So my wife’s ironing a shirt and skirt for a formal occasion we’re attending tomorrow, and, quite naturally, the subject of wrinkles comes up. We discuss for a while, but come up with more questions than answers. I offer up, to those of you who are natural fiber experts, the following:
Why do fabrics wrinkle? What is happening at the fiber level?
Why does ironing unwrinkle fabrics? Again, what is happening at the fiber level?
Why do some fabrics require higher heat to iron out wrinkles? (Specifically, my wife remarked upon the fact that her linen skirt was nearly impossible to iron on the “cotton” setting; it required the higher heat “linen” setting.)
Natural fibers are mostly cellulose, which tend to expand when they absorb moisture. (Each thread contains many cellulose chains side-by-side). With cellulose fibers expanding at different rates, the relatively weak hydrogen-hydrogen bonds that hold two neighboring cellulose fibers together break and reform at different points, which causes wrinkling. Linen wrinkles worse than cotton because it absorbs water more readily. Fabric made with thick thread wrinkles worse than fabric made with fine thread, because the thick thread has more cellulose chains in it. Natural fibers wrinkle worse than polyester because polyester absorbs very little moisture.
Ironing with steam is helpful because the hydrogen in the cellulose will bond with the water from the steam rather than with the hydrogen of neighboring cellulose chains, breaking the bonds long enough to allow the fabric to be ironed flat. As the fabric dries, the hydrogen-hydrogen bonds reform at the proper places.
Many synthetic fibers, like polyester and nylon, absorb much less water and so wrinkling is less of a problem.