Sk. Mohammad Raafi,
Research Assistant, Textile Focus
All over the world, environmental considerations are now becoming vital factors during the selection of consumer products including textiles. Dyeing of cotton textiles requires massive amounts of water, energy, and chemicals. Yearly, more than 400,000 tons of reactive dyes are used to dye cellulosic fibers (mainly cotton). Reactive dyeing of 1 kg cotton could generate 200 kg of wastewater containing up to 50% of initial input of dyes in the dyebath and up to 100 g/L of salts which has severe carcinogenic effects on biological lives. Dyes and chemicals released into waterways also block sunlight and raise Biological Oxygen Demand (BOD) resulting in deadly impact on aquatic lives along with toxicity to vegetation. By using developed dyes having extra reactive groups, one-quarter to one-third water and energy can be saved but the effluent problem is alarming for all time. Ultrafiltration and sorption could remove nearly 100% of dyes from textile effluents but the cost for the wastewater treatments is always very high, accounting for half of the current cost of textile wet processing.
Non-aqueous Dyeing: The Appropriate Innovation in Sustainable Dyeing Technique
Research has been going on to improve the textile dyeing techniques in terms of process efficiency and environmental impacts. Use of water has been a major concern for dyeing technology. Cationization of cotton fabrics with polyacrylamide (chitosan) is a well-known concept which can improve the bond between dyes and cotton textiles; facilitating dyeing at neutral pH in the absence of electrolyte and alkali. But due to numerous disadvantages like light shades, ring dyeing and lack of lightfastness, it has been restricted commercially. Increasing environmental consciousness in textile processing has forced innovative efforts to search for the safe methods for textile chemical processing. Recently, the focus is shifted on non-aqueous dyeing of cotton textiles.
Non-aqueous dyeing media cannot dissolve water-soluble dye but when the dyes and chemical agents are dissolved firstly and fiber is swollen with a little amount of water, the system can transport material and transfer energy efficiently. Moreover, all the aqueous solution can be completely absorbed by the cotton textiles without using any accelerating salts during dyeing. Non-aqueous media can be washed easily from the cotton surface using surfactants and reused by flotation.
Recent research has been conducted with soybean oil/water dual-phase solvents system (SWDS) to achieve high dye fixation in addition to minimal discharge of waste effluents. Through an outstanding increase in chemical potential of reactive dyes in dyeing medium, SWDS remarkably resulted in 100%exhaustion and 92% fixation of dye. Besides, comparing to conventional aqueous dyeing, final discharges of dyes and salts from SWDS were decreased by 85% and 100%, respectively. Moreover, above 99.5% of initially added biodegradable soybean oil could be recycled easily because after 100% dye uptake, the dyeing medium is only left with soybean oil, fatty acid salts and glycerol from saponification.
Another study concerned with the influence of non-aqueous media on the adsorption of reactive dye, is performed with a clear, colorless, odorless and non-oily cyclic siloxane fluid. With the aid of a surfactant and co-surfactant, the dye solution can be evenly emulsified in siloxane non-aqueous medium. The pure incompatibility of reactive dyes with siloxane contributes to a strong affinity between the fiber and the dye, resulting in nearly 100% diffusion of dyes to the surface of the cotton textile under mechanical force.
In both SWDS and siloxane system, cotton fabric was swollen first by dipping in sodium carbonate (Na2CO3) aqueous solution and with proper liquor pick-ups, it was added to reactive dye –non-aqueous suspension bath. Vinyl sulphone reactive dye (blue 19) and a combination of cyanuric chloride and vinyl sulphone reactive dye (red 195) were selected for siloxane dyeing system and Reactive Red 120 was used in SWDS. For comparison, cotton fabrics were dyed also in a conventional aqueous system using salt and alkali.
The SWDS dyeing started at 50°C and after strong agitation for 1 hour, dyes in external phase (soybean oil) completely moved to internal phase (water) in the fabric. Then the bath was heated to 80°C for 30 minutes to diffuse and fix dyes in fabric. Then, soybean oil was easily removed from wet dyed cotton fabrics via centrifuge (vacuum slot), as it could only loosely attach to wet cotton fibers due to its low polarity and high surface tension (32 mN/m). Whereas in the siloxane dyeing system, the adsorption equilibrium time of reactive dye is only 5–10 min at 25°C, and then the temperature was increased to 70°C for 30 min, with a heating rate of 2°C/minute. At last, after completing one cycle of 4- step washing including cold wash, soaping with sodium bicarbonate and standard detergent at 95°C, hot wash at 95°C and cold wash the fabric was dried at ambient temperature.
The adsorption rate of reactive dye is governed by surface tension of non-aqueous media. The lower the surface tension, the faster the adsorption rates of reactive dye and the higher the ultimate uptake of dye. Thus, in addition to making large water savings, non-aqueous dyeing technology provides an innovative approach to increase dye uptake.
Since reactive dyes are insoluble in soybean oil, the difference in chemical potential of dyes between external and internal phases is very high. So, 100% of dyes could migrate into internal phase (water within the fabric). As a result, dyeing fixation is remarkably increased. Whereas in the aqueous system, highly water soluble dyes generate low chemical potentiality to dyes and even after adding a large quantity of salts into dyeing medium, dye concentration in internal phase is still found lower than that for SWDS system.
