Why do clothes sometimes shrink in the wash? 

Clothes shrink during... drying 

In reality, the fibers that make up your sweater's fabric have loosened under hot water and then reorganized into a denser network. So, the garment did not shrink during washing but during drying! With subsequent washes, the shrinkage is less marked or absent.

Shrunk clothes during drying: hydrogen bonds to blame

Natural fibers, such as wool or cotton, are susceptible to the phenomenon due to attraction between molecules called hydrogen bonds. Indeed, these fibers are formed of long molecules:

• cellulose (a sugar) in the case of plant fibers from cotton;
• proteins in sheep's hair (analogous to our hair).

These molecules attach to each other at the time of weaving, in particular thanks to hydrogen bonds.

During washing, the presence of water and heat energy detach some of these bonds. The fibers are partially released and rub against each other. During drying, the water evaporates, and new hydrogen bonds are formed, tightening the network of fibers: they are finer and better attached to each other. The intensity of the shrinkage depends on the nature of the fibers and also on weaving techniques. 

In addition to natural fibers, synthetic fibers such as polyester and nylon are also susceptible to shrinking, but for a different reason. These fibers are made from polymers that are stretched and set into a particular shape during manufacturing. When exposed to high temperatures or heat, the polymer chains can relax and return to their original, unstretched shape, causing the garment to shrink.

How to avoid shrunk clothes?

The best way to avoid shrinkage is to follow the care instructions on the garment label, which often include specific temperature settings for washing and drying. It's also important to avoid overloading the washing machine, as this can prevent the garment from moving and rubbing against other items, which is necessary for removing dirt and stains.

If your garment has already shrunk, there may be ways to restore its original size. One method is to soak the garment in lukewarm water mixed with a hair conditioner, which can relax the fibers and restore their original shape. However, this method may not work for all fabrics, and it's best to test on a small, inconspicuous area first.

In conclusion, shrinking clothes during washing and drying is a common phenomenon that various factors, such as the nature of the fibers, weaving techniques, and temperature, can cause. By following the care instructions and taking proper precautions, you can minimize the risk of shrinkage and keep your favorite garments looking and feeling their best.