Textile Dyeing Methods That Make Fashion More Sustainable

Machinery and Technology

Textile Dyeing Methods That Make Fashion More Sustainable

In early June, Dalton Cheng realized something big was afoot. Cheng, who is head of technology for the textile printing firm Intech Digital, heard from customers that Chinese government authorities in Jiangsu province had shut down massive factories that produce synthetic dyes used by the textile industry. It was just the latest in a series of actions that started in the summer of 2017, when tens of thousands of China's factories were forced to close and undergo environmental inspections.

Overall, as much as 60% of China's denim-dyeing chemical capacity has been shuttered, Cheng says, equal to roughly 30% of global capacity. And that's why his phone was ringing. Intech, headquartered in Hong Kong, might be in a position to help apparel industry customers out of a critical supply bind.

Intech's specialty is digital printing on textiles, including cotton and other cellulosic fabrics like rayon. Printing textiles with pigments rather than dyes uses very little water, Cheng says, & produces much less waste than traditional methods.

Digital printing is one example from a growing list of new, more sustainable fabric-colouring technologies from both major suppliers and smaller chemical and biotech start-ups. The companies see business opportunity in tackling dyeing's wasteful water and energy practices and its reliance on toxic chemicals that can give rivers shocking hues and harm human health.

But the barriers facing those working to promote a more sustainable textile technology are quite high. The industry's sheer scale makes it hard to have an impact: Textiles are a US$ 3 trillion-per-year business that employs nearly 60 million workers worldwide, according to economic research firm Euler Hermes and FashionUnited, an industry information resource.

It's also a manufacturing industry under pressure. Price competition is fierce, and profits are shrinking thanks to volatile raw material costs and rising wages. Despite public commitments by apparel brands to become more sustainable, suppliers contacted by C&EN say their customers will not buy anything that could raise the cost of a finished garment by as little as a penny.

The factory shutdowns have disrupted the textile supply chain, says Holger Schlaefke, global marketing manager at Huntsman's textile effects segment. Huntsman, Archroma, and DyStar are the world's largest suppliers of dyes and textile chemicals.

"Shutdowns are a concern for companies like Huntsman and also for retailers," Schlaefke says. "You can imagine retailers have contracted with a fabric-producing mill - they negotiated six months ago and agreed on a price - but suddenly that price is not certain anymore. It's not a crisis, but it makes business a little more complicated for everyone."

The largest impact of the factory shutdowns has been in the supply and price of so-called disperse dyes, which are used to colour synthetic fibres like polyester, a specialty of Chinese producers, Schlaefke says. Availability of reactive dyes, which are used on cotton, was also reduced, though manufacturers in cotton-rich India are likely to take up the slack. In India as well, the government is taking steps to reel in the textile industry to save precious water resources. "In India, a trend is huge investment in wastewater treatment for liquid discharge," Schlaefke says. It's now common for factories to reuse 90% of their water. While both cotton and polyester are normally coloured with synthetic dyes, dyeing cotton is a more water- and heat-intensive process. The surface of cotton fibres is negatively charged and doesn't readily react with negatively charged dye compounds.

Even with an assist from salts and alkali added to the dye solution, cotton takes up only about 75% of the dye. To ensure colour fastness, dyed fabric or yarn is washed over and over again in hot water, creating large amounts of wastewater.

All told, about 200 litres of water is used to produce 1 kg of fabric. A review of wastewater treatment steps found that textile effluent contains high concentrations of dyes and chemicals, including chromium, arsenic, copper, and zinc. Dyes and chemicals released into waterways also block sunlight and increase biological oxygen demand.

To reduce this burden, Huntsman has developed a line of dyes for cotton called Avitera that bonds to the fibre more readily. According to the company, the colours require one-quarter to one-third less water and one-third less energy. Three reactive groups are attached to the dye formula's chromophore - or colour-providing molecule - compared with the one or two reactive groups common for cotton dyes. Thanks to these extra reactive groups, the dye step lasts about four hours, compared with seven hours for conventional dyes. Still, it takes a lot of legwork to sell customers on a new suite of dyes. "It can be hard to show cost savings when the savings comes from water use or energy," Schlaefke says. Different regions and countries have different cost structures, he says. For example, Bangladesh is now the biggest cotton-producing country, "and as we know, they have no shortage of water," he notes.

Another way to improve the bond between dyes and cotton fibres is a process called cationization. In North Carolina, textile industry veteran Tony Leonard is taking that approach. Leonard is the inventor and technical director behind ColorZen, a start-up that has developed a cotton pretreatment step.

