Eli5: Why does laundry shrink when dried too hot?

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Why does laundry shrink when put in the dryer? And why are some Materials more prone to shrinking than others?

In: Chemistry

Molecules in clothes react differently to heat, so when something is high in a specific material, like polyester or cotton, it’s prone to shrinking.

Clothes are made from thousands of fibers – usually taken from animal hair or plants. The fibers are harvested by sheeting sheep (wool) or picked from the fiberous flowers of the cotton plant.

Now, those curly ringlets of raw material have to be combed out and straightened so that people can weave them into cloth. All those little strands are pulled and combed just like your hair, but with special brushes in a factory. All that combing and straightening causes the natural curl to get straightened.

Once your clothes go loosely into a dryer, and not stretched on the special combs, they return to that curly state and ‘shrink’. The clothes weigh the same, but you can’t fit in them any more because the fibers are curly and locked up in the weave of the cloth.

So why does the fiber get stuck curly or straight? The fiber is just a long group of proteins, the building blocks of our bodies. These little tiny chain-links lock together end to end to end. Some of the proteins have an angle built in, they kink just a bit. So after millions and millions of links, that protein chain isn’t a straight line. It curves and curls.

Adding heat can straighten the proteins, and that makes it easier for the clothing manufacturer to work with. Imagine ironing a shirt to get the wrinkles out. This is the at-home version of straightening those protein chains. Throwing a delicate material just loose in the dryer resets their work, so they put a label on saying “do not dry” or “may shrink”.

Now plastics, like polyester or nylon, also work in a similar way. The synthetic materials are man made out of carbon chains, just like a protein, but made in a chemical plant. These materials are much less likely to shrink, but it is still possible in some cases. The carbon chains looks like this: C-C-C-C-C- and on and on. Sometimes the C- chain gets stuck to itself, especially when hot, and can be forced to curl up. If you have ever had a long piece of string or hair get something sticky in it, and it starts to wad up as it sticks together, this is close to what happens in synthetic clothes. Most of the time these man-made fibers will melt or burn before they shrink, so don’t get them too hot!