The New York Times reports that by May 2008, the U.S. Army will have completely phased out its old Battle Dress Uniform in favor of the newer Army Combat Uniform. The ACU includes such technical enhancements as infrared shoulder flags for nighttime identification and a new digital-pixel camouflage. But the innovation most likely to change soldiers' daily lives is that the new uniform, unlike its predecessor, is wrinkle-free. What causes clothes to wrinkle, anyway?
Heat and water. Some fabrics *—like wool, nylon, and polyester—have what is known as a glass transition temperature. Below this temperature, the material is in its "glass" phase, with a crystal-like structure holding its individual polymers, long chains of repeating molecules, in place within the fiber. At temperatures above the glass transition threshold, some of the bonds holding these molecules together break. This allows the polymers to shift in relation to one another and form new bonds as they cool down. When the fabric returns to the glass phase—after it's been taken out of the dryer, for example—the altered structure gets locked in place in the form of wrinkles.
Another mechanism is at work with cellulose-based fabrics—like cotton, linen, and rayon—which can wrinkle if you throw your laundry into a cold wash, enter a wet T-shirt contest, or simply begin to sweat. That's because these materials are highly absorbent and their polymers are linked by hydrogen bonds—the same bonds that hold together molecules of water. Add moisture to a cotton T-shirt and H2O will penetrate the regions between the long stringy polymers, bringing the fabric into a condition that resembles another material's plastic phase. As the water evaporates, new hydrogen bonds lock in place any creases that formed when the shirt was wet.
In the 1950s, a researcher with the Department of Agriculture named Ruth Rogan Benerito found a way to make a wrinkle-free fabric, in which the hydrogen bonds between polymers were replaced with stronger crosslinking bonds that were water-resistant. Early "permanent press" garments, however, were plagued with problems. The treatment weakened the fabrics by eliminating some natural elasticity. More alarmingly, the crosslinking agent that made the bonds waterproof was formaldehyde, which would sometimes leave clothes itchy and smelly.
Chemists developed an improved treatment in 1992 that eliminated almost all of the formaldehyde from the surface of a garment. Using this technology, Haggar initiated the modern generation of wash-and-wear clothing with its wrinkle-free all-cotton pants.
Modern wrinkle-free fabrics still pose issues of durability, which is why they are often combined with sturdy, flexible synthetic materials like nylon. The ACU, for instance, is a nylon-cotton blend. Another potential downside is cost, though this can cut two ways. The ACU goes for about $30 more than the old uniform. But as the Army points out, the wrinkle-free fabric will save soldiers money at the dry cleaners.
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Explainer thanks Janet Brady and Nancy Howard of Philadelphia University, and Brent Smith of North Carolina State University.
Correction, Feb. 26:The original version of this piece contained several misleading or inaccurate statements. First, it said that all fabrics (including cotton) have a "glass" phase and a "plastic" phase," and suggested that "crosslinking" bonds break at high temperatures. Not all fabrics exhibit this behavior; in those that do, the broken bonds are not called "crosslinks." Wrinkles in cellulose-based materials are caused by moisture, not heat. The original version also described formaldehyde as the catalyst for a reaction that makes bonds between cellulose fibers water-resistant. In fact, it works by replacing hydrogen bonds with stronger crosslinking bonds. (Return to the piece.)