Polyester is an oil based product and when woven into yarn it has a far greater degree of elasticity than cotton yarn. This helps to prevent creasing after washing. The polyester yarn helps hold the bed linen in shape and doesn't shrink. The problem is, it can be quite uncomfortable to sleep under and hot in the summer. The truth is, if a high quality cotton yarn is used and the finish is done properly, 100% Cotton bed linen shouldn't crease that badly and any creases should fall out after a few hours use.