The Effect Of Hair Dye On Hair Strength
Prince of Wales Secondary
Floor Location : J 229 H
In this project, I measured and compared the tensile strength of un-dyed hair to hair dyed with different types of dyes. This project is important because 75% of women dye their hair. My hypothesis was that ammonia-based dyes weaken hair more than ammonia-free dyes. After dying hair samples with six types of dye (two ammonia-based dyes, two ammonia-free dyes, henna and bleach), I initially attempted to count the number of breaks in each hair visible through a microscope. However, no breaks were visible so I changed my testing methodology and calculated the tensile strength of hair dyed with the different dye types instead. To calculate tensile strength, I first measured the breakage weight of multiple hair samples from each dye type and then divided that by the cross-sectional area of the hair. The cross-sectional area was measured using a technique involving a laser pointer and diffraction. I found that, on average, ammonia-based dyes cause a 35.5% reduction in tensile strength, bleach causes a 31% reduction in tensile strength, ammonia-free dyes cause a 17% reduction in tensile strength and henna causes a 16% reduction in tensile strength. These results are explained through a discussion of how the chemicals in each dye interact with the cuticle and cortex layers of hair at a molecular level.