected on the basis of nSw values and the Environmental Health and Safety (EHS) indices. Swelling of cotton by the chosen solvent, N,N-dimethylacetamide (DMAc), was further investigated under various temperatures. Second, the preswollen Received: Revised: Accepted: Published: 2439

November 27, 2014 February 13, 2015 February 13, 2015 February 25, 2015 DOI: 10.1021/ie504677z Ind. Eng. Chem. Res. 2015, 54, 2439−2446

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Industrial & Engineering Chemistry Research

transferred to a 150 mL stainless steel vessel containing 0.15 g of RR24 and a predetermined amount of a base in 60 mL of a 40/60 mixture of DMAc/DMC (liquor ratio = 20:1). The vessel was sealed, placed in a Daelim Starlet DL-6000 Plus laboratory IR dyeing machine, heated at a rate of 2 °C/min to 95 °C, and maintained for 2.5 h with mechanical agitation (60 rpm). After dyeing was completed, the sample was cooled, removed from the vessel, and centrifuged at 1600g for 3 min. The solvent dyed fabric sample was subjected to the rinsing procedure described in Section 2.4 prior to being tested. General Procedure for Aqueous Dyeing. Cotton fabrics were cut into pieces of approximately 3 g. Each piece of fabric was transferred to a 150 mL vessel containing 0.15 g of RR24, 100 g/L NaCl, and10 g/L Na2CO3 in 60 mL of water (liquor ratio = 20:1). The vessel was sealed, placed in a Daelim Starlet DL-6000 Plus laboratory IR dyeing machine, heated at a rate of 2 °C/min to 95 °C, and maintained for 2.5 h with mechanical agitation (60 rpm). After dyeing was completed, the sample was cooled, removed from the vessel, centrifuged at 1600g for 3 min, and subjected to soaping as described in Section 2.5 prior to being tested. 2.4. Removal of Unfixed Dyes from Solvent-Dyed Samples. The dyeing solvent mixture (40/60 DMAc/DMC), pure DMAc, and D.I. water were investigated as the rinsing solvent for the solvent-dyed cotton samples. A dyed sample was placed in a 125 mL Erlenmeyer flask containing 60 mL of a rinsing solvent. The flask was shaken at either ambient temperature or 95 °C on a Daclim DL-2003 water bath shaker for 30 min. The solvent was decanted and collected. The fabric sample was removed from the flask and centrifuged at 1600g for 3 min. The centrifugate was combined with the spent rinsing solvent. The amount of RR24 removed from a single rinse was determined spectroscopically. The process was repeated for five times. 2.5. Soaping Procedure. A dyed sample was rinsed with cold deionized (D.I.) water at the liquor ratio of 1:20. The sample was then placed in a 125 mL Erlenmeyer flask containing a 75 mL aqueous solution of 2 g/L MA soap. The flask was shaken at 95 °C on a Daclim DL-2003 water bath shaker for 15 min. The fabric sample was removed from the flask and rinsed again with cold D.I. water at the liquor ratio of 1:20 for 2 times. The sample was air-dried to constant weight. 2.6. Recyclable Solvent Dyeing. Solvent dyeing was carried out as described in Section 2.3. The fabric sample was then subjected to four rinses with pure DMAc at the liquor ratio of 1:20 and 95 °C. The fabric was then centrifuged at 1600g for 3 min and finally dried with heating under reduced pressure (20 mbar, 80 °C) to constant weight. The centrifugate and rinse baths were combined and evaporated under reduced pressure (20 mbar, 80 °C) to yield a paste consisting of unreacted RR24, K2CO3, and KCl. The paste was added to the spent dyebath, which was filtered, and its volume was measured. The organic contents of the combination was subsequently determined by 1H NMR and UV−vis analysis. It was assumed that the inorganic base, K2CO3, was only consumed by the fixation reaction. 0.047 g of RR24 and 0.008 g of K2CO3 were added to replenish the dyebath. The replenished dyebath was then used to dye the next batch of fabric without further treatment. All solvent vapors were collected using a coldfinger solvent trap, combined, measured, and reused for rinsing in the next dyeing cycle. The process was illustrated in Figure 1. 2.7. Measurements. The microscopic images of cotton fibers were obtained using a Nikon LV100ND transmission optical microscope. Cotton fabrics were preswollen as

cotton fabrics were subjected to solvent reactive dyeing using monochlorotriazine (MCT) model dye, namely, C.I. Reactive Red 24 (RR24). A 40/60 mixture of DMAc and dimethylcarbonate (DMC), a green exhaustion cosolvent, was employed as the dyeing medium. Nonaqueous rinsing of the solvent-dyed fabrics was investigated. Finally, a 10-cycle dyeing sequence that reuses all spent baths was carried out to demonstrate the recyclability of the process.

