Why cotton shrinks

Those long chains of molecules

To: Matmail: Once again we mere mortals approach the sacred oracle at “Straight from the Hip” with a question that has no real purpose except our great thirst for your knowledge. We have a question posed by a six-year-old that a whole office full of attorneys and such cannot seem to answer. Why does cotton shrink? It seems that I remember most items expand when heated, so why does cotton shrink when you heat it? The best any of us could come up with was it having something to do with loss of moisture in the fiber. Are we close, or are our minds shrinking faster than denim in a hot dryer? — [email protected]

This six-year-old — a client? Another one of those danged kindergarten kissing bandits? Well, anyway, the little felon asked a pretty good question. Cotton shrinks because of the pummeling it takes before it’s made into a T-shirt or jockeys or whatever. When it finally hits wash water or a hot dryer, it sort of craps out.

Cotton fibers are made of long chains of molecules, with the chains linked end-to-end by weak hydrogen bonds — like gluing rebar with chewing gum. When the fiber is spun into thread, then put on a loom, the molecules and hydrogen bonds are twisted and stretched until the old bonds pop. New hydrogen bonds form to keep the stressed-out cotton linked up. When you finally launder those tense Ts and distraught denims, the chemical action of water and heat break the hydrogen bonds, and the molecule chains shrink back to their original, untwisted, unstretched, preloomed, prespun state.

Shrink-resistant fabrics are doused with chemicals that make the molecular chains sturdier; some special milling techniques put less strain on the fibers and shrink less when the garment’s washed. The newest unshrinkable cotton is actually a heat-treated cotton fiber with a hidden polyester core. Say, is this some sneaky way of getting us to wear polyester again — disguise it as cotton, then before we know it, haul out those shiny stretch pants and disco shirts? I wouldn’t put it past those fashion fascists.

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