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Inside Outdoor Winter Issue 2015
2015 Outdoor Industry Directory
Deadline November 27, 2014
Place your brand in front of more than 20,000 retailers, for FREE
INSIDE OUTDOOR Magazine’s 2015 OUTDOOR INDUSTRY DIRECTORY is a complete and updated listing of outdoor product, services and component suppliers serving the outdoor specialty retail market. The annual directory is posted online and mailed each December to more than 24,000 retail buyers and suppliers of outdoor products and services.
Supercritical CO2 Colors Textile Production Greener
By Ernest Shiwanov
Color is something most people tend not to think about very often. For the most part, it is a silent attribute surrounding us with blue skies, bright white snowflakes, multi-colored skis and the golden color of an IPA. Yet the business of color is huge – as in coatings, paints, dyes, stains, pigments and ink.
In the outdoor retail world, color plays an important role in all products, especially apparel. Many apparel designers use color services on whose prediction of the next color trend hang the sell-through of their new lines. Other color selections are based on practicality or tradition, such as very bright colors for safety through enhanced visibility. Psychologically, color communicates part of the product’s identification and therefore marketability.
Regrettably, there is a part of the color process that until recently has not received the kind scrutiny it deserved. It is the significantly negative impact the textile dyeing process is having on the environment, the workers and the bottom line. Fortunately, new technologies are starting to leave their mark on this resource-intensive process. One, referred to as supercritical fluid dyeing using carbon dioxide (CO2), has manufacturers lining up to adopt this process. In order to know how it works and how it contrasts to more traditional dyeing, it is important to first gain a basic understanding of the nature of dyes and the process thereof.
The intent of dyeing is to impart a color to a substrate. Dyes have been designed to achieve that in different ways. By solution, color can pass by adherence to compatible surfaces. They also can be mechanically retained. Sometimes dyes are adsorbed (not to be confused with absorbed), physically attaching to the surface. They also can use salts and metals as agents with which they can chemically bond.
Because the end goal of most dyeing is to retain color integrity over time, dye chemistry has driven itself into solutions often coming with environmental consequences. On a small scale, this would not be such a problem. However, on an industrial scale, there are dire costs, most of which are patently or indirectly apparent:large scale dyeing requires copious amounts of water, dye chemicals and power.“In addition, the increased demand for textile products and the proportional increase in their production, and the use of synthetic dyes have together contributed to dye wastewater becoming one of the substantial sources of severe pollution problems in current times,” according to the 2013 report Textile Dyes: Dyeing Process and Environmental Impact from a group of Brazilian scientists led by Farah Chequer, Ph.D.
Dyeing starts at the fiber, yarn, fabric or finished product level. It is most commonly done in batches, as explained here, or in long continuous lengths. Batch dyeing begins by placing the items to be processed in large metal kiers (vats) where, in various steps, they are washed, dyed and rinsed. They are first washed or scoured to remove oils, additives and other accumulated impurities from manufacture. This eliminates any contamination known to hinder dye uptake and color uniformity. Next, the type of material (e.g. wool, polyester, cotton, nylon, etc) to be dyed is matched to the appropriate dye class. Polyester, the most widely dyed textile category, uses disperse dyes (pigment with chemical enhancers). To help speed up the polyester dyeing process, heat (>100C/212F) and pressure is applied to the tank’s aqueous solution of disperse dye. A typical batch processing time is three to four hours, which includes draining and refilling the vessel multiple times for each step, drying and possibly heat-setting the color on the finished product.
Dyeing polyester in this way consumes a lot of resources, is not efficient and pollutes the environment. One-tenth to 50 percent of dye and its constituents not used during the process are washed away as effluent. Microbiologists at Aristotle University of Thessaloniki, Greece (CJ, Ogugbue, et al.), estimated that 200 million tons of dyes are lost annually in such a way. Compounding the problem, wastewater treatment plants have demonstrated ineffectiveness in removing colorant and/or toxicity from the waste stream.
Because of the nature of dyes, they “escape conventional wastewater treatment processes and persist in the environment as a result of their high stability to light, temperature, water, detergents, chemicals, soap and other parameters such as bleach and perspiration,” wrote Chequer and her team in the 2013 study. “In addition, anti-microbial agents resistant to biological degradation are frequently used in the manufacture of textiles, particularly for natural fibers such as cotton.”
The use of water in dyeing is another issue of serious concern. In batch dyeing, every time the kiers are drained and refilled, the contents use five to 10 times their weight in water, according to research published in the Journal of Cleaner Production.
“The oft quoted use of water in cotton production is worth repeating: it has been calculated that approximately 200 liters of water are needed for each kilogram of cotton produced. These effluents are complex mixtures of many pollutants, ranging from original colors lost during the dyeing process, to associated pesticides and heavy metals, and when not properly treated, can cause serious contamination of the water sources,” says the Chequer study.
The impact of water use goes beyond the factory gates, extending into the neighboring communities. As water demand increases and the supply decreases, the price of treated water goes up affecting everyone; particularly those who can least afford it. Some of the communities, such as those mentioned in the above sidebar, are in more arid regions or in drought-affected areas. This adds stress to the already extended water supply.
