Chemists are figuring out how to recycle our clothes | Popgen Tech
Sonja Salmon is a big fan of cellulose, and that’s why she wants to destroy it. “I love pulp,” she says. “I rip pulp because I like it.”
She also disassembles it because the polymer, which occurs naturally in wood and cotton, makes up one quarter of all fibers used in textile production. This means that any effort to recycle clothing and fabric to remain part of the circular economy for as long as possible must include ways to deal with all that pulp.
Salmon, a polymer scientist at North Carolina State University’s Wilson College of Textiles in Raleigh, is working to break down cellulose from discarded textiles and reuse it. Many clothing fabrics are made from a half-polyester/half-cotton blend—the individual fibers of cotton and polyester are tightly twisted around each other to create a yarn that is then woven or knitted into clothing. This structure is difficult to mechanically disassemble, so Salmon treats it with cellulases, a group of enzymes that break down cellulose. “We can chew it up into small enough molecules and fragments that it actually falls out of the rest of the tissue structure,” Salmon says.
Her focus is on characterizing the material that comes out of the destruction process and determining what it is best used for. For example, enzymes break down cellulose into glucose, which can be used as a raw material for biofuel production. They also leave behind tiny bits of cotton fiber that can provide lightweight reinforcement for concrete. “Even though the cotton fiber will no longer be long enough to be spun back into yarn, we believe there is value in this material,” says Salmon.
This way of thinking is very different from how old clothing and textiles such as upholstery fabrics and carpets are currently handled. According to the Ellen MacArthur Foundation, an organization in Cowes, UK that promotes the circular economy, only 13% of the material that goes into clothing is recycled globally. Most of the textile waste—an estimated 92 million tons from the fashion industry alone—produced each year is landfilled or incinerated. “We throw things in the dumpster and treat them like trash,” says Salmon. “We don’t see it as a raw material that can be reused.” The US Environmental Protection Agency estimated that in the United States in 2018, the average person threw away 47 kilograms of textiles. About three-quarters of them – 36 kg – are clothes and shoes, the rest – mainly towels, bed linen, furniture fabrics and carpets. Meanwhile, resources are used to create the starting material (see Thread Count) — water and land to grow more cotton, and oil to produce more polyester (see Polyester Recovery).
To counter all this waste, researchers and start-ups are developing methods to recover and reuse the material. Like Salmon, they mainly focus on chemical recycling, where material is broken down into building blocks and used to create new materials, including fibers that can be woven into new clothing. The challenges lie in developing processes for such treatment. They need to be practical, but they also need to be at least as cost-effective as simply making new fibers.
Spinning new threads
In addition to the natural cellulose fibers of cotton, some textiles include man-made cellulose fibers. These fibers are derived from wood pulp and can be used to produce materials such as viscose (rayon) and a similar material called lyocell. Cellulose fibers make up about 6% of all textile fibers produced, according to Textile Exchange in Lamesa, Texas, a nonprofit organization that promotes environmentally friendly materials.
Seattle, Washington-based startup Evrnu is applying a variation of the lyocell manufacturing process to the problem of textile waste. One major change the company has made to the process is that it uses discarded textiles instead of wood as a source of cellulose. It also revolutionized the process of producing a fiber that, according to the firm’s co-founder and president Christopher Stanev, is superior to both other types of cellulose and cotton, and can be recycled more times. “We can make fiber from cotton much stronger than fiber from wood pulp,” says Stanev, a textile engineer.
In the same way as with the standard lyocell process, the raw material is processed N– methylmorpholine N-oxide (NMMO), an organic compound that dissolves cellulose. The result is a thick pulp, which is then filtered. At the moment, the usual process would involve extruding the pulp through a device called a die – first into air and then into a coagulation bath, mostly water, in which the material solidifies into fiber. Evrnu, however, turns the cellulose molecules into liquid crystals before they are extruded, allowing them to align with each other and create a more crystalline fiber structure.
Recovery of polyester
Cellulose isn’t the only polymer researchers want to reuse—they also have polyester
Polyester is a general term for a number of petroleum-derived polymers, but it mainly refers to polyethylene terephthalate (PET). Worldwide, PET polyester makes up about half of all fiber in all textiles. Cotton makes up another quarter, and the rest is made up of other plant fibers like flax and hemp; animal products such as wool and alpaca; other synthetics, including acrylic and nylon; and man-made cellulose fibers.
