I commonly see clothes made of a 95% cotton, 5% elastane blend, and it’s pretty stretchy. How does that work?
Like, if I make a rope out of 19 cotton strings and one rubber band, that rope is not going to be any stretchier than if it was all cotton. Once the cotton strands are pulled taut, it won’t stretch any further. Adding some stretchy strands alongside the non-stretchy ones doesn’t change that. Even if it was only one cotton strand, that’d still be the limiting factor in its stretchiness, and it wouldn’t stretch beyond that unless the cotton strand broke.
So what’s the trick to make fabric elastic with only a few elastic fibers?
It’s blended into the cotton fibers, so evenly dispersed which helps. For every x cotton fibers there is y elastic fibers.
The natural fibers also have stretch and crimp in them to assist with moving in conjunction with the elastic fibers.
The construction method of the base fabric can help as well. For example, a knit shirt designed to stretch, will be knitted on a machine in a way that encourages movement. This could mean wider loops to allow for porosity and drape. If the fabric is woven, both the warp and weft yarns will include a blended yarn instead of just the weft.