TLDR PEI diffuses into hair at a constant rate, and urea speeds up this process.
The study developed a novel method to analyze the diffusion behavior of poly(ethylene imine) (PEI) into bleached human hair using microspectrophotometry. The diffusion pattern of PEI, which was undetectable by optical microscopy, was successfully measured and showed Fickian type characteristics at pH 11.1, indicating that the diffusion coefficient was independent of PEI concentration. The diffusion coefficient for PEI with molecular weights of 300 and 600 was approximately 10^-10 cm^2/s. Additionally, the presence of urea doubled the diffusion coefficient for PEI with a molecular weight of 600, demonstrating that urea acts as a penetration accelerator for PEI.
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October 2003 in “Journal of applied polymer science” A new method using poly(ethylene imine) improves hair dyeing at lower temperatures with better color retention.
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