Tuesday, February 13, 2007

Why Do Clothes Wrinkle? (by David Grosz, the Slate.com)

explainer: Answers to your questions about the news.
Why Do Clothes Wrinkle?The U.S. Army faces its most pressing question.
By David Grosz
Posted Monday, Feb. 12, 2007, at 6:02 PM ET

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. Every fabric has what is known as a glass transition temperature. Below this temperature, the material is in its "glass" phase, with a crystallike structure holding its fibers in place. Most fabrics are made of long chains of repeating molecules called polymers, held together by crosslinking bonds, like the rungs of a ladder. However, at temperatures above the glass transition threshold, the fabric enters a "plastic" phase in which the crosslinking bonds break. This allows the polymers to shift in relation to one another and form new crosslinks 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.

Some fabrics—like cotton, linen, and rayon—can also 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 crosslinks are 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 its 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 crosslinks between polymers 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 catalyst for the chemical reaction 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.

Got a question about today's news? Ask the Explainer.

Explainer thanks Janet Brady of Philadelphia University.

David Grosz is a writer living in New York.

Article URL: http://www.slate.com/id/2159363/

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