Chemical texture services (perms and relaxers) permanently change the hair by:

Chemical texture services such as permanent waves and chemical relaxers create a permanent (chemical) change by breaking the hair's internal disulfide bonds, restructuring the hair, and then reforming those bonds in a new configuration — curl for a perm, or straightened for a relaxer. Because these are chemical changes within the cortex, they are long-lasting and only grow out or are cut off. Their strength and permanence are exactly why careful analysis, strand testing, and following directions are critical. This is core chemical-service theory.