2. EXPERIMENTAL SECTION 2.1. Materials. Bleached cotton poplin (40 × 40, 133 × 72, 123 g/m2) was kindly supplied by the Esquel Group (Guangdong, China) and used as is. The multifiber adjacent fabrics used for colorfastness assessment were purchased from Testfabrics Inc. Standard detergent was purchased from Shanghai Textile Industry Institute of Technical Supervision. Acetonitrile (ACN), N,N-dimethylformamide (DMF), N,Ndimethylacetamide (DMAc), tetrahydrofuran (THF), pyridine, dioxane, acetone, ethyl acetate, dimethylcarbonate (DMC), tetrabutylammonium bromide, potassium carbonate, sodium carbonate, sulfuric acid, and ammonium acetate were obtained from Sino Reagent Co. and used as received. HPLC grade ACN was purchased from Fischer Scientific. C.I. Reactive Red 24 (RR24) was provided by Taoyuan Dyestuff Co. (Wu Jiang, China). The dye was without auxiliaries and was used as received. 2.2. Swelling of Cotton Fabrics by Solvents. A modified procedure reported by Seoud and co-workers25 was followed to quantify the swelling of cotton fabrics by various solvents. Cotton fabrics were cut into pieces of roughly 2 g each and conditioned at 105 °C for 2 h. The samples were allowed to cool to room temperature in a sealed desiccator. Each sample was then accurately weighed and placed into a stainless steel vessel. Twenty mL of the solvent to be tested was introduced, and the vessel was sealed and kept at 20 °C for 24 h. The sample was removed from the vessel, placed into a centrifuge tube equipped with a perforated separator to hold the sample, and centrifuged using a Beckman Coulter. The Allegra 25R Centrifuge was equipped with a TA-14-50 rotor. Preliminary experiments were carried out in order to determine the optimum combination of centrifugation conditions, including value of g (from 100 to 2400g) and centrifugation time (from 2 to 10 min). The conditions selected were centrifugations using 1600g for 3 min. Swelling in terms of percent mass gain (Sw%) and solvent molecules gained by each anhydroglucose unit (nSw) was calculated according to eq 1 and eq 2, respectively. For each solvent, the centrifugation experiment was repeated 3 times. Sw% = (Msc − Mcc)/Mcc × 100%

(1)

nSw = (Sw% × 100)/(MWsolvent /MWAGU) = (Sw% × 1.62)/MWsolvent

(2)

where Mcc and Msc represent the mass of conditioned cellulose and swollen cellulose, respectively; MWsolvent and MWAGU are the molecular weights of the solvent and the anhydroglucose unit, respectively. For DMAc, the swelling experiment was also carried out at 40, 60, 100, 120, and 150 °C for 1, 12, and 24 h. 2.3. Dyeing Cotton Cellulose in DMAc/DMC and in Water. General Procedure for Dyeing in DMAc/DMC. Cotton fabrics were cut into pieces of approximately 3 g and preswollen in DMAc at 150 °C for 1 h. Each preswollen fabric was 2440

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Figure 1. Illustration of the fully recyclable reactive dyeing of cotton in non-nucleophilic solvents developed in this study.

