By Brian Tsui, Website Coordinator for the GCI
The rise of fast-fashion trends in our global economy has led to a booming textiles manufacturing industry. According to the Ellen MacArthur Foundation, globally the industry consumes over 98 million tonnes of non-renewable resources per year.1 The majority of these items have seen less utilization compared with early 2000s. With an increasing consumer awareness of eco-friendly and sustainable practices in manufacturing, pressures at the consumer level have led to changes to these labour-intensive processes. Of note is the dyeing process, which historically has been a wet process. The dyeing, printing, and washing steps consume large amounts of energy and generate large volumes of wastewater, which contain the excess dyes, surfactants, and salts. In 2014 alone, coloration of textiles generated over 1.5 billion tons of wastewater.1 Much of this wastewater is neither environmentally benign nor biodegradable and requires further energy investment during the remediation step. Smaller eco-conscious manufacturers such as Ikeuchi Organic, a company located in Japan, have been tackling these issues by utilizing 100% wind power for their operations.2 The smaller scale allows for easier implementation of wastewater treatment and a final product which passes Class 1 certification as set by OEKO-TEX STANDARD 100, a global leader in textiles regulations.3 The underutilization of sustainable textiles dyeing processes on a large scale, however, continues to be a problem. The search for highly competitive, as well as energy and cost-efficient processes along with bridging the gap between laboratory and industrial scales to deliver the benefits of eco-friendly manufacturing is an ongoing goal in the textile research community.
Figure 1. Growth of clothing sales and decline in clothing utilization since 2000.1
Traditionally, the dyeing process involves an interaction between a dye molecule and a fibre (adsorption). This is accompanied by the movement of the dye to the interior of the fibre (diffusion). Consider cotton, which comprises an estimated 21% of global fibre usage.4 Cellulose is the primary component of cotton fibers, totalling up to 95% by mass. The most common method for dyeing cotton involves reactive dyes. As their name suggests, a reactive dye is a high energy molecule which can react with natural cellulose and form a covalent bond. The reactive molecule is low cost, easily tuned to a variety of vibrant colors, and simple to apply.
Figure 2. Types of reactions involving reactive dye and cellulose. Top is substitution, bottom is addition.
At each step of the process, there are several factors that determine the quality of the final product.5 Temperature has a pronounced effect on increasing dye penetration and more rapid diffusion, but at the cost of increased dye hydrolysis. Tight cotton fibres require higher temperature for effective permeation of the dye into the fibre, while loose cotton fibres require lower temperatures for the same effect. The alkalinity of the solution also plays an important role, as the concentration of the cellulose anion is proportional to the pH of the solution. During the dyeing process, some alkali is consumed, and therefore the pH of the final dye solution is lower than the initial pH. Electrolytes are also important to the dyeing solution, acting to overcome the electrostatic repulsions between anionic cellulose and the anionic dye linkers. Finally, the concentration of these additives also impacts the dyeing process. There is a fine line between rate of dyeing and negative impacts on production costs and the environment. These are just some examples of challenges textiles research scientists face when developing a new dye process.
Kim et al highlighted the use of nanofibrillated cellulose (NFC) as a supplement to normal cotton cellulose.6 The large surface area of NFC resulted in an abundance of surface cellulose hydroxyls. Consequently, the salt, alkali, and water amounts could be reduced by an order of magnitude, generating significantly less wastewater. Most importantly, the dyeing process was not significantly impacted by these changes and the resulting fabric is largely indistinguishable from traditional dyeing methods. A life-cycle assessment based on the experimental data suggested that the NFC-based process would result in a significant reduction on wastewater load.
Figure 3. Comparison of various dyeing methods. a) conventional dyeing method; b) conventional dyeing method with decreased salt and alkali concentration; c) NFC-based dyeing with decreased salt and alkali concentration.
Xia et al demonstrated the use of a co-solvent, ethanol, effective for eliminating the need for electrolytes.7 The ethanol was found to impact the dyeing process in four main ways: improving the solubility of the reactive dyes, decreasing the dielectric constant and electrostatic repulsion of the solution, increased aggregation of dyes leading to enhanced dyeing kinetics, and decreasing the surface tension improving dyeing efficiency. These efforts, too, improved the clarity of the resulting waste solution compared to commercial wastewater without loss of fabric colour.
Figure 4. Comparison of fabric wastewater with ethanol-water salt-free method and traditional electrolyte-added dyeing method.
Ding et al developed a method for dyeing without the use of reactive organic dyes, instead opting for base metal nanoparticles.8 Carbon black nanoparticles are first functionalized onto cotton fibres via radiation grafting. Subsequent immobilization of iron oxide red, cobalt green, or cobalt blue for red, green, and blue colors respectively, give rise to colorized cotton fabric. Various approximate colors can be created by varying the concentration of each nanoparticle, and the colour is retained even after 20 wash cycles. Most importantly, the lack of reactive dyes results in pronounced clarity of the resulting dye solution compared to real wastewater.
Figure 5. Images of wastewater samples. W0 is real wastewater from a dyeing factory. WCB1–WCB5, WCoG1–WCoG5, WCoG1–WCoG5, WFeR1–WFeR5 are samples from various concentrations of nanoparticle dyes.
Textiles and clothing play an important part of everyday life. The current clothing manufacture system is wasteful and polluting to the environment. With material and process scientists at the forefront, a new textiles economy built from sustainable methodologies will be vital as we continue to deplete our non-renewable resources.
- Ellen MacArthur Foundation, A new textiles economy: Redesigning fashion’s future, (2017, http://www.ellenmacarthurfoundation.org/publications).
- Ikeuchi Organic – Our Philosophy. https://www.ikeuchi.org/about-us/en/concept/ (accessed Mar 15, 2020)
- STANDARD 100 by OEKO-TEX. https://www.oeko-tex.com/en/our-standards/standard-100-by-oeko-tex (accessed Mar 15, 2020)
- Yang, Q. M. Global Fibres Overview, Synthetic Fibres Raw Materials Committee Meeting at APIC, Pattaya, 16 May 2014.
- Shang, S. M. Process control in dyeing of textiles. Process Control in Textile Manufacturing, 300–338. (2013) doi:10.1533/9780857095633.3.300
- Kim, Y.; McCoy, L. T.; Lee, E.; Lee, H.; Saremi, R.; Feit, C.; Hardin, I. R.; Sharma, S.; Mani, S.; Minko, S. Green Chem. 2017, 19, 4031.
- Xia, L.; Wang, A.; Zhang, C.; Liu, Y.; Guo, H.; Ding, C.; Wang, Y.; Xu, W. Green Chem. 2018, 20, 4473.
- Ding, X.; Yu, M.; Wang, Z.; Zhang, B.; Li, L.; Li, J. Green Chem. 2019, 21, 6611