Letter to the Editor pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Comment on “Sustainable Cotton Dyeing in Nonaqueous Medium Applying Protic Ionic Liquids” Jürgen Andreaus* and Laís Feltrin Sidou Departamento de Química, Universidade Regional de Blumenau, 89030-903 Blumenau, SC, Brazil
ACS Sustainable Chem. Eng. 2017, 5 (10), 8756−8765. DOI: 10.1021/acssuschemeng.7b01555 ACS Sustainable Chem. Eng. 2019, 7 (9). DOI: 10.1021/acssuschemeng.9b01809 °C (cold washing) and 57 °C (hot washing) were lower than 0.07, which means that fabrics remained practically uncolored, and differences between the different dyeings using water or PILs are insignificant. It is also surprising that the authors do not present standard deviations for color measurements. The reference value used by the authors is that from a dyeing with only water without the addition of alkali, which is expected to lead to almost zero dye fixation. Color yield or strength obtained with a specific dye through dyeing is related to the dye concentration in the bath or dye quantity used per fabric, usually expressed as % owf (on weight fabric; g dye/100 g fabric). The correlation between the quantity of applied dye and the resulting color strength is commonly used in textile industries for calibration and development of different colors. According to Broadbent,8 reactive dyes with maximum absorptions at 580 and 620 nm, applied at concentrations between 0.1% and 0.4% owf yielded K S−1 values between 1 to above 4 and 2 to above 8, respectively. Since Andrade et. al used an initial dye concentration of 2% owf, much higher K S−1 values than those reported by Broadbent should be expected. As the K S−1 values obtained for all the tested PILs were not greater than 0.07, it can be concluded that very little dye was fixed to the cotton fabrics, and probably (the K S−1 values before washing are not shown in the original paper) most of the dye, if initially on the fabric, was washed off during the washing processes. Although some ionic liquids tested granted a higher K S−1 value than the control process (using water) like 2-HEAPr and 2-HEAAd, the obtained K S−1 value is still too low for a satisfactory and viable dyeing process. In addition, the authors did not show any photos of the dyed fabrics. According to the available literature about the CIE L*a*b* color space, a* and b* coordinate values between −10 and 10 can be read as gray colored and/or without a distinguishable color.8,9 In the work of Andrade et al.,7 all the a* and b* values fall under this spectrum, which indicate a great loss of color after the washing process. Broadbent8 indicates that a CIE L*a*b* color difference greater than 5.8 points between before and after washing corresponds to a fastness grade of 2 or lower in standard color fastness tests, which indicates poor fixation of the dye to the fabric. Another debatable point is the fact that the authors did not compare their results to a standard dyed sample using alkali. As thoroughly explored by many authors,8,10−12 the fixation of
ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by 5.62.157.190 on 04/28/19. For personal use only.
O
ne way to approach sustainability in industrial processes such as textile dyeing can be the selection of adequate alternative green solvents. Andrade et al.7 reported the use of protic ionic liquids as promising nonaqueous solvents for the dyeing of cotton textiles. A critical analysis of the results presented in their article was made and leads to the conclusion that the proposed alternative dyeing procedure did not lead to satisfactory dyeing results and is not viable. The textile industry relies on several relatively harsh wet processes that have a negative impact on the environment not only because of the amount of consumed water but also because of the generated wastewater contaminated with chemicals. Ionic liquids (ILs), organic salts with interesting properties such as negligible vapor pressure, high solvation power, and thermostability, have gained increasing interest as industrial solvents and been investigated for different textile applications,1−3 but little has been reported on their use in textile dyeing.4−6 In the article “Sustainable Cotton Dyeing in Nonaqueous Medium Applying Protic Ionic Liquids”, Andrade et al.7 describe the application of 13 different protic ionic liquids (PILs) as solvents in cotton reactive dyeing, evaluating color yield (K S−1), change in L*a*b* color coordinates, tensile strength, and surface morphology of the dyed fabrics. The authors claim that their alternative exhaustion dyeing process uses only dye and solvent, but no additional chemicals such as alkali or salt, and allows the recycling of the dyeing bath without appreciable loss of efficiency since the unfixed dye remains reactive and can be reused in another dyeing process. Detailed analyses of the presented spectrophotometric data and the claims reveal some critical issues: (i) K S−1 values are very low, which means that attained color intensities are too low for acceptable dyeings. (ii) Values for a* and b* are too low and not distinguishable from gray. (iii) Dyeing results with PILs were not compared to standard dyeing procedures with water as solvent and alkali as auxiliary. (iv) No results on the reuse of the spent dyeing baths were presented. In this Letter to the Editor, we intend to discuss these issues using available data from the literature in order to contribute to the original work and open a dialogue with the research community. Color intensities (K S−1 values) obtained by Andrade et al.7 after dyeing at 60 °C for 60 min and postwashing with an aqueous standard detergent solution (1 g L−1) for 30 min at 25 © XXXX American Chemical Society
