Semi-permanent hair dye washes off after several months. What mechanism(s) keep the dye in the hair? (I assume it's not primary chemical bonds, otherwise it would be permanent)

According to this: http://questions.sci-toys.com/node/72, hair dyes work by creating pockets within the hair. The colored dye molecules then get stuck in these pockets - requiring multiple washes to escape. Short term hair dyes use larger dye molecules so that they cannot get stuck in the hair. On the other hand, longer term dyes actually react with you hair to stay in there for an extended duration of time. For the most part, all of these hair-dye interactions are the result of intermolecular forces.

Semi permanent hair dyes tend to be small compounds that get stuck in your hair. This means that they are not permanently bound to your hair, but they will require a lot of washes to escape. I hope this helps!

Hair is built as a structured, layer design architecture. To keep the dye (semi)permament in the hair, the colour altering dye has to be installed below the top layer.

  • One route to achive this by using reactive dyes, that upon contact with the keratine (that exhibits thiol-groups, $\ce{R-SH}$) indeed to form chemical bonds with the dye, the later -- for example -- decorated with sulfonic acids ($\ce{R-SO3H}$) or its salts. See, for example, patent US3415606 (A)
  • A second route consists of using two solutions that contains fragments of the dye -- aiming that the dye molecule are created by these two (or more) components in the inner of the hair, but once created (within the hair) too large to escape the hair by simple washing.

These processes may assisted by alteration of the pH value: in basic media (i.e. high pH value), the inner of the hair becomes accessible as the "covering shingles" of the cuticula open. In slightly acidic medium (i.e. at low pH-value), the hair's cuticula closes.

Anyway, the dyes have to be applied again and again. Beside the natural reason (the continuously delivered hair), the dyes -- exposed to day light -- degrade and fade. Indeed, lightfastness is one of the parameters tested regularly for these dyes.

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