Both treatments use chemicals to give you straighter hair. However, hair relaxing doesn’t completely straighten your hair. The process typically involves an alkaline chemical like sodium hydroxide (lye) to break down the bonds in your hair. As a result, your curls and kinks get softened, leaving you with straighter-looking hair.On the other hand, hair rebonding breaks down the bonds of your hair and resets them using a flat iron to make your hair straight. Hair rebonding is also called Japanese straightening or thermal reconditioning. The process takes longer than hair relaxing (and is also usually more expensive), but the result is completely straight hair once the treatment is done.