Accordingly, a wide number of dye aggregates are formed in the internal phase. Both for slower diffusion rate of large size aggregates than single molecular form particles and insufficiently exposed reactive groups in aggregates, instant reaction with cellulose was much slower in low temperature. But increase in temperature from 70°C to 90°C, would increase the solubility of dyes in the internal phase consequently increasing the fixation rates.
Varied number of reactive groups present in the molecular structure of dyes and, the molar extinction coefficients of dye might be a reasonable guide to the color depth. Cotton fabrics dyed in SWDS had colorfastness to light, laundering, and dry/wet crocking equal to those dyed in a conventional aqueous system which implicates its success to meet the actual use requirements.
Furthermore, comparing to siloxane dyeing, SWDS holds potentiality to be industrialized as implementation of SWDS in pilot-scale jet dyeing had no significant difference from that from lab-scale dyeing. Via the reuse of soybean oil for 50 times, SWDS could save up to $0.26 per kg of fabric compared to aqueous dyeings in terms of materials cost. SWDS could save 33–55% energy in heating the dyeing medium since, the specific heat capacity value of soybean oil is 0.45–0.67 caL/g/°C, considerably lower than that of water (1 caL/g/°C).
Innovation in Sustainable Dyestuffs
Inspired from biological sensor program, U.K. based Colorifix has come with an idea that microbes grown by proper fermentation can produce high-performance, renewable dyes suitable for mainstream textiles. Microorganisms engineered with advanced synthetic biology processes convert agricultural by-products, such as sugar molasses, into colorants suitable for textile dyeing. French firm- Pili is also using microorganisms to fabricate colorful dyes at the most environmentally friendly way. First the microbes go into a regular dye solution and get implanted in the textile fiber. Then added nutrients cause them to grow. Upon heating, the organisms’ membranes burst. That causes the color to chemically fix to the fiber with help from metal ions and salts in the microbes’ cytoplasm. Stable colors produced by microbes require only a single finish wash. According to Colorifix, a water savings of 90% and an energy savings of 20% can be achieved through standard processes.
Global apparel retailer Gap has announced a scheme called Dry Indigo to manufacture denim using a waterless, indigo foam-dyeing technique, starting with a partnership between Banana Republic and Spanish denim mill, TejidosRoyo. This innovative new process can eliminate water discharge tightly via a tremendous saving in water, energy and chemicals when compared to the traditional slasher indigo process.
Gap Inc.’s manufacturing goal to conserve 10 billion litres of water by the end of 2020 will certainly be accelerated by this innovative technique. The Dry Indigo technique utilizes a foam dye that adheres to denim yarn producing fabric with hand-feel, aesthetic, performance, and washability. This ground-breaking process required ten years of joint research with Gaston Foam Systems and Indigo Mills Designs before its launch. The foam-dyeing technique takes place in a space of less than 65 feet – compared to the hundreds of feet that is typically necessary for a traditional dyeing machine.
Apart from this revolutionary dyeing, sustainable pocketing and trims, such as 100% Global Recycle Standard (GRS) certified recycled poly zipper tape will be used to manufacture the denim garment. Also, the manufacturing will be done at Saitex – a sustainable industry recycling 98% of the water it uses. The Banana Republic Dry Indigo denim will be available through a special collection for both men and women in Spring 2020.
Italian textile chemical company Officina+39 has developed a revolutionary sustainable dyestuffs range- Recycrom, which is made from 100 % recycled used clothing, fibrous material and textile scraps. The scraps are re-processed via a sophisticated eight-step system (patent pending) in which all the fabric fibers are crystallized into an exceptionally fine powder that can be used as a pigment dye for fabrics and garments made of cotton, wool, nylon, or any natural fiber. Since the dye is applied as a suspension, so it can be easily filtered from water; thus reducing the environmental impact. Recycrom is compatible with exhaust dyeing, dipping, spraying, screen printing, and coating.
It is high time, every textile dyeing industry undertook sustainable transformation policy and innovative technologies both for surviving the market by successfully meeting consumer satisfaction and achieving competitive advantages.Figure 4: Officina +39’s Concept of Transforming Textile Fiber into Colored Powder.
- Mu, L. Liu, W. Li, and Y. Yang, “High sorption of reactive dyes onto cotton controlled by chemical potential gradient for reduction of dyeing e ffl uents,” J. Environ. Manage., vol. 239, no. February, pp. 271–278, 2019.
- Pei, J. Wang, Y. Gao, L. Zhu, X. Gu, H. Dou, “Dyeing Property and Adsorption Kinetics of Reactive Dyes for Cotton Textiles in Salt-Free Non-Aqueous Dyeing Systems.”
- Sarker, S. A. Rahman, A.B.M. Foisal, “Treatment of Cotton Fabric with Cationic Polyacrylamide – An Initiative to Salt Free Reactive Dyeing,” no. December 2015, 2016.