"With conventional cotton dyeing, salt is used to negate the charge on the surface of the cotton," Leonard explains. "ColorZen technology uses a quaternary ammonium compound to permanently attach a positively charged amino site on the cellulose molecule." That makes for a natural attraction between dye and fibre, he says.

ColorZen treats raw cotton fibre right from the field after the seeds are removed. "North Carolina is still the heart of the cotton and textile industry here in the United States, and there are ready supplies of baled US cotton here that our customers are tapping into," Leonard notes. After treatment, cotton is spun into yarn at customer facilities.

Leonard contends that ColorZen's pretreatment makes the dyeing process faster while using 90% less water, 75% less energy, and 90% fewer auxiliary chemicals. It also cuts out almost half the dye compared with processes that call for salts in the dye bath.

The company has a partnership with the manufacturing technology firm Jabil to help it scale up its plant in Mebane, N.C. It is also in a programme run by the apparel start-up incubator Fashion for Good. Leonard says the company is developing a supply chain by educating dye houses, spinners, and retailers about ColorZen's benefits.

Even the best pretreatment process can't eliminate the health effects of the dyes and the chemicals used to make them. That's the focus of many of the textile industry's eco-certification programmes.

"With advances in measurement instruments, we're seeing more evidence of contaminants and degradation products than ever before," observes John Frazier, technical director at Hohenstein Institute America, a textile research organisation. Hohenstein developed Oeko-Tex, a series of standards and tools for certifying non-toxic textiles. The first version of the standard was called Oeko-Tex 100 for the number of chemicals it tracked. Oeko-Tex certification is now up to more than 300 chemicals.

The industry needs to both develop "better chemistry and ensure there is less water going out the back," Frazier says. He says he's seeing "a ton of innovation happening. It's very exciting." Schlaefke says Huntsman's Avitera dyes were formulated to be free from p-chloroaniline (PCA), a hazardous chemical used as an intermediate in the manufacture of azo dyes and pigments. "What we see now is a bit more retailers are looking at PCA," he notes. Currently, brands including REI and Levi Strauss & Co. restrict the use of PCA along with a list of other amines from colorants.

Synthetic indigo, used to make blue jeans blue, is an example of a dye that can release unreacted chemicals downstream of manufacturing. A small number of Chinese manufacturers produce most of the world's indigo using aniline as a key raw material. Indigo is unlike most dyes in that in its unreduced form it is not soluble. So companies like Archroma upgrade it into easier-to-use, prereduced solutions that are more water soluble.

The company became concerned after seeing published reports that about 400 metric tons of aniline per year escapes the dyeing process from 70,000 metric tons of indigo. Two-thirds of the escaped chemical ends up in wastewater, on workers, and in the air, while one-third stays on the denim that goes to stores, says James Carnahan, Archroma's global sustainability manager for textiles.

Archroma developed a technology for prereducing indigo to prevent aniline from carrying through as a contaminant. Finished textiles coloured with the dye contain a non=detectable amount of aniline, whereas competitor dyes can contain up to 2,000 ppm of the chemical, according to Archroma.

Brands' awareness of aniline contamination is growing, and the compound has started to appear on lists that brands send to manufacturers restricting the chemicals they can use.

Carnahan acknowledges differing views about how big a problem aniline is in the textile industry. It has a better reputation than the category 1 carcinogenic amines that cleave off of azo dyes and were an early target for elimination by clothing brands. "Aniline is category 2, instead-which is not good, though," Carnahan says.

Of course, in the beginning, indigo came from a plant, not a factory. The very first pair of modern-style blue jeans, made by Levi Strauss, debuted in 1873. That was about 25 years before chemists developed the synthetic route to indigo dye - with its unappetising starting materials of aniline, formaldehyde, and hydrogen cyanide.

The ambition at Stony Creek Colors is to return to those early days. Founder Sarah Bellos says a complete lifecycle review of the production and use of synthetic indigo provides plenty of reasons to look again at indigo from plants.

Stony Creek is developing varieties of leguminous indigo plants that can provide a high-yield, high-profit crop for Tennessee farmers looking for an alternative to tobacco. The company is selling all the dye it can make; its goal is to expand US indigo production to 6,000 hectares in the next five years. That could displace 2.8% of global synthetic indigo production.

The indigo molecule itself is exactly the same as the synthetic version, with one small difference: "Synthetic indigo has a tighter crystal formation that makes it more difficult to reduce," Bellos says. Dye houses can reduce natural indigo in bacterial fermentation vats or use more common reducing systems, she adds.

