Scientists find a ‘greener’ way to make jeans blue

Dying denim their signature blue can release a lot of pollution. A new chemical treatment could prove “greener” to the environment.

Kranich17/Pixabay

By Shi En Kim

November 3, 2021 at 6:30 am

Making jeans takes a toll on the environment. Dyeing denim its signature blue guzzles water and uses toxic chemicals. But a new technology could lower the cost of blue denim and pollute less. The trick: Add an all-natural plant-based chemical to the dye. It’s known as nanocellulose.

“Our research was dedicated [to finding] sustainable technologies for better processing of textiles,” says Smriti Rai. She’s a textile researcher at the University of Georgia in Athens. Her team showed nanocellulose can cut water and chemical consumption during dyeing. They shared the details in the October 21 issue of Green Chemistry.

Jeans’ blue color comes from a pigment known as indigo. Indigo does not dissolve in water. Textile makers must treat indigo with harsh chemicals to make it soluble. Then, they dip denim in a vat of this solution. But even now the dissolved indigo doesn’t want to stick. It takes multiple dips to turn the cloth blue.

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All of this pigment-treated water is also full of hazardous chemicals. Many of these pollutants may not be removed by water-treatment plants. Later, when that treated water is released into the environment, it can pollute waterways.

But the team’s innovative new dyeing technique “totally eliminated this chemistry,” says Rai. “We just mixed [solid] indigo particles with nanocellulose.” No toxic chemicals needed.

Cellulose is a tough organic polymer found in plants cells and wood. It’s also the material that makes up paper. Nanocellulose consists of the same fibers, only on a billionth-of-a-meter scale. They’re shaped like eyelashes, but only a thousandth their size.

To give denim its blue hue, the researchers add indigo powder to a hydrogel containing a small amount of nanocellulose. Hydrogels are a type of polymer that absorbs water. The researchers make theirs just runny enough to smear onto denim. Then they screen-printed the colored goo onto the fabric (see video). This step does away with need for a vat of dye. It also eliminates all but maybe 3 or 4 percent of the water needed for dyeing.

Those nanocellulose rods form a mesh that traps the dye molecules. The mesh also has a large surface area. At the nanoscale, its tiny bumps and ridges collectively add up to more surface area than the bare denim had to start with. So more dye will stick to fabric coated with nanocellulose. And more dye means a deeper blue.

“Because of the very high surface area, we can use less chemicals” to get the same shade, says Sergiy Minko. He’s a University of Georgia chemist who works with Rai. Denin absorbed more indigo in one pass with the new dye than it would have picked up after being dipped in the traditional vat of dye eight times.

But the hydrogel coating swells and unravels when it gets wet again, such as in the wash. This may cause the mesh to release some dye. That would cause the fabric to fade. To avoid this, the researchers treat their colored cloth with chitosan (KY-toh-san). It’s a chemical byproduct of food-industry wastes. (It come from shrimp or crab shells.) Chitosan strengthens nanocellulose by reinforcing the contact points between individual fibers. It also helps nanocellulose glom onto the cotton used to make denim. So chitosan-treated fabric can hold its hue through far more washings.

Nanocellulose and chitosan come from an all-natural materials. Indigo dye can, too. But long ago chemists figured out how to create a low-cost synthetic version, and that’s what most denim-producers now use. The new dyeing process works with both natural and synthetic indigo. The researchers would like to see more people use the natural dye.

Nanocellulose means the new dye process needs less dye, water and labor, Rai’s team says. Minko and Rai hope that this will motivate jeans makers to use natural indigo again. It would also give consumers the chance to opt for more environmentally sustainable fashion. “This cultural aspect is important,” says Minko.

The dyeing process is “a wonderful potential technological advancement,” says Robert O. Vos. He’s an industrial ecologist who works at the University of Southern California. It’s in Los Angeles. Denim fashions are popular the world over. So any advance in denim-making could have a large positive impact on fashion’s environmental footprint, he says. He predicts companies will be eager to adopt the new dye technology.

However, he points out, the denim-making step that uses the most water isn’t dyeing. It’s growing the cotton itself. So even with this innovation, he argues, making jeans will still require a lot of water.