Like cotton, PET polyester can be spun into new fibers, but the respun fibers become shorter and weaker with repeated cycles. However, unlike cotton, a polymer can be broken down into its simpler constituent molecules, and these monomers can be regenerated into new polymers. By starting with PET waste, says Sonya Salman, a polymer scientist at North Carolina State University in Raleigh, it’s possible to create what is essentially a pristine material — one that’s indistinguishable from PET made from petroleum. However, PET is very stable, so converting it to monomers is difficult.
Some scientists are developing enzymes that could fight these molecules. In 2016, a team discovered a bacterium that could break down PET (S. Yoshida and others. Science 351, 1196–1199; 2016), and scientists have since developed other enzymes to degrade it (J. Egan & S. Salmon SN Appl. of science 4, 22; 2022). Christopher Stanev, co-founder of Seattle, Washington-based Evrnu, says that in addition to its primary focus on breaking down cellulose, the startup is also working on processes for breaking down PET and polyurethane and separating polyester and cotton blends.
“By doing that and having a fairly crystalline organization, you can increase the strength and improve the performance of that fiber,” says Stanev. He says the fiber is about 20% stronger than standard lyocell, which is itself stronger than cotton.
This quality means a longer life for the fabric made from the fiber, as well as a fiber that can be regenerated multiple times. Each time the molecules go through the recycling process, they become shorter and thinner. But because they start out stronger, Stanev says, the same material must be regenerated at least five times before it becomes weaker than cotton fiber; some tests in the company’s laboratory show that the material can be recycled up to ten times. This is more than is possible for paper, which can be recycled 5-7 times before the fibers become too short to make a new viable product.
Evrnu is running a pilot project at partner companies in Germany and elsewhere in the United States to show that its process can produce fabric. He hopes a larger textile company will want to license the technology. For now, he uses NMMO because the compound is readily available, but Stanev hopes to eventually switch to an ionic liquid – a salt that is liquid at temperatures below 100 °C – which is more chemically stable than NMMO and more stable to pollution. The firm has not yet optimized such fluids for the production process.
The Finnish company, however, is working with an ionic liquid developed by one of its founders, physicist-chemist Herbert Sixto of Aalto University in Espoo, Finland. The liquid used by Ioncell—the name of both the company and the process—is a superbase, a strongly alkaline substance that breaks the hydrogen bonds in cellulose molecules. In the same way as using NMMO, this process creates a pulp that can be fed through a spinneret to produce new cellulosic fiber. NMMO is generally unstable and requires the addition of buffer solutions, but ionic liquid does not. Sixto says his ionic liquid is also fully recyclable, making the process environmentally friendly, while also producing fibers with better mechanical properties than cotton.
Ioncell’s process can use wood pulp, which Sixta says is part of the circular economy because the raw material comes from Finland’s sustainable forests—managed in a way that growth outpaces the amount of forest removed. “We have a great textile design group at our university, so we can process wood, make pulp, turn it into fiber, turn it into yarn, turn it into fabric, design clothes and show clothes in fashion shows,” Sixto says. The process can also accept textile waste, turning old clothes into new ones. Ioncell built a pilot plant with the goal of evaluating in about two years how well its process works in the real world.
A question of value
Despite the many technical challenges, the main obstacle to widespread textile recycling may be economic, says materials engineer Yujiang Wang of the Georgia Institute of Technology in Atlanta. “Most materials are not that valuable,” Wang says. Polyester, cotton and other fabrics are so cheap to produce that there is little profitability unless the recycling processes are very expensive.
There is also a lack of infrastructure to collect and sort used textiles, except for a few private clothing donation groups. And the complex mix of materials in a garment – not only the various natural and synthetic fibers, but also dyes and chemical coatings, buttons and zippers, and any non-woven additions such as leather or latex – must be separated into individual components to be processed.
Policymakers should consider recycling, which turns used clothing not into new clothing, but into other useful — albeit less valuable — products, Wang argues. Fibers can be ground up for use as soil stabilizers, for example, or cellulose is broken down into glucose, which can be turned into fuel. Even burning polyester for energy is preferable to pulling more oil out of the ground to generate energy. “It doesn’t sound very high-tech, but overall you get a lot out of it,” Wang says.
The circular economy should be seen as a way to reduce as much as possible the creation of the original material when other products can be reused, Wang says. “If you really want to make recycling better for the environment and not just for advertising, then we need to develop more technology so that you can use as much of what you collect as possible,” he says. “It will make the overall circle rounder.”
This article is a part Perspectives of Nature: Circular Economyan editor-independent application created with the financial support of Google. About this content.