How do dyes adhere to fabric

To ensure colorfastness, dyed fabric or yarn is washed over and over again in hot water, creating large amounts of wastewater.

All told, about L 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 J.

Process Technol. Bleeding and crocking: Two components of colorfastness. Bleeding happens when dye comes off a fabric in contact with liquid. Crocking occurs when a dye on a dry fabric rubs off on another dry fabric. Colorfastness: The ability of a dye to preserve the original color during industrial processing and subsequent customer use. Digital textile printing: Directly printing colors and patterns onto fabric using design software, large-format printers, and specialty inks made with pigments or dyes.

Digital printing is an alternative to standard screen printing, which uses a constrained color palette and requires separate stencils and production steps for each color. Dye: Soluble chemicals that contain chromophores, or color-containing compounds. Dyes are mixed with other additives in a color solution. They can be derived from natural sources, such as plants, but are more commonly human made. Different classes of dyes are used for different fibers and stages of the textile production process.

Direct dye: A class of dye that can be applied directly to cotton or other cellulosic fabrics such as rayon, silk, and wool. Direct dyes are applied in a neutral or alkaline bath of hot water.

They do not require mordant or fixatives for fastness; instead, they attach with hydrogen bonds and van der Waals forces. Direct dyes are soluble salts of complex sulfonic acids, including diazo or polyazo chemicals.

Disperse dye: A category of nonionic dyes used to color synthetic yarns and fabrics such as polyester. These organic chemicals, mostly monoazo dyes, are nonsoluble and rely on dispersing agents to spread the color molecules in water. Reactive dye: A class of colored synthetic organic chemicals that attach to textile fibers via a chemical reaction that forms a covalent bond.

Reactive dyes are the most permanent of all dye types and are the most common type of dye used on cotton and other cellulose fibers. They are categorized by their functional group, such as dichlorotriazine or vinyl sulfone. Dye exhaustion or dye fixation: The mass of dye taken up by the yarn or fabric divided by the total initial mass of dye in the water bath. Once the dyeing process reaches equilibrium, a portion of the dye remains in the dye bath and becomes part of the dye process wastewater.

The exhaustion ratio depends on the quality of the dye and the characteristics of the fiber. Leveling agent: Used in disperse dyeing to regulate or slow the uptake of dye onto synthetic fibers to ensure that the color level is uniform. Leveling agents are often nonionic surfactants that increase the solubility of the dye and slow adsorption. Mordant: Also called a dye fixative, a substance used to chemically bond a dye to natural fibers to ensure fastness.

Mordant chemicals include alum, caustic soda, and metal salts. The mordant forms a coordination complex with the dye, increasing its molecular weight and making it insoluble. Pigment: Insoluble materials, usually in powder form, that add color to inks, paints, plastics, cosmetics, and foods.

When used on textiles, they require binders or other additives to attach to the fibers. Pigments can be derived from minerals but can also be made synthetically. Because they are not soluble in water, they can last longer than dyes.

To reduce this burden, Huntsman has developed a line of dyes for cotton called Avitera that bonds to the fiber more readily. According to the company, the colors require one-quarter to one-third less water and one-third less energy. 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. Different regions and countries have different cost structures, he says. Another way to improve the bond between dyes and cotton fibers 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.

ColorZen treats raw cotton fiber right from the field after the seeds are removed. After treatment, cotton is spun into yarn at customer facilities. 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. It is also in a program run by the apparel start-up incubator Fashion for Good.

Hohenstein developed Oeko-Tex, a series of standards and tools for certifying nontoxic textiles. The first version of the standard was called Oeko-Tex for the number of chemicals it tracked. Oeko-Tex certification is now up to more than chemicals. Synthetic indigo, used to make blue jeans blue, is an example of a dye that can release unreacted chemicals downstream of manufacturing.

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 metric tons of aniline per year escapes the dyeing process from 70, metric tons of indigo. Archroma developed a technology for prereducing indigo to prevent aniline from carrying through as a contaminant.

