What were some important discoveries in cosmetic science?

For an assignment I am writing about certain scientific discoveries / innovations and the methods that were used to get there. I am looking to write about something related to hair (maybe the structure or properties). 
What are some of the most important scientific discoveries that are still useful to cosmetic chemists today? I'm looking for something backed up with the original articles showing the methods and steps taken to get there. I was looking into the discovery of ppd and how it led to the creation of L'Oreal's first synthetic hair dye, but I couldn't find any methods or journal articles. 
Any help would be appreciated!


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