In the USA, an estimated 15 million tons of textiles end up in landfills or are burned every year, amounting to 85% of the textiles produced in a year. Sorting by hand is labor intensive, made harder by worn-out or missing labels. More advanced techniques that analyze a fabric’s chemistry often are not precise enough to identify materials in fabric blends, which make up most clothing.
To improve this sorting process, a team from the MIT Lincoln Laboratory and the University of Michigan, Ann Arbor, MI/USA, have found a new way to label fabrics: by weaving fibers with engineered reflectivity into them. These fibers are only reflective under certain infrared light. Depending on the wavelengths of light that the fibers reflect when scanned, recyclers would know which type of fabric is represented. This means that the fiber works like an optical barcode to identify a product.
A structural-color fiber, a type of photonic fiber first developed at the Massachusetts Institute of Technology (MIT), Cambridge, MA/USA, more than 20 years ago, acts like a mirror. By layering certain materials, this mirror can be designed to reflect specific wavelengths. In this case, reflections are wanted at wavelengths that stand out from the optical signatures of the other materials in the fabric, which tend to be dark because common fabric materials absorb infrared radiation.
The fiber starts out as a block of polymer called a preform. The team constructed the preform to contain more than 50 alternating layers of acrylic and polycarbonate. The preform is then heated and pulled like toffee from the top of a tower. Each layer ends up being less than a micron (µm) thick, and in combination produce a fiber that is the same size as a conventional yarn in fabric.
While each individual layer is clear, the pairing of the 2 materials reflects and absorbs light to create an optical effect that can look like color. It is the same effect that gives butterfly wings their rich, shimmering colors.
By controlling the speed at which the fibers are drawn, researchers can “tune” them to reflect and absorb specific, periodic ranges of wavelengths — creating a unique optical barcode in each fiber. This barcode can then be assigned to corresponding fabric types, one symbolizing cotton, for example, and another polyester. The fibers would be woven into fabrics when the fabrics are manufactured, before being put to use in a garment and eventually recycled.
A detector could be adapted from the kind used to sort plastics in the recycling industry, the researchers say. Those detectors similarly use infrared sensing to identify the unique optical signatures of different polymers.
The fibers produced in this study are still slightly thick relative to clothing fibers, so thinning them more while retaining their reflectivity at the desired wavelengths is a continued area of research.
Another avenue to explore is making the fibers more akin to sewing thread. This way, they could be sewn into a garment in cases when weaving them into a certain fabric type could affect its look or feel.
The researchers are also thinking about how structural-color fibers could help tackle other environmental problems in the textile industry, like toxic waste from dyes. For example, such fibers could be used to make fabrics that are inherently imbued with color that never fades.
In 2022, Massachusetts became the first state in the USA to enact a law banning the disposal of textiles in the trash, aiming to up recycling percentages.