Other start-ups have also turned to biology - in particular, engineered microbes - to reduce the use of chemicals in textile dyes. UK-based Colorifix and the French firm Pili say microbes can produce high-performance, renewable dyes suitable for mainstream textiles. All that is required to scale up are fermentation tanks and sugar.

The idea for Colorifix came out of a biological sensor programme in Nepal and Bangladesh. David Nugent and colleagues were in the region to test drinking water wells for arsenic. They asked local village governments what other substances in their water concerned them. "We got a large list of chemicals," Nugent recalls. "When we asked, 'Where do they come from?' the answer we got was textiles, again and again." The team was already using colour made by microorganisms to act as a sensor for water contaminants. Soon, Nugent says, it became clear the researchers could engineer them to produce natural colours, including anthocyanins and carotenoids.

"Once you start looking at how nature makes colours, you see a lot of similarities in the sequences of the proteins and enzymes," Nugent says. With more resources, he'd like to go prospecting to find new molecules. "There are beautiful colours in the ocean, in insects - you can crack open a wide palette of colours."

Not all the colours that engineered microbes can make meet textile industry requirements for lightfastness and temperature stability. Chlorophyll, the secret to nature's abundance of green, can turn an unstylish brown in factory conditions, Nugent says. But microbes that produce stable colours can be adopted by dye houses with very little change to their normal processes. First the microbes go into a solution like a regular dye and get embedded in the textile fibre. Then they are given nutrients that cause them to grow. When heat is applied, the organisms' membranes burst. That causes the colour to chemically attach to the fibre with help from metal ions and salts in the microbes' cytoplasm.

Nugent says the process works like a very efficient reactive dye that requires only a single finish wash. He claims a water savings of 90% and an energy savings of 20% over standard processes. Colorifix is setting up pilot operations in Italy and France; partner dye houses first have to get a certification to work with genetically modified microbes. Pili got its start as a biology outreach programme. "We were doing workshops with kids, growing bacteria and making colours with them, painting with them," Chief Science Officer Guillaume Boissonnat recalls. In 2015, he and his partners realised they could make industrially useful colors that way.

Biology is more efficient than the chemical industry at making dye structures, Boissonnat argues. "Dyes are usually the aromatic molecules from heavy fractions of petroleum. We have made some calculations that show to produce 1 kg of dye, you need 100 kg of petroleum, 1,000 litres of water, & 10 kg of other chemicals."

But the major portion of water used for textile dyeing comes after dyeing, when fabrics, particularly cotton, have to be washed over and over again to remove unfixed dye. Instead, manufacturers can skip dyes and use pigments. As at Intech Digital, the enabling technology for that move is large-scale printers that take the place of dyeing vats. The printers use special versions of ink-jet printheads designed to work with textile inks.

The use of digital textile printing is growing rapidly, thanks in large part to demand from fast-fashion purveyors, says Tim Check, a textile product manager at Epson, which makes printers, printheads, and inks for textiles. Check says retailers such as H&M or Zara demand short runs of styles to appear in stores in as little as two weeks, rather than the traditional design schedule of huge runs delivered after several months.

Patterned textiles for such garments were historically made by screen printing, which is expensive to set up, requires custom templates for each colour, and uses a limited set of hues. Digital printers, which give designers almost unlimited choices, are taking over some of that market.

"The big reason we're really excited is it's having huge growth, well over 20% per year," Check says. But that increase is from a small base of about 2-3% of total textile output.

Digital printing on polyester uses a two-step dye sublimation process that is almost waterless. The pattern is first printed on transfer paper; heat then turns the colour into a cloud of gas, which bonds with the softened polyester. This process doesn't work for cotton, silk, and other natural fabrics, Intech's Cheng points out. For them, his firm offers a new specialty ink made with powdered, insoluble pigments plus a polymer-like binder designed to make the colour stick.

Epson's Check says the digital printing approach can allow apparel manufacturers to operate in regions closer to their customers, such as in North America. "All you need is a printer and a heat system to fix the ink. That means you can do that here in the US without the environmental concerns or cost of managing wastewater," he says. Huntsman's Schlaefke agrees that digital printing is a trend that could have an impact on how much and what types of colorants are used. "The market is moving in the right direction for printing," he says.

Since Huntsman sells both traditional dyes and specialty inks, it stands to gain whichever way the wind blows. Schlaefke says the segment's growth will depend in large part on how economical the machines become.