Textile chemicals – beginning with the one used the most

16 01 2013

saltLet’s begin our review of chemicals used in textile processing with the one chemical that is used most often and in far greater quantity than any other: salt. That’s right. Common table salt, the kind you probably use every day. But in the quantities used by this industry it becomes a monster – we’ll get to that in a minute.

Salt is used in the dye process. The way the dyestuff bonds to the fibers is very important – and the most permanent, colorfast dyes are the ones that are most tightly attached to the fiber molecules (called reactive dyes). Here’s how salt comes into the picture:

When fabrics made of cellulosic (i.e., cotton, linen, hemp or viscose) are dyed, they’re immersed in water which contains dyes which have been dissolved in the water. The surface of the fabric gets covered in negative ionic charges. The reactive dyes used most often to dye cellulosic fabrics also develop a negative charge, so the fibers actually repel the dye – like two magnets repelling each other. If we try to dye a cellulosic fabric without using salt, the dye molecules just roll off the surface of the fibers and the fabric does not show much color change. So these reactive dyes need the addition of salt to “push” the dyes out of solution and into the cloth by neutralizing the negative charge.

The salt acts like a glue to hold the dye molecules in place, and with the addition of alkali, a certain percentage of the dyestuff (called the “fixation rate”) will permanently grab hold of the fiber and become a part of the fiber molecule rather than remaining as an independent chemical entity. For conventional reactive dyes, the fixation rate is often less than 80%, resulting in waste of dyestuff, and also the need to remove that 20% (which is not fixed) from the fabric.(1) But this is incredibly difficult when the “unreacted” dyes are still “glued” onto the fabric by salt. So vast amounts of water are required to simply dilute the salt concentrations to a point where it no longer acts as glue.

That means the textile effluent contains both dyestuff and salt (lots of salt!) The concentrations of salt in the dye bath can be as high as 100 gm per liter. In the worst cases, equal weights of salt to fabric is used to apply reactive dye (i.e., if dyeing 10 lbs of fabric, you need 10 lbs of salt). Think of the billions of yards of fabric that’s produced each year: In Europe alone, 1 million tons of salt is discharged into waterways each year.(2) In areas where salt is discharged into the ecosystem, it takes a long, long time for affected areas to recover, especially in areas of sparse rainfall – such as Tirupur, India.

Tirupur is one of the world’s centers for clothing production , home of 765 dyeing and bleaching industries. These dyehouses had been dumping untreated effluent into the Noyyal River for years, rendering the water unsuitable or irrigation – or drinking. In 2005, the government shut down 571 dyehouses because of the effluent being discharged into the Noyyal. The mill owners said they simply couldn’t afford to put pollution measures into place. The industry is too important to India to keep the mills closed for long, so the government banned the discharge of salt and asked for an advance from the mills before allowing them to re-open. But … on February 4, 2011, the Madras high court ordered 700 dye plants to be shut down because of the damage the effluent was doing to the local environment. Sigh. (Read more about Tirupur here.)

Unfortunately, the salt in textile effluent is not made harmless by treatment plants and can pass straight through to our rivers even if it has been treated. The salt expelled into waterways (untreated) coupled with salt from roadway de-icing has led to the increase in salt in our waters in the United States – salt levels in Lake George have nearly tripled since 1980,(3) which mirrors many other parts of the U.S. Highest levels occur during the annual ice-out and snowmelt where high salt concentrations in streams flowing into Lake George have been linked to die offs of fish, and is known as “spring shock”. A study in Toronto found that half the wells tested exceeded the limit of 20mg of salt per liter of water, 20% exceeded 100mg/liter and 6% exceeded 250mg/liter. (4) It becomes a public health concern for people who drink this water, because it can exacerbate high blood pressure and hypertension in humans. This increase in our drinking water can also cause problems with water balance in the human body. Salt in water is also responsible for the release of mercury into the water system.

Recycling the salt used during the dye process is possible, and this has been used by many of the dyers in Tirupur, and elsewhere, who operate zero discharge facilities. The effluent is cleaned and then the salt is recovered using an energy intensive process to evaporate the water and leave the solid, re-useable salt. This sounds like a good idea – it reduces the pollution levels – but the carbon footprint goes through the roof, so salt recovery isn’t necessarily the best option. In fact, in some areas of the world where water is plentiful and the salt can be diluted in the rivers adequately, it may be better to simply discharge salt than to recover it.

There are some new “low salt” dyes that require only half the amount of “glue”: Ciba Specialty Chemicals, a Swiss manufacturer of textile dyes (now part of BASF) produces a dyestuff which requires less salt. As the company brochure puts it: “Textile companies using the new dyes are able to reduce their costs for salt by up to 2 percent of revenues, a significant drop in an industry with razor-thin profit margins” but these dyes are not widely used because they’re expensive – and manufacturers are following our lead in demanding ever cheaper costs. There are also new low-liquor-ratio (LLR) jet dyeing machines – but that doesn’t mean zero salt, so there is still salt infused effluent which must be treated. And these new ultra low liquor ratio machines are very expensive.

The best option is to avoid salt altogether. Though the salt itself is not expensive, using less salt delivers substantial benefits to the mill because the fabric requires less rinsing in hot water (and hence reductions in energy and water) as well as cost savings of up to 10% of the total process costs.(5) So what about using no salt at all?

There are two ways to dye fabrics without salt: “continuous dyeing” and “cold pad batch dyeing”. Continuous dyeing means that the dye is applied with alkali to activate the dye fixation; the fabric is then steamed for a few minutes to completely fix the dyestuff. Cold pad batch dyeing applies the dyestuff with alkali and the fabric is simply left at room temperature for 24 hours to fix the dye.

Both of these methods don’t use salt, so the unfixed dye chemicals are easier to remove because there is no salt acting as the “glue” – and therefore less water is used. An additional benefit is having a lower salt content in the effluent. So why don’t companies use this method? Continuous dyeing requires investment in big, expensive machines that only make environmental sense if they can be filled with large orders – because they use lots of energy even during downtime.

Cold pad batch machines are relatively inexpensive to buy and run, they are highly productive and can be used for a wide range of fabrics. Yet only 3% of knitted cotton fabric is dyed in Asia using cold pad batch machines.
Why on earth don’t these mills use cold pad batch dyeing? I would love to hear from any mill owners who might let us know more about the economics of dyeing operations.

(1) http://lifestylemonitor.cottoninc.com/Supply-Chain-Insights/Sustainable-Dyeing-Solutions-02-10/
(2) Dyeing for a change: Current Conventions and New Futures in the Textile Color Industry (2006, July) http://www.betterthinking.co.uk
(3) http://www.fundforlakegeorge.org/assets/pdf_files/Fact%20Sheet%2011%20Salt.pdf
(4) http://www.digitaltermpapers.com/a2206.htm
(5) “A Practical Guide For Responsible Sourcing”, The National Resources Defense Council (NRDC), February 2010.


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