Why are some dyes so long-lasting?

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Why are some dyes so long-lasting?
« on: 29/11/2011 05:01:02 »
@nravinad asked the Naked Scientists:
   How do dyes stay so long, like henna on skin, or hair-dyes?
What do you think?
« Last Edit: 29/11/2011 05:01:02 by _system »


Offline bumblebee

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Why are some dyes so long-lasting?
« Reply #1 on: 11/12/2011 18:48:03 »
It works by staining the upper layers of the epidermis. The pigments of the dyes/henna seep through to the first layer of the skin.  Dyes/henna are recommended to be taken off only after at least 2 hours, the longer it is left it, the longer the dye/henna will last.

Don't get me wrong, but if you wash your hair every day , you are more likely to make the henna/hair dye fade faster, compared to another person who washes their hair once a week. (as an example)
« Last Edit: 11/12/2011 18:51:14 by bumblebee »


Offline Sprool

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Re: Why are some dyes so long-lasting?
« Reply #2 on: 13/01/2012 14:30:46 »
Permament hair dyes come in 2 parts - a base and an oxidative dye. Both parts are small enough to penetrate the hair follicle separately. When they come together they react into the coloured dye moilecule which is a much bigger size. The plan is to get the reaction to take place within the hair shaft once the 2 parts have penetrated the hair shaft. Then the dye molecule is too big to exit or wash out, it is locked within the shaft which is why it remains as hair colour much longer. You basically have to grow it out, though UV light does fade it somewhat after a few weeks.
Semi-permanent and temporary hair colours just coat the shaft of the hair without penetrating, so although they can stain, they soon get washed off.