described in Section 2.2. The filament samples were then withdrawn from the corresponding fabric samples and placed between two glass slides. One end of the filament sample was attached to the bottom plate by an adhesive tape. The optical observations were made. Diameter measurements were performed randomly on the microscopic image of a fiber using the tool package that comes with the microscope. The mean value of 10 measurements was used. Dye fixation was measured according to a procedure reported by Kissa.26 After removal of the unfixed dye by solvent rinses or soaping, a reactively dyed fabric sample was cut into small pieces, conditioned at 105 °C for 2 h, and allowed to cool to room temperature in a desiccator. 0.100 g of the cut fabric was weighed out and placed in 5 mL of 70% sulfuric acid. The mixture was allowed to stand at room temperature until completion of the dissolution. The solution was diluted to 10 mL with D.I. water in a volumetric flask. The amount of RR24 on fabric was determined spectroscopically using a Shimadzu UV-1800 spectrophotometer. HPLC analysis was done on an Elite P230 II liquid chromatography equipped with a C-18 reversed phase column (ODS-BP 5 μm, 205 mm × 4.6 mm, Elite, Dalian, China) and a UV−vis detector set to 530 nm. Twenty μL of a sample solution was injected for each analysis. A binary gradient elution was employed for separation. ACN was used as the strong eluting solvent and an aqueous solution of 0.025 M TBABr and 0.05 M NH4Ac was used as the weak eluting solvent. The gradient program started with an initial 30:70 ratio of the strong to weak eluting solvents. The ratio was changed to 55:45 at 3 min. After all peaks had been eluted out, a step of 10 min re-equilibration to the initial conditions was applied. The flow rate was set to be 0.9 mL/min. Color measurements of the dyed cotton fabrics were performed using a Datacolor 650 benchtop spectrophotometer. The Kubelka−Munk equation (eq 3) was used to determine the K/S values of the colored cotton fabrics at maximum absorption wavelength (λmax) of 530 nm for RR24. K /S = (1 − R min)2 /2R min

solvents at room temperature was investigated. Swelling by water was also investigated as a reference. To avoid solvolysis, the solvents chosen were all non-nucleophilic and unreactive to monochlorotriazine dyes. The results were summarized in Table 1. Table 1 also listed the dielectric constant (ε) and Table 1. Swelling of Cotton Fabrics by Selected NonNucleophilic Solvents and Their Related Properties solvent

%Swa

nSwb

εc

Ra (MPa1/2)d

water DMF DMAc diglyme pyridine acetonitrile dimethylcarbonate THF ethyl acetate dioxane

38.8 15.2 9.64 8.92 8.84 6.64 5.82 5.15 3.62 3.1

3.49 0.34 0.18 0.11 0.18 0.26 0.10 0.12 0.07 0.06

80.1 36.7 38.9 7.23 (25 °C) 13.3 36.6 3.09 (25 °C) 7.52 6.08 2.22

21.4 39.8 41.6 45.3 47.7 41.0 46.8 51.0 46.3 47.3

a

Swelling as measured by percentage of weight gain. bSwelling as measured by the number of solvent molecules per anhydroglucose units. cDielectric constants of the solvents at 20 °C.27 dHSP distance calculated according to eq 4 using the corresponding partial HSPs for the solvents27 and cellulose.30

Hansen Solubility Parameter (HSP) distance (Ra)27 of cellulose to the respective solvents. Ra can be used to predict the solubility of cellulose in a given solvent and is calculated according to eq 4: Ra = [4(δ D ‐ Solv − δ D ‐ Cell)2 + (δ P ‐ Solv − δ P ‐ Cell)2 1/2

+ (δ H ‐ Solv − δ H ‐ Cell)2 ]

(4)

where δD‑Solv, δP‑Solv, and δH‑Solv are the partial HSPs of the solvents for dispersion, polar, and hydrogen bonding, respectively;28 δD‑Cell, δP‑Cell, and δH‑Cell are those of cellulose.29 Weight gain measured for the lower-boiling solvents tended to have larger error caused by the more significant background evaporation. The absolute weight gain measured in our study was much smaller compared with a previous swelling study done with grounded cotton fabrics.30 Albeit, the rankings of the solvent’s swelling capacity agree well for both studies. This is understandable considering cellulose powder has much larger surface area compared to cotton fabrics. Since dyeing is performed on intact cotton fabrics, the data collected in this study should be a better representation of swelling in the actual dyeing process. In an attempt to rationalize the swelling data, %Sw and nSw were plotted against Ra (Figure 2A) and ε (Figure 2B). It was

(3)

Rmin is the minimum value of the reflectance curve. Colorfastness of the dyed cotton fabrics to laundering was determined according to AATCC test method 61-1986(2A) using a Darong SW-12 washing colorfastness tester. Dry and wet colorfastness to crocking was examined according to AATCC test method 8-1988 using an Atlas AATCC Mar Tester CM-5.