Vos, Rai and Minko are all fans of jeans. They appreciate their comfort and durability. But ultimately, Vos says, owning fewer jeans would be the greenest option of all. Buy only as many pairs of you need, he says. And wash them less often. Treat these jeans, he says, like the hardy garments they are.

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

biodegradable: Adjective for something that is able to break down into simpler materials, based on the activity of microbes. This usually occurs in the presence of water, sunlight or other conditions that help nurture those organisms.

cellulose: A type of fiber found in plant cell walls. It is formed by chains of glucose molecules.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemistry: The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances.

consumer: (n.) Term for someone who buys something or uses something. (adj.) A person who uses goods and services that must be paid for.

dissolve: To turn a solid into a liquid and disperse it into that starting liquid. (For instance, sugar or salt crystals, which are solids, will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.)

ecologist: A scientist who works in a branch of biology that deals with the relations of organisms to one another and to their physical surroundings.

environment: The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

fabric: Any flexible material that is woven, knitted or can be fused into a sheet by heat.

fiber: Something whose shape resembles a thread or filament. (in nutrition) Components of many fibrous plant-based foods. These so-called non-digestible fibers tend to come from cellulose, lignin, and pectin — all plant constituents that resist breakdown by the body’s digestive enzymes.

green: (in chemistry and environmental science) An adjective to describe products and processes that will pose little or no harm to living things or the environment.

green chemistry: A rapidly growing field of chemistry that seeks to develop products and processes that will pose little or no harm to living things or the environment.

hue: A color or shade of some color.

hydrogel: A “smart” material that can change its structure in response to its environment, such as the local temperature, pH, salt or water concentration. The material is made from a polymer — a chain made from links of identical units — that have free, water-attracting ends sticking out. So in the presence of water, it may hold (bond) those water molecules for quite a while. Some hydrogels are used in baby diapers to hold urine, in potting soils to hold water near to plants until they need it and in wound dressings to keep a sore from drying out.

indigo: A deep blue dye made from Indigofera, a genus of plants belonging to the pea family. One of this dye’s best known contemporary uses: tinting the denim used to make blue jeans. It is also the name of the hue created by these plants or other, synthetic dyes.

innovation: (v. to innovate; adj. innovative) An adaptation or improvement to an existing idea, process or product that is new, clever, more effective or more practical.

journal: (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.

molecule: An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

organic: (in chemistry) An adjective that indicates something is carbon-containing; also a term that relates to the basic chemicals that make up living organisms. (in agriculture) Farm products grown without the use of non-natural and potentially toxic chemicals, such as pesticides.

particle: A minute amount of something.

pigment: A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.

polymer: A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

recycle: To find new uses for something — or parts of something — that might otherwise be discarded, or treated as waste.

shell: The normally hard, protective outer covering of something. It could cover a mollusk or crustacean (such as a mussel or crab), a bird’s egg or some other relatively soft tissue that needs protection (such as a tree nut or peanut). (in munitions) An explosive bullet, bomb, grenade or other projectile.

soluble: Some chemical that is able to dissolve some liquid. The resulting combo becomes a solution.

surface area: The area of some material’s surface. In general, smaller materials and ones with rougher or more convoluted surfaces have a greater exterior surface area — per unit mass — than larger items or ones with smoother exteriors. That becomes important when chemical, biological or physical processes occur on a surface.

sustainable: An adjective to describe the use of resources in a such a way that they will continue to be available long into the future.

synthetic: An adjective that describes something that did not arise naturally, but was instead created by people. Many synthetic materials have been developed to stand in for natural materials, such as synthetic rubber, synthetic diamond or a synthetic hormone. Some may even have a chemical makeup and structure identical to the original.

technology: The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

textile: Cloth or fabric that can be woven of nonwoven (such as when fibers are pressed and bonded together).

toxic: Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

Journal:​ ​​S. Rai​ ​et​ ​al.​ ​Environment-friendly nanocellulose-indigo dyeing of textiles. Green Chemistry.​ Vol. 23, October 21, 2021, p. 7937. doi:​ 10.1039/d1gc02043a.

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