A perm and relaxer are different in that perms usually add curl or wave, while relaxers straighten out the waves or curls already present. They also are separated by the chemicals typically used and application, although the line between relaxer and perm types sometimes blurs. The difference in chemicals makes a perm and relaxer distinct based on pH levels and results, as well.
In terms of mechanical manipulation of the hair, a perm and relaxer are very similar. Both work by lifting up the cuticle or outermost layer of the hair strand and breaking the disulfide bonds deeper within the hair that help provide shape. At this point, the reshaping of the hair depends on the position a cosmetologist puts the hair. For a perm, the cosmetologist wraps the hair around rods or rollers. The cosmetologist puts the hair straight with a relaxer, typically through combing, guiding the hair with the relaxer applicator brush or running the product through the hair with her fingers.
Once a perm solution is on the hair for the recommended amount of time, the cosmetologist must apply a neutralizing solution, which typically contains hydrogen peroxide. The neutralizing solution helps most, but not all, of the broken disulfide bonds reform, causing the hair to reshape based on the structure of the rod or roller. With a relaxer, although a "neutralizing" shampoo is used to restore proper pH to the hair, the neutralizing shampoo does not reform the broken disulfide bonds. This means that a relaxer usually creates weaker hair compared to a perm, although this depends on the exact pH and the condition and type of the hair when starting the chemical process.
Another difference between a perm and a relaxer is chemical makeup. A traditional "cold" perm uses ammonium thioglycolate, sometimes called thio or theo. Some perms that are designed to be gentler use glyceryl monothioglycolate instead; these are called acid or acid-balanced perms, depending on the pH. Traditional relaxers use sodium hydroxide (lye), while no-lye relaxers contain other hydroxides such as calcium hydroxide, lithium hydroxide or potassium hydroxide. Some relaxers use ammonium thioglycolate similar to cold perms, simply omitting the rods and rollers to get a straight effect, so technically, there can be overlap between perm and relaxer products.
Application between a perm and relaxer varies, as well. Perm solutions are liquid in nature, and cosmetologists squirt the product onto the hair from a bottle in most cases. By comparison, a relaxer is usually a paste, which the cosmetologist mixes just prior to starting the application. The cosmetologist brushes the paste into the client's hair.
In terms of pH, relaxers generally are much more alkaline than perms. A regular lye relaxer has a pH between 12 and 14, while a no-lye relaxer has a pH of 9 to 11. The pH of a traditional or alkaline cold perm is 9.0 and 9.6, which is about the same as the pH for a thio relaxer. Acid-balanced perms weigh in with a pH between 7.8 and 8.2, which technically is still basic. Perms in the acid category have a pH of 4.5 and 7.0.
The pH of a perm or relaxer is important for three reasons. First, the higher the pH, the more damage occurs to the hair and the shorter time usually is necessary for the hair to process, forcing the cosmetologist to work very quickly for even results. Secondly, the higher the pH, the greater the chances are of the product irritating or burning the scalp. Lastly, products with lower pH levels are not suitable for resistant or hard-to-process hair and provide gentler, looser results. Those with higher pH levels are not good for previously-processed, dry, very fine or damaged hair and provide tighter results.