3. RESULTS AND DISCUSSION 3.1. Swelling of Cotton at Ambient Temperature in Various Solvents. Swelling of cotton fabrics in various organic 2441

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Figure 3. Effect of temperature on swelling of cotton by DMAc and water. (Samples were swollen for 24 h at designated temperatures.)

workers32 indicated that swelling of cellulose fiber pellets appeared to obey the classical Arrhenius equation. The “activation energy” for swelling by DMAc appeared to be much higher than that for water. Heating the DMAc sample to 80 °C resulted in a 180% increase of nSw from 0.18 to 0.51. Although a higher temperature did not lead to further increase of the final nSw value, it accelerated the rate of swelling. At 150 °C, which is close to the boiling point of DMAc, the time to reach the maximum nSw of 0.51 was shortened to 1 h. The apparent promotion of swelling by heat was also evidenced by the increment in filament diameters. As shown by the microscopic images in Figure 4, the

Figure 2. Correlation of percentage weight gain (%Sw) and molar solvent per AGU (nSw) with (A) Hansen solubility parameter distance (Ra) and (B) dielectric constants (ε) of the organic solvents investigated.

found that nSw had a moderate correlation to both Ra (R2 = 0.729) and ε (R2 = 0.754). On the other hand, %Sw was only poorly correlated to either Ra (R2 = 0.554) or ε (R2 = 0.457). %Sw had been extensively employed for correlating cellulose swelling with solvent properties. However, our results indicate that it is more proper to compare the efficiency of swelling in terms of nSw. The moderate R2 values implied swelling could not be adequately described by a single descriptor of the solvent. Judging by the nSw value, none of the organic solvents came close to water (nSw = 3.49) in terms of the ability to swell cellulose. DMF, ACN, DMAc, and Pyr were found to be able to moderately swell cellulose with nSw values of 0.34, 0.26, 0.18, and 0.18, respectively. These four solvents were considered as candidates to swell cotton fabrics. The results of a comprehensive environmental assessment31 on all of the solvents investigated (see Tables S1 and S2 and Figure S1 in the Supporting Information for details) show that DMAc (scored 3.17) has better EHS (environmental, health, and safety) properties than DMF (scored 3.69), ACN (scored 4.55), and pyridine (scored 5.65). Therefore, DMAc was chosen as the swelling solvent for being the most environmental friendly option. DMAc is also a regular solvent for the textile and fiber industry used in solvent spinning. 3.2. Effect of Temperature on Swelling of Cotton by DMAc. Since swelling of cotton fabrics with organic solvents was not sufficient at room temperature, the effect of temperature on swelling of cotton by DMAc was investigated. The results were plotted in Figure 3, which also showed the temperature effect when water was used as the swelling solvent for comparison. The nSw of DMAc-swollen fabric samples showed a significant dependence on temperature. In contrast, the nSw of water-swollen samples remained constant regardless of temperature changes. Although swelling of cellulose does not involve chemical transition, a study done by Young and co-

Figure 4. Microscopic images of cotton filaments swollen at 150 °C (A) and 25 °C (B) for 24 h.

average diameter of a filament swollen in hot DMAc was 17.4 μm, a 63% improvement compared with a control sample swollen in room temperature DMAc (10.4 μm). As shown in Figure 5, the dye sorption rates also unambiguously distinguished the heat-swollen fabric from the room-temperature-swollen control. The uptake of RR24 by the heat-swollen sample leveled off in less than 2 h, while the control took over 20 h to reach the sorption equilibrium.33 The final percentage sorption of both samples showed no significant difference. The fact that sorption equilibrium is hardly affected by the preswelling treatment is understandable considering it is determined by the difference in chemical potentials of RR24 in solution and on fiber. The dyeing solvent used in the above experiment was a mixture of DMAc and DMC. DMC was used as a sorption-promoting 2442

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cold-water rinses. In solvent dyeing, the rinsing solvent also has to be non-nucleophilic to guarantee full recyclability. The dyeing solvent mixture (60/40, DMC/DMAc) and pure DMAc were investigated as rinsing solvent for the solvent-dyed cotton samples. The rinsing efficiency is expressed as the percentage removal of the unfixed RR24 after a certain number of rinses and presented in Figure 6. As shown in Figure 6, the

