As a lover of soft fabrics, I am curious why even 100% cotton feels stiff or crunchy when hung to dry. Some fabrics are more susceptible to this, others are fine.
In: Chemistry
To cut at the underlying cause, the reason a fabric can become stiff (and incidentally also the reason it can become wrinkled and why you you wake up with stubborn bedhead in the morning) is down to hydrogen bonds.
Natural fibers of cellulose (cotton, linen) contain hydrogen atoms and can weakly polymerize with their neighbors. Polymerize means the molecules link up with each other. Polymerization is the basis for things like plastics as well as oils that “dry” like when you season a skillet or find it’s gummed up inside the oven.
In the case of fabrics, the polymerizing is reversible and based on weak hydrogen bonds that can be broken either through mechanical action or moisture.
Through the action of wearing a garment or tumbling it (in a dryer), the fabric will become less stiff as these bonds are broken.
Moisture works because water strongly likes to make hydrogen bonds of its own, so it swells the fabric, breaking existing bonds and replacing them with its own. Water’s affinity to make these bonds with the fabric and itself can make drying take a long time, which is why “cotton is rotten” when hiking in the backcountry.
Anyway, this water action is how steaming and ironing work to relax wrinkles, by destroying these bonds and allowing new ones to form in the desired shape. As an aside, this is why high humidity can thwart hair styling and make your do frizz.
Fabric softeners work in part because they inhibit this polymerization.
So you can see why tumble drying makes fabrics soft, as it both tumbles and steams.
Permanent press fabrics attempt to replace the weak hydrogen bonds with something that similarly polymerizes the fabric but is more stable and durable. For a while, the agent for this was formaldehyde, which is toxic.
Synthetic fabrics don’t do any of this, so they don’t get stiff regardless.
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