Finished textiles colored with the dye contain a nondetectable amount of aniline, whereas competitor dyes can contain up to 2, ppm of the chemical, according to Archroma.

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.

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 That was about 25 years before chemists developed the synthetic route to indigo dye—with its unappetizing 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 life-cycle review of the production and use of synthetic indigo provides plenty of reasons to look again at indigo from plants. Dyeing : Warm temperature, long process time, requires addition of large amounts of salt and alkali fixatives.

Washing : Long, energy- and water-intensive process using multiple baths, with at least one at boiling temperature. Washing : Shorter process requiring less energy, water, and chemicals than cotton. Uses alkali and chemical reducing agent. Washing : Similar to cotton but shorter process, possibly due to less unfixed dye to be removed.

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 U. That could displace 2. 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. All that is required to scale up are fermentation tanks and sugar. The idea for Colorifix came out of a biological sensor program 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. The team was already using color 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 colors, including anthocyanins and carotenoids. Not all the colors that engineered microbes can make meet textile industry requirements for lightfastness and temperature stability. But microbes that produce stable colors 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 fiber. Then they are given nutrients that cause them to grow. Nugent says the process works like a very efficient reactive dye that requires only a single finish wash. 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 program. In , he and his partners realized they could make industrially useful colors that way. Biology is more efficient than the chemical industry at making dye structures, Boissonnat argues. We have made some calculations that show to produce 1 kg of dye, you need kg of petroleum, 1, L of water, and 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 most permanent, wash-fast dyes are the most tightly attached to the fiber molecules. In addition, the requirements for forming the chemical bonds are what determine the ingredients and method for each recipe for dyeing.

Fiber Reactive Dyes Fiber reactive dyes, such as the dichlorotriazines Procion MX type dyes , are attached to the fiber by the same strong covalent bonds that hold the atoms within the dye molecule together. The dye actually becomes one molecule with the fiber molecule to which it attaches. Covalent bonds are formed by electron sharing between atoms.

Direct Dyes Direct dyes, including that portion of "all purpose" dyes that is left in cotton that has been dyed with them, are only loosely associated with the fiber molecule through the property called substantivity, which is the tendency of the dye to associate with the dye without strong bonds. This substantivity is increased by increasing the size of the dye molecule, so direct dyes tend to be large.

Small dye molecules tend to be bright, while large dye molecules tend to be duller as there are more parts that can absorb additional wavelengths of light , so direct dyes are generally much less bright in color than fiber reactive dyes. Substantivity is said to result from a combination of the relatively weak Van der Waals forces and some hydrogen bonding.

Disperse Dyes Disperse dyes, used for synthetic fibers such as polyester, work by being vaporized by the heat of an iron or heat transfer press and then condensing onto and into the fiber. They can also by induced to migrate into the fiber by boiling with a special carrier chemical. Unsurprisingly, such loosely associated dye may rub off, though not visibly so. The fact that the dye can rub off on the wearer makes this class of dye more allergenic than other types of dye, though this is a problem for only a minority of people.

Such people are happier wearing natural fibers that have been dyed with fiber reactive dye. Acid Dyes The attachment between acid dyes and the protein fibers that they join to are the most complex of all. Proteins are made of up to twenty different amino acids, each of which has a different side chain. At different pHs, different dyes can form rather strong hydrogen bonding to various of these side chains.

Many acid dyes contain a sulfonic group, or, in some cases, a carboxylic group, which can form a strong 'salt linkage' to a basic group in the wool molecule. See Ingamells, under "Further Reading", below. A salt linkage is an ionic bond between fixed ions of opposite charges, due to Coulombic interaction. The strength of this bond is considered to be responsible for the washfastness of good acid dyes. Note that certain specialized fiber reactive dyes are also used for wool, though the fiber reactive dyes commonly used for cotton, such as Procion MX dye, is said to not react appreciably with wool under acid conditions; instead, the types of fiber reactive dyes used on cotton can be used simply as acid dyes with wool, even if the reactive groups on the dye have 'gone bad' by reacting with water.