The challenge is to balance the dwindling water supplies with all levels of industry, domestic needs, agricultural demands and an overtaxed wastewater treatment infrastructure. The dye industry knows the obvious way to help is to reduce its water consumption. Well, it appears a promising new technology is poised to change how some materials are dyed, all without a single drop of water.
Called supercritical fluid dyeing with carbon dioxide (CO2), the initial idea to dye with this method was outlined in a patent application some 24 years ago. A Swiss team headed by Wolfgang Schlenker developed a way to dye hydrophobic materials without leaving much, if any, in the way of dye-waste formation. Today, as it was then, cleaning and recycling the remaining dye bath effluent is problematic and costly. No doubt accelerated by the pace of global warming, the interest in this technology re-emerged. To understand how it works, it helps to recall a simplified version of the phases of matter.
For example, by adjusting the temperature at sea level’s atmospheric pressure, you can transform carbon dioxide to a solid (dry ice), liquid, gas (CO2 fizz in a carbonated drink) or plasma. To make CO2 supercritical, you need to tweak its temperature up to a warm summer’s day or 31C (88F). Then pressurize it to 7.3 MPa (1,059 lb/in2), roughly two-thirds less pressure than a standard 80 cubic foot SCUBA tank. The result is a phase in which the CO2 behaves like a gas and a liquid.
Therein lies the magic-taking advantage of the gas-like properties of the supercritical fluid CO2. Suffusion through the fibers is way easier, faster and more complete than an aqueous dye solution could ever be. Since the polyester fibers swell in this state, the dye mixed within the supercritical CO2 is more profoundly diffused into the fibers. When the dyeing is done and the pressurized supercritical CO2 removed, the dye molecules are trapped within the shrinking polyester fiber, according to the report Textile Dyeing in Supercritical Carbon Dioxide out of Delft University of Technology, in The Netherlands (Van der Kraan, M. et al).
Super critical CO2 pressure vessel for rolled textiles. Hasaka Works, LTD, Osaka, Japan
Partially surveying the advantages of supercritical fluid CO2 makes for a strong argument for its adoption. For one, it does not use water. So the scouring process in traditional dyeing is eliminated. Without washing, there is no drying or wastewater treatment, reducing power consumption and the inescapable pollution. Pigments, not disperse dyes, are used since the additional surfactants and chemical dispersing agents are not necessary. In fact, once the dyeing process is concluded, the remaining unused pigment can be recycled. Most all of the CO2 is recycled, ready to be changed back to its supercritical self. There is no post-dye drying cycle or heat-setting stage, so no need for additional power consumption there, as well.
The dye processing time and energy is reduced from 30 percent to 50 percent, according to research from Yeh Group. Dye quality itself yields a more consistent color and the dyed product’s physical characteristics match those of legacy dyeing methods. On top of that, carbon dioxide itself is basically inert, cheap and overly abundant. Another plus is its use does not require or yield volatile organic compounds (VOCs).
|Comparative energy requirements* (kJ)|
|Process||Conventional||Supercritical Fluid CO2|
|Total energy used||53,605||35,180|
|*Actual results will vary by country and by dyeing equipment.|
Source: Yeh Group’s DryDye comparison to conventional dyeing methods
To dye for?
Indeed, supercritical CO2 dyeing looks pretty good compared to the current industrial methods. Yet it is always important to take a critical look before leaping into any new technology, and commissioning LCAs (life cycle assessments) are one way of examining the impact of any new process for hidden or potential disrupters.
The Dutch company DyeCoo Textile Systems, BV did just that, having a LCIA (life cycle impact assessment) on its supercritical CO2 technology. LCIAs are similar to LCAs but look at specific emissions, expressing them in terms of their potential impact. In this study, Eutrophication, ozone depletion, global warming, acidification and photochemical ozone creation potential were quantified. Primary energy demand, human and eco-toxicity and process water used also were scrutinized. The study compared DyeCoo’s supercritical CO2 process to jet dyeing rather than kier dyeing, per this article. Although there is more energy used in jet dyeing, the results would remain the same: supercritical fluid dyeing with CO2 is a categorically superior method for dyeing polyester. For other types of materials such as cotton that do not swell in supercritical CO2, adding an electrical field (electrophoresis) to the dye solution looks to solve that problem.
Dyeing has the unenviable distinction of being the locus of waste production in textiles. Excessive water usage in areas with stressed water supplies, overburdened wastewater treatment systems and persistent, hard-to-remove effluents from the waste stream are hallmarks of this industry’s problem. With the event of supercritical fluid dyeing with CO2, the tide could very well be turning many shades greener.
China’s Dye – In the center of this industry is China
As one can imagine, China, the world’s leading exporter of textiles, is also the world’s dyeing center. Since 1995, China has been increasing its market share of the textile production business. In 2010, Shaoxing County National Economy and Social Development Journal reports of producing more than 17 billion meters (18,591,426,072 of yards) dyed fabric and more than 130 million items of clothing in 2010.
In comparative terms, the dyed textile’s length is equivalent in distance to driving around the earth 424.5 times. And that is just Shaoxing County, which is the biggest textile center in China, representing one third of China’s textile manufacturing. In Zhejiang Provence, Shaoxing County boasts having 30 percent or more of the dyeing business and has upward of 9,000 textile mills (Source: Toxic Threads: Putting Pollution on Parade. How textile manufacturers are hiding their toxic trail. Greenpeace International Nov 2012).
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