Figure 5. Effect of preswelling temperature on sorption of RR24. Preswelling condition: 100% DMAc, 1 h; dyeing condition: 3% owf, DMAc/DMC = 40/60, 95 °C.

cosolvent for being a poor solvent for the dye. The percentage uptake of RR24 increased with the DMC content in the mixture. Our preliminary study showed that the optimum ratio for DMC to DMAc was 60 to 40. The maximum dye uptake was as high as 96.9%. Further increasing the DMC content resulted in agglomeration of RR24. DMC was selected from the 9 non-nucleophilic solvents mentioned above for being the most environmentally friendly (EHS scored 2.70).34,35 DMC is also inexpensive and has relatively lower vapor pressure than other poor solvents for the dye. Albeit being an inefficient swelling solvent for cellulose (nSw = 0.10), DMC did not induce collapse of the preswollen fabrics even when used at high content. According to the literature, solvent molecules trapped in the gross structure of cotton are not easily replaced.36 Therefore, the expanded inner surface of swollen fabrics remained unchanged upon switching of solvents. The above results showed that preswelling of cotton fabrics by hot DMAc sufficiently opened up the micropores in cellulose to allow rapid dye sorption. Pretreatment of the cotton fabrics by DMAc for 1 h at 150 °C led to a 180% increase in nSw and a 63% increase in fiber diameter, respectively. The time needed to reach dye sorption equilibrium was shortened from 20 to 2 h. The preswelling treatment in 150 °C DMAc for 1 h was applied to all samples subjected to solvent dyeing for the rest of the study. 3.3. Efficient Removal of Unfixed Dyes by Organic Solvents. Similar to aqueous reactive dyeing, the nucleophilic substitution of MCT dyes by cellulose hydroxyls, a.k.a. dye fixation, was also facilitated by adding inorganic carbonate bases such as K2CO3 in the organic dyeing media. Using K2CO3 simplified dyebath recycling because the KCl formed from this reaction could be easily removed by filtration. For RR24, a fixation rate of 52.3% was achieved at the owf of 3% with optimized solvent dyeing conditions. The K/S of the solventdyed fabrics was as high as 19.5. The dye fixation rate and color yield in solvent dyeing were better compared with aqueous dyeing (fixation % = 44.3%, K/S = 15.1). This is a significant achievement in nonaqueous reactive dyeing of cotton. The reported K/S values typically range from 0.1 to 5 for nonaqueous dyeing with commercial reactive dyes at owf up to 10%.13,18,37 Nevertheless, considering the high sorption rate of 96.9%, the relative fixation rate was roughly only 54%. RR24 sorbed but unreacted needs to be thoroughly removed from the fabric to guarantee excellent colorfastness. In aqueous dyeing, unfixed dyes are removed with multiple soaping and hot- and

Figure 6. Effect of number of rinses on unfixed dye removal.

dyeing solvent mixture was not efficient in removing unfixed dyes. Five rinses with the dyeing solvent mixture at 95 °C removed 27.3% of the unfixed RR24. In contrast, rinsing with pure DMAc at 95 °C was able to remove over 99% of the unfixed dyes after 4 cycles. Temperature also played a crucial part in promoting dye desorption from cellulose. Room temperature rinsing with pure DMAc was found to be far less effective. A removal rate of only 58.3% was observed after 5 rinses. The rinsing efficiency depends on the solvent’s ability to solubilize the dye and to swell cotton. To compare the relevance of these two factors, the solubility of RR24 in the rinsing solvents (SolRR24) and the Sw% of the solvents were measured and correlated to the percentage of RR24 removed in the first rinse (RM1‑RR24). Hot and cold D.I. water was also included as rinsing solvents in the analysis. The data are presented in Table 2. The linear regression results (Table S3 in Table 2. Efficiency of Various Rinsing Conditions in Terms of Dye Removed in the First Rinsea rinsing conditions