Received: February 26, 2019
A
DOI: 10.1021/acssuschemeng.9b01112 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
ACS Sustainable Chemistry & Engineering
■
reactive dyes on cellulosic fibers is only possible through nucleophilic substitution and addition reactions, in which the alkali causes acidic dissociation of the hydroxyl groups on the surface of cellulose molecules, forming cellulosate ions that react with the dye. The dye used by Andrade et al.,7 Marinho Sidercron, has a very similar structure to the widely used C.I. Reactive Black 5, with two reactive groups of the sulfatoethylsulfone type. Dye fixation occurs through a nucleophilic addition reaction, but first, the sulfatoethylsulfone groups need to be activated by alkali, generating a vinylsulfone group, consuming one mole of hydroxyl ions and releasing one mole of sulfate ions per mole of reactive group. It appears therefore to be quite unlikely that one part of the dye molecules in the dyeing bath are activated to form the vinyl intermediate and react with the cotton cellulose, while the other part remains in the sulfatoethylsulfone form, so that it can be reused. Conventional reactive cotton dyeing leads to exhausted dyeing baths containing unfixed hydrolyzed dye molecules, which cannot be reused in another reactive dyeing procedure. Despite their claim for reuse of the exhausted dyeing baths, the authors did not present any related results. The differences in the K S−1 values obtained after the two washing procedures (25 and 57 °C) have been also discussed by the authors. They attributed the lower K S−1 values of the hot washed water-dyed fabrics to the higher solubility of the dye in the hot detergent solution. On the other hand, they argued that some of the hot washed PIL-dyed fabrics (2HEAAd, 2-HDEASa, and 2-HDEAPr) revealed higher K S−1 values because of an additional fixation of the dye due to the effect of the PILs on the cellulose structure and an improved fixation. We consider this very unlikely since there are no reports in the literature of significant structure changes in cellulose due to PIL treatment at low temperatures (60 °C).13 Furthermore, differences are quite small, and without standard deviation, it is impossible to verify the statistical significance of these differences. It is also not clear from the presented data if the ionic liquids were completely removed from the cotton fabrics through the washing procedures. Residual PILs can have an influence on some of the evaluated fabric properties. From the above presented analyses and discussions of the data presented by Andrade et al.,7 we conclude that none of the 13 PILs investigated by Andrade et al. serves as a promising solvent for reactive cotton dyeing and that results of new alternative dyeing processes have to be compared to conventional dyeing results, otherwise erroneous conclusions may be drawn.
■
Letter to the Editor
REFERENCES
(1) De Silva, R. D.; Wang, X.; Byrne, N. Recycling Textiles: The Use of Ionic Liquids in the Separation of Cotton Polyester Blends. RSC Adv. 2014, 4 (55), 29094−29098. (2) Kantouch, A.; Khalil, E. M.; Mowafi, S.; El-Sayed, H. Antimicrobial Finishing of Wool Fabric Using Ionic Liquids. J. Text. Inst. 2013, 104 (4), 363−369. (3) Tavanaie, M. A. Ionic Liquids as New Solvents for Textile Fiber Formation and Modification. Chem. Eng. Technol. 2013, 36 (11), 1823−1837. (4) Knittel, D.; Schollmeyer, E. Ionic Liquids for Textile Finishing, Part 1: Dyeing of Textiles. Melliand Textilberichte Int. Text. Rep. 2007, 88 (1/2), E14. (5) Bianchini, R.; Cevasco, G.; Chiappe, C.; Pomelli, C. S.; Rodríguez Douton, M. J. Ionic Liquids Can Significantly Improve Textile Dyeing: An Innovative Application Assuring Economic and Environmental Benefits. ACS Sustainable Chem. Eng. 2015, 3 (9), 2303−2308. (6) Earle, M.; Seddon, K. Dyeing Process in Ionic Liquid Solvents. Patent WO/2009/024766, February 27, 2009. (7) Andrade, R. S.; Torres, D.; Ribeiro, F. R.; Chiari-Andréo, B. G.; Oshiro Junior, J. A.; Iglesias, M. Sustainable Cotton Dyeing in Nonaqueous Medium Applying Protic Ionic Liquids. ACS Sustainable Chem. Eng. 2017, 5 (10), 8756−8765. (8) Broadbent, A. D. Basic Principles of Textile Coloration; Society of Dyers and Colourists: Bradford, West Yorkshire, England, 2001. (9) Schanda, J.Colorimetry: Understanding the CIE System; Wiley: Hoboken, NJ, 2007. (10) The Theory of Coloration of Textiles, 2nd ed.; Johnson, A., Ed.; Society of Dyers and Colourists: Bradford, West Yorkshire, England, 1995. (11) Mahapatra, N. N. Textile Dyes and Dyeing; Woodhead Publishing India in Textiles; CRC Press, 2016. (12) Burkinshaw, S. M. Physico-Chemical Aspects of Textile Coloration; Wiley: Hoboken, NJ, 2016. (13) Pinkert, A.; Marsh, K. N.; Pang, S. Alkanolamine Ionic Liquids and Their Inability To Dissolve Crystalline Cellulose. Ind. Eng. Chem. Res. 2010, 49 (22), 11809−11813.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] ORCID
Jürgen Andreaus: 0000-0001-8124-6528
■
ACKNOWLEDGMENTS The authors are grateful to National Council for Scientific and Technological Development (CNPq) for financing Project 461555/2014-0 and wish to thank CNPq for Grant 313458/ 2017-0 of JA and CAPES (Coordination for the Improvement of Higher Education Personnel) for the scholarship of L.F.S. B
DOI: 10.1021/acssuschemeng.9b01112 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
