The chemistry difference between a thio relaxer (ammonium thioglycolate-based) and a hydroxide relaxer (sodium hydroxide or 'lye') is best described as:
Explanation
Thio relaxers (ammonium or sodium thioglycolate, pH around 9 to 10) break disulfide bonds reductively by donating hydrogen; the bonds can be rebuilt in a new position by an oxidizing neutralizer, the same as in a cold perm. Hydroxide relaxers (sodium, lithium, potassium, or guanidine hydroxide at pH 12 to 14) act by lanthionization: the disulfide bond loses a sulfur atom and becomes a single lanthionine bond that is permanent and irreversible, so an acid-balancing neutralizing shampoo (not peroxide) is used afterward to lower pH. Knowing the difference is critical: NEVER apply a thio over a hydroxide-relaxed head or vice versa, because the chemistries are incompatible and cause breakage. Cal/OSHA §5194 SDS hazard disclosure covers both classes.
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