It’s happened to the best of us: We mindlessly throw a new cotton shirt into the dryer and voilà – now we have a shirt that fits a toddler.
Cotton is susceptible to these types of washing defects, which is not the case with synthetic fibers such as polyester. Much of this vulnerability is due to the individual fibers of cotton clothing. Jillian Goldfarbassociate professor of chemical and biomolecular engineering at Cornell University, told Live Science in an email.
“Cotton fabric is made by weaving together fibers from a cotton plant, which themselves are mostly made of cellulose, a natural biopolymer,” she said. “Cotton … is prone to shrinkage because its fibers swell when they get wet and then contract when they dry.”
If you’ve ever sweated in cotton clothing, you know firsthand how well it absorbs moisture. Synthetic fabrics – like polyester, nylon and spandex – are more sweat- and shrink-resistant because their tightly woven fibers don’t swell in water.
At a chemical level, weaving cotton fibers for clothing creates tension that creates a hydrogen bond network, says Erika Milczek, a chemist and CEO of a biotechnology company CurieCosaid Live Science.
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When variables like heat and water are introduced, this hydrogen bonding network can change, causing the fabric to either relax or contract. This is also the scientific reason for wrinkles in your clothes, Milczek said.
The Science of Shrinking
When it comes to accidentally shrinking your cotton clothes, not all garments are the same, Goldfarb said.
“Even when made of the same material, some cotton fabrics tend to shrink more than others, depending on how the fibers are assembled to form a fabric,” she said. “While woven cotton certainly shrinks, it shrinks significantly less than knitted cotton.”
Think of the crossing of woven cotton fibers like a hashtag, where some fibers are woven under others, Goldfarb said. Horizontally woven yarn is called the “warp,” vertically woven yarn is called the “weft.”
“When the yarns swell when they’re wet, they push the weft threads closer together and shrink in one direction,” Goldfarb explains. “When the moisture is removed from the fabric, the fibers contract.” This means that shrinkage actually begins before the clothing even goes into the dryer. The shrinkage is the dual result of water-soaked fibers and high heat.
Exactly how much your clothes shrink depends on a number of factors, says Milczek. For example, it depends on whether you wash your clothes with just water or add detergent—detergent destroys the hydrogen bonds even further—and whether you dry your clothes at a high or low temperature or hang them to dry.
“The temperature [when line drying] is significantly lower, so evaporation is much slower and the fibers aren’t ‘stressed’ by the heat as they shrink,” Goldfarb explained. A line-dried shirt also experiences more consistent humidity between outside and in the closet, which can lead to less shrinkage, she said.
Save a shrunken shirt
For some, this realization may come a little too late. But don’t worry, there may still be hope for your shrunken clothes.
One obvious solution, says Milczek, is to look for clothes that are shrink-proof from the start. These include cotton clothes with synthetic blends or pre-washed cotton clothes.
If that doesn’t help, you can try a scientifically based method to stop your clothes from shrinking.
“Depending on the quality of the yarn and the weave … it is possible to restore the shrinkage of some cotton fabrics, at least temporarily, if we allow the fibers to swell and let them dry under tension,” Goldfarb said.
One way to do this at home is to use a steam iron, she said. This reintroduces moisture into the garment to expand the fibers, while also applying mechanical force to stretch them back out. But proceed with caution — this method can also easily swing too far in the opposite direction.
“Of course, you can easily ‘overstretch’ the cotton this way, and if you do it unevenly, the result can be a pretty distorted garment,” Goldfarb said.