Clothes made from natural fibers such as cotton, unlike clothes made from artificial fibers such as polyester often shrink when washed at high temperatures  The reason for this shrinkage is that natural fibers are tightly curled in their original state  Since the manufacturer of cloth requires straight fibers, natural fibers are artificially straightened prior to being made into cloth
High temperatures cause all fibers in cloth to return to their original states
High temperatures have no effect on the straightness of fibers in clothes made from a blend of natural and artificial fibers.