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Sustainable Dyeing Solution

Published: 12 May 2010 19:06:43 PST

Throughout the supply chain, there is interest in conserving resources and reducing the textile industry's environmental footprint. Every stage of a textile Product's life cycle has environmental impacts !a fro fiber production through manufacturing and retailing to laundering and disposal by consumers. Of particular concern is the use of water, energy, and chemicals (WEC) in textile processing !a an area where technological advances offer significant savings in resources and environmental benefits. In cotton textile processing, and therefore offer the greatest scope for reductions.

Interest in sustainable technologies

Research by Cotton Incorporated indicates that while consumers have become more environmentally aware, their understanding of textile manufacturing and its effects on the environment is limited, as is their willingness to pay more for environmentally friendly textile products.

According to Cotton Incorporated's 2010 Environment Survey, only 36% of consumers said they would be willing to pay extra for environmentally friendly clothing. Despite limits on consumer awareness and motivation, it is in the textile industry's interest to adopt practices to reduce the use of water, energy, and chemicals, especially in dyeing and inishing. Cotton Incorporated recently surveyed representatives of global mills, brands, and retailers about their motivations for adopting sustainable cotton technologies and practices.

About two-thirds of these companies are currently implementing sustainable practices or technologies (61%) and/or working with supply chain partners that are implementing them (66%). When asked to identify the main reason for their interest in sustainability, the most common response was concern for the environment !a 42% said that it was better for the environment.

Another 14% cited supply chain customer demand, and 11% said it would provide a competitive advantage; 16% said they were still exploring the potential beneits.During the winter of 2008¨C09, Cotton Incorporated conducted in-depth interviews with more than 40 global cotton textile processing companies that account for over 75% of global textile processing. These companies manufacture a wide range of woven, knit, denim, and yarn products and have implemented changes in their processes, dyes and chemicals, equipment, and control systems that significantly reduce requirements for water, energy, and chemicals. In this issue, we highlight two proven commercial technologies that survey respondents identified as having a potentially high impact on WEC reduction: one is the high-ixation reactive dyeing with reduced salt, and the other is the low-liquor-ratio jet dyeing machines.

High-fixation reactive dyeing, reduced salt

Reactive dyes contain a reactive group that forms a chemical bond with cotton iber under alkaline conditions. Reactive dyes give bright, fast colors, and account for over 70% of the dyes used for cotton. However, large quantities of salt are needed to cause the dye to move from the dye bath to the fiber, and the exhaustion and fixation rates for reactive dyes (the percentage of the dye that moves from the dye bath onto the iber and the percentage that bonds permanently to the iber) are relatively low. For conventional reactive dyes, the ixation rate is often less than 80%, resulting in waste of dye, and removing the unixed dye requires extensive rinsing and washing with heated water.

Dye suppliers are now offering improved dyes that enable much higher exhaustion and ixation rates while requiring less than half the salt needed with standard reactive dyes. These high-fixation dyes usually incorporate two different reactive groups within the molecular structure of the dye. Much progress has been made in commercializing higher-ixation reactive dyes for dyeing yarns, wovens, knits, and garments. Some mills have been able to boost their average ixation rates from below 70% to over 85%, and ixation rates of over 90% have been reported. However, these higher-value dyes often are more expensive than conventional dyes. Also, because these dyes have higher affinity for fiber than do conventional dyes, they can be more dificult to apply uniformly, and more water may be required for removal of unixed dye.

As mills gain experience with these new dyes and develop confidence that they deliver savings in WEC reduction and mill cost, their use should increase signiicantly.

Low-liquor-ratio jet dyeing machines

High-ixation reactive dyes most often are used with conventional equipment, but their benefits in WEC reduction are magniied when they are used in low-liquor-ratio (LLR) jet dyeing machines. Jet dyeing machines are based on the principle of accelerating water through a nozzle to transport fabrics through the machine. They are designed to operate efficiently and at high quality with a very low ratio of water to material. Jet dyeing machines have been used commercially for 40 years; however, technological advances have reduced water requirements, and machines of newer designs operate at a liquor ratio of less than 8:1.

These machines usually use low-friction Teflon internal coatings and advanced spray systems to speed rinsing. !°Ultra low liquor ratio! jet dyein machines operate at a liquor ratio of less than 6:1 and almost always depend on forced airlow to convey the fabric through the machine.LLR jet dyeing is widely used in high volume for piece-dyed knits, as well as some wovens, depending on fabric weight. Compared with conventional machines, LLR machines usually enable reduced cycle times and increased productivity, while requiring less than half as much water. Some plants achieve four batches in 24 hours (depending on depth of shade) and average water consumption of less than 50 liters per kilogram of knit fabric. Plants using machines with the newest airflow technology report processing with liquor ratios of less than 4:1. One factor limiting implementation is the high cost of the new machines, which favors use at new facilities rather than as replacements for older machines.

Toward a sustainable future

Advances in making dyeing technology more environmentally friendly have not been limited to improvements in dyestuffs and equipment. In addition, the plants that are the most advanced in reducing WEC pursue a combination of options including high-eficiency management practices, process control, special processes, and treatment and recycling of wastewater. Each plant adopts options compatible with its particular product offerings, economic circumstances, environmental regulations, and supply-chain requirements.

Achieving these WEC reductions has required plants to learn and adapt to new processes and practices and to implement creative means for ensuring acceptable economic returns.

The cotton textile industry can reduce its WEC environmental footprint at least 50% by employing technologies currently used in modern plants (as described in Cotton Incorporated's recent A World of Ideas: publication Technologies for Sustainable Cotton ). Cooperation Textile Manufacturing throughout the supply chain from iber to inished product is critical to encouraging and supporting these efforts.

Source form China Textile Magazine

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