Sw%

SolRR24 (g/L)b

RM1‑RR24 (%)c

DMAc 95 °C DMAc 25 °C DMAc/DMC 95 °C water 25 °C water 95 °C

27.5 9.6 11.0 36.5 36.5

61.97 50.19 7.75 79.88 86.83

69.09 24.6 6.4 82.9 85.7

Rinsing was carried out with pure DMAc at 95 and 25 °C, with DMAc/DMC 40/60 at 95 °C, and with pure water at 95 and 25 °C for 0.5 h. The liquor ratio was 20:1. bThe solubility of RR24 in the rinsing solvents at rinsing temperature. cThe percentage of RR24 removed in the first rinse. a

the Supporting Information) indicated a strong positive correlation of RM1‑RR24 and Sw%. The adjusted R2 was 0.9236, meaning 92.4% of the variation in RM1‑RR24 could be explained by the change in the degree of swelling. The correlation between RM1‑RR24 and SolRR24 was found to be moderate (R2 = 0.8390). A multiple linear regression was also 2443

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Industrial & Engineering Chemistry Research Table 3. K/S and Colorfastness after Solvent Rinsinga fastness to laundering stainingb rinsing conditions

number of rinses

K/S

color change

C

N

P

A1

W

A2

3 4 5 5 5

20.61 19.50 19.15 22.45 24.91 19.30

4−5 5 5 4−5 4−5 5

4−5 5 5 5 4−5 5

5 5 5 5 5 5

5 5 5 5 5 5

5 5 5 5 5 5

5 5 5 4−5 4 5

5 5 5 5 5 5

DMAc 95 °C

DMAc 25 °C DMAc/DMC 95 °C soapingc

a Rinsing was carried out with pure DMAc at 95 and 25 °C and with DMAc/DMC 40/60 at 95 °C for 0.5 h. The liquor ratio was 20:1. bStain fabrics: C, cotton; N, nylon; P, polyester; A1, acrylic; w, wool; A2, acetate. cAqueous soaping was performed after solvent dyeing as described in Section 2.5.

Table 4. K/S and Colorfastness of Fabrics Dyed Repeatedly with Recovered Dyes and Solventsa fastness to laundering stainingc number of dyeing cycles

K/S

1 2 3 4 5 10 aqueous dyeingd

19.3 19.5 19.6 19.4 19.5 19.3 15.1

CMC ΔE 0.66 0.72 0.51 0.55 0.36

b

fastness to crocking

color change

C

N

P

A1

W

A2

dry

wet

5 5 5 5 5 5 5

5 5 5 5 5 5 5

5 5 5 5 5 5 5

5 5 5 5 5 5 5

5 5 5 5 5 5 5

5 5 5 5 5 5 5

5 5 5 5 5 5 5

5 5 5 5 5 5 5

4−5 4−5 4−5 4−5 4−5 4−5 4−5

a Dyeing was carried out with 3% owf of the dye at 95 °C for 2.5 h. The liquor ratio was 20:1. Fabric was presoaked in pure DMAc. bColor differences were measured against the sample dyed in the first cycle. cStain fabrics: C, cotton; N, nylon; P, polyester; A1, acrylic; W, wool; A2, acetate. d Aqueous dyeing carried out with 3% owf of the dye at 90 °C according to recommended procedures.

Therefore, recycling is easier with MCT dyes due to the simpler composition of the spent baths. An exhausted dyebath from aqueous dyeing with RR24 was also subjected to HPLC analysis. The result indicated that all dyes remaining in the spent bath had been hydrolyzed. On the basis of the HPLC results, it is justified to conclude that switching to a nonnucleophilic organic dyeing medium effectively preserves the dyes from hydrolysis and renders the process recyclable. A fully recyclable reactive dyeing procedure was carried out as described in Section 2.7. The dyeing process was repeated for 10 times, and the results are summarized in Table 4. The fabrics dyed with the replenished dyebath and rinsed with the recovered solvents repeatedly exhibited good shade consistence. The color differences measured were all below 1.0. The color depth and colorfastness were also consistently high. The results clearly demonstrated the practicability of a fully recyclable reactive dyeing process. Material consumption and disposal to dye 1 ton of cotton fabrics with RR24 were compared for a 10-cycle solvent dyeing sequence and a typical aqueous batch dyeing procedure (10 batches) (Table S4, Supporting Information). It was assumed that salts and unfixed RR24 were disposed as solid wastes after the final dyeing cycle, while all solvents were distilled and recovered. The detailed calculations and the results can be found in the supplementary data, Supporting Information. Compared with aqueous dyeing, dye consumption and disposal are estimated to be 43% and 90% lower, respectively, in solvent dyeing. Moreover, the amount of salts and bases involved in solvent dyeing is only a mere fraction of that in aqueous dyeing (