Final answer:
In chemical permanent waving, a longer processing time is necessary for thick hair, because it has a greater number of disulfide bonds and a higher density of keratin strands that require more time for chemicals to penetrate and reform the hair's structure.
Step-by-step explanation:
In chemical permanent waving, a longer processing time is required for thick hair. The chemical permanent waving process involves using a reducing substance, usually ammonium thioglycolate, to break and reform the disulfide bonds within the hair's keratin structure. This process allows for the hair to take on a new shape, such as curls or waves.
Thick hair has more disulfide bonds due to its larger volume and greater density of keratin strands. Consequently, it requires more time for the solution to penetrate and effectively break the disulfide bonds. This is in contrast to straight, curly, or wavy hair types, which may not always be as dense.
During a perm, the hair is typically wrapped around forms to produce the desired curl or wave. The initial reduction of disulfide bonds allows the alpha-helices in the hair strands to slip past each other. After shaping, an oxidizing agent is used to reform the disulfide bonds, securing the new shape in place, which can last for several months.