Shrinkage is the process in which a fabric becomes smaller than its original size, usually through the process of laundry. Novice users of modern laundry machines sometimes experience accidental shrinkage of garments, especially when applying heat. Others may intentionally shrink a garment to their size. Some may purchase a garment one or more sizes larger in anticipation of shrinkage.
Cause: For wool garments, shrinkage is due to scales on the fibers which heat, water and agitation cause to stick together. Other fabrics are stretched by mechanical forces during production, and can shrink slightly when heated (though to a lesser degree than wool). Some clothes are “pre-shrunk” to avoid this problem.
Cotton and nylon and other common textiles used in clothing are all polymers. A polymer completely stretched into a linear arrangement has a much lower entropy than a polymer “wound up”. This is because there’s only one way for a chain to be fully extended linearly, while there are many, many possible configurations for it be to “crumpled up.”
However, if crystallization or crosslinking occurs to meaningful extent, this decrease in entropy can be offset and the polymer can be prevented from “collapsing”. Throughout normal use, the fabric is stretched out and perturbed, resulting in a configuration that doesn’t correspond to this highest entropy state.
All chemical systems try to minimize free energy. This equation gives the change in Gibbs free energy given enthalpy, temperature, and entropy. You can see that increased entropy *decreases* the free energy, while increased enthalpy (heat) *increases* the free energy. The drive to lower free energy therefore leads the system to seek the state of highest entropy. When the material is put in the dryer, the heat energy breaks many of the crosslinking bonds. When it slowly cools, it winds up into a higher entropy state, associated with shorter chain lengths, and tighter clothes.
If it was the heat from the dryer, why wouldn’t they shrink when you iron them?
They shrink when you wash them in warm water.
Natural fibers like cotton are only shrinking because they are returning to their natural size. During the fabric-making process, fibers are spun and drawn in order to make thread, which is then used to produce your clothes. When that thread is created, tension is applied which extends the fibers beyond their natural shape. Then when they are submerged in warm water and then agitated in a washing machine, the fibers try to return to their original shape, which causes shrinkage of your clothes. The reason that more expensive “pre-shrunk” cotton does not shrink in the dryer is because either the thread is treated to return the fibers their natural length before use or the clothes are pre-washed.
Shirts shorten upwards with subsequent washes because there is no vertical tension on the threads when you wear them.
Some more thought…
It is the enthalpy that drives the shrinkage.
But, the reason why the clothes can shrinkage after washed rather than heated is that the water plays the role of plasticizer. Small molecular could enter the clothes and reduce the friction between molecular chains.
So, after the water entered, the molecular chains get the ability to move. Thus the clothes shrink.